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 1.281 2018/07/23 19:02:49 kn Exp $
38 .Dd $Mdocdate: July 23 2018 $
43 .Nd OpenSSH SSH client configuration files
46 obtains configuration data from the following sources in
49 .Bl -enum -offset indent -compact
53 user's configuration file
56 system-wide configuration file
57 .Pq Pa /etc/ssh/ssh_config
60 For each parameter, the first obtained value
62 The configuration files contain sections separated by
64 specifications, and that section is only applied for hosts that
65 match one of the patterns given in the specification.
66 The matched host name is usually the one given on the command line
68 .Cm CanonicalizeHostname
69 option for exceptions).
71 Since the first obtained value for each parameter is used, more
72 host-specific declarations should be given near the beginning of the
73 file, and general defaults at the end.
75 The file contains keyword-argument pairs, one per line.
78 and empty lines are interpreted as comments.
79 Arguments may optionally be enclosed in double quotes
81 in order to represent arguments containing spaces.
82 Configuration options may be separated by whitespace or
83 optional whitespace and exactly one
85 the latter format is useful to avoid the need to quote whitespace
86 when specifying configuration options using the
95 keywords and their meanings are as follows (note that
96 keywords are case-insensitive and arguments are case-sensitive):
99 Restricts the following declarations (up to the next
103 keyword) to be only for those hosts that match one of the patterns
104 given after the keyword.
105 If more than one pattern is provided, they should be separated by whitespace.
108 as a pattern can be used to provide global
109 defaults for all hosts.
110 The host is usually the
112 argument given on the command line
114 .Cm CanonicalizeHostname
115 keyword for exceptions).
117 A pattern entry may be negated by prefixing it with an exclamation mark
119 If a negated entry is matched, then the
121 entry is ignored, regardless of whether any other patterns on the line
123 Negated matches are therefore useful to provide exceptions for wildcard
128 for more information on patterns.
130 Restricts the following declarations (up to the next
134 keyword) to be used only when the conditions following the
136 keyword are satisfied.
137 Match conditions are specified using one or more criteria
140 which always matches.
141 The available criteria keywords are:
151 criteria must appear alone or immediately after
153 Other criteria may be combined arbitrarily.
159 Criteria may be negated by prepending an exclamation mark
164 keyword matches only when the configuration file is being re-parsed
165 after hostname canonicalization (see the
166 .Cm CanonicalizeHostname
168 This may be useful to specify conditions that work with canonical host
172 keyword executes the specified command under the user's shell.
173 If the command returns a zero exit status then the condition is considered true.
174 Commands containing whitespace characters must be quoted.
177 accept the tokens described in the
181 The other keywords' criteria must be single entries or comma-separated
182 lists and may use the wildcard and negation operators described in the
187 keyword are matched against the target hostname, after any substitution
191 .Cm CanonicalizeHostname
195 keyword matches against the hostname as it was specified on the command-line.
198 keyword matches against the target username on the remote host.
201 keyword matches against the name of the local user running
203 (this keyword may be useful in system-wide
206 .It Cm AddKeysToAgent
207 Specifies whether keys should be automatically added to a running
209 If this option is set to
211 and a key is loaded from a file, the key and its passphrase are added to
212 the agent with the default lifetime, as if by
214 If this option is set to
217 will require confirmation using the
219 program before adding a key (see
222 If this option is set to
224 each use of the key must be confirmed, as if the
226 option was specified to
228 If this option is set to
230 no keys are added to the agent.
239 Specifies which address family to use when connecting.
250 passphrase/password querying will be disabled.
251 This option is useful in scripts and other batch jobs where no user
252 is present to supply the password.
259 Use the specified address on the local machine as the source address of
261 Only useful on systems with more than one address.
263 Use the address of the specified interface on the local machine as the
264 source address of the connection.
265 .It Cm CanonicalDomains
267 .Cm CanonicalizeHostname
268 is enabled, this option specifies the list of domain suffixes in which to
269 search for the specified destination host.
270 .It Cm CanonicalizeFallbackLocal
271 Specifies whether to fail with an error when hostname canonicalization fails.
274 will attempt to look up the unqualified hostname using the system resolver's
281 .Cm CanonicalizeHostname
282 is enabled and the target hostname cannot be found in any of the domains
284 .Cm CanonicalDomains .
285 .It Cm CanonicalizeHostname
286 Controls whether explicit hostname canonicalization is performed.
289 is not to perform any name rewriting and let the system resolver handle all
293 then, for connections that do not use a
296 will attempt to canonicalize the hostname specified on the command line
300 .Cm CanonicalizePermittedCNAMEs
303 .Cm CanonicalizeHostname
306 then canonicalization is applied to proxied connections too.
