# sshd

## NAME



           **sshd** — OpenSSH daemon
    


## SYNOPSIS



           **sshd**[**-46DdeGiqTtV**] [**-C** _connection_spec_]
           [**-c** _host_certificate_file_] [**-E** _log_file_] [**-f** _config_file_]
           [**-g** _login_grace_time_] [**-h** _host_key_file_] [**-o** _option_] [**-p** _port_]
           [**-u** _len_]
    


## DESCRIPTION



           **sshd**(OpenSSH Daemon) is the daemon program for _ssh_(1).  It
           provides secure encrypted communications between two untrusted
           hosts over an insecure network.
    
           **sshd** listens for connections from clients.  It is normally started
           at boot from _/etc/rc_.  It forks a new daemon for each incoming
           connection.  The forked daemons handle key exchange, encryption,
           authentication, command execution, and data exchange.
    
           **sshd** can be configured using command-line options or a
           configuration file (by default _sshd_config_(5)); command-line
           options override values specified in the configuration file.  **sshd**
           rereads its configuration file when it receives a hangup signal,
           SIGHUP, by executing itself with the name and options it was
           started with, e.g. _/usr/sbin/sshd_.
    
           The options are as follows:
    
           **-4** Forces **sshd** to use IPv4 addresses only.
    
           **-6** Forces **sshd** to use IPv6 addresses only.
    
           **-C** _connection_spec_
                   Specify the connection parameters to use for the **-T**
                   extended test mode.  If provided, any **Match** directives in
                   the configuration file that would apply are applied before
                   the configuration is written to standard output.  The
                   connection parameters are supplied as keyword=value pairs
                   and may be supplied in any order, either with multiple **-C**
                   options or as a comma-separated list.  The keywords are
                   “addr”, “user”, “host”, “laddr”, “lport”, and “rdomain”
                   and correspond to source address, user, resolved source
                   host name, local address, local port number and routing
                   domain respectively.  Additionally the “invalid-user” flag
                   (which does not take a value argument) may be specified to
                   simulate a connection from an unrecognised username.
    
           **-c** _host_certificate_file_
                   Specifies a path to a certificate file to identify **sshd**
                   during key exchange.  The certificate file must match a
                   host key file specified using the **-h** option or the **HostKey**
                   configuration directive.
    
           **-D** When this option is specified, **sshd** will not detach and
                   does not become a daemon.  This allows easy monitoring of
                   **sshd**.
    
           **-d** Debug mode.  The server sends verbose debug output to
                   standard error, and does not put itself in the background.
                   The server also will not _fork_(2) and will only process one
                   connection.  This option is only intended for debugging
                   for the server.  Multiple **-d** options increase the
                   debugging level.  Maximum is 3.
    
           **-E** _log_file_
                   Append debug logs to _log_file_ instead of the system log.
    
           **-e** Write debug logs to standard error instead of the system
                   log.
    
           **-f** _config_file_
                   Specifies the name of the configuration file.  The default
                   is _/etc/ssh/sshd_config_.  **sshd** refuses to start if there
                   is no configuration file.
    
           **-G** Parse and print configuration file.  Check the validity of
                   the configuration file, output the effective configuration
                   to stdout and then exit.  Optionally, **Match** rules may be
                   applied by specifying the connection parameters using one
                   or more **-C** options.
    
           **-g** _login_grace_time_
                   Gives the grace time for clients to authenticate
                   themselves (default 120 seconds).  If the client fails to
                   authenticate the user within this many seconds, the server
                   disconnects and exits.  A value of zero indicates no
                   limit.
    
           **-h** _host_key_file_
                   Specifies a file from which a host key is read.  This
                   option must be given if **sshd** is not run as root (as the
                   normal host key files are normally not readable by anyone
                   but root).  The default is _/etc/ssh/ssh_host_ecdsa_key_ ,
                   _/etc/ssh/ssh_host_ed25519_key_ and
                   _/etc/ssh/ssh_host_rsa_key_.  It is possible to have
                   multiple host key files for the different host key
                   algorithms.
    
