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 1.286 2018/10/03 06:38:35 djm Exp $
38 .Dd $Mdocdate: October 3 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
298 will attempt to canonicalize the hostname specified on the command line
302 .Cm CanonicalizePermittedCNAMEs
305 .Cm CanonicalizeHostname
308 then canonicalization is applied to proxied connections too.
310 If this option is enabled, then the configuration files are processed
311 again using the new target name to pick up any new configuration in matching
316 .It Cm CanonicalizeMaxDots
317 Specifies the maximum number of dot characters in a hostname before
318 canonicalization is disabled.
320 allows a single dot (i.e. hostname.subdomain).
321 .It Cm CanonicalizePermittedCNAMEs
322 Specifies rules to determine whether CNAMEs should be followed when
323 canonicalizing hostnames.
324 The rules consist of one or more arguments of
325 .Ar source_domain_list : Ns Ar target_domain_list ,
327 .Ar source_domain_list
328 is a pattern-list of domains that may follow CNAMEs in canonicalization,
330 .Ar target_domain_list
331 is a pattern-list of domains that they may resolve to.
334 .Qq *.a.example.com:*.b.example.com,*.c.example.com
335 will allow hostnames matching
337 to be canonicalized to names in the
342 .It Cm CASignatureAlgorithms
343 Specifies which algorithms are allowed for signing of certificates
344 by certificate authorities (CAs).
346 .Bd -literal -offset indent
347 ecdsa-sha2-nistp256.ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
348 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
352 will not accept host certificates signed using algorithms other than those
354 .It Cm CertificateFile
355 Specifies a file from which the user's certificate is read.
356 A corresponding private key must be provided separately in order
357 to use this certificate either
371 may use the tilde syntax to refer to a user's home directory
372 or the tokens described in the
376 It is possible to have multiple certificate files specified in
377 configuration files; these certificates will be tried in sequence.
380 directives will add to the list of certificates used for
382 .It Cm ChallengeResponseAuthentication
383 Specifies whether to use challenge-response authentication.
384 The argument to this keyword must be
393 will additionally check the host IP address in the
396 This allows it to detect if a host key changed due to DNS spoofing
397 and will add addresses of destination hosts to
398 .Pa ~/.ssh/known_hosts
399 in the process, regardless of the setting of
400 .Cm StrictHostKeyChecking .
401 If the option is set to
403 the check will not be executed.
407 Specifies the ciphers allowed and their order of preference.
408 Multiple ciphers must be comma-separated.
409 If the specified value begins with a
411 character, then the specified ciphers will be appended to the default set
412 instead of replacing them.
413 If the specified value begins with a
415 character, then the specified ciphers (including wildcards) will be removed
416 from the default set instead of replacing them.
418 The supported ciphers are:
419 .Bd -literal -offset indent
427 aes128-gcm@openssh.com
428 aes256-gcm@openssh.com
429 chacha20-poly1305@openssh.com
433 .Bd -literal -offset indent
434 chacha20-poly1305@openssh.com,
435 aes128-ctr,aes192-ctr,aes256-ctr,
436 aes128-gcm@openssh.com,aes256-gcm@openssh.com
439 The list of available ciphers may also be obtained using
441 .It Cm ClearAllForwardings
442 Specifies that all local, remote, and dynamic port forwardings
443 specified in the configuration files or on the command line be
445 This option is primarily useful when used from the
447 command line to clear port forwardings set in
448 configuration files, and is automatically set by
458 Specifies whether to use compression.
464 .It Cm ConnectionAttempts
465 Specifies the number of tries (one per second) to make before exiting.
466 The argument must be an integer.
467 This may be useful in scripts if the connection sometimes fails.
469 .It Cm ConnectTimeout
470 Specifies the timeout (in seconds) used when connecting to the
471 SSH server, instead of using the default system TCP timeout.
472 This value is used only when the target is down or really unreachable,
473 not when it refuses the connection.
475 Enables the sharing of multiple sessions over a single network connection.
479 will listen for connections on a control socket specified using the
482 Additional sessions can connect to this socket using the same
489 These sessions will try to reuse the master instance's network connection
490 rather than initiating new ones, but will fall back to connecting normally
491 if the control socket does not exist, or is not listening.
497 to listen for control connections, but require confirmation using
503 will continue without connecting to a master instance.
