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NIST Special Publication 800-10

Keeping Your Site Comfortably Secure: An Introduction to Internet

Firewalls

December 1994

Computer Security Division (Information Technology Lab)

http://csrc.nist.gov/
PUBLICATIONS

Publication 800-10 js^ccping Your Sitc Comfortably

Secure: An Introduction to

Internet Firewalls

U.S. DEPARTMENT OF

COMMERCE

Technology Administration

National Institute of Standards

and Technology John R Wack

Lisa J. Carnahan

COMPUTER SECURITY

QC

100

,U57

). 800-10
The National Institute of Standards and Technology was established in 1988 by Congress to "assist industry

in the development of technology . . . needed to improve product quality, to modernize manufacturing processes,

to ensure product reliability . . . and to facilitate rapid commercialization ... of products based on new scientific discoveries."

NIST, originally founded as the National Bureau of Standards in 1901, works to strengthen U.S. industry's

competitiveness; advance science and engineering; and improve public health, safety, and the environment. One of the

agency's basic functions is to develop, maintain, and retain custody of the national standards of measurement, and

provide the means and methods for comparing standards used in science, engineering, manufacturing, commerce,

industry, and education with the standards adopted or recognized by the Federal Government.

As an agency of the U.S. Commerce Department's Technology Administration, NIST conducts basic and applied

research in the physical sciences and engineering, and develops measurement techniques, test methods, standards, and

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'At Boulder, CO 80303. ^Some elements at Boulder, CO 80303.
NisT Special Publication 800-10 Keeping Your Site Comfortably

Secure: An Introduction to

Internet Firewalls

John P. Wack

Lisa J. Carnahan

COMPUTER SECURITY

Computer Systems Laboratory

National Institute of Standards

and Technology

Gaithersburg, MD 20899-0001

December 1994

U.S. Department of Commerce

Ronald H. Brown, Secretary

Technology Administration

Mary L. Good, Under Secretary for Technology

National Institute of Standards and Technology

Arati Prabhakar, Director
Reports on Computer Systems Technology

The National Institute of Standards and Technology (NIST) has a unique responsibility for computer

systems technology within the Federal government. NlST's Computer Systems Laboratory (CSL) develops standards and guidelines, provides technical assistance, and conducts research for computers and

related telecommunications systems to achieve more effective utilization of Federal information technology resources. CSL's responsibilities include development of technical, management, physical, and administrative standards and guidelines for the cost-effective security and privacy of sensitive unclassified

information processed in Federal computers. CSL assists agencies in developing security plans and in improving computer security awareness training. This Special Publication 800 series reports CSL re- search and guidelines to Federal agencies as well as to organizations in industry, government, and

academia.

National Institute of Standards and Technology Special Publication 800-10

Natl. Inst. Stand. Technol. Spec. Publ. 800-10, 70 pages (Dec. 1994)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1994

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402
Ill

Abstract

This document provides an overview of the Internet and security-related problems. It then provides an overview of firewall components and the general reasoning behind firewall usage. Several types of network access policies are described, as well as technical

implementations of those policies. Lastly, the document contains pointers and references

for more detailed information.

The document is designed to assist users in understanding the nature of Internet-related

security problems and what types of firewalls will solve or alleviate specific problems.

Users can then use this document to assist in purchasing or planning a firewall.
iv

This work is a contribution

of the National Institute of Standards and Technology,

and is not subject to copyright.

Because of the nature of this report, it is necessary to mention vendors and commercial

products. The presence or absence of a particular trade name product does not imply

criticism or endorsement by the National Institute of Standards and Technology, nor does

it imply that the products identified are necessarily the best available.

Acknowledgments

The National Institute of Standards and Technology would like to thank the following

individuals who reviewed drafts of this document and advised on document structure

and content: David Curry of Purdue University, Uwe Ellermann of the DFN-CERT

in Germany, and Stephen Weeber of the Department of Energy's Computer Incident

Advisory Capability (CIAC).
Contents

Preface ix

1 Introduction to the Internet and Internet Security 1

1.1 The Internet 1

1.1.1 Common Services 2

1.1.2 Internet Hosts 3

1.2 Overview of TCP/IP Internals 3

1.2.1 IP , 4

1.2.2 TCP 5

1.2.3 UDP 6

1.2.4 ICMP 6

1.2.5 TCP and UDP Port Structure 6

1.3 Security-Related Problems 8

1.3.1 Security Incidents on the Internet 8

1.3.2 Weak Authentication 9

1.3.3 Ease of Spying/Monitoring 10

1.3.4 Ease of Spoofing 10

1.3.5 Flawed LAN Services and Mutually Trusting Hosts 12

1.3.6 Complex Configuration and Controls 12

1.3.7 Host-based Security Does Not Scale 13

1.4 How Vulnerable Are Internet Sites? 13

2 Introduction to Firewalls 15

2.1 The Firewall Concept 15

2.2 Why Firewalls 16

2.2.1 Protection from Vulnerable Services 16

2.2.2 Controlled Access to Site Systems 17

2.2.3 Concentrated Security 17

2.2.4 Enhanced Privacy 17

2.2.5 Logging and Statistics on Network Use, Misuse 18

2.2.6 Policy Enforcement 18

2.3 Issues and Problems with Firewalls 18

2.3.1 Restricted Access to Desirable Services 18

2.3.2 Large Potential for Back Doors 19

2.3.3 Little Protection from Insider Attacks 19

V
vi CONTENTS

2.3.4 Other Issues 19

2.4 Firewall Components 20

2.4.1 Network Policy 21

2.4.2 Advanced Authentication 22

2.4.3 Packet Filtering 24

2.4.4 Application Gateways 29

3 Putting the Pieces Together: Firewall Examples 33

3.1 Packet Filtering Firewall 33

3.2 Dual-homed Gateway Firewall 34

3.3 Screened Host Firewall 36

3.4 Screened Subnet Firewall 38

3.5 Integrating Modem Pools with Firewalls 40

4 Next Steps 43

4.1 Firewall Policy 43

4.1.1 Steps in Creating a Service Access Policy 43

4.1.2 Flexibility in Policy 45

4.1.3 Remote User Advanced Authentication Policy 45

4.1.4 Dial-in/out Policy 46

4.1.5 Remote Network Connections 46

4.1.6 Information Server Policy 46

4.2 Procuring a Firewall 47

4.2.1 What Should a Firewall Contain? 47

4.2.2 To Buy or Build a Firewall 49

4.3 Administration Issues with Firewalls 50

4.3.1 System Management Expertise 50

4.3.2 Site System Administration 50

4.3.3 Incident Handling Contacts 51

Bibliography 53

A On-Line Sources for More Information 55

A.l Firewall-Specific Information 55

A. 2 NIST Computer Security Resource Clearinghouse 55

A. 3 Forum of Incident Response and Security Teams 56

B Internet Firewalls Frequently Asked Questions 59
List of Figures

1.1 Conceptual View of Services and Layers in TCP/IP 4

1.2 TELNET Port, IP Interaction 7

2.1 Router and Application Gateway Firewall Example 15

2.2 Use of Advanced Authentication on a Firewall to Preauthenticate TELNET, FTP Traffic 23

2.3 Representation of Packet Filtering on TELNET and SMTP 25

2.4 Virtual Connection Implemented by an Application Gateway and Proxy

Services 29

3.1 Packet Filtering Firewall 34

3.2 Dual-homed Gateway Firewall with Router 35

3.3 Screened Host Firewall 37

3.4 Screened Subnet Firewall with Additional Systems 39

3.5 Modem Pool Placement with Screened Host Firewall 41

3.6 Modem Pool Placement with Screened Subnet and Dual-Homed Firewalls. 42

vii

Preface

The Internet is a world-wide collection of networks that all use a common protocol for

communications. Many organizations are in the process of connecting to the Internet to

take advantage of Internet services and resources. Businesses and agencies are now using

the Internet or considering Internet access for a variety of purposes, including exchanging

e-mail, distributing agency information to the public, and conducting research. Many

organizations are connecting their existing internal local area networks to the Internet so that local area network workstations can have direct access to Internet services.

Internet connectivity can offer enormous advantages, however security needs to be a major

consideration when planning an Internet connection. There are significant security risks

associated with the Internet that often are not obvious to new (and existing) users. In

particular, intruder activity as well as vulnerabilities that could assist intruder activity are widespread. Intruder activity is difficult to predict and at times can be difficult to

discover and correct. Many organizations already have lost productive time and money

in dealing with intruder activity; some organizations have had their reputations suffer as a result of intruder activity at their sites being publicized.

This publication focuses on security considerations for organizations considering Internet

connections as well as for organizations already connected to the Internet. In particular,

this document focuses on Internet firewalls as one of the mechanisms and methods used

for protecting sites against Internet-borne threats. This document recommends that

organizations use firewall technology and other related tools to filter connections and

limit access. This document is an expansion of the issues and guidance contained in

NIST CSL Bulletin, Connecting to the Internet: Security Considerations [NIST93].

Purpose

The purpose of this document is to provide a basis of understanding of how firewalls work

and the steps necessary for implementing firewalls. Users can then use this document to

assist in planning or purchasing a firewall. This document does not explain how to build

a firewall; references are provided for more detailed information.

ix
X PREFACE

Audience

The intended audience of this pubhcation is technical-level management, i.e., those individuals who may be responsible for implementing or maintaining Internet connections.

This document would also be appropriate for other management who wish to learn more

about Internet security issues.

Some technical background in computer security and computer network communications

is assumed. However, this document is intended to be a starting point; more detailed

information about Internet security and firewalls can be found in the references section.

Document Structure

This document begins with an overview of the Internet and common services. It describes

Internet-related security problems in detail by examining problems with various TCP/IP

services and by examining other factors that have caused the Internet to grow less secure. Chapter 2 discusses firewalls, their benefits as well as their disadvantages, and then the

various firewall components, including advanced authentication measures and network

access policy. Chapter 3 describes several firewall configurations that illustrate how the

firewall components fit together and can be used to implement various policies. Chapter

4 discusses procurement, administrative issues, and other actions sites should take to

secure their Internet-connected systems. Appendix A provides pointers to other books

and information about firewalls and Internet security. Appendix B contains a collection

of frequently asked questions about firewalls that is available on-line (see Appendix B for

more information).

Terminology

Internet firewalls are often referred to as secure Internet gateways in other literature.

This document uses firewall to refer to a secure Internet gateway.

A firewall, as defined in this document, includes a number of items such as policy, network

arrangement, and technical controls and procedures. This document uses firewall system

when referring to the hosts or routers that implement the firewall.

This document, when referring to a network protected by a firewall, uses protected subnet

or protected LAN (Local Area Network).

Some people dispute whether TCP/IP protocols should be referred to as protocols or

services. It could be argued, for example, that TELNET is a protocol, a service, or a

command. Where it makes obvious sense, this document uses protocol, otherwise it uses
PREFACE xi

service.

This document uses application gateways to refer to some firewall systems as opposed to

bastion hosts.

As much as possible, this document avoids using terms such as hacker and cracker, and

uses instead the less ambiguous intruder and attacker.

Background

The Internet is a vital and growing network that is changing the way many organizations

and individuals communicate and do business. However, the Internet suffers from significant and widespread security problems. Many agencies and organizations have been

attacked or probed} by intruders, with resultant high losses to productivity and reputation. In some cases, organizations have had to disconnect from the Internet temporarily,

and have invested significant resources in correcting problems with system and network

configuration. Sites that are unaware of or ignorant of these problems face a significant

risk that they will be attacked by network intruders. Even sites that do observe good

security practices face problems with new vulnerabilities in networking software and the

persistence of some intruders.

A number of factors have contributed to this state of affairs. The fundamental problem may be that the Internet was not designed to be very secure, i.e., open access for

the purposes of research was the prime consideration at the time the Internet was implemented. However, the phenomenal success of the Internet in combination with the

introduction of diff"erent types of users, including unethical users, has aggravated existing

security deficiencies to the extent that wide-open Internet sites risk inevitable break-ins

and resultant damages. Other factors include the following:

• vulnerable TCP/IP services - a number of the TCP/IP services are not secure

and can be compromised by knowledgeable intruders; services used in the local

area networking environment for improving network management are especially

vulnerable,

• ease of spying and spoofing - the majority of Internet traffic is unencrypted;

e-mail, passwords, and file transfers can be monitored and captured using readilyavailable software, intruders can then reuse passwords to break into systems,

• lack of policy - many sites are configured unintentionally for wide-open Internet

access without regard for the potential for abuse from the Internet; many sites

^Intruders have been observed to target specific sites for intrusions by methodically scanning host

systems for vulnerabilities. Intruders often use automated probes, i.e., software that scans all host

systems connected to a site's network. This is sometimes referred to as probing a site.
xii PREFACE

permit more TCP/IP services than they require for their operations and do not

attempt to limit access to information about their computers that could prove

valuable to intruders, and

• complexity of configuration - host security access controls are often complex to

configure and monitor; controls that are accidentally misconfigured often result in

unauthorized access.

Solutions

Fortunately, there are readily-available solutions that can be used to improve site security.

A firewall system is one technique that has proven highly effective for improving the

overall level of site security. A firewall system is a collection of systems, routers, and

policy placed at a site's central connection to a network. A firewall forces all network

connections to pass through the gateway where they can be examined and evaluated,

and provides other services such as advanced authentication measures to replace simple

passwords. The firewall may then restrict access to or from selected systems, or block

certain TCP/IP services, or provide other security features. A well-configured firewall

system can act also as an organization's "public-relations vehicle" and can help to present

a favorable image of the organization to other Internet users. A simple network usage policy that can be implemented by a firewall system is to provide

access from internal to external systems, but little or no access from external to internal

systems. However, a firewall does not negate the need for stronger system security.

There are many tools available for system administrators to enhance system security and

provide additional logging capability. Such tools can check for strong passwords, log

connection information, detect changes in system files, and provide other features that

will help administrators detect signs of intruders and break-ins.

Recommendations

NIST recommends that agencies and organizations, prior to connecting to the Internet,

develop policy that clearly identifies the Internet services they will be using and how those

services will be used. The policy should be clear, concise, and understandable, with a

built-in mechanisms for changing the policy. Organizations should strongly consider using

firewall systems as part of the implementation of that policy. NIST recommends also

that agencies and organizations use advanced authentication measures, i.e., smartcards,

or authentication tokens, or other one-time password mechanisms, as an integral part of

firewalls for authenticating connections to site systems.
1

Introduction to the Internet and

Internet Security

While Internet connectivity offers enormous benefits in terms of increased access to information, Internet connectivity is not necessarily a good thing for sites with low levels of

security. The Internet suffers from glaring security problems that, if ignored, could have

disastrous results for unprepared sites. Inherent problems with TCP/IP services, the

complexity of host configuration, vulnerabilities introduced in the software development

process, and a variety of other factors have all contributed to making unprepared sites

open to intruder activity and related problems.

The following sections present a brief overview of the Internet, TCP/IP, and then explain

what some of the Internet security related problems are and what factors have contributed

to their seriousness.

1.1 The Internet

The Internet is a world-wide "network of networks" that use the TCP/IP (Transmission

Control Protocol/Internet Protocol) protocol suite for communications. The Internet was

created initially to help foster communication among government-sponsored researchers.

Throughout the 1980's, the Internet grew steadily to include educational institutions,

government agencies, commercial organizations, and international organizations. In the

1990's, the Internet has undergone phenomenal growth, with connections increasing faster

than any other network ever created (including the telephone network). Many millions of

users are now connected to the Internet, with roughly half being business users [Cerf93].

The Internet is being used as the basis for the National Information Infrastructure (Nil).

