noble (5) ssh_config.5.gz

Provided by: openssh-client_9.6p1-3ubuntu13.8_amd64 bug

NAME

     ssh_config — OpenSSH client configuration file

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)

     Unless noted otherwise, 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 usually the one given on the
     command line (see the CanonicalizeHostname option for exceptions).

     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.

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

              Include /etc/ssh/ssh_config.d/*.conf
              SendEnv LANG LC_*
              HashKnownHosts yes
              GSSAPIAuthentication yes

     /etc/ssh/ssh_config.d/*.conf files are included at the start of the system-wide configuration file, so
     options set there will override those in /etc/ssh/ssh_config.

     The file contains keyword-argument pairs, one per line.  Lines starting with ‘#’ and empty lines are
     interpreted as comments.  Arguments may optionally be enclosed in double quotes (") in order to represent
     arguments containing spaces.  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.

     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 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 usually the hostname argument given on the command line
             (see the CanonicalizeHostname keyword for exceptions).

             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.

     Match   Restricts the following declarations (up to the next Host or Match keyword) to be used only when
             the conditions following the Match keyword are satisfied.  Match conditions are specified using one
             or more criteria or the single token all which always matches.  The available criteria keywords
             are: canonical, final, exec, localnetwork, host, originalhost, Tag, user, and localuser.  The all
             criteria must appear alone or immediately after canonical or final.  Other criteria may be combined
             arbitrarily.  All criteria but all, canonical, and final require an argument.  Criteria may be
             negated by prepending an exclamation mark (‘!’).

             The canonical keyword matches only when the configuration file is being re-parsed after hostname
             canonicalization (see the CanonicalizeHostname option).  This may be useful to specify conditions
             that work with canonical host names only.

             The final keyword requests that the configuration be re-parsed (regardless of whether
             CanonicalizeHostname is enabled), and matches only during this final pass.  If CanonicalizeHostname
             is enabled, then canonical and final match during the same pass.

             The exec keyword executes the specified command under the user's shell.  If the command returns a
             zero exit status then the condition is considered true.  Commands containing whitespace characters
             must be quoted.  Arguments to exec accept the tokens described in the TOKENS section.

             The localnetwork keyword matches the addresses of active local network interfaces against the
             supplied list of networks in CIDR format.  This may be convenient for varying the effective
             configuration on devices that roam between networks.  Note that network address is not a
             trustworthy criteria in many situations (e.g. when the network is automatically configured using
             DHCP) and so caution should be applied if using it to control security-sensitive configuration.

             The other keywords' criteria must be single entries or comma-separated 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 substitution by the Hostname or
             CanonicalizeHostname options.  The originalhost keyword matches against the hostname as it was
             specified on the command-line.  The tagged keyword matches a tag name specified by a prior Tag
             directive or on the ssh(1) command-line using the -P flag.  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).

     AddKeysToAgent
             Specifies whether keys should be automatically added to a running ssh-agent(1).  If this option is
             set to yes and a key is loaded from a file, the key and its passphrase are added to the agent with
             the default lifetime, as if by ssh-add(1).  If this option is set to ask, ssh(1) will require
             confirmation using the SSH_ASKPASS program before adding a key (see ssh-add(1) for details).  If
             this option is set to confirm, each use of the key must be confirmed, as if the -c option was
             specified to ssh-add(1).  If this option is set to no, no keys are added to the agent.
             Alternately, this option may be specified as a time interval using the format described in the TIME
             FORMATS section of sshd_config(5) to specify the key's lifetime in ssh-agent(1), after which it
             will automatically be removed.  The argument must be no (the default), yes, confirm (optionally
             followed by a time interval), ask or a time interval.

     AddressFamily
             Specifies which address family to use when connecting.  Valid arguments are any (the default), inet
             (use IPv4 only), or inet6 (use IPv6 only).

     BatchMode
             If set to yes, user interaction such as password prompts and host key confirmation requests will be
             disabled.  In addition, the ServerAliveInterval option will be set to 300 seconds by default
             (Debian-specific).  This option is useful in scripts and other batch jobs where no user is present
             to interact with ssh(1), and where it is desirable to detect a broken network swiftly.  The
             argument must be yes or no (the default).

     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.

     BindInterface
             Use the address of the specified interface on the local machine as the source address of the
             connection.

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

     CanonicalizeFallbackLocal
             Specifies whether to fail with an error when hostname canonicalization 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.