308 If this option is enabled, then the configuration files are processed
309 again using the new target name to pick up any new configuration in matching
314 .It Cm CanonicalizeMaxDots
315 Specifies the maximum number of dot characters in a hostname before
316 canonicalization is disabled.
318 allows a single dot (i.e. hostname.subdomain).
319 .It Cm CanonicalizePermittedCNAMEs
320 Specifies rules to determine whether CNAMEs should be followed when
321 canonicalizing hostnames.
322 The rules consist of one or more arguments of
323 .Ar source_domain_list : Ns Ar target_domain_list ,
325 .Ar source_domain_list
326 is a pattern-list of domains that may follow CNAMEs in canonicalization,
328 .Ar target_domain_list
329 is a pattern-list of domains that they may resolve to.
332 .Qq *.a.example.com:*.b.example.com,*.c.example.com
333 will allow hostnames matching
335 to be canonicalized to names in the
340 .It Cm CertificateFile
341 Specifies a file from which the user's certificate is read.
342 A corresponding private key must be provided separately in order
343 to use this certificate either
357 may use the tilde syntax to refer to a user's home directory
358 or the tokens described in the
362 It is possible to have multiple certificate files specified in
363 configuration files; these certificates will be tried in sequence.
366 directives will add to the list of certificates used for
368 .It Cm ChallengeResponseAuthentication
369 Specifies whether to use challenge-response authentication.
370 The argument to this keyword must be
379 will additionally check the host IP address in the
382 This allows it to detect if a host key changed due to DNS spoofing
383 and will add addresses of destination hosts to
384 .Pa ~/.ssh/known_hosts
385 in the process, regardless of the setting of
386 .Cm StrictHostKeyChecking .
387 If the option is set to
389 the check will not be executed.
393 Specifies the ciphers allowed and their order of preference.
394 Multiple ciphers must be comma-separated.
395 If the specified value begins with a
397 character, then the specified ciphers will be appended to the default set
398 instead of replacing them.
399 If the specified value begins with a
401 character, then the specified ciphers (including wildcards) will be removed
402 from the default set instead of replacing them.
404 The supported ciphers are:
405 .Bd -literal -offset indent
413 aes128-gcm@openssh.com
414 aes256-gcm@openssh.com
415 chacha20-poly1305@openssh.com
419 .Bd -literal -offset indent
420 chacha20-poly1305@openssh.com,
421 aes128-ctr,aes192-ctr,aes256-ctr,
422 aes128-gcm@openssh.com,aes256-gcm@openssh.com
425 The list of available ciphers may also be obtained using
427 .It Cm ClearAllForwardings
428 Specifies that all local, remote, and dynamic port forwardings
429 specified in the configuration files or on the command line be
431 This option is primarily useful when used from the
433 command line to clear port forwardings set in
434 configuration files, and is automatically set by
444 Specifies whether to use compression.
450 .It Cm ConnectionAttempts
451 Specifies the number of tries (one per second) to make before exiting.
452 The argument must be an integer.
453 This may be useful in scripts if the connection sometimes fails.
455 .It Cm ConnectTimeout
456 Specifies the timeout (in seconds) used when connecting to the
457 SSH server, instead of using the default system TCP timeout.
458 This value is used only when the target is down or really unreachable,
459 not when it refuses the connection.
461 Enables the sharing of multiple sessions over a single network connection.
465 will listen for connections on a control socket specified using the
468 Additional sessions can connect to this socket using the same
475 These sessions will try to reuse the master instance's network connection
476 rather than initiating new ones, but will fall back to connecting normally
477 if the control socket does not exist, or is not listening.
483 to listen for control connections, but require confirmation using
489 will continue without connecting to a master instance.
493 forwarding is supported over these multiplexed connections, however the
494 display and agent forwarded will be the one belonging to the master
495 connection i.e. it is not possible to forward multiple displays or agents.
497 Two additional options allow for opportunistic multiplexing: try to use a
498 master connection but fall back to creating a new one if one does not already
504 The latter requires confirmation like the
508 Specify the path to the control socket used for connection sharing as described
511 section above or the string
513 to disable connection sharing.
516 may use the tilde syntax to refer to a user's home directory
517 or the tokens described in the
520 It is recommended that any
522 used for opportunistic connection sharing include
523 at least %h, %p, and %r (or alternatively %C) and be placed in a directory
524 that is not writable by other users.
525 This ensures that shared connections are uniquely identified.
526 .It Cm ControlPersist
527 When used in conjunction with
529 specifies that the master connection should remain open
530 in the background (waiting for future client connections)
531 after the initial client connection has been closed.
534 then the master connection will not be placed into the background,
535 and will close as soon as the initial client connection is closed.
