path: root/sshd_config.0

SSHD_CONFIG(5)            OpenBSD Programmer's Manual           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):

     AcceptEnv
             Specifies what environment variables sent by the client will be
             copied into the session's environ(7).  See SendEnv in
             ssh_config(5) for how to configure the client.  Note that envi-
             ronment passing is only supported for protocol 2.  Variables are
             specified by name, which may contain the wildcard characters `*'
             and `?'.  Multiple environment variables may be separated by
             whitespace or spread across multiple AcceptEnv directives.  Be
             warned that some environment variables could be used to bypass
             restricted user environments.  For this reason, care should be
             taken in the use of this directive.  The default is not to accept
             any environment variables.

     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 se-
             curity unless users are also denied shell access, as they can al-
             ways 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 us-
             er names that match one of the patterns.  `*' and `?' can be used
             as wildcards in the patterns.  Only user names are valid; a nu-
             merical 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 au-
             thenticated and %u is replaced by the username of that user.  Af-
             ter 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
             Specifies whether challenge response authentication is allowed.
             All authentication styles from login.conf(5) are supported.  The
             default is ``yes''.

     Ciphers
             Specifies the ciphers allowed for protocol version 2.  Multiple
             ciphers must be comma-separated.  The supported ciphers are
             ``3des-cbc'', ``aes128-cbc'', ``aes192-cbc'', ``aes256-cbc'',
             ``aes128-ctr'', ``aes192-ctr'', ``aes256-ctr'', ``arcfour'',
             ``blowfish-cbc'', and ``cast128-cbc''.  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 be-
             ing 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 de-
             fault, 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
             forwarded ports.  The argument must be ``yes'' or ``no''.  The
             default is ``no''.

     GSSAPIAuthentication
             Specifies whether user authentication based on GSSAPI is allowed.
             The default is ``no''.  Note that this option applies to protocol
             version 2 only.

     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 to-
             gether 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_rsa_key and /etc/ssh/ssh_host_dsa_key for pro-
             tocol 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/shosts.equiv are still used.  The de-
             fault 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-
             sions by later breaking into the machine and stealing the keys.
             The key is never stored anywhere.  If the value is 0, the key is
             never regenerated.  The default is 3600 (seconds).

     ListenAddress
             Specifies the local addresses sshd should listen on.  The follow-
             ing forms may be used:

                   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-separat-
             ed.  The default is ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-
             sha1-96,hmac-md5-96''.

     MaxAuthTries
             Specifies the maximum number of authentication attempts permitted
             per connection.  Once the number of failures reaches half this
             value, additional failures are logged.  The default is 6.

     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 ex-
             pires 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 de-
             fault is ``yes''.

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

     PermitRootLogin
             Specifies whether root can login using ssh(1).  The argument must
             be ``yes'', ``without-password'', ``forced-commands-only'' or
             ``no''.  The default is ``yes''.

             If this option is set to ``without-password'' password authenti-
             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 by-
             pass access restrictions in some configurations using mechanisms
             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 ``2,1''.  Note that the order of the pro-
             tocol list does not indicate preference, because the client se-
             lects among multiple protocol versions offered by the server.
             Specifying ``2,1'' is identical to ``1,2''.

     PubkeyAuthentication
             Specifies whether public key authentication is allowed.  The de-
             fault is ``yes''.  Note that this option applies to protocol ver-
             sion 2 only.

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

     RSAAuthentication
             Specifies whether pure RSA authentication is allowed.  The de-
             fault is ``yes''.  This option applies to protocol version 1 on-
             ly.

     ServerKeyBits
             Defines the number of bits in the ephemeral protocol version 1
             server key.  The minimum value is 512, and the default is 768.

     StrictModes
             Specifies whether sshd should check file modes and ownership of
             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 up-
             on 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 on-
             ly.

     SyslogFacility
             Gives the facility code that is used when logging messages from
             sshd.  The possible values are: DAEMON, USER, AUTH, LOCAL0, LO-
             CAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.  The de-
             fault 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 tem-
             porarily, 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 re-
             sources.

             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 en-
             abled, 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 the Pluggable Authentication Module interface.  If set to
             ``yes'' this will enable PAM authentication using
             ChallengeResponseAuthentication and PAM account and session mod-
             ule processing for all authentication types.

             Because PAM challenge-response authentication usually serves an
             equivalent role to password authentication, you should disable
             either PasswordAuthentication or ChallengeResponseAuthentication.

             If UsePAM is enabled, you will not be able to run sshd(8) as a
             non-root user.  The default is ``no''.

     UsePrivilegeSeparation
             Specifies whether sshd separates privileges by creating an un-
             privileged 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
             privilege separation is to prevent privilege escalation by con-
             taining any corruption within the unprivileged processes.  The
             default is ``yes''.

     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 ``no''.

             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 au-
             thentication spoofing and authentication data verification and
             substitution occur on the client side.  The security risk of us-
             ing 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 de-
             fault 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
     value.

     Time format examples:

           600     600 seconds (10 minutes)
           10m     10 minutes
           1h30m   1 hour 30 minutes (90 minutes)

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.

OpenBSD 3.6                   September 25, 1999                             8