Manual Pages: sshd_config(5)

    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):

         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 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 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 inac-
                 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 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 de-

                 fault is ``no''.

         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
                 RhostsAuthentication, 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''.

         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 tem-
                 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 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 iden-
                 tity.  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 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 lo-
                 cal addresses.  Multiple ListenAddress options are permitted. Ad-
                 ditionally, 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''.

         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.

                 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-separat-
                 ed.  The default is ``2,1''. Note that the order of the protocol
                 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 de-
                 fault is ``yes''. Note that this option applies to protocol ver-
                 sion 2 only.

         RhostsAuthentication
                 Specifies whether authentication using rhosts or /etc/hosts.equiv
                 files is sufficient.  Normally, this method should not be permit-
                 ted because it is insecure.  RhostsRSAAuthentication should be
                 used instead, because it performs RSA-based host authentication
                 in addition to normal rhosts or /etc/hosts.equiv authentication.
                 The default is ``no''. This option applies to protocol version 1
                 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 only.

         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.

         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.

         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''.

         VerifyReverseMapping
                 Specifies whether sshd should try to verify 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 ``no''.

         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.

    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.

    SEE ALSO
         sshd(8)

    OpenBSD 3.3                   September 25, 1999                             8

NAME | SYNOPSIS | DESCRIPTION | FILES | AUTHORS | SEE ALSO