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 1.292 2019/03/01 02:16:47 djm Exp $
37 .Dd $Mdocdate: March 1 2019 $
42 .Nd OpenSSH SSH client configuration files
45 obtains configuration data from the following sources in
48 .Bl -enum -offset indent -compact
52 user's configuration file
55 system-wide configuration file
56 .Pq Pa /etc/ssh/ssh_config
59 For each parameter, the first obtained value
61 The configuration files contain sections separated by
63 specifications, and that section is only applied for hosts that
64 match one of the patterns given in the specification.
65 The matched host name is usually the one given on the command line
67 .Cm CanonicalizeHostname
68 option for exceptions).
70 Since the first obtained value for each parameter is used, more
71 host-specific declarations should be given near the beginning of the
72 file, and general defaults at the end.
74 The file contains keyword-argument pairs, one per line.
77 and empty lines are interpreted as comments.
78 Arguments may optionally be enclosed in double quotes
80 in order to represent arguments containing spaces.
81 Configuration options may be separated by whitespace or
82 optional whitespace and exactly one
84 the latter format is useful to avoid the need to quote whitespace
85 when specifying configuration options using the
94 keywords and their meanings are as follows (note that
95 keywords are case-insensitive and arguments are case-sensitive):
98 Restricts the following declarations (up to the next
102 keyword) to be only for those hosts that match one of the patterns
103 given after the keyword.
104 If more than one pattern is provided, they should be separated by whitespace.
107 as a pattern can be used to provide global
108 defaults for all hosts.
109 The host is usually the
111 argument given on the command line
113 .Cm CanonicalizeHostname
114 keyword for exceptions).
116 A pattern entry may be negated by prefixing it with an exclamation mark
118 If a negated entry is matched, then the
120 entry is ignored, regardless of whether any other patterns on the line
122 Negated matches are therefore useful to provide exceptions for wildcard
127 for more information on patterns.
129 Restricts the following declarations (up to the next
133 keyword) to be used only when the conditions following the
135 keyword are satisfied.
136 Match conditions are specified using one or more criteria
139 which always matches.
140 The available criteria keywords are:
151 criteria must appear alone or immediately after
155 Other criteria may be combined arbitrarily.
162 Criteria may be negated by prepending an exclamation mark
167 keyword matches only when the configuration file is being re-parsed
168 after hostname canonicalization (see the
169 .Cm CanonicalizeHostname
171 This may be useful to specify conditions that work with canonical host
176 keyword requests that the configuration be re-parsed (regardless of whether
177 .Cm CanonicalizeHostname
178 is enabled), and matches only during this final pass.
180 .Cm CanonicalizeHostname
185 match during the same pass.
189 keyword executes the specified command under the user's shell.
190 If the command returns a zero exit status then the condition is considered true.
191 Commands containing whitespace characters must be quoted.
194 accept the tokens described in the
198 The other keywords' criteria must be single entries or comma-separated
199 lists and may use the wildcard and negation operators described in the
204 keyword are matched against the target hostname, after any substitution
208 .Cm CanonicalizeHostname
212 keyword matches against the hostname as it was specified on the command-line.
215 keyword matches against the target username on the remote host.
218 keyword matches against the name of the local user running
220 (this keyword may be useful in system-wide
223 .It Cm AddKeysToAgent
224 Specifies whether keys should be automatically added to a running
226 If this option is set to
228 and a key is loaded from a file, the key and its passphrase are added to
229 the agent with the default lifetime, as if by
231 If this option is set to
234 will require confirmation using the
236 program before adding a key (see
239 If this option is set to
241 each use of the key must be confirmed, as if the
243 option was specified to
245 If this option is set to
247 no keys are added to the agent.
256 Specifies which address family to use when connecting.
267 passphrase/password querying will be disabled.
268 This option is useful in scripts and other batch jobs where no user
269 is present to supply the password.
276 Use the specified address on the local machine as the source address of
278 Only useful on systems with more than one address.
280 Use the address of the specified interface on the local machine as the
281 source address of the connection.
282 .It Cm CanonicalDomains
284 .Cm CanonicalizeHostname
285 is enabled, this option specifies the list of domain suffixes in which to
286 search for the specified destination host.
287 .It Cm CanonicalizeFallbackLocal
288 Specifies whether to fail with an error when hostname canonicalization fails.
291 will attempt to look up the unqualified hostname using the system resolver's
298 .Cm CanonicalizeHostname
299 is enabled and the target hostname cannot be found in any of the domains
301 .Cm CanonicalDomains .
302 .It Cm CanonicalizeHostname
303 Controls whether explicit hostname canonicalization is performed.
306 is not to perform any name rewriting and let the system resolver handle all
310 then, for connections that do not use a
315 will attempt to canonicalize the hostname specified on the command line
319 .Cm CanonicalizePermittedCNAMEs
322 .Cm CanonicalizeHostname
325 then canonicalization is applied to proxied connections too.
