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 1.11 2003/05/20 12:09:32 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.
126 .It Cm AFSTokenPassing
127 Specifies whether to pass AFS tokens to remote host.
128 The argument to this keyword must be
132 This option applies to protocol version 1 only.
136 passphrase/password querying will be disabled.
137 This option is useful in scripts and other batch jobs where no user
138 is present to supply the password.
146 Specify the interface to transmit from on machines with multiple
147 interfaces or aliased addresses.
148 Note that this option does not work if
149 .Cm UsePrivilegedPort
152 .It Cm ChallengeResponseAuthentication
153 Specifies whether to use challenge response authentication.
154 The argument to this keyword must be
161 If this flag is set to
163 ssh will additionally check the host IP address in the
166 This allows ssh to detect if a host key changed due to DNS spoofing.
167 If the option is set to
169 the check will not be executed.
173 Specifies the cipher to use for encrypting the session
174 in protocol version 1.
182 is only supported in the
184 client for interoperability with legacy protocol 1 implementations
185 that do not support the
188 Its use is strongly discouraged due to cryptographic weaknesses.
192 Specifies the ciphers allowed for protocol version 2
193 in order of preference.
194 Multiple ciphers must be comma-separated.
198 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
199 aes192-cbc,aes256-cbc''
201 .It Cm ClearAllForwardings
202 Specifies that all local, remote and dynamic port forwardings
203 specified in the configuration files or on the command line be
205 This option is primarily useful when used from the
207 command line to clear port forwardings set in
208 configuration files, and is automatically set by
219 Specifies whether to use compression.
226 .It Cm CompressionLevel
227 Specifies the compression level to use if compression is enabled.
228 The argument must be an integer from 1 (fast) to 9 (slow, best).
229 The default level is 6, which is good for most applications.
230 The meaning of the values is the same as in
232 Note that this option applies to protocol version 1 only.
233 .It Cm ConnectionAttempts
234 Specifies the number of tries (one per second) to make before exiting.
235 The argument must be an integer.
236 This may be useful in scripts if the connection sometimes fails.
238 .It Cm ConnectTimeout
239 Specifies the timeout (in seconds) used when connecting to the ssh
240 server, instead of using the default system TCP timeout.
241 This value is used only when the target is down or really unreachable,
242 not when it refuses the connection.
243 .It Cm DynamicForward
244 Specifies that a TCP/IP port on the local machine be forwarded
245 over the secure channel, and the application
246 protocol is then used to determine where to connect to from the
248 The argument must be a port number.
249 Currently the SOCKS4 protocol is supported, and
251 will act as a SOCKS4 server.
252 Multiple forwardings may be specified, and
253 additional forwardings can be given on the command line.
254 Only the superuser can forward privileged ports.
256 Sets the escape character (default:
258 The escape character can also
259 be set on the command line.
260 The argument should be a single character,
262 followed by a letter, or
264 to disable the escape
265 character entirely (making the connection transparent for binary
268 Specifies whether the connection to the authentication agent (if any)
269 will be forwarded to the remote machine.
277 Agent forwarding should be enabled with caution.
278 Users with the ability to bypass file permissions on the remote host
279 (for the agent's Unix-domain socket)
280 can access the local agent through the forwarded connection.
281 An attacker cannot obtain key material from the agent,
282 however they can perform operations on the keys that enable them to
283 authenticate using the identities loaded into the agent.
285 Specifies whether X11 connections will be automatically redirected
286 over the secure channel and
296 X11 forwarding should be enabled with caution.
297 Users with the ability to bypass file permissions on the remote host
298 (for the user's X authorization database)
299 can access the local X11 display through the forwarded connection.
300 An attacker may then be able to perform activities such as keystroke monitoring.
302 Specifies whether remote hosts are allowed to connect to local
306 binds local port forwardings to the loopback address.
307 This prevents other remote hosts from connecting to forwarded ports.
309 can be used to specify that
311 should bind local port forwardings to the wildcard address,
312 thus allowing remote hosts to connect to forwarded ports.
319 .It Cm GlobalKnownHostsFile
320 Specifies a file to use for the global
321 host key database instead of
322 .Pa /etc/ssh/ssh_known_hosts .