1
2 1.1 THE INTERNET

1.1.1 Common Services

There are a number of services associated with TCP/IP and the Internet. The most commonly used service is electronic mail {e-mail), implemented by the Simple Mail Transfer

Protocol (SMTP). Also, TELNET (terminal emulation), for remote terminal access, and

FTP (file transfer protocol) are used widely. Beyond that, there are a number of services

and protocols used for remote printing, remote file and disk sharing, management of

distributed databases, and for information services. Following is a brief list of the most

common services:

• SMTP - Simple Mail Transfer Protocol, used for sending and receiving electronic

mail,

• TELNET - used for connecting to remote systems connected via the network, uses

basic terminal emulation features,

• FTP - File Transfer Protocol, used to retrieve or store files on networked systems,

• DNS - Domain Name Service, used by TELNET, FTP, and other services for

translating host names to IP addresses,

• information-based services, such as

— gopher - a menu-oriented information browser and server that can provide a

user-friendly interface to other information-based services, — WAIS - Wide Area Information Service, used for indexing and searching with

databases of files, and — WWW/http - World Wide Web, a superset of FTP, gopher, WAIS, other

information services, using the hypertext transfer protocol (http), with Mosaic

being a popular WWW client,

• RPC-based services - Remote Procedure Call services, such as

— NFS - Network File System, allows systems to share directories and disks,

causes a remote directory or disk to appear to be local, and — NIS - Network Information Services, allows multiple systems to share

databases, e.g., the password file, to permit centralized management,

• X Window System - a graphical windowing system and set of application libraries

for use on workstations, and

• rlogin, rsh, and other "r" services - employs a concept of mutually trusting

hosts, for executing commands on other systems without requiring a password.
1.1.2 Internet Hosts 3

Although TCP/IP can be used equally well in a local area or wide area networking

environment, a common use is for file and printer sharing at the local area networking

level and for electronic mail and remote terminal access at both the local and the wide area

networking levels. Gopher and Mosaic are increasingly popular; both present problems

to firewall designers as will be discussed in later sections.

1.1.2 Internet Hosts

Many host systems connected to the Internet run a version of the UNIX operating system.

TCP/IP was first implemented in the early 1980's for the version of UNIX written at

the University of California at Berkeley known as the Berkeley Software Distribution

(BSD). Many modern versions of UNIX derive their networking code directly from the

BSD releases, thus UNIX provides a more-or-less standard set of TCP/IP services. This

standard of sorts has resulted in many different versions of UNIX suff"ering from the same

vulnerabilities, however it has also provided a common means for implementing firewall

strategies such as IP packet filtering. It is important to note that BSD UNIX source

code is fairly easy to obtain free from Internet sites, thus many good and bad people

have been able to study the code for potential flaws and exploitable vulnerabilities.

Although UNIX is the predominant Internet host operating system, many other types

of operating systems and computers are connected to the Internet, including systems

running Digital Equipment Corporation's VMS, NeXT, mainframe operating systems,

and personal computer operating systems such as for DOS, Microsoft Windows, and for

Apple systems. Although personal computer systems often provide only client services,

i.e., one can use TELNET to connect from but not to a personal computer, increasingly

powerful personal computers are also beginning to provide, at low cost, the same services as larger hosts. Versions of UNIX for the personal computer, including Linux, FreeBSD,

and BSDi, and other operating systems such as Microsoft Windows NT, can provide the

same services and applications that were, until recently, found only on larger systems.

The ramifications of this are that more people are able to utilize a wider array of TCP/IP

services than ever before. While this is good in that the benefits of networking are more

available, it has negative consequences in that there is more potential for harm from

intruders (as well as uneducated but well-intentioned users who, to some sites, may

appear to be intruders).

1.2 Overview of TCP/IP Internals

This section provides a simplified overview of TCP/IP for the purposes of later discussion on Internet-related security problems. [Com91a], [Com91b], [Ford94], [Hunt92],
4 1 . 2 OVERVIEW OF TCP/IP INTERNALS

and [Bel89] provide more complete descriptions; readers who wish to learn more should

consult these references.

Part of the popularity of the TCP/IP protocol suite is due to its ability to be implemented

on top of a variety of communications channels and lower-level protocols such as Tl and

X.25, Ethernet, and RS-232-controlled serial lines. Most sites use Ethernet connections

at local area networks to connect hosts and client systems, and then connect that network

via a Tl line to a regional network (i.e., a regional TCP/IP backbone) that connects to

other organizational networks and h)ackbones. Sites customarily have one connection

to the Internet, but large sites often have two or more connections. Modem speeds are increasing as new communications standards are being approved, thus versions of TCP/IP

that operate over the switched telephone network are becoming more popular. Many sites

and individuals use PPP (Point- to- Point Protocol) and SLIP (Serial Line IP), to connect

networks and workstations to other networks using the switched telephone network.

TCP/IP is more correctly a suite of protocols including TCP and IP, UDP (User Datagram Protocol), ICMP (Internet Control Message Protocol), and several others. The

TCP/IP protocol suite does not conform exactly to the Open Systems Interconnection's

seven layer model, but rather could be pictured as shown in figure LI.

APPLICATION LAYER

TRANSPORT LAYER

IP ICMP NETWORK. LAYER

IP Packet PHYSICAL LAYER

Figure LI: Conceptual View of Services and Layers in TCP/IP.

1.2.1 IP

The IP layer receives packets delivered by lower-level layers, e.g., an Ethernet device

driver, and passes the packets "up" to the higher-layer TCP or UDP layers. Conversely,
1.2.2 TCP 5

IP transmits packets that have been received from the TCP or UDP layers to the lowerlevel layer.

IP packets are unreliable datagrams in that IP does nothing to ensure that IP packets

are delivered in sequential order or are not damaged by errors. The IP packets contain

the address of the host from which the packet was sent, referred to as the source address,

and the address of the host that is to receive the packet, referred to as the destination

address.

The higher-level TCP and UDP services generally assume that the source address in a

packet is valid when accepting a packet. In other words, the IP address forms the basis

of authentication for many services; the services trust that the packet has been sent from

a valid host and that host is indeed who it says it is. IP does contain an option known

as IP Source Routing, which can be used to specify a direct route to a destination and

return path back to the origination. The route could involve the use of other routers or hosts that normally would not be used to forward packets to the destination. A

source routed IP packet, to some TCP and UDP services, appears to come from the last

system in the route as opposed to coming from the true origination. This option exists

for testing purposes, however [Bel89] points out that source routing can be used to trick

systems into permitting connections from systems that otherwise would not be permitted

to connect. Thus, that a number of services trust and rely on the authenticity of the IP

source address is problematic and can lead to breakins and intruder activity.

1.2.2 TCP

If the IP packets contain encapsulated TCP packets, the IP software will pass them

"up" to the TCP software layer. TCP sequentially orders the packets and performs error

correction, and implements virtual circuits, or connections between hosts. The TCP

packets contain sequence numbers and acknowledgements of received packets so that

packets received out of order can be reordered and damaged packets can be retransmitted.

TCP passes its information up to higher-layer applications, e.g., a TELNET client or

server. The applications, in turn, pass information back to the TCP layer, which passes

information down to the IP layer and device drivers and the physical medium, and back

to the receiving host.

Connection oriented services, such as TELNET, FTP, rlogin, X Windows, and SMTP,

require a high degree of reliability and therefore use TCP. DNS uses TCP in some cases

(for transmitting and receiving domain name service databases), but uses UDP for transmitting information about individual hosts.
6 1 . 2 OVERVIEW OF TCP/IP INTERNALS

1.2.3 UDP

As shown in figure 1.1, UDP interacts with application programs at the same relative

layer as TCP. However, there is no error correction or retransmission of misordered or lost

packets. UDP is therefore not used for connection-oriented services that need a virtual

circuit. It is used for services that are query-response oriented, such as NFS, where the

number of messages with regard to the exchange is small compared to TELNET or FTP

sessions. Services that use UDP include RPC-based services such as NTS and NFS, NTP

(Network Time Protocol), and DNS (DNS also uses TCP).

It is easier to spoof UDP packets than TCP packets, since there is no initial connection

setup (handshake) involved (since there is no virtual circuit between the two systems)

[Ches94]. Thus, there is a higher risk associated with UDP-based services.

1.2.4 ICMP

ICMP (Internet Control Message Protocol) is at the same relative layer as IP; its purpose is to transmit information needed to control IP traffic. It is used mainly to provide

information about routes to destination addresses. ICMP redirect messages inform hosts

about more accurate routes to other systems, whereas ICMP unreachable messages indicate problems with a route. Additionally, ICMP can cause TCP connections to terminate

"gracefully" if the route becomes unavailable, ping is a commonly-used ICMP-based service.

[Bel89] discusses two problems with ICMP: older versions of UNIX could drop all connections between two hosts even if only one connection was experiencing network problems.

Also, ICMP redirect messages can be used to trick routers and hosts acting as routers

into using "false" routes; these false routes would aid in directing traffic to an attacker's

system instead of a legitimate trusted system. This could in turn lead to an attacker

gaining access to systems that normally would not permit connections to the attacker's

system or network.

1.2.5 TCP and UDP Port Structure

TCP and UDP services generally have a client-server relationship. For example, a TELNET server process initially sits idle at a system, waiting for an incoming connection.

A user then interacts with a TELNET client process, which initiates a connection with

the TELNET server. The client writes to the server, the server reads from the client

and sends back its response. The client reads the response and reports back to the user. Thus, the connection is bidirectional and can be used for reading and writing.
1.2.5 TCP and UDP Port Structure 7

How are multiple TELNET connections between two systems identified and coordinated?

A TCP or UDP connection is uniquely identified by the following four items present in

each message:

• source IP address - the address of the system that sent the packet,

• destination IP address - the address of the system that receives the packet,

• source port - the connection's port at the source system, and

• destination port - the connection's port at the destination system.

The port is a software construct that is used by the client or server for sending or receiving messages; a port is identified by a 16-bit number. Server processes are usually

associated with a fixed port, e.g., 25 for SMTP or 6000 for X Windows; the port number

is "well-known" because it, along with the destination IP address, needs to be used when

initiating a connection to a particular host and service. Client processes, on the other

hand, request a port number from the operating system when they begin execution; the

port number is random although in some cases it is the next available port number.

As an example of how ports are used for sending and receiving messages, consider the

TELNET protocol. The TELNET server listens for incoming messages on port 23, and

sends outgoing messages to port 23. A TELNET client, on the same or different system,

would first request an unused port number from the operating system, and then use this

port when sending and receiving messages. It would place this port number, say 3097,

in packets destined for the TELNET server so that the server, when responding to the

client, could place the client's port number in its TCP packets. The client's host, upon

receiving a message, would examine the port and know which TELNET client should

receive the message. This is shown conceptually in figure 1.2.

123.4.5.6-3097 to 1234.5.30-23,

123.4,5.6' - Client 123.4.5.30 - Server

Figure 1.2: TELNET Port, IP Interaction.

There is a somewhat-uniform rule that only privileged server processes, i.e., those processes that operate with UNIX superuser privileges, can use port numbers less than

1024 (referred to as privileged ports). Servers mostly use ports numbered less than 1024,
8 1.3 SECURITY-RELATED PROBLEMS

whereas clients generally must request unprivileged port numbers from the operating

system. Although this rule is not firm and is not required in the TCP/IP protocol

specifications, BSD-based systems adhere to it. As an accidental but fortuitous result,

firewalls can block or filter access to services by examining the port numbers in TCP or UDP packets and then routing or dropping the packet based on a policy that specifies

which services are permitted or denied (this is covered in more detail in Chapter 2).

Not all TCP and UDP servers and clients use ports in as straightforward a fashion as TELNET, but in general the procedure described here is useful in the firewalls context.

For example, many personal computer operating systems have no UNIX superuser concept, but still use ports as described (although there is no standard that requires this).

1.3 Security-Related Problems

As stated earlier, the Internet suffers from severe security-related problems. Sites that

ignore these problems face some significant risk that they will be attacked by intruders

and that they may provide intruders with a staging ground for attacks on other networks.

Even sites that do observe good security practices face problems with new vulnerabilities

in networking software and the persistence of some intruders.

Some of the problems with Internet security are a result of inherent vulnerabilities in

the services (and the protocols that the services implement), while others are a result of

host configuration and access controls that are poorly implemented or overly complex to

administer. Additionally, the role and importance of system management is often shortchanged in job descriptions, resulting in many administrators being, at best, part-time

and poorly prepared. This is further aggravated by the tremendous growth of the Internet

and how the Internet is used; businesses and agencies now depend on the Internet (often

more than they realize) for communications and research and thus have much more to

lose if their sites are attacked. The following sections describe problems on the Internet

and factors that contribute to these problems.

1.3.1 Security Incidents on the Internet

As evidence of the above, three problems have occurred within months of each other. In

the first, persistent vulnerabilities in the UNIX sendmaiP program were discussed openly

on Internet discussion lists. Sites that had not corrected their sendmail programs were

^sendmail is the mail transport software for most UNIX hosts. It is a very large, complex program

that has been found repeatedly to contain vulnerabilities that have permitted intruder access to systems

that run sendmail.
1.3.2 Weak Authentication 9

forced to scramble to correct the programs before attackers used the vulnerabilities to

attack the sites. However, due to the complexity of the sendmail program and networking

software in general, three subsequent versions of sendmail were found to still contain

significant vulnerabilities [CIAC94a]. The sendmail program is used widely, and sites

without firewalls to limit access to sendmail are forced to react quickly whenever problems

are found and vulnerabilities revealed.

In the second, a version of a popular and free FTP server was found to contain a Trojan

Horse that permitted privileged access to the server. Sites using this FTP server, but not

necessarily the contaminated version, were again forced to react very carefully and quickly

to this situation [CIAC94c]. Many sites rely on the wealth of free software available on

the Internet, especially security-related software that adds capability for logging, access

control, and integrity checking that vendors often do not provide as part of the operating

system. While the software is often high quality, sites may have little recourse other

than to rely on the authors of the software if it is found to have vulnerabilities and other

problems.^

The third problem has the strongest implications: [CERT94] and [CIAC94b] reported

that intruders had broken into potentially thousands of systems throughout the Internet,

including gateways between major networks, and installed sniffer programs to monitor

network traffic for usernames and static passwords typed in by users to connect to networked systems. The intruders had used various known techniques for breaking into

systems, as well as using passwords that had been "sniff'ed." One of the implications of

this incident is that static or reusable passwords are obsolete for protecting access to user

accounts. In fact, a user connecting to a remote" system across the Internet may be unintentionally placing that system at risk of attack by intruders who could be monitoring

the network traffic to the remote system.

The following sections go into more detail on problems with Internet security. [Garf92],

[Cur92], [Bel89], [Ches94], and [Farm93] all provide more background and detail; readers

are encouraged to consult these references.

1.3.2 Weak Authentication

Incident handling teams estimate that many incidents stem from use of weak, static passwords. Passwords on the Internet can be "cracked" a number of different ways, however

the two most common methods are by cracking the encrypted form of the password and

by monitoring communications channels for password packets. The UNIX operating system usually stores an encrypted form of passwords in a file that can be read by normal

^It should be pointed out that even vendor-supported software has such problems and may be even

harder to get fixed in a timely fashion.
10 1 . 3 SECURITY-RELATED PROBLEMS

users. The password file can be obtained by simply copying it or via a number of other

intruder methods. Once the file is obtained, an intruder can run readily-available password cracking programs against the passwords. If the passwords are weak, e.g., less that

8 characters, English words, etc., they could be cracked and used to gain access into the

system.

Another problem with authentication results from some TCP or UDP services being able

to authenticate only to the granularity of host addresses and not to specific users. For

example, an NFS (UDP) server cannot grant access to a specific user on a host, it must

grant access to the entire host. The administrator of a server may trust a specific user on a host and wish to grant access to that user, but the administrator has no control

over other users on that host and is thus forced to grant access to all users (or grant no

access at all).

1.3.3 Ease of Spying/Monitoring

It is important to note that when a user connects to her account on a remote host using

TELNET or FTP, the user's password travels across the Internet unencrypted, or in

plaintext. Thus, another method for breaking into systems is to monitor connections for

IP packets bearing a username and password, and then using them on the system to

login normally. If the captured password is to an administrator account, then the job

of obtaining privileged access is made much easier. As noted previously, hundreds and

possibly thousands of systems across the Internet have been penetrated as a result of

monitoring for usernames and passwords.

Electronic mail, as well as the contents of TELNET and FTP sessions, can be monitored

and used to learn information about a site and its business transactions. Most users do

not encrypt e-mail, yet many assume that e-mail is secure and thus safe for transmitting

sensitive information.

The X Window System is an increasingly popular service that is also vulnerable to

spying and monitoring. X permits multiple windows to be opened at a workstation,

along with display of graphics and multi-media applications (for example, the WWW

browser Mosaic). Intruders can sometimes open windows on other systems and read

keystrokes that could contain passwords or sensitive information.