     CanonicalizeHostname
             Controls whether explicit hostname canonicalization is performed.  The default, no, is not to
             perform any name rewriting and let the system resolver handle all hostname lookups.  If set to yes
             then, for connections that do not use a ProxyCommand or ProxyJump, ssh(1) will attempt to
             canonicalize the hostname specified on the command line using the CanonicalDomains suffixes and
             CanonicalizePermittedCNAMEs rules.  If CanonicalizeHostname is set to always, then canonicalization
             is applied to proxied connections too.

             If this option is enabled, then the configuration files are processed again using the new target
             name to pick up any new configuration in matching Host and Match stanzas.  A value of none disables
             the use of a ProxyJump host.

     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 canonicalized to names in the "*.b.example.com" or "*.c.example.com"
             domains.

             A single argument of "none" causes no CNAMEs to be considered for canonicalization.  This is the
             default behaviour.

     CASignatureAlgorithms
             Specifies which algorithms are allowed for signing of certificates by certificate authorities
             (CAs).  The default is:

                   ssh-ed25519,ecdsa-sha2-nistp256,
                   ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
                   sk-ssh-ed25519@openssh.com,
                   sk-ecdsa-sha2-nistp256@openssh.com,
                   rsa-sha2-512,rsa-sha2-256

             If the specified list begins with a ‘+’ character, then the specified algorithms will be appended
             to the default set instead of replacing them.  If the specified list begins with a ‘-’ character,
             then the specified algorithms (including wildcards) will be removed from the default set instead of
             replacing them.

             ssh(1) will not accept host certificates signed using algorithms other than those specified.

     CertificateFile
             Specifies a file from which the user's certificate is read.  A corresponding private key must be
             provided separately in order to use this certificate either from an IdentityFile directive or -i
             flag to ssh(1), via ssh-agent(1), or via a PKCS11Provider or SecurityKeyProvider.

             Arguments to CertificateFile may use the tilde syntax to refer to a user's home directory, the
             tokens described in the TOKENS section and environment variables as described in the ENVIRONMENT
             VARIABLES section.

             It is possible to have multiple certificate files specified in configuration files; these
             certificates will be tried in sequence.  Multiple CertificateFile directives will add to the list
             of certificates used for authentication.

     ChannelTimeout
             Specifies whether and how quickly ssh(1) should close inactive channels.  Timeouts are specified as
             one or more “type=interval” pairs separated by whitespace, where the “type” must be a channel type
             name (as described in the table below), optionally containing wildcard characters.

             The timeout value “interval” is specified in seconds or may use any of the units documented in the
             TIME FORMATS section.  For example, “session=5m” would cause the interactive session to terminate
             after five minutes of inactivity.  Specifying a zero value disables the inactivity timeout.

             The available channel types include:

             agent-connection
                     Open connections to ssh-agent(1).

             direct-tcpip, direct-streamlocal@openssh.com
                     Open TCP or Unix socket (respectively) connections that have been established from a ssh(1)
                     local forwarding, i.e. LocalForward or DynamicForward.

             forwarded-tcpip, forwarded-streamlocal@openssh.com
                     Open TCP or Unix socket (respectively) connections that have been established to a sshd(8)
                     listening on behalf of a ssh(1) remote forwarding, i.e. RemoteForward.

             session
                     The interactive main session, including shell session, command execution, scp(1), sftp(1),
                     etc.

             tun-connection
                     Open TunnelForward connections.

             x11-connection
                     Open X11 forwarding sessions.

             Note that in all the above cases, terminating an inactive session does not guarantee to remove all
             resources associated with the session, e.g. shell processes or X11 clients relating to the session
             may continue to execute.

             Moreover, terminating an inactive channel or session does not necessarily close the SSH connection,
             nor does it prevent a client from requesting another channel of the same type.  In particular,
             expiring an inactive forwarding session does not prevent another identical forwarding from being
             subsequently created.

             The default is not to expire channels of any type for inactivity.

     CheckHostIP
             If set to yes, ssh(1) will additionally check the host IP address in the known_hosts file.  This
             allows it to detect if a host key changed due to DNS spoofing and will add addresses of destination
             hosts to ~/.ssh/known_hosts in the process, regardless of the setting of StrictHostKeyChecking.  If
             the option is set to no (the default), the check will not be executed.

     Ciphers
             Specifies the ciphers allowed and their order of preference.  Multiple ciphers must be comma-
             separated.  If the specified list begins with a ‘+’ character, then the specified ciphers will be
             appended to the default set instead of replacing them.  If the specified list begins with a ‘-’
             character, then the specified ciphers (including wildcards) will be removed from the default set
             instead of replacing them.  If the specified list begins with a ‘^’ character, then the specified
             ciphers will be placed at the head of the default set.

