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 1.20 2003/09/02 18:50:06 jmc 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.
191 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
192 aes192-cbc,aes256-cbc''
194 .It Cm ClearAllForwardings
195 Specifies that all local, remote and dynamic port forwardings
196 specified in the configuration files or on the command line be
198 This option is primarily useful when used from the
200 command line to clear port forwardings set in
201 configuration files, and is automatically set by
212 Specifies whether to use compression.
219 .It Cm CompressionLevel
220 Specifies the compression level to use if compression is enabled.
221 The argument must be an integer from 1 (fast) to 9 (slow, best).
222 The default level is 6, which is good for most applications.
223 The meaning of the values is the same as in
225 Note that this option applies to protocol version 1 only.
226 .It Cm ConnectionAttempts
227 Specifies the number of tries (one per second) to make before exiting.
228 The argument must be an integer.
229 This may be useful in scripts if the connection sometimes fails.
231 .It Cm ConnectTimeout
232 Specifies the timeout (in seconds) used when connecting to the ssh
233 server, instead of using the default system TCP timeout.
234 This value is used only when the target is down or really unreachable,
235 not when it refuses the connection.
236 .It Cm DynamicForward
237 Specifies that a TCP/IP port on the local machine be forwarded
238 over the secure channel, and the application
239 protocol is then used to determine where to connect to from the
241 The argument must be a port number.
242 Currently the SOCKS4 and SOCKS5 protocols are supported, and
244 will act as a SOCKS server.
245 Multiple forwardings may be specified, and
246 additional forwardings can be given on the command line.
247 Only the superuser can forward privileged ports.
248 .It Cm EnableSSHKeysign
249 Setting this option to
251 in the global client configuration file
252 .Pa /etc/ssh/ssh_config
253 enables the use of the helper program
256 .Cm HostbasedAuthentication .
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 X 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 Specifies whether remote hosts are allowed to connect to local
317 binds local port forwardings to the loopback address.
318 This prevents other remote hosts from connecting to forwarded ports.
320 can be used to specify that
322 should bind local port forwardings to the wildcard address,
323 thus allowing remote hosts to connect to forwarded ports.
330 .It Cm GlobalKnownHostsFile
331 Specifies a file to use for the global
332 host key database instead of
333 .Pa /etc/ssh/ssh_known_hosts .
334 .It Cm GSSAPIAuthentication
335 Specifies whether authentication based on GSSAPI may be used, either using
336 the result of a successful key exchange, or using GSSAPI user
340 Note that this option applies to protocol version 2 only.
341 .It Cm GSSAPIDelegateCredentials
342 Forward (delegate) credentials to the server.
345 Note that this option applies to protocol version 2 only.
346 .It Cm HostbasedAuthentication
347 Specifies whether to try rhosts based authentication with public key
355 This option applies to protocol version 2 only and
357 .Cm RhostsRSAAuthentication .
358 .It Cm HostKeyAlgorithms
359 Specifies the protocol version 2 host key algorithms
360 that the client wants to use in order of preference.
361 The default for this option is:
362 .Dq ssh-rsa,ssh-dss .
364 Specifies an alias that should be used instead of the
365 real host name when looking up or saving the host key
366 in the host key database files.
367 This option is useful for tunneling ssh connections
368 or for multiple servers running on a single host.
370 Specifies the real host name to log into.
371 This can be used to specify nicknames or abbreviations for hosts.
372 Default is the name given on the command line.
373 Numeric IP addresses are also permitted (both on the command line and in
377 Specifies a file from which the user's RSA or DSA authentication identity
380 .Pa $HOME/.ssh/identity
381 for protocol version 1, and
382 .Pa $HOME/.ssh/id_rsa
384 .Pa $HOME/.ssh/id_dsa
385 for protocol version 2.
386 Additionally, any identities represented by the authentication agent
387 will be used for authentication.
388 The file name may use the tilde
389 syntax to refer to a user's home directory.
390 It is possible to have
391 multiple identity files specified in configuration files; all these
392 identities will be tried in sequence.
394 Specifies whether the system should send TCP keepalive messages to the
396 If they are sent, death of the connection or crash of one
397 of the machines will be properly noticed.
398 However, this means that
399 connections will die if the route is down temporarily, and some people
404 (to send keepalives), and the client will notice
405 if the network goes down or the remote host dies.