1.3.4 Ease of Spoofing

As noted in section 1.2.1, the IP address of a host is presumed to be valid and is therefore

trusted by TCP and UDP services. A problem is that, using IP source routing, an

attacker's host can masquerade as a trusted host or client. Briefly, IP source routing is
1.3.4 Ease of Spoofing 11

an option that can be used to specify a direct route to a destination and return path

back to the origination. The route can involve the use of other routers or hosts that

normally would not be used to forward packets to the destination. An example of how

this can be used such that an attacker's system could masquerade as the trusted client

of a particular server is as follows:

1. the attacker would change her host's IP address to match that of the trusted client,

2. the attacker would then construct a source route to the server that specifies the

direct path the IP packets should take to the server and should take from the server back to the attacker's host, using the trusted client as the last hop in the route to

the server,

3. the attacker sends a client request to the server using the source route,

4. the server accepts the client request as if it came directly from the trusted client

and returns a reply to the trusted client,

5. the trusted client, using the source route, forwards the packet on to the attacker's

host.

Many UNIX hosts accept source routed packets and will pass them on as the source route

indicates. Many routers will accept source routed packets as well, whereas some routers

can be configured to block source routed packets.

An even simpler method for spoofing a client is to wait until the client system is turned

off and then impersonate the client's system. In many organizations, staff members use

personal computers and TCP/IP network software to connect to and utilize UNIX hosts

as a local area network server. The personal computers often use NFS to obtain access to server directories and files (NFS uses IP addresses only to authenticate clients). An

attacker could, after hours, configure a personal computer with the same name and IP

address as another's, and then initiate connections to the UNIX host as if it were the

"real" client. This is very simple to accomplish and likely would be an insider attack.

Electronic mail on the Internet is particularly easy to spoof and, without enhancements

such as digital signatures[NIST94a], generally cannot be trusted. As a brief example,

consider the exchange that takes place when Internet hosts exchange mail. The exchange

takes place using a simple protocol consisting of ASCII-character commands. An intruder

easily could enter these commands by hand by using TELNET to connect directly to a

system's Simple Mail Transfer Protocol (SMTP) port. The receiving host trusts that

the sending host is who it says it is, thus the origin of the mail can be spoofed easily by

entering a sender address that is different from the true address. As a result, any user,

without privileges, can falsify or spoof e-mail.
12 1.3 SECURITY-RELATED PROBLEMS

Other services, such as Domain Name Service, can be spoofed, but with more difficulty

than electronic mail. These services still represent a threat that needs to be considered

when using them.

1.3.5 Flawed LAN Services and Mutually Trusting Hosts

Host systems are difficult and time consuming to manage securely. To ease management

demands and to enhance local area networking, some sites use services such as Network

Information Services (NIS) and Network File System (NFS). These services can greatly

reduce the amount of redundant management by permitting certain databases such as the password files to be managed in a distributed manner and by permitting systems to

share files and data. Ironically, these services are inherently insecure and can be exploited

to gain access by knowledgeable intruders. If a central server system is compromised,

then the other systems trusting the central system could be compromised rather easily.

Some services such as rlogin allow for hosts to "trust" each other for the purposes of user convenience and enhanced sharing of systems and devices. If a system is penetrated or spoofed, and that system is trusted by other systems, it is simple for the intruder to then

gain access to the other systems. As an example, a user with an account on more than

one system can eliminate the need to enter a password at every system by configuring the

accounts to trust connections from the user's primary system. When the user uses the

rlogin command to connect to a host, the destination system will not ask for a password

or account name, and the user's connection will be accepted. While this has a positive

aspect in that the user's password does not get transmitted and could not be monitored

and captured, it has a negative aspect in that if the user's primary account were to be

penetrated, the intruder could simply use rlogin to penetrate the accounts on the other

systems. For this reason, use of mutually-trusting hosts is discouraged [Bel89], [Ches94].

1.3.6 Complex Configuration and Controls

Host system access controls are often complex to configure and test for correctness. As a

result, controls that are accidentally misconfigured can result in intruders gaining access. Some major UNIX vendors still ship host systems with access controls configured for

maximum (i.e., least secure) access, which can result in unauthorized access if left as is. A number of security incidents have occurred on the Internet due in part to vulnerabilities

discovered by intruders (and subsequently, users, incident response teams, and vendors).

Since most modern variants of UNIX derive their networking code from the BSD releases,

and since the source code to the BSD releases is widely available, intruders have been

able to study the code for bugs and conditions that can be exploited to gain access
1.3.7 Host-based Security Does Not Scale 13

to systems. The bugs exist in part because of the complexity of the software and the

inability to test it in all the environments in which it must operate. Sometimes the bugs

are easily discovered and corrected, other times little can be done except to rewrite the

application, which is usually the option of last resort (the sendmail program may be an

example of the latter).

1.3.7 Host-based Security Does Not Scale

Host-based security does not scale well: as the number of hosts at a site increases, the

ability to ensure that security is at a high level for each host decreases. Given that secure

management of just one system can be demanding, managing many such systems could

easily result in mistakes and omissions. A contributing factor is that the role of system

management is often short-changed and performed in haste. As a result, some systems

will be less secure than other systems, and these systems could be the weak links that

ultimately will "break" the overall security chain.

If a vulnerability is discovered in networking software, a site that is not protected by a

firewall needs to correct the vulnerability on all exposed systems as quickly as possible.

As discussed in section 1.3.2, some vulnerabilities have permitted easy access to the UNIX

root account; a site with many UNIX hosts would be particularly at risk to intruders in

such a situation. Patching vulnerabilities on many systems in a short amount of time

may not be practical and, if different versions of the operating system are in use, may

not be possible. Such a site would be a "sitting duck" to intruder activity.

1.4 How Vulnerable Are Internet Sites?

As noted in the preceding sections, a number of the TCP and UDP services provide poor

levels of security in today's Internet environment. With millions of users connected to the

Internet, and governments and industry placing more reliance on Internet availability, the

flaws in these services, as well as the availability of source code and tools to automate

breaking into systems, can be devastating to sites that suflfer break-ins. However, it

is difficult to know or assess the true risks of using the Internet and, following, how

vulnerable a site is to some form of attack from intruders and related activity. There are no firm statistics.

The Computer Emergency Response Team Coordination Center (CERT/CC) has maintained some base statistics on the number of incidents they have handled since their

inception in 1988. The numbers have climbed quite steeply as each year has progressed,

however at the same time, the Internet has also grown dramatically. In some cases, CERT

counts multiple break-ins of the same pattern as all part of a single incident, thus a sin-
14 1.4 HOW VULNERABLE ARE INTERNET SITES?

gle incident could be comprised of hundreds of break-ins at different sites. It is difficult

to draw strong conclusions as to whether the number of incidents and break-ins has re- mained proportionally the same. Further complicating this is that more people are aware

of the existence of incident response teams and may be more likely to report incidents,

thus one wonders whether there are more incidents or just more incidents reported.

NIST asserts that the Internet, while a useful and vital network, is at the same time

very vulnerable to attacks. Sites that are connected to the Internet face some risk that

site systems will be attacked or affected in some form by intruders, and that the risk is

significant. The following factors would influence the level of risk:

• the number of systems at the site,

• what services the site uses,

• how interconnected the site is to the Internet,

• the site's profile, or how well-known the site is, and

• how prepared the site is to handle computer security incidents.

The more systems that are connected, obviously the harder it is to control their security.

Equally, if a site is connected to the Internet at several points, it likely would be more

vulnerable to attacks than a site with a single gateway. At the same time, though, how

well prepared a site is, and the degree to which the site relies on the Internet, can increase or decrease the risk. A site's high profile could attract more potential intruders who wish

to do some harm to the site's image. It should be mentioned, though, that "quiet,"

less-frequently used sites are also attractive to intruders since they can more easily hide

their activity.

NIST asserts that sites that use recommended procedures and controls for increasing

computer security have significantly lower risks of attack. Firewalls, combined with onetime passwords that are immune from monitoring or guessing, can increase greatly a site's

overall level of security and make using the Internet quite safe. The following chapters

contain more detail on firewalls and how they can. be used to protect against many of

the threats and vulnerabilities mentioned and referenced in this chapter.
2

Introduction to Firewalls

A number of the security problems with the Internet discussed in Chapter 1 can be

remedied or made less serious through the use of existing and well-known techniques and

controls for host security. A firewall can significantly improve the level of site security

while at the same time permitting access to vital Internet services. This chapter provides

an overview of firewalls, including how they protect against the vulnerabilities described

in Chapter 1, what firewalls don't protect against, and the components that make up

a firewall. This chapter gives special emphasis to the use of advanced authentication

and the importance of policy for determining how a firewall will implement a protection

scheme.

Packet Filtering

Router

Internet

Site Systems

Application Gateway

Figure 2.1: Router and Application Gateway Firewall Example.

2.1 The Firewall Concept

Perhaps it is best to describe first what a firewall is not a firewall is not simply a

router, host system, or collection of systems that provides security to a network. Rather,

a firewall is an approach to security; it helps implement a larger security policy that

defines the services and access to be permitted, and it is an implementation of that

policy in terms of a network configuration, one or more host systems and routers, and

other security measures such as advanced authentication in place of static passwords.

The main purpose of a firewall system is to control access to or from a protected network

(i.e., a site). It implements a network access policy by forcing connections to pass through

15
16 2.2 WKY FIREWALLS

the firewall, where they can be examined and evaluated.

A firewall system can be a router, a personal computer, a host, or a collection of hosts,

set up specifically to shield a site or subnet from protocols and services that can be

abused from hosts outside the subnet. A firewall system is usually located at a higherlevel gateway, such as a site's connection to the Internet, however firewall systems can be

located at lower-level gateways to provide protection for some smaller collection of hosts

or subnets.

2.2 Why Firewalls

The general reasoning behind firewall usage is that without a firewall, a subnet's systems

expose themselves to inherently insecure services such as NFS or NIS and to probes and

attacks from hosts elsewhere on the network. In a firewall-less environment, network

security relies totally on host security and all hosts must, in a sense, cooperate to achieve

a uniformly high level of security. The larger the subnet, the less manageable it is to

maintain all hosts at the same level of security. As mistakes and lapses in security become

more common, break-ins occur not as the result of complex attacks, but because of simple

errors in configuration and inadequate passwords.

A firewall approach provides numerous advantages to sites by helping to increase overall

host security. The following sections summarize the primary benefits of using a firewall.

2.2.1 Protection from Vulnerable Services

A firewall can greatly improve network security and reduce risks to hosts on the subnet

by filtering inherently insecure services. As a result, the subnet network environment

is exposed to fewer risks, since only selected protocols will be able to pass through the

firewall.

For example, a firewall could prohibit certain vulnerable services such as NFS from

entering or leaving a protected subnet. This provides the benefit of preventing the services

from being exploited by outside attackers, but at the same time permits the use of these

services with greatly reduced risk to exploitation. Services such as NIS or NFS that are

particularly useful on a local area network basis can thus be enjoyed and used to reduce

the host management burden.

Firewalls can also provide protection from routing-based attacks, such as source routing and attempts to redirect routing paths to compromised sites via ICMP redirects.

A firewall could reject all source-routed packets and ICMP redirects and then inform

administrators of the incidents.
2.2.2 Controlled Access to Site Systems 17

2.2.2 Controlled Access to Site Systems

A firewall also provides the ability to control access to site systems. For example, some

hosts can be made reachable from outside networks, whereas others can be effectively

sealed off from unwanted access. A site could prevent outside access to its hosts except

for special cases such as mail servers or information servers.

This brings to the fore an access policy that firewalls are particularly adept at enforcing:

do not provide access to hosts or services that do not require access. Put differently, why

provide access to hosts and services that could be exploited by attackers when the access

is not used or required? If, for example, a user requires little or no network access to her

desktop workstation, then a firewall can enforce this policy.

2.2.3 Concentrated Security

A firewall can actually be less expensive for an organization in that all or most modified

software and additional security software could be located on the firewall systems as opposed to being distributed on many hosts. In particular, one-time password systems

and other add-on authentication software could be located at the firewall as opposed to

each system that needed to be accessed from the Internet.

Other solutions to network security such as Kerberos [NIST94c] involve modifications at

each host system. While Kerberos and other techniques should be considered for their

advantages and may be more appropriate than firewalls in certain situations, firewalls

tend to be simpler to implement in that only the firewall need run specialized software.

2.2.4 Enhanced Privacy

Privacy is of great concern to certain sites, since what would normally be considered

innocuous information might actually contain clues that would be useful to an attacker.

Using a firewall, some sites wish to block services such as finger and Domain Name

Service, finger displays information about users such as their last login time, whether

they've read mail, and other items. But, finger could leak information to attackers about

how often a system is used, whether the system has active users connected, and whether

the system could be attacked without drawing attention.

Firewalls can also be used to block DNS information about site systems, thus the names

and IP addresses of site systems would not be available to Internet hosts. Some sites feel

that by blocking this information, they are hiding information that would otherwise be

useful to attackers.
18 2.3 ISSUES AND PROBLEMS WITH FIREWALLS

2.2.5 Logging and Statistics on Network Use, Misuse

If all access to and from the Internet passes through a firewall, the firewall can log accesses and provide valuable statistics about network usage. A firewall, with appropriate alarms

that sound when suspicious activity occurs can also provide details on whether the firewall

and network are being probed or attacked.

It is important to collect network usage statistics and evidence of probing for a number of

reasons. Of primary importance is knowing whether the firewall is withstanding probes

and attacks, and determining whether the controls on the firewall are adequate. Network

usage statistics are also important as input into network requirements studies and risk

analysis activities.

2.2.6 Policy Enforcement

Lastly, but perhaps most importantly, a firewall provides the means for implementing

and enforcing a network access policy. In eff'ect, a firewall provides access control to users and services. Thus, a network access policy can be enforced by a firewall, whereas

without a firewall, such a policy depends entirely on the cooperation of users. A site may

be able to depend on its own users for their cooperation, however it cannot nor should

not depend on Internet users in general.

2.3 Issues and Problems with Firewalls

Given these benefits to the firewall approach, there are also a number of disadvantages,

and there are a number of things that firewalls cannot protect against. A firewall is not

by any means a panacea for Internet security problems.

2.3.1 Restricted Access to Desirable Services

The most obvious disadvantage of a firewall is that it may likely block certain services that

users want, such as TELNET, FTP, X Windows, NFS, etc. However, these disadvantage

are not unique to firewalls; network access could be restricted at the host level as well,

depending on a site's security policy. A well-planned security policy that balances security

requirements with user needs can help greatly to alleviate problems with reduced access

to services.

Some sites may have a topology that does not lend itself to a firewall, or may use services

such as NFS in such a manner that using a firewall would require a major restructuring
2.3.2 Large Potential for Back Doors 19

of network use. For example, a site might depend on using NFS and NIS across major

gateways. In such a situation, the relative costs of adding a firewall would need to be

compared against the cost of the vulnerabilities associated with not using a firewall,

i.e., a risk analysis, and then a decision made on the outcome of the analysis. Other

solutions such as Kerberos may be more appropriate, however these solutions carry their

own disadvantages as well. [NIST94c] contains more information on Kerberos and other

potential solutions.

2.3.2 Large Potential for Back Doors

Secondly, firewalls do not protect against back doors into the site. For example, if

unrestricted modem access is still permitted into a site protected by a firewall, attackers

could effectively jump around the firewall [Haf91]. Modem speeds are now fast enough to

make running SLIP (Serial Line IP) and PPP (Point-to-Point Protocol) practical; a SLIP

or PPP connection inside a protected subnet is in essence another network connection

and a potential backdoor. Why have a firewall if unrestricted modem access is permitted?

2.3.3 Little Protection from Insider Attacks

Firewalls generally do not provide protection from insider threats. While a firewall may

be designed to prevent outsiders from obtaining sensitive data, the firewall does not

prevent an insider from copying the data onto a tape and taking it out of the facility.

Thus, it is faulty to assume that the existence of a firewall provides protection from

insider attacks or attacks in general that do not need to use the firewall. It is perhaps

unwise to invest significant resources in a firewall if other avenues for stealing data or attacking systems are neglected.

•i

2.3.4 Other Issues

Other problems or issues with firewalls are as follows:

• WWW, gopher - Newer information servers and clients such as those for World

Wide Web (WWW), gopher, WAIS, and others were not designed to work well

with firewall policies and, due to their newness, are generally considered risky. The

potential exists for data-driven attacks, in which data processed by the clients can

contain instructions to the clients; the instructions could tell the client to alter access controls and important security-related files on the host.
20 2.4 FIREWALL COMPONENTS

• MBONE - Multicast IP transmissions (MBONE) for video and voice are encapsulated in other packets; firewalls generally forward the packets without examining

the packet contents. MBONE transmissions represent a potential threat if the

packets were to contain commands to alter security controls and permit intruders.