           **-i** Specifies that **sshd** is being run from _inetd_(8).
    
           **-o** _option_
                   Can be used to give options in the format used in the
                   configuration file.  This is useful for specifying options
                   for which there is no separate command-line flag.  For
                   full details of the options, and their values, see
                   _sshd_config_(5).
    
           **-p** _port_
                   Specifies the port on which the server listens for
                   connections (default 22).  Multiple port options are
                   permitted.  Ports specified in the configuration file with
                   the **Port** option are ignored when a command-line port is
                   specified.  Ports specified using the **ListenAddress** option
                   override command-line ports.
    
           **-q** Quiet mode.  Nothing is sent to the system log.  Normally
                   the beginning, authentication, and termination of each
                   connection is logged.
    
           **-T** Extended test mode.  Check the validity of the
                   configuration file, output the effective configuration to
                   stdout and then exit.  Optionally, **Match** rules may be
                   applied by specifying the connection parameters using one
                   or more **-C** options.  This is similar to the **-G** flag, but
                   it includes the additional testing performed by the **-t**
                   flag.
    
           **-t** Test mode.  Only check the validity of the configuration
                   file and sanity of the keys.  This is useful for updating
                   **sshd** reliably as configuration options may change.
    
           **-u** _len_  This option is used to specify the size of the field in
                   the _utmp_ structure that holds the remote host name.  If
                   the resolved host name is longer than _len_ , the dotted
                   decimal value will be used instead.  This allows hosts
                   with very long host names that overflow this field to
                   still be uniquely identified.  Specifying **-u0** indicates
                   that only dotted decimal addresses should be put into the
                   _utmp_ file.  **-u0** may also be used to prevent **sshd** from
                   making DNS requests unless the authentication mechanism or
                   configuration requires it.  Authentication mechanisms that
                   may require DNS include **HostbasedAuthentication** and using
                   a **from="pattern-list"** option in a key file.  Configuration
                   options that require DNS include using a USER@HOST pattern
                   in **AllowUsers** or **DenyUsers**.
    
           **-V** Display the version number and exit.
    


## AUTHENTICATION



           The OpenSSH SSH daemon supports SSH protocol 2 only.  Each host
           has a host-specific key, used to identify the host.  Whenever a
           client connects, the daemon responds with its public host key.
           The client compares the host key against its own database to
           verify that it has not changed.  Forward secrecy is provided
           through a Diffie-Hellman key agreement.  This key agreement
           results in a shared session key.  The rest of the session is
           encrypted using a symmetric cipher.  The client selects the
           encryption algorithm to use from those offered by the server.
           Additionally, session integrity is provided through a
           cryptographic message authentication code (MAC).
    
           Finally, the server and the client enter an authentication dialog.
           The client tries to authenticate itself using host-based
           authentication, public key authentication, challenge-response
           authentication, or password authentication.
    
           Regardless of the authentication type, the account is checked to
           ensure that it is accessible.  An account is not accessible if it
           is locked, listed in **DenyUsers** or its group is listed in
           **DenyGroups**.  The definition of a locked account is system
           dependent. Some platforms have their own account database (eg AIX)
           and some modify the passwd field ( ‘*LK*’ on Solaris and UnixWare,
           ‘*’ on HP-UX, containing ‘Nologin’ on Tru64, a leading ‘*LOCKED*’
           on FreeBSD and a leading ‘!’ on most Linuxes).  If there is a
           requirement to disable password authentication for the account
           while allowing still public-key, then the passwd field should be
           set to something other than these values (eg ‘NP’ or ‘*NP*’ ).
    
           If the client successfully authenticates itself, a dialog for
           preparing the session is entered.  At this time the client may
           request things like allocating a pseudo-tty, forwarding X11
           connections, forwarding TCP connections, or forwarding the
           authentication agent connection over the secure channel.
    
