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 1.24 2003/11/12 16:39:58 jakob 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 authentication based on GSSAPI may be used, either using
354 the result of a successful key exchange, or using GSSAPI user
358 Note that this option applies to protocol version 2 only.
359 .It Cm GSSAPIDelegateCredentials
360 Forward (delegate) credentials to the server.
363 Note that this option applies to protocol version 2 only.
364 .It Cm HostbasedAuthentication
365 Specifies whether to try rhosts based authentication with public key
373 This option applies to protocol version 2 only and
375 .Cm RhostsRSAAuthentication .
376 .It Cm HostKeyAlgorithms
377 Specifies the protocol version 2 host key algorithms
378 that the client wants to use in order of preference.
379 The default for this option is:
380 .Dq ssh-rsa,ssh-dss .
382 Specifies an alias that should be used instead of the
383 real host name when looking up or saving the host key
384 in the host key database files.
385 This option is useful for tunneling ssh connections
386 or for multiple servers running on a single host.
388 Specifies the real host name to log into.
389 This can be used to specify nicknames or abbreviations for hosts.
390 Default is the name given on the command line.
391 Numeric IP addresses are also permitted (both on the command line and in
395 Specifies a file from which the user's RSA or DSA authentication identity
398 .Pa $HOME/.ssh/identity
399 for protocol version 1, and
400 .Pa $HOME/.ssh/id_rsa
402 .Pa $HOME/.ssh/id_dsa
403 for protocol version 2.
404 Additionally, any identities represented by the authentication agent
405 will be used for authentication.
406 The file name may use the tilde
407 syntax to refer to a user's home directory.
408 It is possible to have
409 multiple identity files specified in configuration files; all these
410 identities will be tried in sequence.
412 Specifies whether the system should send TCP keepalive messages to the
414 If they are sent, death of the connection or crash of one
415 of the machines will be properly noticed.
416 However, this means that
417 connections will die if the route is down temporarily, and some people
422 (to send keepalives), and the client will notice
423 if the network goes down or the remote host dies.
424 This is important in scripts, and many users want it too.
426 To disable keepalives, the value should be set to
429 Specifies that a TCP/IP port on the local machine be forwarded over
430 the secure channel to the specified host and port from the remote machine.
431 The first argument must be a port number, and the second must be
433 IPv6 addresses can be specified with an alternative syntax:
435 Multiple forwardings may be specified, and additional
436 forwardings can be given on the command line.
437 Only the superuser can forward privileged ports.
439 Gives the verbosity level that is used when logging messages from
441 The possible values are:
442 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
444 DEBUG and DEBUG1 are equivalent.
445 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
447 Specifies the MAC (message authentication code) algorithms
448 in order of preference.
449 The MAC algorithm is used in protocol version 2
450 for data integrity protection.
451 Multiple algorithms must be comma-separated.
453 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
454 .It Cm NoHostAuthenticationForLocalhost
455 This option can be used if the home directory is shared across machines.
456 In this case localhost will refer to a different machine on each of
457 the machines and the user will get many warnings about changed host keys.
458 However, this option disables host authentication for localhost.
459 The argument to this keyword must be
463 The default is to check the host key for localhost.
464 .It Cm NumberOfPasswordPrompts
465 Specifies the number of password prompts before giving up.
466 The argument to this keyword must be an integer.
468 .It Cm PasswordAuthentication
469 Specifies whether to use password authentication.
470 The argument to this keyword must be
477 Specifies the port number to connect on the remote host.
479 .It Cm PreferredAuthentications
480 Specifies the order in which the client should try protocol 2
481 authentication methods.
482 This allows a client to prefer one method (e.g.
483 .Cm keyboard-interactive )
484 over another method (e.g.
486 The default for this option is:
487 .Dq hostbased,publickey,keyboard-interactive,password .
489 Specifies the protocol versions
491 should support in order of preference.
492 The possible values are
496 Multiple versions must be comma-separated.
501 tries version 2 and falls back to version 1
502 if version 2 is not available.
504 Specifies the command to use to connect to the server.
506 string extends to the end of the line, and is executed with
508 In the command string,
510 will be substituted by the host name to
514 The command can be basically anything,
515 and should read from its standard input and write to its standard output.
