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 1.109 2008/06/12 16:35:31 dtucker 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 not only the host IP address will be checked, but also a fingerprint
166 along with an ASCII art
167 representation of the key will be printed.
168 If the option is set to
170 the check will not be executed.
174 Specifies the cipher to use for encrypting the session
175 in protocol version 1.
183 is only supported in the
185 client for interoperability with legacy protocol 1 implementations
186 that do not support the
189 Its use is strongly discouraged due to cryptographic weaknesses.
193 Specifies the ciphers allowed for protocol version 2
194 in order of preference.
195 Multiple ciphers must be comma-separated.
196 The supported ciphers are
211 .Bd -literal -offset 3n
212 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
213 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
214 aes192-ctr,aes256-ctr
216 .It Cm ClearAllForwardings
217 Specifies that all local, remote, and dynamic port forwardings
218 specified in the configuration files or on the command line be
220 This option is primarily useful when used from the
222 command line to clear port forwardings set in
223 configuration files, and is automatically set by
234 Specifies whether to use compression.
241 .It Cm CompressionLevel
242 Specifies the compression level to use if compression is enabled.
243 The argument must be an integer from 1 (fast) to 9 (slow, best).
244 The default level is 6, which is good for most applications.
245 The meaning of the values is the same as in
247 Note that this option applies to protocol version 1 only.
248 .It Cm ConnectionAttempts
249 Specifies the number of tries (one per second) to make before exiting.
250 The argument must be an integer.
251 This may be useful in scripts if the connection sometimes fails.
253 .It Cm ConnectTimeout
254 Specifies the timeout (in seconds) used when connecting to the
255 SSH server, instead of using the default system TCP timeout.
256 This value is used only when the target is down or really unreachable,
257 not when it refuses the connection.
259 Enables the sharing of multiple sessions over a single network connection.
263 will listen for connections on a control socket specified using the
266 Additional sessions can connect to this socket using the same
273 These sessions will try to reuse the master instance's network connection
274 rather than initiating new ones, but will fall back to connecting normally
275 if the control socket does not exist, or is not listening.
280 to listen for control connections, but require confirmation using the
282 program before they are accepted (see
288 ssh will continue without connecting to a master instance.
292 forwarding is supported over these multiplexed connections, however the
293 display and agent forwarded will be the one belonging to the master
294 connection i.e. it is not possible to forward multiple displays or agents.
296 Two additional options allow for opportunistic multiplexing: try to use a
297 master connection but fall back to creating a new one if one does not already
303 The latter requires confirmation like the
307 Specify the path to the control socket used for connection sharing as described
310 section above or the string
312 to disable connection sharing.
315 will be substituted by the local host name,
317 will be substituted by the target host name,
321 by the remote login username.
322 It is recommended that any
324 used for opportunistic connection sharing include
325 at least %h, %p, and %r.
326 This ensures that shared connections are uniquely identified.
327 .It Cm DynamicForward
328 Specifies that a TCP port on the local machine be forwarded
329 over the secure channel, and the application
330 protocol is then used to determine where to connect to from the
335 .Oo Ar bind_address : Oc Ar port .
337 IPv6 addresses can be specified by enclosing addresses in square brackets or
338 by using an alternative syntax:
339 .Oo Ar bind_address Ns / Oc Ns Ar port .
340 By default, the local port is bound in accordance with the
345 may be used to bind the connection to a specific address.
350 indicates that the listening port be bound for local use only, while an
353 indicates that the port should be available from all interfaces.
355 Currently the SOCKS4 and SOCKS5 protocols are supported, and
357 will act as a SOCKS server.
358 Multiple forwardings may be specified, and
359 additional forwardings can be given on the command line.
360 Only the superuser can forward privileged ports.
361 .It Cm EnableSSHKeysign
362 Setting this option to
364 in the global client configuration file
365 .Pa /etc/ssh/ssh_config
366 enables the use of the helper program
369 .Cm HostbasedAuthentication .
376 This option should be placed in the non-hostspecific section.
379 for more information.
