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 1.9 2003/05/15 14:55:25 djm 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).
118 .It Cm AFSTokenPassing
119 Specifies whether to pass AFS tokens to remote host.
120 The argument to this keyword must be
124 This option applies to protocol version 1 only.
128 passphrase/password querying will be disabled.
129 This option is useful in scripts and other batch jobs where no user
130 is present to supply the password.
138 Specify the interface to transmit from on machines with multiple
139 interfaces or aliased addresses.
140 Note that this option does not work if
141 .Cm UsePrivilegedPort
144 .It Cm ChallengeResponseAuthentication
145 Specifies whether to use challenge response authentication.
146 The argument to this keyword must be
153 If this flag is set to
155 ssh will additionally check the host IP address in the
158 This allows ssh to detect if a host key changed due to DNS spoofing.
159 If the option is set to
161 the check will not be executed.
165 Specifies the cipher to use for encrypting the session
166 in protocol version 1.
174 is only supported in the
176 client for interoperability with legacy protocol 1 implementations
177 that do not support the
180 Its use is strongly discouraged due to cryptographic weaknesses.
184 Specifies the ciphers allowed for protocol version 2
185 in order of preference.
186 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. This value is
233 used only when the target is down or really unreachable, not when it
234 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 protocol is supported, and
243 will act as a SOCKS4 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.
248 Sets the escape character (default:
250 The escape character can also
251 be set on the command line.
252 The argument should be a single character,
254 followed by a letter, or
256 to disable the escape
257 character entirely (making the connection transparent for binary
260 Specifies whether the connection to the authentication agent (if any)
261 will be forwarded to the remote machine.
269 Agent forwarding should be enabled with caution.
270 Users with the ability to bypass file permissions on the remote host
271 (for the agent's Unix-domain socket)
272 can access the local agent through the forwarded connection.
273 An attacker cannot obtain key material from the agent,
274 however they can perform operations on the keys that enable them to
275 authenticate using the identities loaded into the agent.
277 Specifies whether X11 connections will be automatically redirected
278 over the secure channel and
288 X11 forwarding should be enabled with caution.
289 Users with the ability to bypass file permissions on the remote host
290 (for the user's X authorization database)
291 can access the local X11 display through the forwarded connection.
292 An attacker may then be able to perform activities such as keystroke monitoring.
294 Specifies whether remote hosts are allowed to connect to local
298 binds local port forwardings to the loopback address.
299 This prevents other remote hosts from connecting to forwarded ports.
301 can be used to specify that
303 should bind local port forwardings to the wildcard address,
304 thus allowing remote hosts to connect to forwarded ports.
311 .It Cm GlobalKnownHostsFile
312 Specifies a file to use for the global
313 host key database instead of
314 .Pa /etc/ssh/ssh_known_hosts .
315 .It Cm HostbasedAuthentication
316 Specifies whether to try rhosts based authentication with public key
324 This option applies to protocol version 2 only and
326 .Cm RhostsRSAAuthentication .
327 .It Cm HostKeyAlgorithms
328 Specifies the protocol version 2 host key algorithms
329 that the client wants to use in order of preference.
330 The default for this option is:
331 .Dq ssh-rsa,ssh-dss .
333 Specifies an alias that should be used instead of the
334 real host name when looking up or saving the host key
335 in the host key database files.
336 This option is useful for tunneling ssh connections
337 or for multiple servers running on a single host.
339 Specifies the real host name to log into.
340 This can be used to specify nicknames or abbreviations for hosts.
341 Default is the name given on the command line.
342 Numeric IP addresses are also permitted (both on the command line and in
346 Specifies a file from which the user's RSA or DSA authentication identity
347 is read. The default is
348 .Pa $HOME/.ssh/identity
349 for protocol version 1, and
350 .Pa $HOME/.ssh/id_rsa
352 .Pa $HOME/.ssh/id_dsa
353 for protocol version 2.
354 Additionally, any identities represented by the authentication agent
355 will be used for authentication.
356 The file name may use the tilde
357 syntax to refer to a user's home directory.
358 It is possible to have
359 multiple identity files specified in configuration files; all these
360 identities will be tried in sequence.
362 Specifies whether the system should send TCP keepalive messages to the
364 If they are sent, death of the connection or crash of one
365 of the machines will be properly noticed.
366 However, this means that
367 connections will die if the route is down temporarily, and some people
372 (to send keepalives), and the client will notice
373 if the network goes down or the remote host dies.
374 This is important in scripts, and many users want it too.
376 To disable keepalives, the value should be set to
378 .It Cm KerberosAuthentication
379 Specifies whether Kerberos authentication will be used.
380 The argument to this keyword must be
384 .It Cm KerberosTgtPassing
385 Specifies whether a Kerberos TGT will be forwarded to the server.
386 This will only work if the Kerberos server is actually an AFS kaserver.
387 The argument to this keyword must be
392 Specifies that a TCP/IP port on the local machine be forwarded over
393 the secure channel to the specified host and port from the remote machine.
394 The first argument must be a port number, and the second must be
396 IPv6 addresses can be specified with an alternative syntax:
398 Multiple forwardings may be specified, and additional
399 forwardings can be given on the command line.
400 Only the superuser can forward privileged ports.
402 Gives the verbosity level that is used when logging messages from
404 The possible values are:
405 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
407 DEBUG and DEBUG1 are equivalent.
408 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
410 Specifies the MAC (message authentication code) algorithms
411 in order of preference.
412 The MAC algorithm is used in protocol version 2
413 for data integrity protection.
