IABSD.fr/src/usr.bin/ssh/ssh_config.5

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• Author : djm
  Date : 2021-11-10 06:29:25
  Hash : 2eee802d
  Message : add the sntrup761x25519-sha512@openssh.com hybrid ECDH/x25519 + Streamlined NTRU Prime post-quantum KEX to the default KEXAlgorithms list (after the ECDH methods but before the prime-group DH ones). ok markus@

• usr.bin/ssh/ssh_config.5

• .\"
  .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
  .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
  .\"                    All rights reserved
  .\"
  .\" As far as I am concerned, the code I have written for this software
  .\" can be used freely for any purpose.  Any derived versions of this
  .\" software must be clearly marked as such, and if the derived work is
  .\" incompatible with the protocol description in the RFC file, it must be
  .\" called by a name other than "ssh" or "Secure Shell".
  .\"
  .\" Copyright (c) 1999,2000 Markus Friedl.  All rights reserved.
  .\" Copyright (c) 1999 Aaron Campbell.  All rights reserved.
  .\" Copyright (c) 1999 Theo de Raadt.  All rights reserved.
  .\"
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  .\" $OpenBSD: ssh_config.5,v 1.367 2021/11/10 06:29:25 djm Exp $
  .Dd $Mdocdate: November 10 2021 $
  .Dt SSH_CONFIG 5
  .Os
  .Sh NAME
  .Nm ssh_config
  .Nd OpenSSH client configuration file
  .Sh DESCRIPTION
  .Xr ssh 1
  obtains configuration data from the following sources in
  the following order:
  .Pp
  .Bl -enum -offset indent -compact
  .It
  command-line options
  .It
  user's configuration file
  .Pq Pa ~/.ssh/config
  .It
  system-wide configuration file
  .Pq Pa /etc/ssh/ssh_config
  .El
  .Pp
  For each parameter, the first obtained value
  will be used.
  The configuration files contain sections separated by
  .Cm 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
  .Cm CanonicalizeHostname
  option for exceptions).
  .Pp
  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.
  .Pp
  The file contains keyword-argument pairs, one per line.
  Lines starting with
  .Ql #
  and empty lines are interpreted as comments.
  Arguments may optionally be enclosed in double quotes
  .Pq \&"
  in order to represent arguments containing spaces.
  Configuration options may be separated by whitespace or
  optional whitespace and exactly one
  .Ql = ;
  the latter format is useful to avoid the need to quote whitespace
  when specifying configuration options using the
  .Nm ssh ,
  .Nm scp ,
  and
  .Nm sftp
  .Fl o
  option.
  .Pp
  The possible
  keywords and their meanings are as follows (note that
  keywords are case-insensitive and arguments are case-sensitive):
  .Bl -tag -width Ds
  .It Cm Host
  Restricts the following declarations (up to the next
  .Cm Host
  or
  .Cm 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
  .Ql *
  as a pattern can be used to provide global
  defaults for all hosts.
  The host is usually the
  .Ar hostname
  argument given on the command line
  (see the
  .Cm CanonicalizeHostname
  keyword for exceptions).
  .Pp
  A pattern entry may be negated by prefixing it with an exclamation mark
  .Pq Sq !\& .
  If a negated entry is matched, then the
  .Cm 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.
  .Pp
  See
  .Sx PATTERNS
  for more information on patterns.
  .It Cm Match
  Restricts the following declarations (up to the next
  .Cm Host
  or
  .Cm Match
  keyword) to be used only when the conditions following the
  .Cm Match
  keyword are satisfied.
  Match conditions are specified using one or more criteria
  or the single token
  .Cm all
  which always matches.
  The available criteria keywords are:
  .Cm canonical ,
  .Cm final ,
  .Cm exec ,
  .Cm host ,
  .Cm originalhost ,
  .Cm user ,
  and
  .Cm localuser .
  The
  .Cm all
  criteria must appear alone or immediately after
  .Cm canonical
  or
  .Cm final .
  Other criteria may be combined arbitrarily.
  All criteria but
  .Cm all ,
  .Cm canonical ,
  and
  .Cm final
  require an argument.
  Criteria may be negated by prepending an exclamation mark
  .Pq Sq !\& .
  .Pp
  The
  .Cm canonical
  keyword matches only when the configuration file is being re-parsed
  after hostname canonicalization (see the
  .Cm CanonicalizeHostname
  option).
  This may be useful to specify conditions that work with canonical host
  names only.
  .Pp
  The
  .Cm final
  keyword requests that the configuration be re-parsed (regardless of whether
  .Cm CanonicalizeHostname
  is enabled), and matches only during this final pass.
  If
  .Cm CanonicalizeHostname
  is enabled, then
  .Cm canonical
  and
  .Cm final
  match during the same pass.
  .Pp
  The
  .Cm 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
  .Cm exec
  accept the tokens described in the
  .Sx TOKENS
  section.
  .Pp
  The other keywords' criteria must be single entries or comma-separated
  lists and may use the wildcard and negation operators described in the
  .Sx PATTERNS
  section.
  The criteria for the
  .Cm host
  keyword are matched against the target hostname, after any substitution
  by the
  .Cm Hostname
  or
  .Cm CanonicalizeHostname
  options.
  The
  .Cm originalhost
  keyword matches against the hostname as it was specified on the command-line.
  The
  .Cm user
  keyword matches against the target username on the remote host.
  The
  .Cm localuser
  keyword matches against the name of the local user running
  .Xr ssh 1
  (this keyword may be useful in system-wide
  .Nm
  files).
  .It Cm AddKeysToAgent
  Specifies whether keys should be automatically added to a running
  .Xr ssh-agent 1 .
  If this option is set to
  .Cm 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
  .Xr ssh-add 1 .
  If this option is set to
  .Cm ask ,
  .Xr ssh 1
  will require confirmation using the
  .Ev SSH_ASKPASS
  program before adding a key (see
  .Xr ssh-add 1
  for details).
  If this option is set to
  .Cm confirm ,
  each use of the key must be confirmed, as if the
  .Fl c
  option was specified to
  .Xr ssh-add 1 .
  If this option is set to
  .Cm no ,
  no keys are added to the agent.
