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 1.211 2015/06/02 09:10:40 djm Exp $
37 .Dd $Mdocdate: June 2 2015 $
42 .Nd OpenSSH SSH client configuration files
45 .Nm /etc/ssh/ssh_config
48 obtains configuration data from the following sources in
51 .Bl -enum -offset indent -compact
55 user's configuration file
58 system-wide configuration file
59 .Pq Pa /etc/ssh/ssh_config
62 For each parameter, the first obtained value
64 The configuration files contain sections separated by
66 specifications, and that section is only applied for hosts that
67 match one of the patterns given in the specification.
68 The matched host name is usually the one given on the command line
70 .Cm CanonicalizeHostname
71 option for exceptions.)
73 Since the first obtained value for each parameter is used, more
74 host-specific declarations should be given near the beginning of the
75 file, and general defaults at the end.
77 The configuration file has the following format:
79 Empty lines and lines starting with
82 Otherwise a line is of the format
83 .Dq keyword arguments .
84 Configuration options may be separated by whitespace or
85 optional whitespace and exactly one
87 the latter format is useful to avoid the need to quote whitespace
88 when specifying configuration options using the
95 Arguments may optionally be enclosed in double quotes
97 in order to represent arguments containing spaces.
100 keywords and their meanings are as follows (note that
101 keywords are case-insensitive and arguments are case-sensitive):
104 Restricts the following declarations (up to the next
108 keyword) to be only for those hosts that match one of the patterns
109 given after the keyword.
110 If more than one pattern is provided, they should be separated by whitespace.
113 as a pattern can be used to provide global
114 defaults for all hosts.
115 The host is usually the
117 argument given on the command line
119 .Cm CanonicalizeHostname
120 option for exceptions.)
122 A pattern entry may be negated by prefixing it with an exclamation mark
124 If a negated entry is matched, then the
126 entry is ignored, regardless of whether any other patterns on the line
128 Negated matches are therefore useful to provide exceptions for wildcard
133 for more information on patterns.
135 Restricts the following declarations (up to the next
139 keyword) to be used only when the conditions following the
141 keyword are satisfied.
142 Match conditions are specified using one or more critera
145 which always matches.
146 The available criteria keywords are:
156 criteria must appear alone or immediately after
158 Other criteria may be combined arbitrarily.
164 Criteria may be negated by prepending an exclamation mark
169 keyword matches only when the configuration file is being re-parsed
170 after hostname canonicalization (see the
171 .Cm CanonicalizeHostname
173 This may be useful to specify conditions that work with canonical host
177 keyword executes the specified command under the user's shell.
178 If the command returns a zero exit status then the condition is considered true.
179 Commands containing whitespace characters must be quoted.
180 The following character sequences in the command will be expanded prior to
183 will be substituted by the first component of the local host name,
185 will be substituted by the local host name (including any domain name),
187 will be substituted by the target host name,
189 will be substituted by the original target host name
190 specified on the command-line,
192 the destination port,
194 by the remote login username, and
196 by the username of the user running
199 The other keywords' criteria must be single entries or comma-separated
200 lists and may use the wildcard and negation operators described in the
205 keyword are matched against the target hostname, after any substitution
209 .Cm CanonicalizeHostname
213 keyword matches against the hostname as it was specified on the command-line.
216 keyword matches against the target username on the remote host.
219 keyword matches against the name of the local user running
221 (this keyword may be useful in system-wide
225 Specifies which address family to use when connecting.
235 passphrase/password querying will be disabled.
236 This option is useful in scripts and other batch jobs where no user
237 is present to supply the password.
245 Use the specified address on the local machine as the source address of
247 Only useful on systems with more than one address.
248 Note that this option does not work if
249 .Cm UsePrivilegedPort
252 .It Cm CanonicalDomains
254 .Cm CanonicalizeHostname
255 is enabled, this option specifies the list of domain suffixes in which to
256 search for the specified destination host.
257 .It Cm CanonicalizeFallbackLocal
258 Specifies whether to fail with an error when hostname canonicalization fails.
261 will attempt to look up the unqualified hostname using the system resolver's
268 .Cm CanonicalizeHostname
269 is enabled and the target hostname cannot be found in any of the domains
271 .Cm CanonicalDomains .
