Manual de Seguridad de DebianJavier Fernández-Sanguino Peña <jfs@debian.org>

Manual de Seguridad de Debianpor Javier Fernández-Sanguino Peña
ResumenEste documento trata sobre la seguridad dentro del proyecto y del sistema operativo Debian. Comienza con el procesode protección y fortalecimiento de la instalación de la distribución predeterminada de Debian GNU/Linux. También
cubre algunas de las tareas comunes para configurar un entorno de red seguro utilizando Debian GNU/Linux, dainformación adicional sobre las herramientas de seguridad disponibles y habla sobre cómo el equipo de seguridad y
auditoría hace valer la seguridad en Debian.
Copyright © 2012 The Debian Project
GNU General Public License Notice:This work is free documentation: you can redistribute it and/or modify it under the terms of the GNU
General Public License as published by the Free Software Foundation, either version 2 of the License, or (at your option) any later version.
This work is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.

Tabla de contenidos
1. Introducción ................................................................................................................... 1
Authors ..................................................................................................................... 1
Lugares donde encontrar este manual (diversos formatos) .................................................. 2
Notas/Retroalimentación/Organización ............................................................................ 2
Conocimiento previo ................................................................................................... 2
Things that need to be written (FIXME/TODO) ............................................................... 3
Credits and thanks! ..................................................................................................... 5
2. Antes de empezar ............................................................................................................ 7¿Para qué quiere su sistema? ........................................................................................ 7
Sea consciente de los problemas de seguridad general ....................................................... 7¿Cómo maneja Debian la seguridad? .............................................................................. 9
3. Antes y durante la instalación .......................................................................................... 10
Elegir una contraseña para el BIOS .............................................................................. 10
Partitioning the system ............................................................................................... 10
Elegir un esquema de particiones inteligente .......................................................... 10
Selecting the appropriate file systems ................................................................... 11
No se conecte a Internet hasta que no esté preparado ....................................................... 11
Establecer una contraseña para «root» .......................................................................... 12
Ejecute el mínimo número de servicios requeridos .......................................................... 12
Deshabilitar servicios ......................................................................................... 13
Disabling inetd or its services ............................................................................. 14
Install the minimum amount of software required ........................................................... 14
Removing Perl .................................................................................................. 16
Lea las listas de correo de seguridad de Debian .............................................................. 17
4. Después de la instalación ................................................................................................ 18
Subscribe to the Debian Security Announce mailing list ................................................... 18
Ejecute una actualización de seguridad ......................................................................... 18
Security update of libraries ................................................................................. 19
Security update of the kernel ............................................................................... 19
Change the BIOS (again) ........................................................................................... 21
Colocar una contraseña a lilo o grub ............................................................................ 21
Disable root prompt on the initramfs ............................................................................ 22
Eliminar el prompt de root del núcleo .......................................................................... 22
Restricción del acceso a la consola .............................................................................. 23
Restricting system reboots through the console ............................................................... 23
Restricting the use of the Magic SysRq key ................................................................... 24
Montando particiones de manera correcta ...................................................................... 25
Setting /tmp noexec ......................................................................................... 26
Serie /usr leer-únicamente ................................................................................... 26
Acceso seguro para los usuarios .................................................................................. 26
Uso de la autentificación: PAM ........................................................................... 26
Password security in PAM .................................................................................. 27
Control de acceso de usuarios con PAM ............................................................... 28
Limites para los usuarios mediante PAM .............................................................. 28
Control de 'su' mediante PAM ............................................................................. 29
Carpetas temporales en PAM .............................................................................. 29
Configuración de aplicaciones genéricas de PAM ................................................... 29
Limitación de recursos: archivo limits.conf .................................................... 30
User login actions: edit /etc/login.defs ........................................................ 31
User login actions: edit /etc/pam.d/login ...................................................... 32
Restricting ftp: editing /etc/ftpusers ............................................................ 33iii

Manual de Seguridad de DebianUso de su ........................................................................................................ 33
Uso de sudo ..................................................................................................... 33
Prohibición administración remota ....................................................................... 33
Restringiendo usuarios ....................................................................................... 34
Auditoría de usuarios ......................................................................................... 34
Repasando los perfiles del usuario ....................................................................... 36
Setting users umasks .......................................................................................... 36
Limitar el acceso a los usuarios ........................................................................... 37
Generación de contraseñas de usuario ................................................................... 38
Comprobación de contraseñas de usuarios ............................................................. 38
Sacando usuarios inactivos ................................................................................. 39
Uso de tcpwrappers ................................................................................................... 39
La importancia de logs y alarmas ................................................................................ 40
Uso y personalización de logcheck ...................................................................... 41
Configurando el sitio donde las alertas son enviadas ................................................ 41
Usar un servidor de registro ................................................................................ 42
Permisos para el archivo de registro ..................................................................... 43
Añadiendo parches al kernel ....................................................................................... 43
Protección contra desbordamiento de búfer .................................................................... 44
Parches contra el desbordamiento de búfer para el núcleo ......................................... 45
Testing programs for overflows ........................................................................... 45
Transferencia segura de archivos ................................................................................. 45
Límites y control de los sistemas de archivos ................................................................. 46
Uso de Quotas .................................................................................................. 46
The ext2 filesystem specific attributes (chattr/lsattr) .............................................. 47
Integridad de su sistema de archivos ..................................................................... 48
Configuración de revisión de setuid ...................................................................... 48
Conectividad de red segura ......................................................................................... 49
Características de la red configurando kernel ......................................................... 49
Configuring syncookies ...................................................................................... 49
Securing the network on boot-time ....................................................................... 50
Configuarción de las características de los cortafuegos ............................................. 53
Disabling weak-end hosts issues .......................................................................... 53
Protección contra ataques ARP ............................................................................ 54
Taking a snapshot of the system .................................................................................. 55
Otras recomendaciones ............................................................................................... 56
No use software que dependa de svgalib ............................................................... 56
5. Asegurando los servicios que se ejecutan en su sistema ........................................................ 57
Asegurando ssh ......................................................................................................... 57
Chrooting ssh ................................................................................................... 58
Ssh clients ....................................................................................................... 59
Disallowing file transfers .................................................................................... 59
Restricing access to file transfer only ................................................................... 59
Asegurando Squid ..................................................................................................... 59
Asegurando FTP ....................................................................................................... 61
Asegurando el acceso al sistema X Window .................................................................. 61
Revisar su administrador visual ........................................................................... 62
Seguridad en el acceso de impresión (El asunto de lpd y lprng) ......................................... 63
Reforzando el servidor de correo ................................................................................. 64
Configuring a Nullmailer .................................................................................... 64
Providing secure access to mailboxes ................................................................... 65
Recibiendo Correo de forma segura ...................................................................... 65
Asegurando BIND ..................................................................................................... 66
Bind configuration to avoid misuse ...................................................................... 66iv

Manual de Seguridad de Debian

Changing BIND's user ....................................................................................... 68
Chrooting the name server .................................................................................. 70
Asegurando Apache ................................................................................................... 71
Disabling users from publishing web contents ........................................................ 72
Permisos para el archivo de registro ..................................................................... 72
Published web files ........................................................................................... 72
Asegurando finger ..................................................................................................... 73
Cambio general de directorio raíz y paranoia suid ........................................................... 73
Making chrooted environments automatically ......................................................... 74
Texto claro general con el password paranoia ................................................................ 74
Incapacitar NIS ......................................................................................................... 74
Securing RPC services ............................................................................................... 75
Desactivar los servicios RPC ............................................................................... 75
Limites al acceso de servicios RPC ...................................................................... 75
Añadir capacidades al cortafuegos ................................................................................ 76
El sistema local corta fuegos ............................................................................... 76
Usar otros corta fuegos para proteger otros sistemas ................................................ 76
Configurando un cortafuegos ............................................................................... 77
6. Fortalecimiento automático de sistemas Debian .................................................................. 84
Harden .................................................................................................................... 84
Bastille Linux ........................................................................................................... 85
7. Infrastructura de seguridad de Debian ............................................................................... 86
Equipo de seguridad .................................................................................................. 86
Avisos de seguridad de Debian .................................................................................... 86
Referencias cruzadas de vulnerabilidades .............................................................. 87
CVE compatibility ............................................................................................. 87
Security Tracker ........................................................................................................ 88
Debian Security Build Infrastructure ............................................................................. 88
Developer's guide to security updates ................................................................... 89
Firma de paquete en Debian ....................................................................................... 89
El esquema propuesto para revisiones de firma de paquete ....................................... 89
Secure apt ........................................................................................................ 90
Per distribution release check .............................................................................. 90
Release check of non Debian sources .................................................................. 100
Alternativa firmar esquema por paquete .............................................................. 101
8. Herramientas de seguridad en Debian .............................................................................. 102
Herramientas de evaluación de vulnerabilidades remotas. ............................................... 102
Herramientas de escáner de red .................................................................................. 102
Auditorías internas ................................................................................................... 103
Auditorías de código fuente ...................................................................................... 103
Redes virtuales privadas ........................................................................................... 104
Point to Point tunneling .................................................................................... 104
Infraestructura de clave pública (ICP). ........................................................................ 105
SSL Infrastructure ................................................................................................... 105
Herramientas antivirus .............................................................................................. 105
GPG agent ............................................................................................................. 107
9. Developer's Best Practices for OS Security ....................................................................... 109
Best practices for security review and design ............................................................... 109
Creating users and groups for software daemons ........................................................... 110
10. Antes del compromiso ................................................................................................ 113
Sistema seguro ........................................................................................................ 113
Tracking security vulnerabilities ......................................................................... 113
Continuously update the system ......................................................................... 114
Avoid using the unstable branch ........................................................................ 116v

Manual de Seguridad de DebianSecurity support for the testing branch ................................................................ 116
Automatic updates in a Debian GNU/Linux system ............................................... 117
Do periodic integrity checks ...................................................................................... 118
Montar el descubrimiento de intrusión ........................................................................ 119
Detección de intrusos basadas en la máquina ........................................................ 119
Detección de intrusos basadas en la máquina ........................................................ 120
Evitando rootkits ..................................................................................................... 120
LKM - Loadable Kernel Modules (módulos cargables en el núcleo) .......................... 120
Detector de rootkits. ......................................................................................... 120
Ideas geniales/paranóicas — qué debe hacer ................................................................ 121
Construyendo un equipo trampa ......................................................................... 122
11. After the compromise (incident response) ....................................................................... 124
Comportamiento general ........................................................................................... 124
Realizar copia de seguridad del sistema ....................................................................... 124
Contact your local CERT .......................................................................................... 125
Análisis forense ....................................................................................................... 125
Analysis of malware ................................................................................................ 126
12. Preguntas Frecuentes .................................................................................................. 127
La seguridad en el sistema operativo Debian ................................................................ 127¿Es más seguro Debian que X? .......................................................................... 127
My system is vulnerable! (Are you sure?) ............................................................ 137
Software específico .................................................................................................. 139
Proftpd es vulnerable a un ataque en el servicio Denial. ......................................... 139
After installing portsentry, there are a lot of ports open. ......................................... 139
Preguntas con respecto al equipo de seguridad Debian ................................................... 140
A. Historia de revisiones .................................................................................................. 141
B. Appendix ................................................................................................................... 153
El proceso de fortalecimiento es manejado paso a paso .................................................. 153
Lista de chequeo de la configuración .......................................................................... 155
Montar un IDS aislado ............................................................................................. 157
Setting up a bridge firewall ....................................................................................... 158
A bridge providing NAT and firewall capabilities ................................................. 159
Añadir capacidades al cortafuegos ...................................................................... 160
Basic IPtables rules .......................................................................................... 160
Sample script to change the default Bind installation. ..................................................... 161
Security update protected by a firewall ........................................................................ 164
Chroot environment for SSH ..................................................................................... 166
Chrooting the ssh users .................................................................................... 166
Chrooting the ssh server ................................................................................... 170
Chroot environment for Apache ......................................................................... 179
See also ......................................................................................................... 184

vi

Lista de ejemplos
B.1. Reglas Iptables ......................................................................................................... 160

vii

Capítulo 1. Introducción
Una de las cosas más dificiles sobre los documentos de seguridad es que cada caso es único. Dos cosas alas que se debe prestar atención son la amenaza del entorno y las necesidades de seguridad, tanto de cadaparte individual como del servidor o de la red. Por ejemplo, las necesidades de seguridad de un usuario
local son completamente diferentes a las de la red de un banco. Mientras que un usuario local necesita
defenderse contra el cracker script-kiddie, un banco tiene que preocuparse de ataques dirigidos. Además,
el banco tiene que proteger los datos de sus clientes con precisión milimétrica. En resumen, todo usuario
debe considerar el equilibrio entre utilización y seguridad/paranoia.
Observe que este manual solamente trata de asuntos relacionados con el software. Ni el mejor software delmundo podría protegerlo si alguien tuviera acceso físico a la máquina. Usted puede colocarla bajo su mesao puede ponerla en un búnker con un ejército que la protega. Sin embargo, un ordenador de escritorio puede
ser muchísimo más seguro (desde el punto de vista del software) que un sistema protegido físicamente siel primero de ellos se configura de la manera apropiada y el segundo está lleno de agujeros de seguridad.
Lógicamente, usted debe considerar ambos casos.
Este documento da una apreciación global de lo que usted puede hacer para incrementar la seguridad
de su sistema Debian GNU/Linux. Si usted ha leido otros documentos con respecto a la seguridad en
Linux, encontrará que describen problemas comunes, los cuales pueden solaparse con este documento.
Sin embargo este documento no intenta ser la única fuente de información que usted debería usar, sólo
intenta adaptar esa misma información para su aplicación sobre un sistema Debian GNU/Linux. La forma
de trabajar de distintas distribuciones es diferente (el ejemplo habitual es la forma de arrancar y para losdemonios del sistema); aquí usted encontrará material apropiado para los procedimientos y herramientas
utilizadas por Debian.

Authors
The current maintainer of this document is Javier Fernández-Sanguino Peña. Please forward him any
comments, additions or suggestions, and they will be considered for inclusion in future releases of thismanual.
This manual was started as a HOWTO by Alexander Reelsen. After it was published on the Internet,
Javier Fernández-Sanguino Peña incorporated it into the Debian Documentation Project [http://www.debian.org/doc]. A number of people have contributed to this manual (all contributions are listed in the changelog) but the following deserve special mention since they have provided significant contributions (fullsections, chapters or appendices):• Stefano Canepa• Era Eriksson• Carlo Perassi• Alexandre Ratti• Jaime Robles• Yotam Rubin• Frederic Schutz• Pedro Zorzenon Neto
• Oohara Yuuma

1

Introducción• Davor Ocelic

Lugares donde encontrar este manual (diversos formatos)
You can download or view the latest version of the Securing Debian Manual from the Debian Documentation Project [https://www.debian.org/doc/user-manuals#securing]. If you are reading a copy from anothersite, please check the primary copy in case it provides new information. If you are reading a translation,please review the version the translation refers to to the latest version available. If you find that the version
is behind please consider using the original copy or review the to see what has changed.
If you want a full copy of the manual you can either download the text version [https://www.debian.org/doc/manuals/securing-debian-manual/securing-debian-manual.en.txt] or the PDF version[https://www.debian.org/doc/manuals/securing-debian-manual/securing-debian-manual.en.pdf] from the
Debian Documentation Project's site. These versions might be more useful if you intend to copy the document over to a portable device for offline reading or you want to print it out. Be forewarned, the manualis over two hundred pages long and some of the code fragments, due to the formatting tools used, are notwrapped in the PDF version and might be printed incomplete.
The document is also provided in text, html and PDF formats in the harden-doc [http://packages.debian.org/harden-doc] package. Notice, however, that the package maybe not be completely up to date withthe document provided on the Debian site (but you can always use the source package to build an updated
version yourself).
This document is part of the documents distributed by the Debian Documentation Project [https://www.debian.org/doc/ddp]. You can review the changes introduced in the document using a web browser and obtaining information from the version control logs online [https://salsa.debian.org/ddp-team/securing-debian-manual]. You can also checkout the code using Git with the following call in the command line:$ git clone https://salsa.debian.org/ddp-team/securing-debian-manual.git

Notas/Retroalimentación/Organización
Ahora, la parte oficial. Tanto Alexander Reelsen como Javier Fernández-Sanguino escribieron la mayoríade párrafos de este manual, pero en opinión de ambos éste no debería ser el caso. Ambos han crecido yvivido con el software libre, es algo que usan a diario y supongo que usted también. Por eso animamosa todo el mundo a enviar todo tipo de retroalimentación, añadidos o cualquier otra sugerencia que usted
pueda tener.
Si desea mantener una cierta sección o mejor un párrafo, escriba a quien mantiene el documento y serábien recibido. Especialmente si encuentra una sección marcada como ARREGLAME, lo que significa quelos autores no tienen el tiempo para hacerlo o el conocimiento total necesario sobre el tema, escríbales
un correo inmediatamente.
Por el tema de este manual está claro que es muy importante mantenerlo actualizado y usted puede hacersu parte. Por favor, contribuya.

Conocimiento previo
La instalación de Debian GNU/Linux no es muy difícil y usted mismo debe haber sido capaz de instalarlo.
Si tiene algún conocimiento sobre Linux u otro Unix y está familiarizado con la seguridad básica, le será2

Introducción

más fácil entender este manual, dado que este documento no puede explicar cada pequeño detalle o característica (de lo contrario hubiera sido un libro en lugar de un manual). Si usted no está tan familiarizado,
probablemente debería mirar references para saber como encontrar información más detallada.

Things that need to be written (FIXME/TODO)This section describes all the things that need to be fixed in this manual. Some paragraphs include FIXME
or TODO tags describing what content is missing (or what kind of work needs to be done). The purpose ofthis section is to describe all the things that could be included in the future in the manual, or enhancementsthat need to be done (or would be interesting to add).
If you feel you can provide help in contributing content fixing any element of this list (or the inline annotations), contact the main author (“Authors”).
• This document has yet to be updated based on the latest Debian releases. The default configuration ofsome packages need to be adapted as they have been modified since this document was written.
• Expand the incident response information, maybe add some ideas derived from Red Hat's Security
Guide's chapter on incident response [https://web.archive.org/web/20100412191348/http://www.redhat.com/docs/manuals/linux/RHL-9-Manual/security-guide/ch-response.html].
• Write about remote monitoring tools (to check for system availability) such as monit, daemontools and mon. See Sysamin Guide [https://web.archive.org/web/20100110040204/http://linuxdevcenter.com/pub/a/linux/2002/05/09/sysadminguide.html].
• Consider writing a section on how to build Debian-based network appliances (with information suchas the base system, equivs and FAI).
• Check if this site [https://web.archive.org/web/20040731082209/http://www.giac.org/practical/gsec/
Chris_Koutras_GSEC.pdf] has relevant info not yet covered here.
• Add information on how to set up a laptop with Debian, look here [https://web.archive.org/web/20040725013857/http://www.giac.org/practical/gcux/Stephanie_Thomas_GCUX.pdf].
• Add information on how to set up a firewall using Debian GNU/Linux. The section regarding firewallingis oriented currently towards a single system (not protecting others...) also talk on how to test the setup.
• Add information on setting up a proxy firewall with Debian GNU/Linux stating specifically which packages provide proxy services (like xfwp, ftp-proxy, redir, smtpd, dnrd, jftpgw, oops, pdnsd, perdition,transproxy, tsocks). Should point to the manual for any other info. Note that zorp is now available as a
Debian package and is a proxy firewall (they also provide Debian packages upstream).
• Information on service configuration with file-rc.
• Check all the reference URLs and remove/fix those no longer available.
• Add information on available replacements (in Debian) for common servers which are useful for limited
functionality. Examples:• local lpr with cups (package)?
• remote lrp with lpr• bind with dnrd/maradns• apache with dhttpd/thttpd/wn (tux?)3

Introducción• exim/sendmail with ssmtpd/smtpd/postfix• squid with tinyproxy• ftpd with oftpd/vsftp• ...
• More information regarding security-related kernel patches in Debian, including the ones shown above
and specific information on how to enable these patches in a Debian system.
• Linux Intrusion Detection (kernel-patch-2.4-lids)• Confianza en Linux (dentro del paquete trustees)• NSA Enhanced Linux [http://wiki.debian.org/SELinux]• ipac-ng• ...
• Details of turning off unnecessary network services (besides inetd), it is partly in the hardening procedure but could be broadened a bit.
• Information regarding password rotation which is closely related to policy.
• Policy, and educating users about policy.
• More about tcpwrappers, and wrappers in general?
• hosts.equiv and other major security holes.
• Issues with file sharing servers such as Samba and NFS?
• suidmanager/dpkg-statoverrides.
• lpr and lprng.
• Switching off the GNOME IP things.
• Talk about pam_chroot (see http://lists.debian.org/debian-security/2002/05/msg00011.html) and
its usefulness to limit users. Introduce information related to https://web.archive.org/web/20031204060940/http://www.securityfocus.com/infocus/1575. pdmenu, for example isavailable in Debian (whereas flash is not).
• Talk about chrooting services, some more info on this Linux Focus article [http://www.linuxfocus.org/English/January2002/article225.shtml].
• Talk about programs to make chroot jails. compartment and chrootuid are waiting in incoming. Someothers (makejail, jailer) could also be introduced.
• More information regarding log analysis software (i.e. logcheck and logcolorise).
• 'advanced' routing (traffic policing is security related).
• limiting ssh access to running certain commands.
• using dpkg-statoverride.

4

Introducción• secure ways to share a CD burner among users.
• secure ways of providing networked sound in addition to network display capabilities (so that X clients'
sounds are played on the X server's sound hardware).
• securing web browsers.
• setting up ftp over ssh.
• using crypto loopback file systems.
• encrypting the entire file system.
• steganographic tools.
• setting up a PKA for an organization.
• using LDAP to manage users. There is a HOWTO of ldap+kerberos for Debian at http://www.bayour.com written by Turbo Fredrikson.
• How to remove information of reduced utility in production systems such as /usr/share/doc, /usr/share/man (yes, security by obscurity).
• More information on lcap based on the packages README file (well, not there yet, see http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=169465) and from the article from LWN: http://lwn.net/1999/1202/
kernel.php3.
• Add Colin's article on how to setup a chroot environment for a full sid system (https://web.archive.org/web/20030204012846/https://people.debian.org/~walters/chroot.html).
• Add information on running multiple snort sensors in a given system (check bug reports sent to snort).
• Add information on setting up a honeypot (honeyd).
• Describe situation wrt to FreeSwan (orphaned) and OpenSwan. VPN section needs to be rewritten.
• Add a specific section about databases, current installation defaults and how to secure access.
• Add a section about the usefulness of virtual servers (Xen et al).
• Explain how to use some integrity checkers (AIDE, integrit or samhain). The basics are simple andcould even explain some configuration improvements.

Credits and thanks!
• Alexander Reelsen escribió el documento original.
• added more info to the original doc.
• Robert van der Meulen aportó los párrafos de quota y muchas buenas ideas.
• Ethan Benson corrigió los párrafos de PAM y sugirió buenas ideas.
• Dariusz Puchalak hizo contribuciones a muchos capítulos.
• Gaby Schilders contribuyó a una buena idea de Genio/Paranoia.

5

Introducción• Era Eriksson resolvió problemas de idioma en muchos lugares y contribuyó al apéndice de la lista decomprobaciones.
• Philipe Gaspar escribió la información de LKM.
• Yotam Rubin contribuyó a los ajustes de muchos fallos ortográficos así como a la información con
respecto a las versiones de bind y las contraseñas md5.
• Francois Bayart provided the appendix describing how to set up a bridge firewall.
• Joey Hess wrote the section describing how Secure Apt works on the Debian Wiki [http://wiki.debian.org/SecureApt].
• Martin F. Krafft wrote some information on his blog regarding fingerprint verification which was also
reused for the Secure Apt section.
• Francesco Poli did an extensive review of the manual and provided quite a lot of bug reports and typo
fixes which improved and helped update the document.
• All the people who made suggestions for improvements that (eventually) were included here (see “Lugares donde encontrar este manual (diversos formatos)”).
• (de Alexander) A todas las personas que me animaron a escribir, este COMO (El cual posteriormente
se convirtió en el manual) .
• La totalidad del proyecto Debian.

6

Capítulo 2. Antes de empezar¿Para qué quiere su sistema?
No hay muchas diferencias entre proteger Debian y proteger cualquier otro sistema; para hacerlo adecuadamente tiene primero que decidir qué es lo que piensa hacer con él. Tras esto, la siguiente consideración
será preguntarse si quiere un sistema realmente seguro.
Encontrará que este manual está escrito empezando desde abajo, esto es, leerá alguna información sobre
tareas a realizar antes, durante y después de la instalación de su sistema Debian. Se puede pensar en tareascomo:• Decidir qué servicios necesita y limitar el sistema a éstos. Esto incluye desactivar/desinstalar servicios
innecesarios, y añadir filtros como cortafuegos, o «tcpwrappers».
• Limitar los usuarios y permisos en su sistema.
• Fortalecer los servicios ofrecidos de forma que, en el caso de que un servicio resulte comprometido, el
impacto en el sistema sea mínimo.
• Utilizar herramientas apropiadas para garantizar que se detecte el uso no autorizado, de forma que pueda
usted tomar las medidas oportunas.

Sea consciente de los problemas de seguridad
generalEl siguiente manual no entrará en detalles (normalmente) sobre por qué algunos asuntos se consideranriesgos de seguridad. Sin embargo, usted podría querer tener una mejor formación relativa a la seguridad
general de UNIX y específica de Linux. Tómese su tiempo en leer documentos relacionados con la seguridad, con objeto de tomar decisiones bien fundamentadas cuando se encuentre con diferentes alternativas.
Debian GNU/Linux está basado en el núcleo de Linux, por ello también se puede aplicar mucha de la
información relacionada con la seguridad de Linux, así como de otras distribuciones y de UNIX en general(incluso si las herramientas utilizadas, o los programas disponibles, difieren).
Entre los documentos útiles se incluyen (N. del T., algunos de los documentos en inglés pueden encontrarse
en internet traducidos al español):• The http://www.tldp.org/HOWTO/Security-HOWTO/ is one of the best references regarding general
Linux security.
• The http://www.tldp.org/HOWTO/Security-Quickstart-HOWTO/ is also a very good starting point for
novice users (both to Linux and security).
• The http://seifried.org/lasg/ is a complete guide that touches all the issues related to security in Linux,from kernel security to VPNs. Note that it has not been updated since 2001, but some information isstill relevant. 1• Kurt Seifried's http://seifried.org/security/os/linux/20020324-securing-linux-step-by-step.html.
1

At a given time it was superseded by the "Linux Security Knowledge Base". This documentation is also provided in Debian through the lskb
package. Now it's back as the Lasg again.

7Antes de empezar• En http://www.tldp.org/links/p_books.html#securing_linux puede encontrar un documento similar a este manual, pero relacionado con RedHat, algunos de los temas no son específicos de la distribución yson también aplicables a Debian.
• Another Red Hat related document is https://web.archive.org/web/20050520170309/https://ltp.sourceforge.net/docs/RHEL-EAL3-Configuration-Guide.pdf.
• IntersectAlliance has published some documents that can be used as reference cards on how
to harden Linux servers (and their services), the documents are available at https://web.archive.org/web/20030210231943/http://www.intersectalliance.com/projects/index.html.
• For network administrators, a good reference for building a secure network is the https://web.archive.org/web/20030418093551/http://www.linuxsecurity.com/docs/LDP/Securing-Domain-HOWTO/.
• If you want to evaluate the programs you are going to use (or want to build up some new ones) you should
read the http://www.tldp.org/HOWTO/Secure-Programs-HOWTO/ (master copy is available at http://
www.dwheeler.com/secure-programs/, it includes slides and talks from the author, David Wheeler)• If you are considering installing firewall capabilities, you should read the http://www.tldp.org/HOWTO/Firewall-HOWTO.html and the http://www.tldp.org/HOWTO/IPCHAINS-HOWTO.html (for kernels previous to 2.4).
• Finally, a good card to keep handy is the https://web.archive.org/web/20030308013020/http://www.linuxsecurity.com/docs/QuickRefCard.pdf.
En cualquier caso, hay mas información concerniente a los servicios explicados aquí (NFS, NIS, SMB...)en muchos de los CÓMOs del http://es.tldp.org/. Alguno de estos documentos habla sobre seguridad vistaésta desde el lado de un servicio, por tanto, asegúrese de echar un vistazo también allí.
Los documentos de CÓMO del Proyecto de documentación de Linux están disponibles en Debian GNU/Linux por medio de la instalación de doc-linux-text (versión de texto) o doc-linux-html (versión html). Tras
la instalación, estos documentos estarán disponibles en los directorios /usr/share/doc/HOWTO/entxt y /usr/share/doc/HOWTO/en-html, respectivamente.
Otros libros recomendados de Linux:• Maximum Linux Security : A Hacker's Guide to Protecting Your Linux Server and Network. Anonymous. Paperback - 829 páginas. Sams Publishing. ISBN: 0672313413. Julio 1999.
• Linux Security de John S. Flowers. New Riders; ISBN: 0735700354. Marzo 1999
• https://web.archive.org/web/20030202131658/https://www.linux.org/books/ISBN_0072127732.html
By Brian Hatch. McGraw-Hill Higher Education. ISBN 0072127732. April, 2001
Otros libros (que podrían estar relacionados con asuntos generales relativos a UNIX y la seguridad, y no
ser específicos de Linux):• https://web.archive.org/web/20030206231652/http://www.oreilly.com/catalog/puis/ Garfinkel, Simpson, and Spafford, Gene; O'Reilly Associates; ISBN 0-56592-148-8; 1004pp; 1996.
• Firewalls and Internet Security Cheswick, William R. y Bellovin, Steven M.; Addison-Wesley; 1994;
ISBN 0-201-63357-4; 320pp.
Algunas páginas Web para conservar actualizadas relativas a seguridad:• http://csrc.nist.gov/.

8

Antes de empezar• https://cve.mitre.org/data/refs/refmap/source-BUGTRAQ.html CVE Reference Map for Source BUGTRAQ• http://www.linuxsecurity.com/. General information regarding Linux security (tools, news...). Most useful is the https://linuxsecurity.com/howtos page.

¿Cómo maneja Debian la seguridad?
Simplemente con objeto de tener una visión general sobre seguridad en Debian GNU/Linux, debería tomar
nota de los diferentes asuntos que Debian trata de resolver para proporcionar un sistema global seguro:• Debian problems are always handled openly, even security related. Security issues are discussed openly
on the debian-security mailing list. Debian Security Advisories (DSAs) are sent to public mailing lists(both internal and external) and are published on the public server. As the http://www.debian.org/social_contract states: We will not hide problems. We will keep our entire bug report database open for
public view at all times. Reports that people file online will promptly become visible to others.
• Debian sigue atentamente los asuntos de seguridad. El equipo de seguridad revisa muchas fuentes a lacaza de paquetes relacionados con el tema de la seguridad, que pudieran incluirse en Debian, siendo la
más importante http://www.securityfocus.com/cgi-bin/vulns.pl.
• Las actualizaciones de seguridad son la primera prioridad. Cuando aparece un problema de seguridad en
un paquete de Debian, la actualización de seguridad se prepara tan rápido como es posible y se distribuye
para las versiones estable e inestable, incluyendo todas las arquitecturas.
• La información relativa a la seguridad se centraliza en un único punto, http://security.debian.org/.
• Debian está continuamente intentando mejorar la seguridad global de la distribución mediante nuevos
proyectos, tales como los mecanismos automáticos de verificación de firma de los paquetes.
• Debian proporciona un número de herramientas útiles, relacionadas con la seguridad, para la administración y supervisión del sistema. Los desarrolladores intentan integrar bien estas herramientas en la
distribución, con objeto de que formen un paquete integrado que fortalezca las directrices de seguridad
local. Las herramientas incluyen: comprobadores de integridad, herramientas de auditoría, herramientas
de endurecimiento, herramientas de cortafuegos, herramientas de detección de intrusos, etc.
• Package maintainers are aware of security issues. This leads to many "secure by default" service installations which could impose certain restrictions on their normal use. Debian does, however, try tobalance security and ease of administration - the programs are not de-activated when you install them(as it is the case with say, the BSD family of operating systems). In any case, prominent security issues(such as setuid programs) are part of the http://www.debian.org/doc/debian-policy/.
By publishing security information specific to Debian and complementing other information-security documents related to Debian (see “Conocimiento previo”), this document aims to produce better system installations security-wise.

9Capítulo 3. Antes y durante lainstalación
Elegir una contraseña para el BIOS
Antes de instalar cualquier sistema operativo en su ordenador, establezca la contraseña del BIOS. Tras lainstalación (una vez que haya habilitado el arranque desde el disco duro) debería volver al BIOS y cambiar
la secuencia de arranque para deshabilitar el arranque desde la disquetera, la unidad de cdrom o cualquierotro dispositivo desde el que no se debería arrancar. De otra forma un «cracker» únicamente necesitaráacceso físico y un disquete de arranque para acceder a todo su sistema.
Incluso mejor es deshabilitar el arranque a menos que se proporcione una contraseña. Esto puede ser muy
efectivo en un servidor, ya que no se rearranca muy a menudo. El inconveniente de esta táctica es que
el rearranque requiere intervención humana, lo que puede causar problemas si no hay un acceso fácil ala máquina.
Note: many BIOSes have well known default master passwords, and applications also exist to retrieve thepasswords from the BIOS. Corollary: don't depend on this measure to secure console access to system.

Partitioning the system
Elegir un esquema de particiones inteligente
Un esquema de particiones inteligente depende del uso que vaya a tener la máquina. Una buena regla
general es ser bastante liberal con las particiones y prestar atención a los siguientes factores:• Any directory tree which a user has write permissions to, such as e.g. /home, /tmp and /var/tmp/,
should be on a separate partition. This reduces the risk of a user DoS by filling up your "/" mountpoint and rendering the system unusable (Note: this is not strictly true, since there is always some spacereserved for root which a normal user cannot fill), and it also prevents hardlink attacks. 1• Any partition which can fluctuate, e.g. /var (especially /var/log) should also be on a separate
partition. On a Debian system, you should create /var a little bit bigger than on other systems, because
downloaded packages (the apt cache) are stored in /var/cache/apt/archives.
• Cualquier partición en la que quiera instalar software que no pertenezca a la distribución debería estar
aparte. De acuerdo con la jerarquía estándar de archivos, debería hacerse con /opt o /usr/local.
Si éstos están en particiones separadas, no se borrarán en caso de tener que reinstalar Debian.
• Desde el punto de vista de la seguridad, cobra sentido intentar mover los datos estáticos a su propia
partición, y luego montar esa partición como de sólo lectura. O mejor todavía, poner los datos en un
medio de sólo lectura. Mire m~s adelante si quiere m~s detalles.
In the case of a mail server it is important to have a separate partition for the mail spool. Remote users(either knowingly or unknowingly) can fill the mail spool (/var/mail and/or /var/spool/mail).
1

A very good example of this kind of attacks using /tmp is detailed in http://www.hackinglinuxexposed.com/articles/20031111.html and http://
www.hackinglinuxexposed.com/articles/20031214.html (notice that the incident is Debian-related). It is basicly an attack in which a local user
stashes away a vulnerable setuid application by making a hard link to it, effectively avoiding any updates (or removal) of the binary itself madeby the system administrator. Dpkg was recently fixed to prevent this (see http://bugs.debian.org/225692) but other setuid binaries (not controlled
by the package manager) are at risk if partitions are not setup correctly.

10Antes y durante la instalación

If the spool is on a separate partition, this situation will not render the system unusable. Otherwise (if thespool directory is on the same partition as /var) the system might have important problems: log entries
will not be created, packages cannot be installed, and some programs might even have problems startingup (if they use /var/run).
Also, for partitions in which you cannot be sure of the needed space, installing Logical Volume Manager(lvm-common and the needed binaries for your kernel, this might be either lvm10, lvm6, or lvm5). Usinglvm, you can create volume groups that expand multiple physical volumes.

Selecting the appropriate file systems
During the system partitioning you also have to decide which file system you want to use. The default
file system2 selected in the Debian installation for Linux partitions is ext3, a journaling file system. It is
recommended that you always use a journaling file system, such as ext3, reiserfs, jfs or xfs, tominimize the problems derived from a system crash in the following cases:• for laptops in all the file systems installed. That way if you run out of battery unexpectedly or the systemfreezes due to a hardware issue (such as X configuration which is somewhat common) you will be lesslikely to lose data during a hardware reboot.
• for production systems which store large amounts of data (like mail servers, ftp servers, network filesystems...) it is recommended on these partitions. That way, in the event of a system crash, the server
will take less time to recover and check the file systems, and data loss will be less likely.
Leaving aside the performance issues regarding journalling file systems (since this can sometimes turn intoa religious war), it is usually better to use the ext3 file system. The reason for this is that it is backwards
compatible with ext2, so if there are any issues with the journalling you can disable it and still have aworking file system. Also, if you need to recover the system with a bootdisk (or CD-ROM) you do notneed a custom kernel. If the kernel is 2.4 or 2.6 ext3 support is already available, if it is a 2.2 kernel
you will be able to boot the file system even if you lose journalling capabilities. If you are using otherjournalling file systems you will find that you might not be able to recover unless you have a 2.4 or 2.6
kernel with the needed modules built-in. If you are stuck with a 2.2 kernel on the rescue disk, it might beeven more difficult to have it access reiserfs or xfs.
In any case, data integrity might be better under ext3 since it does file-data journalling while others do
only meta-data journalling, see http://lwn.net/2001/0802/a/ext3-modes.php3.
Notice, however, that there are some partitions that might not benefit from using a journaling filesystem.
For example, if you are using a separate partition for /tmp/ you might be better off using a standard
ext2 filesystem as it will be cleaned up when the system boots.

No se conecte a Internet hasta que no esté preparado
No debería conectarse a Internet de forma inmediata durante la instalación. Esto puede sonar como algo
estúpido, pero la instalación por la red es un método común. Puesto que el sistema instalará y activará
servicios de forma inmediata, si está conectado a Internet y los servicios no están configurados de forma
apropiada, el sistema está abierto a un ataque.
Observe también que algunos servicios podrían tener vulnerabilidades de seguridad no corregidas, en lospaquetes que esté utilizando para la instalación. Esto es normalmente cierto si usted está realizando la2

Since Debian GNU/Linux 4.0, codename etch

11

Antes y durante la instalación

instalación desde un medio antiguo (como CD-ROMs). En este caso, ¡el sistema podría estar comprometido
incluso antes de terminar la instalación!
Como la instalación y las actualizaciones de Debian pueden hacerse por Internet, podría usted pensar que
es una buena idea utilizar esta característica en la instalación. Si el sistema se va a conectar directamente a
Internet (sin la protección de un cortafuegos o NAT), es mejor instalar sin conexión a Internet, utilizando
una réplica local de los paquetes tanto para las fuentes de Debian como para las actualizaciones de seguridad. Puede configurar las réplicas de paquetes, utilizando otro sistema conectado a Internet con herramientas específicas de Debian (si es un sistema Debian) como apt-move ó apt-proxy, u otras herramientas
de replicación comunes, para proporcionar el archivo al sistema instalado. Si no puede hacer esto, puede
configurar las reglas del cortafuegos para limitar el acceso al sistema mientras hace la actualización (vea“Security update protected by a firewall”).

Establecer una contraseña para «root»
Setting a good root password is the most basic requirement for having a secure system. See passwd(1) forsome hints on how to create good passwords. You can also use an automatic password generation program
to do this for you (see “Generación de contraseñas de usuario”).
Plenty of information on choosing good passwords can be found on the Internet; two that provide a decent
summary and rationale are Eric Wolfram's http://wolfram.org/writing/howto/password.html and Walter
Belgers' https://web.archive.org/web/20030218000949/http://www.belgers.com/write/pwseceng.txt

Ejecute el mínimo número de servicios requeridos
Los servicios son programas como los servidores de ftp y los servidores de web. Puesto que éstos tienen
que estar escuchando las conexiones entrantes para detectar solicitudes de servicio, ordenadores externos
pueden conectarse a los suyos. Los servicios son algunas veces vulnerables (p. ejem. pueden resultar comprometidos por un ataque determinado) y presentan por tanto un riesgo para la seguridad.
No debería instalar servicios que su máquina no necesite. Cada servicio instalado podría introducir nuevos,
quizás no obvios (o conocidos), agujeros de seguridad en su ordenador.
As you may already know, when you install a given service the default behavior is to activate it. In adefault Debian installation, with no services installed, the number of running services is quite low and thenumber of network-oriented services is even lower. In a default Debian 3.1 standard installation you will
end up with OpenSSH, Exim (depending on how you configured it) and the RPC portmapper available
as network services3. If you did not go through a standard installation but selected an expert installation
you can end up with no active network services. The RPC portmapper is installed by default because it isneeded for many services, for example NFS, to run on a given system. However, it can be easily removed,
see “Securing RPC services” for more information on how to secure or disable RPC services.
Cuando instala un nuevo servicio relacionado con la red (demonio) en su sistema Debian GNU/Linux,
puede habilitarlo de dos formas: por medio del superdemonio inetd (esto es, añadiendo una línea a /etc/inetd.conf) o por medio de un programa autónomo que enlace con sus interfaces de red. Los
programas autónomos se controlan a través de los archivos de /etc/init.d, a los que se llama durante el arranque por medio de los mecanismos de SysV (o un alternativo) utilizando enlaces simbólicos
en /etc/rc?.d/* (para más información sobre cómo se hace esto, lea /usr/share/doc/sysvinit/README.runlevels.gz).
3

The footprint in Debian 3.0 and earlier releases wasn't as tight, since some inetd services were enabled by default. Also standard installations of
Debian 2.2 installed the NFS server as well as the telnet server.

12Antes y durante la instalación

If you want to keep some services but use them rarely, use the update-* commands, e.g. update-inetd andupdate-rc.d to remove them from the startup process. For more information on how to disable network
services read “Deshabilitar servicios”. If you want to change the default behaviour of starting up services
on installation of their associated packages4 use policy-rc.d, please read /usr/share/doc/sysvrc/README.policy-rc.d.gz for more information.
invoke-rc.d support is mandatory in Debian, which means that for Debian 4.0 etch and later releases you
can write a policy-rc.d file that forbids starting new daemons before you configure them. Although nosuch scripts are packaged yet, they are quite simple to write. See policyrcd-script-zg2.

Deshabilitar servicios
Disabling a daemon service is quite simple. You either remove the package providing the program for that
service or you remove or rename the startup links under /etc/rc${runlevel}.d/. If you rename
them make sure they do not begin with 'S' so that they don't get started by /etc/init.d/rc. Do not remove all
the available links or the package management system will regenerate them on package upgrades, makesure you leave at least one link (typically a 'K', i.e. kill, link). For more information read http://www.debian.org/doc/manuals/reference/ch-system.en.html#s-custombootscripts section of the Debian Reference(Chapter 2 - Debian fundamentals).
You can remove these links manually or using update-rc.d (see update-rc.d(8)). For example, you
can disable a service from executing in the multi-user runlevels by doing:# update-rc.d name stop XX 2 3 4 5 .
Donde XX es un número que determinará cuándo se ejecutará la acción de parada de ese servicio. Por favorobserve que, si no utiliza file-rc, update-rc.d -f service remove no funcionará correctamente,
puesto que se eliminarán todos los enlaces, y éstos se regenerarán con la reinstalación o actualización delpaquete (que probablemente no sea lo que desea). Si piensa que esto no es intuitivo, probablemente tenga
razón (vea http://bugs.debian.org/67095). De la página del manual:

Si alguno de los archivos /etc/rcrunlevel.d/[SK]??nombre ya existe, entonces
update-rc.d no hará nada. Esto es así para que el administrador del sistema
pueda reorganizar los enlaces, y siempre que deje al menos un enlace sin
eliminar, su configuración no se sobreescribirá.
Si utiliza file-rc toda la información relativa al arranque de servicios se manipula por un archivo de configuración común y se mantiene incluso si los paquetes se eliminan del sistema.
You can use the TUI (Text User Interface) provided by sysv-rc-conf to do all these changes easily (sysvrc-conf works both for file-rc and normal System V runlevels). You will also find similar GUIs for desktop
systems. You can also use the command line interface of sysv-rc-conf:# sysv-rc-conf foobar off
The advantage of using this utility is that the rc.d links are returned to the status they had before the 'off'call if you re-enable the service with:# sysv-rc-conf foobar on
4

This is desirable if you are setting up a development chroot, for example.

13Antes y durante la instalación

Other (less recommended) methods of disabling services are:
• Removing the /etc/init.d/service_name script and removing the startup links using:# update-rc.d name remove• Move the script file (/etc/init.d/service_name) to another name (for example /etc/
init.d/OFF.service_name). This will leave dangling symlinks under /etc/rc${runlevel}.d/ and will generate error messages when booting up the system.
• Remove the execute permission from the /etc/init.d/service_name file. That will also generate error messages when booting.
• Edit the /etc/init.d/service_name script to have it stop immediately once it is executed (byadding an exit 0 line at the beginning or commenting out the start-stop-daemon part in it). If you
do this, you will not be able to use the script to startup the service manually later on.
Nevertheless, the files under /etc/init.d are configuration files and should not get overwritten due
to package upgrades if you have made local changes to them.
A diferencia de otros sistemas operativos (UNIX), los servicios en Debian no pueden deshabilitarse mediante la modificación de los archivos en /etc/default/service_name.
ARRÉGLEME: Añada más información sobre la manipulación de demonios utilizando file-rc

Disabling inetd or its services
You should check if you really need the inetd daemon nowadays. Inetd was always a way to compensate
for kernel deficiencies, but those have been taken care of in modern Linux kernels. Denial of Service
possibilities exist against inetd (which can increase the machine's load tremendously), and many people
always preferred using stand-alone daemons instead of calling services via inetd. If you still want to runsome kind of inetd service, then at least switch to a more configurable Inet daemon like xinetd, rlinetd
or openbsd-inetd.
You should stop all unneeded Inetd services on your system, like echo, chargen, discard, daytime, time,talk, ntalk and r-services (rsh, rlogin and rcp) which are considered HIGHLY insecure (use ssh instead).
Puede deshabilitar servicios editando directamente /etc/inetd.conf, pero Debian proporciona una
mejor alternativa: update-inetd (que comenta los servicios en una forma que hace fácil volver a activarlos). Podría detener el demonio de telnet mediante la ejecución de estos comandos para cambiar el archivo de configuración y volver a arrancar el demonio (en este caso el servicio de telnet está deshabilitado):/usr/sbin/update-inetd --disable telnet
Si quiere tener servicios escuchando, pero no quiere tenerlos escuchando en todas las direcciones IP desu ordenador principal, podría querer utilizar una característica no documentada de inetd (remplazar elnombre del servicio por la sintaxis servicio@ip) o utilizar un demonio alternativo como xinetd.

Install the minimum amount of software required
Debian comes with a lot of software, for example the Debian 3.0 woody release includes 6 or 7 (depending
on architecture) CD-ROMs of software and thousands of packages, and the Debian 3.1 sarge release ships

14Antes y durante la instalaciónwith around 13 CD-ROMs of software. With so much software, and even if the base system installation isquite reduced 5 you might get carried away and install more than is really needed for your system.
Since you already know what the system is for (don't you?) you should only install software that is reallyneeded for it to work. Any unnecessary tool that is installed might be used by a user that wants to compromise the system or by an external intruder that has gotten shell access (or remote code execution throughan exploitable service).
The presence, for example, of development utilities (a C compiler) or interpreted languages (such as perl
- but see below -, python, tcl...) may help an attacker compromise the system even further:• allowing him to do privilege escalation. It's easier, for example, to run local exploits in the system if
there is a debugger and compiler ready to compile and test them!
• providing tools that could help the attacker to use the compromised system as a base of attack against
other systems. 6
Of course, an intruder with local shell access can download his own set of tools and execute them, andeven the shell itself can be used to make complex programs. Removing unnecessary software will not help
prevent the problem but will make it slightly more difficult for an attacker to proceed (and some might
give up in this situation looking for easier targets). So, if you leave tools in a production system that could
be used to remotely attack systems (see “Herramientas de evaluación de vulnerabilidades remotas.”) you
can expect an intruder to use them too if available.
Please notice that a default installation of Debian sarge (i.e. an installation where no individual packages
are selected) will install a number of development packages that are not usually needed. This is becausesome development packages are of Standard priority. If you are not going to do any development you cansafely remove the following packages from your system, which will also help free up some space:
PackageSize------------------------+-------gdb
2,766,822
gcc-3.3
1,570,284
dpkg-dev
166,800
libc6-dev
2,531,564
cpp-3.3
1,391,346
manpages-dev
1,081,408
flex
257,678g++
1,384 (Note: virtual package)
linux-kernel-headers
1,377,022
bin86
82,090cpp
29,446gcc
4,896 (Note: virtual package)g++-3.3
1,778,880
bison
702,830
5For example, in Debian woody it is around 400-500 Mbs, try this:$ size=0$ for i in `grep -A 1 -B 1 "^Section: base" /var/lib/dpkg/available |grep -A 2 "^Priority: required" |grep "^Installed-Size" |cut -d : -f 2`; do size=$(($size+$i)); done
$ echo $size
47762
6
Many intrusions are made just to get access to resources to do illegitimate activity (denial of service attacks, spam, rogue ftp servers, dns pollution...)rather than to obtain confidential data from the compromised system.

15

Antes y durante la instalación

makelibstdc++5-3.3-dev

366,138
774,982

This is something that is fixed in releases post-sarge, see http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=301273 and http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=301138. Due to a bug in the
installation system this did not happen when installing with the installation system of the Debian 3.0 woody
release.

Removing Perl
You must take into account that removing perl might not be too easy (as a matter of fact it can be quitedifficult) in a Debian system since it is used by many system utilities. Also, the perl-base is Priority:
required (that about says it all). It's still doable, but you will not be able to run any perl application in thesystem; you will also have to fool the package management system to think that the perl-base is installedeven if it's not. 7
Which utilities use perl? You can see for yourself:$ for i in /bin/* /sbin/* /usr/bin/* /usr/sbin/*; do [ -f $i ] && {type=`file $i | grep -il perl`; [ -n "$type" ] && echo $i; }; done
These include the following utilities in packages with priority required or important:• /usr/bin/chkdupexe of package util-linux.
• /usr/bin/replay of package bsdutils.
• /usr/sbin/cleanup-info of package dpkg.
• /usr/sbin/dpkg-divert of package dpkg.
• /usr/sbin/dpkg-statoverride of package dpkg.
• /usr/sbin/install-info of package dpkg.
• /usr/sbin/update-alternatives of package dpkg.
• /usr/sbin/update-rc.d of package sysvinit.
• /usr/bin/grog of package groff-base.
• /usr/sbin/adduser of package adduser.
• /usr/sbin/debconf-show of package debconf.
• /usr/sbin/deluser of package adduser.
• /usr/sbin/dpkg-preconfigure of package debconf.
• /usr/sbin/dpkg-reconfigure of package debconf.
• /usr/sbin/exigrep of package exim.
• /usr/sbin/eximconfig of package exim.
7

You can make (on another system) a dummy package with equivs.

16

Antes y durante la instalación• /usr/sbin/eximstats of package exim.
• /usr/sbin/exim-upgrade-to-r3 of package exim.
• /usr/sbin/exiqsumm of package exim.
• /usr/sbin/keytab-lilo of package lilo.
• /usr/sbin/liloconfig of package lilo.
• /usr/sbin/lilo_find_mbr of package lilo.
• /usr/sbin/syslogd-listfiles of package sysklogd.
• /usr/sbin/syslog-facility of package sysklogd.
• /usr/sbin/update-inetd of package netbase.
So, without Perl and, unless you remake these utilities in shell script, you will probably not be able tomanage any packages (so you will not be able to upgrade the system, which is not a Good Thing).
If you are determined to remove Perl from the Debian base system, and you have spare time, submit bug
reports to the previous packages including (as a patch) replacements for the utilities above written in shell
script.
If you wish to check out which Debian packages depend on Perl you can use$ grep-available -s Package,Priority -F Depends perlor$ apt-cache rdepends perl

Lea las listas de correo de seguridad de Debian
Nunca está de más echar un vistazo a la lista de correo debian-security-announce, donde el equipo deseguridad de Debian anuncia los avisos y reparaciones para los paquetes publicados, o a la lista mailto:debian-security@lists.debian.org, donde puede participar en discusiones sobre asuntos relacionados con laseguridad en Debian.
Con objeto de recibir alertas sobre actualizaciones de seguridad importantes, envíe un correo a mailto:debian-security-announce-request@lists.debian.org con la palabra "subscribe" en el asunto. También puede suscribirse a esta lista de correo moderada a través de la página web http://www.debian.org/MailingLists/subscribe
Esta lista de correo tiene un volumen de mensajes muy bajo, y al suscribirse comenzará inmediatamentea recibir alertas de actualizaciones de seguridad para la distribución Debian. Esto le permitirá descargarse rápidamente nuevos paquetes con los errores de seguridad reparados, lo que es muy importante paramantener un sistema seguro. (Vea “Ejecute una actualización de seguridad” para obtener detalles sobre
cómo hacer esto.)17

Capítulo 4. Después de la instalación
Once the system is installed you can still do more to secure the system; some of the steps described in thischapter can be taken. Of course this really depends on your setup but for physical access prevention you
should read “Change the BIOS (again)”,“Colocar una contraseña a lilo o grub”, “Eliminar el prompt de rootdel núcleo”, “Restricción del acceso a la consola”, and “Restricting system reboots through the console”.
Before connecting to any network, especially if it's a public one you should, at the very least, execute a
security update (see “Ejecute una actualización de seguridad”). Optionally, you could take a snapshot of
your system (see “Taking a snapshot of the system”).

Subscribe to the Debian Security Announce
mailing list
In order to receive information on available security updates you should subscribe yourself to the debian-security-announce mailing list in order to receive the Debian Security Advisories (DSAs). See “Equipo de seguridad” for more information on how the Debian security team works. For information on how
to subscribe to the Debian mailing lists read http://lists.debian.org.
DSAs are signed with the Debian Security Team's signature which can be retrieved from http://security.debian.org.
You should consider, also, subscribing to the http://lists.debian.org/debian-security for general discussion
on security issues in the Debian operating system. You will be able to contact other fellow system administrators in the list as well as Debian developers and upstream developers of security tools who cananswer your questions and offer advice.
FIXME: Add the key here too?

Ejecute una actualización de seguridad
As soon as new security bugs are detected in packages, Debian maintainers and upstream authors generally patch them within days or even hours. After the bug is fixed, a new package is provided on http://
security.debian.org.
If you are installing a Debian release you must take into account that since the release was made there
might have been security updates after it has been determined that a given package is vulnerable. Also,
there might have been minor releases (there have been four for the Debian 3.0 sarge release) which include
these package updates.
During installation security updates are configured for your system and pending updates downloaded andapplied, unless you specifically opt out of this or the system was not connected to the Internet. The updates
are applied even before the first boot, so the new system starts its life as up to date as possible.
To manually update the system, put the following line in your sources.list and you will get security
updates automatically, whenever you update your system. Replace [CODENAME] with the release codename, e.g. squeeze.
deb http://security.debian.org/ [CODENAME]/updates main contrib non-free
Note: If you are using the testing branch use the security testing mirror sources as described in “Security
support for the testing branch”.

18Después de la instalación

Once you've done this you can use multiple tools to upgrade your system. If you are running a desktop
system you will have1 an application called update-notifier that will make it easy to check if new updates
are available, by selecting it you can make a system upgrade from the desktop (using update-manager).
For more information see “Checking for updates at the Desktop”. In desktop environments you can also
use synaptic (GNOME), kpackage or adept (KDE) for more advanced interfaces. If you are running a textonly terminal you can use aptitude, apt or dselect (deprecated) to upgrade:• If you want to use aptitude's text interface you just have to press u (update) followed by g (to upgrade).
Or just do the following from the command line (as root):# aptitude update# aptitude upgrade• If you want to use apt do just like with aptitude but substitute the aptitude lines above with apt-get.
• If you want to use dselect then first [U]pdate, then [I]nstall and finally, [C]onfigure the installed/upgraded packages.
If you like, you can add the deb-src lines to /etc/apt/sources.list as well. See apt(8) for further
details.

Security update of libraries
Once you have executed a security update you might need to restart some of the system services. If you
do not do this, some services might still be vulnerable after a security upgrade. The reason for this is that
daemons that are running before an upgrade might still be using the old libraries before the upgrade 2.
From Debian Jessie and up, you can install the needrestart package, which will run automatically after
each APT upgrade and prompt you to restart services that are affected by the just-installed updates. In
earlier releases, you can run the checkrestart program (available in the debian-goodies package) manually
after your APT upgrade.
Some packages (like libc6) will do this check in the postinst phase for a limited set of services speciallysince an upgrade of essential libraries might break some applications (until restarted)3.
Bringing the system to run level 1 (single user) and then back to run level 3 (multi user) should take careof the restart of most (if not all) system services. But this is not an option if you are executing the security
upgrade from a remote connection (like ssh) since it will be severed.
Excercise caution when dealing with security upgrades if you are doing them over a remote connection
like ssh. A suggested procedure for a security upgrade that involves a service restart is to restart the SSH
daemon and then, immediately, attempt a new ssh connection without breaking the previous one. If theconnection fails, revert the upgrade and investigate the issue.

