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sshd_config(5)

NAME

     sshd_config -- OpenSSH SSH daemon configuration file

SYNOPSIS

     /etc/ssh/sshd_config

DESCRIPTION

     sshd reads configuration data from /etc/ssh/sshd_config (or the file
     specified with -f on the command line).  The file contains keyword-argu-
     ment pairs, one per line.	Lines starting with `#' and empty lines are
     interpreted as comments.

     The possible keywords and their meanings are as follows (note that key-
     words are case-insensitive and arguments are case-sensitive):

     AllowGroups
	     This keyword can be followed by a list of group name patterns,
	     separated by spaces.  If specified, login is allowed only for
	     users whose primary group or supplementary group list matches one
	     of the patterns.  `*' and `?' can be used as wildcards in the
	     patterns.	Only group names are valid; a numerical group ID is
	     not recognized.  By default, login is allowed for all groups.

     AllowTcpForwarding
	     Specifies whether TCP forwarding is permitted.  The default is
	     ``yes''.  Note that disabling TCP forwarding does not improve
	     security unless users are also denied shell access, as they can
	     always install their own forwarders.

     AllowUsers
	     This keyword can be followed by a list of user name patterns,
	     separated by spaces.  If specified, login is allowed only for
	     user names that match one of the patterns.  `*' and `?' can be
	     used as wildcards in the patterns.  Only user names are valid; a
	     numerical user ID is not recognized.  By default, login is
	     allowed for all users.  If the pattern takes the form USER@HOST
	     then USER and HOST are separately checked, restricting logins to
	     particular users from particular hosts.

     AuthorizedKeysFile
	     Specifies the file that contains the public keys that can be used
	     for user authentication.  AuthorizedKeysFile may contain tokens
	     of the form %T which are substituted during connection set-up.
	     The following tokens are defined: %% is replaced by a literal
	     '%', %h is replaced by the home directory of the user being
	     authenticated and %u is replaced by the username of that user.
	     After expansion, AuthorizedKeysFile is taken to be an absolute
	     path or one relative to the user's home directory.  The default
	     is ``.ssh/authorized_keys''.

     Banner  In some jurisdictions, sending a warning message before authenti-
	     cation may be relevant for getting legal protection.  The con-
	     tents of the specified file are sent to the remote user before
	     authentication is allowed.  This option is only available for
	     protocol version 2.  By default, no banner is displayed.

     ChallengeResponseAuthentication
	     ciphers must be comma-separated.  The default is

	       ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
		 aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr''

     ClientAliveInterval
	     Sets a timeout interval in seconds after which if no data has
	     been received from the client, sshd will send a message through
	     the encrypted channel to request a response from the client.  The
	     default is 0, indicating that these messages will not be sent to
	     the client.  This option applies to protocol version 2 only.

     ClientAliveCountMax
	     Sets the number of client alive messages (see above) which may be
	     sent without sshd receiving any messages back from the client.
	     If this threshold is reached while client alive messages are
	     being sent, sshd will disconnect the client, terminating the ses-
	     sion.  It is important to note that the use of client alive mes-
	     sages is very different from TCPKeepAlive (below).  The client
	     alive messages are sent through the encrypted channel and there-
	     fore will not be spoofable.  The TCP keepalive option enabled by
	     TCPKeepAlive is spoofable.  The client alive mechanism is valu-
	     able when the client or server depend on knowing when a connec-
	     tion has become inactive.

	     The default value is 3.  If ClientAliveInterval (above) is set to
	     15, and ClientAliveCountMax is left at the default, unresponsive
	     ssh clients will be disconnected after approximately 45 seconds.

     Compression
	     Specifies whether compression is allowed.	The argument must be
	     ``yes'' or ``no''.  The default is ``yes''.

     DenyGroups
	     This keyword can be followed by a list of group name patterns,
	     separated by spaces.  Login is disallowed for users whose primary
	     group or supplementary group list matches one of the patterns.
	     `*' and `?' can be used as wildcards in the patterns.  Only group
	     names are valid; a numerical group ID is not recognized.  By
	     default, login is allowed for all groups.

     DenyUsers
	     This keyword can be followed by a list of user name patterns,
	     separated by spaces.  Login is disallowed for user names that
	     match one of the patterns.  `*' and `?' can be used as wildcards
	     in the patterns.  Only user names are valid; a numerical user ID
	     is not recognized.  By default, login is allowed for all users.
	     If the pattern takes the form USER@HOST then USER and HOST are
	     separately checked, restricting logins to particular users from
	     particular hosts.

