ssh_config

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

DESCRIPTION
     ssh(1) obtains configuration data from the following sources in the fol-
     lowing 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 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.

     Note that the Debian openssh-client package sets several options as stan-
     dard in /etc/ssh/ssh_config which are not the default in ssh(1):

           o   SendEnv LANG LC_*
           o   HashKnownHosts yes
           o   GSSAPIAuthentication yes

     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 speci-
     fying 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 key-
     words are case-insensitive and arguments are case-sensitive):

     Host    Restricts the following declarations (up to the next Host or
             Match 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 exclama-
             tion 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
             shell.  If the command returns a zero exit status then the condi-
             tion is considered true.  Commands containing whitespace charac-
             ters must be quoted.  The following character sequences in the
             command will be expanded prior to execution: '%L' will be substi-
             tuted 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 destination port, '%r' by the remote login
             username, and '%u' by the username of the user running ssh(1).

             The other keywords' criteria must be single entries or comma-sep-
             arated lists and may use the wildcard and negation operators
             described in the PATTERNS section.  The criteria for the host
             keyword are matched against the target hostname, after any sub-
             stitution by the Hostname option.  The originalhost keyword
             matches against the hostname as it was specified on the command-
             line.  The user keyword matches against the target username on
             the remote host.  The localuser keyword matches against the name
             of the local user running ssh(1) (this keyword may be useful in
             system-wide ssh_config files).

     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.
             In addition, the ServerAliveInterval option will be set to 300
             seconds by default.  This option is useful in scripts and other
             batch jobs where no user is present to supply the password, and
             where it is desirable to detect a broken network swiftly.  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''.

     CanonicalDomains
             When CanonicalizeHostname is enabled, this option specifies the
             list of domain suffixes in which to search for the specified des-
             tination host.

     CanonicalizeFallbackLocal
             Specifies whether to fail with an error when hostname canonical-
             ization fails.  The default, ``yes'', will attempt to look up the
             unqualified hostname using the system resolver's search rules.  A
             value of ``no'' will cause ssh(1) to fail instantly if
             CanonicalizeHostname is enabled and the target hostname cannot be
             found in any of the domains specified by CanonicalDomains.

             target hostname changing, then the configuration files are pro-
             cessed again using the new target name to pick up any new config-
             uration in matching Host stanzas.

     CanonicalizeMaxDots
             Specifies the maximum number of dot characters in a hostname
             before canonicalization is disabled.  The default, ``1'', allows
             a single dot (i.e. hostname.subdomain).

     CanonicalizePermittedCNAMEs
             Specifies rules to determine whether CNAMEs should be followed
             when canonicalizing hostnames.  The rules consist of one or more
             arguments of source_domain_list:target_domain_list, where
             source_domain_list is a pattern-list of domains that may follow
             CNAMEs in canonicalization, and target_domain_list is a pattern-
             list of domains that they may resolve to.

             For example, ``*.a.example.com:*.b.example.com,*.c.example.com''
             will allow hostnames matching ``*.a.example.com'' to be canoni-
             calized to names in the ``*.b.example.com'' or
             ``*.c.example.com'' domains.

     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 proto-
             col 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 sup-
             ported 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'',
             ``cast128-cbc'', and ``chacha20-poly1305@openssh.com''.

             The default is:

             cleared.  This option is primarily useful when used from the
             ssh(1) command line to clear port forwardings set in configura-
             tion 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 connec-
             tions on a control socket specified using the ControlPath argu-
             ment.  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 con-
             nections, 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 connect-
             ing to a master instance.

             X11 and ssh-agent(1) forwarding is supported over these multi-
             plexed connections, however the display and agent forwarded will
             be the one belonging to the master connection i.e. it is not pos-
             sible 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.
             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 has been closed.  If set to ``no'', then the master connec-
             tion 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 speci-
             fied 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 set-
             ting.  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 dis-
             able the escape character entirely (making the connection trans-
             parent for binary data).

