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SSH_CONFIG(5)               BSD File Formats 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 configura-tion configuration
     tion 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).

     AskPassGUI
             Specifies whether to display the graphical password dialog when publickey authentication is
             used and a password is required to decrypt the private key.  This keyword is predicated on
             KeychainIntegration being enabled.  The argument to this keyword must be ``yes'' or ``no''.
             The default is ``yes''.  Note that the graphical password dialog will automatically add identi-ties identities
             ties to the authentication agent (if any).

     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'',
             ``aes128-gcm@openssh.com'', ``aes256-gcm@openssh.com'', ``arcfour128'', ``arcfour256'',
             ``arcfour'', ``blowfish-cbc'', and ``cast128-cbc''.  The default is:

                aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
                aes128-gcm@openssh.com,aes256-gcm@openssh.com,
                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 appli-cations. applications.
             cations.  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 unreach-able, unreachable,
             able, 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 confirma-tion confirmation
             tion 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 substi-tuted substituted
             tuted by the local host name (including any domain name), `%h' will be substituted by the tar-get target
             get host name, `%n' will be substituted by the original target host name specified on the com-mand command
             mand 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 connec-tion connection
             tion 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 connec-tion connection
             tion 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 argu-ment argument
             ment 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 disable
             able 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 permis-sions permissions
             sions 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, how-ever however
             ever they can perform operations on the keys that enable them to authenticate using the identi-ties identities
             ties 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 permis-sions permissions
             sions 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 for-warded forwarded
             warded 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.

     GSSAPIKeyExchange
             Specifies whether key exchange based on GSSAPI may be used. When using GSSAPI key exchange the
             server need not have a host key.  The default is ``no''.  Note that this option applies to pro-tocol protocol
             tocol version 2 only.

     GSSAPIClientIdentity
             If set, specifies the GSSAPI client identity that ssh should use when connecting to the server.
             The default is unset, which means that the default identity will be used.

     GSSAPIServerIdentity
             If set, specifies the GSSAPI server identity that ssh should expect when connecting to the
             server. The default is unset, which means that the expected GSSAPI server identity will be
             determined from the target hostname.

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

     GSSAPIRenewalForcesRekey
             If set to ``yes'' then renewal of the client's GSSAPI credentials will force the rekeying of
             the ssh connection. With a compatible server, this can delegate the renewed credentials to a
             session on the server.  The default is ``no''.

     GSSAPITrustDns
             Set to ``yes to indicate that the DNS is trusted to securely canonicalize'' the name of the
             host being connected to. If ``no, the hostname entered on the'' command line will be passed
             untouched to the GSSAPI library.  The default is ``no''.  This option only applies to protocol
             version 2 connections using GSSAPI.

     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 argu-ment argument
             ment must be ``yes'' or ``no''.  The default is ``no''.  This option applies to protocol ver-sion version
             sion 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 connec-tions connections
             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 abbrevia-tions abbreviations
             tions 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) or a PKCS11Provider offers more identities.  The argu-ment argument
             ment 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 authen-tication authentication
             tication agent will be used for authentication.  ssh(1) will try to load certificate informa-tion information
             tion 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 fol-lowing following
             lowing 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''.

     KeychainIntegration
             Specifies whether to enable Keychain support on Mac OS X.  If Keychain support is enabled, then
             passwords for identities can be managed via the Mac OS X Keychain.  The value for this keyword
             must be ``yes'' or ``no''.  The default is ``yes''.

     KexAlgorithms
             Specifies the available KEX (Key Exchange) algorithms.  Multiple algorithms must be comma-sepa-rated. comma-separated.
             rated.  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 spec-ified specified
             ified 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 privi-leged privileged
             leged 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 inter-faces. interfaces.
             faces.

     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 ver-bose verbose
             bose 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 algorithms that contain ``-etm'' calculate the MAC after encryp-tion encryption
             tion (encrypt-then-mac).  These are considered safer and their use recommended.  The default
             is:

                   hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
                   umac-64-etm@openssh.com,umac-128-etm@openssh.com,
                   hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
                   hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com,
                   hmac-md5-96-etm@openssh.com,
                   hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@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 local-host localhost
             host will refer to a different machine on each of the machines and the user will get many warn-ings warnings
             ings 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 renego-tiated. renegotiated.
             tiated.  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 forward-ings forwardings
             ings 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 argu-ment argument
             ment must be ``yes'' or ``no''.  The default is ``no''.  This option applies to protocol ver-sion version
             sion 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 authenti-cation authentication
             cation 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 vari-ables. variables.
             ables.

             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 spoofa-ble. spoofable.
             ble.  The TCP keepalive option enabled by TCPKeepAlive is spoofable.  The server alive mecha-nism mechanism
             nism is valuable when the client or server depend on knowing when a connection has become inac-tive. inactive.
             tive.

             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 dis-connect disconnect
             connect 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 pro-vides provides
             vides 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 fre-quently frequently
             quently 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.  How-ever, However,
             ever, 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 net-work network
             work 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 speci-fied, specified,
             fied, 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 dif-ferent different
             ferent 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 argu-ment argument
             ment 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 fin-gerprint fingerprint
             gerprint 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 xauth (which causes ssh to
             search $PATH).

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 spec-ified specified
             ified 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.

BSD                            October 11, 2013                            BSD

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