ssh_config.0

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SSH_CONFIG(5)             OpenBSD Programmer's Manual            SSH_CONFIG(5)

NAME
     ssh_config - OpenSSH SSH client configuration files

SYNOPSIS
     $HOME/.ssh/config
     /etc/ssh/ssh_config

DESCRIPTION
     ssh obtains configuration data from the following sources in the follow-
     ing order:
           1.   command-line options
           2.   user's configuration file ($HOME/.ssh/config)
           3.   system-wide configuration file (/etc/ssh/ssh_config)

     For each parameter, the first obtained value will be used.  The configu-
     ration files contain sections separated by ``Host'' specifications, and
     that section is only applied for hosts that match one of the patterns
     given in the specification.  The matched host name is the one given on
     the command line.

     Since the first obtained value for each parameter is used, more host-spe-
     cific declarations should be given near the beginning of the file, and
     general defaults at the end.

     The configuration file has the following format:

     Empty lines and lines starting with `#' are comments.

     Otherwise a line is of the format ``keyword arguments''.  Configuration
     options may be separated by whitespace or optional whitespace and exactly
     one `='; the latter format is useful to avoid the need to quote whites-
     pace when specifying configuration options using the ssh, scp and sftp -o
     option.

     The possible keywords and their meanings are as follows (note that key-
     words are case-insensitive and arguments are case-sensitive):

     Host    Restricts the following declarations (up to the next Host key-
             word) to be only for those hosts that match one of the patterns
             given after the keyword.  `*' and `?' can be used as wildcards in
             the patterns.  A single `*' as a pattern can be used to provide
             global defaults for all hosts.  The host is the hostname argument
             given on the command line (i.e., the name is not converted to a
             canonicalized host name before matching).

     AddressFamily
             Specifies which address family to use when connecting.  Valid ar-
             guments are ``any'', ``inet'' (use IPv4 only) or ``inet6'' (use
             IPv6 only).

     BatchMode
             If set to ``yes'', passphrase/password querying will be disabled.
             This option is useful in scripts and other batch jobs where no
             user is present to supply the password.  The argument must be
             ``yes'' or ``no''.  The default is ``no''.

     BindAddress
             Specify the interface to transmit from on machines with multiple
             interfaces or aliased addresses.  Note that this option does not
             work if UsePrivilegedPort is set to ``yes''.

     ChallengeResponseAuthentication
             Specifies whether to use challenge response authentication.  The
             argument to this keyword must be ``yes'' or ``no''.  The default
             is ``yes''.

     CheckHostIP
             If this flag is set to ``yes'', ssh will additionally check the
             host IP address in the known_hosts file.  This allows ssh to de-
             tect if a host key changed due to DNS spoofing.  If the option is
             set to ``no'', the check will not be executed.  The default is
             ``yes''.

     Cipher  Specifies the cipher to use for encrypting the session in proto-
             col version 1.  Currently, ``blowfish'', ``3des'', and ``des''
             are supported.  des is only supported in the ssh client for in-
             teroperability with legacy protocol 1 implementations that do not
             support the 3des cipher.  Its use is strongly discouraged due to
             cryptographic weaknesses.  The default is ``3des''.

     Ciphers
             Specifies the ciphers allowed for protocol version 2 in order of
             preference.  Multiple ciphers must be comma-separated.  The sup-
             ported ciphers are ``3des-cbc'', ``aes128-cbc'', ``aes192-cbc'',
             ``aes256-cbc'', ``aes128-ctr'', ``aes192-ctr'', ``aes256-ctr'',
             ``arcfour'', ``blowfish-cbc'', and ``cast128-cbc''.  The default
             is

               ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
                 aes192-cbc,aes256-cbc''

     ClearAllForwardings
             Specifies that all local, remote and dynamic port forwardings
             specified in the configuration files or on the command line be
             cleared.  This option is primarily useful when used from the ssh
             command line to clear port forwardings set in configuration
             files, and is automatically set by scp(1) and sftp(1).  The argu-
             ment must be ``yes'' or ``no''.  The default is ``no''.

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

     CompressionLevel
             Specifies the compression level to use if compression is enabled.
             The argument must be an integer from 1 (fast) to 9 (slow, best).
             The default level is 6, which is good for most applications.  The
             meaning of the values is the same as in gzip(1).  Note that this
             option applies to protocol version 1 only.

     ConnectionAttempts
             Specifies the number of tries (one per second) to make before ex-
             iting.  The argument must be an integer.  This may be useful in
             scripts if the connection sometimes fails.  The default is 1.

     ConnectTimeout
             Specifies the timeout (in seconds) used when connecting to the
             ssh server, instead of using the default system TCP timeout.
             This value is used only when the target is down or really un-
             reachable, not when it refuses the connection.

