2008-07-23 09:15:38 +00:00
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SSHD_CONFIG(5) OpenBSD Programmer's Manual SSHD_CONFIG(5)
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NAME
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sshd_config - OpenSSH SSH daemon configuration file
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SYNOPSIS
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/etc/ssh/sshd_config
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DESCRIPTION
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sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file
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specified with -f on the command line). The file contains keyword-argu-
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ment pairs, one per line. Lines starting with `#' and empty lines are
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interpreted as comments. Arguments may optionally be enclosed in double
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quotes (") in order to represent arguments containing spaces.
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The possible keywords and their meanings are as follows (note that key-
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words are case-insensitive and arguments are case-sensitive):
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AcceptEnv
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Specifies what environment variables sent by the client will be
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copied into the session's environ(7). See SendEnv in
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ssh_config(5) for how to configure the client. Note that envi-
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ronment passing is only supported for protocol 2. Variables are
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specified by name, which may contain the wildcard characters `*'
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and `?'. Multiple environment variables may be separated by
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whitespace or spread across multiple AcceptEnv directives. Be
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warned that some environment variables could be used to bypass
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restricted user environments. For this reason, care should be
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taken in the use of this directive. The default is not to accept
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any environment variables.
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AddressFamily
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Specifies which address family should be used by sshd(8). Valid
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arguments are ``any'', ``inet'' (use IPv4 only), or ``inet6''
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(use IPv6 only). The default is ``any''.
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2008-07-23 09:33:08 +00:00
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AllowAgentForwarding
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Specifies whether ssh-agent(1) forwarding is permitted. The de-
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fault is ``yes''. Note that disabling agent forwarding does not
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improve security unless users are also denied shell access, as
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they can always install their own forwarders.
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2008-07-23 09:15:38 +00:00
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AllowGroups
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This keyword can be followed by a list of group name patterns,
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separated by spaces. If specified, login is allowed only for
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users whose primary group or supplementary group list matches one
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of the patterns. Only group names are valid; a numerical group
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ID is not recognized. By default, login is allowed for all
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groups. The allow/deny directives are processed in the following
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order: DenyUsers, AllowUsers, DenyGroups, and finally
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AllowGroups.
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See PATTERNS in ssh_config(5) for more information on patterns.
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AllowTcpForwarding
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Specifies whether TCP forwarding is permitted. The default is
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``yes''. Note that disabling TCP forwarding does not improve se-
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curity unless users are also denied shell access, as they can al-
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ways install their own forwarders.
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AllowUsers
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This keyword can be followed by a list of user name patterns,
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separated by spaces. If specified, login is allowed only for us-
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er names that match one of the patterns. Only user names are
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valid; a numerical user ID is not recognized. By default, login
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is allowed for all users. If the pattern takes the form US-
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ER@HOST then USER and HOST are separately checked, restricting
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logins to particular users from particular hosts. The allow/deny
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directives are processed in the following order: DenyUsers,
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AllowUsers, DenyGroups, and finally AllowGroups.
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See PATTERNS in ssh_config(5) for more information on patterns.
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AuthorizedKeysFile
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Specifies the file that contains the public keys that can be used
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for user authentication. AuthorizedKeysFile may contain tokens
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of the form %T which are substituted during connection setup.
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The following tokens are defined: %% is replaced by a literal
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'%', %h is replaced by the home directory of the user being au-
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thenticated, and %u is replaced by the username of that user.
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After expansion, AuthorizedKeysFile is taken to be an absolute
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path or one relative to the user's home directory. The default
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is ``.ssh/authorized_keys''.
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2008-07-23 09:28:49 +00:00
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Banner The contents of the specified file are sent to the remote user
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before authentication is allowed. If the argument is ``none''
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then no banner is displayed. This option is only available for
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2008-07-23 09:15:38 +00:00
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protocol version 2. By default, no banner is displayed.
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ChallengeResponseAuthentication
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Specifies whether challenge-response authentication is allowed.