272 .It Cm CanonicalizeHostname
273 Controls whether explicit hostname canonicalization is performed.
276 is not to perform any name rewriting and let the system resolver handle all
280 then, for connections that do not use a
283 will attempt to canonicalize the hostname specified on the command line
287 .Cm CanonicalizePermittedCNAMEs
290 .Cm CanonicalizeHostname
293 then canonicalization is applied to proxied connections too.
295 If this option is enabled, then the configuration files are processed
296 again using the new target name to pick up any new configuration in matching
301 .It Cm CanonicalizeMaxDots
302 Specifies the maximum number of dot characters in a hostname before
303 canonicalization is disabled.
306 allows a single dot (i.e. hostname.subdomain).
307 .It Cm CanonicalizePermittedCNAMEs
308 Specifies rules to determine whether CNAMEs should be followed when
309 canonicalizing hostnames.
310 The rules consist of one or more arguments of
311 .Ar source_domain_list : Ns Ar target_domain_list ,
313 .Ar source_domain_list
314 is a pattern-list of domains that may follow CNAMEs in canonicalization,
316 .Ar target_domain_list
317 is a pattern-list of domains that they may resolve to.
320 .Dq *.a.example.com:*.b.example.com,*.c.example.com
321 will allow hostnames matching
323 to be canonicalized to names in the
328 .It Cm ChallengeResponseAuthentication
329 Specifies whether to use challenge-response authentication.
330 The argument to this keyword must be
337 If this flag is set to
340 will additionally check the host IP address in the
343 This allows ssh to detect if a host key changed due to DNS spoofing
344 and will add addresses of destination hosts to
345 .Pa ~/.ssh/known_hosts
346 in the process, regardless of the setting of
347 .Cm StrictHostKeyChecking .
348 If the option is set to
350 the check will not be executed.
354 Specifies the cipher to use for encrypting the session
355 in protocol version 1.
363 is only supported in the
365 client for interoperability with legacy protocol 1 implementations
366 that do not support the
369 Its use is strongly discouraged due to cryptographic weaknesses.
373 Specifies the ciphers allowed for protocol version 2
374 in order of preference.
375 Multiple ciphers must be comma-separated.
376 The supported ciphers are:
378 .Bl -item -compact -offset indent
394 aes128-gcm@openssh.com
396 aes256-gcm@openssh.com
408 chacha20-poly1305@openssh.com
412 .Bd -literal -offset indent
413 aes128-ctr,aes192-ctr,aes256-ctr,
414 aes128-gcm@openssh.com,aes256-gcm@openssh.com,
415 chacha20-poly1305@openssh.com,
416 arcfour256,arcfour128,
417 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,
418 aes192-cbc,aes256-cbc,arcfour
421 The list of available ciphers may also be obtained using the
427 .It Cm ClearAllForwardings
428 Specifies that all local, remote, and dynamic port forwardings
429 specified in the configuration files or on the command line be
431 This option is primarily useful when used from the
433 command line to clear port forwardings set in
434 configuration files, and is automatically set by
445 Specifies whether to use compression.
452 .It Cm CompressionLevel
453 Specifies the compression level to use if compression is enabled.
454 The argument must be an integer from 1 (fast) to 9 (slow, best).
455 The default level is 6, which is good for most applications.
456 The meaning of the values is the same as in
458 Note that this option applies to protocol version 1 only.
459 .It Cm ConnectionAttempts
460 Specifies the number of tries (one per second) to make before exiting.
461 The argument must be an integer.
462 This may be useful in scripts if the connection sometimes fails.
464 .It Cm ConnectTimeout
465 Specifies the timeout (in seconds) used when connecting to the
466 SSH server, instead of using the default system TCP timeout.
467 This value is used only when the target is down or really unreachable,
468 not when it refuses the connection.
470 Enables the sharing of multiple sessions over a single network connection.
474 will listen for connections on a control socket specified using the
477 Additional sessions can connect to this socket using the same
484 These sessions will try to reuse the master instance's network connection
485 rather than initiating new ones, but will fall back to connecting normally
486 if the control socket does not exist, or is not listening.
491 to listen for control connections, but require confirmation using
496 ssh will continue without connecting to a master instance.
500 forwarding is supported over these multiplexed connections, however the
501 display and agent forwarded will be the one belonging to the master
502 connection i.e. it is not possible to forward multiple displays or agents.
