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 1.7 2003/03/28 10:11:43 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 GSSAPI configuration file
61 .Pq Pa $HOME/.ssh/config.gssapi
63 Kerberos configuration file
64 .Pq Pa $HOME/.ssh/config.krb
66 AFS configuration file
67 .Pq Pa $HOME/.ssh/config.afs
69 system-wide configuration file
70 .Pq Pa /etc/ssh/ssh_config
73 For each parameter, the first obtained value
75 The configuration files contain sections bracketed by
77 specifications, and that section is only applied for hosts that
78 match one of the patterns given in the specification.
79 The matched host name is the one given on the command line.
81 Since the first obtained value for each parameter is used, more
82 host-specific declarations should be given near the beginning of the
83 file, and general defaults at the end.
85 The configuration file has the following format:
87 Empty lines and lines starting with
91 Otherwise a line is of the format
92 .Dq keyword arguments .
93 Configuration options may be separated by whitespace or
94 optional whitespace and exactly one
96 the latter format is useful to avoid the need to quote whitespace
97 when specifying configuration options using the
106 keywords and their meanings are as follows (note that
107 keywords are case-insensitive and arguments are case-sensitive):
110 Restricts the following declarations (up to the next
112 keyword) to be only for those hosts that match one of the patterns
113 given after the keyword.
117 can be used as wildcards in the
121 as a pattern can be used to provide global
122 defaults for all hosts.
125 argument given on the command line (i.e., the name is not converted to
126 a canonicalized host name before matching).
127 .It Cm AFSTokenPassing
128 Specifies whether to pass AFS tokens to remote host.
129 The argument to this keyword must be
133 This option applies to protocol version 1 only.
137 passphrase/password querying will be disabled.
138 This option is useful in scripts and other batch jobs where no user
139 is present to supply the password.
147 Specify the interface to transmit from on machines with multiple
148 interfaces or aliased addresses.
149 Note that this option does not work if
150 .Cm UsePrivilegedPort
153 .It Cm ChallengeResponseAuthentication
154 Specifies whether to use challenge response authentication.
155 The argument to this keyword must be
162 If this flag is set to
164 ssh will additionally check the host IP address in the
167 This allows ssh to detect if a host key changed due to DNS spoofing.
168 If the option is set to
170 the check will not be executed.
174 Specifies the cipher to use for encrypting the session
175 in protocol version 1.
183 is only supported in the
185 client for interoperability with legacy protocol 1 implementations
186 that do not support the
189 Its use is strongly discouraged due to cryptographic weaknesses.
193 Specifies the ciphers allowed for protocol version 2
194 in order of preference.
195 Multiple ciphers must be comma-separated.
199 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
200 aes192-cbc,aes256-cbc''
202 .It Cm ClearAllForwardings
203 Specifies that all local, remote and dynamic port forwardings
204 specified in the configuration files or on the command line be
206 This option is primarily useful when used from the
208 command line to clear port forwardings set in
209 configuration files, and is automatically set by
220 Specifies whether to use compression.
227 .It Cm CompressionLevel
228 Specifies the compression level to use if compression is enabled.
229 The argument must be an integer from 1 (fast) to 9 (slow, best).
230 The default level is 6, which is good for most applications.
231 The meaning of the values is the same as in
233 Note that this option applies to protocol version 1 only.
234 .It Cm ConnectionAttempts
235 Specifies the number of tries (one per second) to make before exiting.
236 The argument must be an integer.
237 This may be useful in scripts if the connection sometimes fails.
239 .It Cm DynamicForward
240 Specifies that a TCP/IP port on the local machine be forwarded
241 over the secure channel, and the application
242 protocol is then used to determine where to connect to from the
244 The argument must be a port number.
245 Currently the SOCKS4 protocol is supported, and
247 will act as a SOCKS4 server.
248 Multiple forwardings may be specified, and
249 additional forwardings can be given on the command line.
250 Only the superuser can forward privileged ports.
252 Sets the escape character (default:
254 The escape character can also
255 be set on the command line.
256 The argument should be a single character,
258 followed by a letter, or
260 to disable the escape
261 character entirely (making the connection transparent for binary
264 Specifies whether the connection to the authentication agent (if any)
265 will be forwarded to the remote machine.