  Alternately, this option may be specified as a time interval
  using the format described in the
  .Sx TIME FORMATS
  section of
  .Xr sshd_config 5
  to specify the key's lifetime in
  .Xr ssh-agent 1 ,
  after which it will automatically be removed.
  The argument must be
  .Cm no
  (the default),
  .Cm yes ,
  .Cm confirm
  (optionally followed by a time interval),
  .Cm ask
  or a time interval.
  .It Cm AddressFamily
  Specifies which address family to use when connecting.
  Valid arguments are
  .Cm any
  (the default),
  .Cm inet
  (use IPv4 only), or
  .Cm inet6
  (use IPv6 only).
  .It Cm BatchMode
  If set to
  .Cm yes ,
  user interaction such as password prompts and host key confirmation requests
  will be disabled.
  This option is useful in scripts and other batch jobs where no user
  is present to interact with
  .Xr ssh 1 .
  The argument must be
  .Cm yes
  or
  .Cm no
  (the default).
  .It Cm 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.
  .It Cm BindInterface
  Use the address of the specified interface on the local machine as the
  source address of the connection.
  .It Cm CanonicalDomains
  When
  .Cm CanonicalizeHostname
  is enabled, this option specifies the list of domain suffixes in which to
  search for the specified destination host.
  .It Cm CanonicalizeFallbackLocal
  Specifies whether to fail with an error when hostname canonicalization fails.
  The default,
  .Cm yes ,
  will attempt to look up the unqualified hostname using the system resolver's
  search rules.
  A value of
  .Cm no
  will cause
  .Xr ssh 1
  to fail instantly if
  .Cm CanonicalizeHostname
  is enabled and the target hostname cannot be found in any of the domains
  specified by
  .Cm CanonicalDomains .
  .It Cm CanonicalizeHostname
  Controls whether explicit hostname canonicalization is performed.
  The default,
  .Cm no ,
  is not to perform any name rewriting and let the system resolver handle all
  hostname lookups.
  If set to
  .Cm yes
  then, for connections that do not use a
  .Cm ProxyCommand
  or
  .Cm ProxyJump ,
  .Xr ssh 1
  will attempt to canonicalize the hostname specified on the command line
  using the
  .Cm CanonicalDomains
  suffixes and
  .Cm CanonicalizePermittedCNAMEs
  rules.
  If
  .Cm CanonicalizeHostname
  is set to
  .Cm always ,
  then canonicalization is applied to proxied connections too.
  .Pp
  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
  .Cm Host
  and
  .Cm Match
  stanzas.
  A value of
  .Cm none
  disables the use of a
  .Cm ProxyJump
  host.
  .It Cm 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).
  .It Cm CanonicalizePermittedCNAMEs
  Specifies rules to determine whether CNAMEs should be followed when
  canonicalizing hostnames.
  The rules consist of one or more arguments of
  .Ar source_domain_list : Ns Ar target_domain_list ,
  where
  .Ar source_domain_list
  is a pattern-list of domains that may follow CNAMEs in canonicalization,
  and
  .Ar target_domain_list
  is a pattern-list of domains that they may resolve to.
  .Pp
  For example,
  .Qq *.a.example.com:*.b.example.com,*.c.example.com
  will allow hostnames matching
  .Qq *.a.example.com
  to be canonicalized to names in the
  .Qq *.b.example.com
  or
  .Qq *.c.example.com
  domains.
  .Pp
  A single argument of
  .Qq none
  causes no CNAMEs to be considered for canonicalization.
  This is the default behaviour.
  .It Cm CASignatureAlgorithms
  Specifies which algorithms are allowed for signing of certificates
  by certificate authorities (CAs).
  The default is:
  .Bd -literal -offset indent
  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
  .Ed
  .Pp
  If the specified list begins with a
  .Sq +
  character, then the specified algorithms will be appended to the default set
  instead of replacing them.
  If the specified list begins with a
  .Sq -
  character, then the specified algorithms (including wildcards) will be removed
  from the default set instead of replacing them.
  .Pp
  .Xr ssh 1
  will not accept host certificates signed using algorithms other than those
  specified.
  .It Cm 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
  .Cm IdentityFile
  directive or
  .Fl i
  flag to
  .Xr ssh 1 ,
  via
  .Xr ssh-agent 1 ,
  or via a
  .Cm PKCS11Provider
  or
  .Cm SecurityKeyProvider .
  .Pp
  Arguments to
  .Cm CertificateFile
  may use the tilde syntax to refer to a user's home directory,
  the tokens described in the
  .Sx TOKENS
  section and environment variables as described in the
  .Sx ENVIRONMENT VARIABLES
  section.
  .Pp
  It is possible to have multiple certificate files specified in
  configuration files; these certificates will be tried in sequence.
  Multiple
  .Cm CertificateFile
  directives will add to the list of certificates used for
  authentication.
  .It Cm CheckHostIP
  If set to
  .Cm yes
  .Xr ssh 1
  will additionally check the host IP address in the
  .Pa 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
  .Pa ~/.ssh/known_hosts
  in the process, regardless of the setting of
  .Cm StrictHostKeyChecking .
  If the option is set to
  .Cm no
  (the default),
  the check will not be executed.
  .It Cm Ciphers
  Specifies the ciphers allowed and their order of preference.
  Multiple ciphers must be comma-separated.
  If the specified list begins with a
  .Sq +
  character, then the specified ciphers will be appended to the default set
  instead of replacing them.
  If the specified list begins with a
  .Sq -
  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
  .Sq ^
  character, then the specified ciphers will be placed at the head of the
  default set.
  .Pp
  The supported ciphers are:
  .Bd -literal -offset indent
  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
  .Ed
  .Pp
  The default is:
  .Bd -literal -offset indent
  chacha20-poly1305@openssh.com,
  aes128-ctr,aes192-ctr,aes256-ctr,
  aes128-gcm@openssh.com,aes256-gcm@openssh.com
  .Ed
  .Pp
  The list of available ciphers may also be obtained using
  .Qq ssh -Q cipher .
  .It Cm 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
  .Xr ssh 1
  command line to clear port forwardings set in
  configuration files, and is automatically set by
  .Xr scp 1
  and
  .Xr sftp 1 .
  The argument must be
  .Cm yes
  or
  .Cm no
  (the default).
