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 1.205 2015/02/20 22:17:21 djm Exp $
37 .Dd $Mdocdate: February 20 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 keywork 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 If the option is set to
346 the check will not be executed.
350 Specifies the cipher to use for encrypting the session
351 in protocol version 1.
359 is only supported in the
361 client for interoperability with legacy protocol 1 implementations
362 that do not support the
365 Its use is strongly discouraged due to cryptographic weaknesses.
369 Specifies the ciphers allowed for protocol version 2
370 in order of preference.
371 Multiple ciphers must be comma-separated.
372 The supported ciphers are:
374 .Bl -item -compact -offset indent
390 aes128-gcm@openssh.com
392 aes256-gcm@openssh.com
404 chacha20-poly1305@openssh.com
408 .Bd -literal -offset indent
409 aes128-ctr,aes192-ctr,aes256-ctr,
410 aes128-gcm@openssh.com,aes256-gcm@openssh.com,
411 chacha20-poly1305@openssh.com,
412 arcfour256,arcfour128,
413 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,
414 aes192-cbc,aes256-cbc,arcfour
417 The list of available ciphers may also be obtained using the
423 .It Cm ClearAllForwardings
424 Specifies that all local, remote, and dynamic port forwardings
425 specified in the configuration files or on the command line be
427 This option is primarily useful when used from the
429 command line to clear port forwardings set in
430 configuration files, and is automatically set by
441 Specifies whether to use compression.
448 .It Cm CompressionLevel
449 Specifies the compression level to use if compression is enabled.
450 The argument must be an integer from 1 (fast) to 9 (slow, best).
451 The default level is 6, which is good for most applications.
452 The meaning of the values is the same as in
454 Note that this option applies to protocol version 1 only.
455 .It Cm ConnectionAttempts
456 Specifies the number of tries (one per second) to make before exiting.
457 The argument must be an integer.
458 This may be useful in scripts if the connection sometimes fails.
460 .It Cm ConnectTimeout
461 Specifies the timeout (in seconds) used when connecting to the
462 SSH server, instead of using the default system TCP timeout.
463 This value is used only when the target is down or really unreachable,
464 not when it refuses the connection.
466 Enables the sharing of multiple sessions over a single network connection.
470 will listen for connections on a control socket specified using the
473 Additional sessions can connect to this socket using the same
480 These sessions will try to reuse the master instance's network connection
481 rather than initiating new ones, but will fall back to connecting normally
482 if the control socket does not exist, or is not listening.
487 to listen for control connections, but require confirmation using the
489 program before they are accepted (see
495 ssh will continue without connecting to a master instance.
499 forwarding is supported over these multiplexed connections, however the
500 display and agent forwarded will be the one belonging to the master
501 connection i.e. it is not possible to forward multiple displays or agents.
503 Two additional options allow for opportunistic multiplexing: try to use a
504 master connection but fall back to creating a new one if one does not already
510 The latter requires confirmation like the
514 Specify the path to the control socket used for connection sharing as described
517 section above or the string
519 to disable connection sharing.
522 will be substituted by the first component of the local host name,
524 will be substituted by the local host name (including any domain name),
526 will be substituted by the target host name,
528 will be substituted by the original target host name
529 specified on the command line,
531 the destination port,
533 by the remote login username,
535 by the username of the user running
538 by a hash of the concatenation: %l%h%p%r.
539 It is recommended that any
541 used for opportunistic connection sharing include
542 at least %h, %p, and %r (or alternatively %C) and be placed in a directory
543 that is not writable by other users.
544 This ensures that shared connections are uniquely identified.
545 .It Cm ControlPersist
546 When used in conjunction with
548 specifies that the master connection should remain open
549 in the background (waiting for future client connections)
550 after the initial client connection has been closed.
553 then the master connection will not be placed into the background,
554 and will close as soon as the initial client connection is closed.