539 then the master connection will remain in the background indefinitely
540 (until killed or closed via a mechanism such as the
542 If set to a time in seconds, or a time in any of the formats documented in
544 then the backgrounded master connection will automatically terminate
545 after it has remained idle (with no client connections) for the
547 .It Cm DynamicForward
548 Specifies that a TCP port on the local machine be forwarded
549 over the secure channel, and the application
550 protocol is then used to determine where to connect to from the
555 .Oo Ar bind_address : Oc Ar port .
557 IPv6 addresses can be specified by enclosing addresses in square brackets.
558 By default, the local port is bound in accordance with the
563 may be used to bind the connection to a specific address.
568 indicates that the listening port be bound for local use only, while an
571 indicates that the port should be available from all interfaces.
573 Currently the SOCKS4 and SOCKS5 protocols are supported, and
575 will act as a SOCKS server.
576 Multiple forwardings may be specified, and
577 additional forwardings can be given on the command line.
578 Only the superuser can forward privileged ports.
579 .It Cm EnableSSHKeysign
580 Setting this option to
582 in the global client configuration file
583 .Pa /etc/ssh/ssh_config
584 enables the use of the helper program
587 .Cm HostbasedAuthentication .
593 This option should be placed in the non-hostspecific section.
596 for more information.
598 Sets the escape character (default:
600 The escape character can also
601 be set on the command line.
602 The argument should be a single character,
604 followed by a letter, or
606 to disable the escape
607 character entirely (making the connection transparent for binary
609 .It Cm ExitOnForwardFailure
612 should terminate the connection if it cannot set up all requested
613 dynamic, tunnel, local, and remote port forwardings, (e.g.\&
614 if either end is unable to bind and listen on a specified port).
616 .Cm ExitOnForwardFailure
617 does not apply to connections made over port forwardings and will not,
620 to exit if TCP connections to the ultimate forwarding destination fail.
626 .It Cm FingerprintHash
627 Specifies the hash algorithm used when displaying key fingerprints.
634 Specifies whether the connection to the authentication agent (if any)
635 will be forwarded to the remote machine.
642 Agent forwarding should be enabled with caution.
643 Users with the ability to bypass file permissions on the remote host
644 (for the agent's Unix-domain socket)
645 can access the local agent through the forwarded connection.
646 An attacker cannot obtain key material from the agent,
647 however they can perform operations on the keys that enable them to
648 authenticate using the identities loaded into the agent.
650 Specifies whether X11 connections will be automatically redirected
651 over the secure channel and
660 X11 forwarding should be enabled with caution.
661 Users with the ability to bypass file permissions on the remote host
662 (for the user's X11 authorization database)
663 can access the local X11 display through the forwarded connection.
664 An attacker may then be able to perform activities such as keystroke monitoring
666 .Cm ForwardX11Trusted
667 option is also enabled.
668 .It Cm ForwardX11Timeout
669 Specify a timeout for untrusted X11 forwarding
670 using the format described in the
674 X11 connections received by
676 after this time will be refused.
677 The default is to disable untrusted X11 forwarding after twenty minutes has
679 .It Cm ForwardX11Trusted
680 If this option is set to
682 remote X11 clients will have full access to the original X11 display.
684 If this option is set to
687 remote X11 clients will be considered untrusted and prevented
688 from stealing or tampering with data belonging to trusted X11
692 token used for the session will be set to expire after 20 minutes.
693 Remote clients will be refused access after this time.
695 See the X11 SECURITY extension specification for full details on
696 the restrictions imposed on untrusted clients.
698 Specifies whether remote hosts are allowed to connect to local
702 binds local port forwardings to the loopback address.
703 This prevents other remote hosts from connecting to forwarded ports.
705 can be used to specify that ssh
706 should bind local port forwardings to the wildcard address,
707 thus allowing remote hosts to connect to forwarded ports.
713 .It Cm GlobalKnownHostsFile
714 Specifies one or more files to use for the global
715 host key database, separated by whitespace.
717 .Pa /etc/ssh/ssh_known_hosts ,
718 .Pa /etc/ssh/ssh_known_hosts2 .
719 .It Cm GSSAPIAuthentication
720 Specifies whether user authentication based on GSSAPI is allowed.
723 .It Cm GSSAPIDelegateCredentials
724 Forward (delegate) credentials to the server.
727 .It Cm HashKnownHosts
730 should hash host names and addresses when they are added to
731 .Pa ~/.ssh/known_hosts .
732 These hashed names may be used normally by
736 but they do not reveal identifying information should the file's contents
740 Note that existing names and addresses in known hosts files
741 will not be converted automatically,
742 but may be manually hashed using
744 .It Cm HostbasedAuthentication
745 Specifies whether to try rhosts based authentication with public key
752 .It Cm HostbasedKeyTypes
753 Specifies the key types that will be used for hostbased authentication
754 as a comma-separated list of patterns.
