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 1.29 2004/03/05 10:53:58 markus 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.
190 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
191 aes192-cbc,aes256-cbc''
193 .It Cm ClearAllForwardings
194 Specifies that all local, remote and dynamic port forwardings
195 specified in the configuration files or on the command line be
197 This option is primarily useful when used from the
199 command line to clear port forwardings set in
200 configuration files, and is automatically set by
211 Specifies whether to use compression.
218 .It Cm CompressionLevel
219 Specifies the compression level to use if compression is enabled.
220 The argument must be an integer from 1 (fast) to 9 (slow, best).
221 The default level is 6, which is good for most applications.
222 The meaning of the values is the same as in
224 Note that this option applies to protocol version 1 only.
225 .It Cm ConnectionAttempts
226 Specifies the number of tries (one per second) to make before exiting.
227 The argument must be an integer.
228 This may be useful in scripts if the connection sometimes fails.
230 .It Cm ConnectTimeout
231 Specifies the timeout (in seconds) used when connecting to the ssh
232 server, instead of using the default system TCP timeout.
233 This value is used only when the target is down or really unreachable,
234 not when it refuses the connection.
235 .It Cm DynamicForward
236 Specifies that a TCP/IP port on the local machine be forwarded
237 over the secure channel, and the application
238 protocol is then used to determine where to connect to from the
240 The argument must be a port number.
241 Currently the SOCKS4 and SOCKS5 protocols are supported, and
243 will act as a SOCKS server.
244 Multiple forwardings may be specified, and
245 additional forwardings can be given on the command line.
246 Only the superuser can forward privileged ports.
247 .It Cm EnableSSHKeysign
248 Setting this option to
250 in the global client configuration file
251 .Pa /etc/ssh/ssh_config
252 enables the use of the helper program
255 .Cm HostbasedAuthentication .
262 This option should be placed in the non-hostspecific section.
265 for more information.
267 Sets the escape character (default:
269 The escape character can also
270 be set on the command line.
271 The argument should be a single character,
273 followed by a letter, or
275 to disable the escape
276 character entirely (making the connection transparent for binary
279 Specifies whether the connection to the authentication agent (if any)
280 will be forwarded to the remote machine.
288 Agent forwarding should be enabled with caution.
289 Users with the ability to bypass file permissions on the remote host
290 (for the agent's Unix-domain socket)
291 can access the local agent through the forwarded connection.
292 An attacker cannot obtain key material from the agent,
293 however they can perform operations on the keys that enable them to
294 authenticate using the identities loaded into the agent.
296 Specifies whether X11 connections will be automatically redirected
297 over the secure channel and
307 X11 forwarding should be enabled with caution.
308 Users with the ability to bypass file permissions on the remote host
309 (for the user's X11 authorization database)
310 can access the local X11 display through the forwarded connection.
311 An attacker may then be able to perform activities such as keystroke monitoring
313 .Cm ForwardX11Trusted
314 option is also enabled.
315 .It Cm ForwardX11Trusted
316 If the this option is set to
318 then remote X11 clients will have full access to the original X11 display.
319 If this option is set to
321 then remote X11 clients will be considered untrusted and prevented
322 from stealing or tampering with data belonging to trusted X11
328 See the X11 SECURITY extension specification for full details on
329 the restrictions imposed on untrusted clients.
331 Specifies whether remote hosts are allowed to connect to local
335 binds local port forwardings to the loopback address.
336 This prevents other remote hosts from connecting to forwarded ports.
338 can be used to specify that
340 should bind local port forwardings to the wildcard address,
341 thus allowing remote hosts to connect to forwarded ports.
348 .It Cm GlobalKnownHostsFile
349 Specifies a file to use for the global
350 host key database instead of
351 .Pa /etc/ssh/ssh_known_hosts .
352 .It Cm GSSAPIAuthentication
353 Specifies whether user authentication based on GSSAPI is allowed.
356 Note that this option applies to protocol version 2 only.
357 .It Cm GSSAPIDelegateCredentials
358 Forward (delegate) credentials to the server.
361 Note that this option applies to protocol version 2 only.
362 .It Cm HostbasedAuthentication
363 Specifies whether to try rhosts based authentication with public key
371 This option applies to protocol version 2 only and
373 .Cm RhostsRSAAuthentication .
374 .It Cm HostKeyAlgorithms
375 Specifies the protocol version 2 host key algorithms
376 that the client wants to use in order of preference.