327 If this option is enabled, then the configuration files are processed
328 again using the new target name to pick up any new configuration in matching
333 .It Cm CanonicalizeMaxDots
334 Specifies the maximum number of dot characters in a hostname before
335 canonicalization is disabled.
337 allows a single dot (i.e. hostname.subdomain).
338 .It Cm CanonicalizePermittedCNAMEs
339 Specifies rules to determine whether CNAMEs should be followed when
340 canonicalizing hostnames.
341 The rules consist of one or more arguments of
342 .Ar source_domain_list : Ns Ar target_domain_list ,
344 .Ar source_domain_list
345 is a pattern-list of domains that may follow CNAMEs in canonicalization,
347 .Ar target_domain_list
348 is a pattern-list of domains that they may resolve to.
351 .Qq *.a.example.com:*.b.example.com,*.c.example.com
352 will allow hostnames matching
354 to be canonicalized to names in the
359 .It Cm CASignatureAlgorithms
360 Specifies which algorithms are allowed for signing of certificates
361 by certificate authorities (CAs).
363 .Bd -literal -offset indent
364 ecdsa-sha2-nistp256.ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
365 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
369 will not accept host certificates signed using algorithms other than those
371 .It Cm CertificateFile
372 Specifies a file from which the user's certificate is read.
373 A corresponding private key must be provided separately in order
374 to use this certificate either
388 may use the tilde syntax to refer to a user's home directory
389 or the tokens described in the
393 It is possible to have multiple certificate files specified in
394 configuration files; these certificates will be tried in sequence.
397 directives will add to the list of certificates used for
399 .It Cm ChallengeResponseAuthentication
400 Specifies whether to use challenge-response authentication.
401 The argument to this keyword must be
411 will additionally check the host IP address in the
414 This allows it to detect if a host key changed due to DNS spoofing
415 and will add addresses of destination hosts to
416 .Pa ~/.ssh/known_hosts
417 in the process, regardless of the setting of
418 .Cm StrictHostKeyChecking .
419 If the option is set to
421 the check will not be executed.
423 Specifies the ciphers allowed and their order of preference.
424 Multiple ciphers must be comma-separated.
425 If the specified value begins with a
427 character, then the specified ciphers will be appended to the default set
428 instead of replacing them.
429 If the specified value begins with a
431 character, then the specified ciphers (including wildcards) will be removed
432 from the default set instead of replacing them.
434 The supported ciphers are:
435 .Bd -literal -offset indent
443 aes128-gcm@openssh.com
444 aes256-gcm@openssh.com
445 chacha20-poly1305@openssh.com
449 .Bd -literal -offset indent
450 chacha20-poly1305@openssh.com,
451 aes128-ctr,aes192-ctr,aes256-ctr,
452 aes128-gcm@openssh.com,aes256-gcm@openssh.com
455 The list of available ciphers may also be obtained using
457 .It Cm ClearAllForwardings
458 Specifies that all local, remote, and dynamic port forwardings
459 specified in the configuration files or on the command line be
461 This option is primarily useful when used from the
463 command line to clear port forwardings set in
464 configuration files, and is automatically set by
474 Specifies whether to use compression.
480 .It Cm ConnectionAttempts
481 Specifies the number of tries (one per second) to make before exiting.
482 The argument must be an integer.
483 This may be useful in scripts if the connection sometimes fails.
485 .It Cm ConnectTimeout
486 Specifies the timeout (in seconds) used when connecting to the
487 SSH server, instead of using the default system TCP timeout.
488 This value is used only when the target is down or really unreachable,
489 not when it refuses the connection.
491 Enables the sharing of multiple sessions over a single network connection.
495 will listen for connections on a control socket specified using the
498 Additional sessions can connect to this socket using the same
505 These sessions will try to reuse the master instance's network connection
506 rather than initiating new ones, but will fall back to connecting normally
507 if the control socket does not exist, or is not listening.
513 to listen for control connections, but require confirmation using
519 will continue without connecting to a master instance.
523 forwarding is supported over these multiplexed connections, however the
524 display and agent forwarded will be the one belonging to the master
525 connection i.e. it is not possible to forward multiple displays or agents.
527 Two additional options allow for opportunistic multiplexing: try to use a
528 master connection but fall back to creating a new one if one does not already
534 The latter requires confirmation like the
538 Specify the path to the control socket used for connection sharing as described
541 section above or the string
543 to disable connection sharing.
546 may use the tilde syntax to refer to a user's home directory
547 or the tokens described in the
550 It is recommended that any
552 used for opportunistic connection sharing include
553 at least %h, %p, and %r (or alternatively %C) and be placed in a directory
554 that is not writable by other users.
555 This ensures that shared connections are uniquely identified.
556 .It Cm ControlPersist
557 When used in conjunction with
559 specifies that the master connection should remain open
560 in the background (waiting for future client connections)
561 after the initial client connection has been closed.
