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 1.61 2005/07/08 12:53:10 jmc Exp $
39 .Dd September 25, 1999
44 .Nd OpenSSH SSH client configuration files
46 .Bl -tag -width Ds -compact
48 .It Pa /etc/ssh/ssh_config
52 obtains configuration data from the following sources in
54 .Bl -enum -offset indent -compact
58 user's configuration file
61 system-wide configuration file
62 .Pq Pa /etc/ssh/ssh_config
65 For each parameter, the first obtained value
67 The configuration files contain sections separated by
69 specifications, and that section is only applied for hosts that
70 match one of the patterns given in the specification.
71 The matched host name is the one given on the command line.
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
83 Otherwise a line is of the format
84 .Dq keyword arguments .
85 Configuration options may be separated by whitespace or
86 optional whitespace and exactly one
88 the latter format is useful to avoid the need to quote whitespace
89 when specifying configuration options using the
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.
109 can be used as wildcards in the
113 as a pattern can be used to provide global
114 defaults for all hosts.
117 argument given on the command line (i.e., the name is not converted to
118 a canonicalized host name before matching).
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
158 ssh will additionally check the host IP address in the
161 This allows ssh to detect if a host key changed due to DNS spoofing.
162 If the option is set to
164 the check will not be executed.
168 Specifies the cipher to use for encrypting the session
169 in protocol version 1.
177 is only supported in the
179 client for interoperability with legacy protocol 1 implementations
180 that do not support the
183 Its use is strongly discouraged due to cryptographic weaknesses.
187 Specifies the ciphers allowed for protocol version 2
188 in order of preference.
189 Multiple ciphers must be comma-separated.
190 The supported ciphers are
206 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
207 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
208 aes192-ctr,aes256-ctr''
210 .It Cm ClearAllForwardings
211 Specifies that all local, remote and dynamic port forwardings
212 specified in the configuration files or on the command line be
214 This option is primarily useful when used from the
216 command line to clear port forwardings set in
217 configuration files, and is automatically set by
228 Specifies whether to use compression.
235 .It Cm CompressionLevel
236 Specifies the compression level to use if compression is enabled.
237 The argument must be an integer from 1 (fast) to 9 (slow, best).
238 The default level is 6, which is good for most applications.
239 The meaning of the values is the same as in
241 Note that this option applies to protocol version 1 only.
242 .It Cm ConnectionAttempts
243 Specifies the number of tries (one per second) to make before exiting.
244 The argument must be an integer.
245 This may be useful in scripts if the connection sometimes fails.
247 .It Cm ConnectTimeout
248 Specifies the timeout (in seconds) used when connecting to the ssh
249 server, instead of using the default system TCP timeout.
250 This value is used only when the target is down or really unreachable,
251 not when it refuses the connection.
253 Enables the sharing of multiple sessions over a single network connection.
257 will listen for connections on a control socket specified using the
260 Additional sessions can connect to this socket using the same
267 These sessions will reuse the master instance's network connection rather
268 than initiating new ones.
273 to listen for control connections, but require confirmation using the
275 program before they are accepted (see
282 will continue without connecting to a master instance.
286 forwarding is supported over these multiplexed connections, however the
287 display and agent fowarded will be the one belonging to the master
288 connection i.e. it is not possible to forward multiple displays or agents.
290 Two additional options allow for opportunistic multiplexing: try to use a
291 master connection but fall back to creating a new one if one does not already
297 The latter requires confirmation like the
301 Specify the path to the control socket used for connection sharing as described
304 section above or the string
306 to disable connection sharing.
309 will be substituted by the target host name,
313 by the remote login username.
314 It is recommended that any
316 used for opportunistic connection sharing include
317 all three of these escape sequences.
318 This ensures that shared connections are uniquely identified.
319 .It Cm DynamicForward
320 Specifies that a TCP/IP port on the local machine be forwarded
321 over the secure channel, and the application
322 protocol is then used to determine where to connect to from the
324 The argument must be a port number.
325 Currently the SOCKS4 and SOCKS5 protocols are supported, and
327 will act as a SOCKS server.
328 Multiple forwardings may be specified, and
329 additional forwardings can be given on the command line.
330 Only the superuser can forward privileged ports.
331 .It Cm EnableSSHKeysign
332 Setting this option to
334 in the global client configuration file
335 .Pa /etc/ssh/ssh_config
336 enables the use of the helper program
339 .Cm HostbasedAuthentication .
346 This option should be placed in the non-hostspecific section.
349 for more information.
