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 1.54 2005/05/23 23:32:46 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 Specify the path to the control socket used for connection sharing.
286 .It Cm DynamicForward
287 Specifies that a TCP/IP port on the local machine be forwarded
288 over the secure channel, and the application
289 protocol is then used to determine where to connect to from the
291 The argument must be a port number.
292 Currently the SOCKS4 and SOCKS5 protocols are supported, and
294 will act as a SOCKS server.
295 Multiple forwardings may be specified, and
296 additional forwardings can be given on the command line.
297 Only the superuser can forward privileged ports.
298 .It Cm EnableSSHKeysign
299 Setting this option to
301 in the global client configuration file
302 .Pa /etc/ssh/ssh_config
303 enables the use of the helper program
306 .Cm HostbasedAuthentication .
313 This option should be placed in the non-hostspecific section.
316 for more information.
318 Sets the escape character (default:
320 The escape character can also
321 be set on the command line.
322 The argument should be a single character,
324 followed by a letter, or
326 to disable the escape
327 character entirely (making the connection transparent for binary
330 Specifies whether the connection to the authentication agent (if any)
331 will be forwarded to the remote machine.
339 Agent forwarding should be enabled with caution.
340 Users with the ability to bypass file permissions on the remote host
341 (for the agent's Unix-domain socket)
342 can access the local agent through the forwarded connection.
343 An attacker cannot obtain key material from the agent,
344 however they can perform operations on the keys that enable them to
345 authenticate using the identities loaded into the agent.
347 Specifies whether X11 connections will be automatically redirected
348 over the secure channel and
358 X11 forwarding should be enabled with caution.
359 Users with the ability to bypass file permissions on the remote host
360 (for the user's X11 authorization database)
361 can access the local X11 display through the forwarded connection.
362 An attacker may then be able to perform activities such as keystroke monitoring
364 .Cm ForwardX11Trusted
365 option is also enabled.
366 .It Cm ForwardX11Trusted
367 If this option is set to
369 then remote X11 clients will have full access to the original X11 display.
371 If this option is set to
373 then remote X11 clients will be considered untrusted and prevented
374 from stealing or tampering with data belonging to trusted X11
378 token used for the session will be set to expire after 20 minutes.
379 Remote clients will be refused access after this time.
384 See the X11 SECURITY extension specification for full details on
385 the restrictions imposed on untrusted clients.
387 Specifies whether remote hosts are allowed to connect to local
391 binds local port forwardings to the loopback address.
392 This prevents other remote hosts from connecting to forwarded ports.
394 can be used to specify that
396 should bind local port forwardings to the wildcard address,
397 thus allowing remote hosts to connect to forwarded ports.
404 .It Cm GlobalKnownHostsFile
405 Specifies a file to use for the global
406 host key database instead of
407 .Pa /etc/ssh/ssh_known_hosts .
408 .It Cm GSSAPIAuthentication
409 Specifies whether user authentication based on GSSAPI is allowed.
412 Note that this option applies to protocol version 2 only.
413 .It Cm GSSAPIDelegateCredentials
414 Forward (delegate) credentials to the server.
417 Note that this option applies to protocol version 2 only.
418 .It Cm HashKnownHosts
421 should hash host names and addresses when they are added to
422 .Pa ~/.ssh/known_hosts .
423 These hashed names may be used normally by
427 but they do not reveal identifying information should the file's contents
431 Note that hashing of names and addresses will not be retrospectively applied
432 to existing known hosts files, but these may be manually hashed using
434 .It Cm HostbasedAuthentication
435 Specifies whether to try rhosts based authentication with public key
443 This option applies to protocol version 2 only and
445 .Cm RhostsRSAAuthentication .
446 .It Cm HostKeyAlgorithms
447 Specifies the protocol version 2 host key algorithms
448 that the client wants to use in order of preference.
449 The default for this option is:
450 .Dq ssh-rsa,ssh-dss .
452 Specifies an alias that should be used instead of the
453 real host name when looking up or saving the host key
454 in the host key database files.
455 This option is useful for tunneling ssh connections
456 or for multiple servers running on a single host.
458 Specifies the real host name to log into.
459 This can be used to specify nicknames or abbreviations for hosts.
460 Default is the name given on the command line.
461 Numeric IP addresses are also permitted (both on the command line and in
465 Specifies a file from which the user's RSA or DSA authentication identity
469 for protocol version 1, and
473 for protocol version 2.
474 Additionally, any identities represented by the authentication agent
475 will be used for authentication.
476 The file name may use the tilde
477 syntax to refer to a user's home directory.
478 It is possible to have
479 multiple identity files specified in configuration files; all these
480 identities will be tried in sequence.