516 It should eventually connect an
518 server running on some machine, or execute
521 Host key management will be done using the
522 HostName of the host being connected (defaulting to the name typed by
524 Setting the command to
526 disables this option entirely.
529 is not available for connects with a proxy command.
531 .It Cm PubkeyAuthentication
532 Specifies whether to try public key authentication.
533 The argument to this keyword must be
539 This option applies to protocol version 2 only.
541 Specifies that a TCP/IP port on the remote machine be forwarded over
542 the secure channel to the specified host and port from the local machine.
543 The first argument must be a port number, and the second must be
545 IPv6 addresses can be specified with an alternative syntax:
547 Multiple forwardings may be specified, and additional
548 forwardings can be given on the command line.
549 Only the superuser can forward privileged ports.
550 .It Cm RhostsRSAAuthentication
551 Specifies whether to try rhosts based authentication with RSA host
559 This option applies to protocol version 1 only and requires
562 .It Cm RSAAuthentication
563 Specifies whether to try RSA authentication.
564 The argument to this keyword must be
568 RSA authentication will only be
569 attempted if the identity file exists, or an authentication agent is
573 Note that this option applies to protocol version 1 only.
574 .It Cm SmartcardDevice
575 Specifies which smartcard device to use.
576 The argument to this keyword is the device
578 should use to communicate with a smartcard used for storing the user's
580 By default, no device is specified and smartcard support is not activated.
581 .It Cm StrictHostKeyChecking
582 If this flag is set to
585 will never automatically add host keys to the
586 .Pa $HOME/.ssh/known_hosts
587 file, and refuses to connect to hosts whose host key has changed.
588 This provides maximum protection against trojan horse attacks,
589 however, can be annoying when the
590 .Pa /etc/ssh/ssh_known_hosts
591 file is poorly maintained, or connections to new hosts are
593 This option forces the user to manually
595 If this flag is set to
598 will automatically add new host keys to the
599 user known hosts files.
600 If this flag is set to
603 will be added to the user known host files only after the user
604 has confirmed that is what they really want to do, and
606 will refuse to connect to hosts whose host key has changed.
608 known hosts will be verified automatically in all cases.
616 .It Cm UsePrivilegedPort
617 Specifies whether to use a privileged port for outgoing connections.
628 Note that this option must be set to
631 .Cm RhostsRSAAuthentication
634 Specifies the user to log in as.
635 This can be useful when a different user name is used on different machines.
636 This saves the trouble of
637 having to remember to give the user name on the command line.
638 .It Cm UserKnownHostsFile
639 Specifies a file to use for the user
640 host key database instead of
641 .Pa $HOME/.ssh/known_hosts .
642 .It Cm VerifyHostKeyDNS
643 Specifies whether to verify the remote key using DNS and SSHFP resource
645 If this option is set to
647 the client will implicitly trust keys that matches a secure fingerprint
649 Insecure fingerprints will be handled as if this option was set to
651 If this option is set to
653 information on fingerprint match will be displayed, but the user will still
654 need to confirm new host keys according to the
655 .Cm StrictHostKeyChecking
664 Note that this option applies to protocol version 2 only.
666 Specifies the full pathname of the
670 .Pa /usr/X11R6/bin/xauth .
674 .It Pa $HOME/.ssh/config
675 This is the per-user configuration file.
676 The format of this file is described above.
677 This file is used by the
680 This file does not usually contain any sensitive information,
681 but the recommended permissions are read/write for the user, and not
682 accessible by others.
683 .It Pa /etc/ssh/ssh_config
684 Systemwide configuration file.
685 This file provides defaults for those
686 values that are not specified in the user's configuration file, and
687 for those users who do not have a configuration file.
688 This file must be world-readable.
693 OpenSSH is a derivative of the original and free
694 ssh 1.2.12 release by Tatu Ylonen.
695 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
696 Theo de Raadt and Dug Song
697 removed many bugs, re-added newer features and
699 Markus Friedl contributed the support for SSH
700 protocol versions 1.5 and 2.0.