559 then the master connection will remain in the background indefinitely
560 (until killed or closed via a mechanism such as the
564 If set to a time in seconds, or a time in any of the formats documented in
566 then the backgrounded master connection will automatically terminate
567 after it has remained idle (with no client connections) for the
569 .It Cm DynamicForward
570 Specifies that a TCP port on the local machine be forwarded
571 over the secure channel, and the application
572 protocol is then used to determine where to connect to from the
577 .Oo Ar bind_address : Oc Ar port .
579 IPv6 addresses can be specified by enclosing addresses in square brackets.
580 By default, the local port is bound in accordance with the
585 may be used to bind the connection to a specific address.
590 indicates that the listening port be bound for local use only, while an
593 indicates that the port should be available from all interfaces.
595 Currently the SOCKS4 and SOCKS5 protocols are supported, and
597 will act as a SOCKS server.
598 Multiple forwardings may be specified, and
599 additional forwardings can be given on the command line.
600 Only the superuser can forward privileged ports.
601 .It Cm EnableSSHKeysign
602 Setting this option to
604 in the global client configuration file
605 .Pa /etc/ssh/ssh_config
606 enables the use of the helper program
609 .Cm HostbasedAuthentication .
616 This option should be placed in the non-hostspecific section.
619 for more information.
621 Sets the escape character (default:
623 The escape character can also
624 be set on the command line.
625 The argument should be a single character,
627 followed by a letter, or
629 to disable the escape
630 character entirely (making the connection transparent for binary
632 .It Cm ExitOnForwardFailure
635 should terminate the connection if it cannot set up all requested
636 dynamic, tunnel, local, and remote port forwardings.
643 .It Cm FingerprintHash
644 Specifies the hash algorithm used when displaying key fingerprints.
652 Specifies whether the connection to the authentication agent (if any)
653 will be forwarded to the remote machine.
661 Agent forwarding should be enabled with caution.
662 Users with the ability to bypass file permissions on the remote host
663 (for the agent's Unix-domain socket)
664 can access the local agent through the forwarded connection.
665 An attacker cannot obtain key material from the agent,
666 however they can perform operations on the keys that enable them to
667 authenticate using the identities loaded into the agent.
669 Specifies whether X11 connections will be automatically redirected
670 over the secure channel and
680 X11 forwarding should be enabled with caution.
681 Users with the ability to bypass file permissions on the remote host
682 (for the user's X11 authorization database)
683 can access the local X11 display through the forwarded connection.
684 An attacker may then be able to perform activities such as keystroke monitoring
686 .Cm ForwardX11Trusted
687 option is also enabled.
688 .It Cm ForwardX11Timeout
689 Specify a timeout for untrusted X11 forwarding
690 using the format described in the
691 TIME FORMATS section of
693 X11 connections received by
695 after this time will be refused.
696 The default is to disable untrusted X11 forwarding after twenty minutes has
698 .It Cm ForwardX11Trusted
699 If this option is set to
701 remote X11 clients will have full access to the original X11 display.
703 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.
716 See the X11 SECURITY extension specification for full details on
717 the restrictions imposed on untrusted clients.
719 Specifies whether remote hosts are allowed to connect to local
723 binds local port forwardings to the loopback address.
724 This prevents other remote hosts from connecting to forwarded ports.
726 can be used to specify that ssh
727 should bind local port forwardings to the wildcard address,
728 thus allowing remote hosts to connect to forwarded ports.
735 .It Cm GlobalKnownHostsFile
736 Specifies one or more files to use for the global
737 host key database, separated by whitespace.
739 .Pa /etc/ssh/ssh_known_hosts ,
740 .Pa /etc/ssh/ssh_known_hosts2 .
741 .It Cm GSSAPIAuthentication
742 Specifies whether user authentication based on GSSAPI is allowed.
745 Note that this option applies to protocol version 2 only.
746 .It Cm GSSAPIDelegateCredentials
747 Forward (delegate) credentials to the server.
750 Note that this option applies to protocol version 2 only.
751 .It Cm HashKnownHosts
754 should hash host names and addresses when they are added to
755 .Pa ~/.ssh/known_hosts .
