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 1.374 2022/09/17 10:33:18 djm Exp $
37 .Dd $Mdocdate: September 17 2022 $
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.
248 Alternately, this option may be specified as a time interval
249 using the format described in the
253 to specify the key's lifetime in
255 after which it will automatically be removed.
261 (optionally followed by a time interval),
265 Specifies which address family to use when connecting.
276 user interaction such as password prompts and host key confirmation requests
278 This option is useful in scripts and other batch jobs where no user
279 is present to interact with
287 Use the specified address on the local machine as the source address of
289 Only useful on systems with more than one address.
291 Use the address of the specified interface on the local machine as the
292 source address of the connection.
293 .It Cm CanonicalDomains
295 .Cm CanonicalizeHostname
296 is enabled, this option specifies the list of domain suffixes in which to
297 search for the specified destination host.
298 .It Cm CanonicalizeFallbackLocal
299 Specifies whether to fail with an error when hostname canonicalization fails.
302 will attempt to look up the unqualified hostname using the system resolver's
309 .Cm CanonicalizeHostname
310 is enabled and the target hostname cannot be found in any of the domains
312 .Cm CanonicalDomains .
313 .It Cm CanonicalizeHostname
314 Controls whether explicit hostname canonicalization is performed.
317 is not to perform any name rewriting and let the system resolver handle all
321 then, for connections that do not use a
326 will attempt to canonicalize the hostname specified on the command line
330 .Cm CanonicalizePermittedCNAMEs
333 .Cm CanonicalizeHostname
336 then canonicalization is applied to proxied connections too.
338 If this option is enabled, then the configuration files are processed
339 again using the new target name to pick up any new configuration in matching
346 disables the use of a
349 .It Cm CanonicalizeMaxDots
350 Specifies the maximum number of dot characters in a hostname before
351 canonicalization is disabled.
353 allows a single dot (i.e. hostname.subdomain).
354 .It Cm CanonicalizePermittedCNAMEs
355 Specifies rules to determine whether CNAMEs should be followed when
356 canonicalizing hostnames.
357 The rules consist of one or more arguments of
358 .Ar source_domain_list : Ns Ar target_domain_list ,
360 .Ar source_domain_list
361 is a pattern-list of domains that may follow CNAMEs in canonicalization,
363 .Ar target_domain_list
364 is a pattern-list of domains that they may resolve to.
367 .Qq *.a.example.com:*.b.example.com,*.c.example.com
368 will allow hostnames matching
370 to be canonicalized to names in the
378 causes no CNAMEs to be considered for canonicalization.
379 This is the default behaviour.
380 .It Cm CASignatureAlgorithms
381 Specifies which algorithms are allowed for signing of certificates
382 by certificate authorities (CAs).
384 .Bd -literal -offset indent
385 ssh-ed25519,ecdsa-sha2-nistp256,
386 ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
387 sk-ssh-ed25519@openssh.com,
388 sk-ecdsa-sha2-nistp256@openssh.com,
389 rsa-sha2-512,rsa-sha2-256
392 If the specified list begins with a
394 character, then the specified algorithms will be appended to the default set
395 instead of replacing them.
396 If the specified list begins with a
398 character, then the specified algorithms (including wildcards) will be removed
399 from the default set instead of replacing them.
402 will not accept host certificates signed using algorithms other than those
404 .It Cm CertificateFile
405 Specifies a file from which the user's certificate is read.
406 A corresponding private key must be provided separately in order
407 to use this certificate either
419 .Cm SecurityKeyProvider .
423 may use the tilde syntax to refer to a user's home directory,
424 the tokens described in the
426 section and environment variables as described in the
427 .Sx ENVIRONMENT VARIABLES
430 It is possible to have multiple certificate files specified in
431 configuration files; these certificates will be tried in sequence.
434 directives will add to the list of certificates used for
440 will additionally check the host IP address in the
443 This allows it to detect if a host key changed due to DNS spoofing
444 and will add addresses of destination hosts to
445 .Pa ~/.ssh/known_hosts
446 in the process, regardless of the setting of
447 .Cm StrictHostKeyChecking .
448 If the option is set to
451 the check will not be executed.
453 Specifies the ciphers allowed and their order of preference.
454 Multiple ciphers must be comma-separated.
455 If the specified list begins with a
457 character, then the specified ciphers will be appended to the default set
458 instead of replacing them.
459 If the specified list begins with a
461 character, then the specified ciphers (including wildcards) will be removed
462 from the default set instead of replacing them.
463 If the specified list begins with a
465 character, then the specified ciphers will be placed at the head of the
468 The supported ciphers are:
469 .Bd -literal -offset indent
477 aes128-gcm@openssh.com
478 aes256-gcm@openssh.com
479 chacha20-poly1305@openssh.com
483 .Bd -literal -offset indent
484 chacha20-poly1305@openssh.com,
485 aes128-ctr,aes192-ctr,aes256-ctr,
486 aes128-gcm@openssh.com,aes256-gcm@openssh.com
489 The list of available ciphers may also be obtained using
491 .It Cm ClearAllForwardings
492 Specifies that all local, remote, and dynamic port forwardings
493 specified in the configuration files or on the command line be
495 This option is primarily useful when used from the
497 command line to clear port forwardings set in
498 configuration files, and is automatically set by
508 Specifies whether to use compression.
514 .It Cm ConnectionAttempts
515 Specifies the number of tries (one per second) to make before exiting.
516 The argument must be an integer.
517 This may be useful in scripts if the connection sometimes fails.
519 .It Cm ConnectTimeout
520 Specifies the timeout (in seconds) used when connecting to the
521 SSH server, instead of using the default system TCP timeout.
522 This timeout is applied both to establishing the connection and to performing
523 the initial SSH protocol handshake and key exchange.
