path: root/ssh_config.0

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-
             col version 1, and $HOME/.ssh/id_rsa and $HOME/.ssh/id_dsa for
             protocol version 2.  Additionally, any identities represented by
             the authentication agent will be used for authentication.  The
             file name may use the tilde syntax to refer to a user's home di-
             rectory.  It is possible to have multiple identity files speci-
             fied in configuration files; all these identities will be tried
             in sequence.

     IdentitiesOnly
             Specifies that ssh should only use the authentication identity
             files configured in the ssh_config files, even if the ssh-agent
             offers more identities.  The argument to this keyword must be
             ``yes'' or ``no''.  This option is intented for situations where
             ssh-agent offers many different identities.  The default is
             ``no''.

     KbdInteractiveDevices
             Specifies the list of methods to use in keyboard-interactive au-
             thentication.  Multiple method names must be comma-separated.
             The default is to use the server specified list.

     LocalForward
             Specifies that a TCP/IP port on the local machine be forwarded
             over the secure channel to the specified host and port from the
             remote machine.  The first argument must be [bind_address:]port
             and the second argument must be host:hostport.  IPv6 addresses
             can be specified by enclosing addresses in square brackets or by
             using an alternative syntax: [bind_address/]port and
             host/hostport.  Multiple forwardings may be specified, and addi-
             tional forwardings can be given on the command line.  Only the
             superuser can forward privileged ports.  By default, the local
             port is bound in accordance with the GatewayPorts setting.  How-
             ever, an explicit bind_address may be used to bind the connection
             to a specific address.  The bind_address of ``localhost'' indi-
             cates that the listening port be bound for local use only, while
             an empty address or `*' indicates that the port should be avail-
             able from all interfaces.

     LogLevel
             Gives the verbosity level that is used when logging messages from
             ssh.  The possible values are: QUIET, FATAL, ERROR, INFO, VER-
             BOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.  The default is INFO.
             DEBUG and DEBUG1 are equivalent.  DEBUG2 and DEBUG3 each specify
             higher levels of verbose output.

     MACs    Specifies the MAC (message authentication code) algorithms in or-
             der of preference.  The MAC algorithm is used in protocol version
             2 for data integrity protection.  Multiple algorithms must be
             comma-separated.  The default is ``hmac-md5,hmac-sha1,hmac-
             ripemd160,hmac-sha1-96,hmac-md5-96''.

     NoHostAuthenticationForLocalhost
             This option can be used if the home directory is shared across
             machines.  In this case localhost will refer to a different ma-
             chine on each of the machines and the user will get many warnings
             about changed host keys.  However, this option disables host au-
             thentication for localhost.  The argument to this keyword must be
             ``yes'' or ``no''.  The default is to check the host key for lo-
             calhost.

     NumberOfPasswordPrompts
             Specifies the number of password prompts before giving up.  The
             argument to this keyword must be an integer.  Default is 3.

     PasswordAuthentication
             Specifies whether to use password authentication.  The argument
             to this keyword must be ``yes'' or ``no''.  The default is
             ``yes''.

     Port    Specifies the port number to connect on the remote host.  Default
             is 22.

     PreferredAuthentications
             Specifies the order in which the client should try protocol 2 au-
             thentication methods.  This allows a client to prefer one method
             (e.g. keyboard-interactive) over another method (e.g. password)
             The default for this option is: ``hostbased,publickey,keyboard-
             interactive,password''.

     Protocol
             Specifies the protocol versions ssh should support in order of
             preference.  The possible values are ``1'' and ``2''.  Multiple
             versions must be comma-separated.  The default is ``2,1''.  This
             means that ssh tries version 2 and falls back to version 1 if
             version 2 is not available.

     ProxyCommand
             Specifies the command to use to connect to the server.  The com-
             mand string extends to the end of the line, and is executed with
             /bin/sh.  In the command string, `%h' will be substituted by the
             host name to connect and `%p' by the port.  The command can be
             basically anything, and should read from its standard input and
             write to its standard output.  It should eventually connect an
             sshd(8) server running on some machine, or execute sshd -i some-
             where.  Host key management will be done using the HostName of
             the host being connected (defaulting to the name typed by the us-
             er).  Setting the command to ``none'' disables this option en-
             tirely.  Note that CheckHostIP is not available for connects with
             a proxy command.

     PubkeyAuthentication
             Specifies whether to try public key authentication.  The argument
             to this keyword must be ``yes'' or ``no''.  The default is
             ``yes''.  This option applies to protocol version 2 only.

     RemoteForward
             Specifies that a TCP/IP port on the remote machine be forwarded
             over the secure channel to the specified host and port from the
             local machine.  The first argument must be [bind_address:]port
             and the second argument must be host:hostport.  IPv6 addresses
             can be specified by enclosing addresses in square brackets or by
             using an alternative syntax: [bind_address/]port and
             host/hostport.  Multiple forwardings may be specified, and addi-
             tional forwardings can be given on the command line.  Only the
             superuser can forward privileged ports.

             If the bind_address is not specified, the default is to only bind
             to loopback addresses.  If the bind_address is `*' or an empty
             string, then the forwarding is requested to listen on all inter-
             faces.  Specifying a remote bind_address will only succeed if the
             server's GatewayPorts option is enabled (see sshd_config(5)).