  .It Cm Compression
  Specifies whether to use compression.
  The argument must be
  .Cm yes
  or
  .Cm no
  (the default).
  .It Cm 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.
  .It Cm 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.
  .It Cm ControlMaster
  Enables the sharing of multiple sessions over a single network connection.
  When set to
  .Cm yes ,
  .Xr ssh 1
  will listen for connections on a control socket specified using the
  .Cm ControlPath
  argument.
  Additional sessions can connect to this socket using the same
  .Cm ControlPath
  with
  .Cm ControlMaster
  set to
  .Cm 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.
  .Pp
  Setting this to
  .Cm ask
  will cause
  .Xr ssh 1
  to listen for control connections, but require confirmation using
  .Xr ssh-askpass 1 .
  If the
  .Cm ControlPath
  cannot be opened,
  .Xr ssh 1
  will continue without connecting to a master instance.
  .Pp
  X11 and
  .Xr 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.
  .Pp
  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:
  .Cm auto
  and
  .Cm autoask .
  The latter requires confirmation like the
  .Cm ask
  option.
  .It Cm ControlPath
  Specify the path to the control socket used for connection sharing as described
  in the
  .Cm ControlMaster
  section above or the string
  .Cm none
  to disable connection sharing.
  Arguments to
  .Cm ControlPath
  may use the tilde syntax to refer to a user's home directory,
  the tokens described in the
  .Sx TOKENS
  section and environment variables as described in the
  .Sx ENVIRONMENT VARIABLES
  section.
  It is recommended that any
  .Cm 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.
  .It Cm ControlPersist
  When used in conjunction with
  .Cm 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
  .Cm 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
  .Cm yes
  or 0,
  then the master connection will remain in the background indefinitely
  (until killed or closed via a mechanism such as the
  .Qq ssh -O exit ) .
  If set to a time in seconds, or a time in any of the formats documented in
  .Xr sshd_config 5 ,
  then the backgrounded master connection will automatically terminate
  after it has remained idle (with no client connections) for the
  specified time.
  .It Cm 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.
  .Pp
  The argument must be
  .Sm off
  .Oo Ar bind_address : Oc Ar port .
  .Sm on
  IPv6 addresses can be specified by enclosing addresses in square brackets.
  By default, the local port is bound in accordance with the
  .Cm GatewayPorts
  setting.
  However, an explicit
  .Ar bind_address
  may be used to bind the connection to a specific address.
  The
  .Ar bind_address
  of
  .Cm localhost
  indicates that the listening port be bound for local use only, while an
  empty address or
  .Sq *
  indicates that the port should be available from all interfaces.
  .Pp
  Currently the SOCKS4 and SOCKS5 protocols are supported, and
  .Xr 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.
  .It Cm EnableSSHKeysign
  Setting this option to
  .Cm yes
  in the global client configuration file
  .Pa /etc/ssh/ssh_config
  enables the use of the helper program
  .Xr ssh-keysign 8
  during
  .Cm HostbasedAuthentication .
  The argument must be
  .Cm yes
  or
  .Cm no
  (the default).
  This option should be placed in the non-hostspecific section.
  See
  .Xr ssh-keysign 8
  for more information.
  .It Cm EscapeChar
  Sets the escape character (default:
  .Ql ~ ) .
  The escape character can also
  be set on the command line.
  The argument should be a single character,
  .Ql ^
  followed by a letter, or
  .Cm none
  to disable the escape
  character entirely (making the connection transparent for binary
  data).
  .It Cm ExitOnForwardFailure
  Specifies whether
  .Xr 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
  .Cm ExitOnForwardFailure
  does not apply to connections made over port forwardings and will not,
  for example, cause
  .Xr ssh 1
  to exit if TCP connections to the ultimate forwarding destination fail.
  The argument must be
  .Cm yes
  or
  .Cm no
  (the default).
  .It Cm FingerprintHash
  Specifies the hash algorithm used when displaying key fingerprints.
  Valid options are:
  .Cm md5
  and
  .Cm sha256
  (the default).
  .It Cm ForkAfterAuthentication
  Requests
  .Nm ssh
  to go to background just before command execution.
  This is useful if
  .Nm ssh
  is going to ask for passwords or passphrases, but the user
  wants it in the background.
  This implies the
  .Cm StdinNull
  configuration option being set to
  .Dq yes .
  The recommended way to start X11 programs at a remote site is with
  something like
  .Ic ssh -f host xterm ,
  which is the same as
  .Ic ssh host xterm
  if the
  .Cm ForkAfterAuthentication
  configuration option is set to
  .Dq yes .
  .Pp
  If the
  .Cm ExitOnForwardFailure
  configuration option is set to
  .Dq yes ,
  then a client started with the
  .Cm ForkAfterAuthentication
  configuration option being set to
  .Dq 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
  .Cm yes
  (same as the
  .Fl f
  option) or
  .Cm no
  (the default).
  .It Cm ForwardAgent
  Specifies whether the connection to the authentication agent (if any)
  will be forwarded to the remote machine.
  The argument may be
  .Cm yes ,
  .Cm no
  (the default),
  an explicit path to an agent socket or the name of an environment variable
  (beginning with
  .Sq $ )
  in which to find the path.
  .Pp
  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.
  .It Cm ForwardX11
  Specifies whether X11 connections will be automatically redirected
  over the secure channel and
  .Ev DISPLAY
  set.
  The argument must be
  .Cm yes
  or
  .Cm no
  (the default).
  .Pp
  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
  .Cm ForwardX11Trusted
  option is also enabled.
  .It Cm ForwardX11Timeout
  Specify a timeout for untrusted X11 forwarding
  using the format described in the
  .Sx TIME FORMATS
  section of
  .Xr sshd_config 5 .
  X11 connections received by
  .Xr ssh 1
  after this time will be refused.
  Setting
  .Cm 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.
  .It Cm ForwardX11Trusted
  If this option is set to
  .Cm yes ,
  remote X11 clients will have full access to the original X11 display.
  .Pp
  If this option is set to
  .Cm no
  (the default),
  remote X11 clients will be considered untrusted and prevented
  from stealing or tampering with data belonging to trusted X11
  clients.
