path: root/ssh_config.0

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

NAME
     ssh_config - OpenSSH SSH client configuration files

SYNOPSIS
     ~/.ssh/config
     /etc/ssh/ssh_config

DESCRIPTION
     ssh(1) obtains configuration data from the following sources in the
     following order:

           1.   command-line options
           2.   user's configuration file (~/.ssh/config)
           3.   system-wide configuration file (/etc/ssh/ssh_config)

     For each parameter, the first obtained value will be used.  The
     configuration 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-
     specific 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 whitespace when
     specifying configuration options using the ssh, scp, and sftp -o option.
     Arguments may optionally be enclosed in double quotes (") in order to
     represent arguments containing spaces.

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

     Host    Restricts the following declarations (up to the next Host
             keyword) to be only for those hosts that match one of the
             patterns given after the keyword.  If more than one pattern is
             provided, they should be separated by whitespace.  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).

             A pattern entry may be negated by prefixing it with an
             exclamation mark (`!').  If a negated entry is matched, then the
             Host entry is ignored, regardless of whether any other patterns
             on the line match.  Negated matches are therefore useful to
             provide exceptions for wildcard matches.

             See PATTERNS for more information on patterns.

     AddressFamily
             Specifies which address family to use when connecting.  Valid
             arguments 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
             Use the specified address on the local machine as the source
             address of the connection.  Only useful on systems with more than
             one address.  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(1) will additionally check
             the host IP address in the known_hosts file.  This allows ssh to
             detect 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
             protocol version 1.  Currently, ``blowfish'', ``3des'', and
             ``des'' are supported.  des is only supported in the ssh(1)
             client for interoperability 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
             supported ciphers are ``3des-cbc'', ``aes128-cbc'',
             ``aes192-cbc'', ``aes256-cbc'', ``aes128-ctr'', ``aes192-ctr'',
             ``aes256-ctr'', ``arcfour128'', ``arcfour256'', ``arcfour'',
             ``blowfish-cbc'', and ``cast128-cbc''.  The default is:

                aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
                aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
                aes256-cbc,arcfour

     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(1) command line to clear port forwardings set in
             configuration files, and is automatically set by scp(1) and
             sftp(1).  The argument 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
             exiting.  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
             unreachable, not when it refuses the connection.

     ControlMaster
             Enables the sharing of multiple sessions over a single network
             connection.  When set to ``yes'', ssh(1) 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 try to reuse the master instance's
             network connection rather than initiating new ones, but will fall
             back to connecting normally if the control socket does not exist,
             or is not listening.

             Setting this to ``ask'' will cause ssh to listen for control
             connections, but require confirmation using the SSH_ASKPASS
             program before they are accepted (see ssh-add(1) for details).
             If the ControlPath cannot be opened, ssh will continue without
             connecting to a master instance.

             X11 and ssh-agent(1) forwarding is supported over these
             multiplexed connections, however the display and agent forwarded
             will be the one belonging to the master connection i.e. it is not
             possible to forward multiple displays or agents.

             Two additional options allow for opportunistic multiplexing: try
             to use a master connection but fall back to creating a new one if
             one does not already exist.  These options are: ``auto'' and
             ``autoask''.  The latter requires confirmation like the ``ask''
             option.

     ControlPath
             Specify the path to the control socket used for connection
             sharing as described in the ControlMaster section above or the
             string ``none'' to disable connection sharing.  In the path, `%L'
             will be substituted by the first component of the local host
             name, `%l' will be substituted by the local host name (including
             any domain name), `%h' will be substituted by the target host
             name, `%n' will be substituted by the original target host name
             specified on the command line, `%p' the port, `%r' by the remote
             login username, and `%u' by the username of the user running
             ssh(1).  It is recommended that any ControlPath used for
             opportunistic connection sharing include at least %h, %p, and %r.
             This ensures that shared connections are uniquely identified.

     ControlPersist
             When used in conjunction with ControlMaster, specifies that the
             master connection should remain open in the background (waiting
             for future client connections) after the initial client
             connection has been closed.  If set to ``no'', then the master
             connection will not be placed into the background, and will close
             as soon as the initial client connection is closed.  If set to
             ``yes'', then the master connection will remain in the background
             indefinitely (until killed or closed via a mechanism such as the
             ssh(1) ``-O exit'' option).  If set to a time in seconds, or a
             time in any of the formats documented in sshd_config(5), then the
             backgrounded master connection will automatically terminate after
             it has remained idle (with no client connections) for the
             specified time.