755 Alternately if the specified value begins with a
757 character, then the specified key types will be appended to the default set
758 instead of replacing them.
759 If the specified value begins with a
761 character, then the specified key types (including wildcards) will be removed
762 from the default set instead of replacing them.
763 The default for this option is:
764 .Bd -literal -offset 3n
765 ecdsa-sha2-nistp256-cert-v01@openssh.com,
766 ecdsa-sha2-nistp384-cert-v01@openssh.com,
767 ecdsa-sha2-nistp521-cert-v01@openssh.com,
768 ssh-ed25519-cert-v01@openssh.com,
769 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
770 ssh-rsa-cert-v01@openssh.com,
771 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
772 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
779 may be used to list supported key types.
780 .It Cm HostKeyAlgorithms
781 Specifies the host key algorithms
782 that the client wants to use in order of preference.
783 Alternately if the specified value begins with a
785 character, then the specified key types will be appended to the default set
786 instead of replacing them.
787 If the specified value begins with a
789 character, then the specified key types (including wildcards) will be removed
790 from the default set instead of replacing them.
791 The default for this option is:
792 .Bd -literal -offset 3n
793 ecdsa-sha2-nistp256-cert-v01@openssh.com,
794 ecdsa-sha2-nistp384-cert-v01@openssh.com,
795 ecdsa-sha2-nistp521-cert-v01@openssh.com,
796 ssh-ed25519-cert-v01@openssh.com,
797 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
798 ssh-rsa-cert-v01@openssh.com,
799 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
800 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
803 If hostkeys are known for the destination host then this default is modified
804 to prefer their algorithms.
806 The list of available key types may also be obtained using
809 Specifies an alias that should be used instead of the
810 real host name when looking up or saving the host key
811 in the host key database files and when validating host certificates.
812 This option is useful for tunneling SSH connections
813 or for multiple servers running on a single host.
815 Specifies the real host name to log into.
816 This can be used to specify nicknames or abbreviations for hosts.
819 accept the tokens described in the
822 Numeric IP addresses are also permitted (both on the command line and in
825 The default is the name given on the command line.
826 .It Cm IdentitiesOnly
829 should only use the authentication identity and certificate files explicitly
840 offers more identities.
841 The argument to this keyword must be
846 This option is intended for situations where ssh-agent
847 offers many different identities.
851 socket used to communicate with the authentication agent.
853 This option overrides the
855 environment variable and can be used to select a specific agent.
856 Setting the socket name to
858 disables the use of an authentication agent.
861 is specified, the location of the socket will be read from the
863 environment variable.
867 may use the tilde syntax to refer to a user's home directory
868 or the tokens described in the
872 Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
876 .Pa ~/.ssh/id_ecdsa ,
877 .Pa ~/.ssh/id_ed25519
880 Additionally, any identities represented by the authentication agent
881 will be used for authentication unless
884 If no certificates have been explicitly specified by
885 .Cm CertificateFile ,
887 will try to load certificate information from the filename obtained by
890 to the path of a specified
895 may use the tilde syntax to refer to a user's home directory
896 or the tokens described in the
900 It is possible to have
901 multiple identity files specified in configuration files; all these
902 identities will be tried in sequence.
905 directives will add to the list of identities tried (this behaviour
906 differs from that of other configuration directives).
909 may be used in conjunction with
911 to select which identities in an agent are offered during authentication.
913 may also be used in conjunction with
915 in order to provide any certificate also needed for authentication with
918 Specifies a pattern-list of unknown options to be ignored if they are
919 encountered in configuration parsing.
920 This may be used to suppress errors if
922 contains options that are unrecognised by
924 It is recommended that
926 be listed early in the configuration file as it will not be applied
927 to unknown options that appear before it.
929 Include the specified configuration file(s).
930 Multiple pathnames may be specified and each pathname may contain
932 wildcards and, for user configurations, shell-like
934 references to user home directories.
935 Files without absolute paths are assumed to be in
937 if included in a user configuration file or
939 if included from the system configuration file.
941 directive may appear inside a
946 to perform conditional inclusion.
948 Specifies the IPv4 type-of-service or DSCP class for connections.
976 to use the operating system default.
977 This option may take one or two arguments, separated by whitespace.
978 If one argument is specified, it is used as the packet class unconditionally.
979 If two values are specified, the first is automatically selected for
980 interactive sessions and the second for non-interactive sessions.
984 for interactive sessions and
987 for non-interactive sessions.
988 .It Cm KbdInteractiveAuthentication
989 Specifies whether to use keyboard-interactive authentication.
990 The argument to this keyword must be
995 .It Cm KbdInteractiveDevices
996 Specifies the list of methods to use in keyboard-interactive authentication.
997 Multiple method names must be comma-separated.
998 The default is to use the server specified list.
999 The methods available vary depending on what the server supports.
1000 For an OpenSSH server,
1001 it may be zero or more of:
1005 .It Cm KexAlgorithms
1006 Specifies the available KEX (Key Exchange) algorithms.
1007 Multiple algorithms must be comma-separated.
1008 Alternately if the specified value begins with a
1010 character, then the specified methods will be appended to the default set
1011 instead of replacing them.