• viruses - Firewalls do not protect against users downloading virus-infected personal computer programs from Internet archives or transferring such programs in

attachments to e-mail. Because these programs can be encoded or compressed in

any number of ways, a firewall cannot scan such programs to search for virus signatures with any degree of accuracy. The virus problem still exists and must be

handled with other policy and anti-viral controls.

• throughput - Firewalls represent a potential bottleneck, since all connections must

pass through the firewall and, in some cases, be examined by the firewall. However, this is generally not a problem today, as firewalls can pass data at Tl (1.5

Megabits/second) rates and most Internet sites are at connection rates less than or equal to Tl.

• all eggs in single basket - A firewall system concentrates security in one spot

as opposed to distributing it among systems. A compromise of the firewall could

be disastrous to other less-protected systems on the subnet. This weakness can

be countered, however, with the argument that lapses and weaknesses in security are more likely to be found as the number of systems in a subnet increase, thereby

multiplying the ways in which subnets can be exploited.

Despite these disadvantages, NIST strongly recommends that sites protect their resources

with firewalls and other security tools and techniques.

2.4 Firewall Components

The primary components (or aspects) of a firewall are:

• network policy,

• advanced authentication mechanisms,

• packet filtering, and

• application gateways.

The following sections describe each of these components more fully.
2.4.1 Network Policy 21

2.4.1 Network Policy

There are two levels of network policy that directly influence the design, installation and

use of a firewall system. The higher-level policy is an issue-specific, network access policy

that defines those services that will be allowed or explicitly denied from the restricted

network, how these services will be used, and the conditions for exceptions to this policy.

The lower-level policy describes how the firewall will actually go about restricting the

access and filtering the services that were defined in the higher level policy. The following

sections describe these policies in brief.

Service Access Policy

The service access policy should focus on Internet-specific use issues as defined above,

and perhaps all outside network access (i.e., dial-in policy, and SLIP and PPP connections) as well. This policy should be an extension of an overall organizational policy

regarding the protection of information resources in the organization. For a firewall to

be successful, the service access policy must be realistic and sound and should be drafted

before implementing a firewall. A realistic policy is one that provides a balance between

protecting the network from known risks, while still providing users access to network

resources. If a firewall system denies or restricts services, it usually requires the strength

of the service access policy to prevent the firewall's access controls from being modified

on an ad hoc basis. Only a management-backed, sound policy can provide this.

A firewall can implement a number of service access policies, however a typical policy

may be to allow no access to a site from the Internet, but allow access from the site to the

Internet. Another typical policy would be to allow some access from the Internet, but

perhaps only to selected systems such as information servers and e-mail servers. Firewalls

often implement service access policies that allow some user access from the Internet to

selected internal hosts, but this access would be granted only if necessary and only if it

could be combined with advanced authentication.

Firewall Design Policy

The firewall design policy is specific to the firewall. It defines the rules used to implement the service access policy. One cannot design this policy in a vacuum isolated

from understanding issues such as firewall capabilities and limitations, and threats and

vulnerabilities associated with TCP/IP. Firewalls generally implement one of two basic

design policies:

1. permit any service unless it is expressly denied, and
22 2.4 FIREWALL COMPONENTS

2. deny any service unless it is expressly permitted.

A firewall that implements the first policy allows all services to pass into the site by

default, with the exception of those services that the service access policy has identified as disallowed. A firewall that implements the second poHcy denies all services by default,

but then passes those services that have been identified as allowed. This second policy

follows the classic access model used in all areas of information security.

The first policy is less desirable, since it offers more avenues for getting around the

firewall, e.g., users could access new services currently not denied by the policy (or even

addressed by the policy) or run denied services at non-standard TCP/UDP ports that

aren't denied by the policy. Certain services such as X Windows, FTP, Archie, and RPC

cannot be filtered easily [Chap92], [Ches94], and are better accommodated by a firewall

that implements the first policy. The second policy is stronger and safer, but it is more

difficult to implement and may impact users more in that certain services such as those

just mentioned may have to be blocked or restricted more heavily.

The relationship between the high level service access policy and its lower level counterpart is reflected in the discussion above. This relationship exists because the implementation of the service access policy is so heavily dependent upon the capabilities and

limitations of the firewall system, as well as the inherent security problems associated

with the wanted Internet services. For example, wanted services defined in the service access policy may have to be denied if the inherent security problems in these services

cannot be effectively controlled by the lower level policy and if the security of the network

takes precedence over other factors. On the other hand, an organization that is heavily

dependent on these services to meet its mission may have to accept higher risk and allow access to these services. This relationship between the service access policy and its

lower level counterpart allows for an iterative process in defining both, thus producing

the realistic and sound policy initially described.

The service access policy is the most significant component of the four described here.

The other three components are used to implement and enforce the policy. (And as noted

above, the service access policy should be a reflection of a strong overall organization se- curity policy.) The effectiveness of the flrewall system in protecting the network depends

on the type of firewall implementation used, the use of proper firewall procedures, and

the service access policy.

2.4.2 Advanced Authentication

Sections 1.3, 1.3.1, and 1.3.2 describe incidents on the Internet that have occurred in

part due to the weaknesses associated with traditional passwords. For years, users have

been advised to choose passwords that would be difficult to guess and to not reveal their
2.4.2 Advanced Authentication 23

passwords. However, even if users follow this advice (and many do not), the fact that

intruders can and do monitor the Internet for passwords that are transmitted in the clear

has rendered traditional passwords obsolete.

Advanced authentication measures such as smartcards, authentication tokens, biometrics, and software-based mechanisms are designed to counter the weaknesses of traditional

passwords. While the authentication techniques vary, they are similar in that the passwords generated by advanced authentication devices cannot be reused by an attacker

who has monitored a connection. Given the inherent problems with passwords on the

Internet, an Internet-accessible firewall that does not use or does not contain the hooks

to use advanced authentication makes little sense.

Some of the more popular advanced authentication devices in use today are called onetime password systems. A smartcard or authentication token, for example, generates a

response that the host system can use in place of a traditional password. Because the

token or card works in conjunction with software or hardware on the host, the generated

response is unique for every login. The result is a one-time password that, if monitored,

cannot be reused by an intruder to gain access to an account. [NIST94a] and [NISTQla]

contain more detail on advanced authentication devices and measures.

Unauthenticated TELNET,

FTP Traffic

Authenticated TELNET,

FTP Traffic

Firewall System

w/Advanced Authentication SW

Figure 2.2: Use of Advanced Authentication on a Firewall to Preauthenticate TELNET,

FTP Traffic.

Since firewalls can centralize and control site access, the firewall is the logical place for

the advanced authentication software or hardware to be located. Although advanced

authentication measures could be used at each host, it is more practical and manageable

to centralize the measures at the firewall. Figure 2.2 illustrates that a site without a

firewall using advanced authentication permits unauthenticated application traffic such

as TELNET or FTP directly to site systems. If the hosts do not use advanced authentication, then intruders could attempt to crack passwords or could monitor the network

for login sessions that would include the passwords. Figure 2.2 also shows a site with a

firewall using advanced authentication, such that TELNET or FTP sessions originating

from the Internet to site systems must pass the advanced authentication before being

permitted to the site systems. The site systems may still require static passwords before
24 2.4 FIREWALL COMPONENTS

permitting access, however these passwords would be immune from exploitation, even if

the passwords are monitored, as long as the advanced authentication measures and other

firewall components prevent intruders from penetrating or bypassing the firewall.

Sections 2.4.4 and 3 contain more information on using advanced authentication measures

with firewalls. See [NIST94b] for more information on using advanced authentication

measures with hosts.

2.4.3 Packet Filtering

IP packet filtering is done usually using a packet filtering router designed for filtering

packets as they pass between the router's interfaces. A packet filtering router usually

can filter IP packets based on some or all of the following fields:

• source IP address,

• destination IP address,

• TCP/UDP source port, and

• TCP/UDP destination port.

Not all packet filtering routers currently filter the source TCP/UDP port, however more

vendors are starting to incorporate this capability. Some routers examine which of the

router's network interfaces a packet arrived at, and then use this as an additional filtering

criterion. Some UNIX hosts provide packet filtering capability, although most do not.

Filtering can be used in a variety of ways to block connections from or to specific hosts

or networks, and to block connections to specific ports. A site might wish to block

connections from certain addresses, such as from hosts or sites that it considers to be

hostile or untrustworthy. Alternatively, a site may wish to block connections from all

addresses external to the site (with certain exceptions, such as with SMTP for receiving

e-mail)

.

Adding TCP or UDP port filtering to IP address filtering results in a great deal of

flexibility. Recall from Chapter 1 that servers such as the TELNET daemon reside

usually at specific ports, such as port 23 for TELNET. If a firewall can block TCP or UDP connections to or from specific ports, then one can implement policies that call

for certain types of connections to be made to specific hosts, but not other hosts. For

example, a site may wish to block all incoming connections to all hosts except for several

firewalls-related systems. At those systems, the site may wish to allow only specific

services, such as SMTP for one system and TELNET or FTP connections to another
2.4.3 Packet Filtering 25

system. With filtering on TCP or UDP ports, this policy can be implemented in a

straightforward fashion by a packet filtering router or by a host with packet filtering

capability.

Other Traffic

Packet Filtering Router

SMTP Traffic Only

Internet

TELNET Traffic Only

D

Figure 2.3: Representation of Packet Filtering on TELNET and SMTP.

As an example of packet filtering, consider a policy to allow only certain connections to

a network of address 123.4.*.*. TELNET connections will be allowed to only one host,

123.4.5.6, which may be the site's TELNET application gateway, and SMTP connections

will be allowed to two hosts, 123.4.5.7 and 123.4.5.8, which may be the site's two electronic

mail gateways. NNTP (Network News Transfer Protocol) is allowed only from the site's

NNTP feed system, 129.6.48.254, and only to the site's NNTP server, 123.4.5.9, and

NTP (Network Time Protocol) is allowed to all hosts. All other services and packets are to be blocked. An example of the ruleset would be as follows:

Type Source Addr Dest Addr Source Port Dest Port Action

tcp * 123.4.5.6 > 1023 23 permit

tcp * 123.4.5.7 > 1023 25 permit

tcp * 123.4.5.8 > 1023 25 permit

tcp 129.6.48.254 123.4.5.9 > 1023 119 permit

udp * 123.4.*.* > 1023 123 permit

* * * * * deny

The first rule allows TCP packets from any source address and port greater than 1023

on the Internet to the destination address of 123.4.5.6 and port of 23 at the site. Port

23 is the port associated with the TELNET server, and all TELNET clients should have

unprivileged source ports of 1024 or higher. The second and third rules work in a similar

fashion, except packets to destination addresses 123.4.5.7 and 123.4.5.8, and port 25

for SMTP, are permitted. The fourth rule permits packets to the site's NNTP server,

but only from source address 129.6.48.254 to destination address 123.4.5.9 and port 119
26 2.4 FIREWALL COMPONENTS

(129.6.48.254 is the only NNTP server that the site should receive news from, thus access

to the site for NNTP is restricted to only that system). The fifth rule permits NT?

traffic, which uses UDP as opposed to TCP, from any source to any destination address

at the site. Finally, the sixth rule denies all other packets - if this rule weren't present,

the router may or may not deny all subsequent packets. This is a very basic example of

packet filtering. Actual rules permit more complex filtering and greater flexibility.

Which Protocols to Filter

The decision to filter certain protocols and fields depends on the network access policy,

i.e., which systems should have Internet access and the type of access to permit. The

following services are inherently vulnerable to abuse and are usually blocked at a firewall

from entering or leaving the site [Chap92], [Garf92]:

• tftp, port 69, trivial FTP, used for booting diskless workstations, terminal servers and routers, can also be used to read any file on the system if set up incorrectly,

• X Windows, OpenWindows, ports 6000+, port 2000, can leak information from

X window displays including all keystrokes,

• RPC, port 111, Remote Procedure Call services including NIS and NFS, which

can be used to steal system information such as passwords and read and write to

files, and

• rlogin, rsh, and rexec, ports 513, 514, and 512, services that if improperly

configured can permit unauthorized access to accounts and commands.

Other services, whether inherently dangerous or not, are usually filtered and possibly

restricted to only those systems that need them. These would include:

• TELNET, port 23, often restricted to only certain systems,

• FTP, ports 20 and 21, like TELNET, often restricted to only certain systems,

• SMTP, port 25, often restricted to a central e-mail server,

• RIP, port 520, routing information protocol, can be spoofed to redirect packet

routing,

• DNS, port 53, domain names service zone transfers, contains names of hosts and

information about hosts that could be helpful to attackers, could be spoofed.
2.4.3 Packet Filtering 27

• UUCP, port 540, UNIX-to-UNIX CoPy, if improperly configured can be used for

unauthorized access,

• NNTP, port 119, Network News Transfer Protocol, for accessing and reading

network news, and

• gopher, http (for Mosaic), ports 70 and 80, information servers and client

programs for gopher and WWW clients, should be restricted to an application

gateway that contains proxy services.

While some of these services such as TELNET or FTP are inherently risky, blocking

access to these services completely may be too drastic a policy for many sites. Not

all systems, though, generally require access to all services. For example, restricting

TELNET or FTP access from the Internet to only those systems that require the access can improve security at no cost to user convenience. Services such as NNTP may seem

to pose little threat, but restricting these services to only those systems that need them

helps to create a cleaner network environment and reduces the likelihood of exploitation

from yet-to-be-discovered vulnerabilities and threats.

Problems with Packet Filtering Routers

Packet filtering routers suffer from a number of weaknesses, as described in [Chap92].

Packet filtering rules are complex to specify and usually no testing facility exists for

verifying the correctness of the rules (other than by exhaustive testing by hand). Some

routers do not provide any logging capability, so that if a router's rules still let dangerous

packets through, the packets may not be detected until a break-in has occurred.

Often times, exceptions to rules need to be made to allow certain types of access that

normally would be blocked. But, exceptions to packet filtering rules sometimes can

make the filtering rules so complex as to be unmanageable. For example, it is relatively

straightforward to specify a rule to block all inbound connections to port 23 (the TELNET

server). If exceptions are made, i.e., if certain site systems need to accept TELNET

connections directly, then a rule for each system must be added. Sometimes the addition

of certain rules may complicate the entire filtering scheme. As noted previously, testing

a complex set of rules for correctness may be so diflScult as to be impractical.

Some packet filtering routers do not filter on the TCP/UDP source port, which can

make the filtering ruleset more complex and can open up "holes" in the filtering scheme.

[Chap92] describes such a problem with sites that wish to allow inbound and outbound

SMTP connections. As described in section 1.2.5, TCP connections include a source and

destination port. In the case of a system initiating an SMTP connection to a server,

the source port would be a randomly chosen port at or above 1024 and the destination
28 2.4 FIREWALL COMPONENTS

port would be 25, the port that the SMTP server "Ustens" at. The server would return

packets with source port of 25 and destination port equal to the randomly-chosen port at

the client. If a site permits both inbound and outbound SMTP connections, the router

must allow destination ports and source ports > 1023 in both directions. If the router can

filter on source port, it can block all packets coming into the site that have a destination

port > 1023 and a source port other than 25. Without the ability to filter on source

port, the router must permit connections that use source and destination ports > 1024.

Users could conceivably run servers at ports > 1023 and thus get "around" the filtering

policy (i.e., a site system's telnet server that normally listens at port 23 could be told to

listen at port 9876 instead; users on the Internet could then telnet to this server even if

the router blocks destination port 23).

Another problem is that a number of RPC (Remote Procedure Call) services are very

difficult to filter effectively because the associated servers listen at ports that are assigned

randomly at system startup. A service known as portmapper maps initial calls to RPC

services to the assigned service numbers, but there is no such equivalent for a packet

filtering router. Since the router cannot be told which ports the services reside at, it

isn't possible to block completely these services unless one blocks all UDP packets (RPC

services mostly use UDP). Blocking all UDP would block potentially necessary services

such as DNS. Thus, blocking RPC results in a dilemma.

Packet filtering routers with more than two interfaces sometimes do not have the ca- pability to filter packets according to which interface the packets arrived at and which

interface the packet is bound for. Filtering inbound and outbound packets simplifies the

packet filtering rules and permits the router to more easily determine whether an IP address is valid or being spoofed. Routers without this capability offer more impediments

to implementing filtering strategies.

Related to this, packet filtering routers can implement both of the design policies discussed in section 2.4.1. A ruleset that is less flexible, i.e., that does not filter on source

port or on inbound and outbound interfaces, reduces the ability of the router to implement the second and more stringent policy, deny all services except those expressly

permitted, without having to curtail the types of services permitted through the router.