Security update of the kernelFirst, make sure your kernel is being managed through the packaging system. If you have installed using the
installation system from Debian 3.0 or previous releases, your kernel is not integrated into the packaging
system and might be out of date. You can easily confirm this by running:
1

In Etch and later releases
Even though the libraries have been removed from the filesystem the inodes will not be cleared up until no program has an open file descriptor
pointing to them.
3
This happened, for example, in the upgrade from libc6 2.2.x to 2.3.x due to NSS authentication issues, see http://lists.debian.org/debian-glibc/2003/03/msg00276.html.
219Después de la instalación$ dpkg -S `readlink -f /vmlinuz`
linux-image-2.6.18-4-686: /boot/vmlinuz-2.6.18-4-686
If your kernel is not being managed you will see a message saying that the package manager did not findthe file associated to any package instead of the message above, which says that the file associated tothe current running kernel is being provided by the linux-image-2.6.18-4-686. So first, you will need tomanually install a kernel image package. The exact kernel image you need to install depends on yourarchitecture and your prefered kernel version. Once this is done, you will be able to manage the security
updates of the kernel just like those of any other package. In any case, notice that the kernel updates will
only be done for kernel updates of the same kernel version you are using, that is, apt will not automaticallyupgrade your kernel from the 2.4 release to the 2.6 release (or from the 2.4.26 release to the 2.4.27 release4).
The installation system of recent Debian releases will handle the selected kernel as part of the package
system. You can review which kernels you have installed by running:$ COLUMNS=150 dpkg -l 'linux-image*' | awk '$1 ~ /ii/ { print $0 }'To see if your kernel needs to be updated run:$ kernfile=`readlink -f /vmlinuz`$ kernel=`dpkg -S $kernfile | awk -F : '{print $1}'`$ apt-cache policy $kernel
linux-image-2.6.18-4-686:
Installed: 2.6.18.dfsg.1-12
Candidate: 2.6.18.dfsg.1-12
Version table:*** 2.6.18.dfsg.1-12 0
100 /var/lib/dpkg/status
If you are doing a security update which includes the kernel image you need to reboot the system in orderfor the security update to be useful. Otherwise, you will still be running the old (and vulnerable) kernel
image.
If you need to do a system reboot (because of a kernel upgrade) you should make sure that the kernel willboot up correctly and network connectivity will be restored, specially if the security upgrade is done overa remote connection like ssh. For the former you can configure your boot loader to reboot to the original
kernel in the event of a failure (for more detailed information read Remotely rebooting Debian GNU/Linux machines [http://www.debian-administration.org/?article=70]). For the latter you have to introduce
a network connectivity test script that will check if the kernel has started up the network subsystem properly
and reboot the system if it did not5. This should prevent nasty surprises like updating the kernel and thenrealizing, after a reboot, that it did not detect or configure the network hardware properly and you needto travel a long distance to bring the system up again. Of course, having the system serial console 6 in thesystem connected to a console or terminal server should also help debug reboot issues remotely.

4

Unless you have installed a kernel metapackage like linux-image-2.6-686 which will always pull in the latest kernel minor revision for a kernel
release and a given architecture.
5
A sample script called testnet [http://www.debian-administration.org/articles/70/testnet] is available in the Remotely rebooting Debian GNU/Linux machines [http://www.debian-administration.org/?article=70] article. A more elaborate network connectivity testing script is available in thisTesting network connectivity article. [http://www.debian-administration.org/?article=128]
6
Setting up a serial console is beyond the scope of this document, for more information read the Serial HOWTO [http://www.tldp.org/HOWTO/Serial-HOWTO.html] and the Remote Serial Console HOWTO [http://www.tldp.org/HOWTO/Remote-Serial-Console-HOWTO/index.html].

20

Después de la instalación

Change the BIOS (again)
Remember “Elegir una contraseña para el BIOS”? Well, then you should now, once you do not need toboot from removable media, to change the default BIOS setup so that it only boots from the hard drive.
Make sure you will not lose the BIOS password, otherwise, in the event of a hard disk failure you will not
be able to return to the BIOS and change the setup so you can recover it using, for example, a CD-ROM.
Another less secure but more convenient way is to change the setup to have the system boot up from thehard disk and, if it fails, try removable media. By the way, this is often done because most people don't
use the BIOS password that often; it's easily forgotten.

Colocar una contraseña a lilo o grub
Anybody can easily get a root-shell and change your passwords by entering<name-of-your-bootimage> init=/bin/shEs muy fácil entrar a una shell con el usuario root y cambiar las contraseñas simplemente tecleando "<name-of-your-bootimage> init=/bin/sh". Luego de cambiar las contraseñas y reingresar al sistema, la persona
ha tiene acceso ilimitado (como root) y puede hacer cualquier cosa que el/ella quiera en el sistema. Despuésde este procedimiento, usted no tendrá acceso a su sistema, porque usted no conoce la contraseña de root.
Asegúrese que esto no pueda suceder, usted debería colocar una contraseña para el cargador de linux.
Usted puede escoger entre una contraseña global y una contraseña para una imagen.
Para LILO usted necesita editar el archivo /etc/lilo.conf y agregar una contraseña y restringirlo
como en el siguiente ejemplo:
image=/boot/2.2.14-vmlinuz
label=Linux
read-only
password=hackme
restricted
Then, make sure that the configuration file is not world readable to prevent local users from reading the
password. When done, rerun lilo. Omitting the restricted line causes lilo to always prompt for apassword, regardless of whether LILO was passed parameters. The default permissions for /etc/lilo.conf grant read and write permissions to root, and enable read-only access for lilo.conf's group,root.
Si usted usa GRUB en lugar de LILO, edite /boot/grub/menu.lst y agregue las siguientes dos líneas
al inicio (sustituyendo, por supuesto 'hackme' con la contraseña deseada). Esto previene a los usuarios deeditar los ítems de entrada. 'timeout3' especifica tres segundos antes del arranque del sistema por defecto.
timeout 3
password hackme
Para asegurar mas la integridad de la contraseña, usted podría guardarla una forma encriptada. La utilidad
de grub-d5-crypt es que genera una contraseña la cual es compatible con el algorítmo (md5) de encripciónde grub. Para especificar en GRUB que el formato de la contraseña md5 será usado, use la siguiente
instrucción:21Después de la instalación

timeout 3
password --md5 $1$bw0ez$tljnxxKLfMzmnDVaQWgjP0
El parámetro --md5 fue agregado para instruir a grub a realizar el proceso de autenticación. La contraseña
proporcionada es la versión encriptada en md5 de "hackme". Usar el método de encripción md5 es preferible a su contraparte en solo texto. Mas información acera de la contraseña GRUB puede ser encontrada
en el paquete de grub-doc.

Disable root prompt on the initramfs
Note: This applies to the default kernels provided for releases after Debian 3.1
Los núcleos de Linux 2.6 proporcionan una forma para tener acceso a la línea de comandos del administrador que será presentada justo después de cargar el sistema de archivos cramfs. Un mensaje aparecerá para
permitir al administrador entrar en una línea de comandos con permisos de root, esta línea de comandos
puede ser usada manualmente para cargar módulos cuando la autodetección falla. Este comportamiento es
el predeterminado para initrd's linuxrc. El siguiente mensaje aparecerá:"ALERT!

/dev/sda1 does not exist.

Dropping to a shell!

In order to remove this behavior you need to set the following boot argument:panic=0. Add this to thevariable GRUB_CMDLINE_LINUX in /etc/default/grub and issue update-grub or to the append
section of /etc/lilo.conf.

Eliminar el prompt de root del núcleo
Note: This does not apply to the kernels provided for Debian 3.1 as the timeout for the kernel delay has
been changed to 0.
Los núcleos de Linux 2.6 proporcionan una forma para tener acceso a la línea de comandos del administrador que será presentada justo después de cargar el sistema de archivos cramfs. Un mensaje aparecerá para
permitir al administrador entrar en una línea de comandos con permisos de root, esta línea de comandos
puede ser usada manualmente para cargar módulos cuando la autodetección falla. Este comportamiento es
el predeterminado para initrd's linuxrc. El siguiente mensaje aparecerá:

Press ENTER to obtain a shell (waits 5 seconds)
Para eliminar este comportamiento usted necesita cambiar /etc/mkinitrd/mkinitrd.conf y colocar:# DELAY define lo segundos que el script linuxrc debe esperar para
# que el ususario pueda interrunpir el inicio del sistema
DELAY=0
Luego regenera su imagen del disco RAM. Usted puede hacer esto por ejemplo con:# cd /boot# mkinitrd -o initrd.img-2.4.18-k7 /lib/modules/2.4.18-k7
O hacer (preferir):

22

Después de la instalación# dpkg-reconfigure kernel-image-2.4.x-yz

Restricción del acceso a la consola
Algunas políticas de seguridad quieren forzar a los administradores para registrarse en el sistema a travésde la consola con su usuario/contraseña y luego llegar a ser un superusuario (consu o sudo). Esta política
es implementada en Debian al editar el archivo /etc/login.defs o /etc/securetty cuando seusa PAM. En:/etc/pam.d/login In older Debian releases you would need to edit login.defs, and use the CONSOLE variable which defines a file or list of terminals on which root logins are allowed. enables the
pam_securetty.so module. This module, when properly configured will not ask for a password when theroot user tries to login on an insecure console, rejecting access as this user.
securetty The /etc/securetty is a configuration file that belongs to the login package. by adding/removing the terminals to which root access will be allowed. If you wish to allow only local console
access then you need console, ttyX Or ttyvX in GNU/FreeBSD, and ttyE0 in GNU/KNetBSD. and vc/X (ifusing devfs devices), you might want to add also ttySX Or comX in GNU/Hurd, cuaaX in GNU/FreeBSD,
and ttyXX in GNU/KNetBSD. if you are using a serial console for local access (where X is an integer, you
might want to have multiple instances. The default configuration for Wheezy The default configuration
in woody includes 12 local tty and vc consoles, as well as the console device but does not allow remotelogins. In sarge the default configuration provides 64 consoles for tty and vc consoles. includes many tty
devices, serial ports, vc consoles as well as the X server and the console device. You can safely adjust this
if you are not using that many consoles. You can confirm the virtual consoles and the tty devices you haveby reviewing /etc/inittab Look for the getty calls. . For more information on terminal devices readthe Text-Terminal-HOWTO [http://tldp.org/HOWTO/Text-Terminal-HOWTO-6.html]
Cuando use PAM se hacen otros cambios para el proceso de registro, los cuales pueden incluir restricciones
para usuarios y grupos a tiempos dados, puede ser configurado en /etc/pam.d/login. Una interesantecaracterística que puede ser incapacitada es la posibilidad de registrar con contraseñas sin efecto (nulas).
Esta característica puede ser limitada removiendo el nullok de la linea:

auth required pam_unix.so nullok

Restricting system reboots through the console
If your system has a keyboard attached to it anyone (yes anyone) with physical access to the system canreboot the system through it without login in just pressing the Ctrl+Alt+Delete keyboard combination,
also known as the three finger salute. This might, or might not, adhere to your security policy.
This is aggravated in environments in which the operating system is running virtualised. In these environments, the possibility extends to users that have access to the virtual console (which might be accessedover the network). Also note that, in these environments, this keyboard combination is used constantly(to open a login shell in some GUI operating systems) and an administrator might virtually send it andforce a system reboot.
There are two ways to restrict this:• configure it so that only allowed users can reboot the system,

23

Después de la instalación• disable this feature completely.
If you want to restrict this, you must check the /etc/inittab so that the line that includes ctrlaltdel calls shutdown with the -a switch.
The default in Debian includes this switch:

ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now
The -a switch, as the shutdown(8) manpage describes,makes it possible to allow some users to shutdown
the system. For this the file /etc/shutdown.allow must be created and the administrator has toinclude there the name of users which can boot the system. When the three finger salute combination ispressed in a console the program will check if any of the users listed in the file are logged in. If none ofthem is, shutdown will not reboot the system.
If you want to disable the Ctrl+Alt+Del combination you just need to comment the line with the ctrlaltdel
definition in the /etc/inittab.
Remember to run init q after making any changes to the /etc/inittab file for the changes to takeeffect.

Restricting the use of the Magic SysRq key
The Magic SysRq key is a key combination that allows users connected to the system console of a Linuxkernel to perform some low-level commands. These low-level commands are sent by pressing simultaneously Alt+SysRq and a command key. The SysRq key in many keyboards is labeled as the Print Screen key.
Since the Etch release, the Magic SysRq key feature is enabled in the Linux kernel to allow console users
certain privileges. You can confirm this by checking if the /proc/sys/kernel/sysrq exists andreviewing its value:$ cat /proc/sys/kernel/sysrq
438
The default value shown above allows all of the SysRq functions except for the possibility of sending
signals to processes. For example, it allow users connected to the console to remount all systems readonly, reboot the system or cause a kernel panic. In all the features are enabled, or in older kernels (earlier
than 2.6.12) the value will be just 1.
You should disable this functionality ifaccess to the console is not restricted to authorised users: the console
is connected to a modem line, there is easy physical access to the system or it is running in a virtualised
environment and other users access the console. To do this edit the /etc/sysctl.conf and add thefollowing lines:# Disables the magic SysRq key
kernel.sysrq = 0
For more information, read security chapter in the Remote Serial Console HOWTO [http://tldp.org/
HOWTO/Remote-Serial-Console-HOWTO/security-sysrq.html], Kernel SysRQ documentation [https://www.kernel.org/doc/Documentation/admin-guide/sysrq.rst]. and the Magic_SysRq_key wikipedia entry[http://en.wikipedia.org/wiki/Magic_SysRq_key].

24

Después de la instalación

Montando particiones de manera correcta
When mounting an Ext file system (ext2, ext3 or ext4), there are several additional options you canapply to the mount call or to /etc/fstab. For instance, this is my fstab entry for the /tmp partition:/dev/hda7

/tmp

ext2

defaults,nosuid,noexec,nodev

02

usted ve la diferencia a las secciones de opciones . La opción nosuid ignora los bits setuid y setgid
completamente , mientras que noexec prohibe la ejecución de programas en ese punto de montaje, ynodev, ignora los dispositivos.Esto suena grandioso , pero esto
• only applies to ext2 or ext3 file systems• puede ser evitado fácilmente
La opción noexec previene los binarios de ejecutarse directamente, pero se engaña fácilmente:

alex@joker:/tmp# mount | grep tmp/dev/hda7 on /tmp type ext2 (rw,noexec,nosuid,nodev)
alex@joker:/tmp# ./datebash: ./date: Permission denied
alex@joker:/tmp# /lib/ld-linux.so.2 ./date
Sun Dec 3 17:49:23 CET 2000
Newer versions of the kernel do however handle the noexec flag properly:angrist:/tmp# mount | grep /tmp
/dev/hda3 on /tmp type ext3 (rw,noexec,nosuid,nodev)
angrist:/tmp# ./datebash: ./tmp: Permission denied
angrist:/tmp# /lib/ld-linux.so.2 ./date./date: error while loading shared libraries: ./date: failed to map segmentfrom shared object: Operation not permitted
Sin embargo, muchos "script kiddies" cuentan con "xploits" que intentan crear y ejecutar los archivos en/tmp.Si ellos no tienen una pista, ellos entrarán en esta trampa. En otros términos, un usuario no puedeengañarse en ejecutar un binario troyanizado en /tmp e.g. por ejemplo cuando él agrega a propósito /tmpdentro de su PATH.
También se previene de algún programa que podría depender en que /tmp sea ejecutable. Más notablemente, Debconf tiene (¿tenía?) algunos problemas que consideran esto, para más información vea Bug
http://bugs.debian.org/116448.
The following is a more thorough example. A note, though: /var could be set noexec, but some software7
keeps its programs under in /var. The same applies to the nosuid option.

/dev/sda6/dev/sda12/usr/usr/share

ext2
ext2

7

defaults,ro,nodev
0
defaults,ro,nodev,nosuid

Some of this includes the package manager dpkg since the installation (post,pre) and removal (post,pre) scripts are at /var/lib/dpkg/ andSmartlist

25

2
0

2

Después de la instalación/dev/sda7/dev/sda8/dev/sda9/dev/sda10/dev/sda11/dev/sda13/dev/fd0/dev/fd0/dev/hda/var/tmp/var/tmp/var/log/var/account/home/mnt/fd0/mnt/floppy/mnt/cdrom

ext2
defaults,nodev,usrquota,grpquota
0
ext2
defaults,nodev,nosuid,noexec,usrquota,grpqu
ext2
defaults,nodev,nosuid,noexec,usrquota,grpqu
ext2
defaults,nodev,nosuid,noexec
02
ext2
defaults,nodev,nosuid,noexec
02
ext2
rw,nosuid,nodev,exec,auto,nouser,async,usrq
ext2
defaults,users,nodev,nosuid,noexec
0
vfat
defaults,users,nodev.nosuid,noexec0
iso9660 ro,users,nodev.nosuid,noexec
0

Setting /tmp noexec
Tenga cuidado si esta poniendo /tmpy usted quiere instalar el nuevo software, desde que alguno podría
usarlo para la instalación. Apt es uno de esos programas (vea http://bugs.debian.org/116448) si no configuró propiamente APT::ExtractTemplates::TempDir (vea apt-extracttemplates(1)). Usted puede poner esta variable en /etc/apt/apt.conf a otro directorio con privilegios exec que no sea /tmp

Serie /usr leer-únicamente
Si usted pusiera /usr leer - únicamente usted no podrá instalar los nuevos paquetes en su Debian GNU /sistema Linux. Usted tendrá, primero que remontar leer -escribir, instale los paquetes y entonces remónteloleer-únicamente. La última versión apt (en Debian 3.0ŽwoodyŽ) puede configurarse para ejecutar lasórdenes antes y después de instalar los paquetes, para que usted pueda propiamente querer configurarlo.
Hacer esto modifica /etc/apt/apt.conf y agrega:

DPkg{
Pre-Invoke { "mount /usr -o remount,rw" };Post-Invoke { "mount /usr -o remount,ro" };};Note that the Post-Invoke may fail with a "/usr busy" error message. This happens mainly when you areusing files during the update that got updated. You can find these programs by running# lsof +L1
Stop or restart these programs and run the Post-Invoke manually. Beware! This means you'll likely needto restart your X session (if you're running one) every time you do a major upgrade of your system. Youmight want to reconsider whether a read-only /usr is suitable for your system. See also this discussion
on debian-devel about read-only [http://lists.debian.org/debian-devel/2001/11/threads.html#00212].

Acceso seguro para los usuarios
Uso de la autentificación: PAM
PAM (Pluggable Authentication Modules) allows system administrators to choose how applications authenticate users. Note that PAM can do nothing unless an application is compiled with support for PAM.
Most of the applications that are shipped with Debian have this support built in (Debian did not have PAM
support before 2.2). The current default configuration for any PAM-enabled service is to emulate UNIX

26

Después de la instalación

authentication (read /usr/share/doc/libpam0g/Debian-PAM-MiniPolicy.gz for more information on how PAM services should work in Debian).
Each application with PAM support provides a configuration file in /etc/pam.d/ which can be used
to modify its behavior:• what backend is used for authentication.
• what backend is used for sessions.
• how do password checks behave.
The following description is far from complete, for more information you might want to read the Linux-PAM Guides [https://packages.debian.org/sid/libpam-doc] as a reference. This documentation is available in the system if you install the libpam-doc at /usr/share/doc/libpam-doc/html/.
PAM le ofrece a usted la posibilidad a ir por varios pasos de autenticación una vez, sin el uso de conocimientos .Usted puede autenticar de nuevo una base de datos Berkeley y de nuevo el archivo de password
normal y el uso únicamente de registros en si correctamente autenticos en ambos. Usted puede limitar amuchos con PAM , así como usted puede abrir sus puertas del sistema muy extensamente. Así que tenga
cuidado. Una línea de la típica configuración tiene un campo de mando como su segundo elemento.

Password security in PAM
Review the /etc/pam.d/common-password, included by /etc/pam.d/passwd8 This file is included by other files in /etc/pam.d/ to define the behaviour of password use in subsystems that grant
access to services in the machine, like the console login (login), graphical login managers (such as gdm
or lightdm), and remote login (such as sshd). This definition is
You have to make sure that the pam_unix.so module uses the "sha512" option to use encrypted passwords.
This is the default in Debian Squeeze.
The line with the definition of the pam_unix module will look something like:

password[success=1 default=ignore]

pam_unix.so nullok obscure minlen=8 sh

Esta definición• Enforces password encryption when storing passwords, using the SHA-512 hash function (option
sha512),• Enables password complexity checks (option obscure) as defined in the pam_unix(8) manpage,• Imposes a minimum password length (option min) of 8.
You have to ensure that encrypted passwords are used in PAM applications, since this helps protect against
dictionary cracks. Using encryption also makes it possible to use passwords longer than 8 characters.
Since this module is also used to define how passwords are changed (it is included by chpasswd) you canstrengthen the password security in the system by installing libpam-cracklib and introducing this definitionin the /etc/pam.d/common-password configuration file:8

In old Debian releases the configuration of the modules was defined directly in /etc/pam.d/passwd.

27

Después de la instalación# Asegúrese de tener instalado libpam-cracklib, sino no podrá entrar en el sistema
password
required
pam_cracklib.so retry=3 minlen=12 difok=3
password[success=1 default=ignore]
pam_unix.so obscure minlen=8 sha512 us
So, what does this incantation do? The first line loads the cracklib PAM module, which provides password
strength-checking, prompts for a new password with a minimum size 9 of 12 characters, and difference ofat least 3 characters from the old password, and allows 3 retries. Cracklib depends on a wordlist package(such as wenglish, wspanish, wbritish, ...), so make sure you install one that is appropriate for your language
or cracklib might not be useful to you at all.
The second line (using the pam_unix.so module) is the default configuration in Debian, as described above, save for the use_authok option. The use_authok option is required if pam_unix.so is stacked after
pam_cracklib.so, and is used to hand over the password from the previous module. Otherwise, the user
would be prompted for the password twice.
For more information about setting up Cracklib, read the pam_cracklib(8) manpage and the article Linux Password Security with pam_cracklib [http://www.deer-run.com/~hal/sysadmin/pam_cracklib.html]
by Hal Pomeranz.
By enabling the cracklib PAM module you setup a policy that forces uses to use strong passwords.
Alternatively, you can setup and configure PAM modules to use double factor authentication such as:libpam-barada, libpam-google-authenticator, libpam-oath, libpam-otpw, libpam-poldi, libpam-usb or libpam-yubico. The configuration of these modules would make it possible to access the system using external authentication mechanisms such as smartcards, external USB keys, or One-Time-Passwords generated
by external applications running, for example, in the user's mobile phone.
Please note that these restrictions apply to all users but not to the password changes done by the root user.
The root user will be able to set up any password (any length or complexity) for personal use or others
regardless of the restrictions defined here.

Control de acceso de usuarios con PAM
Para asegurarse que el root (administrador de Linux) del usuario sólo puede anotarse en el sistema de lostérminos locales, la línea siguiente debe habilitarse en /etc/pam.d/login:authrequisite

pam_securetty.soThen you should modify the list of terminals on which direct root login is allowed in /etc/securetty(as described in “Restricción del acceso a la consola”). Alternatively, you could enable the pam_accessmodule and modify /etc/security/access.conf which allows for a more general and fine-tuned
access control, but (unfortunately) lacks decent log messages (logging within PAM is not standardized
and is particularly unrewarding problem to deal with). We'll return to access.conf a little later.

Limites para los usuarios mediante PAM
The following line should be enabled in /etc/pam.d/login to set up user resource limits.

sessionrequired

pam_limits.so

9

The minlen option is not entirely straightforward and is not exactly the number of characters in the password. A tradeoff can be defined between
complexity and length by adjusting the "credit" parameters of different character classes. For more information read the pam_cracklib(8) manpage.

28

Después de la instalación

Esto restringe los recursos del sistema que se permiten a los usuarios (vea en la siguiente pagina userlimits. Por ejemplo, usted podría restringir el número de logins coexistente (de un grupo dado de usuarios,o sistema-ancho) usted puede tener, el número de procesos, el tamaño de memoria......

Control de 'su' mediante PAM
Si usted quiere proteger su (un comando), para que sólo algunas personas puedan usarlo para volverse aroot en su sistema, usted necesita agregar uno nuevo para agregar un nuevo "wheel" de grupo a su sistema(ésa es la manera más limpia, desde que ningún archivo tiene tal un permiso de grupo todavía). Agregue elroot y los otros usuarios que deberian ser capaces de ejecutar su a el usuario de root a este grupo. Entonces
agregue la línea siguiente a /etc/pam.d/su:

authrequisite

pam_wheel.so group=wheel debug

Esto asegura que sólo personas de el grupo wheel pueden usar su para volverse root. Otros usuarios noseran capaces de volverse root. De hecho ellos conseguirán un mensaje negado si ellos intentan volverse
volverse root.
Si usted quiere que sólo ciertos usuarios autentiquen a un servicio de PAM, esto es bastante fácil de lograr usando los archivos dónde los usuarios que son permitidos al login (o no) se guarden. Sólo imagine
que usted quiere permitirle el login de ŽrefŽto al usuario vía ssh. Así que usted lo pone en /etc/sshusers-allowed y le escribe lo siguiente en /etc/pam.d/ssh:authrequired

pam_listfile.so item=user sense=allow file=/etc/sshusers-

Carpetas temporales en PAM
Since there have been a number of so called insecure tempfile vulnerabilities, thttpd is one example (seeDSA-883-1 [http://www.debian.org/security/2005/dsa-883]), the libpam-tmpdir is a good package to install. All you have to do is add the following to /etc/pam.d/common-session:

sessionoptional

pam_tmpdir.so

There has also been a discussion about adding this by default in Debian configuration, but it s. See http://
lists.debian.org/debian-devel/2005/11/msg00297.html for more information.

Configuración de aplicaciones genéricas de PAM
Por último, pero no menos importante, cree /etc/pam.d/other y coloque las líneas siguientes:

authrequiredauthrequiredauthrequiredauthrequiredaccount required
account required
account required
password required
password required

pam_securetty.so
pam_unix_auth.so
pam_warn.so
pam_deny.so
pam_unix_acct.so
pam_warn.so
pam_deny.so
pam_unix_passwd.so
pam_warn.so

29

Después de la instalación

password required
session required
session required
session required

pam_deny.so
pam_unix_session.so
pam_warn.so
pam_deny.so

Estas líneas mantendrán una buena configuración predefinida en todas las aplicaciones que apoyan PAM(se niega el acceso por el valor predeterminado).

Limitación de recursos: archivo limits.conf
You should really take a serious look into this file. Here you can define user resource limits. In old releases
this configuration file was /etc/limits.conf, but in newer releases (with PAM) the /etc/security/limits.conf configuration file should be used instead.
If you do not restrict resource usage, any user with a valid shell in your system (or even an intruder who
compromised the system through a service or a daemon going awry) can use up as much CPU, memory,
stack, etc. as the system can provide. This resource exhaustion problem can be fixed by the use of PAM.
There is a way to add resource limits to some shells (for example, bash has ulimit, see bash(1)), but since
not all of them provide the same limits and since the user can change shells (see chsh(1)) it is better toplace the limits on the PAM modules as they will apply regardless of the shell used and will also apply
to PAM modules that are not shell-oriented.
Resource limits are imposed by the kernel, but they need to be configured through the limits.conf
and the PAM configuration of the different services need to load the appropriate PAM. You can check
which services are enforcing limits by running:$ find /etc/pam.d/ \! -name "*.dpkg*" | xargs -- grep limits |grep -v ":#"
Commonly, login, ssh and the graphic session managers (gdm, kdm or xdm) should enforce user limits
but you might want to do this in other PAM configuration files, such as cron, to prevent system daemonsfrom taking over all system resources.
The specific limits settings you might want to enforce depend on your system's resources, that's one of themain reasons why no limits are enforced in the default installation.
For example, the configuration example below enforces a 100 process limit for all users (to prevent fork
bombs) as well as a limit of 10MB of memory per process and a limit of 10 simultaneous logins. Users in
the adm group have higher limits and can produce core files if they want to (there is only a soft limit).

*
*
*
*
*
*
*
*
@adm
@adm
@adm
@adm
@adm

soft
hard
hard
hard
hard
hard
hard
hard
hard
soft
hard
hard

core
core
rss
memlock
nproc
maxlogins
data
fsize
core
rss
nproc
nproc
fsize

30

0
0
1000
1000
100
1
1024002048
100000100000
2000
3000
100000Después de la instalación

@adm

-

maxlogins

10

These would be the limits a default user (including system daemons) would have:$ ulimit -acore file size
(blocks, -c) 0
data seg size
(kbytes, -d) 102400
file size(blocks, -f) 2048
max locked memory(kbytes, -l) 10000
max memory size
(kbytes, -m) 10000
open files(-n) 1024
pipe size(512 bytes, -p) 8
stack size
(kbytes, -s) 8192
cpu time(seconds, -t) unlimited
max user processes(-u) 100
virtual memory(kbytes, -v) unlimited
estos son los limites para un usuarios administrador:$ ulimit -acore file size
(blocks, -c) 0
data seg size
(kbytes, -d) 102400
file size(blocks, -f) 100000
max locked memory(kbytes, -l) 100000
max memory size
(kbytes, -m) 100000
open files(-n) 1024
pipe size(512 bytes, -p) 8
stack size
(kbytes, -s) 8192
cpu time(seconds, -t) unlimited
max user processes(-u) 2000
virtual memory(kbytes, -v) unlimited
For more information read:• PAM reference guide for available modules [https://web.archive.org/web/20030601112932/http://
www.kernel.org/pub/linux/libs/pam/Linux-PAM-html/pam-6.html]• PAM configuration article [https://web.archive.org/web/20030217012148/http://www.samag.com/documents/s=1161/sam0009a/0009a.htm].
• Seifried's Securing Linux Step by Step [http://seifried.org/security/os/linux/20020324-securing-linux-step-by-step.html] on the Limiting users overview section.
• LASG [http://seifried.org/lasg/users/] in the Limiting and monitoring users section.

User login actions: edit /etc/login.defs
The next step is to edit the basic configuration and action upon user login. Note that this file is not partof the PAM configuration, it's a configuration file honored by login and su programs, so it doesn't makesense tuning it for cases where neither of the two programs are at least indirectly called (the getty program
which sits on the consoles and offers the initial login prompt does invoke login).
FAILLOG_ENAB

yes

31

Después de la instalación

Si usted habilita esta variable, se anotarán los logins fallados. Es importante guardar huella de ellos para
coger a alguien que pruebe un ataque de fuerza bruta.
LOG_UNKFAIL_ENABno

If you set this variable to 'yes' it will record unknown usernames if the login failed. It is best if you use'no' (the default) since, otherwise, user passwords might be inadvertenly logged here (if a user mistypes
and they enter their password as the username). If you set it to 'yes', make sure the logs have the proper
permissions (640 for example, with an appropriate group setting such as adm).
SYSLOG_SU_ENAB

yes

Este uno habilita el logging de la pueba su a syslog. Bastante importante en serias maquinas pero noteque esto puede crear el retiro de los resultados a medida que esten bien.
SYSLOG_SG_ENAByes

The same as SYSLOG_SU_ENAB but applies to the sg program.
ENCRYPT_METHODSHA512

As stated above, encrypted passwords greatly reduce the problem of dictionary attacks, since you can
use longer passwords. This definition has to be consistent with the value defined in /etc/pam.d/common-password.

User login actions: edit /etc/pam.d/login
You can adjust the login configuration file to implement an stricter policy. For example, you can change
the default configuration and increase the delay time between login prompts. The default configuration
sets a 3 seconds delay:authoptional

pam_faildelay.sodelay=3000000

Increasing the delay value to a higher value to make it harder to use the terminal to log in using brute
force. If a wrong password is typed in, the possible attacker (or normal user!) has to wait longer seconds
to get a new login prompt, which is quite time consuming when you test passwords. For example, if you
set delay=10000000, users will have to wait 10 seconds if they type a wrong password.
In this file you can also set the system to present a message to users before a user logs in. The default isdisabled, as shown below:# authrequired

pam_issue.so issue=/etc/issue

If required by your security policy, this file can be used to show a standard message indicating that accessto the system is restricted and user acess is logged. This kind of disclaimer might be required in someenvironments and jurisdictions. To enable it, just include the relevant information in the /etc/issue10
file and uncomment the line enabling the pam_issue.so module in /etc/pam.d/login. In this file you
can also enable additional features which might be relevant to apply local security policies such as:
10

The default content of this file provides information about the operating system and version run by the system, which you might not want toprovide to anonymous users.

32

Después de la instalación• setting rules for which users can access at which times, by enabling the pam_time.so module and configuring /etc/security/time.conf accordingly (disabled by default),• setup login sessions to use user limits as defined in /etc/security/limits.conf (enabled bydefault),• present the user with the information of previous login information (enabled by default),• print a message (/etc/motd and /run/motd.dynamic) to users after login in (enabled by default),

Restricting ftp: editing /etc/ftpusers
Este archivo contiene una lista de usuarios no autorizados a entrar en el sistema mediante ftp. S#o debería
emplear este archivo si desea prpoprcionar ftp (lo cual es -en general- poco aconsejable debido al uso
de contraseñas sin cifrar). Si incorpora soporte para PAM, puede autorizar o denegar el uso de ciertos
servicios a los usuarios.
FIXME (BUG): Is it a bug that the default ftpusers in Debian does not include all the administrative
users (in base-passwd).
A convenient way to add all system accounts to the /etc/ftpusers is to run$ awk -F : '{if ($3<1000) print $1}' /etc/passwd > /etc/ftpusers

Uso de su
Si usted realmente necesita que los usuarios se vuelvan el super usuario en su sistema,e.g. por instalarlos paquetes o agregar usuarios, usted puede usar el comando su para cambiar su identidad. Usted debe
intentar evitar cualquier login como root del usuario y en cambio usar su. Realmente, la mejor solución
es quitar su y cambiar a sudo, como él tiene más rasgos que su. Sin embargo,su es más común como se
usa en muchos otro Unixes.

Uso de sudo
sudo le permite al usuario ejecutar los comandos definidos bajo la identidad de otro usuario, así como root.
Si el usuario agrega a /etc/sudoers y se autentica correctamente, él es capaz de avanzar comandos
en que se ha definido /etc/sudoers. Las Violaciones, como las contraseñas incorrectas o intentos deejecutar un programa usted no tienen permiso para ser anotado y mandado por correo a root.

Prohibición administración remota
You should also modify /etc/security/access.conf to disallow remote logins to administrative
accounts. This way users need to invoke su (or sudo) to use any administrative powers and the appropriate
audit trace will always be generated.
You need to add the following line to /etc/security/access.conf, the default Debian configuration file has a sample line commented out:-:wheel:ALL EXCEPT LOCAL
Remember to enable the pam_access module for every service (or default configuration) in /etc/
pam.d/ if you want your changes to /etc/security/access.conf honored.

33

Después de la instalación

Restringiendo usuarios
A veces usted podría pensar que necesita tener los usuarios creados en su sistema local para proporcionar unservicio (pop3 manda por correo el servicio o ftp). Antes de hacer eso, primero recuerde que la aplicaciónde PAM en Debian GNU/Linux le permite validar a los usuarios con una variedad ancha de el servicio dedirectorio externo (el radio, el ldap, etc.) con tal de que por el ,el libpam sea empacado.
Si los usuarios necesitan ser creados y el sistema puede ser remotamente de acceso tome en cuenta que losusuarios sean capaces al login al sistema. Usted puede arreglar esto dando a los usuarios una nula (/dev/null) interfaz de comandos (él necesitaría ser listada en /etc/shells). Si usted quiere permítalesa los usuarios acceder a el sistema pero limitar sus movimientos, usted puede usar el /bin/rbash,
equivalente a agregar la opción -r en bash (RESTICTED SHELL ver bash(1)). Por favor note que incluso
con la interfaz de comandos restringido, un usuario que entra en acceso a un programa interactivo (esopodría permitirle la ejecución de un subshell) podría poder desviar los límites de el shell.
Debian currently provides in the unstable release (and might be included in the next stable releases) the
pam_chroot module (in the libpam-chroot). An alternative to it is to chroot the service that provides
remote logging (ssh, telnet). 11
Si usted desea restringirlo when los usuarios pueden acceder a el sistema que usted quiere tener personalizado /etc/security/access.conf para sus necesidades.
Information on how to chroot users accessing the system through the ssh service is described in “Chroot
environment for SSH”.

Auditoría de usuarios
If you are really paranoid you might want to add a system-wide configuration to audit what the users are
doing in your system. This sections presents some tips using diverse utilities you can use.

Input and output audit with script
You can use the script command to audit both what the users run and what are the results of those commands. You cannot setup script as a shell (even if you add it to /etc/shells). But you can have theshell initialization file run the following:umask 077
exec script -q -a "/var/log/sessions/$USER"
Of course, if you do this system wide it means that the shell would not continue reading personal initialization files (since the shell gets overwritten by script). An alternative is to do this in the user's initialization
files (but then the user could remove this, see the comments about this below)
You also need to setup the files in the audit directory (in the example /var/log/sessions/) so thatusers can write to it but cannot remove the file. This could be done, for example, by creating the user
session files in advance and setting them with the append-only flag using chattr.
A useful alternative for sysadmins, which includes date information would be:

umask 077
exec script -q -a "/var/log/sessions/$USER-`date +%Y%m%d`"11

libpam-chroot has not been yet thoroughly tested, it does work for login but it might not be easy to set up the environment for other programs

34

Después de la instalación

Using the shell history fileIf you want to review what does the user type in the shell (but not what the result of that is) you can setup
a system-wide /etc/profile that configures the environment so that all commands are saved into ahistory file. The system-wide configuration needs to be setup in such a way that users cannot remove audit
capabilities from their shell. This is somewhat shell specific so make sure that all users are using a shellthat supports this.
For example, for bash, the /etc/profile could be set as follows 12 :HISTFILE=~/.bash_history
HISTSIZE=10000
HISTFILESIZE=999999# Don't let the users enter commands that are ignored# in the history file
HISTIGNORE=""HISTCONTROL=""
readonly HISTFILE
readonly HISTSIZE
readonly HISTFILESIZE
readonly HISTIGNORE
readonly HISTCONTROL
export HISTFILE HISTSIZE HISTFILESIZE HISTIGNORE HISTCONTROLFor this to work, the user can only append information to .bash_history file. You need also to set
the append-only option using chattr program for .bash_history for all users. 13.
Note that you could introduce the configuration above in the user's .profile. But then you would needto setup permissions properly in such a way that prevents the user from modifying this file. This includes:
having the user's home directories not belong to the user (since the user would be able to remove the fileotherwise) but at the same time allow the user to read the .profile configuration file and write on the.bash_history. It would be good to set the immutable flag (also using chattr) for .profile too
if you do it this way.

Auditoría total de usuarios mediante contabilidad de utilidades
El ejemplo anterior es una manera simple de configurar el usuario interviniendo el cual no podría ser útilpara los sistemas complejos. Si éste es su caso, usted necesita mirar a acct, la contabilidad de utilidades.
Éstos anotarán todos los comandos corridos por usuarios o procesos en el sistema, al gasto de espaciodel disco.
Al activar la contabilidad, toda la información sobre los procesos y el usuario se guarda bajo /var/account/, más específicamente en el pacct. El paquete de contabilidad incluye algunas herramientas(sa y ac) para analizar estos datos.

Otros métodos de auditoría
If you are completely paranoid and want to audit every user's command, you could take bash source code, edit it and have it send all that the user typed into another file. Or have ttysnoop constantly monitor
any new ttys 14 and dump the output into a file. Other useful program is snoopy (see also github: https://
12

Setting HISTSIZE to a very large number can cause issues under some shells since the history is kept in memory for every user session. Youmight be safer if you set this to a high-enough value and backup user's history files (if you need all of the user's history for some reason)
13
Without the append-only flag users would be able to empty the contents of the history file running > .bash_history
14
Ttys are spawned for local logins and remote logins through ssh and telnet

35

Después de la instalación

github.com/a2o/snoopy) which is a user-transparent program that hooks in as a library providing a wrapper around execve() calls, any command executed is logged to syslogd using the authpriv facility(usually stored at /var/log/auth.log).

Repasando los perfiles del usuario
Si usted quiere normalmente see a los usuarios qué están haciendo, cuando esten ellos conectándos usted
pueden usar la base de datos de wtmp que incluye toda la información del login. Este archivo puede
procesarse con varias utilidades, entre ellos sac el cual puede hacer un profile en cada usuario que muestra
en que estructura de tiempo ellos normalmente anotan adelante en el sistema.
En caso de que usted tiene la contabilidad activada, usted también puede usar las herramientas con tal deque por esto en el comando determine cuando los usuarios acceden a el sistema y qué ellos ejecuten.

Setting users umasks
Depending on your user policy you might want to change how information is shared between users, thatis, what the default permissions of new files created by users are.
Debian's default umask setting is 022 this means that files (and directories) can be read and accessed by
the user's group and by any other users in the system. This definition is set in the standard configuration
file /etc/profile which is used by all shells.
If Debian's default value is too permissive for your system you will have to change the umask setting for
all the shells. More restrictive umask settings include 027 (no access is allowed to new files for the othergroup, i.e. to other users in the system) or 077 (no access is allowed to new files to the members the user'sgroup). Debian (by default15) creates one group per user so that only the user is included in its group.
Consequently 027 and 077 are equivalent as the user's group contains only the user.
This change is set by defining a proper umask setting for all users. You can change this by introducing an umask call in the shell configuration files: /etc/profile (source by all Bourne-compatible
shells), /etc/csh.cshrc, /etc/csh.login, /etc/zshrc and probably some others (dependingon the shells you have installed on your system). You can also change the UMASK setting in /etc/login.defs, Of all of these the last one that gets loaded by the shell takes precedence. The order is: thedefault system configuration for the user's shell (i.e. /etc/profile and other system-wide configuration files) and then the user's shell (his ~/.profile, ~/.bash_profile, etc...). Some shells, however, can be executed with a nologin value which might skip sourcing some of those files. See your shell's
manpage for additional information.
For connections that make use of login the UMASK definition in /etc/login.defs is used before
any of the others. However, that value does not apply to user executed programs that do not use login suchas those run through su, cron or ssh.
Don't forget to review and maybe modify the dotfiles under /etc/skel/ since these will be new user's
defaults when created with the adduser command. Debian default dotfiles do not include any umask callbut if there is any in the dotfiles newly created users might a different value.
Note, however that users can modify their own umask setting if they want to, making it more permissive
or more restricted, by changing their own dotfiles.
The libpam-umask package adjusts the users' default umask using PAM. Add the following, after installing the package, to /etc/pam.d/common-session:15

As defined in /etc/adduser.conf (USERGROUPS=yes). You can change this behaviour if you set this value to no, although it is notrecommended36Después de la instalación

sessionoptional

pam_umask.so umask=077

Finally, you should consider changing root's default 022 umask (as defined in /root/.bashrc) to amore strict umask. That will prevent the system administrator from inadvertenly dropping sensitive files
when working as root to world-readable directories (such as /tmp) and having them available for youraverage user.

Limitar el acceso a los usuarios
FIXME: Content needed. Describe the consequences of changing packages permissions when upgrading(an admin this paranoid should chroot his users BTW) if not using dpkg-statoverride.
If you need to grant users access to the system with a shell think about it very carefully. A user can, by
default unless in a severely restricted environment (like a chroot jail), retrieve quite a lot of informationfrom your system including:• some configuration files in /etc. However, Debian's default permissions for some sensitive files(which might, for example, contain passwords), will prevent access to critical information. To see whichfiles are only accessible by the root user for example
find /etc -type f -a -perm 600 -a -uid 0
as superuser.
• your installed packages, either by looking at the package database, at the /usr/share/doc directoryor by guessing by looking at the binaries and libraries installed in your system.
• some log files at /var/log. Note also that some log files are only accessible to root and the admgroup (tryfind /var/log -type f -a -perm 640) and some are even only available to the root user (tryfind /var/log -type f -a -perm
600 -a -uid 0).
What can a user see in your system? Probably quite a lot of things, try this (take a deep breath):find / -type f -a -perm +006 2>/dev/null
find / -type d -a -perm +007 2>/dev/null
The output is the list of files that a user can see and the accessable directories.

Limitar el acceso a los datos de otros usuarios
If you still grant shell access to users you might want to limit what information they can view from otherusers. Users with shell access have a tendency to create quite a number of files under their $HOMEs:
mailboxes, personal documents, configuration of X/GNOME/KDE applications...
In Debian each user is created with one associated group, and no two users belong to the same group. Thisis the default behavior: when an user account is created, a group of the same name is created too, and the37Después de la instalación

user is assigned to it. This avoids the concept of a common users group which might make it more difficult
for users to hide information from other users.
However, users' $HOME directories are created with 0755 permissions (group-readable and world-readable). The group permissions is not an issue since only the user belongs to the group, however the world
permissions might (or might not) be an issue depending on your local policy.
You can change this behavior so that user creation provides different $HOME permissions. To change
the behavior for new users when they get created, change DIR_MODE in the configuration file /etc/
adduser.conf to 0750 (no world-readable access).
Users can still share information, but not directly in their $HOME directories unless they change its permissions.
Note that disabling world-readable home directories will prevent users from creating their personal web
pages in the ~/public_html directory, since the web server will not be able to read one component
in the path - namely their $HOME directory. If you want to permit users to publish HTML pages in their~/public_html, then change DIR_MODE to 0751. This will allow the web server to access the final
public_html directory (which itself should have a mode of 0755) and provide the content publishedby users. Of course, we are only talking about a default configuration here; users can generally tune modesof their own files completely to their liking, or you could keep content intended for the web in a separate
location which is not a subdirectory of user's $HOME directory.

Generación de contraseñas de usuario
There are many cases when an administrator needs to create many user accounts and provide passwords
for all of them. Of course, the administrator could easily just set the password to be the same as the user's
account name, but that would not be very sensitive security-wise. A better approach is to use a password
generating program. Debian provides makepasswd, apg and pwgen packages which provide programs (thename is the same as the package) that can be used for this purpose. Makepasswd will generate true random
passwords with an emphasis on security over pronounceability while pwgen will try to make meaninglessbut pronounceable passwords (of course this might depend on your mother language). Apg has algorithms
to provide for both (there is a client/server version for this program but it is not included in the Debianpackage).
Passwd does not allow non-interactive assignation of passwords (since it uses direct tty access). If you
want to change passwords when creating a large number of users you can create them using adduser withthe --disabled-login option and then use usermod or chpasswd16 (both from the passwd package
so you already have them installed). If you want to use a file with all the information to make users as abatch process you might be better off using newusers.

Comprobación de contraseñas de usuarios
User passwords can sometimes become the weakest link in the security of a given system. This is due
to some users choosing weak passwords for their accounts (and the more of them that have access to it
the greater the chances of this happening). Even if you established checks with the cracklib PAM moduleand password limits as described in “Uso de la autentificación: PAM” users will still be able to use weakpasswords. Since user access might include remote shell access (over ssh, hopefully) it's important to makepassword guessing as hard as possible for the remote attackers, especially if they were somehow able tocollect important information such as usernames or even the passwd and shadow files themselves.
16

Chpasswd cannot handle MD5 password generation so it needs to be given the password in encrypted form before using it, with the-eoption.

38

Después de la instalación

A system administrator must, given a big number of users, check if the passwords they have are consistent
with the local security policy. How to check? Try to crack them as an attacker would if having access tothe hashed passwords (the /etc/shadow file).
An administrator can use john or crack (both are brute force password crackers) together with an appropriate wordlist to check users' passwords and take appropriate action when a weak password is detected.
You can search for Debian GNU packages that contain word lists using apt-cache search wordlist, orvisit some Internet wordlist sites.

Sacando usuarios inactivos
Idle users are usually a security problem, a user might be idle maybe because he's out to lunch or because
a remote connection hung and was not re-established. For whatever the reason, idle users might lead to
a compromise:• because the user's console might be unlocked and can be accessed by an intruder.
• because an attacker might be able to re-attach to a closed network connection and send commands to
the remote shell (this is fairly easy if the remote shell is not encrypted as in the case of telnet).
Some remote systems have even been compromised through an idle (and detached) screen.
Automatic disconnection of idle users is usually a part of the local security policy that must be enforced.
There are several ways to do this:• If bash is the user shell, a system administrator can set a default TMOUT value (see bash(1)) which will
make the shell automatically log off remote idle users. Note that it must be set with the -o option orusers will be able to change (or unset) it.
• Install timeoutd and configure /etc/timeouts according to your local security policy. The daemon
will watch for idle users and time out their shells accordingly.
• Install autolog and configure it to remove idle users.
The timeoutd or autolog daemons are the preferred method since, after all, users can change their default
shell or can, after running their default shell, switch to another (uncontrolled) shell.

Uso de tcpwrappers
TCP wrappers were developed when there were no real packet filters available and access control wasneeded. Nevertheless, they're still very interesting and useful. The TCP wrappers allow you to allow ordeny a service for a host or a domain and define a default allow or deny rule (all performed on the application level). If you want more information take a look at hosts_access(5) manual page.
Muchos servicios instalados en Debian son cualquiera de estos dos:
• lanzó a través del servicio del tcpwrapper (tcpd)• compiló con el soporte libwrapper incorporado.
On the one hand, for services configured in /etc/inetd.conf (this includes telnet, ftp, netbios, swat
and finger) you will see that the configuration file executes /usr/sbin/tcpd first. On the other hand, even if
a service is not launched by the inetd superdaemon, support for the tcp wrappers rules can be compiled into
it. Services compiled with tcp wrappers in Debian include ssh, portmap, in.talk, rpc.statd, rpc.mountd,
gdm, oaf (the GNOME activator daemon), nessus and many others.

39Después de la instalación
To see which packages use tcpwrappers 17 try:

$ apt-cache rdepends libwrap0
Tenga en cuenta esto cuando el tcpchk está avanzando. Usted puede agregar servicios en que se unen a labiblioteca de la envoltura de los archivos host.deny y hosts.allow pero los tcpchk advertirá queeste no puede encontrar esos servicios desde que parece para ellos en /etc/inetd.conf (el manpage
no es totalmente exacto aquí).
Ahora, aquí viene un truco pequeño, y probablemente la intrusión más pequeña del sistema de descubrimiento disponible. En general, usted debe tener una política decente del cortafuego como una primera
línea, y envolturas del tcp como la segunda línea de defensa. Un truco pequeño es poner un comando
SPAWN 18en /etc/hosts.deny que envía correos a root siempre que hay un servicio negado en las envolturas de los gatillos:

ALL: ALL: SPAWN ( \echo -e "\n\
TCP Wrappers\: Connection refused\n\By\: $(uname -n)\n\Process\: %d (pid %p)\n\User\: %u\n\Host\: %c\n\Date\: $(date)\n\" | /usr/bin/mail -s "Connection to %d blocked" root) &Beware (tenga cuidado): El ejemplo anterior impreso puede fácilmente ser DoSed por estar haciendo las
muchas conexiones en un período corto de tiempo. Muchos correos electrónicos significan mucho delarchivo I/O para enviar únicamente unos correos.

La importancia de logs y alarmas
Cómo las bitácoras y alarmas son tratadas es un problema importante en un sistema seguro. Es fácil verque, aun cuando el sistema está perfectamente configurado y, supuestamente, 99% asegurado. Si el 1%
sucede, y no hay seguridad midiendo en tales situaciones, primero, descubra esto y, segundo, las alarmasdel aumento, el sistema no está en absoluto seguro.
Debian GNU/Linux provides some tools to perform log analysis, most notably swatch, 19 logcheck or loganalysis (all will need some customisation to remove unnecessary things from the report). It might also be
useful, if the system is nearby, to have the system logs printed on a virtual console. This is useful since
you can (from a distance) see if the system is behaving properly. Debian's /etc/syslog.conf comeswith a commented default configuration; to enable it uncomment the lines and restart syslogd (/etc/
init.d/syslogd restart):

daemon,mail.*;\17

On older Debian releases you might need to do this:$ apt-cache showpkg libwrap0 | egrep '^[[:space:]]' | sort -u | \sed 's/,libwrap0$//;s/^[[:space:]]\+//'
beware of the case here since spawn will not work
19
there's a very good article on it written by http://www.spitzner.net/swatch.html
1840

Después de la instalación

news.=crit;news.=err;news.=notice;\*.=debug;*.=info;\*.=notice;*.=warn/dev/tty8
To colorize the logs, you could take a look at colorize, ccze or glark. There is a lot to log analysis thatcannot be fully covered here, so a good information resource would be books should as http://books.google.com/books?id=UyktqN6GnWEC. In any case, even automated tools are no match for the best analysis
tool: your brain.

Uso y personalización de logcheck
The logcheck package in Debian is divided into the three packages logcheck (the main program), logcheck-database (a database of regular expressions for the program) and logtail (prints loglines that have
not yet been read). The Debian default (in /etc/cron.d/logcheck) is that logcheck is run every
hour and after reboots.
Hay también un número de registros de auditorias de herramientas, en el site, como logcheck. Estas herramientas pueden ser absolutamente usables si se garantiza propiamente para alertar al administrador
sobre eventos inusuales en el sistema de archivos locales. logcheck. pude ser enteramente garantizado,
pude enviar mensajes desde eventos recuperados y desde los registros que son meritorios de atención.
El abandono de instalación incluye perfiles para eventos ignorados y violaciones políticas para tres diferentes montajes (estación de trabajo, servidor y paranoia). Los paquetes de Debian incluyen un archivo de configuración /etc/logcheck/logcheck.conf, dirigido por el programa, que define alusuario y que también revisa sus envios. También suministra una forma de paquete que provee servicios para implementar nuevas políticas en los directorios: /etc/logcheck/hacking.d/_packagename_, /etc/logcheck/violations.d/_packagename_, /etc/logcheck/violations.ignore.d/_packagename_, /etc/logcheck/ignore.d.paranoid/_packagename_, /etc/logcheck/ignore.d.server/_packagename_, and /etc/logcheck/ignore.d.workstation/_packagename_. Sin embargo, no muchos paquetes lo hacen actualmente.
Si usted tiene una política que puede ser útil para otros usuarios , por favor envielo como un pequeño reporte para los paquetes apropiados, mire mas información en /usr/share/doc/logcheck/README.Debian
The best way to configure logcheck is to edit its main configuration file /etc/logcheck/logcheck.conf after installation. Change the default user (root) to whom reports should be mailed. You
should set the reportlevel in there, too. logcheck-database has three report levels of increasing verbosity:
workstation, server, paranoid. "server" being the default level, paranoid is only recommended for highsecurity machines running as few services as possible and workstation for relatively sheltered, non-criticalmachines. If you wish to add new log files just add them to /etc/logcheck/logcheck.logfiles.
It is tuned for default syslog install.
Once this is done you might want to check the mails that are sent, for the first few days/weeks/months. If
you find you are sent messages you do not wish to receive, just add the regular expressions (see regex(7)and egrep(1)) that correspond to these messages to the /etc/logcheck/ignore.d.reportlevel/local. Try to match the whole logline. Details on howto write rules are explained in /usr/share/doc/logcheck-database/README.logcheck-database.gz. It's an ongoing tuning process; once the messages that are sent are always relevant you can consider the tuning finished. Note that
if logcheck does not find anything relevant in your system it will not mail you even if it does run (so you
might get a mail only once a week, if you are lucky).

Configurando el sitio donde las alertas son enviadas
Debian viene con una configuración de syslog estándard dentro de (etc/syslog.conf)que anota mensajes
para apropiar archivos dependiendo de la facilidad del sistema. Usted debería familiarizarse con ésto,41

Después de la instalación

debe mirar el archivo syslog.conf o sino la documentación. Si usted pretende mantener un sistema
seguro usted podrá estar precavido de a dónde se mandan los mensajes de registro de manera que no pasen
inadvertidos.
Por ejemplo, enviar mensajes a la consola es una configuración interesante ya que es útil para muchos
sistemas de nivel de producción. Pero para muchos sistemas también es importante añadir una nueva
máquina que podría servir como servidor de registro (i.e. esto recibe los registros desde todos los otros
sistemas).
El correo de Root también deberia ser considerado, muchos controles de seguridad como snort) envían
alarmas al buzón de Root. Este buzón normalmente apunta al primer usuario que se creó en el sistema(compruebe /etc/aliases). Tenga cuidado de enviar correo de root a cualquier lugar donde pueda ser
leído (ya sea local ó remotamente)
Hay otros informes y alianzas en su sistema. En un pequeño sistema, ésto probablemente lo más simple
para asegurarse de que todas las alianzas apunten hacia la cuenta de root, y que el correo para root este
dispuesto para el sistema de buzón personal del administrador.
ARREGLAME: it would be interesting to tell how a Debian system can send/receive SNMP traps related
to security problems (jfs). Check: snmptraglogd, snmp and snmpd.