377 The default for this option is:
378 .Dq ssh-rsa,ssh-dss .
380 Specifies an alias that should be used instead of the
381 real host name when looking up or saving the host key
382 in the host key database files.
383 This option is useful for tunneling ssh connections
384 or for multiple servers running on a single host.
386 Specifies the real host name to log into.
387 This can be used to specify nicknames or abbreviations for hosts.
388 Default is the name given on the command line.
389 Numeric IP addresses are also permitted (both on the command line and in
393 Specifies a file from which the user's RSA or DSA authentication identity
396 .Pa $HOME/.ssh/identity
397 for protocol version 1, and
398 .Pa $HOME/.ssh/id_rsa
400 .Pa $HOME/.ssh/id_dsa
401 for protocol version 2.
402 Additionally, any identities represented by the authentication agent
403 will be used for authentication.
404 The file name may use the tilde
405 syntax to refer to a user's home directory.
406 It is possible to have
407 multiple identity files specified in configuration files; all these
408 identities will be tried in sequence.
409 .It Cm IdentitiesOnly
412 should only use the authentication identity files configured in the
417 offers more identities.
418 The argument to this keyword must be
422 This option is intented for situations where
424 offers many different identities.
428 Specifies that a TCP/IP port on the local machine be forwarded over
429 the secure channel to the specified host and port from the remote machine.
430 The first argument must be a port number, and the second must be
432 IPv6 addresses can be specified with an alternative syntax:
434 Multiple forwardings may be specified, and additional
435 forwardings can be given on the command line.
436 Only the superuser can forward privileged ports.
438 Gives the verbosity level that is used when logging messages from
440 The possible values are:
441 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
443 DEBUG and DEBUG1 are equivalent.
444 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
446 Specifies the MAC (message authentication code) algorithms
447 in order of preference.
448 The MAC algorithm is used in protocol version 2
449 for data integrity protection.
450 Multiple algorithms must be comma-separated.
452 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
453 .It Cm NoHostAuthenticationForLocalhost
454 This option can be used if the home directory is shared across machines.
455 In this case localhost will refer to a different machine on each of
456 the machines and the user will get many warnings about changed host keys.
457 However, this option disables host authentication for localhost.
458 The argument to this keyword must be
462 The default is to check the host key for localhost.
463 .It Cm NumberOfPasswordPrompts
464 Specifies the number of password prompts before giving up.
465 The argument to this keyword must be an integer.
467 .It Cm PasswordAuthentication
468 Specifies whether to use password authentication.
469 The argument to this keyword must be
476 Specifies the port number to connect on the remote host.
478 .It Cm PreferredAuthentications
479 Specifies the order in which the client should try protocol 2
480 authentication methods.
481 This allows a client to prefer one method (e.g.
482 .Cm keyboard-interactive )
483 over another method (e.g.
485 The default for this option is:
486 .Dq hostbased,publickey,keyboard-interactive,password .
488 Specifies the protocol versions
490 should support in order of preference.
491 The possible values are
495 Multiple versions must be comma-separated.
500 tries version 2 and falls back to version 1
501 if version 2 is not available.
503 Specifies the command to use to connect to the server.
505 string extends to the end of the line, and is executed with
507 In the command string,
509 will be substituted by the host name to
513 The command can be basically anything,
514 and should read from its standard input and write to its standard output.
515 It should eventually connect an
517 server running on some machine, or execute
520 Host key management will be done using the
521 HostName of the host being connected (defaulting to the name typed by
523 Setting the command to
525 disables this option entirely.
528 is not available for connects with a proxy command.
530 .It Cm PubkeyAuthentication
531 Specifies whether to try public key authentication.
532 The argument to this keyword must be
538 This option applies to protocol version 2 only.
540 Specifies that a TCP/IP port on the remote machine be forwarded over
541 the secure channel to the specified host and port from the local machine.
542 The first argument must be a port number, and the second must be
544 IPv6 addresses can be specified with an alternative syntax:
546 Multiple forwardings may be specified, and additional
547 forwardings can be given on the command line.
548 Only the superuser can forward privileged ports.
549 .It Cm RhostsRSAAuthentication
550 Specifies whether to try rhosts based authentication with RSA host
558 This option applies to protocol version 1 only and requires
561 .It Cm RSAAuthentication
562 Specifies whether to try RSA authentication.
563 The argument to this keyword must be
567 RSA authentication will only be
568 attempted if the identity file exists, or an authentication agent is
572 Note that this option applies to protocol version 1 only.
573 .It Cm ServerAliveInterval
574 Sets a timeout interval in seconds after which if no data has been received
577 will send a message through the encrypted
578 channel to request a response from the server.
