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 1.35 2004/06/13 14:01:42 dtucker Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH client configuration files
45 .Bl -tag -width Ds -compact
46 .It Pa $HOME/.ssh/config
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
58 .Pq Pa $HOME/.ssh/config
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 bracketed 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
202 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
203 aes192-cbc,aes256-cbc''
205 .It Cm ClearAllForwardings
206 Specifies that all local, remote and dynamic port forwardings
207 specified in the configuration files or on the command line be
209 This option is primarily useful when used from the
211 command line to clear port forwardings set in
212 configuration files, and is automatically set by
223 Specifies whether to use compression.
230 .It Cm CompressionLevel
231 Specifies the compression level to use if compression is enabled.
232 The argument must be an integer from 1 (fast) to 9 (slow, best).
233 The default level is 6, which is good for most applications.
234 The meaning of the values is the same as in
236 Note that this option applies to protocol version 1 only.
237 .It Cm ConnectionAttempts
238 Specifies the number of tries (one per second) to make before exiting.
239 The argument must be an integer.
240 This may be useful in scripts if the connection sometimes fails.
242 .It Cm ConnectTimeout
243 Specifies the timeout (in seconds) used when connecting to the ssh
244 server, instead of using the default system TCP timeout.
245 This value is used only when the target is down or really unreachable,
246 not when it refuses the connection.
247 .It Cm DynamicForward
248 Specifies that a TCP/IP port on the local machine be forwarded
249 over the secure channel, and the application
250 protocol is then used to determine where to connect to from the
252 The argument must be a port number.
253 Currently the SOCKS4 and SOCKS5 protocols are supported, and
255 will act as a SOCKS server.
256 Multiple forwardings may be specified, and
257 additional forwardings can be given on the command line.
258 Only the superuser can forward privileged ports.
259 .It Cm EnableSSHKeysign
260 Setting this option to
262 in the global client configuration file
263 .Pa /etc/ssh/ssh_config
264 enables the use of the helper program
267 .Cm HostbasedAuthentication .
274 This option should be placed in the non-hostspecific section.
277 for more information.
279 Sets the escape character (default:
281 The escape character can also
282 be set on the command line.
283 The argument should be a single character,
285 followed by a letter, or
287 to disable the escape
288 character entirely (making the connection transparent for binary
291 Specifies whether the connection to the authentication agent (if any)
292 will be forwarded to the remote machine.
300 Agent forwarding should be enabled with caution.
301 Users with the ability to bypass file permissions on the remote host
302 (for the agent's Unix-domain socket)
303 can access the local agent through the forwarded connection.
304 An attacker cannot obtain key material from the agent,
305 however they can perform operations on the keys that enable them to
306 authenticate using the identities loaded into the agent.
308 Specifies whether X11 connections will be automatically redirected
309 over the secure channel and
319 X11 forwarding should be enabled with caution.
320 Users with the ability to bypass file permissions on the remote host
321 (for the user's X11 authorization database)
322 can access the local X11 display through the forwarded connection.
323 An attacker may then be able to perform activities such as keystroke monitoring
325 .Cm ForwardX11Trusted
326 option is also enabled.
327 .It Cm ForwardX11Trusted
328 If this option is set to
330 then remote X11 clients will have full access to the original X11 display.
331 If this option is set to
333 then remote X11 clients will be considered untrusted and prevented
334 from stealing or tampering with data belonging to trusted X11
340 See the X11 SECURITY extension specification for full details on
341 the restrictions imposed on untrusted clients.
343 Specifies whether remote hosts are allowed to connect to local
347 binds local port forwardings to the loopback address.
348 This prevents other remote hosts from connecting to forwarded ports.
350 can be used to specify that
352 should bind local port forwardings to the wildcard address,
353 thus allowing remote hosts to connect to forwarded ports.
360 .It Cm GlobalKnownHostsFile
361 Specifies a file to use for the global
362 host key database instead of
363 .Pa /etc/ssh/ssh_known_hosts .
364 .It Cm GSSAPIAuthentication
365 Specifies whether user authentication based on GSSAPI is allowed.
368 Note that this option applies to protocol version 2 only.
369 .It Cm GSSAPIDelegateCredentials
370 Forward (delegate) credentials to the server.
373 Note that this option applies to protocol version 2 only.
374 .It Cm HostbasedAuthentication
375 Specifies whether to try rhosts based authentication with public key
383 This option applies to protocol version 2 only and
385 .Cm RhostsRSAAuthentication .
386 .It Cm HostKeyAlgorithms
387 Specifies the protocol version 2 host key algorithms
388 that the client wants to use in order of preference.