504 Two additional options allow for opportunistic multiplexing: try to use a
505 master connection but fall back to creating a new one if one does not already
511 The latter requires confirmation like the
515 Specify the path to the control socket used for connection sharing as described
518 section above or the string
520 to disable connection sharing.
523 will be substituted by the first component of the local host name,
525 will be substituted by the local host name (including any domain name),
527 will be substituted by the target host name,
529 will be substituted by the original target host name
530 specified on the command line,
532 the destination port,
534 by the remote login username,
536 by the username of the user running
539 by a hash of the concatenation: %l%h%p%r.
540 It is recommended that any
542 used for opportunistic connection sharing include
543 at least %h, %p, and %r (or alternatively %C) and be placed in a directory
544 that is not writable by other users.
545 This ensures that shared connections are uniquely identified.
546 .It Cm ControlPersist
547 When used in conjunction with
549 specifies that the master connection should remain open
550 in the background (waiting for future client connections)
551 after the initial client connection has been closed.
554 then the master connection will not be placed into the background,
555 and will close as soon as the initial client connection is closed.
560 then the master connection will remain in the background indefinitely
561 (until killed or closed via a mechanism such as the
565 If set to a time in seconds, or a time in any of the formats documented in
567 then the backgrounded master connection will automatically terminate
568 after it has remained idle (with no client connections) for the
570 .It Cm DynamicForward
571 Specifies that a TCP port on the local machine be forwarded
572 over the secure channel, and the application
573 protocol is then used to determine where to connect to from the
578 .Oo Ar bind_address : Oc Ar port .
580 IPv6 addresses can be specified by enclosing addresses in square brackets.
581 By default, the local port is bound in accordance with the
586 may be used to bind the connection to a specific address.
591 indicates that the listening port be bound for local use only, while an
594 indicates that the port should be available from all interfaces.
596 Currently the SOCKS4 and SOCKS5 protocols are supported, and
598 will act as a SOCKS server.
599 Multiple forwardings may be specified, and
600 additional forwardings can be given on the command line.
601 Only the superuser can forward privileged ports.
602 .It Cm EnableSSHKeysign
603 Setting this option to
605 in the global client configuration file
606 .Pa /etc/ssh/ssh_config
607 enables the use of the helper program
610 .Cm HostbasedAuthentication .
617 This option should be placed in the non-hostspecific section.
620 for more information.
622 Sets the escape character (default:
624 The escape character can also
625 be set on the command line.
626 The argument should be a single character,
628 followed by a letter, or
630 to disable the escape
631 character entirely (making the connection transparent for binary
633 .It Cm ExitOnForwardFailure
636 should terminate the connection if it cannot set up all requested
637 dynamic, tunnel, local, and remote port forwardings.
644 .It Cm FingerprintHash
645 Specifies the hash algorithm used when displaying key fingerprints.
653 Specifies whether the connection to the authentication agent (if any)
654 will be forwarded to the remote machine.
662 Agent forwarding should be enabled with caution.
663 Users with the ability to bypass file permissions on the remote host
664 (for the agent's Unix-domain socket)
665 can access the local agent through the forwarded connection.
666 An attacker cannot obtain key material from the agent,
667 however they can perform operations on the keys that enable them to
668 authenticate using the identities loaded into the agent.
670 Specifies whether X11 connections will be automatically redirected
671 over the secure channel and
681 X11 forwarding should be enabled with caution.
682 Users with the ability to bypass file permissions on the remote host
683 (for the user's X11 authorization database)
684 can access the local X11 display through the forwarded connection.
685 An attacker may then be able to perform activities such as keystroke monitoring
687 .Cm ForwardX11Trusted
688 option is also enabled.
689 .It Cm ForwardX11Timeout
690 Specify a timeout for untrusted X11 forwarding
691 using the format described in the
692 TIME FORMATS section of
694 X11 connections received by
696 after this time will be refused.
697 The default is to disable untrusted X11 forwarding after twenty minutes has
699 .It Cm ForwardX11Trusted
700 If this option is set to
702 remote X11 clients will have full access to the original X11 display.
704 If this option is set to
706 remote X11 clients will be considered untrusted and prevented
707 from stealing or tampering with data belonging to trusted X11
711 token used for the session will be set to expire after 20 minutes.
