3 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5 .\" All rights reserved
7 .\" As far as I am concerned, the code I have written for this software
8 .\" can be used freely for any purpose. Any derived versions of this
9 .\" software must be clearly marked as such, and if the derived work is
10 .\" incompatible with the protocol description in the RFC file, it must be
11 .\" called by a name other than "ssh" or "Secure Shell".
13 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
17 .\" Redistribution and use in source and binary forms, with or without
18 .\" modification, are permitted provided that the following conditions
20 .\" 1. Redistributions of source code must retain the above copyright
21 .\" notice, this list of conditions and the following disclaimer.
22 .\" 2. Redistributions in binary form must reproduce the above copyright
23 .\" notice, this list of conditions and the following disclaimer in the
24 .\" documentation and/or other materials provided with the distribution.
26 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
37 .\" $OpenBSD: ssh_config.5,v 1.110 2008/06/12 19:10:09 jmc Exp $
43 .Nd OpenSSH SSH client configuration files
46 .Nm /etc/ssh/ssh_config
49 obtains configuration data from the following sources in
52 .Bl -enum -offset indent -compact
56 user's configuration file
59 system-wide configuration file
60 .Pq Pa /etc/ssh/ssh_config
63 For each parameter, the first obtained value
65 The configuration files contain sections separated by
67 specifications, and that section is only applied for hosts that
68 match one of the patterns given in the specification.
69 The matched host name is the one given on the command line.
71 Since the first obtained value for each parameter is used, more
72 host-specific declarations should be given near the beginning of the
73 file, and general defaults at the end.
75 The configuration file has the following format:
77 Empty lines and lines starting with
80 Otherwise a line is of the format
81 .Dq keyword arguments .
82 Configuration options may be separated by whitespace or
83 optional whitespace and exactly one
85 the latter format is useful to avoid the need to quote whitespace
86 when specifying configuration options using the
93 Arguments may optionally be enclosed in double quotes
95 in order to represent arguments containing spaces.
98 keywords and their meanings are as follows (note that
99 keywords are case-insensitive and arguments are case-sensitive):
102 Restricts the following declarations (up to the next
104 keyword) to be only for those hosts that match one of the patterns
105 given after the keyword.
106 If more than one pattern is provided, they should be separated by whitepsace.
109 as a pattern can be used to provide global
110 defaults for all hosts.
113 argument given on the command line (i.e. the name is not converted to
114 a canonicalized host name before matching).
118 for more information on patterns.
120 Specifies which address family to use when connecting.
130 passphrase/password querying will be disabled.
131 This option is useful in scripts and other batch jobs where no user
132 is present to supply the password.
140 Use the specified address on the local machine as the source address of
142 Only useful on systems with more than one address.
143 Note that this option does not work if
144 .Cm UsePrivilegedPort
147 .It Cm ChallengeResponseAuthentication
148 Specifies whether to use challenge-response authentication.
149 The argument to this keyword must be
156 If this flag is set to
159 will additionally check the host IP address in the
162 This allows ssh to detect if a host key changed due to DNS spoofing.
163 If the option is set to
165 a fingerprint and an ASCII art representation of the key are printed,
166 in addition to the host IP address check.
167 If the option is set to
169 the check will not be executed.
173 Specifies the cipher to use for encrypting the session
174 in protocol version 1.
182 is only supported in the
184 client for interoperability with legacy protocol 1 implementations
185 that do not support the
188 Its use is strongly discouraged due to cryptographic weaknesses.
192 Specifies the ciphers allowed for protocol version 2
193 in order of preference.
194 Multiple ciphers must be comma-separated.
195 The supported ciphers are
210 .Bd -literal -offset 3n
211 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
212 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
213 aes192-ctr,aes256-ctr
215 .It Cm ClearAllForwardings
216 Specifies that all local, remote, and dynamic port forwardings
217 specified in the configuration files or on the command line be
219 This option is primarily useful when used from the
221 command line to clear port forwardings set in
222 configuration files, and is automatically set by
233 Specifies whether to use compression.
240 .It Cm CompressionLevel
241 Specifies the compression level to use if compression is enabled.
242 The argument must be an integer from 1 (fast) to 9 (slow, best).
243 The default level is 6, which is good for most applications.
244 The meaning of the values is the same as in
246 Note that this option applies to protocol version 1 only.
247 .It Cm ConnectionAttempts
248 Specifies the number of tries (one per second) to make before exiting.
249 The argument must be an integer.
250 This may be useful in scripts if the connection sometimes fails.
252 .It Cm ConnectTimeout
253 Specifies the timeout (in seconds) used when connecting to the
254 SSH server, instead of using the default system TCP timeout.
255 This value is used only when the target is down or really unreachable,
256 not when it refuses the connection.