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All authentication styles from login.conf(5) are supported. The
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default is ``yes''.
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2008-07-23 09:28:49 +00:00
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ChrootDirectory
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Specifies a path to chroot(2) to after authentication. This
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path, and all its components, must be root-owned directories that
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are not writable by any other user or group.
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The path may contain the following tokens that are expanded at
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runtime once the connecting user has been authenticated: %% is
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replaced by a literal '%', %h is replaced by the home directory
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of the user being authenticated, and %u is replaced by the user-
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name of that user.
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The ChrootDirectory must contain the necessary files and directo-
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ries to support the users' session. For an interactive session
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this requires at least a shell, typically sh(1), and basic /dev
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nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4),
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arandom(4) and tty(4) devices. For file transfer sessions using
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``sftp'', no additional configuration of the environment is nec-
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essary if the in-process sftp server is used (see Subsystem for
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details).
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The default is not to chroot(2).
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2008-07-23 09:15:38 +00:00
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Ciphers
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Specifies the ciphers allowed for protocol version 2. Multiple
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ciphers must be comma-separated. The supported ciphers are
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``3des-cbc'', ``aes128-cbc'', ``aes192-cbc'', ``aes256-cbc'',
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``aes128-ctr'', ``aes192-ctr'', ``aes256-ctr'', ``arcfour128'',
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``arcfour256'', ``arcfour'', ``blowfish-cbc'', and
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``cast128-cbc''. The default is:
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2009-02-24 18:49:27 +00:00
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aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
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aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
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aes256-cbc,arcfour
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2008-07-23 09:15:38 +00:00
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ClientAliveCountMax
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Sets the number of client alive messages (see below) which may be
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sent without sshd(8) receiving any messages back from the client.
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If this threshold is reached while client alive messages are be-
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ing sent, sshd will disconnect the client, terminating the ses-
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sion. It is important to note that the use of client alive mes-
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sages is very different from TCPKeepAlive (below). The client
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alive messages are sent through the encrypted channel and there-
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fore will not be spoofable. The TCP keepalive option enabled by
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TCPKeepAlive is spoofable. The client alive mechanism is valu-
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able when the client or server depend on knowing when a connec-
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tion has become inactive.
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The default value is 3. If ClientAliveInterval (see below) is
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set to 15, and ClientAliveCountMax is left at the default, unre-
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sponsive SSH clients will be disconnected after approximately 45
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seconds. This option applies to protocol version 2 only.
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ClientAliveInterval
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Sets a timeout interval in seconds after which if no data has
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been received from the client, sshd(8) will send a message
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through the encrypted channel to request a response from the
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client. The default is 0, indicating that these messages will
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not be sent to the client. This option applies to protocol ver-
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sion 2 only.
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Compression
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Specifies whether compression is allowed, or delayed until the
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user has authenticated successfully. The argument must be
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``yes'', ``delayed'', or ``no''. The default is ``delayed''.
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DenyGroups
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This keyword can be followed by a list of group name patterns,
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separated by spaces. Login is disallowed for users whose primary
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group or supplementary group list matches one of the patterns.
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Only group names are valid; a numerical group ID is not recog-
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nized. By default, login is allowed for all groups. The al-
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low/deny directives are processed in the following order:
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DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
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See PATTERNS in ssh_config(5) for more information on patterns.
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DenyUsers
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This keyword can be followed by a list of user name patterns,
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separated by spaces. Login is disallowed for user names that
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match one of the patterns. Only user names are valid; a numeri-
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cal user ID is not recognized. By default, login is allowed for
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all users. If the pattern takes the form USER@HOST then USER and
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HOST are separately checked, restricting logins to particular
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users from particular hosts. The allow/deny directives are pro-
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cessed in the following order: DenyUsers, AllowUsers, DenyGroups,
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and finally AllowGroups.
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See PATTERNS in ssh_config(5) for more information on patterns.