For example, problematic services such as those that are RPC-based become even more

diflficult to filter with a less-flexible ruleset; no filtering on source port forces one to permit

connections between ports > 1023. With a less-flexible ruleset, the router is less able to

express a stringent policy, and the first policy, permit all services except those expressly

permitted, is usually followed.

Readers are advised to consult [Chap92], which provides a concise overview of packet

filtering and associated problems. While packet filtering is a vital and important tool, it

is very important to understand the problems and how they can be addressed.
2.4.4 Application Gateways 29

2.4.4 Application Gateways

To counter some of the weaknesses associated with packet filtering routers, firewalls

need to use software applications to forward and filter connections for services such as TELNET and FTP. Such an application is referred to as a proxy service, while the host

running the proxy service is referred to as an application gateway. Application gateways

and packet filtering routers can be combined to provide higher levels of security and

flexibility than if either were used alone.

As an example, consider a site that blocks all incoming TELNET and FTP connections

using a packet filtering router. The router allows TELNET and FTP packets to go to

one host only, the TELNET/FTP application gateway. A user who wishes to connect

inbound to a site system would have to connect first to the application gateway, and then

to the destination host, as follows:

1. a user first telnets to the application gateway and enters the name of an internal

host,

2. the gateway checks the user's source IP address and accepts or rejects it according

to any access criteria in place,

3. the user may need to authenticate herself (possibly using a one-time password

device),

4. the proxy service creates a TELNET connection between the gateway and the

internal host,

5. the proxy service then passes bytes between the two connections, and

6. the application gateway logs the connection.

Application Gateway

Figure 2.4: Virtual Connection Implemented by an Application Gateway and Proxy

Services.
30 2.4 FIREWALL COMPONENTS

This example points out several benefits to using proxy services. First, proxy services

allow only those services through for which there is a proxy. In other words, if an application gateway contains proxies for FTP and TELNET, then only FTP and TELNET

may be allowed into the protected subnet, and all other services are completely blocked.

For some sites, this degree of security is important, as it guarantees that only those

services that are deemed "trustworthy" are allowed through the firewall. It also prevents other untrusted services from being implemented behind the backs of the firewall

administrators.

Another benefit to using proxy services is that the protocol can be filtered. Some firewalls,

for example, can filter FTP connections and deny use of the FTP put command, which

is useful if one wants to guarantee that users cannot write to, say, an anonymous FTP

server.

Application gateways have a number of general advantages over the default mode of

permitting application traffic directly to internal hosts. These include:

• information hiding, in which the names of internal systems need not necessarily

be made known via DNS to outside systems, since the application gateway may be

the only host whose name must be made known to outside systems,

• robust authentication and logging, in which the application traffic can be preauthenticated before it reaches internal hosts and can be logged more effectively

than if logged with standard host logging,

• cost-effectiveness, because third-party software or hardware for authentication

or logging need be located only at the application gateway, and

• less-complex filtering rules, in which the rules at the packet filtering router will

be less complex than they would if the router needed to filter application traffic and

direct it to a number of specific systems. The router need only allow application

traffic destined for the application gateway and reject the rest.

A disadvantage of application gateways is that, in the case of client-server protocols such

as TELNET, two steps are required to connect inbound or outbound. Some application

gateways require modified clients, which can be viewed as a disadvantage or an advantage,

depending on whether the modified clients make it easier to use the firewall. A TELNET

application gateway would not necessarily require a modified TELNET client, however

it would require a modification in user behavior: the user has to connect (but not login)

to the firewall as opposed to connecting directly to the host. But a modified TELNET

client could make the firewall transparent by permitting a user to specify the destination

system (as opposed to the firewall) in the TELNET command. The firewall would serve

as the route to the destination system and thereby intercept the connection, and then
2.4.4 Application Gateways 31

perform additional steps as necessary such as querying for a one-time password. User

behavior stays the same, however at the price of requiring a modified client on each

system.

In addition to TELNET, application gateways are used generally for FTP and e-mail, as well as for X Windows and some other services. Some FTP application gateways

include the capability to deny put and get command to specific hosts. For example, an

outside user who has established an FTP session (via the FTP application gateway) to

an internal system such as an anonymous FTP server might try to upload files to the

server. The application gateway can filter the FTP protocol and deny all puts to the

anonymous FTP server; this would ensure that nothing can be uploaded to the server and would provide a higher degree of assurance than relying only on file permissions at

the anonymous FTP server to be set correctly.'*

An e-mail application gateway serves to centralize e-mail collection and distribution to

internal hosts and users. To outside users, all internal users would have e-mail addresses

of the form:

user@emailhost

where emailhost is the name of the e-mail gateway. The gateway would accept mail from

outside users and then forward mail along to other internal systems as necessary. Users

sending e-mail from internal systems could send it directly from their hosts, or in the case

where internal system names are not known outside the protected subnet, the mail would

be sent to the application gateway, which could then forward the mail to the destination

host. Some e-mail gateways use a more secure version of the sendmail program to accept

e-mail.

Circuit-Level Gateways

[Ches94] defines another firewall component that other authors sometimes include under

the category of application gateway. A circuit-level gateway relays TCP connections but

does no extra processing or filtering of the protocol. For example, the TELNET application gateway example provided here would be an example of a circuit-level gateway,

since once the connection between the source and destination is established, the firewall

''Some sites have instituted policies that deny put and get commands in certain directions; having a

firewall that can filter FTP commands is especially useful in such a situation. Some sites have disallowed

get commands outbound, thus no users could retrieve information or software from outside sources. Other

sites have disallowed put commands outbound, thus no users could store information on FTP servers external to the site. More common has been to allow no put commands inbound, thus no external users can write to FTP servers at the site.
32 2.4 FIREWALL COMPONENTS

simply passes bytes between the systems. Another example of a circuit-level gateway

would be for NNTP, in which the NNTP server would connect to the firewall, and then

internal systems' NNTP clients would connect to the firewall. The firewall would, again,

simply pass bytes.
3

Putting the Pieces Together:

Firewall Examples

Now that the basic components of firewalls have been examined, some examples of different firewall configurations are provided to give a more concrete understanding of firewall

implementation. The firewall examples shown here are:

• Packet Filtering Firewall,

• Dual-homed Gateway Firewall,

• Screened Host Firewall, and

• Screened Subnet Firewall.

Additionally, a section is provided that discusses methods for integrating dial-in modem

access with firewalls. The examples are based loosely on [Ran93], which provides concise

but detailed guidance on firewall definition and design. In the examples, assumptions

about policy are kept to a minimum, but policy issues that affect the firewall design are pointed out where appropriate. Readers should note that there are many other types

of firewalls that are not illustrated here; their absence does not indicate that they are

less secure, only that it is impractical to illustrate every potential design. The examples

shown here were chosen primarily because they are covered by other literature in more

detail and thus serve well as a basis for more study.

3.1 Packet Filtering Firewall

The packet filtering firewall (fig. 3.1) is perhaps most common and easiest to employ

for small, uncomplicated sites. However, it suffers from a number of disadvantages and

is less desirable as a firewall than the other example firewalls discussed in this chapter.

33
34 3.2 DUAL-HOMED GATEWAY FIREWALL

Basically, one installs a packet filtering router at the Internet (or any subnet) gateway

and then configures the packet filtering rules in the router to block or filter protocols

and addresses. The site systems usually have direct access to the Internet while all or most access to site systems from the Internet is blocked. However, the router could allow

selective access to systems and services, depending on the policy. Usually, inherentlydangerous services such as NIS, NFS, and X Windows are blocked.

Internet

Site systems

Figure 3.1: Packet Filtering Firewall.

A packet filtering firewall suff"ers from the same disadvantages as a packet filtering router,

however they can become magnified as the security needs of a protected site becomes

more complex and stringent. These would include the following:

• there is little or no logging capability, thus an administrator may not easily determine whether the router has been compromised or is under attack,

• packet filtering rules are often difficult to test thoroughly, which may leave a site

open to untested vulnerabilities,

• if complex filtering rules are required, the filtering rules may become unmanageable,

and

• each host directly accessible from the Internet will require its own copy of advanced

authentication measures.

A packet filtering router can implement either of the design policies discussed in section

2.4.1. However, if the router does not filter on source port or filter on inbound as well as outbound packets, it may be more difficult to implement the second policy, i.e., deny

everything unless specifically permitted. If the goal is to implement the second policy, a

router that provides the most fiexibility in the filtering strategy is desirable. Again, see [Chap92] as well as [Ches94] for more information.

3.2 Dual-homed Gateway Firewall

The dual-homed gateway (fig. 3.2) is a better alternative to packet filtering router firewalls. It consists of a host system with two network interfaces, and with the host's IP
35

forwarding capability disabled (i.e., the default condition is that the host can no longer

route packets between the two connected networks). In addition, a packet filtering router

can be placed at the Internet connection to provide additional protection. This would

create an inner, screened subnet that could be used for locating specialized systems such

as information servers and modem pools.

Unlike the packet filtering firewall, the dual-homed gateway is a complete block to IP

traffic between the Internet and protected site. Services and access is provided by proxy

servers on the gateway. It is a simple firewall, yet very secure.^

Info Server

Internet

^ HTTP/Gopher/FTP Application Traffic

Figure 3.2: Dual-homed Gateway Firewall with Router.

This type of firewall implements the second design policy, i.e., deny all services unless

they are specifically permitted, since no services pass except those for which proxies exist.

The ability of the host to accept source-routed packets would be disabled, so that no other

packets could be passed by the host to the protected subnet. It can be used to achieve

a high degree of privacy since routes to the protected subnet need to be known only to

the firewall and not to Internet systems (because Internet systems cannot route packets

directly to the protected systems). The names and IP addresses of site systems would

be hidden from Internet systems, because the firewall would not pass DNS information.

A simple setup for a dual-homed gateway would be to provide proxy services for TELNET

and FTP, and centralized e-mail service in which the firewall would accept all site mail

and then forward it to site systems. Because it uses a host system, the firewall can house

software to require users to use authentication tokens or other advanced authentication

measures. The firewall can also log access and log attempts or probes to the system that

might indicate intruder activity.

The dual-homed gateway firewall, as well as the screened subnet firewall mentioned later

^Some dual-homed gateway firewalls do not use proxy services but require users to have accounts on the gateway for access to the Internet. This firewall is not recommended, as maintaining multiple

accounts on a firewall can lead to user mistakes, which can lead to intruder attacks and break-ins.
36 3.3 SCREENED HOST FIREWALL

in this chapter, provides the ability to segregate traffic concerned with an information

server from other traffic to and from the site. An information server could be located on

the subnet between the gateway and the router, as shown in figure 3.2. Assuming that

the gateway provides the appropriate proxy services for the information server (e.g., ftp,

gopher, or http), the router can prevent direct Internet access to the firewall and force access to go through the firewall. If direct access is permitted to the server (which is

the less secure alternative), then the server's name and IP address can be advertised by

DNS. Locating the information server there also adds to the security of the site, as any

intruder penetration of the information server would still be prevented from reaching site

systems by the dual-homed gateway.

The inflexibility of the dual-homed gateway could be a disadvantage to some sites. Since

all services are blocked except those for which proxies exist, access to other services

cannot be opened up; systems that require the access would need to be placed on the

Internet side of the gateway. However, a router could be used as shown in figure 3.2 to

create a subnet between the gateway and the router, and the systems that require extra

services could be located there (this is discussed more in section 3.4 with screened subnet

firewalls)

.

Another important consideration is that the security of the host system used for the

firewall must be very secure, as the use of any vulnerable services or techniques on the

host could lead to break-ins. If the firewall is compromised, an intruder could potentially

subvert the firewall and perform some activity such as to re-enable IP routing.

[Garf92], [Ran93], and [Ches94] discuss advantages and disadvantages of dual-homed

gateways used as firewalls.

3.3 Screened Host Firewall

The screened host firewall (fig. 3.3) is a more flexible firewall than the dual-homed

gateway firewall, however the flexibility is achieved with some cost to security. The

screened host firewall is often appropriate for sites that need more flexibility than that

provided by the dual-homed gateway flrewall.

The screened host firewall combines a packet-filtering router with an application gateway

located on the protected subnet side of the router.^ The application gateway needs only

one network interface. The application gateway's proxy services would pass TELNET,

FTP, and other services for which proxies exist, to site systems. The router filters or

^The application gateway could also be located on the Internet side of the router with no apparent

loss in security. Locating the application gateway on the outside may help to reinforce the understanding

that it is subject to Internet attacks and should not necessarily be trusted.
37

screens inherently dangerous protocols from reaching the application gateway and site

systems. It rejects (or accepts) application traffic according to the following rules:

• application traffic from Internet sites to the application gateway gets routed,

• all other traffic from Internet sites gets rejected, and

• the router rejects any application traffic originating from the inside unless it came

from the application gateway.

Info Server

Application Gateway

Figure 3.3: Screened Host Firewall.

Unlike the dual-homed gateway firewall, the application gateway needs only one network

interface and does not require a separate subnet between the application gateway and

the router. This permits the firewall to be made more flexible but perhaps less secure by

permitting the router to pass certain trusted services "around" the application gateway

and directly to site systems. The trusted services might be those for which proxy services

don't exist, and might be trusted in the sense that the risk of using the services has been

considered and found acceptable. For example, less-risky services such as NTP could be

permitted to pass through the router to site systems. If the site systems require DNS

access to Internet systems, DNS could be permitted to site systems. In this configuration,

the firewall could implement a mixture of the two design policies, the proportions of which

depend on how many and what types of services are routed directly to site systems.

The additional flexibility of the screened host firewall is cause for two concerns. First,

there are now two systems, the router and the application gateway, that need to be conflgured carefully. As noted before, packet filtering router rules can be complex to configure,

difficult to test, and prone to mistakes that lead to holes through the router. However,
38 3.4 SCREENED SUBNET FIREWALL

since the router needs to limit application traffic only to the application gateway, the

ruleset may not be as complex as for a typical site using a packet filtering firewall (which

may restrict application traffic to multiple systems).

The second disadvantage is that the flexibility opens up the possibility that the policy

can be violated (as with the packet filtering firewall). This is less of a problem than with

the dual-homed gateway firewall, since it is technically impossible to pass traffic through

the dual-homed gateway unless there is a corresponding proxy service. Again, a strong

policy is essential.

[Garf92], [Ran93], and [Ches94] provide more details on screened host firewalls.

3.4 Screened Subnet Firewall

The screened subnet firewall is a variation of the dual-homed gateway and screened host

firewalls. It can be used to locate each component of the firewall on a separate system,

thereby achieving greater throughput and flexibility, although at some cost to simplicity.

But, each component system of the firewall needs to implement only a specific task,

making the systems less complex to configure.

In figure 3.4, two routers are used to create an inner, screened subnet. This subnet

(sometimes referred to in other literature as the "DMZ" ) houses the application gateway,

however it could also house information servers, modem pools, and other systems that

require carefully-controlled access. The router shown as the connection point to the

Internet would route traffic according to the following rules:

• application traffic from the application gateway to Internet systems gets routed,

• e-mail traffic from the e-mail server to Internet sites gets routed,

• application traffic from Internet sites to the application gateway gets routed,

• e-mail traffic from Internet sites to the e-mail server gets routed,

• ftp, gopher, etc., traffic from Internet sites to the information server gets routed,

and

• all other traffic gets rejected.

The outer router restricts Internet access to specific systems on the screened subnet,

and blocks all other traffic to the Internet originating from systems that should not

be originating connections (such as the modem pool, the information server, and site
39

systems). The router would be used as well to block packets such as NFS, NIS, or any

other vulnerable protocols that do not need to pass to or from hosts on the screened

subnet.

The inner router passes traffic to and from systems on the screened subnet according to

the following rules:

• application traffic from the application gateway to site systems gets routed,

• e-mail traffic from the e-mail server to site systems gets routed,

• application traffic to the application gateway from site systems get routed,

• e-mail traffic from site systems to the e-mail server gets routed,

• ftp, gopher, etc., traffic from site systems to the information server gets routed,

• all other traffic gets rejected.

Info Server

Application Gateway

Figure 3.4: Screened Subnet Firewall with Additional Systems.