381 Sets the escape character (default:
383 The escape character can also
384 be set on the command line.
385 The argument should be a single character,
387 followed by a letter, or
389 to disable the escape
390 character entirely (making the connection transparent for binary
392 .It Cm ExitOnForwardFailure
395 should terminate the connection if it cannot set up all requested
396 dynamic, tunnel, local, and remote port forwardings.
404 Specifies whether the connection to the authentication agent (if any)
405 will be forwarded to the remote machine.
413 Agent forwarding should be enabled with caution.
414 Users with the ability to bypass file permissions on the remote host
415 (for the agent's Unix-domain socket)
416 can access the local agent through the forwarded connection.
417 An attacker cannot obtain key material from the agent,
418 however they can perform operations on the keys that enable them to
419 authenticate using the identities loaded into the agent.
421 Specifies whether X11 connections will be automatically redirected
422 over the secure channel and
432 X11 forwarding should be enabled with caution.
433 Users with the ability to bypass file permissions on the remote host
434 (for the user's X11 authorization database)
435 can access the local X11 display through the forwarded connection.
436 An attacker may then be able to perform activities such as keystroke monitoring
438 .Cm ForwardX11Trusted
439 option is also enabled.
440 .It Cm ForwardX11Trusted
441 If this option is set to
443 remote X11 clients will have full access to the original X11 display.
445 If this option is set to
447 remote X11 clients will be considered untrusted and prevented
448 from stealing or tampering with data belonging to trusted X11
452 token used for the session will be set to expire after 20 minutes.
453 Remote clients will be refused access after this time.
458 See the X11 SECURITY extension specification for full details on
459 the restrictions imposed on untrusted clients.
461 Specifies whether remote hosts are allowed to connect to local
465 binds local port forwardings to the loopback address.
466 This prevents other remote hosts from connecting to forwarded ports.
468 can be used to specify that ssh
469 should bind local port forwardings to the wildcard address,
470 thus allowing remote hosts to connect to forwarded ports.
477 .It Cm GlobalKnownHostsFile
478 Specifies a file to use for the global
479 host key database instead of
480 .Pa /etc/ssh/ssh_known_hosts .
481 .It Cm GSSAPIAuthentication
482 Specifies whether user authentication based on GSSAPI is allowed.
485 Note that this option applies to protocol version 2 only.
486 .It Cm GSSAPIDelegateCredentials
487 Forward (delegate) credentials to the server.
490 Note that this option applies to protocol version 2 only.
491 .It Cm HashKnownHosts
494 should hash host names and addresses when they are added to
495 .Pa ~/.ssh/known_hosts .
496 These hashed names may be used normally by
500 but they do not reveal identifying information should the file's contents
504 Note that existing names and addresses in known hosts files
505 will not be converted automatically,
506 but may be manually hashed using
508 .It Cm HostbasedAuthentication
509 Specifies whether to try rhosts based authentication with public key
517 This option applies to protocol version 2 only and
519 .Cm RhostsRSAAuthentication .
520 .It Cm HostKeyAlgorithms
521 Specifies the protocol version 2 host key algorithms
522 that the client wants to use in order of preference.
523 The default for this option is:
524 .Dq ssh-rsa,ssh-dss .
526 Specifies an alias that should be used instead of the
527 real host name when looking up or saving the host key
528 in the host key database files.
529 This option is useful for tunneling SSH connections
530 or for multiple servers running on a single host.
532 Specifies the real host name to log into.
533 This can be used to specify nicknames or abbreviations for hosts.
534 The default is the name given on the command line.
535 Numeric IP addresses are also permitted (both on the command line and in
538 .It Cm IdentitiesOnly
541 should only use the authentication identity files configured in the
546 offers more identities.
547 The argument to this keyword must be
551 This option is intended for situations where ssh-agent
552 offers many different identities.
556 Specifies a file from which the user's RSA or DSA authentication identity
560 for protocol version 1, and
564 for protocol version 2.