Usar un servidor de registro
A loghost is a host which collects syslog data remotely over the network. If one of your machines iscracked, the intruder is not able to cover the tracks, unless hacking the loghost as well. So, the loghost
should be especially secure. Making a machine a loghost is simple. Just start the syslogd withsyslogd -r
and a new loghost is born. In order to do this permanently in Debian, edit /etc/default/syslogd
and change the line

SYSLOGD=""aSYSLOGD="-r"
Luego, configure las otras máquinas al enviar los datos al servidor de registro. Agrege una entrada comola siguiente /etc/syslog.conf:

facility.level@your_loghost

Mire la documentación para saber que usar en lugar de facility y level (ellos no deben ser introducirse deforma literal como se hace aquí). Si usted quiere registrar todo remotamente, escriba:*.*@your_loghost

into your syslog.conf. Logging remotely as well as locally is the best solution (the attacker might
presume to have covered his tracks after deleting the local log files). See the syslog(3), syslogd(8) andsyslog.conf(5) manpages for additional information.

42

Después de la instalación

Permisos para el archivo de registro
It is not only important to decide how alerts are used, but also who has read/modify access to the log files(if not using a remote loghost). Security alerts which the attacker can change or disable are not worth muchin the event of an intrusion. Also, you have to take into account that log files might reveal quite a lot of
information about your system to an intruder who has access to them.
Algunos permisos para el achivo de registro no son perfectos despúes de la instalación. Primero /var/
log/lastlog y /var/log/faillogno necesitan tener un permiso de lectura para un usuario normal. En el archivo lastlog usted puede ver quien entró recientemente y en faillog usted mira un resumen delas entradas fallidas. El autor recomienda cambiar permisos a 660.Haga una breve revisión en sus archivosde registro y decida muy cuidadosamente cuales logfile deben tener permiso de lectura y escritura para unusuario con UID distinto a 0 y un grupo aparte de 'adm' o 'root'.

# find /var/log -type f -exec ls -l {} \; | cut -c 17-35 |sort -u(see to what users do files in /var/log belong)# find /var/log -type f -exec ls -l {} \; | cut -c 26-34 |sort -u(see to what groups do files in /var/log belong)# find /var/log -perm +004
(files which are readable by any user)# find /var/log \! -group root \! -group adm -exec ls -ld {} \;(files which belong to groups not root or adm)
To customize how log files are created you will probably have to customize the program that generates
them. If the log file gets rotated, however, you can customize the behavior of creation and rotation.

Añadiendo parches al kernel
Debian GNU/Linux suministra algunos de los parches para el kernel de Linux que aumentan su aseguramiento. Estos incluyen:• Linux Intrusion Detection [http://www.lids.org] provided in the kernel-patch-2.4-lids package. This kernel patch makes the process of hardening your Linux system easier by allowing you to restrict, hide andprotect processes, even from root. It implements mandatory access control capabilities.
• Linux Trustees [http://trustees.sourceforge.net/], provided in package trustees. This patch adds a decent
advanced permissions management system to your Linux kernel. Special objects (called trustees) arebound to every file or directory, and are stored in kernel memory, which allows fast lookup of all permissions.
• NSA Enhanced Linux (in package selinux). Backports of the SElinux-enabled packages are available
at https://salsa.debian.org/selinux-team. More information available at SElinux in Debian Wiki page[http://wiki.debian.org/SELinux], at Manoj Srivastava's [http://www.golden-gryphon.com/software/security/selinux.xhtml] and Russell Cookers's [http://www.coker.com.au/selinux/] SElinux websites.
• The kernel patch http://people.redhat.com/mingo/exec-shield provided in the kernel-patch-exec-shieldpackage. This patch provides protection against some buffer overflows (stack smashing attacks).
• The Grsecurity patch [http://www.grsecurity.net/], provided by the kernel-patch-2.4-grsecurity and kernel-patch-grsecurity2 packages 20 implements Mandatory Access Control through RBAC, provides buf20

Notice that this patch conflicts with patches already included in Debian's 2.4 kernel source package. You will need to use the stock vanilla kernel.
You can do this with the following steps:

43Después de la instalación

fer overflow protection through PaX, ACLs, network randomness (to make OS fingerprinting more difficult) and many more features [http://www.grsecurity.net/features.php].
• The kernel-patch-adamantix provides the patches developed for Adamantix [http://www.adamantix.org/], a Debian-based distribution. This kernel patch for the 2.4.x kernel releases introduces somesecurity features such as a non-executable stack through the use of http://pageexec.virtualave.net/ andmandatory access control based on http://www.rsbac.org/. Other features include: http://www.vanheusden.com/Linux/sp/, AES encrypted loop device, MPPE support and an IPSEC v2.6 backport.
• cryptoloop-source. This patches allows you to use the functions of the kernel crypto API to create encrypted filesystems using the loopback device.
• IPSEC kernel support (in package linux-patch-openswan). If you want to use the IPsec protocol withLinux, you need this patch. You can create VPNs with this quite easily, even to Windows machines, as
IPsec is a common standard. IPsec capabilities have been added to the 2.5 development kernel, so thisfeature will be present by default in the future Linux Kernel 2.6. Homepage: http://www.openswan.org.
FIXME: The latest 2.4 kernels provided in Debian include a backport of the IPSEC code from 2.5.
Comment on this.
The following security kernel patches are only available for old kernel versions in woody and are deprecated:• http://acl.bestbits.at/ (ACLs) for Linux provided in the package kernel-patch-acl. This kernel patch adds
access control lists, an advanced method for restricting access to files. It allows you to control fine-grain
access to files and directory.
• The http://www.openwall.com/linux/ linux kernel patch by Solar Designer, provided in the kernel-patch-2.2.18-openwall package. This is a useful set of kernel restrictions, like restricted links, FIFOs
in /tmp, a restricted /proc file system, special file descriptor handling, non-executable user stackarea and other features. Note: This package applies to the 2.2 release, no packages are available for the
2.4 release patches provided by Solar.
• kernel-patch-int. This patch also adds cryptographic capabilities to the Linux kernel, and was usefulwith Debian releases up to Potato. It doesn't work with Woody, and if you are using Sarge or a newer
version, you should use a more recent kernel which includes these features already.
However, some patches have not been provided in Debian yet. If you feel that some of these should be
included please ask for it at the http://www.debian.org/devel/wnpp/.

Protección contra desbordamiento de búfer
Buffer overflow is the name of a common attack to software 21 which makes use of insufficient boundary
checking (a programming error, most commonly in the C language) in order to execute machine code
through program inputs. These attacks, against server software which listen to connections remotely and# apt-get install kernel-source-2.4.22 kernel-patch-debian-2.4.22# tar xjf /usr/src/kernel-source-2.4.22.tar.bz2# cd kernel-source-2.4.22# /usr/src/kernel-patches/all/2.4.22/unpatch/debian
For more information see http://bugs.debian.org/194225, http://bugs.debian.org/199519, http://bugs.debian.org/206458, http://bugs.debian.org/203759, http://bugs.debian.org/204424, http://bugs.debian.org/210762, http://bugs.debian.org/211213, and the http://lists.debian.org/debian-devel/2003/09/msg01133.html
21
So common, in fact, that they have been the basis of 20% of the reported security vulnerabilities every year, as determined by http://icat.nist.gov/
icat.cfm?function=statistics

44Después de la instalación

against local software which grant higher privileges to users (setuid or setgid) can result in the compromise of any given system.
There are mainly four methods to protect against buffer overflows:• patch the kernel to prevent stack execution. You can use either: Exec-shield, OpenWall or PaX (includedin the Grsecurity and Adamantix patches).
• fix the source code by using tools to find fragments of it that might introduce this vulnerability.
• recompile the source code to introduce proper checks that prevent overflows, using the http://www.research.ibm.com/trl/projects/security/ssp/ patch for GCC (which is used by http://www.adamantix.org)
Debian GNU/Linux, as of the 3.0 release, provides software to introduce all of these methods except forthe protection on source code compilation (but this has been requested in http://bugs.debian.org/213994).
Notice that even if Debian provided a compiler which featured stack/buffer overflow protection all packages would need to be recompiled in order to introduce this feature. This is, in fact, what the Adamantix
distribution does (among other features). The effect of this new feature on the stability of software is yet
to be determined (some programs or some processor architectures might break due to it).
In any case, be aware that even these workarounds might not prevent buffer overflows since there areways to circumvent these, as described in phrack's magazine http://packetstorm.linuxsecurity.com/mag/phrack/phrack58.tar.gz or in CORE's Advisory http://online.securityfocus.com/archive/1/269246.
If you want to test out your buffer overflow protection once you have implemented it (regardless of themethod) you might want to install the paxtest and run the tests it provides.

Parches contra el desbordamiento de búfer para el núcleo
Kernel patches related to buffer overflows include the Openwall patch provides protection against buffer
overflows in 2.2 linux kernels. For 2.4 or newer kernels, you need to use the Exec-shield implementation,or the PaX implementation (provided in the grsecurity patch, kernel-patch-2.4-grsecurity, and in the Adamantix patch, kernel-patch-adamantix). For more information on using these patches read the the section“Añadiendo parches al kernel”.

Testing programs for overflows
The use of tools to detect buffer overflows requires, in any case, of programming experience in order
to fix (and recompile) the code. Debian provides, for example: bfbtester (a buffer overflow tester thatbrute-forces binaries through command line and environment overflows). Other packages of interest would
also be rats, pscan, flawfinder and splint.

Transferencia segura de archivos
During normal system administration one usually needs to transfer files in and out from the installedsystem. Copying files in a secure manner from a host to another can be achieved by using the ssh server
package. Another possibility is the use of ftpd-ssl, a ftp server which uses the Secure Socket Layer toencrypt the transmissions.
Any of these methods need special clients. Debian does provide client software, such as scp from the sshpackage, which works like rcp but is encrypted completely, so the bad guys cannot even find out WHAT
you copy. There is also a ftp-ssl package for the equivalent server. You can find clients for these software

45

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even for other operating systems (non-UNIX), putty and winscp provide secure copy implementations
for any version of Microsoft's operating system.
Note that using scp provides access to the users to all the file system unless chroot'ed as described in“Chrooting ssh”. FTP access can be chroot'ed, probably easier depending on you chosen daemon, as
described in “Asegurando FTP”. If you are worried about users browsing your local files and want to haveencrypted communication you can either use an ftp daemon with SSL support or combine clear-text ftp
and a VPN setup (see “Redes virtuales privadas”).

Límites y control de los sistemas de archivos
Uso de Quotas
Tener una buena política de quotas es importante, esto abstiene a los usuarios de llenar el disco duro.
Usted puede usar dos sistemas diferentes de quotas: quota de usuario y quota de grupo. Como usted provablemente dedujo, la quota del usuario límita la cantidad de espacio del que un usuario puede disponer,la quota del grupo hace lo equivalente para los grupos. Tenga en cuenta ésto cuando esté organizando eltamaño de quotas.
Hay algunos puntos importantes para considerar acerca de la configuración del sistema de quotas:• Mantener las quotas suficientemente pequeñas, para que los usuarios no ocupen el espacio de su disco.
• Mantener las quotas lo suficientemente grandes, para que los usuarios no se quejen o su quota de correo
les impida aceptar un correo por un periodo de tiempo largo.
• Use las quotas para todas las áreas en las que los usuarios puedan escribir, en /home como también
en /tmp.
Every partition or directory to which users have full write access should be quota enabled. Calculate andassign a workable quota size for those partitions and directories which combines usability and security.
Ahora que usted quiere usar quotas. Primero que todo usted necesita revisar que habilito el uso de quotas
en el kernel. Si no, usted necesitará recompilarla. Después de ésto dése cuenta que el paquete 'quota' esté
instalado. Si no está usted necesitará este.
Habilitar la quota para los respectivos sistemas de archivos es tan fácil como modificar la configuración
inicial ajustándola a defaults,usrquota en su archivo /etc/fstab . Si usted necesita unquota para grupos, sustituya usrquota por grpquota. Puede usar ambos. Luego cree unos archivos
quota.user y quota.group vacios en la raíz de los sistemas de archivos en los que usted quiera usar quotas(Ej. con touch /home/quota.user /home/quota.group para el sistema de archivos /home).
touch/home/quota.user /home/quota.groupfor a /home file system).
Restart quota by doing/etc/init.d/quota stop;/etc/init.d/quota
start. Now quota should be running, and quota sizes can be set.
Editing quotas for a specific user can be done by46

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edquota -u <user>. Group quotas can be modified withedquota -g <group>. Then set the soft and hard quota and/or inode quotas as needed.
Para más información acerca de las quotas, lea el manual de páginas sobre las quotas, y el mini-howto (/usr/share/doc/HOWTO/en-html/mini/Quota.html).

The ext2 filesystem specific attributes (chattr/lsattr)
In addition to the usual Unix permissions, the ext2 and ext3 filesystems offer a set of specific attributes
that give you more control over the files on your system. Unlike the basic permissions, these attributes are
not displayed by the usual ls -l command or changed using chmod, and you need two other utilities, lsattrand chattr (in package e2fsprogs) to manage them. Note that this means that these attributes will usually
not be saved when you backup your system, so if you change any of them, it may be worth saving the
successive chattr commands in a script so that you can set them again later if you have to restore a backup.
Among all available attributes, the two that are most important for increasing security are referenced by
the letters 'i' and 'a', and they can only be set (or removed) by the superuser:• The 'i' attribute ('immutable'): a file with this attribute can neither be modified nor deleted or renamed
and no link can be created to it, even by the superuser.
• The 'a' attribute ('append'): this attribute has the same effect that the immutable attribute, except thatyou can still open the file in append mode. This means that you can still add more content to it but it
is impossible to modify previous content. This attribute is especially useful for the log files stored in /var/log/, though you should consider that they get moved sometimes due to the log rotation scripts.
These attributes can also be set for directories, in which case everyone is denied the right to modify thecontents of a directory list (e.g. rename or remove a file, ...). When applied to a directory, the append
attribute only allows file creation.
It is easy to see how the 'a' attribute improves security, by giving to programs that are not running as thesuperuser the ability to add data to a file without modifying its previous content. On the other hand, the'i' attribute seems less interesting: after all, the superuser can already use the basic Unix permissions to
restrict access to a file, and an intruder that would get access to the superuser account could always use thechattr program to remove the attribute. Such an intruder may first be confused when noticing not being
able to remove a file, but you should not assume blindness - after all, the intruder got into your system!
Some manuals (including a previous version of this document) suggest to simply remove the chattr andlsattr programs from the system to increase security, but this kind of strategy, also known as "security by
obscurity", is to be absolutely avoided, since it provides a false sense of security.
A secure way to solve this problem is to use the capabilities of the Linux kernel, as described in “Defensa
proactiva.”. The capability of interest here is called CAP_LINUX_IMMUTABLE: if you remove it fromthe capabilities bounding set (using for example the command lcap CAP_LINUX_IMMUTABLE) it won't
be possible to change any 'a' or 'i' attribute on your system anymore, even by the superuser ! A complete
strategy could be as follows:• Set the attributes 'a' and 'i' on any file you want;• Add the command lcap CAP_LINUX_IMMUTABLE (as well as lcap CAP_SYS_MODULE, as suggested
in “Defensa proactiva.”) to one of the startup scripts;• Set the 'i' attribute on this script and other startup files, as well as on the lcap binary itself;

47

Después de la instalación• Execute the above command manually (or reboot your system to make sure everything works as planned).
Now that the capability has been removed from the system, an intruder cannot change any attribute on
the protected files, and thus cannot change or remove the files. If the machine is forced to reboot (whichis the only way to restore the capabilities bounding set), it will easily be detected, and the capability will
be removed again as soon as the system restarts anyway. The only way to change a protected file would
be to boot the system in single-user mode or using another bootdisk, two operations that require physicalaccess to the machine !

Integridad de su sistema de archivos¿Está usted seguro de que el /bin/login en su disco duro es todavía el binario que instaló allí haceunos meses? ¿Qué pasaría si es una versión hackeada, que guarda la contraseña introducida en un archivo
oculto o la envía por un correo claro pro todoel internet?
El único método para tener alguna protección es comprobar sus archivos cada día/hora/mes (yo prefierocada día) comparando la vieja md5sum y la actual. Dos archivos no pueden tener la misma md5sum, demodo que anda sobre seguro aquí, excepto alquien que hackeó el algoroitmo para crear md5sums un lamáquina. Esto es bueno, extremadamente difícil y muy improbable. Realmente usted debería considerar
que auditar sus binarios es muy importante, ya que es un modo fácil para reconocer los cambios en sus
binarios. Las herramientas que comúnmente se uaan para ésto son sXid, AIDE (Ambientación Avanzada
de Detección de Intrusos), TripWire (no es libre; la nueva versió será GPL), integrit y samhain.
Common tools used for this are sxid, aide (Advanced Intrusion Detection Environment), tripwire, integrit
and samhain. Installing debsums will also help you to check the file system integrity, by comparing the
md5sums of every file against the md5sums used in the Debian package archive. But beware: those files
can easily be changed by an attacker and not all packages provide md5sums listings for the binaries they
provided. For more information please read “Do periodic integrity checks” and “Taking a snapshot of thesystem”.
You might want to use locate to index the whole filesystem, if so, consider the implications of that. The
Debian findutils package contains locate which runs as user nobody, and so it only indexes files which
are visible to everybody. However, if you change it's behaviour you will make all file locations visible
to all users. If you want to index all the filesystem (not the bits that the user nobody can see) you canreplace locate with the package slocate. slocate is labeled as a security enhanced version of GNU locate,but it actually provides additional file-locating functionality. When using slocate, the user only sees theactually accessible files and you can exclude any files or directories on the system. The slocate package
runs its update process with higher privledges than locate, and indexes every file. Users are then able toquickly search for every file which they are able to see. slocate doesn't let them see new files; it filters
the output based on your UID.
You might want to use bsign or elfsign. elfsign provides an utility to add a digital signature to an ELF binary
and a second utility to verify that signature. The current implementation uses PKI to sign the checksumof the binary. The benefits of doing this are that it enables one to determine if a binary has been modified
and who created it. bsign uses GPG, elfsign uses PKI (X.509) certificates (OpenSSL).

Configuración de revisión de setuid
The Debian checksecurity package provides a cron job that runs daily in /etc/cron.daily/checksecurity22. This cron job will run the /usr/sbin/checksecurity script that will store information of thischanges.
22

In previous releases, checksecurity was integrated into cron and the file was /etc/cron.daily/standard

48

Después de la instalación

El comportamiento por defecto no manda la información al superusuario, pero en cambio guarda diariamente copias de los cambios dentro de /var/log/setuid.changes. Usted debe colocar el CHECKSECURITY_EMAIL (dentro de /etc/checksecurity.conf) a 'root'. Mire checksecurity(8) para
mas información de configuración.

Conectividad de red segura
FIXME: More (Debian-specific) content needed.

Características de la red configurando kernel
Many features of the kernel can be modified while running by echoing something into the /proc filesystem or by using sysctl. By entering /sbin/sysctl -A you can see what you can configure and what theoptions are, and it can be modified running/sbin/sysctl -w variable=value(see sysctl(8)). Only in rare cases do you need to edit something here, but you can increase security thatway as well. For example:
net/ipv4/icmp_echo_ignore_broadcasts = 1
This is a Windows emulator because it acts like Windows on broadcast ping if this option is set to 1. Thatis, ICMP echo requests sent to the broadcast address will be ignored. Otherwise, it does nothing.
If you want to prevent you system from answering ICMP echo requests, just enable this configuration
option:net/ipv4/icmp_echo_ignore_all = 0
Los paquetes con direcciones imposibles (debido a las rutas incorectas) sobre el registro que obtuvo su red.
/proc/sys/net/ipv4/conf/all/log_martians = 1
For more information on what things can be done with /proc/sys/net/ipv4/* read /usr/src/
linux/Documentation/filesystems/proc.txt. All the options are described thoroughly under /usr/src/linux/Documentation/networking/ip-sysctl.txt23.

Configuring syncookies
This option is a double-edged sword. On the one hand it protects your system against syn packet flooding;on the other hand it violates defined standards (RFCs).
net/ipv4/tcp_syncookies = 1
If you want to change this option each time the kernel is working you need to change it in /etc/network/options by setting syncookies=yes. This will take effect when ever /etc/init.d/networking is run (which is typically done at boot time) while the following will have a one-time effectuntil the reboot:23

In Debian the kernel-source-version packages copy the sources to /usr/src/kernel-source-version.tar.bz2, just substituteversion to whatever kernel version sources you have installed

49

Después de la instalación

echo 1 > /proc/sys/net/ipv4/tcp_syncookies
This option will only be available if the kernel is compiled with the CONFIG_SYNCOOKIES. All Debiankernels are compiled with this option builtin but you can verify it running:$ sysctl -A |grep syncookies
net/ipv4/tcp_syncookies = 1
For more information on TCP syncookies read http://cr.yp.to/syncookies.html.

Securing the network on boot-time
When setting configuration options for the kernel networking you need configure it so that it's loaded
every time the system is restarted. The following example enables many of the previous options as wellas other useful options.
There are actually two ways to configure your network at boot time. You can configure /etc/sysctl.conf (see: sysctl.conf(5)) or introduce a script that is called when the interface is enabled. The first
option will be applied to all interfaces, whileas the second option allows you to configure this on a perinterface basis.
An example of a /etc/sysctl.conf configuration that will secure some network options at the kernel
level is shown below. Notice the comment in it, /etc/network/options might override some values
if they contradict those in this file when the /etc/init.d/networking is run (which is later thanprocps on the startup sequence).
## /etc/sysctl.conf - Configuration file for setting system variables# See sysctl.conf (5) for information. Also see the files under# Documentation/sysctl/, Documentation/filesystems/proc.txt, and# Documentation/networking/ip-sysctl.txt in the kernel sources# (/usr/src/kernel-$version if you have a kernel-package installed)# for more information of the values that can be defined here.
## Be warned that /etc/init.d/procps is executed to set the following# variables. However, after that, /etc/init.d/networking sets some# network options with builtin values. These values may be overridden# using /etc/network/options.
##kernel.domainname = example.com# Additional settings - adapted from the script contributed# by Dariusz Puchala (see below)# Ignore ICMP broadcasts
net/ipv4/icmp_echo_ignore_broadcasts = 1## Ignore bogus ICMP errors
net/ipv4/icmp_ignore_bogus_error_responses = 1## Do not accept ICMP redirects (prevent MITM attacks)
net/ipv4/conf/all/accept_redirects = 050

Después de la instalación# _or_# Accept ICMP redirects only for gateways listed in our default# gateway list (enabled by default)# net/ipv4/conf/all/secure_redirects = 1## Do not send ICMP redirects (we are not a router)
net/ipv4/conf/all/send_redirects = 0## Do not forward IP packets (we are not a router)# Note: Make sure that /etc/network/options has 'ip_forward=no'
net/ipv4/conf/all/forwarding = 0## Enable TCP Syn Cookies# Note: Make sure that /etc/network/options has 'syncookies=yes'
net/ipv4/tcp_syncookies = 1## Log Martian Packets
net/ipv4/conf/all/log_martians = 1## Turn on Source Address Verification in all interfaces to# prevent some spoofing attacks# Note: Make sure that /etc/network/options has 'spoofprotect=yes'
net/ipv4/conf/all/rp_filter = 1## Do not accept IP source route packets (we are not a router)
net/ipv4/conf/all/accept_source_route = 0
To use the script you need to first create the script, for example, in /etc/network/interface-secure (the name is given as an example) and call it from /etc/network/interfaces like this:
auto eth0
iface eth0 inet static
address xxx.xxx.xxx.xxx
netmask 255.255.255.xxx
broadcast xxx.xxx.xxx.xxx
gateway xxx.xxx.xxx.xxx
pre-up /etc/network/interface-secure
In this example, before the interface eth0 is enabled the script will be called to secure all network interfaces
as shown below.
#!/bin/sh -e# Script-name: /etc/network/interface-secure## Modifies some default behavior in order to secure against# some TCP/IP spoofing & attacks for all interfaces.
## Contributed by Dariusz Puchalak.
#echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts# Broadcast echo protection enabled.
echo 0 > /proc/sys/net/ipv4/conf/all/forwarding# IP forwarding disabled.

51

Después de la instalación

echo 1 > /proc/sys/net/ipv4/tcp_syncookies # TCP syn cookies protection enabled.
echo 1 >/proc/sys/net/ipv4/conf/all/log_martians # Log strange packets.
# (this includes spoofed packets, source routed packets, redirect packets)# but be careful with this on heavy loaded web servers.
echo 1 > /proc/sys/net/ipv4/icmp_ignore_bogus_error_responses# Bad error message protection enabled.
# IP spoofing protection.
echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter# Disable ICMP redirect acceptance.
echo 0 > /proc/sys/net/ipv4/conf/all/accept_redirects
echo 0 > /proc/sys/net/ipv4/conf/all/send_redirects# Disable source routed packets.
echo 0 > /proc/sys/net/ipv4/conf/all/accept_source_route
exit 0
Notice that you can actually have per-interface scripts that will enable different network options for different interfaces (if you have more than one), just change the pre-up line to:pre-up /etc/network/interface-secure $IFACE
And use a script which will only apply changes to a specific interface, not to all of the interfaces available.
Notice that some networking options can only be enabled globally, however. A sample script is this one:#!/bin/sh -e# Script-name: /etc/network/interface-secure## Modifies some default behavior in order to secure against# some TCP/IP spoofing & attacks for a given interface.
## Contributed by Dariusz Puchalak.
#
IFACE=$1
if [ -z "$IFACE" ] ; thenecho "$0: Must give an interface name as argument!"echo "Usage: $0 <interface>"
exit 1fi

if [ ! -e /proc/sys/net/ipv4/conf/$IFACE/ ]; thenecho "$0: Interface $IFACE does not exit (cannot find /proc/sys/net/ipv4/conf/)"
exit 1fiecho 0 > /proc/sys/net/ipv4/conf/$IFACE/forwarding # IP forwarding disabled.
echo 1 >/proc/sys/net/ipv4/conf/$IFACE/log_martians # Log strange packets.
# (this includes spoofed packets, source routed packets, redirect packets)# but be careful with this on heavy loaded web servers.

52

Después de la instalación# Protección contra falsificación de IP (IP spoofing).
echo 1 > /proc/sys/net/ipv4/conf/$IFACE/rp_filter# Disable ICMP redirect acceptance.
echo 0 > /proc/sys/net/ipv4/conf/$IFACE/accept_redirects
echo 0 > /proc/sys/net/ipv4/conf/$IFACE/send_redirects# Desativa el ruteo en origen (source routing).
echo 0 > /proc/sys/net/ipv4/conf/$IFACE/accept_source_route
exit 0An alternative solution is to create an init.d script and have it run on bootup (using update-rc.d tocreate the appropriate rc.d links).

Configuarción de las características de los cortafuegos
Para tener la capacidad del cortafuego, para proteger el sistema local u otros detrás de este, el kernel necesita estar compidalo con las capacidades del cortefuego. El Debian normal 2.2 kernel (también 2.2) susmistra el paquete de filtro del cortafuego ipchains, el kernel normal de Debian 3.0 (kernel 2.4) suministra
el poderoso paquete de filtros de cortafuegos iptables (filtro de la red). Las distribucines más viejas deDebian necesitan el parche apropiado del kernel (Debian 2.1 usa el kernel 2.0.34).
En todo caso, es bastante fácil usar un kernel diferente al suministrado por Debian. Usted puede encontrar
paquetes de kernel pre-compilados que puede instalar fácilmente en el sistemas de Debian. Usted tambiém
puede obtener las fuentes del kernel usando kernel-source-X y armar paquetes de kernel personalizados
con make-kpkg.
Configurando los cortafuegos en Debian se discute más a fondo en “Añadir capacidades al cortafuegos”.

Disabling weak-end hosts issues
Systems with more than one interface on different networks can have services configured so that they willbind only to a given IP address. This usually prevents access to services when requested through any otheraddress. However, this does not mean (although it is a common misconception) that the service is boundto a given hardware address (interface card). 24
It seems, however, not to work with services bound to 127.0.0.1, you might need to write the tests using
raw sockets.
This is not an ARP issue and it's not an RFC violation (it's called weak end host in RFC1122 [ftp://ftp.isi.edu/in-notes/rfc1122.txt], (in the section 3.3.4.2). Remember, IP addresses have nothing to do with physical interfaces.
24

To reproduce this (example provided by Felix von Leitner on the Bugtraq mailing list):

host a (eth0 connected to eth0 of host b):
ifconfig eth0 10.0.0.1
ifconfig eth1 23.0.0.1
tcpserver -RHl localhost 23.0.0.1 8000 echo fnord
host b:ifconfig eth0 10.0.0.2
route add 23.0.0.1 gw 10.0.0.1
telnet 23.0.0.1 8000

53

Después de la instalación

On 2.2 (and previous) kernels this can be fixed with:# echo 1 > /proc/sys/net/ipv4/conf/all/hidden# echo 1 > /proc/sys/net/ipv4/conf/eth0/hidden# echo 1 > /proc/sys/net/ipv4/conf/eth1/hidden.....
On later kernels this can be fixed either with:• Reglas Iptables• properly configured routing. 25• kernel patching. 26
Along this text there will be many occasions in which it is shown how to configure some services (sshd
server, apache, printer service...) in order to have them listening on any given address, the reader shouldtake into account that, without the fixes given here, the fix would not prevent accesses from within thesame (local) network. 27
FIXME: Comments on Bugtraq indicate there is a Linux specific method to bind to a given interface.
FIXME: Submit a bug against netbase so that the routing fix is standard behavior in Debian?

Protección contra ataques ARPWhen you don't trust the other boxes on your LAN (which should always be the case, because it's the safest
attitude) you should protect yourself from the various existing ARP attacks.
As you know the ARP protocol is used to link IP addresses to MAC addresses (see ftp://ftp.isi.edu/innotes/rfc826.txt for all the details). Every time you send a packet to an IP address an ARP resolution isdone (first by looking into the local ARP cache then if the IP isn't present in the cache by broadcasting an
ARP query) to find the target's hardware address. All the ARP attacks aim to fool your box into thinking
that box B's IP address is associated to the intruder's box's MAC address; Then every packet that you want
to send to the IP associated to box B will be send to the intruder's box...
Those attacks (ARP cache poisoning, ARP spoofing...) allow the attacker to sniff the traffic even on switched networks, to easily hijack connections, to disconnect any host from the network... ARP attacks are
powerful and simple to implement, and several tools exists, such as arpspoof from the dsniff package or
http://arpoison.sourceforge.net/.
However, there is always a solution:
25

The fact that this behavior can be changed through routing was described by Matthew G. Marsh in the Bugtraq thread:eth0 = 1.1.1.1/24
eth1 = 2.2.2.2/24
ip rule add from 1.1.1.1/32 dev lo table 1 prio 15000
ip rule add from 2.2.2.2/32 dev lo table 2 prio 16000
ip route add default dev eth0 table 1
ip route add default dev eth1 table 2
26
There are some patches available for this behavior as described in Bugtraq's thread at http://www.linuxvirtualserver.org/~julian/#hidden andhttp://www.fefe.de/linux-eth-forwarding.diff.
27
An attacker might have many problems pulling the access through after configuring the IP-address binding while not being on the same broadcast
domain (same network) as the attacked host. If the attack goes through a router it might be quite difficult for the answers to return somewhere.

54

Después de la instalación• Use a static ARP cache. You can set up "static" entries in your ARP cache with:arp -s host_name hdwr_addr
By setting static entries for each important host in your network you ensure that nobody will create/modify a (fake) entry for these hosts (static entries don't expire and can't be modified) and spoofed ARP
replies will be ignored.
• Detect suspicious ARP traffic. You can use arpwatch, karpski or more general IDS that can also detectsuspicious ARP traffic (snort, http://www.prelude-ids.org...).
• Implement IP traffic filtering validating the MAC address.

Taking a snapshot of the system
Before putting the system into production system you could take a snapshot of the whole system. Thissnapshot could be used in the event of a compromise (see Capítulo 11, After the compromise (incident
response)). You should remake this upgrade whenever the system is upgraded, especially if you upgrade
to a new Debian release.
For this you can use a writable removable-media that can be set up read-only, this could be a floppy disk
(read protected after use), a CD on a CD-ROM unit (you could use a rewritable CD-ROM so you could
even keep backups of md5sums in different dates), or a USB disk or MMC card (if your system can accessthose and they can be write protected).
The following script creates such a snapshot:#!/bin/bash/bin/mount /dev/fd0 /mnt/floppy
trap "/bin/umount /dev/fd0" 0 1 2 3 9 13 15
if [ ! -f /usr/bin/md5sum ] ; thenecho "Cannot find md5sum. Aborting."
exit 1fi/bin/cp /usr/bin/md5sum /mnt/floppy
echo "Calculating md5 database">/mnt/floppy/md5checksums.txt
for dir in /bin/ /sbin/ /usr/bin/ /usr/sbin/ /lib/ /usr/lib/
dofind $dir -type f | xargs /usr/bin/md5sum >>/mnt/floppy/md5checksums-lib.txt
doneecho "post installation md5 database calculated"
if [ ! -f /usr/bin/sha1sum ] ; thenecho "Cannot find sha1sum"
echo "WARNING: Only md5 database will be stored"
else/bin/cp /usr/bin/sha1sum /mnt/floppy
echo "Calculating SHA-1 database">/mnt/floppy/sha1checksums.txt
for dir in /bin/ /sbin/ /usr/bin/ /usr/sbin/ /lib/ /usr/lib/
dofind $dir -type f | xargs /usr/bin/sha1sum >>/mnt/floppy/sha1checksums-lib.txt

55

Después de la instalación

done
echo "post installation sha1 database calculated"fiexit 0Note that the md5sum binary (and sha1sum, if available) is placed on the floppy drive so it can be usedlater on to check the binaries of the system (just in case it gets trojaned). However, if you want to makesure that you are running a legitimate binary, you might want to either compile a static copy of the md5sum
binary and use that one (to prevent a trojaned libc library from interfering with the binary) or to use thesnapshot of md5sums only from a clean environment such as a rescue CD-ROM or a Live-CD (to preventa trojaned kernel from interfering). I cannot stress this enough: if you are on a compromised system you
cannot trust its output, see Capítulo 11, After the compromise (incident response).
The snapshot does not include the files under /var/lib/dpkg/info which includes the MD5 hashesof installed packages (in files ending with .md5sums). You could copy this information along too, however you should notice:• the md5sums files include the md5sum of all files provided by the Debian packages, not just systembinaries. As a consequence, that database is bigger (5 Mb versus 600 Kb in a Debian GNU/Linux systemwith a graphical system and around 2.5 Gb of software installed) and will not fit in small removable
media (like a single floppy disk, but would probably fit in a removable USB memory).
• not all Debian packages provide md5sums for the files installed since it is not (currently) mandated
policy. Notice, however, that you can generate the md5sums for all packages using debsums after you've
finished the system installation:# debsums --generate=missing,keep
Once the snapshot is done you should make sure to set the medium read-only. You can then store it for
backup or place it in the drive and use it to drive a cron check nightly comparing the original md5sums
against those on the snapshot.
If you do not want to setup a manual check you can always use any of the integrity systems available thatwill do this and more, for more information please read “Do periodic integrity checks”.

Otras recomendaciones
No use software que dependa de svgalib
SVGAlib es muy bueno para los amantes de la consola como yo, pero durante mucho tiempo se ha comprobado que esto ha sido muy inseguro. Han sido liberadas fallas en contra de zgv y era sencillo convertirseen root. Intente evitar el uso de programas que usen SVGAlib siempre que sea posible.

56

Capítulo 5. Asegurando los servicios
que se ejecutan en su sistema
Los servicios que corren en su sistema pueden ser asegurados de dos maneras:• Haciéndolos accequibles dentro de los puntos (interfaces) en los que tienen que estar.
• Configurándolos de una manera apropiada para que puedan ser debidamente usados por los usuarios
legítimos de una manera autorizada.
Restringir los servicios de modo que solamente puedan ser accedidos desde un lugar dado puede ser hecho
restringiendo el acceso al nivel del kernel (i.e. cortafuego), configúrelos sólo para escuchar en un interfaz
dada (algunos servicios no pueden suministrar ésta característica) o usando otros métodos, por ejemplo elparche linux vserver (para 2.4.16) puede ser usado para forzar procesos de forma que usen solo una interfaz.
Regarding the services running from inetd (telnet, ftp, finger, pop3...) it is worth noting that inetd canbe configured so that services only listen on a given interface (using service@ip syntax) but that's an
undocumented feature. One of its substitutes, the xinetd meta-daemon includes a bind option just forthis matter. See ixnetd.conf(5) manual page.

service nntp{
socket_type = stream
protocol = tcp
wait = nouser = news
group = news
server = /usr/bin/env
server_args = POSTING_OK=1 PATH=/usr/sbin/:/usr/bin:/sbin/:/bin+/usr/sbin/snntpd logger -p news.info
bind = 127.0.0.1}
Laa siguientes secciones detallan como cada servicio determinado puede ser configurado debidamente
dependiendo de los usos que se quieran dar.

Asegurando ssh
Si aún está usando telnet en vez de ssh, debe detener la lectura de este manual y cambiar esto. Ssh debería
ser usado para todas las entradas remotas en vez de telnet. En una época donde es fácil husmear el tráfico
de internet y obtener contraseñas en texto plano, debe usar sólo protocólos que usen criptografía. De unavez, ejecute un apt-get install ssh en su sistema.
Anime a todos los usuarios de su sistema para usar ssh en vez de telnet, o mejor aún, desinstale telnet/telnetd. Además, debe evitar las entradas al sistema usando ssh como root y use métodos alternativos envez de root, como su o sudo. Finalmente, el archivo sshd_config, dentro de /etc/ssh, debe sermodificado para aumentar la seguridad así:• Haga que ssh escuche solo la interfaz dada, sólo en un caso de que haya más de uno (y no necesite un ssh
disponible sobre éste) o que en un futuro agrege una nueva tarjeta de red (y no necesite una conexión
desde ssh en ésta).

57

Asegurando los servicios
que se ejecutan en su sistema• Intente no permitir al Root entrar tanto como sea posible. Si alguien quiere volverse root por vía ssh,
dos logins serán necesarios y la contraseña root no puede ser obtenida a fuerza bruta por vía SSH.
• Port 666 or ListenAddress 192.168.0.1:666 Change the listen port, so the intruder cannot
be completely sure whether a sshd daemon runs (be forewarned, this is security by obscurity).
• PermitEmptyPasswords no Empty passwords make a mockery of system security.
• AllowUsers alex ref me@somewhere Allow only certain users to have access via ssh to thismachine. user@host can also be used to restrict a given user from accessing only at a given host.
• Permita que solamente los miembros de ciertos grupos tengan acceso vía a ssh a esta máquina. AllowGroups y AllowUsers tienen directivas equivalentes para denegar el acceso a una máquina. Predeciblemente se llaman "DenyUsers" y "DenyGroups".
• Queda completamente a su elección lo que usted quiera hacer. Es más seguro permitir el acceso a lamáquina solamente a usuarios con llaves ssh en el archivo ~/.ssh/authorized_keys. Si es lo que quieredéle el valor "no".
• Disable any form of authentication you do not really need, if you do not use, for example RhostsRSAAuthentication, HostbasedAuthentication, KerberosAuthentication or
RhostsAuthentication you should disable them, even if they are already by default (see themanpage sshd_config(5) manual page).
• Protocol 2 Disable the protocol version 1, since it has some design flaws that make it easier to crack
passwords. For more information read http://earthops.net/ssh-timing.pdf or the http://xforce.iss.net/static/6449.php.
• Banner /etc/some_file Add a banner (it will be retrieved from the file) to users connecting to
the ssh server. In some countries sending a warning before access to a given system about unauthorizedaccess or user monitoring should be added to have legal protection.
You can also restrict access to the ssh server using pam_listfile or pam_wheel in the PAM control
file. For example, you could keep anyone not listed in /etc/loginusers away by adding this line to/etc/pam.d/ssh:authrequired

pam_listfile.so sense=allow onerr=fail item=user file=/etc/

Como nota final, dese cuenta que estas directivas son de los archivos de la configuración de OpenSSH.
Ahora mismo hay tres demonios SSH usadados habitualmente, ssh1, ssh2, y el OpenSSH de la gente de
OpenBSD. Ssh1 fue el primer domonio ssh diaponible y aún es el más comunmente usado (hay rumoresde que existe incluso un porte a windows). Ssh2 tiene muchas ventajas sobre ssh1, pero se distribuye con
una licencia mixta de código abierto-cerrado. OpenSSH es un demonio completamente libre que soportatanto ssh1 como ssh2. La versión instalada en Debian cuando se escoge el paquete 'ssh' es OpenSSH.
You can read more information on how to set up SSH with PAM support in the http://lists.debian.org/debian-security/2001/11/msg00395.html.

Chrooting ssh
Currently OpenSSH does not provide a way to chroot automatically users upon connection (the commercial version does provide this functionality). However there is a project to provide this functionality for
OpenSSH too, see http://chrootssh.sourceforge.net, it is not currently packaged for Debian, though. You
could use, however, the pam_chroot module as described in “Restringiendo usuarios”.

58Asegurando los servicios
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In “Chroot environment for SSH” you can find several options to make a chroot environment for SSH.

Ssh clients
If you are using an SSH client against the SSH server you must make sure that it supports the same protocols
that are enforced on the server. For example, if you use the mindterm package, it only supports protocol
version 1. However, the sshd server is, by default, configured to only accept version 2 (for security reasons).

Disallowing file transfers
If you do not want users to transfer files to and from the ssh server you need to restrict access to the sftpserverand the scp access. You can restrict sftp-server by configuring the proper Subsystem in the /etc/ssh/sshd_config.
You can also chroot users (using libpam-chroot so that, even if file transfer is allowed, they are limited toan environment which does not include any system files.

Restricing access to file transfer only
You might want to restrict access to users so that they can only do file transfers and cannot have interactive
shells. In order to do this you can either:• disallow users from login to the ssh server (as described above either through the configuration file or
PAM configuration).
• give users a restricted shell such as scponly or rssh. These shells restrict the commands available to theusers so that they are not provided any remote execution priviledges.

Asegurando Squid
Squid is one of the most popular proxy/cache server, and there are some security issues that should
be taken into account. Squid's default configuration file denies all users requests. However the Debian package allows access from 'localhost', you just need to configure your browser properly. You
should configure Squid to allow access to trusted users, hosts or networks defining an Access Control List on /etc/squid/squid.conf, see the https://web.archive.org/web/20061206052115/http://
www.deckle.co.za/squid-users-guide/Main_Page for more information about defining ACLs rules. Noticethat Debian provides a minimum configuration for Squid that will prevent anything, except from localhost
to connect to your proxy server (which will run in the default port 3128). You will need to customize your/etc/squid/squid.conf as needed.
The recommended minimum configuration (provided with the package) is shown below:

acl all src 0.0.0.0/0.0.0.0
acl manager proto cache_object
acl localhost src 127.0.0.1/255.255.255.255
acl SSL_ports port 443 563
acl Safe_ports port 80# httpacl Safe_ports port 21# ftp
acl Safe_ports port 443 563# https, snews
acl Safe_ports port 70# gopheracl Safe_ports port 210# waisacl Safe_ports port 1025-65535 # unregistered ports

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Asegurando los servicios
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acl Safe_ports port 280# http-mgmt
acl Safe_ports port 488# gss-httpacl Safe_ports port 591# filemakeracl Safe_ports port 777# multiling httpacl Safe_ports port 901# SWATacl purge method PURGE
acl CONNECT method CONNECT(...)# Only allow cachemgr access from localhost
http_access allow manager localhost
http_access deny manager# Only allow purge requests from localhost
http_access allow purge localhost
http_access deny purge# Deny requests to unknown ports
http_access deny !Safe_ports# Deny CONNECT to other than SSL ports
http_access deny CONNECT !SSL_ports## INSERT YOUR OWN RULE(S) HERE TO ALLOW ACCESS FROM YOUR CLIENTS#
http_access allow localhost# And finally deny all other access to this proxy
http_access deny all#Default:# icp_access deny all##Allow ICP queries from everyone
icp_access allow all
You should also configure Squid based on your system resources, including cache memory (option cache_mem), location of the cached files and the amount of space they will take up on disk (option cache_dir).
Además, si no configuró debidamente, algúien puede enviar correo a través de Squid, puesto que el diseño
de los protocolos HTTP y SMTP es semejante. El archivo de configuración Squid niega por defecto elacceso al puerto 25. Si desea permitir las conexiones del puerto 25 adiciónelo a la lista Safe_ports. Sin
embargo, esto NO is recomendado.
Ajustar y configurar debidamente el proxy/cache es solamente una parte para mantener su sitio seguro.
Otra tarea necesaria es analizar los registros de Squid asegurándose que todas las cosas que están trabajando, deben hacerlo como se espera. Hay algunos paquetes en Debian GNU/Linux que pueden ayudar al
administrador a hacer esto. Los siguientes paquetes estan disponibles en woody (Debian 3.0):• calamaris - Analizar de las bitácoras de los proxy Squid y Oops.
• modlogan - Analizador modular de bitácoras.
• sarg - Generador de Reportes de Análisis de Squid.
• squidtaild - Squid log monitoring program.
When using Squid in Accelerator Mode it acts as a web server too. Turning on this option increases code complexity, making it less reliable. By default Squid is not configured to act as a web server, so you don't need to worry about this. Note that if you want to use this feature be sure that it is60

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really necessary. To find more information about Accelerator Mode on Squid see the https://web.archive.org/web/20070104164802/http://www.deckle.co.za/squid-users-guide/Accelerator_Mode

Asegurando FTP
Si realmente tiene que usar FTP (sin enmascararlo con sslwrap o dentro de un tunel ssl o ssh), debería
hacer cambio del directorio raíz de FTP hacia el directorio de los usuarios ftp, de modo que que el usuario
sea incapaz de mirar cualquier otra cosa que su propio derectorio. De otra manera ellos pueden atravesar
su sistema de archivos tal como si tuvieran una línea de comandos. Usted puede añadir la siguiente línea
en su proftpd.conf en la sección global para habilitar esta característica del cambio de directorio raíz:
feature:DefaultRoot ~Reinicie proftpd con /etc/init.d/proftpd restart y revise si puede escapar desde su directorioraíz ahora.
Para impedir los ataques de Proftp DoS use ../../.., y adicione la siguiente línea en /etc/
proftpd.conf: DenyFilter \*.*/
No olvide que FTP envía login y contraseñas de autenticación en el texto plano (esto no es un problema
si usted está proporcionando un servicio público anónimo) y hay buenas alternativas en Debian para ésto.
Por elemplo, sftp (sumistrado por ssh). También hay implementaciones libres de SSH para otros sistemas
operativos, por ejemplo: http://www.chiark.greenend.org.uk/~sgtatham/putty/ y http://www.cygwin.com.
Sin, embargo, si aún mantiene el servidor de FTP mientras los usuarios acceden a SSH podría encontrar unproblema típico. Usuarios que acceden a los servidores Anónimos de FTP dentro de un sistema aseguradocon SSH es el camino intentar entrar en el servidor FTP. Mientras el acceso se niegue, la contraseña nunca
se enviará por la red en texto plano. Para evitar esto, el desarrollador de ProFTPd, TJ Saunders, creó unparche que impide a los usuarios anónimos del servidor FTP intentar contraseñas con cuentas SSH válidas.
Más información y parches disponibles en: http://www.castaglia.org/proftpd/#Patches.

Asegurando el acceso al sistema X Window
Hoy en día, más y más empresas usan las terminales X cuando necesitan un servicio para muchas estaciones
de trabajo, ésto puede ser peligroso porque necesita permitir que un servidor de archivos se conecte con
los clientes (el servicio X, desde el punto de vista X. X intercambia la definición de cliente y servidor)
Si sigue la (muy mala) sugerencia de muchos documentos, tecleé xhost + en su máquina. Esto permite
conenctar con su sistema a cualquier cliente X. Para tener una seguridad ligeramente mejor, puede usar elcomando xhost +hostname en vez de la anterior para permitir un acceso desde servidores específicos.
A much more secure solution, though, is to use ssh to tunnel X and encrypt the whole session. This is done
automatically when you ssh to another machine. For this to work, you have to configure both the ssh client
and the ssh server. On the ssh client, ForwardX11 should be set to yes in /etc/ssh/ssh_config.
On the ssh server, X11Forwarding should be set to yes in /etc/ssh/sshd_config and the package xbase-clients should be installed because the ssh server uses /usr/X11R6/bin/xauth (/usr/bin/xauth on Debian unstable) when setting up the pseudo X display. In times of SSH, you shoulddrop the xhost based access control completely.
Para mayor seguridad, si no necesita acceso a X desde otras máquinas, dehabilite el enlace con el puerto
tcp 6000 tecleando simplemente: startx -- -nolisten tcp

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Asegurando los servicios
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Este es el comportamiento original en XFree 4.0 (el servidor X suministrado en Debian 3.0). Si está usando
XFree 3.3.6 (i.e. tiene un Debian 2.2 instalado) puede editar /etc/X11/xinit/xserverrcc paraque tenga unas líneas como las siguientes:#!/bin/sh
exec /usr/bin/X11/X -dpi 100 -nolisten tcp
If you are using XDM set /etc/X11/xdm/Xservers to: :0 local /usr/bin/X11/X vt7 dpi 100 -nolisten tcp. If you are using Gdm make sure that the DisallowTCP=true option
is set in the /etc/gdm/gdm.conf (which is the default in Debian). This will basically append -nolisten tcp to every X command line 1.
You can also set the default's system timeout for xscreensaver locks. Even if the user can override it,
you should edit the /etc/X11/app-defaults/XScreenSaver configuration file and change thelock line:*lock:

False(which is the default in Debian) to:*lock:True

FIXME: Add information on how to disable the screensavers which show the user desktop (which might
have sensitive information).
Lea mas sobre la seguridad X Window en http://www.linuxdoc.org/HOWTO/XWindow-User-HOWTO.html (/usr/share/doc/HOWTO/en-txt/XWindow-User-HOWTO.txt.gz).
ARREGLAME: Add info on thread of debian-security on how to change config files of XFree 3.3.6 todo this.

Revisar su administrador visual
Si usted solamente quiere tener un administrador visual instalado para el uso local (teniendo un bonito login
grafico), asegurarse que el material seguro XDMCP (control de protocolo de administrador visual X) este
inhabilitado. En XDM usted puede hacer esto con la siguiente linea. /etc/X11/xdm/xdm-config:

DisplayManager.requestPort: 0
For GDM there should be in your gdm.conf:[xdmcp]Enable=false
Normalmente, todos los administradores visuales estan configurados para no iniciar los servicios de
XDMCP por defecto en Debian.
1

Gdm will not append -nolisten tcp if it finds a -query or -indirect on the command line since the query wouldn't work.

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Seguridad en el acceso de impresión (El asunto de lpd y lprng)
Imagine, que usted llega al trabajo, y la impresora está botando interminables cantidades de papel porquealguien está negando el servicio de linea de su demonio de impresión. ¿No es terrible?
En cualquier arquitectura de impresión Unix, tiene que haber la forma de enviar los datos de los clientes alos servidores de impresión. En el lpr ylp tradicional, el comando del cliente es copiado o se hace un enlacesimbólico de los datos en el directorio de cola (por lo cual usualmente estos programas son SUID o SGID).
Para evitar algunos asuntos usted debe mantener seguros, los servidores de impresión. Esto significa que
usted necesita configurar su servicio de impresión para que solo se permita la conexión del conjunto deservidores confiables. Para hacer esto es necesario, añadir los servidores a los que se les va a permitir
imprimir en /etc/hosts.lpd.
Sin embargo, incluso si usted hace esto, el demonio lpr acepta las conexiones entrantes en el puerto 515 decualquier interfaz. Deberia considerar hacer una regla de cortafuegos para las conexiones de red/servidor
a las cuales no se permite la impresión (el demoniolpr no puede ser limitado a escuchar únicamente a
una dirección IP dada).
Lprng se prefiere en lugar de lpr porque este puede ser configurado para hacer el control de acceso a IP,
ademas se puede especificar cual interfaz va a emplear (aunque sea un poco extraño).
Si está usando el servicio de impresión de su sistema, pero solo localmente, no querrá compartir este
servicio en la red. Puede considerar el uso de otros sistemas de impresión, como el servicio proporcionado
en cups http://pdq.sourceforge.net/ el cual se basa en el permiso de un usuario del dispositivo/dev/lp0
En cups, los datos de impresión se transfieren al servidor vía el protocolo http. Esto significa que el programa del cliente no necesita ningún privilegio especial, solamente requiere que el servidor esté escuchando
sobre un puerto cualquiera.
Sin embargo, si usted quiere usar cups, pero solo localmente usted puede configurar esto para escuchar ala interfaz loopback cambiando /etc/cups/cupsd.conf:

Listen 127.0.0.1:631
Hay muchas otras opciones de seguridad, como por ejemplo permitir o negar redes y servidores en estearchivo de configuración. Sin embargo si no los necesita, debería limitar posibilidad de escuchar el puerto.
Cups también ofrece documentación a través del puerto HTTP, si no quiere revelar información potencialmente útil para agresores externos (estando abierto el puerto), también agregue:<Location />Order Deny,Allow
Deny From All
Allow From 127.0.0.1</Location>
Este archivo de configuración puede ser modificado para añadir muchas caracteristicas incluyendo certficados SSL/TLS y criptografía. Los manuales estan disponibles en http://localhost:631/ or at cups.org.
ARREGLAME: Add more content (the article on http://www.rootprompt.org provides some very interesting views).

63Asegurando los servicios
que se ejecutan en su sistema
ARREGLAME: Check if PDG is available in Debian, and if so,suggest this as the preferred printing
system.
ARREGLAME: Check if Farmer/Wietse has a replacement for printer daemon and if it's available in
Debian.

Reforzando el servidor de correo
Si su servidor no es un sistema de correo, usted realmente no necesita tener un demonio de correo escuchando conexiones entrantes, pero usted podría querer envío de correo local, por ejemplo para recibir elcorreo del usuario Root desde cualquier sistema de alerta que usted tenga en algún lugar.
If you have exim you do not need the daemon to be working in order to do this since the standard cronjob flushes the mail queue. See “Deshabilitar servicios” on how to do this.

Configuring a Nullmailer
You might want to have a local mailer daemon so that it can relay the mails sent locally to another system.
This is common when you have to administer a number of systems and do not want to connect to each ofthem to read the mail sent locally. Just as all logging of each individual system can be centralized by using
a central syslog server, mail can be sent to a central mailserver.
Such a relay-only system should be configured properly for this. The daemon could, as well, be configured
to only listen on the loopback address.
The following configuration steps only need to be taken to configure the exim package in the Debian 3.0
release. If you are using a later release (such as 3.1 which uses exim4) the installation system has been
improved so that if the mail transport agent is configured to only deliver local mail it will automatically
only allow connections from the local host and will not permit remote connections.
In a Debian 3.0 system using exim, you will have to remove the SMTP daemon from inetd:$ update-inetd --disable smtp
y configurar el demonio de correo solo para escuchar en la interfaz loopback. En exim (el MTA por
defecto) usted puede hacer esto añadiendo la siguiente línea editando: /etc/exim.conf y añadiendola siguiente linea:

local_interfaces = "127.0.0.1"
Reinicie ambos demonios (inetd y exim) y estarán escuchando en el socket 127.0.0.1:25 solamente. Sea
cuidadoso, y primero desconecte inetd, de lo contrario, exim no iniciara ya que el demonio inetd estámanejando las conexiones entrantes.
Para usar postfix edite /etc/postfix/main.conf:

inet_interfaces = localhost
Si usted solo quiere un correo local, este metodo es mejor que usar la cubierta tcp-wrapping al demonio decorreo o añadir las reglas del cortafuego para limitar el acceso de cualquier persona a este. Sin embargo, si
necesita que escuche en otras interfaces, debería considerar lanzarlo desde inetd y añadir un tcp-wraping deforma que las conexiones sean revisadas contra /etc/hosts.allow y /etc/hosts.deny también

64

Asegurando los servicios
que se ejecutan en su sistemaserá advertido cuando un acceso no autorizado está atentando en contra de su demonio de correo, usted
debe instaurar un registrador apropiado para cualquiera de los metodos mencionados anteriormente.
In any case, to reject mail relay attempts at the SMTP level, you can change /etc/exim/exim.conf
to include:

receiver_verify = true
Even if your mail server will not relay the message, this kind of configuration is needed for the relay tester
at http://www.abuse.net/relay.html to determine that your server is not relay capable.
If you want a relay-only setup, however, you can consider changing the mailer daemon to programs thatcan only be configured to forward the mail to a remote mail server. Debian provides currently both ssmtp
and nullmailer for this purpose. In any case, you can evaluate for yourself any of the mail transport agents
2
provided by Debian and see which one suits best to the system's purposes.