481 .It Cm IdentitiesOnly
484 should only use the authentication identity files configured in the
489 offers more identities.
490 The argument to this keyword must be
494 This option is intented for situations where
496 offers many different identities.
499 .It Cm KbdInteractiveDevices
500 Specifies the list of methods to use in keyboard-interactive authentication.
501 Multiple method names must be comma-separated.
502 The default is to use the server specified list.
504 Specifies that a TCP/IP port on the local machine be forwarded over
505 the secure channel to the specified host and port from the remote machine.
506 The first argument must be
508 .Oo Ar bind_address : Oc Ar port
510 and the second argument must be
511 .Ar host : Ns Ar hostport .
512 IPv6 addresses can be specified by enclosing addresses in square brackets or
513 by using an alternative syntax:
514 .Oo Ar bind_address Ns / Oc Ns Ar port
516 .Ar host Ns / Ns Ar hostport .
517 Multiple forwardings may be specified, and additional forwardings can be
518 given on the command line.
519 Only the superuser can forward privileged ports.
520 By default, the local port is bound in accordance with the
525 may be used to bind the connection to a specific address.
530 indicates that the listening port be bound for local use only, while an
533 indicates that the port should be available from all interfaces.
535 Gives the verbosity level that is used when logging messages from
537 The possible values are:
538 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
540 DEBUG and DEBUG1 are equivalent.
541 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
543 Specifies the MAC (message authentication code) algorithms
544 in order of preference.
545 The MAC algorithm is used in protocol version 2
546 for data integrity protection.
547 Multiple algorithms must be comma-separated.
549 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
550 .It Cm NoHostAuthenticationForLocalhost
551 This option can be used if the home directory is shared across machines.
552 In this case localhost will refer to a different machine on each of
553 the machines and the user will get many warnings about changed host keys.
554 However, this option disables host authentication for localhost.
555 The argument to this keyword must be
559 The default is to check the host key for localhost.
560 .It Cm NumberOfPasswordPrompts
561 Specifies the number of password prompts before giving up.
562 The argument to this keyword must be an integer.
564 .It Cm PasswordAuthentication
565 Specifies whether to use password authentication.
566 The argument to this keyword must be
573 Specifies the port number to connect on the remote host.
575 .It Cm PreferredAuthentications
576 Specifies the order in which the client should try protocol 2
577 authentication methods.
578 This allows a client to prefer one method (e.g.\&
579 .Cm keyboard-interactive )
580 over another method (e.g.\&
582 The default for this option is:
583 .Dq hostbased,publickey,keyboard-interactive,password .
585 Specifies the protocol versions
587 should support in order of preference.
588 The possible values are
592 Multiple versions must be comma-separated.
597 tries version 2 and falls back to version 1
598 if version 2 is not available.
600 Specifies the command to use to connect to the server.
602 string extends to the end of the line, and is executed with
604 In the command string,
606 will be substituted by the host name to
610 The command can be basically anything,
611 and should read from its standard input and write to its standard output.
612 It should eventually connect an
614 server running on some machine, or execute
617 Host key management will be done using the
618 HostName of the host being connected (defaulting to the name typed by
620 Setting the command to
622 disables this option entirely.
625 is not available for connects with a proxy command.
627 This directive is useful in conjunction with
629 and its proxy support.
630 For example, the following directive would connect via an HTTP proxy at
632 .Bd -literal -offset 3n
633 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
635 .It Cm PubkeyAuthentication
636 Specifies whether to try public key authentication.
637 The argument to this keyword must be
643 This option applies to protocol version 2 only.
645 Specifies that a TCP/IP port on the remote machine be forwarded over
646 the secure channel to the specified host and port from the local machine.
647 The first argument must be
649 .Oo Ar bind_address : Oc Ar port
651 and the second argument must be
652 .Ar host : Ns Ar hostport .
653 IPv6 addresses can be specified by enclosing addresses in square brackets
654 or by using an alternative syntax:
655 .Oo Ar bind_address Ns / Oc Ns Ar port
657 .Ar host Ns / Ns Ar hostport .
658 Multiple forwardings may be specified, and additional
659 forwardings can be given on the command line.
660 Only the superuser can forward privileged ports.
664 is not specified, the default is to only bind to loopback addresses.
669 or an empty string, then the forwarding is requested to listen on all
673 will only succeed if the server's
675 option is enabled (see
676 .Xr sshd_config 5 ) .
677 .It Cm RhostsRSAAuthentication
678 Specifies whether to try rhosts based authentication with RSA host
686 This option applies to protocol version 1 only and requires
689 .It Cm RSAAuthentication
690 Specifies whether to try RSA authentication.
691 The argument to this keyword must be
695 RSA authentication will only be
696 attempted if the identity file exists, or an authentication agent is
700 Note that this option applies to protocol version 1 only.
702 Specifies what variables from the local
704 should be sent to the server.