507 forwarding is supported over these multiplexed connections, however the
508 display and agent forwarded will be the one belonging to the master
509 connection i.e. it is not possible to forward multiple displays or agents.
511 Two additional options allow for opportunistic multiplexing: try to use a
512 master connection but fall back to creating a new one if one does not already
518 The latter requires confirmation like the
522 Specify the path to the control socket used for connection sharing as described
525 section above or the string
527 to disable connection sharing.
530 may use the tilde syntax to refer to a user's home directory
531 or the tokens described in the
534 It is recommended that any
536 used for opportunistic connection sharing include
537 at least %h, %p, and %r (or alternatively %C) and be placed in a directory
538 that is not writable by other users.
539 This ensures that shared connections are uniquely identified.
540 .It Cm ControlPersist
541 When used in conjunction with
543 specifies that the master connection should remain open
544 in the background (waiting for future client connections)
545 after the initial client connection has been closed.
548 then the master connection will not be placed into the background,
549 and will close as soon as the initial client connection is closed.
553 then the master connection will remain in the background indefinitely
554 (until killed or closed via a mechanism such as the
556 If set to a time in seconds, or a time in any of the formats documented in
558 then the backgrounded master connection will automatically terminate
559 after it has remained idle (with no client connections) for the
561 .It Cm DynamicForward
562 Specifies that a TCP port on the local machine be forwarded
563 over the secure channel, and the application
564 protocol is then used to determine where to connect to from the
569 .Oo Ar bind_address : Oc Ar port .
571 IPv6 addresses can be specified by enclosing addresses in square brackets.
572 By default, the local port is bound in accordance with the
577 may be used to bind the connection to a specific address.
582 indicates that the listening port be bound for local use only, while an
585 indicates that the port should be available from all interfaces.
587 Currently the SOCKS4 and SOCKS5 protocols are supported, and
589 will act as a SOCKS server.
590 Multiple forwardings may be specified, and
591 additional forwardings can be given on the command line.
592 Only the superuser can forward privileged ports.
593 .It Cm EnableSSHKeysign
594 Setting this option to
596 in the global client configuration file
597 .Pa /etc/ssh/ssh_config
598 enables the use of the helper program
601 .Cm HostbasedAuthentication .
607 This option should be placed in the non-hostspecific section.
610 for more information.
612 Sets the escape character (default:
614 The escape character can also
615 be set on the command line.
616 The argument should be a single character,
618 followed by a letter, or
620 to disable the escape
621 character entirely (making the connection transparent for binary
623 .It Cm ExitOnForwardFailure
626 should terminate the connection if it cannot set up all requested
627 dynamic, tunnel, local, and remote port forwardings, (e.g.\&
628 if either end is unable to bind and listen on a specified port).
630 .Cm ExitOnForwardFailure
631 does not apply to connections made over port forwardings and will not,
634 to exit if TCP connections to the ultimate forwarding destination fail.
640 .It Cm FingerprintHash
641 Specifies the hash algorithm used when displaying key fingerprints.
648 Specifies whether the connection to the authentication agent (if any)
649 will be forwarded to the remote machine.
656 Agent forwarding should be enabled with caution.
657 Users with the ability to bypass file permissions on the remote host
658 (for the agent's Unix-domain socket)
659 can access the local agent through the forwarded connection.
660 An attacker cannot obtain key material from the agent,
661 however they can perform operations on the keys that enable them to
662 authenticate using the identities loaded into the agent.
664 Specifies whether X11 connections will be automatically redirected
665 over the secure channel and
674 X11 forwarding should be enabled with caution.
675 Users with the ability to bypass file permissions on the remote host
676 (for the user's X11 authorization database)
677 can access the local X11 display through the forwarded connection.
678 An attacker may then be able to perform activities such as keystroke monitoring
680 .Cm ForwardX11Trusted
681 option is also enabled.
682 .It Cm ForwardX11Timeout
683 Specify a timeout for untrusted X11 forwarding
684 using the format described in the
688 X11 connections received by
690 after this time will be refused.
692 .Cm ForwardX11Timeout
693 to zero will disable the timeout and permit X11 forwarding for the life
695 The default is to disable untrusted X11 forwarding after twenty minutes has
697 .It Cm ForwardX11Trusted
698 If this option is set to
700 remote X11 clients will have full access to the original X11 display.
