xenial (5) ssh_config.5.gz

Provided by: openssh-client_7.2p2-4ubuntu2.10_amd64 bug

NAME

     ssh_config — OpenSSH SSH client configuration files

SYNOPSIS

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

DESCRIPTION

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

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

     For each parameter, the first obtained value will be used.  The configuration files contain sections
     separated by “Host” specifications, and that section is only applied for hosts that match one of the
     patterns given in the specification.  The matched host name is 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):

              SendEnv LANG LC_*
              HashKnownHosts yes
              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 specifying configuration
     options using the ssh, scp, and sftp -o option.  Arguments may optionally be enclosed in double quotes (")
     in order to represent arguments containing spaces.

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

     Host    Restricts the following declarations (up to the next Host 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 option 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, exec, host, originalhost, user, and localuser.  The all criteria must appear alone
             or immediately after canonical.  Other criteria may be combined arbitrarily.  All criteria but all
             and canonical 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 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.  The following character sequences in the
             command will be expanded prior to execution: ‘%L’ will be substituted by the first component of the
             local host name, ‘%l’ will be substituted by the local host name (including any domain name), ‘%h’
             will be substituted by the target host name, ‘%n’ will be substituted by the original target host
             name specified on the command-line, ‘%p’ the 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-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 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 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.  The
             argument must be “yes”, “confirm”, “ask”, or “no”.  The default is “no”.

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

     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 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, 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.

     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.

     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.

             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 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.

     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 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 check will not be executed.  The default
             is “yes”.

     Cipher  Specifies the cipher to use for encrypting the session in protocol version 1.  Currently,
             “blowfish”, “3des”, and “des” are supported.  des is only supported in the ssh(1) client for
             interoperability with legacy protocol 1 implementations that do not support the 3des cipher.  Its
             use is strongly discouraged due to cryptographic weaknesses.  The default is “3des”.

     Ciphers
             Specifies the ciphers allowed for protocol version 2 in order of preference.  Multiple ciphers must
             be comma-separated.  If the specified value begins with a ‘+’ character, then the specified ciphers
             will be appended to the default set instead of replacing them.

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

             The list of available ciphers may also be obtained using the -Q option of ssh(1) with an argument
             of “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 is “no”.

     Compression
             Specifies whether to use compression.  The argument must be “yes” or “no”.  The default is “no”.

     CompressionLevel
             Specifies the compression level to use if compression is enabled.  The argument must be an integer
             from 1 (fast) to 9 (slow, best).  The default level is 6, which is good for most applications.  The
             meaning of the values is the same as in gzip(1).  Note that this option applies to protocol version
             1 only.

     ConnectionAttempts
             Specifies the number of tries (one per second) to make before exiting.  The argument must be an
             integer.  This may be useful in scripts if the connection sometimes fails.  The default is 1.

     ConnectTimeout
             Specifies the timeout (in seconds) used when connecting to the SSH server, instead of using the
             default system TCP timeout.  This value is used only when the target is down or really unreachable,
             not when it refuses the connection.

     ControlMaster
             Enables the sharing of multiple sessions over a single network connection.  When set to “yes”,
             ssh(1) will listen for connections on a control socket specified using the ControlPath argument.
             Additional sessions can connect to this socket using the same ControlPath with ControlMaster set to
             “no” (the default).  These sessions will try to reuse the master instance's network connection
             rather than initiating new ones, but will fall back to connecting normally if the control socket
             does not exist, or is not listening.

             Setting this to “ask” will cause ssh to listen for control connections, but require confirmation
             using ssh-askpass(1).  If the ControlPath cannot be opened, ssh will continue without connecting to
             a master instance.

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

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

     ControlPath
             Specify the path to the control socket used for connection sharing as described in the
             ControlMaster section above or the string “none” to disable connection sharing.  In the path, ‘%L’
             will be substituted by the first component of the local host name, ‘%l’ will be substituted by the
             local host name (including any domain name), ‘%h’ will be substituted by the target host name, ‘%n’
             will be substituted by the original target host name specified on the command line, ‘%p’ the
             destination port, ‘%r’ by the remote login username, ‘%u’ by the username and ‘%i’ by the numeric
             user ID (uid) of the user running ssh(1), and ‘%C’ by a hash of the concatenation: %l%h%p%r.  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”, 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(1) “-O exit” option).  If set to a time in seconds, or a time in
             any of the formats documented in sshd_config(5), then the backgrounded master connection will
             automatically terminate after it has remained idle (with no client connections) for the specified
             time.

     DynamicForward
             Specifies that a TCP port on the local machine be forwarded over the secure channel, and the
             application protocol is then used to determine where to connect to from the remote machine.

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

             Currently the SOCKS4 and SOCKS5 protocols are supported, and ssh(1) will act as a SOCKS server.
             Multiple forwardings may be specified, and additional forwardings can be given on the command line.
             Only the superuser can forward privileged ports.