           After this, the client either requests an interactive shell or
           execution of a non-interactive command, which **sshd** will execute
           via the user's shell using its **-c** option.  The sides then enter
           session mode.  In this mode, either side may send data at any
           time, and such data is forwarded to/from the shell or command on
           the server side, and the user terminal in the client side.
    
           When the user program terminates and all forwarded X11 and other
           connections have been closed, the server sends command exit status
           to the client, and both sides exit.
    


## LOGIN PROCESS



           When a user successfully logs in, **sshd** does the following:
    
                 1.   If the login is on a tty, and no command has been
                      specified, prints last login time and _/etc/motd_ (unless
                      prevented in the configuration file or by _~/.hushlogin_ ;
                      see the “FILES” section).
    
                 2.   If the login is on a tty, records login time.
    
                 3.   Checks _/etc/nologin_ ; if it exists, prints contents and
                      quits (unless root).
    
                 4.   Changes to run with normal user privileges.
    
                 5.   Sets up basic environment.
    
                 6.   Reads the file _~/.ssh/environment_ , if it exists, and
                      users are allowed to change their environment.  See the
                      **PermitUserEnvironment** option in _sshd_config_(5).
    
                 7.   Changes to user's home directory.
    
                 8.   If _~/.ssh/rc_ exists and the _sshd_config_(5) **PermitUserRC**
                      option is set, runs it; else if _/etc/ssh/sshrc_ exists,
                      runs it; otherwise runs _xauth_(1).  The “rc” files are
                      given the X11 authentication protocol and cookie in
                      standard input.  See “SSHRC”, below.
    
                 9.   Runs user's shell or command.  All commands are run
                      under the user's login shell as specified in the system
                      password database.
    


## SSHRC



           If the file _~/.ssh/rc_ exists, _sh_(1) runs it after reading the
           environment files but before starting the user's shell or command.
           It must not produce any output on stdout; stderr must be used
           instead.  If X11 forwarding is in use, it will receive the "proto
           cookie" pair in its standard input (and DISPLAY in its
           environment).  The script must call _xauth_(1) because **sshd** will not
           run xauth automatically to add X11 cookies.
    
           The primary purpose of this file is to run any initialization
           routines which may be needed before the user's home directory
           becomes accessible; AFS is a particular example of such an
           environment.
    
           This file will probably contain some initialization code followed
           by something similar to:
    
              if read proto cookie && [ -n "$DISPLAY" ]; then
                      if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
                              # X11UseLocalhost=yes
                              echo add unix:`echo $DISPLAY |
                                  cut -c11-` $proto $cookie
                      else
                              # X11UseLocalhost=no
                              echo add $DISPLAY $proto $cookie
                      fi | xauth -q -
              fi
    
           If this file does not exist, _/etc/ssh/sshrc_ is run, and if that
           does not exist either, xauth is used to add the cookie.
    


## SSH_KNOWN_HOSTS FILE FORMAT



           The _/etc/ssh/ssh_known_hosts_ and _~/.ssh/known_hosts_ files contain
           host public keys for all known hosts.  The global file should be
           prepared by the administrator (optional), and the per-user file is
           maintained automatically: whenever the user connects to an unknown
           host, its key is added to the per-user file.
    
           Each line in these files contains the following fields: marker
           (optional), hostnames, keytype, base64-encoded key, comment.  The
           fields are separated by spaces.
    
           The marker is optional, but if it is present then it must be one
           of “@cert-authority”, to indicate that the line contains a
           certification authority (CA) key, or “@revoked”, to indicate that
           the key contained on the line is revoked and must not ever be
           accepted.  Only one marker should be used on a key line.
    
           Hostnames is a comma-separated list of patterns (‘*’ and ‘?’ act
           as wildcards); each pattern in turn is matched against the host
           name.  When **sshd** is authenticating a client, such as when using
           **HostbasedAuthentication** , this will be the canonical client host
           name.  When _ssh_(1) is authenticating a server, this will be the
           host name given by the user, the value of the _ssh_(1) **HostkeyAlias**
           if it was specified, or the canonical server hostname if the
           _ssh_(1) **CanonicalizeHostname** option was used.
    
