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 1.57 2005/06/18 04:30:36 djm Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH client configuration files
45 .Bl -tag -width Ds -compact
47 .It Pa /etc/ssh/ssh_config
51 obtains configuration data from the following sources in
53 .Bl -enum -offset indent -compact
57 user's configuration file
60 system-wide configuration file
61 .Pq Pa /etc/ssh/ssh_config
64 For each parameter, the first obtained value
66 The configuration files contain sections separated by
68 specifications, and that section is only applied for hosts that
69 match one of the patterns given in the specification.
70 The matched host name is the one given on the command line.
72 Since the first obtained value for each parameter is used, more
73 host-specific declarations should be given near the beginning of the
74 file, and general defaults at the end.
76 The configuration file has the following format:
78 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
97 keywords and their meanings are as follows (note that
98 keywords are case-insensitive and arguments are case-sensitive):
101 Restricts the following declarations (up to the next
103 keyword) to be only for those hosts that match one of the patterns
104 given after the keyword.
108 can be used as wildcards in the
112 as a pattern can be used to provide global
113 defaults for all hosts.
116 argument given on the command line (i.e., the name is not converted to
117 a canonicalized host name before matching).
119 Specifies which address family to use when connecting.
129 passphrase/password querying will be disabled.
130 This option is useful in scripts and other batch jobs where no user
131 is present to supply the password.
139 Specify the interface to transmit from on machines with multiple
140 interfaces or aliased addresses.
141 Note that this option does not work if
142 .Cm UsePrivilegedPort
145 .It Cm ChallengeResponseAuthentication
146 Specifies whether to use challenge response authentication.
147 The argument to this keyword must be
154 If this flag is set to
156 ssh will additionally check the host IP address in the
159 This allows ssh to detect if a host key changed due to DNS spoofing.
160 If the option is set to
162 the check will not be executed.
166 Specifies the cipher to use for encrypting the session
167 in protocol version 1.
175 is only supported in the
177 client for interoperability with legacy protocol 1 implementations
178 that do not support the
181 Its use is strongly discouraged due to cryptographic weaknesses.
185 Specifies the ciphers allowed for protocol version 2
186 in order of preference.
187 Multiple ciphers must be comma-separated.
188 The supported ciphers are
204 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
205 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
206 aes192-ctr,aes256-ctr''
208 .It Cm ClearAllForwardings
209 Specifies that all local, remote and dynamic port forwardings
210 specified in the configuration files or on the command line be
212 This option is primarily useful when used from the
214 command line to clear port forwardings set in
215 configuration files, and is automatically set by
226 Specifies whether to use compression.
233 .It Cm CompressionLevel
234 Specifies the compression level to use if compression is enabled.
235 The argument must be an integer from 1 (fast) to 9 (slow, best).
236 The default level is 6, which is good for most applications.
237 The meaning of the values is the same as in
239 Note that this option applies to protocol version 1 only.
240 .It Cm ConnectionAttempts
241 Specifies the number of tries (one per second) to make before exiting.
242 The argument must be an integer.
243 This may be useful in scripts if the connection sometimes fails.
245 .It Cm ConnectTimeout
246 Specifies the timeout (in seconds) used when connecting to the ssh
247 server, instead of using the default system TCP timeout.
248 This value is used only when the target is down or really unreachable,
249 not when it refuses the connection.
251 Enables the sharing of multiple sessions over a single network connection.
255 will listen for connections on a control socket specified using the
258 Additional sessions can connect to this socket using the same
265 These sessions will reuse the master instance's network connection rather
266 than initiating new ones.
271 to listen for control connections, but require confirmation using the
273 program before they are accepted (see
280 will continue without connecting to a master instance.
282 Two additional options allow for opportunistic multiplexing: try to use a
283 master connection but fall back to creating a new one if one does not already
289 The latter requires confirmation like the
293 Specify the path to the control socket used for connection sharing as described
296 section above or the string
298 to disable connection sharing.
301 will be substituted by the target host name,
305 by the remote login username.
306 It is recommended that any
308 used for opportunistic connection sharing include
309 all three of these escape sequences.
310 This ensures that shared connections are uniquely identified.
311 .It Cm DynamicForward
312 Specifies that a TCP/IP port on the local machine be forwarded
313 over the secure channel, and the application
314 protocol is then used to determine where to connect to from the
316 The argument must be a port number.
317 Currently the SOCKS4 and SOCKS5 protocols are supported, and
319 will act as a SOCKS server.
320 Multiple forwardings may be specified, and
321 additional forwardings can be given on the command line.