702 If this option is set to
705 remote X11 clients will be considered untrusted and prevented
706 from stealing or tampering with data belonging to trusted X11
710 token used for the session will be set to expire after 20 minutes.
711 Remote clients will be refused access after this time.
713 See the X11 SECURITY extension specification for full details on
714 the restrictions imposed on untrusted clients.
716 Specifies whether remote hosts are allowed to connect to local
720 binds local port forwardings to the loopback address.
721 This prevents other remote hosts from connecting to forwarded ports.
723 can be used to specify that ssh
724 should bind local port forwardings to the wildcard address,
725 thus allowing remote hosts to connect to forwarded ports.
731 .It Cm GlobalKnownHostsFile
732 Specifies one or more files to use for the global
733 host key database, separated by whitespace.
735 .Pa /etc/ssh/ssh_known_hosts ,
736 .Pa /etc/ssh/ssh_known_hosts2 .
737 .It Cm GSSAPIAuthentication
738 Specifies whether user authentication based on GSSAPI is allowed.
741 .It Cm GSSAPIDelegateCredentials
742 Forward (delegate) credentials to the server.
745 .It Cm HashKnownHosts
748 should hash host names and addresses when they are added to
749 .Pa ~/.ssh/known_hosts .
750 These hashed names may be used normally by
754 but they do not reveal identifying information should the file's contents
758 Note that existing names and addresses in known hosts files
759 will not be converted automatically,
760 but may be manually hashed using
762 .It Cm HostbasedAuthentication
763 Specifies whether to try rhosts based authentication with public key
770 .It Cm HostbasedKeyTypes
771 Specifies the key types that will be used for hostbased authentication
772 as a comma-separated list of patterns.
773 Alternately if the specified value begins with a
775 character, then the specified key types will be appended to the default set
776 instead of replacing them.
777 If the specified value begins with a
779 character, then the specified key types (including wildcards) will be removed
780 from the default set instead of replacing them.
781 The default for this option is:
782 .Bd -literal -offset 3n
783 ecdsa-sha2-nistp256-cert-v01@openssh.com,
784 ecdsa-sha2-nistp384-cert-v01@openssh.com,
785 ecdsa-sha2-nistp521-cert-v01@openssh.com,
786 ssh-ed25519-cert-v01@openssh.com,
787 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
788 ssh-rsa-cert-v01@openssh.com,
789 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
790 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
797 may be used to list supported key types.
798 .It Cm HostKeyAlgorithms
799 Specifies the host key algorithms
800 that the client wants to use in order of preference.
801 Alternately if the specified value begins with a
803 character, then the specified key types will be appended to the default set
804 instead of replacing them.
805 If the specified value begins with a
807 character, then the specified key types (including wildcards) will be removed
808 from the default set instead of replacing them.
809 The default for this option is:
810 .Bd -literal -offset 3n
811 ecdsa-sha2-nistp256-cert-v01@openssh.com,
812 ecdsa-sha2-nistp384-cert-v01@openssh.com,
813 ecdsa-sha2-nistp521-cert-v01@openssh.com,
814 ssh-ed25519-cert-v01@openssh.com,
815 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
816 ssh-rsa-cert-v01@openssh.com,
817 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
818 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
821 If hostkeys are known for the destination host then this default is modified
822 to prefer their algorithms.
824 The list of available key types may also be obtained using
827 Specifies an alias that should be used instead of the
828 real host name when looking up or saving the host key
829 in the host key database files and when validating host certificates.
830 This option is useful for tunneling SSH connections
831 or for multiple servers running on a single host.
833 Specifies the real host name to log into.
834 This can be used to specify nicknames or abbreviations for hosts.
837 accept the tokens described in the
840 Numeric IP addresses are also permitted (both on the command line and in
843 The default is the name given on the command line.
844 .It Cm IdentitiesOnly
847 should only use the authentication identity and certificate files explicitly
858 offers more identities.
859 The argument to this keyword must be
864 This option is intended for situations where ssh-agent
865 offers many different identities.
869 socket used to communicate with the authentication agent.
871 This option overrides the
873 environment variable and can be used to select a specific agent.
874 Setting the socket name to
876 disables the use of an authentication agent.
879 is specified, the location of the socket will be read from the
881 environment variable.