351 Sets the escape character (default:
353 The escape character can also
354 be set on the command line.
355 The argument should be a single character,
357 followed by a letter, or
359 to disable the escape
360 character entirely (making the connection transparent for binary
363 Specifies whether the connection to the authentication agent (if any)
364 will be forwarded to the remote machine.
372 Agent forwarding should be enabled with caution.
373 Users with the ability to bypass file permissions on the remote host
374 (for the agent's Unix-domain socket)
375 can access the local agent through the forwarded connection.
376 An attacker cannot obtain key material from the agent,
377 however they can perform operations on the keys that enable them to
378 authenticate using the identities loaded into the agent.
380 Specifies whether X11 connections will be automatically redirected
381 over the secure channel and
391 X11 forwarding should be enabled with caution.
392 Users with the ability to bypass file permissions on the remote host
393 (for the user's X11 authorization database)
394 can access the local X11 display through the forwarded connection.
395 An attacker may then be able to perform activities such as keystroke monitoring
397 .Cm ForwardX11Trusted
398 option is also enabled.
399 .It Cm ForwardX11Trusted
400 If this option is set to
402 then remote X11 clients will have full access to the original X11 display.
404 If this option is set to
406 then remote X11 clients will be considered untrusted and prevented
407 from stealing or tampering with data belonging to trusted X11
411 token used for the session will be set to expire after 20 minutes.
412 Remote clients will be refused access after this time.
417 See the X11 SECURITY extension specification for full details on
418 the restrictions imposed on untrusted clients.
420 Specifies whether remote hosts are allowed to connect to local
424 binds local port forwardings to the loopback address.
425 This prevents other remote hosts from connecting to forwarded ports.
427 can be used to specify that
429 should bind local port forwardings to the wildcard address,
430 thus allowing remote hosts to connect to forwarded ports.
437 .It Cm GlobalKnownHostsFile
438 Specifies a file to use for the global
439 host key database instead of
440 .Pa /etc/ssh/ssh_known_hosts .
441 .It Cm GSSAPIAuthentication
442 Specifies whether user authentication based on GSSAPI is allowed.
445 Note that this option applies to protocol version 2 only.
446 .It Cm GSSAPIDelegateCredentials
447 Forward (delegate) credentials to the server.
450 Note that this option applies to protocol version 2 only.
451 .It Cm HashKnownHosts
454 should hash host names and addresses when they are added to
455 .Pa ~/.ssh/known_hosts .
456 These hashed names may be used normally by
460 but they do not reveal identifying information should the file's contents
464 Note that hashing of names and addresses will not be retrospectively applied
465 to existing known hosts files, but these may be manually hashed using
467 .It Cm HostbasedAuthentication
468 Specifies whether to try rhosts based authentication with public key
476 This option applies to protocol version 2 only and
478 .Cm RhostsRSAAuthentication .
479 .It Cm HostKeyAlgorithms
480 Specifies the protocol version 2 host key algorithms
481 that the client wants to use in order of preference.
482 The default for this option is:
483 .Dq ssh-rsa,ssh-dss .
485 Specifies an alias that should be used instead of the
486 real host name when looking up or saving the host key
487 in the host key database files.
488 This option is useful for tunneling ssh connections
489 or for multiple servers running on a single host.
491 Specifies the real host name to log into.
492 This can be used to specify nicknames or abbreviations for hosts.
493 Default is the name given on the command line.
494 Numeric IP addresses are also permitted (both on the command line and in
498 Specifies a file from which the user's RSA or DSA authentication identity
502 for protocol version 1, and
506 for protocol version 2.
507 Additionally, any identities represented by the authentication agent
508 will be used for authentication.
509 The file name may use the tilde
510 syntax to refer to a user's home directory.
511 It is possible to have
512 multiple identity files specified in configuration files; all these
513 identities will be tried in sequence.
514 .It Cm IdentitiesOnly
517 should only use the authentication identity files configured in the
522 offers more identities.
523 The argument to this keyword must be
527 This option is intented for situations where
529 offers many different identities.
532 .It Cm KbdInteractiveDevices
533 Specifies the list of methods to use in keyboard-interactive authentication.
534 Multiple method names must be comma-separated.
535 The default is to use the server specified list.
537 Specifies that a TCP/IP port on the local machine be forwarded over
538 the secure channel to the specified host and port from the remote machine.
539 The first argument must be
541 .Oo Ar bind_address : Oc Ar port
543 and the second argument must be
544 .Ar host : Ns Ar hostport .
