2 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4 .\" All rights reserved
6 .\" As far as I am concerned, the code I have written for this software
7 .\" can be used freely for any purpose. Any derived versions of this
8 .\" software must be clearly marked as such, and if the derived work is
9 .\" incompatible with the protocol description in the RFC file, it must be
10 .\" called by a name other than "ssh" or "Secure Shell".
12 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
16 .\" Redistribution and use in source and binary forms, with or without
17 .\" modification, are permitted provided that the following conditions
19 .\" 1. Redistributions of source code must retain the above copyright
20 .\" notice, this list of conditions and the following disclaimer.
21 .\" 2. Redistributions in binary form must reproduce the above copyright
22 .\" notice, this list of conditions and the following disclaimer in the
23 .\" documentation and/or other materials provided with the distribution.
25 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
36 .\" $OpenBSD: ssh_config.5,v 1.325 2020/04/11 20:20:09 jmc Exp $
37 .Dd $Mdocdate: April 11 2020 $
42 .Nd OpenSSH client configuration file
45 obtains configuration data from the following sources in
48 .Bl -enum -offset indent -compact
52 user's configuration file
55 system-wide configuration file
56 .Pq Pa /etc/ssh/ssh_config
59 For each parameter, the first obtained value
61 The configuration files contain sections separated by
63 specifications, and that section is only applied for hosts that
64 match one of the patterns given in the specification.
65 The matched host name is usually the one given on the command line
67 .Cm CanonicalizeHostname
68 option for exceptions).
70 Since the first obtained value for each parameter is used, more
71 host-specific declarations should be given near the beginning of the
72 file, and general defaults at the end.
74 The file contains keyword-argument pairs, one per line.
77 and empty lines are interpreted as comments.
78 Arguments may optionally be enclosed in double quotes
80 in order to represent arguments containing spaces.
81 Configuration options may be separated by whitespace or
82 optional whitespace and exactly one
84 the latter format is useful to avoid the need to quote whitespace
85 when specifying configuration options using the
94 keywords and their meanings are as follows (note that
95 keywords are case-insensitive and arguments are case-sensitive):
98 Restricts the following declarations (up to the next
102 keyword) to be only for those hosts that match one of the patterns
103 given after the keyword.
104 If more than one pattern is provided, they should be separated by whitespace.
107 as a pattern can be used to provide global
108 defaults for all hosts.
109 The host is usually the
111 argument given on the command line
113 .Cm CanonicalizeHostname
114 keyword for exceptions).
116 A pattern entry may be negated by prefixing it with an exclamation mark
118 If a negated entry is matched, then the
120 entry is ignored, regardless of whether any other patterns on the line
122 Negated matches are therefore useful to provide exceptions for wildcard
127 for more information on patterns.
129 Restricts the following declarations (up to the next
133 keyword) to be used only when the conditions following the
135 keyword are satisfied.
136 Match conditions are specified using one or more criteria
139 which always matches.
140 The available criteria keywords are:
151 criteria must appear alone or immediately after
155 Other criteria may be combined arbitrarily.
162 Criteria may be negated by prepending an exclamation mark
167 keyword matches only when the configuration file is being re-parsed
168 after hostname canonicalization (see the
169 .Cm CanonicalizeHostname
171 This may be useful to specify conditions that work with canonical host
176 keyword requests that the configuration be re-parsed (regardless of whether
177 .Cm CanonicalizeHostname
178 is enabled), and matches only during this final pass.
180 .Cm CanonicalizeHostname
185 match during the same pass.
189 keyword executes the specified command under the user's shell.
190 If the command returns a zero exit status then the condition is considered true.
191 Commands containing whitespace characters must be quoted.
194 accept the tokens described in the
198 The other keywords' criteria must be single entries or comma-separated
199 lists and may use the wildcard and negation operators described in the
204 keyword are matched against the target hostname, after any substitution
208 .Cm CanonicalizeHostname
212 keyword matches against the hostname as it was specified on the command-line.
215 keyword matches against the target username on the remote host.
218 keyword matches against the name of the local user running
220 (this keyword may be useful in system-wide
223 .It Cm AddKeysToAgent
224 Specifies whether keys should be automatically added to a running
226 If this option is set to
228 and a key is loaded from a file, the key and its passphrase are added to
229 the agent with the default lifetime, as if by
231 If this option is set to
234 will require confirmation using the
236 program before adding a key (see
239 If this option is set to
241 each use of the key must be confirmed, as if the
243 option was specified to
245 If this option is set to
247 no keys are added to the agent.
256 Specifies which address family to use when connecting.
267 user interaction such as password prompts and host key confirmation requests
269 This option is useful in scripts and other batch jobs where no user
270 is present to interact with
278 Use the specified address on the local machine as the source address of
280 Only useful on systems with more than one address.
282 Use the address of the specified interface on the local machine as the
283 source address of the connection.
284 .It Cm CanonicalDomains
286 .Cm CanonicalizeHostname
287 is enabled, this option specifies the list of domain suffixes in which to
288 search for the specified destination host.
289 .It Cm CanonicalizeFallbackLocal
290 Specifies whether to fail with an error when hostname canonicalization fails.
293 will attempt to look up the unqualified hostname using the system resolver's
300 .Cm CanonicalizeHostname
301 is enabled and the target hostname cannot be found in any of the domains
303 .Cm CanonicalDomains .