756 These hashed names may be used normally by
760 but they do not reveal identifying information should the file's contents
764 Note that existing names and addresses in known hosts files
765 will not be converted automatically,
766 but may be manually hashed using
768 .It Cm HostbasedAuthentication
769 Specifies whether to try rhosts based authentication with public key
777 This option applies to protocol version 2 only and
779 .Cm RhostsRSAAuthentication .
780 .It Cm HostbasedKeyTypes
781 Specifies the key types that will be used for hostbased authentication
782 as a comma-separated pattern list.
785 will allow all key types.
790 may be used to list supported key types.
791 .It Cm HostKeyAlgorithms
792 Specifies the protocol version 2 host key algorithms
793 that the client wants to use in order of preference.
794 The default for this option is:
795 .Bd -literal -offset 3n
796 ecdsa-sha2-nistp256-cert-v01@openssh.com,
797 ecdsa-sha2-nistp384-cert-v01@openssh.com,
798 ecdsa-sha2-nistp521-cert-v01@openssh.com,
799 ssh-ed25519-cert-v01@openssh.com,
800 ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
801 ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
802 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
803 ssh-ed25519,ssh-rsa,ssh-dss
806 If hostkeys are known for the destination host then this default is modified
807 to prefer their algorithms.
809 The list of available key types may also be obtained using the
816 Specifies an alias that should be used instead of the
817 real host name when looking up or saving the host key
818 in the host key database files.
819 This option is useful for tunneling SSH connections
820 or for multiple servers running on a single host.
822 Specifies the real host name to log into.
823 This can be used to specify nicknames or abbreviations for hosts.
824 If the hostname contains the character sequence
826 then this will be replaced with the host name specified on the command line
827 (this is useful for manipulating unqualified names).
828 The character sequence
830 will be replaced by a single
832 character, which may be used when specifying IPv6 link-local addresses.
834 The default is the name given on the command line.
835 Numeric IP addresses are also permitted (both on the command line and in
838 .It Cm IdentitiesOnly
841 should only use the authentication identity files configured in the
848 offers more identities.
849 The argument to this keyword must be
853 This option is intended for situations where ssh-agent
854 offers many different identities.
858 Specifies a file from which the user's DSA, ECDSA, Ed25519 or RSA authentication
862 for protocol version 1, and
864 .Pa ~/.ssh/id_ecdsa ,
865 .Pa ~/.ssh/id_ed25519
868 for protocol version 2.
869 Additionally, any identities represented by the authentication agent
870 will be used for authentication unless
874 will try to load certificate information from the filename obtained by
877 to the path of a specified
880 The file name may use the tilde
881 syntax to refer to a user's home directory or one of the following
884 (local user's home directory),
890 (remote host name) or
894 It is possible to have
895 multiple identity files specified in configuration files; all these
896 identities will be tried in sequence.
899 directives will add to the list of identities tried (this behaviour
900 differs from that of other configuration directives).
903 may be used in conjunction with
905 to select which identities in an agent are offered during authentication.
907 Specifies a pattern-list of unknown options to be ignored if they are
908 encountered in configuration parsing.
909 This may be used to suppress errors if
911 contains options that are unrecognised by
913 It is recommended that
915 be listed early in the configuration file as it will not be applied
916 to unknown options that appear before it.
918 Specifies the IPv4 type-of-service or DSCP class for connections.
945 This option may take one or two arguments, separated by whitespace.
946 If one argument is specified, it is used as the packet class unconditionally.
947 If two values are specified, the first is automatically selected for
948 interactive sessions and the second for non-interactive sessions.
951 for interactive sessions and
953 for non-interactive sessions.
954 .It Cm KbdInteractiveAuthentication
955 Specifies whether to use keyboard-interactive authentication.
956 The argument to this keyword must be
962 .It Cm KbdInteractiveDevices
963 Specifies the list of methods to use in keyboard-interactive authentication.
964 Multiple method names must be comma-separated.
965 The default is to use the server specified list.
966 The methods available vary depending on what the server supports.
967 For an OpenSSH server,
968 it may be zero or more of:
974 Specifies the available KEX (Key Exchange) algorithms.
975 Multiple algorithms must be comma-separated.
977 .Bd -literal -offset indent
978 curve25519-sha256@libssh.org,
979 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
980 diffie-hellman-group-exchange-sha256,
981 diffie-hellman-group14-sha1,
982 diffie-hellman-group-exchange-sha1,
983 diffie-hellman-group1-sha1
986 The list of available key exchange algorithms may also be obtained using the
993 Specifies a command to execute on the local machine after successfully
994 connecting to the server.