Providing secure access to mailboxes
If you want to give remote access to mailboxes there are a number of POP3 and IMAP daemons available.3
However, if you provide IMAP access note that it is a general file access protocol, it can become theequivalent of a shell access because users might be able to retrieve any file that they can through it.
Try, for example, to configure as your inbox path {server.com}/etc/passwd if it succeeds your
IMAP daemon is not properly configured to prevent this kind of access.
Of the IMAP servers in Debian the cyrus server (in the cyrus-imapd package) gets around this by having
all access to a database in a restricted part of the file system. Also, uw-imapd (either install the uwimapd or better, if your IMAP clients support it, uw-imapd-ssl) can be configured to chroot the users maildirectory but this is not enabled by default. The documentation provided gives more information on how
to configure it.
Also, you might want to run an IMAP server that does not need valid users to be created on the local system(which would grant shell access too), courier-imap (for IMAP) and courier-pop, teapop (for POP3) andcyrus-imapd (for both POP3 and IMAP) provide servers with authentication methods beside the local user
accounts. cyrus can use any authentication method that can be configured through PAM while teapop
might use databases (such as postgresql and mysql) for user authentication.
FIXME: Check: uw-imapd might be configured with user authentication through PAM too.

Recibiendo Correo de forma segura
Leer/recibir correo es el protocolo más común de texto plano. Si usted usa POP3 o IMAP para obtener su
correo, la contraseña es enviada en texto plano a través de la red, de modo que casi cualquiera podría leersu correo a partir de ahora. En lugar de esto, use SSL (Capa segura de Sockets) para recibir su correo. La
2To retrieve the list of mailer daemons available in Debian try:

$ apt-cache search mail-transport-agent
The list will not include qmail, which is distributed only as source code in the qmail-src package.
3
A list of servers/daemons which support these protocols in Debian can be retrieved with:$ apt-cache search pop3-server$ apt-cache search imap-server

65Asegurando los servicios
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otra alternativa es ssh, si tiene una cuenta shell en la máquina que actua como el servidor POP o IMAP.
Este es un ejemplo básico fetchmailrc para demostrar esto:

poll my-imap-mailserver.org via "localhost"with proto IMAP port 1236
user "ref" there with password "hackme" is alex here warnings 3600
folders.Mail/debianpreconnect 'ssh -f -P -C -L 1236:my-imap-mailserver.org:143 -l ref
my-imap-mailserver.org sleep 15 </dev/null > /dev/null'
La preconexión es la línea más importante. Este lanza una sesión ssh y crea el tunel necesario, el cualautomaticamente envía las conexiones para tener acceso a localhost puerto 1236 al servidor de correo
IMAP, pero codificado. Otra posibilidad seria, usar el fetchmail con la caracteristica ssl.
Si usted quiere suministrar un servicio de correo codificado como POP e IMAP,apt-get install
stunnel e inicie sus demonios de esta es la forma:

stunnel -p /etc/ssl/certs/stunnel.pem -d pop3s -l /usr/sbin/popd
Este comando encapsula al demonio proveido (-l) en el puerto (-d) y usa el certificado ssl especificado (-p).

Asegurando BIND
Hay diferentes consideraciones que puede implementar para asegurar el demonio de servidor de nombres,las cuales son similares a las mismas que cuando se asegura cualquier servicio dado:• Configurar el demonio por si solo apropiadamente para que este no pueda ser afectado desde afuera.
Esto abarca limitar las posibles dudas de los clientes: zona transferida y consultas recursivas.
• Limitar el acceso del demonio al servidor mismo, de modo que si este es usado para entrar, el daño enel sistema esté limitado. Esto incluye correr el demonio como un usuario no privilegiado y cambiarle
el directorio raiz.

Bind configuration to avoid misuse
Deberia restringir alguna de la información que es dada por el servidor DNS para clientes externos paraque no pueda ser usado para acceder a información valiosa de su organización que usted no quiere dar.
Esto incluye añadir las siguientes opciones: allow-transfer, allow-query, allow-recursive y version.Puede
limitar en una sección global (para que se aplica a todas las zonas presentes) o sobre una base por zona.
Esta información esta documentada en el paquete bind-doc, lea más sobre esto en /usr/share/doc/
bind/html/index.html una vez el paquete este instalado.
Imagine que su servidor está conectado a Internet y a su red interna (su IP interno es 192.168.1.2)(unservicio de multi domicilio basico). Usted no quiere dar ningun servicio para Internet y solo quiere permitir
el lookups DNS desde su servidor interno. Usted podria restringir esto para incluirlo en: /etc/bind/
named.conf:

options {\t allow-query { 192.168.1/24; } ;\t allow-transfer { none; } ;66

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allow-recursion { 192.168.1/24; } ;\t listen-on { 192.168.1.2; } ;\t forward { only; } ;\t forwarders { A.B.C.D; } ;};La opciónlisten-onhace el bind DNS solo para la interfaz que tiene la dirección interna, pero si esta interfaz
es la misma como la interfaz que se conecta a Internet (por ejemplo, si usted está usando NAT), las dudas
seran solamente aceptadas si llegan desde su servidor interno. Si el sistema tiene multiples interfaces y ellisten-on no está presente, solamente los usuarios internos podrian pregutar, ya que el puerto seria accesible
para los atacantes exteriores, ellos podrian tratar de arrojarlo al servidor DNS (o explotar el amortiguador
desbordandose agresivamente). Usted aun podría leer esto en 127.0.0.1 si usted no está dando el servicio
DNS por ningun otro sistema que el de usted mismo.
El registro version.bind en la clase caos contiene la versión del proceso bind que se está ejecutando. Esta
información es frecuentemente usada por dispositivos automaticos e individuos maliciosos que desean
determinar si el bind de uno es vulnerable a un ataque específico. Para proporcionar falsa o negativa
información en el registro de la version.bind, uno limita la probabilidad que un servidor pueda ser atacado
basandonos en la versión publicitaria.Para suministrar su proia versión, utilice la version dirigida de lasiguiente manera:
options { ... various options here ...
version "Not available."; };Cambiar el registro de la version.bind que no proporciona una protección actual en contra de los ataques,pero este debería ser considerado un salva guardia útil.
A sample named.conf configuration file might be the following:acl internal {127.0.0.1/32;
10.0.0.0/8;
aa.bb.cc.dd;};// localhost// internal// eth0 IP

acl friendly {ee.ff.gg.hh;
aa.bb.cc.dd;
127.0.0.1/32;
10.0.0.0/8;};// slave DNS// eth0 IP// localhost// internal

options {directory "/var/cache/bind";allow-query { internal; };allow-recursion { internal; };allow-transfer { none; };};// From here to the mysite.bogus zone// is basically unmodified from the debian default
logging {category lame-servers { null; };category cname { null; };};67

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zone "." {type hint;
file "/etc/bind/db.root";};zone "localhost" {type master;
file "/etc/bind/db.local";};zone "127.in-addr.arpa" {type master;
file "/etc/bind/db.127";};
zone "0.in-addr.arpa" {type master;
file "/etc/bind/db.0";};zone "255.in-addr.arpa" {type master;
file "/etc/bind/db.255";};// zones I added myself
zone "mysite.bogus" {type master;
file "/etc/bind/named.mysite";allow-query { any; };allow-transfer { friendly; };};Please (again) check the Bug Tracking System regarding Bind, specifically http://bugs.debian.org/94760.
Feel free to contribute to the bug report if you think you can add useful information.

Changing BIND's user
Con respecto a limitar los privilegios de BIND, usted debe darse cuenta que si un usuario del non-rootrecorre Bind, Bind no podra detectar las nuevas interfaces automaticamente. Como por ejemplo si usted
pone en un portatil una tarjeta PCMCIA. Cambie el archivo README Debian en el directorio nombrado (/usr/share/doc/bind/README.Debian)para mas información acerca de este uso. Recientemente
han habido muchos problemas de seguridad en lo que concierne a BIND, y por esto es necesario cambiar
el usuario util cuando sea posible.
Notice, in any case, that this only applies to BIND version 8. In the Debian packages for BIND version 9(since the 9.2.1-5 version, available since sarge) the bind user is created and used by setting the OPTIONS
variable in /etc/default/bind9. If you are using BIND version 9 and your name server daemon is
not running as the bind user verify the settings on that file.
Para correr BIND bajo un usuario diferente, primero cree un usuario separado y un grupo para esto (noes buena idea usar,not nobody o nogroup para todo sevicio que no corra como raiz). En este ejemplo, el
usuario y el grupo namedserán usados. Usted puede hacer esto entrando a:68

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addgroup named
adduser --system --home /home/named --no-create-home --ingroup named \--disabled-password --disabled-login named
Notice that the user named will be quite restricted. If you want, for whatever reason, to have a less restrictive setup use:
adduser --system --ingroup named named
Ahora edite /etc/init.d/bind con su editor favorito y cambie la linea comenzando con:start-stop-daemon --start
to4
start-stop-daemon --start --quiet --exec /usr/sbin/named -- -g named -u named
Or you can change (create it if it does not exit) the default configuration file (/etc/default/bindfor BIND version 8) and introduce the following:
OPTIONS="-u named -g named"
Change the permissions of files that are used by Bind, including /etc/bind/rndc.key:-rw-r-----

1 rootnamed

77 Jan

4 01:02 rndc.key

and where bind creates its pidfile, using, for example, /var/run/named instead of /var/run:$ mkdir /var/run/named$ chown named.named /var/run/named$ vi /etc/named.conf[ ... update the configuration file to use this new location ...]
options { ...
pid-file "/var/run/named/named.pid";};[ ... ]
Also, in order to avoid running anything as root, change the reload line in the init.d script by substituting:reload)/usr/sbin/ndc reloadpara:
reload)$0 stop
sleep 1
4Note that depending on your bind version you might not have the -g option, most notably if you are using bind9 in sarge (9.2.4 version).

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Asegurando los servicios
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Note: Depending on your Debian version you might have to change the restart line too. This was fixedin Debian's bind version 1:8.3.1-2.
Todo lo que usted necesita hacer ahora es reiniciar Bind'/etc/init.d/bind, y luego cambiar su syslog por
dos entradas como estas:
Sep
Sep

4 15:11:08 nexus named[13439]: group = named
4 15:11:08 nexus named[13439]: user = named

Voilà! Your named now does not run as root. If you want to read more information on why BIND does
not run as non-root user on Debian systems, please check the Bug Tracking System regarding Bind, specifically http://bugs.debian.org/50013 and http://bugs.debian.org/132582, http://bugs.debian.org/53550,
http://bugs.debian.org/52745, and http://bugs.debian.org/128129. Feel free to contribute to the bug reports
if you think you can add useful information.

Chrooting the name server
To achieve maximum BIND security, now build a chroot jail (see “Cambio general de directorio raíz yparanoia suid”) around your daemon. There is an easy way to do this: the -t option (see the named(8)manual page or page 100 of http://www.nominum.com/content/documents/bind9arm.pdf). This will make
Bind chroot itself into the given directory without you needing to set up a chroot jail and worry about
dynamic libraries. The only files that need to be in the chroot jail are:dev/null
etc/bind/- should hold named.conf and all the server zones
sbin/named-xfer - if you do name transfers
var/run/named/ - should hold the pid and the name server cache (ifany) this directory needs to be writable by named
user
var/log/named- if you set up logging to a file, needs to be writable
for the named user
dev/log- syslogd should be listening here if named is configured tolog through it
Para que su denmonio BIND trabeje apropiedamente, este necesita permiso en los archivos nombrados.
Ésta es una tarea fácil ya que los archivos de configuarción estan siempre en /etc/named/. Tenga encuenta que esto solamente necesita acceso de lectura para los archivos de la zona, a menos que este sea unsecundario o un servidor llamado cache. Si este es su caso usted tendra que dar permiso de lecto-escritura
a las zonas necesarias (asi como la zona transferida desde los tarbajos del servidor primario).
Also, you can find more information regarding Bind chrooting in the http://www.tldp.org/HOWTO/Chroot-BIND-HOWTO.html (regarding Bind 9) and http://www.tldp.org/HOWTO/Chroot-BIND8HOWTO.html (regarding Bind 8). This same documents should be available through the installation of
the doc-linux-text (text version) or doc-linux-html (HTML version). Another useful document is http://
web.archive.org/web/20011024064030/http://www.psionic.com/papers/dns/dns-linux.
If you are setting up a full chroot jail (i.e. not just -t) for Bind in Debian, make sure you have the following
files in it5:5This setup has not been tested for new release of Bind yet.

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dev/log - syslogd should be listening here
dev/null
etc/bind/named.conf
etc/localtime
etc/group - with only a single line: "named:x:GID:"
etc/ld.so.cache - generated with ldconfig
lib/ld-2.1.3.so
lib/libc-2.1.3.so
lib/ld-linux.so.2 - symlinked to ld-2.1.3.so
lib/libc.so.6 - symlinked to libc-2.1.3.sosbin/ldconfig - may be deleted after setting up the chroot
sbin/named-xfer - if you do name transfers
var/run/And modify also syslogd listen on $CHROOT/dev/log so the named server can write syslog entries
into the local system log.
If you want to avoid problems with dynamic libraries, you can compile bind statically. You can use aptget for this, with the source option. It can even download the packages you need to properly compile
it. You would need to do something similar to:$ apt-get source bind
# apt-get build-dep bind
$ cd bind-8.2.5-2(edit src/port/linux/Makefile so CFLAGS includes the '-static'
option)$ dpkg-buildpackage -rfakeroot -uc -us$ cd ..
# dpkg -i bind-8.2.5-2*deb
After installation, you will need to move around the files to the chroot jail6 you can keep the init.d
scripts in /etc/init.d so that the system will automatically start the name server, but edit them to add--chroot /location_of_chroot in the calls to start-stop-daemon in those scripts or use the -t
option for BIND by setting it in the OPTIONS argument at the /etc/default/bind (for version 8)
or /etc/default/bind9 (for version 9) configuration file.
For more information on how to set up chroots see “Cambio general de directorio raíz y paranoia suid”.
FIXME:
Merge
info
from
http://people.debian.org/~pzn/howto/chroot-bind.sh.txt,
http://
www.cryptio.net/~ferlatte/config/ (Debian-specific), http://web.archive.org/web/20021216104548/
http://www.psionic.com/papers/whitep01.html and http://csrc.nist.gov/fasp/FASPDocs/NISTSecuringDNS.htm.

Asegurando Apache
FIXME: Add content: modules provided with the normal Apache installation (under /usr/lib/apache/X.X/
mod_*) and modules that can be installed separately in libapache-mod-XXX packages.
Usted puede limitar el acceso a el servidor Apache si si usted quiere usar esto solo internamente (paraobjetivos de prueba, para tener acceso al archivodoc-central etc..) y si no quiere que extraños tengan esto.
Para hacer esto use el Listen o BindAddress dirigidos en /etc/apache/http.conf.
Usando Listen:
6

Unless you use the instdir option when calling dpkg but then the chroot jail might be a little more complex.

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Listen 127.0.0.1:80
Usando BindAddress:
BindAddress 127.0.0.1
Luego reinicie Apache con /etc/init.d/apache restart y vera que esto es de solo Audición
en la interfaz loopback.
De todos modos, que usted no este usando todo lo funcionamiento suministrado por Apache, usted podria
querer dar un vistazo a otro servicio de la web proporcionados en Debian como dhttpd.
La http://httpd.apache.org/docs/misc/security_tips.html proporciona información relacxionada con las medidas de seguridad que deben ser tomadas en el servidor web del Apache (esta misma información estásuministrada en Debian por el paqueteapache-doc).
More information on further restricting Apache by setting up a chroot jail is provided in “Chroot environment for Apache”.

Disabling users from publishing web contents
The default Apache installation in Debian permits users to publish content under the $HOME/public_html. This content can be retrieved remotely using an URL such as: http://your_apache_server/~user.
If you do not want to permit this you must change the /etc/apache/http.conf configuration filecommenting out (in Apache 1.3) the following module:
LoadModule userdir_module /usr/lib/apache/1.3/mod_userdir.so
If you are using Apache 2.0 you must remove the file /etc/apache2/mods-enabled/userdir.load or restrict the default configuration by modifying /etc/apache2/mods-enabled/
userdir.conf.
However, if the module was linked statically (you can list the modules that are compiled in running apache -l) you must add the following to the Apache configuration file:
Userdir disabled
An attacker might still do user enumeration, since the answer of the web server will be a 403 Permission
Denied and not a 404 Not available. You can avoid this if you use the Rewrite module.

Permisos para el archivo de registro
Apache logfiles, since 1.3.22-1, are owned by user 'root' and group 'adm' with permissions 640. Thesepermissions are changed after rotation. An intruder that accessed the system through the web server would
not be able (without privilege escalation) to remove old log file entries.

Published web files
Apache files are located under /var/www. Just after installation the default file provides some informationon the system (mainly that it's a Debian system running Apache). The default webpages are owned by72

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user root and group root by default, while the Apache process runs as user www-data and group wwwdata. This should make attackers that compromise the system through the web server harder to deface thesite. You should, of course, substitute the default web pages (which might provide information you do notwant to show to outsiders) with your own.

Asegurando finger
Si usted quiere recorrer el servicio de finger, primero preguntese si usted necesita realizar esto. Si lo
hace, usted mismo descubrira que Debian proporciona muchos demonios finger (Puest fuera de apt-cachesearch fingerd):• cfingerd - Demonio finger configurable• efingerd - Es otro demonio finger para unix capaz de una fina-sintonización de su rendimiento.
• ffingerd - Un demonio seguro.
• fingerd - Remoto servidor de la información del usuario• BSD- Demonio finger con soporte qmail.
ffingerd es el demonio finger recomendado para si usted va ausar esto para un servicio publico. De todos
modos usted se fortalece, cuando establece este a traves de inetd, xinetd o tcpserver para: limitar el numero
de procesos que estaran corriendo al mismo tiempo, limitar el acceso para el demonio finger a partir deun numero dado por los servidores (usando el wrappers tcp) y teniendo esto solamente por audición parala interfaz en la que usted necesita estar.

Cambio general de directorio raíz y paranoia
suidchroot es una de las posibilidades más poderosas para restringir un demonio, un ususario u otro servicio.
Sólo imagine una cárcel alrededor de su objetivo, del cual no puede escapar (normalmente, hay sin embargo
muchas condiciones que permiten un escape fuera de su cárcel). Si usted no confía en un usuario, puede
crear un cambio en el ambiente. Ésto puede usar un pequeño espacio adicional de disco, puesto que senecesita copiar todos los ejecutables necesarios, así como las biblioteca dentro de la cárcel. Aún si elusuario hace algo malicioso, el alcance de un daño es limitado al aseguramiento.
Many services running as daemons could benefit from this sort of arrangement. The daemons that you
install with your Debian distribution will not come, however, chrooted7 per default.
This includes: name servers (such as bind), web servers (such as apache), mail servers (such as sendmail)and ftp servers (such as wu-ftpd). It is probably fair to say that the complexity of BIND is the reason why
it has been exposed to a lot of attacks in recent years (see “Asegurando BIND”).
However, Debian does provide some software that can help set up chroot environments. See “Making
chrooted environments automatically”.
Anyway, if you run any service on your system, you should consider running them as secure as possible.
This includes: revoking root privileges, running in a restricted environment (such as a chroot jail) or replacing them with a more secure equivalent.
7

It does try to run them under minimum priviledge which includes running daemons with their own users instead of having them run as root.

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Sin embargo, esté prevenido que el seguro chroot puede estar dañado si el usuario entra en éste es elsuperusuario. Así que usted necesita que el servicio corra como un usuario no privilegiado. Límitando su
ambien usted está límitando la palabra leíbles que el servicio de archivos ejecutables puede acceder, así,usted límita las posibilidades de una subida del privilegio por el uso de vulnerabilidades de seguridad delos sistemas locales. Incluso en ésta situación usted no puede estar copmpletamente seguro de que no hay
ninguna manera para que un atacante hábil se escape de algún modo del aseguramiento. Usando solamente
un servidor de programa, el cual tiene una reputación de medida de aseguramiento que es buena. Incluso lacavidad minusiosa de archivos manuales puede ser abierta por un atacante hábil interrumpiendo el sistema
por dentro. Despues de todo, chroot no fue diseñado como una herramienta de comprobación.

Making chrooted environments automatically
There are several programs to chroot automatically servers and services. Debian currently (accepted in May
2002) provides Wietse Venema's chrootuid in the chrootuid package, as well as compartment and makejail. These programs can be used to set up a restricted environment for executing any program (chrootuid
enables you to even run it as a restricted user).
Some of these tools can be used to set up the chroot environment easily. The makejail program for example, can create and update a chroot jail with short configuration files (it provides sample configuration
files for bind, apache, postgresql and mysql). It attempts to guess and install into the jail all files required by the daemon using strace, stat and Debian's package dependencies. More information at http://
www.floc.net/makejail/. Jailer is a similar tool which can be retrieved from http://www.balabit.hu/downloads/jailer/ and is also available as a Debian package.

Texto claro general con el password paranoia
Usted deberia tratar de evitar cualquier servicio de red el cual envia y recibe contraseñas en un texto claro
sobre una red como FTP/Telnet/NIS/RPC. El autor recomienda para todos el uso de ssh en cambio detelnet y ftp.
Mantenga en mente que migrar de telenet a ssh pero usando otros protocolos de texto claro no aumentan su
seguridad de NINGUNA forma! lo mejor seria eliminar ftp, telnet, pop, imap, http y suplantarlos con sus
respectivos servicios codificados. Usted debe considerar moverse desde otros servicios hasta sus versiones
SSL, ftp-ssl, telnet-ssl, pop-ssl, https...
Muchos de estas indicaciones numeradas en la parte superior se aplican a documentos en todo el sistema
Unix (Usted los encontrara si lee cualquier otro hardening-related relacionado con lo que tiene que ver
con Linux y otros Unix).

Incapacitar NIS
Es posible que usted no tenga que usar NIS, en el servicio de información de la red, porque este permite
que la contraseña actue. Este puede ser demasiado inseguro si su organización está rota.
Si usted necesita que la contraseña actue entre maquinas, usted deberia considerar usar otras alternativas. Por ejemplo usted puede colocar un servidor LDAP y configurar PAM en su sistema para contactar el servidor LDAP para la autenticación del usuario. Usted puede encontrar una detallada organización en el http://www.linuxdoc.org/HOWTO/LDAP-HOWTO.html (/usr/share/doc/HOWTO/entxt/LDAP-HOWTO.txt.gz).
Lea mas sobre la seguridad en http://www.linuxdoc.org/HOWTO/NIS-HOWTO.html (/usr/share/doc/HOWTO/en-txt/NIS-HOWTO.txt.gz).

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ARREGLAME (jfs): Add info on how to setup this in DebianSecuring RPC services
You should disable RPC if you do not need it.
Remote Procedure Call (RPC) is a protocol that programs can use to request services from other programs
located on different computers. The portmap service controls RPC services by mapping RPC program
numbers into DARPA protocol port numbers; it must be running in order to make RPC calls.
RPC-based services have had a bad record of security holes, although the portmapper itself hasn't (but still
provides information to a remote attacker). Notice that some of the DDoS (distributed denial of service)attacks use RPC exploits to get into the system and act as a so called agent/handler.
You only need RPC if you are using an RPC-based service. The most common RPC-based services are
NFS (Network File System) and NIS (Network Information System). See the previous section for more
information about NIS. The File Alteration Monitor (FAM) provided by the package fam is also an RPC
service, and thus depends on portmap.
NFS services are quite important in some networks. If that is the case for you, then you will needto find a balance of security and usability for your network (you can read more about NFS security in the http://www.tldp.org/HOWTO/NFS-HOWTO.html (/usr/share/doc/HOWTO/en-txt/
NFS-HOWTO.txt.gz)).

Desactivar los servicios RPC
Inhabilitar el paquete portmap es super sencillo. Hay diferentes mátodos. Uno de los más sencillos en unsistema Debian 3.0 es hacer un desinstalamiento del paquete portmap. Si usted está usando otra versión,
tendrá que desactivar el servicio como se ve en disableserv, esto es debido a el programa que forma partedel paquete net-base (el cual no puede ser desinstalado sin que el sistema se haya destruido).
Notice that some desktop environments (notably, GNOME) use RPC services and need the portmapper
for some of the file management features. If this is your case, you can limit the access to RPC services
as described below.

Limites al acceso de servicios RPC
Unfortunately, in some cases removing RPC services from the system is not an option. Some local desktop
services (notably SGI's fam) are RPC based and thus need a local portmapper. This means that under somesituations, users installing a desktop environment (like GNOME) will install the portmapper too.
There are several ways to limit access to the portmapper and to RPC services:• Block access to the ports used by these services with a local firewall (see “Añadir capacidades al cortafuegos”).
• Block access to these services using tcp wrappers, since the portmapper (and some RPC services) arecompiled with libwrap (see “Uso de tcpwrappers”). This means that you can block access to themthrough the hosts.allow and hosts.deny tcp wrappers configuration.
• Since version 5-5, the portmap package can be configured to listen only on the loopback interface.
To do this, modify /etc/default/portmap, uncomment the following line: #OPTIONS="-i127.0.0.1" and restart the portmapper. This is sufficient to allow local RPC services to work while75

Asegurando los servicios
que se ejecutan en su sistemaat the same time prevents remote systems from accessing them (see, however, “Disabling weak-end
hosts issues”).

Añadir capacidades al cortafuegos
The Debian GNU/Linux operating system has the built-in capabilities provided by the Linux kernel. If you
install a recent Debian release (default kernel installed is 2.6) you will have iptables (netfilter) firewalling
available8.

El sistema local corta fuegos
Usted puede usar las reglas de corta fuegos cono una forma para asegurar el acceso en un sistema local,
invluso para limitaer la salida de comunicación hecha por este. Las reglas corta fuegos pueden ser usadas
también para protejer procesos que no pueden ser configurados apropiadamente ni proveer servicios para
algunas redes, direcciones, IP, etc...
Sin embargo, este paso se presentara despues en el manual, basicamente porque es mucho mejor para nodepender únicamente de la capacidad del corta fuegos para protejer un sistema dado. La seguridad en
un sistema no puede ser hecho de cubiertas, el corta fuegos deberia ser el ultimo en incluirse, una vez
todos los servicios hayan sido fortalecidos. Usted puede fácilmente imaginar un plan en el cual el sistema
está protegido solamente por un corta fuegos incorporadoy un administrador blissfully que remueve lasreglas del corta fuegos por cualquiera que sea la razón (problemas con la instalación, molestias, errores
humanos...), este sistema abierto ampliamnete para un ataque.
On the other hand, having firewall rules on the local system also prevents some bad things from happening.
Even if the services provided are configured securely, a firewall can protect from misconfigurations orfrom fresh installed services that have not yet been properly configured. Also, a tight configuration will
prevent trojans calling home from working unless the firewalling code is removed. Note that an intruder
does not need superuser access to install a trojan locally that could be remotely controlled (since binding
on ports is allowed if they are not priviledged ports and capabilities have not been removed).
Thus, a proper firewall setup would be one with a default deny policy, that is:• incoming connections are allowed only to local services by allowed machines.
• outgoing connections are only allowed to services used by your system (DNS, web browsing, POP,email...).9• the forward rule denies everything (unless you are protecting other systems, see below).
• el resto de conexiones entrantes o salientes son denegadas.

Usar otros corta fuegos para proteger otros sistemas
A Debian firewall can also be installed in order to protect, with filtering rules, access to systems behind it,
limiting their exposure to the Internet. A firewall can be configured to prevent access from systems outsideof the local network to internal services (ports) that are not public. For example, on a mail server, only
8

Available since the kernel version 2.4 (which was the default kernel in Debian 3.0). Previous kernel versions (2.2, available in even older Debianreleases) used ipchains. The main difference between ipchains and iptables is that the latter is based on stateful packet inspection which provides
for more secure (and easier to build) filtering configurations. Older (and now unsupported) Debian distributions using the 2.0 kernel series needed
the appropriate kernel patch.
9
Unlike personal firewalls in other operating systems, Debian GNU/Linux does not (yet) provide firewall generation interfaces that can make rules
limiting them per process or user. However, the iptables code can be configured to do this (see the owner module in the iptables(8) manual page).

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port 25 (where the mail service is being given) needs to be accessible from the outside. A firewall can be
configured to, even if there are other network services besides the public ones running in the mail server,throw away packets (this is known as filtering) directed towards them.
You can even set up a Debian GNU/Linux box as a bridge firewall, i.e. a filtering firewall completelytransparent to the network that lacks an IP address and thus cannot be attacked directly. Depending on
the kernel you have installed, you might need to install the bridge firewall patch and then go to 802.1d
Ethernet Bridging when configuring the kernel and a new option netfilter ( firewalling ) support. See the“Setting up a bridge firewall ” for more information on how to set this up in a Debian GNU/Linux system.

Configurando un cortafuegos
The default Debian installation, unlike other Linux distributions, does not yet provide a way for the administrator to setup a firewall configuration throughout the default installation but you can install a numberof firewall configuration packages (see “Paquetes del Corta Fuegos”).
Of course, the configuration of the firewall is always system and network dependant. An administrator
must know beforehand what is the network layout and the systems to protect, the services that need to beaccessed, and whether or not other network considerations (like NAT or routing) need to be taken into
account. Be careful when configuring your firewall, as Laurence J. Lane says in the iptables package:
The tools can easily be misused, causing enormous amounts of grief by completely crippling network
access to a system. It is not terribly uncommon for a remote system administrator to accidentally get locked
out of a system hundreds or thousands of miles away. You can even manage to get locked out of a computer
who's keyboard is under your own fingers. Please, use due caution.
Remember this: just installing the iptables (or the older firewalling code) does not give you any protection,
just provides the software. In order to have a firewall you need to configure it!
Si usted no tiene un indicio sobre como colocar sus reglas al corta fuegos consulte el Paquete Filtrador
HOWTO proporcionado por iptables al leer fuera de la linea en /usr/share/doc/iptables/html/
If you do not know much about firewalling you should start by reading the http://www.tldp.org/HOWTO/Firewall-HOWTO.html, install the doc-linux-text package if you want to read it offline. If you wantto ask questions or need help setting up a firewall you can use the debian-firewall mailing list, see http://
lists.debian.org/debian-firewall. Also see “Conocimiento previo” for more (general) pointers on firewalls.
Another good iptables tutorial is http://iptables-tutorial.frozentux.net/iptables-tutorial.html.

Paquetes del Corta Fuegos
Setting up manually a firewall can be complicated for novice (and sometimes even expert) administrators.
However, the free software community has created a number of tools that can be used to easily configure
a local firewall. Be forewarned that some of these tools are oriented more towards local-only protection(also known as personal firewall) and some are more versatile and can be used to configure complex rules
to protect whole networks.
Hay un software completo que pueden ser usados para colocar reglas de corta fuegos en un sistema Debian• Para entornos gráficos• firestarter, a GNOME application oriented towards end-users that includes a wizard useful to quickly
setup firewall rules. The application includes a GUI to be able to monitor when a firewall rule blocks
traffic.
• guarddog, a KDE based firewall configuration package oriented both to novice and advanced users.

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Asegurando los servicios
que se ejecutan en su sistema• knetfilter, a KDE GUI to manage firewall and NAT rules for iptables (alternative/competitor to theguarddog tool although slightly oriented towards advanced users).
• fireflier, an interactive tool to create iptables rules based on traffic seen on the system and applications.
It has a server-client model so you have to install both the server (fireflier-server) and one of theavailable clients, with one client available for different desktop environments: fireflier-client-gtk (Gtk
+ client), fireflier-client-kde (KDE client) and fireflier-client-qt (QT client).
• For servers (headless) systems:• fwbuilder, an object oriented GUI which includes policy compilers for various firewall platforms
including Linux' netfilter, BSD's pf (used in OpenBSD, NetBSD, FreeBSD and MacOS X) as well as
router's access-lists. It is similar to enterprise firewall management software. Complete fwbuilder'sfunctionality is also available from the command line.
• shorewall, a firewall configuration tool which provides support for IPsec as well as limited support for
traffic shaping as well as the definition of the firewall rules. Configuration is done through a simple
set of files that are used to generate the iptables rules.
• bastille, this hardening application is described in Capítulo 6, Fortalecimiento automático de sistemas
Debian. One of the hardening steps that the administrator can configure is a definition of the allowed
and disallowed network traffic that is used to generate a set of firewall rules that the system will
execute on startup.
Lots of other iptables frontends come with Debian; an extensive list comparing the different packages in
Debian is maintained at the http://wiki.debian.org/Firewalls.
Notice that some of the packages outlined previously will introduce firewalling scripts to be run when thesystem boots. Test them extensively before rebooting or you might find yourself locked from the box. If
you mix different firewalling packages you can have undesired effects, usually, the firewalling script thatruns last will be the one that configures the system (which might not be what you intend). Consult the
package documentation and use either one of these setups.
As mentioned before, some programs, like firestarter, guarddog and knetfilter, are administration GUIs
using either GNOME or KDE (last two). These applications are much more user-oriented (i.e. for homeusers) than some of the other packages in the list which might be more administrator-oriented. Some ofthe programs mentioned before (like bastille) are focused at setting up firewall rules to protect the host
they run in but are not necessarily designed to setup firewall rules for firewall hosts that protect a network(like shorewall or fwbuilder).
There is yet another type of firewall application: application proxies. If you are looking into setting up
an enterprise-level firewall that does packet filtering and provides a number of transparent proxies thatcan do fine-grain traffic analysis you should consider using zorp, which provides this in a single program.
You can also manually setup this type of firewall host using the proxies available in Debian for different
services like for DNS using bind (properly configured), dnsmasq, pdnsd or totd for FTP using frox or ftpproxy, for X11 using xfwp, for IMAP using imapproxy, for mail using smtpd, or for POP3 using p3scan.
For other protocols you can either use a generic TCP proxy like simpleproxy or a generic SOCKS proxy
like dante-server, tsocks or socks4-server. Typically, you will also use a web caching system (like squid)
and a web filtering system (like squidguard or dansguardian).

Manual init.d configuration
Another possibility is to manually configure your firewall rules through an init.d script that will run all
the iptables commands. Take the following steps:

78

Asegurando los servicios
que se ejecutan en su sistema• Review the script below and adapt it to your needs.
• Test the script and review the syslog messages to see which traffic is being dropped. If you are testingfrom the network you will want to either run the sample shell snippet to remove the firewall (if you don't
type anything in 20 seconds) or you might want to comment out the default deny policy definitions (-P
INPUT DROP and -P OUTPUT DROP) and check that the system will not drop any legitimate traffic.
• Move the script to /etc/init.d/myfirewall• The below script takes advantage of Debian's use (since Squeeze) of dependency based boot sequencing.
For more information see: https://wiki.debian.org/LSBInitScripts/DependencyBasedBoot and https://wiki.debian.org/LSBInitScripts. With the LSB headers set as they are in the script, insserv will automatically configure the system to start the firewall before any network is brought up, and stop the firewall
after any network is brought down.

# insserv myfirewall
A continuación tiene un ejemplo de script cortafuegos:#!/bin/sh### BEGIN INIT INFO
# Provides:
myfirewall# Required-Start:$local_fs# Required-Stop:$local_fs# Default-Start:
S# Default-Stop:
0 6# X-Start-Before:$network# X-Stop-After:$network# Short-Description: My custom firewall.
### END INIT INFO#
# Sencillo ejemplo de cofiguración de un cortafuegos.
## Tenga en cuenta que:# - Esta configuraciñon se aplica en todas las interfaces de red# - Si desea restringirlo a una única interfaz utlice '-i INTERFAZ'#
en cada ejecución del comando iptables# - Se permite el acceso TCP/UDP a cualquier máquina#
Probablemente desee restringirlo mediante el uso de '--source'.
## chkconfig: 2345 9 91
# descripción: activa/desactiva el cortafuegos durante el inicio.
## Puede probar este script antes de instalarlo con este código.
# Simplemente hace que si no escribe nada durante 10 segundos,# se borran todas las reglas del cortafuegos.
#--------------------------------------------------------------# while true; do test=""; read -t 20 -p "OK? " test ; \# [ -z "$test" ] && /etc/init.d/myfirewall clear ; done#--------------------------------------------------------------PATH=/bin:/sbin:/usr/bin:/usr/sbin

79

Asegurando los servicios
que se ejecutan en su sistema# Services que el sistema proporciona en red
TCP_SERVICES="22" # sólo SSH
UDP_SERVICES=""# Services que el sistema necesita de la red.k
REMOTE_TCP_SERVICES="80" # Navegación web.
REMOTE_UDP_SERVICES="53" # DNS# Red mediante la que se accederá remotamente a la máquina# (si se deja en blanco, no se definirá ninguna regla)# NETWORK_MGMT=192.168.0.0/24# Si desea tener dicha red (es decir, ha descomentado la# anterior línea), también deberá definir el puerto para SSH# descomentando la siguiente línea. Recuerde elminar el puerto SSH# en la variable TCP_SERVICES# SSH_PORT="22"
if ! [ -x /sbin/iptables ]; thenexit 0fi
fw_start () {# Tráfico de entrada:/sbin/iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT# Servicios
if [ -n "$TCP_SERVICES" ] ; thenfor PORT in $TCP_SERVICES; do/sbin/iptables -A INPUT -p tcp --dport ${PORT} -j ACCEPT
done
fi
if [ -n "$UDP_SERVICES" ] ; thenfor PORT in $UDP_SERVICES; do/sbin/iptables -A INPUT -p udp --dport ${PORT} -j ACCEPT
donefi# Acceso remotoif [ -n "$NETWORK_MGMT" ] ; then/sbin/iptables -A INPUT -p tcp --src ${NETWORK_MGMT} --dport ${SSH_PORT} -j ACC
fi# Pruebas remotas/sbin/iptables -A INPUT -p icmp -j ACCEPT/sbin/iptables -A INPUT -i lo -j ACCEPT/sbin/iptables -P INPUT DROP
/sbin/iptables -A INPUT -j LOG# Tráfico de salida:/sbin/iptables -A OUTPUT -j ACCEPT -o lo/sbin/iptables -A OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT# Se permiteel tráfico de ICMP:/sbin/iptables -A OUTPUT -p icmp -j ACCEPT# También las actualizaciones de seguridad:# Puede añadir directamente la IP del servidor para evitar ataques de DNS# pero no verá cambios de IP de este servidor/sbin/iptables -A OUTPUT -p tcp -d security.debian.org --dport 80 -j ACCEPT# Igualmente para los servicios que definimos:

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Asegurando los servicios
que se ejecutan en su sistema
if [ -n "$REMOTE_TCP_SERVICES" ] ; thenfor PORT in $REMOTE_TCP_SERVICES; do/sbin/iptables -A OUTPUT -p tcp --dport ${PORT} -j ACCEPT
done
fi
if [ -n "$REMOTE_UDP_SERVICES" ] ; thenfor PORT in $REMOTE_UDP_SERVICES; do/sbin/iptables -A OUTPUT -p udp --dport ${PORT} -j ACCEPT
done
fi# Cualquier otra conexión se registrará mediante syslog/sbin/iptables -A OUTPUT -j LOG/sbin/iptables -A OUTPUT -j REJECT/sbin/iptables -P OUTPUT DROP
# Protecciones adicionales para la red# (algunas sólo funcionan en ciertas versiones del núcleo)
echo 1 > /proc/sys/net/ipv4/tcp_syncookies
echo 0 > /proc/sys/net/ipv4/ip_forward
echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts
echo 1 > /proc/sys/net/ipv4/conf/all/log_martians
echo 1 > /proc/sys/net/ipv4/ip_always_defragecho 1 > /proc/sys/net/ipv4/icmp_ignore_bogus_error_responses
echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter
echo 0 > /proc/sys/net/ipv4/conf/all/send_redirects
echo 0 > /proc/sys/net/ipv4/conf/all/accept_source_route}
fw_stop () {/sbin/iptables -F/sbin/iptables -t nat -F/sbin/iptables -t mangle -F/sbin/iptables -P INPUT DROP
/sbin/iptables -P FORWARD DROP
/sbin/iptables -P OUTPUT ACCEPT}
fw_clear () {/sbin/iptables -F/sbin/iptables -t nat -F/sbin/iptables -t mangle -F/sbin/iptables -P INPUT ACCEPT/sbin/iptables -P FORWARD ACCEPT/sbin/iptables -P OUTPUT ACCEPT}case "$1" instart|restart)echo -n "Iniciando el cortafuegos.."
fw_stopfw_startecho "Iniciado.";;81

Asegurando los servicios
que se ejecutan en su sistema
stop)echo -n "Deteniendo el cortafuegos.."
fw_stopecho "Detenido.";;
clear)
echo -n "Borrando las reglas del cortafuegos."
fw_clearecho "Borradas.";;*)echo "Uso: $0 {start|stop|restart|clear}"exit 1;;esac
exit 0Instead of including all of the iptables rules in the init.d script you can use the iptables-restore program
to restore the rules saved using iptables-save. In order to do this you need to setup your rules, save theruleset under a static location (such as /etc/default/firewall)

Configuring firewall rules through ifup
You can use also the network configuration in /etc/network/interfaces to setup your firewall
rules. For this you will need to:• Create your firewalling ruleset for when the interface is active.
• Save your ruleset with iptables-save to a file in /etc, for example /etc/iptables.up.rules• Configure /etc/network/interfaces to use the configured ruleset:

iface eth0 inet static
address x.x.x.x[.. configuración de la interfaz..]
pre-up iptables-restore < /etc/iptables.up.rules
You can optionally also setup a set of rules to be applied when the network interface is down creating
a set of rules, saving it in /etc/iptables.down.rules and adding this directive to the interface
configuration:

post-down iptables-restore < /etc/iptables.down.rules
For more advanced firewall configuration scripts through ifupdown you can use the hooks available toeach interface as in the *.d/ directories called with run-parts (see run-parts(8) manual page).

Probar las configuración del cortafuegos
Testing your firewall configuration is as easy, and as dangerous, as just running your firewall script (orenabling the configuration you defined in your firewall configuration application). However, if you are
not careful enough and you are configuring your firewall remotely (like through an SSH connection) you
could lock yourself out.

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Asegurando los servicios
que se ejecutan en su sistema
There are several ways to prevent this. One is running a script in a separate terminal that will remove thefirewall configuration if you don't feed it input. An example of this is:$while true; do test=""; read -t 20 -p "OK? " test ; \[ -z "$test" ] && /etc/init.d/firewall clear ; done

Another one is to introduce a backdoor in your system through an alternate mechanism that allows you
to either clear the firewall system or punch a hole in it if something goes awry. For this you can use
knockd and configure it so that a certain port connection attempt sequence will clear the firewall (or add
a temporary rule). Even though the packets will be dropped by the firewall, since knockd binds to theinterface and sees you will be able to work around the problem.
Testing a firewall that is protecting an internal network is a different issue, you will want to look at some of
the tools used for remote vulnerability assessment (see “Herramientas de evaluación de vulnerabilidades
remotas.”) to probe the network from the outside in (or from any other direction) to test the effectivenessof the firewall configuation.

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Capítulo 6. Fortalecimiento automático
de sistemas DebianTras leer toda la información de los capítulos anteriores podría pensar «Tengo que hacer una gran cantidad
de cosas para reforzar mi sistema, ¿no podrían hacerse estas cosas de forma automática?». La respuesta essí, pero sea cuidadoso con las herramientas de automatización. Hay gente que cree que una herramienta defortalecimiento no elimina la necesidad de una buena administración. Por tanto, no se engañe pensando quepuede automatizar el proceso completo y con eso solucionar todos los asuntos relacionados. La seguridad
es siempre un proceso activo en el cual el administrador tiene que participar y no limitarse a esperar a quelas herramientas hagan todo el trabajo, puesto que ninguna herramienta individual puede arreglárselas por
sí misma: con todas las posibles implementaciones de las directrices de seguridad, con todos los ataquesy en todos los entornos.
Desde woody (Debian 3.0) hay dos paquetes específicos, útiles para reforzar la seguridad. El paquete
harden que realiza una aproximación basada en las dependencias del paquete para instalar rápidamente
valiosos paquetes de seguridad y eliminar otros defectuosos, la configuración de los paquetes tiene que
hacerla el administrador. El paquete bastille que implementa una política de seguridad dada en el sistema
local basada en configuraciones anteriores del administrador (la configuración puede hacerse mediante unprocedimiento guiado en el que se deberá ir contestando sí/no a preguntas sencillas).

HardenThe harden package tries to make it more easy to install and administer hosts that need good security.
This package should be used by people that want some quick help to enhance the security of the system.
It automatically installs some tools that should enhance security in some way: intrusion detection tools,
security analysis tools, etc. Harden installs the following virtual packages (i.e. no contents, just dependencies or recommendations on others):• harden-tools: herramientas para mejorar la seguridad del sistema (comprobadores de integridad, detección de intrusos, parches del núcleo...)• harden-environment: ayuda a configurar un sistema fortalecido (actualmente vacío).
• harden-servers: elimina servidores considerados inseguros por alguna razón.
• harden-clients: elimina clientes considerados inseguros por alguna razón.
• harden-remoteaudit: herramientas para realizar auditorías de un sistema de forma remota.
• harden-nids: helps to install a network intrusion detection system.
• harden-surveillance: helps to install tools for monitoring of networks and services.
Useful packages which are not a dependence:• harden-doc: provides this same manual and other security-related documentation packages.
• harden-development: development tools for creating more secure programs.
Vaya con cuidado, porque si tiene software que le resulte necesario (y no desea desinstalarlo por alguna
razón) pero entra en conflicto con alguno de los paquetes mencionados anteriormente, podría implicar
limitaciones en el uso de harden. Los paquetes «harden» no hacen nada (directamente). Sin embargo,84Fortalecimiento automático de sistemas Debiantienen conflictos intencionados con paquetes conocidos como no seguros. De esta forma, el sistema degestión de paquetes de Debian no aprobará la instalación de estos paquetes. Por ejemplo, si intenta instalar
un demonio de telnet con harden-servers, apt dirá:# apt-get install telnetd
Se ELIMINARÁN los siguientes paquetes:
harden-servers
Se instalarán los siguientes paquetes NUEVOS:
telnetd¿Quiere continuar? [S/n]Esto debería hacer saltar la alarma en la cabeza del administrador, llevándole a reconsiderar sus acciones.

Bastille Linux
http://www.bastille-unix.org es una herramienta de fortalecimiento automático orientada originalmente alas distribuciones RedHat y Mandrake. Sin embargo, el paquete bastille suministrado con Debian (desde
woody) viene parcheado con objeto de proporcionar la misma funcionalidad en los sistemas Debian GNU/Linux.
«Bastille» puede utilizarse con diferentes interfaces (todas documentadas en su correspondiente página demanual en el paquete de Debian) que permiten al administrador:• Answer questions step by step regarding the desired security of your system (using InteractuveBastille(8)• Utilizar un ajuste de seguridad predeterminado (entre tres opciones: Relajada, Moderada o Paranoia)
en una configuración dada (servidor o estación de trabajo) y dejar que «Bastille» decida qué política deseguridad implementar (utilizar BastilleChooser(8))• Take a predefined configuration file (could be provided by Bastille or made by the administrator) and
implement a given security policy (using AutomatedBastille(8)).

85Capítulo 7. Infrastructura de seguridad
de DebianEquipo de seguridad
Debian has a Security Team, that handles security in the stable distribution. Handling security means theykeep track of vulnerabilities that arise in software (watching forums such as Bugtraq, or vuln-dev) anddetermine if the stable distribution is affected by it.
Also, the Debian Security Team is the contact point for problems that are coordinated by upstream developers or organizations such as http://www.cert.org which might affect multiple vendors. That is, when problems are not Debian-specific. The contact point of the Security Team is mailto:team@security.debian.org
which only the members of the security team read.
Sensitive information should be sent to the first address and, in some cases, should be encrypted with the
Debian Security Contact key (as found in the Debian keyring).
Once a probable problem is received by the Security Team it will investigate if the stable distribution isaffected and if it is, a fix is made for the source code base. This fix will sometimes include backporting
the patch made upstream (which usually is some versions ahead of the one distributed by Debian). After
testing of the fix is done, new packages are prepared and published in the http://security.debian.org site
so they can be retrieved through apt (see “Ejecute una actualización de seguridad”). At the same time a
Debian Security Advisory (DSA) is published on the web site and sent to public mailing lists including
http://lists.debian.org/debian-security-announce and Bugtraq.
Puede encontrar algunas preguntas frecuentes acerca del Equipo de Seguridad en “Preguntas con respecto
al equipo de seguridad Debian”.

Avisos de seguridad de Debian
Debian Security Advisories (DSAs) are made whenever a security vulnerability is discovered that affects
a Debian package. These advisories, signed by one of the Security Team members, include information of
the versions affected as well as the location of the updates. This information is:• version number for the fix.
• problem type.
• whether it is remote or locally exploitable.
• short description of the package.
• descripciñon del problema• descripción de la forma de explotarlo.
• descripción de la solución
DSAs are published both on http://www.debian.org/ and in the http://www.debian.org/security/. Usually
this does not happen until the website is rebuilt (every four hours) so they might not be present immediately.
The preferred channel is the debian-security-announce mailing list.
Interested users can, however (and this is done in some Debian-related portals) use the RDF channel
to download automatically the DSAs to their desktop. Some applications, such as Evolution (an email

86

Infrastructura de seguridad de Debianclient and personal information assistant) and Multiticker (a GNOME applet), can be used to retrieve theadvisories automatically. The RDF channel is available at http://www.debian.org/security/dsa.rdf.
DSAs published on the website might be updated after being sent to the public-mailing lists. A common
update is adding cross references to security vulnerability databases. Also, translations1 of DSAs are notsent to the security mailing lists but are directly included in the website.

Referencias cruzadas de vulnerabilidades
Debian provides a fully http://www.debian.org/security/crossreferences including all the references available for all the advisories published since 1998. This table is provided to complement the http://cve.mitre.org/cve/refs/refmap/source-DEBIAN.html.
You will notice that this table provides references to security databases such as http://www.securityfocus.com/bid, http://www.cert.org/advisories/ and http://www.kb.cert.org/vuls as well as CVE names (seebelow). These references are provided for convenience use, but only CVE references are periodically reviewed and included.
Advantages of adding cross references to these vulnerability databases are:• it makes it easier for Debian users to see and track which general (published) advisories have alreadybeen covered by Debian.
• system administrators can learn more about the vulnerability and its impact by following the crossreferences.
• this information can be used to cross-check output from vulnerability scanners that include references
to CVE to remove false positives (see “Vulnerability assessment scanner X says my Debian system isvulnerable!”).

CVE compatibility
Debian Security Advisories were http://www.debian.org/security/CVE-certificate.jpg2 in February 24,
2004.
Debian developers understand the need to provide accurate and up to date information of the security
status of the Debian distribution, allowing users to manage the risk associated with new security vulnerabilities. CVE enables us to provide standardized references that allow users to develop a https://cve.mitre.org/compatible/enterprise.html.
The http://cve.mitre.org project is maintained by the MITRE Corporation and provides a list of standardized names for vulnerabilities and security exposures.
Debian believes that providing users with additional information related to security issues that affect the
Debian distribution is extremely important. The inclusion of CVE names in advisories help users associate
generic vulnerabilities with specific Debian updates, which reduces the time spent handling vulnerabilities
that affect our users. Also, it eases the management of security in an environment where CVE-enabled
security tools -such as network or host intrusion detection systems, or vulnerability assessment tools- arealready deployed regardless of whether or not they are based on the Debian distribution.
Debian provides CVE names for all DSAs released since September 1998. All of the advisories can be
retrieved on the Debian web site, and announcements related to new vulnerabilities include CVE names
if available at the time of their release. Advisories associated with a given CVE name can be searched
directly through the Debian Security Tracker (see below).
1
2

Translations are available in up to ten different languages.
The full http://cve.mitre.org/compatible/phase2/SPI_Debian.html is available at CVE

87

Infrastructura de seguridad de Debian

In some cases you might not find a given CVE name in published advisories, for example because:• No Debian products are affected by that vulnerability.
• There is not yet an advisory covering that vulnerability (the security issue might have been reported asa http://bugs.debian.org/cgi-bin/pkgreport.cgi?tag=security but a fix has not been tested and uploaded).
• An advisory was published before a CVE name was assigned to a given vulnerability (look for an update
at the web site).

Security Tracker
The central database of what the Debian security teams know about vulnerabilities is the http://security-tracker.debian.org. It cross references packages, vulnerable and fixed versions for different suites,
CVE names, Debian bug numbers, DSA's and miscellaneous notes. It can be searched, e.g. by CVE nameto see which Debian packages are affected or fixed, or by package to show unresolved security issues.
The only information missing from the tracker is confidential information that the security team received
under embargo.
The package debsecan uses the information in the tracker to report to the administrator of a system whichof the installed packages are vulnerable, and for which updates are available to fix security issues.

Debian Security Build Infrastructure
Since Debian is currently supported in a large number of architectures, administrators sometimes wonder
if a given architecture might take more time to receive security updates than another. As a matter of fact,
except for rare circumstances, updates are available to all architectures at the same time.
Packages in the security archive are autobuilt, just like the regular archive. However, security updates area little more different than normal uploads sent by package maintainers since, in some cases, before being
published they need to wait until they can be tested further, an advisory written, or need to wait for a weekor more to avoid publicizing the flaw until all vendors have had a reasonable chance to fix it.
Thus, the security upload archive works with the following procedure:• Someone finds a security problem.
• Someone fixes the problem, and makes an upload to security-master.debian.org's incoming (this someone is usually a Security Team member but can be also a package maintainer with an appropriate fix thathas contacted the Security Team previously). The Changelog includes a testing-security or stable-security as target distribution.
• The upload gets checked and processed by a Debian system and moved into queue/accepted, and thebuildds are notified. Files in here can be accessed by the security team and (somewhat indirectly) by
the buildds.
• Security-enabled buildds pick up the source package (prioritized over normal builds), build it, and send
the logs to the security team.
• The security team reply to the logs, and the newly built packages are uploaded to queue/unchecked,
where they're processed by a Debian system, and moved into queue/accepted.
• When the security team find the source package acceptable (i.e., that it's been correctly built for all
applicable architectures and that it fixes the security hole and doesn't introduce new problems of itsown) they run a script which:88

Infrastructura de seguridad de Debian• installs the package into the security archive.
• updates the Packages, Sources and Release files of security.debian.org in the usual way(dpkg-scanpackages, dpkg-scansources, ...).
• sets up a template advisory that the security team can finish off.
• forwards the packages to the appropriate proposed-updates so that it can be included in the real archive
as soon as possible.
This procedure, previously done by hand, was tested and put through during the freezing stage of Debian
3.0 woody (July 2002). Thanks to this infrastructure the Security Team was able to have updated packages
ready for the apache and OpenSSH issues for all the supported (almost twenty) architectures in less thana day.

Developer's guide to security updates
Debian developers that need to coordinate with the security team on fixing in issue in their packages,
can refer to the Developer's Reference section http://www.debian.org/doc/manuals/developers-reference/pkgs.html#bug-security.

Firma de paquete en DebianThis section could also be titled "how to upgrade/update safely your Debian GNU/Linux system" and it
deserves its own section basically because it is an important part of the Security Infrastructure. Package
signing is an important issue since it avoids tampering of packages distributed in mirrors and of downloads
with man-in-the-middle attacks. Automatic software update is an important feature but it's also importantto remove security threats that could help the distribution of trojans and the compromise of systems during
updates 3
FIXME: probably the Internet Explorer vulnerability handling. certificate chains has an impact on security
updates on Microsoft Windows.
Debian does not provide signed packages but provides a mechanism available since Debian 4.0 (codenameetch) to check for downloaded package's integrity4. For more information, see “Secure apt”.
This issue is better described in the http://www.cryptnet.net/fdp/crypto/strong_distro.html by V. Alex
Brennen.