1012 If the specified value begins with a
1014 character, then the specified methods (including wildcards) will be removed
1015 from the default set instead of replacing them.
1017 .Bd -literal -offset indent
1018 curve25519-sha256,curve25519-sha256@libssh.org,
1019 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1020 diffie-hellman-group-exchange-sha256,
1021 diffie-hellman-group16-sha512,
1022 diffie-hellman-group18-sha512,
1023 diffie-hellman-group-exchange-sha1,
1024 diffie-hellman-group14-sha256,
1025 diffie-hellman-group14-sha1
1028 The list of available key exchange algorithms may also be obtained using
1031 Specifies a command to execute on the local machine after successfully
1032 connecting to the server.
1033 The command string extends to the end of the line, and is executed with
1037 accept the tokens described in the
1041 The command is run synchronously and does not have access to the
1045 It should not be used for interactive commands.
1047 This directive is ignored unless
1048 .Cm PermitLocalCommand
1051 Specifies that a TCP port on the local machine be forwarded over
1052 the secure channel to the specified host and port from the remote machine.
1053 The first argument must be
1055 .Oo Ar bind_address : Oc Ar port
1057 and the second argument must be
1058 .Ar host : Ns Ar hostport .
1059 IPv6 addresses can be specified by enclosing addresses in square brackets.
1060 Multiple forwardings may be specified, and additional forwardings can be
1061 given on the command line.
1062 Only the superuser can forward privileged ports.
1063 By default, the local port is bound in accordance with the
1066 However, an explicit
1068 may be used to bind the connection to a specific address.
1073 indicates that the listening port be bound for local use only, while an
1076 indicates that the port should be available from all interfaces.
1078 Gives the verbosity level that is used when logging messages from
1080 The possible values are:
1081 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1082 The default is INFO.
1083 DEBUG and DEBUG1 are equivalent.
1084 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1086 Specifies the MAC (message authentication code) algorithms
1087 in order of preference.
1088 The MAC algorithm is used for data integrity protection.
1089 Multiple algorithms must be comma-separated.
1090 If the specified value begins with a
1092 character, then the specified algorithms will be appended to the default set
1093 instead of replacing them.
1094 If the specified value begins with a
1096 character, then the specified algorithms (including wildcards) will be removed
1097 from the default set instead of replacing them.
1099 The algorithms that contain
1101 calculate the MAC after encryption (encrypt-then-mac).
1102 These are considered safer and their use recommended.
1105 .Bd -literal -offset indent
1106 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1107 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1108 hmac-sha1-etm@openssh.com,
1109 umac-64@openssh.com,umac-128@openssh.com,
1110 hmac-sha2-256,hmac-sha2-512,hmac-sha1
1113 The list of available MAC algorithms may also be obtained using
1115 .It Cm NoHostAuthenticationForLocalhost
1116 Disable host authentication for localhost (loopback addresses).
1117 The argument to this keyword must be
1122 .It Cm NumberOfPasswordPrompts
1123 Specifies the number of password prompts before giving up.
1124 The argument to this keyword must be an integer.
1126 .It Cm PasswordAuthentication
1127 Specifies whether to use password authentication.
1128 The argument to this keyword must be
1133 .It Cm PermitLocalCommand
1134 Allow local command execution via the
1137 .Ic !\& Ns Ar command
1140 The argument must be
1145 .It Cm PKCS11Provider
1146 Specifies which PKCS#11 provider to use.
1147 The argument to this keyword is the PKCS#11 shared library
1149 should use to communicate with a PKCS#11 token providing the user's
1152 Specifies the port number to connect on the remote host.
1154 .It Cm PreferredAuthentications
1155 Specifies the order in which the client should try authentication methods.
1156 This allows a client to prefer one method (e.g.\&
1157 .Cm keyboard-interactive )
1158 over another method (e.g.\&
1161 .Bd -literal -offset indent
1162 gssapi-with-mic,hostbased,publickey,
1163 keyboard-interactive,password
1166 Specifies the command to use to connect to the server.
1168 string extends to the end of the line, and is executed
1169 using the user's shell
1171 directive to avoid a lingering shell process.
1175 accept the tokens described in the
1178 The command can be basically anything,
1179 and should read from its standard input and write to its standard output.
1180 It should eventually connect an
1182 server running on some machine, or execute
1185 Host key management will be done using the
1186 HostName of the host being connected (defaulting to the name typed by
1188 Setting the command to
1190 disables this option entirely.