545 IPv6 addresses can be specified by enclosing addresses in square brackets or
546 by using an alternative syntax:
547 .Oo Ar bind_address Ns / Oc Ns Ar port
549 .Ar host Ns / Ns Ar hostport .
550 Multiple forwardings may be specified, and additional forwardings can be
551 given on the command line.
552 Only the superuser can forward privileged ports.
553 By default, the local port is bound in accordance with the
558 may be used to bind the connection to a specific address.
563 indicates that the listening port be bound for local use only, while an
566 indicates that the port should be available from all interfaces.
568 Gives the verbosity level that is used when logging messages from
570 The possible values are:
571 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
573 DEBUG and DEBUG1 are equivalent.
574 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
576 Specifies the MAC (message authentication code) algorithms
577 in order of preference.
578 The MAC algorithm is used in protocol version 2
579 for data integrity protection.
580 Multiple algorithms must be comma-separated.
582 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
583 .It Cm NoHostAuthenticationForLocalhost
584 This option can be used if the home directory is shared across machines.
585 In this case localhost will refer to a different machine on each of
586 the machines and the user will get many warnings about changed host keys.
587 However, this option disables host authentication for localhost.
588 The argument to this keyword must be
592 The default is to check the host key for localhost.
593 .It Cm NumberOfPasswordPrompts
594 Specifies the number of password prompts before giving up.
595 The argument to this keyword must be an integer.
597 .It Cm PasswordAuthentication
598 Specifies whether to use password authentication.
599 The argument to this keyword must be
606 Specifies the port number to connect on the remote host.
608 .It Cm PreferredAuthentications
609 Specifies the order in which the client should try protocol 2
610 authentication methods.
611 This allows a client to prefer one method (e.g.\&
612 .Cm keyboard-interactive )
613 over another method (e.g.\&
615 The default for this option is:
616 .Dq hostbased,publickey,keyboard-interactive,password .
618 Specifies the protocol versions
620 should support in order of preference.
621 The possible values are
625 Multiple versions must be comma-separated.
630 tries version 2 and falls back to version 1
631 if version 2 is not available.
633 Specifies the command to use to connect to the server.
635 string extends to the end of the line, and is executed with
637 In the command string,
639 will be substituted by the host name to
643 The command can be basically anything,
644 and should read from its standard input and write to its standard output.
645 It should eventually connect an
647 server running on some machine, or execute
650 Host key management will be done using the
651 HostName of the host being connected (defaulting to the name typed by
653 Setting the command to
655 disables this option entirely.
658 is not available for connects with a proxy command.
660 This directive is useful in conjunction with
662 and its proxy support.
663 For example, the following directive would connect via an HTTP proxy at
665 .Bd -literal -offset 3n
666 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
668 .It Cm PubkeyAuthentication
669 Specifies whether to try public key authentication.
670 The argument to this keyword must be
676 This option applies to protocol version 2 only.
678 Specifies that a TCP/IP port on the remote machine be forwarded over
679 the secure channel to the specified host and port from the local machine.
680 The first argument must be
682 .Oo Ar bind_address : Oc Ar port
684 and the second argument must be
685 .Ar host : Ns Ar hostport .
686 IPv6 addresses can be specified by enclosing addresses in square brackets
687 or by using an alternative syntax:
688 .Oo Ar bind_address Ns / Oc Ns Ar port
690 .Ar host Ns / Ns Ar hostport .
691 Multiple forwardings may be specified, and additional
692 forwardings can be given on the command line.
693 Only the superuser can forward privileged ports.
697 is not specified, the default is to only bind to loopback addresses.
702 or an empty string, then the forwarding is requested to listen on all
706 will only succeed if the server's
708 option is enabled (see
709 .Xr sshd_config 5 ) .
710 .It Cm RhostsRSAAuthentication
711 Specifies whether to try rhosts based authentication with RSA host
719 This option applies to protocol version 1 only and requires
722 .It Cm RSAAuthentication
723 Specifies whether to try RSA authentication.
724 The argument to this keyword must be
728 RSA authentication will only be
729 attempted if the identity file exists, or an authentication agent is
733 Note that this option applies to protocol version 1 only.
735 Specifies what variables from the local
737 should be sent to the server.
738 Note that environment passing is only supported for protocol 2, the
739 server must also support it, and the server must be configured to
740 accept these environment variables.
745 for how to configure the server.