Thus, no site system is directly reachable from the Internet and vice versa, as with the

dual-homed gateway firewall. A big difference, though, is that the routers are used to

direct traffic to specific systems, thereby eliminating the need for the application gateway

to be dual-homed. Greater throughput can be achieved, then, if a router is used as the

gateway to the protected subnet. Consequently, the screened subnet firewall may be more

appropriate for sites with large amounts of traffic or sites that need very high-speed traffic.
40 3.5 INTEGRATING MODEM POOLS WITH FIREWALLS

The two routers provide redundancy in that an attacker would have to subvert both

routers to reach site systems directly. The application gateway, e-mail server, and information server could be set up such that they would be the only systems "known"

from the Internet; no other system name need be known or used in a DNS database that

would be accessible to outside systems. The application gateway can house advanced

authentication software to authenticate all inbound connections. It is, obviously, more

involved to configure, however the use of separate systems for application gateways and

packet filters keeps the configuration more simple and manageable.

The screened subnet firewall, like the screened host firewall, can be made more flexible by

permitting certain "trusted" services to pass between the Internet and the site systems.

However, this flexibility may open the door to exceptions to the policy, thus weakening the

effect of the firewall. In many ways, the dual-homed gateway firewall is more desireable

because the policy cannot be weakened (because the dual-homed gateway cannot pass

services for which there is no proxy). However, where throughput and flexibility are important, the screened subnet firewall may be more preferable.

As an alternative to passing services directly between the Internet and site systems, one

could locate the systems that need these services directly on the screened subnet. For

example, a site that does not permit X Windows or NFS traffic between Internet and

site systems, but needs to anyway, could locate the systems that need the access on the

screened subnet. The systems could still maintain access to site systems by connecting

to the application gateway and reconfiguring the inner router as necessary. This is not a

perfect solution, but an option for sites that require a high degree of security.

There are two disadvantages to the screened subnet firewall. First, the firewall can be

made to pass "trusted" services around the application gateway (s), thereby subverting

the policy. This is true also with the screened host firewall, however the screened subnet

firewall provides a location to house systems that need direct access to those services.

With the screened host firewall, the "trusted" services that get passed around the application gateway end up being in contact with site systems. The second disadvantage

is that more emphasis is placed on the routers for providing security. As noted, packet

filtering routers are sometimes quite complex to configure and mistakes could open the

entire site to security holes.

[Ran93] and [Ches94] provide more details on screened subnet firewalls.

3.5 Integrating Modem Pools with Firewalls

Many sites permit dial-in access to modems located at various points throughout the

site. As discussed in section 2.3.2, this is a potential backdoor and could negate all the
41

protection provided by the firewall. A much better method for handling modems is to

concentrate them into a modem pool, and then secure connections from that pool.

The modem pool likely would consist of modems connected to a terminal server, which

is a specialized computer designed for connecting modems to a network. A dial-in user

connects to the terminal server, and then connects (e.g., telnets) from there to other

host systems. Some terminal servers provide security features that can restrict connections to specific systems, or require users to authenticate using an authentication token.

Alternatively, the terminal server can be a host system with modems connected to it.

Application Gateivay

Figure 3.5: Modem Pool Placement with Screened Host Firewall.

Figure 3.5 shows a modem pool located on the Internet side of the screened host firewall.

Since the connections from modems need to be treated with the same suspicion as connections from the Internet, locating the modem pool on the outside of the firewall forces

the modem connections to pass through the firewall.

The application gateway's advanced authentication measures can be used then to authenticate users who connect from modems as well as from the Internet. The packet

filtering router could be used to prevent inside systems from connecting directly to the

modem pool.

A disadvantage to this, though, is that the modem pool is connected directly to the Internet and thus more exposed to attack. If an intruder managed to penetrate the modem

pool, the intruder might use it as a basis for connecting to and attacking other Internet

systems. Thus, a terminal server with security features to reject dial-in connections to

any system but the application gateway should be used.

The dual-homed gateway and screened subnet firewalls provide a more secure method

for handling modem pools. In figure 3.6, the terminal server gets located on the inner,

screened subnet, where access to and from the modem pool can be carefully controlled

by the routers and application gateways. The router on the Internet side protects the

modem pool from any direct Internet access except from the application gateway.
42 3.5 INTEGRATING MODEM POOLS WITH FIREWALLS

Site Systems

Internet

Application Gateway

Dial-In

Figure 3.6: Modem Pool Placement with Screened Subnet and Dual-Homed Firewalls.

With the dual-homed gateway and screened subnet firewalls, the router connected to the

Internet would prevent routing between Internet systems and the modem pool. With the

screened subnet firewall, the router connected to the site would prevent routing between

site systems and the modem pool; with the dual-homed gateway firewall, the application

gateway would prevent the routing. Users dialing into the modem pool could connect to

site systems or the Internet only by connecting to the application gateway, which would

use advanced authentication measures.

If a site uses any of these measures to protect dial-in access, it must rigidly enforce

a policy that prevents any users from connecting modems elsewhere on the protected

subnet. Even if the modems contain security features, this adds more complexity to the

firewall protection scheme and adds another "weak link" to the chain.
4

Next Steps

Up to this point, this document has provided a basic vocabulary of threats and risks

associated with the Internet, how Internet firewalls can be used to address those problems,

and some examples of firewall implementations. This chapter provides basic guidance

on designing a network service policy and choosing a firewall design policy, and then

discusses next steps in obtaining a firewall. It closes with a discussion of issues involved

in maintaining a firewall and other steps for improving overall network security. The

discussion is brief and serves only to raise issues; readers are urged to consult more

complete discussions such as [RFC1244] and [Ches94], and specific examples of policies

such as in [Avol94].

4.1 Firewall Policy

Policy was discussed in 2.4.1 in terms of a service access policy and a firewall design

policy. This section discusses these policies in relationship to overall site policy, and

offers guidance on how to identify needs, risks, and then policies.

Policy decisions regarding the use of firewall technology should be made in conjunction

with the policy decisions needed to secure the whole site. This includes decisions concerning host systems security, dial-in access, off-site Internet access, protection of information

off-site, data communications security and others. A stand-alone policy concerning only

the firewall is not effective; it needs to be incorporated into a strong site security policy.

Refer to [RFC1244] for information on creating a site security policy geared towards the

needs of Internet sites.

4.1.1 Steps in Creating a Service Access Policy

A firewall is a direct implementation of the network service access and design policies, as discussed in section 2.4.1. There are a number of service access policies that may be

43
44 4.1 FIREWALL POLICY

implemented, such as no inbound access and full outbound access or restricted inbound

access and restricted outbound access. The firewall design policy determines to a large

degree the service access policy: the more robust the firewall design policy, the more

stringent the service access policy. Thus, the firewall design policy needs to be decided

upon first.

As explained in section 2.4.1, the firewall design policy is generally to deny all services

except those that are explicitly permitted or to permit all services except those that are

explicitly denied. The former is more secure and is therefore preferred, but it is also more

stringent and causes fewer services to be permitted by the service access policy.

Chapter 3 provided several firewall examples, and showed that certain firewalls can implement either design policy whereas one, the dual-homed gateway, is inherently a "deny

all" firewall. However, the examples also showed that systems needing certain services

that shouldn't be passed through the firewalls could be located on screened subnets separate from other site systems. The point here is that depending on security and flexibility

requirements, certain types of firewalls are more appropriate than others. This shows

also the importance of choosing a policy first before implementing the firewall; doing the

opposite could result in a clumsy fit.

To arrive at a firewall design policy and then ultimately a firewall system that implements

the policy, NIST recommends that the firewall design policy start with the most secure,

i.e., deny all services except those that are explicitly permitted. The policy designer then

should understand and document the following:

• which Internet services the organization plans to use, e.g., TELNET, Mosaic, and

NFS,

• where the services will be used, e.g., on a local basis, across the Internet, dial-in

from home, or from remote organizations,

• additional needs, such as encryption or dial-in support,

• what are the risks associated with providing these services and access,

• what is the cost in terms of controls and impact on network usability to provide

protection, and

• assumptions about security versus usability: does security win out if a particular

service is too risky or too expensive to secure.

The creation of these items is straightforward, however at the same time highly iterative.

For example, a site may wish to use NFS across two remote sites, however the "deny all"

design policy may not permit NFS (as explained in sec. 2.4.1). If the risks associated
4.1.2 Flexibili ty in Policy 45

with NFS are acceptable to the organization, it may require changing the design policy

to the less secure approach of permitting all services except those specifically denied and

passing NFS through the firewall to site systems. Or, it may require obtaining a firewall

that can locate the systems that require NFS on a screened subnet, thus preserving the

"deny all" design policy for the rest of the site systems. Or, the risks of using NFS may

prove too great; NFS would have to be dropped from the list of services to use remotely.

The aim of this exercise, then, is to arrive at a service access policy and the firewall

design policy.

To assist in this process, the following sections present some common issues that need to

be addressed in the policies associated with firewall use.

4.1.2 Flexibility in Policy

Any security policy that deals with Internet access, Internet services, and network access

in general, should be fiexible. This flexibility must exist for two reasons: the Internet itself

is in flux, and the organization's needs may change as the Internet offers new services and

methods for doing business. New protocols and services are emerging on the Internet,

which off"ers more benefits to organizations using the Internet, but may also result in new

security concerns. Thus, a policy needs to be able to reflect and incorporate these new

concerns. The other reason for the flexibility is that the risk of the organization also does

not remain static. The change in risk may be a reflection of major changes such as new

responsibilities being assigned to the organization, or smaller changes such as a network

configuration change.

4.1.3 Remote User Advanced Authentication Policy

Remote users are those who originate connections to site system from elsewhere on the

Internet. These connections could come from any location on the Internet, from dial-in

lines, or from authorized users on travel or working from home. Regardless, all such

connections should use the advanced authentication service of the firewall to access systems at the site. Policy should reflect that remote users may not access systems through

unauthorized modems placed behind the firewall. There must be no exceptions to this

policy, as it may take only one captured password or one uncontrolled modem line to

enable a backdoor around the firewall.

Such a policy has its drawbacks: increased user training for using advanced authentication

measures, increased expense if remote users must be supplied with authentication tokens

or smartcards, and increased overhead in administering remote access. But, it does not

make sense to install a firewall and at the same time not control remote access.
46 4.1 FIREWALL POLICY

4.1.4 Dial-in/out Policy

A useful feature for authorized users is to have remote access to the systems when these

users are not on site. A dial-in capability allows them to access systems from locations

where Internet access is not available. However as discussed in section 2.3.2, dial-in

capabilities add another avenue for intruder access.

Authorized users may also wish to have a dial-out capability to access those systems that

cannot be reached through the Internet. These users need to recognize the vulnerabilities

they may be creating if they are careless with modem access. A dial-out capability may

easily become a dial-in capability if proper precautions are not taken.

The dial-in and dial-out capabilities should be considered in the design of the firewall and

incorporated into it. Forcing outside users to go through the advanced authentication

of the firewall should be strongly reflected in policy. Policy can also prohibit the use of

unauthorized modems attached to host systems and personal computers at the site if the

modem capability is offered through the firewall. A strong policy and effective modem

service may limit the number of unauthorized modems throughout the site, thus limiting

this dangerous vulnerability as well.

4.1.5 Remote Network Connections

In addition to dial-in/dial-out connections, the use of Serial Line IP (SLIP) and Pointto-Point Protocol (PPP) connections need to be considered as part of the policy. Users

could use SLIP or PPP to create new network connections into a site protected by a

firewall. Such a connection is potentially a backdoor around the firewall, and may be an

even larger backdoor than a simple dial-in connection.

Section 3 provided several examples for locating dial-in capability such that dial-in connections would pass first through the firewall. This sort of arrangement could be used as well for SLIP and PPP connections, however this would need to be set forth in policy.

As usual, the policy would have to be very strong with regard to these connections.

4.1.6 Information Server Policy

A site that is providing public access to an information server must incorporate this access into the firewall design. While the information server itself creates specific security

concerns, the information server should not become a vulnerability to the security of the

protected site. Policy should reflect the philosophy that the security of the site will not

be compromised in order to provide an information service.
47

One can make a useful distinction that information server traffic, i.e., the traffic concerned

with retrieving information from an organization's information server, is fundamentally

different from other "conduct of business" traffic such as e-mail (or other information

server traffic for the purposes of business research). The two types of traffic have their

own risks and do not necessarily need to be mixed with each other.

Section 3 discusses incorporating an information server into the firewall design. The

screened subnet and dual-homed gateway firewall examples show information servers that

can be located on a screened subnet and in effect be isolated from other site systems.

This reduces the chance that an information server could be compromised and then used

to attack site systems.

4.2 Procuring a Firewall

After policy has been decided, there are a number of issues to be considered in procuring

a firewall. Many of these issues are the same as for procuring other software systems,

thus familiar steps such as requirements definition, analysis, and design specification are standard. The following sections describe some additional considerations, including

minimal criteria for a firewall and whether to build or purchase a firewall.

4.2.1 What Should a Firewall Contain?

Once the decision is made to use firewall technology to implement an organization's

security policy, the next step is to procure a firewall that provides the appropriate level

of protection and is cost-effective. However, what features should a firewall have, at a

minimum, to provide effective protection? One cannot answer this question entirely with

specifics, but it is possible to recommend that, in general, a firewall have the following

features or attributes:

• The firewall should be able to support a "deny all services except those specifically

permitted" design policy, even if that is not the policy used.

• The firewall should support your security policy, not impose one.

• The firewall should be flexible; it should be able to accommodate new services and

needs if the security policy of the organization changes.

• The firewall should contain advanced authentication measures or should contain

the hooks for installing advanced authentication measures.
48 4.2 PROCURING A FIREWALL

• The firewall should employ filtering techniques to permit or deny services to specified host systems as needed.

• The IP filtering language should be flexible, user-friendly to program, and should

filter on as many attributes as possible, including source and destination IP address,

protocol type, source and destination TCP/UDP port, and inbound and outbound

interface.

• The firewall should use proxy services for services such as FTP and TELNET, so that advanced authentication measures can be employed and centralized at the

firewall. If services such as NNTP, X, http, or gopher are required, the firewall

should contain the corresponding proxy services.

• The firewall should contain the ability to centralize SMTP access, to reduce direct

SMTP connections between site and remote systems. This results in centralized

handling of site e-mail.

• The firewall should accomodate public access to the site, such that public information servers can be protected by the firewall but can be segregated from site

systems that do not require the public access.

• The firewall should contain the ability to concentrate and filter dial-in access.

• The firewall should contain mechanisms for logging traffic and suspicious activity,

and should contain mechanisms for log reduction so that logs are readable and

understandable.

• If the firewall requires an operating system such as UNIX, a secured version of

the operating system should be part of the firewall, with other security tools as necessary to ensure firewall host integrity. The operating system should have all

patches installed.

• The firewall should be developed in a manner that its strength and correctness is

verifiable. It should be simple in design so that it can be understood and maintained.

• The firewall and any corresponding operating system should be updated with

patches and other bug fixes in a timely manner.

There are undoubtably more issues and requirements, however many of them will be

specific to each site's own needs. A thorough requirements definition and high-level risk

assessment will identify most issues and requirements, however it should be emphasized

that the Internet is a constantly changing network. New vulnerabilities can arise, and

new services and enhancements to other services may represent potential difficulties for
4.2.2 To Buy or Build a Firewall 49

any firewall installation. Therefore, flexibility to adapt to changing needs is an important

consideration.

4.2.2 To Buy or Build a Firewall

A number of organizations may have the capability to build a firewall for themselves,

i.e., put together a firewall by using available software components and equipment or by

writing a firewall from scratch. At the same time, there are a number of vendors offering

a wide spectrum of services in firewall technology. Service can be as limited as providing

the necessary hardware and software only, or as broad as providing services to develop

security policy, risk assessments, security reviews and security training.

Whether one buys or builds a firewall it must be reiterated that one should first develop a

policy and related requirements before proceeding. If an organization is having difficulty

developing a policy, it may need to contact a vendor who can assist in this process.

If an organization has the in-house expertise to build a firewall, it may prove more costeflFective to do so. One of the advantages of building a firewall is that in-house personnel

understand the specifics of the design and use of the firewall. This knowledge may not

exist in-house with a vendor supported firewall.

At the same time, an in-house firewall can be expensive in terms of time required to

build and document the firewall, and the time required for maintaining the firewall and

adding features to it as required. These costs are sometimes not considered; organizations

sometimes make the mistake of counting only the costs for the equipment. If a true

accounting is made for all costs associated with building a firewall, it could prove more

economical to purchase a vendor firewall.