406 This is important in scripts, and many users want it too.
408 To disable keepalives, the value should be set to
411 Specifies that a TCP/IP port on the local machine be forwarded over
412 the secure channel to the specified host and port from the remote machine.
413 The first argument must be a port number, and the second must be
415 IPv6 addresses can be specified with an alternative syntax:
417 Multiple forwardings may be specified, and additional
418 forwardings can be given on the command line.
419 Only the superuser can forward privileged ports.
421 Gives the verbosity level that is used when logging messages from
423 The possible values are:
424 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
426 DEBUG and DEBUG1 are equivalent.
427 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
429 Specifies the MAC (message authentication code) algorithms
430 in order of preference.
431 The MAC algorithm is used in protocol version 2
432 for data integrity protection.
433 Multiple algorithms must be comma-separated.
435 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
436 .It Cm NoHostAuthenticationForLocalhost
437 This option can be used if the home directory is shared across machines.
438 In this case localhost will refer to a different machine on each of
439 the machines and the user will get many warnings about changed host keys.
440 However, this option disables host authentication for localhost.
441 The argument to this keyword must be
445 The default is to check the host key for localhost.
446 .It Cm NumberOfPasswordPrompts
447 Specifies the number of password prompts before giving up.
448 The argument to this keyword must be an integer.
450 .It Cm PasswordAuthentication
451 Specifies whether to use password authentication.
452 The argument to this keyword must be
459 Specifies the port number to connect on the remote host.
461 .It Cm PreferredAuthentications
462 Specifies the order in which the client should try protocol 2
463 authentication methods.
464 This allows a client to prefer one method (e.g.
465 .Cm keyboard-interactive )
466 over another method (e.g.
468 The default for this option is:
469 .Dq hostbased,publickey,keyboard-interactive,password .
471 Specifies the protocol versions
473 should support in order of preference.
474 The possible values are
478 Multiple versions must be comma-separated.
483 tries version 2 and falls back to version 1
484 if version 2 is not available.
486 Specifies the command to use to connect to the server.
488 string extends to the end of the line, and is executed with
490 In the command string,
492 will be substituted by the host name to
496 The command can be basically anything,
497 and should read from its standard input and write to its standard output.
498 It should eventually connect an
500 server running on some machine, or execute
503 Host key management will be done using the
504 HostName of the host being connected (defaulting to the name typed by
506 Setting the command to
508 disables this option entirely.
511 is not available for connects with a proxy command.
513 .It Cm PubkeyAuthentication
514 Specifies whether to try public key authentication.
515 The argument to this keyword must be
521 This option applies to protocol version 2 only.
523 Specifies that a TCP/IP port on the remote machine be forwarded over
524 the secure channel to the specified host and port from the local machine.
525 The first argument must be a port number, and the second must be
527 IPv6 addresses can be specified with an alternative syntax:
529 Multiple forwardings may be specified, and additional
530 forwardings can be given on the command line.
531 Only the superuser can forward privileged ports.
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.
558 The argument to this keyword is the device
560 should use to communicate with a smartcard used for storing the user's
562 By default, no device is specified and smartcard support is not activated.
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 RhostsRSAAuthentication
616 Specifies the user to log in as.
617 This can be useful when a different user name is used on different machines.
618 This saves the trouble of
619 having to remember to give the user name on the command line.
620 .It Cm UserKnownHostsFile
621 Specifies a file to use for the user
622 host key database instead of
623 .Pa $HOME/.ssh/known_hosts .
624 .It Cm VerifyHostKeyDNS
625 Specifies whether to verify the remote key using DNS and SSHFP resource
629 Note that this option applies to protocol version 2 only.
631 Specifies the full pathname of the
635 .Pa /usr/X11R6/bin/xauth .
639 .It Pa $HOME/.ssh/config
640 This is the per-user configuration file.
641 The format of this file is described above.
642 This file is used by the
645 This file does not usually contain any sensitive information,
646 but the recommended permissions are read/write for the user, and not
647 accessible by others.
648 .It Pa /etc/ssh/ssh_config
649 Systemwide configuration file.
650 This file provides defaults for those
651 values that are not specified in the user's configuration file, and
652 for those users who do not have a configuration file.
653 This file must be world-readable.
658 OpenSSH is a derivative of the original and free
659 ssh 1.2.12 release by Tatu Ylonen.
660 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
661 Theo de Raadt and Dug Song
662 removed many bugs, re-added newer features and
664 Markus Friedl contributed the support for SSH
665 protocol versions 1.5 and 2.0.