995 The command string extends to the end of the line, and is executed with
997 The following escape character substitutions will be performed:
999 (local user's home directory),
1005 (host name as provided on the command line),
1009 (remote user name) or
1011 (local user name) or
1013 by a hash of the concatenation: %l%h%p%r.
1015 The command is run synchronously and does not have access to the
1019 It should not be used for interactive commands.
1021 This directive is ignored unless
1022 .Cm PermitLocalCommand
1025 Specifies that a TCP port on the local machine be forwarded over
1026 the secure channel to the specified host and port from the remote machine.
1027 The first argument must be
1029 .Oo Ar bind_address : Oc Ar port
1031 and the second argument must be
1032 .Ar host : Ns Ar hostport .
1033 IPv6 addresses can be specified by enclosing addresses in square brackets.
1034 Multiple forwardings may be specified, and additional forwardings can be
1035 given on the command line.
1036 Only the superuser can forward privileged ports.
1037 By default, the local port is bound in accordance with the
1040 However, an explicit
1042 may be used to bind the connection to a specific address.
1047 indicates that the listening port be bound for local use only, while an
1050 indicates that the port should be available from all interfaces.
1052 Gives the verbosity level that is used when logging messages from
1054 The possible values are:
1055 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1056 The default is INFO.
1057 DEBUG and DEBUG1 are equivalent.
1058 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1060 Specifies the MAC (message authentication code) algorithms
1061 in order of preference.
1062 The MAC algorithm is used in protocol version 2
1063 for data integrity protection.
1064 Multiple algorithms must be comma-separated.
1065 The algorithms that contain
1067 calculate the MAC after encryption (encrypt-then-mac).
1068 These are considered safer and their use recommended.
1070 .Bd -literal -offset indent
1071 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1072 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1073 umac-64@openssh.com,umac-128@openssh.com,
1074 hmac-sha2-256,hmac-sha2-512,
1075 hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com,
1076 hmac-ripemd160-etm@openssh.com,
1077 hmac-sha1-96-etm@openssh.com,hmac-md5-96-etm@openssh.com,
1078 hmac-md5,hmac-sha1,hmac-ripemd160,
1079 hmac-sha1-96,hmac-md5-96
1082 The list of available MAC algorithms may also be obtained using the
1088 .It Cm NoHostAuthenticationForLocalhost
1089 This option can be used if the home directory is shared across machines.
1090 In this case localhost will refer to a different machine on each of
1091 the machines and the user will get many warnings about changed host keys.
1092 However, this option disables host authentication for localhost.
1093 The argument to this keyword must be
1097 The default is to check the host key for localhost.
1098 .It Cm NumberOfPasswordPrompts
1099 Specifies the number of password prompts before giving up.
1100 The argument to this keyword must be an integer.
1102 .It Cm PasswordAuthentication
1103 Specifies whether to use password authentication.
1104 The argument to this keyword must be
1110 .It Cm PermitLocalCommand
1111 Allow local command execution via the
1114 .Ic !\& Ns Ar command
1117 The argument must be
1123 .It Cm PKCS11Provider
1124 Specifies which PKCS#11 provider to use.
1125 The argument to this keyword is the PKCS#11 shared library
1127 should use to communicate with a PKCS#11 token providing the user's
1130 Specifies the port number to connect on the remote host.
1132 .It Cm PreferredAuthentications
1133 Specifies the order in which the client should try protocol 2
1134 authentication methods.