564 then the master connection will not be placed into the background,
565 and will close as soon as the initial client connection is closed.
569 then the master connection will remain in the background indefinitely
570 (until killed or closed via a mechanism such as the
572 If set to a time in seconds, or a time in any of the formats documented in
574 then the backgrounded master connection will automatically terminate
575 after it has remained idle (with no client connections) for the
577 .It Cm DynamicForward
578 Specifies that a TCP port on the local machine be forwarded
579 over the secure channel, and the application
580 protocol is then used to determine where to connect to from the
585 .Oo Ar bind_address : Oc Ar port .
587 IPv6 addresses can be specified by enclosing addresses in square brackets.
588 By default, the local port is bound in accordance with the
593 may be used to bind the connection to a specific address.
598 indicates that the listening port be bound for local use only, while an
601 indicates that the port should be available from all interfaces.
603 Currently the SOCKS4 and SOCKS5 protocols are supported, and
605 will act as a SOCKS server.
606 Multiple forwardings may be specified, and
607 additional forwardings can be given on the command line.
608 Only the superuser can forward privileged ports.
609 .It Cm EnableSSHKeysign
610 Setting this option to
612 in the global client configuration file
613 .Pa /etc/ssh/ssh_config
614 enables the use of the helper program
617 .Cm HostbasedAuthentication .
623 This option should be placed in the non-hostspecific section.
626 for more information.
628 Sets the escape character (default:
630 The escape character can also
631 be set on the command line.
632 The argument should be a single character,
634 followed by a letter, or
636 to disable the escape
637 character entirely (making the connection transparent for binary
639 .It Cm ExitOnForwardFailure
642 should terminate the connection if it cannot set up all requested
643 dynamic, tunnel, local, and remote port forwardings, (e.g.\&
644 if either end is unable to bind and listen on a specified port).
646 .Cm ExitOnForwardFailure
647 does not apply to connections made over port forwardings and will not,
650 to exit if TCP connections to the ultimate forwarding destination fail.
656 .It Cm FingerprintHash
657 Specifies the hash algorithm used when displaying key fingerprints.
664 Specifies whether the connection to the authentication agent (if any)
665 will be forwarded to the remote machine.
672 Agent forwarding should be enabled with caution.
673 Users with the ability to bypass file permissions on the remote host
674 (for the agent's Unix-domain socket)
675 can access the local agent through the forwarded connection.
676 An attacker cannot obtain key material from the agent,
677 however they can perform operations on the keys that enable them to
678 authenticate using the identities loaded into the agent.
680 Specifies whether X11 connections will be automatically redirected
681 over the secure channel and
690 X11 forwarding should be enabled with caution.
691 Users with the ability to bypass file permissions on the remote host
692 (for the user's X11 authorization database)
693 can access the local X11 display through the forwarded connection.
694 An attacker may then be able to perform activities such as keystroke monitoring
696 .Cm ForwardX11Trusted
697 option is also enabled.
698 .It Cm ForwardX11Timeout
699 Specify a timeout for untrusted X11 forwarding
700 using the format described in the
704 X11 connections received by
706 after this time will be refused.
708 .Cm ForwardX11Timeout
709 to zero will disable the timeout and permit X11 forwarding for the life
711 The default is to disable untrusted X11 forwarding after twenty minutes has
713 .It Cm ForwardX11Trusted
714 If this option is set to
716 remote X11 clients will have full access to the original X11 display.
718 If this option is set to
721 remote X11 clients will be considered untrusted and prevented
722 from stealing or tampering with data belonging to trusted X11
726 token used for the session will be set to expire after 20 minutes.
727 Remote clients will be refused access after this time.
729 See the X11 SECURITY extension specification for full details on
730 the restrictions imposed on untrusted clients.
732 Specifies whether remote hosts are allowed to connect to local
736 binds local port forwardings to the loopback address.
737 This prevents other remote hosts from connecting to forwarded ports.
739 can be used to specify that ssh
740 should bind local port forwardings to the wildcard address,
741 thus allowing remote hosts to connect to forwarded ports.
747 .It Cm GlobalKnownHostsFile
748 Specifies one or more files to use for the global
749 host key database, separated by whitespace.
751 .Pa /etc/ssh/ssh_known_hosts ,
752 .Pa /etc/ssh/ssh_known_hosts2 .
753 .It Cm GSSAPIAuthentication
754 Specifies whether user authentication based on GSSAPI is allowed.
757 .It Cm GSSAPIDelegateCredentials
758 Forward (delegate) credentials to the server.
761 .It Cm HashKnownHosts
764 should hash host names and addresses when they are added to
765 .Pa ~/.ssh/known_hosts .