1193 is not available for connects with a proxy command.
1195 This directive is useful in conjunction with
1197 and its proxy support.
1198 For example, the following directive would connect via an HTTP proxy at
1200 .Bd -literal -offset 3n
1201 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1204 Specifies one or more jump proxies as either
1213 Multiple proxies may be separated by comma characters and will be visited
1215 Setting this option will cause
1217 to connect to the target host by first making a
1219 connection to the specified
1221 host and then establishing a
1222 TCP forwarding to the ultimate target from there.
1224 Note that this option will compete with the
1226 option - whichever is specified first will prevent later instances of the
1227 other from taking effect.
1228 .It Cm ProxyUseFdpass
1231 will pass a connected file descriptor back to
1233 instead of continuing to execute and pass data.
1236 .It Cm PubkeyAcceptedKeyTypes
1237 Specifies the key types that will be used for public key authentication
1238 as a comma-separated list of patterns.
1239 Alternately if the specified value begins with a
1241 character, then the key types after it will be appended to the default
1242 instead of replacing it.
1243 If the specified value begins with a
1245 character, then the specified key types (including wildcards) will be removed
1246 from the default set instead of replacing them.
1247 The default for this option is:
1248 .Bd -literal -offset 3n
1249 ecdsa-sha2-nistp256-cert-v01@openssh.com,
1250 ecdsa-sha2-nistp384-cert-v01@openssh.com,
1251 ecdsa-sha2-nistp521-cert-v01@openssh.com,
1252 ssh-ed25519-cert-v01@openssh.com,
1253 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
1254 ssh-rsa-cert-v01@openssh.com,
1255 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1256 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
1259 The list of available key types may also be obtained using
1261 .It Cm PubkeyAuthentication
1262 Specifies whether to try public key authentication.
1263 The argument to this keyword must be
1269 Specifies the maximum amount of data that may be transmitted before the
1270 session key is renegotiated, optionally followed a maximum amount of
1271 time that may pass before the session key is renegotiated.
1272 The first argument is specified in bytes and may have a suffix of
1277 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1278 The default is between
1282 depending on the cipher.
1283 The optional second value is specified in seconds and may use any of the
1284 units documented in the
1288 The default value for
1292 which means that rekeying is performed after the cipher's default amount
1293 of data has been sent or received and no time based rekeying is done.
1294 .It Cm RemoteCommand
1295 Specifies a command to execute on the remote machine after successfully
1296 connecting to the server.
1297 The command string extends to the end of the line, and is executed with
1301 accept the tokens described in the
1304 .It Cm RemoteForward
1305 Specifies that a TCP port on the remote machine be forwarded over
1307 The remote port may either be forwarded to a specified host and port
1308 from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote
1309 client to connect to arbitrary destinations from the local machine.
1310 The first argument must be
1312 .Oo Ar bind_address : Oc Ar port
1314 If forwarding to a specific destination then the second argument must be
1315 .Ar host : Ns Ar hostport ,
1316 otherwise if no destination argument is specified then the remote forwarding
1317 will be established as a SOCKS proxy.
1319 IPv6 addresses can be specified by enclosing addresses in square brackets.
1320 Multiple forwardings may be specified, and additional
1321 forwardings can be given on the command line.
1322 Privileged ports can be forwarded only when
1323 logging in as root on the remote machine.
1328 the listen port will be dynamically allocated on the server and reported
1329 to the client at run time.
1333 is not specified, the default is to only bind to loopback addresses.
1338 or an empty string, then the forwarding is requested to listen on all
1342 will only succeed if the server's
1344 option is enabled (see
1345 .Xr sshd_config 5 ) .
1347 Specifies whether to request a pseudo-tty for the session.
1348 The argument may be one of:
1350 (never request a TTY),
1352 (always request a TTY when standard input is a TTY),
1354 (always request a TTY) or
1356 (request a TTY when opening a login session).
1357 This option mirrors the
1363 .It Cm RevokedHostKeys
1364 Specifies revoked host public keys.
1365 Keys listed in this file will be refused for host authentication.
1366 Note that if this file does not exist or is not readable,
1367 then host authentication will be refused for all hosts.
1368 Keys may be specified as a text file, listing one public key per line, or as
1369 an OpenSSH Key Revocation List (KRL) as generated by
1371 For more information on KRLs, see the KEY REVOCATION LISTS section in
1374 Specifies what variables from the local
1376 should be sent to the server.
1377 The server must also support it, and the server must be configured to
1378 accept these environment variables.
1381 environment variable is always sent whenever a
1382 pseudo-terminal is requested as it is required by the protocol.
1387 for how to configure the server.
1388 Variables are specified by name, which may contain wildcard characters.
1389 Multiple environment variables may be separated by whitespace or spread
1396 for more information on patterns.