389 The default for this option is:
390 .Dq ssh-rsa,ssh-dss .
392 Specifies an alias that should be used instead of the
393 real host name when looking up or saving the host key
394 in the host key database files.
395 This option is useful for tunneling ssh connections
396 or for multiple servers running on a single host.
398 Specifies the real host name to log into.
399 This can be used to specify nicknames or abbreviations for hosts.
400 Default is the name given on the command line.
401 Numeric IP addresses are also permitted (both on the command line and in
405 Specifies a file from which the user's RSA or DSA authentication identity
408 .Pa $HOME/.ssh/identity
409 for protocol version 1, and
410 .Pa $HOME/.ssh/id_rsa
412 .Pa $HOME/.ssh/id_dsa
413 for protocol version 2.
414 Additionally, any identities represented by the authentication agent
415 will be used for authentication.
416 The file name may use the tilde
417 syntax to refer to a user's home directory.
418 It is possible to have
419 multiple identity files specified in configuration files; all these
420 identities will be tried in sequence.
421 .It Cm IdentitiesOnly
424 should only use the authentication identity files configured in the
429 offers more identities.
430 The argument to this keyword must be
434 This option is intented for situations where
436 offers many different identities.
440 Specifies that a TCP/IP port on the local machine be forwarded over
441 the secure channel to the specified host and port from the remote machine.
442 The first argument must be a port number, and the second must be
444 IPv6 addresses can be specified with an alternative syntax:
446 Multiple forwardings may be specified, and additional
447 forwardings can be given on the command line.
448 Only the superuser can forward privileged ports.
450 Gives the verbosity level that is used when logging messages from
452 The possible values are:
453 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
455 DEBUG and DEBUG1 are equivalent.
456 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
458 Specifies the MAC (message authentication code) algorithms
459 in order of preference.
460 The MAC algorithm is used in protocol version 2
461 for data integrity protection.
462 Multiple algorithms must be comma-separated.
464 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
465 .It Cm NoHostAuthenticationForLocalhost
466 This option can be used if the home directory is shared across machines.
467 In this case localhost will refer to a different machine on each of
468 the machines and the user will get many warnings about changed host keys.
469 However, this option disables host authentication for localhost.
470 The argument to this keyword must be
474 The default is to check the host key for localhost.
475 .It Cm NumberOfPasswordPrompts
476 Specifies the number of password prompts before giving up.
477 The argument to this keyword must be an integer.
479 .It Cm PasswordAuthentication
480 Specifies whether to use password authentication.
481 The argument to this keyword must be
488 Specifies the port number to connect on the remote host.
490 .It Cm PreferredAuthentications
491 Specifies the order in which the client should try protocol 2
492 authentication methods.
493 This allows a client to prefer one method (e.g.
494 .Cm keyboard-interactive )
495 over another method (e.g.
497 The default for this option is:
498 .Dq hostbased,publickey,keyboard-interactive,password .
500 Specifies the protocol versions
502 should support in order of preference.
503 The possible values are
507 Multiple versions must be comma-separated.
512 tries version 2 and falls back to version 1
513 if version 2 is not available.
515 Specifies the command to use to connect to the server.
517 string extends to the end of the line, and is executed with
519 In the command string,
521 will be substituted by the host name to
525 The command can be basically anything,
526 and should read from its standard input and write to its standard output.
527 It should eventually connect an
529 server running on some machine, or execute
532 Host key management will be done using the
533 HostName of the host being connected (defaulting to the name typed by
535 Setting the command to
537 disables this option entirely.
540 is not available for connects with a proxy command.
542 .It Cm PubkeyAuthentication
543 Specifies whether to try public key authentication.
544 The argument to this keyword must be
550 This option applies to protocol version 2 only.
552 Specifies that a TCP/IP port on the remote machine be forwarded over
553 the secure channel to the specified host and port from the local machine.
554 The first argument must be a port number, and the second must be
556 IPv6 addresses can be specified with an alternative syntax:
558 Multiple forwardings may be specified, and additional
559 forwardings can be given on the command line.
560 Only the superuser can forward privileged ports.
561 .It Cm RhostsRSAAuthentication
562 Specifies whether to try rhosts based authentication with RSA host
570 This option applies to protocol version 1 only and requires
573 .It Cm RSAAuthentication
574 Specifies whether to try RSA authentication.
575 The argument to this keyword must be
579 RSA authentication will only be
580 attempted if the identity file exists, or an authentication agent is
584 Note that this option applies to protocol version 1 only.
586 Specifies what variables from the local
588 should be sent to the server.