In deciding whether to purchase or build a firewall, answers to the following questions

may help an organization gauge whether it has the resources to build and operate a

successful firewall:

• how will the firewall be tested; who will verify that the firewall performs as expected,

• who will perform general maintenance of the firewall, such as backups and repairs,

• who will install updates to the firewall, such as for new proxy servers, new patches,

and other enhancements,

• can security-related patches and problems be corrected in a timely manner, and

• who will perform user support and training.
50 4.3 ADMINISTRATION ISSUES WITH FIREWALLS

Many vendors offer maintenance services along with firewall installation, therefore the

organization should consider whether it has the internal resources to perform the above.

4.3 Administration Issues with Firewalls

It should not be surprising that firewall administration is a critical job role and should

be afforded as much time as possible. In small organizations, it may require less than

a full-time position, however it should take precedence over other duties. The cost of a

firewall should include the cost of administering the firewall; administration should never

be shortchanged.

4.3.1 System Management Expertise

As evidenced by previous discussions concerning the many host system break-ins occurring throughout the Internet, the need for highly trained, quality, full-time host system

administrators is clearly shown. But, there is also indication that this need is not being

met; many sites do not manage systems such that the systems are secure and protected

from intruder attacks. Many system managers are part-time at best and do not upgrade

systems with patches and bug fixes as available.

Firewall management expertise is a highly critical job role, as a firewall can only be

as effective as its administration. If the firewall is not maintained properly, it may

become insecure, and may permit break-ins while providing an illusion that the site is

still secure. A site's security policy should clearly reflect the importance of strong firewall

administration. Management should demonstrate its commitment to this importance in

terms of full-time personnel, proper funding for procurement and maintenance and other

necessary resources.

4.3.2 Site System Administration

A firewall is not an excuse to pay less attention to site system administration. It is in fact

the opposite: if a firewall is penetrated, a poorly administered site could be wide-open to

intrusions and resultant damage. A firewall in no way reduces the need for highly skilled

system administration.

At the same time, a firewall can permit a site to be "proactive" in its system administration as opposed to reactive. Because the firewall provides a barrier, sites can spend more

time on system administration duties and less time reacting to incidents and damage

control. It is recommended that sites
4.3.3 Incident Handling Contacts 51

• standardize operating system versions and software to make installation of patches

and security fixes more manageable,

• institute a program for efficient, site-wide installation of patches and new software,

• use services to assist in centralizing system administration, if this will result in

better administration and better security,

• perform periodic scans and checks of host systems to detect common vulnerabilities

and errors in configuration, and

• ensure that a communications pathway exists between system administrators and

firewall/site security administrators to alert the site about new security problems,

alerts, patches, and other security-related information.

4.3.3 Incident Handling Contacts

An important consideration under firewall and site system administration is incident

handling assistance and contacts. NIST recommends that organizations develop incident

handling capabilities that can deal with suspicious activity and intrusions, and that can

keep an organization up to date with computer security threat and vulnerability information. Because of the changing nature of Internet threats and risks, it is important that

those maintaining firewalls be part of the incident handling process. Firewall administrators need to be aware of new vulnerabilities in products they are using, or if intruder

activity is on-going and can be detected using prescribed techniques. [Cur92], [Garf92],

and [RFC 1244], contain information on developing incident response teams and contacts.

NIST has produced a publication specifically on creating incident response capabilities

[NIST91b].

See Appendix A for more information on incident response team contacts and the Forum

of Incident Response and Security Teams (FIRST).
I

j

I
Bibliography

[Avol94] Frederick Avolio and Marcus Ranum. A Network Perimeter With Secure Internet Access. In Internet Society Symposium on Network and Distributed System

Security, pages 109-119. Internet Society, February 2-4 1994.

[Bel89] Steven M. Bellovin. Security Problems in the TCP/IP Protocol Suite. Computer Communications Review, 9(2):32-48, April 1989.

[Cerf93] Vinton Cerf. A National Information Infrastructure. Connexions, June 1993.

[CERT94] Computer Emergency Response Team/Coordination Center. CA-94:01,

Ongoing Network Monitoring Attacks. Available from FIRST.ORG, file

pub/alerts/cert9401.txt, February 1994.

[Chap92] D. Brent Chapman. Network (In) Security Through IP Packet Filtering. In

USENIX Security Symposium III Proceedings, pages 63-76. USENIX Association, September 14-16 1992.

[Ches94] William R. Cheswick and Steven M. Bellovin. Firewalls and Internet Security.

Addison-Wesley, Reading, MA, 1994.

[CIAC94a] Computer Incident Advisory Capability. Number e-07, unix sendmail vulnerabilities update. Available from FIRST.ORG, file pub/alerts/e-07.txt, January

1994.

[CIAC94b] Computer Incident Advisory Capability. Number e-09, network monitoring

attacks. Available from FIRST.ORG, file pub/alerts/e-09.txt, February 1994.

[CIAC94c] Computer Incident Advisory Capability. Number e-14, wuarchive ftpd trojan

horse. Available from FIRST.ORG, file pub/alerts/e-14.txt, February 1994.

[Com91a] Douglas E. Comer. Internetworking with TCP/IP: Principles, Protocols, and

Architecture. Prentice-Hall, Englewood Cliff's, NJ, 1991.

[Com91b] Douglas E. Comer and David L. Stevens. Internetworking with TCP/IP:

Design, Implementation, and Internals. Prentice-Hall, Englewood Cliffs, NJ,

1991.

53
54 BIBLIOGRAPHY

[Cur92] David Curry. UNIX System Security: A Guide for Users and System Administrators. Addison-Wesley, Reading, MA, 1992.

[Farm93] Dan Farmer and Wietse Venema. Improving the security of your site by breaking into it. Available from FTP.WIN.TUB. NL, file /pub/security/admin-guideto-cracking.lOl.Z, 1993.

[Ford94] Warwick Ford. Computer Communications Security. Prentice-Hall, Englewood

Cliffs, NJ, 1994.

[Garf92] Simpson Garfinkel and Gene Spafford. Practical UNIX Security. O'Reilly and

Associates, Inc., Sebastopol, CA, 1992.

[Haf91] Katie Hafner and John Markoff. Cyberpunk: Outlaws and Hackers on the

Computer Frontier. Simon and Schuster, New York, 1991.

[Hunt92] Craig Hunt. TCP/IP Network Administration. O'Reilly and Associates, Inc.,

Sebastopol, CA, 1992.

[NIST91a] NIST. Advanced Authentication Technology. CSL Bulletin, National Institute of Standards and Technology, November 1991.

[NIST91b] NIST. Establishing a Computer Security Incident Response Capability. Special Publication 800-3, National Institute of Standards and Technology, January 1991.

[NIST93] NIST. Connecting to the Internet: Security Considerations. CSL Bulletin,

National Institute of Standards and Technology, July 1993.

[NIST94a] NIST. Guideline for the use of Advanced Authentication Technology Alternatives. Federal Information Processing Standard 190, National Institute of

Standards and Technology, September 1994.

[NIST94b] NIST. Reducing the Risk of Internet Connection and Use. CSL Bulletin,

National Institute of Standards and Technology, May 1994.

[NIST94c] NIST. Security in Open Systems. Special Publication 800-7, National Institute

of Standards and Technology, September 1994.

[Ran93] Marcus Ranum. Thinking About Firewalls. In SANS-II Conference, April

1993.

[RFC1244] Paul Holbrook and Joyce Reynolds. RFC 1244: Security Policy Handbook,

prepared for the Internet Engineering Task Force, 1991.
A

On-Line Sources for More

Information

Readers who need additional information on firewalls, Internet security, and security

policy issues should consult the references. In addition, there are several on-line sources

for more information.

A.l Firewall-Specific Information

Readers are urged to consult the following ftp site,

ftp.greatcircle.com

for more information on firewalls. This site contains information on firewall vendors,

firewall-related papers and articles, and collections of mailing list postings organized by

topic. This site also maintains a mailing list on firewall issues; information about the

mailing list is available on-line at the site.

In addition to the above, a number of router and firewall vendors maintain e-mail lists

and ftp sites that contain firewall and Internet security-related information. Check with

your vendor for the appropriate address.

A.2 NIST Computer Security Resource Clearinghouse

NIST operates a clearinghouse of computer security-related information. This clearinghouse contains information on a broad range of subjects, including computer security incident response team alerts, papers on Internet security, policy and training information.

55
56 A.3 FORUM OF INCIDENT RESPONSE AND SECURITY TEAMS

privacy, computer viruses, advanced authentication, and firewalls. The clearinghouse can

be accessed via the Internet (http, gopher, and ftp) and dial-in. To connect via gopher

and ftp, use the following:

gopher csrc.ncsl.nist.gov

ftp csrc.ncsl.nist.gov - login as user "anonymous"

To access the clearinghouse via an http client, use the following Uniform Resource Locator

(URL):

http:/ /csrc.ncsl.nist.gov

The clearinghouse can be accessed via modem, at V.34 (28.8), V.32 (14.4), and lower

speeds. The clearinghouse assumes 8 bit characters, no parity, and 1 stop bit. Dial the

following:

301-948-5717

and you will be connected to a lynx http client. This client is self-explanatory to use; online help is included. A full assortment of download protocols are available for transferring

files to your local system.

A.3 Forum of Incident Response and Security

Teams

The Forum of Incident Response and Security Teams (FIRST) is an organization whose

members work together voluntarily to deal with computer security problems and their

prevention. The organization is composed of incident response teams, a steering committee, and a secretariat, which is currently NIST, and ad hoc working groups. Much of the

focus is on Internet security-related threats. The forum meets regularly and conducts

annual workshops on incident handling.

Many businesses, universities, and government organizations are members of FIRST. A

list of members, background information, and information on membership is available

on-line. To connect via gopher and ftp, use the following:

gopher gopher.first.org

ftp first.org - login as user "anonymous"
57

To access the clearinghouse via an http client, use the following Uniform Resource Locator

(URL):

http:/ /www.first.org/first

More information about FIRST can be obtained from any participating member or the

National Institute of Standards and Technology at the following address:

National Institute of Standards and Technology

Forum of Incident Response and Security Teams

A-216, Technology

Gaithersburg, MD 20899

301-975-3359

first@first.org

B

Internet Firewalls Frequently Asked

Questions

This appendix contains a FAQ (Frequently Asked Questions) on Internet firewalls. It available on-line at several locations, including

ftp.greatcircle.com

ftp.tis.com

Internet Firewalls Frequently Asked Questions

About the FAQ

This FAQ is not an advertisement or endorsement for any

product, compainy, or consultant. The maintainor welcomes input

and comments on the contents of this FAQ. Comments related

to the FAQ should be addressed to Fwalls-FAQ@tis.com.

Contents

:

1: What is a network firewall?

2: Why would I want a firewall?

3: What can a firewall protect against?

4: What can't a firewall protect against?

5: What are good sources of print information on firewalls?

6: Where can I get more information on firewalls on the network?

7: What are some commercial products or consultcints who sell/service firewall

8: What are some of the basic design decisions in a firewall?

9: What are proxy servers and how do they work?

10: What are some cheap packet screening tools?

11: What are some reasonable filtering rules for my Cisco?

12: How do I make DNS work with a firewall?

13: How do I make FTP work through my firewall?

14: How do I make Telnet work through my firewall?

15: How do I make Finger and whois work through my firewall?

59
60 A.3 FORUM OF INCIDENT RESPONSE AND SECURITY TEAMS

16: How do I make gopher, archie, and other services work through my firewall?

17: What are the issues about X-Window through a firewall?

18: Glossary of firewall related terms

Date: Thu Mar 3 12:35:59 1994

From: Fwalls-FAQOtis . com

Subject: 1: What is a network firewall?

A firewall is any one of several ways of protecting one

network from another untrusted network. The actual mecheinism

whereby this is accomplished varies widely, but in

principle, the firewall can be thought of as a pair of

mechanisms: one which exists to block traffic, and the other

which exists to permit traffic. Some firewalls place a

greater emphasis on blocking traffic, while others emphasize

permitting traffic.

Date: Thu Mar 3 12:36:15 1994

From: Fwalls-FAQ@tis.com

Subject: 2: Why would I want a firewall?

The Internet, like any other society, is plagued with the

kind of jerks who enjoy the electronic equivalent of writing

on other people's walls with spraypaint, tearing their

mailboxes off, or just sitting in the street blowing their

car horns . Some people try to get real work done over the

Internet, and others have sensitive or proprietary data they

must protect. A firewall's purpose is to keep the jerks out

of your network while still letting you get your job done.

Many traditional-style corporations eind data centers have

computing security policies aind practices that must be

adhered to. In a case where a company's policies dictate how

data must be protected, a firewall is very important, since

it is the embodiment of the corporate policy. Frequently,

the hardest part of hooking to the Internet, if you're a

large company, is not justifying the expense or effort, but

convincing management that it's safe to do so. A firewall

provides not only real security - it often plays an

important role as a security blanket for management.

Lastly, a firewall can act as your corporate "ambassador" to

the Internet. Memy corporations use their firewall systems

as a place to store public information about corporate

products and services, files to download, bug-fixes, emd so forth. Several of these systems have become important parts

of the Internet service structure (e.g.: UUnet.uu.net,

gatekeeper.dec.com) and have reflected well on their

corporate sponsors

.

Date: Thu Mar 3 13:24:13 1994

From: Fwalls-FAQ@tis.com
Subject: 3: What can a firewall protect against?

Some firewalls permit only Email traffic through them,

thereby protecting the network against any attacks other

than attacks against the Email service. Other firewalls

provide less strict protections, and block services that are

known to be problems.

Generally, firewalls are configured to protect against

unauthentic at ed interactive logins from the "outside" world.

This, more than anything, helps prevent vandals from logging

into machines on your network. More elaborate firewalls

block traffic from the outside to the inside, but permit

users on the inside to commiuiicate freely with the outside.

The firewall can protect you against any type of network

borne attack if you unplug it

.

Firewalls are also important since they cein provide a single

"choke point" where security eind audit cam be imposed.

Unlike in a situation where a computer system is being attacked

by someone dialing in with a modem, the firewall can act as

an effective "phone tap" and tracing tool.

Date: Thu Mar 3 14:02:07 1994

From: Fwalls-FAQOtis . com

Subject: 4: What can't a firewall protect against?

Firewalls can't protect against attacks that don't

go through the firewall. Many corporations that connect to

the Internet are very concerned about proprietary data

leaking out of the company through that route. Unfortunately

for those concerned, a magnetic tape can just as effectively

be used to export data. Firewall policies must be realistic,

and reflect the level of security in the entire network. For

example, a site with top secret or classified data doesn't

need a firewall at all: they shouldn't be hooking up to the

internet in the first place, or the systems with the really

secret data should be isolated from the rest of the

corporate network.

Firewalls can't protect very well against things

like viruses. There are too many ways of encoding binary

files for transfer over networks, and too many different

architectures and viruses to try to search for them all.

In other words, a firewall cannot replace securityconsciousness on the part of your users. In general, a firewall

cannot protect against a data-driven attack — attacks in which

something is mailed or copied to an internal host where it is

then executed. This form of attack has occurred in the past

against various versions of Sendmail.

Date: Thu Mar 24 13:46:32 1994

From: Fwalls-FAQ@tis.com

Subject: 5: What are good sources of print information on firewalls
62 A.3 FORUM OF INCIDENT RESPONSE AND SECURITY TEAMS

There are several books that touch on firewalls. The best

known are

Cheswick and Bellovin, "Firewalls and Internet Security:

Repelling the Wily Hacker" Addison-Wesley , ??, 1994

Garfinkel and Spafford, "Practical UNIX Security" O'Reilly

and associates (discusses primarily host security)

Related references are:

Comer and Stevens, "Internetworking with TCP/IP" Prentice Hall, 1991

Curry, "UNIX System Security" Addison Wesley, 1992

Date: Thu Mar 3 13:48:14 1994

From: Fwalls-FAQ@tis.com

Subject: 6: Where can I get more information on firewalls on the network?

Ftp.greatcircle.com - Firewalls mailing list archives.

Directory: pub/firewalls

Ftp.tis.com - Internet firewall toolkit cind papers.

Directory: pub/firewalls

Research.att.com - Papers on firewalls and breakins.

Directory : dist/internet_security

Net.Tamu.edu - Texas AMU security tools.