525 Enables the sharing of multiple sessions over a single network connection.
529 will listen for connections on a control socket specified using the
532 Additional sessions can connect to this socket using the same
539 These sessions will try to reuse the master instance's network connection
540 rather than initiating new ones, but will fall back to connecting normally
541 if the control socket does not exist, or is not listening.
547 to listen for control connections, but require confirmation using
553 will continue without connecting to a master instance.
557 forwarding is supported over these multiplexed connections, however the
558 display and agent forwarded will be the one belonging to the master
559 connection i.e. it is not possible to forward multiple displays or agents.
561 Two additional options allow for opportunistic multiplexing: try to use a
562 master connection but fall back to creating a new one if one does not already
568 The latter requires confirmation like the
572 Specify the path to the control socket used for connection sharing as described
575 section above or the string
577 to disable connection sharing.
580 may use the tilde syntax to refer to a user's home directory,
581 the tokens described in the
583 section and environment variables as described in the
584 .Sx ENVIRONMENT VARIABLES
586 It is recommended that any
588 used for opportunistic connection sharing include
589 at least %h, %p, and %r (or alternatively %C) and be placed in a directory
590 that is not writable by other users.
591 This ensures that shared connections are uniquely identified.
592 .It Cm ControlPersist
593 When used in conjunction with
595 specifies that the master connection should remain open
596 in the background (waiting for future client connections)
597 after the initial client connection has been closed.
601 then the master connection will not be placed into the background,
602 and will close as soon as the initial client connection is closed.
606 then the master connection will remain in the background indefinitely
607 (until killed or closed via a mechanism such as the
609 If set to a time in seconds, or a time in any of the formats documented in
611 then the backgrounded master connection will automatically terminate
612 after it has remained idle (with no client connections) for the
614 .It Cm DynamicForward
615 Specifies that a TCP port on the local machine be forwarded
616 over the secure channel, and the application
617 protocol is then used to determine where to connect to from the
622 .Oo Ar bind_address : Oc Ar port .
624 IPv6 addresses can be specified by enclosing addresses in square brackets.
625 By default, the local port is bound in accordance with the
630 may be used to bind the connection to a specific address.
635 indicates that the listening port be bound for local use only, while an
638 indicates that the port should be available from all interfaces.
640 Currently the SOCKS4 and SOCKS5 protocols are supported, and
642 will act as a SOCKS server.
643 Multiple forwardings may be specified, and
644 additional forwardings can be given on the command line.
645 Only the superuser can forward privileged ports.
646 .It Cm EnableSSHKeysign
647 Setting this option to
649 in the global client configuration file
650 .Pa /etc/ssh/ssh_config
651 enables the use of the helper program
654 .Cm HostbasedAuthentication .
660 This option should be placed in the non-hostspecific section.
663 for more information.
665 Sets the escape character (default:
667 The escape character can also
668 be set on the command line.
669 The argument should be a single character,
671 followed by a letter, or
673 to disable the escape
674 character entirely (making the connection transparent for binary
676 .It Cm ExitOnForwardFailure
679 should terminate the connection if it cannot set up all requested
680 dynamic, tunnel, local, and remote port forwardings, (e.g.\&
681 if either end is unable to bind and listen on a specified port).
683 .Cm ExitOnForwardFailure
684 does not apply to connections made over port forwardings and will not,
687 to exit if TCP connections to the ultimate forwarding destination fail.
693 .It Cm FingerprintHash
694 Specifies the hash algorithm used when displaying key fingerprints.
700 .It Cm ForkAfterAuthentication
703 to go to background just before command execution.
706 is going to ask for passwords or passphrases, but the user
707 wants it in the background.
710 configuration option being set to
712 The recommended way to start X11 programs at a remote site is with
714 .Ic ssh -f host xterm ,
718 .Cm ForkAfterAuthentication
719 configuration option is set to
723 .Cm ExitOnForwardFailure
724 configuration option is set to
726 then a client started with the
727 .Cm ForkAfterAuthentication
728 configuration option being set to
730 will wait for all remote port forwards to be successfully established
731 before placing itself in the background.
732 The argument to this keyword must be
740 Specifies whether the connection to the authentication agent (if any)
741 will be forwarded to the remote machine.
746 an explicit path to an agent socket or the name of an environment variable
749 in which to find the path.
751 Agent forwarding should be enabled with caution.
752 Users with the ability to bypass file permissions on the remote host
753 (for the agent's Unix-domain socket)
754 can access the local agent through the forwarded connection.
755 An attacker cannot obtain key material from the agent,
756 however they can perform operations on the keys that enable them to
757 authenticate using the identities loaded into the agent.
759 Specifies whether X11 connections will be automatically redirected
760 over the secure channel and
769 X11 forwarding should be enabled with caution.
770 Users with the ability to bypass file permissions on the remote host
771 (for the user's X11 authorization database)
772 can access the local X11 display through the forwarded connection.
773 An attacker may then be able to perform activities such as keystroke monitoring
775 .Cm ForwardX11Trusted
776 option is also enabled.
777 .It Cm ForwardX11Timeout
778 Specify a timeout for untrusted X11 forwarding
779 using the format described in the
783 X11 connections received by
785 after this time will be refused.