     EnableSSHKeysign
             Setting this option to “yes” in the global client configuration file /etc/ssh/ssh_config enables
             the use of the helper program ssh-keysign(8) during HostbasedAuthentication.  The argument must be
             “yes” or “no”.  The default is “no”.  This option should be placed in the non-hostspecific section.
             See ssh-keysign(8) for more information.

     EscapeChar
             Sets the escape character (default: ‘~’).  The escape character can also be set on the command
             line.  The argument should be a single character, ‘^’ followed by a letter, or “none” to disable
             the escape character entirely (making the connection transparent for binary data).

     ExitOnForwardFailure
             Specifies whether ssh(1) should terminate the connection if it cannot set up all requested dynamic,
             tunnel, local, and remote port forwardings, (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 is “no”.

     FingerprintHash
             Specifies the hash algorithm used when displaying key fingerprints.  Valid options are: “md5” and
             “sha256”.  The default is “sha256”.

     ForwardAgent
             Specifies whether the connection to the authentication agent (if any) will be forwarded to the
             remote machine.  The argument must be “yes” or “no”.  The default is “no”.

             Agent forwarding should be enabled with caution.  Users with the ability to bypass file permissions
             on the remote host (for the agent's Unix-domain socket) can access the local agent through the
             forwarded connection.  An attacker cannot obtain key material from the agent, however they can
             perform operations on the keys that enable them to authenticate using the identities loaded into
             the agent.

     ForwardX11
             Specifies whether X11 connections will be automatically redirected over the secure channel and
             DISPLAY set.  The argument must be “yes” or “no”.  The default is “no”.

             X11 forwarding should be enabled with caution.  Users with the ability to bypass file permissions
             on the remote host (for the user's X11 authorization database) can access the local X11 display
             through the forwarded connection.  An attacker may then be able to perform activities such as
             keystroke monitoring if the ForwardX11Trusted option is also enabled.

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

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

             If this option is set to “no”, remote X11 clients will be considered untrusted and prevented from
             stealing or tampering with data belonging to trusted X11 clients.  Furthermore, the xauth(1) token
             used for the session will be set to expire after 20 minutes.  Remote clients will be refused access
             after this time.

             The default is “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 specify that ssh should bind local port
             forwardings to the wildcard address, thus allowing remote hosts to connect to forwarded ports.  The
             argument must be “yes” or “no”.  The default is “no”.

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

     GSSAPIAuthentication
             Specifies whether user authentication based on GSSAPI is allowed.  The default is “no”.

     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”.

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

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

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

     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”.

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

     HostbasedKeyTypes
             Specifies the key types that will be used for hostbased authentication as a comma-separated pattern
             list.  Alternately if the specified value begins with a ‘+’ character, then the specified key types
             will be appended to the default set instead of replacing them.  The default for this option is:

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

             The -Q option of ssh(1) may be used to list supported key types.

     HostKeyAlgorithms
             Specifies the host key algorithms that the client wants to use in order of preference.  Alternately
             if the specified value begins with a ‘+’ character, then the specified key types will be appended
             to the default set instead of replacing them.  The default for this option is:

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

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

             The list of available key types may also be obtained using the -Q option of ssh(1) with an argument
             of “key”.

     HostKeyAlias
             Specifies an alias that should be used instead of the real host name when looking up or saving the
             host key in the host key database files.  This option is useful for tunneling SSH connections or
             for multiple servers running on a single host.

     HostName
             Specifies the real host name to log into.  This can be used to specify nicknames or abbreviations
             for hosts.  If the hostname contains the character sequence ‘%h’, then this will be replaced with
             the host name specified on the command line (this is useful for manipulating unqualified names).
             The character sequence ‘%%’ will be replaced by a single ‘%’ character, which may be used when
             specifying IPv6 link-local addresses.

             The default is the name given on the command line.  Numeric IP addresses are also permitted (both
             on the command line and in HostName specifications).

     IdentitiesOnly
             Specifies that ssh(1) should only use the authentication identity and certificate files explicitly
             configured in the ssh_config files or passed on the ssh(1) command-line, 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.  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.

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

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

             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.

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

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

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

     KexAlgorithms
             Specifies the available KEX (Key Exchange) algorithms.  Multiple algorithms must be comma-
             separated.  Alternately if the specified value begins with a ‘+’ character, then the specified
             methods will be appended to the default set instead of replacing them.  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

             The list of available key exchange algorithms may also be obtained using the -Q option of ssh(1)
             with an argument of “kex”.

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

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

             This directive is ignored unless PermitLocalCommand has been enabled.

     LocalForward
             Specifies that a TCP port on the local machine be forwarded over the secure channel to the
             specified host and port from the remote machine.  The first argument must be [bind_address:]port
             and the second argument must be host:hostport.  IPv6 addresses can be specified by enclosing
             addresses in square brackets.  Multiple forwardings may be specified, and additional forwardings
             can be given on the command line.  Only the superuser can forward privileged ports.  By default,
             the local port is bound in accordance with the GatewayPorts setting.  However, an explicit
             bind_address may be used to bind the connection to a specific address.  The bind_address of
             “localhost” indicates that the listening port be bound for local use only, while an empty address
             or ‘*’ indicates that the port should be available from all interfaces.

