sshd_config.0

OpenSSL Source code for SFTP, SSH, and many others

SSHD_CONFIG(5)            System File Formats 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-arguM--
     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 keyM--
     words are case-insensitive and arguments are case-sensitive):

     AFSTokenPassing
             Specifies whether an AFS token may be forwarded to the server.
             Default is ``no''.

     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
             users 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 authentiM--
             cation may be relevant for getting legal protection.  The conM--
             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 default is

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

     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 session.
             It is important to note that the use of client alive messages is
             very different from KeepAlive (below). The client alive messages
             are sent through the encrypted channel and therefore will not be
             spoofable. The TCP keepalive option enabled by KeepAlive is
             spoofable. The client alive mechanism is valuable when the client
             or server depend on knowing when a connection has become inacM--
             tive.

             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
             forwarded ports.  The argument must be ``yes'' or ``no''.  The
             default is ``no''.

     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_rsa_key and /etc/ssh/ssh_host_dsa_key for proM--
             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
             RhostsAuthentication, RhostsRSAAuthentication or
             HostbasedAuthentication.

             /etc/hosts.equiv and /etc/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''.

     KeepAlive
             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 temM--
             porarily, and some people find it annoying.  On the other hand,
             if keepalives are not sent, sessions may hang indefinitely on the
             server, leaving ``ghost'' users and consuming server resources.

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

             To disable keepalives, the value should be set to ``no''.

     KerberosAuthentication
             Specifies whether Kerberos authentication is allowed.  This can
             be in the form of a Kerberos ticket, or if

     PAMAuthenticationViaKbdInt
             Specifies whether PAM challenge response authentication is
             allowed. This allows the use of most PAM challenge response
             authentication modules, but it will allow password authentication
             regardless of whether PasswordAuthentication is yes, the password
             provided by the user 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''.

     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''.

     KerberosTgtPassing
             Specifies whether a Kerberos TGT may be forwarded to the server.
             Default is ``no'', as this only works when the Kerberos KDC is
             actually an AFS kaserver.

     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 sesM--
             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 followM--
             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.