787 .Cm ForwardX11Timeout
788 to zero will disable the timeout and permit X11 forwarding for the life
790 The default is to disable untrusted X11 forwarding after twenty minutes has
792 .It Cm ForwardX11Trusted
793 If this option is set to
795 remote X11 clients will have full access to the original X11 display.
797 If this option is set to
800 remote X11 clients will be considered untrusted and prevented
801 from stealing or tampering with data belonging to trusted X11
805 token used for the session will be set to expire after 20 minutes.
806 Remote clients will be refused access after this time.
808 See the X11 SECURITY extension specification for full details on
809 the restrictions imposed on untrusted clients.
811 Specifies whether remote hosts are allowed to connect to local
815 binds local port forwardings to the loopback address.
816 This prevents other remote hosts from connecting to forwarded ports.
818 can be used to specify that ssh
819 should bind local port forwardings to the wildcard address,
820 thus allowing remote hosts to connect to forwarded ports.
826 .It Cm GlobalKnownHostsFile
827 Specifies one or more files to use for the global
828 host key database, separated by whitespace.
830 .Pa /etc/ssh/ssh_known_hosts ,
831 .Pa /etc/ssh/ssh_known_hosts2 .
832 .It Cm GSSAPIAuthentication
833 Specifies whether user authentication based on GSSAPI is allowed.
836 .It Cm GSSAPIDelegateCredentials
837 Forward (delegate) credentials to the server.
840 .It Cm HashKnownHosts
843 should hash host names and addresses when they are added to
844 .Pa ~/.ssh/known_hosts .
845 These hashed names may be used normally by
849 but they do not visually reveal identifying information if the
850 file's contents are disclosed.
853 Note that existing names and addresses in known hosts files
854 will not be converted automatically,
855 but may be manually hashed using
857 .It Cm HostbasedAcceptedAlgorithms
858 Specifies the signature algorithms that will be used for hostbased
859 authentication as a comma-separated list of patterns.
860 Alternately if the specified list begins with a
862 character, then the specified signature algorithms will be appended
863 to the default set instead of replacing them.
864 If the specified list begins with a
866 character, then the specified signature algorithms (including wildcards)
867 will be removed from the default set instead of replacing them.
868 If the specified list begins with a
870 character, then the specified signature algorithms will be placed
871 at the head of the default set.
872 The default for this option is:
873 .Bd -literal -offset 3n
874 ssh-ed25519-cert-v01@openssh.com,
875 ecdsa-sha2-nistp256-cert-v01@openssh.com,
876 ecdsa-sha2-nistp384-cert-v01@openssh.com,
877 ecdsa-sha2-nistp521-cert-v01@openssh.com,
878 sk-ssh-ed25519-cert-v01@openssh.com,
879 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
880 rsa-sha2-512-cert-v01@openssh.com,
881 rsa-sha2-256-cert-v01@openssh.com,
883 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
884 sk-ssh-ed25519@openssh.com,
885 sk-ecdsa-sha2-nistp256@openssh.com,
886 rsa-sha2-512,rsa-sha2-256
893 may be used to list supported signature algorithms.
894 This was formerly named HostbasedKeyTypes.
895 .It Cm HostbasedAuthentication
896 Specifies whether to try rhosts based authentication with public key
903 .It Cm HostKeyAlgorithms
904 Specifies the host key signature algorithms
905 that the client wants to use in order of preference.
906 Alternately if the specified list begins with a
908 character, then the specified signature algorithms will be appended to
909 the default set instead of replacing them.
910 If the specified list begins with a
912 character, then the specified signature algorithms (including wildcards)
913 will be removed from the default set instead of replacing them.
914 If the specified list begins with a
916 character, then the specified signature algorithms will be placed
917 at the head of the default set.
918 The default for this option is:
919 .Bd -literal -offset 3n
920 ssh-ed25519-cert-v01@openssh.com,
921 ecdsa-sha2-nistp256-cert-v01@openssh.com,
922 ecdsa-sha2-nistp384-cert-v01@openssh.com,
923 ecdsa-sha2-nistp521-cert-v01@openssh.com,
924 sk-ssh-ed25519-cert-v01@openssh.com,
925 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
926 rsa-sha2-512-cert-v01@openssh.com,
927 rsa-sha2-256-cert-v01@openssh.com,
929 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
930 sk-ecdsa-sha2-nistp256@openssh.com,
931 sk-ssh-ed25519@openssh.com,
932 rsa-sha2-512,rsa-sha2-256
935 If hostkeys are known for the destination host then this default is modified
936 to prefer their algorithms.
938 The list of available signature algorithms may also be obtained using
939 .Qq ssh -Q HostKeyAlgorithms .
941 Specifies an alias that should be used instead of the
942 real host name when looking up or saving the host key
943 in the host key database files and when validating host certificates.
944 This option is useful for tunneling SSH connections
945 or for multiple servers running on a single host.
947 Specifies the real host name to log into.
948 This can be used to specify nicknames or abbreviations for hosts.
951 accept the tokens described in the
954 Numeric IP addresses are also permitted (both on the command line and in
957 The default is the name given on the command line.
958 .It Cm IdentitiesOnly
961 should only use the configured authentication identity and certificate files
962 (either the default files, or those explicitly configured in the
973 .Cm SecurityKeyProvider
974 offers more identities.
975 The argument to this keyword must be
980 This option is intended for situations where ssh-agent
981 offers many different identities.
985 socket used to communicate with the authentication agent.
987 This option overrides the
989 environment variable and can be used to select a specific agent.
990 Setting the socket name to
992 disables the use of an authentication agent.
995 is specified, the location of the socket will be read from the
997 environment variable.