     LogLevel
             Gives the verbosity level that is used when logging messages from ssh(1).  The possible values are:
             QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.  The default is INFO.  DEBUG
             and DEBUG1 are equivalent.  DEBUG2 and DEBUG3 each specify higher levels of verbose output.

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

             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 the -Q option of ssh(1) with an
             argument of “mac”.

     NoHostAuthenticationForLocalhost
             This option can be used if the home directory is shared across machines.  In this case localhost
             will refer to a different machine on each of the machines and the user will get many warnings about
             changed host keys.  However, this option disables host authentication for localhost.  The argument
             to this keyword must be “yes” or “no”.  The default is to check the host key for localhost.

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

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

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

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

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

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

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

     Protocol
             Specifies the protocol versions ssh(1) should support in order of preference.  The possible values
             are ‘1’ and ‘2’.  Multiple versions must be comma-separated.  When this option is set to “2,1” ssh
             will try version 2 and fall back to version 1 if version 2 is not available.  The default is ‘2’.
             Protocol 1 suffers from a number of cryptographic weaknesses and should not be used.  It is only
             offered to support legacy devices.

     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.

             In the command string, any occurrence of ‘%h’ will be substituted by the host name to connect, ‘%p’
             by the port, and ‘%r’ by the remote user name.  The command can be basically anything, and should
             read from its standard input and write to its standard output.  It should eventually connect an
             sshd(8) server running on some machine, or execute sshd -i somewhere.  Host key management will be
             done using the HostName of the host being connected (defaulting to the name typed by the user).
             Setting the command to “none” disables this option entirely.  Note that CheckHostIP is not
             available for connects with a proxy command.

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

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

     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”.

     PubkeyAcceptedKeyTypes
             Specifies the key types that will be used for public key authentication as a comma-separated
             pattern list.  Alternately if the specified value begins with a ‘+’ character, then the key types
             after it will be appended to the default instead of replacing it.  The default for this option is:

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

             The -Q option of ssh(1) may be used to list supported key types.

     PubkeyAuthentication
             Specifies whether to try public key authentication.  The argument to this keyword must be “yes” or
             “no”.  The default is “yes”.

     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 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.

     RemoteForward
             Specifies that a TCP port on the remote machine be forwarded over the secure channel to the
             specified host and port from the local machine.  The first argument must be [bind_address:]port and
             the second argument must be host:hostport.  IPv6 addresses can be specified by enclosing addresses
             in square brackets.  Multiple forwardings may be specified, and additional forwardings can be given
             on the command line.  Privileged ports can be forwarded only when logging in as root on the remote
             machine.

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

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

     RequestTTY
             Specifies whether to request a pseudo-tty for the session.  The argument may be one of: “no” (never
             request a TTY), “yes” (always request a TTY when standard input is a TTY), “force” (always request
             a TTY) or “auto” (request a TTY when opening a login session).  This option mirrors the -t and -T
             flags for ssh(1).

     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).

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

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

     SendEnv
             Specifies what variables from the local environ(7) should be sent to the server.  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.  The default
             is not to send any environment variables.

             See PATTERNS for more information on patterns.

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

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

     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.  ProtocolKeepAlives and SetupTimeOut are Debian-specific compatibility
             aliases for this option.

     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 is “no”.

     StrictHostKeyChecking
             If this flag is set to “yes”, ssh(1) will never automatically add host keys to the
             ~/.ssh/known_hosts file, and refuses to connect to hosts whose host key has changed.  This provides
             maximum protection against trojan horse attacks, though it can be annoying when the
             /etc/ssh/ssh_known_hosts file is poorly maintained or when connections to new hosts are frequently
             made.  This option forces the user to manually add all new hosts.  If this flag is set to “no”, ssh
             will automatically add new host keys to the user known hosts files.  If this flag is set to “ask”,
             new host keys will be added to the user known host files only after the user has confirmed that is
             what they really want to do, and ssh will refuse to connect to hosts whose host key has changed.
             The host keys of known hosts will be verified automatically in all cases.  The argument must be
             “yes”, “no”, or “ask”.  The default is “ask”.

     TCPKeepAlive
             Specifies whether the system should send TCP keepalive messages to the other side.  If they are
             sent, death of the connection or crash of one of the machines will be properly noticed.  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”.

     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” (the default) or “ask”.  Enabling 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.  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.

     UsePrivilegedPort
             Specifies whether to use a privileged port for outgoing connections.  The argument must be “yes” or
             “no”.  The default is “no”.  If set to “yes”, ssh(1) must be setuid root.  Note that this option
             must be set to “yes” for RhostsRSAAuthentication with older servers.

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

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

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

             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”, no fingerprint strings are printed at login and only the 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
     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"

FILES

     ~/.ssh/config
             This is the per-user configuration file.  The format of this file is described above.  This file is
             used by the SSH client.  Because of the potential for abuse, this file must have strict
             permissions: read/write for the user, and not accessible by others.  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.