322 Only the superuser can forward privileged ports.
323 .It Cm EnableSSHKeysign
324 Setting this option to
326 in the global client configuration file
327 .Pa /etc/ssh/ssh_config
328 enables the use of the helper program
331 .Cm HostbasedAuthentication .
338 This option should be placed in the non-hostspecific section.
341 for more information.
343 Sets the escape character (default:
345 The escape character can also
346 be set on the command line.
347 The argument should be a single character,
349 followed by a letter, or
351 to disable the escape
352 character entirely (making the connection transparent for binary
355 Specifies whether the connection to the authentication agent (if any)
356 will be forwarded to the remote machine.
364 Agent forwarding should be enabled with caution.
365 Users with the ability to bypass file permissions on the remote host
366 (for the agent's Unix-domain socket)
367 can access the local agent through the forwarded connection.
368 An attacker cannot obtain key material from the agent,
369 however they can perform operations on the keys that enable them to
370 authenticate using the identities loaded into the agent.
372 Specifies whether X11 connections will be automatically redirected
373 over the secure channel and
383 X11 forwarding should be enabled with caution.
384 Users with the ability to bypass file permissions on the remote host
385 (for the user's X11 authorization database)
386 can access the local X11 display through the forwarded connection.
387 An attacker may then be able to perform activities such as keystroke monitoring
389 .Cm ForwardX11Trusted
390 option is also enabled.
391 .It Cm ForwardX11Trusted
392 If this option is set to
394 then remote X11 clients will have full access to the original X11 display.
396 If this option is set to
398 then remote X11 clients will be considered untrusted and prevented
399 from stealing or tampering with data belonging to trusted X11
403 token used for the session will be set to expire after 20 minutes.
404 Remote clients will be refused access after this time.
409 See the X11 SECURITY extension specification for full details on
410 the restrictions imposed on untrusted clients.
412 Specifies whether remote hosts are allowed to connect to local
416 binds local port forwardings to the loopback address.
417 This prevents other remote hosts from connecting to forwarded ports.
419 can be used to specify that
421 should bind local port forwardings to the wildcard address,
422 thus allowing remote hosts to connect to forwarded ports.
429 .It Cm GlobalKnownHostsFile
430 Specifies a file to use for the global
431 host key database instead of
432 .Pa /etc/ssh/ssh_known_hosts .
433 .It Cm GSSAPIAuthentication
434 Specifies whether user authentication based on GSSAPI is allowed.
437 Note that this option applies to protocol version 2 only.
438 .It Cm GSSAPIDelegateCredentials
439 Forward (delegate) credentials to the server.
442 Note that this option applies to protocol version 2 only.
443 .It Cm HashKnownHosts
446 should hash host names and addresses when they are added to
447 .Pa ~/.ssh/known_hosts .
448 These hashed names may be used normally by
452 but they do not reveal identifying information should the file's contents
456 Note that hashing of names and addresses will not be retrospectively applied
457 to existing known hosts files, but these may be manually hashed using
459 .It Cm HostbasedAuthentication
460 Specifies whether to try rhosts based authentication with public key
468 This option applies to protocol version 2 only and
470 .Cm RhostsRSAAuthentication .
471 .It Cm HostKeyAlgorithms
472 Specifies the protocol version 2 host key algorithms
473 that the client wants to use in order of preference.
474 The default for this option is:
475 .Dq ssh-rsa,ssh-dss .
477 Specifies an alias that should be used instead of the
478 real host name when looking up or saving the host key
479 in the host key database files.
480 This option is useful for tunneling ssh connections
481 or for multiple servers running on a single host.
483 Specifies the real host name to log into.
484 This can be used to specify nicknames or abbreviations for hosts.
485 Default is the name given on the command line.
486 Numeric IP addresses are also permitted (both on the command line and in
490 Specifies a file from which the user's RSA or DSA authentication identity
494 for protocol version 1, and
498 for protocol version 2.
499 Additionally, any identities represented by the authentication agent
500 will be used for authentication.
501 The file name may use the tilde
502 syntax to refer to a user's home directory.
503 It is possible to have
504 multiple identity files specified in configuration files; all these
505 identities will be tried in sequence.
506 .It Cm IdentitiesOnly
509 should only use the authentication identity files configured in the
514 offers more identities.
515 The argument to this keyword must be
519 This option is intented for situations where
521 offers many different identities.