El esquema propuesto para revisiones de firma de paquete
The current scheme for package signature checking using apt is:• el archivo de publicación incluye el md5sum de Paquetes.gz (este contiene el md5sums de paquetes) yserá firmado. La firma es algo que pertenece a una fuente de confianza.
• This signed Release file is downloaded by 'apt-get update' and stored along with Packages.gz.
• Cuando un paquete va a ser instalado, primero se baja, luego el md5sum es generado.
3

Some operating systems have already been plagued with automatic-updates problems such as the http://www.cunap.com/~hardingr/projects/osx/
exploit.html.
4
Older releases, such as Debian 3.1 sarge can use this feature by using backported versions of this package management tool89

Infrastructura de seguridad de Debian• El archivo de publicación firmado es revisado (firma correcta) y este se extrae del md5sum para el archivo Paquetes.gz, el número de comprobación de Paquetes.gz es generado y (si es correcto) el md5sum
del paquete que se bajó es extraido de este.
• Si el md5sum del paquete que se bajó es el mismo que el del archivo Paquetes.gz, el paquete será instalado o de lo contrario el administrador será alertado y el paquete será dejado en cache (asi el administrador puede decidir si se instala o no). Si el paquete no está en los Paquetes.gz y el administrador haconfigurado el sistema para instalar únicamente los paquetes revisados, éste tampoco será instalado.
Adicional a esto, la cadena de Sums MD5 apt es capaz de verificar si un paquete se origina desde una
publicación específica. Este es menos flexible que firmar paquete por paquete, pero puede ser combinado
con este esquema también (véase más abajo).
This scheme is http://lists.debian.org/debian-devel/2003/12/msg01986.html in apt 0.6 and is available
since the Debian 4.0 release. For more information see “Secure apt”. Packages that provide a front-end to
apt need to be modified to adapt to this new feature; this is the case of aptitude which was http://lists.debian.org/debian-devel/2005/03/msg02641.html to adapt to this scheme. Front-ends currently known to
work properly with this feature include aptitude and synaptic.
La firma de un paquete ha sido discutida en Debian de vez en cuando, para mayor información usted puede leer: http://www.debian.org/News/weekly/2001/8/ y http://www.debian.org/News/weekly/2001/11/.
http://www.debian.org/News/weekly/2001/8/yhttp://www.debian.org/News/weekly/2000/11/.

Secure apt
The apt 0.6 release, available since Debian 4.0 etch and later releases, includes apt-secure (also known
as secure apt) which is a tool that will allow a system administrator to test the integrity of the packages
downloaded through the above scheme. This release includes the tool apt-key for adding new keys to apt's
keyring, which by default includes only the current Debian archive signing key.
These changes are based on the patch for apt (available in http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=203741) which provides this implementation.
Secure apt works by checking the distribution through the Release file, as discussed in “Per distribution
release check”. Typically, this process will be transparent to the administrator although you will need tointervene every year5 to add the new archive key when it is rotated, for more information on the steps anadministrator needs to take a look at “Añadir de forma segura una clave”.
This feature is still under development, if you believe you find bugs in it, please, make first sure you areusing the latest version (as this package might change quite a bit before it is finally released) and, if runningthe latest version, submit a bug against the apt package.
You can find more information at http://wiki.debian.org/SecureApt and the official documentation: http://www.enyo.de/fw/software/apt-secure/ and https://web.archive.org/web/20070206063141/
http://www.syntaxpolice.org/apt-secure/.

Per distribution release check
This section describes how the distribution release check mechanism works, it was written by Joey Hessand is also available at the http://wiki.debian.org/SecureApt.

Conceptos básicos
Here are a few basic concepts that you'll need to understand for the rest of this section.
5

Until an automatic mechanism is developed.

90Infrastructura de seguridad de Debian

A checksum is a method of taking a file and boiling it down to a reasonably short number that uniquely
identifies the content of the file. This is a lot harder to do well than it might seem, and the most commonlyused type of checksum, the MD5 sum, is in the process of being broken.
Public key cryptography is based on pairs of keys, a public key and a private key. The public key is givenout to the world; the private key must be kept a secret. Anyone possessing the public key can encrypta message so that it can only be read by someone possessing the private key. It's also possible to use a
private key to sign a file, not encrypt it. If a private key is used to sign a file, then anyone who has the
public key can check that the file was signed by that key. No one who doesn't have the private key canforge such a signature.
These keys are quite long numbers (1024 to 2048 digits or longer), and to make them easier to work withthey have a key id, which is a shorter, 8 or 16 digit number that can be used to refer to them.
gpg is the tool used in secure apt to sign files and check their signatures.
apt-key is a program that is used to manage a keyring of gpg keys for secure apt. The keyring is kept in
the file /etc/apt/trusted.gpg (not to be confused with the related but not very interesting /etc/
apt/trustdb.gpg). apt-key can be used to show the keys in the keyring, and to add or remove a key.

Release checksums
A Debian archive contains a Release file, which is updated each time any of the packages in the archive
change. Among other things, the Release file contains some MD5 sums of other files in the archive.
An excerpt of an example Release file:

MD5Sum:6b05b392f792ba5a436d590c129de21f
1356479a23edda7a69f24eb8d6f4a14b
2a5167881adc9ad1a8864f281b1eb959
88de3533bf6e054d1799f8e49b6aed8b3453 Packages
1131 Packages.gz
1715 Sources
658 Sources.gz

The Release files also include SHA-1 checksums, which will be useful once MD5 sums become fully
broken, however apt doesn't use them yet.
Now if we look inside a Packages file, we'll find more MD5 sums, one for each package listed in it.
For example:

Package: uqm
Priority: optional...
Filename: unstable/uqm_0.4.0-1_i386.deb
Size: 580558
MD5sum: 864ec6157c1eea88acfef44d0f34d219
These two checksums can be used to verify that you have downloaded a correct copy of the Packages
file, with a md5sum that matches the one in the Release file. And when it downloads an individual
package, it can also check its md5sum against the content of the Packages file. If apt fails at either ofthese steps, it will abort.
None of this is new in secure apt, but it does provide the foundation. Notice that so far there is one file thatapt doesn't have a way to check: The Release file. Secure apt is all about making apt verify the Release
file before it does anything else with it, and plugging this hole, so that there is a chain of verification fromthe package that you are going to install all the way back to the provider of the package.

91

Infrastructura de seguridad de DebianVerification of the Release file
To verify the Release file, a gpg signature is added for the Release file. This is put in a file named
Release.gpg that is shipped alongside the Release file. It looks something like this 6 , although only
gpg actually looks at its contents normally:-----BEGIN PGP SIGNATURE----Version: GnuPG v1.4.1 (GNU/Linux)
iD8DBQBCqKO1nukh8wJbxY8RAsfHAJ9hu8oGNRAl2MSmP5+z2RZb6FJ8kACfWvEx
UBGPVc7jbHHsg78EhMBlV/U==x6og-----END PGP SIGNATURE-----

Check of Release.gpg by apt
Secure apt always downloads Release.gpg files when it's downloading Release files, and if it cannot
download the Release.gpg, or if the signature is bad, it will complain, and will make note that the
Packages files that the Release file points to, and all the packages listed therein, are from an untrusted
source. Here's how it looks during an apt-get update:

W: GPG error: http://ftp.us.debian.org testing Release: The following signatures
couldn't be verified because the public key is not available: NO_PUBKEY 010908312D
Note that the second half of the long number is the key id of the key that apt doesn't know about, in thiscase that's 2D230C5F.
If you ignore that warning and try to install a package later, apt will warn again:
WARNING: The following packages cannot be authenticated!
libglib-perl libgtk2-perl
Install these packages without verification [y/N]?
If you say Y here you have no way to know if the file you're getting is the package you're supposed to
install, or if it's something else entirely that somebody that can intercept the communication against theserver7 has arranged for you, containing a nasty suprise.
Note that you can disable these checks by running apt with --allow-unauthenticated.
It's also worth noting that newer versions of the Debian installer use the same signed Release file mechanism during their debootstrap of the Debian base system, before apt is available, and that the installer
even uses this system to verify pieces of itself that it downloads from the net. Also, Debian does not currently sign the Release files on its CDs; apt can be configured to always trust packages from CDs sothis is not a large problem.

How to tell apt what to trust
So the security of the whole system depends on there being a Release.gpg file, which signs a Release
file, and of apt checking that signature using gpg. To check the signature, it has to know the public key ofthe person who signed the file. These keys are kept in apt's own keyring (/etc/apt/trusted.gpg),
and managing the keys is where secure apt comes in.
6
7

Technically speaking, this is an ASCII-armored detached gpg signature.
Or has poisoned your DNS, or is spoofing the server, or has replaced the file in the mirror you are using, etc.

92Infrastructura de seguridad de Debian

By default, Debian systems come preconfigured with the Debian archive key in the keyring.

# apt-key list
/etc/apt/trusted.gpg-------------------pub
1024D/4F368D5D 2005-01-31 [expires: 2006-01-31]
uid
Debian Archive Automatic Signing Key (2005) <ftpmaster@debian.
Here 4F368D5D is the key id, and notice that this key was only valid for a one year period. Debian rotatesthese keys as a last line of defense against some sort of security breach breaking a key.
That will make apt trust the official Debian archive, but if you add some other apt repository to /etc/
apt/sources.list, you'll also have to give apt its key if you want apt to trust it. Once you have the
key and have verified it, it's a simple matter of running apt-key add file to add it. Getting the key andverifying it are the trickier parts.

Finding the key for a repository
The debian-archive-keyring package is used to distribute keys to apt. Upgrades to this package can add(or remove) gpg keys for the main Debian archive.
For other archives, there is not yet a standard location where you can find the key for a given apt repository.
There's a rough standard of putting the key up on the web page for the repository or as a file in the repository
itself, but no real standard, so you might have to hunt for it.
The Debian archive signing key is available at https://ftp-master.debian.org/keys.html.8
gpg itself has a standard way to distribute keys, using a keyserver that gpg can download a key from andadd it to its keyring. For example:$ gpg --keyserver pgpkeys.mit.edu --recv-key 2D230C5F
gpg: solicitando clave 2D230C5F de servidor hkp pgpkeys.mit.edu
gpg: key 2D230C5F: clave pública "Debian Archive Automatic Signing Key (2006) <ftpm
aster@debian.org>" importada
gpg: cantidad total procesada: 1
gpg:
importada: 1
You can then export that key from your own keyring and feed it to apt-key:$ gpg -a --export 2D230C5F | sudo apt-key add gpg: no se encuentran claves absolutamente fiables
OK
The "gpg: no ultimately trusted keys found" warning means that gpg was not configured to ultimately trust
a specific key. Trust settings are part of OpenPGPs Web-of-Trust which does not apply here. So there is
no problem with this warning. In typical setups the user's own key is ultimately trusted.

Añadir de forma segura una clave
By adding a key to apt's keyring, you're telling apt to trust everything signed by the key, and this lets you
know for sure that apt won't install anything not signed by the person who possesses the private key. But
8"ziyi" is the name of the tool used for signing on the Debian servers, the name is based on the name of a http://en.wikipedia.org/wiki/Zhang_Ziyi.

93

Infrastructura de seguridad de Debian

if you're sufficiently paranoid, you can see that this just pushes things up a level, now instead of having
to worry if a package, or a Release file is valid, you can worry about whether you've actually gotten
the right key. Is the key file from https://ftp-master.debian.org/keys.html mentioned above really Debian'sarchive signing key, or has it been modified (or this document lies).
It's good to be paranoid in security, but verifying things from here is harder. gpg has the concept of a chainof trust, which can start at someone you're sure of, who signs someone's key, who signs some other key,etc., until you get to the archive key. If you're sufficiently paranoid you'll want to check that your archive
key is signed by a key that you can trust, with a trust chain that goes back to someone you know personally.
If you want to do this, visit a Debian conference or perhaps a local LUG for a key signing 9.
If you can't afford this level of paranoia, do whatever feels appropriate to you when adding a new apt
source and a new key. Maybe you'll want to mail the person providing the key and verify it, or maybe
you're willing to take your chances with downloading it and assuming you got the real thing. The important
thing is that by reducing the problem to what archive keys to trust, secure apt lets you be as careful andsecure as it suits you to be.

Verifiación de la integridad de la clave
You can verify the fingerprint as well as the signatures on the key. Retrieving the fingerprint can be done
for multiple sources, you can talk to Debian Developers on IRC, read the mailing list where the key change
will be announced or any other additional means to verify the fingerprint. For example you can do this:$ GET http://ftp-master.debian.org/ziyi_key_2006.asc | gpg --import
gpg: key 2D230C5F: public key "Debian Archive Automatic Signing Key (2006)<ftpmaster&debian.org>" imported
gpg: Total number processed: 1
gpg:
imported: 1$ gpg --check-sigs --fingerprint 2D230C5F
pub
1024D/2D230C5F 2006-01-03 [expires: 2007-02-07]
Key fingerprint = 0847 50FC 01A6 D388 A643 D869 0109 0831 2D23 0C5F
uid
Debian Archive Automatic Signing Key (2006) <ftpmaster@debian.org>
sig!3
2D230C5F 2006-01-03 Debian Archive Automatic Signing Key(2006) <ftpmaster@debian.org>
sig!
2A4E3EAA 2006-01-03 Anthony Towns <aj@azure.humbug.org.au>
sig!
4F368D5D 2006-01-03 Debian Archive Automatic Signing Key(2005) <ftpmaster@debian.org>
sig!
29982E5A 2006-01-04 Steve Langasek <vorlon@dodds.net>
sig!
FD6645AB 2006-01-04 Ryan Murray <rmurray@cyberhqz.com>
sig!
AB2A91F5 2006-01-04 James Troup <james@nocrew.org>
and then as in “Firma de paquete en Debian” check the trust path from your key (or a key you trust) toat least one of the keys used to sign the archive key. If you are sufficiently paranoid you will tell apt totrust the key only if you find an acceptable path:$ gpg --export -a 2D230C5F | sudo apt-key add Ok
Note that the key is signed with the previous archive key, so theoretically you can just build on yourprevious trust.
9

Not all apt repository keys are signed at all by another key. Maybe the person setting up the repository doesn't have another key, or maybe they
don't feel comfortable signing such a role key with their main key. For information on setting up a key for a repository see “Release check of non
Debian sources”.

94Infrastructura de seguridad de Debian

Debian archive key yearly rotation
As mentioned above, the Debian archive signing key is changed each year, in January. Since secure apt isyoung, we don't have a great deal of experience with changing the key and there are still rough spots.
In January 2006, a new key for 2006 was made and the Release file began to be signed by it, but to try
to avoid breaking systems that had the old 2005 key, the Release file was signed by that as well. The
intent was that apt would accept one signature or the other depending on the key it had, but apt turned out
to be buggy and refused to trust the file unless it had both keys and was able to check both signatures. Thiswas fixed in apt version 0.6.43.1. There was also confusion about how the key was distributed to users
who already had systems using secure apt; initially it was uploaded to the web site with no announcement
and no real way to verify it and users were forced to download it by hand.
In January 2006, a new key for 2006 was made and the Release file began to be signed by it, but to try to
avoid breaking systems that had the old 2005 key, the Release file was signed by that as well. In orderto prevent confusion on the best distribution mechanism for users who already have systems using secure
apt, the debian-archive-keyring package was introduced, which manages apt keyring updates.

Known release checking problems
One not so obvious problem is that if your clock is very far off, secure apt will not work. If it's set to adate in the past, such as 1999, apt will fail with an unhelpful message such as this:

W: GPG error: http://archive.progeny.com sid Release: Unknown error executing gpg
Although apt-key list will make the problem plain:

gpg: la clave 2D230C5F fue creada 192324901 segundos en el futuro (viaje en el tiem
gpg: la clave 2D230C5F fue creada 192324901 segundos en el futuro (viaje en el tiem
pub
1024D/2D230C5F 2006-01-03
uid
Debian Archive Automatic Signing Key (2006) <ftpmaster@debian.
If it's set to a date too far in the future, apt will treat the keys as expired.
Another problem you may encouter if using testing or unstable is that if you have not run apt-get update
lately and apt-get install a package, apt might complain that it cannot be authenticated (why does it dothis?). apt-get update will fix this.

Manual per distribution release check
In case you want to add now the additional security checks and don't want or cannot run the latest apt
version10 you can use the script below, provided by Anthony Towns. This script can automatically dosome new security checks to allow the user to be sure that the software s/he's downloading matches thesoftware Debian's distributing. This stops Debian developers from hacking into someone's system without
the accountability provided by uploading to the main archive, or mirrors mirroring something almost, but
not quite like Debian, or mirrors providing out of date copies of unstable with known security problems.
Esta muestra de código renombrada como apt-release-check, debería ser usada de la siguiente manera:# apt-get update
10

Either because you are using the stable, sarge, release or an older release or because you don't want to use the latest apt version, although wewould really appreciate testing of it.

95

Infrastructura de seguridad de Debian# apt-release-check(...resultados...)# apt-get dist-upgrade
Primero usted necesita:• get the keys the archive software uses to sign Release files from https://ftp-master.debian.org/keys.html and add them to ~/.gnupg/trustedkeys.gpg (which is what gpgv uses bydefault).

gpg --no-default-keyring --keyring trustedkeys.gpg --import ziyi_key_2006.asc• remover algunas /etc/apt/sources.list líneas que no utilizanla estructura normal de distribuciones, o cambie el script de modo que este trabaje con ellas.
• estar preparado para ignorar que las actualizaciones de seguridad Debian no hayan firmado archivosde publicaciones, y que los archivos de Fuente no tengan la suma de comprobaciones en el archivo dePublicación (aun).
• prepárese para verificar que las fuentes apropiadas son firmadas con las llaves propicias.
This is the example code for apt-check-sigs, the latest version can be retrieved from http://people.debian.org/~ajt/apt-check-sigs. This code is currently in beta, for more information read http://lists.debian.org/debian-devel/2002/07/msg00421.html.

#!/bin/bash# This script is copyright (c) 2001, Anthony Towns## This program is free software; you can redistribute it and/or modify# it under the terms of the GNU General Public License as published by# the Free Software Foundation; either version 2 of the License, or# (at your option) any later version.
## This program is distributed in the hope that it will be useful,# but WITHOUT ANY WARRANTY; without even the implied warranty of# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the# GNU General Public License for more details.
rm -rf /tmp/apt-release-checkmkdir /tmp/apt-release-check || exit 1
cd /tmp/apt-release-check/>OK/>MISSING/>NOCHECK/>BAD
arch=`dpkg --print-installation-architecture`am_root () {[ `id -u` -eq 0 ]}96Infrastructura de seguridad de Debian

get_md5sumsize () {cat "$1" | awk '/^MD5Sum:/,/^SHA1:/' |MYARG="$2" perl -ne '@f = split /\s+/; if ($f[3] eq $ENV{"MYARG"}) {print "$f[1] $f[2]\n"; exit(0); }'}checkit () {local FILE="$1"
local LOOKUP="$2"Y="`get_md5sumsize Release "$LOOKUP"`"Y="`echo "$Y" | sed 's/^ *//;s/ */ /g'`"
if [ ! -e "/var/lib/apt/lists/$FILE" ]; thenif [ "$Y" = "" ]; then
# No file, but not needed anyway
echo "OK"returnfiecho "$FILE" >>MISSING
echo "MISSING $Y"
returnfi
if [ "$Y" = "" ]; thenecho "$FILE" >>NOCHECK
echo "NOCHECK"
returnfi
X="`md5sum < /var/lib/apt/lists/$FILE` `wc -c </var/lib/apt/lists/$FILE`" X="`echo "$X" | sed 's/^ *//;s/ */ /g'`"
if [ "$X" != "$Y" ]; thenecho "$FILE" >>BAD
echo "BAD"
returnfiecho "$FILE" >>OK
echo "OK"}echo
echo "Checking sources in /etc/apt/sources.list:"echo "~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~"echo(echo "You should take care to ensure that the distributions you're downloading"echo "are the ones you think you are downloading, and that they are as up to"echo "date as you would expect (testing and unstable should be no more than"echo "two or three days out of date, stable-updates no more than a few weeks"
echo "or a month).") | fmtecho
cat /etc/apt/sources.list |sed 's/^ *//' | grep '^[^#]' |while read ty url dist comps; doif [ "${url%%:*}" = "http" -o "${url%%:*}" = "ftp" ]; thenbaseurl="${url#*://}"

97

Infrastructura de seguridad de Debian

elsecontinuefiecho "Source: ${ty} ${url} ${dist} ${comps}"
rm -f Release Release.gpg
wget -q -O Release "${url}/dists/${dist}/Release"
if ! grep -q '^' Release; thenecho " * NO TOP-LEVEL Release FILE"
else
origline=`sed -n 's/^Origin: *//p' Release | head -1`
lablline=`sed -n 's/^Label: *//p' Release | head -1`
suitline=`sed -n 's/^Suite: *//p' Release | head -1`
codeline=`sed -n 's/^Codename: *//p' Release | head -1`
dateline=`grep "^Date:" Release | head -1`
dscrline=`grep "^Description:" Release | head -1`
echo " o Origin: $origline/$lablline"
echo " o Suite: $suitline/$codeline"
echo " o $dateline"
echo " o $dscrline"
if [ "${dist%%/*}" != "$suitline" -a "${dist%%/*}" !="$codeline" ]; then echo " * WARNING: asked for $dist,
got $suitline/$codeline" fiwget -q -O Release.gpg "${url}/dists/${dist}/Release.gpg"
sigline="`gpgv --status-fd 3 Release.gpg Release 3>&1 >/dev/null
2>&1 | sed -n "s/^\[GNUPG:\] GOODSIG [0-9A-Fa-f]* //p"`" if ["$sigline" ]; then echo " o Signed by: $sigline"
else
echo " * NO VALID SIGNATURE">Releasefi
fi
okaycomps=""for comp in $comps; doif [ "$ty" = "deb" ]; then
X=$(checkit "`echo"${baseurl}/dists/${dist}/${comp}/binary-${arch}/Release" | sed 's,//*,_,g'`""${comp}/binary-${arch}/Release") Y=$(checkit "`echo"${baseurl}/dists/${dist}/${comp}/binary-${arch}/Packages" | sed 's,//*,_,g'`""${comp}/binary-${arch}/Packages") if [ "$X $Y" = "OK OK"]; then okaycomps="$okaycomps $comp"else echo " * PROBLEMS WITH $comp ($X,
$Y)" fi elif [ "$ty" = "deb-src" ]; then
X=$(checkit "`echo"${baseurl}/dists/${dist}/${comp}/source/Release" | sed 's,//*,_,g'`""${comp}/source/Release") Y=$(checkit "`echo"${baseurl}/dists/${dist}/${comp}/source/Sources" | sed 's,//*,_,g'`""${comp}/source/Sources") if [ "$X $Y" = "OK OK" ]; thenokaycomps="$okaycomps $comp"else echo " * PROBLEMS WITH component $comp($X, $Y)" fi fi

98

Infrastructura de seguridad de Debiandone[ "$okaycomps" = "" ] || echo " o Okay:$okaycomps"
echo
done
echo "Results"
echo "~~~~~~~"
echo
allokay=true
cd /tmp/apt-release-checkdiff <(cat BAD MISSING NOCHECK OK | sort) <(cd /var/lib/apt/lists && find .
-type f -maxdepth 1 | sed 's,^\./,,g' | grep '_' | sort) | sed -n 's/^> //p'/>UNVALIDATEDcd /tmp/apt-release-checkif grep -q ^ UNVALIDATED; thenallokay=false(echo "The following files in /var/lib/apt/lists have not been validated."echo "This could turn out to be a harmless indication that this script"
echo "is buggy or out of date, or it could let trojaned packages get onto"
echo "your system.") | fmtecho
sed 's/^/ /' < UNVALIDATED
echofi
if grep -q ^ BAD; thenallokay=false(echo "The contents of the following files in /var/lib/apt/lists does not"echo "match what was expected. This may mean these sources are out of date,"echo "that the archive is having problems, or that someone is actively"
echo "using your mirror to distribute trojans."
if am_root; thenecho "The files have been renamed to have the extension .FAILED and"echo "will be ignored by apt."
cat BAD | while read a; do
mv /var/lib/apt/lists/$a /var/lib/apt/lists/${a}.FAILED
donefi) | fmt
echo
sed 's/^/ /' < BAD
echofi
if grep -q ^ MISSING; thenallokay=false(echo "The following files from /var/lib/apt/lists were missing. This"echo "may cause you to miss out on updates to some vulnerable packages.") | fmtecho
sed 's/^/ /' < MISSING
echo
fi

99

Infrastructura de seguridad de Debianif grep -q ^ NOCHECK; thenallokay=false(echo "The contents of the following files in /var/lib/apt/lists could not"echo "be validated due to the lack of a signed Release file, or the lack"echo "of an appropriate entry in a signed Release file. This probably"
echo "means that the maintainers of these sources are slack, but may mean"echo "these sources are being actively used to distribute trojans."
if am_root; thenecho "The files have been renamed to have the extension .FAILED and"echo "will be ignored by apt."
cat NOCHECK | while read a; do
mv /var/lib/apt/lists/$a /var/lib/apt/lists/${a}.FAILED
donefi) | fmt
echo
sed 's/^/ /' < NOCHECK
echofi
if $allokay; thenecho 'Everything seems okay!'echofi
rm -rf /tmp/apt-release-check
You might need to apply the following patch for sid since md5sum adds an '-' after the sum when the
input is stdin:@@ -37,7 +37,7 @@local LOOKUP="$2"+Y="`get_md5sumsize Release "$LOOKUP"`"Y="`echo "$Y" | sed 's/^ *//;s/ */ /g'`"Y="`echo "$Y" | sed 's/-//;s/^ *//;s/ */ /g'`"

if [ ! -e "/var/lib/apt/lists/$FILE" ]; thenif [ "$Y" = "" ]; then@@ -55,7 +55,7 @@returnfi
X="`md5sum < /var/lib/apt/lists/$FILE` `wc -c < /var/lib/apt/lists/$FILE`"
X="`echo "$X" | sed 's/^ *//;s/ */ /g'`"+
X="`echo "$X" | sed 's/-//;s/^ *//;s/ */ /g'`"
if [ "$X" != "$Y" ]; thenecho "$FILE" >>BAD
echo "BAD"

Release check of non Debian sources
Notice that, when using the latest apt version (with secure apt) no extra effort should be required on yourpart unless you use non-Debian sources, in which case an extra confirmation step will be required by apt-100

Infrastructura de seguridad de Debianget. This is avoided by providing Release and Release.gpg files in the non-Debian sources. The
Release file can be generated with apt-ftparchive (available in apt-utils 0.5.0 and later), the Release.gpg is just a detached signature. To generate both follow this simple procedure:$ rm -f dists/unstable/Release$ apt-ftparchive release dists/unstable > dists/unstable/Release$ gpg --sign -ba -o dists/unstable/Release.gpg dists/unstable/Release

Alternativa firmar esquema por paquete
El esquema adicional de firmar cada uno y todos los paquetes, permite que estos sean revisados cuando no
son tan referenciados por un archivo de Paquetes existentes, además, los paquetes tercera-persona donde
nunca existieron Paquetes para que estos también puedan ser usados en Debian, sin embargo, no serán
un esquema por defecto.
This package signing scheme can be implemented using debsig-verify and debsigs. These two packages
can sign and verify embedded signatures in the .deb itself. Debian already has the capability to do this
now, but there is no feature plan to implement the policy or other tools since the archive signing scheme isprefered. These tools are available for users and archive administrators that would rather use this scheme
instead.
Latest dpkg versions (since 1.9.21) incorporate a http://lists.debian.org/debian-dpkg/2001/03/
msg00024.html that provides this functionality as soon as debsig-verify is installed.
NOTA: Normalmente /etc/dpkg/dpkg.cfg se desmonta con "no-debsig" como por defecto.
NOTE2: Signatures from developers are currently stripped when they enter off the package archive since
the currently preferred method is release checks as described previously.

101Capítulo 8. Herramientas de seguridad
en Debian
ARRÉGLAME: Se necesita más contenido.
Debian provides also a number of security tools that can make a Debian box suited for security purposes. These purposes include protection of information systems through firewalls (either packet or application-level), intrusion detection (both network and host based), vulnerability assessment, antivirus, private
networks, etc.
Since Debian 3.0 (woody), the distribution features cryptographic software integrated into the main distribution. OpenSSH and GNU Privacy Guard are included in the default install, and strong encryption is now
present in web browsers and web servers, databases, and so forth. Further integration of cryptography isplanned for future releases. This software, due to export restrictions in the US, was not distributed alongwith the main distribution but included only in non-US sites.

Herramientas de evaluación de vulnerabilidades remotas.
The tools provided by Debian to perform remote vulnerability assessment are: 1• nessus• raccess• nikto (whisker's replacement)
De lejos, la herramienta más completa y actualizada es nessus, que se compone de un cliente (nessus), que
es el IGU, y un servidor (nessusd), el cual lanza los ataques programados. Nessus incluye vulnerabilidades
remotas para un amplio número de sistemas incluyendo aplicaciones de red, servidores ftp, servidores
www, etc. Los últimos accesorios de seguridad son capaces de analizar un sitio web y tratar de descubrir,de las páginas interactivas disponibles, las que se podrían atacar. También hay clientes Java y para Win32(no se incluyen en Debian) que se pueden usar para conectar con el servidor.
nikto is a web-only vulnerability assessment scanner including anti-IDS tactics (most of which are notanti-IDS anymore). It is one of the best cgi-scanners available, being able to detect a WWW server andlaunch only a given set of attacks against it. The database used for scanning can be easily modified toprovide for new information.

Herramientas de escáner de red
Debian proporciona algunas herramientas utilizadas para el análisis remoto de anfitriones (pero sin evaluación de vulnerabilidades). Estas herramientas, en algunos casos, las utilizan los escáneres de evaluaciónde vulnerabilidades como el primer tipo de «ataque» ejecutado contra anfitriones remotos en un intento
por determinar los servicios remotos disponibles. Actualmente Debian proporciona:• nmap• xprobe
1Some of them are provided when installing the harden-remoteaudit package.

102

Herramientas de seguridad en Debian• p0f• knocker• isic• hping2• icmpush• nbtscan (for SMB /NetBIOS audits)• fragrouter• strobe (in the netdiag package)• irpasMientras que queso y xprobe proporcionan únicamente detección de sistemas operativos remotos (utilizando TCP/IP fingerprinting2), nmap y knocker realizan detección de sistemas operativos y escaneo depuertos de anfitriones remotos. Por otro lado, hping2 y icmpush se pueden utilizar en técnicas de ataques
remotos ICMP.
Diseñado específicamente para redes SMB, nbtscan puede utilizarse para escanear redes IP y obtener
información sobre servidores con SMB habilitado, incluyendo: nombres de usuarios, nombres de red,
direcciones MAC...
On the other hand, fragrouter can be used to test network intrusion detection systems and see if the NIDS
can be eluded by fragmentation attacks.
FIXME: Check http://bugs.debian.org/153117 (ITP fragrouter) to see if it's included.
FIXME add information based on https://web.archive.org/web/20040725013857/http://www.giac.org/
practical/gcux/Stephanie_Thomas_GCUX.pdf which describes how to use Debian and a laptop to scan for
wireless (803.1) networks (link not there any more).

Auditorías internas
En la actualidad, tiger es la única herramienta utilizada en Debian que sirve para ejecutar auditorías internas(también llamadas «White box») de anfitriones con el fin de determinar si el sistema de ficheros está
configurado correctamente, qué procesos están escuchando en el anfitrión, etc.

Auditorías de código fuente
Debian proporciona tres paquetes que sirven para auditar programas con código fuente C/C++ y encontrar
errores de programación que pueden conducir a potenciales grietas de seguridad:• flawfinder• rats• splint• pscan2

Sin embargo, la base de datos de fingerprinting de Queso es bastante antigua

103Herramientas de seguridad en DebianRedes virtuales privadas
A virtual private network (VPN) is a group of two or more computer systems, typically connected to a
private network with limited public network access, that communicate securely over a public network.
VPNs may connect a single computer to a private network (client-server), or a remote LAN to a private
network (server-server). VPNs often include the use of encryption, strong authentication of remote users
or hosts, and methods for hiding the private network's topology.
Debian proporciona unos cuantos paquetes para configurar redes privadas virtuales cifradas:• vtun• tunnelv (non-US section)• cipe-source, cipe-common• tinc• secvpn• pptpd• openvpn• openswan (http://www.openswan.org/)
FIXME: Update the information here since it was written with FreeSWAN in mind. Check Bug #237764
and Message-Id: <200412101215.04040.rmayr@debian.org>.
The OpenSWAN package is probably the best choice overall, since it promises to interoperate with almost
anything that uses the IP security protocol, IPsec (RFC 2411). However, the other packages listed above
can also help you get a secure tunnel up in a hurry. The point to point tunneling protocol (PPTP) is a
proprietary Microsoft protocol for VPN. It is supported under Linux, but is known to have serious security
issues.
For more information see the http://www.tldp.org/HOWTO/VPN-Masquerade-HOWTO.html (covers IPsec and PPTP), http://www.tldp.org/HOWTO/VPN-HOWTO.html (covers PPP over SSH),
http://www.tldp.org/HOWTO/mini/Cipe+Masq.html, and http://www.tldp.org/HOWTO/mini/ppp-ssh/index.html.
Also worth checking out is http://yavipin.sourceforge.net/, but no Debian packages seem to be available
yet.

Point to Point tunneling
If you want to provide a tunneling server for a mixed environment (both Microsoft operating systems andLinux clients) and IPsec is not an option (since it's only provided for Windows 2000 and Windows XP),you can use PoPToP (Point to Point Tunneling Server), provided in the pptpd package.
If you want to use Microsoft's authentication and encryption with the server provided in the ppp package,
note the following from the FAQ:

It is only necessary to use PPP 2.3.8 if you want Microsoft compatible
MSCHAPv2/MPPE authentication and encryption. The reason for this is that
the MSCHAPv2/MPPE patch currently supplied (19990813) is against PPP

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Herramientas de seguridad en Debian

2.3.8. If you don't need Microsoft compatible authentication/encryption
any 2.3.x PPP source will be fine.
However, you also have to apply the kernel patch provided by the kernel-patch-mppe package, which
provides the pp_mppe module for pppd.
Take into account that the encryption in ppptp forces you to store user passwords in clear text, and that theMS-CHAPv2 protocol contains http://mopo.informatik.uni-freiburg.de/pptp_mschapv2/.

Infraestructura de clave pública (ICP).
Public Key Infrastructure (PKI) is a security architecture introduced to provide an increased level of confidence for exchanging information over insecure networks. It makes use of the concept of public andprivate cryptographic keys to verify the identity of the sender (signing) and to ensure privacy (encryption).
Cuando se considera una ICP, usted se enfrenta a un amplia variedad de cuestiones:• una Autoridad certificadora (AC) que pueda resolver y verificar certificados, y que trabaje bajo una
jerarquía dada.
• un directorio para soportar los certificados públicos de usuarios• una base de datos (?) para mantener las listas de revocación de certificados (LRC)• dispositivos que interactúen con el AC con el fin de imprimir en tarjetas inteligentes/unidades USB/
cualquier cosa que almacene certificados de forma segura• programas listos para usar certificados emitidos por una AC para unirse en comunicaciones cifradas ycomprobar certificados contra LRC (para autenticación y soluciones completas de inicio de sesión)• una autoridad de marcas de tiempo para firmar documentos digitalmente• una consola de administración desde la cual se puedan utilizar apropiadamente todos estos (generaciónde certificados, control de lista de revocaciones, etc...)Debian GNU/Linux has software packages to help you with some of these PKI issues. They include OpenSSL (for certificate generation), OpenLDAP (as a directory to hold the certificates), gnupg and openswan (with X.509 standard support). However, as of the Woody release (Debian 3.0), Debian does nothave any of the freely available Certificate Authorities such as pyCA, http://www.openca.org or the CA
samples from OpenSSL. For more information read the http://ospkibook.sourceforge.net/.

SSL Infrastructure
Debian does provide some SSL certificates with the distribution so that they can be installed locally. They
are found in the ca-certificates package. This package provides a central repository of certificates thathave been submitted to Debian and approved (that is, verified) by the package maintainer, useful for any
OpenSSL applications which verify SSL connections.
FIXME: read debian-devel to see if there was something added to this.

Herramientas antivirus
There are not many anti-virus tools included with Debian GNU/Linux, probably because GNU/Linux users
are not plagued by viruses. The Unix security model makes a distinction between privileged (root) processes and user-owned processes, therefore a "hostile" executable that a non-root user receives or creates and105Herramientas de seguridad en Debianthen executes cannot "infect" or otherwise manipulate the whole system. However, GNU/Linux worms
and viruses do exist, although there has not (yet, hopefully) been any that has spread in the wild over any
Debian distribution. In any case, administrators might want to build up anti-virus gateways that protect
against viruses arising on other, more vulnerable systems in their network.
Debian GNU/Linux currently provides the following tools for building antivirus environments:• http://www.clamav.net, provided since Debian sarge (3.1 release). Packages are provided both for thevirus scanner (clamav) for the scanner daemon (clamav-daemon) and for the data files needed for thescanner. Since keeping an antivirus up-to-date is critical for it to work properly there are two different
ways to get this data: clamav-freshclam provides a way to update the database through the Internet
automatically and clamav-data which provides the data files directly. 3• mailscanner an e-mail gateway virus scanner and spam detector. Using sendmail or exim as its basis, it
can use more than 17 different virus scanning engines (including clamav).
• libfile-scan-perl which provides File::Scan, a Perl extension for scanning files for viruses. This modules
can be used to make platform independent virus scanners.
• http://www.sourceforge.net/projects/amavis, provided in the package amavis-ng and available in sarge,
which is a mail virus scanner which integrates with different MTA (Exim, Sendmail, Postfix, or Qmail)and supports over 15 virus scanning engines (including clamav, File::Scan and openantivirus).
• http://packages.debian.org/sanitizer, a tool that uses the procmail package, which can scan email attachments for viruses, block attachments based on their filenames, and more.
• http://packages.debian.org/amavis-postfix, a script that provides an interface from a mail transport agent
to one or more commercial virus scanners (this package is built with support for the postfix MTA only).
• exiscan, an e-mail virus scanner written in Perl that works with Exim.
• blackhole-qmail a spam filter for Qmail with built-in support for Clamav.
Some gateway daemons support already tools extensions to build antivirus environments including exim4daemon-heavy (the heavy version of the Exim MTA), frox (a transparent caching ftp proxy server), messagewall (an SMTP proxy daemon) and pop3vscan (a transparent POP3 proxy).
Debian currently provide clamav as the only antivirus scanning software in the main official distribution
and it also provides multiple interfaces to build gateways with antivirus capabilities for different protocols.
Some other free software antivirus projects which might be included in future Debian GNU/Linux releases:http://sourceforge.net/projects/openantivirus/ (see http://bugs.debian.org/150698 and http://bugs.debian.org/150695 ).
FIXME: Is there a package that provides a script to download the latest virus signatures from http://www.openantivirus.org/latest.php?
FIXME: Check if scannerdaemon is the same as the open antivirus scanner daemon (read ITPs).
However, Debian will never provide propietary (non-free and undistributable) antivirus software
such as: Panda Antivirus, NAI Netshield, http://www.sophos.com/, http://www.antivirus.com, or http://3

If you use this last package and are running an official Debian, the database will not be updated with security updates. You should either use
clamav-freshclam, clamav-getfiles to generate new clamav-data packages or update from the maintainers location:

deb http://people.debian.org/~zugschlus/clamav-data/ /deb-src http://people.debian.org/~zugschlus/clamav-data/ /106

Herramientas de seguridad en Debianwww.ravantivirus.com. For more pointers see the http://www.computer-networking.de/~link/security/avlinux_e.txt. This does not mean that this software cannot be installed properly in a Debian system4.
For more information on how to set up a virus detection system read Dave Jones' article https://web.archive.org/web/20120509212938/http://www.linuxjournal.com/article/4882.

GPG agent
It is very common nowadays to digitally sign (and sometimes encrypt) e-mail. You might, for example,
find that many people participating on mailing lists sign their list e-mail. Public key signatures are currently
the only means to verify that an e-mail was sent by the sender and not by some other person.
Debian GNU/Linux provides a number of e-mail clients with built-in e-mail signing capabilities that interoperate either with gnupg or pgp:
• evolution.
• mutt.
• kmail.
• icedove (rebranded version of Mozilla's Thunderbird) through the http://enigmail.mozdev.org/ plugin.
This plugin is provided by the enigmail package.
• sylpheed. Depending on how the stable version of this package evolves, you may need to use the bleedingedge version, sylpheed-claws.
• gnus, which when installed with the mailcrypt package, is an emacs interface to gnupg.
• kuvert, which provides this functionality independently of your chosen mail user agent (MUA) by interacting with the mail transport agent (MTA).
Key servers allow you to download published public keys so that you may verify signatures. One such key
server is http://wwwkeys.pgp.net. gnupg can automatically fetch public keys that are not already in yourpublic keyring. For example, to configure gnupg to use the above key server, edit the file ~/.gnupg/
options and add the following line: 5

keyserver wwwkeys.pgp.net
Most key servers are linked, so that when your public key is added to one server, the addition is propagated to all the other public key servers. There is also a Debian GNU/Linux package debian-keyring, thatprovides all the public keys of the Debian developers. The gnupg keyrings are installed in /usr/share/keyrings/.
Más información en:• http://www.gnupg.org/faq.html.
• http://www.gnupg.org/gph/en/manual.html.
• https://web.archive.org/web/20080201103530/http://
www.dewinter.com/gnupg_howto/english/GPGMiniHowto.html.
4

Actually, there is an installer package for the F-prot antivirus, which is non-free but gratis for home users, called f-prot-installer. This installer,
however, just downloads http://www.f-prot.com/products/home_use/linux/ and installs it in the system.
5
For more examples of how to configure gnupg check /usr/share/doc/mutt/examples/gpg.rc.

107Herramientas de seguridad en Debian• https://web.archive.org/web/20080513095235/http://www.uk.pgp.net/pgpnet/pgp-faq/.
• https://web.archive.org/web/20060222110131/http://www.cryptnet.net/fdp/crypto/gpg-party.html.

108

Capítulo 9. Developer's Best Practices
for OS Security
This chapter introduces some best secure coding practices for developers writing Debian packages. If
you are really interested in secure coding I recommend you read David Wheeler's http://www.dwheeler.com/secure-programs/ and http://www.securecoding.org by Mark G. Graff and Kenneth R. van Wyk(O'Reilly, 2003).

Best practices for security review and design
Developers that are packaging software should make a best effort to ensure that the installation of thesoftware, or its use, does not introduce security risks to either the system it is installed on or its users.
In order to do so, they should make their best to review the source code of the package and detect any
flaws that might introduce security bugs before releasing the software or distributing a new version. It is
acknowledged that the cost of fixing bugs grows for different stages of its development, so it is easier (andcheaper) to fix bugs when designing than when the software has been deployed and is in maintenance mode
(some studies say that the cost in this later phase is sixty times higher). Although there are some tools that
try to automatically detect these flaws, developers should strive to learn about the different kind of security
flaws in order to understand them and be able to spot them in the code they (or others) have written.
The programming bugs which lead to security bugs typically include: http://en.wikipedia.org/wiki/Buffer_overflow, format string overflows, heap overflows and integer overflows (in C/C++ programs), temporary http://en.wikipedia.org/wiki/Symlink_race (in scripts), http://en.wikipedia.org/wiki/Directory_traversal and command injection (in servers) and http://en.wikipedia.org/wiki/Cross_site_scripting, andhttp://en.wikipedia.org/wiki/SQL_injection (in the case of web-oriented applications). For a more complete information on security bugs review Fortify's http://vulncat.fortifysoftware.com/.
Some of these issues might not be easy to spot unless you are an expert in the programming language
the software uses, but some security problems are easy to detect and fix. For example, finding temporary
race conditions due to misuse of temporary directories can easily be done just by running grep -r "/tmp/" .. Those calls can be reviewed and replace the hardcoded filenames using temporary directories
to calls to either mktemp or tempfile in shell scripts, File::Temp(3perl) in Perl scripts, or tmpfile(3) in
C/C++.
There are a set of tools available to assist to the security code review phase. These include rats, flawfinderand pscan. For more information, read the http://www.debian.org/security/audit/tools.
When packaging software developers have to make sure that they follow common security principles,
including:• The software runs with the minimum privileges it needs:• The package does install binaries setuid or setgid. Lintian will warn of http://lintian.debian.org/reports/Tsetuid-binary.html, http://lintian.debian.org/reports/Tsetgid-binary.html and http://lintian.debian.org/reports/Tsetuid-gid-binary.html binaries.
• The daemons the package provide run with a low privilege user (see “Creating users and groups forsoftware daemons”)• Programmed (i.e., cron) tasks running in the system do NOT run as root or, if they do, do not implement
complex tasks.

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Developer's Best Practices for OS Security
If you have to do any of the above make sure the programs that might run with higher privileges havebeen audited for security bugs. If you are unsure, or need help, contact the http://www.debian.org/security/audit/. In the case of setuid/setgid binaries, follow the Debian policy section regarding http://www.debian.org/doc/debian-policy/ch-files.html#s10.9
For more information, specific to secure programming, make sure you read (or point your upstream to)
http://www.dwheeler.com/secure-programs/ and the https://buildsecurityin.us-cert.gov/portal/ portal.

Creating users and groups for software daemons
If your software runs a daemon that does not need root privileges, you need to create a user for it. There
are two kind of Debian users that can be used by packages: static uids (assigned by base-passwd, for a listof static users in Debian see “Usuarios y grupos del sistema operativo”) and dynamic uids in the range
assigned to system users.
In the first case, you need to ask for a user or group id to the base-passwd. Once the user is available there
the package needs to be distributed including a proper versioned depends to the base-passwd package.
In the second case, you need to create the system user either in the preinst or in the postinst and make thepackage depend on adduser (>= 3.11).
The following example code creates the user and group the daemon will run as when the package is installed
or upgraded:[...]case "$1" ininstall|upgrade)# If the package has default file it could be sourced, so that# the local admin can overwrite the defaults[ -f "/etc/default/packagename" ] && . /etc/default/packagename# Sane defaults:[ -z "$SERVER_HOME" ] && SERVER_HOME=server_dir[ -z "$SERVER_USER" ] && SERVER_USER=server_user[ -z "$SERVER_NAME" ] && SERVER_NAME="Server description"[ -z "$SERVER_GROUP" ] && SERVER_GROUP=server_group# Groups that the user will be added to, if undefined, then none.
ADDGROUP=""# create user to avoid running server as root# 1. create group if not existing
if ! getent group | grep -q "^$SERVER_GROUP:" ; thenecho -n "Adding group $SERVER_GROUP.."
addgroup --quiet --system $SERVER_GROUP 2>/dev/null ||true
echo "..done"fi# 2. create homedir if not existing110Developer's Best Practices for OS Security
test -d $SERVER_HOME || mkdir $SERVER_HOME# 3. create user if not existing
if ! getent passwd | grep -q "^$SERVER_USER:"; thenecho -n "Adding system user $SERVER_USER.."adduser --quiet \--system \--ingroup $SERVER_GROUP \--no-create-home \--disabled-password \$SERVER_USER 2>/dev/null || trueecho "..done"fi# 4. adjust passwd entry
usermod -c "$SERVER_NAME" \-d $SERVER_HOME\-g $SERVER_GROUP \$SERVER_USER# 5. adjust file and directory permissions
if ! dpkg-statoverride --list $SERVER_HOME >/dev/null
thenchown -R $SERVER_USER:adm $SERVER_HOME
chmod u=rwx,g=rxs,o= $SERVER_HOMEfi# 6. Add the user to the ADDGROUP group
if test -n $ADDGROUP
then
if ! groups $SERVER_USER | cut -d: -f2 | \grep -qw $ADDGROUP; thenadduser $SERVER_USER $ADDGROUP
fi
fi;;
configure)[...]You have to make sure that the init.d script file:• Starts the daemon dropping privileges: if the software does not do the setuid(2) or seteuid(2) call itself,
you can use the --chuid call of start-stop-daemon.
• Stops the daemon only if the user id matches, you can use the start-stop-daemon --user option for
this.
• Does not run if either the user or the group do not exist:

if ! getent passwd | grep -q "^server_user:"; thenecho "Server user does not exist. Aborting" >&2
exit 1fi
if ! getent group | grep -q "^server_group:" ; thenecho "Server group does not exist. Aborting" >&2
exit 1fi

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Developer's Best Practices for OS Security
If the package creates the system user it can remove it when it is purged in its postrm. This has somedrawbacks, however. For example, files created by it will be orphaned and might be taken over by a new
system user in the future if it is assigned the same uid1. Consequently, removing system users on purge
is not yet mandatory and depends on the package needs. If unsure, this action could be handled by asking
the administrator for the prefered action when the package is installed (i.e. through debconf).
Maintainers that want to remove users in their postrm scripts are referred to the deluser/deluser --system option.
Running programs with a user with limited privileges makes sure that any security issue will not beable to damage the full system. It also follows the principle of least privilege. Also consider you canlimit privileges in programs through other mechanisms besides running as non-root2. For more information, read the http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/minimize-privileges.html chapter of the Secure Programming for Linux and Unix HOWTO book.

1

Some relevant threads discussing these drawbacks include http://lists.debian.org/debian-mentors/2004/10/msg00338.html and http://lists.debian.org/debian-devel/2004/05/msg01156.html
2
You can even provide a SELinux policy for it

112

Capítulo 10. Antes del compromiso
Sistema seguro
You should strive to keep your system secure by monitoring its usage and also the vulnerabilities thatmight affect it, patching them as soon as patches are available. Even though you might have installed areally secure system initially you have to remember that security in a system degrades with time, security
vulnerabilities might be found for exposed system services and users might expose the system security
either because of lack of understanding (e.g. accessing a system remotely with a clear-text protocol orusing easy to guess passwords) or because they are actively trying to subvert the system's security (e.g.
install additional services locally on their accounts).

Tracking security vulnerabilities
Although most administrators are aware of security vulnerabilities affecting their systems when they see
a patch that is made available you can strive to keep ahead of attacks and introduce temporary countermeasures for security vulnerabilities by detecting when your system is vulnerable. This is specially true
when running an exposed system (i.e. connected to the Internet) and providing a service. In such case thesystem's administrators should take care to monitor known information sources to be the first to know
when a vulnerability is detected that might affect a critical service.
This typically includes subscribing to the announcement mailing lists, project websites or bug tracking
systems provided by the software developers for a specific piece of code. For example, Apache users
should regularly review Apache's http://httpd.apache.org/security_report.html and subscribe to the http://
httpd.apache.org/lists.html#http-announce mailing list.
In order to track known vulnerabilities affecting the Debian distribution, the Debian Testing Security Team provides a https://security-tracker.debian.org/ that lists all the known vulnerabilities
which have not been yet fixed in Debian packages. The information in that tracker is obtained through different public channels and includes known vulnerabilities which are available either through security vulnerability databases or http://www.debian.org/Bugs/. Administrators can search
for the known security issues being tracked for https://security-tracker.debian.org/tracker/status/release/stable, https://security-tracker.debian.org/tracker/status/release/oldstable, https://security-tracker.debian.org/tracker/status/release/testing, or https://security-tracker.debian.org/tracker/status/release/unstable.
The tracker has searchable interfaces (by http://cve.mitre.org/ name and package name) and some tools(such as debsecan, see “Automatically checking for security issues with debsecan”) use that database toprovide information of vulnerabilities affecting a given system which have not yet been addressed (i.e.
those who are pending a fix).
Concious administrators can use that information to determine which security bugs might affect the systemthey are managing, determine the severity of the bug and apply (if available) temporary countermeasures
before a patch is available fixing this issue.
Security issues tracked for releases supported by the Debian Security Team should eventually be handled
through Debian Security Advisories (DSA) and will be available for all users (see “Continuously update
the system”). Once security issues are fixed through an advisory they will not be available in the tracker,
but you will be able to search security vulnerabilities (by CVE name) using the http://www.debian.org/security/crossreferences available for published DSAs.
Notice, however, that the information tracked by the Debian Testing Security Team only involves disclosed vulnerabilities (i.e. those already public). In some occasions the Debian Security Team might be hand-113

Antes del compromiso

ling and preparing DSAs for packages based on undisclosed information provided to them (for example,
through closed vendor mailing lists or by upstream maintainers of software). So do not be surprised to findsecurity issues that only show up as an advisory but never get to show up in the security tracker.

Continuously update the system
You should conduct security updates frequently. The vast majority of exploits result from known vulnerabilities that have not been patched in time, as this http://www.cs.umd.edu/~waa/vulnerability.html (presented at the 2001 IEEE Symposium on Security and Privacy) explains. Updates are described under “Ejecute una actualización de seguridad”.

Manually checking which security updates are available
Debian does have a specific tool to check if a system needs to be updated but many users will just wantto manually check if any security updates are available for their system.
If you have configured your system as described in “Ejecute una actualización de seguridad” you justneed to do:# apt-get update# apt-get upgrade -s[ ... review packages to be upgraded ... ]# apt-get upgrade# checkrestart[ ... restart services that need to be restarted ... ]And restart those services whose libraries have been updated if any. Note: Read “Ejecute una actualizaciónde seguridad” for more information on library (and kernel) upgrades.
The first line will download the list of packages available from your configured package sources. The s will do a simulation run, that is, it will not download or install the packages but rather tell you which
ones should be downloaded/installed. From the output you can derive which packages have been fixed by
Debian and are available as a security update. Sample:# apt-get upgrade -s
Reading Package Lists... Done
Building Dependency Tree... Done
2 packages upgraded, 0 newly installed, 0 to remove and 0
Inst cvs (1.11.1p1debian-8.1 Debian-Security:3.0/stable)
Inst libcupsys2 (1.1.14-4.4 Debian-Security:3.0/stable)
Conf cvs (1.11.1p1debian-8.1 Debian-Security:3.0/stable)
Conf libcupsys2 (1.1.14-4.4 Debian-Security:3.0/stable)

not upgraded.

In this example, you can see that the system needs to be updated with new cvs and cupsys packages which
are being retrieved from woody's security update archive. If you want to understand why these packages are needed, you should go to http://security.debian.org and check which recent Debian Security Advisories have been published related to these packages. In this case, the related DSAs are https://lists.debian.org/debian-security-announce/2003/msg00014.html (for cvs) and https://lists.debian.org/debian-security-announce/2003/msg00013.html (for cupsys).
Notice that you will need to reboot your system if there has been a kernel upgrade.

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Checking for updates at the Desktop
Since Debian 4.0 lenny Debian provides and installs in a default installation update-notifier. This is a
GNOME application that will startup when you enter your Desktop and can be used to keep track of
updates available for your system and install them. It uses update-manager for this.
In a stable system updates are only available when a security patch is available or at point releases. Consequently, if the system is properly configured to receive security updates as described in “Ejecute una
actualización de seguridad” and you have a cron task running to update the package information you will
be notified through an icon in the desktop notifcation area.
The notification is not intrusive and users are not forced to install updates. From the notification icon adesktop user (with the administrator's password) can access a simple GUI to show available updates andinstall them.
This application works by checking the package database and comparing the system with its contents. If
the package database is updated periodically through a cron task then the contents of the database will benewer than the packages installed in the system and the application will notify you.
Apt installs such a task (/etc/cron.d/apt) which will run based on Apt's configuration (more specifically APT::Periodic). In the GNOME environment this configuration value can be adjusted by going
to System > Admin > Software origins > Updates, or running /usr/bin/software-properties.
If the system is set to download the packages list daily but not download the packages themselves your /etc/apt/apt.conf.d/10periodic should look like this:APT::Periodic::Update-Package-Lists "1";
APT::Periodic::Download-Upgradeable-Packages "0";You can use a different cron task, such as the one installed by cron-apt (see “Automatically checking for
updates with cron-apt”). You can also just manually check for upgrades using this application.
Users of the KDE desktop environment will probably prefer to install adept and adept-notifier insteadwhich offers a similar functionality but is not part of the standard installation.

Automatically checking for updates with cron-apt
Another method for automatic security updates is the use of cron-apt. This package provides a tool to
update the system at regular intervals (using a cron job), and can also be configured to send mails to thesystem administrator using the local mail transport agent. It will just update the package list and download
new packages by default but it can be configured to automatically install new updates.
Notice that you might want to check the distribution release, as described in “Per distribution releasecheck”, if you intend to automatically updated your system (even if only downloading the packages).
Otherwise, you cannot be sure that the downloaded packages really come from a trusted source.
More information is available at the http://www.debian-administration.org/articles/162.