  Furthermore, the
  .Xr xauth 1
  token used for the session will be set to expire after 20 minutes.
  Remote clients will be refused access after this time.
  .Pp
  See the X11 SECURITY extension specification for full details on
  the restrictions imposed on untrusted clients.
  .It Cm GatewayPorts
  Specifies whether remote hosts are allowed to connect to local
  forwarded ports.
  By default,
  .Xr ssh 1
  binds local port forwardings to the loopback address.
  This prevents other remote hosts from connecting to forwarded ports.
  .Cm 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
  .Cm yes
  or
  .Cm no
  (the default).
  .It Cm GlobalKnownHostsFile
  Specifies one or more files to use for the global
  host key database, separated by whitespace.
  The default is
  .Pa /etc/ssh/ssh_known_hosts ,
  .Pa /etc/ssh/ssh_known_hosts2 .
  .It Cm GSSAPIAuthentication
  Specifies whether user authentication based on GSSAPI is allowed.
  The default is
  .Cm no .
  .It Cm GSSAPIDelegateCredentials
  Forward (delegate) credentials to the server.
  The default is
  .Cm no .
  .It Cm HashKnownHosts
  Indicates that
  .Xr ssh 1
  should hash host names and addresses when they are added to
  .Pa ~/.ssh/known_hosts .
  These hashed names may be used normally by
  .Xr ssh 1
  and
  .Xr sshd 8 ,
  but they do not visually reveal identifying information if the
  file's contents are disclosed.
  The default is
  .Cm no .
  Note that existing names and addresses in known hosts files
  will not be converted automatically,
  but may be manually hashed using
  .Xr ssh-keygen 1 .
  .It Cm 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
  .Sq +
  character, then the specified signature algorithms will be appended
  to the default set instead of replacing them.
  If the specified list begins with a
  .Sq -
  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
  .Sq ^
  character, then the specified signature algorithms will be placed
  at the head of the default set.
  The default for this option is:
  .Bd -literal -offset 3n
  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
  .Ed
  .Pp
  The
  .Fl Q
  option of
  .Xr ssh 1
  may be used to list supported signature algorithms.
  This was formerly named HostbasedKeyTypes.
  .It Cm HostbasedAuthentication
  Specifies whether to try rhosts based authentication with public key
  authentication.
  The argument must be
  .Cm yes
  or
  .Cm no
  (the default).
  .It Cm 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
  .Sq +
  character, then the specified signature algorithms will be appended to
  the default set instead of replacing them.
  If the specified list begins with a
  .Sq -
  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
  .Sq ^
  character, then the specified signature algorithms will be placed
  at the head of the default set.
  The default for this option is:
  .Bd -literal -offset 3n
  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
  .Ed
  .Pp
  If hostkeys are known for the destination host then this default is modified
  to prefer their algorithms.
  .Pp
  The list of available signature algorithms may also be obtained using
  .Qq ssh -Q HostKeyAlgorithms .
  .It Cm 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.
  .It Cm Hostname
  Specifies the real host name to log into.
  This can be used to specify nicknames or abbreviations for hosts.
  Arguments to
  .Cm Hostname
  accept the tokens described in the
  .Sx TOKENS
  section.
  Numeric IP addresses are also permitted (both on the command line and in
  .Cm Hostname
  specifications).
  The default is the name given on the command line.
  .It Cm IdentitiesOnly
  Specifies that
  .Xr ssh 1
  should only use the configured authentication identity and certificate files
  (either the default files, or those explicitly configured in the
  .Nm
  files
  or passed on the
  .Xr ssh 1
  command-line),
  even if
  .Xr ssh-agent 1
  or a
  .Cm PKCS11Provider
  or
  .Cm SecurityKeyProvider
  offers more identities.
  The argument to this keyword must be
  .Cm yes
  or
  .Cm no
  (the default).
  This option is intended for situations where ssh-agent
  offers many different identities.
  .It Cm IdentityAgent
  Specifies the
  .Ux Ns -domain
  socket used to communicate with the authentication agent.
  .Pp
  This option overrides the
  .Ev SSH_AUTH_SOCK
  environment variable and can be used to select a specific agent.
  Setting the socket name to
  .Cm none
  disables the use of an authentication agent.
  If the string
  .Qq SSH_AUTH_SOCK
  is specified, the location of the socket will be read from the
  .Ev SSH_AUTH_SOCK
  environment variable.
  Otherwise if the specified value begins with a
  .Sq $
  character, then it will be treated as an environment variable containing
  the location of the socket.
  .Pp
  Arguments to
  .Cm IdentityAgent
  may use the tilde syntax to refer to a user's home directory,
  the tokens described in the
  .Sx TOKENS
  section and environment variables as described in the
  .Sx ENVIRONMENT VARIABLES
  section.
  .It Cm IdentityFile
  Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA,
  Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read.
  The default is
  .Pa ~/.ssh/id_dsa ,
  .Pa ~/.ssh/id_ecdsa ,
  .Pa ~/.ssh/id_ecdsa_sk ,
  .Pa ~/.ssh/id_ed25519 ,
  .Pa ~/.ssh/id_ed25519_sk
  and
  .Pa ~/.ssh/id_rsa .
  Additionally, any identities represented by the authentication agent
  will be used for authentication unless
  .Cm IdentitiesOnly
  is set.
  If no certificates have been explicitly specified by
  .Cm CertificateFile ,
  .Xr ssh 1
  will try to load certificate information from the filename obtained by
  appending
  .Pa -cert.pub
  to the path of a specified
  .Cm IdentityFile .
  .Pp
  Arguments to
  .Cm IdentityFile
  may use the tilde syntax to refer to a user's home directory
  or the tokens described in the
  .Sx TOKENS
  section.
  .Pp
  It is possible to have
  multiple identity files specified in configuration files; all these
  identities will be tried in sequence.
  Multiple
  .Cm IdentityFile
  directives will add to the list of identities tried (this behaviour
  differs from that of other configuration directives).
  .Pp
  .Cm IdentityFile
  may be used in conjunction with
  .Cm IdentitiesOnly
  to select which identities in an agent are offered during authentication.
  .Cm IdentityFile
  may also be used in conjunction with
  .Cm CertificateFile
  in order to provide any certificate also needed for authentication with
  the identity.
