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SSHD_CONFIG(5)		    BSD 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-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.

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

     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 secu-
	     rity 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
	     Specifies whether challenge response authentication is allowed.
	     All authentication styles from login.conf(5) are supported.  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.

	     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 users' 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 neces-
	     sary if the in-process sftp server is used (see "internal-sftp"
	     will force the use of an in-process sftp server that requires no
	     support files when used with ChrootDirectory.  Subsystem for
	     details).

	     The default is not to chroot(2).

     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", "arcfour128",
	     "arcfour256", "arcfour", "blowfish-cbc", and "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 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.

     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.

     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".

     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.	Specifying a command of "internal-sftp" will
	     force the use of an in-process sftp server that requires no sup-
	     port files when used with ChrootDirectory.

     ForceCommand
	     Forces the execution of the command specified by ForceCommand,
	     ignoring any command supplied by the client.  The command is
	     invoked by using the user's login shell with the -c option.  This
	     applies to shell, command, or subsystem execution.	 The command
	     originally supplied by the client is available in the
	     SSH_ORIGINAL_COMMAND environment variable.

     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 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 forwardings
	     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.

     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_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/ssh/shosts.equiv are still used.	 The
	     default is "yes".

     IgnoreUserKnownHosts
	     Specifies whether sshd 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.  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.
	     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-sepa-
	     rated.  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
	     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 linearly
	     and all connection attempts are refused if the number of unau-
	     thenticated connections reaches "full" (60).

     PasswordAuthentication
	     Specifies whether password authentication is allowed.  The
	     default 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 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 authentica-
	     tion 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 log in.

     PermitTunnel
	     Specifies whether tun(4) device forwarding is allowed.  The argu-
	     ment must be "yes", "point-to-point", "ethernet" or "no".	The
	     default is "no".

     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 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 of the last
	     user login when a user logs in interactively.  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-separated.
	     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.	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 ver-
	     sion 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 768.

     ShowPatchLevel
	     Specifies whether sshd will display the patch level of the binary
	     in the identification string.  The patch level is set at compile-
	     time.  The default is "no".  This option applies to protocol ver-
	     sion 1 only.

     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 (with optional
	     arguments) to execute upon subsystem request.

	     The command sftp-server(8) implements the "sftp" file transfer
	     subsystem.

	     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.  The possible values are: DAEMON, USER, AUTH, AUTHPRIV,
	     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 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 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 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 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
	     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
	     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
	     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/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.

BSD			      September 25, 1999			   BSD
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