1398 It is possible to clear previously set
1400 variable names by prefixing patterns with
1402 The default is not to send any environment variables.
1403 .It Cm ServerAliveCountMax
1404 Sets the number of server alive messages (see below) which may be
1407 receiving any messages back from the server.
1408 If this threshold is reached while server alive messages are being sent,
1409 ssh will disconnect from the server, terminating the session.
1410 It is important to note that the use of server alive messages is very
1414 The server alive messages are sent through the encrypted channel
1415 and therefore will not be spoofable.
1416 The TCP keepalive option enabled by
1419 The server alive mechanism is valuable when the client or
1420 server depend on knowing when a connection has become inactive.
1422 The default value is 3.
1424 .Cm ServerAliveInterval
1425 (see below) is set to 15 and
1426 .Cm ServerAliveCountMax
1427 is left at the default, if the server becomes unresponsive,
1428 ssh will disconnect after approximately 45 seconds.
1429 .It Cm ServerAliveInterval
1430 Sets a timeout interval in seconds after which if no data has been received
1433 will send a message through the encrypted
1434 channel to request a response from the server.
1436 is 0, indicating that these messages will not be sent to the server.
1438 Directly specify one or more environment variables and their contents to
1439 be sent to the server.
1442 the server must be prepared to accept the environment variable.
1443 .It Cm StreamLocalBindMask
1444 Sets the octal file creation mode mask
1446 used when creating a Unix-domain socket file for local or remote
1448 This option is only used for port forwarding to a Unix-domain socket file.
1450 The default value is 0177, which creates a Unix-domain socket file that is
1451 readable and writable only by the owner.
1452 Note that not all operating systems honor the file mode on Unix-domain
1454 .It Cm StreamLocalBindUnlink
1455 Specifies whether to remove an existing Unix-domain socket file for local
1456 or remote port forwarding before creating a new one.
1457 If the socket file already exists and
1458 .Cm StreamLocalBindUnlink
1461 will be unable to forward the port to the Unix-domain socket file.
1462 This option is only used for port forwarding to a Unix-domain socket file.
1464 The argument must be
1469 .It Cm StrictHostKeyChecking
1470 If this flag is set to
1473 will never automatically add host keys to the
1474 .Pa ~/.ssh/known_hosts
1475 file, and refuses to connect to hosts whose host key has changed.
1476 This provides maximum protection against man-in-the-middle (MITM) attacks,
1477 though it can be annoying when the
1478 .Pa /etc/ssh/ssh_known_hosts
1479 file is poorly maintained or when connections to new hosts are
1481 This option forces the user to manually
1484 If this flag is set to
1486 then ssh will automatically add new host keys to the user
1487 known hosts files, but will not permit connections to hosts with
1489 If this flag is set to
1493 ssh will automatically add new host keys to the user known hosts files
1494 and allow connections to hosts with changed hostkeys to proceed,
1495 subject to some restrictions.
1496 If this flag is set to
1500 will be added to the user known host files only after the user
1501 has confirmed that is what they really want to do, and
1502 ssh will refuse to connect to hosts whose host key has changed.
1504 known hosts will be verified automatically in all cases.
1505 .It Cm SyslogFacility
1506 Gives the facility code that is used when logging messages from
1508 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1509 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1510 The default is USER.
1512 Specifies whether the system should send TCP keepalive messages to the
1514 If they are sent, death of the connection or crash of one
1515 of the machines will be properly noticed.
1516 However, this means that
1517 connections will die if the route is down temporarily, and some people
1522 (to send TCP keepalive messages), and the client will notice
1523 if the network goes down or the remote host dies.
1524 This is important in scripts, and many users want it too.
1526 To disable TCP keepalive messages, the value should be set to
1529 .Cm ServerAliveInterval
1530 for protocol-level keepalives.
1534 device forwarding between the client and the server.
1535 The argument must be
1546 requests the default tunnel mode, which is
1547 .Cm point-to-point .
1551 devices to open on the client
1556 The argument must be
1558 .Ar local_tun Op : Ar remote_tun .
1560 The devices may be specified by numerical ID or the keyword
1562 which uses the next available tunnel device.
1565 is not specified, it defaults to
1569 .It Cm UpdateHostKeys
1572 should accept notifications of additional hostkeys from the server sent
1573 after authentication has completed and add them to
1574 .Cm UserKnownHostsFile .
1575 The argument must be
1580 Enabling this option allows learning alternate hostkeys for a server
1581 and supports graceful key rotation by allowing a server to send replacement
1582 public keys before old ones are removed.
1583 Additional hostkeys are only accepted if the key used to authenticate the
1584 host was already trusted or explicitly accepted by the user.
1589 then the user is asked to confirm the modifications to the known_hosts file.