304 .It Cm CanonicalizeHostname
305 Controls whether explicit hostname canonicalization is performed.
308 is not to perform any name rewriting and let the system resolver handle all
312 then, for connections that do not use a
317 will attempt to canonicalize the hostname specified on the command line
321 .Cm CanonicalizePermittedCNAMEs
324 .Cm CanonicalizeHostname
327 then canonicalization is applied to proxied connections too.
329 If this option is enabled, then the configuration files are processed
330 again using the new target name to pick up any new configuration in matching
335 .It Cm CanonicalizeMaxDots
336 Specifies the maximum number of dot characters in a hostname before
337 canonicalization is disabled.
339 allows a single dot (i.e. hostname.subdomain).
340 .It Cm CanonicalizePermittedCNAMEs
341 Specifies rules to determine whether CNAMEs should be followed when
342 canonicalizing hostnames.
343 The rules consist of one or more arguments of
344 .Ar source_domain_list : Ns Ar target_domain_list ,
346 .Ar source_domain_list
347 is a pattern-list of domains that may follow CNAMEs in canonicalization,
349 .Ar target_domain_list
350 is a pattern-list of domains that they may resolve to.
353 .Qq *.a.example.com:*.b.example.com,*.c.example.com
354 will allow hostnames matching
356 to be canonicalized to names in the
361 .It Cm CASignatureAlgorithms
362 Specifies which algorithms are allowed for signing of certificates
363 by certificate authorities (CAs).
365 .Bd -literal -offset indent
366 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
367 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
371 will not accept host certificates signed using algorithms other than those
373 .It Cm CertificateFile
374 Specifies a file from which the user's certificate is read.
375 A corresponding private key must be provided separately in order
376 to use this certificate either
388 .Cm SecurityKeyProvider .
392 may use the tilde syntax to refer to a user's home directory
393 or the tokens described in the
397 It is possible to have multiple certificate files specified in
398 configuration files; these certificates will be tried in sequence.
401 directives will add to the list of certificates used for
403 .It Cm ChallengeResponseAuthentication
404 Specifies whether to use challenge-response authentication.
405 The argument to this keyword must be
415 will additionally check the host IP address in the
418 This allows it to detect if a host key changed due to DNS spoofing
419 and will add addresses of destination hosts to
420 .Pa ~/.ssh/known_hosts
421 in the process, regardless of the setting of
422 .Cm StrictHostKeyChecking .
423 If the option is set to
425 the check will not be executed.
427 Specifies the ciphers allowed and their order of preference.
428 Multiple ciphers must be comma-separated.
429 If the specified list begins with a
431 character, then the specified ciphers will be appended to the default set
432 instead of replacing them.
433 If the specified list begins with a
435 character, then the specified ciphers (including wildcards) will be removed
436 from the default set instead of replacing them.
437 If the specified list begins with a
439 character, then the specified ciphers will be placed at the head of the
442 The supported ciphers are:
443 .Bd -literal -offset indent
451 aes128-gcm@openssh.com
452 aes256-gcm@openssh.com
453 chacha20-poly1305@openssh.com
457 .Bd -literal -offset indent
458 chacha20-poly1305@openssh.com,
459 aes128-ctr,aes192-ctr,aes256-ctr,
460 aes128-gcm@openssh.com,aes256-gcm@openssh.com
463 The list of available ciphers may also be obtained using
465 .It Cm ClearAllForwardings
466 Specifies that all local, remote, and dynamic port forwardings
467 specified in the configuration files or on the command line be
469 This option is primarily useful when used from the
471 command line to clear port forwardings set in
472 configuration files, and is automatically set by
482 Specifies whether to use compression.
488 .It Cm ConnectionAttempts
489 Specifies the number of tries (one per second) to make before exiting.
490 The argument must be an integer.
491 This may be useful in scripts if the connection sometimes fails.
493 .It Cm ConnectTimeout
494 Specifies the timeout (in seconds) used when connecting to the
495 SSH server, instead of using the default system TCP timeout.
496 This timeout is applied both to establishing the connection and to performing
497 the initial SSH protocol handshake and key exchange.
499 Enables the sharing of multiple sessions over a single network connection.
503 will listen for connections on a control socket specified using the
506 Additional sessions can connect to this socket using the same
513 These sessions will try to reuse the master instance's network connection
514 rather than initiating new ones, but will fall back to connecting normally
515 if the control socket does not exist, or is not listening.
521 to listen for control connections, but require confirmation using
527 will continue without connecting to a master instance.
531 forwarding is supported over these multiplexed connections, however the
532 display and agent forwarded will be the one belonging to the master
533 connection i.e. it is not possible to forward multiple displays or agents.
535 Two additional options allow for opportunistic multiplexing: try to use a
536 master connection but fall back to creating a new one if one does not already
542 The latter requires confirmation like the
546 Specify the path to the control socket used for connection sharing as described
549 section above or the string
551 to disable connection sharing.
554 may use the tilde syntax to refer to a user's home directory
555 or the tokens described in the
558 It is recommended that any
560 used for opportunistic connection sharing include
561 at least %h, %p, and %r (or alternatively %C) and be placed in a directory
562 that is not writable by other users.