524 .It Cm KbdInteractiveDevices
525 Specifies the list of methods to use in keyboard-interactive authentication.
526 Multiple method names must be comma-separated.
527 The default is to use the server specified list.
529 Specifies that a TCP/IP port on the local machine be forwarded over
530 the secure channel to the specified host and port from the remote machine.
531 The first argument must be
533 .Oo Ar bind_address : Oc Ar port
535 and the second argument must be
536 .Ar host : Ns Ar hostport .
537 IPv6 addresses can be specified by enclosing addresses in square brackets or
538 by using an alternative syntax:
539 .Oo Ar bind_address Ns / Oc Ns Ar port
541 .Ar host Ns / Ns Ar hostport .
542 Multiple forwardings may be specified, and additional forwardings can be
543 given on the command line.
544 Only the superuser can forward privileged ports.
545 By default, the local port is bound in accordance with the
550 may be used to bind the connection to a specific address.
555 indicates that the listening port be bound for local use only, while an
558 indicates that the port should be available from all interfaces.
560 Gives the verbosity level that is used when logging messages from
562 The possible values are:
563 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
565 DEBUG and DEBUG1 are equivalent.
566 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
568 Specifies the MAC (message authentication code) algorithms
569 in order of preference.
570 The MAC algorithm is used in protocol version 2
571 for data integrity protection.
572 Multiple algorithms must be comma-separated.
574 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
575 .It Cm NoHostAuthenticationForLocalhost
576 This option can be used if the home directory is shared across machines.
577 In this case localhost will refer to a different machine on each of
578 the machines and the user will get many warnings about changed host keys.
579 However, this option disables host authentication for localhost.
580 The argument to this keyword must be
584 The default is to check the host key for localhost.
585 .It Cm NumberOfPasswordPrompts
586 Specifies the number of password prompts before giving up.
587 The argument to this keyword must be an integer.
589 .It Cm PasswordAuthentication
590 Specifies whether to use password authentication.
591 The argument to this keyword must be
598 Specifies the port number to connect on the remote host.
600 .It Cm PreferredAuthentications
601 Specifies the order in which the client should try protocol 2
602 authentication methods.
603 This allows a client to prefer one method (e.g.\&
604 .Cm keyboard-interactive )
605 over another method (e.g.\&
607 The default for this option is:
608 .Dq hostbased,publickey,keyboard-interactive,password .
610 Specifies the protocol versions
612 should support in order of preference.
613 The possible values are
617 Multiple versions must be comma-separated.
622 tries version 2 and falls back to version 1
623 if version 2 is not available.
625 Specifies the command to use to connect to the server.
627 string extends to the end of the line, and is executed with
629 In the command string,
631 will be substituted by the host name to
635 The command can be basically anything,
636 and should read from its standard input and write to its standard output.
637 It should eventually connect an
639 server running on some machine, or execute
642 Host key management will be done using the
643 HostName of the host being connected (defaulting to the name typed by
645 Setting the command to
647 disables this option entirely.
650 is not available for connects with a proxy command.
652 This directive is useful in conjunction with
654 and its proxy support.
655 For example, the following directive would connect via an HTTP proxy at
657 .Bd -literal -offset 3n
658 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
660 .It Cm PubkeyAuthentication
661 Specifies whether to try public key authentication.
662 The argument to this keyword must be
668 This option applies to protocol version 2 only.
670 Specifies that a TCP/IP port on the remote machine be forwarded over
671 the secure channel to the specified host and port from the local machine.
672 The first argument must be
674 .Oo Ar bind_address : Oc Ar port
676 and the second argument must be
677 .Ar host : Ns Ar hostport .
678 IPv6 addresses can be specified by enclosing addresses in square brackets
679 or by using an alternative syntax:
680 .Oo Ar bind_address Ns / Oc Ns Ar port
682 .Ar host Ns / Ns Ar hostport .
683 Multiple forwardings may be specified, and additional
684 forwardings can be given on the command line.
685 Only the superuser can forward privileged ports.
689 is not specified, the default is to only bind to loopback addresses.
694 or an empty string, then the forwarding is requested to listen on all
698 will only succeed if the server's
700 option is enabled (see
701 .Xr sshd_config 5 ) .
702 .It Cm RhostsRSAAuthentication
703 Specifies whether to try rhosts based authentication with RSA host
711 This option applies to protocol version 1 only and requires
714 .It Cm RSAAuthentication
715 Specifies whether to try RSA authentication.
716 The argument to this keyword must be
720 RSA authentication will only be
721 attempted if the identity file exists, or an authentication agent is
725 Note that this option applies to protocol version 1 only.
727 Specifies what variables from the local
729 should be sent to the server.