             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
                   chacha20-poly1305@openssh.com

             The default is:

                   chacha20-poly1305@openssh.com,
                   aes128-ctr,aes192-ctr,aes256-ctr,
                   aes128-gcm@openssh.com,aes256-gcm@openssh.com

             The list of available ciphers may also be obtained using "ssh -Q cipher".

     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).

     Compression
             Specifies whether to use compression.  The argument must be yes or no (the default).

     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 timeout is applied both to establishing the connection and to
             performing the initial SSH protocol handshake and key exchange.

     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(1) to listen for control connections, but require confirmation
             using ssh-askpass(1).  If the ControlPath cannot be opened, ssh(1) 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.  Arguments to
             ControlPath may use the tilde syntax to refer to a user's home directory, the tokens described in
             the TOKENS section and environment variables as described in the ENVIRONMENT VARIABLES section.  It
             is recommended that any ControlPath used for opportunistic connection sharing include at least %h,
             %p, and %r (or alternatively %C) and be placed in a directory that is not writable by other users.
             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 (the default), 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
             or 0, then the master connection will remain in the background indefinitely (until killed or closed
             via a mechanism such as the "ssh -O exit").  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.

     EnableEscapeCommandline
             Enables the command line option in the EscapeChar menu for interactive sessions (default ‘~C’).  By
             default, the command line is disabled.

     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).  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, (e.g. if either end is unable to bind and listen on a
             specified port).  Note that ExitOnForwardFailure does not apply to connections made over port
             forwardings and will not, for example, cause ssh(1) to exit if TCP connections to the ultimate
             forwarding destination fail.  The argument must be yes or no (the default).

     FingerprintHash
             Specifies the hash algorithm used when displaying key fingerprints.  Valid options are: md5 and
             sha256 (the default).

     ForkAfterAuthentication
             Requests ssh to go to background just before command execution.  This is useful if ssh is going to
             ask for passwords or passphrases, but the user wants it in the background.  This implies the
             StdinNull configuration option being set to “yes”.  The recommended way to start X11 programs at a
             remote site is with something like ssh -f host xterm, which is the same as ssh host xterm if the
             ForkAfterAuthentication configuration option is set to “yes”.

             If the ExitOnForwardFailure configuration option is set to “yes”, then a client started with the
             ForkAfterAuthentication configuration option being set to “yes” will wait for all remote port
             forwards to be successfully established before placing itself in the background.  The argument to
             this keyword must be yes (same as the -f option) or no (the default).

     ForwardAgent
             Specifies whether the connection to the authentication agent (if any) will be forwarded to the
             remote machine.  The argument may be yes, no (the default), an explicit path to an agent socket or
             the name of an environment variable (beginning with ‘$’) in which to find the path.

             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).

             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.
             Setting ForwardX11Timeout to zero will disable the timeout and permit X11 forwarding for the life
             of the connection.  The default is to disable untrusted X11 forwarding after twenty minutes has
             elapsed.

     ForwardX11Trusted
             If this option is set to yes, (the Debian-specific default), remote X11 clients will have full
             access to the original X11 display.

             If this option is set to no (the upstream default), 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.

             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).

     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.

     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.

     GSSAPIDelegateCredentials
             Forward (delegate) credentials to the server.  The default is no.

     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”.

     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 will delegate the renewed credentials to a session on
             the server.

             Checks are made to ensure that credentials are only propagated when the new credentials match the
             old ones on the originating client and where the receiving server still has the old set in its
             cache.

             The default is “no”.

             For this to work GSSAPIKeyExchange needs to be enabled in the server and also used by the client.

     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.

     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”.

     GSSAPIKexAlgorithms
             The list of key exchange algorithms that are offered for GSSAPI key exchange. Possible values are

                gss-gex-sha1-,
                gss-group1-sha1-,
                gss-group14-sha1-,
                gss-group14-sha256-,
                gss-group16-sha512-,
                gss-nistp256-sha256-,

                gss-curve25519-sha256-
             The default is
             “gss-group14-sha256-,gss-group16-sha512-,gss-nistp256-sha256-,gss-curve25519-sha256-,gss-gex-sha1-,gss-group14-sha1-”.
             This option only applies to 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
             visually reveal identifying information if the file's contents are 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.