712 Remote clients will be refused access after this time.
717 See the X11 SECURITY extension specification for full details on
718 the restrictions imposed on untrusted clients.
720 Specifies whether remote hosts are allowed to connect to local
724 binds local port forwardings to the loopback address.
725 This prevents other remote hosts from connecting to forwarded ports.
727 can be used to specify that ssh
728 should bind local port forwardings to the wildcard address,
729 thus allowing remote hosts to connect to forwarded ports.
736 .It Cm GlobalKnownHostsFile
737 Specifies one or more files to use for the global
738 host key database, separated by whitespace.
740 .Pa /etc/ssh/ssh_known_hosts ,
741 .Pa /etc/ssh/ssh_known_hosts2 .
742 .It Cm GSSAPIAuthentication
743 Specifies whether user authentication based on GSSAPI is allowed.
746 Note that this option applies to protocol version 2 only.
747 .It Cm GSSAPIDelegateCredentials
748 Forward (delegate) credentials to the server.
751 Note that this option applies to protocol version 2 only.
752 .It Cm HashKnownHosts
755 should hash host names and addresses when they are added to
756 .Pa ~/.ssh/known_hosts .
757 These hashed names may be used normally by
761 but they do not reveal identifying information should the file's contents
765 Note that existing names and addresses in known hosts files
766 will not be converted automatically,
767 but may be manually hashed using
769 .It Cm HostbasedAuthentication
770 Specifies whether to try rhosts based authentication with public key
778 This option applies to protocol version 2 only and
780 .Cm RhostsRSAAuthentication .
781 .It Cm HostbasedKeyTypes
782 Specifies the key types that will be used for hostbased authentication
783 as a comma-separated pattern list.
786 will allow all key types.
791 may be used to list supported key types.
792 .It Cm HostKeyAlgorithms
793 Specifies the protocol version 2 host key algorithms
794 that the client wants to use in order of preference.
795 The default for this option is:
796 .Bd -literal -offset 3n
797 ecdsa-sha2-nistp256-cert-v01@openssh.com,
798 ecdsa-sha2-nistp384-cert-v01@openssh.com,
799 ecdsa-sha2-nistp521-cert-v01@openssh.com,
800 ssh-ed25519-cert-v01@openssh.com,
801 ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
802 ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
803 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
804 ssh-ed25519,ssh-rsa,ssh-dss
807 If hostkeys are known for the destination host then this default is modified
808 to prefer their algorithms.
810 The list of available key types may also be obtained using the
817 Specifies an alias that should be used instead of the
818 real host name when looking up or saving the host key
819 in the host key database files.
820 This option is useful for tunneling SSH connections
821 or for multiple servers running on a single host.
823 Specifies the real host name to log into.
824 This can be used to specify nicknames or abbreviations for hosts.
825 If the hostname contains the character sequence
827 then this will be replaced with the host name specified on the command line
828 (this is useful for manipulating unqualified names).
829 The character sequence
831 will be replaced by a single
833 character, which may be used when specifying IPv6 link-local addresses.
835 The default is the name given on the command line.
836 Numeric IP addresses are also permitted (both on the command line and in
839 .It Cm IdentitiesOnly
842 should only use the authentication identity files configured in the
849 offers more identities.
850 The argument to this keyword must be
854 This option is intended for situations where ssh-agent
855 offers many different identities.
859 Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
863 for protocol version 1, and
865 .Pa ~/.ssh/id_ecdsa ,
866 .Pa ~/.ssh/id_ed25519
869 for protocol version 2.
870 Additionally, any identities represented by the authentication agent
871 will be used for authentication unless
875 will try to load certificate information from the filename obtained by
878 to the path of a specified
881 The file name may use the tilde
882 syntax to refer to a user's home directory or one of the following
885 (local user's home directory),
891 (remote host name) or
895 It is possible to have
896 multiple identity files specified in configuration files; all these
897 identities will be tried in sequence.
900 directives will add to the list of identities tried (this behaviour
901 differs from that of other configuration directives).
904 may be used in conjunction with
906 to select which identities in an agent are offered during authentication.
908 Specifies a pattern-list of unknown options to be ignored if they are
909 encountered in configuration parsing.
910 This may be used to suppress errors if
912 contains options that are unrecognised by
914 It is recommended that
916 be listed early in the configuration file as it will not be applied
917 to unknown options that appear before it.