     RhostsRSAAuthentication
             Specifies whether to try rhosts based authentication with RSA
             host authentication.  The argument must be ``yes'' or ``no''.
             The default is ``no''.  This option applies to protocol version 1
             only and requires ssh to be setuid root.

     RSAAuthentication
             Specifies whether to try RSA authentication.  The argument to
             this keyword must be ``yes'' or ``no''.  RSA authentication will
             only be attempted if the identity file exists, or an authentica-
             tion agent is running.  The default is ``yes''.  Note that this
             option applies to protocol version 1 only.

     SendEnv
             Specifies what variables from the local environ(7) should be sent
             to the server.  Note that environment passing is only supported
             for protocol 2, the server must also support it, and the server
             must be configured to accept these environment variables.  Refer
             to AcceptEnv in sshd_config(5) for how to configure the server.
             Variables are specified by name, which may contain the wildcard
             characters `*' and `?'.  Multiple environment variables may be
             separated by whitespace or spread across multiple SendEnv direc-
             tives.  The default is not to send any environment variables.

     ServerAliveInterval
             Sets a timeout interval in seconds after which if no data has
             been received from the server, ssh will send a message through
             the encrypted channel to request a response from the server.  The
             default is 0, indicating that these messages will not be sent to
             the server.  This option applies to protocol version 2 only.

     ServerAliveCountMax
             Sets the number of server alive messages (see above) which may be
             sent without ssh receiving any messages back from the server.  If
             this threshold is reached while server alive messages are being
             sent, ssh will disconnect from the server, terminating the ses-
             sion.  It is important to note that the use of server alive mes-
             sages is very different from TCPKeepAlive (below).  The server
             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 server 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, for example, ServerAliveInterval
             (above) is set to 15, and ServerAliveCountMax is left at the de-
             fault, if the server becomes unresponsive ssh will disconnect af-
             ter approximately 45 seconds.

     SmartcardDevice
             Specifies which smartcard device to use.  The argument to this
             keyword is the device ssh should use to communicate with a smart-
             card used for storing the user's private RSA key.  By default, no
             device is specified and smartcard support is not activated.

     StrictHostKeyChecking
             If this flag is set to ``yes'', ssh will never automatically add
             host keys to the $HOME/.ssh/known_hosts file, and refuses to con-
             nect to hosts whose host key has changed.  This provides maximum
             protection against trojan horse attacks, however, can be annoying
             when the /etc/ssh/ssh_known_hosts file is poorly maintained, or
             connections to new hosts are frequently made.  This option forces
             the user to manually add all new hosts.  If this flag is set to
             ``no'', ssh will automatically add new host keys to the user
             known hosts files.  If this flag is set to ``ask'', new host keys
             will be added to the user known host files only after the user
             has confirmed that is what they really want to do, and ssh will
             refuse to connect to hosts whose host key has changed.  The host
             keys of known hosts will be verified automatically in all cases.
             The argument must be ``yes'', ``no'' or ``ask''.  The default is
             ``ask''.

     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.

             The default is ``yes'' (to send TCP keepalive messages), and the
             client will notice if the network goes down or the remote host
             dies.  This is important in scripts, and many users want it too.

             To disable TCP keepalive messages, the value should be set to
             ``no''.

     UsePrivilegedPort
             Specifies whether to use a privileged port for outgoing connec-
             tions.  The argument must be ``yes'' or ``no''.  The default is
             ``no''.  If set to ``yes'' ssh must be setuid root.  Note that
             this option must be set to ``yes'' for RhostsRSAAuthentication
             with older servers.

     User    Specifies the user to log in as.  This can be useful when a dif-
             ferent user name is used on different machines.  This saves the
             trouble of having to remember to give the user name on the com-
             mand line.

     UserKnownHostsFile
             Specifies a file to use for the user host key database instead of
             $HOME/.ssh/known_hosts.

     VerifyHostKeyDNS
             Specifies whether to verify the remote key using DNS and SSHFP
             resource records.  If this option is set to ``yes'', the client
             will implicitly trust keys that match a secure fingerprint from
             DNS.  Insecure fingerprints will be handled as if this option was
             set to ``ask''.  If this option is set to ``ask'', information on
             fingerprint match will be displayed, but the user will still need
             to confirm new host keys according to the StrictHostKeyChecking
             option.  The argument must be ``yes'', ``no'' or ``ask''.  The
             default is ``no''.  Note that this option applies to protocol
             version 2 only.

     XAuthLocation
             Specifies the full pathname of the xauth(1) program.  The default
             is /usr/X11R6/bin/xauth.

FILES
     $HOME/.ssh/config
             This is the per-user configuration file.  The format of this file
             is described above.  This file is used by the ssh client.  Be-
             cause of the potential for abuse, this file must have strict per-
             missions: read/write for the user, and not accessible by others.

     /etc/ssh/ssh_config
             Systemwide configuration file.  This file provides defaults for
             those values that are not specified in the user's configuration
             file, and for those users who do not have a configuration file.
             This file must be world-readable.

SEE ALSO
     ssh(1)

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.

OpenBSD 3.7                   September 25, 1999                             8