414 Multiple algorithms must be comma-separated.
416 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
417 .It Cm NoHostAuthenticationForLocalhost
418 This option can be used if the home directory is shared across machines.
419 In this case localhost will refer to a different machine on each of
420 the machines and the user will get many warnings about changed host keys.
421 However, this option disables host authentication for localhost.
422 The argument to this keyword must be
426 The default is to check the host key for localhost.
427 .It Cm NumberOfPasswordPrompts
428 Specifies the number of password prompts before giving up.
429 The argument to this keyword must be an integer.
431 .It Cm PasswordAuthentication
432 Specifies whether to use password authentication.
433 The argument to this keyword must be
440 Specifies the port number to connect on the remote host.
442 .It Cm PreferredAuthentications
443 Specifies the order in which the client should try protocol 2
444 authentication methods. This allows a client to prefer one method (e.g.
445 .Cm keyboard-interactive )
446 over another method (e.g.
448 The default for this option is:
449 .Dq hostbased,publickey,keyboard-interactive,password .
451 Specifies the protocol versions
453 should support in order of preference.
454 The possible values are
458 Multiple versions must be comma-separated.
463 tries version 2 and falls back to version 1
464 if version 2 is not available.
466 Specifies the command to use to connect to the server.
468 string extends to the end of the line, and is executed with
470 In the command string,
472 will be substituted by the host name to
476 The command can be basically anything,
477 and should read from its standard input and write to its standard output.
478 It should eventually connect an
480 server running on some machine, or execute
483 Host key management will be done using the
484 HostName of the host being connected (defaulting to the name typed by
486 Setting the command to
488 disables this option entirely.
491 is not available for connects with a proxy command.
493 .It Cm PubkeyAuthentication
494 Specifies whether to try public key authentication.
495 The argument to this keyword must be
501 This option applies to protocol version 2 only.
503 Specifies that a TCP/IP port on the remote machine be forwarded over
504 the secure channel to the specified host and port from the local machine.
505 The first argument must be a port number, and the second must be
507 IPv6 addresses can be specified with an alternative syntax:
509 Multiple forwardings may be specified, and additional
510 forwardings can be given on the command line.
511 Only the superuser can forward privileged ports.
512 .It Cm RhostsAuthentication
513 Specifies whether to try rhosts based authentication.
515 declaration only affects the client side and has no effect whatsoever
517 Most servers do not permit RhostsAuthentication because it
519 .Cm RhostsRSAAuthentication ) .
520 The argument to this keyword must be
526 This option applies to protocol version 1 only and requires
528 to be setuid root and
529 .Cm UsePrivilegedPort
532 .It Cm RhostsRSAAuthentication
533 Specifies whether to try rhosts based authentication with RSA host
541 This option applies to protocol version 1 only and requires
544 .It Cm RSAAuthentication
545 Specifies whether to try RSA authentication.
546 The argument to this keyword must be
550 RSA authentication will only be
551 attempted if the identity file exists, or an authentication agent is
555 Note that this option applies to protocol version 1 only.
556 .It Cm SmartcardDevice
557 Specifies which smartcard device to use. The argument to this keyword is
560 should use to communicate with a smartcard used for storing the user's
561 private RSA key. By default, no device is specified and smartcard support
563 .It Cm StrictHostKeyChecking
564 If this flag is set to
567 will never automatically add host keys to the
568 .Pa $HOME/.ssh/known_hosts
569 file, and refuses to connect to hosts whose host key has changed.
570 This provides maximum protection against trojan horse attacks,
571 however, can be annoying when the
572 .Pa /etc/ssh/ssh_known_hosts
573 file is poorly maintained, or connections to new hosts are
575 This option forces the user to manually
577 If this flag is set to
580 will automatically add new host keys to the
581 user known hosts files.
582 If this flag is set to
585 will be added to the user known host files only after the user
586 has confirmed that is what they really want to do, and
588 will refuse to connect to hosts whose host key has changed.
590 known hosts will be verified automatically in all cases.
598 .It Cm UsePrivilegedPort
599 Specifies whether to use a privileged port for outgoing connections.
610 Note that this option must be set to
613 .Cm RhostsAuthentication
615 .Cm RhostsRSAAuthentication
616 authentications are needed with older servers.
618 Specifies the user to log in as.
619 This can be useful when a different user name is used on different machines.
620 This saves the trouble of
621 having to remember to give the user name on the command line.
622 .It Cm UserKnownHostsFile
623 Specifies a file to use for the user
624 host key database instead of
625 .Pa $HOME/.ssh/known_hosts .
626 .It Cm VerifyHostKeyDNS
627 Specifies whether to verify the remote key using DNS and SSHFP resource
632 Specifies the full pathname of the
636 .Pa /usr/X11R6/bin/xauth .
640 .It Pa $HOME/.ssh/config
641 This is the per-user configuration file.
642 The format of this file is described above.
643 This file is used by the
646 This file does not usually contain any sensitive information,
647 but the recommended permissions are read/write for the user, and not
648 accessible by others.
649 .It Pa /etc/ssh/ssh_config
650 Systemwide configuration file.
651 This file provides defaults for those
652 values that are not specified in the user's configuration file, and
653 for those users who do not have a configuration file.
654 This file must be world-readable.
657 OpenSSH is a derivative of the original and free
658 ssh 1.2.12 release by Tatu Ylonen.
659 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
660 Theo de Raadt and Dug Song
661 removed many bugs, re-added newer features and
663 Markus Friedl contributed the support for SSH
664 protocol versions 1.5 and 2.0.