730 Note that environment passing is only supported for protocol 2, the
731 server must also support it, and the server must be configured to
732 accept these environment variables.
737 for how to configure the server.
738 Variables are specified by name, which may contain the wildcard characters
742 Multiple environment variables may be separated by whitespace or spread
746 The default is not to send any environment variables.
747 .It Cm ServerAliveInterval
748 Sets a timeout interval in seconds after which if no data has been received
751 will send a message through the encrypted
752 channel to request a response from the server.
754 is 0, indicating that these messages will not be sent to the server.
755 This option applies to protocol version 2 only.
756 .It Cm ServerAliveCountMax
757 Sets the number of server alive messages (see above) which may be
760 receiving any messages back from the server.
761 If this threshold is reached while server alive messages are being sent,
763 will disconnect from the server, terminating the session.
764 It is important to note that the use of server alive messages is very
768 The server alive messages are sent through the encrypted channel
769 and therefore will not be spoofable.
770 The TCP keepalive option enabled by
773 The server alive mechanism is valuable when the client or
774 server depend on knowing when a connection has become inactive.
776 The default value is 3.
778 .Cm ServerAliveInterval
779 (above) is set to 15, and
780 .Cm ServerAliveCountMax
781 is left at the default, if the server becomes unresponsive ssh
782 will disconnect after approximately 45 seconds.
783 .It Cm SmartcardDevice
784 Specifies which smartcard device to use.
785 The argument to this keyword is the device
787 should use to communicate with a smartcard used for storing the user's
789 By default, no device is specified and smartcard support is not activated.
790 .It Cm StrictHostKeyChecking
791 If this flag is set to
794 will never automatically add host keys to the
795 .Pa ~/.ssh/known_hosts
796 file, and refuses to connect to hosts whose host key has changed.
797 This provides maximum protection against trojan horse attacks,
798 however, can be annoying when the
799 .Pa /etc/ssh/ssh_known_hosts
800 file is poorly maintained, or connections to new hosts are
802 This option forces the user to manually
804 If this flag is set to
807 will automatically add new host keys to the
808 user known hosts files.
809 If this flag is set to
812 will be added to the user known host files only after the user
813 has confirmed that is what they really want to do, and
815 will refuse to connect to hosts whose host key has changed.
817 known hosts will be verified automatically in all cases.
826 Specifies whether the system should send TCP keepalive messages to the
828 If they are sent, death of the connection or crash of one
829 of the machines will be properly noticed.
830 However, this means that
831 connections will die if the route is down temporarily, and some people
836 (to send TCP keepalive messages), and the client will notice
837 if the network goes down or the remote host dies.
838 This is important in scripts, and many users want it too.
840 To disable TCP keepalive messages, the value should be set to
842 .It Cm UsePrivilegedPort
843 Specifies whether to use a privileged port for outgoing connections.
854 Note that this option must be set to
857 .Cm RhostsRSAAuthentication
860 Specifies the user to log in as.
861 This can be useful when a different user name is used on different machines.
862 This saves the trouble of
863 having to remember to give the user name on the command line.
864 .It Cm UserKnownHostsFile
865 Specifies a file to use for the user
866 host key database instead of
867 .Pa ~/.ssh/known_hosts .
868 .It Cm VerifyHostKeyDNS
869 Specifies whether to verify the remote key using DNS and SSHFP resource
871 If this option is set to
873 the client will implicitly trust keys that match a secure fingerprint
875 Insecure fingerprints will be handled as if this option was set to
877 If this option is set to
879 information on fingerprint match will be displayed, but the user will still
880 need to confirm new host keys according to the
881 .Cm StrictHostKeyChecking
890 Note that this option applies to protocol version 2 only.
892 Specifies the full pathname of the
896 .Pa /usr/X11R6/bin/xauth .
901 This is the per-user configuration file.
902 The format of this file is described above.
903 This file is used by the
906 Because of the potential for abuse, this file must have strict permissions:
907 read/write for the user, and not accessible by others.
908 .It Pa /etc/ssh/ssh_config
909 Systemwide configuration file.
910 This file provides defaults for those
911 values that are not specified in the user's configuration file, and
912 for those users who do not have a configuration file.
913 This file must be world-readable.
918 OpenSSH is a derivative of the original and free
919 ssh 1.2.12 release by Tatu Ylonen.
920 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
921 Theo de Raadt and Dug Song
922 removed many bugs, re-added newer features and
924 Markus Friedl contributed the support for SSH
925 protocol versions 1.5 and 2.0.