     ExitOnForwardFailure
             Specifies whether ssh(1) should terminate the connection if it
             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 redi-
             rected 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.

     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 con-
             sidered 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 min-
             utes.  Remote clients will be refused access after this time.

             The default is ``yes'' (Debian-specific).

             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 spec-
             ify 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 data-
             base, separated by whitespace.  The default is
             /etc/ssh/ssh_known_hosts, /etc/ssh/ssh_known_hosts2.

             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 con-
             nections 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 GSS-
             API.

     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 iden-
             tifying 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).  Use of this option may
             break facilities such as tab-completion that rely on being able
             to read unhashed host names from ~/.ssh/known_hosts.

     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,

             base 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 permit-
             ted (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 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, ED25519 or RSA
             authentication identity is read.  The default is ~/.ssh/identity
             for protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa,
             ~/.ssh/id_ed25519 and ~/.ssh/id_rsa for protocol version 2.
             Additionally, any identities represented by the authentication
             agent will be used for authentication unless IdentitiesOnly is
             set.  ssh(1) will try to load certificate information from the
             filename obtained by appending -cert.pub to the path of a speci-
             fied 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 con-
             figuration files; all these identities will be tried in sequence.
             Multiple IdentityFile directives will add to the list of identi-
             ties tried (this behaviour differs from that of other configura-
             tion directives).

             IdentityFile may be used in conjunction with IdentitiesOnly to
             select which identities in an agent are offered during authenti-
             cation.

     IgnoreUnknown
             Specifies a pattern-list of unknown options to be ignored if they
             are encountered in configuration parsing.  This may be used to
             suppress errors if ssh_config contains options that are unrecog-
             nised by ssh(1).  It is recommended that IgnoreUnknown be listed
             early in the configuration file as it will not be applied to
             unknown options that appear before it.


     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:

                   curve25519-sha256@libssh.org,
                   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 suc-
             cessfully 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 privi-
             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
             sion 2 for data integrity protection.  Multiple algorithms must
             be comma-separated.  The algorithms that contain ``-etm'' calcu-
             late the MAC after encryption (encrypt-then-mac).  These are con-
             sidered 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 localhost will refer to a different
             machine on each of the machines and the user will get many warn-
             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 commu-
             nicate 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

             port, and '%r' by the remote user name.  The command can be basi-
             cally 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).  Set-
             ting the command to ``none'' disables this option entirely.  Note
             that CheckHostIP is not available for connects with a proxy com-
             mand.

             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

     ProxyUseFdpass
             Specifies that ProxyCommand will pass a connected file descriptor
             back to ssh(1) instead of continuing to execute and pass data.
             The default is ``no''.

     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, optionally followed a
             maximum amount of time that may pass before the session key is
             renegotiated.  The first argument is specified in bytes and may
             have a suffix of 'K', 'M', or 'G' to indicate Kilobytes,
             Megabytes, or Gigabytes, respectively.  The default is between
             '1G' and '4G', depending on the cipher.  The optional second
             value is specified in seconds and may use any of the units docu-
             mented in the TIME FORMATS section of sshd_config(5).  The
             default value for RekeyLimit is ``default none'', which means
             that rekeying is performed after the cipher's default amount of
             data has been sent or received and no time based rekeying is
             done.  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.

             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 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 wildcard char-
             acters.  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 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
             (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; in protocol version 1 there is no mechanism to
             request a response from the server to the server alive messages,
             so disconnection is the responsibility of the TCP stack.

     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
             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 automati-
             cally 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.  This
             option only uses TCP keepalives (as opposed to using ssh level
             keepalives), so takes a long time to notice when the connection
             dies.  As such, you probably want the ServerAliveInterval option
             as well.  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 connec-
             tions.  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 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.

             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/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 decla-
     rations for any host in the ``.co.uk'' set of domains, the following pat-
     tern 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
     organization 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 oth-
             ers.  It may be group-writable provided that the group in ques-
             tion contains only the user.

     /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.

BSD                           September 21, 2018                           BSD
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