919 Specifies the IPv4 type-of-service or DSCP class for connections.
946 This option may take one or two arguments, separated by whitespace.
947 If one argument is specified, it is used as the packet class unconditionally.
948 If two values are specified, the first is automatically selected for
949 interactive sessions and the second for non-interactive sessions.
952 for interactive sessions and
954 for non-interactive sessions.
955 .It Cm KbdInteractiveAuthentication
956 Specifies whether to use keyboard-interactive authentication.
957 The argument to this keyword must be
963 .It Cm KbdInteractiveDevices
964 Specifies the list of methods to use in keyboard-interactive authentication.
965 Multiple method names must be comma-separated.
966 The default is to use the server specified list.
967 The methods available vary depending on what the server supports.
968 For an OpenSSH server,
969 it may be zero or more of:
975 Specifies the available KEX (Key Exchange) algorithms.
976 Multiple algorithms must be comma-separated.
978 .Bd -literal -offset indent
979 curve25519-sha256@libssh.org,
980 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
981 diffie-hellman-group-exchange-sha256,
982 diffie-hellman-group-exchange-sha1,
983 diffie-hellman-group14-sha1,
984 diffie-hellman-group1-sha1
987 The list of available key exchange algorithms may also be obtained using the
994 Specifies a command to execute on the local machine after successfully
995 connecting to the server.
996 The command string extends to the end of the line, and is executed with
998 The following escape character substitutions will be performed:
1000 (local user's home directory),
1006 (host name as provided on the command line),
1010 (remote user name) or
1012 (local user name) or
1014 by a hash of the concatenation: %l%h%p%r.
1016 The command is run synchronously and does not have access to the
1020 It should not be used for interactive commands.
1022 This directive is ignored unless
1023 .Cm PermitLocalCommand
1026 Specifies that a TCP port on the local machine be forwarded over
1027 the secure channel to the specified host and port from the remote machine.
1028 The first argument must be
1030 .Oo Ar bind_address : Oc Ar port
1032 and the second argument must be
1033 .Ar host : Ns Ar hostport .
1034 IPv6 addresses can be specified by enclosing addresses in square brackets.
1035 Multiple forwardings may be specified, and additional forwardings can be
1036 given on the command line.
1037 Only the superuser can forward privileged ports.
1038 By default, the local port is bound in accordance with the
1041 However, an explicit
1043 may be used to bind the connection to a specific address.
1048 indicates that the listening port be bound for local use only, while an
1051 indicates that the port should be available from all interfaces.
1053 Gives the verbosity level that is used when logging messages from
1055 The possible values are:
1056 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1057 The default is INFO.
1058 DEBUG and DEBUG1 are equivalent.
1059 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1061 Specifies the MAC (message authentication code) algorithms
1062 in order of preference.
1063 The MAC algorithm is used in protocol version 2
1064 for data integrity protection.
1065 Multiple algorithms must be comma-separated.
1066 The algorithms that contain
1068 calculate the MAC after encryption (encrypt-then-mac).
1069 These are considered safer and their use recommended.
1071 .Bd -literal -offset indent
1072 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1073 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1074 umac-64@openssh.com,umac-128@openssh.com,
1075 hmac-sha2-256,hmac-sha2-512,
1076 hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
1077 hmac-ripemd160-etm@openssh.com,
1078 hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com,
1079 hmac-md5,hmac-sha1,hmac-ripemd160,
1080 hmac-sha1-96,hmac-md5-96
1083 The list of available MAC algorithms may also be obtained using the
1089 .It Cm NoHostAuthenticationForLocalhost
1090 This option can be used if the home directory is shared across machines.
1091 In this case localhost will refer to a different machine on each of
1092 the machines and the user will get many warnings about changed host keys.