  .It Cm 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
  .Nm
  contains options that are unrecognised by
  .Xr ssh 1 .
  It is recommended that
  .Cm IgnoreUnknown
  be listed early in the configuration file as it will not be applied
  to unknown options that appear before it.
  .It Cm Include
  Include the specified configuration file(s).
  Multiple pathnames may be specified and each pathname may contain
  .Xr glob 7
  wildcards and, for user configurations, shell-like
  .Sq ~
  references to user home directories.
  Wildcards will be expanded and processed in lexical order.
  Files without absolute paths are assumed to be in
  .Pa ~/.ssh
  if included in a user configuration file or
  .Pa /etc/ssh
  if included from the system configuration file.
  .Cm Include
  directive may appear inside a
  .Cm Match
  or
  .Cm Host
  block
  to perform conditional inclusion.
  .It Cm IPQoS
  Specifies the IPv4 type-of-service or DSCP class for connections.
  Accepted values are
  .Cm af11 ,
  .Cm af12 ,
  .Cm af13 ,
  .Cm af21 ,
  .Cm af22 ,
  .Cm af23 ,
  .Cm af31 ,
  .Cm af32 ,
  .Cm af33 ,
  .Cm af41 ,
  .Cm af42 ,
  .Cm af43 ,
  .Cm cs0 ,
  .Cm cs1 ,
  .Cm cs2 ,
  .Cm cs3 ,
  .Cm cs4 ,
  .Cm cs5 ,
  .Cm cs6 ,
  .Cm cs7 ,
  .Cm ef ,
  .Cm le ,
  .Cm lowdelay ,
  .Cm throughput ,
  .Cm reliability ,
  a numeric value, or
  .Cm 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
  .Cm af21
  (Low-Latency Data)
  for interactive sessions and
  .Cm cs1
  (Lower Effort)
  for non-interactive sessions.
  .It Cm KbdInteractiveAuthentication
  Specifies whether to use keyboard-interactive authentication.
  The argument to this keyword must be
  .Cm yes
  (the default)
  or
  .Cm no .
  .Cm ChallengeResponseAuthentication
  is a deprecated alias for this.
  .It Cm 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:
  .Cm bsdauth ,
  .Cm pam ,
  and
  .Cm skey .
  .It Cm KexAlgorithms
  Specifies the available KEX (Key Exchange) algorithms.
  Multiple algorithms must be comma-separated.
  If the specified list begins with a
  .Sq +
  character, then the specified algorithms will be appended to the default set
  instead of replacing them.
  If the specified list begins with a
  .Sq -
  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
  .Sq ^
  character, then the specified algorithms will be placed at the head of the
  default set.
  The default is:
  .Bd -literal -offset indent
  curve25519-sha256,curve25519-sha256@libssh.org,
  ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
  sntrup761x25519-sha512@openssh.com,
  diffie-hellman-group-exchange-sha256,
  diffie-hellman-group16-sha512,
  diffie-hellman-group18-sha512,
  diffie-hellman-group14-sha256
  .Ed
  .Pp
  The list of available key exchange algorithms may also be obtained using
  .Qq ssh -Q kex .
  .It Cm KnownHostsCommand
  Specifies a command to use to obtain a list of host keys, in addition to
  those listed in
  .Cm UserKnownHostsFile
  and
  .Cm 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
  .Sx VERIFYING HOST KEYS
  section in
  .Xr ssh 1 ) .
  Arguments to
  .Cm KnownHostsCommand
  accept the tokens described in the
  .Sx 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
  .Cm 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.
  .It Cm 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
  .Cm LocalCommand
  accept the tokens described in the
  .Sx TOKENS
  section.
  .Pp
  The command is run synchronously and does not have access to the
  session of the
  .Xr ssh 1
  that spawned it.
  It should not be used for interactive commands.
  .Pp
  This directive is ignored unless
  .Cm PermitLocalCommand
  has been enabled.
  .It Cm 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
  .Sm off
  .Oo Ar bind_address : Oc Ar port
  .Sm on
  or a Unix domain socket path.
  The second argument is the destination and may be
  .Ar host : Ns Ar hostport
  or a Unix domain socket path if the remote host supports it.
  .Pp
  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
  .Cm GatewayPorts
  setting.
  However, an explicit
  .Ar bind_address
  may be used to bind the connection to a specific address.
  The
  .Ar bind_address
  of
  .Cm localhost
  indicates that the listening port be bound for local use only, while an
  empty address or
  .Sq *
  indicates that the port should be available from all interfaces.
  Unix domain socket paths may use the tokens described in the
  .Sx TOKENS
  section and environment variables as described in the
  .Sx ENVIRONMENT VARIABLES
  section.
  .It Cm LogLevel
  Gives the verbosity level that is used when logging messages from
  .Xr 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.
  .It Cm 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:
  .Bd -literal -offset indent
  kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
  .Ed
  .Pp
  would enable detailed logging for line 1000 of
  .Pa kex.c ,
  everything in the
  .Fn kex_exchange_identification
  function, and all code in the
  .Pa packet.c
  file.
  This option is intended for debugging and no overrides are enabled by default.
  .It Cm 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
  .Sq +
  character, then the specified algorithms will be appended to the default set
  instead of replacing them.
  If the specified list begins with a
  .Sq -
  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
  .Sq ^
  character, then the specified algorithms will be placed at the head of the
  default set.
  .Pp
  The algorithms that contain
  .Qq -etm
  calculate the MAC after encryption (encrypt-then-mac).
  These are considered safer and their use recommended.
  .Pp
  The default is:
  .Bd -literal -offset indent
  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
  .Ed
  .Pp
  The list of available MAC algorithms may also be obtained using
  .Qq ssh -Q mac .
  .It Cm NoHostAuthenticationForLocalhost
  Disable host authentication for localhost (loopback addresses).
  The argument to this keyword must be
  .Cm yes
  or
  .Cm no
  (the default).
  .It Cm NumberOfPasswordPrompts
  Specifies the number of password prompts before giving up.
  The argument to this keyword must be an integer.
  The default is 3.
  .It Cm PasswordAuthentication
  Specifies whether to use password authentication.
  The argument to this keyword must be
  .Cm yes
  (the default)
  or
  .Cm no .