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ForceCommand
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Forces the execution of the command specified by ForceCommand,
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2008-07-23 09:28:49 +00:00
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ignoring any command supplied by the client and ~/.ssh/rc if pre-
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sent. The command is invoked by using the user's login shell
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with the -c option. This applies to shell, command, or subsystem
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execution. It is most useful inside a Match block. The command
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originally supplied by the client is available in the
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SSH_ORIGINAL_COMMAND environment variable. Specifying a command
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of ``internal-sftp'' will force the use of an in-process sftp
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server that requires no support files when used with
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ChrootDirectory.
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2008-07-23 09:15:38 +00:00
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GatewayPorts
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Specifies whether remote hosts are allowed to connect to ports
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forwarded for the client. By default, sshd(8) binds remote port
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forwardings to the loopback address. This prevents other remote
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hosts from connecting to forwarded ports. GatewayPorts can be
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used to specify that sshd should allow remote port forwardings to
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bind to non-loopback addresses, thus allowing other hosts to con-
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nect. The argument may be ``no'' to force remote port forward-
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ings to be available to the local host only, ``yes'' to force re-
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mote port forwardings to bind to the wildcard address, or
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``clientspecified'' to allow the client to select the address to
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which the forwarding is bound. The default is ``no''.
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GSSAPIAuthentication
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Specifies whether user authentication based on GSSAPI is allowed.
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The default is ``no''. Note that this option applies to protocol
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version 2 only.
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GSSAPICleanupCredentials
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Specifies whether to automatically destroy the user's credentials
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cache on logout. The default is ``yes''. Note that this option
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applies to protocol version 2 only.
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HostbasedAuthentication
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Specifies whether rhosts or /etc/hosts.equiv authentication to-
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gether with successful public key client host authentication is
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allowed (host-based authentication). This option is similar to
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RhostsRSAAuthentication and applies to protocol version 2 only.
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The default is ``no''.
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HostbasedUsesNameFromPacketOnly
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Specifies whether or not the server will attempt to perform a re-
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verse name lookup when matching the name in the ~/.shosts,
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~/.rhosts, and /etc/hosts.equiv files during
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HostbasedAuthentication. A setting of ``yes'' means that sshd(8)
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uses the name supplied by the client rather than attempting to
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resolve the name from the TCP connection itself. The default is
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``no''.
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HostKey
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Specifies a file containing a private host key used by SSH. The
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default is /etc/ssh/ssh_host_key for protocol version 1, and
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/etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for pro-
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tocol version 2. Note that sshd(8) will refuse to use a file if
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it is group/world-accessible. It is possible to have multiple
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host key files. ``rsa1'' keys are used for version 1 and ``dsa''
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or ``rsa'' are used for version 2 of the SSH protocol.
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IgnoreRhosts
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Specifies that .rhosts and .shosts files will not be used in
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RhostsRSAAuthentication or HostbasedAuthentication.
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/etc/hosts.equiv and /etc/shosts.equiv are still used. The de-
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fault is ``yes''.
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IgnoreUserKnownHosts
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Specifies whether sshd(8) should ignore the user's
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~/.ssh/known_hosts during RhostsRSAAuthentication or
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HostbasedAuthentication. The default is ``no''.
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KerberosAuthentication
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Specifies whether the password provided by the user for
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PasswordAuthentication will be validated through the Kerberos
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KDC. To use this option, the server needs a Kerberos servtab
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which allows the verification of the KDC's identity. The default
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is ``no''.
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KerberosGetAFSToken
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If AFS is active and the user has a Kerberos 5 TGT, attempt to
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acquire an AFS token before accessing the user's home directory.
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The default is ``no''.
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KerberosOrLocalPasswd
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If password authentication through Kerberos fails then the pass-
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word will be validated via any additional local mechanism such as
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/etc/passwd. The default is ``yes''.
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KerberosTicketCleanup
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Specifies whether to automatically destroy the user's ticket
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cache file on logout. The default is ``yes''.
