sshd_config

     /etc/ssh/sshd_config

DESCRIPTION
     sshd(8) 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.  Arguments may optionally be enclosed in double
     quotes (") in order to represent arguments containing spaces.

     Note that the Debian openssh-server package sets several options as stan-
     dard in /etc/ssh/sshd_config which are not the default in sshd(8).  The
     exact list depends on whether the package was installed fresh or upgraded
     from various possible previous versions, but includes at least the fol-
     lowing:

           o   Protocol 2
           o   ChallengeResponseAuthentication no
           o   X11Forwarding yes
           o   PrintMotd no
           o   AcceptEnv LANG LC_*
           o   Subsystem sftp /usr/lib/openssh/sftp-server
           o   UsePAM yes

     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.

     AddressFamily
             Specifies which address family should be used by sshd(8).  Valid
             arguments are ``any'', ``inet'' (use IPv4 only), or ``inet6''
             (use IPv6 only).  The default is ``any''.

     AllowAgentForwarding
             Specifies whether ssh-agent(1) forwarding is permitted.  The
             default is ``yes''.  Note that disabling agent forwarding does
             not improve security unless users are also denied shell access,
             as they can always install their own forwarders.

     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
             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.  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.  The allow/deny
             directives are processed in the following order: DenyUsers,
             AllowUsers, DenyGroups, and finally AllowGroups.

             See PATTERNS in ssh_config(5) for more information on patterns.

     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 setup.
             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  The contents of the specified file are sent to the remote user
             before authentication is allowed.  If the argument is ``none''
             then no banner is displayed.  This option is only available for
             protocol version 2.  By default, no banner is displayed.

     ChallengeResponseAuthentication
             Specifies whether challenge-response authentication is allowed
             (e.g. via PAM).  The default is ``yes''.

     ChrootDirectory
             Specifies a path to chroot(2) to after authentication.  This
             path, and all its components, must be root-owned directories that
             are not writable by any other user or group.  After the chroot,
             sshd(8) changes the working directory to the user's home direc-
             tory.

             The path may contain the following tokens that are expanded at
             runtime once the connecting user has been authenticated: %% is
             replaced by a literal '%', %h is replaced by the home directory
             of the user being authenticated, and %u is replaced by the user-
             name of that user.

             The ChrootDirectory must contain the necessary files and directo-
             ries to support the user's session.  For an interactive session
             this requires at least a shell, typically sh(1), and basic /dev
             nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4),
             arandom(4) and tty(4) devices.  For file transfer sessions using
             ``sftp'', no additional configuration of the environment is nec-
             ``cast128-cbc''.  The default is:

                aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
                aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
                aes256-cbc,arcfour

     ClientAliveCountMax
             Sets the number of client alive messages (see below) which may be
             sent without sshd(8) 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 (see below) is
             set to 15, and ClientAliveCountMax is left at the default, unre-
             sponsive SSH clients will be disconnected after approximately 45
             seconds.  This option applies to protocol version 2 only.

     ClientAliveInterval
             Sets a timeout interval in seconds after which if no data has
             been received from the client, sshd(8) 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 ver-
             sion 2 only.

     Compression
             Specifies whether compression is allowed, or delayed until the
             user has authenticated successfully.  The argument must be
             ``yes'', ``delayed'', or ``no''.  The default is ``delayed''.

     DebianBanner
             Specifies whether the distribution-specified extra version suffix
             is included during initial protocol handshake.  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.
             Only group names are valid; a numerical group ID is not recog-
             nized.  By default, login is allowed for all groups.  The
             allow/deny directives are processed in the following order:
             DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.

             See PATTERNS in ssh_config(5) for more information on patterns.

     DenyUsers
     ForceCommand
             Forces the execution of the command specified by ForceCommand,
             ignoring any command supplied by the client and ~/.ssh/rc if
             present.  The command is invoked by using the user's login shell
             with the -c option.  This applies to shell, command, or subsystem
             execution.  It is most useful inside a Match block.  The command
             originally supplied by the client is available in the
             SSH_ORIGINAL_COMMAND environment variable.  Specifying a command
             of ``internal-sftp'' will force the use of an in-process sftp
             server that requires no support files when used with
             ChrootDirectory.

     GatewayPorts
             Specifies whether remote hosts are allowed to connect to ports
             forwarded for the client.  By default, sshd(8) 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 allow remote port forwardings to
             bind to non-loopback addresses, thus allowing other hosts to con-
             nect.  The argument may be ``no'' to force remote port forward-
             ings to be available to the local host only, ``yes'' to force
             remote port forwardings to bind to the wildcard address, or
             ``clientspecified'' to allow the client to select the address to
             which the forwarding is bound.  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.

     GSSAPIKeyExchange
             Specifies whether key exchange based on GSSAPI is allowed. GSSAPI
             key exchange doesn't rely on ssh keys to verify host identity.
             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.

     GSSAPIStrictAcceptorCheck
             Determines whether to be strict about the identity of the GSSAPI
             acceptor a client authenticates against. If ``yes'' then the
             client must authenticate against the host service on the current
             hostname. If ``no'' then the client may authenticate against any
             service key stored in the machine's default store. This facility
             is provided to assist with operation on multi homed machines.
             The default is ``yes''.  Note that this option applies only to
             protocol version 2 GSSAPI connections, and setting it to ``no''
             may only work with recent Kerberos GSSAPI libraries.