     GatewayPorts
	     Specifies whether remote hosts are allowed to connect to ports
	     forwarded for the client.	By default, sshd binds remote port
	     forwardings to the loopback address.  This prevents other remote
	     hosts from connecting to forwarded ports.	GatewayPorts can be
	     used to specify that sshd should bind remote port forwardings to
	     the wildcard address, thus allowing remote hosts to connect to

     GSSAPICleanupCredentials
	     Specifies whether to automatically destroy the user's credentials
	     cache on logout.  The default is ``yes''.	Note that this option
	     applies to protocol version 2 only.

     HostbasedAuthentication
	     Specifies whether rhosts or /etc/hosts.equiv authentication
	     together with successful public key client host authentication is
	     allowed (hostbased authentication).  This option is similar to
	     RhostsRSAAuthentication and applies to protocol version 2 only.
	     The default is ``no''.

     HostKey
	     Specifies a file containing a private host key used by SSH.  The
	     default is /etc/ssh/ssh_host_key for protocol version 1, and
	     /etc/ssh/ssh_host_dsa_key for protocol version 2.	Note that sshd
	     will refuse to use a file if it is group/world-accessible.  It is
	     possible to have multiple host key files.	``rsa1'' keys are used
	     for version 1 and ``dsa'' or ``rsa'' are used for version 2 of
	     the SSH protocol.

     IgnoreRhosts
	     Specifies that .rhosts and .shosts files will not be used in
	     RhostsRSAAuthentication or HostbasedAuthentication.

	     /etc/hosts.equiv and /etc/ssh/shosts.equiv are still used.  The
	     default is ``yes''.

     IgnoreUserKnownHosts
	     Specifies whether sshd should ignore the user's
	     $HOME/.ssh/known_hosts during RhostsRSAAuthentication or
	     HostbasedAuthentication.  The default is ``no''.

     KerberosAuthentication
	     Specifies whether the password provided by the user for
	     PasswordAuthentication will be validated through the Kerberos
	     KDC.  To use this option, the server needs a Kerberos servtab
	     which allows the verification of the KDC's identity.  Default is
	     ``no''.

     KerberosGetAFSToken
	     If AFS is active and the user has a Kerberos 5 TGT, attempt to
	     aquire an AFS token before accessing the user's home directory.
	     Default is ``no''.

     KerberosOrLocalPasswd
	     If set then if password authentication through Kerberos fails
	     then the password will be validated via any additional local
	     mechanism such as /etc/passwd.  Default is ``yes''.

     KerberosTicketCleanup
	     Specifies whether to automatically destroy the user's ticket
	     cache file on logout.  Default is ``yes''.

     KeyRegenerationInterval
	     In protocol version 1, the ephemeral server key is automatically
	     regenerated after this many seconds (if it has been used).  The
	     purpose of regeneration is to prevent decrypting captured ses-

		   ListenAddress host|IPv4_addr|IPv6_addr
		   ListenAddress host|IPv4_addr:port
		   ListenAddress [host|IPv6_addr]:port

	     If port is not specified, sshd will listen on the address and all
	     prior Port options specified.  The default is to listen on all
	     local addresses.  Multiple ListenAddress options are permitted.
	     Additionally, any Port options must precede this option for non
	     port qualified addresses.

     LoginGraceTime
	     The server disconnects after this time if the user has not suc-
	     cessfully logged in.  If the value is 0, there is no time limit.
	     The default is 120 seconds.

     LogLevel
	     Gives the verbosity level that is used when logging messages from
	     sshd.  The possible values are: QUIET, FATAL, ERROR, INFO, VER-
	     BOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.  The default is INFO.
	     DEBUG and DEBUG1 are equivalent.  DEBUG2 and DEBUG3 each specify
	     higher levels of debugging output.  Logging with a DEBUG level
	     violates the privacy of users and is not recommended.

     MACs    Specifies the available MAC (message authentication code) algo-
	     rithms.  The MAC algorithm is used in protocol version 2 for data
	     integrity protection.  Multiple algorithms must be comma-sepa-
	     rated.  The default is
	     ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96''.

     MaxStartups
	     Specifies the maximum number of concurrent unauthenticated con-
	     nections to the sshd daemon.  Additional connections will be
	     dropped until authentication succeeds or the LoginGraceTime
	     expires for a connection.	The default is 10.