563 This ensures that shared connections are uniquely identified.
564 .It Cm ControlPersist
565 When used in conjunction with
567 specifies that the master connection should remain open
568 in the background (waiting for future client connections)
569 after the initial client connection has been closed.
573 then the master connection will not be placed into the background,
574 and will close as soon as the initial client connection is closed.
578 then the master connection will remain in the background indefinitely
579 (until killed or closed via a mechanism such as the
581 If set to a time in seconds, or a time in any of the formats documented in
583 then the backgrounded master connection will automatically terminate
584 after it has remained idle (with no client connections) for the
586 .It Cm DynamicForward
587 Specifies that a TCP port on the local machine be forwarded
588 over the secure channel, and the application
589 protocol is then used to determine where to connect to from the
594 .Oo Ar bind_address : Oc Ar port .
596 IPv6 addresses can be specified by enclosing addresses in square brackets.
597 By default, the local port is bound in accordance with the
602 may be used to bind the connection to a specific address.
607 indicates that the listening port be bound for local use only, while an
610 indicates that the port should be available from all interfaces.
612 Currently the SOCKS4 and SOCKS5 protocols are supported, and
614 will act as a SOCKS server.
615 Multiple forwardings may be specified, and
616 additional forwardings can be given on the command line.
617 Only the superuser can forward privileged ports.
618 .It Cm EnableSSHKeysign
619 Setting this option to
621 in the global client configuration file
622 .Pa /etc/ssh/ssh_config
623 enables the use of the helper program
626 .Cm HostbasedAuthentication .
632 This option should be placed in the non-hostspecific section.
635 for more information.
637 Sets the escape character (default:
639 The escape character can also
640 be set on the command line.
641 The argument should be a single character,
643 followed by a letter, or
645 to disable the escape
646 character entirely (making the connection transparent for binary
648 .It Cm ExitOnForwardFailure
651 should terminate the connection if it cannot set up all requested
652 dynamic, tunnel, local, and remote port forwardings, (e.g.\&
653 if either end is unable to bind and listen on a specified port).
655 .Cm ExitOnForwardFailure
656 does not apply to connections made over port forwardings and will not,
659 to exit if TCP connections to the ultimate forwarding destination fail.
665 .It Cm FingerprintHash
666 Specifies the hash algorithm used when displaying key fingerprints.
673 Specifies whether the connection to the authentication agent (if any)
674 will be forwarded to the remote machine.
679 an explicit path to an agent socket or the name of an environment variable
682 in which to find the path.
684 Agent forwarding should be enabled with caution.
685 Users with the ability to bypass file permissions on the remote host
686 (for the agent's Unix-domain socket)
687 can access the local agent through the forwarded connection.
688 An attacker cannot obtain key material from the agent,
689 however they can perform operations on the keys that enable them to
690 authenticate using the identities loaded into the agent.
692 Specifies whether X11 connections will be automatically redirected
693 over the secure channel and
702 X11 forwarding should be enabled with caution.
703 Users with the ability to bypass file permissions on the remote host
704 (for the user's X11 authorization database)
705 can access the local X11 display through the forwarded connection.
706 An attacker may then be able to perform activities such as keystroke monitoring
708 .Cm ForwardX11Trusted
709 option is also enabled.
710 .It Cm ForwardX11Timeout
711 Specify a timeout for untrusted X11 forwarding
712 using the format described in the
716 X11 connections received by
718 after this time will be refused.
720 .Cm ForwardX11Timeout
721 to zero will disable the timeout and permit X11 forwarding for the life
723 The default is to disable untrusted X11 forwarding after twenty minutes has
725 .It Cm ForwardX11Trusted
726 If this option is set to
728 remote X11 clients will have full access to the original X11 display.
730 If this option is set to
733 remote X11 clients will be considered untrusted and prevented
734 from stealing or tampering with data belonging to trusted X11
738 token used for the session will be set to expire after 20 minutes.
739 Remote clients will be refused access after this time.
741 See the X11 SECURITY extension specification for full details on
742 the restrictions imposed on untrusted clients.
744 Specifies whether remote hosts are allowed to connect to local
748 binds local port forwardings to the loopback address.
749 This prevents other remote hosts from connecting to forwarded ports.
751 can be used to specify that ssh
752 should bind local port forwardings to the wildcard address,
753 thus allowing remote hosts to connect to forwarded ports.
759 .It Cm GlobalKnownHostsFile
760 Specifies one or more files to use for the global
761 host key database, separated by whitespace.
763 .Pa /etc/ssh/ssh_known_hosts ,
764 .Pa /etc/ssh/ssh_known_hosts2 .
765 .It Cm GSSAPIAuthentication
766 Specifies whether user authentication based on GSSAPI is allowed.
769 .It Cm GSSAPIDelegateCredentials
770 Forward (delegate) credentials to the server.
773 .It Cm HashKnownHosts
776 should hash host names and addresses when they are added to
777 .Pa ~/.ssh/known_hosts .
778 These hashed names may be used normally by
782 but they do not visually reveal identifying information if the
783 file's contents are disclosed.
786 Note that existing names and addresses in known hosts files
787 will not be converted automatically,
788 but may be manually hashed using
790 .It Cm HostbasedAuthentication
791 Specifies whether to try rhosts based authentication with public key
798 .It Cm HostbasedKeyTypes
799 Specifies the key types that will be used for hostbased authentication
800 as a comma-separated list of patterns.