746 Variables are specified by name, which may contain the wildcard characters
750 Multiple environment variables may be separated by whitespace or spread
754 The default is not to send any environment variables.
755 .It Cm ServerAliveInterval
756 Sets a timeout interval in seconds after which if no data has been received
759 will send a message through the encrypted
760 channel to request a response from the server.
762 is 0, indicating that these messages will not be sent to the server.
763 This option applies to protocol version 2 only.
764 .It Cm ServerAliveCountMax
765 Sets the number of server alive messages (see above) which may be
768 receiving any messages back from the server.
769 If this threshold is reached while server alive messages are being sent,
771 will disconnect from the server, terminating the session.
772 It is important to note that the use of server alive messages is very
776 The server alive messages are sent through the encrypted channel
777 and therefore will not be spoofable.
778 The TCP keepalive option enabled by
781 The server alive mechanism is valuable when the client or
782 server depend on knowing when a connection has become inactive.
784 The default value is 3.
786 .Cm ServerAliveInterval
787 (above) is set to 15, and
788 .Cm ServerAliveCountMax
789 is left at the default, if the server becomes unresponsive ssh
790 will disconnect after approximately 45 seconds.
791 .It Cm SmartcardDevice
792 Specifies which smartcard device to use.
793 The argument to this keyword is the device
795 should use to communicate with a smartcard used for storing the user's
797 By default, no device is specified and smartcard support is not activated.
798 .It Cm StrictHostKeyChecking
799 If this flag is set to
802 will never automatically add host keys to the
803 .Pa ~/.ssh/known_hosts
804 file, and refuses to connect to hosts whose host key has changed.
805 This provides maximum protection against trojan horse attacks,
806 however, can be annoying when the
807 .Pa /etc/ssh/ssh_known_hosts
808 file is poorly maintained, or connections to new hosts are
810 This option forces the user to manually
812 If this flag is set to
815 will automatically add new host keys to the
816 user known hosts files.
817 If this flag is set to
820 will be added to the user known host files only after the user
821 has confirmed that is what they really want to do, and
823 will refuse to connect to hosts whose host key has changed.
825 known hosts will be verified automatically in all cases.
834 Specifies whether the system should send TCP keepalive messages to the
836 If they are sent, death of the connection or crash of one
837 of the machines will be properly noticed.
838 However, this means that
839 connections will die if the route is down temporarily, and some people
844 (to send TCP keepalive messages), and the client will notice
845 if the network goes down or the remote host dies.
846 This is important in scripts, and many users want it too.
848 To disable TCP keepalive messages, the value should be set to
850 .It Cm UsePrivilegedPort
851 Specifies whether to use a privileged port for outgoing connections.
862 Note that this option must be set to
865 .Cm RhostsRSAAuthentication
868 Specifies the user to log in as.
869 This can be useful when a different user name is used on different machines.
870 This saves the trouble of
871 having to remember to give the user name on the command line.
872 .It Cm UserKnownHostsFile
873 Specifies a file to use for the user
874 host key database instead of
875 .Pa ~/.ssh/known_hosts .
876 .It Cm VerifyHostKeyDNS
877 Specifies whether to verify the remote key using DNS and SSHFP resource
879 If this option is set to
881 the client will implicitly trust keys that match a secure fingerprint
883 Insecure fingerprints will be handled as if this option was set to
885 If this option is set to
887 information on fingerprint match will be displayed, but the user will still
888 need to confirm new host keys according to the
889 .Cm StrictHostKeyChecking
898 Note that this option applies to protocol version 2 only.
899 .It Cm VersionAddendum
900 Specifies a string to append to the regular version string to identify
901 OS- or site-specific modifications.
903 .Dq FreeBSD-20050903 .
905 Specifies the full pathname of the
909 .Pa /usr/X11R6/bin/xauth .
914 This is the per-user configuration file.
915 The format of this file is described above.
916 This file is used by the
919 Because of the potential for abuse, this file must have strict permissions:
920 read/write for the user, and not accessible by others.
921 .It Pa /etc/ssh/ssh_config
922 Systemwide configuration file.
923 This file provides defaults for those
924 values that are not specified in the user's configuration file, and
925 for those users who do not have a configuration file.
926 This file must be world-readable.
931 OpenSSH is a derivative of the original and free
932 ssh 1.2.12 release by Tatu Ylonen.
933 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
934 Theo de Raadt and Dug Song
935 removed many bugs, re-added newer features and
937 Markus Friedl contributed the support for SSH
938 protocol versions 1.5 and 2.0.