258 Enables the sharing of multiple sessions over a single network connection.
262 will listen for connections on a control socket specified using the
265 Additional sessions can connect to this socket using the same
272 These sessions will try to reuse the master instance's network connection
273 rather than initiating new ones, but will fall back to connecting normally
274 if the control socket does not exist, or is not listening.
279 to listen for control connections, but require confirmation using the
281 program before they are accepted (see
287 ssh will continue without connecting to a master instance.
291 forwarding is supported over these multiplexed connections, however the
292 display and agent forwarded will be the one belonging to the master
293 connection i.e. it is not possible to forward multiple displays or agents.
295 Two additional options allow for opportunistic multiplexing: try to use a
296 master connection but fall back to creating a new one if one does not already
302 The latter requires confirmation like the
306 Specify the path to the control socket used for connection sharing as described
309 section above or the string
311 to disable connection sharing.
314 will be substituted by the local host name,
316 will be substituted by the target host name,
320 by the remote login username.
321 It is recommended that any
323 used for opportunistic connection sharing include
324 at least %h, %p, and %r.
325 This ensures that shared connections are uniquely identified.
326 .It Cm DynamicForward
327 Specifies that a TCP port on the local machine be forwarded
328 over the secure channel, and the application
329 protocol is then used to determine where to connect to from the
334 .Oo Ar bind_address : Oc Ar port .
336 IPv6 addresses can be specified by enclosing addresses in square brackets or
337 by using an alternative syntax:
338 .Oo Ar bind_address Ns / Oc Ns Ar port .
339 By default, the local port is bound in accordance with the
344 may be used to bind the connection to a specific address.
349 indicates that the listening port be bound for local use only, while an
352 indicates that the port should be available from all interfaces.
354 Currently the SOCKS4 and SOCKS5 protocols are supported, and
356 will act as a SOCKS server.
357 Multiple forwardings may be specified, and
358 additional forwardings can be given on the command line.
359 Only the superuser can forward privileged ports.
360 .It Cm EnableSSHKeysign
361 Setting this option to
363 in the global client configuration file
364 .Pa /etc/ssh/ssh_config
365 enables the use of the helper program
368 .Cm HostbasedAuthentication .
375 This option should be placed in the non-hostspecific section.
378 for more information.
380 Sets the escape character (default:
382 The escape character can also
383 be set on the command line.
384 The argument should be a single character,
386 followed by a letter, or
388 to disable the escape
389 character entirely (making the connection transparent for binary
391 .It Cm ExitOnForwardFailure
394 should terminate the connection if it cannot set up all requested
395 dynamic, tunnel, local, and remote port forwardings.
403 Specifies whether the connection to the authentication agent (if any)
404 will be forwarded to the remote machine.
412 Agent forwarding should be enabled with caution.
413 Users with the ability to bypass file permissions on the remote host
414 (for the agent's Unix-domain socket)
415 can access the local agent through the forwarded connection.
416 An attacker cannot obtain key material from the agent,
417 however they can perform operations on the keys that enable them to
418 authenticate using the identities loaded into the agent.
420 Specifies whether X11 connections will be automatically redirected
421 over the secure channel and
431 X11 forwarding should be enabled with caution.
432 Users with the ability to bypass file permissions on the remote host
433 (for the user's X11 authorization database)
434 can access the local X11 display through the forwarded connection.
435 An attacker may then be able to perform activities such as keystroke monitoring
437 .Cm ForwardX11Trusted
438 option is also enabled.
439 .It Cm ForwardX11Trusted
440 If this option is set to
442 remote X11 clients will have full access to the original X11 display.
444 If this option is set to
446 remote X11 clients will be considered untrusted and prevented
447 from stealing or tampering with data belonging to trusted X11
451 token used for the session will be set to expire after 20 minutes.
452 Remote clients will be refused access after this time.
457 See the X11 SECURITY extension specification for full details on
458 the restrictions imposed on untrusted clients.
460 Specifies whether remote hosts are allowed to connect to local
464 binds local port forwardings to the loopback address.
465 This prevents other remote hosts from connecting to forwarded ports.
467 can be used to specify that ssh
468 should bind local port forwardings to the wildcard address,
469 thus allowing remote hosts to connect to forwarded ports.
476 .It Cm GlobalKnownHostsFile
477 Specifies a file to use for the global
478 host key database instead of
479 .Pa /etc/ssh/ssh_known_hosts .