  .It Cm PermitLocalCommand
  Allow local command execution via the
  .Ic LocalCommand
  option or using the
  .Ic !\& Ns Ar command
  escape sequence in
  .Xr ssh 1 .
  The argument must be
  .Cm yes
  or
  .Cm no
  (the default).
  .It Cm PermitRemoteOpen
  Specifies the destinations to which remote TCP port forwarding is permitted when
  .Cm RemoteForward
  is used as a SOCKS proxy.
  The forwarding specification must be one of the following forms:
  .Pp
  .Bl -item -offset indent -compact
  .It
  .Cm PermitRemoteOpen
  .Sm off
  .Ar host : port
  .Sm on
  .It
  .Cm PermitRemoteOpen
  .Sm off
  .Ar IPv4_addr : port
  .Sm on
  .It
  .Cm PermitRemoteOpen
  .Sm off
  .Ar \&[ IPv6_addr \&] : port
  .Sm on
  .El
  .Pp
  Multiple forwards may be specified by separating them with whitespace.
  An argument of
  .Cm any
  can be used to remove all restrictions and permit any forwarding requests.
  An argument of
  .Cm none
  can be used to prohibit all forwarding requests.
  The wildcard
  .Sq *
  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.
  .It Cm PKCS11Provider
  Specifies which PKCS#11 provider to use or
  .Cm 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
  .Xr ssh 1
  should use to communicate with a PKCS#11 token providing keys for user
  authentication.
  .It Cm Port
  Specifies the port number to connect on the remote host.
  The default is 22.
  .It Cm PreferredAuthentications
  Specifies the order in which the client should try authentication methods.
  This allows a client to prefer one method (e.g.\&
  .Cm keyboard-interactive )
  over another method (e.g.\&
  .Cm password ) .
  The default is:
  .Bd -literal -offset indent
  gssapi-with-mic,hostbased,publickey,
  keyboard-interactive,password
  .Ed
  .It Cm 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
  .Ql exec
  directive to avoid a lingering shell process.
  .Pp
  Arguments to
  .Cm ProxyCommand
  accept the tokens described in the
  .Sx 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
  .Xr sshd 8
  server running on some machine, or execute
  .Ic sshd -i
  somewhere.
  Host key management will be done using the
  .Cm Hostname
  of the host being connected (defaulting to the name typed by the user).
  Setting the command to
  .Cm none
  disables this option entirely.
  Note that
  .Cm CheckHostIP
  is not available for connects with a proxy command.
  .Pp
  This directive is useful in conjunction with
  .Xr nc 1
  and its proxy support.
  For example, the following directive would connect via an HTTP proxy at
  192.0.2.0:
  .Bd -literal -offset 3n
  ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
  .Ed
  .It Cm ProxyJump
  Specifies one or more jump proxies as either
  .Xo
  .Sm off
  .Op Ar user No @
  .Ar host
  .Op : Ns Ar port
  .Sm on
  or an ssh URI
  .Xc .
  Multiple proxies may be separated by comma characters and will be visited
  sequentially.
  Setting this option will cause
  .Xr ssh 1
  to connect to the target host by first making a
  .Xr ssh 1
  connection to the specified
  .Cm ProxyJump
  host and then establishing a
  TCP forwarding to the ultimate target from there.
  Setting the host to
  .Cm none
  disables this option entirely.
  .Pp
  Note that this option will compete with the
  .Cm ProxyCommand
  option - whichever is specified first will prevent later instances of the
  other from taking effect.
  .Pp
  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.
  .Pa ~/.ssh/config
  should be used if specific configuration is required for jump hosts.
  .It Cm ProxyUseFdpass
  Specifies that
  .Cm ProxyCommand
  will pass a connected file descriptor back to
  .Xr ssh 1
  instead of continuing to execute and pass data.
  The default is
  .Cm no .
  .It Cm 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
  .Sq +
  character, then the algorithms after it will be appended to the default
  instead of replacing it.
  If the specified list begins with a
  .Sq -
  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
  .Sq ^
  character, then the specified algorithms will be placed at the head of the
  default set.
  The default for this option is:
  .Bd -literal -offset 3n
  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
  .Ed
  .Pp
  The list of available signature algorithms may also be obtained using
  .Qq ssh -Q PubkeyAcceptedAlgorithms .
  .It Cm PubkeyAuthentication
  Specifies whether to try public key authentication.
  The argument to this keyword must be
  .Cm yes
  (the default)
  or
  .Cm no .
  .It Cm RekeyLimit
  Specifies the maximum amount of data that may be transmitted 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
  .Sq K ,
  .Sq M ,
  or
  .Sq G
  to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
  The default is between
  .Sq 1G
  and
  .Sq 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
  .Xr sshd_config 5 .
  The default value for
  .Cm RekeyLimit
  is
  .Cm 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.
  .It Cm 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
  .Cm RemoteCommand
  accept the tokens described in the
  .Sx TOKENS
  section.
  .It Cm 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
  .Sm off
  .Oo Ar bind_address : Oc Ar port
  .Sm on
  or, if the remote host supports it, a Unix domain socket path.
  If forwarding to a specific destination then the second argument must be
  .Ar host : Ns Ar 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
  .Cm PermitRemoteOpen .
  .Pp
  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
  .Sx TOKENS
  section and environment variables as described in the
  .Sx ENVIRONMENT VARIABLES
  section.
  .Pp
  If the
  .Ar port
  argument is 0,
  the listen port will be dynamically allocated on the server and reported
  to the client at run time.
  .Pp
  If the
  .Ar bind_address
  is not specified, the default is to only bind to loopback addresses.
  If the
  .Ar bind_address
  is
  .Ql *
  or an empty string, then the forwarding is requested to listen on all
  interfaces.
  Specifying a remote
  .Ar bind_address
  will only succeed if the server's
  .Cm GatewayPorts
  option is enabled (see
  .Xr sshd_config 5 ) .
  .It Cm RequestTTY
  Specifies whether to request a pseudo-tty for the session.
  The argument may be one of:
  .Cm no
  (never request a TTY),
  .Cm yes
  (always request a TTY when standard input is a TTY),
  .Cm force
  (always request a TTY) or
  .Cm auto
  (request a TTY when opening a login session).