766 These hashed names may be used normally by
770 but they do not reveal identifying information should the file's contents
774 Note that existing names and addresses in known hosts files
775 will not be converted automatically,
776 but may be manually hashed using
778 .It Cm HostbasedAuthentication
779 Specifies whether to try rhosts based authentication with public key
786 .It Cm HostbasedKeyTypes
787 Specifies the key types that will be used for hostbased authentication
788 as a comma-separated list of patterns.
789 Alternately if the specified value begins with a
791 character, then the specified key types will be appended to the default set
792 instead of replacing them.
793 If the specified value begins with a
795 character, then the specified key types (including wildcards) will be removed
796 from the default set instead of replacing them.
797 The default for this option is:
798 .Bd -literal -offset 3n
799 ecdsa-sha2-nistp256-cert-v01@openssh.com,
800 ecdsa-sha2-nistp384-cert-v01@openssh.com,
801 ecdsa-sha2-nistp521-cert-v01@openssh.com,
802 ssh-ed25519-cert-v01@openssh.com,
803 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
804 ssh-rsa-cert-v01@openssh.com,
805 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
806 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
813 may be used to list supported key types.
814 .It Cm HostKeyAlgorithms
815 Specifies the host key algorithms
816 that the client wants to use in order of preference.
817 Alternately if the specified value begins with a
819 character, then the specified key types will be appended to the default set
820 instead of replacing them.
821 If the specified value begins with a
823 character, then the specified key types (including wildcards) will be removed
824 from the default set instead of replacing them.
825 The default for this option is:
826 .Bd -literal -offset 3n
827 ecdsa-sha2-nistp256-cert-v01@openssh.com,
828 ecdsa-sha2-nistp384-cert-v01@openssh.com,
829 ecdsa-sha2-nistp521-cert-v01@openssh.com,
830 ssh-ed25519-cert-v01@openssh.com,
831 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
832 ssh-rsa-cert-v01@openssh.com,
833 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
834 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
837 If hostkeys are known for the destination host then this default is modified
838 to prefer their algorithms.
840 The list of available key types may also be obtained using
843 Specifies an alias that should be used instead of the
844 real host name when looking up or saving the host key
845 in the host key database files and when validating host certificates.
846 This option is useful for tunneling SSH connections
847 or for multiple servers running on a single host.
849 Specifies the real host name to log into.
850 This can be used to specify nicknames or abbreviations for hosts.
853 accept the tokens described in the
856 Numeric IP addresses are also permitted (both on the command line and in
859 The default is the name given on the command line.
860 .It Cm IdentitiesOnly
863 should only use the authentication identity and certificate files explicitly
874 offers more identities.
875 The argument to this keyword must be
880 This option is intended for situations where ssh-agent
881 offers many different identities.
885 socket used to communicate with the authentication agent.
887 This option overrides the
889 environment variable and can be used to select a specific agent.
890 Setting the socket name to
892 disables the use of an authentication agent.
895 is specified, the location of the socket will be read from the
897 environment variable.
898 Otherwise if the specified value begins with a
900 character, then it will be treated as an environment variable containing
901 the location of the socket.
905 may use the tilde syntax to refer to a user's home directory
906 or the tokens described in the
910 Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
914 .Pa ~/.ssh/id_ecdsa ,
915 .Pa ~/.ssh/id_ed25519
918 Additionally, any identities represented by the authentication agent
919 will be used for authentication unless
922 If no certificates have been explicitly specified by
923 .Cm CertificateFile ,
925 will try to load certificate information from the filename obtained by
928 to the path of a specified
933 may use the tilde syntax to refer to a user's home directory
934 or the tokens described in the
938 It is possible to have
939 multiple identity files specified in configuration files; all these
940 identities will be tried in sequence.
943 directives will add to the list of identities tried (this behaviour
944 differs from that of other configuration directives).
947 may be used in conjunction with
949 to select which identities in an agent are offered during authentication.
951 may also be used in conjunction with
953 in order to provide any certificate also needed for authentication with
956 Specifies a pattern-list of unknown options to be ignored if they are
957 encountered in configuration parsing.
958 This may be used to suppress errors if
960 contains options that are unrecognised by
962 It is recommended that
964 be listed early in the configuration file as it will not be applied
965 to unknown options that appear before it.
967 Include the specified configuration file(s).
968 Multiple pathnames may be specified and each pathname may contain
970 wildcards and, for user configurations, shell-like
972 references to user home directories.
973 Files without absolute paths are assumed to be in
975 if included in a user configuration file or
977 if included from the system configuration file.
979 directive may appear inside a
984 to perform conditional inclusion.
986 Specifies the IPv4 type-of-service or DSCP class for connections.
1014 to use the operating system default.
1015 This option may take one or two arguments, separated by whitespace.
1016 If one argument is specified, it is used as the packet class unconditionally.
1017 If two values are specified, the first is automatically selected for
1018 interactive sessions and the second for non-interactive sessions.