273 Agent forwarding should be enabled with caution.
274 Users with the ability to bypass file permissions on the remote host
275 (for the agent's Unix-domain socket)
276 can access the local agent through the forwarded connection.
277 An attacker cannot obtain key material from the agent,
278 however they can perform operations on the keys that enable them to
279 authenticate using the identities loaded into the agent.
281 Specifies whether X11 connections will be automatically redirected
282 over the secure channel and
292 X11 forwarding should be enabled with caution.
293 Users with the ability to bypass file permissions on the remote host
294 (for the user's X authorization database)
295 can access the local X11 display through the forwarded connection.
296 An attacker may then be able to perform activities such as keystroke monitoring.
298 Specifies whether remote hosts are allowed to connect to local
302 binds local port forwardings to the loopback address.
303 This prevents other remote hosts from connecting to forwarded ports.
305 can be used to specify that
307 should bind local port forwardings to the wildcard address,
308 thus allowing remote hosts to connect to forwarded ports.
315 .It Cm GlobalKnownHostsFile
316 Specifies a file to use for the global
317 host key database instead of
318 .Pa /etc/ssh/ssh_known_hosts .
319 .It Cm GssapiAuthentication
320 Specifies whether authentication based on GSSAPI may be used, either using
321 the result of a successful key exchange, or using GSSAPI user
325 .It Cm GssapiKeyExchange
326 Specifies whether key exchange based on GSSAPI may be used. When using
327 GSSAPI key exchange the server need not have a host key.
330 .It Cm GssapiDelegateCredentials
331 Specifies whether GSSAPI credentials will be delegated (forwarded) to
335 .It Cm HostbasedAuthentication
336 Specifies whether to try rhosts based authentication with public key
344 This option applies to protocol version 2 only and
346 .Cm RhostsRSAAuthentication .
347 .It Cm HostKeyAlgorithms
348 Specifies the protocol version 2 host key algorithms
349 that the client wants to use in order of preference.
350 The default for this option is:
351 .Dq ssh-rsa,ssh-dss .
353 Specifies an alias that should be used instead of the
354 real host name when looking up or saving the host key
355 in the host key database files.
356 This option is useful for tunneling ssh connections
357 or for multiple servers running on a single host.
359 Specifies the real host name to log into.
360 This can be used to specify nicknames or abbreviations for hosts.
361 Default is the name given on the command line.
362 Numeric IP addresses are also permitted (both on the command line and in
366 Specifies a file from which the user's RSA or DSA authentication identity
367 is read. The default is
368 .Pa $HOME/.ssh/identity
369 for protocol version 1, and
370 .Pa $HOME/.ssh/id_rsa
372 .Pa $HOME/.ssh/id_dsa
373 for protocol version 2.
374 Additionally, any identities represented by the authentication agent
375 will be used for authentication.
376 The file name may use the tilde
377 syntax to refer to a user's home directory.
378 It is possible to have
379 multiple identity files specified in configuration files; all these
380 identities will be tried in sequence.
382 Specifies whether the system should send TCP keepalive messages to the
384 If they are sent, death of the connection or crash of one
385 of the machines will be properly noticed.
386 However, this means that
387 connections will die if the route is down temporarily, and some people
392 (to send keepalives), and the client will notice
393 if the network goes down or the remote host dies.
394 This is important in scripts, and many users want it too.
396 To disable keepalives, the value should be set to
398 .It Cm KerberosAuthentication
399 Specifies whether Kerberos authentication will be used.
400 The argument to this keyword must be
404 .It Cm KerberosTgtPassing
405 Specifies whether a Kerberos TGT will be forwarded to the server.
406 This will only work if the Kerberos server is actually an AFS kaserver.
407 The argument to this keyword must be
412 Specifies that a TCP/IP port on the local machine be forwarded over
413 the secure channel to the specified host and port from the remote machine.
414 The first argument must be a port number, and the second must be
416 IPv6 addresses can be specified with an alternative syntax:
418 Multiple forwardings may be specified, and additional
419 forwardings can be given on the command line.
420 Only the superuser can forward privileged ports.
422 Gives the verbosity level that is used when logging messages from
424 The possible values are:
425 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
427 DEBUG and DEBUG1 are equivalent.
428 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
430 Specifies the MAC (message authentication code) algorithms
431 in order of preference.