     HostbasedAcceptedAlgorithms
             Specifies the signature algorithms that will be used for hostbased authentication as a comma-
             separated list of patterns.  Alternately if the specified list begins with a ‘+’ character, then
             the specified signature algorithms will be appended to the default set instead of replacing them.
             If the specified list begins with a ‘-’ character, then the specified signature algorithms
             (including wildcards) will be removed from the default set instead of replacing them.  If the
             specified list begins with a ‘^’ character, then the specified signature algorithms will be placed
             at the head of the default set.  The default for this option is:

                ssh-ed25519-cert-v01@openssh.com,
                ecdsa-sha2-nistp256-cert-v01@openssh.com,
                ecdsa-sha2-nistp384-cert-v01@openssh.com,
                ecdsa-sha2-nistp521-cert-v01@openssh.com,
                sk-ssh-ed25519-cert-v01@openssh.com,
                sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
                rsa-sha2-512-cert-v01@openssh.com,
                rsa-sha2-256-cert-v01@openssh.com,
                ssh-ed25519,
                ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
                sk-ssh-ed25519@openssh.com,
                sk-ecdsa-sha2-nistp256@openssh.com,
                rsa-sha2-512,rsa-sha2-256

             The -Q option of ssh(1) may be used to list supported signature algorithms.  This was formerly
             named HostbasedKeyTypes.

     HostbasedAuthentication
             Specifies whether to try rhosts based authentication with public key authentication.  The argument
             must be yes or no (the default).

     HostKeyAlgorithms
             Specifies the host key signature algorithms that the client wants to use in order of preference.
             Alternately if the specified list begins with a ‘+’ character, then the specified signature
             algorithms will be appended to the default set instead of replacing them.  If the specified list
             begins with a ‘-’ character, then the specified signature algorithms (including wildcards) will be
             removed from the default set instead of replacing them.  If the specified list begins with a ‘^’
             character, then the specified signature algorithms will be placed at the head of the default set.
             The default for this option is:

                ssh-ed25519-cert-v01@openssh.com,
                ecdsa-sha2-nistp256-cert-v01@openssh.com,
                ecdsa-sha2-nistp384-cert-v01@openssh.com,
                ecdsa-sha2-nistp521-cert-v01@openssh.com,
                sk-ssh-ed25519-cert-v01@openssh.com,
                sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
                rsa-sha2-512-cert-v01@openssh.com,
                rsa-sha2-256-cert-v01@openssh.com,
                ssh-ed25519,
                ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
                sk-ecdsa-sha2-nistp256@openssh.com,
                sk-ssh-ed25519@openssh.com,
                rsa-sha2-512,rsa-sha2-256

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

             The list of available signature algorithms may also be obtained using "ssh -Q HostKeyAlgorithms".

     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 and when validating host certificates.  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.  Arguments to Hostname accept the tokens described in the TOKENS section.  Numeric IP
             addresses are also permitted (both on the command line and in Hostname specifications).  The
             default is the name given on the command line.

     IdentitiesOnly
             Specifies that ssh(1) should only use the configured authentication identity and certificate files
             (either the default files, or those explicitly configured in the ssh_config files or passed on the
             ssh(1) command-line), even if ssh-agent(1) or a PKCS11Provider or SecurityKeyProvider offers more
             identities.  The argument to this keyword must be yes or no (the default).  This option is intended
             for situations where ssh-agent offers many different identities.

     IdentityAgent
             Specifies the UNIX-domain socket used to communicate with the authentication agent.

             This option overrides the SSH_AUTH_SOCK environment variable and can be used to select a specific
             agent.  Setting the socket name to none disables the use of an authentication agent.  If the string
             "SSH_AUTH_SOCK" is specified, the location of the socket will be read from the SSH_AUTH_SOCK
             environment variable.  Otherwise if the specified value begins with a ‘$’ character, then it will
             be treated as an environment variable containing the location of the socket.

             Arguments to IdentityAgent may use the tilde syntax to refer to a user's home directory, the tokens
             described in the TOKENS section and environment variables as described in the ENVIRONMENT VARIABLES
             section.

     IdentityFile
             Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA, Ed25519,
             authenticator-hosted Ed25519 or RSA authentication identity is read.  You can also specify a public
             key file to use the corresponding private key that is loaded in ssh-agent(1) when the private key
             file is not present locally.  The default is ~/.ssh/id_rsa, ~/.ssh/id_ecdsa, ~/.ssh/id_ecdsa_sk,
             ~/.ssh/id_ed25519, ~/.ssh/id_ed25519_sk and ~/.ssh/id_dsa.  Additionally, any identities
             represented by the authentication agent will be used for authentication unless IdentitiesOnly is
             set.  If no certificates have been explicitly specified by CertificateFile, ssh(1) will try to load
             certificate information from the filename obtained by appending -cert.pub to the path of a
             specified IdentityFile.

             Arguments to IdentityFile may use the tilde syntax to refer to a user's home directory or the
             tokens described in the TOKENS section.  Alternately an argument of none may be used to indicate no
             identity files should be loaded.

             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).

             IdentityFile may be used in conjunction with IdentitiesOnly to select which identities in an agent
             are offered during authentication.  IdentityFile may also be used in conjunction with
             CertificateFile in order to provide any certificate also needed for authentication with the
             identity.