	     Alternatively, random early drop can be enabled by specifying the
	     three colon separated values ``start:rate:full'' (e.g.,
	     "10:30:60").  sshd will refuse connection attempts with a proba-
	     bility of ``rate/100'' (30%) if there are currently ``start''
	     (10) unauthenticated connections.	The probability increases lin-
	     early and all connection attempts are refused if the number of
	     unauthenticated connections reaches ``full'' (60).

     PasswordAuthentication
	     Specifies whether password authentication is allowed.  The
	     default is ``no'', unless sshd was built without PAM support, in
	     which case the default is ``yes''.  Note that if
	     ChallengeResponseAuthentication is ``yes'', and the PAM authenti-
	     cation policy for sshd includes pam_unix(8), password authentica-
	     tion will be allowed through the challenge-response mechanism
	     regardless of the value of PasswordAuthentication.

     PermitEmptyPasswords
	     When password authentication is allowed, it specifies whether the
	     server allows login to accounts with empty password strings.  The
	     default is ``no''.

     PermitRootLogin
	     cation is disabled for root.  Note that other authentication
	     methods (e.g., keyboard-interactive/PAM) may still allow root to
	     login using a password.

	     If this option is set to ``forced-commands-only'' root login with
	     public key authentication will be allowed, but only if the
	     command option has been specified (which may be useful for taking
	     remote backups even if root login is normally not allowed).  All
	     other authentication methods are disabled for root.

	     If this option is set to ``no'' root is not allowed to login.

     PermitUserEnvironment
	     Specifies whether ~/.ssh/environment and environment= options in
	     ~/.ssh/authorized_keys are processed by sshd.  The default is
	     ``no''.  Enabling environment processing may enable users to
	     bypass access restrictions in some configurations using mecha-
	     nisms such as LD_PRELOAD.

     PidFile
	     Specifies the file that contains the process ID of the sshd dae-
	     mon.  The default is /var/run/sshd.pid.

     Port    Specifies the port number that sshd listens on.  The default is
	     22.  Multiple options of this type are permitted.	See also
	     ListenAddress.

     PrintLastLog
	     Specifies whether sshd should print the date and time when the
	     user last logged in.  The default is ``yes''.

     PrintMotd
	     Specifies whether sshd should print /etc/motd when a user logs in
	     interactively.  (On some systems it is also printed by the shell,
	     /etc/profile, or equivalent.)  The default is ``yes''.

     Protocol
	     Specifies the protocol versions sshd supports.  The possible val-
	     ues are ``1'' and ``2''.  Multiple versions must be comma-sepa-
	     rated.  The default is ``2''.  Note that the order of the proto-
	     col list does not indicate preference, because the client selects
	     among multiple protocol versions offered by the server.  Specify-
	     ing ``2,1'' is identical to ``1,2''.

     PubkeyAuthentication
	     Specifies whether public key authentication is allowed.  The
	     default is ``yes''.  Note that this option applies to protocol
	     version 2 only.

     RhostsRSAAuthentication
	     Specifies whether rhosts or /etc/hosts.equiv authentication
	     together with successful RSA host authentication is allowed.  The
	     default is ``no''.  This option applies to protocol version 1
	     only.

     RSAAuthentication
	     Specifies whether pure RSA authentication is allowed.  The
	     default is ``yes''.  This option applies to protocol version 1
	     the user's files and home directory before accepting login.  This
	     is normally desirable because novices sometimes accidentally
	     leave their directory or files world-writable.  The default is
	     ``yes''.

     Subsystem
	     Configures an external subsystem (e.g., file transfer daemon).
	     Arguments should be a subsystem name and a command to execute
	     upon subsystem request.  The command sftp-server(8) implements
	     the ``sftp'' file transfer subsystem.  By default no subsystems
	     are defined.  Note that this option applies to protocol version 2
	     only.

     SyslogFacility
	     Gives the facility code that is used when logging messages from
	     sshd.  The possible values are: DAEMON, USER, AUTH, LOCAL0,
	     LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.  The
	     default is AUTH.

     TCPKeepAlive
	     Specifies whether the system should send TCP keepalive messages
	     to the other side.  If they are sent, death of the connection or
	     crash of one of the machines will be properly noticed.  However,
	     this means that connections will die if the route is down tempo-
	     rarily, and some people find it annoying.	On the other hand, if
	     TCP keepalives are not sent, sessions may hang indefinitely on
	     the server, leaving ``ghost'' users and consuming server
	     resources.