882 Otherwise if the specified value begins with a
884 character, then it will be treated as an environment variable containing
885 the location of the socket.
889 may use the tilde syntax to refer to a user's home directory
890 or the tokens described in the
894 Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
898 .Pa ~/.ssh/id_ecdsa ,
899 .Pa ~/.ssh/id_ed25519
902 Additionally, any identities represented by the authentication agent
903 will be used for authentication unless
906 If no certificates have been explicitly specified by
907 .Cm CertificateFile ,
909 will try to load certificate information from the filename obtained by
912 to the path of a specified
917 may use the tilde syntax to refer to a user's home directory
918 or the tokens described in the
922 It is possible to have
923 multiple identity files specified in configuration files; all these
924 identities will be tried in sequence.
927 directives will add to the list of identities tried (this behaviour
928 differs from that of other configuration directives).
931 may be used in conjunction with
933 to select which identities in an agent are offered during authentication.
935 may also be used in conjunction with
937 in order to provide any certificate also needed for authentication with
940 Specifies a pattern-list of unknown options to be ignored if they are
941 encountered in configuration parsing.
942 This may be used to suppress errors if
944 contains options that are unrecognised by
946 It is recommended that
948 be listed early in the configuration file as it will not be applied
949 to unknown options that appear before it.
951 Include the specified configuration file(s).
952 Multiple pathnames may be specified and each pathname may contain
954 wildcards and, for user configurations, shell-like
956 references to user home directories.
957 Files without absolute paths are assumed to be in
959 if included in a user configuration file or
961 if included from the system configuration file.
963 directive may appear inside a
968 to perform conditional inclusion.
970 Specifies the IPv4 type-of-service or DSCP class for connections.
998 to use the operating system default.
999 This option may take one or two arguments, separated by whitespace.
1000 If one argument is specified, it is used as the packet class unconditionally.
1001 If two values are specified, the first is automatically selected for
1002 interactive sessions and the second for non-interactive sessions.
1006 for interactive sessions and
1009 for non-interactive sessions.
1010 .It Cm KbdInteractiveAuthentication
1011 Specifies whether to use keyboard-interactive authentication.
1012 The argument to this keyword must be
1017 .It Cm KbdInteractiveDevices
1018 Specifies the list of methods to use in keyboard-interactive authentication.
1019 Multiple method names must be comma-separated.
1020 The default is to use the server specified list.
1021 The methods available vary depending on what the server supports.
1022 For an OpenSSH server,
1023 it may be zero or more of:
1027 .It Cm KexAlgorithms
1028 Specifies the available KEX (Key Exchange) algorithms.
1029 Multiple algorithms must be comma-separated.
1030 Alternately if the specified value begins with a
1032 character, then the specified methods will be appended to the default set
1033 instead of replacing them.
1034 If the specified value begins with a
1036 character, then the specified methods (including wildcards) will be removed
1037 from the default set instead of replacing them.
1039 .Bd -literal -offset indent
1040 curve25519-sha256,curve25519-sha256@libssh.org,
1041 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1042 diffie-hellman-group-exchange-sha256,
1043 diffie-hellman-group16-sha512,
1044 diffie-hellman-group18-sha512,
1045 diffie-hellman-group-exchange-sha1,
1046 diffie-hellman-group14-sha256,
1047 diffie-hellman-group14-sha1
1050 The list of available key exchange algorithms may also be obtained using
1053 Specifies a command to execute on the local machine after successfully
1054 connecting to the server.
1055 The command string extends to the end of the line, and is executed with
1059 accept the tokens described in the
1063 The command is run synchronously and does not have access to the
1067 It should not be used for interactive commands.
1069 This directive is ignored unless
1070 .Cm PermitLocalCommand
1073 Specifies that a TCP port on the local machine be forwarded over
1074 the secure channel to the specified host and port from the remote machine.
1075 The first argument must be
1077 .Oo Ar bind_address : Oc Ar port
1079 and the second argument must be
1080 .Ar host : Ns Ar hostport .
1081 IPv6 addresses can be specified by enclosing addresses in square brackets.
1082 Multiple forwardings may be specified, and additional forwardings can be
1083 given on the command line.
1084 Only the superuser can forward privileged ports.
1085 By default, the local port is bound in accordance with the
1088 However, an explicit
1090 may be used to bind the connection to a specific address.