1022 for interactive sessions and
1025 for non-interactive sessions.
1026 .It Cm KbdInteractiveAuthentication
1027 Specifies whether to use keyboard-interactive authentication.
1028 The argument to this keyword must be
1033 .It Cm KbdInteractiveDevices
1034 Specifies the list of methods to use in keyboard-interactive authentication.
1035 Multiple method names must be comma-separated.
1036 The default is to use the server specified list.
1037 The methods available vary depending on what the server supports.
1038 For an OpenSSH server,
1039 it may be zero or more of:
1043 .It Cm KexAlgorithms
1044 Specifies the available KEX (Key Exchange) algorithms.
1045 Multiple algorithms must be comma-separated.
1046 Alternately if the specified value begins with a
1048 character, then the specified methods will be appended to the default set
1049 instead of replacing them.
1050 If the specified value begins with a
1052 character, then the specified methods (including wildcards) will be removed
1053 from the default set instead of replacing them.
1055 .Bd -literal -offset indent
1056 curve25519-sha256,curve25519-sha256@libssh.org,
1057 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1058 diffie-hellman-group-exchange-sha256,
1059 diffie-hellman-group16-sha512,
1060 diffie-hellman-group18-sha512,
1061 diffie-hellman-group14-sha256,
1062 diffie-hellman-group14-sha1
1065 The list of available key exchange algorithms may also be obtained using
1068 Specifies a command to execute on the local machine after successfully
1069 connecting to the server.
1070 The command string extends to the end of the line, and is executed with
1074 accept the tokens described in the
1078 The command is run synchronously and does not have access to the
1082 It should not be used for interactive commands.
1084 This directive is ignored unless
1085 .Cm PermitLocalCommand
1088 Specifies that a TCP port on the local machine be forwarded over
1089 the secure channel to the specified host and port from the remote machine.
1090 The first argument must be
1092 .Oo Ar bind_address : Oc Ar port
1094 and the second argument must be
1095 .Ar host : Ns Ar hostport .
1096 IPv6 addresses can be specified by enclosing addresses in square brackets.
1097 Multiple forwardings may be specified, and additional forwardings can be
1098 given on the command line.
1099 Only the superuser can forward privileged ports.
1100 By default, the local port is bound in accordance with the
1103 However, an explicit
1105 may be used to bind the connection to a specific address.
1110 indicates that the listening port be bound for local use only, while an
1113 indicates that the port should be available from all interfaces.
1115 Gives the verbosity level that is used when logging messages from
1117 The possible values are:
1118 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1119 The default is INFO.
1120 DEBUG and DEBUG1 are equivalent.
1121 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1123 Specifies the MAC (message authentication code) algorithms
1124 in order of preference.
1125 The MAC algorithm is used for data integrity protection.
1126 Multiple algorithms must be comma-separated.
1127 If the specified value begins with a
1129 character, then the specified algorithms will be appended to the default set
1130 instead of replacing them.
1131 If the specified value begins with a
1133 character, then the specified algorithms (including wildcards) will be removed
1134 from the default set instead of replacing them.
1136 The algorithms that contain
1138 calculate the MAC after encryption (encrypt-then-mac).
1139 These are considered safer and their use recommended.
1142 .Bd -literal -offset indent
1143 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1144 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1145 hmac-sha1-etm@openssh.com,
1146 umac-64@openssh.com,umac-128@openssh.com,
1147 hmac-sha2-256,hmac-sha2-512,hmac-sha1
1150 The list of available MAC algorithms may also be obtained using
1152 .It Cm NoHostAuthenticationForLocalhost
1153 Disable host authentication for localhost (loopback addresses).
1154 The argument to this keyword must be
1159 .It Cm NumberOfPasswordPrompts
1160 Specifies the number of password prompts before giving up.
1161 The argument to this keyword must be an integer.
1163 .It Cm PasswordAuthentication
1164 Specifies whether to use password authentication.
1165 The argument to this keyword must be
1170 .It Cm PermitLocalCommand
1171 Allow local command execution via the
1174 .Ic !\& Ns Ar command
1177 The argument must be
1182 .It Cm PKCS11Provider
1183 Specifies which PKCS#11 provider to use or
1185 to indicate that no provider should be used (the default).
1186 The argument to this keyword is a path to the PKCS#11 shared library
1188 should use to communicate with a PKCS#11 token providing keys for user
1191 Specifies the port number to connect on the remote host.
1193 .It Cm PreferredAuthentications
1194 Specifies the order in which the client should try authentication methods.
1195 This allows a client to prefer one method (e.g.\&
1196 .Cm keyboard-interactive )
1197 over another method (e.g.\&
1200 .Bd -literal -offset indent
1201 gssapi-with-mic,hostbased,publickey,
1202 keyboard-interactive,password
1205 Specifies the command to use to connect to the server.
1207 string extends to the end of the line, and is executed
1208 using the user's shell
1210 directive to avoid a lingering shell process.