565 Additionally, any identities represented by the authentication agent
566 will be used for authentication.
568 The file name may use the tilde
569 syntax to refer to a user's home directory or one of the following
572 (local user's home directory),
578 (remote host name) or
582 It is possible to have
583 multiple identity files specified in configuration files; all these
584 identities will be tried in sequence.
585 .It Cm KbdInteractiveAuthentication
586 Specifies whether to use keyboard-interactive authentication.
587 The argument to this keyword must be
593 .It Cm KbdInteractiveDevices
594 Specifies the list of methods to use in keyboard-interactive authentication.
595 Multiple method names must be comma-separated.
596 The default is to use the server specified list.
597 The methods available vary depending on what the server supports.
598 For an OpenSSH server,
599 it may be zero or more of:
605 Specifies a command to execute on the local machine after successfully
606 connecting to the server.
607 The command string extends to the end of the line, and is executed with
609 The following escape character substitutions will be performed:
611 (local user's home directory),
617 (host name as provided on the command line),
621 (remote user name) or
624 This directive is ignored unless
625 .Cm PermitLocalCommand
628 Specifies that a TCP port on the local machine be forwarded over
629 the secure channel to the specified host and port from the remote machine.
630 The first argument must be
632 .Oo Ar bind_address : Oc Ar port
634 and the second argument must be
635 .Ar host : Ns Ar hostport .
636 IPv6 addresses can be specified by enclosing addresses in square brackets or
637 by using an alternative syntax:
638 .Oo Ar bind_address Ns / Oc Ns Ar port
640 .Ar host Ns / Ns Ar hostport .
641 Multiple forwardings may be specified, and additional forwardings can be
642 given on the command line.
643 Only the superuser can forward privileged ports.
644 By default, the local port is bound in accordance with the
649 may be used to bind the connection to a specific address.
654 indicates that the listening port be bound for local use only, while an
657 indicates that the port should be available from all interfaces.
659 Gives the verbosity level that is used when logging messages from
661 The possible values are:
662 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
664 DEBUG and DEBUG1 are equivalent.
665 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
667 Specifies the MAC (message authentication code) algorithms
668 in order of preference.
669 The MAC algorithm is used in protocol version 2
670 for data integrity protection.
671 Multiple algorithms must be comma-separated.
673 .Bd -literal -offset indent
674 hmac-md5,hmac-sha1,umac-64@openssh.com,
675 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
677 .It Cm NoHostAuthenticationForLocalhost
678 This option can be used if the home directory is shared across machines.
679 In this case localhost will refer to a different machine on each of
680 the machines and the user will get many warnings about changed host keys.
681 However, this option disables host authentication for localhost.
682 The argument to this keyword must be
686 The default is to check the host key for localhost.
687 .It Cm NumberOfPasswordPrompts
688 Specifies the number of password prompts before giving up.
689 The argument to this keyword must be an integer.
691 .It Cm PasswordAuthentication
692 Specifies whether to use password authentication.
693 The argument to this keyword must be
699 .It Cm PermitLocalCommand
700 Allow local command execution via the
703 .Ic !\& Ns Ar command
713 Specifies the port number to connect on the remote host.
715 .It Cm PreferredAuthentications
716 Specifies the order in which the client should try protocol 2
717 authentication methods.
718 This allows a client to prefer one method (e.g.\&
719 .Cm keyboard-interactive )
720 over another method (e.g.\&
722 The default for this option is:
723 .Do gssapi-with-mic ,
726 keyboard-interactive,
730 Specifies the protocol versions
732 should support in order of preference.
733 The possible values are
737 Multiple versions must be comma-separated.
741 tries version 2 and falls back to version 1
742 if version 2 is not available.
744 Specifies the command to use to connect to the server.
746 string extends to the end of the line, and is executed with
748 In the command string,
750 will be substituted by the host name to
754 The command can be basically anything,
755 and should read from its standard input and write to its standard output.