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KeyRegenerationInterval
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In protocol version 1, the ephemeral server key is automatically
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regenerated after this many seconds (if it has been used). The
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purpose of regeneration is to prevent decrypting captured ses-
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sions by later breaking into the machine and stealing the keys.
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The key is never stored anywhere. If the value is 0, the key is
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never regenerated. The default is 3600 (seconds).
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ListenAddress
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Specifies the local addresses sshd(8) should listen on. The fol-
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lowing forms may be used:
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ListenAddress host|IPv4_addr|IPv6_addr
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ListenAddress host|IPv4_addr:port
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ListenAddress [host|IPv6_addr]:port
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If port is not specified, sshd will listen on the address and all
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prior Port options specified. The default is to listen on all
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local addresses. Multiple ListenAddress options are permitted.
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Additionally, any Port options must precede this option for non-
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port qualified addresses.
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LoginGraceTime
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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-separat-
|
2008-07-23 09:23:42 +00:00
|
|
|
ed. The default is:
|
|
|
|
|
|
|
|
|
|
hmac-md5,hmac-sha1,umac-64@openssh.com,
|
|
|
|
|
hmac-ripemd160,hmac-sha1-96,hmac-md5-96
|
2008-07-23 09:15:38 +00:00
|
|
|
|
|
|
|
|
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, un-
|
2008-07-23 09:33:08 +00:00
|
|
|
til 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 ad-
|
|
|
|
|
dresses 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
|
2009-02-24 18:49:27 +00:00
|
|
|
Match keyword. Available keywords are AllowAgentForwarding,
|
|
|
|
|
AllowTcpForwarding, Banner, ChrootDirectory, ForceCommand,
|
|
|
|
|
GatewayPorts, GSSAPIAuthentication, HostbasedAuthentication,
|
2008-07-23 09:15:38 +00:00
|
|
|
KbdInteractiveAuthentication, KerberosAuthentication,
|
2009-02-24 18:49:27 +00:00
|
|
|
MaxAuthTries, MaxSessions, PasswordAuthentication,
|
|
|
|
|
PermitEmptyPasswords, PermitOpen, PermitRootLogin,
|
|
|
|
|
RhostsRSAAuthentication, RSAAuthentication, X11DisplayOffset,
|
|
|
|
|
X11Forwarding and X11UseLocalHost.
|
2008-07-23 09:15:38 +00:00
|
|
|
|
|
|
|
|
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.
|
|
|
|
|
|
2008-07-23 09:33:08 +00:00
|
|
|
MaxSessions
|
|
|
|
|
Specifies the maximum number of open sessions permitted per net-
|
|
|
|
|
work connection. The default is 10.
|
|
|
|
|
|
2008-07-23 09:15:38 +00:00
|
|
|
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 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(8) will refuse connection attempts with a
|
|
|
|
|
probability of ``rate/100'' (30%) if there are currently
|
|
|
|
|
``start'' (10) unauthenticated connections. The probability in-
|
|
|
|
|
creases linearly 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''.
|
|
|
|
|
|
|
|
|
|
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
|
|
|
|
|
whitespace. An argument of ``any'' can be used to remove all re-
|
|
|
|
|
strictions and permit any forwarding requests. By default all
|
|
|
|
|
port forwarding requests are permitted.
|
|
|
|
|
|
|
|
|
|
PermitRootLogin
|
|
|
|
|
Specifies whether root can log in using ssh(1). The argument
|
|
|
|
|
must be ``yes'', ``without-password'', ``forced-commands-only'',
|
|
|
|
|
or ``no''. The default is ``yes''.
|
|
|
|
|
|
|
|
|
|
If this option is set to ``without-password'', password authenti-
|
|
|
|
|
cation is disabled for root.
|
|
|
|
|
|
|
|
|
|
If this option is set to ``forced-commands-only'', root login
|
|
|
|
|
with public key authentication will be allowed, but only if the
|
|
|
|
|
command option has been specified (which may be useful for taking
|
|
|
|
|
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 by-
|
|
|
|
|
pass 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-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.