589 Note that environment passing is only supported for protocol 2, the
590 server must also support it, and the server must be configured to
591 accept these environment variables.
596 for how to configure the server.
597 Variables are specified by name, which may contain the wildcard characters
601 Multiple environment variables may be separated by whitespace or spread
605 The default is not to send any environment variables.
606 .It Cm ServerAliveInterval
607 Sets a timeout interval in seconds after which if no data has been received
610 will send a message through the encrypted
611 channel to request a response from the server.
613 is 0, indicating that these messages will not be sent to the server.
614 This option applies to protocol version 2 only.
615 .It Cm ServerAliveCountMax
616 Sets the number of server alive messages (see above) which may be
619 receiving any messages back from the server.
620 If this threshold is reached while server alive messages are being sent,
622 will disconnect from the server, terminating the session.
623 It is important to note that the use of server alive messages is very
627 The server alive messages are sent through the encrypted channel
628 and therefore will not be spoofable.
629 The TCP keepalive option enabled by
632 The server alive mechanism is valuable when the client or
633 server depend on knowing when a connection has become inactive.
635 The default value is 3.
637 .Cm ServerAliveInterval
638 (above) is set to 15, and
639 .Cm ServerAliveCountMax
640 is left at the default, if the server becomes unresponsive ssh
641 will disconnect after approximately 45 seconds.
642 .It Cm SmartcardDevice
643 Specifies which smartcard device to use.
644 The argument to this keyword is the device
646 should use to communicate with a smartcard used for storing the user's
648 By default, no device is specified and smartcard support is not activated.
649 .It Cm StrictHostKeyChecking
650 If this flag is set to
653 will never automatically add host keys to the
654 .Pa $HOME/.ssh/known_hosts
655 file, and refuses to connect to hosts whose host key has changed.
656 This provides maximum protection against trojan horse attacks,
657 however, can be annoying when the
658 .Pa /etc/ssh/ssh_known_hosts
659 file is poorly maintained, or connections to new hosts are
661 This option forces the user to manually
663 If this flag is set to
666 will automatically add new host keys to the
667 user known hosts files.
668 If this flag is set to
671 will be added to the user known host files only after the user
672 has confirmed that is what they really want to do, and
674 will refuse to connect to hosts whose host key has changed.
676 known hosts will be verified automatically in all cases.
685 Specifies whether the system should send TCP keepalive messages to the
687 If they are sent, death of the connection or crash of one
688 of the machines will be properly noticed.
689 However, this means that
690 connections will die if the route is down temporarily, and some people
695 (to send TCP keepalive messages), and the client will notice
696 if the network goes down or the remote host dies.
697 This is important in scripts, and many users want it too.
699 To disable TCP keepalive messages, the value should be set to
701 .It Cm UsePrivilegedPort
702 Specifies whether to use a privileged port for outgoing connections.
713 Note that this option must be set to
716 .Cm RhostsRSAAuthentication
719 Specifies the user to log in as.
720 This can be useful when a different user name is used on different machines.
721 This saves the trouble of
722 having to remember to give the user name on the command line.
723 .It Cm UserKnownHostsFile
724 Specifies a file to use for the user
725 host key database instead of
726 .Pa $HOME/.ssh/known_hosts .
727 .It Cm VerifyHostKeyDNS
728 Specifies whether to verify the remote key using DNS and SSHFP resource
730 If this option is set to
732 the client will implicitly trust keys that match a secure fingerprint
734 Insecure fingerprints will be handled as if this option was set to
736 If this option is set to
738 information on fingerprint match will be displayed, but the user will still
739 need to confirm new host keys according to the
740 .Cm StrictHostKeyChecking
749 Note that this option applies to protocol version 2 only.
751 Specifies the full pathname of the
755 .Pa /usr/X11R6/bin/xauth .
759 .It Pa $HOME/.ssh/config
760 This is the per-user configuration file.
761 The format of this file is described above.
762 This file is used by the
765 Because of the potential for abuse, this file must have strict permissions:
766 read/write for the user, and not accessible by others.
767 .It Pa /etc/ssh/ssh_config
768 Systemwide configuration file.
769 This file provides defaults for those
770 values that are not specified in the user's configuration file, and
771 for those users who do not have a configuration file.
772 This file must be world-readable.
777 OpenSSH is a derivative of the original and free
778 ssh 1.2.12 release by Tatu Ylonen.
779 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
780 Theo de Raadt and Dug Song
781 removed many bugs, re-added newer features and
783 Markus Friedl contributed the support for SSH
784 protocol versions 1.5 and 2.0.