     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 unrecognised
             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.

     Include
             Include the specified configuration file(s).  Multiple pathnames may be specified and each pathname
             may contain glob(7) wildcards and, for user configurations, shell-like ‘~’ references to user home
             directories.  Wildcards will be expanded and processed in lexical order.  Files without absolute
             paths are assumed to be in ~/.ssh if included in a user configuration file or /etc/ssh if included
             from the system configuration file.  Include directive may appear inside a Match or Host block to
             perform conditional inclusion.

     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, le, lowdelay, throughput, reliability, a numeric value, or none to use the operating system
             default.  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 (the default) or no.  ChallengeResponseAuthentication is a deprecated alias for this.

     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 and pam.

     KexAlgorithms
             Specifies the available KEX (Key Exchange) algorithms.  Multiple algorithms must be comma-
             separated.  If the specified list begins with a ‘+’ character, then the specified algorithms will
             be appended to the default set instead of replacing them.  If the specified list begins with a ‘-’
             character, then the specified algorithms (including wildcards) will be removed from the default set
             instead of replacing them.  If the specified list begins with a ‘^’ character, then the specified
             algorithms will be placed at the head of the default set.  The default is:

                   sntrup761x25519-sha512@openssh.com,
                   curve25519-sha256,curve25519-sha256@libssh.org,
                   ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
                   diffie-hellman-group-exchange-sha256,
                   diffie-hellman-group16-sha512,
                   diffie-hellman-group18-sha512,
                   diffie-hellman-group14-sha256

             The list of available key exchange algorithms may also be obtained using "ssh -Q kex".

     KnownHostsCommand
             Specifies a command to use to obtain a list of host keys, in addition to those listed in
             UserKnownHostsFile and GlobalKnownHostsFile.  This command is executed after the files have been
             read.  It may write host key lines to standard output in identical format to the usual files
             (described in the VERIFYING HOST KEYS section in ssh(1)).  Arguments to KnownHostsCommand accept
             the tokens described in the TOKENS section.  The command may be invoked multiple times per
             connection: once when preparing the preference list of host key algorithms to use, again to obtain
             the host key for the requested host name and, if CheckHostIP is enabled, one more time to obtain
             the host key matching the server's address.  If the command exits abnormally or returns a non-zero
             exit status then the connection is terminated.

     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.
             Arguments to LocalCommand accept the tokens described in the TOKENS section.

             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 specifies the listener and may
             be [bind_address:]port or a Unix domain socket path.  The second argument is the destination and
             may be host:hostport or a Unix domain socket path if the remote host supports it.

             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.  Unix domain socket paths may use the tokens described in the TOKENS section and
             environment variables as described in the ENVIRONMENT VARIABLES section.

     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.

     LogVerbose
             Specify one or more overrides to LogLevel.  An override consists of a pattern lists that matches
             the source file, function and line number to force detailed logging for.  For example, an override
             pattern of:

                   kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*

             would enable detailed logging for line 1000 of kex.c, everything in the
             kex_exchange_identification() function, and all code in the packet.c file.  This option is intended
             for debugging and no overrides are enabled by default.

     MACs    Specifies the MAC (message authentication code) algorithms in order of preference.  The MAC
             algorithm is used for data integrity protection.  Multiple algorithms must be comma-separated.  If
             the specified list begins with a ‘+’ character, then the specified algorithms will be appended to
             the default set instead of replacing them.  If the specified list begins with a ‘-’ character, then
             the specified algorithms (including wildcards) will be removed from the default set instead of
             replacing them.  If the specified list begins with a ‘^’ character, then the specified algorithms
             will be placed at the head of the default set.

             The algorithms that contain "-etm" calculate the MAC after encryption (encrypt-then-mac).  These
             are considered safer and their use recommended.

             The default is:

                   umac-64-etm@openssh.com,umac-128-etm@openssh.com,
                   hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
                   hmac-sha1-etm@openssh.com,
                   umac-64@openssh.com,umac-128@openssh.com,
                   hmac-sha2-256,hmac-sha2-512,hmac-sha1

             The list of available MAC algorithms may also be obtained using "ssh -Q mac".

     NoHostAuthenticationForLocalhost
             Disable host authentication for localhost (loopback addresses).  The argument to this keyword must
             be yes or no (the default).

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

     ObscureKeystrokeTiming
             Specifies whether ssh(1) should try to obscure inter-keystroke timings from passive observers of
             network traffic.  If enabled, then for interactive sessions, ssh(1) will send keystrokes at fixed
             intervals of a few tens of milliseconds and will send fake keystroke packets for some time after
             typing ceases.  The argument to this keyword must be yes, no or an interval specifier of the form
             interval:milliseconds (e.g. interval:80 for 80 milliseconds).  The default is to obscure keystrokes
             using a 20ms packet interval.  Note that smaller intervals will result in higher fake keystroke
             packet rates.