756 It should eventually connect an
758 server running on some machine, or execute
761 Host key management will be done using the
762 HostName of the host being connected (defaulting to the name typed by
764 Setting the command to
766 disables this option entirely.
769 is not available for connects with a proxy command.
771 This directive is useful in conjunction with
773 and its proxy support.
774 For example, the following directive would connect via an HTTP proxy at
776 .Bd -literal -offset 3n
777 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
779 .It Cm PubkeyAuthentication
780 Specifies whether to try public key authentication.
781 The argument to this keyword must be
787 This option applies to protocol version 2 only.
789 Specifies the maximum amount of data that may be transmitted before the
790 session key is renegotiated.
791 The argument is the number of bytes, with an optional suffix of
796 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
797 The default is between
801 depending on the cipher.
802 This option applies to protocol version 2 only.
804 Specifies that a TCP port on the remote machine be forwarded over
805 the secure channel to the specified host and port from the local machine.
806 The first argument must be
808 .Oo Ar bind_address : Oc Ar port
810 and the second argument must be
811 .Ar host : Ns Ar hostport .
812 IPv6 addresses can be specified by enclosing addresses in square brackets
813 or by using an alternative syntax:
814 .Oo Ar bind_address Ns / Oc Ns Ar port
816 .Ar host Ns / Ns Ar hostport .
817 Multiple forwardings may be specified, and additional
818 forwardings can be given on the command line.
819 Only the superuser can forward privileged ports.
823 is not specified, the default is to only bind to loopback addresses.
828 or an empty string, then the forwarding is requested to listen on all
832 will only succeed if the server's
834 option is enabled (see
835 .Xr sshd_config 5 ) .
836 .It Cm RhostsRSAAuthentication
837 Specifies whether to try rhosts based authentication with RSA host
845 This option applies to protocol version 1 only and requires
848 .It Cm RSAAuthentication
849 Specifies whether to try RSA authentication.
850 The argument to this keyword must be
854 RSA authentication will only be
855 attempted if the identity file exists, or an authentication agent is
859 Note that this option applies to protocol version 1 only.
861 Specifies what variables from the local
863 should be sent to the server.
864 Note that environment passing is only supported for protocol 2.
865 The server must also support it, and the server must be configured to
866 accept these environment variables.
871 for how to configure the server.
872 Variables are specified by name, which may contain wildcard characters.
873 Multiple environment variables may be separated by whitespace or spread
877 The default is not to send any environment variables.
881 for more information on patterns.
882 .It Cm ServerAliveCountMax
883 Sets the number of server alive messages (see below) which may be
886 receiving any messages back from the server.
887 If this threshold is reached while server alive messages are being sent,
888 ssh will disconnect from the server, terminating the session.
889 It is important to note that the use of server alive messages is very
893 The server alive messages are sent through the encrypted channel
894 and therefore will not be spoofable.
895 The TCP keepalive option enabled by
898 The server alive mechanism is valuable when the client or
899 server depend on knowing when a connection has become inactive.
901 The default value is 3.
903 .Cm ServerAliveInterval
904 (see below) is set to 15 and
905 .Cm ServerAliveCountMax
906 is left at the default, if the server becomes unresponsive,
907 ssh will disconnect after approximately 45 seconds.
908 This option applies to protocol version 2 only.
909 .It Cm ServerAliveInterval
910 Sets a timeout interval in seconds after which if no data has been received
913 will send a message through the encrypted
914 channel to request a response from the server.
916 is 0, indicating that these messages will not be sent to the server.
917 This option applies to protocol version 2 only.
918 .It Cm SmartcardDevice
919 Specifies which smartcard device to use.
920 The argument to this keyword is the device
922 should use to communicate with a smartcard used for storing the user's
924 By default, no device is specified and smartcard support is not activated.
925 .It Cm StrictHostKeyChecking
926 If this flag is set to
929 will never automatically add host keys to the
930 .Pa ~/.ssh/known_hosts
931 file, and refuses to connect to hosts whose host key has changed.