1093 However, this option disables host authentication for localhost.
1094 The argument to this keyword must be
1098 The default is to check the host key for localhost.
1099 .It Cm NumberOfPasswordPrompts
1100 Specifies the number of password prompts before giving up.
1101 The argument to this keyword must be an integer.
1103 .It Cm PasswordAuthentication
1104 Specifies whether to use password authentication.
1105 The argument to this keyword must be
1111 .It Cm PermitLocalCommand
1112 Allow local command execution via the
1115 .Ic !\& Ns Ar command
1118 The argument must be
1124 .It Cm PKCS11Provider
1125 Specifies which PKCS#11 provider to use.
1126 The argument to this keyword is the PKCS#11 shared library
1128 should use to communicate with a PKCS#11 token providing the user's
1131 Specifies the port number to connect on the remote host.
1133 .It Cm PreferredAuthentications
1134 Specifies the order in which the client should try protocol 2
1135 authentication methods.
1136 This allows a client to prefer one method (e.g.\&
1137 .Cm keyboard-interactive )
1138 over another method (e.g.\&
1141 .Bd -literal -offset indent
1142 gssapi-with-mic,hostbased,publickey,
1143 keyboard-interactive,password
1146 Specifies the protocol versions
1148 should support in order of preference.
1149 The possible values are
1153 Multiple versions must be comma-separated.
1154 When this option is set to
1157 will try version 2 and fall back to version 1
1158 if version 2 is not available.
1162 Specifies the command to use to connect to the server.
1164 string extends to the end of the line, and is executed
1165 using the user's shell
1167 directive to avoid a lingering shell process.
1169 In the command string, any occurrence of
1171 will be substituted by the host name to
1176 by the remote user name.
1177 The command can be basically anything,
1178 and should read from its standard input and write to its standard output.
1179 It should eventually connect an
1181 server running on some machine, or execute
1184 Host key management will be done using the
1185 HostName of the host being connected (defaulting to the name typed by
1187 Setting the command to
1189 disables this option entirely.
1192 is not available for connects with a proxy command.
1194 This directive is useful in conjunction with
1196 and its proxy support.
1197 For example, the following directive would connect via an HTTP proxy at
1199 .Bd -literal -offset 3n
1200 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1202 .It Cm ProxyUseFdpass
1205 will pass a connected file descriptor back to
1207 instead of continuing to execute and pass data.
1210 .It Cm PubkeyAuthentication
1211 Specifies whether to try public key authentication.
1212 The argument to this keyword must be
1218 This option applies to protocol version 2 only.
1220 Specifies the maximum amount of data that may be transmitted before the
1221 session key is renegotiated, optionally followed a maximum amount of
1222 time that may pass before the session key is renegotiated.
1223 The first argument is specified in bytes and may have a suffix of
1228 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1229 The default is between
1233 depending on the cipher.
1234 The optional second value is specified in seconds and may use any of the
1235 units documented in the
1236 TIME FORMATS section of
1238 The default value for
1242 which means that rekeying is performed after the cipher's default amount
1243 of data has been sent or received and no time based rekeying is done.
1244 This option applies to protocol version 2 only.
1245 .It Cm RemoteForward
1246 Specifies that a TCP port on the remote machine be forwarded over
1247 the secure channel to the specified host and port from the local machine.
1248 The first argument must be
1250 .Oo Ar bind_address : Oc Ar port
1252 and the second argument must be
1253 .Ar host : Ns Ar hostport .
1254 IPv6 addresses can be specified by enclosing addresses in square brackets.
1255 Multiple forwardings may be specified, and additional
1256 forwardings can be given on the command line.
1257 Privileged ports can be forwarded only when
1258 logging in as root on the remote machine.
1264 the listen port will be dynamically allocated on the server and reported
1265 to the client at run time.
1269 is not specified, the default is to only bind to loopback addresses.
1274 or an empty string, then the forwarding is requested to listen on all
1278 will only succeed if the server's
1280 option is enabled (see
1281 .Xr sshd_config 5 ) .
1283 Specifies whether to request a pseudo-tty for the session.
1284 The argument may be one of:
1286 (never request a TTY),
1288 (always request a TTY when standard input is a TTY),
1290 (always request a TTY) or
1292 (request a TTY when opening a login session).
1293 This option mirrors the
1299 .It Cm RevokedHostKeys
1300 Specifies revoked host public keys.
1301 Keys listed in this file will be refused for host authentication.
1302 Note that if this file does not exist or is not readable,
1303 then host authentication will be refused for all hosts.