1590 Confirmation is currently incompatible with
1591 .Cm ControlPersist ,
1592 and will be disabled if it is enabled.
1596 from OpenSSH 6.8 and greater support the
1597 .Qq hostkeys@openssh.com
1598 protocol extension used to inform the client of all the server's hostkeys.
1600 Specifies the user to log in as.
1601 This can be useful when a different user name is used on different machines.
1602 This saves the trouble of
1603 having to remember to give the user name on the command line.
1604 .It Cm UserKnownHostsFile
1605 Specifies one or more files to use for the user
1606 host key database, separated by whitespace.
1608 .Pa ~/.ssh/known_hosts ,
1609 .Pa ~/.ssh/known_hosts2 .
1610 .It Cm VerifyHostKeyDNS
1611 Specifies whether to verify the remote key using DNS and SSHFP resource
1613 If this option is set to
1615 the client will implicitly trust keys that match a secure fingerprint
1617 Insecure fingerprints will be handled as if this option was set to
1619 If this option is set to
1621 information on fingerprint match will be displayed, but the user will still
1622 need to confirm new host keys according to the
1623 .Cm StrictHostKeyChecking
1627 if compiled with LDNS and
1632 .Sx VERIFYING HOST KEYS
1635 .It Cm VersionAddendum
1636 Specifies a string to append to the regular version string to identify
1637 OS- or site-specific modifications.
1639 .Dq FreeBSD-20180909 .
1642 may be used to disable this.
1643 .It Cm VisualHostKey
1644 If this flag is set to
1646 an ASCII art representation of the remote host key fingerprint is
1647 printed in addition to the fingerprint string at login and
1648 for unknown host keys.
1649 If this flag is set to
1652 no fingerprint strings are printed at login and
1653 only the fingerprint string will be printed for unknown host keys.
1654 .It Cm XAuthLocation
1655 Specifies the full pathname of the
1659 .Pa /usr/local/bin/xauth .
1664 consists of zero or more non-whitespace characters,
1666 (a wildcard that matches zero or more characters),
1669 (a wildcard that matches exactly one character).
1670 For example, to specify a set of declarations for any host in the
1673 the following pattern could be used:
1677 The following pattern
1678 would match any host in the 192.168.0.[0-9] network range:
1680 .Dl Host 192.168.0.?
1684 is a comma-separated list of patterns.
1685 Patterns within pattern-lists may be negated
1686 by preceding them with an exclamation mark
1689 to allow a key to be used from anywhere within an organization
1693 the following entry (in authorized_keys) could be used:
1695 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1697 Note that a negated match will never produce a positive result by itself.
1698 For example, attempting to match
1700 against the following pattern-list will fail:
1702 .Dl from=\&"!host1,!host2\&"
1704 The solution here is to include a term that will yield a positive match,
1707 .Dl from=\&"!host1,!host2,*\&"
1709 Arguments to some keywords can make use of tokens,
1710 which are expanded at runtime:
1712 .Bl -tag -width XXXX -offset indent -compact
1719 Local user's home directory.
1721 The remote hostname.
1727 The local hostname, including the domain name.
1729 The original remote hostname, as given on the command line.
1733 The remote username.
1739 network interface assigned if
1740 tunnel forwarding was requested, or
1748 accepts the tokens %%, %h, %i, %L, %l, %n, %p, %r, and %u.
1751 accepts the tokens %%, %d, %h, %i, %l, %r, and %u.
1754 accepts the tokens %%, %C, %h, %i, %L, %l, %n, %p, %r, and %u.
1757 accepts the tokens %% and %h.
1762 accept the tokens %%, %d, %h, %i, %l, %r, and %u.
1765 accepts the tokens %%, %C, %d, %h, %i, %l, %n, %p, %r, %T, and %u.
1768 accepts the tokens %%, %h, %p, and %r.
1771 accepts the tokens %%, %C, %d, %h, %i, %l, %n, %p, %r, and %u.
1774 .It Pa ~/.ssh/config
1775 This is the per-user configuration file.
1776 The format of this file is described above.
1777 This file is used by the SSH client.
1778 Because of the potential for abuse, this file must have strict permissions:
1779 read/write for the user, and not accessible by others.
1780 .It Pa /etc/ssh/ssh_config
1781 Systemwide configuration file.
1782 This file provides defaults for those
1783 values that are not specified in the user's configuration file, and
1784 for those users who do not have a configuration file.
1785 This file must be world-readable.
1791 OpenSSH is a derivative of the original and free
1792 ssh 1.2.12 release by
1794 .An Aaron Campbell , Bob Beck , Markus Friedl ,
1795 .An Niels Provos , Theo de Raadt
1798 removed many bugs, re-added newer features and
1801 contributed the support for SSH protocol versions 1.5 and 2.0.
projects / FreeBSD / FreeBSD.git / blob