998 Otherwise if the specified value begins with a
1000 character, then it will be treated as an environment variable containing
1001 the location of the socket.
1005 may use the tilde syntax to refer to a user's home directory,
1006 the tokens described in the
1008 section and environment variables as described in the
1009 .Sx ENVIRONMENT VARIABLES
1012 Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA,
1013 Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read.
1016 .Pa ~/.ssh/id_ecdsa ,
1017 .Pa ~/.ssh/id_ecdsa_sk ,
1018 .Pa ~/.ssh/id_ed25519 ,
1019 .Pa ~/.ssh/id_ed25519_sk
1022 Additionally, any identities represented by the authentication agent
1023 will be used for authentication unless
1026 If no certificates have been explicitly specified by
1027 .Cm CertificateFile ,
1029 will try to load certificate information from the filename obtained by
1032 to the path of a specified
1037 may use the tilde syntax to refer to a user's home directory
1038 or the tokens described in the
1042 It is possible to have
1043 multiple identity files specified in configuration files; all these
1044 identities will be tried in sequence.
1047 directives will add to the list of identities tried (this behaviour
1048 differs from that of other configuration directives).
1051 may be used in conjunction with
1053 to select which identities in an agent are offered during authentication.
1055 may also be used in conjunction with
1057 in order to provide any certificate also needed for authentication with
1059 .It Cm IgnoreUnknown
1060 Specifies a pattern-list of unknown options to be ignored if they are
1061 encountered in configuration parsing.
1062 This may be used to suppress errors if
1064 contains options that are unrecognised by
1066 It is recommended that
1068 be listed early in the configuration file as it will not be applied
1069 to unknown options that appear before it.
1071 Include the specified configuration file(s).
1072 Multiple pathnames may be specified and each pathname may contain
1074 wildcards and, for user configurations, shell-like
1076 references to user home directories.
1077 Wildcards will be expanded and processed in lexical order.
1078 Files without absolute paths are assumed to be in
1080 if included in a user configuration file or
1082 if included from the system configuration file.
1084 directive may appear inside a
1089 to perform conditional inclusion.
1091 Specifies the IPv4 type-of-service or DSCP class for connections.
1120 to use the operating system default.
1121 This option may take one or two arguments, separated by whitespace.
1122 If one argument is specified, it is used as the packet class unconditionally.
1123 If two values are specified, the first is automatically selected for
1124 interactive sessions and the second for non-interactive sessions.
1128 for interactive sessions and
1131 for non-interactive sessions.
1132 .It Cm KbdInteractiveAuthentication
1133 Specifies whether to use keyboard-interactive authentication.
1134 The argument to this keyword must be
1139 .Cm ChallengeResponseAuthentication
1140 is a deprecated alias for this.
1141 .It Cm KbdInteractiveDevices
1142 Specifies the list of methods to use in keyboard-interactive authentication.
1143 Multiple method names must be comma-separated.
1144 The default is to use the server specified list.
1145 The methods available vary depending on what the server supports.
1146 For an OpenSSH server,
1147 it may be zero or more of:
1151 .It Cm KexAlgorithms
1152 Specifies the available KEX (Key Exchange) algorithms.
1153 Multiple algorithms must be comma-separated.
1154 If the specified list begins with a
1156 character, then the specified algorithms will be appended to the default set
1157 instead of replacing them.
1158 If the specified list begins with a
1160 character, then the specified algorithms (including wildcards) will be removed
1161 from the default set instead of replacing them.
1162 If the specified list begins with a
1164 character, then the specified algorithms will be placed at the head of the
1167 .Bd -literal -offset indent
1168 sntrup761x25519-sha512@openssh.com,
1169 curve25519-sha256,curve25519-sha256@libssh.org,
1170 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1171 diffie-hellman-group-exchange-sha256,
1172 diffie-hellman-group16-sha512,
1173 diffie-hellman-group18-sha512,
1174 diffie-hellman-group14-sha256
1177 The list of available key exchange algorithms may also be obtained using
1179 .It Cm KnownHostsCommand
1180 Specifies a command to use to obtain a list of host keys, in addition to
1182 .Cm UserKnownHostsFile
1184 .Cm GlobalKnownHostsFile .
1185 This command is executed after the files have been read.
1186 It may write host key lines to standard output in identical format to the
1187 usual files (described in the
1188 .Sx VERIFYING HOST KEYS
1192 .Cm KnownHostsCommand
1193 accept the tokens described in the
1196 The command may be invoked multiple times per connection: once when preparing
1197 the preference list of host key algorithms to use, again to obtain the
1198 host key for the requested host name and, if
1200 is enabled, one more time to obtain the host key matching the server's
1202 If the command exits abnormally or returns a non-zero exit status then the
1203 connection is terminated.
1205 Specifies a command to execute on the local machine after successfully
1206 connecting to the server.
1207 The command string extends to the end of the line, and is executed with
1211 accept the tokens described in the
1215 The command is run synchronously and does not have access to the
1219 It should not be used for interactive commands.
1221 This directive is ignored unless
1222 .Cm PermitLocalCommand
1225 Specifies that a TCP port on the local machine be forwarded over
1226 the secure channel to the specified host and port from the remote machine.
1227 The first argument specifies the listener and may be
1229 .Oo Ar bind_address : Oc Ar port
1231 or a Unix domain socket path.
1232 The second argument is the destination and may be
1233 .Ar host : Ns Ar hostport
1234 or a Unix domain socket path if the remote host supports it.