1304 Keys may be specified as a text file, listing one public key per line, or as
1305 an OpenSSH Key Revocation List (KRL) as generated by
1307 For more information on KRLs, see the KEY REVOCATION LISTS section in
1309 .It Cm RhostsRSAAuthentication
1310 Specifies whether to try rhosts based authentication with RSA host
1312 The argument must be
1318 This option applies to protocol version 1 only and requires
1321 .It Cm RSAAuthentication
1322 Specifies whether to try RSA authentication.
1323 The argument to this keyword must be
1327 RSA authentication will only be
1328 attempted if the identity file exists, or an authentication agent is
1332 Note that this option applies to protocol version 1 only.
1334 Specifies what variables from the local
1336 should be sent to the server.
1337 Note that environment passing is only supported for protocol 2.
1338 The server must also support it, and the server must be configured to
1339 accept these environment variables.
1342 environment variable is always sent whenever a
1343 pseudo-terminal is requested as it is required by the protocol.
1348 for how to configure the server.
1349 Variables are specified by name, which may contain wildcard characters.
1350 Multiple environment variables may be separated by whitespace or spread
1354 The default is not to send any environment variables.
1358 for more information on patterns.
1359 .It Cm ServerAliveCountMax
1360 Sets the number of server alive messages (see below) which may be
1363 receiving any messages back from the server.
1364 If this threshold is reached while server alive messages are being sent,
1365 ssh will disconnect from the server, terminating the session.
1366 It is important to note that the use of server alive messages is very
1370 The server alive messages are sent through the encrypted channel
1371 and therefore will not be spoofable.
1372 The TCP keepalive option enabled by
1375 The server alive mechanism is valuable when the client or
1376 server depend on knowing when a connection has become inactive.
1378 The default value is 3.
1380 .Cm ServerAliveInterval
1381 (see below) is set to 15 and
1382 .Cm ServerAliveCountMax
1383 is left at the default, if the server becomes unresponsive,
1384 ssh will disconnect after approximately 45 seconds.
1385 This option applies to protocol version 2 only.
1386 .It Cm ServerAliveInterval
1387 Sets a timeout interval in seconds after which if no data has been received
1390 will send a message through the encrypted
1391 channel to request a response from the server.
1393 is 0, indicating that these messages will not be sent to the server.
1394 This option applies to protocol version 2 only.
1395 .It Cm StreamLocalBindMask
1396 Sets the octal file creation mode mask
1398 used when creating a Unix-domain socket file for local or remote
1400 This option is only used for port forwarding to a Unix-domain socket file.
1402 The default value is 0177, which creates a Unix-domain socket file that is
1403 readable and writable only by the owner.
1404 Note that not all operating systems honor the file mode on Unix-domain
1406 .It Cm StreamLocalBindUnlink
1407 Specifies whether to remove an existing Unix-domain socket file for local
1408 or remote port forwarding before creating a new one.
1409 If the socket file already exists and
1410 .Cm StreamLocalBindUnlink
1413 will be unable to forward the port to the Unix-domain socket file.
1414 This option is only used for port forwarding to a Unix-domain socket file.
1416 The argument must be
1422 .It Cm StrictHostKeyChecking
1423 If this flag is set to
1426 will never automatically add host keys to the
1427 .Pa ~/.ssh/known_hosts
1428 file, and refuses to connect to hosts whose host key has changed.
1429 This provides maximum protection against trojan horse attacks,
1430 though it can be annoying when the
1431 .Pa /etc/ssh/ssh_known_hosts
1432 file is poorly maintained or when connections to new hosts are
1434 This option forces the user to manually
1436 If this flag is set to
1438 ssh will automatically add new host keys to the
1439 user known hosts files.
1440 If this flag is set to
1443 will be added to the user known host files only after the user
1444 has confirmed that is what they really want to do, and
1445 ssh will refuse to connect to hosts whose host key has changed.
1447 known hosts will be verified automatically in all cases.
1448 The argument must be
1456 Specifies whether the system should send TCP keepalive messages to the
1458 If they are sent, death of the connection or crash of one
1459 of the machines will be properly noticed.
1460 However, this means that
1461 connections will die if the route is down temporarily, and some people
1466 (to send TCP keepalive messages), and the client will notice
1467 if the network goes down or the remote host dies.
1468 This is important in scripts, and many users want it too.
1470 To disable TCP keepalive messages, the value should be set to
1475 device forwarding between the client and the server.
1476 The argument must be
1486 requests the default tunnel mode, which is
1487 .Dq point-to-point .