     PasswordAuthentication
             Specifies whether to use password authentication.  The argument to this keyword must be yes (the
             default) or no.

     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).

     PermitRemoteOpen
             Specifies the destinations to which remote TCP port forwarding is permitted when RemoteForward is
             used as a SOCKS proxy.  The forwarding specification must be one of the following forms:

                   PermitRemoteOpen host:port
                   PermitRemoteOpen IPv4_addr:port
                   PermitRemoteOpen [IPv6_addr]:port

             Multiple forwards may be specified by separating them with whitespace.  An argument of any can be
             used to remove all restrictions and permit any forwarding requests.  An argument of none can be
             used to prohibit all forwarding requests.  The wildcard ‘*’ can be used for host or port to allow
             all hosts or ports respectively.  Otherwise, no pattern matching or address lookups are performed
             on supplied names.

     PKCS11Provider
             Specifies which PKCS#11 provider to use or none to indicate that no provider should be used (the
             default).  The argument to this keyword is a path to the PKCS#11 shared library ssh(1) should use
             to communicate with a PKCS#11 token providing keys for user authentication.

     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 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

     ProxyCommand
             Specifies the command to use to connect to the server.  The command string extends to the end of
             the line, and is executed using the user's shell ‘exec’ directive to avoid a lingering shell
             process.

             Arguments to ProxyCommand accept the tokens described in the TOKENS section.  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

     ProxyJump
             Specifies one or more jump proxies as either [user@]host[:port] or an ssh URI.  Multiple proxies
             may be separated by comma characters and will be visited sequentially.  Setting this option will
             cause ssh(1) to connect to the target host by first making a ssh(1) connection to the specified
             ProxyJump host and then establishing a TCP forwarding to the ultimate target from there.  Setting
             the host to none disables this option entirely.

             Note that this option will compete with the ProxyCommand option - whichever is specified first will
             prevent later instances of the other from taking effect.

             Note also that the configuration for the destination host (either supplied via the command-line or
             the configuration file) is not generally applied to jump hosts.  ~/.ssh/config should be used if
             specific configuration is required for jump hosts.

     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.

     PubkeyAcceptedAlgorithms
             Specifies the signature algorithms that will be used for public key authentication as a comma-
             separated list of patterns.  If the specified list begins with a ‘+’ character, then the algorithms
             after it will be appended to the default instead of replacing it.  If the specified list begins
             with a ‘-’ character, then the specified algorithms (including wildcards) will be removed from the
             default set instead of replacing them.  If the specified list begins with a ‘^’ character, then the
             specified algorithms will be placed at the head of the default set.  The default for this option
             is:

                ssh-ed25519-cert-v01@openssh.com,
                ecdsa-sha2-nistp256-cert-v01@openssh.com,
                ecdsa-sha2-nistp384-cert-v01@openssh.com,
                ecdsa-sha2-nistp521-cert-v01@openssh.com,
                sk-ssh-ed25519-cert-v01@openssh.com,
                sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
                rsa-sha2-512-cert-v01@openssh.com,
                rsa-sha2-256-cert-v01@openssh.com,
                ssh-ed25519,
                ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
                sk-ssh-ed25519@openssh.com,
                sk-ecdsa-sha2-nistp256@openssh.com,
                rsa-sha2-512,rsa-sha2-256

             The list of available signature algorithms may also be obtained using "ssh -Q
             PubkeyAcceptedAlgorithms".

     PubkeyAuthentication
             Specifies whether to try public key authentication.  The argument to this keyword must be yes (the
             default), no, unbound or host-bound.  The final two options enable public key authentication while
             respectively disabling or enabling the OpenSSH host-bound authentication protocol extension
             required for restricted ssh-agent(1) forwarding.

     RekeyLimit
             Specifies the maximum amount of data that may be transmitted or received before the session key is
             renegotiated, optionally followed by 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 documented 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.

     RemoteCommand
             Specifies a command to execute on the remote 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.
             Arguments to RemoteCommand accept the tokens described in the TOKENS section.

     RemoteForward
             Specifies that a TCP port on the remote machine be forwarded over the secure channel.  The remote
             port may either be forwarded to a specified host and port from the local machine, or may act as a
             SOCKS 4/5 proxy that allows a remote client to connect to arbitrary destinations from the local
             machine.  The first argument is the listening specification and may be [bind_address:]port or, if
             the remote host supports it, a Unix domain socket path.  If forwarding to a specific destination
             then the second argument must be host:hostport or a Unix domain socket path, otherwise if no
             destination argument is specified then the remote forwarding will be established as a SOCKS proxy.
             When acting as a SOCKS proxy, the destination of the connection can be restricted by
             PermitRemoteOpen.