801 Alternately if the specified list begins with a
803 character, then the specified key types will be appended to the default set
804 instead of replacing them.
805 If the specified list begins with a
807 character, then the specified key types (including wildcards) will be removed
808 from the default set instead of replacing them.
809 If the specified list begins with a
811 character, then the specified key types will be placed at the head of the
813 The default for this option is:
814 .Bd -literal -offset 3n
815 ecdsa-sha2-nistp256-cert-v01@openssh.com,
816 ecdsa-sha2-nistp384-cert-v01@openssh.com,
817 ecdsa-sha2-nistp521-cert-v01@openssh.com,
818 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
819 ssh-ed25519-cert-v01@openssh.com,
820 sk-ssh-ed25519-cert-v01@openssh.com,
821 rsa-sha2-512-cert-v01@openssh.com,
822 rsa-sha2-256-cert-v01@openssh.com,
823 ssh-rsa-cert-v01@openssh.com,
824 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
825 sk-ecdsa-sha2-nistp256@openssh.com,
826 ssh-ed25519,sk-ssh-ed25519@openssh.com,
827 rsa-sha2-512,rsa-sha2-256,ssh-rsa
834 may be used to list supported key types.
835 .It Cm HostKeyAlgorithms
836 Specifies the host key algorithms
837 that the client wants to use in order of preference.
838 Alternately if the specified list begins with a
840 character, then the specified key types will be appended to the default set
841 instead of replacing them.
842 If the specified list begins with a
844 character, then the specified key types (including wildcards) will be removed
845 from the default set instead of replacing them.
846 If the specified list begins with a
848 character, then the specified key types will be placed at the head of the
850 The default for this option is:
851 .Bd -literal -offset 3n
852 ecdsa-sha2-nistp256-cert-v01@openssh.com,
853 ecdsa-sha2-nistp384-cert-v01@openssh.com,
854 ecdsa-sha2-nistp521-cert-v01@openssh.com,
855 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
856 ssh-ed25519-cert-v01@openssh.com,
857 sk-ssh-ed25519-cert-v01@openssh.com,
858 rsa-sha2-512-cert-v01@openssh.com,
859 rsa-sha2-256-cert-v01@openssh.com,
860 ssh-rsa-cert-v01@openssh.com,
861 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
862 sk-ecdsa-sha2-nistp256@openssh.com,
863 ssh-ed25519,sk-ssh-ed25519@openssh.com,
864 rsa-sha2-512,rsa-sha2-256,ssh-rsa
867 If hostkeys are known for the destination host then this default is modified
868 to prefer their algorithms.
870 The list of available key types may also be obtained using
871 .Qq ssh -Q HostKeyAlgorithms .
873 Specifies an alias that should be used instead of the
874 real host name when looking up or saving the host key
875 in the host key database files and when validating host certificates.
876 This option is useful for tunneling SSH connections
877 or for multiple servers running on a single host.
879 Specifies the real host name to log into.
880 This can be used to specify nicknames or abbreviations for hosts.
883 accept the tokens described in the
886 Numeric IP addresses are also permitted (both on the command line and in
889 The default is the name given on the command line.
890 .It Cm IdentitiesOnly
893 should only use the configured authentication identity and certificate files
894 (either the default files, or those explicitly configured in the
905 .Cm SecurityKeyProvider
906 offers more identities.
907 The argument to this keyword must be
912 This option is intended for situations where ssh-agent
913 offers many different identities.
917 socket used to communicate with the authentication agent.
919 This option overrides the
921 environment variable and can be used to select a specific agent.
922 Setting the socket name to
924 disables the use of an authentication agent.
927 is specified, the location of the socket will be read from the
929 environment variable.
930 Otherwise if the specified value begins with a
932 character, then it will be treated as an environment variable containing
933 the location of the socket.
937 may use the tilde syntax to refer to a user's home directory
938 or the tokens described in the
942 Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA,
943 Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read.
946 .Pa ~/.ssh/id_ecdsa ,
947 .Pa ~/.ssh/id_ecdsa_sk ,
948 .Pa ~/.ssh/id_ed25519 ,
949 .Pa ~/.ssh/id_ed25519_sk
952 Additionally, any identities represented by the authentication agent
953 will be used for authentication unless
956 If no certificates have been explicitly specified by
957 .Cm CertificateFile ,
959 will try to load certificate information from the filename obtained by
962 to the path of a specified
967 may use the tilde syntax to refer to a user's home directory
968 or the tokens described in the
972 It is possible to have
973 multiple identity files specified in configuration files; all these
974 identities will be tried in sequence.
977 directives will add to the list of identities tried (this behaviour
978 differs from that of other configuration directives).
981 may be used in conjunction with
983 to select which identities in an agent are offered during authentication.
985 may also be used in conjunction with
987 in order to provide any certificate also needed for authentication with
990 Specifies a pattern-list of unknown options to be ignored if they are
991 encountered in configuration parsing.
992 This may be used to suppress errors if
994 contains options that are unrecognised by
996 It is recommended that
998 be listed early in the configuration file as it will not be applied
999 to unknown options that appear before it.