1135 This allows a client to prefer one method (e.g.\&
1136 .Cm keyboard-interactive )
1137 over another method (e.g.\&
1140 .Bd -literal -offset indent
1141 gssapi-with-mic,hostbased,publickey,
1142 keyboard-interactive,password
1145 Specifies the protocol versions
1147 should support in order of preference.
1148 The possible values are
1152 Multiple versions must be comma-separated.
1153 When this option is set to
1156 will try version 2 and fall back to version 1
1157 if version 2 is not available.
1161 Specifies the command to use to connect to the server.
1163 string extends to the end of the line, and is executed
1164 using the user's shell
1166 directive to avoid a lingering shell process.
1168 In the command string, any occurrence of
1170 will be substituted by the host name to
1175 by the remote user name.
1176 The command can be basically anything,
1177 and should read from its standard input and write to its standard output.
1178 It should eventually connect an
1180 server running on some machine, or execute
1183 Host key management will be done using the
1184 HostName of the host being connected (defaulting to the name typed by
1186 Setting the command to
1188 disables this option entirely.
1191 is not available for connects with a proxy command.
1193 This directive is useful in conjunction with
1195 and its proxy support.
1196 For example, the following directive would connect via an HTTP proxy at
1198 .Bd -literal -offset 3n
1199 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1201 .It Cm ProxyUseFdpass
1204 will pass a connected file descriptor back to
1206 instead of continuing to execute and pass data.
1209 .It Cm PubkeyAuthentication
1210 Specifies whether to try public key authentication.
1211 The argument to this keyword must be
1217 This option applies to protocol version 2 only.
1219 Specifies the maximum amount of data that may be transmitted before the
1220 session key is renegotiated, optionally followed a maximum amount of
1221 time that may pass before the session key is renegotiated.
1222 The first argument is specified in bytes and may have a suffix of
1227 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1228 The default is between
1232 depending on the cipher.
1233 The optional second value is specified in seconds and may use any of the
1234 units documented in the
1235 TIME FORMATS section of
1237 The default value for
1241 which means that rekeying is performed after the cipher's default amount
1242 of data has been sent or received and no time based rekeying is done.
1243 This option applies to protocol version 2 only.
1244 .It Cm RemoteForward
1245 Specifies that a TCP port on the remote machine be forwarded over
1246 the secure channel to the specified host and port from the local machine.
1247 The first argument must be
1249 .Oo Ar bind_address : Oc Ar port
1251 and the second argument must be
1252 .Ar host : Ns Ar hostport .
1253 IPv6 addresses can be specified by enclosing addresses in square brackets.
1254 Multiple forwardings may be specified, and additional
1255 forwardings can be given on the command line.
1256 Privileged ports can be forwarded only when
1257 logging in as root on the remote machine.
1263 the listen port will be dynamically allocated on the server and reported
1264 to the client at run time.
1268 is not specified, the default is to only bind to loopback addresses.
1273 or an empty string, then the forwarding is requested to listen on all
1277 will only succeed if the server's
1279 option is enabled (see
1280 .Xr sshd_config 5 ) .
1282 Specifies whether to request a pseudo-tty for the session.
1283 The argument may be one of:
1285 (never request a TTY),
1287 (always request a TTY when standard input is a TTY),
1289 (always request a TTY) or
1291 (request a TTY when opening a login session).
1292 This option mirrors the
1298 .It Cm RevokedHostKeys
1299 Specifies revoked host public keys.
1300 Keys listed in this file will be refused for host authentication.
1301 Note that if this file does not exist or is not readable,
1302 then host authentication will be refused for all hosts.
1303 Keys may be specified as a text file, listing one public key per line, or as
1304 an OpenSSH Key Revocation List (KRL) as generated by
1306 For more information on KRLs, see the KEY REVOCATION LISTS section in
1308 .It Cm RhostsRSAAuthentication
1309 Specifies whether to try rhosts based authentication with RSA host
1311 The argument must be
1317 This option applies to protocol version 1 only and requires
1320 .It Cm RSAAuthentication
1321 Specifies whether to try RSA authentication.
1322 The argument to this keyword must be
1326 RSA authentication will only be
1327 attempted if the identity file exists, or an authentication agent is
1331 Note that this option applies to protocol version 1 only.
1333 Specifies what variables from the local
1335 should be sent to the server.