1236 IPv6 addresses can be specified by enclosing addresses in square brackets.
1237 Multiple forwardings may be specified, and additional forwardings can be
1238 given on the command line.
1239 Only the superuser can forward privileged ports.
1240 By default, the local port is bound in accordance with the
1243 However, an explicit
1245 may be used to bind the connection to a specific address.
1250 indicates that the listening port be bound for local use only, while an
1253 indicates that the port should be available from all interfaces.
1254 Unix domain socket paths may use the tokens described in the
1256 section and environment variables as described in the
1257 .Sx ENVIRONMENT VARIABLES
1260 Gives the verbosity level that is used when logging messages from
1262 The possible values are:
1263 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1264 The default is INFO.
1265 DEBUG and DEBUG1 are equivalent.
1266 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1268 Specify one or more overrides to LogLevel.
1269 An override consists of a pattern lists that matches the source file, function
1270 and line number to force detailed logging for.
1271 For example, an override pattern of:
1272 .Bd -literal -offset indent
1273 kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
1276 would enable detailed logging for line 1000 of
1279 .Fn kex_exchange_identification
1280 function, and all code in the
1283 This option is intended for debugging and no overrides are enabled by default.
1285 Specifies the MAC (message authentication code) algorithms
1286 in order of preference.
1287 The MAC algorithm is used for data integrity protection.
1288 Multiple algorithms must be comma-separated.
1289 If the specified list begins with a
1291 character, then the specified algorithms will be appended to the default set
1292 instead of replacing them.
1293 If the specified list begins with a
1295 character, then the specified algorithms (including wildcards) will be removed
1296 from the default set instead of replacing them.
1297 If the specified list begins with a
1299 character, then the specified algorithms will be placed at the head of the
1302 The algorithms that contain
1304 calculate the MAC after encryption (encrypt-then-mac).
1305 These are considered safer and their use recommended.
1308 .Bd -literal -offset indent
1309 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1310 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1311 hmac-sha1-etm@openssh.com,
1312 umac-64@openssh.com,umac-128@openssh.com,
1313 hmac-sha2-256,hmac-sha2-512,hmac-sha1
1316 The list of available MAC algorithms may also be obtained using
1318 .It Cm NoHostAuthenticationForLocalhost
1319 Disable host authentication for localhost (loopback addresses).
1320 The argument to this keyword must be
1325 .It Cm NumberOfPasswordPrompts
1326 Specifies the number of password prompts before giving up.
1327 The argument to this keyword must be an integer.
1329 .It Cm PasswordAuthentication
1330 Specifies whether to use password authentication.
1331 The argument to this keyword must be
1336 .It Cm PermitLocalCommand
1337 Allow local command execution via the
1340 .Ic !\& Ns Ar command
1343 The argument must be
1348 .It Cm PermitRemoteOpen
1349 Specifies the destinations to which remote TCP port forwarding is permitted when
1351 is used as a SOCKS proxy.
1352 The forwarding specification must be one of the following forms:
1354 .Bl -item -offset indent -compact
1356 .Cm PermitRemoteOpen
1361 .Cm PermitRemoteOpen
1363 .Ar IPv4_addr : port
1366 .Cm PermitRemoteOpen
1368 .Ar \&[ IPv6_addr \&] : port
1372 Multiple forwards may be specified by separating them with whitespace.
1375 can be used to remove all restrictions and permit any forwarding requests.
1378 can be used to prohibit all forwarding requests.
1381 can be used for host or port to allow all hosts or ports respectively.
1382 Otherwise, no pattern matching or address lookups are performed on supplied
1384 .It Cm PKCS11Provider
1385 Specifies which PKCS#11 provider to use or
1387 to indicate that no provider should be used (the default).
1388 The argument to this keyword is a path to the PKCS#11 shared library
1390 should use to communicate with a PKCS#11 token providing keys for user
1393 Specifies the port number to connect on the remote host.
1395 .It Cm PreferredAuthentications
1396 Specifies the order in which the client should try authentication methods.
1397 This allows a client to prefer one method (e.g.\&
1398 .Cm keyboard-interactive )
1399 over another method (e.g.\&
1402 .Bd -literal -offset indent
1403 gssapi-with-mic,hostbased,publickey,
1404 keyboard-interactive,password
1407 Specifies the command to use to connect to the server.
1409 string extends to the end of the line, and is executed
1410 using the user's shell
1412 directive to avoid a lingering shell process.
1416 accept the tokens described in the
1419 The command can be basically anything,
1420 and should read from its standard input and write to its standard output.
1421 It should eventually connect an
1423 server running on some machine, or execute
1426 Host key management will be done using the
1428 of the host being connected (defaulting to the name typed by the user).
1429 Setting the command to
1431 disables this option entirely.
1434 is not available for connects with a proxy command.
1436 This directive is useful in conjunction with
1438 and its proxy support.
1439 For example, the following directive would connect via an HTTP proxy at
1441 .Bd -literal -offset 3n
1442 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1445 Specifies one or more jump proxies as either
1454 Multiple proxies may be separated by comma characters and will be visited
1456 Setting this option will cause
1458 to connect to the target host by first making a
1460 connection to the specified
1462 host and then establishing a
1463 TCP forwarding to the ultimate target from there.
1466 disables this option entirely.
1468 Note that this option will compete with the
1470 option - whichever is specified first will prevent later instances of the
1471 other from taking effect.
1473 Note also that the configuration for the destination host (either supplied
1474 via the command-line or the configuration file) is not generally applied
1477 should be used if specific configuration is required for jump hosts.