1001 Include the specified configuration file(s).
1002 Multiple pathnames may be specified and each pathname may contain
1004 wildcards and, for user configurations, shell-like
1006 references to user home directories.
1007 Files without absolute paths are assumed to be in
1009 if included in a user configuration file or
1011 if included from the system configuration file.
1013 directive may appear inside a
1018 to perform conditional inclusion.
1020 Specifies the IPv4 type-of-service or DSCP class for connections.
1049 to use the operating system default.
1050 This option may take one or two arguments, separated by whitespace.
1051 If one argument is specified, it is used as the packet class unconditionally.
1052 If two values are specified, the first is automatically selected for
1053 interactive sessions and the second for non-interactive sessions.
1057 for interactive sessions and
1060 for non-interactive sessions.
1061 .It Cm KbdInteractiveAuthentication
1062 Specifies whether to use keyboard-interactive authentication.
1063 The argument to this keyword must be
1068 .It Cm KbdInteractiveDevices
1069 Specifies the list of methods to use in keyboard-interactive authentication.
1070 Multiple method names must be comma-separated.
1071 The default is to use the server specified list.
1072 The methods available vary depending on what the server supports.
1073 For an OpenSSH server,
1074 it may be zero or more of:
1078 .It Cm KexAlgorithms
1079 Specifies the available KEX (Key Exchange) algorithms.
1080 Multiple algorithms must be comma-separated.
1081 If the specified list begins with a
1083 character, then the specified methods will be appended to the default set
1084 instead of replacing them.
1085 If the specified list begins with a
1087 character, then the specified methods (including wildcards) will be removed
1088 from the default set instead of replacing them.
1089 If the specified list begins with a
1091 character, then the specified methods will be placed at the head of the
1094 .Bd -literal -offset indent
1095 curve25519-sha256,curve25519-sha256@libssh.org,
1096 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1097 diffie-hellman-group-exchange-sha256,
1098 diffie-hellman-group16-sha512,
1099 diffie-hellman-group18-sha512,
1100 diffie-hellman-group14-sha256
1103 The list of available key exchange algorithms may also be obtained using
1106 Specifies a command to execute on the local machine after successfully
1107 connecting to the server.
1108 The command string extends to the end of the line, and is executed with
1112 accept the tokens described in the
1116 The command is run synchronously and does not have access to the
1120 It should not be used for interactive commands.
1122 This directive is ignored unless
1123 .Cm PermitLocalCommand
1126 Specifies that a TCP port on the local machine be forwarded over
1127 the secure channel to the specified host and port from the remote machine.
1128 The first argument specifies the listener and may be
1130 .Oo Ar bind_address : Oc Ar port
1132 or a Unix domain socket path.
1133 The second argument is the destination and may be
1134 .Ar host : Ns Ar hostport
1135 or a Unix domain socket path if the remote host supports it.
1137 IPv6 addresses can be specified by enclosing addresses in square brackets.
1138 Multiple forwardings may be specified, and additional forwardings can be
1139 given on the command line.
1140 Only the superuser can forward privileged ports.
1141 By default, the local port is bound in accordance with the
1144 However, an explicit
1146 may be used to bind the connection to a specific address.
1151 indicates that the listening port be bound for local use only, while an
1154 indicates that the port should be available from all interfaces.
1155 Unix domain socket paths accept the tokens described in the
1159 Gives the verbosity level that is used when logging messages from
1161 The possible values are:
1162 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1163 The default is INFO.
1164 DEBUG and DEBUG1 are equivalent.
1165 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1167 Specifies the MAC (message authentication code) algorithms
1168 in order of preference.
1169 The MAC algorithm is used for data integrity protection.
1170 Multiple algorithms must be comma-separated.
1171 If the specified list begins with a
1173 character, then the specified algorithms will be appended to the default set
1174 instead of replacing them.
1175 If the specified list begins with a
1177 character, then the specified algorithms (including wildcards) will be removed
1178 from the default set instead of replacing them.
1179 If the specified list begins with a
1181 character, then the specified algorithms will be placed at the head of the
1184 The algorithms that contain
1186 calculate the MAC after encryption (encrypt-then-mac).
1187 These are considered safer and their use recommended.
1190 .Bd -literal -offset indent
1191 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1192 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1193 hmac-sha1-etm@openssh.com,
1194 umac-64@openssh.com,umac-128@openssh.com,
1195 hmac-sha2-256,hmac-sha2-512,hmac-sha1
1198 The list of available MAC algorithms may also be obtained using
1200 .It Cm NoHostAuthenticationForLocalhost
1201 Disable host authentication for localhost (loopback addresses).
1202 The argument to this keyword must be
1207 .It Cm NumberOfPasswordPrompts
1208 Specifies the number of password prompts before giving up.
1209 The argument to this keyword must be an integer.
1211 .It Cm PasswordAuthentication
1212 Specifies whether to use password authentication.
1213 The argument to this keyword must be
1218 .It Cm PermitLocalCommand
1219 Allow local command execution via the
1222 .Ic !\& Ns Ar command
1225 The argument must be
1230 .It Cm PKCS11Provider
1231 Specifies which PKCS#11 provider to use or
1233 to indicate that no provider should be used (the default).