1336 Note that environment passing is only supported for protocol 2.
1337 The server must also support it, and the server must be configured to
1338 accept these environment variables.
1343 for how to configure the server.
1344 Variables are specified by name, which may contain wildcard characters.
1345 Multiple environment variables may be separated by whitespace or spread
1349 The default is not to send any environment variables.
1353 for more information on patterns.
1354 .It Cm ServerAliveCountMax
1355 Sets the number of server alive messages (see below) which may be
1358 receiving any messages back from the server.
1359 If this threshold is reached while server alive messages are being sent,
1360 ssh will disconnect from the server, terminating the session.
1361 It is important to note that the use of server alive messages is very
1365 The server alive messages are sent through the encrypted channel
1366 and therefore will not be spoofable.
1367 The TCP keepalive option enabled by
1370 The server alive mechanism is valuable when the client or
1371 server depend on knowing when a connection has become inactive.
1373 The default value is 3.
1375 .Cm ServerAliveInterval
1376 (see below) is set to 15 and
1377 .Cm ServerAliveCountMax
1378 is left at the default, if the server becomes unresponsive,
1379 ssh will disconnect after approximately 45 seconds.
1380 This option applies to protocol version 2 only.
1381 .It Cm ServerAliveInterval
1382 Sets a timeout interval in seconds after which if no data has been received
1385 will send a message through the encrypted
1386 channel to request a response from the server.
1388 is 0, indicating that these messages will not be sent to the server.
1389 This option applies to protocol version 2 only.
1390 .It Cm StreamLocalBindMask
1391 Sets the octal file creation mode mask
1393 used when creating a Unix-domain socket file for local or remote
1395 This option is only used for port forwarding to a Unix-domain socket file.
1397 The default value is 0177, which creates a Unix-domain socket file that is
1398 readable and writable only by the owner.
1399 Note that not all operating systems honor the file mode on Unix-domain
1401 .It Cm StreamLocalBindUnlink
1402 Specifies whether to remove an existing Unix-domain socket file for local
1403 or remote port forwarding before creating a new one.
1404 If the socket file already exists and
1405 .Cm StreamLocalBindUnlink
1408 will be unable to forward the port to the Unix-domain socket file.
1409 This option is only used for port forwarding to a Unix-domain socket file.
1411 The argument must be
1417 .It Cm StrictHostKeyChecking
1418 If this flag is set to
1421 will never automatically add host keys to the
1422 .Pa ~/.ssh/known_hosts
1423 file, and refuses to connect to hosts whose host key has changed.
1424 This provides maximum protection against trojan horse attacks,
1425 though it can be annoying when the
1426 .Pa /etc/ssh/ssh_known_hosts
1427 file is poorly maintained or when connections to new hosts are
1429 This option forces the user to manually
1431 If this flag is set to
1433 ssh will automatically add new host keys to the
1434 user known hosts files.
1435 If this flag is set to
1438 will be added to the user known host files only after the user
1439 has confirmed that is what they really want to do, and
1440 ssh will refuse to connect to hosts whose host key has changed.
1442 known hosts will be verified automatically in all cases.
1443 The argument must be
1451 Specifies whether the system should send TCP keepalive messages to the
1453 If they are sent, death of the connection or crash of one
1454 of the machines will be properly noticed.
1455 However, this means that
1456 connections will die if the route is down temporarily, and some people
1461 (to send TCP keepalive messages), and the client will notice
1462 if the network goes down or the remote host dies.
1463 This is important in scripts, and many users want it too.
1465 To disable TCP keepalive messages, the value should be set to
1470 device forwarding between the client and the server.
1471 The argument must be
1481 requests the default tunnel mode, which is
1482 .Dq point-to-point .
1488 devices to open on the client
1493 The argument must be
1495 .Ar local_tun Op : Ar remote_tun .
1497 The devices may be specified by numerical ID or the keyword
1499 which uses the next available tunnel device.