480 .It Cm GSSAPIAuthentication
481 Specifies whether user authentication based on GSSAPI is allowed.
484 Note that this option applies to protocol version 2 only.
485 .It Cm GSSAPIDelegateCredentials
486 Forward (delegate) credentials to the server.
489 Note that this option applies to protocol version 2 only.
490 .It Cm HashKnownHosts
493 should hash host names and addresses when they are added to
494 .Pa ~/.ssh/known_hosts .
495 These hashed names may be used normally by
499 but they do not reveal identifying information should the file's contents
503 Note that existing names and addresses in known hosts files
504 will not be converted automatically,
505 but may be manually hashed using
507 .It Cm HostbasedAuthentication
508 Specifies whether to try rhosts based authentication with public key
516 This option applies to protocol version 2 only and
518 .Cm RhostsRSAAuthentication .
519 .It Cm HostKeyAlgorithms
520 Specifies the protocol version 2 host key algorithms
521 that the client wants to use in order of preference.
522 The default for this option is:
523 .Dq ssh-rsa,ssh-dss .
525 Specifies an alias that should be used instead of the
526 real host name when looking up or saving the host key
527 in the host key database files.
528 This option is useful for tunneling SSH connections
529 or for multiple servers running on a single host.
531 Specifies the real host name to log into.
532 This can be used to specify nicknames or abbreviations for hosts.
533 The default is the name given on the command line.
534 Numeric IP addresses are also permitted (both on the command line and in
537 .It Cm IdentitiesOnly
540 should only use the authentication identity files configured in the
545 offers more identities.
546 The argument to this keyword must be
550 This option is intended for situations where ssh-agent
551 offers many different identities.
555 Specifies a file from which the user's RSA or DSA authentication identity
559 for protocol version 1, and
563 for protocol version 2.
564 Additionally, any identities represented by the authentication agent
565 will be used for authentication.
567 The file name may use the tilde
568 syntax to refer to a user's home directory or one of the following
571 (local user's home directory),
577 (remote host name) or
581 It is possible to have
582 multiple identity files specified in configuration files; all these
583 identities will be tried in sequence.
584 .It Cm KbdInteractiveAuthentication
585 Specifies whether to use keyboard-interactive authentication.
586 The argument to this keyword must be
592 .It Cm KbdInteractiveDevices
593 Specifies the list of methods to use in keyboard-interactive authentication.
594 Multiple method names must be comma-separated.
595 The default is to use the server specified list.
596 The methods available vary depending on what the server supports.
597 For an OpenSSH server,
598 it may be zero or more of:
604 Specifies a command to execute on the local machine after successfully
605 connecting to the server.
606 The command string extends to the end of the line, and is executed with
608 The following escape character substitutions will be performed:
610 (local user's home directory),
616 (host name as provided on the command line),
620 (remote user name) or
623 This directive is ignored unless
624 .Cm PermitLocalCommand
627 Specifies that a TCP port on the local machine be forwarded over
628 the secure channel to the specified host and port from the remote machine.
629 The first argument must be
631 .Oo Ar bind_address : Oc Ar port
633 and the second argument must be
634 .Ar host : Ns Ar hostport .
635 IPv6 addresses can be specified by enclosing addresses in square brackets or
636 by using an alternative syntax:
637 .Oo Ar bind_address Ns / Oc Ns Ar port
639 .Ar host Ns / Ns Ar hostport .
640 Multiple forwardings may be specified, and additional forwardings can be
641 given on the command line.
642 Only the superuser can forward privileged ports.
643 By default, the local port is bound in accordance with the
648 may be used to bind the connection to a specific address.
653 indicates that the listening port be bound for local use only, while an
656 indicates that the port should be available from all interfaces.
658 Gives the verbosity level that is used when logging messages from
660 The possible values are:
661 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
663 DEBUG and DEBUG1 are equivalent.
664 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
666 Specifies the MAC (message authentication code) algorithms
667 in order of preference.
668 The MAC algorithm is used in protocol version 2
669 for data integrity protection.
670 Multiple algorithms must be comma-separated.
672 .Bd -literal -offset indent
673 hmac-md5,hmac-sha1,umac-64@openssh.com,
674 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
676 .It Cm NoHostAuthenticationForLocalhost
677 This option can be used if the home directory is shared across machines.
678 In this case localhost will refer to a different machine on each of
679 the machines and the user will get many warnings about changed host keys.