1478 .It Cm ProxyUseFdpass
1481 will pass a connected file descriptor back to
1483 instead of continuing to execute and pass data.
1486 .It Cm PubkeyAcceptedAlgorithms
1487 Specifies the signature algorithms that will be used for public key
1488 authentication as a comma-separated list of patterns.
1489 If the specified list begins with a
1491 character, then the algorithms after it will be appended to the default
1492 instead of replacing it.
1493 If the specified list begins with a
1495 character, then the specified algorithms (including wildcards) will be removed
1496 from the default set instead of replacing them.
1497 If the specified list begins with a
1499 character, then the specified algorithms will be placed at the head of the
1501 The default for this option is:
1502 .Bd -literal -offset 3n
1503 ssh-ed25519-cert-v01@openssh.com,
1504 ecdsa-sha2-nistp256-cert-v01@openssh.com,
1505 ecdsa-sha2-nistp384-cert-v01@openssh.com,
1506 ecdsa-sha2-nistp521-cert-v01@openssh.com,
1507 sk-ssh-ed25519-cert-v01@openssh.com,
1508 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1509 rsa-sha2-512-cert-v01@openssh.com,
1510 rsa-sha2-256-cert-v01@openssh.com,
1512 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1513 sk-ssh-ed25519@openssh.com,
1514 sk-ecdsa-sha2-nistp256@openssh.com,
1515 rsa-sha2-512,rsa-sha2-256
1518 The list of available signature algorithms may also be obtained using
1519 .Qq ssh -Q PubkeyAcceptedAlgorithms .
1520 .It Cm PubkeyAuthentication
1521 Specifies whether to try public key authentication.
1522 The argument to this keyword must be
1529 The final two options enable public key authentication while respectively
1530 disabling or enabling the OpenSSH host-bound authentication protocol
1531 extension required for restricted
1535 Specifies the maximum amount of data that may be transmitted or received
1536 before the session key is renegotiated, optionally followed by a maximum
1537 amount of time that may pass before the session key is renegotiated.
1538 The first argument is specified in bytes and may have a suffix of
1543 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1544 The default is between
1548 depending on the cipher.
1549 The optional second value is specified in seconds and may use any of the
1550 units documented in the TIME FORMATS section of
1552 The default value for
1556 which means that rekeying is performed after the cipher's default amount
1557 of data has been sent or received and no time based rekeying is done.
1558 .It Cm RemoteCommand
1559 Specifies a command to execute on the remote machine after successfully
1560 connecting to the server.
1561 The command string extends to the end of the line, and is executed with
1565 accept the tokens described in the
1568 .It Cm RemoteForward
1569 Specifies that a TCP port on the remote machine be forwarded over
1571 The remote port may either be forwarded to a specified host and port
1572 from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote
1573 client to connect to arbitrary destinations from the local machine.
1574 The first argument is the listening specification and may be
1576 .Oo Ar bind_address : Oc Ar port
1578 or, if the remote host supports it, a Unix domain socket path.
1579 If forwarding to a specific destination then the second argument must be
1580 .Ar host : Ns Ar hostport
1581 or a Unix domain socket path,
1582 otherwise if no destination argument is specified then the remote forwarding
1583 will be established as a SOCKS proxy.
1584 When acting as a SOCKS proxy, the destination of the connection can be
1586 .Cm PermitRemoteOpen .
1588 IPv6 addresses can be specified by enclosing addresses in square brackets.
1589 Multiple forwardings may be specified, and additional
1590 forwardings can be given on the command line.
1591 Privileged ports can be forwarded only when
1592 logging in as root on the remote machine.
1593 Unix domain socket paths may use the tokens described in the
1595 section and environment variables as described in the
1596 .Sx ENVIRONMENT VARIABLES
1602 the listen port will be dynamically allocated on the server and reported
1603 to the client at run time.
1607 is not specified, the default is to only bind to loopback addresses.
1612 or an empty string, then the forwarding is requested to listen on all
1616 will only succeed if the server's
1618 option is enabled (see
1619 .Xr sshd_config 5 ) .
1621 Specifies whether to request a pseudo-tty for the session.
1622 The argument may be one of:
1624 (never request a TTY),
1626 (always request a TTY when standard input is a TTY),
1628 (always request a TTY) or
1630 (request a TTY when opening a login session).
1631 This option mirrors the
1637 .It Cm RequiredRSASize
1638 Specifies the minimum RSA key size (in bits) that
1641 User authentication keys smaller than this limit will be ignored.
1642 Servers that present host keys smaller than this limit will cause the
1643 connection to be terminated.
1647 Note that this limit may only be raised from the default.
1648 .It Cm RevokedHostKeys
1649 Specifies revoked host public keys.
1650 Keys listed in this file will be refused for host authentication.
1651 Note that if this file does not exist or is not readable,
1652 then host authentication will be refused for all hosts.
1653 Keys may be specified as a text file, listing one public key per line, or as
1654 an OpenSSH Key Revocation List (KRL) as generated by
1656 For more information on KRLs, see the KEY REVOCATION LISTS section in
1658 .It Cm SecurityKeyProvider
1659 Specifies a path to a library that will be used when loading any
1660 FIDO authenticator-hosted keys, overriding the default of using
1661 the built-in USB HID support.
1663 If the specified value begins with a
1665 character, then it will be treated as an environment variable containing
1666 the path to the library.
1668 Specifies what variables from the local
1670 should be sent to the server.
1671 The server must also support it, and the server must be configured to
1672 accept these environment variables.
1675 environment variable is always sent whenever a
1676 pseudo-terminal is requested as it is required by the protocol.