     ControlMaster
             Enables the sharing of multiple sessions over a single network
             connection.  When set to ``yes'' ssh will listen for connections
             on a control socket specified using the ControlPath argument.
             Additional sessions can connect to this socket using the same
             ControlPath with ControlMaster set to ``no'' (the default).
             These sessions will reuse the master instance's network connec-
             tion rather than initiating new ones.  Setting this to ``ask''
             will cause ssh to listen for control connections, but require
             confirmation using the SSH_ASKPASS program before they are ac-
             cepted (see ssh-add(1) for details).

     ControlPath
             Specify the path to the control socket used for connection shar-
             ing.  See ControlMaster above.

     DynamicForward
             Specifies that a TCP/IP port on the local machine be forwarded
             over the secure channel, and the application protocol is then
             used to determine where to connect to from the remote machine.
             The argument must be a port number.  Currently the SOCKS4 and
             SOCKS5 protocols are supported, and ssh will act as a SOCKS serv-
             er.  Multiple forwardings may be specified, and additional for-
             wardings can be given on the command line.  Only the superuser
             can forward privileged ports.

     EnableSSHKeysign
             Setting this option to ``yes'' in the global client configuration
             file /etc/ssh/ssh_config enables the use of the helper program
             ssh-keysign(8) during HostbasedAuthentication.  The argument must
             be ``yes'' or ``no''.  The default is ``no''.  This option should
             be placed in the non-hostspecific section.  See ssh-keysign(8)
             for more information.

     EscapeChar
             Sets the escape character (default: `~').  The escape character
             can also be set on the command line.  The argument should be a
             single character, `^' followed by a letter, or ``none'' to dis-
             able the escape character entirely (making the connection trans-
             parent for binary data).

     ForwardAgent
             Specifies whether the connection to the authentication agent (if
             any) will be forwarded to the remote machine.  The argument must
             be ``yes'' or ``no''.  The default is ``no''.

             Agent forwarding should be enabled with caution.  Users with the
             ability to bypass file permissions on the remote host (for the
             agent's Unix-domain socket) can access the local agent through
             the forwarded connection.  An attacker cannot obtain key material
             from the agent, however they can perform operations on the keys
             that enable them to authenticate using the identities loaded into
             the agent.

     ForwardX11
             Specifies whether X11 connections will be automatically redirect-
             ed over the secure channel and DISPLAY set.  The argument must be
             ``yes'' or ``no''.  The default is ``no''.

             X11 forwarding should be enabled with caution.  Users with the
             ability to bypass file permissions on the remote host (for the
             user's X11 authorization database) can access the local X11 dis-
             play through the forwarded connection.  An attacker may then be
             able to perform activities such as keystroke monitoring if the
             ForwardX11Trusted option is also enabled.

     ForwardX11Trusted
             If this option is set to ``yes'' then remote X11 clients will
             have full access to the original X11 display.

             If this option is set to ``no'' then remote X11 clients will be
             considered untrusted and prevented from stealing or tampering
             with data belonging to trusted X11 clients.  Furthermore, the
             xauth(1) token used for the session will be set to expire after
             20 minutes.  Remote clients will be refused access after this
             time.

             The default is ``no''.

             See the X11 SECURITY extension specification for full details on
             the restrictions imposed on untrusted clients.

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

     GlobalKnownHostsFile
             Specifies a file to use for the global host key database instead
             of /etc/ssh/ssh_known_hosts.

     GSSAPIAuthentication
             Specifies whether user authentication based on GSSAPI is allowed.
             The default is ``no''.  Note that this option applies to protocol
             version 2 only.

     GSSAPIDelegateCredentials
             Forward (delegate) credentials to the server.  The default is
             ``no''.  Note that this option applies to protocol version 2 on-
             ly.

     HashKnownHosts
             Indicates that ssh should hash host names and addresses when they
             are added to $HOME/.ssh/known_hosts.  These hashed names may be
             used normally by ssh and sshd, but they do not reveal identifying
             information should the file's contents be disclosed.  The default
             is ``no''.  Note that hashing of names and addresses will not be
             retrospectively applied to existing known hosts files, but these
             may be manually hashed using ssh-keygen(1).

     HostbasedAuthentication
             Specifies whether to try rhosts based authentication with public
             key authentication.  The argument must be ``yes'' or ``no''.  The
             default is ``no''.  This option applies to protocol version 2 on-
             ly and is similar to RhostsRSAAuthentication.

     HostKeyAlgorithms
             Specifies the protocol version 2 host key algorithms that the
             client wants to use in order of preference.  The default for this
             option is: ``ssh-rsa,ssh-dss''.

     HostKeyAlias
             Specifies an alias that should be used instead of the real host
             name when looking up or saving the host key in the host key
             database files.  This option is useful for tunneling ssh connec-
             tions or for multiple servers running on a single host.

     HostName
             Specifies the real host name to log into.  This can be used to
             specify nicknames or abbreviations for hosts.  Default is the
             name given on the command line.  Numeric IP addresses are also
             permitted (both on the command line and in HostName specifica-
             tions).

     IdentityFile
             Specifies a file from which the user's RSA or DSA authentication
             identity is read.  The default is $HOME/.ssh/identity for proto-