1214 accept the tokens described in the
1217 The command can be basically anything,
1218 and should read from its standard input and write to its standard output.
1219 It should eventually connect an
1221 server running on some machine, or execute
1224 Host key management will be done using the
1225 HostName of the host being connected (defaulting to the name typed by
1227 Setting the command to
1229 disables this option entirely.
1232 is not available for connects with a proxy command.
1234 This directive is useful in conjunction with
1236 and its proxy support.
1237 For example, the following directive would connect via an HTTP proxy at
1239 .Bd -literal -offset 3n
1240 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1243 Specifies one or more jump proxies as either
1252 Multiple proxies may be separated by comma characters and will be visited
1254 Setting this option will cause
1256 to connect to the target host by first making a
1258 connection to the specified
1260 host and then establishing a
1261 TCP forwarding to the ultimate target from there.
1263 Note that this option will compete with the
1265 option - whichever is specified first will prevent later instances of the
1266 other from taking effect.
1268 Note also that the configuration for the destination host (either supplied
1269 via the command-line or the configuration file) is not generally applied
1272 should be used if specific configuration is required for jump hosts.
1273 .It Cm ProxyUseFdpass
1276 will pass a connected file descriptor back to
1278 instead of continuing to execute and pass data.
1281 .It Cm PubkeyAcceptedKeyTypes
1282 Specifies the key types that will be used for public key authentication
1283 as a comma-separated list of patterns.
1284 Alternately if the specified value begins with a
1286 character, then the key types after it will be appended to the default
1287 instead of replacing it.
1288 If the specified value begins with a
1290 character, then the specified key types (including wildcards) will be removed
1291 from the default set instead of replacing them.
1292 The default for this option is:
1293 .Bd -literal -offset 3n
1294 ecdsa-sha2-nistp256-cert-v01@openssh.com,
1295 ecdsa-sha2-nistp384-cert-v01@openssh.com,
1296 ecdsa-sha2-nistp521-cert-v01@openssh.com,
1297 ssh-ed25519-cert-v01@openssh.com,
1298 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
1299 ssh-rsa-cert-v01@openssh.com,
1300 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1301 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
1304 The list of available key types may also be obtained using
1306 .It Cm PubkeyAuthentication
1307 Specifies whether to try public key authentication.
1308 The argument to this keyword must be
1314 Specifies the maximum amount of data that may be transmitted before the
1315 session key is renegotiated, optionally followed a maximum amount of
1316 time that may pass before the session key is renegotiated.
1317 The first argument is specified in bytes and may have a suffix of
1322 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1323 The default is between
1327 depending on the cipher.
1328 The optional second value is specified in seconds and may use any of the
1329 units documented in the
1333 The default value for
1337 which means that rekeying is performed after the cipher's default amount
1338 of data has been sent or received and no time based rekeying is done.
1339 .It Cm RemoteCommand
1340 Specifies a command to execute on the remote machine after successfully
1341 connecting to the server.
1342 The command string extends to the end of the line, and is executed with
1346 accept the tokens described in the
1349 .It Cm RemoteForward
1350 Specifies that a TCP port on the remote machine be forwarded over
1352 The remote port may either be forwarded to a specified host and port
1353 from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote
1354 client to connect to arbitrary destinations from the local machine.
1355 The first argument must be
1357 .Oo Ar bind_address : Oc Ar port
1359 If forwarding to a specific destination then the second argument must be
1360 .Ar host : Ns Ar hostport ,
1361 otherwise if no destination argument is specified then the remote forwarding
1362 will be established as a SOCKS proxy.
1364 IPv6 addresses can be specified by enclosing addresses in square brackets.
1365 Multiple forwardings may be specified, and additional
1366 forwardings can be given on the command line.
1367 Privileged ports can be forwarded only when
1368 logging in as root on the remote machine.
1373 the listen port will be dynamically allocated on the server and reported
1374 to the client at run time.
1378 is not specified, the default is to only bind to loopback addresses.
1383 or an empty string, then the forwarding is requested to listen on all
1387 will only succeed if the server's
1389 option is enabled (see
1390 .Xr sshd_config 5 ) .
1392 Specifies whether to request a pseudo-tty for the session.
1393 The argument may be one of:
1395 (never request a TTY),
1397 (always request a TTY when standard input is a TTY),
1399 (always request a TTY) or
1401 (request a TTY when opening a login session).
1402 This option mirrors the
1408 .It Cm RevokedHostKeys
1409 Specifies revoked host public keys.
1410 Keys listed in this file will be refused for host authentication.
1411 Note that if this file does not exist or is not readable,
1412 then host authentication will be refused for all hosts.
1413 Keys may be specified as a text file, listing one public key per line, or as
1414 an OpenSSH Key Revocation List (KRL) as generated by
1416 For more information on KRLs, see the KEY REVOCATION LISTS section in
1419 Specifies what variables from the local
1421 should be sent to the server.