|
|
|
|
|
|
|
|
|
|
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 on-
|
|
|
|
|
ly.
|
|
|
|
|
|
|
|
|
|
ServerKeyBits
|
|
|
|
|
Defines the number of bits in the ephemeral protocol version 1
|
2008-07-23 09:33:08 +00:00
|
|
|
server key. The minimum value is 512, and the default is 1024.
|
2008-07-23 09:15:38 +00:00
|
|
|
|
|
|
|
|
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''.
|
|
|
|
|
|
|
|
|
|
Subsystem
|
|
|
|
|
Configures an external subsystem (e.g. file transfer daemon).
|
|
|
|
|
Arguments should be a subsystem name and a command (with optional
|
2008-07-23 09:28:49 +00:00
|
|
|
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.
|
|
|
|
|
|
2008-07-23 09:15:38 +00:00
|
|
|
By default no subsystems are defined. Note that this option ap-
|
|
|
|
|
plies 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 de-
|
|
|
|
|
fault 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 re-
|
|
|
|
|
sources.
|
|
|
|
|
|
|
|
|
|
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(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 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.
|
|
|
|
|
|
|
|
|
|
UsePAM Enables the Pluggable Authentication Module interface. If set to
|
|
|
|
|
``yes'' this will enable PAM authentication using
|
|
|
|
|
ChallengeResponseAuthentication and PasswordAuthentication in ad-
|
|
|
|
|
dition to PAM account and session module processing for all au-
|
|
|
|
|
thentication 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(8) 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''.
|
|
|
|
|
|
|
|
|
|
X11DisplayOffset
|
|
|
|
|
Specifies the first display number available for sshd(8)'s X11
|
|
|
|
|
forwarding. This prevents sshd from interfering with real X11
|
|
|
|
|
servers. The default is 10.
|
|
|
|
|
|
|
|
|
|
X11Forwarding
|
|
|
|
|
Specifies whether X11 forwarding is permitted. The argument must
|
|
|
|
|
be ``yes'' or ``no''. The default is ``no''.
|
|
|
|
|
|
|
|
|
|
When X11 forwarding is enabled, there may be additional exposure
|
|
|
|
|
to the server and to client displays if the sshd(8) proxy display
|
|
|
|
|
is configured to listen on the wildcard address (see
|
|
|
|
|
X11UseLocalhost below), though this is not the default. Addi-
|
|
|
|
|
tionally, the authentication spoofing and authentication data
|
|
|
|
|
verification and substitution occur on the client side. The se-
|
|
|
|
|
curity risk of using X11 forwarding is that the client's X11 dis-
|
|
|
|
|
play 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 pro-
|
|
|
|
|
tect 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(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 de-
|
|
|
|
|
fault is ``yes''.
|
|
|
|
|
|
|
|
|
|
XAuthLocation
|
|
|
|
|
Specifies the full pathname of the xauth(1) program. The default
|
|
|
|
|
is /usr/X11R6/bin/xauth.
|
|
|
|
|
|
|
|
|
|
TIME FORMATS
|
|
|
|
|
sshd(8) command-line arguments and configuration file options that speci-
|
|
|
|
|
fy 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.
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Time format examples:
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600 600 seconds (10 minutes)
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10m 10 minutes
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1h30m 1 hour 30 minutes (90 minutes)
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FILES
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/etc/ssh/sshd_config
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Contains configuration data for sshd(8). This file should be
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writable by root only, but it is recommended (though not neces-
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sary) that it be world-readable.
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SEE ALSO
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sshd(8)
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AUTHORS
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OpenSSH is a derivative of the original and free ssh 1.2.12 release by
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Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
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de Raadt and Dug Song removed many bugs, re-added newer features and cre-
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ated OpenSSH. Markus Friedl contributed the support for SSH protocol
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versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
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for privilege separation.
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2009-02-24 18:49:27 +00:00
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OpenBSD 4.5 February 22, 2009 10
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