	     The default is ``yes'' (to send TCP keepalive messages), and the
	     server will notice if the network goes down or the client host
	     crashes.  This avoids infinitely hanging sessions.

	     To disable TCP keepalive messages, the value should be set to
	     ``no''.

     UseDNS  Specifies whether sshd should lookup the remote host name and
	     check that the resolved host name for the remote IP address maps
	     back to the very same IP address.	The default is ``yes''.

     UseLogin
	     Specifies whether login(1) is used for interactive login ses-
	     sions.  The default is ``no''.  Note that login(1) is never used
	     for remote command execution.  Note also, that if this is
	     enabled, X11Forwarding will be disabled because login(1) does not
	     know how to handle xauth(1) cookies.  If UsePrivilegeSeparation
	     is specified, it will be disabled after authentication.

     UsePAM  Enables PAM authentication (via challenge-response) and session
	     set up.  If you enable this, you should probably disable
	     PasswordAuthentication.  If you enable then you will not be able
	     to run sshd as a non-root user.  The default is ``yes''.

     UsePrivilegeSeparation
	     Specifies whether sshd separates privileges by creating an
	     unprivileged child process to deal with incoming network traffic.
	     After successful authentication, another process will be created
	     that has the privilege of the authenticated user.	The goal of

     X11DisplayOffset
	     Specifies the first display number available for sshd's X11 for-
	     warding.  This prevents sshd from interfering with real X11
	     servers.  The default is 10.

     X11Forwarding
	     Specifies whether X11 forwarding is permitted.  The argument must
	     be ``yes'' or ``no''.  The default is ``yes''.

	     When X11 forwarding is enabled, there may be additional exposure
	     to the server and to client displays if the sshd proxy display is
	     configured to listen on the wildcard address (see X11UseLocalhost
	     below), however this is not the default.  Additionally, the
	     authentication spoofing and authentication data verification and
	     substitution occur on the client side.  The security risk of
	     using X11 forwarding is that the client's X11 display server may
	     be exposed to attack when the ssh client requests forwarding (see
	     the warnings for ForwardX11 in ssh_config(5)).  A system adminis-
	     trator may have a stance in which they want to protect clients
	     that may expose themselves to attack by unwittingly requesting
	     X11 forwarding, which can warrant a ``no'' setting.

	     Note that disabling X11 forwarding does not prevent users from
	     forwarding X11 traffic, as users can always install their own
	     forwarders.  X11 forwarding is automatically disabled if UseLogin
	     is enabled.

     X11UseLocalhost
	     Specifies whether sshd should bind the X11 forwarding server to
	     the loopback address or to the wildcard address.  By default,
	     sshd binds the forwarding server to the loopback address and sets
	     the hostname part of the DISPLAY environment variable to
	     ``localhost''.  This prevents remote hosts from connecting to the
	     proxy display.  However, some older X11 clients may not function
	     with this configuration.  X11UseLocalhost may be set to ``no'' to
	     specify that the forwarding server should be bound to the wild-
	     card address.  The argument must be ``yes'' or ``no''.  The
	     default is ``yes''.

     XAuthLocation
	     Specifies the full pathname of the xauth(1) program.  The default
	     is /usr/X11R6/bin/xauth.

   Time Formats
     sshd command-line arguments and configuration file options that specify
     time may be expressed using a sequence of the form: time[qualifier],
     where time is a positive integer value and qualifier is one of the fol-
     lowing:

	   <none>  seconds
	   s | S   seconds
	   m | M   minutes
	   h | H   hours
	   d | D   days
	   w | W   weeks

     Each member of the sequence is added together to calculate the total time

FILES

     /etc/ssh/sshd_config
	     Contains configuration data for sshd.  This file should be
	     writable by root only, but it is recommended (though not neces-
	     sary) that it be world-readable.

SEE ALSO

     sshd(8)

AUTHORS

     OpenSSH is a derivative of the original and free ssh 1.2.12 release by
     Tatu Ylonen.  Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
     de Raadt and Dug Song removed many bugs, re-added newer features and cre-
     ated OpenSSH.  Markus Friedl contributed the support for SSH protocol
     versions 1.5 and 2.0.  Niels Provos and Markus Friedl contributed support
     for privilege separation.

FreeBSD 5.4		      September 25, 1999		   FreeBSD 5.4

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