680 However, this option disables host authentication for localhost.
681 The argument to this keyword must be
685 The default is to check the host key for localhost.
686 .It Cm NumberOfPasswordPrompts
687 Specifies the number of password prompts before giving up.
688 The argument to this keyword must be an integer.
690 .It Cm PasswordAuthentication
691 Specifies whether to use password authentication.
692 The argument to this keyword must be
698 .It Cm PermitLocalCommand
699 Allow local command execution via the
702 .Ic !\& Ns Ar command
712 Specifies the port number to connect on the remote host.
714 .It Cm PreferredAuthentications
715 Specifies the order in which the client should try protocol 2
716 authentication methods.
717 This allows a client to prefer one method (e.g.\&
718 .Cm keyboard-interactive )
719 over another method (e.g.\&
721 The default for this option is:
722 .Do gssapi-with-mic ,
725 keyboard-interactive,
729 Specifies the protocol versions
731 should support in order of preference.
732 The possible values are
736 Multiple versions must be comma-separated.
740 tries version 2 and falls back to version 1
741 if version 2 is not available.
743 Specifies the command to use to connect to the server.
745 string extends to the end of the line, and is executed with
747 In the command string,
749 will be substituted by the host name to
753 The command can be basically anything,
754 and should read from its standard input and write to its standard output.
755 It should eventually connect an
757 server running on some machine, or execute
760 Host key management will be done using the
761 HostName of the host being connected (defaulting to the name typed by
763 Setting the command to
765 disables this option entirely.
768 is not available for connects with a proxy command.
770 This directive is useful in conjunction with
772 and its proxy support.
773 For example, the following directive would connect via an HTTP proxy at
775 .Bd -literal -offset 3n
776 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
778 .It Cm PubkeyAuthentication
779 Specifies whether to try public key authentication.
780 The argument to this keyword must be
786 This option applies to protocol version 2 only.
788 Specifies the maximum amount of data that may be transmitted before the
789 session key is renegotiated.
790 The argument is the number of bytes, with an optional suffix of
795 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
796 The default is between
800 depending on the cipher.
801 This option applies to protocol version 2 only.
803 Specifies that a TCP port on the remote machine be forwarded over
804 the secure channel to the specified host and port from the local machine.
805 The first argument must be
807 .Oo Ar bind_address : Oc Ar port
809 and the second argument must be
810 .Ar host : Ns Ar hostport .
811 IPv6 addresses can be specified by enclosing addresses in square brackets
812 or by using an alternative syntax:
813 .Oo Ar bind_address Ns / Oc Ns Ar port
815 .Ar host Ns / Ns Ar hostport .
816 Multiple forwardings may be specified, and additional
817 forwardings can be given on the command line.
818 Only the superuser can forward privileged ports.
822 is not specified, the default is to only bind to loopback addresses.
827 or an empty string, then the forwarding is requested to listen on all
831 will only succeed if the server's
833 option is enabled (see
834 .Xr sshd_config 5 ) .
835 .It Cm RhostsRSAAuthentication
836 Specifies whether to try rhosts based authentication with RSA host
844 This option applies to protocol version 1 only and requires
847 .It Cm RSAAuthentication
848 Specifies whether to try RSA authentication.
849 The argument to this keyword must be
853 RSA authentication will only be
854 attempted if the identity file exists, or an authentication agent is
858 Note that this option applies to protocol version 1 only.
860 Specifies what variables from the local
862 should be sent to the server.
863 Note that environment passing is only supported for protocol 2.
864 The server must also support it, and the server must be configured to
865 accept these environment variables.
870 for how to configure the server.
871 Variables are specified by name, which may contain wildcard characters.
872 Multiple environment variables may be separated by whitespace or spread
876 The default is not to send any environment variables.
880 for more information on patterns.
881 .It Cm ServerAliveCountMax
882 Sets the number of server alive messages (see below) which may be
885 receiving any messages back from the server.
886 If this threshold is reached while server alive messages are being sent,
887 ssh will disconnect from the server, terminating the session.
888 It is important to note that the use of server alive messages is very
892 The server alive messages are sent through the encrypted channel
893 and therefore will not be spoofable.
894 The TCP keepalive option enabled by
897 The server alive mechanism is valuable when the client or
898 server depend on knowing when a connection has become inactive.
900 The default value is 3.
902 .Cm ServerAliveInterval
903 (see below) is set to 15 and
904 .Cm ServerAliveCountMax
905 is left at the default, if the server becomes unresponsive,
906 ssh will disconnect after approximately 45 seconds.
907 This option applies to protocol version 2 only.
908 .It Cm ServerAliveInterval
909 Sets a timeout interval in seconds after which if no data has been received
912 will send a message through the encrypted
913 channel to request a response from the server.