432 The MAC algorithm is used in protocol version 2
433 for data integrity protection.
434 Multiple algorithms must be comma-separated.
436 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
437 .It Cm NoHostAuthenticationForLocalhost
438 This option can be used if the home directory is shared across machines.
439 In this case localhost will refer to a different machine on each of
440 the machines and the user will get many warnings about changed host keys.
441 However, this option disables host authentication for localhost.
442 The argument to this keyword must be
446 The default is to check the host key for localhost.
447 .It Cm NumberOfPasswordPrompts
448 Specifies the number of password prompts before giving up.
449 The argument to this keyword must be an integer.
451 .It Cm PasswordAuthentication
452 Specifies whether to use password authentication.
453 The argument to this keyword must be
460 Specifies the port number to connect on the remote host.
462 .It Cm PreferredAuthentications
463 Specifies the order in which the client should try protocol 2
464 authentication methods. 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,external-keyx,gssapi,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 RhostsAuthentication
533 Specifies whether to try rhosts based authentication.
535 declaration only affects the client side and has no effect whatsoever
537 Most servers do not permit RhostsAuthentication because it
539 .Cm RhostsRSAAuthentication ) .
540 The argument to this keyword must be
546 This option applies to protocol version 1 only and requires
548 to be setuid root and
549 .Cm UsePrivilegedPort
552 .It Cm RhostsRSAAuthentication
553 Specifies whether to try rhosts based authentication with RSA host
561 This option applies to protocol version 1 only and requires
564 .It Cm RSAAuthentication
565 Specifies whether to try RSA authentication.
566 The argument to this keyword must be
570 RSA authentication will only be
571 attempted if the identity file exists, or an authentication agent is
575 Note that this option applies to protocol version 1 only.
576 .It Cm SmartcardDevice
577 Specifies which smartcard device to use. The argument to this keyword is
580 should use to communicate with a smartcard used for storing the user's
581 private RSA key. By default, no device is specified and smartcard support
583 .It Cm StrictHostKeyChecking
584 If this flag is set to
587 will never automatically add host keys to the
588 .Pa $HOME/.ssh/known_hosts
589 file, and refuses to connect to hosts whose host key has changed.
590 This provides maximum protection against trojan horse attacks,
591 however, can be annoying when the
592 .Pa /etc/ssh/ssh_known_hosts
593 file is poorly maintained, or connections to new hosts are
595 This option forces the user to manually
597 If this flag is set to
600 will automatically add new host keys to the
601 user known hosts files.
602 If this flag is set to
605 will be added to the user known host files only after the user
606 has confirmed that is what they really want to do, and
608 will refuse to connect to hosts whose host key has changed.
610 known hosts will be verified automatically in all cases.
618 .It Cm UsePrivilegedPort
619 Specifies whether to use a privileged port for outgoing connections.
630 Note that this option must be set to
633 .Cm RhostsAuthentication
635 .Cm RhostsRSAAuthentication
636 authentications are needed with older servers.
638 Specifies the user to log in as.
639 This can be useful when a different user name is used on different machines.
640 This saves the trouble of
641 having to remember to give the user name on the command line.
642 .It Cm UserKnownHostsFile
643 Specifies a file to use for the user
644 host key database instead of
645 .Pa $HOME/.ssh/known_hosts .
647 Specifies the full pathname of the
651 .Pa /usr/X11R6/bin/xauth .
655 .It Pa $HOME/.ssh/config
656 This is the per-user configuration file.
657 The format of this file is described above.
658 This file is used by the
661 This file does not usually contain any sensitive information,
662 but the recommended permissions are read/write for the user, and not
663 accessible by others.
664 .It Pa /etc/ssh/ssh_config
665 Systemwide configuration file.
666 This file provides defaults for those
667 values that are not specified in the user's configuration file, and
668 for those users who do not have a configuration file.
669 This file must be world-readable.
672 OpenSSH is a derivative of the original and free
673 ssh 1.2.12 release by Tatu Ylonen.
674 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
675 Theo de Raadt and Dug Song
676 removed many bugs, re-added newer features and
678 Markus Friedl contributed the support for SSH
679 protocol versions 1.5 and 2.0.