1095 indicates that the listening port be bound for local use only, while an
1098 indicates that the port should be available from all interfaces.
1100 Gives the verbosity level that is used when logging messages from
1102 The possible values are:
1103 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1104 The default is INFO.
1105 DEBUG and DEBUG1 are equivalent.
1106 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1108 Specifies the MAC (message authentication code) algorithms
1109 in order of preference.
1110 The MAC algorithm is used for data integrity protection.
1111 Multiple algorithms must be comma-separated.
1112 If the specified value begins with a
1114 character, then the specified algorithms will be appended to the default set
1115 instead of replacing them.
1116 If the specified value begins with a
1118 character, then the specified algorithms (including wildcards) will be removed
1119 from the default set instead of replacing them.
1121 The algorithms that contain
1123 calculate the MAC after encryption (encrypt-then-mac).
1124 These are considered safer and their use recommended.
1127 .Bd -literal -offset indent
1128 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1129 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1130 hmac-sha1-etm@openssh.com,
1131 umac-64@openssh.com,umac-128@openssh.com,
1132 hmac-sha2-256,hmac-sha2-512,hmac-sha1
1135 The list of available MAC algorithms may also be obtained using
1137 .It Cm NoHostAuthenticationForLocalhost
1138 Disable host authentication for localhost (loopback addresses).
1139 The argument to this keyword must be
1144 .It Cm NumberOfPasswordPrompts
1145 Specifies the number of password prompts before giving up.
1146 The argument to this keyword must be an integer.
1148 .It Cm PasswordAuthentication
1149 Specifies whether to use password authentication.
1150 The argument to this keyword must be
1155 .It Cm PermitLocalCommand
1156 Allow local command execution via the
1159 .Ic !\& Ns Ar command
1162 The argument must be
1167 .It Cm PKCS11Provider
1168 Specifies which PKCS#11 provider to use.
1169 The argument to this keyword is the PKCS#11 shared library
1171 should use to communicate with a PKCS#11 token providing the user's
1174 Specifies the port number to connect on the remote host.
1176 .It Cm PreferredAuthentications
1177 Specifies the order in which the client should try authentication methods.
1178 This allows a client to prefer one method (e.g.\&
1179 .Cm keyboard-interactive )
1180 over another method (e.g.\&
1183 .Bd -literal -offset indent
1184 gssapi-with-mic,hostbased,publickey,
1185 keyboard-interactive,password
1188 Specifies the command to use to connect to the server.
1190 string extends to the end of the line, and is executed
1191 using the user's shell
1193 directive to avoid a lingering shell process.
1197 accept the tokens described in the
1200 The command can be basically anything,
1201 and should read from its standard input and write to its standard output.
1202 It should eventually connect an
1204 server running on some machine, or execute
1207 Host key management will be done using the
1208 HostName of the host being connected (defaulting to the name typed by
1210 Setting the command to
1212 disables this option entirely.
1215 is not available for connects with a proxy command.
1217 This directive is useful in conjunction with
1219 and its proxy support.
1220 For example, the following directive would connect via an HTTP proxy at
1222 .Bd -literal -offset 3n
1223 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1226 Specifies one or more jump proxies as either
1235 Multiple proxies may be separated by comma characters and will be visited
1237 Setting this option will cause
1239 to connect to the target host by first making a
1241 connection to the specified
1243 host and then establishing a
1244 TCP forwarding to the ultimate target from there.
1246 Note that this option will compete with the
1248 option - whichever is specified first will prevent later instances of the
1249 other from taking effect.
1250 .It Cm ProxyUseFdpass
1253 will pass a connected file descriptor back to
1255 instead of continuing to execute and pass data.
1258 .It Cm PubkeyAcceptedKeyTypes
1259 Specifies the key types that will be used for public key authentication
1260 as a comma-separated list of patterns.
1261 Alternately if the specified value begins with a
1263 character, then the key types after it will be appended to the default
1264 instead of replacing it.
1265 If the specified value begins with a
1267 character, then the specified key types (including wildcards) will be removed
1268 from the default set instead of replacing them.