1234 The argument to this keyword is a path to the PKCS#11 shared library
1236 should use to communicate with a PKCS#11 token providing keys for user
1239 Specifies the port number to connect on the remote host.
1241 .It Cm PreferredAuthentications
1242 Specifies the order in which the client should try authentication methods.
1243 This allows a client to prefer one method (e.g.\&
1244 .Cm keyboard-interactive )
1245 over another method (e.g.\&
1248 .Bd -literal -offset indent
1249 gssapi-with-mic,hostbased,publickey,
1250 keyboard-interactive,password
1253 Specifies the command to use to connect to the server.
1255 string extends to the end of the line, and is executed
1256 using the user's shell
1258 directive to avoid a lingering shell process.
1262 accept the tokens described in the
1265 The command can be basically anything,
1266 and should read from its standard input and write to its standard output.
1267 It should eventually connect an
1269 server running on some machine, or execute
1272 Host key management will be done using the
1274 of the host being connected (defaulting to the name typed by the user).
1275 Setting the command to
1277 disables this option entirely.
1280 is not available for connects with a proxy command.
1282 This directive is useful in conjunction with
1284 and its proxy support.
1285 For example, the following directive would connect via an HTTP proxy at
1287 .Bd -literal -offset 3n
1288 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1291 Specifies one or more jump proxies as either
1300 Multiple proxies may be separated by comma characters and will be visited
1302 Setting this option will cause
1304 to connect to the target host by first making a
1306 connection to the specified
1308 host and then establishing a
1309 TCP forwarding to the ultimate target from there.
1311 Note that this option will compete with the
1313 option - whichever is specified first will prevent later instances of the
1314 other from taking effect.
1316 Note also that the configuration for the destination host (either supplied
1317 via the command-line or the configuration file) is not generally applied
1320 should be used if specific configuration is required for jump hosts.
1321 .It Cm ProxyUseFdpass
1324 will pass a connected file descriptor back to
1326 instead of continuing to execute and pass data.
1329 .It Cm PubkeyAcceptedKeyTypes
1330 Specifies the key types that will be used for public key authentication
1331 as a comma-separated list of patterns.
1332 If the specified list begins with a
1334 character, then the key types after it will be appended to the default
1335 instead of replacing it.
1336 If the specified list begins with a
1338 character, then the specified key types (including wildcards) will be removed
1339 from the default set instead of replacing them.
1340 If the specified list begins with a
1342 character, then the specified key types will be placed at the head of the
1344 The default for this option is:
1345 .Bd -literal -offset 3n
1346 ecdsa-sha2-nistp256-cert-v01@openssh.com,
1347 ecdsa-sha2-nistp384-cert-v01@openssh.com,
1348 ecdsa-sha2-nistp521-cert-v01@openssh.com,
1349 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1350 ssh-ed25519-cert-v01@openssh.com,
1351 sk-ssh-ed25519-cert-v01@openssh.com,
1352 rsa-sha2-512-cert-v01@openssh.com,
1353 rsa-sha2-256-cert-v01@openssh.com,
1354 ssh-rsa-cert-v01@openssh.com,
1355 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1356 sk-ecdsa-sha2-nistp256@openssh.com,
1357 ssh-ed25519,sk-ssh-ed25519@openssh.com,
1358 rsa-sha2-512,rsa-sha2-256,ssh-rsa
1361 The list of available key types may also be obtained using
1362 .Qq ssh -Q PubkeyAcceptedKeyTypes .
1363 .It Cm PubkeyAuthentication
1364 Specifies whether to try public key authentication.
1365 The argument to this keyword must be
1371 Specifies the maximum amount of data that may be transmitted before the
1372 session key is renegotiated, optionally followed a maximum amount of
1373 time that may pass before the session key is renegotiated.
1374 The first argument is specified in bytes and may have a suffix of
1379 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1380 The default is between
1384 depending on the cipher.
1385 The optional second value is specified in seconds and may use any of the
1386 units documented in the TIME FORMATS section of
1388 The default value for
1392 which means that rekeying is performed after the cipher's default amount
1393 of data has been sent or received and no time based rekeying is done.
1394 .It Cm RemoteCommand
1395 Specifies a command to execute on the remote machine after successfully
1396 connecting to the server.
1397 The command string extends to the end of the line, and is executed with
1401 accept the tokens described in the
1404 .It Cm RemoteForward
1405 Specifies that a TCP port on the remote machine be forwarded over
1407 The remote port may either be forwarded to a specified host and port
1408 from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote
1409 client to connect to arbitrary destinations from the local machine.
1410 The first argument is the listening specification and may be
1412 .Oo Ar bind_address : Oc Ar port
1414 or, if the remote host supports it, a Unix domain socket path.
1415 If forwarding to a specific destination then the second argument must be
1416 .Ar host : Ns Ar hostport
1417 or a Unix domain socket path,
1418 otherwise if no destination argument is specified then the remote forwarding
1419 will be established as a SOCKS proxy.
1421 IPv6 addresses can be specified by enclosing addresses in square brackets.
1422 Multiple forwardings may be specified, and additional
1423 forwardings can be given on the command line.
1424 Privileged ports can be forwarded only when
1425 logging in as root on the remote machine.
1426 Unix domain socket paths accept the tokens described in the
1433 the listen port will be dynamically allocated on the server and reported
1434 to the client at run time.
1438 is not specified, the default is to only bind to loopback addresses.
