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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(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.

     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
	     of the patterns.  Only group names are valid; a numerical group
	     ID is not recognized.  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.

     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.	 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 proto‐
	     col version 2.  By default, no banner is displayed.

     ChallengeResponseAuthentication
	     Specifies whether challenge-response authentication is allowed
	     (e.g. via PAM or though authentication styles supported in
	     login.conf(5)) The default is “yes”.

     ChrootDirectory
	     Specifies the pathname of a directory to chroot(2) to after
	     authentication.  All components of the pathname 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 directory.

	     The pathname 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 neces‐
	     sary if the in-process sftp server is used, though sessions which
	     use logging do require /dev/log inside the chroot directory (see
	     sftp-server(8) 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(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”.

     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
	     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.	 Only user names are valid; a numeri‐
	     cal 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 pro‐
	     cessed in the following order: DenyUsers, AllowUsers, DenyGroups,
	     and finally AllowGroups.

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

     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 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 (host-based authentication).  This option is similar to
	     RhostsRSAAuthentication and applies to protocol version 2 only.
	     The default is “no”.

     HostbasedUsesNameFromPacketOnly
	     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”.

     HostCertificate
	     Specifies a file containing a public host certificate.  The cer‐
	     tificate's public key must match a private host key already spec‐
	     ified by HostKey.	The default behaviour of sshd(8) is not to
	     load any certificates.

     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‐
	     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(8) should listen on.  The fol‐
	     lowing 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(8).  The possible values are: 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,
	     PubkeyAuthentication, RhostsRSAAuthentication, RSAAuthentication,
	     X11DisplayOffset, X11Forwarding and X11UseLocalHost.

     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.

     MaxSessions
	     Specifies the maximum number of open sessions permitted per net‐
	     work connection.  The default is 10.

     MaxStartups
	     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) unauthenti‐
	     cated 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.  See also
	     UsePAM.  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 “no”.  Note that if
	     ChallengeResponseAuthentication is “yes”, the root user may be
	     allowed in with its password even if PermitRootLogin is set to
	     “without-password”.

	     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” (layer 3), “ethernet” (layer
	     2), or “no”.  Specifying “yes” permits both “point-to-point” and
	     “ethernet”.  The default is “no”.

     PermitUserEnvironment
	     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 mechanisms 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’.  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
	     default is “yes”.	Note that this option applies to protocol ver‐
	     sion 2 only.

     RevokedKeys
	     Specifies a list of revoked public keys.  Keys listed in this
	     file will be refused for public key authentication.  Note that if
	     this file is not readable, then public key authentication will be
	     refused for all users.

     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
	     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”.  Note that this does not apply to ChrootDirectory, whose
	     permissions and ownership are checked unconditionally.

     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(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”.

     TrustedUserCAKeys
	     Specifies a file containing public keys of certificate authori‐
	     ties that are trusted to sign user certificates for authentica‐
	     tion.  Keys are listed one per line; empty lines and comments
	     starting with ‘#’ are allowed.  If a certificate is presented for
	     authentication and has its signing CA key listed in this file,
	     then it may be used for authentication for any user listed in the
	     certificate's principals list.  Note that certificates that lack
	     a list of principals will not be permitted for authentication
	     using TrustedUserCAKeys.  For more details on certificates, see
	     the CERTIFICATES section in ssh-keygen(1).

     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.

	     If UsePAM is enabled, you will not be able to run sshd(8) as a
	     non-root user.  The default is “yes”.

     UsePrivilegeSeparation
	     Specifies whether sshd(8) 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”.

     VersionAddendum
	     Specifies a string to append to the regular version string to
	     identify OS- or site-specific modifications.  The default is
	     “FreeBSD-20100308”.

     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 “yes”.

	     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/local/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
	   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(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				 March 4, 2010				   BSD
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