1269 The default for this option is:
1270 .Bd -literal -offset 3n
1271 ecdsa-sha2-nistp256-cert-v01@openssh.com,
1272 ecdsa-sha2-nistp384-cert-v01@openssh.com,
1273 ecdsa-sha2-nistp521-cert-v01@openssh.com,
1274 ssh-ed25519-cert-v01@openssh.com,
1275 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
1276 ssh-rsa-cert-v01@openssh.com,
1277 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1278 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
1281 The list of available key types may also be obtained using
1283 .It Cm PubkeyAuthentication
1284 Specifies whether to try public key authentication.
1285 The argument to this keyword must be
1291 Specifies the maximum amount of data that may be transmitted before the
1292 session key is renegotiated, optionally followed a maximum amount of
1293 time that may pass before the session key is renegotiated.
1294 The first argument is specified in bytes and may have a suffix of
1299 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1300 The default is between
1304 depending on the cipher.
1305 The optional second value is specified in seconds and may use any of the
1306 units documented in the
1310 The default value for
1314 which means that rekeying is performed after the cipher's default amount
1315 of data has been sent or received and no time based rekeying is done.
1316 .It Cm RemoteCommand
1317 Specifies a command to execute on the remote machine after successfully
1318 connecting to the server.
1319 The command string extends to the end of the line, and is executed with
1323 accept the tokens described in the
1326 .It Cm RemoteForward
1327 Specifies that a TCP port on the remote machine be forwarded over
1329 The remote port may either be forwarded to a specified host and port
1330 from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote
1331 client to connect to arbitrary destinations from the local machine.
1332 The first argument must be
1334 .Oo Ar bind_address : Oc Ar port
1336 If forwarding to a specific destination then the second argument must be
1337 .Ar host : Ns Ar hostport ,
1338 otherwise if no destination argument is specified then the remote forwarding
1339 will be established as a SOCKS proxy.
1341 IPv6 addresses can be specified by enclosing addresses in square brackets.
1342 Multiple forwardings may be specified, and additional
1343 forwardings can be given on the command line.
1344 Privileged ports can be forwarded only when
1345 logging in as root on the remote machine.
1350 the listen port will be dynamically allocated on the server and reported
1351 to the client at run time.
1355 is not specified, the default is to only bind to loopback addresses.
1360 or an empty string, then the forwarding is requested to listen on all
1364 will only succeed if the server's
1366 option is enabled (see
1367 .Xr sshd_config 5 ) .
1369 Specifies whether to request a pseudo-tty for the session.
1370 The argument may be one of:
1372 (never request a TTY),
1374 (always request a TTY when standard input is a TTY),
1376 (always request a TTY) or
1378 (request a TTY when opening a login session).
1379 This option mirrors the
1385 .It Cm RevokedHostKeys
1386 Specifies revoked host public keys.
1387 Keys listed in this file will be refused for host authentication.
1388 Note that if this file does not exist or is not readable,
1389 then host authentication will be refused for all hosts.
1390 Keys may be specified as a text file, listing one public key per line, or as
1391 an OpenSSH Key Revocation List (KRL) as generated by
1393 For more information on KRLs, see the KEY REVOCATION LISTS section in
1396 Specifies what variables from the local
1398 should be sent to the server.
1399 The server must also support it, and the server must be configured to
1400 accept these environment variables.
1403 environment variable is always sent whenever a
1404 pseudo-terminal is requested as it is required by the protocol.
1409 for how to configure the server.
1410 Variables are specified by name, which may contain wildcard characters.
1411 Multiple environment variables may be separated by whitespace or spread
1418 for more information on patterns.
1420 It is possible to clear previously set
1422 variable names by prefixing patterns with
1424 The default is not to send any environment variables.
1425 .It Cm ServerAliveCountMax
1426 Sets the number of server alive messages (see below) which may be
1429 receiving any messages back from the server.
1430 If this threshold is reached while server alive messages are being sent,
1431 ssh will disconnect from the server, terminating the session.
1432 It is important to note that the use of server alive messages is very
1436 The server alive messages are sent through the encrypted channel
1437 and therefore will not be spoofable.
1438 The TCP keepalive option enabled by
1441 The server alive mechanism is valuable when the client or
1442 server depend on knowing when a connection has become inactive.
1444 The default value is 3.
1446 .Cm ServerAliveInterval
1447 (see below) is set to 15 and
1448 .Cm ServerAliveCountMax
1449 is left at the default, if the server becomes unresponsive,
1450 ssh will disconnect after approximately 45 seconds.