1422 The server must also support it, and the server must be configured to
1423 accept these environment variables.
1426 environment variable is always sent whenever a
1427 pseudo-terminal is requested as it is required by the protocol.
1432 for how to configure the server.
1433 Variables are specified by name, which may contain wildcard characters.
1434 Multiple environment variables may be separated by whitespace or spread
1441 for more information on patterns.
1443 It is possible to clear previously set
1445 variable names by prefixing patterns with
1447 The default is not to send any environment variables.
1448 .It Cm ServerAliveCountMax
1449 Sets the number of server alive messages (see below) which may be
1452 receiving any messages back from the server.
1453 If this threshold is reached while server alive messages are being sent,
1454 ssh will disconnect from the server, terminating the session.
1455 It is important to note that the use of server alive messages is very
1459 The server alive messages are sent through the encrypted channel
1460 and therefore will not be spoofable.
1461 The TCP keepalive option enabled by
1464 The server alive mechanism is valuable when the client or
1465 server depend on knowing when a connection has become inactive.
1467 The default value is 3.
1469 .Cm ServerAliveInterval
1470 (see below) is set to 15 and
1471 .Cm ServerAliveCountMax
1472 is left at the default, if the server becomes unresponsive,
1473 ssh will disconnect after approximately 45 seconds.
1474 .It Cm ServerAliveInterval
1475 Sets a timeout interval in seconds after which if no data has been received
1478 will send a message through the encrypted
1479 channel to request a response from the server.
1481 is 0, indicating that these messages will not be sent to the server.
1483 Directly specify one or more environment variables and their contents to
1484 be sent to the server.
1487 the server must be prepared to accept the environment variable.
1488 .It Cm StreamLocalBindMask
1489 Sets the octal file creation mode mask
1491 used when creating a Unix-domain socket file for local or remote
1493 This option is only used for port forwarding to a Unix-domain socket file.
1495 The default value is 0177, which creates a Unix-domain socket file that is
1496 readable and writable only by the owner.
1497 Note that not all operating systems honor the file mode on Unix-domain
1499 .It Cm StreamLocalBindUnlink
1500 Specifies whether to remove an existing Unix-domain socket file for local
1501 or remote port forwarding before creating a new one.
1502 If the socket file already exists and
1503 .Cm StreamLocalBindUnlink
1506 will be unable to forward the port to the Unix-domain socket file.
1507 This option is only used for port forwarding to a Unix-domain socket file.
1509 The argument must be
1514 .It Cm StrictHostKeyChecking
1515 If this flag is set to
1518 will never automatically add host keys to the
1519 .Pa ~/.ssh/known_hosts
1520 file, and refuses to connect to hosts whose host key has changed.
1521 This provides maximum protection against man-in-the-middle (MITM) attacks,
1522 though it can be annoying when the
1523 .Pa /etc/ssh/ssh_known_hosts
1524 file is poorly maintained or when connections to new hosts are
1526 This option forces the user to manually
1529 If this flag is set to
1531 then ssh will automatically add new host keys to the user
1532 known hosts files, but will not permit connections to hosts with
1534 If this flag is set to
1538 ssh will automatically add new host keys to the user known hosts files
1539 and allow connections to hosts with changed hostkeys to proceed,
1540 subject to some restrictions.
1541 If this flag is set to
1545 will be added to the user known host files only after the user
1546 has confirmed that is what they really want to do, and
1547 ssh will refuse to connect to hosts whose host key has changed.
1549 known hosts will be verified automatically in all cases.
1550 .It Cm SyslogFacility
1551 Gives the facility code that is used when logging messages from
1553 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1554 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1555 The default is USER.
1557 Specifies whether the system should send TCP keepalive messages to the
1559 If they are sent, death of the connection or crash of one
1560 of the machines will be properly noticed.
1561 However, this means that
1562 connections will die if the route is down temporarily, and some people
1567 (to send TCP keepalive messages), and the client will notice
1568 if the network goes down or the remote host dies.
1569 This is important in scripts, and many users want it too.
1571 To disable TCP keepalive messages, the value should be set to
1574 .Cm ServerAliveInterval
1575 for protocol-level keepalives.
1579 device forwarding between the client and the server.
1580 The argument must be
1591 requests the default tunnel mode, which is
1592 .Cm point-to-point .
1596 devices to open on the client
1601 The argument must be
1603 .Ar local_tun Op : Ar remote_tun .
1605 The devices may be specified by numerical ID or the keyword
1607 which uses the next available tunnel device.
1610 is not specified, it defaults to
1614 .It Cm UpdateHostKeys
1617 should accept notifications of additional hostkeys from the server sent
1618 after authentication has completed and add them to
1619 .Cm UserKnownHostsFile .