Automatically checking for security issues with debsecan
The debsecan program evaluates the security status of by reporting both missing security updates and security vulnerabilities. Unlike cron-apt, which only provides information related to security updates available, but this tool obtains information from the security vulnerability database maintained by the Debian115

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Security Team which includes also information on vulnerabilities which are not yet fixed through a security update. Consequently, it is more efficient at helping administrators track security vulnerabilities (asdescribed in “Tracking security vulnerabilities”).
Upon installing the Debian package debsecan, and if the administrator consents to it, it will generate a crontask that will make it run and send the output to a specific user whenever it finds a vulnerable package.
It will also download the information from the Internet. The location of the security database is also partof the questions ask on installation and are later defined /etc/default/debsecan, it can be easilyadjusted for systems that do not have Internet access so that they all pull from a local mirror so that there
is a single point that access the vulnerability database.
Notice, however, that the Security Team tracks many vulnerabilities including low-risk issues which might
not be fixed through a security update and some vulnerabilities initially reported as affecting Debian might,
later on, upon investigation, be dismissed. Debsecan will report on all the vulnerabilities, which makes it
a quite more verbose than the other tools described above.
More information is available at the http://www.enyo.de/fw/software/debsecan/.

Other methods for security updates
There is also the apticron, which, similarly to cron-apt will check for updates and send mails to the administrator. More information on apticron is available at the http://www.debian-administration.org/articles/491.
You might also want to take a look at http://clemens.endorphin.org/secpack/ which is an unofficial program to do security updates from security.debian.org with signature checking written by Fruhwirth Clemens. Or to the Nagios Plugin http://www.unixdaemon.net/nagios_plugins.html#check_debian_packages
written by Dean Wilson.

Avoid using the unstable branch
Unless you want to dedicate time to patch packages yourself when a vulnerability arises, you should not
use Debian's unstable branch for production-level systems. The main reason for this is that there are no
security updates for unstable.
The fact is that some security issues might appear in unstable and not in the stable distribution. This is due
to new functionality constantly being added to the applications provided there, as well as new applications
being included which might not yet have been thoroughly tested.
In order to do security upgrades in the unstable branch, you might have to do full upgrades to new versions (which might update much more than just the affected package). Although there have been some
exceptions, security patches are usually only back ported into the stable branch. The main idea being thatbetween updates, no new code should be added, just fixes for important issues.
Notice, however, that you can use the security tracker (as described in “Tracking security vulnerabilities”)
to track known security vulnerabilities affecting this branch.

Security support for the testing branch
If you are using the testing branch, there are some issues that you must take into account regarding theavailability of security updates:• When a security fix is prepared, the Security Team backports the patch to stable (since stable is usuallysome minor or major versions behind). Package maintainers are responsible for preparing packages forthe unstable branch, usually based on a new upstream release. Sometimes the changes happen at nearly116

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the same time and sometimes one of the releases gets the security fix before. Packages for the stable
distribution are more thoroughly tested than unstable, since the latter will in most cases provide thelatest upstream release (which might include new, unknown bugs).
• Security updates are available for the unstable branch usually when the package maintainer makes a
new package and for the stable branch when the Security Team make a new upload and publish a DSA.
Notice that neither of these change the testing branch.
• If no (new) bugs are detected in the unstable version of the package, it moves to testing after several
days. The time this takes is usually ten days, although that depends on the upload priority of the change
and whether the package is blocked from entering testing by its dependency relationships. Note that if
the package is blocked from entering testing the upload priority will not change the time it takes to enter.
This behavior might change based on the release state of the distribution. When a release is almost imminent, the Security Team or package maintainers might provide updates directly to testing.
Additionally, the http://secure-testing-master.debian.net can issue Debian Testing Security Advisories(DTSAs) for packages in the testing branch if there is an immediate need to fix a security issue in thatbranch and cannot wait for the normal procedure (or the normal procedure is being blocked by some otherpackages).
Users willing to take advantage of this support should add the following lines to their /etc/apt/sources.list (instead of the lines described in “Ejecute una actualización de seguridad”):

deb http://security.debian.org testing/updates main contrib non-free# This line makes it possible to donwload source packages too
deb-src http://security.debian.org testing/updates main contrib non-free
For additional information on this support please read the http://lists.debian.org/debian-devel-announce/2006/05/msg00006.html. This support officially started in http://lists.debian.org/debian-devel-announce/2005/09/msg00006.html in a separate repository and was later integrated into the main security archive.

Automatic updates in a Debian GNU/Linux system
First of all, automatic updates are not fully recommended, since administrators should review the DSAs
and understand the impact of any given security update.
If you want to update your system automatically you should:• Configure apt so that those packages that you do not want to update stay at their current version, eitherwith apt's pinning feature or marking them as hold with aptitude or dpkg.
To pin the packages under a given release, you must edit /etc/apt/preferences (see apt_preferences(5)) and add:

Package: *Pin: release a=stable
Pin-Priority: 100
FIXME: verify if this configuration is OK.
• Either use cron-apt as described in “Automatically checking for updates with cron-apt” and enable it to
install downloaded packages or add a cron entry yourself so that the update is run daily, for example:
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apt-get update && apt-get -y upgrade
The -y option will have apt assume 'yes' for all the prompts that might arise during the update. In
some cases, you might want to use the --trivial-only option instead of the --assume-yes(equivalent to -y).1• Configure debconf so no questions will be asked during upgrades, so that they can be done non-interactively. 2• Check the results of the cron execution, which will be mailed to the superuser (unless changed with
MAILTO environment variable in the script).
A safer alternative might be to use the -d (or --download-only) option, which will download but
not install the necessary packages. Then if the cron execution shows that the system needs to be updated,
it can be done manually.
In order to accomplish any of these tasks, the system must be properly configured to download security
updates as discussed in “Ejecute una actualización de seguridad”.
However, this is not recommended for unstable without careful analysis, since you might bring your system
into an unusable state if some serious bug creeps into an important package and gets installed in yoursystem. Testing is slightly more secure with regard to this issue, since serious bugs have a better chance ofbeing detected before the package is moved into the testing branch (although, you may have no security
updates available whatsoever).
If you have a mixed distribution, that is, a stable installation with some packages updated to testing or
unstable, you can fiddle with the pinning preferences as well as the --target-release option in aptget to update only those packages that you have updated.3

Do periodic integrity checks
Based on the baseline information you generated after installation (i.e. the snapshot described in “Taking
a snapshot of the system”), you should be able to do an integrity check from time to time. An integrity
check will be able to detect filesystem modifications made by an intruder or due to a system administrators
mistake.
Integrity checks should be, if possible, done offline.4 That is, without using the operating system of thesystem to review, in order to avoid a false sense of security (i.e. false negatives) produced by, for example,
installed rootkits. The integrity database that the system is checked against should also be used from readonly media.
You can consider doing integrity checks online using any of the filesystem integrity tools available (described in “Integridad de su sistema de archivos”) if taking offline the system is not an option. However,precaution should be taken to use a read-only integrity database and also assure that the integrity checking
tool (and the operating system kernel) has not been tampered with.
1

You may also want to use the --quiet (-q) option to reduce the output of apt-get, which will stop the generation of any output if no packages
are installed.
2
Note that some packages might not use debconf and updates will stall due to packages asking for user input during configuration.
3
This is a common issue since many users want to maintain a stable system while updating some packages to unstable to gain the latest functionality.
This need arises due to some projects evolving faster than the time between Debian's stable releases.
4
An easy way to do this is using a Live CD, such as http://www.knoppix-std.org/ which includes both the file integrity tools and the integrity
database for your system.

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Some of the tools mentioned in the integrity tools section, such as aide, integrit or samhain are alreadyprepared to do periodic reviews (through the crontab in the first two cases and through a standalone daemon
in samhain) and can warn the administrator through different channels (usually e-mail, but samhain canalso send pages, SNMP traps or syslog alerts) when the filesystem changes.
Of course, if you execute a security update of the system, the snapshot taken for the system should be retaken to accommodate the changes done by the security update.

Montar el descubrimiento de intrusión
Debian GNU/Linux includes tools for intrusion detection, which is the practice of detecting inappropriate
or malicious activity on your local system, or other systems in your private network. This kind of defense isimportant if the system is very critical or you are truly paranoid. The most common approaches to intrusion
detection are statistical anomaly detection and pattern-matching detection.
Siempre debe darse cuenta que para mejorar realmente el sistema de seguridad con la introducción dealgunas de estas herramientas, usted necesitara tener un mecanismo de alerta+respuesta, pero no use el
descubrimiento de intrusión si usted no va a alertar a nadie (i.e. no malgaste su tiempo configurando cosasque mas tarde no usara).
When a particular attack has been detected, most intrusion detection tools will either log the event withsyslogd or send e-mail to the root user (the mail recipient is usually configurable). An administrator has
to properly configure the tools so that false positives do not trigger alerts. Alerts may also indicate an
ongoing attack and might not be useful, say, one day later, since the attack might have already succeeded.
So be sure that there is a proper policy on handling alerts and that the technical mechanisms to implement
this policy are in place.
An interesting source of information is http://www.cert.org/tech_tips/intruder_detection_checklist.html

Detección de intrusos basadas en la máquina
Network based intrusion detection tools monitor the traffic on a network segment and use this information
as a data source. Specifically, the packets on the network are examined, and they are checked to see ifthey match a certain signature.
snort is a flexible packet sniffer or logger that detects attacks using an attack signature dictionary. It detects
a variety of attacks and probes, such as buffer overflows, stealth port scans, CGI attacks, SMB probes, andmuch more. snort also has real-time alerting capability. You can use snort for a range of hosts on yournetwork as well as for your own host. This is a tool which should be installed on every router to keep an
eye on your network. Just install it with apt-get install snort, follow the questions, and watch
it log. For a little broader security framework, see http://www.prelude-ids.org.
Snort en Debian está habilitado con muchos chequeos de seguridad los cuales usted debe solicitar, sinembargo, usted debe personalizar el montaje para tomarlo dentro de las consideraciones de servicios particulares en donde usted avanza sobre su sistema. Usted también tiene que solicitarlo para recuperar loschequeos adicionales y asi especificar estos servicios.
There are other, simpler tools that can be used to detect network attacks. portsentry is an interesting packagethat can tip you off to port scans against your hosts. Other tools like ippl or iplogger will also detect someIP (TCP and ICMP) attacks, even if they do not provide the kind of advanced techniques snort does.
You can test any of these tools with the Debian package idswakeup, a shell script which generates false
alarms, and includes many common attack signatures.

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Detección de intrusos basadas en la máquina
Host based intrusion detection involves loading software on the system to be monitored which uses log
files and/or the systems auditing programs as a data source. It looks for suspicious processes, monitors
host access, and may even monitor changes to critical system files.
tiger is an older intrusion detection tool which has been ported to Debian since the Woody branch. tiger
provides checks of common issues related to security break-ins, like password strength, file system problems, communicating processes, and other ways root might be compromised. This package includes new
Debian-specific security checks including: MD5sums checks of installed files, locations of files not belonging to packages, and analysis of local listening processes. The default installation sets up tiger to runeach day, generating a report that is sent to the superuser about possible compromises of the system.
Log analysis tools, such as logcheck can also be used to detect intrusion attempts. See “Uso y personalización de logcheck”.
In addition, packages which monitor file system integrity (see “Integridad de su sistema de archivos”) canbe quite useful in detecting anomalies in a secured environment. It is most likely that an effective intrusion
will modify some files in the local file system in order to circumvent local security policy, install Trojans,or create users. Such events can be detected with file system integrity checkers.

Evitando rootkits
LKM - Loadable Kernel Modules (módulos cargables en
el núcleo)
Loadable kernel modules are files containing dynamically loadable kernel components used to expand
the functionality of the kernel. The main benefit of using modules is the ability to add additional devices,like an Ethernet or sound card, without patching the kernel source and recompiling the entire kernel.
However, crackers are now using LKMs for root-kits (knark and adore), opening up back doors in GNU/Linux systems.
LKM back doors are more sophisticated and less detectable than traditional root-kits. They can hide processes, files, directories and even connections without modifying the source code of binaries. For example,
a malicious LKM can force the kernel into hiding specific processes from procfs, so that even a known
good copy of the binary ps would not list accurate information about the current processes on the system.

Detector de rootkits.
There are two approaches to defending your system against LKM root-kits, a proactive defense and areactive defense. The detection work can be simple and painless, or difficult and tiring, depending on theapproach taken.

Defensa proactiva.
The advantage of this kind of defense is that it prevents damage to the system in the first place. One suchstrategy is getting there first, that is, loading an LKM designed to protect the system from other malicious
LKMs. A second strategy is to remove capabilities from the kernel itself. For example, you can remove
the capability of loadable kernel modules entirely. Note, however, that there are rootkits which mightwork even in this case, there are some that tamper with /dev/kmem (kernel memory) directly to makethemselves undetectable.
Debian GNU/Linux has a few packages that can be used to mount a proactive defense:120

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lcap - A user friendly interface to remove capabilities (kernel-based access control) in the kernel, making
the system more secure. For example, executing lcap CAP_SYS_MODULE5 will remove module loading
capabilities (even for the root user).6 There is some (old) information on capabilities at Jon Corbet's http://
lwn.net/1999/1202/kernel.php3 section on LWN (dated December 1999).
If you don't really need many kernel features on your GNU/Linux system, you may want to disable loadable modules support during kernel configuration. To disable loadable module support, just set CONFIG_MODULES=n during the configuration stage of building your kernel, or in the .config file. This
will prevent LKM root-kits, but you lose this powerful feature of the Linux kernel. Also, disabling loadable
modules can sometimes overload the kernel, making loadable support necessary.

Defensa Reactiva.
The advantage of a reactive defense is that it does not overload system resources. It works by comparing
the system call table with a known clean copy in a disk file, System.map. Of course, a reactive defense
will only notify the system administrator after the system has already been compromised.
Detection of some root-kits in Debian can be accomplished with the chkrootkit package. The http://
www.chkrootkit.org program checks for signs of several known root-kits on the target system, but is nota definitive test.

Ideas geniales/paranóicas — qué debe hacer
This is probably the most unstable and funny section, since I hope that some of the "duh, that sounds
crazy" ideas might be realized. The following are just some ideas for increasing security - maybe genius,paranoid, crazy or inspired depending on your point of view.
• Jugando alrededor con PAM. Como se dijo en el articulo phrack 56 PAM , lo agradable con PAM esque "usted está limitado únicamente por lo que pueda pensar" es verdad, imagine la raíz del inicio desesión únicamente posible con revisión de impresión o eyescan o cryptocard (porque yo aqui hago una
conjunción OR y no AND).
• Iniciación de sesión fascista. Yo diria que que todo lo que nosotros hemos hablado acerca de login
es "un sueve inicio de sesión". si usted quiere ejecutar una sesión real, tome una impresora con papel
fanfold y registre todo lo complicado para imprimir sobre el.Los sonidos divertidos, son confiables yno pueden ser removidos.
• CD distribution. This idea is very easy to realize and offers pretty good security. Create a hardened
Debian distribution, with proper firewall rules. Turn it into a boot-able ISO image, and burn it on a
CDROM. Now you have a read-only distribution, with about 600 MB space for services. Just makesure all data that should get written is done over the network. It is impossible for intruders to get read/
write access on this system, and any changes an intruder does make can be disabled with a reboot of
the system.
• El Switch de capacidad del modulo apagado. Cuando desconecta el uso de modulos del núcleo en un
tiempo compilado del núcleo, muchos núcleo se basan en puertas traseras imposibles para poder implementarlas, ya que muchos de ellos estan basados en la instalación de modulos modificados del núcleo.
5

There are over 28 capabilities including: CAP_BSET, CAP_CHOWN, CAP_FOWNER, CAP_FSETID, CAP_FS_MASK, CAP_FULL_SET,
CAP_INIT_EFF_SET, CAP_INIT_INH_SET, CAP_IPC_LOCK, CAP_IPC_OWNER, CAP_KILL, CAP_LEASE, CAP_LINUX_IMMUTABLE, CAP_MKNOD, CAP_NET_ADMIN, CAP_NET_BIND_SERVICE, CAP_NET_RAW, CAP_SETGID, CAP_SETPCAP, CAP_SETUID,CAP_SYS_ADMIN, CAP_SYS_BOOT, CAP_SYS_CHROOT, CAP_SYS_MODULE, CAP_SYS_NICE, CAP_SYS_PACCT, CAP_SYS_PTRACE,CAP_SYS_RAWIO, CAP_SYS_RESOURCE, CAP_SYS_TIME, and CAP_SYS_TTY_CONFIG. All of them can be de-activated to harden yourkernel.
6
You don't need to install lcap to do this, but it's easier than setting /proc/sys/kernel/cap-bound by hand.

121Antes del compromiso• Entrando a través del cable serial (contribuido por Gaby Schilders). Dado que que los servidores auntienen puertos en serie, imagínese tener una máquina de registro de bitácoras desconectada de su reden la mitad con un puerto serial multiplexor (antiquisimo o algo similar). Ahora todos sus servidores
registrando a sus puertos seriales. Con sólo escritura. la máquina de registo únicamente acepta texto
plano como entrada sobre sus puertos seriales y únicamente escribe en un archivo de registro. Enganche
un cd/dvd writer. Cuando el registro del archivo está cerca de 600 MB lo copia al cd-rom. Ahora si
pudieran hacer quemadoras con auto-cambiadores ... No copia tan dura como la impreosra, pero quepuede manejar largos volúmenes y los cd no toman mucho espacio de almacenamiento.
• Change file attributes using chattr (taken from the Tips-HOWTO, written by Jim Dennis). After a clean
install and initial configuration, use the chattr program with the +i attribute to make files unmodifiable(the file cannot be deleted, renamed, linked or written to). Consider setting this attribute on all the files
in /bin, /sbin/, /usr/bin, /usr/sbin, /usr/lib and the kernel files in root. You can also
make a copy of all files in /etc/, using tar or the like, and mark the archive as immutable.
La razón para todo es limitar el daño que usted pueda ocasionar cuando se registyra como root. Usted no
podra sobreescribir archivos con un desviado operador de redirecciones, usted no podra hacer del sitemaalgo inusual con un desviado espacio dentro de un comando rm -fr (usted puede permancer haciendolo suficiente con los daños de sus datos - pero sus binarios y librerías estarán seguros).
Ésta también emplea una variedad de seguridad y rechazo de servicios de cualquier imposible explosión
o algo de mayor dificultad (ya que muchos de ellos confian en sobre copiar archivos a través de lasacciones de algun programa SUIDque no suministra arbitrariamente una interfase de comandos)El único inconveniente de este es cuando se construye y se hace su make install sobre varias clases desistemas binarios. Sobre la otra mano también previene la instalación desde los archivos sobre escritos.
Cuando usted olvida leer el Makefile y chattr -i los archivos que pueden ser sobre escritos fallan con elmake (y los directorios para los cuales usted necesita para añadir archivos), usted solo use el comando
chattr y regrese. Usted también puede tener la oportuinidad de mover sus viejos bins, libs o lo que sea
dentro de un old/directory o puede renombrar, marcar o lo que sea.
Note que esto lo previene de hacer una actualización de los paquetes de su sistema. Dado que los archivos
que ellos suministran no pueden ser sobre escritos, y usted debe tener un mecanismo para desactivar la
bandera de inmutable sobre todos los binarios antes de un apt-get update.
• Play with UTP cabling in a way that you cut 2 or 4 wires and make the cable one-way traffic only. Then
use UDP packets to send information to the destination machine which can act as a secure log server
or a credit card storage system.

Construyendo un equipo trampa
A honeypot is a system designed to teach system administrators how crackers probe for and exploit asystem. It is a system setup with the expectation and goal that the system will be probed, attacked andpotentially exploited. By learning the tools and methods employed by the cracker, a system administrator
can learn to better protect their own systems and network.
Debian GNU/Linux systems can easily be used to setup a honeynet, if you dedicate the time to implement
and monitor it. You can easily setup the fake honeypot server as well as the firewall7 that controls thehoneynet and some sort of network intrusion detector, put it on the Internet, and wait. Do take care that if
the system is exploited, you are alerted in time (see “La importancia de logs y alarmas”) so that you cantake appropriate measures and terminate the compromise when you've seen enough. Here are some of the
packages and issues to consider when setting up your honeypot:• la tecnología del cortafuegos usted la debera necesitar (suministrado por Linux Kernel).
7

You will typically use a bridge firewall so that the firewall itself is not detectable, see “Setting up a bridge firewall ”.

122Antes del compromiso• syslog-ng, useful for sending logs from the honeypot to a remote syslog server.
• snort para montar la captura de todo la llegada del trafico de red para honeypot y para detectar ataques.
• osh, a SETUID root, security enhanced, restricted shell with logging (see Lance Spitzner's article below).
• Of course, all the daemons you will be using for your fake server honeypot. Depending on what typeof attacker you want to analyse you will or will not harden the honeypot and keep it up to date withsecurity patches.
• Chequeadores integrales (vea check-integ) y los toolkit de Coroners y (tct) para hacer una auditoria depost ataque.
• honeyd and farpd to setup a honeypot that will listen to connections to unused IP addresses and forward
them to scripts simulating live services. Also check out iisemulator.
• tinyhoneypot to setup a simple honeypot server with fake services.
If you cannot use spare systems to build up the honeypots and the network systems to protect and control
it you can use the virtualisation technology available in xen or uml (User-Mode-Linux). If you take thisroute you will need to patch your kernel with either kernel-patch-xen or kernel-patch-uml.
You can read more about building honeypots in Lanze Spitzner's excellent article http://www.net-security.org/text/articles/spitzner/honeypot.shtml (from the Know your Enemy series). Also, the http://project.honeynet.org/ provides valuable information about building honeypots and auditing the attacks madeon them.

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Capítulo 11. After the compromise(incident response)
Comportamiento generalIf you are physically present when an attack is happening, your first response should be to remove themachine from the network by unplugging the network card (if this will not adversely affect any business
transactions). Disabling the network at layer 1 is the only true way to keep the attacker out of the compromised box (Phillip Hofmeister's wise advice).
However, some tools installed by rootkits, trojans and, even, a rogue user connected through a back door,
might be capable of detecting this event and react to it. Seeing a rm -rf / executed when you unplug
the network from the system is not really much fun. If you are unwilling to take the risk, and you are surethat the system is compromised, you should unplug the power cable (all of them if more than one) andcross your fingers. This may be extreme but, in fact, will avoid any logic-bomb that the intruder might
have programmed. In this case, the compromised system should not be re-booted. Either the hard disks
should be moved to another system for analysis, or you should use other media (a CD-ROM) to boot thesystem and analyze it. You should not use Debian's rescue disks to boot the system, but you can use theshell provided by the installation disks (remember, Alt+F2 will take you to it) to analyze 1 the system.
The most recommended method for recovering a compromised system is to use a live-filesystem on CDROM with all the tools (and kernel modules) you might need to access the compromised system. You can
use the mkinitrd-cd package to build such a CD-ROM2. You might find the http://www.caine-live.net/(Computer Aided Investigative Environment) CD-ROM useful here too, since it's also a live CD-ROM
under active development with forensic tools useful in these situations. There is not (yet) a Debian-based
tool such as this, nor an easy way to build the CD-ROM using your own selection of Debian packages andmkinitrd-cd (so you'll have to read the documentation provided with it to make your own CD-ROMs).
If you really want to fix the compromise quickly, you should remove the compromised host from yournetwork and re-install the operating system from scratch. Of course, this may not be effective because you
will not learn how the intruder got root in the first place. For that case, you must check everything: firewall,
file integrity, log host, log files and so on. For more information on what to do following a break-in, see
http://www.cert.org/tech_tips/root_compromise.html or SANS's https://www.sans.org/white-papers/.
Some common questions on how to handle a compromised Debian GNU/Linux system are also available
in.

Realizar copia de seguridad del sistema
Recuerde que si está seguro de que el sistema ha estado comprometido, no puede fiarse del software
instalado ni de ninguna información que este le devuelva. Las aplicaciones podrían contener un troyano,podrían instalarse módulos del núcleo, etc.
Lo mejor que puede hacer es una copia de seguridad del sistema de archivos completo (utilizando dd)
tras arrancar desde un medio seguro. Los CDROMs de Debian GNU/Linux pueden venir bien para esto,
puesto que proporcionan una shell en consola cuando arranca la instalación (acceda a ella utilizando Alt+2
1>If you are adventurous, you can login to the system and save information on all running processes (you'll get a lot from /proc/nnn/). It is possible
to get the whole executable code from memory, even if the attacker has deleted the executable files from disk. Then pull the power cord.
2>In fact, this is the tool used to build the CD-ROMs for the http://www.gibraltar.at/ project (a firewall on a live CD-ROM based on the Debian
distribution).

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After the compromise (incident response)
y pulsando Enter). Desde esta shell, copie la información que quiera salvar en otro ordenador, si es posible(quizás a un servidor de archivos de red por medio de NFS/FTP). Luego puede llevar a cabo cualquieranálisis del compromiso o la reinstalación mientras el sistema afectado está desconectado.
Si está seguro de que el único compromiso es un módulo troyano del núcleo, puede intentar arrancar laimagen del núcleo desde el CDROM de Debian en modo rescue (N. del T.: modo rescate). Asegúrese dearrancar en modo monousuario, de modo que no se ejecute otro proceso troyano después del núcleo.

Contact your local CERTThe CERT (Computer and Emergency Response Team) is an organization that can help you recover froma system compromise. There are CERTs worldwide 3 and you should contact your local CERT in the eventof a security incident which has lead to a system compromise. The people at your local CERT can help
you recover from it.
Providing your local CERT (or the CERT coordination center) with information on the compromise even
if you do not seek assistance can also help others since the aggregate information of reported incidents
is used in order to determine if a given vulnerability is in wide spread use, if there is a new worm aloft,
which new attack tools are being used. This information is used in order to provide the Internet community
with information on the http://www.cert.org/current/, and to publish http://www.cert.org/incident_notes/
and even http://www.cert.org/advisories/. For more detailed information read on how (and why) to reportan incident read http://www.cert.org/tech_tips/incident_reporting.html.
You can also use less formal mechanisms if you need help for recovering from a compromise or want todiscuss incident information. This includes the http://marc.theaimsgroup.com/?l=incidents and the http://
marc.theaimsgroup.com/?l=intrusions.

Análisis forense
If you wish to gather more information, the tct (The Coroner's Toolkit from Dan Farmer and Wietse Venema) package contains utilities which perform a post mortem analysis of a system. tct allows the user
to collect information about deleted files, running processes and more. See the included documentation
for more information. These same utilities and some others can be found in http://www.sleuthkit.org/ by
Brian Carrier, which provides a web front-end for forensic analysis of disk images. In Debian you can findboth sleuthkit (the tools) and autopsy (the graphical front-end).
Recuerde también que los análisis forenses deberían hacerse siempre sobre una copia de seguridad de losdatos, nunca sobre los datos mismos, por si se alteran los datos durante el análisis y se pierde la evidencia.
You will find more information on forensic analysis in Dan Farmer's and Wietse Venema's http://www.porcupine.org/forensics/forensic-discovery/ book (available online), as well as in their http://www.porcupine.org/forensics/column.html and their http://www.porcupine.org/forensics/handouts.html. Brian Carrier's newsletter http://www.sleuthkit.org/informer/index.php is also a very good resource on forensic
analysis tips. Finally, the http://www.honeynet.org/misc/chall.html are an excellent way to hone your forensic analysis skills as they include real attacks against honeypot systems and provide challenges thatvary from forensic analysis of disks to firewall logs and packet captures. For information about available
forensics packages in Debian visit https://salsa.debian.org and search for forensic.
3> This is a list of some CERTs, for a full list look at the http://www.first.org/about/organization/teams/index.html (FIRST is the Forum of Incident Response and Security Teams): http://www.auscert.org.au (Australia), http://www.unam-cert.unam.mx/ (Mexico) http://www.cert.funet.fi(Finland), http://www.dfn-cert.de (Germany), http://cert.uni-stuttgart.de/ (Germany), http://security.dico.unimi.it/ (Italy), http://www.jpcert.or.jp/(Japan), http://cert.uninett.no (Norway), http://www.cert.hr (Croatia) http://www.cert.pl (Poland), http://www.cert.ru (Russia), http://www.arnes.si/si-cert/ (Slovenia) http://www.rediris.es/cert/ (Spain), http://www.switch.ch/cert/ (Switzerland), http://www.cert.org.tw (Taiwan), and http://
www.cert.org (US).

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After the compromise (incident response)
FIXME: This paragraph will hopefully provide more information about forensics in a Debian system inthe coming future.
FIXME: Talk on how to do a debsums on a stable system with the MD5sums on CD and with the recovered
file system restored on a separate partition.
FIXME: Add pointers to forensic analysis papers (like the Honeynet's reverse challenge or http://staff.washington.edu/dittrich/).

Analysis of malware
Some other tools that can be used for forensic analysis provided in the Debian distribution are: strace andltraceAny of these packages can be used to analyze rogue binaries (such as back doors), in order to determine
how they work and what they do to the system. Some other common tools include ldd (in libc6), stringsand objdump (both in binutils).
If you try to do forensic analysis with back doors or suspected binaries retrieved from compromised systems, you should do so in a secure environment (for example in a bochs or xen image or a chroot'ed environment using a user with low privileges4). Otherwise your own system can be back doored/r00ted too!
If you are interested in malware analysis then you should read the http://www.porcupine.org/forensics/forensic-discovery/chapter6.html chapter of Dan Farmer's and Wietse Venema's forensics book.

4>Be very careful if using chroots, since if the binary uses a kernel-level exploit to increase its privileges it might still be able to infect your system

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Capítulo 12. Preguntas Frecuentes
Este capítulo introduce algunas de las preguntas más comunes de la lista de seguridad de Debian. Debería
leerlas antes de preguntar o la gente posiblemente le diga RTFM (N.T. Read The Fucking Manual - Leael P*to Manual).

La seguridad en el sistema operativo Debian¿Es más seguro Debian que X?
Un sistema es sólo tan seguro como su administrador es capaz de hacerlo. La instalación predeterminadade Debian de servicios trata de ser segura, pero puede no ser tan paranoica como la de otros sistemas
operativos que instalan todos los servicios deshabilitados de manera predeterminada. En cualquier caso,
el administrador del sistema necesita adaptar la seguridad del sistema a su política de seguridad local. Paraver una recopilación de datos acerca de vulnerabilidades de seguridad de muchos sistemas operativos mireen http://securityfocus.com/vulns/stats.shtml. ¿Le son útiles estos datos? El servidor lista varios factoresa considerar cuando se interpretan los datos y avisa de que los datos no pueden usarse para compararlas vulnerabilidades de un sistema operativo frente a otro.1Tenga también en mente que alguna de lasvulnerabilidades de BugTraq que afectan a Debian se aplican sólo a la rama unstable.
For a collection of data regarding security vulnerabilities for many operating systems, see the http://
www.cert.org/stats/cert_stats.html or generate stats using the http://nvd.nist.gov/statistics.cfm (formerly
ICAT) Is this data useful? There are several factors to consider when interpreting the data, and it is worth
noticing that the data cannot be used to compare the vulnerabilities of one operating system versus another.2 Also, keep in mind that some reported vulnerabilities regarding Debian apply only to the unstable(i.e. unreleased) branch.

Is Debian more secure than other Linux distributions (such as Red
Hat, SuSE...)?
There are not really many differences between Linux distributions, with exceptions to the base installationand package management system. Most distributions share many of the same applications, with differences mainly in the versions of these applications that are shipped with the distribution's stable release. For
example, the kernel, Bind, Apache, OpenSSH, Xorg, gcc, zlib, etc. are all common across Linux distributions.
For example, Red Hat was unlucky and shipped when foo 1.2.3 was current, which was then later foundto have a security hole. Debian, on the other hand, was lucky enough to ship foo 1.2.4, which incorporated
the bug fix. That was the case in the big http://www.cert.org/advisories/CA-2000-17.html problem froma couple years ago.
There is a lot of collaboration between the respective security teams for the major Linux distributions.
Known security updates are rarely, if ever, left unfixed by a distribution vendor. Knowledge of a security
vulnerability is never kept from another distribution vendor, as fixes are usually coordinated upstream, orby http://www.cert.org. As a result, necessary security updates are usually released at the same time, andthe relative security of the different distributions is very similar.
1

Por ejemplo, teniendo en cuenta los datos de Securityfocus, puede parecer que Windows NT es más seguro que Linux, lo que es una afirmación
cuestionable. Después de todo, las distribuciones de Linux proporcionan habitualmente muchas más aplicaciones comparadas con Windows NT
de Microsoft.
2
For example, based on some data, it might seem that Windows NT is more secure than Linux, which is a questionable assertion. After all, Linux
distributions usually provide many more applications compared to Microsoft's Windows NT. This counting vulnerabilities issues are better describedin http://www.dwheeler.com/oss_fs_why.html#security by David A. Wheeler

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Preguntas Frecuentes

One of Debian's main advantages with regards to security is the ease of system updates through the use of
apt. Here are some other aspects of security in Debian to consider:• Debian provides more security tools than other distributions, see Capítulo 8, Herramientas de seguridad
en Debian.
• Debian's standard installation is smaller (less functionality), and thus more secure. Other distributions,in the name of usability, tend to install many services by default, and sometimes they are not properly
configured (remember the http://www.sophos.com/virusinfo/analyses/linuxlion.html http://www.sophos.com/virusinfo/analyses/linuxramen.html). Debian's installation is not as limited as OpenBSD (nodaemons are active per default), but it's a good compromise. 3• Debian documents best security practices in documents like this one.

Hay muchos errores de Debian en Bugtraq. ¿Significa eso que es
muy vulnerable?
The Debian distribution boasts a large and growing number of software packages, probably more thanprovided by many proprietary operating systems. The more packages installed, the greater the potential
for security issues in any given system.
Más y más personas están examinando el código fuente en busca de debilidades. Hay muchos avisos acerca
de auditorías del código fuente de los componentes más importantes de Debian. Cuando esas auditorías
de código fuente descubren vulnerabilidades, se solucionan y se envía un aviso a las listas y a Bugtraq.
Bugs that are present in the Debian distribution usually affect other vendors and distributions as well.
Check the "Debian specific: yes/no" section at the top of each advisory (DSA).

Does Debian have any certification related to security?
Short answer: no.
Long answer: certification costs money (specially a serious security certification), nobody has dedicatedthe resources in order to certify Debian GNU/Linux to any level of, for example, the http://niap.nist.gov/
cc-scheme/st/. If you are interested in having a security-certified GNU/Linux distribution, try to provide
the resources needed to make it possible.
There are currently at least two linux distributions certified at different http://en.wikipedia.org/wiki/Evaluation_Assurance_Level levels. Notice that some of the CC tests are being integrated into the http://
ltp.sourceforge.net which is available in Debian in the ltp.

¿Hay algún programa de securización para Debian?
Sí. http://www.bastille-unix.org, originalmente orientado hacia otras distribuciones de Linux (RedHat y
Mandrake), actualmente funciona para Debian. Los pasos están siendo tomados para integrar los cambios
hechos a la versión original al paquete Debian llamado bastille.
Sin embargo, algunas personas creen que una herramienta de securización no elimina la necesidad de una
buena administración.

I want to run XYZ service, which one should I choose?
One of Debian's great strengths is the wide variety of choice available between packages that provide thesame functionality (DNS servers, mail servers, ftp servers, web servers, etc.). This can be confusing to the3>Without diminishing the fact that some distributions, such as Red Hat or Mandrake, are also taking into account security in their standard
installations by having the user select security profiles, or using wizards to help with configuration of personal firewalls.

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Preguntas Frecuentes

novice administrator when trying to determine which package is right for you. The best match for a given
situation depends on a balance between your feature and security needs. Here are some questions to ask
yourself when deciding between similar packages:• Is the software maintained upstream? When was the last release?
• Is the package mature? The version number really does not tell you about its maturity. Try to trace thesoftware's history.
• Is the software bug-ridden? Have there been security advisories related to it?
• Does the software provide all the functionality you need? Does it provide more than you really need?

Quiero ejecutar el servicio XYZ, ¿cuál debería elegir?
Una de las mayores fortalezas de Debian es la gran variedad de elecciones posibles entre paquetes que
proporcionan la misma funcionalidad (servidores de DNS, servidores de correo, servidores de FTP, servidores WEB, etc.). Eso puede resultar confuso para el administrador novel al tratar de determinar que
paquete es el adecuado. La mejor opción para una situación concreta se basa en el compromiso entre su
funcionalidad y los requerimientos de seguridad. Hay algunas preguntas que debe contestar antes de decidir entre paquetes similares:

How can I remove all the banners for services?
If you do not like users connecting to your POP3 daemon, for example, and retrieving information about
your system, you might want to remove (or change) the banner the service shows to users. 4 Doing so
depends on the software you are running for a given service. For example, in postfix, you can set your
SMTP banner in /etc/postfix/main.cf:

smtpd_banner = $myhostname ESMTP $mail_name (Debian/GNU)
Other software is not as easy to change. ssh will need to be recompiled in order to change the version thatit prints. Take care not to remove the first part (SSH-2.0) of the banner, which clients use to identify
which protocol(s) is supported by your package.

¿Son seguros todos los paquetes de Debian?
The Debian security team cannot possibly analyze all the packages included in Debian for potential security
vulnerabilities, since there are just not enough resources to source code audit the whole project. However,Debian does benefit from the source code audits made by upstream developers.
De todas formas un desarrollador Debian podría distribuir un troyano en un paquete y no habría forma
posible de comprobarlo. Incluso si se introduce en una rama de Debian, podría ser imposible cubrir todaslas posibles situaciones en las que un troyano puede ejecutarse. Esa es la razón por la que Debian tiene
una cláusula de "no garantías" en su licencia.
Aun así, los usuarios de Debian tiene confianza en el hecho de que el código estable tiene una gran audiencia y la mayoría de los problemas pueden descubrirse con el uso. Instalar programas no probados noes recomendable en un sistema crítico (si no puede hacer la auditoría de código necesaria). En cualquiercaso, si se introduce una vulnerabilidad de seguridad en una distribución, el proceso que se usa para incluir
paquetes (usando firma digital) asegura que el problema puede seguirse hasta el desarrollador. El proyecto
Debian no se ha tomado a la ligera este tema.
4>Note that this is 'security by obscurity', and will probably not be worth the effort in the long term.

129

Preguntas Frecuentes

Why are some log files/configuration files world-readable, isn't thisinsecure?
Of course, you can change the default Debian permissions on your system. The current policy regarding
log files and configuration files is that they are world readable unless they provide sensitive information.
Be careful if you do make changes since:• Processes might not be able to write to log files if you restrict their permissions.
• Some applications may not work if the configuration file they depend on cannot be read. For example, if
you remove the world-readable permission from /etc/samba/smb.conf, the smbclient program
will not work when run by a normal user.
FIXME: Check if this is written in the Policy. Some packages (i.e. ftp daemons) seem to enforce different
permissions.

Why does /root/ (or UserX) have 755 permissions?
As a matter of fact, the same questions stand for any other user. Since Debian's installation does not place
any file under that directory, there's no sensitive information to protect there. If you feel these permissions
are too broad for your system, consider tightening them to 750. For users, read “Limitar el acceso a losdatos de otros usuarios”.
This Debian security mailing list http://lists.debian.org/debian-devel/2000/11/msg00783.html has moreon this issue.

After installing a grsec/firewall, I started receiving many console
messages! How do I remove them?
If you are receiving console messages, and have configured /etc/syslog.conf to redirect them toeither files or a special TTY, you might be seeing messages sent directly to the console.
The default console log level for any given kernel is 7, which means that any message with lower priority
will appear in the console. Usually, firewalls (the LOG rule) and some other security tools log lower that
this priority, and thus, are sent directly to the console.
To reduce messages sent to the console, you can use dmesg (-n option, see dmseg(8)), which examines
and controls the kernel ring buffer. To fix this after the next reboot, change /etc/init.d/klogd from:KLOGD=""
para:
KLOGD="-c 4"
Use a lower number for -c if you are still seeing them. A description of the different log levels can befound in /usr/include/sys/syslog.h:#define LOG_EMERG#define LOG_ALERT#define LOG_CRIT#define LOG_ERR

0
1
2
3

/* system is unusable *//* action must be taken immediately *//* critical conditions *//* error conditions */130

Preguntas Frecuentes#define LOG_WARNING#define LOG_NOTICE#define LOG_INFO#define LOG_DEBUG

4
5
6
7

/* warning conditions *//* normal but significant condition *//* informational *//* debug-level messages */Usuarios y grupos del sistema operativo¿Son necesarios todos los usuarios del sistema?
Yes and no. Debian comes with some predefined users (user id (UID) < 99 as described in http://www.debian.org/doc/debian-policy/ or /usr/share/doc/base-passwd/README) to ease the installationof some services that require that they run under an appropriate user/UID. If you do not intend to install
new services, you can safely remove those users who do not own any files in your system and do not run
any services. In any case, the default behavior is that UID's from 0 to 99 are reserved in Debian, and UID'sfrom 100 to 999 are created by packages on install (and deleted when the package is purged).
To easily find users who don't own any files, execute the following command5 (run it as root, since acommon user might not have enough permissions to go through some sensitive directories):
cut -f 1 -d : /etc/passwd | \while read i; do find / -user "$i" | grep -q . || echo "$i"; done
Estos usuarios son del paquete base-passwd. Mire en su documentación si quiere más información acerca de cómo se gestionan estos usuarios en Debian. La lista de usuarios predeterminados (con su grupo
correspondiente) es la siguiente:• root: Root es (típicamente) el super usuario.
• daemon: Algunos demonios sin privilegios que necesitan escribir en archivos del del disco se ejecutan
como daemon.daemon (p.e., portmap, atd, y probablemente otros). Los demonios que no necesitan
tener ningún archivo corren como nobody.nogroup, algunos otros demonios más complejos se ejecutan
con usuarios dedicados. El usuario del demonio es también útil para demonios instalados localmente.
• bin: mantenido por razones históricas.
• sys: igual que bin. Aun así, /dev/vcs* y /var/spool/cups son del usuario y grupo sys.
• sync: La shell del usuario sync es /bin/sync. Aun así si su contraseña se pone fácil de adivinar (como""), cualquiera puede sincronizar el sistema en la consola incluso si no tiene cuenta.
• games: Muchos juegos tienen SETGID de manera que puedan escribir sus propios archivos de puntuación más alta. Se explica en la política.
• man: El programa man (a veces) se ejecuta con el usuario man, para que pueda escribir páginas en /var/cache/man.
• lp: Usado por los demonios de impresión.
• mail: los buzones de /var/mail son del grupo mail, como se explica en la política. El usuario y grupo
se usan también con otros propósitos por varios MTAs.
• news: Varios servidores de noticias y programas asociados (como suck) usan el usuario y grupo newsde varias formas. Los archivos en la cola news son habitualmente del usuario y grupo news. Programas
como inews que pueden usarse para enviar noticias son típicamente noticias con SETGID.
5

Be careful, as this will traverse your whole system. If you have a lot of disk and partitions you might want to reduce it in scope.

131

Preguntas Frecuentes• uucp: El usuario y grupo uucp es usado por el subsistema UUCP. Tiene su propia cola y archivos deconfiguración. Los usuarios en el grupo uucp pueden ejecutar uucico.
• proxy: Como daemon, este usuario y grupo lo usan algunos demonios (especialmente demonios deproxy) que no tienen usuarios dedicados y necesitan tener archivos. Por ejemplo el grupo proxy lo usan
pdnsd, y squid.
• majordom: Majordomo tiene un UID estático en sistemas Debian por razones históricas. No se instalaen sistemas nuevos.
• postgres: Las bases de datos de Postgresql son de este usuario y grupo. Todos los archvos de /var/
lib/postgresql son de este usuario para reforzar la seguridad.
• www-data: Algunos servidores web corren como www-data. El contenido *no* debe ser de este usuarioo un servidor comprometido podría reescribir el web. Los datos escritos por el servidor web incluyendo
los archivos de registro, deben ser de www-data.
• backup: Muchas funciones de copia/restauración pueden ser delegadas a alguien sin privilegios completos de root.
• operator: Operator es históricamente (y prácticamente) el único 'usuario' que puede acceder al sistemade forma remota y no depende de NIS/NFS.
• list: Los archivos de listas de correo y sus datos son de este usuario y grupo. Algunos programas delistas de correo pueden ejecutarse también con este usuario.
• irc: Usado por demonios de IRC. Un usuario determinado se necesita únicamente por un error de ircd,
que hace SETUID()s de si mismo a un UID determinado en el arranque.
• gnats.
• nobody, nogroup: Los demonios que no necesitan ningún archivo se ejecutan con el usuario nobody y
grupo nogroup. Así que no debería existir ningún archivo de este usuario o grupo en el sistema.
Otros grupos que no tienen un usuario asociado:• adm: El grupo adm se usa para tareas de monitorización del sistema. Los miembros de este grupo pueden
leer muchos de los archivos de /var/log, y pueden usar xconsole. Históricamente /var/log eran/usr/adm (y más tarde /var/adm), del mismo grupo.
• tty: Los dispositivos TTY son de este grupo. Lo usan write y wall para escribir en los TTYs de otras
personas.
• disk: Acceso directo a disco. Mayormente equivalente al acceso de root.
• kmem: /dev/kmem y archivos similares son accesibles en modo lectura por este grupo. Esto es, mayormente, una reliquia de BSD pero algunos programas siguen necesitando acceso directo de lectura a lamemoria del sistema que se hace con SETGID kmem.
• dialout: Acceso directo y completo a los puertos serie. Los miembros de este grupo pueden reconfigurar
el módem, llamar a cualquier parte, etc.
• dip: El nombre del grupo viene de "Dial-up IP", y ser de ese grupo le permite usar herramientas como
ppp, dip, wvdial, etc. para comenzar una conexión. La mayoría de los usuarios de este grupo no pueden
configurar el módem, pero pueden ejecutar programas que lo usa.
• fax: Permite a los miembros usar el programa de fax para enviar / recibir faxes.

132Preguntas Frecuentes• voice: Voicemail, usado por sistemas que usan módems como contestadores.
• cdrom: Este grupo puede ser usado localmente para dar acceso al CDROM a los usuarios.
• floppy: Este grupo puede ser usado localmente para dar acceso a la disquetera a los usuarios.
• tape: Este grupo puede ser usado localmente para dar acceso a la cinta a los usuarios.
• sudo: Los miembros de este grupo no necesitan teclear la contraseña cuando usen el programa sudo.
Mire /usr/share/doc/sudo/OPTIONS.
• audio: Este grupo puede ser usado localmente para dar acceso a los usuarios al dispositivo de audio.
• src: Este grupo tiene el código fuente incluyendo los archivos de /usr/src. Puede usarse para darle
al usuario la capacidad de gestionar el sistema de código fuente.
• shadow: /etc/shadow es de lectura para este grupo. Algunos programas que necesitan acceso a estearchivo tienen SETGID shadow.
• utmp: Este grupo puede escribir en /var/run/utmp y archivos similares. Los programas que necesitan escribir ahí tienen SETGID utmp.
• video: Este grupo puede ser usado para dar a los usuarios acceso al dispositivo de video.
• staff: Permite a los usuarios añadir modificaciones locales al sistema (/usr/local, /home) sin necesidad de tener privilegios de root. Comparar con el grupo "adm", que está más relacionado con monitorización/seguridad.
• users: Mientras los sistemas Debian usan el sistema de usuarios privados de forma predeterminada (cadausuario tiene su propio grupo), algunos prefieren un sistema de grupos más tradicional, en el que cadausuario es miembro de este grupo.

I removed a system user! How can I recover?
If you have removed a system user and have not made a backup of your password and group files you
can try recovering from this issue using update-passwd (see update-passwd(8)).

¿Qué diferencia hay entre el grupo adm y el staff?
The 'adm' group are usually administrators, and this group permission allows them to read log files without
having to su. The 'staff' group are usually help-desk/junior sysadmins, allowing them to work in /usr/
local and create directories in /home.

Why is there a new group when I add a new user? (or Why does Debian give each user one group?)
The default behavior in Debian is that each user has its own, private group. The traditional UN*X scheme
assigned all users to the users group. Additional groups were created and used to restrict access to sharedfiles associated with different project directories. Managing files became difficult when a single user worked on multiple projects because when someone created a file, it was associated with the primary groupto which they belong (e.g. 'users').
Debian's scheme solves this problem by assigning each user to their own group; so that with a proper umask(0002) and the SETGID bit set on a given project directory, the correct group is automatically assigned tofiles created in that directory. This makes it easier for people who work on multiple projects, because they
will not have to change groups or umasks when working on shared files.

133

Preguntas Frecuentes

You can, however, change this behavior by modifying /etc/adduser.conf. Change the USERGROUPS variable to 'no', so that a new group is not created when a new user is created. Also, set USERS_GID to the GID of the users group which all users will belong to.

Cuestiones sobre servicios en ejecución y puertos abiertos.
¿Por qué están todos los servicios activados tras la instalación?
Es una aproximación al problema de, por una parte la seguridad y por otra la usabilidad. Al contrario que
como OpenBSD, deshabilita los servicios a no ser que los habilite el administrador, Debian GNU/Linux
habilitará todos los servicios instalados a no ser que se desactiven (mire en “Deshabilitar servicios” paraver más información). Después de todo usted instaló el servicio ¿no es así?
There has been much discussion on Debian mailing lists (both at debian-devel and at debian-security)
regarding which is the better approach for a standard installation. However, as of this writing (March
2002), there still isn't a consensus.

Can I remove inetd?
Inetd no es fácil de eliminar porque netbase depende del paquete que lo provee (netkit-inetd). Si quiere
deshabilitarlo (mire disableserv o elimine el paquete usando el paquete equivs).

¿Por qué tengo abierto el puerto 111?
El puerto 111 es el portmapper de sunrpc y se instala de manera predeterminada como parte del sistema
base de Debian porque no se sabe cuando un programa de usuario necesitará usar RPC para funcionar
correctamente. En cualquier caso, se usa mayormente en NFS. Si no lo necesita, elimínelo tal y como se
explica en rpc.
In versions of the portmap package later than 5-5 you can actually have the portmapper installed but
listening only on localhost (by modifying /etc/default/portmap)

What use is identd (port 113) for?
Identd service is an authentication service that identifies the owner of a specific TCP/IP connection to
the remote server accepting the connection. Typically, when a user connects to a remote host, inetd on
the remote host sends back a query to port 113 to find the owner information. It is often used by mail,FTP and IRC servers, and can also be used to track down which user in your local system is attacking
a remote system.
There has been extensive discussion on the security of identd (See http://lists.debian.org/debian-security/2001/08/msg00297.html). In general, identd is more helpful on a multi-user system than on a single
user workstation. If you don't have a use for it, disable it, so that you are not leaving a service open to theoutside world. If you decide to firewall the identd port, please use a reject policy and not a deny policy,
otherwise a connection to a server utilizing identd will hang until a timeout expires (see http://logi.cc/linux/reject_or_deny.php3).

I have services using port 1 and 6, what are they and how can I remove them?
If you have run the command netstat -an and receive:
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address
Foreign Address
PID/Program nameraw
0
0 0.0.0.0:1
0.0.0.0:*

134

State
7

Preguntas Frecuentes

raw
-

0

0 0.0.0.0:6

0.0.0.0:*

7

You are not seeing processes listening on TCP/UDP port 1 and 6. In fact, you are seeing a process listening
on a raw socket for protocols 1 (ICMP) and 6 (TCP). Such behavior is common to both legitimate software
like intrustion detection systems, such as iplogger and portsentry, but some trojans have also been known
yo use them. If you have the mentioned packages simply remove them to close the port. If you do not, try
netstat's -p (process) option to see which process is running these listeners.

I found the port XYZ open, can I close it?
Por supuesto que puede, usted puede ir dejando los portales abiertos en su sitio de política, fortaleciendo
los servicios públicos disponibles para otros sistemas. Revise si estos son abiertos por inetd (observe “Disabling inetd or its services”) o por otros paquetes instalados y tómelo en medidas apropiadas (configure
inetd, remueva el paquete, anule el funcionamiento en bootup...).

Will removing services from /etc/services help secure my box?
No, /etc/services solo suministra un mapping desde un nombre virtual a un acceso numérico dado,
removiendo nombres desde (usualmente) donde no haya que impedir servicios al ser iniciados. Algunos
demonios no podrán funcionar si /etc/services ha sido modificado, pero ésta no es la norma y noes la vía recomendable para hacerlo, observe disableserv.

Common security issues¡¡He perdido mi password y no puedo tener acceso al sistema!!
Usted necesita seguir unos pasos para recuperar su password y este depende desi usted ha aplicado o noel procedimiento para limitar el acceso a Lilo y BIOS.
Si usted ha limitado a ambos. Necesita desactivar las características de BIOS (hágalo solo desde el discoduro) antes de proceder, además, si usted olvida el password de BIOS, tendrá que abrir el sistema y eliminarla batería BIOS manualmente.
Once you have enabled booting from a CD-ROM or diskette enable, try the following:• inicie desde un disco de rescate e inicie el kernel.
• desplácese hasta la consola virtual (Alt+F2)• monte el disco duro, en donde está su /root• edite (El disco de rescate Debian 2.2, viene con ae, Debian 3.0 viene con nano-tiny, el cual es similara vi) /etc/shadow y modifique la línea:root:asdfjl290341274075:XXXX:X:XXXX:X::: (X=cualquier número)para:root::XXXX:X:XXXX:X:::
This will remove the forgotten root password, contained in the first colon separated field after the username. Save the file, reboot the system and login with root using an empty password. Remember to reset the135

Preguntas Frecuentes

password. This will work unless you have configured the system more tightly, i.e. if you have not allowed
users to have null passwords or not allowed root to login from the console.
Si usted ha introducido también estas características, usted necesitará entrar en el modo single. LILO nonecesita ser limitado si usted ha hecho esto también, necesitará reiniciar lilo justo después que el administrador lo reajusta. Esto es totalmente difícil desde que su /etc/lilo.conf necesite ser atrapado por
tener un / sistema de archivo, que es un disco de la memoria ram y no del verdadero disco duro.
Once LILO is unrestricted, try the following:• Presionar Alt, shift o la tecla Control, antes que el sistema BIOS finalice, usted debería obtener la entrada
LILO.
• Type linux single, linux init=/bin/sh or linux 1 at the prompt.
• usted debería obtener una entrada a interfaz de comando en el modo singleuser (este solicitará la clave,aunque usted ya la conozca).
• Re-mount read/write the root (/) partition, using the mount command.

# mount -o remount,rw /• Cambiar la clave del superusuario con passwd (desde que usted sea el superusuario, no se le solicitarála clave anterior).

How do I accomplish setting up a service for my users without giving out shell accounts?
For example, if you want to set up a POP service, you don't need to set up a user account for each user
accessing it. It's best to set up directory-based authentication through an external service (like Radius,
LDAP or an SQL database). Just install the appropriate PAM library (libpam-radius-auth, libpam-ldap, libpam-pgsql or libpam-mysql), read the documentation (for starters, see “Uso de la autentificación: PAM”)
and configure the PAM-enabled service to use the back end you have chosen. This is done by editing thefiles under /etc/pam.d/ for your service and modifying theauthrequired

pam_unix_auth.so shadow nullok use_first_pass

to, for example, ldap:

authrequired

pam_ldap.so

In the case of LDAP directories, some services provide LDAP schemas to be included in your directorythat are required in order to use LDAP authentication. If you are using a relational database, a useful trick
is to use the where clause when configuring the PAM modules. For example, if you have a database withthe following table attributes:(user_id, user_name, realname, shell, password, UID, GID, homedir, sys, pop, imap
By making the services attributes boolean fields, you can use them to enable or disable access to thedifferent services just by inserting the appropriate lines in the following files:• /etc/pam.d/imap:where=imap=1.

136

Preguntas Frecuentes• /etc/pam.d/qpopper:where=pop=1.
• /etc/nss-mysql*.conf:users.where_clause = user.sys = 1;.
• /etc/proftpd.conf: SQLWhereClause "ftp=1".

My system is vulnerable! (Are you sure?)
Vulnerability assessment scanner X says my Debian system is vulnerable!
Many vulnerability assessment scanners give false positives when used on Debian systems, since they
only use version checks to determine if a given software package is vulnerable, but do not really test the
security vulnerability itself. Since Debian does not change software versions when fixing a package (manytimes the fix made for newer releases is back ported), some tools tend to think that an updated Debiansystem is vulnerable when it is not.
If you think your system is up to date with security patches, you might want to use the cross references tosecurity vulnerability databases published with the DSAs (see “Avisos de seguridad de Debian”) to weedout false positives, if the tool you are using includes CVE references.

I've seen an attack in my system's logs. Is my system compromised?
A trace of an attack does not always mean that your system has been compromised, and you should takethe usual steps to determine if the system is indeed compromised (see Capítulo 11, After the compromise(incident response)). Even if your system was not vulnerable to the attack that was logged, a determinedattacker might have used some other vulnerability besides the ones you have detected.

I have found strange 'MARK' lines in my logs: Am I compromised?
You might find the following lines in your system logs:
Dec 30 07:33:36 debian -- MARK -Dec 30 07:53:36 debian -- MARK -Dec 30 08:13:36 debian -- MARK -This does not indicate any kind of compromise, and users changing between Debian releases might findit strange. If your system does not have high loads (or many active services), these lines might appear
throughout your logs. This is an indication that your syslogd daemon is running properly. From syslogd(8):-m interval
The syslogd logs a mark timestamp regularly.
The
default interval between two -- MARK -- lines is 20
minutes. This can be changed with this option.
Setting the interval to zero turns it off entirely.