     DynamicForward
             Specifies that a TCP 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 [bind_address:]port.  IPv6 addresses can be
             specified by enclosing addresses in square brackets.  By default,
             the local port is bound in accordance with the GatewayPorts
             setting.  However, an explicit bind_address may be used to bind
             the connection to a specific address.  The bind_address of
             ``localhost'' indicates that the listening port be bound for
             local use only, while an empty address or `*' indicates that the
             port should be available from all interfaces.

             Currently the SOCKS4 and SOCKS5 protocols are supported, and
             ssh(1) will act as a SOCKS server.  Multiple forwardings may be
             specified, and additional forwardings 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
             disable the escape character entirely (making the connection
             transparent for binary data).

     ExitOnForwardFailure
             Specifies whether ssh(1) should terminate the connection if it
             cannot set up all requested dynamic, tunnel, local, and remote
             port forwardings.  The argument must be ``yes'' or ``no''.  The
             default is ``no''.

     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
             redirected 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
             display through the forwarded connection.  An attacker may then
             be able to perform activities such as keystroke monitoring if the
             ForwardX11Trusted option is also enabled.

     ForwardX11Timeout
             Specify a timeout for untrusted X11 forwarding using the format
             described in the TIME FORMATS section of sshd_config(5).  X11
             connections received by ssh(1) after this time will be refused.
             The default is to disable untrusted X11 forwarding after twenty
             minutes has elapsed.

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

             If this option is set to ``no'', 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(1) binds local port forwardings
             to the loopback address.  This prevents other remote hosts from
             connecting to forwarded ports.  GatewayPorts can be used to
             specify that ssh should bind local 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''.

     GlobalKnownHostsFile
             Specifies one or more files to use for the global host key
             database, separated by whitespace.  The default is
             /etc/ssh/ssh_known_hosts, /etc/ssh/ssh_known_hosts2.

     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
             only.

     HashKnownHosts
             Indicates that ssh(1) should hash host names and addresses when
             they are added to ~/.ssh/known_hosts.  These hashed names may be
             used normally by ssh(1) and sshd(8), but they do not reveal
             identifying information should the file's contents be disclosed.
             The default is ``no''.  Note that existing names and addresses in
             known hosts files will not be converted automatically, but 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
             only 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:

                ecdsa-sha2-nistp256-cert-v01@openssh.com,
                ecdsa-sha2-nistp384-cert-v01@openssh.com,
                ecdsa-sha2-nistp521-cert-v01@openssh.com,
                ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
                ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
                ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
                ssh-rsa,ssh-dss

             If hostkeys are known for the destination host then this default
             is modified to prefer their algorithms.

     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
             connections 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.  If the hostname
             contains the character sequence `%h', then this will be replaced
             with the host name specified on the command line (this is useful
             for manipulating unqualified names).  The default is the name
             given on the command line.  Numeric IP addresses are also
             permitted (both on the command line and in HostName
             specifications).

     IdentitiesOnly
             Specifies that ssh(1) should only use the authentication identity
             files configured in the ssh_config files, even if ssh-agent(1)
             offers more identities.  The argument to this keyword must be
             ``yes'' or ``no''.  This option is intended for situations where
             ssh-agent offers many different identities.  The default is
             ``no''.

     IdentityFile
             Specifies a file from which the user's DSA, ECDSA or RSA
             authentication identity is read.  The default is ~/.ssh/identity
             for protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa and
             ~/.ssh/id_rsa for protocol version 2.  Additionally, any
             identities represented by the authentication agent will be used
             for authentication.  ssh(1) will try to load certificate
             information from the filename obtained by appending -cert.pub to
             the path of a specified IdentityFile.

             The file name may use the tilde syntax to refer to a user's home
             directory or one of the following escape characters: `%d' (local
             user's home directory), `%u' (local user name), `%l' (local host
             name), `%h' (remote host name) or `%r' (remote user name).

             It is possible to have multiple identity files specified in
             configuration files; all these identities will be tried in
             sequence.  Multiple IdentityFile directives will add to the list
             of identities tried (this behaviour differs from that of other
             configuration directives).

     IPQoS   Specifies the IPv4 type-of-service or DSCP class for connections.
             Accepted values are ``af11'', ``af12'', ``af13'', ``af21'',
             ``af22'', ``af23'', ``af31'', ``af32'', ``af33'', ``af41'',
             ``af42'', ``af43'', ``cs0'', ``cs1'', ``cs2'', ``cs3'', ``cs4'',
             ``cs5'', ``cs6'', ``cs7'', ``ef'', ``lowdelay'', ``throughput'',
             ``reliability'', or a numeric value.  This option may take one or
             two arguments, separated by whitespace.  If one argument is
             specified, it is used as the packet class unconditionally.  If
             two values are specified, the first is automatically selected for
             interactive sessions and the second for non-interactive sessions.
             The default is ``lowdelay'' for interactive sessions and
             ``throughput'' for non-interactive sessions.