915 is 0, indicating that these messages will not be sent to the server.
916 This option applies to protocol version 2 only.
917 .It Cm SmartcardDevice
918 Specifies which smartcard device to use.
919 The argument to this keyword is the device
921 should use to communicate with a smartcard used for storing the user's
923 By default, no device is specified and smartcard support is not activated.
924 .It Cm StrictHostKeyChecking
925 If this flag is set to
928 will never automatically add host keys to the
929 .Pa ~/.ssh/known_hosts
930 file, and refuses to connect to hosts whose host key has changed.
931 This provides maximum protection against trojan horse attacks,
932 though it can be annoying when the
933 .Pa /etc/ssh/ssh_known_hosts
934 file is poorly maintained or when connections to new hosts are
936 This option forces the user to manually
938 If this flag is set to
940 ssh will automatically add new host keys to the
941 user known hosts files.
942 If this flag is set to
945 will be added to the user known host files only after the user
946 has confirmed that is what they really want to do, and
947 ssh will refuse to connect to hosts whose host key has changed.
949 known hosts will be verified automatically in all cases.
958 Specifies whether the system should send TCP keepalive messages to the
960 If they are sent, death of the connection or crash of one
961 of the machines will be properly noticed.
962 However, this means that
963 connections will die if the route is down temporarily, and some people
968 (to send TCP keepalive messages), and the client will notice
969 if the network goes down or the remote host dies.
970 This is important in scripts, and many users want it too.
972 To disable TCP keepalive messages, the value should be set to
977 device forwarding between the client and the server.
988 requests the default tunnel mode, which is
995 devices to open on the client
1000 The argument must be
1002 .Ar local_tun Op : Ar remote_tun .
1004 The devices may be specified by numerical ID or the keyword
1006 which uses the next available tunnel device.
1009 is not specified, it defaults to
1013 .It Cm UsePrivilegedPort
1014 Specifies whether to use a privileged port for outgoing connections.
1015 The argument must be
1024 must be setuid root.
1025 Note that this option must be set to
1028 .Cm RhostsRSAAuthentication
1031 Specifies the user to log in as.
1032 This can be useful when a different user name is used on different machines.
1033 This saves the trouble of
1034 having to remember to give the user name on the command line.
1035 .It Cm UserKnownHostsFile
1036 Specifies a file to use for the user
1037 host key database instead of
1038 .Pa ~/.ssh/known_hosts .
1039 .It Cm VerifyHostKeyDNS
1040 Specifies whether to verify the remote key using DNS and SSHFP resource
1042 If this option is set to
1044 the client will implicitly trust keys that match a secure fingerprint
1046 Insecure fingerprints will be handled as if this option was set to
1048 If this option is set to
1050 information on fingerprint match will be displayed, but the user will still
1051 need to confirm new host keys according to the
1052 .Cm StrictHostKeyChecking
1054 The argument must be
1061 Note that this option applies to protocol version 2 only.
1064 .Sx VERIFYING HOST KEYS
1067 .It Cm XAuthLocation
1068 Specifies the full pathname of the
1072 .Pa /usr/X11R6/bin/xauth .
1077 consists of zero or more non-whitespace characters,
1079 (a wildcard that matches zero or more characters),
1082 (a wildcard that matches exactly one character).
1083 For example, to specify a set of declarations for any host in the
1086 the following pattern could be used:
1090 The following pattern
1091 would match any host in the 192.168.0.[0-9] network range:
1093 .Dl Host 192.168.0.?
1097 is a comma-separated list of patterns.
1098 Patterns within pattern-lists may be negated
1099 by preceding them with an exclamation mark
1102 to allow a key to be used from anywhere within an organisation
1106 the following entry (in authorized_keys) could be used:
1108 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1111 .It Pa ~/.ssh/config
1112 This is the per-user configuration file.
1113 The format of this file is described above.
1114 This file is used by the SSH client.
1115 Because of the potential for abuse, this file must have strict permissions:
1116 read/write for the user, and not accessible by others.
1117 .It Pa /etc/ssh/ssh_config
1118 Systemwide configuration file.
1119 This file provides defaults for those
1120 values that are not specified in the user's configuration file, and
1121 for those users who do not have a configuration file.
1122 This file must be world-readable.
1127 OpenSSH is a derivative of the original and free
1128 ssh 1.2.12 release by Tatu Ylonen.
1129 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1130 Theo de Raadt and Dug Song
1131 removed many bugs, re-added newer features and
1133 Markus Friedl contributed the support for SSH
1134 protocol versions 1.5 and 2.0.
andersk / openssh.git / blob