             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.  Unix domain socket paths may
             use the tokens described in the TOKENS section and environment variables as described in the
             ENVIRONMENT VARIABLES section.

             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).

     RequiredRSASize
             Specifies the minimum RSA key size (in bits) that ssh(1) will accept.  User authentication keys
             smaller than this limit will be ignored.  Servers that present host keys smaller than this limit
             will cause the connection to be terminated.  The default is 1024 bits.  Note that this limit may
             only be raised from the default.

     RevokedHostKeys
             Specifies revoked host public keys.  Keys listed in this file will be refused for host
             authentication.  Note that if this file does not exist or is not readable, then host authentication
             will be refused for all hosts.  Keys may be specified as a text file, listing one public key per
             line, or as an OpenSSH Key Revocation List (KRL) as generated by ssh-keygen(1).  For more
             information on KRLs, see the KEY REVOCATION LISTS section in ssh-keygen(1).  Arguments to
             RevokedHostKeys may use the tilde syntax to refer to a user's home directory, the tokens described
             in the TOKENS section and environment variables as described in the ENVIRONMENT VARIABLES section.

     SecurityKeyProvider
             Specifies a path to a library that will be used when loading any FIDO authenticator-hosted keys,
             overriding the default of using the built-in USB HID support.

             If the specified value begins with a ‘$’ character, then it will be treated as an environment
             variable containing the path to the library.

     SendEnv
             Specifies what variables from the local environ(7) should be sent to the server.  The server must
             also support it, and the server must be configured to accept these environment variables.  Note
             that the TERM environment variable is always sent whenever a pseudo-terminal is requested as it is
             required by the protocol.  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.

             See PATTERNS for more information on patterns.

             It is possible to clear previously set SendEnv variable names by prefixing patterns with -.  The
             default is not to send any environment variables.

     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 unresponsive.

             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.

     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, or 300 if the
             BatchMode option is set (Debian-specific).  ProtocolKeepAlives and SetupTimeOut are Debian-specific
             compatibility aliases for this option.

     SessionType
             May be used to either request invocation of a subsystem on the remote system, or to prevent the
             execution of a remote command at all.  The latter is useful for just forwarding ports.  The
             argument to this keyword must be none (same as the -N option), subsystem (same as the -s option) or
             default (shell or command execution).

     SetEnv  Directly specify one or more environment variables and their contents to be sent to the server.
             Similarly to SendEnv, with the exception of the TERM variable, the server must be prepared to
             accept the environment variable.

     StdinNull
             Redirects stdin from /dev/null (actually, prevents reading from stdin).  Either this or the
             equivalent -n option must be used when ssh is run in the background.  The argument to this keyword
             must be yes (same as the -n option) or no (the default).

     StreamLocalBindMask
             Sets the octal file creation mode mask (umask) used when creating a Unix-domain socket file for
             local or remote port forwarding.  This option is only used for port forwarding to a Unix-domain
             socket file.

             The default value is 0177, which creates a Unix-domain socket file that is readable and writable
             only by the owner.  Note that not all operating systems honor the file mode on Unix-domain socket
             files.

     StreamLocalBindUnlink
             Specifies whether to remove an existing Unix-domain socket file for local or remote port forwarding
             before creating a new one.  If the socket file already exists and StreamLocalBindUnlink is not
             enabled, ssh will be unable to forward the port to the Unix-domain socket file.  This option is
             only used for port forwarding to a Unix-domain socket file.

             The argument must be yes or no (the default).

     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 man-in-the-middle (MITM) 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 accept-new then ssh will automatically add new host keys to the user's
             known_hosts file, but will not permit connections to hosts with changed host keys.  If this flag is
             set to no or off, ssh will automatically add new host keys to the user known hosts files and allow
             connections to hosts with changed hostkeys to proceed, subject to some restrictions.  If this flag
             is set to ask (the default), 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.

     SyslogFacility
             Gives the facility code that is used when logging messages from ssh(1).  The possible values are:
             DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.  The default is
             USER.

     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.  See also ServerAliveInterval for
             protocol-level keepalives.

     Tag     Specify a configuration tag name that may be later used by a Match directive to select a block of
             configuration.

     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 (the default).  Specifying yes requests the
             default tunnel mode, which is point-to-point.

     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.