     KbdInteractiveAuthentication
             Specifies whether to use keyboard-interactive authentication.
             The argument to this keyword must be ``yes'' or ``no''.  The
             default is ``yes''.

     KbdInteractiveDevices
             Specifies the list of methods to use in keyboard-interactive
             authentication.  Multiple method names must be comma-separated.
             The default is to use the server specified list.  The methods
             available vary depending on what the server supports.  For an
             OpenSSH server, it may be zero or more of: ``bsdauth'', ``pam'',
             and ``skey''.

     KexAlgorithms
             Specifies the available KEX (Key Exchange) algorithms.  Multiple
             algorithms must be comma-separated.  The default is:

                   ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
                   diffie-hellman-group-exchange-sha256,
                   diffie-hellman-group-exchange-sha1,
                   diffie-hellman-group14-sha1,
                   diffie-hellman-group1-sha1

     LocalCommand
             Specifies a command to execute on the local machine after
             successfully connecting to the server.  The command string
             extends to the end of the line, and is executed with the user's
             shell.  The following escape character substitutions will be
             performed: `%d' (local user's home directory), `%h' (remote host
             name), `%l' (local host name), `%n' (host name as provided on the
             command line), `%p' (remote port), `%r' (remote user name) or
             `%u' (local user name).

             The command is run synchronously and does not have access to the
             session of the ssh(1) that spawned it.  It should not be used for
             interactive commands.

             This directive is ignored unless PermitLocalCommand has been
             enabled.

     LocalForward
             Specifies that a TCP 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.  Multiple
             forwardings may be specified, and additional 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.  However, an explicit
             bind_address may be used to bind the connection to a specific
             address.  The bind_address of ``localhost'' indicates that the
             listening port be bound for local use only, while an empty
             address or `*' indicates that the port should be available from
             all interfaces.

     LogLevel
             Gives the verbosity level that is used when logging messages from
             ssh(1).  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 verbose output.

     MACs    Specifies the MAC (message authentication code) algorithms in
             order 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,umac-64@openssh.com,
                   hmac-sha2-256,hmac-sha2-512,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
             machine on each of the machines and the user will get many
             warnings about changed host keys.  However, this option disables
             host authentication for localhost.  The argument to this keyword
             must be ``yes'' or ``no''.  The default is to check the host key
             for localhost.

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

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

     PermitLocalCommand
             Allow local command execution via the LocalCommand option or
             using the !command escape sequence in ssh(1).  The argument must
             be ``yes'' or ``no''.  The default is ``no''.

     PKCS11Provider
             Specifies which PKCS#11 provider to use.  The argument to this
             keyword is the PKCS#11 shared library ssh(1) should use to
             communicate with a PKCS#11 token providing the user's private RSA
             key.

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

     PreferredAuthentications
             Specifies the order in which the client should try protocol 2
             authentication methods.  This allows a client to prefer one
             method (e.g. keyboard-interactive) over another method (e.g.
             password).  The default is:

                   gssapi-with-mic,hostbased,publickey,
                   keyboard-interactive,password

     Protocol
             Specifies the protocol versions ssh(1) should support in order of
             preference.  The possible values are `1' and `2'.  Multiple
             versions must be comma-separated.  When this option is set to
             ``2,1'' ssh will try version 2 and fall back to version 1 if
             version 2 is not available.  The default is `2'.

     ProxyCommand
             Specifies the command to use to connect to the server.  The
             command string extends to the end of the line, and is executed
             with the user's shell.  In the command string, any occurrence of
             `%h' will be substituted by the host name to connect, `%p' by the
             port, and `%r' by the remote user name.  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
             somewhere.  Host key management will be done using the HostName
             of the host being connected (defaulting to the name typed by the
             user).  Setting the command to ``none'' disables this option
             entirely.  Note that CheckHostIP is not available for connects
             with a proxy command.

             This directive is useful in conjunction with nc(1) and its proxy
             support.  For example, the following directive would connect via
             an HTTP proxy at 192.0.2.0:

                ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p

     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.

     RekeyLimit
             Specifies the maximum amount of data that may be transmitted
             before the session key is renegotiated.  The argument is the
             number of bytes, with an optional suffix of `K', `M', or `G' to
             indicate Kilobytes, Megabytes, or Gigabytes, respectively.  The
             default is between `1G' and `4G', depending on the cipher.  This
             option applies to protocol version 2 only.

     RemoteForward
             Specifies that a TCP 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.  Multiple
             forwardings may be specified, and additional forwardings can be
             given on the command line.  Privileged ports can be forwarded
             only when logging in as root on the remote machine.