I found users using 'su' in my logs: Am I compromised?
Usted puede encontrar líneas en sus bitácoras como:Apr

1 09:25:01 server su[30315]: + ??? root-nobody

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Preguntas Frecuentes

Apr

1 09:25:01 server PAM_unix[30315]: (su) session opened for user nobody by (UNo se preocupe tanto, revise si esto se debe a un trabajo en funcionamiento a través de cron (usualmente
con /etc/cron.daily/find o logrotate):$ grep 25 /etc/crontab
25 6 * * * root test -e /usr/sbin/anacron || run-parts --report/etc/cron.daily$ grep nobody /etc/cron.daily/*find:cd / || updatedb --localuser=nobody 2>/dev/null

I have found 'possible SYN flooding' in my logs: Am I under attack?
If you see entries like these in your logs:
May 1 12:35:25 linux kernel: possible SYN flooding on port X. Sending cookies.
May 1 12:36:25 linux kernel: possible SYN flooding on port X. Sending cookies.
May 1 12:37:25 linux kernel: possible SYN flooding on port X. Sending cookies.
May 1 13:43:11 linux kernel: possible SYN flooding on port X. Sending cookies.
Check if there is a high number of connections to the server using netstat, for example:
linux:~# netstat -ant | grep SYN_RECV | wc -l
9000
This is an indication of a denial of service (DoS) attack against your system's X port (most likely against
a public service such as a web server or mail server). You should activate TCP syncookies in your kernel,
see “Configuring syncookies”. Note, however, that a DoS attack might flood your network even if you
can stop it from crashing your systems (due to file descriptors being depleted, the system might becomeunresponsive until the TCP connections timeout). The only effective way to stop this attack is to contact
your network provider.

I have found strange root sessions in my logs: Am I compromised?
You might see these kind of entries in your /var/log/auth.log file:May 2 11:55:02 linux PAM_unix[1477]: (cron) session closed for user root
May 2 11:55:02 linux PAM_unix[1476]: (cron) session closed for user root
May 2 12:00:01 linux PAM_unix[1536]: (cron) session opened for user root by(UID=0)
May 2 12:00:02 linux PAM_unix[1536]: (cron) session closed for user root
These are due to a cron job being executed (in this example, every five minutes). To determine which
program is responsible for these jobs, check entries under: /etc/crontab, /etc/cron.d, /etc/
crond.daily and root's crontab under /var/spool/cron/crontabs.

He sufrido una interrupción, ¿qué debo hacer?
There are several steps you might want to take in case of a break-in:• Check if your system is up to date with security patches for published vulnerabilities. If your system
is vulnerable, the chances that the system is in fact compromised are increased. The chances increase138

Preguntas Frecuentes

further if the vulnerability has been known for a while, since there is usually more activity related toolder vulnerabilities. Here is a link to http://www.sans.org/top20/.
• Read this document, especially the Capítulo 11, After the compromise (incident response) section.
• Ask for assistance. You might use the debian-security mailing list and ask for advice on how to recover/patch your system.
• Notify your local http://www.cert.org (if it exists, otherwise you may want to consider contacting CERT
directly). This might or might not help you, but, at the very least, it will inform CERT of ongoingattacks. This information is very valuable in determining which tools and attacks are being used by theblackhat community.

¿Cómo puedo encontrar el origen de un ataque?
Observando las bitácoras (si ellas no han sido cambiadas), usando sistemas de detección de intrusión(observe intrusion-detect), traceroute, whois y herramientas similares (incluyendo análisis forense), usted
puede encontrar un ataque a la fuente. La manera como usted debería reaccionar frente a esta información
depende, solamente del uso apropiado que usted le de a la política de seguridad y que usted considere loque es un ataque. ¿Es un scanner remoto un ataque? ¿Un ataque es una prueba de vulnerabilidad?

Cualquier programa en Debian es vulnerable ¿Qué debo hacer?
Tómese un momento. Primero, observe si la vulnerabilidad ha sido anunciada en listas de correo de seguridad pública (como Bugtraq) u otros foros, el equipo de seguridad Debian permanece actualizado con
estas listas, de manera que ellos ya pueden estar enterados del problema. No ejecute ningunas acciones
remotas si usted ya observa algún anuncio en http://security.debian.org.
If no information seems to be published, please send e-mail about the affected package(s), as well as adetailed description of the vulnerability (proof of concept code is also OK), to mailto:team@security.debian.org. This will get you in touch with Debian's security team.

El número de versión para un paquete indica que todavía estoy corriendo una versión vulnerable
En lugar de actualizar una nueva descarga, podemos fijar la seguridad en backport, a la versión que fue
enviada en la descarga establecida. La razón de esto es asegurarnos que una descarga cambie posiblemente
un poco algunas cosas o que se interrumpan repentinamente, como un resultado de la seguridad fija. Usted
puede chequear si está corriendo una versión segura de un paquete, observando el paquete changelog
o comparando su número de versión exacta (versión upstream -slash- descargar debian)el número de laversión exacta con la versión indicada en el Asesor de seguridad Debian.

Software específico
Proftpd es vulnerable a un ataque en el servicio Denial.
Agregue DenyFilter \*.*/ a su archivo de configuración, para más información observe http://
www.proftpd.org/critbugs.html.

After installing portsentry, there are a lot of ports open.
That's just the way portsentry works. It opens about twenty unused ports to try to detect port scans.

139Preguntas Frecuentes

Preguntas con respecto al equipo de seguridad
DebianThe security team keeps its list of Frequently Asked Questions at the http://www.debian.org/security/faq.
Please refer to that web page for up to date information.

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Apéndice A. Historia de revisiones
Historial de revisiones
Revisión 3-19.2

Sun May 19 2024

Translation files synchronised with XML sources 3-19
Revisión 3-19.1
Mon May 1 2017
Translation files synchronised with XML sources 3-19
Revisión 3-19
April 2017

HolgerWansing<hwansing@mailbox.org>
MarcosFouces<marcos.fouces@gmail.com>
MarcosFouces<marcos.fouces@gmail.com>

Migrate to Docbook XML.
Build with Publican. No longer use custom Makefile.
Migrate svn repository to git.
Import chinese, italian, spanish, portuguese, japanese, russian, french and german translations to PO format.
Revisión 3-18
February 2015
ThijsKinkhorst<thijs@debian.org>
Clarify FAQ on raw sockets.
Update section 4.5 on GRUB2.
Replace example postrm user removal code with advice to use deluser/delgroup --system
Revisión 3-17
January 2015
Thijs
Kinkhorst<thijs@debian.org>
Remove mention of MD5 shadow passwords.
Do not recommend dselect for holding packages.
No longer include the Security Team FAQ verbatim, because it duplicates information documented elsewhere and is hence perpetually out of date.
Update section on restart after library upgrades to mention needrestart.
Avoid gender-specific language. Patch by Myriam.
Use LSB headers for firewall script. Patch by Dominic Walden.
Revisión 3-16
January 2013
JavierFernández-SanguinoPeña.<jfs@debian.org>
Indicate that the document is not updated with latest versions.
Update pointers to current location of sources.
Update information on security updates for newer releases.
Point information for Developers to online sources instead of keeping the information in the document,
to prevent duplication.
Extend the information regarding securing console access, including limiting the Magic SysRq key.
Update the information related to PAM modules including how to restrict console logins, use cracklib
and use the features avialable in /etc/pam.d/login. Remove the references to obsolete variables in /etc/
login.defs.
Reference some of the PAM modules available to use double factor authentication, for administrators thatwant to stop using passwords altogether.
Fix shell script example in Appendix.
Fix reference errors.
Point to the Basille sourceforge project instead of the bastille-unix.org site as it is not responding.
Revisión 3-15
December 2010
JavierFernández-Sanguino
Peña<jfs@debian.org>
Change reference to Log Analysis' website as this is no longer available.
Revisión 3-14
March 2009
JavierFernández-Sanguino
Peña<jfs@debian.org>
Change the section related to choosing a filesystem: note that ext3 is now the default.

141

Historia de revisiones

Change the name of the packages related to enigmail to reflect naming changes introduced in Debian.
Revisión 3-13
February 2008
JavierFernández-Sanguino
Peña<jfs@debian.org>
Change URLs pointing to Bastille Linux to www.Bastille-UNIX.org since the domain has been http://bastille-linux.sourceforge.net/press-release-newname.html.
Fix pointers to Linux Ramen and Lion worms.
Use linux-image in the examples instead of the (old) kernel-image packages.
Fix typos spotted by Francesco Poli.
Revisión 3-12August 2007
JavierFernández-Sanguino
Peña<jfs@debian.org>
Update the information related to security updates. Drop the text talking about Tiger and include information on the update-notifier and adept tools (for Desktops) as well as debsecan. Also include some pointers
to other tools available.
Divide the firewall applications based on target users and add fireflier to the Desktop firewall applications
list.
Remove references to libsafe, it's not in the archive any longer (was removed January 2006).
Fix the location of syslog's configuration, thanks to John Talbut.
Revisión 3-11
January 2007
JavierFernández-Sanguino
Peña<jfs@debian.org>
Thanks go to Francesco Poli for his extensive review of the document.
Remove most references to the woody release as it is no longer available (in the archive) and security
support for it is no longer available.
Describe how to restrict users so that they can only do file transfers.
Added a note regarding the debian-private declasiffication decision.
Updated link of incident handling guides.
Added a note saying that development tools (compilers, etc.) are not installed now in the default 'etch'
installation.
Added a note saying that development tools (compilers, etc.) are not installed now in the default 'etch'
installation.
Fix references to the master security server.
Add pointers to additional APT-secure documentation.
Improve the description of APT signatures.
Comment out some things which are not yet final related to the mirror's official public keys.
Fixed name of the Debian Testing Security Team.
Remove reference to sarge in an example.
Update the antivirus section, clamav is now available on the release. Also mention the f-prot installer.
Removes all references to freeswan as it is obsolete.
Describe issues related to ruleset changes to the firewall if done remotely and provide some tips (in footnotes).
Update the information related to the IDS installation, mention BASE and the need to setup a logging
database.
Rewrite the "running bind as a non-root user" section as this no longer applies to Bind9. Also remove thereference to the init.d script since the changes need to be done through /etc/default.
Remove the obsolete way to setup iptables rulesets as woody is no longer supported.
Revert the advice regarding LOG_UNKFAIL_ENAB it should be set to 'no' (as per default).
Added more information related to updating the system with desktop tools (including update-notifier) anddescribe aptitude usage to update the system. Also note that dselect is deprecated.
Updated the contents of the FAQ and remove redundant paragraphs.
Review and update the section related to forensic analysis of malware.
Remove or fix some dead links.
Fix many typos and gramatical errors reported by Francesco Poli.
Revisión 3-10
November 2006
JavierFernández-Sanguino
Peña<jfs@debian.org>
Provide examples using apt-cache's rdepends as suggested by Ozer Sarilar.

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Historia de revisiones

Fix location of Squid's user's manual because of its relocation as notified by Oskar Pearson (its maintainer).
Fix information regarding umask, it's logins.defs (and not limits.conf) where this can be configured for all
login connections. Also state what is Debian's default and what would be a more restrictive value for bothusers and root. Thanks to Reinhard Tartler for spotting the bug.
Revisión 3-9
October 2006
JavierFernández-Sanguino
Peña<jfs@debian.org>
Add information on how to track security vulnerabilities and add references to the Debian Testing Security
Tracker.
Add more information on the security support for testing.
Fix a large number of typos with a patch provided by Simon Brandmair.
Added section on how to disable root prompt on initramfs provided by Max Attems.
Remove references to queso.
Note that testing is now security-supported in the introduction.
Revisión 3-8
July 2006
JavierFernández-Sanguino
Peña<jfs@debian.org>
Rewrote the information on how to setup ssh chroots to clarify the different options available, thank to
Bruce Park for bringing up the different mistakes in this appendix.
Fix lsof call as suggested by Christophe Sahut.
Include patches for typo fixes from Uwe Hermann.
Fix typo in reference spotted by Moritz Naumann.
Revisión 3-7
April 2006
JavierFernández-Sanguino
Peña<jfs@debian.org>
Add a section on Debian Developer's best practices for security.
Ammended firewall script with comments from WhiteGhost.
Revisión 3-6
March 2006
JavierFernández-Sanguino
Peña<jfs@debian.org>
Included a patch from Thomas Sjögren which describes that noexec works as expected with "new" kernels, adds information regarding tempfile handling, and some new pointers to external documentation.
Add a pointer to Dan Farmer's and Wietse Venema's forensic discovery web site, as suggested by Freek
Dijkstra, and expanded a little bit the forensic analysis section with more pointers.
Fixed URL of Italy's CERT, thanks to Christoph Auer.
Reuse Joey Hess' information at the wiki on secure apt and introduce it in the infrastructure section.
Review sections referring to old versions (woody or potato).
Fix some cosmetic issues with patch from Simon Brandmair.
Included patches from Carlo Perassi: acl patches are obsolete, openwall patches are obsolete too, removed
fixme notes about 2.2 and 2.4 series kernels, hap is obsolete (and not present in WNPP), remove references
to Immunix (StackGuard is now in Novell's hands), and fix a FIXME about the use of bsign or elfsign.
Updated references to SElinux web pages to point to the Wiki (currently the most up to date source of
information).
Include file tags and make a more consistent use of "MD5 sum" with a patch from Jens Seidel.
Patch from Joost van Baal improving the information on the firewall section (pointing to the wiki insteadof listing all firewall packages available) (Closes: #339865).
Review the FAQ section on vulnerability stats, thanks to Carlos Galisteo de Cabo for pointing out that
it was out of date.
Use the quote from the Social Contract 1.1 instead of 1.0 as suggested by Francesco Poli.
Revisión 3-5
November 2005
JavierFernández-Sanguino
Peña<jfs@debian.org>
Note on the SSH section that the chroot will not work if using the nodev option in the partition and point
to the latest ssh packages with the chroot patch, thanks to Lutz Broedel for pointing these issues out.
Fix typo spotted by Marcos Roberto Greiner (md5sum should be sha1sum in code snippet).
Included Jens Seidel's patch fixing a number of package names and typos.
Slightly update of the tools section, removed tools no longer available and added some new ones.

143

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Rewrite parts of the section related to where to find this document and what formats are available (thewebsite does provide a PDF version). Also note that copies on other sites and translations might be obsolete(many of the Google hits for the manual in other sites are actually out of date).
Revisión 3-4
August-September 2005
JavierFernández-Sanguino
Peña<jfs@debian.org>
Improved the after installation security enhancements related to kernel configuration for network levelprotection with a sysctl.conf file provided by Will Moy.
Improved the gdm section, thanks to Simon Brandmair.
Typo fixes from Frédéric Bothamy and Simon Brandmair.
Improvements in the after installation sections related to how to generate the MD5 (or SHA-1) sums ofbinaries for periodic review.
Updated the after installation sections regarding checksecurity configuration (was out of date).
Revisión 3-3
June 2005
JavierFernández-Sanguino
Peña<jfs@debian.org>
Added a code snippet to use grep-available to generate the list of packages depending on Perl. As requested
in #302470.
Rewrite of the section on network services (which ones are installed and how to disable them).
Added more information to the honeypot deployment section mentioning useful Debian packages.
Revisión 3-2
March 2005
JavierFernández-Sanguino
Peña<jfs@debian.org>
Expanded the PAM configuration limits section.
Added information on how to use pam_chroot for openssh (based on pam_chroot's README).
Fixed some minor issues reported by Dan Jacobson.
Updated the kernel patches information partially based on a patch from Carlo Perassi and also by adding
deprecation notes and new kernel patches available (adamantix).
Included patch from Simon Brandmair that fixes a sentence related to login failures in terminal.
Added Mozilla/Thunderbird to the valid GPG agents as suggested by Kapolnai Richard.
Expanded the section on security updates mentioning library and kernel updates and how to detect whenservices need to be restarted.
Rewrote the firewall section, moved the information that applies to woody down and expand the othersections including some information on how to manually set the firewall (with a sample script) and how
to test the firewall configuration.
Added some information preparing for the 3.1 release.
Added more detailed information on kernel upgrades, specifically targeted at those that used the old installation system.
Added a small section on the experimental apt 0.6 release which provides package signing checks. Movedold content to the section and also added a pointer to changes made in aptitude.
Typo fixes spotted by Frédéric Bothamy.
Revisión 3-1
January 2005
JavierFernández-Sanguino
Peña<jfs@debian.org>
Added clarification to ro /usr with patch from Joost van Baal.
Apply patch from Jens Seidel fixing many typos.
FreeSWAN is dead, long live OpenSWAN.
Added information on restricting access to RPC services (when they cannot be disabled) also included
patch provided by Aarre Laakso.
Update aj's apt-check-sigs script.
Apply patch Carlo Perassi fixing URLs.
Apply patch from Davor Ocelic fixing many errors, typos, urls, grammar and FIXMEs. Also adds some
additional information to some sections.
Rewrote the section on user auditing, highlight the usage of script which does not have some of the issues
associated to shell history.
Revisión 3-0
December 2004
JavierFernández-Sanguino
Peña<jfs@debian.org>
Rewrote the user-auditing information and include examples on how to use script.

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Revisión 2-99

March 2004

JavierFernández-Sanguinoña<jfs@debian.org>

Pe-

Added information on references in DSAs and CVE-Compatibility.
Added information on apt 0.6 (apt-secure merge in experimental).
Fixed location of Chroot daemons HOWTO as suggested by Shuying Wang.
Changed APACHECTL line in the Apache chroot example (even if its not used at all) as suggested by
Leonard Norrgard.
Added a footnote regarding hardlink attacks if partitions are not setup properly.
Added some missing steps in order to run bind as named as provided by Jeffrey Prosa.
Added notes about Nessus and Snort out-of-dateness in woody and availability of backported packages.
Added a chapter regarding periodic integrity test checks.
Clarified the status of testing regarding security updates (Debian bug 233955).
Added more information regarding expected contents in securetty (since it's kernel specific).
Added pointer to snoopylogger (Debian bug 179409).
Added reference to guarddog (Debian bug 170710).
apt-ftparchive is in apt-utils, not in apt (thanks to Emmanuel Chantreau for pointing this out).
Removed jvirus from AV list.
Revisión 2-98
JavierFernández-Sanguino
Peña<jfs@debian.org>
Fixed URL as suggested by Frank Lichtenheld.
Fixed PermitRootLogin typo as suggested by Stefan Lindenau.
Revisión 2-97
September 2003
JavierFernández-Sanguino
Peña<jfs@debian.org>
Added those that have made the most significant contributions to this manual (please mail me if you think
you should be in the list and are not).
Added some blurb about FIXME/TODOs.
Moved the information on security updates to the beginning of the section as suggested by Elliott Mitchell.
Added grsecurity to the list of kernel-patches for security but added a footnote on the current issues with
it as suggested by Elliott Mitchell.
Removed loops (echo to 'all') in the kernel's network security script as suggested by Elliott Mitchell.
Added more (up-to-date) information in the antivirus section.
Rewrote the buffer overflow protection section and added more information on patches to the compiler
to enable this kind of protection.
Revisión 2-96
August 2003
JavierFernández-Sanguino
Peña<jfs@debian.org>
Removed (and then re-added) appendix on chrooting Apache. The appendix is now dual-licensed.
Revisión 2-95
June 2003
JavierFernández-Sanguino
Peña<jfs@debian.org>
Fixed typos spotted by Leonard Norrgard.
Added a section on how to contact CERT for incident handling (Capítulo 11, After the compromise (incident response)).
More information on setting up a Squid proxy.
Added a pointer and removed a FIXME thanks to Helge H. F.
Fixed a typo (save_inactive) spotted by Philippe Faes.
Fixed several typos spotted by Jaime Robles.
Revisión 2-94
April 2003
JavierFernández-Sanguino
Peña<jfs@debian.org>
Following Maciej Stachura's suggestions I've expanded the section on limiting users.
Fixed typo spotted by Wolfgang Nolte.
Fixed links with patch contributed by Ruben Leote Mendes
Added a link to David Wheeler's excellent document on the footnote about counting security vulnerabilities.
Revisión 2-93
March 2003
FrédéricSchütz<schutz@mathgen.ch>145

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rewrote entirely the section of ext2 attributes (lsattr/chattr)
Revisión 2-92
February 2003

JavierFernández-Sanguino
Peña<jfs@debian.org>, FrédéricSchütz<schutz@mathgen.ch>
Merge section 9.3 ("useful kernel patches") into section 4.13 ("Adding kernel patches"), and added somecontent.
Added a few more TODOs.
Added information on how to manually check for updates and also about cron-apt. That way Tiger is not
perceived as the only way to do automatic update checks.
Slightly rewrite of the section on executing a security updates due to Jean-Marc Ranger comments.
Added a note on Debian's installation (which will suggest the user to execute a security update right after
installation).
Revisión 2-91
January/February 2003
JavierFernández-Sanguino
Peña<jfs@debian.org>
Added a patch contributed by Frédéric Schütz.
Added a few more references on capabilities thanks to Frédéric.
Slight changes in the bind section adding a reference to BIND's 9 online documentation and proper references in the first area (Hi Pedro!).
Fixed the changelog date - new year :-).
Added a reference to Colin's articles for the TODOs.
Removed reference to old ssh+chroot patches.
More patches from Carlo Perassi.
Typo fixes (recursive in Bind is recursion), pointed out by Maik Holtkamp.
Revisión 2-9
December 2002
JavierFernández-Sanguino
Peña<jfs@debian.org>
Reorganized the information on chroot (merged two sections, it didn't make much sense to have themseparated).
Added the notes on chrooting Apache provided by Alexandre Ratti.
Applied patches contributed by Guillermo Jover.
Revisión 2-8
JavierFernández-Sanguino
Peña<jfs@debian.org>
Applied patches from Carlo Perassi, fixes include: re-wrapping the lines, URL fixes, and fixed some FIXMEs.
Updated the contents of the Debian security team FAQ.
Added a link to the Debian security team FAQ and the Debian Developer's reference, the duplicated sections might (just might) be removed in the future.
Fixed the hand-made auditing section with comments from Michal Zielinski.
Added links to wordlists (contributed by Carlo Perassi).
Fixed some typos (still many around).
Fixed TDP links as suggested by John Summerfield.
Revisión 2-7
JavierFernández-Sanguino
Peña<jfs@debian.org>
Some typo fixes contributed by Tuyen Dinh, Bartek Golenko and Daniel K. Gebhart.
Note regarding /dev/kmem rootkits contributed by Laurent Bonnaud.
Fixed typos and FIXMEs contributed by Carlo Perassi.
Revisión 2-6
September 2002
CrisTillman<tillman@voicetrak.com>
Changed around to improve grammar/spelling.
s/host.deny/hosts.deny/ (1 place).
Applied Larry Holish's patch (quite big, fixes a lot of FIXMEs).
Revisión 2-5.1
September 2002
JavierFernández-Sanguino
Peña<jfs@debian.org>
Fixed minor typos submitted by Thiemo Nagel.

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Added a footnote suggested by Thiemo Nagel.
Fixed an URL link.
Revisión 2-5.0
August 2002

JavierFernández-Sanguino
Peña<jfs@debian.org>
Applied a patch contributed by Philipe Gaspar regarding the Squid which also kills a FIXME.
Yet another FAQ item regarding service banners taken from the debian-security mailing list (thread "Telnet
information" started 26th July 2002).
Added a note regarding use of CVE cross references in the How much time does the Debian security
team... FAQ item.
Added a new section regarding ARP attacks contributed by Arnaud "Arhuman" Assad.
New FAQ item regarding dmesg and console login by the kernel.
Small tidbits of information to the signature-checking issues in packages (it seems to not have gotten pastbeta release).
New FAQ item regarding vulnerability assessment tools false positives.
Added new sections to the chapter that contains information on package signatures and reorganized it asa new Debian Security Infrastructure chapter.
New FAQ item regarding Debian vs. other Linux distributions.
New section on mail user agents with GPG/PGP functionality in the security tools chapter.
Clarified how to enable MD5 passwords in woody, added a pointer to PAM as well as a note regarding
the max definition in PAM.
Added a new appendix on how to create chroot environments (after fiddling a bit with makejail and fixing,
as well, some of its bugs), integrated duplicate information in all the appendix.
Added some more information regarding SSH chrooting and its impact on secure file transfers. Some information has been retrieved from the debian-security mailing list (June 2002 thread: secure file transfers).
New sections on how to do automatic updates on Debian systems as well as the caveats of using testingor unstable regarding security updates.
New section regarding keeping up to date with security patches in the Before compromise section as well
as a new section about the debian-security-announce mailing list.
Added information on how to automatically generate strong passwords.
New section regarding login of idle users.
Reorganized the securing mail server section based on the Secure/hardened/minimal Debian (or "Why isthe base system the way it is?") thread on the debian-security mailing list (May 2002).
Reorganized the section on kernel network parameters, with information provided in the debian-security
mailing list (May 2002, syn flood attacked? thread) and added a new FAQ item as well.
New section on how to check users passwords and which packages to install for this.
New section on PPTP encryption with Microsoft clients discussed in the debian-security mailing list (April
2002).
Added a new section describing what problems are there when binding any given service to a specific
IP address, this information was written based on the Bugtraq mailing list in the thread: Linux kernel 2.4"weak end host" issue (previously discussed on debian-security as "arp problem") (started on May 9th
2002 by Felix von Leitner).
Added information on ssh protocol version 2.
Added two subsections related to Apache secure configuration (the things specific to Debian, that is).
Added a new FAQ related to raw sockets, one related to /root, an item related to users' groups and anotherone related to log and configuration files permissions.
Added a pointer to a bug in libpam-cracklib that might still be open... (need to check).
Added more information regarding forensics analysis (pending more information on packet inspection
tools such as tcpflow).
Changed the "what should I do regarding compromise" into a bullet list and included some more stuff.
Added some information on how to set up the Xscreensaver to lock the screen automatically after theconfigured timeout.
Added a note related to the utilities you should not install in the system. Included a note regarding Perl andwhy it cannot be easily removed in Debian. The idea came after reading Intersect's documents regarding
Linux hardening.

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Added information on lvm and journalling file systems, ext3 recommended. The information there might
be too generic, however.
Added a link to the online text version (check).
Added some more stuff to the information on firewalling the local system, triggered by a comment madeby Hubert Chan in the mailing list.
Added more information on PAM limits and pointers to Kurt Seifried's documents (related to a post by
him to Bugtraq on April 4th 2002 answering a person that had ``discovered'' a vulnerability in Debian
GNU/Linux related to resource starvation).
As suggested by Julián Muñoz, provided more information on the default Debian umask and what a user
can access if given a shell in the system (scary, huh?).
Included a note in the BIOS password section due to a comment from Andreas Wohlfeld.
Included patches provided by Alfred E. Heggestad fixing many of the typos still present in the document.
Added a pointer to the changelog in the Credits section since most people who contribute are listed here
(and not there).
Added a few more notes to the chattr section and a new section after installation talking about system
snapshots. Both ideas were contributed by Kurt Pomeroy.
Added a new section after installation just to remind users to change the boot-up sequence.
Added some more TODO items provided by Korn Andras.
Added a pointer to the NIST's guidelines on how to secure DNS provided by Daniel Quinlan.
Added a small paragraph regarding Debian's SSL certificates infrastructure.
Added Daniel Quinlan's suggestions regarding ssh authentication and exim's relay configuration.
Added more information regarding securing bind including changes suggested by Daniel Quinlan and an
appendix with a script to make some of the changes commented on in that section.
Added a pointer to another item regarding Bind chrooting (needs to be merged).
Added a one liner contributed by Cristian Ionescu-Idbohrn to retrieve packages with tcpwrappers support.
Added a little bit more info on Debian's default PAM setup.
Included a FAQ question about using PAM to provide services without shell accounts.
Moved two FAQ items to another section and added a new FAQ regarding attack detection (and compromised systems).
Included information on how to set up a bridge firewall (including a sample Appendix). Thanks to Francois
Bayart who sent this to me in March.
Added a FAQ regarding the syslogd's MARK heartbeat from a question answered by Noah Meyerhans
and Alain Tesio in December 2001.
Included information on buffer overflow protection as well as some information on kernel patches.
Added more information (and reorganized) the firewall section. Updated the information regarding theiptables package and the firewall generators available.
Reorganized the information regarding log checking, moved logcheck information from host intrusion
detection to that section.
Added some information on how to prepare a static package for bind for chrooting (untested).
Added a FAQ item regarding some specific servers/services (could be expanded with some of the recommendations from the debian-security list).
Added some information on RPC services (and when it's necessary).
Added some more information on capabilities (and what lcap does). Is there any good documentation on
this? I haven't found any documentation on my 2.4 kernel.
Se corrigieron algunos errores ortográficos.
Revisión 2-4
June 2002
JavierFernández-Sanguino
Peña<jfs@debian.org>
Reescrita la parte de la sección BIOS.
Revisión 2-3.1
April 2002
JavierFernández-Sanguino
Peña<jfs@debian.org>
La mayoría de los archivos se encuentran marcados con la etiqueta file.
Fallo de ortografía observado por Edi Stojicevi.
La sección de herramientas de auditoría remota se ha modificado ligeramente.
Se añadieron algunas piezas de PORHACER.

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Se añadió más información con respecto a impresoras y los archivos de configuración de cups (tomado
de un hilo en debian-security).
Se añadió un parche suministrado por Jesus Climent relacionado con el acceso de usuarios válidos delsistema en Proftpd cuando se ha configurado como servidor anónimo.
Pequeños cambios sobre divisiones de esquemas para el caso especial de servidores de correo.
Se añadió Hacking Linux Exposed para la sección de los libros.
Error en directorio notificado por Eduardo Pérez Ureta.
Error ortográfico /etc/ssh en la checklist notificado por Edi Stojicevi.
Revisión 2-3.0
April 2002
JavierFernández-Sanguino
Peña<jfs@debian.org>
Cambio de ubicación del fichero de configuración de dpkg.
Alexander eliminado de la información de contacto.
Se añadieron direcciones de correo alternativas.
Se arregló la dirección de correo de Alexander (aún entre comentarios).
Se arregló la ubicación de la llave publicada de la distribución (gracias a Pedro Zorzenon por señalarlo).
Revisión 2-2
April 2002
JavierFernández-Sanguino
Peña<jfs@debian.org>
Se arreglaron errores ortográficos gracias a Jamin W. Collins.
Se añadió una referencia a la página de manual de apt-extracttemplate (documenta la configuración
APT::ExtracTemplate).
Se añadió la sección sobre SSH restringido. Información basada en los correos enviados por Mark Janssen,
Christian G. Warden y Emmanuel Lacour en la lista de correo debian-security.
Se añadió información sobre programas antivirus.
Se añadió un FAQ: las bitácoras de su debido al cron que se ejecuta como root.
Revisión 2-1
April 2002
JavierFernández-Sanguino
Peña<jfs@debian.org>
Se eliminó el ARREGLAME de lshell gracias a Oohara Yuuma.
Se agregó un paquete para sXid y se eliminaron comentarios desde que éste se encuentra disponible.
Se corrigieron algunos fallos ortográficos descubiertos por Oohara Yuuma.
ACID está ahora disponible en Debian (en el paquete acidlab). Gracias a Oohara Yuuma por notificarlo.
Se arreglaron los URLs de seguridad de Linux (gracias a Dave Wreski por comentarlo). versión 2.0 cuando
todos los ARREGLAMEs estaban cambiados, pero los eliminé de los números 1.9X :(Revisión 2-0
March 2002
JavierFernández-Sanguino
Peña<jfs@debian.org>
Se convirtió el HOWTO a un manual (ahora puedo decir apropiadamente LEJM).
Se añadió más información con respecto a los tcpwrappers y a Debian (ahora muchos servicios estáncompilados con soporte para ellos, así que ya no es problema de inetd).
Se aclaró la información sobre como deshabilitar el servicio rpc para hacerlo más consistente (la información rpc hacía referencia a update-rc.d).
Se añadieron pequeñas notas sobre lprng.
Se agregó alguna información sobre servidores comprometidos (aún muy rústico).
Se corrigieron fallos ortográficos detectados por Mark Bucciarelli.
Se añadieron algunos pasos en la recuperación de password para proteger los casos en que el administrador
tiene paranoid-mode=on.
Se añadió información para colocar paranoid-mode=on cuando el login está en la consola.
Nuevo párrafo para introducir las configuraciones de servicios.
Se reorganizó la sección Después de la instalación. Además ésta se descompone en varios temas más,facilitando la lectura.
Se escribió información sobre como montar un cortafuegos con el montaje estándar de Debian 3.0 (paquete
iptables).
Un pequeño párrafo explicando por qué la instalación estando conectado a Internet no es buena idea ycómo evitar esto usando las herramientas Debian.
Un pequeño párrafo referenciando a un trabajo publicado en el IEEE sobre como aplicar a tiempo parchesde seguridad.

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Un apéndice sobre como montar una máquina snort Debian basada en lo que Vladimir envió a la lista deseguridad de debian-security (3 de septiembre de 2001).
Información sobre como logcheck se monta en Debian y como puede ser usado en el sistema HIDS.
Información sobre la contabilidad del usuario y los beneficios de los análisis.
Se incluyó la configuración apt.conf para leer únicamente /usr copiado del correo de Olaf Meeuwissen ala lista de correos debian-security.
Nueva sección en VPN con algunas indicaciones y paquetes disponibles en Debian (se necesita contenido
de como establecer VPNs y problemas específicos de Debian), basado en los envíos de Jaroslaw Tabor ySamuli Suonpaa a la lista debian-security.
Una corta nota con respecto a algún programa que automáticamente construye jaulas para el cambio dedirectorio raíz.
Nuevo artículo FAQ con respecto a identd basado en una discusión en la lista de correo debian-security(febrero 2002, empezado por Johannes Weiss).
Nuevo artículo FAQ con respecto al inetd basada en una discusión en la lista de correo debian-security(febrero 2002).
Se introdujo una nota en rcconf en la sección "deshabilitar servicios".
Varió el enfoque con respecto a LKM, gracias a Philipe Gaspar.
Se añadieron enlaces a documentos del CERT y fuentes de información de Couterpane.
Revisión 1-99
January 2002
JavierFernández-Sanguino
Peña<jfs@debian.org>
Se añadió un nuevo FAQ con respecto al tiempo de arreglo de vulnerabilidades de seguridad.
Secciones FAQ reorganizadas.
Se comenzó a escribir la sección con respecto al firewalling en Debian GNU/Linux (podría ser ampliado
un poco).
Eliminados errores ortográficos detectados por Matt Kraai.
Cambiada la información de DNS.
Se agregó información sobre whisker y nbtscan para la sección de auditoría.
Se modificó algún URL erróneo.
Revisión 1-98
January 2002
JavierFernández-Sanguino
Peña<jfs@debian.org>
Se añadió una nueva sección con respecto a la auditoría usando Debian GNU/Linux.
Se añadió información con respecto al demonio finger tomada de la lista de correo de seguridad.
Revisión 1-97
January 2002
JavierFernández-Sanguino
Peña<jfs@debian.org>
Se cambió el enlace a Linux Trustees.
Se corrigieron fallos ortográficos (parches de Oohara Yuuma y Pedro Zorzenon).
Revisión 1-96
December 2001
JavierFernández-Sanguino
Peña<jfs@debian.org>
Se reorganizó el servicio de instalación y se añadieron y eliminaron algunas notas.
Se añadieron algunas notas con respecto al uso de sistemas de comprobación de integridad como herramientas de detección de intrusos.
Se añadió un capítulo con respecto firmas de paquetes.
Revisión 1-95
December 2001
JavierFernández-Sanguino
Peña<jfs@debian.org>
Se añadieron notas con respecto a la seguridad de Squid enviadas por Philipe Gaspar.
Cambios de enlaces sobre rookits gracias a Philipe Gaspar.
Revisión 1-94
November 2001
JavierFernández-Sanguino
Peña<jfs@debian.org>
Se añadieron algunas notas con respecto a Apache y Lpr/lpng.
Se añadió alguna información con respecto a noexec y particiones de acceso aleatorio.
Reescritura de como puede el usuario ayudar en los asuntos de seguridad Debian (FAQ).
Revisión 1-93
November 2001
JavierFernández-Sanguino
Peña<jfs@debian.org>
Se arregló el sitio donde se encuentra el programa de correo.

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Se añadieron algunos nuevos elementos a las FAQ.
Revisión 1-92
October 2001

JavierFernández-Sanguino
Peña<jfs@debian.org>
Añadió una pequeña sección de como se maneja la seguridad en Debian.
Clarificación sobre las contraseñas MD5 (gracias a `rocky').
Añadida un poco más de información con respecto a harden-X de Stephen Egmond.
Se añadieron algunos nuevos elementos a las FAQ.
Revisión 1-91
October 2001
JavierFernández-Sanguino
Peña<jfs@debian.org>
Añadida un poco de información forense enviada por Yotam Rubin.
Añadió información de como construir una red trampa con Debian GNU/Linux.
Añadidas unas cosas a hacer más.
Corrección de más errores ortográficos (gracias a Yotam).
Revisión 1-9
October 2001
JavierFernández-Sanguino
Peña<jfs@debian.org>
Se añadió un parche para arreglar errores de ortografía y un poco de nueva información (contribuido por
Yotam Rubin).
Se añadieron referencias a otra documentación en línea (y no en línea) tanto en una única sección (vea
“Sea consciente de los problemas de seguridad general”) como dentro de algunas secciones.
Añadida alguna información sobre como configurar opciones de bind para restringir el acceso al servidor
de DNS.
Agregada información de como bastionar un sistema de Debian automáticamente (con respecto al paquete
harden y bastille).
Eliminados algunos PORHACER hechos y añadidos otros nuevos.
Revisión 1-8
October 2001
JavierFernández-Sanguino
Peña<jfs@debian.org>
Se añadió la lista de usuario/grupo por defecto proporcionada por Joey Hess (enviada a la lista de correo
debian-security).
Se añadió información sobre los rootkits LKM (“LKM - Loadable Kernel Modules (módulos cargables
en el núcleo)”) contribuida por Philipe Gaspar.
Se agregó información sobre Proftp contribuida por Emmanuel Lacour.
Se recuperó el apéndice checklist de Era Eriksson.
Se añadieron algunos artículos nuevos al PORHACER y se arreglaron otros.
Se incluyeron manualmente los parches de Era dado que no se habían incluido en la versión anterior.
Revisión 1-7
September 2001
JavierFernández-Sanguino
Peña<jfs@debian.org>, EraEriksson<era@iki.fi>
Se arreglaron errores ortográficos y se cambiaron algunas palabras.
Cambios menores de las etiquetas para seguir removiendo las tt, y sustituirlas por las etiquetas de prgn/
package.
Revisión 1-6
August 2001
JavierFernández-Sanguino
Peña<jfs@debian.org>
Se añadió el enlace al documento como se publicó en el DDP (debería reemplazar el original en el futuro
cercano).
Comenzó un mini-FAQ (debería extenderse) con algunas preguntas recuperadas de mi buzón.
Se añadió información general a considerar cuando se está bastionando.
Se añadió un párrafo con respecto al envío de correo local (entrante).
Se añadieron enlaces de información.
Se añadió información con respecto al servicio de impresión.
Se añadió una lista de chequeo de bastionado.
Se reorganizó información de NIS y RPC.
Se añadieron algunas notas tomadas mientras está leyendo este documento en mi nuevo visor :)Se arreglaron algunas líneas mal formateadas.
Se corrigieron algunos errores ortográficos.

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Se añadieron ideas Geniales/Paranoícas contribuidas por Gaby Schilders.
Revisión 1-5
May 2001
JavierFernández-Sanguino
Peña<jfs@debian.org>, JosipRodin<joy@debian.org>
Se añadieron párrafos relacionados con bind y algunos ARREGLAMEs.
Revisión 1-4
May 2001
JavierFernández-Sanguino
Peña<jfs@debian.org>
Se revisaron algunos setuid pequeños.
Cambios menores.
Se averiguó como usar sgml2txt -f para la versión txt.
Revisión 1-3
March 2001
JavierFernández-Sanguino
Peña<jfs@debian.org>
Se añadió una actualización de seguridad después del párrafo de la instalación.
Se añadió un párrafo del proftpd.
En ésta ocasión se escribió algo sobre XDM, disculpas por el anterior.
Revisión 1-2
December 2000
JavierFernández-Sanguino
Peña<jfs@debian.org>
Muchas correcciones de gramática por James Treacy, nuevo párrafo de XDM.
Revisión 1-1
December 2000
JavierFernández-Sanguino
Peña<jfs@debian.org>
Errores ortográficos, cambios varios.
Revisión 1-0
December 2000
JavierFernández-Sanguino
Peña<jfs@debian.org>
Versión inicial.

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Apéndice B. Appendix
El proceso de fortalecimiento es manejado paso a paso
Below is a post-installation, step-by-step procedure for hardening a Debian 2.2 GNU/Linux system. Thisis one possible approach to such a procedure and is oriented toward the hardening of network services.
It is included to show the entire process you might use during configuration. Also, see “Lista de chequeode la configuración”.
• Install the system, taking into account the information regarding partitioning included earlier in thisdocument. After base installation, go into custom install. Do not select task packages.
• Vaya através de dselect y remueva lo que no es necesario, si no selecciono paquetes antesv de ser(I)instalados. Deje la menor cantidad de programas necesarios en el servidor.
• Actualice todo el software desde los utimos paquetes disponibles en security.debian.org como se expuso
previamente en update.
• implemente las sugerencias presentadas en este manual , considerando las cuotas del usuario, definiciones de login y lilo.
• Make a list of services currently running on your system. Try:

$ ps -aux$ netstat -pn -l -A inet
# /usr/sbin/lsof -i | grep LISTEN
Necesitará instalar lsof-2.2 para que el tercer comando funcione (corralo como root). Debería ser consiente que lsof puede transladar la palabra LISTEN a su configuraciones de los locales.
• In order to remove unnecessary services, first determine what package provides the service and how it
is started. This can be accomplished by checking the program that listens in the socket. The following
shell script, which uses the programs lsof and dpkg, does just that:#!/bin/sh# ARREGLAME: this is quick and dirty; replace with a more robust script snippet
for i in `sudo lsof -i | grep LISTEN | cut -d " " -f 1 |sort -u` ; dopack=`dpkg -S $i |grep bin |cut -f 1 -d : | uniq`
echo "Service $i is installed by $pack";
init=`dpkg -L $pack |grep init.d/ `if [ ! -z "$init" ]; thenecho "and is run by $init"fidone• Once you find any unwanted services, remove the associated package (with dpkg --purge), or disable
the service from starting automatically at boot time using update-rc.d (see “Deshabilitar servicios”).
• For inetd services (launched by the superdaemon), check which services are enabled in /etc/
inetd.conf using:153

Appendix$ grep -v "^#" /etc/inetd.conf | sort -u
Then disable those services that are not needed by commenting out the line that includes them in /etc/
inetd.conf, removing the package, or using update-inetd.
• Si se tienen servicios de cubierta (usando estos /usr/sbin/tcpd) revise que los /etc/hosts.allow
y /etc/hosts.deny estén configurados acorde a su política de servicios.
• If the server uses more than one external interface, depending on the service, you may want to limit theservice to listen on a specific interface. For example, if you want internal FTP access only, make theFTP daemon listen only on your management interface, not on all interfaces (i.e, 0.0.0.0:21).
• Reinicie la máquina, o cámbiela para entrar a single user y vuelva a multiusuario con# init 1(....)# init 2• Revise los servicios disponibles actualmente, y si es necesario repita estos pasos de nuevo.
• Instale, ahora los servicios necesarios, si usted todavia no lo ha hecho, y configurelos apropiadamente.
• Use the following shell command to determine what user each available service is running as:# for i in `/usr/sbin/lsof -i |grep LISTEN |cut -d " " -f 1 |sort -u`; \> do user=`ps ef |grep $i |grep -v grep |cut -f 1 -d " "` ; \> echo "Service $i is running as user $user"; done
Consider changing these services to a specific user/group and maybe chroot'ing them for increased security. You can do this by changing the /etc/init.d scripts which start the service. Most services
in Debian use start-stop-daemon, which has options (--change-uid and --chroot) for accomplishing this. A word of warning regarding the chroot'ing of services: you may need to put all the files
installed by the package (use dpkg -L) providing the service, as well as any packages it depends on, inthe chroot'ed environment. Information about setting up a chroot environment for the ssh program canbe found in “Chroot environment for SSH”.
• Repita los pasos anteriores para revisar que los solamente los servicios deseados corran, y que lo hagancomo el usuario o grupo desea.
• Pruebe la instalación de servicios para saber si trabajan como se esperaba.
• Compruebe el sistema usando un revisor de aseguramiento de vulnerabilidades (como nessus) para
determinar las vulnerabilidades del sistema (configuraciones erróneas, servicios viejos o innecesarios)• Install network and host intrusion measures like snort and logcheck.
• Repita el paso del revisor de red y verifique que los sistemas de detección de intrusiónd trabajan correctamente.
para los verdaderos paranoicos, considere también lo siguiente:• Agregar capacidades de cortafuegos al sistema, aceptando conexiones entrantes solamente para los servicios ofrecidos y limite conexiones salientes para los autorizados.

154

Appendix• Vuelva a revisar la instalación con una nueva herramienta de revisión de vulnerabilidades.
• Using a network scanner, check outbound connections from the system to an outside host and verify
that unwanted connections do not find their way out.
ARREGLAME: Este procedimiento considera el servicio de fortelecimiento, pero no el sistema de fortalecimiento a nivel de usuario, incluir informaciones con respecto al chequeo de permisos del usuario,
archivos setuid y paros en el sistema usando el sistema de archivos.

Lista de chequeo de la configuración
Este apéndice reitera puntos de otras secciones de este manual condensando en un formato de lista de chequeo. El propósito es ser un resumen para quienes ya han leído el manual. También hay otras buenas listas
de chequeo disponibles, Kurt Seifried tiene una configuración basada en un curso en http://seifried.org/security/os/linux/20020324-securing-linux-step-by-step.h tml.
ARREGLAME: esto es basado en v1.4 del manual, y podría necesitar actualizarse.
• límitar la entrada para un acceso físico.
• Enable a password in the BIOS.
• Disable floppy/cdrom/... booting in the system's BIOS.
• enviar una contraseña LILO o GRUB (/etc/lilo.conf o /boot/grub/menu.lst, respectivamente); revisar que la configuración de los archivos LILO o GRUB sea de lectura-protegida.
• partitura• Separe los datos del suscriptor, no sistema de datos, y cambiar rápaidamente los datos del tiempo derecorrido de sus datos de particón.
• enviar nosuid,noexec,nodev montar opciones en /etc/fstab sobre la partición ext2 tal
como /tmp.
• Higiene de las contraseñas y aseguramiento a la entrada• Defina una buena contraseña para el administrador• Instale y use PAM• Agregue MD5 para soportar PAM y asegúrese que: (en términos generales) las entradas en el archivo /etc/pam.d/ que permiten acceso a la máquina, tengan en el segundo campo del archivo
pam.d definidoc com "requisite" o "required".
• Cambie /etc/pam.d/login para permitir solamente entradas locales al administrador.
• también marque las tty: atuorizadas en /etc/security/access.conf y generalmente configurar este archivo para limitar la entrada del administrador tanto como sea posible.
• Agregue pam_limits.so si usted desea habilitar límites para usuarios.
• Cambie /etc/pam.d/passwd: especifique un tamaño mínimo de contraseña.(podrían ser 6
caracteres) y habilite md5• agregue un grupo wheel a /etc/group si desea, agregue pam_wheel. para entrar a /etc/
pam.d/su155

Appendix• para especificar controles por usuario iniciales, use pam_listfile cuando sea apropiado.
• tenga un archivo /etc/pam.d/othery montarlo con seguridad alta.
• ponga límites en /etc/security/limits.conf (note que /etc/limits no es usado si usted
está utilizando PAM)
• Restrinja /etc/login.defs; también, si usted habilita MD5 y/o PAM, asegurese que usted hace
los cambios carrespondientes ahí también.
• Tighten up /etc/pam.d/login• inhabilite el acceso a root por ftp en /etc/ftpusers• Disable network root login; use su(1) or sudo(1). (consider installing sudo)• Use PAM para hacer cumplir las restrinciones adicionales sobre logins?
• Otros asuntos de la seguridad local.
• Cambios de Kernel (ver kernel-conf)• Parches del Kernel (ver kernel-patches)• restrinja los permisos de los archivos de bitácora (/var/log/{last,fail}log, Apache logs)• Verifique que la revisión de setuid está habilitada en /etc/checksecurity.conf• Considere hacer algunos archivos log append-only y configurar archivos inmutables usando chattr(ext2 filesystems únicamente)• Active integridad de archivos (vea check-integ). instale debsums• Enviar bitácoras a una impresora local?
• Queme su configuración sobre un CD de arranque y e inicie la máquina desde este?
• incapacitar módulos del kernel?
• Limite el acceso a la red• Instale y configure ssh (se sugiere PermitRootLogin No /etc/ssh/sshd_config, PermitEmptyPasswords No; note que hay otras sugerencias en este texto)• Disable or remove in.telnetd, if installed• Generalmente, inhabilite los servicios gratuitos en /etc/inetd.conf usando update-inetd--disable (or inhabilite todo inetd, o use un reemplazo como xinetd or rlinetd)• Disable other gratuitous network services; ftp, DNS, WWW etc should not be running if you do notneed them and monitor them regularly. In most cases mail should be running but configured for local
delivery only.
• Para los servicios que necesite, no use solamente los programas comunes, busque más versiones
seguras enviadas por Debian (o busque otros recursos). Para cualquier servicio que usted termine
usando, asegúrese de entender los riesgos.
156

Appendix• Monte celdas de directorio raíz distintos para usuarios externos y demonios.
• Configure firewall and tcpwrappers (i.e. hosts_access(5)); note trick for /etc/hosts.deny intext.
• si usted corre ftp, monte su servidor ftpd y siempre corra chrooted para el directorio raíz del usuario• Si usted corre X, inhabilite la autenticación xhost y use ssh a cambio, mejor aún, inhabilite X remotosi usted puede (adicione -nolisten tcp a X desde la línea de comandos y apague XDMCP en /etc/
X11/xdm/xdm-config configurando requestPort a 0)
• Inhabilite el acceso externo a impresoras.
• Use tunel para sesiones de IMAP o POP a través de SSL o ssh; instale stunnel si usted quiere proporcionar este servicio a usuarios de correo remoto• Monte un loghost y configure otras máquinas para enviar logs a ésta (/etc/syslog.conf)• asegure BIND, Sendmail, y otros demonios complejos (Configure una celda de cambio de directorio ;corra como non-root pseudo-user)• Install tiger or a similar network intrusion detection tool.
• Install snort or a similar network intrusion detection tool.v• Prescinda de NIS y RPC si usted puede (inhabilite portmap).
• Políticas• Eduque a los usuarios acerca de los porque y los como de sus políticas. Cuando usted ha prohibido
algo que normalmente está disponible en otros sistemas, suministre documenteción que expliquecomo conseguir resultados similares usando otros medios más seguros.
• Prohiba el uso de protocolos que usa claves en texto plano (telnet, rsh y friends; ftp, imap, http, ...).
• Prohiba programas comoVGAlib.
• Use quotas de disco.
• manténegase informado sobre asuntos de seguridad• Subscríbase a la lista de correo de seguridad• Subscríbase a las actualizaciones de seguridad, adicione a /etc/apt/sources.list una entrada (o entradas) a http://security.debian.org/debian-security• Además recuerde correr periódicamente apt-get update ; apt-get upgrade (tal vez instalar un cronjob?) como se explicó en “Ejecute una actualización de seguridad”.

Montar un IDS aislado
You can easily set up a dedicated Debian system as a stand-alone Intrusion Detection System using snort
and a web-based interface to analyse the intrusion detection alerts:• Instale una base de sistema Debian y no seleccione paquetes adicionales.
157

Appendix• Install one of the Snort versions with database support and configure the IDS to log alerts into the
database.
• Download and install BASE (Basic Analysis and Security Engine), or ACID (Analysis Console for
Intrusion Databases). Configure it to use the same database than Snort.
• Download and install the necessary packages1.
BASE is currently packaged for Debian in acidbase and ACID is packaged as acidlab2. Both provide agraphical WWW interface to Snort's output.
Besides the base installation you will also need a web server (such as apache), a PHP interpreter and a
relational database (such postgresql or mysql) where Snort will store its alerts.
This system should be set up with at least two interfaces: one interface connected to a management LAN(for accessing the results and maintaining the system), and one interface with no IP address attached to
the network segment being analyzed. You should configure the web server to listen only on the interface
connected to the management LAN.
You should configure both interfaces in the standard Debian /etc/network/interfaces configuration file. One (the management LAN) address can be configured as you would normally do. The otherinterface needs to be configured so that it is started up when the system boots, but with no interface address.
You can use the following interface definition:

auto eth0
iface eth0 inet manual
up ifconfig $IFACE 0.0.0.0 up
up ip link set $IFACE promisc on
down ip link set $IFACE promisc off
down ifconfig $IFACE down
The above configures an interface to read all the traffic on the network in a stealth-type configuration. This
prevents the NIDS system to be a direct target in a hostile network since the sensors have no IP addresson the network. Notice, however, that there have been known bugs over time in sensors part of NIDS (forexample see https://lists.debian.org/debian-security-announce/2003/msg00087.html related to Snort) andremote buffer overflows might even be triggered by network packet processing.
You might also want to read the http://www.faqs.org/docs/Linux-HOWTO/Snort-Statistics-HOWTO.html
and the documentation available at the https://www.snort.org/#documents.

Setting up a bridge firewall
This information was contributed by Francois Bayart in order to help users set up a Linux bridge/firewallwith the 2.4.x kernel and iptables. Kernel patches are no more needed as the code was made standard partof the Linux kernel distribution.
To configure the kernel with necessary support, run make menuconfig or make xconfig. In thesection Networking options, enable the following options:[*] Network packet filtering (replaces ipchains)[ ]Network packet filtering debugging (NEW)1
2

Typically the needed packages will be installed through the dependencies
It can also be downloaded from http://www.cert.org/kb/acid/, http://acidlab.sourceforge.net or http://www.andrew.cmu.edu/~rdanyliw/snort/.

158

Appendix<*> 802.1d Ethernet Bridging[*]
netfilter (firewalling) support (NEW)Caution: you must disable this if you want to apply some firewalling rules or else iptables will not work:[ ]Network packet filtering debugging (NEW)

Next, add the correct options in the section IP: Netfilter Configuration. Then, compile and install thekernel. If you want to do it the Debian way, install kernel-package and run make-kpkg to create a custom
Debian kernel package you can install on your server using dpkg. Once the new kernel is compiled andinstalled, install the bridge-utils package.
Once these steps are complete, you can complete the configuration of your bridge. The next section presents two different possible configurations for the bridge, each with a hypothetical network map and thenecessary commands.

A bridge providing NAT and firewall capabilities
The first configuration uses the bridge as a firewall with network address translation (NAT) that protects
a server and internal LAN clients. A diagram of the network configuration is shown below:

Internet ---- router ( 62.3.3.25 ) ---- bridge (62.3.3.26 gw 62.3.3.25 / 192.168.0.
|||---- WWW Server (62.3.3.27 gw 62.3.3.25)||
LAN --- Zipowz (192.168.0.2 gw 192.168.0.1
The following commands show how this bridge can be configured.

# Create the interface br0/usr/sbin/brctl addbr br0# Add the Ethernet interface to use with the bridge/usr/sbin/brctl addif br0 eth0
/usr/sbin/brctl addif br0 eth1
# Start up the Ethernet interface/sbin/ifconfig eth0 0.0.0.0/sbin/ifconfig eth1 0.0.0.0# Configure the bridge ethernet# The bridge will be correct and invisible ( transparent firewall ).
# It's hidden in a traceroute and you keep your real gateway on the# other computers. Now if you want you can config a gateway on your# bridge and choose it as your new gateway for the other computers.
/sbin/ifconfig br0 62.3.3.26 netmask 255.255.255.248 broadcast 62.3.3.31# I have added this internal IP to create my NAT
ip addr add 192.168.0.1/24 dev br0

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Appendix/sbin/route add default gw 62.3.3.25

Añadir capacidades al cortafuegos
A second possible configuration is a system that is set up as a transparent firewall for a LAN with a public
IP address space.
Internet ---- router (62.3.3.25) ---- bridge (62.3.3.26)|||---- WWW Server (62.3.3.28 gw 62.3.3.25)|||---- Mail Server (62.3.3.27 gw 62.3.3.25)
The following commands show how this bridge can be configured.
# Create the interface br0/usr/sbin/brctl addbr br0# Add the Ethernet interface to use with the bridge/usr/sbin/brctl addif br0 eth0
/usr/sbin/brctl addif br0 eth1
# Start up the Ethernet interface/sbin/ifconfig eth0 0.0.0.0/sbin/ifconfig eth1 0.0.0.0# Configure the bridge Ethernet# The bridge will be correct and invisible ( transparent firewall ).
# It's hidden in a traceroute and you keep your real gateway on the# other computers. Now if you want you can config a gateway on your# bridge and choose it as your new gateway for the other computers.
/sbin/ifconfig br0 62.3.3.26 netmask 255.255.255.248 broadcast 62.3.3.31
If you traceroute the Linux Mail Server, you won't see the bridge. If you want access to the bridge withssh, you must have a gateway or you must first connect to another server, such as the "Mail Server", andthen connect to the bridge through the internal network card.