1451 .It Cm ServerAliveInterval
1452 Sets a timeout interval in seconds after which if no data has been received
1455 will send a message through the encrypted
1456 channel to request a response from the server.
1458 is 0, indicating that these messages will not be sent to the server.
1460 Directly specify one or more environment variables and their contents to
1461 be sent to the server.
1464 the server must be prepared to accept the environment variable.
1465 .It Cm StreamLocalBindMask
1466 Sets the octal file creation mode mask
1468 used when creating a Unix-domain socket file for local or remote
1470 This option is only used for port forwarding to a Unix-domain socket file.
1472 The default value is 0177, which creates a Unix-domain socket file that is
1473 readable and writable only by the owner.
1474 Note that not all operating systems honor the file mode on Unix-domain
1476 .It Cm StreamLocalBindUnlink
1477 Specifies whether to remove an existing Unix-domain socket file for local
1478 or remote port forwarding before creating a new one.
1479 If the socket file already exists and
1480 .Cm StreamLocalBindUnlink
1483 will be unable to forward the port to the Unix-domain socket file.
1484 This option is only used for port forwarding to a Unix-domain socket file.
1486 The argument must be
1491 .It Cm StrictHostKeyChecking
1492 If this flag is set to
1495 will never automatically add host keys to the
1496 .Pa ~/.ssh/known_hosts
1497 file, and refuses to connect to hosts whose host key has changed.
1498 This provides maximum protection against man-in-the-middle (MITM) attacks,
1499 though it can be annoying when the
1500 .Pa /etc/ssh/ssh_known_hosts
1501 file is poorly maintained or when connections to new hosts are
1503 This option forces the user to manually
1506 If this flag is set to
1508 then ssh will automatically add new host keys to the user
1509 known hosts files, but will not permit connections to hosts with
1511 If this flag is set to
1515 ssh will automatically add new host keys to the user known hosts files
1516 and allow connections to hosts with changed hostkeys to proceed,
1517 subject to some restrictions.
1518 If this flag is set to
1522 will be added to the user known host files only after the user
1523 has confirmed that is what they really want to do, and
1524 ssh will refuse to connect to hosts whose host key has changed.
1526 known hosts will be verified automatically in all cases.
1527 .It Cm SyslogFacility
1528 Gives the facility code that is used when logging messages from
1530 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1531 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1532 The default is USER.
1534 Specifies whether the system should send TCP keepalive messages to the
1536 If they are sent, death of the connection or crash of one
1537 of the machines will be properly noticed.
1538 However, this means that
1539 connections will die if the route is down temporarily, and some people
1544 (to send TCP keepalive messages), and the client will notice
1545 if the network goes down or the remote host dies.
1546 This is important in scripts, and many users want it too.
1548 To disable TCP keepalive messages, the value should be set to
1551 .Cm ServerAliveInterval
1552 for protocol-level keepalives.
1556 device forwarding between the client and the server.
1557 The argument must be
1568 requests the default tunnel mode, which is
1569 .Cm point-to-point .
1573 devices to open on the client
1578 The argument must be
1580 .Ar local_tun Op : Ar remote_tun .
1582 The devices may be specified by numerical ID or the keyword
1584 which uses the next available tunnel device.
1587 is not specified, it defaults to
1591 .It Cm UpdateHostKeys
1594 should accept notifications of additional hostkeys from the server sent
1595 after authentication has completed and add them to
1596 .Cm UserKnownHostsFile .
1597 The argument must be
1602 Enabling this option allows learning alternate hostkeys for a server
1603 and supports graceful key rotation by allowing a server to send replacement
1604 public keys before old ones are removed.
1605 Additional hostkeys are only accepted if the key used to authenticate the
1606 host was already trusted or explicitly accepted by the user.
1611 then the user is asked to confirm the modifications to the known_hosts file.
1612 Confirmation is currently incompatible with
1613 .Cm ControlPersist ,
1614 and will be disabled if it is enabled.
1618 from OpenSSH 6.8 and greater support the
1619 .Qq hostkeys@openssh.com
1620 protocol extension used to inform the client of all the server's hostkeys.
1622 Specifies the user to log in as.
1623 This can be useful when a different user name is used on different machines.
1624 This saves the trouble of
1625 having to remember to give the user name on the command line.
1626 .It Cm UserKnownHostsFile
1627 Specifies one or more files to use for the user
1628 host key database, separated by whitespace.