1681 for how to configure the server.
1682 Variables are specified by name, which may contain wildcard characters.
1683 Multiple environment variables may be separated by whitespace or spread
1690 for more information on patterns.
1692 It is possible to clear previously set
1694 variable names by prefixing patterns with
1696 The default is not to send any environment variables.
1697 .It Cm ServerAliveCountMax
1698 Sets the number of server alive messages (see below) which may be
1701 receiving any messages back from the server.
1702 If this threshold is reached while server alive messages are being sent,
1703 ssh will disconnect from the server, terminating the session.
1704 It is important to note that the use of server alive messages is very
1708 The server alive messages are sent through the encrypted channel
1709 and therefore will not be spoofable.
1710 The TCP keepalive option enabled by
1713 The server alive mechanism is valuable when the client or
1714 server depend on knowing when a connection has become unresponsive.
1716 The default value is 3.
1718 .Cm ServerAliveInterval
1719 (see below) is set to 15 and
1720 .Cm ServerAliveCountMax
1721 is left at the default, if the server becomes unresponsive,
1722 ssh will disconnect after approximately 45 seconds.
1723 .It Cm ServerAliveInterval
1724 Sets a timeout interval in seconds after which if no data has been received
1727 will send a message through the encrypted
1728 channel to request a response from the server.
1730 is 0, indicating that these messages will not be sent to the server.
1732 May be used to either request invocation of a subsystem on the remote system,
1733 or to prevent the execution of a remote command at all.
1734 The latter is useful for just forwarding ports.
1735 The argument to this keyword must be
1745 (shell or command execution).
1747 Directly specify one or more environment variables and their contents to
1748 be sent to the server.
1751 with the exception of the
1753 variable, the server must be prepared to accept the environment variable.
1755 Redirects stdin from
1757 (actually, prevents reading from stdin).
1758 Either this or the equivalent
1760 option must be used when
1762 is run in the background.
1763 The argument to this keyword must be
1770 .It Cm StreamLocalBindMask
1771 Sets the octal file creation mode mask
1773 used when creating a Unix-domain socket file for local or remote
1775 This option is only used for port forwarding to a Unix-domain socket file.
1777 The default value is 0177, which creates a Unix-domain socket file that is
1778 readable and writable only by the owner.
1779 Note that not all operating systems honor the file mode on Unix-domain
1781 .It Cm StreamLocalBindUnlink
1782 Specifies whether to remove an existing Unix-domain socket file for local
1783 or remote port forwarding before creating a new one.
1784 If the socket file already exists and
1785 .Cm StreamLocalBindUnlink
1788 will be unable to forward the port to the Unix-domain socket file.
1789 This option is only used for port forwarding to a Unix-domain socket file.
1791 The argument must be
1796 .It Cm StrictHostKeyChecking
1797 If this flag is set to
1800 will never automatically add host keys to the
1801 .Pa ~/.ssh/known_hosts
1802 file, and refuses to connect to hosts whose host key has changed.
1803 This provides maximum protection against man-in-the-middle (MITM) attacks,
1804 though it can be annoying when the
1805 .Pa /etc/ssh/ssh_known_hosts
1806 file is poorly maintained or when connections to new hosts are
1808 This option forces the user to manually
1811 If this flag is set to
1813 then ssh will automatically add new host keys to the user's
1815 file, but will not permit connections to hosts with
1817 If this flag is set to
1821 ssh will automatically add new host keys to the user known hosts files
1822 and allow connections to hosts with changed hostkeys to proceed,
1823 subject to some restrictions.
1824 If this flag is set to
1828 will be added to the user known host files only after the user
1829 has confirmed that is what they really want to do, and
1830 ssh will refuse to connect to hosts whose host key has changed.
1832 known hosts will be verified automatically in all cases.
1833 .It Cm SyslogFacility
1834 Gives the facility code that is used when logging messages from
1836 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1837 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1838 The default is USER.
1840 Specifies whether the system should send TCP keepalive messages to the
1842 If they are sent, death of the connection or crash of one
1843 of the machines will be properly noticed.
1844 However, this means that
1845 connections will die if the route is down temporarily, and some people
1850 (to send TCP keepalive messages), and the client will notice
1851 if the network goes down or the remote host dies.
1852 This is important in scripts, and many users want it too.
1854 To disable TCP keepalive messages, the value should be set to
1857 .Cm ServerAliveInterval
1858 for protocol-level keepalives.
1862 device forwarding between the client and the server.
1863 The argument must be
1874 requests the default tunnel mode, which is
1875 .Cm point-to-point .
1879 devices to open on the client
1884 The argument must be
1886 .Ar local_tun Op : Ar remote_tun .
1888 The devices may be specified by numerical ID or the keyword
1890 which uses the next available tunnel device.
1893 is not specified, it defaults to
1897 .It Cm UpdateHostKeys
1900 should accept notifications of additional hostkeys from the server sent
1901 after authentication has completed and add them to
1902 .Cm UserKnownHostsFile .
1903 The argument must be
1908 This option allows learning alternate hostkeys for a server
1909 and supports graceful key rotation by allowing a server to send replacement
1910 public keys before old ones are removed.
1912 Additional hostkeys are only accepted if the key used to authenticate the
1913 host was already trusted or explicitly accepted by the user, the host was
1915 .Cm UserKnownHostsFile
1917 .Cm GlobalKnownHostsFile )
1918 and the host was authenticated using a plain key and not a certificate.