580 is 0, indicating that these messages will not be sent to the server.
581 This option applies to protocol version 2 only.
582 .It Cm ServerAliveCountMax
583 Sets the number of server alive messages (see above) which may be
586 receiving any messages back from the server.
587 If this threshold is reached while server alive messages are being sent,
589 will disconnect from the server, terminating the session.
590 It is important to note that the use of server alive messages is very
594 The server alive messages are sent through the encrypted channel
595 and therefore will not be spoofable.
596 The TCP keepalive option enabled by
599 The server alive mechanism is valuable when the client or
600 server depend on knowing when a connection has become inactive.
602 The default value is 3.
604 .Cm ServerAliveInterval
605 (above) is set to 15, and
606 .Cm ServerAliveCountMax
607 is left at the default, if the server becomes unresponsive ssh
608 will disconnect after approximately 45 seconds.
609 .It Cm SmartcardDevice
610 Specifies which smartcard device to use.
611 The argument to this keyword is the device
613 should use to communicate with a smartcard used for storing the user's
615 By default, no device is specified and smartcard support is not activated.
616 .It Cm StrictHostKeyChecking
617 If this flag is set to
620 will never automatically add host keys to the
621 .Pa $HOME/.ssh/known_hosts
622 file, and refuses to connect to hosts whose host key has changed.
623 This provides maximum protection against trojan horse attacks,
624 however, can be annoying when the
625 .Pa /etc/ssh/ssh_known_hosts
626 file is poorly maintained, or connections to new hosts are
628 This option forces the user to manually
630 If this flag is set to
633 will automatically add new host keys to the
634 user known hosts files.
635 If this flag is set to
638 will be added to the user known host files only after the user
639 has confirmed that is what they really want to do, and
641 will refuse to connect to hosts whose host key has changed.
643 known hosts will be verified automatically in all cases.
652 Specifies whether the system should send TCP keepalive messages to the
654 If they are sent, death of the connection or crash of one
655 of the machines will be properly noticed.
656 However, this means that
657 connections will die if the route is down temporarily, and some people
662 (to send TCP keepalive messages), and the client will notice
663 if the network goes down or the remote host dies.
664 This is important in scripts, and many users want it too.
666 To disable TCP keepalive messages, the value should be set to
668 .It Cm UsePrivilegedPort
669 Specifies whether to use a privileged port for outgoing connections.
680 Note that this option must be set to
683 .Cm RhostsRSAAuthentication
686 Specifies the user to log in as.
687 This can be useful when a different user name is used on different machines.
688 This saves the trouble of
689 having to remember to give the user name on the command line.
690 .It Cm UserKnownHostsFile
691 Specifies a file to use for the user
692 host key database instead of
693 .Pa $HOME/.ssh/known_hosts .
694 .It Cm VerifyHostKeyDNS
695 Specifies whether to verify the remote key using DNS and SSHFP resource
697 If this option is set to
699 the client will implicitly trust keys that match a secure fingerprint
701 Insecure fingerprints will be handled as if this option was set to
703 If this option is set to
705 information on fingerprint match will be displayed, but the user will still
706 need to confirm new host keys according to the
707 .Cm StrictHostKeyChecking
716 Note that this option applies to protocol version 2 only.
718 Specifies the full pathname of the
722 .Pa /usr/X11R6/bin/xauth .
726 .It Pa $HOME/.ssh/config
727 This is the per-user configuration file.
728 The format of this file is described above.
729 This file is used by the
732 This file does not usually contain any sensitive information,
733 but the recommended permissions are read/write for the user, and not
734 accessible by others.
735 .It Pa /etc/ssh/ssh_config
736 Systemwide configuration file.
737 This file provides defaults for those
738 values that are not specified in the user's configuration file, and
739 for those users who do not have a configuration file.
740 This file must be world-readable.
745 OpenSSH is a derivative of the original and free
746 ssh 1.2.12 release by Tatu Ylonen.
747 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
748 Theo de Raadt and Dug Song
749 removed many bugs, re-added newer features and
751 Markus Friedl contributed the support for SSH
752 protocol versions 1.5 and 2.0.