1502 is not specified, it defaults to
1506 .It Cm UpdateHostKeys
1509 should accept notifications of additional hostkeys from the server sent
1510 after authentication has completed and add them to
1511 .Cm UserKnownHostsFile .
1512 The argument must be
1517 Enabling this option allows learning alternate hostkeys for a server
1518 and supports graceful key rotation by allowing a server to send replacement
1519 public keys before old ones are removed.
1520 Additional hostkeys are only accepted if the key used to authenticate the
1521 host was already trusted or explicity accepted by the user.
1526 then the user is asked to confirm the modifications to the known_hosts file.
1527 Confirmation is currently incompatible with
1528 .Cm ControlPersist ,
1529 and will be disabled if it is enabled.
1533 from OpenSSH 6.8 and greater support the
1534 .Dq hostkeys@openssh.com
1535 protocol extension used to inform the client of all the server's hostkeys.
1536 .It Cm UsePrivilegedPort
1537 Specifies whether to use a privileged port for outgoing connections.
1538 The argument must be
1547 must be setuid root.
1548 Note that this option must be set to
1551 .Cm RhostsRSAAuthentication
1554 Specifies the user to log in as.
1555 This can be useful when a different user name is used on different machines.
1556 This saves the trouble of
1557 having to remember to give the user name on the command line.
1558 .It Cm UserKnownHostsFile
1559 Specifies one or more files to use for the user
1560 host key database, separated by whitespace.
1562 .Pa ~/.ssh/known_hosts ,
1563 .Pa ~/.ssh/known_hosts2 .
1564 .It Cm VerifyHostKeyDNS
1565 Specifies whether to verify the remote key using DNS and SSHFP resource
1567 If this option is set to
1569 the client will implicitly trust keys that match a secure fingerprint
1571 Insecure fingerprints will be handled as if this option was set to
1573 If this option is set to
1575 information on fingerprint match will be displayed, but the user will still
1576 need to confirm new host keys according to the
1577 .Cm StrictHostKeyChecking
1579 The argument must be
1586 Note that this option applies to protocol version 2 only.
1588 See also VERIFYING HOST KEYS in
1590 .It Cm VisualHostKey
1591 If this flag is set to
1593 an ASCII art representation of the remote host key fingerprint is
1594 printed in addition to the fingerprint string at login and
1595 for unknown host keys.
1596 If this flag is set to
1598 no fingerprint strings are printed at login and
1599 only the fingerprint string will be printed for unknown host keys.
1602 .It Cm XAuthLocation
1603 Specifies the full pathname of the
1607 .Pa /usr/X11R6/bin/xauth .
1612 consists of zero or more non-whitespace characters,
1614 (a wildcard that matches zero or more characters),
1617 (a wildcard that matches exactly one character).
1618 For example, to specify a set of declarations for any host in the
1621 the following pattern could be used:
1625 The following pattern
1626 would match any host in the 192.168.0.[0-9] network range:
1628 .Dl Host 192.168.0.?
1632 is a comma-separated list of patterns.
1633 Patterns within pattern-lists may be negated
1634 by preceding them with an exclamation mark
1637 to allow a key to be used from anywhere within an organization
1641 the following entry (in authorized_keys) could be used:
1643 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1646 .It Pa ~/.ssh/config
1647 This is the per-user configuration file.
1648 The format of this file is described above.
1649 This file is used by the SSH client.
1650 Because of the potential for abuse, this file must have strict permissions:
1651 read/write for the user, and not accessible by others.
1652 .It Pa /etc/ssh/ssh_config
1653 Systemwide configuration file.
1654 This file provides defaults for those
1655 values that are not specified in the user's configuration file, and
1656 for those users who do not have a configuration file.
1657 This file must be world-readable.
1662 OpenSSH is a derivative of the original and free
1663 ssh 1.2.12 release by Tatu Ylonen.
1664 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1665 Theo de Raadt and Dug Song
1666 removed many bugs, re-added newer features and
1668 Markus Friedl contributed the support for SSH
1669 protocol versions 1.5 and 2.0.
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