     GSSAPIStoreCredentialsOnRekey
             Controls whether the user's GSSAPI credentials should be updated
             Specifies whether or not the server will attempt to perform a
             reverse name lookup when matching the name in the ~/.shosts,
             ~/.rhosts, and /etc/hosts.equiv files during
             HostbasedAuthentication.  A setting of ``yes'' means that sshd(8)
             uses the name supplied by the client rather than attempting to
             resolve the name from the TCP connection itself.  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(8) 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(8) should ignore the user's
             ~/.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.  The default
             is ``no''.

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

     KerberosOrLocalPasswd
             If password authentication through Kerberos fails then the pass-
             word will be validated via any additional local mechanism such as
             /etc/passwd.  The default is ``yes''.

     KerberosTicketCleanup
             Specifies whether to automatically destroy the user's ticket
             cache file on logout.  The 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-
             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(8).  The possible values are: SILENT, QUIET, FATAL, ERROR,
             INFO, VERBOSE, 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,umac-64@openssh.com,
                   hmac-ripemd160,hmac-sha1-96,hmac-md5-96

     Match   Introduces a conditional block.  If all of the criteria on the
             Match line are satisfied, the keywords on the following lines
             override those set in the global section of the config file,
             until either another Match line or the end of the file.

             The arguments to Match are one or more criteria-pattern pairs.
             The available criteria are User, Group, Host, and Address.  The
             match patterns may consist of single entries or comma-separated
             lists and may use the wildcard and negation operators described
             in the PATTERNS section of ssh_config(5).

             The patterns in an Address criteria may additionally contain
             addresses to match in CIDR address/masklen format, e.g.
             ``192.0.2.0/24'' or ``3ffe:ffff::/32''.  Note that the mask
             length provided must be consistent with the address - it is an
             error to specify a mask length that is too long for the address
             or one with bits set in this host portion of the address.  For
             example, ``192.0.2.0/33'' and ``192.0.2.0/8'' respectively.

             Only a subset of keywords may be used on the lines following a
             Match keyword.  Available keywords are AllowAgentForwarding,
             AllowTcpForwarding, Banner, ChrootDirectory, ForceCommand,
             GatewayPorts, GSSAPIAuthentication, HostbasedAuthentication,
             KbdInteractiveAuthentication, KerberosAuthentication,
             MaxAuthTries, MaxSessions, PasswordAuthentication,
             PermitEmptyPasswords, PermitOpen, PermitRootLogin,
             RhostsRSAAuthentication, RSAAuthentication, X11DisplayOffset,
             Specifies the maximum number of concurrent unauthenticated con-
             nections to the SSH 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(8) will refuse connection attempts with a
             probability of ``rate/100'' (30%) if there are currently
             ``start'' (10) unauthenticated connections.  The probability
             increases linearly 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 ``yes''.

     PermitBlacklistedKeys
             Specifies whether sshd(8) should allow keys recorded in its
             blacklist of known-compromised keys (see ssh-vulnkey(1)).  If
             ``yes'', then attempts to authenticate with compromised keys will
             be logged but accepted.  If ``no'', then attempts to authenticate
             with compromised keys will be rejected.  The default is ``no''.

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

     PermitOpen
             Specifies the destinations to which TCP port forwarding is per-
             mitted.  The forwarding specification must be one of the follow-
             ing forms:

                   PermitOpen host:port
                   PermitOpen IPv4_addr:port
                   PermitOpen [IPv6_addr]:port

             Multiple forwards may be specified by separating them with white-
             space.  An argument of ``any'' can be used to remove all restric-
             tions and permit any forwarding requests.  By default all port
             forwarding requests are permitted.

     PermitRootLogin
             Specifies whether root can log in 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.

             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
             Specifies whether ~/.ssh/environment and environment= options in
             ~/.ssh/authorized_keys are processed by sshd(8).  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 SSH dae-
             mon.  The default is /var/run/sshd.pid.

     Port    Specifies the port number that sshd(8) listens on.  The default
             is 22.  Multiple options of this type are permitted.  See also
             ListenAddress.

     PrintLastLog
             Specifies whether sshd(8) should print the date and time of the
             last user login when a user logs in interactively.  The default
             is ``yes''.

     PrintMotd
             Specifies whether sshd(8) 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(8) supports.  The possible
             values 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
             selects 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
             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
             only.

     ServerKeyBits
             Defines the number of bits in the ephemeral protocol version 1
             server key.  The minimum value is 512, and the default is 1024.

     StrictModes
             Specifies whether sshd(8) should check file modes and ownership

             Alternately the name ``internal-sftp'' implements an in-process
             ``sftp'' server.  This may simplify configurations using
             ChrootDirectory to force a different filesystem root on clients.

             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(8).  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''.

             This option was formerly called KeepAlive.

     UseDNS  Specifies whether sshd(8) should look up 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 the Pluggable Authentication Module interface.  If set to
             ``yes'' this will enable PAM authentication using
             ChallengeResponseAuthentication and PasswordAuthentication in
             addition to PAM account and session module 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.

     X11DisplayOffset
             Specifies the first display number available for sshd(8)'s X11
             forwarding.  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(8) proxy display
             is configured to listen on the wildcard address (see
             X11UseLocalhost below), though this is not the default.  Addi-
             tionally, 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 administrator may have a stance in
             which they want to protect clients that may expose themselves to
             attack by unwittingly requesting X11 forwarding, which can war-
             rant 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(8) 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/bin/xauth.

TIME FORMATS
     sshd(8) command-line arguments and configuration file options that spec-
     ify 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

FILES
     /etc/ssh/sshd_config
             Contains configuration data for sshd(8).  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.

BSD                             August 20, 2014                            BSD
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