           A pattern may also be preceded by ‘!’ to indicate negation: if the
           host name matches a negated pattern, it is not accepted (by that
           line) even if it matched another pattern on the line.  A hostname
           or address may optionally be enclosed within ‘[’ and ‘]’ brackets
           then followed by ‘:’ and a non-standard port number.
    
           Alternately, hostnames may be stored in a hashed form which hides
           host names and addresses should the file's contents be disclosed.
           Hashed hostnames start with a ‘|’ character.  Only one hashed
           hostname may appear on a single line and none of the above
           negation or wildcard operators may be applied.
    
           The keytype and base64-encoded key are taken directly from the
           host key; they can be obtained, for example, from
           _/etc/ssh/ssh_host_rsa_key.pub_.  The optional comment field
           continues to the end of the line, and is not used.
    
           Lines starting with ‘#’ and empty lines are ignored as comments.
    
           When performing host authentication, authentication is accepted if
           any matching line has the proper key; either one that matches
           exactly or, if the server has presented a certificate for
           authentication, the key of the certification authority that signed
           the certificate.  For a key to be trusted as a certification
           authority, it must use the “@cert-authority” marker described
           above.
    
           The known hosts file also provides a facility to mark keys as
           revoked, for example when it is known that the associated private
           key has been stolen.  Revoked keys are specified by including the
           “@revoked” marker at the beginning of the key line, and are never
           accepted for authentication or as certification authorities, but
           instead will produce a warning from _ssh_(1) when they are
           encountered.
    
           It is permissible (but not recommended) to have several lines or
           different host keys for the same names.  This will inevitably
           happen when short forms of host names from different domains are
           put in the file.  It is possible that the files contain
           conflicting information; authentication is accepted if valid
           information can be found from either file.
    
           Note that the lines in these files are typically hundreds of
           characters long, and you definitely don't want to type in the host
           keys by hand.  Rather, generate them by a script, _ssh-keyscan_(1)
           or by taking, for example, _/etc/ssh/ssh_host_rsa_key.pub_ and
           adding the host names at the front.  _ssh-keygen_(1) also offers
           some basic automated editing for _~/.ssh/known_hosts_ including
           removing hosts matching a host name and converting all host names
           to their hashed representations.
    
           An example ssh_known_hosts file:
    
              # Comments allowed at start of line
              cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....=
              # A hashed hostname
              |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa
              AAAA1234.....=
              # A revoked key
              @revoked * ssh-rsa AAAAB5W...
              # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org
              @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
    


## FILES



           _~/.hushlogin_
                   This file is used to suppress printing the last login time
                   and _/etc/motd_ , if **PrintLastLog** and **PrintMotd** ,
                   respectively, are enabled.  It does not suppress printing
                   of the banner specified by **Banner**.
    
           _~/.rhosts_
                   This file is used for host-based authentication (see
                   _ssh_(1) for more information).  On some machines this file
                   may need to be world-readable if the user's home directory
                   is on an NFS partition, because **sshd** reads it as root.
                   Additionally, this file must be owned by the user, and
                   must not have write permissions for anyone else.  The
                   recommended permission for most machines is read/write for
                   the user, and not accessible by others.
    
           _~/.shosts_
                   This file is used in exactly the same way as _.rhosts_ , but
                   allows host-based authentication without permitting login
                   with rlogin/rsh.
    
           _~/.ssh/_
                   This directory is the default location for all user-
                   specific configuration and authentication information.
                   There is no general requirement to keep the entire
                   contents of this directory secret, but the recommended
                   permissions are read/write/execute for the user, and not
                   accessible by others.
    
           _~/.ssh/authorized_keys_
                   Lists the public keys (ECDSA, Ed25519, RSA) that can be
                   used for logging in as this user.  The format of this file
                   is described above.  The content of the file is not highly
                   sensitive, but the recommended permissions are read/write
                   for the user, and not accessible by others.
    