1493 devices to open on the client
1498 The argument must be
1500 .Ar local_tun Op : Ar remote_tun .
1502 The devices may be specified by numerical ID or the keyword
1504 which uses the next available tunnel device.
1507 is not specified, it defaults to
1511 .It Cm UpdateHostKeys
1514 should accept notifications of additional hostkeys from the server sent
1515 after authentication has completed and add them to
1516 .Cm UserKnownHostsFile .
1517 The argument must be
1522 Enabling this option allows learning alternate hostkeys for a server
1523 and supports graceful key rotation by allowing a server to send replacement
1524 public keys before old ones are removed.
1525 Additional hostkeys are only accepted if the key used to authenticate the
1526 host was already trusted or explicity accepted by the user.
1531 then the user is asked to confirm the modifications to the known_hosts file.
1532 Confirmation is currently incompatible with
1533 .Cm ControlPersist ,
1534 and will be disabled if it is enabled.
1538 from OpenSSH 6.8 and greater support the
1539 .Dq hostkeys@openssh.com
1540 protocol extension used to inform the client of all the server's hostkeys.
1541 .It Cm UsePrivilegedPort
1542 Specifies whether to use a privileged port for outgoing connections.
1543 The argument must be
1552 must be setuid root.
1553 Note that this option must be set to
1556 .Cm RhostsRSAAuthentication
1559 Specifies the user to log in as.
1560 This can be useful when a different user name is used on different machines.
1561 This saves the trouble of
1562 having to remember to give the user name on the command line.
1563 .It Cm UserKnownHostsFile
1564 Specifies one or more files to use for the user
1565 host key database, separated by whitespace.
1567 .Pa ~/.ssh/known_hosts ,
1568 .Pa ~/.ssh/known_hosts2 .
1569 .It Cm VerifyHostKeyDNS
1570 Specifies whether to verify the remote key using DNS and SSHFP resource
1572 If this option is set to
1574 the client will implicitly trust keys that match a secure fingerprint
1576 Insecure fingerprints will be handled as if this option was set to
1578 If this option is set to
1580 information on fingerprint match will be displayed, but the user will still
1581 need to confirm new host keys according to the
1582 .Cm StrictHostKeyChecking
1584 The argument must be
1591 Note that this option applies to protocol version 2 only.
1593 See also VERIFYING HOST KEYS in
1595 .It Cm VisualHostKey
1596 If this flag is set to
1598 an ASCII art representation of the remote host key fingerprint is
1599 printed in addition to the fingerprint string at login and
1600 for unknown host keys.
1601 If this flag is set to
1603 no fingerprint strings are printed at login and
1604 only the fingerprint string will be printed for unknown host keys.
1607 .It Cm XAuthLocation
1608 Specifies the full pathname of the
1612 .Pa /usr/X11R6/bin/xauth .
1617 consists of zero or more non-whitespace characters,
1619 (a wildcard that matches zero or more characters),
1622 (a wildcard that matches exactly one character).
1623 For example, to specify a set of declarations for any host in the
1626 the following pattern could be used:
1630 The following pattern
1631 would match any host in the 192.168.0.[0-9] network range:
1633 .Dl Host 192.168.0.?
1637 is a comma-separated list of patterns.
1638 Patterns within pattern-lists may be negated
1639 by preceding them with an exclamation mark
1642 to allow a key to be used from anywhere within an organization
1646 the following entry (in authorized_keys) could be used:
1648 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1651 .It Pa ~/.ssh/config
1652 This is the per-user configuration file.
1653 The format of this file is described above.
1654 This file is used by the SSH client.
1655 Because of the potential for abuse, this file must have strict permissions:
1656 read/write for the user, and not accessible by others.
1657 .It Pa /etc/ssh/ssh_config
1658 Systemwide configuration file.
1659 This file provides defaults for those
1660 values that are not specified in the user's configuration file, and
1661 for those users who do not have a configuration file.
1662 This file must be world-readable.
1667 OpenSSH is a derivative of the original and free
1668 ssh 1.2.12 release by Tatu Ylonen.
1669 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1670 Theo de Raadt and Dug Song
1671 removed many bugs, re-added newer features and
1673 Markus Friedl contributed the support for SSH
1674 protocol versions 1.5 and 2.0.
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