  This option mirrors the
  .Fl t
  and
  .Fl T
  flags for
  .Xr ssh 1 .
  .It Cm 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
  .Xr ssh-keygen 1 .
  For more information on KRLs, see the KEY REVOCATION LISTS section in
  .Xr ssh-keygen 1 .
  .It Cm 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.
  .Pp
  If the specified value begins with a
  .Sq $
  character, then it will be treated as an environment variable containing
  the path to the library.
  .It Cm SendEnv
  Specifies what variables from the local
  .Xr 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
  .Ev TERM
  environment variable is always sent whenever a
  pseudo-terminal is requested as it is required by the protocol.
  Refer to
  .Cm AcceptEnv
  in
  .Xr 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
  .Cm SendEnv
  directives.
  .Pp
  See
  .Sx PATTERNS
  for more information on patterns.
  .Pp
  It is possible to clear previously set
  .Cm SendEnv
  variable names by prefixing patterns with
  .Pa - .
  The default is not to send any environment variables.
  .It Cm ServerAliveCountMax
  Sets the number of server alive messages (see below) which may be
  sent without
  .Xr 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
  .Cm TCPKeepAlive
  (below).
  The server alive messages are sent through the encrypted channel
  and therefore will not be spoofable.
  The TCP keepalive option enabled by
  .Cm TCPKeepAlive
  is spoofable.
  The server alive mechanism is valuable when the client or
  server depend on knowing when a connection has become unresponsive.
  .Pp
  The default value is 3.
  If, for example,
  .Cm ServerAliveInterval
  (see below) is set to 15 and
  .Cm ServerAliveCountMax
  is left at the default, if the server becomes unresponsive,
  ssh will disconnect after approximately 45 seconds.
  .It Cm ServerAliveInterval
  Sets a timeout interval in seconds after which if no data has been received
  from the server,
  .Xr 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.
  .It Cm 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
  .Cm none
  (same as the
  .Fl N
  option),
  .Cm subsystem
  (same as the
  .Fl s
  option) or
  .Cm default
  (shell or command execution).
  .It Cm SetEnv
  Directly specify one or more environment variables and their contents to
  be sent to the server.
  Similarly to
  .Cm SendEnv ,
  with the exception of the
  .Ev TERM
  variable, the server must be prepared to accept the environment variable.
  .It Cm StdinNull
  Redirects stdin from
  .Pa /dev/null
  (actually, prevents reading from stdin).
  Either this or the equivalent
  .Fl n
  option must be used when
  .Nm ssh
  is run in the background.
  The argument to this keyword must be
  .Cm yes
  (same as the
  .Fl n
  option) or
  .Cm no
  (the default).
  .It Cm StreamLocalBindMask
  Sets the octal file creation mode mask
  .Pq 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.
  .Pp
  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.
  .It Cm 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
  .Cm StreamLocalBindUnlink
  is not enabled,
  .Nm 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.
  .Pp
  The argument must be
  .Cm yes
  or
  .Cm no
  (the default).
  .It Cm StrictHostKeyChecking
  If this flag is set to
  .Cm yes ,
  .Xr ssh 1
  will never automatically add host keys to the
  .Pa ~/.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
  .Pa /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.
  .Pp
  If this flag is set to
  .Cm accept-new
  then ssh will automatically add new host keys to the user's
  .Pa known_hosts
  file, but will not permit connections to hosts with
  changed host keys.
  If this flag is set to
  .Cm no
  or
  .Cm 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
  .Cm 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.
  .It Cm SyslogFacility
  Gives the facility code that is used when logging messages from
  .Xr ssh 1 .
  The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
  LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
  The default is USER.
  .It Cm TCPKeepAlive
  Specifies whether the system should send TCP keepalive messages to the
  other side.
  If they are sent, death of the connection or crash of one
  of the machines will be properly noticed.
  However, this means that
  connections will die if the route is down temporarily, and some people
  find it annoying.
  .Pp
  The default is
  .Cm 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.
  .Pp
  To disable TCP keepalive messages, the value should be set to
  .Cm no .
  See also
  .Cm ServerAliveInterval
  for protocol-level keepalives.
  .It Cm Tunnel
  Request
  .Xr tun 4
  device forwarding between the client and the server.
  The argument must be
  .Cm yes ,
  .Cm point-to-point
  (layer 3),
  .Cm ethernet
  (layer 2),
  or
  .Cm no
  (the default).
  Specifying
  .Cm yes
  requests the default tunnel mode, which is
  .Cm point-to-point .
  .It Cm TunnelDevice
  Specifies the
  .Xr tun 4
  devices to open on the client
  .Pq Ar local_tun
  and the server
  .Pq Ar remote_tun .
  .Pp
  The argument must be
  .Sm off
  .Ar local_tun Op : Ar remote_tun .
  .Sm on
  The devices may be specified by numerical ID or the keyword
  .Cm any ,
  which uses the next available tunnel device.
  If
  .Ar remote_tun
  is not specified, it defaults to
  .Cm any .
  The default is
  .Cm any:any .
  .It Cm UpdateHostKeys
  Specifies whether
  .Xr ssh 1
  should accept notifications of additional hostkeys from the server sent
  after authentication has completed and add them to
  .Cm UserKnownHostsFile .
  The argument must be
  .Cm yes ,
  .Cm no
  or
  .Cm 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.
  .Pp
  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
  .Cm UserKnownHostsFile
  (i.e. not
  .Cm GlobalKnownHostsFile )
  and the host was authenticated using a plain key and not a certificate.
  .Pp
  .Cm UpdateHostKeys
  is enabled by default if the user has not overridden the default
  .Cm UserKnownHostsFile
  setting and has not enabled
  .Cm VerifyHostKeyDNS ,
  otherwise
  .Cm UpdateHostKeys
  will be set to
  .Cm no .
  .Pp
  If
  .Cm UpdateHostKeys
  is set to
  .Cm ask ,
  then the user is asked to confirm the modifications to the known_hosts file.
  Confirmation is currently incompatible with
  .Cm ControlPersist ,
  and will be disabled if it is enabled.
  .Pp
  Presently, only
  .Xr sshd 8
  from OpenSSH 6.8 and greater support the
  .Qq hostkeys@openssh.com
  protocol extension used to inform the client of all the server's hostkeys.