1443 or an empty string, then the forwarding is requested to listen on all
1447 will only succeed if the server's
1449 option is enabled (see
1450 .Xr sshd_config 5 ) .
1452 Specifies whether to request a pseudo-tty for the session.
1453 The argument may be one of:
1455 (never request a TTY),
1457 (always request a TTY when standard input is a TTY),
1459 (always request a TTY) or
1461 (request a TTY when opening a login session).
1462 This option mirrors the
1468 .It Cm RevokedHostKeys
1469 Specifies revoked host public keys.
1470 Keys listed in this file will be refused for host authentication.
1471 Note that if this file does not exist or is not readable,
1472 then host authentication will be refused for all hosts.
1473 Keys may be specified as a text file, listing one public key per line, or as
1474 an OpenSSH Key Revocation List (KRL) as generated by
1476 For more information on KRLs, see the KEY REVOCATION LISTS section in
1478 .It Cm SecurityKeyProvider
1479 Specifies a path to a library that will be used when loading any
1480 FIDO authenticator-hosted keys, overriding the default of using
1481 the built-in USB HID support.
1483 If the specified value begins with a
1485 character, then it will be treated as an environment variable containing
1486 the path to the library.
1488 Specifies what variables from the local
1490 should be sent to the server.
1491 The server must also support it, and the server must be configured to
1492 accept these environment variables.
1495 environment variable is always sent whenever a
1496 pseudo-terminal is requested as it is required by the protocol.
1501 for how to configure the server.
1502 Variables are specified by name, which may contain wildcard characters.
1503 Multiple environment variables may be separated by whitespace or spread
1510 for more information on patterns.
1512 It is possible to clear previously set
1514 variable names by prefixing patterns with
1516 The default is not to send any environment variables.
1517 .It Cm ServerAliveCountMax
1518 Sets the number of server alive messages (see below) which may be
1521 receiving any messages back from the server.
1522 If this threshold is reached while server alive messages are being sent,
1523 ssh will disconnect from the server, terminating the session.
1524 It is important to note that the use of server alive messages is very
1528 The server alive messages are sent through the encrypted channel
1529 and therefore will not be spoofable.
1530 The TCP keepalive option enabled by
1533 The server alive mechanism is valuable when the client or
1534 server depend on knowing when a connection has become unresponsive.
1536 The default value is 3.
1538 .Cm ServerAliveInterval
1539 (see below) is set to 15 and
1540 .Cm ServerAliveCountMax
1541 is left at the default, if the server becomes unresponsive,
1542 ssh will disconnect after approximately 45 seconds.
1543 .It Cm ServerAliveInterval
1544 Sets a timeout interval in seconds after which if no data has been received
1547 will send a message through the encrypted
1548 channel to request a response from the server.
1550 is 0, indicating that these messages will not be sent to the server.
1552 Directly specify one or more environment variables and their contents to
1553 be sent to the server.
1556 the server must be prepared to accept the environment variable.
1557 .It Cm StreamLocalBindMask
1558 Sets the octal file creation mode mask
1560 used when creating a Unix-domain socket file for local or remote
1562 This option is only used for port forwarding to a Unix-domain socket file.
1564 The default value is 0177, which creates a Unix-domain socket file that is
1565 readable and writable only by the owner.
1566 Note that not all operating systems honor the file mode on Unix-domain
1568 .It Cm StreamLocalBindUnlink
1569 Specifies whether to remove an existing Unix-domain socket file for local
1570 or remote port forwarding before creating a new one.
1571 If the socket file already exists and
1572 .Cm StreamLocalBindUnlink
1575 will be unable to forward the port to the Unix-domain socket file.
1576 This option is only used for port forwarding to a Unix-domain socket file.
1578 The argument must be
1583 .It Cm StrictHostKeyChecking
1584 If this flag is set to
1587 will never automatically add host keys to the
1588 .Pa ~/.ssh/known_hosts
1589 file, and refuses to connect to hosts whose host key has changed.
1590 This provides maximum protection against man-in-the-middle (MITM) attacks,
1591 though it can be annoying when the
1592 .Pa /etc/ssh/ssh_known_hosts
1593 file is poorly maintained or when connections to new hosts are
1595 This option forces the user to manually
1598 If this flag is set to
1600 then ssh will automatically add new host keys to the user
1601 known hosts files, but will not permit connections to hosts with
1603 If this flag is set to
1607 ssh will automatically add new host keys to the user known hosts files
1608 and allow connections to hosts with changed hostkeys to proceed,
1609 subject to some restrictions.
1610 If this flag is set to
1614 will be added to the user known host files only after the user
1615 has confirmed that is what they really want to do, and
1616 ssh will refuse to connect to hosts whose host key has changed.
1618 known hosts will be verified automatically in all cases.
1619 .It Cm SyslogFacility
1620 Gives the facility code that is used when logging messages from
1622 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1623 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1624 The default is USER.
1626 Specifies whether the system should send TCP keepalive messages to the
1628 If they are sent, death of the connection or crash of one
1629 of the machines will be properly noticed.
1630 However, this means that
1631 connections will die if the route is down temporarily, and some people
1636 (to send TCP keepalive messages), and the client will notice
1637 if the network goes down or the remote host dies.