Directory: pub/security/TAMU

The internet firewalls mailing list is a forum for firewall

administrators and implement or s . To subscribe to Firewalls, send

"subscribe firewalls"

in the body of a message (not on the "Subject:" line) to

"Majordomo@GreatCircle.COM". Archives of past Firewalls postings are

available for anonymous FTP from ftp.greatcircle.com in pub/firewalls/archive

Date: Thu Mar 3 12:38:10 1994

From: Fwalls-FAQ@tis.com

Subject: 7: What are some commercial products or consultants who sell/service firewall

We feel this topic is too sensitive to address in a FAQ, as well as being difficult to maintain an up-to-date list.

Date: Thu Mar 3 12:38:31 1994

From: Fwalls-FAQ@tis.com

Subject: 8: What are some of the basic design decisions in a firewall?

There are a number of basic design issues that should be
addressed by the lucky person who has been tasked with the

responsibility of designing, specifying, and implementing or

overseeing the installation of a firewall.

The first and most important is reflects the policy of how

your company or organization wants to operate the system: is

the firewall in place to explicitly deny all services except

those critical to the mission of connecting to the net, or

is the firewall in place to provide a metered and audited

method of "queuing" access in a non-threatening manner.

There are degrees of paranoia between these positions; the

final stance of your firewall may be more the result of a political than an engineering decision.

The second is: what level of monitoring, rediindancy, and

control do you want? Having established the acceptable risk

level (e.g.: how paranoid you are) by resolving the first

issue, you can form a checklist of what should be monitored,

permitted, and denied. In other words, you start by figuring

out your overall objectives, and then combine a needs

analysis with a risk assessment, and sort the almost always

conflicting requirements out into a laiindry list that

specifies what you plan to implement.

The third issue is financial. We can't address this one here

in anything but vague terms, but it's important to try to

quantify any proposed solutions in terms of how much it will

cost either to buy or to implement. For example, a complete

firewall product may cost between $100,000 at the high end,

and free at the low end. The free option, of doing some

fancy configuring on a Cisco or similar router will cost

nothing but staff time and cups of coffee. Implementing a

high end firewall from scratch might cost several manmonths, which may equate to $30,000 worth of staff salary

cind benefits. The systems management overhead is also a consideration. Building a home-brew is fine, but it's

important to build it so that it doesn't require constant

and expensive f iddling-with. It's important, in other words,

to evaluate firewalls not only in terms of what they cost

now, but continuing costs such as support.

On the technical side, there are a couple of decisions to

make, based on the fact that for all practical purposes what

we are talking about is a static traffic routing service

placed between the network service provider's router and

your internal network. The traffic routing service may be

implemented at an IP level via something like screening

rules in a router, or at an application level via proxy

gateways and services.

The decision to make here is whether to place an exposed

stripped-down machine on the outside network to run proxy

services for telnet, ftp, news, etc., or whether to set up a

screening router as a filter, permitting communication with

one or more internal machines. There are plusses amd minuses

to both approaches, with the proxy machine providing a

greater level of audit and potentially security in return

for increased cost in configuration and a decrease in the
64 A. 3 FORUM OF INCIDENT RESPONSE AND SECURITY TEAMS

level of service that may be provided (since a proxy needs

to be developed for each desired service) . The old trade-off

between ease-of-use Eind security comes back to haunt us with

a vengeance.

Date: Thu Mar 10 16:56:35 1994

From: Fwalls-FAQ@tis.com

Subject: 9: What are proxy servers and how do they work?

A proxy server (sometimes referred to as an application

gateway or forwarder) is an application that mediates

traffic between a protected network and the Internet.

Proxies are often used instead of router-based traffic

controls, to prevent traffic from passing directly between

networks. Many proxies contain extra logging or support for

user authentication. Since proxies must "understand" the

application protocol being used, they can also implement

protocol specific security (e.g., an FTP proxy might be

configurable to permit incoming FTP and block outgoing

FTP) .

Proxy servers are application specific. In order to support

a new protocol via a proxy, a proxy must be developed for

it. SOCKS is a generic proxy system that can be compiled

into a client-side application to make it work through a

firewall. Its advaintage is that it's easy to use, but it

doesn't support the addition of authentication hooks or protocol specific logging. For more information on SOCKS,

see ftp.nec.com: /pub/security/socks . cstc Users are

encouraged to check the file "FILES" for a description

of the directory's contents.

Date: Mon Jun 6 10:07:36 1994

From: Fwalls-FAq@tis.com

Subject: 10: What are some cheap packet screening tools?

The Texas AMU security tools include software for

implementing screening routers (FTP net.tamu.edu,

pub/security/TAMU) . Karlbridge is a PC-based screening

router kit (FTP nisca.acs.ohio-state.edu, pub/kbridge) . A

version of the Digital Equipment Corporation "screend"

kernel screening software is available for BSD/386,

NetBSD, and ESDI. Many commercial routers support screening

of various forms.

Date: Mon Jvm 6 10:05:51 1994

From: Fwalls-FAQ@tis.com

Subject: 11: What are some reasonable filtering rules for my Cisco?

The following example shows one possible configuration for

using the Cisco as a filtering router. It is a sample that
shows the implementation of a specific policy. Your policy

will \indoubtedly vary.

In this example, a company has Class B network address of 128.88.0.0

and is using 8 bits for subnets. The Internet connection is on the

"red" subnet 128.88.254.0. All other subnets are considered trusted

or "blue" subnets.

+ + + +

I IP provider I I Gateway I

I 128.88.254.1 | | 128.88.254.2 I

I "Red" net

+ +

I Cisco I

I 128.88.254.3 I

I I

I 128.88.1.1 I + +

I+

I "Blue" net

+ + +

I mail router I

I 128.88.1.2 I + +

Keeping the following points in mind will help in understanding the

configuration fragments:

1. Ciscos applying filtering to output packets only.

2. Rules are tested in order and stop when the first match is found.

3. There is an implicit deny rule at the end of an access list that

denies everything.

The example below concentrates on the filtering parts of a configuration.

Line numbers and formatting have been added for readability.

The policy to be implemented is:

- Anything not explicitly allowed is denied

- Traffic between the external gateway machine emd

blue net hosts is allowed,

- permit services orginating from the blue net

- allow a range of ports for FTP data connections back to the

blue net

.

1 no ip source-route

2 ! 3 interface Ethernet 0

4 ip address 128.88.1.1 255.255.255.0

5 ip access-group 10

6 ! 7 interface Ethernet 1 8 ip address 128.88.254.3 255.255.255.0
66 A.3 FORUM OF INCIDENT RESPONSE AND SECURITY TEAMS

9 ip access-group 11

10 ! 11 access-list 10 permit ip 128.88.254.2 0.0.0.0

128.88.0.0 0.0.255.255

12 access-list 10 deny tcp 0.0.0.0 255.255.255.255

128.88.0.0 0.0.255.255 It 1025

13 access-list 10 deny tcp 0.0.0.0 255.255.255.255

128.88.0.0 0.0.255.255 gt 4999

14 access-list 10 permit tcp 0.0.0.0 255.255.255.255

128.88.0.0 0.0.255.255

15 ! 16 access-list 11 permit ip 128.88.0.0 0.0.255.255

128.88.254.2 0.0.0.0

17 access-list 11 deny tcp 128.88.0.0 0.0.255.255

0.0.0.0 255.255.255.255 eq 25

18 access-list 11 permit tcp 128.88.0.0 0.0.255.255

0.0.0.0 255.255.255.255

Lines Explanation

1 Although this is not a filtering rule, it is good to include here.

5 Ethernet 0 is on the red net. Extended access list 10 will

be applied to output on this interface. You can also

think of output from the red net as input on the blue net.

9 Ethernet 1 is on the blue net. Extended access list 11 will

be applied to output on this interface.

11 Allow all traffic from the gateway machine to the blue net.

12-14 Allow connections originating from the red net that come in

between ports 1024 and 5000. This is to allow ftp data

connections back into the blue net . 5000 was chosen as the

upper limit as it is where OpenView starts.

Note: again, we are assuming this is acceptable for the given policy.

There is no way to tell a Cisco to filter on source port.

Newer versions of the Cisco firmware will apparently support

source port filtering.

Since the rules are tested until the first match we must use this

rather obtuse syntax.

16 Allow all blue net packets to the gateway machine.

17 Deny SMTP (tcp port 25) mail to the red net.

18 Allow all other TCP traffic to the red net.

Cisco. Com has an archive of examples for building firewalls

using Cisco routers, available for FTP from: ftp.cisco.com

in /pub/acl-examples . tar .

Z

Date: Thu Mar 3 13:52:47 1994
From: Fwalls-FAQ@tis.com

Subject: 12: How do I make DNS work with a firewall?

Some orgcLiiizations want to hide DNS names from the outside.

Many experts disagree as to whether or not hiding DNS names

is worthwhile, but if site/corporate policy mandates hiding

domain names, this is one approach that is known to work.

This approach is one of many, cind is useful for

organizations that wish to hide their host names from the

Internet. The success of this approach lies on the fact that

DNS clients on a machine don't have to talk to a DNS server

on that same machine. In other words, just because there's

a DNS server on a machine, there's nothing wrong with (and

there are often advantages to) redirecting that machine's

DNS client activity to a DNS server on another machine.

First , you set up a DNS server on the bastion host that the

outside world can talk to. You set this server up so that it

claims to be authoritative for your domains. In fact, all

this server knows is what you want the outside world to

know; the names and addresses of your gateways, your

wildcard MX records, cind so forth. This is the "public"

server

.

Then, you set up a DNS server on an internal machine. This

server also claims to be authoritiative for your domains;

imlike the public server, this one is telling the truth.

This is your "normal" nameserver, into which you put all

your "normal" DNS stuff. You also set this server up to

forward queries that it can't resolve to the public server

(using a "forwarders" line in /etc/named. boot on a UNIX

machine, for example).

Finally, you set up all your DNS clients (the

/etc/resolv . conf file on a UNIX box, for instance),

including the ones on the machine with the public server, to

use the internal server. This is the key.

An internal client asking about an internal host asks the

internal server, and gets an cinswer; an internal client

asking about an external host asks the internal server,

which asks the public server, which asks the Internet, and

the answer is relayed back. A client on the public server

works just the same way. An external client, however,

asking about ain internal host gets back the "restricted"

answer from the public server.

This approach assumes that there's a packet filtering

firewall between these two servers that will allow them to

talk DNS to each other, but otherwise restricts DNS between

other hosts.

Another trick that's useful in this scheme is to employ

wildcard PTR records in your IN-ADDR.ARPA domains. These

cause an an address-to-name lookup for any of your nonpublic hosts to return something like "unknown. YOUR. DOMAIN"

rather than an error. This satisfies anonymous FTP sites
68 A.3 FORUM OF INCIDENT RESPONSE AND SECURITY TEAMS

like ftp.uu.net that insist on having a name for the

machines they talk to. This may fail when talking to sites

that do a DNS cross-check in which the host name is matched

against its address and vice versa.

Note that hiding neimes in the DNS doesn't address the

problem of host names "leaiking" out in mail headers,

news articles, etc.

Date: Thu Mar 3 21:14:24 1994

From: Fwalls-FAQ@tis.com

Subject: 13: How do I maLke FTP work through my firewall?

Generally, making FTP work through the firewall is done

either using a proxy server or by permitting incoming

connections to the network at a restricted port remge, and

otherwise restricting incoming connections using something

like "established" screening rules. The FTP client is then

modified to bind the data port to a port within that reinge.

This entails being able to modify the FTP client application

on internal hosts.

A different approach is to use the FTP "PASV"

option to indicate that the remote FTP server should permit

the client to initiate connections. The PASV approach

assiimes that the FTP server on the remote system supports

that operation. (See RFC1579 for more information)

Other sites prefer to build client versions of

the FTP program that are linked against a SOCKS library.

Date: Mon Mar 7 13:00:08 1994

From: Fwalls-FAQ@tis.com

Subject: 14: How do I make Telnet work through my firewall?

Telnet is generally supported either by using am application

proxy, or by simply configuring a router to permit outgoing

connections using something like the "established" screening

rules. Application proxies could be in the form of a standalone

proxy running on the bastion host, or in the form of a SOCKS

server and a modified client.

Date: Thu Mar 3 14:16:12 1994

From: Fwalls-FAQ@tis.com

Subject: 15: How do I make Finger and whois work through my firewall?

Permit connections to the finger port from only trusted

machines, which cem issue finger requests in the form of:

finger user@host .domain@f irewall

This approach only works with the stemdard UNIX version of

finger. Some finger servers do not permit user@host@host
69

f ingering

.

Many sites block inbound finger requests for a variety of

reasons, foremost being past security bugs in the finger

server (the Morris internet worm made these bugs famous)

and the risk of proprietary or sensitive information being

revealed in user's finger information.

Date: Thu Mar 3 12:40:54 1994

From: Fwalls-FAQOtis . com

Subject: 16: How do I make gopher, archie, and other services work through my firewall?

This is still an area of active research in the firewall

commiinity. Many firewall administrators support these

services only through the character-cell interface provided

by telnet. Unfortunately, many of the sexier network

services malce connections to multiple remote systems,

without transmitting any inline information that a proxy

could talce advemtage of, aind often the newer information

retrieval systems transmit data to local hosts and disks

with only minimal security. There are risks that (for

example) WAIS clients may request uuencoded files, which

decode and modify security related files in the user's home

directory. At present, there is a lot of head-scratching

going on between the firewall administrators who are

responsible for guarding the network perimeters, eind the

users, who wauit to talce adveintage of these very sexy and

admittedly useful tools.

Date: Mon Jun 6 10:12:03 1994

From: Fwalls-FAQ@tis.com

Subject: 17: What are the issues about X-Window through a firewall?

X Windows is a very useful system, but unfortunately has

some major security flaws. Remote systems that Ccin gain or spoof

access to a workstation's X display can monitor keystrokes that

a user enters, download copies of the contents of their windows,

etc

.

While attempts have been made to overcome them (E.g.,

MIT "Magic Cookie") it is still entirely too easy for an attacker

to interfere with a user's X display. Most firewalls block all X

traffic. Some permit X traffic through application proxies such as

the DEC CRL X proxy (FTP crl.dec.com).

Date: Thu Mar 24 14:05:27 1994

From: Fwalls-FAQ@tis.com

Subject: 18: Glossary of firewall related terms

Host-based Firewall:

A firewall where the security is implemented in software running

on a general-purpose computer of some sort . Security in host-based
70 A.3 FORUM OF INCIDENT RESPONSE AND SECURITY TEAMS

firewalls is generally at the application level, rather than at a network level.

Router-based Firewall:

A firewall where the security is implemented using screening

routers as the primary meeins of protecting the network.

Screening Router:

A router that is used to implement part of the security of a

firewall by configuring it to selectively permit or deny

traffic at a network level.

Bastion Host

A host system that is a "strong point" in the network's security

perimeter. Bastion hosts should be configured to be particularly

resistcint to attack. In a host-based firewall, the bastion host

is the platform on which the firewall software is run.

Bastion hosts are also referred to as "gateway hosts."

Dual-Homed Gateway:

A firewall consisting of a bastion host with 2 network interfaces,

one of which is connected to the protected network, the other of

which is connected to the Internet. IP traffic forwarding is

usually disabled, restricting all traffic between the two networks

to whatever passes through some kind of application proxy.

Application Proxy:

An application that forwards application traffic through a firewall. Proxies tend to be specific to the protocol they

are designed to forward, and may provide increased access

control or audit

.

Screened Subnet

A firewall architecture in which a "sand box" or "demilitarized

zone" network is set up between the protected network aind the

Internet, with traffic between the protected network amd the

Internet blocked. Conceptually, this is similar to a dual-homed

gateway, except that an entire network, rather than a single

host is reachable from the outside.

Contributors

mjr@tis.com - Marcus Rcinum, Trusted Information Systems

leibowa@wl.com - Allen Leibowitz, Waxner Lambert Inc.

brent@greatcircle.com - Brent Chapmem, Great Circle Associates

bdboyle@erenj.com - Brian Boyle, Exxon Research
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collectively constitute the Federal Information Processing Standards Register. The Register serves as the

official source of information in the Federal Government regarding standards issued by NIST pursuant to the Federal Property and Administrative Services Act of 1949 as amended. Public Law 89-306 (79 Stat.

1 127), and as implemented by Executive Order 1 1717 (38 FR 12315, dated May 11, 1973) and Part 6 of

Title 15 CFR (Code of Federal Regulations).

NIST Interagency Reports (NISTIR)—A special series of interim or final reports on work performed by

NIST for outside sponsors (both government and nongovernment). In general, initial distribution is handled

by the sponsor; public distribution is by the National Technical Information Service, Springfield, VA 22161,

in paper copy or microfiche form.
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