                   If this file, the _~/.ssh_ directory, or the user's home
                   directory are writable by other users, then the file could
                   be modified or replaced by unauthorized users.  In this
                   case, **sshd** will not allow it to be used unless the
                   **StrictModes** option has been set to “no”.
    
           _~/.ssh/environment_
                   This file is read into the environment at login (if it
                   exists).  It can only contain empty lines, comment lines
                   (that start with ‘#’), and assignment lines of the form
                   name=value.  The file should be writable only by the user;
                   it need not be readable by anyone else.  Environment
                   processing is disabled by default and is controlled via
                   the **PermitUserEnvironment** option.
    
           _~/.ssh/known_hosts_
                   Contains a list of host keys for all hosts the user has
                   logged into that are not already in the systemwide list of
                   known host keys.  The format of this file is described
                   above.  This file should be writable only by root/the
                   owner and can, but need not be, world-readable.
    
           _~/.ssh/rc_
                   Contains initialization routines to be run before the
                   user's home directory becomes accessible.  This file
                   should be writable only by the user, and need not be
                   readable by anyone else.
    
           _/etc/hosts.equiv_
                   This file is for host-based authentication (see _ssh_(1)).
                   It should only be writable by root.
    
           _/etc/moduli_
                   Contains Diffie-Hellman groups used for the "Diffie-
                   Hellman Group Exchange" key exchange method.  The file
                   format is described in _moduli_(5).  If no usable groups are
                   found in this file then fixed internal groups will be
                   used.
    
           _/etc/motd_
                   See _motd_(5).
    
           _/etc/nologin_
                   If this file exists, **sshd** refuses to let anyone except
                   root log in.  The contents of the file are displayed to
                   anyone trying to log in, and non-root connections are
                   refused.  The file should be world-readable.
    
           _/etc/shosts.equiv_
                   This file is used in exactly the same way as _hosts.equiv_ ,
                   but allows host-based authentication without permitting
                   login with rlogin/rsh.
    
           _/etc/ssh/ssh_host_ecdsa_key_
           _/etc/ssh/ssh_host_ed25519_key_
           _/etc/ssh/ssh_host_rsa_key_
                   These files contain the private parts of the host keys.
                   These files should only be owned by root, readable only by
                   root, and not accessible to others.  Note that **sshd** does
                   not start if these files are group/world-accessible.
    
           _/etc/ssh/ssh_host_ecdsa_key.pub_
           _/etc/ssh/ssh_host_ed25519_key.pub_
           _/etc/ssh/ssh_host_rsa_key.pub_
                   These files contain the public parts of the host keys.
                   These files should be world-readable but writable only by
                   root.  Their contents should match the respective private
                   parts.  These files are not really used for anything; they
                   are provided for the convenience of the user so their
                   contents can be copied to known hosts files.  These files
                   are created using _ssh-keygen_(1).
    
           _/etc/ssh/ssh_known_hosts_
                   Systemwide list of known host keys.  This file should be
                   prepared by the system administrator to contain the public
                   host keys of all machines in the organization.  The format
                   of this file is described above.  This file should be
                   writable only by root/the owner and should be world-
                   readable.
    
           _/etc/ssh/sshd_config_
                   Contains configuration data for **sshd**.  The file format and
                   configuration options are described in _sshd_config_(5).
    
           _/etc/ssh/sshrc_
                   Similar to _~/.ssh/rc_ , it can be used to specify machine-
                   specific login-time initializations globally.  This file
                   should be writable only by root, and should be world-
                   readable.
    
           _/var/empty_
                   _chroot_(2) directory used by **sshd** during privilege
                   separation in the pre-authentication phase.  The directory
                   should not contain any files and must be owned by root and
                   not group or world-writable.
    
           _/var/run/sshd.pid_
                   Contains the process ID of the **sshd** listening for
                   connections (if there are several daemons running
                   concurrently for different ports, this contains the
                   process ID of the one started last).  The content of this
                   file is not sensitive; it can be world-readable.
    


* * *