932 This provides maximum protection against trojan horse attacks,
933 though it can be annoying when the
934 .Pa /etc/ssh/ssh_known_hosts
935 file is poorly maintained or when connections to new hosts are
937 This option forces the user to manually
939 If this flag is set to
941 ssh will automatically add new host keys to the
942 user known hosts files.
943 If this flag is set to
946 will be added to the user known host files only after the user
947 has confirmed that is what they really want to do, and
948 ssh will refuse to connect to hosts whose host key has changed.
950 known hosts will be verified automatically in all cases.
959 Specifies whether the system should send TCP keepalive messages to the
961 If they are sent, death of the connection or crash of one
962 of the machines will be properly noticed.
963 However, this means that
964 connections will die if the route is down temporarily, and some people
969 (to send TCP keepalive messages), and the client will notice
970 if the network goes down or the remote host dies.
971 This is important in scripts, and many users want it too.
973 To disable TCP keepalive messages, the value should be set to
978 device forwarding between the client and the server.
989 requests the default tunnel mode, which is
996 devices to open on the client
1001 The argument must be
1003 .Ar local_tun Op : Ar remote_tun .
1005 The devices may be specified by numerical ID or the keyword
1007 which uses the next available tunnel device.
1010 is not specified, it defaults to
1014 .It Cm UsePrivilegedPort
1015 Specifies whether to use a privileged port for outgoing connections.
1016 The argument must be
1025 must be setuid root.
1026 Note that this option must be set to
1029 .Cm RhostsRSAAuthentication
1032 Specifies the user to log in as.
1033 This can be useful when a different user name is used on different machines.
1034 This saves the trouble of
1035 having to remember to give the user name on the command line.
1036 .It Cm UserKnownHostsFile
1037 Specifies a file to use for the user
1038 host key database instead of
1039 .Pa ~/.ssh/known_hosts .
1040 .It Cm VerifyHostKeyDNS
1041 Specifies whether to verify the remote key using DNS and SSHFP resource
1043 If this option is set to
1045 the client will implicitly trust keys that match a secure fingerprint
1047 Insecure fingerprints will be handled as if this option was set to
1049 If this option is set to
1051 information on fingerprint match will be displayed, but the user will still
1052 need to confirm new host keys according to the
1053 .Cm StrictHostKeyChecking
1055 The argument must be
1062 Note that this option applies to protocol version 2 only.
1065 .Sx VERIFYING HOST KEYS
1068 .It Cm XAuthLocation
1069 Specifies the full pathname of the
1073 .Pa /usr/X11R6/bin/xauth .
1078 consists of zero or more non-whitespace characters,
1080 (a wildcard that matches zero or more characters),
1083 (a wildcard that matches exactly one character).
1084 For example, to specify a set of declarations for any host in the
1087 the following pattern could be used:
1091 The following pattern
1092 would match any host in the 192.168.0.[0-9] network range:
1094 .Dl Host 192.168.0.?
1098 is a comma-separated list of patterns.
1099 Patterns within pattern-lists may be negated
1100 by preceding them with an exclamation mark
1103 to allow a key to be used from anywhere within an organisation
1107 the following entry (in authorized_keys) could be used:
1109 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1112 .It Pa ~/.ssh/config
1113 This is the per-user configuration file.
1114 The format of this file is described above.
1115 This file is used by the SSH client.
1116 Because of the potential for abuse, this file must have strict permissions:
1117 read/write for the user, and not accessible by others.
1118 .It Pa /etc/ssh/ssh_config
1119 Systemwide configuration file.
1120 This file provides defaults for those
1121 values that are not specified in the user's configuration file, and
1122 for those users who do not have a configuration file.
1123 This file must be world-readable.
1128 OpenSSH is a derivative of the original and free
1129 ssh 1.2.12 release by Tatu Ylonen.
1130 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1131 Theo de Raadt and Dug Song
1132 removed many bugs, re-added newer features and
1134 Markus Friedl contributed the support for SSH
1135 protocol versions 1.5 and 2.0.
andersk / openssh.git / blob