             If the port argument is `0', the listen port will be dynamically
             allocated on the server and reported to the client at run time.

             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
             interfaces.  Specifying a remote bind_address will only succeed
             if the server's GatewayPorts option is enabled (see
             sshd_config(5)).

     RequestTTY
             Specifies whether to request a pseudo-tty for the session.  The
             argument may be one of: ``no'' (never request a TTY), ``yes''
             (always request a TTY when standard input is a TTY), ``force''
             (always request a TTY) or ``auto'' (request a TTY when opening a
             login session).  This option mirrors the -t and -T flags for
             ssh(1).

     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(1) 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
             authentication 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 wildcard
             characters.  Multiple environment variables may be separated by
             whitespace or spread across multiple SendEnv directives.  The
             default is not to send any environment variables.

             See PATTERNS for more information on patterns.

     ServerAliveCountMax
             Sets the number of server alive messages (see below) which may be
             sent without ssh(1) 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
             session.  It is important to note that the use of server alive
             messages is very different from TCPKeepAlive (below).  The server
             alive messages are sent through the encrypted channel and
             therefore will not be spoofable.  The TCP keepalive option
             enabled by TCPKeepAlive is spoofable.  The server alive mechanism
             is valuable when the client or server depend on knowing when a
             connection has become inactive.

             The default value is 3.  If, for example, ServerAliveInterval
             (see below) is set to 15 and ServerAliveCountMax is left at the
             default, if the server becomes unresponsive, ssh will disconnect
             after approximately 45 seconds.  This option applies to protocol
             version 2 only.

     ServerAliveInterval
             Sets a timeout interval in seconds after which if no data has
             been received from the server, ssh(1) 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.

     StrictHostKeyChecking
             If this flag is set to ``yes'', ssh(1) will never automatically
             add host keys to the ~/.ssh/known_hosts file, and refuses to
             connect to hosts whose host key has changed.  This provides
             maximum protection against trojan horse attacks, though it can be
             annoying when the /etc/ssh/ssh_known_hosts file is poorly
             maintained or when 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
             temporarily, 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''.

     Tunnel  Request tun(4) device forwarding between the client and the
             server.  The argument must be ``yes'', ``point-to-point'' (layer
             3), ``ethernet'' (layer 2), or ``no''.  Specifying ``yes''
             requests the default tunnel mode, which is ``point-to-point''.
             The default is ``no''.

     TunnelDevice
             Specifies the tun(4) devices to open on the client (local_tun)
             and the server (remote_tun).

             The argument must be local_tun[:remote_tun].  The devices may be
             specified by numerical ID or the keyword ``any'', which uses the
             next available tunnel device.  If remote_tun is not specified, it
             defaults to ``any''.  The default is ``any:any''.

     UsePrivilegedPort
             Specifies whether to use a privileged port for outgoing
             connections.  The argument must be ``yes'' or ``no''.  The
             default is ``no''.  If set to ``yes'', ssh(1) 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
             different user name is used on different machines.  This saves
             the trouble of having to remember to give the user name on the
             command line.

     UserKnownHostsFile
             Specifies one or more files to use for the user host key
             database, separated by whitespace.  The default is
             ~/.ssh/known_hosts, ~/.ssh/known_hosts2.

     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.

             See also VERIFYING HOST KEYS in ssh(1).

     VisualHostKey
             If this flag is set to ``yes'', an ASCII art representation of
             the remote host key fingerprint is printed in addition to the hex
             fingerprint string at login and for unknown host keys.  If this
             flag is set to ``no'', no fingerprint strings are printed at
             login and only the hex fingerprint string will be printed for
             unknown host keys.  The default is ``no''.

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

PATTERNS
     A pattern consists of zero or more non-whitespace characters, `*' (a
     wildcard that matches zero or more characters), or `?' (a wildcard that
     matches exactly one character).  For example, to specify a set of
     declarations for any host in the ``.co.uk'' set of domains, the following
     pattern could be used:

           Host *.co.uk

     The following pattern would match any host in the 192.168.0.[0-9] network
     range:

           Host 192.168.0.?

     A pattern-list is a comma-separated list of patterns.  Patterns within
     pattern-lists may be negated by preceding them with an exclamation mark
     (`!').  For example, to allow a key to be used from anywhere within an
     organisation except from the ``dialup'' pool, the following entry (in
     authorized_keys) could be used:

           from="!*.dialup.example.com,*.example.com"

FILES
     ~/.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.
             Because of the potential for abuse, this file must have strict
             permissions: 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
     created OpenSSH.  Markus Friedl contributed the support for SSH protocol
     versions 1.5 and 2.0.

OpenBSD 5.2                      June 29, 2012                     OpenBSD 5.2