     UpdateHostKeys
             Specifies whether ssh(1) should accept notifications of additional hostkeys from the server sent
             after authentication has completed and add them to UserKnownHostsFile.  The argument must be yes,
             no or ask.  This option allows learning alternate hostkeys for a server and supports graceful key
             rotation by allowing a server to send replacement public keys before old ones are removed.

             Additional hostkeys are only accepted if the key used to authenticate the host was already trusted
             or explicitly accepted by the user, the host was authenticated via UserKnownHostsFile (i.e. not
             GlobalKnownHostsFile) and the host was authenticated using a plain key and not a certificate.

             UpdateHostKeys is enabled by default if the user has not overridden the default UserKnownHostsFile
             setting and has not enabled VerifyHostKeyDNS, otherwise UpdateHostKeys will be set to no.

             If UpdateHostKeys is set to ask, then the user is asked to confirm the modifications to the
             known_hosts file.  Confirmation is currently incompatible with ControlPersist, and will be disabled
             if it is enabled.

             Presently, only sshd(8) from OpenSSH 6.8 and greater support the "hostkeys@openssh.com" protocol
             extension used to inform the client of all the server's hostkeys.

     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.  Each
             filename may use tilde notation to refer to the user's home directory, the tokens described in the
             TOKENS section and environment variables as described in the ENVIRONMENT VARIABLES section.  A
             value of none causes ssh(1) to ignore any user-specific known hosts files.  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 default is no.

             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 fingerprint string at login and for unknown host keys.  If this flag is
             set to no (the default), no fingerprint strings are printed at login and only the fingerprint
             string will be printed for unknown host keys.

     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
     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 organization except from the "dialup" pool, the following entry (in authorized_keys) could be used:

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

     Note that a negated match will never produce a positive result by itself.  For example, attempting to match
     "host3" against the following pattern-list will fail:

           from="!host1,!host2"

     The solution here is to include a term that will yield a positive match, such as a wildcard:

           from="!host1,!host2,*"

TOKENS

     Arguments to some keywords can make use of tokens, which are expanded at runtime:

           %%    A literal ‘%’.
           %C    Hash of %l%h%p%r%j.
           %d    Local user's home directory.
           %f    The fingerprint of the server's host key.
           %H    The known_hosts hostname or address that is being searched for.
           %h    The remote hostname.
           %I    A string describing the reason for a KnownHostsCommand execution: either ADDRESS when looking
                 up a host by address (only when CheckHostIP is enabled), HOSTNAME when searching by hostname,
                 or ORDER when preparing the host key algorithm preference list to use for the destination host.
           %i    The local user ID.
           %j    The contents of the ProxyJump option, or the empty string if this option is unset.
           %K    The base64 encoded host key.
           %k    The host key alias if specified, otherwise the original remote hostname given on the command
                 line.
           %L    The local hostname.
           %l    The local hostname, including the domain name.
           %n    The original remote hostname, as given on the command line.
           %p    The remote port.
           %r    The remote username.
           %T    The local tun(4) or tap(4) network interface assigned if tunnel forwarding was requested, or
                 "NONE" otherwise.
           %t    The type of the server host key, e.g.  ssh-ed25519.
           %u    The local username.

     CertificateFile, ControlPath, IdentityAgent, IdentityFile, KnownHostsCommand, LocalForward, Match exec,
     RemoteCommand, RemoteForward, RevokedHostKeys, and UserKnownHostsFile accept the tokens %%, %C, %d, %h, %i,
     %j, %k, %L, %l, %n, %p, %r, and %u.

     KnownHostsCommand additionally accepts the tokens %f, %H, %I, %K and %t.

     Hostname accepts the tokens %% and %h.

     LocalCommand accepts all tokens.

     ProxyCommand and ProxyJump accept the tokens %%, %h, %n, %p, and %r.

     Note that some of these directives build commands for execution via the shell.  Because ssh(1) performs no
     filtering or escaping of characters that have special meaning in shell commands (e.g. quotes), it is the
     user's responsibility to ensure that the arguments passed to ssh(1) do not contain such characters and that
     tokens are appropriately quoted when used.

ENVIRONMENT VARIABLES

     Arguments to some keywords can be expanded at runtime from environment variables on the client by enclosing
     them in ${}, for example ${HOME}/.ssh would refer to the user's .ssh directory.  If a specified environment
     variable does not exist then an error will be returned and the setting for that keyword will be ignored.

     The keywords CertificateFile, ControlPath, IdentityAgent, IdentityFile, KnownHostsCommand, and
     UserKnownHostsFile support environment variables.  The keywords LocalForward and RemoteForward support
     environment variables only for Unix domain socket paths.

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 writable by others.  It may be group-writable
             provided that the group in question 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.  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.