1638 This is important in scripts, and many users want it too.
1640 To disable TCP keepalive messages, the value should be set to
1643 .Cm ServerAliveInterval
1644 for protocol-level keepalives.
1648 device forwarding between the client and the server.
1649 The argument must be
1660 requests the default tunnel mode, which is
1661 .Cm point-to-point .
1665 devices to open on the client
1670 The argument must be
1672 .Ar local_tun Op : Ar remote_tun .
1674 The devices may be specified by numerical ID or the keyword
1676 which uses the next available tunnel device.
1679 is not specified, it defaults to
1683 .It Cm UpdateHostKeys
1686 should accept notifications of additional hostkeys from the server sent
1687 after authentication has completed and add them to
1688 .Cm UserKnownHostsFile .
1689 The argument must be
1694 This option allows learning alternate hostkeys for a server
1695 and supports graceful key rotation by allowing a server to send replacement
1696 public keys before old ones are removed.
1697 Additional hostkeys are only accepted if the key used to authenticate the
1698 host was already trusted or explicitly accepted by the user.
1701 is enabled by default if the user has not overridden the default
1702 .Cm UserKnownHostsFile
1712 then the user is asked to confirm the modifications to the known_hosts file.
1713 Confirmation is currently incompatible with
1714 .Cm ControlPersist ,
1715 and will be disabled if it is enabled.
1719 from OpenSSH 6.8 and greater support the
1720 .Qq hostkeys@openssh.com
1721 protocol extension used to inform the client of all the server's hostkeys.
1723 Specifies the user to log in as.
1724 This can be useful when a different user name is used on different machines.
1725 This saves the trouble of
1726 having to remember to give the user name on the command line.
1727 .It Cm UserKnownHostsFile
1728 Specifies one or more files to use for the user
1729 host key database, separated by whitespace.
1731 .Pa ~/.ssh/known_hosts ,
1732 .Pa ~/.ssh/known_hosts2 .
1733 .It Cm VerifyHostKeyDNS
1734 Specifies whether to verify the remote key using DNS and SSHFP resource
1736 If this option is set to
1738 the client will implicitly trust keys that match a secure fingerprint
1740 Insecure fingerprints will be handled as if this option was set to
1742 If this option is set to
1744 information on fingerprint match will be displayed, but the user will still
1745 need to confirm new host keys according to the
1746 .Cm StrictHostKeyChecking
1752 .Sx VERIFYING HOST KEYS
1755 .It Cm VisualHostKey
1756 If this flag is set to
1758 an ASCII art representation of the remote host key fingerprint is
1759 printed in addition to the fingerprint string at login and
1760 for unknown host keys.
1761 If this flag is set to
1764 no fingerprint strings are printed at login and
1765 only the fingerprint string will be printed for unknown host keys.
1766 .It Cm XAuthLocation
1767 Specifies the full pathname of the
1771 .Pa /usr/X11R6/bin/xauth .
1776 consists of zero or more non-whitespace characters,
1778 (a wildcard that matches zero or more characters),
1781 (a wildcard that matches exactly one character).
1782 For example, to specify a set of declarations for any host in the
1785 the following pattern could be used:
1789 The following pattern
1790 would match any host in the 192.168.0.[0-9] network range:
1792 .Dl Host 192.168.0.?
1796 is a comma-separated list of patterns.
1797 Patterns within pattern-lists may be negated
1798 by preceding them with an exclamation mark
1801 to allow a key to be used from anywhere within an organization
1805 the following entry (in authorized_keys) could be used:
1807 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1809 Note that a negated match will never produce a positive result by itself.
1810 For example, attempting to match
1812 against the following pattern-list will fail:
1814 .Dl from=\&"!host1,!host2\&"
1816 The solution here is to include a term that will yield a positive match,
1819 .Dl from=\&"!host1,!host2,*\&"
1821 Arguments to some keywords can make use of tokens,
1822 which are expanded at runtime:
1824 .Bl -tag -width XXXX -offset indent -compact
1831 Local user's home directory.
1833 The remote hostname.
1839 The local hostname, including the domain name.
1841 The original remote hostname, as given on the command line.
1845 The remote username.
1851 network interface assigned if
1852 tunnel forwarding was requested, or
1859 .Cm CertificateFile ,
1868 accept the tokens %%, %C, %d, %h, %i, %L, %l, %n, %p, %r, and %u.
1871 accepts the tokens %% and %h.
1877 accepts the tokens %%, %h, %n, %p, and %r.
1880 .It Pa ~/.ssh/config
1881 This is the per-user configuration file.
1882 The format of this file is described above.
1883 This file is used by the SSH client.
1884 Because of the potential for abuse, this file must have strict permissions:
1885 read/write for the user, and not writable by others.
1886 .It Pa /etc/ssh/ssh_config
1887 Systemwide configuration file.
1888 This file provides defaults for those
1889 values that are not specified in the user's configuration file, and
1890 for those users who do not have a configuration file.
1891 This file must be world-readable.
1897 OpenSSH is a derivative of the original and free
1898 ssh 1.2.12 release by
1900 .An Aaron Campbell , Bob Beck , Markus Friedl ,
1901 .An Niels Provos , Theo de Raadt
1904 removed many bugs, re-added newer features and
1907 contributed the support for SSH protocol versions 1.5 and 2.0.