Basic IPtables rules
This is an example of the basic rules that could be used for either of these setups.

Ejemplo B.1. Reglas Iptables
iptables -F FORWARD
iptables -P FORWARD DROP
iptables -A FORWARD -s 0.0.0.0/0.0.0.0 -d 0.0.0.0/0.0.0.0 -m state --state INVALID
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT# Some funny rules but not in a classic Iptables sorry ...

160

Appendix# Limit ICMP
# iptables -A FORWARD -p icmp -m limit --limit 4/s -j ACCEPT# Match string, a good simple method to block some VIRUS very quickly# iptables -I FORWARD -j DROP -p tcp -s 0.0.0.0/0 -m string --string "cmd.exe"# Block all MySQL connection just to be sureiptables -A FORWARD -p tcp -s 0/0 -d 62.3.3.0/24 --dport 3306 -j DROP
# Linux Mail Server Rules# Allow FTP-DATA (20), FTP (21), SSH (22)iptables -A FORWARD -p tcp -s 0.0.0.0/0 -d 62.3.3.27/32 --dport 20:22 -j ACCEPT# Allow the Mail Server to connect to the outside# Note: This is *not* needed for the previous connections# (remember: stateful filtering) and could be removed.
iptables -A FORWARD -p tcp -s 62.3.3.27/32 -d 0/0 -j ACCEPT# WWW Server Rules# Allow HTTP ( 80 ) connections with the WWW server
iptables -A FORWARD -p tcp -s 0.0.0.0/0 -d 62.3.3.28/32 --dport 80 -j ACCEPT# Allow HTTPS ( 443 ) connections with the WWW server
iptables -A FORWARD -p tcp -s 0.0.0.0/0 -d 62.3.3.28/32 --dport 443 -j ACCEPT# Allow the WWW server to go out# Note: This is *not* needed for the previous connections# (remember: stateful filtering) and could be removed.
iptables -A FORWARD -p tcp -s 62.3.3.28/32 -d 0/0 -j ACCEPTSample script to change the default Bind installation.
This script automates the procedure for changing the bind version 8 name server's default installation sothat it does not run as the superuser. Notice that bind version 9 in Debian already does this by default 3 ,and you are much better using that version than bind version 8.
This script is here for historical purposes and to show how you can automate this kind of changes system-wide. The script will create the user and groups defined for the name server and will modify both /etc/default/bind and /etc/init.d/bind so that the program will run with that user. Use withextreme care since it has not been tested thoroughly.
You can also create the users manually and use the patch available for the default init.d script attached tohttp://bugs.debian.org/cgi-bin/bugreport.cgi?bug=157245.

#!/bin/sh# Change the default Debian bind v8 configuration to have it run# with a non-root user and group.
#3

Since version 9.2.1-5. That is, since Debian release sarge.

161

Appendix# DO NOT USER this with version 9, use debconf for configure this instead## WARN: This script has not been tested thoroughly, please# verify the changes made to the INITD script# (c) 2002 Javier Fernandez-Sanguino Pena##
This program is free software; you can redistribute it and/or modify#
it under the terms of the GNU General Public License as published by#
the Free Software Foundation; either version 1, or (at your option)#
any later version.
##
This program is distributed in the hope that it will be useful,#
but WITHOUT ANY WARRANTY; without even the implied warranty of#
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the#
GNU General Public License for more details.
##
Please see the file `COPYING' for the complete copyright notice.
#

restore() {# Just in case, restore the system if the changes failecho "WARN: Restoring to the previous setup since I'm unable to properly changeecho "WARN: Please check the $INITDERR script."
mv $INITD $INITDERR
cp $INITDBAK $INITD}

USER=named
GROUP=named
INITD=/etc/init.d/bind
DEFAULT=/etc/default/bind
INITDBAK=$INITD.preuserchange
INITDERR=$INITD.changeerror
AWKS="awk ' /\/usr\/sbin\/ndc reload/ { print \"stop; sleep 2; start;\"; noprint[ `id -u` -ne 0 ] && {echo "This program must be run by the root user"
exit 1}
RUNUSER=`ps eo user,fname |grep named |cut -f 1 -d " "`if [ "$RUNUSER" = "$USER" ]thenecho "WARN: The name server running daemon is already running as $USER"
echo "ERR: This script will not do any changes to your setup."
exit 1fi
if [ ! -f "$INITD" ]thenecho "ERR: This system does not have $INITD (which this script tries to change
RUNNING=`ps eo fname |grep named`

162

Appendix[ -z "$RUNNING" ] && \echo "ERR: In fact the name server daemon is not even running (is it installecho "ERR: No changes will be made to your system"
exit 1fi# Check if there are options already setup
if [ -e "$DEFAULT" ]thenif grep -q ^OPTIONS $DEFAULT; thenecho "ERR: The $DEFAULT file already has options set."echo "ERR: No changes will be made to your system"fifi# Check if named group exists
if [ -z "`grep $GROUP /etc/group`" ]thenecho "Creating group $GROUP:"
addgroup $GROUP
else
echo "WARN: Group $GROUP already exists. Will not create it"fi# Same for the user
if [ -z "`grep $USER /etc/passwd`" ]thenecho "Creating user $USER:"
adduser --system --home /home/$USER \--no-create-home --ingroup $GROUP \--disabled-password --disabled-login $USER
else
echo "WARN: The user $USER already exists. Will not create it"fi# Change the init.d script# First make a backup (check that there is not already# one there first)
if [ ! -f $INITDBAK ]then
cp $INITD $INITDBAK
fi# Then use it to change it
cat $INITDBAK |eval $AWKS > $INITD# Now put the options in the /etc/default/bind file:cat >>$DEFAULT <<EOF# Make bind run with the user we defined
OPTIONS="-u $USER -g $GROUP"
EOF
echo "WARN: The script $INITD has been changed, trying to test the changes."163

Appendix

echo "Restarting the named daemon (check for errors here)."$INITD restart
if [ $? -ne 0 ]thenecho "ERR: Failed to restart the daemon."restoreexit 1fi
RUNNING=`ps eo fname |grep named`
if [ -z "$RUNNING" ]thenecho "ERR: Named is not running, probably due to a problem with the changes."
restoreexit 1fi# Check if it's running as expected
RUNUSER=`ps eo user,fname |grep named |cut -f 1 -d " "`if [ "$RUNUSER" = "$USER" ]thenecho "All has gone well, named seems to be running now as $USER."
else
echo "ERR: The script failed to automatically change the system."echo "ERR: Named is currently running as $RUNUSER."
restoreexit 1fiexit 0The previous script, run on Woody's (Debian 3.0) custom bind (version 8), will modify the initd file after
creating the 'named' user and group and will

Security update protected by a firewall
After a standard installation, a system may still have some security vulnerabilities. Unless you can download updates for the vulnerable packages on another system (or you have mirrored security.debian.org for
local use), the system will have to be connected to the Internet for the downloads.
However, as soon as you connect to the Internet you are exposing this system. If one of your local services
is vulnerable, you might be compromised even before the update is finished! This may seem paranoid but,in fact, analysis from the http://www.honeynet.org has shown that systems can be compromised in lessthan three days, even if the system is not publicly known (i.e., not published in DNS records).
When doing an update on a system not protected by an external system like a firewall, it is possible toproperly configure your local firewall to restrict connections involving only the security update itself.
The example below shows how to set up such local firewall capabilities, which allow connections fromsecurity.debian.org only, logging all others.
The following example can be use to setup a restricted firewall ruleset. Run this commands from a local
console (not a remote one) to reduce the chances of locking yourself out of the system.

164

Appendix# iptables -F# iptables -L
Chain INPUT (policy ACCEPT)
targetprot opt source

destination

Chain FORWARD (policy ACCEPT)
targetprot opt source

destination

Chain OUTPUT (policy ACCEPT)
targetprot opt source
destination# iptables -A OUTPUT -d security.debian.org --dport 80 -j ACCEPT# iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT# iptables -A INPUT -p icmp -j ACCEPT# iptables -A INPUT -j LOG# iptables -A OUTPUT -j LOG# iptables -P INPUT DROP
# iptables -P FORWARD DROP
# iptables -P OUTPUT DROP
# iptables -L
Chain INPUT (policy DROP)
targetprot opt source
destination
ACCEPT
all -- 0.0.0.0/0
0.0.0.0/0
state RELATED,ESTABLIACCEPT
icmp -- 0.0.0.0/0
0.0.0.0/0
LOG
all -- anywhere
anywhere
LOG level warning
Chain FORWARD (policy DROP)
targetprot opt source

destination

Chain OUTPUT (policy DROP)
targetprot opt source
ACCEPT80-- anywhereLOG
all -- anywhere

destination
security.debian.org
anywhere
LOG level warning

Note: Using a DROP policy in the INPUT chain is the most correct thing to do, but be very careful when
doing this after flushing the chain from a remote connection. When testing firewall rulesets from a remotelocation it is best if you run a script with the firewall ruleset (instead of introducing the ruleset line by line
through the command line) and, as a precaution, keep a backdoor44

Such as knockd. Alternatively, you can open a different console and have the system ask for confirmation that there is somebody on the other side,and reset the firewall chain if no confirmation is given. The following test script could be of use:#!/bin/bash
while true; doread -n 1 -p "Are you there? " -t 30 ayt
if [ -z "$ayt" ] ; then
break
fi
done
# Reset the firewall chain, user is not availableecho
echo "Resetting firewall chain!"iptables -F
iptables -P INPUT ACCEPT
iptables -P FORWARD ACCEPT165

Appendix

Of course, you should disable any backdoors before getting the system into production. configured so thatyou can re-enable access to the system if you make a mistake. That way there would be no need to go toa remote location to fix a firewall ruleset that blocks you.
FIXME: This needs DNS to be working properly since it is required for security.debian.org to work. You
can add security.debian.org to /etc/hosts but now it is a CNAME to several hosts (there is more than one
security mirror)
FIXME: this will only work with HTTP URLs since ftp might need the ip_conntrack_ftp module, or use
passive mode.

Chroot environment for SSH
Creating a restricted environment for SSH is a tough job due to its dependencies and the fact that, unlike
other servers, SSH provides a remote shell to users. Thus, you will also have to consider the applications
users will be allowed to use in the environment.
You have two options to setup a restricted remote shell:• Chrooting the ssh users, by properly configuring the ssh daemon you can ask it to chroot a user after
authentication just before it is provided a shell. Each user can have their own environment.
• Chrooting the ssh server, since you chroot the ssh application itself all users are chrooted to the definedenvironment.
The first option has the advantage of making it possible to have both non-chrooted and chrooted users,
if you don't introduce any setuid application in the user's chroots it is more difficult to break out of it.
However, you might need to setup individual chroots for each user and it is more difficult to setup (as it
requires cooperation from the SSH server). The second option is more easy to setup, and protects from an
exploitation of the ssh server itself (since it's also in the chroot) but it will have the limitation that all users
will share the same chroot environment (you cannot setup a per-user chroot environment).

Chrooting the ssh users
You can setup the ssh server so that it will chroot a set of defined users into a shell with a limited set of
applications available.

Using libpam-chroot
Probably the easiest way is to use the libpam-chroot package provided in Debian. Once you install it you
need to:• Modify /etc/pam.d/ssh to use this PAM module, add as its last line5:

sessionrequired

pam_chroot.so

iptables -P OUTPUT ACCEPT
exit 1
5
You can use the debug option to have it send the progress of the module to the authpriv.notice facility

166

Appendix• set a proper chroot environment for the user. You can try using the scripts available at /usr/share/doc/libpam-chroot/examples/, use the makejail6 program or setup a minimum Debianenvironment with debootstrap. Make sure the environment includes the needed devices 7.
• Configure /etc/security/chroot.conf so that the users you determine are chrooted to thedirectory you setup previously. You might want to have independent directories for different users sothat they will not be able to see neither the whole system nor each other's.
• Configure SSH: Depending on your OpenSSH version the chroot environment might work straight ofthe box or not. Since 3.6.1p2 the do_pam_session() function is called after sshd has dropped privileges,
since chroot() needs root priviledges it will not work with Privilege separation on. In newer OpenSSH
versions, however, the PAM code has been modified and do_pam_session is called before dropping
priviledges so it will work even with Privilege separation is on. If you have to disable it modify /etc/
ssh/sshd_config like this:UsePrivilegeSeparation no
Notice that this will lower the security of your system since the OpenSSH server will then run as root
user. This means that if a remote attack is found against OpenSSH an attacker will get root privileges
instead of sshd, thus compromising the whole system. 8
If you don't disable Privilege Separation you will need an /etc/passwd which includes the user's UID
inside the chroot for Privilege Separation to work properly.
If you have Privilege Separation set to yes and your OpenSSH version does not behave properly you willneed to disable it. If you don't, users that try to connect to your server and would be chrooted by thismodule will see this:$ ssh -l user server
user@server's password:
Connection to server closed by remote host.
Connection to server closed.
This is because the ssh daemon, which is running as 'sshd', is not be able to make the chroot() system call.
To disable Privilege separation you have to modify the /etc/ssh/sshd_config configuration file
as described above.
Notice that if any of the following is missing the users will not be able to logon to the chroot:
6

You can create a very limited bash environment with the following python definition for makejail, just create the directory /var/chroots/
users/foo and a file with the following contents and call it bash.py:

chroot="/var/chroots/users/foo"cleanJailFirst=1
testCommandsInsideJail=["bash ls"]
And then run makejail bash.py to create the user environment at /var/chroots/users/foo. To test the environment run:# chroot /var/chroots/users/foo/ ls
bin dev etc lib proc sbin usr
7
In some occasions you might need the /dev/ptmx and /dev/pty* devices and the /dev/pts/ subdirectory. Running MAKEDEV in the/dev directory of the chrooted environment should be sufficient to create them if they do not exist. If you are using kernels (version 2.6) which
dynamically create device files you will need to create the /dev/pts/ files yourself and grant them the proper privileges.
8
If you are using a kernel that implements Mandatory Access Control (RSBAC/SElinux) you can avoid changing this configuration just by granting
the sshd user privileges to make the chroot() system call.

167

Appendix• The /proc filesystem needs to be mounted in the users' chroot.
• The necessary /dev/pts/ devices need to exist. If the files are generated by your running kernel
automatically then you have to manually create them on the chroot's /dev/.
• The user's home directory has to exist in the chroot, otherwise the ssh daemon will not continue.
You can debug all these issues if you use the debug keyword in the /etc/pam.d/ssh PAM definition.
If you encounter issues you might find it useful to enable the debugging mode on the ssh client too.
Note: This information is also available (and maybe more up to date) in /usr/share/doc/libpam-chroot/README.Debian.gz, please review it for updated information before taking the above
steps.

Patching the ssh server
Debian's sshd does not allow restriction of a user's movement through the server, since it lacks thechroot function that the commercial program sshd2 includes (using 'ChrootGroups' or 'ChrootUsers',
see sshd2_config(5)). However, there is a patch available to add this functionality available from http://chrootssh.sourceforge.net (requested and available in http://bugs.debian.org/139047 in Debian). The patch
may be included in future releases of the OpenSSH package. Emmanuel Lacour has ssh deb packages forsarge with this feature. They are available at http://debian.home-dn.net/sarge/ssh/. Notice that those might
not be up to date so completing the compilation step is recommended.
After applying the patch, modify /etc/passwd by changing the home path of the users (with the special/./ token):joeuser:x:1099:1099:Joe Random User:/home/joe/./:/bin/bash
Esto restringirá both accesos de el interfaz de comandos remoto así como la copia remota a través delcanal ssh.
Asegúrese para tener todos los binarios necesitados y bibliotecas en el el camino del chrooted para losusuarios. Estos archivos deben poseer por root evitar ser manoseados por el usuario (para terminar elchrooted encarcelado). Una muestra podría incluir:./bin:
total 660
drwxr-xr-x
drwxr-xr-x-r-xr-xr-x-r-xr-xr-x-r-xr-xr-x-rwxr-xr-x-r-xr-xr-x-r-xr-xr-x
lrwxrwxrwx

2 root8 guest1 root1 root1 root1 root1 root1 root1 root

root
guest
root
root
root
root
root
root
root

./etc:
total 24drwxr-xr-x

2 rootroot

4096 Mar 18 13:36 .
4096 Mar 15 16:53 ..
531160 Feb 6 22:36 bash
43916 Nov 29 13:19 ls
16684 Nov 29 13:19 mkdir
23960 Mar 18 13:36 more
9916 Jul 26 2001 pwd
24780 Nov 29 13:19 rm
4 Mar 30 16:29 sh -> bash

4096 Mar 15 16:13 .

168

Appendix

drwxr-xr-x-rw-r--r--rw-r--r--rw-r--r--rw-r--r--

8 guest1 root1 root1 root1 root./lib:
total 1848
drwxr-xr-x
2 rootdrwxr-xr-x
8 guest-rwxr-xr-x
1 root-rwxr-xr-x
1 root-rw-r--r-1 root
-rw-r--r-1 root
-rw-r--r-1 root
-rw-r--r-1 root
-rw-r--r-1 root
libnss_files.so.2-rw-r--r-1 root
-rw-r--r-1 root
-rw-r--r-1 root
-rw-r--r-1 root
-rw-r--r-1 root

guest
root
root
root
root

4096 Mar 15 16:53 ..
54 Mar 15 13:23 group
428 Mar 15 15:56 hosts
44 Mar 15 15:53 passwd
52 Mar 15 13:23 shells

root
guest
root
root
root
root
root
root
root

4096 Mar 18 13:37 .
4096 Mar 15 16:53 ..
92511 Mar 15 12:49 ld-linux.so.2
1170812 Mar 15 12:49 libc.so.6
20900 Mar 15 13:01 libcrypt.so.1
9436 Mar 15 12:49 libdl.so.2
248132 Mar 15 12:48 libncurses.so.5
71332 Mar 15 13:00 libnsl.so.1
34144 Mar 15 16:10

root
root
root
root
root

29420 Mar 15 12:57 libpam.so.0
105498 Mar 15 12:51 libpthread.so.0
25596 Mar 15 12:51 librt.so.1
7760 Mar 15 12:59 libutil.so.1
24328 Mar 15 12:57 libwrap.so.0./usr:
total 16
drwxr-xr-x
drwxr-xr-x
drwxr-xr-x
drwxr-xr-x

4 root8 guest2 root2 root

root
guest
root
root

4096 Mar 15 13:00 .
4096 Mar 15 16:53 ..
4096 Mar 15 15:55 bin
4096 Mar 15 15:37 lib./usr/bin:total 340
drwxr-xr-x
drwxr-xr-x-rwxr-xr-x-rwxr-xr-x-r-xr-xr-x-rwxr-xr-x-r-sr-xr-x-rwxr-xr-x

2 root4 root1 root1 root1 root1 root1 root1 root

root
root
root
root
root
root
root
root

4096 Mar 15 15:55 .
4096 Mar 15 13:00 ..
10332 Mar 15 15:55 env
13052 Mar 15 13:13 id
25432 Mar 15 12:40 scp
43768 Mar 15 15:15 sftp
218456 Mar 15 12:40 ssh
9692 Mar 15 13:17 tty

root
root
root

4096 Mar 15 15:37 .
4096 Mar 15 13:00 ..
771088 Mar 15 13:01

root
root

54548 Mar 15 13:00 libz.so.1
23096 Mar 15 15:37 sftp-server./usr/lib:
total 852
drwxr-xr-x
2 rootdrwxr-xr-x
4 root-rw-r--r-1 root
libcrypto.so.0.9.6-rw-r--r-1 root
-rwxr-xr-x
1 root169

Appendix

Chrooting the ssh server
If you create a chroot which includes the SSH server files in, for example /var/chroot/ssh, you
would start the ssh server chroot'ed with this command:# chroot /var/chroot/ssh /sbin/sshd -f /etc/sshd_config
That would make startup the sshd daemon inside the chroot. In order to do that you have to first prepare thecontents of the /var/chroot/ssh directory so that it includes both the SSH server and all the utilities
that the users connecting to that server might need. If you are doing this you should make certain that
OpenSSH uses Privilege Separation (which is the default) having the following line in the configuration
file /etc/ssh/sshd_config:

UsePrivilegeSeparation yes
That way the remote daemon will do as few things as possible as the root user so even if there is a bugin it it will not compromise the chroot. Notice that, unlike the case in which you setup a per-user chroot,
the ssh daemon is running in the same chroot as the users so there is at least one potential process running
as root which could break out of the chroot.
Notice, also, that in order for SSH to work in that location, the partition where the chroot directory resides
cannot be mounted with the nodev option. If you use that option, then you will get the following error:
PRNG is not seeded, because /dev/urandom does not work in the chroot.

Setup a minimal system (the really easy way)
You can use debootstrap to setup a minimal environment that just includes the ssh server. In order todo this you just have to create a chroot as described in the http://www.debian.org/doc/manuals/reference/ch09#_chroot_system document. This method is bound to work (you will get all the necessary componentes for the chroot) but at the cost of disk space (a minimal installation of Debian will amount to several
hundred megabytes). This minimal system might also include setuid files that a user in the chroot could
use to break out of the chroot if any of those could be use for a privilege escalation.

Automatically making the environment (the easy way)
You can easily create a restricted environment with the makejail package, since it automatically takes careof tracing the server daemon (with strace), and makes it run under the restricted environment.
The advantage of programs that automatically generate chroot environments is that they are capable ofcopying any package to the chroot environment (even following the package's dependencies and making
sure it's complete). Thus, providing user applications is easier.
To set up the environment using makejail's provided examples, just create /var/chroot/sshd anduse the command:# makejail /usr/share/doc/makejail/examples/sshd.py
This will setup the chroot in the /var/chroot/sshd directory. Notice that this chroot will not fully
work unless you:•170

Appendix

Mount the procfs filesystem in /var/chroot/sshd/proc. Makejail will mount it for you but if
the system reboots you need to remount it running:# mount -t proc proc /var/chroot/sshd/proc
You can also have it be mounted automatically by editing /etc/fstab and including this line:

proc-ssh /var/chroot/sshd/proc

proc none 0 0• Have syslog listen to the device /dev/log inside the chroot. In order to do this you have modify /etc/default/syslogd and add -a /var/chroot/sshd/dev/log to the SYSLOGD variable definition.
Read the sample file to see what other changes need to be made to the environment. Some of these changes,
such as copying user's home directories, cannot be done automatically. Also, limit the exposure of sensitive
information by only copying the data from a given number of users from the files /etc/shadow or /etc/group. Notice that if you are using Privilege Separation the sshd user needs to exist in those files.
The following sample environment has been (slightly) tested in Debian 3.0 and is built with the configuration file provided in the package and includes the fileutils package:.
|-- bin||-- ash||-- bash||-- chgrp||-- chmod||-- chown||-- cp||-- csh -> /etc/alternatives/csh||-- dd||-- df||-- dir||-- fdflush||-- ksh||-- ln||-- ls||-- mkdir||-- mknod||-- mv||-- rbash -> bash||-- rm||-- rmdir||-- sh -> bash||-- sync||-- tcsh||-- touch||-- vdir||-- zsh -> /etc/alternatives/zsh|`-- zsh4|-- dev||-- null171

Appendix||-- ptmx||-- pts||-- ptya0(...)||-- tty||-- tty0(...)|`-- urandom|-- etc||-- alternatives|||-- csh -> /bin/tcsh||`-- zsh -> /bin/zsh4||-- environment||-- hosts||-- hosts.allow||-- hosts.deny||-- ld.so.conf||-- localtime -> /usr/share/zoneinfo/Europe/Madrid||-- motd||-- nsswitch.conf||-- pam.conf||-- pam.d|||-- other||`-- ssh||-- passwd||-- resolv.conf||-- security|||-- access.conf|||-- chroot.conf|||-- group.conf|||-- limits.conf|||-- pam_env.conf||`-- time.conf||-- shadow||-- shells|`-- ssh||-- moduli||-- ssh_host_dsa_key||-- ssh_host_dsa_key.pub||-- ssh_host_rsa_key||-- ssh_host_rsa_key.pub|`-- sshd_config|-- home|`-- userX|-- lib||-- ld-2.2.5.so||-- ld-linux.so.2 -> ld-2.2.5.so||-- libc-2.2.5.so||-- libc.so.6 -> libc-2.2.5.so||-- libcap.so.1 -> libcap.so.1.10||-- libcap.so.1.10||-- libcrypt-2.2.5.so||-- libcrypt.so.1 -> libcrypt-2.2.5.so||-- libdl-2.2.5.so172

Appendix

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|-- libdl.so.2 -> libdl-2.2.5.so|-- libm-2.2.5.so|-- libm.so.6 -> libm-2.2.5.so|-- libncurses.so.5 -> libncurses.so.5.2|-- libncurses.so.5.2|-- libnsl-2.2.5.so|-- libnsl.so.1 -> libnsl-2.2.5.so|-- libnss_compat-2.2.5.so|-- libnss_compat.so.2 -> libnss_compat-2.2.5.so|-- libnss_db-2.2.so|-- libnss_db.so.2 -> libnss_db-2.2.so|-- libnss_dns-2.2.5.so|-- libnss_dns.so.2 -> libnss_dns-2.2.5.so|-- libnss_files-2.2.5.so|-- libnss_files.so.2 -> libnss_files-2.2.5.so|-- libnss_hesiod-2.2.5.so|-- libnss_hesiod.so.2 -> libnss_hesiod-2.2.5.so|-- libnss_nis-2.2.5.so|-- libnss_nis.so.2 -> libnss_nis-2.2.5.so|-- libnss_nisplus-2.2.5.so|-- libnss_nisplus.so.2 -> libnss_nisplus-2.2.5.so|-- libpam.so.0 -> libpam.so.0.72|-- libpam.so.0.72|-- libpthread-0.9.so|-- libpthread.so.0 -> libpthread-0.9.so|-- libresolv-2.2.5.so|-- libresolv.so.2 -> libresolv-2.2.5.so|-- librt-2.2.5.so|-- librt.so.1 -> librt-2.2.5.so|-- libutil-2.2.5.so|-- libutil.so.1 -> libutil-2.2.5.so|-- libwrap.so.0 -> libwrap.so.0.7.6|-- libwrap.so.0.7.6`-- security|-- pam_access.so|-- pam_chroot.so|-- pam_deny.so|-- pam_env.so|-- pam_filter.so|-- pam_ftp.so|-- pam_group.so|-- pam_issue.so|-- pam_lastlog.so|-- pam_limits.so|-- pam_listfile.so|-- pam_mail.so|-- pam_mkhomedir.so|-- pam_motd.so|-- pam_nologin.so|-- pam_permit.so|-- pam_rhosts_auth.so|-- pam_rootok.so|-- pam_securetty.so|-- pam_shells.so173

Appendix||-- pam_stress.so||-- pam_tally.so||-- pam_time.so||-- pam_unix.so||-- pam_unix_acct.so -> pam_unix.so||-- pam_unix_auth.so -> pam_unix.so||-- pam_unix_passwd.so -> pam_unix.so||-- pam_unix_session.so -> pam_unix.so||-- pam_userdb.so||-- pam_warn.so|`-- pam_wheel.so|-- sbin|`-- start-stop-daemon|-- usr||-- bin|||-- dircolors|||-- du|||-- install|||-- link|||-- mkfifo|||-- shred|||-- touch -> /bin/touch||`-- unlink||-- lib|||-- libcrypto.so.0.9.6|||-- libdb3.so.3 -> libdb3.so.3.0.2|||-- libdb3.so.3.0.2|||-- libz.so.1 -> libz.so.1.1.4||`-- libz.so.1.1.4||-- sbin||`-- sshd|`-- share||-- locale||`-- es|||-- LC_MESSAGES||||-- fileutils.mo||||-- libc.mo|||`-- sh-utils.mo||`-- LC_TIME -> LC_MESSAGES|`-- zoneinfo|`-- Europe|`-- Madrid`-- var`-- run|-- sshd`-- sshd.pid
27 directories, 733 files
For Debian release 3.1 you have to make sure that the environment includes also the common files for
PAM. The following files need to be copied over to the chroot if makejail did not do it for you:$ ls /etc/pam.d/common-*174

Appendix/etc/pam.d/common-account/etc/pam.d/common-auth/etc/pam.d/common-password/etc/pam.d/common-session

Manually creating the environment (the hard way)
It is possible to create an environment, using a trial-and-error method, by monitoring the sshd server traces
and log files in order to determine the necessary files. The following environment, contributed by José
Luis Ledesma, is a sample listing of files in a chroot environment for ssh in Debian woody (3.0): 9.:
total 36
drwxr-xr-x 9 root root 4096 Jun 5 10:05 ./drwxr-xr-x 11 root root 4096 Jun 3 13:43 ../drwxr-xr-x 2 root root 4096 Jun 4 12:13 bin/drwxr-xr-x 2 root root 4096 Jun 4 12:16 dev/
drwxr-xr-x 4 root root 4096 Jun 4 12:35 etc/
drwxr-xr-x 3 root root 4096 Jun 4 12:13 lib/
drwxr-xr-x 2 root root 4096 Jun 4 12:35 sbin/
drwxr-xr-x 2 root root 4096 Jun 4 12:32 tmp/
drwxr-xr-x 2 root root 4096 Jun 4 12:16 usr/./bin:
total 8368
drwxr-xr-x 2 root root 4096 Jun 4 12:13 ./drwxr-xr-x 9 root root 4096 Jun 5 10:05 ../-rwxr-xr-x 1 root root 109855 Jun 3 13:45 a2p*
-rwxr-xr-x 1 root root 387764 Jun 3 13:45 bash*-rwxr-xr-x 1 root root 36365 Jun 3 13:45 c2ph*-rwxr-xr-x 1 root root 20629 Jun 3 13:45 dprofpp*-rwxr-xr-x 1 root root 6956 Jun 3 13:46 env*
-rwxr-xr-x 1 root root 158116 Jun 3 13:45 fax2ps*-rwxr-xr-x 1 root root 104008 Jun 3 13:45 faxalter*-rwxr-xr-x 1 root root 89340 Jun 3 13:45 faxcover*-rwxr-xr-x 1 root root 441584 Jun 3 13:45 faxmail*-rwxr-xr-x 1 root root 96036 Jun 3 13:45 faxrm*-rwxr-xr-x 1 root root 107000 Jun 3 13:45 faxstat*-rwxr-xr-x 1 root root 77832 Jun 4 11:46 grep*-rwxr-xr-x 1 root root 19597 Jun 3 13:45 h2ph*-rwxr-xr-x 1 root root 46979 Jun 3 13:45 h2xs*-rwxr-xr-x 1 root root 10420 Jun 3 13:46 id*-rwxr-xr-x 1 root root 4528 Jun 3 13:46 ldd*
-rwxr-xr-x 1 root root 111386 Jun 4 11:46 less*-r-xr-xr-x 1 root root 26168 Jun 3 13:45 login*-rwxr-xr-x 1 root root 49164 Jun 3 13:45 ls*-rwxr-xr-x 1 root root 11600 Jun 3 13:45 mkdir*-rwxr-xr-x 1 root root 24780 Jun 3 13:45 more*-rwxr-xr-x 1 root root 154980 Jun 3 13:45 pal2rgb*-rwxr-xr-x 1 root root 27920 Jun 3 13:46 passwd*-rwxr-xr-x 1 root root 4241 Jun 3 13:45 pl2pm*-rwxr-xr-x 1 root root 2350 Jun 3 13:45 pod2html*-rwxr-xr-x 1 root root 7875 Jun 3 13:45 pod2latex*
9

Notice that there are no SETUID files. This makes it more difficult for remote users to escape the chroot environment. However, it also preventsusers from changing their passwords, since the passwd program cannot modify the files /etc/passwd or /etc/shadow.

175Appendix-rwxr-xr-x 1 root root 17587 Jun 3 13:45 pod2man*-rwxr-xr-x 1 root root 6877 Jun 3 13:45 pod2text*-rwxr-xr-x 1 root root 3300 Jun 3 13:45 pod2usage*-rwxr-xr-x 1 root root 3341 Jun 3 13:45 podchecker*-rwxr-xr-x 1 root root 2483 Jun 3 13:45 podselect*-r-xr-xr-x 1 root root 82412 Jun 4 11:46 ps*-rwxr-xr-x 1 root root 36365 Jun 3 13:45 pstruct*-rwxr-xr-x 1 root root 7120 Jun 3 13:45 pwd*
-rwxr-xr-x 1 root root 179884 Jun 3 13:45 rgb2ycbcr*-rwxr-xr-x 1 root root 20532 Jun 3 13:45 rm*-rwxr-xr-x 1 root root 6720 Jun 4 10:15 rmdir*-rwxr-xr-x 1 root root 14705 Jun 3 13:45 s2p*
-rwxr-xr-x 1 root root 28764 Jun 3 13:46 scp*
-rwxr-xr-x 1 root root 385000 Jun 3 13:45 sendfax*-rwxr-xr-x 1 root root 67548 Jun 3 13:45 sendpage*-rwxr-xr-x 1 root root 88632 Jun 3 13:46 sftp*-rwxr-xr-x 1 root root 387764 Jun 3 13:45 sh*-rws--x--x 1 root root 744500 Jun 3 13:46 slogin*-rwxr-xr-x 1 root root 14523 Jun 3 13:46 splain*-rws--x--x 1 root root 744500 Jun 3 13:46 ssh*
-rwxr-xr-x 1 root root 570960 Jun 3 13:46 ssh-add*-rwxr-xr-x 1 root root 502952 Jun 3 13:46 ssh-agent*-rwxr-xr-x 1 root root 575740 Jun 3 13:46 ssh-keygen*-rwxr-xr-x 1 root root 383480 Jun 3 13:46 ssh-keyscan*-rwxr-xr-x 1 root root 39 Jun 3 13:46 ssh_europa*-rwxr-xr-x 1 root root 107252 Jun 4 10:14 strace*-rwxr-xr-x 1 root root 8323 Jun 4 10:14 strace-graph*-rwxr-xr-x 1 root root 158088 Jun 3 13:46 thumbnail*-rwxr-xr-x 1 root root 6312 Jun 3 13:46 tty*
-rwxr-xr-x 1 root root 55904 Jun 4 11:46 useradd*-rwxr-xr-x 1 root root 585656 Jun 4 11:47 vi*-rwxr-xr-x 1 root root 6444 Jun 4 11:45 whoami*./dev:
total 8drwxr-xr-x 2 root root 4096 Jun 4 12:16 ./drwxr-xr-x 9 root root 4096 Jun 5 10:05 ../crw-r--r-- 1 root root 1, 9 Jun 3 13:43 urandom./etc:
total 208
drwxr-xr-x 4 root root 4096 Jun 4 12:35 ./drwxr-xr-x 9 root root 4096 Jun 5 10:05 ../-rw------- 1 root root 0 Jun 4 11:46 .pwd.lock-rw-r--r-- 1 root root 653 Jun 3 13:46 group-rw-r--r-- 1 root root 242 Jun 4 11:33 host.conf-rw-r--r-- 1 root root 857 Jun 4 12:04 hosts-rw-r--r-- 1 root root 1050 Jun 4 11:29 ld.so.cache-rw-r--r-- 1 root root 304 Jun 4 11:28 ld.so.conf-rw-r--r-- 1 root root 235 Jun 4 11:27 ld.so.conf~-rw-r--r-- 1 root root 88039 Jun 3 13:46 moduli-rw-r--r-- 1 root root 1342 Jun 4 11:34 nsswitch.conf
drwxr-xr-x 2 root root 4096 Jun 4 12:02 pam.d/-rw-r--r-- 1 root root 28 Jun 4 12:00 pam_smb.conf-rw-r--r-- 1 root root 2520 Jun 4 11:57 passwd-rw-r--r-- 1 root root 7228 Jun 3 13:48 profile

176

Appendix-rw-r--r-- 1 root root 1339 Jun 4 11:33 protocols-rw-r--r-- 1 root root 274 Jun 4 11:44 resolv.conf
drwxr-xr-x 2 root root 4096 Jun 3 13:43 security/-rw-r----- 1 root root 1178 Jun 4 11:51 shadow-rw------- 1 root root 80 Jun 4 11:45 shadow-rw-r----- 1 root root 1178 Jun 4 11:48 shadow.old-rw-r--r-- 1 root root 161 Jun 3 13:46 shells-rw-r--r-- 1 root root 1144 Jun 3 13:46 ssh_config-rw------- 1 root root 668 Jun 3 13:46 ssh_host_dsa_key-rw-r--r-- 1 root root 602 Jun 3 13:46 ssh_host_dsa_key.pub-rw------- 1 root root 527 Jun 3 13:46 ssh_host_key-rw-r--r-- 1 root root 331 Jun 3 13:46 ssh_host_key.pub-rw------- 1 root root 883 Jun 3 13:46 ssh_host_rsa_key-rw-r--r-- 1 root root 222 Jun 3 13:46 ssh_host_rsa_key.pub-rw-r--r-- 1 root root 2471 Jun 4 12:15 sshd_config./etc/pam.d:
total 24drwxr-xr-x 2 root root 4096 Jun 4 12:02 ./drwxr-xr-x 4 root root 4096 Jun 4 12:35 ../lrwxrwxrwx 1 root root 4 Jun 4 12:02 other -> sshd
-rw-r--r-- 1 root root 318 Jun 3 13:46 passwd-rw-r--r-- 1 root root 546 Jun 4 11:36 ssh-rw-r--r-- 1 root root 479 Jun 4 12:02 sshd
-rw-r--r-- 1 root root 370 Jun 3 13:46 su./etc/security:
total 32
drwxr-xr-x 2 root root 4096 Jun 3 13:43 ./drwxr-xr-x 4 root root 4096 Jun 4 12:35 ../-rw-r--r-- 1 root root 1971 Jun 3 13:46 access.conf-rw-r--r-- 1 root root 184 Jun 3 13:46 chroot.conf-rw-r--r-- 1 root root 2145 Jun 3 13:46 group.conf-rw-r--r-- 1 root root 1356 Jun 3 13:46 limits.conf-rw-r--r-- 1 root root 2858 Jun 3 13:46 pam_env.conf-rw-r--r-- 1 root root 2154 Jun 3 13:46 time.conf./lib:
total 8316
drwxr-xr-x 3 root root 4096 Jun 4 12:13 ./drwxr-xr-x 9 root root 4096 Jun 5 10:05 ../-rw-r--r-- 1 root root 1024 Jun 4 11:51 cracklib_dict.hwm-rw-r--r-- 1 root root 214324 Jun 4 11:51 cracklib_dict.pwd-rw-r--r-- 1 root root 11360 Jun 4 11:51 cracklib_dict.pwi-rwxr-xr-x 1 root root 342427 Jun 3 13:46 ld-linux.so.2*-rwxr-xr-x 1 root root 4061504 Jun 3 13:46 libc.so.6*
lrwxrwxrwx 1 root root 15 Jun 4 12:11 libcrack.so -> libcrack.so.2.7*
lrwxrwxrwx 1 root root 15 Jun 4 12:11 libcrack.so.2 -> libcrack.so.2.7*-rwxr-xr-x 1 root root 33291 Jun 4 11:39 libcrack.so.2.7*-rwxr-xr-x 1 root root 60988 Jun 3 13:46 libcrypt.so.1*-rwxr-xr-x 1 root root 71846 Jun 3 13:46 libdl.so.2*-rwxr-xr-x 1 root root 27762 Jun 3 13:46 libhistory.so.4.0*
lrwxrwxrwx 1 root root 17 Jun 4 12:12 libncurses.so.4 -> libncurses.so.4.2*-rwxr-xr-x 1 root root 503903 Jun 3 13:46 libncurses.so.4.2*
lrwxrwxrwx 1 root root 17 Jun 4 12:12 libncurses.so.5 -> libncurses.so.5.0*-rwxr-xr-x 1 root root 549429 Jun 3 13:46 libncurses.so.5.0*-rwxr-xr-x 1 root root 369801 Jun 3 13:46 libnsl.so.1*

177

Appendix-rwxr-xr-x 1 root root 142563 Jun 4 11:49 libnss_compat.so.1*-rwxr-xr-x 1 root root 215569 Jun 4 11:49 libnss_compat.so.2*-rwxr-xr-x 1 root root 61648 Jun 4 11:34 libnss_dns.so.1*-rwxr-xr-x 1 root root 63453 Jun 4 11:34 libnss_dns.so.2*-rwxr-xr-x 1 root root 63782 Jun 4 11:34 libnss_dns6.so.2*-rwxr-xr-x 1 root root 205715 Jun 3 13:46 libnss_files.so.1*-rwxr-xr-x 1 root root 235932 Jun 3 13:49 libnss_files.so.2*-rwxr-xr-x 1 root root 204383 Jun 4 11:33 libnss_nis.so.1*-rwxr-xr-x 1 root root 254023 Jun 4 11:33 libnss_nis.so.2*-rwxr-xr-x 1 root root 256465 Jun 4 11:33 libnss_nisplus.so.2*
lrwxrwxrwx 1 root root 14 Jun 4 12:12 libpam.so.0 -> libpam.so.0.72*-rwxr-xr-x 1 root root 31449 Jun 3 13:46 libpam.so.0.72*
lrwxrwxrwx 1 root root 19 Jun 4 12:12 libpam_misc.so.0 ->libpam_misc.so.0.72*-rwxr-xr-x 1 root root 8125 Jun 3 13:46 libpam_misc.so.0.72*
lrwxrwxrwx 1 root root 15 Jun 4 12:12 libpamc.so.0 -> libpamc.so.0.72*-rwxr-xr-x 1 root root 10499 Jun 3 13:46 libpamc.so.0.72*-rwxr-xr-x 1 root root 176427 Jun 3 13:46 libreadline.so.4.0*-rwxr-xr-x 1 root root 44729 Jun 3 13:46 libutil.so.1*-rwxr-xr-x 1 root root 70254 Jun 3 13:46 libz.a*lrwxrwxrwx 1 root root 13 Jun 4 12:13 libz.so -> libz.so.1.1.3*
lrwxrwxrwx 1 root root 13 Jun 4 12:13 libz.so.1 -> libz.so.1.1.3*-rwxr-xr-x 1 root root 63312 Jun 3 13:46 libz.so.1.1.3*
drwxr-xr-x 2 root root 4096 Jun 4 12:00 security/./lib/security:
total 668
drwxr-xr-x 2 root root 4096 Jun 4 12:00 ./drwxr-xr-x 3 root root 4096 Jun 4 12:13 ../-rwxr-xr-x 1 root root 10067 Jun 3 13:46 pam_access.so*-rwxr-xr-x 1 root root 8300 Jun 3 13:46 pam_chroot.so*-rwxr-xr-x 1 root root 14397 Jun 3 13:46 pam_cracklib.so*-rwxr-xr-x 1 root root 5082 Jun 3 13:46 pam_deny.so*-rwxr-xr-x 1 root root 13153 Jun 3 13:46 pam_env.so*-rwxr-xr-x 1 root root 13371 Jun 3 13:46 pam_filter.so*-rwxr-xr-x 1 root root 7957 Jun 3 13:46 pam_ftp.so*-rwxr-xr-x 1 root root 12771 Jun 3 13:46 pam_group.so*-rwxr-xr-x 1 root root 10174 Jun 3 13:46 pam_issue.so*-rwxr-xr-x 1 root root 9774 Jun 3 13:46 pam_lastlog.so*-rwxr-xr-x 1 root root 13591 Jun 3 13:46 pam_limits.so*-rwxr-xr-x 1 root root 11268 Jun 3 13:46 pam_listfile.so*-rwxr-xr-x 1 root root 11182 Jun 3 13:46 pam_mail.so*-rwxr-xr-x 1 root root 5923 Jun 3 13:46 pam_nologin.so*-rwxr-xr-x 1 root root 5460 Jun 3 13:46 pam_permit.so*-rwxr-xr-x 1 root root 18226 Jun 3 13:46 pam_pwcheck.so*-rwxr-xr-x 1 root root 12590 Jun 3 13:46 pam_rhosts_auth.so*-rwxr-xr-x 1 root root 5551 Jun 3 13:46 pam_rootok.so*-rwxr-xr-x 1 root root 7239 Jun 3 13:46 pam_securetty.so*-rwxr-xr-x 1 root root 6551 Jun 3 13:46 pam_shells.so*-rwxr-xr-x 1 root root 55925 Jun 4 12:00 pam_smb_auth.so*-rwxr-xr-x 1 root root 12678 Jun 3 13:46 pam_stress.so*-rwxr-xr-x 1 root root 11170 Jun 3 13:46 pam_tally.so*-rwxr-xr-x 1 root root 11124 Jun 3 13:46 pam_time.so*-rwxr-xr-x 1 root root 45703 Jun 3 13:46 pam_unix.so*-rwxr-xr-x 1 root root 45703 Jun 3 13:46 pam_unix2.so*

178

Appendix-rwxr-xr-x 1 root root 45386 Jun 3 13:46 pam_unix_acct.so*-rwxr-xr-x 1 root root 45386 Jun 3 13:46 pam_unix_auth.so*-rwxr-xr-x 1 root root 45386 Jun 3 13:46 pam_unix_passwd.so*-rwxr-xr-x 1 root root 45386 Jun 3 13:46 pam_unix_session.so*-rwxr-xr-x 1 root root 9726 Jun 3 13:46 pam_userdb.so*-rwxr-xr-x 1 root root 6424 Jun 3 13:46 pam_warn.so*-rwxr-xr-x 1 root root 7460 Jun 3 13:46 pam_wheel.so*./sbin:total 3132
drwxr-xr-x 2 root root 4096 Jun 4 12:35 ./drwxr-xr-x 9 root root 4096 Jun 5 10:05 ../-rwxr-xr-x 1 root root 178256 Jun 3 13:46 choptest*-rwxr-xr-x 1 root root 184032 Jun 3 13:46 cqtest*-rwxr-xr-x 1 root root 81096 Jun 3 13:46 dialtest*-rwxr-xr-x 1 root root 1142128 Jun 4 11:28 ldconfig*-rwxr-xr-x 1 root root 2868 Jun 3 13:46 lockname*-rwxr-xr-x 1 root root 3340 Jun 3 13:46 ondelay*-rwxr-xr-x 1 root root 376796 Jun 3 13:46 pagesend*-rwxr-xr-x 1 root root 13950 Jun 3 13:46 probemodem*-rwxr-xr-x 1 root root 9234 Jun 3 13:46 recvstats*-rwxr-xr-x 1 root root 64480 Jun 3 13:46 sftp-server*-rwxr-xr-x 1 root root 744412 Jun 3 13:46 sshd*-rwxr-xr-x 1 root root 30750 Jun 4 11:46 su*-rwxr-xr-x 1 root root 194632 Jun 3 13:46 tagtest*-rwxr-xr-x 1 root root 69892 Jun 3 13:46 tsitest*-rwxr-xr-x 1 root root 43792 Jun 3 13:46 typetest*./tmp:
total 8drwxr-xr-x 2 root root 4096 Jun 4 12:32 ./drwxr-xr-x 9 root root 4096 Jun 5 10:05 .././usr:
total 8drwxr-xr-x 2 root root 4096 Jun 4 12:16 ./drwxr-xr-x 9 root root 4096 Jun 5 10:05 ../lrwxrwxrwx 1 root root 7 Jun 4 12:14 bin -> ../bin//lrwxrwxrwx 1 root root 7 Jun 4 11:33 lib -> ../lib//
lrwxrwxrwx 1 root root 8 Jun 4 12:13 sbin -> ../sbin//

Chroot environment for Apache
Introducción
The chroot utility is often used to jail a daemon in a restricted tree. You can use it to insulate services
from one another, so that security issues in a software package do not jeopardize the whole server. Whenusing the makejail script, setting up and updating the chrooted tree is much easier.
FIXME: Apache can also be chrooted using http://www.modsecurity.org which is available in libapache-mod-security (for Apache 1.x) and libapache2-mod-security (for Apache 2.x).

LicensingThis document is copyright 2002 Alexandre Ratti. It has been dual-licensed and released under the GPL
version 2 (GNU General Public License) the GNU-FDL 1.2 (GNU Free Documentation Licence) and isincluded in this manual with his explicit permission.

179

Appendix

Installing the serverThis procedure was tested on Debian GNU/Linux 3.0 (Woody) with makejail 0.0.4-1 (in Debian/testing).
•
Log in as root and create a new jail directory:$ mkdir -p /var/chroot/apache• Create a new user and a new group. The chrooted Apache server will run as this user/group, which isn't
used for anything else on the system. In this example, both user and group are called chrapach.

$ adduser --home /var/chroot/apache --shell /bin/false \--no-create-home --system --group chrapach
FIXME: is a new user needed? (Apache already runs as the apache user)• Install Apache as usual on Debian: apt-get install apache• Set up Apache (e.g. define your subdomains, etc.). In the /etc/apache/httpd.conf configuration
file, set the Group and User options to chrapach. Restart Apache and make sure the server is working
correctly. Now, stop the Apache daemon.
• Install makejail (available in Debian/testing for now). You should also install wget and lynx as they
will be used by makejail to test the chrooted server: apt-get install makejail wget lynx
• Copy the sample configuration file for Apache to the /etc/makejail directory:# cp /usr/share/doc/makejail/examples/apache.py /etc/makejail/• Edit /etc/makejail/apache.py. You need to change the chroot, users and groups options. To
run this version of makejail, you can also add a packages option. See the http://www.floc.net/makejail/current/doc/. A sample is shown here:chroot="/var/chroot/apache"
testCommandsInsideJail=["/usr/sbin/apachectl start"]
processNames=["apache"]testCommandsOutsideJail=["wget -r --spider http://localhost/","lynx --source https://localhost/"]preserve=["/var/www","/var/log/apache","/dev/log"]
users=["chrapach"]groups=["chrapach"]packages=["apache", "apache-common"]userFiles=["/etc/password","/etc/shadow"]groupFiles=["/etc/group","/etc/gshadow"]
forceCopy=["/etc/hosts","/etc/mime.types"]
180

Appendix

FIXME: some options do not seem to work properly. For instance, /etc/shadow and /etc/gshadow are not copied, whereas /etc/password and /etc/group are fully copied instead of being
filtered.
• Create the chroot tree: makejail /etc/makejail/apache.py• If /etc/password and /etc/group were fully copied, type:$ grep chrapach /etc/passwd > /var/chroot/apache/etc/passwd$ grep chrapach /etc/group > /var/chroot/apache/etc/groupto replace them with filtered copies.
• Copy the Web site pages and the logs into the jail. These files are not copied automatically (see thepreserve option in makejail's configuration file).

# cp -Rp /var/www /var/chroot/apache/var# cp -Rp /var/log/apache/*.log /var/chroot/apache/var/log/apache• Edit the startup script for the system logging daemon so that it also listen to the /var/chroot/apache/dev/log socket. In /etc/default/syslogd, replace: SYSLOGD="" with SYSLOGD="-a /var/chroot/apache/dev/log" and restart the daemon (/etc/init.d/sysklogd
restart).
• Edit the Apache startup script (/etc/init.d/apache). You might need to make some changes tothe default startup script for it to run properly with a chrooted tree. Such as:• set a new CHRDIR variable at the top of the file;• edit the start, stop, reload, etc. sections;• add a line to mount and unmount the /proc filesystem within the jail.

#! /bin/bash## apache#Start the apache HTTP server.

CHRDIR=/var/chroot/apache
NAME=apache
PATH=/bin:/usr/bin:/sbin:/usr/sbin
DAEMON=/usr/sbin/apache
SUEXEC=/usr/lib/apache/suexec
PIDFILE=/var/run/$NAME.pid
CONF=/etc/apache/httpd.conf
APACHECTL=/usr/sbin/apachectl
trap "" 1export LANG=C
export PATH

181

Appendix

test -f $DAEMON || exit 0
test -f $APACHECTL || exit 0# ensure we don't leak environment vars into apachectl
APACHECTL="env -i LANG=${LANG} PATH=${PATH} chroot $CHRDIR $APACHECTL"
if egrep -q -i "^[[:space:]]*ServerType[[:space:]]+inet" $CONF
thenexit 0fi
case "$1" instart)
echo -n "Starting web server: $NAME"
mount -t proc proc /var/chroot/apache/proc
start-stop-daemon --start --pidfile $PIDFILE --exec $DAEMON \--chroot $CHRDIR;;
stop)echo -n "Stopping web server: $NAME"
start-stop-daemon --stop --pidfile "$CHRDIR/$PIDFILE" --oknodo
umount /var/chroot/apache/proc;;
reload)echo -n "Reloading $NAME configuration"
start-stop-daemon --stop --pidfile "$CHRDIR/$PIDFILE" \--signal USR1 --startas $DAEMON --chroot $CHRDIR;;
reload-modules)
echo -n "Reloading $NAME modules"
start-stop-daemon --stop --pidfile "$CHRDIR/$PIDFILE" --oknodo \--retry 30
start-stop-daemon --start --pidfile $PIDFILE \--exec $DAEMON --chroot $CHRDIR;;
restart)$0 reload-modules
exit $?
;;
force-reload)$0 reload-modules
exit $?
;;*)

echo "Usage: /etc/init.d/$NAME {start|stop|reload|reload-modules|force-reload
exit 1
;;
esac

182

Appendix

if [ $? == 0 ]; thenecho .
exit 0else
echo failed
exit 1fi
FIXME: should the first Apache process be run as another user than root (i.e. add --chuid chrapach:chrapach)? Cons: chrapach will need write access to the logs, which is awkward.
• Replace in /etc/logrotate.d/apache/var/log/apache/*.log with /var/chroot/
apache/var/log/apache/*.log• Start Apache (/etc/init.d/apache start) and check what is it reported in the jail log (/var/chroot/
apache/var/log/apache/error.log). If your setup is more complex, (e.g. if you also use PHP
and MySQL), files will probably be missing. if some files are not copied automatically by makejail,you can list them in the forceCopy (to copy files directly) or packages (to copy full packages and theirdependencies) option the /etc/makejail/apache.py configuration file.
• Type ps aux | grep apache to make sure Apache is running. You should see something like:root 180 0.0 1.1 2936 1436 ? S 04:03 0:00 /usr/sbin/apachechrapach 189 0.0 1.1 2960 1456 ? S 04:03 0:00 /usr/sbin/apachechrapach 190 0.0 1.1 2960 1456 ? S 04:03 0:00 /usr/sbin/apachechrapach 191 0.0 1.1 2960 1456 ? S 04:03 0:00 /usr/sbin/apachechrapach 192 0.0 1.1 2960 1456 ? S 04:03 0:00 /usr/sbin/apachechrapach 193 0.0 1.1 2960 1456 ? S 04:03 0:00 /usr/sbin/apache• Make sure the Apache processes are running chrooted by looking in the /proc filesystem: ls -la /proc/process_number/root/. where process_number is one of the PID numbers listed
above (2nd column; 189 for instance). The entries for a restricted tree should be listed:

drwxr-sr-x 10 root staff 240 Dec 2 16:06 .
drwxrwsr-x 4 root staff 72 Dec 2 08:07 ..
drwxr-xr-x 2 root root 144 Dec 2 16:05 bin
drwxr-xr-x 2 root root 120 Dec 3 04:03 dev
drwxr-xr-x 5 root root 408 Dec 3 04:03 etc
drwxr-xr-x 2 root root 800 Dec 2 16:06 lib
dr-xr-xr-x 43 root root 0 Dec 3 05:03 proc
drwxr-xr-x 2 root root 48 Dec 2 16:06 sbin
drwxr-xr-x 6 root root 144 Dec 2 16:04 usr
drwxr-xr-x 7 root root 168 Dec 2 16:06 var
To automate this test, you can type:ls -la /proc/`cat /var/chroot/apache/var/run/apache.pid`/root/.
FIXME: Add other tests that can be run to make sure the jail is closed?
The reason I like this is because setting up the jail is not very difficult and the server can be updated injust two lines:

183

Appendix

apt-get update && apt-get install apache
makejail /etc/makejail/apache.py

See also
If you are looking for more information you can consider these sources of information in which the information presented is based: http://www.floc.net/makejail/, this program was written by Alain Tesio

184