1921 is enabled by default if the user has not overridden the default
1922 .Cm UserKnownHostsFile
1923 setting and has not enabled
1924 .Cm VerifyHostKeyDNS ,
1934 then the user is asked to confirm the modifications to the known_hosts file.
1935 Confirmation is currently incompatible with
1936 .Cm ControlPersist ,
1937 and will be disabled if it is enabled.
1941 from OpenSSH 6.8 and greater support the
1942 .Qq hostkeys@openssh.com
1943 protocol extension used to inform the client of all the server's hostkeys.
1945 Specifies the user to log in as.
1946 This can be useful when a different user name is used on different machines.
1947 This saves the trouble of
1948 having to remember to give the user name on the command line.
1949 .It Cm UserKnownHostsFile
1950 Specifies one or more files to use for the user
1951 host key database, separated by whitespace.
1952 Each filename may use tilde notation to refer to the user's home directory,
1953 the tokens described in the
1955 section and environment variables as described in the
1956 .Sx ENVIRONMENT VARIABLES
1959 .Pa ~/.ssh/known_hosts ,
1960 .Pa ~/.ssh/known_hosts2 .
1961 .It Cm VerifyHostKeyDNS
1962 Specifies whether to verify the remote key using DNS and SSHFP resource
1964 If this option is set to
1966 the client will implicitly trust keys that match a secure fingerprint
1968 Insecure fingerprints will be handled as if this option was set to
1970 If this option is set to
1972 information on fingerprint match will be displayed, but the user will still
1973 need to confirm new host keys according to the
1974 .Cm StrictHostKeyChecking
1980 .Sx VERIFYING HOST KEYS
1983 .It Cm VisualHostKey
1984 If this flag is set to
1986 an ASCII art representation of the remote host key fingerprint is
1987 printed in addition to the fingerprint string at login and
1988 for unknown host keys.
1989 If this flag is set to
1992 no fingerprint strings are printed at login and
1993 only the fingerprint string will be printed for unknown host keys.
1994 .It Cm XAuthLocation
1995 Specifies the full pathname of the
1999 .Pa /usr/X11R6/bin/xauth .
2004 consists of zero or more non-whitespace characters,
2006 (a wildcard that matches zero or more characters),
2009 (a wildcard that matches exactly one character).
2010 For example, to specify a set of declarations for any host in the
2013 the following pattern could be used:
2017 The following pattern
2018 would match any host in the 192.168.0.[0-9] network range:
2020 .Dl Host 192.168.0.?
2024 is a comma-separated list of patterns.
2025 Patterns within pattern-lists may be negated
2026 by preceding them with an exclamation mark
2029 to allow a key to be used from anywhere within an organization
2033 the following entry (in authorized_keys) could be used:
2035 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
2037 Note that a negated match will never produce a positive result by itself.
2038 For example, attempting to match
2040 against the following pattern-list will fail:
2042 .Dl from=\&"!host1,!host2\&"
2044 The solution here is to include a term that will yield a positive match,
2047 .Dl from=\&"!host1,!host2,*\&"
2049 Arguments to some keywords can make use of tokens,
2050 which are expanded at runtime:
2052 .Bl -tag -width XXXX -offset indent -compact
2059 Local user's home directory.
2061 The fingerprint of the server's host key.
2065 hostname or address that is being searched for.
2067 The remote hostname.
2069 A string describing the reason for a
2070 .Cm KnownHostsCommand
2073 when looking up a host by address (only when
2077 when searching by hostname, or
2079 when preparing the host key algorithm preference list to use for the
2084 The base64 encoded host key.
2086 The host key alias if specified, otherwise the original remote hostname given
2087 on the command line.
2091 The local hostname, including the domain name.
2093 The original remote hostname, as given on the command line.
2097 The remote username.
2103 network interface assigned if
2104 tunnel forwarding was requested, or
2108 The type of the server host key, e.g.
2114 .Cm CertificateFile ,
2118 .Cm KnownHostsCommand ,
2124 .Cm UserKnownHostsFile
2125 accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u.
2127 .Cm KnownHostsCommand
2128 additionally accepts the tokens %f, %H, %I, %K and %t.
2131 accepts the tokens %% and %h.
2139 accept the tokens %%, %h, %n, %p, and %r.
2140 .Sh ENVIRONMENT VARIABLES
2141 Arguments to some keywords can be expanded at runtime from environment
2142 variables on the client by enclosing them in
2146 would refer to the user's .ssh directory.
2147 If a specified environment variable does not exist then an error will be
2148 returned and the setting for that keyword will be ignored.
2151 .Cm CertificateFile ,
2155 .Cm KnownHostsCommand ,
2157 .Cm UserKnownHostsFile
2158 support environment variables.
2163 support environment variables only for Unix domain socket paths.
2166 .It Pa ~/.ssh/config
2167 This is the per-user configuration file.
2168 The format of this file is described above.
2169 This file is used by the SSH client.
2170 Because of the potential for abuse, this file must have strict permissions:
2171 read/write for the user, and not writable by others.
2172 .It Pa /etc/ssh/ssh_config
2173 Systemwide configuration file.
2174 This file provides defaults for those
2175 values that are not specified in the user's configuration file, and
2176 for those users who do not have a configuration file.
2177 This file must be world-readable.
2183 OpenSSH is a derivative of the original and free
2184 ssh 1.2.12 release by
2186 .An Aaron Campbell , Bob Beck , Markus Friedl ,
2187 .An Niels Provos , Theo de Raadt
2190 removed many bugs, re-added newer features and
2193 contributed the support for SSH protocol versions 1.5 and 2.0.