  .It Cm 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.
  .It Cm 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
  .Sx TOKENS
  section and environment variables as described in the
  .Sx ENVIRONMENT VARIABLES
  section.
  The default is
  .Pa ~/.ssh/known_hosts ,
  .Pa ~/.ssh/known_hosts2 .
  .It Cm VerifyHostKeyDNS
  Specifies whether to verify the remote key using DNS and SSHFP resource
  records.
  If this option is set to
  .Cm 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
  .Cm ask .
  If this option is set to
  .Cm ask ,
  information on fingerprint match will be displayed, but the user will still
  need to confirm new host keys according to the
  .Cm StrictHostKeyChecking
  option.
  The default is
  .Cm no .
  .Pp
  See also
  .Sx VERIFYING HOST KEYS
  in
  .Xr ssh 1 .
  .It Cm VisualHostKey
  If this flag is set to
  .Cm 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
  .Cm no
  (the default),
  no fingerprint strings are printed at login and
  only the fingerprint string will be printed for unknown host keys.
  .It Cm XAuthLocation
  Specifies the full pathname of the
  .Xr xauth 1
  program.
  The default is
  .Pa /usr/X11R6/bin/xauth .
  .El
  .Sh PATTERNS
  A
  .Em pattern
  consists of zero or more non-whitespace characters,
  .Sq *
  (a wildcard that matches zero or more characters),
  or
  .Sq ?\&
  (a wildcard that matches exactly one character).
  For example, to specify a set of declarations for any host in the
  .Qq .co.uk
  set of domains,
  the following pattern could be used:
  .Pp
  .Dl Host *.co.uk
  .Pp
  The following pattern
  would match any host in the 192.168.0.[0-9] network range:
  .Pp
  .Dl Host 192.168.0.?
  .Pp
  A
  .Em pattern-list
  is a comma-separated list of patterns.
  Patterns within pattern-lists may be negated
  by preceding them with an exclamation mark
  .Pq Sq !\& .
  For example,
  to allow a key to be used from anywhere within an organization
  except from the
  .Qq dialup
  pool,
  the following entry (in authorized_keys) could be used:
  .Pp
  .Dl from=\&"!*.dialup.example.com,*.example.com\&"
  .Pp
  Note that a negated match will never produce a positive result by itself.
  For example, attempting to match
  .Qq host3
  against the following pattern-list will fail:
  .Pp
  .Dl from=\&"!host1,!host2\&"
  .Pp
  The solution here is to include a term that will yield a positive match,
  such as a wildcard:
  .Pp
  .Dl from=\&"!host1,!host2,*\&"
  .Sh TOKENS
  Arguments to some keywords can make use of tokens,
  which are expanded at runtime:
  .Pp
  .Bl -tag -width XXXX -offset indent -compact
  .It %%
  A literal
  .Sq % .
  .It \&%C
  Hash of %l%h%p%r.
  .It %d
  Local user's home directory.
  .It %f
  The fingerprint of the server's host key.
  .It %H
  The
  .Pa known_hosts
  hostname or address that is being searched for.
  .It %h
  The remote hostname.
  .It \%%I
  A string describing the reason for a
  .Cm KnownHostsCommand
  execution: either
  .Cm ADDRESS
  when looking up a host by address (only when
  .Cm CheckHostIP
  is enabled),
  .Cm HOSTNAME
  when searching by hostname, or
  .Cm ORDER
  when preparing the host key algorithm preference list to use for the
  destination host.
  .It %i
  The local user ID.
  .It %K
  The base64 encoded host key.
  .It %k
  The host key alias if specified, otherwise the original remote hostname given
  on the command line.
  .It %L
  The local hostname.
  .It %l
  The local hostname, including the domain name.
  .It %n
  The original remote hostname, as given on the command line.
  .It %p
  The remote port.
  .It %r
  The remote username.
  .It \&%T
  The local
  .Xr tun 4
  or
  .Xr tap 4
  network interface assigned if
  tunnel forwarding was requested, or
  .Qq NONE
  otherwise.
  .It %t
  The type of the server host key, e.g.
  .Cm ssh-ed25519 .
  .It %u
  The local username.
  .El
  .Pp
  .Cm CertificateFile ,
  .Cm ControlPath ,
  .Cm IdentityAgent ,
  .Cm IdentityFile ,
  .Cm KnownHostsCommand ,
  .Cm LocalForward ,
  .Cm Match exec ,
  .Cm RemoteCommand ,
  .Cm RemoteForward ,
  and
  .Cm UserKnownHostsFile
  accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u.
  .Pp
  .Cm KnownHostsCommand
  additionally accepts the tokens %f, %H, %I, %K and %t.
  .Pp
  .Cm Hostname
  accepts the tokens %% and %h.
  .Pp
  .Cm LocalCommand
  accepts all tokens.
  .Pp
  .Cm ProxyCommand
  accepts the tokens %%, %h, %n, %p, and %r.
  .Sh ENVIRONMENT VARIABLES
  Arguments to some keywords can be expanded at runtime from environment
  variables on the client by enclosing them in
  .Ic ${} ,
  for example
  .Ic ${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.
  .Pp
  The keywords
  .Cm CertificateFile ,
  .Cm ControlPath ,
  .Cm IdentityAgent ,
  .Cm IdentityFile ,
  .Cm KnownHostsCommand ,
  and
  .Cm UserKnownHostsFile
  support environment variables.
  The keywords
  .Cm LocalForward
  and
  .Cm RemoteForward
  support environment variables only for Unix domain socket paths.
  .Sh FILES
  .Bl -tag -width Ds
  .It Pa ~/.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 Pa /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.
  .El
  .Sh SEE ALSO
  .Xr ssh 1
  .Sh AUTHORS
  .An -nosplit
  OpenSSH is a derivative of the original and free
  ssh 1.2.12 release by
  .An Tatu Ylonen .
  .An Aaron Campbell , Bob Beck , Markus Friedl ,
  .An Niels Provos , Theo de Raadt
  and
  .An Dug Song
  removed many bugs, re-added newer features and
  created OpenSSH.
  .An Markus Friedl
  contributed the support for SSH protocol versions 1.5 and 2.0.