1620 The argument must be
1625 Enabling this option allows learning alternate hostkeys for a server
1626 and supports graceful key rotation by allowing a server to send replacement
1627 public keys before old ones are removed.
1628 Additional hostkeys are only accepted if the key used to authenticate the
1629 host was already trusted or explicitly accepted by the user.
1634 then the user is asked to confirm the modifications to the known_hosts file.
1635 Confirmation is currently incompatible with
1636 .Cm ControlPersist ,
1637 and will be disabled if it is enabled.
1641 from OpenSSH 6.8 and greater support the
1642 .Qq hostkeys@openssh.com
1643 protocol extension used to inform the client of all the server's hostkeys.
1645 Specifies the user to log in as.
1646 This can be useful when a different user name is used on different machines.
1647 This saves the trouble of
1648 having to remember to give the user name on the command line.
1649 .It Cm UserKnownHostsFile
1650 Specifies one or more files to use for the user
1651 host key database, separated by whitespace.
1653 .Pa ~/.ssh/known_hosts ,
1654 .Pa ~/.ssh/known_hosts2 .
1655 .It Cm VerifyHostKeyDNS
1656 Specifies whether to verify the remote key using DNS and SSHFP resource
1658 If this option is set to
1660 the client will implicitly trust keys that match a secure fingerprint
1662 Insecure fingerprints will be handled as if this option was set to
1664 If this option is set to
1666 information on fingerprint match will be displayed, but the user will still
1667 need to confirm new host keys according to the
1668 .Cm StrictHostKeyChecking
1674 .Sx VERIFYING HOST KEYS
1677 .It Cm VisualHostKey
1678 If this flag is set to
1680 an ASCII art representation of the remote host key fingerprint is
1681 printed in addition to the fingerprint string at login and
1682 for unknown host keys.
1683 If this flag is set to
1686 no fingerprint strings are printed at login and
1687 only the fingerprint string will be printed for unknown host keys.
1688 .It Cm XAuthLocation
1689 Specifies the full pathname of the
1693 .Pa /usr/X11R6/bin/xauth .
1698 consists of zero or more non-whitespace characters,
1700 (a wildcard that matches zero or more characters),
1703 (a wildcard that matches exactly one character).
1704 For example, to specify a set of declarations for any host in the
1707 the following pattern could be used:
1711 The following pattern
1712 would match any host in the 192.168.0.[0-9] network range:
1714 .Dl Host 192.168.0.?
1718 is a comma-separated list of patterns.
1719 Patterns within pattern-lists may be negated
1720 by preceding them with an exclamation mark
1723 to allow a key to be used from anywhere within an organization
1727 the following entry (in authorized_keys) could be used:
1729 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1731 Note that a negated match will never produce a positive result by itself.
1732 For example, attempting to match
1734 against the following pattern-list will fail:
1736 .Dl from=\&"!host1,!host2\&"
1738 The solution here is to include a term that will yield a positive match,
1741 .Dl from=\&"!host1,!host2,*\&"
1743 Arguments to some keywords can make use of tokens,
1744 which are expanded at runtime:
1746 .Bl -tag -width XXXX -offset indent -compact
1753 Local user's home directory.
1755 The remote hostname.
1761 The local hostname, including the domain name.
1763 The original remote hostname, as given on the command line.
1767 The remote username.
1773 network interface assigned if
1774 tunnel forwarding was requested, or
1782 accepts the tokens %%, %h, %i, %L, %l, %n, %p, %r, and %u.
1785 accepts the tokens %%, %d, %h, %i, %l, %r, and %u.
1788 accepts the tokens %%, %C, %h, %i, %L, %l, %n, %p, %r, and %u.
1791 accepts the tokens %% and %h.
1796 accept the tokens %%, %d, %h, %i, %l, %r, and %u.
1799 accepts the tokens %%, %C, %d, %h, %i, %l, %n, %p, %r, %T, and %u.
1802 accepts the tokens %%, %h, %p, and %r.
1805 accepts the tokens %%, %C, %d, %h, %i, %l, %n, %p, %r, and %u.
1808 .It Pa ~/.ssh/config
1809 This is the per-user configuration file.
1810 The format of this file is described above.
1811 This file is used by the SSH client.
1812 Because of the potential for abuse, this file must have strict permissions:
1813 read/write for the user, and not writable by others.
1814 .It Pa /etc/ssh/ssh_config
1815 Systemwide configuration file.
1816 This file provides defaults for those
1817 values that are not specified in the user's configuration file, and
1818 for those users who do not have a configuration file.
1819 This file must be world-readable.
1825 OpenSSH is a derivative of the original and free
1826 ssh 1.2.12 release by
1828 .An Aaron Campbell , Bob Beck , Markus Friedl ,
1829 .An Niels Provos , Theo de Raadt
1832 removed many bugs, re-added newer features and
1835 contributed the support for SSH protocol versions 1.5 and 2.0.