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RedHat 9 (Linux i386) - man page for sshd_config (redhat section 5)

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-argument 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 keywords are case-insen-
     sitive and arguments are case-sensitive):

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

     AllowGroups
	     This keyword can be followed by a list of group name patterns, separated by spaces.
	     If specified, login is allowed only for users whose primary group or supplementary
	     group list matches one of the patterns.  '*' and ''?  can be used as wildcards in
	     the patterns.  Only group names are valid; a numerical group ID is not recognized.
	     By default, login is allowed for all groups.

     AllowTcpForwarding
	     Specifies whether TCP forwarding is permitted.  The default is ``yes''.  Note that
	     disabling TCP forwarding does not improve security unless users are also denied
	     shell access, as they can always install their own forwarders.

     AllowUsers
	     This keyword can be followed by a list of user name patterns, separated by spaces.
	     If specified, login is allowed only for users names that match one of the patterns.
	     '*' and ''?  can be used as wildcards in the patterns.  Only user names are valid; a
	     numerical user ID is not recognized.  By default, login is allowed for all users.
	     If the pattern takes the form USER@HOST then USER and HOST are separately checked,
	     restricting logins to particular users from particular hosts.

     AuthorizedKeysFile
	     Specifies the file that contains the public keys that can be used for user authenti-
	     cation.  AuthorizedKeysFile may contain tokens of the form %T which are substituted
	     during connection set-up. The following tokens are defined: %% is replaced by a lit-
	     eral '%', %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 authentication may be rele-
	     vant for getting legal protection.  The contents 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 inactive.

	     The default value is 3. If ClientAliveInterval (above) is set to 15, and
	     ClientAliveCountMax is left at the default, unresponsive ssh clients will be discon-
	     nected after approximately 45 seconds.

     Compression
	     Specifies whether compression is allowed.	The argument must be ``yes'' or ``no''.
	     The default is ``yes''.

     DenyGroups
	     This keyword can be followed by a list of group name patterns, separated by spaces.
	     Login is disallowed for users whose primary group or supplementary group list
	     matches one of the patterns.  '*' and ''?	can be used as wildcards in the patterns.
	     Only group names are valid; a numerical group ID is not recognized.  By default,
	     login is allowed for all groups.

     DenyUsers
	     This keyword can be followed by a list of user name patterns, separated by spaces.
	     Login is disallowed for user names that match one of the patterns.  '*' and ''?  can
	     be used as wildcards in the patterns.  Only user names are valid; a numerical user
	     ID is not recognized.  By default, login is allowed for all users.  If the pattern
	     takes the form USER@HOST then USER and HOST are separately checked, restricting
	     logins to particular users from particular hosts.

     GatewayPorts
	     Specifies whether remote hosts are allowed to connect to ports forwarded for the
	     client.  By default, sshd binds remote port forwardings to the loopback address.
	     This prevents other remote hosts from connecting to forwarded ports.  GatewayPorts
	     can be used to specify that sshd should bind remote port forwardings to the wildcard
	     address, thus allowing remote hosts to connect to forwarded ports.  The argument
	     must be ``yes'' or ``no''.  The default is ``no''.

     HostbasedAuthentication
	     Specifies whether rhosts or /etc/hosts.equiv authentication together with successful
	     public key client host authentication is allowed (hostbased authentication).  This
	     option is similar to RhostsRSAAuthentication and applies to protocol version 2 only.
	     The default is ``no''.

     HostKey
	     Specifies a file containing a private host key used by SSH.  The default is
	     /etc/ssh/ssh_host_key for protocol version 1, and /etc/ssh/ssh_host_rsa_key and
	     /etc/ssh/ssh_host_dsa_key for protocol 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/ssh/shosts.equiv are still used.  The default is ``yes''.

     IgnoreUserKnownHosts
	     Specifies whether sshd should ignore the user's $HOME/.ssh/known_hosts during
	     RhostsRSAAuthentication or HostbasedAuthentication.  The default is ``no''.

     KeepAlive
	     Specifies whether the system should send TCP keepalive messages to the other side.
	     If they are sent, death of the connection or crash of one of the machines will be
	     properly noticed.	However, this means that connections will die if the route is
	     down temporarily, 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 net-
	     work 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 identity.
	     Default is ``no''.

     KerberosOrLocalPasswd
	     If set then if password authentication through Kerberos fails then the password will
	     be validated via any additional local mechanism such as /etc/passwd.  Default is
	     ``yes''.

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

     KerberosTicketCleanup
	     Specifies whether to automatically destroy the user's ticket cache file on logout.
	     Default is ``yes''.

     KeyRegenerationInterval
	     In protocol version 1, the ephemeral server key is automatically regenerated after
	     this many seconds (if it has been used).  The purpose of regeneration is to prevent
	     decrypting captured sessions 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 regen-
	     erated.  The default is 3600 (seconds).

     ListenAddress
	     Specifies the local addresses sshd should listen on.  The following 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 successfully 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 possi-
	     ble 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) algorithms.  The MAC algo-
	     rithm is used in protocol version 2 for data integrity protection.  Multiple algo-
	     rithms must be comma-separated.  The default is
	     ``hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96''.

     MaxStartups
	     Specifies the maximum number of concurrent unauthenticated connections 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 sepa-
	     rated values ``start:rate:full'' (e.g., "10:30:60").  sshd 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 con-
	     nection attempts are refused if the number of unauthenticated connections reaches
	     ``full'' (60).

     PAMAuthenticationViaKbdInt
	     Specifies whether PAM challenge response authentication is allowed. This allows the
	     use of most PAM challenge response authentication modules, but it will allow pass-
	     word authentication regardless of whether PasswordAuthentication is enabled.

     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 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 authentication is disabled
	     for root.

	     If this option is set to ``forced-commands-only'' root login with public key authen-
	     tication 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 bypass access restrictions in some con-
	     figurations using mechanisms such as LD_PRELOAD.

     PidFile
	     Specifies the file that contains the process ID of the sshd daemon.  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 values 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 version 2 only.

     RhostsAuthentication
	     Specifies whether authentication using rhosts or /etc/hosts.equiv files is suffi-
	     cient.  Normally, this method should not be permitted 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 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 min-
	     imum 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 upon subsystem request.	The command
	     sftp-server(8) implements the ``sftp'' file transfer subsystem.  By default no sub-
	     systems 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, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5,
	     LOCAL6, LOCAL7.  The default is AUTH.

     UseLogin
	     Specifies whether login(1) is used for interactive login sessions.  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.

     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 containing
	     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 forwarding.  This pre-
	     vents 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 administra-
	     tor may have a stance in which they want to protect clients that may expose them-
	     selves 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 traf-
	     fic, as users can always install their own forwarders.  X11 forwarding is automati-
	     cally 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 wildcard 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/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 following:

	   <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 necessary) 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 created 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)

BSD					September 25, 1999				      BSD


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