1630 .Pa ~/.ssh/known_hosts ,
1631 .Pa ~/.ssh/known_hosts2 .
1632 .It Cm VerifyHostKeyDNS
1633 Specifies whether to verify the remote key using DNS and SSHFP resource
1635 If this option is set to
1637 the client will implicitly trust keys that match a secure fingerprint
1639 Insecure fingerprints will be handled as if this option was set to
1641 If this option is set to
1643 information on fingerprint match will be displayed, but the user will still
1644 need to confirm new host keys according to the
1645 .Cm StrictHostKeyChecking
1649 if compiled with LDNS and
1654 .Sx VERIFYING HOST KEYS
1657 .It Cm VersionAddendum
1658 Specifies a string to append to the regular version string to identify
1659 OS- or site-specific modifications.
1661 .Dq FreeBSD-20180909 .
1664 may be used to disable this.
1665 .It Cm VisualHostKey
1666 If this flag is set to
1668 an ASCII art representation of the remote host key fingerprint is
1669 printed in addition to the fingerprint string at login and
1670 for unknown host keys.
1671 If this flag is set to
1674 no fingerprint strings are printed at login and
1675 only the fingerprint string will be printed for unknown host keys.
1676 .It Cm XAuthLocation
1677 Specifies the full pathname of the
1681 .Pa /usr/local/bin/xauth .
1686 consists of zero or more non-whitespace characters,
1688 (a wildcard that matches zero or more characters),
1691 (a wildcard that matches exactly one character).
1692 For example, to specify a set of declarations for any host in the
1695 the following pattern could be used:
1699 The following pattern
1700 would match any host in the 192.168.0.[0-9] network range:
1702 .Dl Host 192.168.0.?
1706 is a comma-separated list of patterns.
1707 Patterns within pattern-lists may be negated
1708 by preceding them with an exclamation mark
1711 to allow a key to be used from anywhere within an organization
1715 the following entry (in authorized_keys) could be used:
1717 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1719 Note that a negated match will never produce a positive result by itself.
1720 For example, attempting to match
1722 against the following pattern-list will fail:
1724 .Dl from=\&"!host1,!host2\&"
1726 The solution here is to include a term that will yield a positive match,
1729 .Dl from=\&"!host1,!host2,*\&"
1731 Arguments to some keywords can make use of tokens,
1732 which are expanded at runtime:
1734 .Bl -tag -width XXXX -offset indent -compact
1741 Local user's home directory.
1743 The remote hostname.
1749 The local hostname, including the domain name.
1751 The original remote hostname, as given on the command line.
1755 The remote username.
1761 network interface assigned if
1762 tunnel forwarding was requested, or
1770 accepts the tokens %%, %h, %i, %L, %l, %n, %p, %r, and %u.
1773 accepts the tokens %%, %d, %h, %i, %l, %r, and %u.
1776 accepts the tokens %%, %C, %h, %i, %L, %l, %n, %p, %r, and %u.
1779 accepts the tokens %% and %h.
1784 accept the tokens %%, %d, %h, %i, %l, %r, and %u.
1787 accepts the tokens %%, %C, %d, %h, %i, %l, %n, %p, %r, %T, and %u.
1790 accepts the tokens %%, %h, %p, and %r.
1793 accepts the tokens %%, %C, %d, %h, %i, %l, %n, %p, %r, and %u.
1796 .It Pa ~/.ssh/config
1797 This is the per-user configuration file.
1798 The format of this file is described above.
1799 This file is used by the SSH client.
1800 Because of the potential for abuse, this file must have strict permissions:
1801 read/write for the user, and not accessible by others.
1802 .It Pa /etc/ssh/ssh_config
1803 Systemwide configuration file.
1804 This file provides defaults for those
1805 values that are not specified in the user's configuration file, and
1806 for those users who do not have a configuration file.
1807 This file must be world-readable.
1813 OpenSSH is a derivative of the original and free
1814 ssh 1.2.12 release by
1816 .An Aaron Campbell , Bob Beck , Markus Friedl ,
1817 .An Niels Provos , Theo de Raadt
1820 removed many bugs, re-added newer features and
1823 contributed the support for SSH protocol versions 1.5 and 2.0.
projects / FreeBSD / FreeBSD.git / blob