705 Note that environment passing is only supported for protocol 2, the
706 server must also support it, and the server must be configured to
707 accept these environment variables.
712 for how to configure the server.
713 Variables are specified by name, which may contain the wildcard characters
717 Multiple environment variables may be separated by whitespace or spread
721 The default is not to send any environment variables.
722 .It Cm ServerAliveInterval
723 Sets a timeout interval in seconds after which if no data has been received
726 will send a message through the encrypted
727 channel to request a response from the server.
729 is 0, indicating that these messages will not be sent to the server.
730 This option applies to protocol version 2 only.
731 .It Cm ServerAliveCountMax
732 Sets the number of server alive messages (see above) which may be
735 receiving any messages back from the server.
736 If this threshold is reached while server alive messages are being sent,
738 will disconnect from the server, terminating the session.
739 It is important to note that the use of server alive messages is very
743 The server alive messages are sent through the encrypted channel
744 and therefore will not be spoofable.
745 The TCP keepalive option enabled by
748 The server alive mechanism is valuable when the client or
749 server depend on knowing when a connection has become inactive.
751 The default value is 3.
753 .Cm ServerAliveInterval
754 (above) is set to 15, and
755 .Cm ServerAliveCountMax
756 is left at the default, if the server becomes unresponsive ssh
757 will disconnect after approximately 45 seconds.
758 .It Cm SmartcardDevice
759 Specifies which smartcard device to use.
760 The argument to this keyword is the device
762 should use to communicate with a smartcard used for storing the user's
764 By default, no device is specified and smartcard support is not activated.
765 .It Cm StrictHostKeyChecking
766 If this flag is set to
769 will never automatically add host keys to the
770 .Pa ~/.ssh/known_hosts
771 file, and refuses to connect to hosts whose host key has changed.
772 This provides maximum protection against trojan horse attacks,
773 however, can be annoying when the
774 .Pa /etc/ssh/ssh_known_hosts
775 file is poorly maintained, or connections to new hosts are
777 This option forces the user to manually
779 If this flag is set to
782 will automatically add new host keys to the
783 user known hosts files.
784 If this flag is set to
787 will be added to the user known host files only after the user
788 has confirmed that is what they really want to do, and
790 will refuse to connect to hosts whose host key has changed.
792 known hosts will be verified automatically in all cases.
801 Specifies whether the system should send TCP keepalive messages to the
803 If they are sent, death of the connection or crash of one
804 of the machines will be properly noticed.
805 However, this means that
806 connections will die if the route is down temporarily, and some people
811 (to send TCP keepalive messages), and the client will notice
812 if the network goes down or the remote host dies.
813 This is important in scripts, and many users want it too.
815 To disable TCP keepalive messages, the value should be set to
817 .It Cm UsePrivilegedPort
818 Specifies whether to use a privileged port for outgoing connections.
829 Note that this option must be set to
832 .Cm RhostsRSAAuthentication
835 Specifies the user to log in as.
836 This can be useful when a different user name is used on different machines.
837 This saves the trouble of
838 having to remember to give the user name on the command line.
839 .It Cm UserKnownHostsFile
840 Specifies a file to use for the user
841 host key database instead of
842 .Pa ~/.ssh/known_hosts .
843 .It Cm VerifyHostKeyDNS
844 Specifies whether to verify the remote key using DNS and SSHFP resource
846 If this option is set to
848 the client will implicitly trust keys that match a secure fingerprint
850 Insecure fingerprints will be handled as if this option was set to
852 If this option is set to
854 information on fingerprint match will be displayed, but the user will still
855 need to confirm new host keys according to the
856 .Cm StrictHostKeyChecking
865 Note that this option applies to protocol version 2 only.
867 Specifies the full pathname of the
871 .Pa /usr/X11R6/bin/xauth .
876 This is the per-user configuration file.
877 The format of this file is described above.
878 This file is used by the
881 Because of the potential for abuse, this file must have strict permissions:
882 read/write for the user, and not accessible by others.
883 .It Pa /etc/ssh/ssh_config
884 Systemwide configuration file.
885 This file provides defaults for those
886 values that are not specified in the user's configuration file, and
887 for those users who do not have a configuration file.
888 This file must be world-readable.
893 OpenSSH is a derivative of the original and free
894 ssh 1.2.12 release by Tatu Ylonen.
895 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
896 Theo de Raadt and Dug Song
897 removed many bugs, re-added newer features and
899 Markus Friedl contributed the support for SSH
900 protocol versions 1.5 and 2.0.