323 .It Cm HostbasedAuthentication
324 Specifies whether to try rhosts based authentication with public key
332 This option applies to protocol version 2 only and
334 .Cm RhostsRSAAuthentication .
335 .It Cm HostKeyAlgorithms
336 Specifies the protocol version 2 host key algorithms
337 that the client wants to use in order of preference.
338 The default for this option is:
339 .Dq ssh-rsa,ssh-dss .
341 Specifies an alias that should be used instead of the
342 real host name when looking up or saving the host key
343 in the host key database files.
344 This option is useful for tunneling ssh connections
345 or for multiple servers running on a single host.
347 Specifies the real host name to log into.
348 This can be used to specify nicknames or abbreviations for hosts.
349 Default is the name given on the command line.
350 Numeric IP addresses are also permitted (both on the command line and in
354 Specifies a file from which the user's RSA or DSA authentication identity
357 .Pa $HOME/.ssh/identity
358 for protocol version 1, and
359 .Pa $HOME/.ssh/id_rsa
361 .Pa $HOME/.ssh/id_dsa
362 for protocol version 2.
363 Additionally, any identities represented by the authentication agent
364 will be used for authentication.
365 The file name may use the tilde
366 syntax to refer to a user's home directory.
367 It is possible to have
368 multiple identity files specified in configuration files; all these
369 identities will be tried in sequence.
371 Specifies whether the system should send TCP keepalive messages to the
373 If they are sent, death of the connection or crash of one
374 of the machines will be properly noticed.
375 However, this means that
376 connections will die if the route is down temporarily, and some people
381 (to send keepalives), and the client will notice
382 if the network goes down or the remote host dies.
383 This is important in scripts, and many users want it too.
385 To disable keepalives, the value should be set to
387 .It Cm KerberosAuthentication
388 Specifies whether Kerberos authentication will be used.
389 The argument to this keyword must be
393 .It Cm KerberosTgtPassing
394 Specifies whether a Kerberos TGT will be forwarded to the server.
395 This will only work if the Kerberos server is actually an AFS kaserver.
396 The argument to this keyword must be
401 Specifies that a TCP/IP port on the local machine be forwarded over
402 the secure channel to the specified host and port from the remote machine.
403 The first argument must be a port number, and the second must be
405 IPv6 addresses can be specified with an alternative syntax:
407 Multiple forwardings may be specified, and additional
408 forwardings can be given on the command line.
409 Only the superuser can forward privileged ports.
411 Gives the verbosity level that is used when logging messages from
413 The possible values are:
414 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
416 DEBUG and DEBUG1 are equivalent.
417 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
419 Specifies the MAC (message authentication code) algorithms
420 in order of preference.
421 The MAC algorithm is used in protocol version 2
422 for data integrity protection.
423 Multiple algorithms must be comma-separated.
425 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
426 .It Cm NoHostAuthenticationForLocalhost
427 This option can be used if the home directory is shared across machines.
428 In this case localhost will refer to a different machine on each of
429 the machines and the user will get many warnings about changed host keys.
430 However, this option disables host authentication for localhost.
431 The argument to this keyword must be
435 The default is to check the host key for localhost.
436 .It Cm NumberOfPasswordPrompts
437 Specifies the number of password prompts before giving up.
438 The argument to this keyword must be an integer.
440 .It Cm PasswordAuthentication
441 Specifies whether to use password authentication.
442 The argument to this keyword must be
449 Specifies the port number to connect on the remote host.
451 .It Cm PreferredAuthentications
452 Specifies the order in which the client should try protocol 2
453 authentication methods.
454 This allows a client to prefer one method (e.g.
455 .Cm keyboard-interactive )
456 over another method (e.g.
458 The default for this option is:
459 .Dq hostbased,publickey,keyboard-interactive,password .
461 Specifies the protocol versions
463 should support in order of preference.
464 The possible values are
468 Multiple versions must be comma-separated.
473 tries version 2 and falls back to version 1
474 if version 2 is not available.
476 Specifies the command to use to connect to the server.
478 string extends to the end of the line, and is executed with
480 In the command string,
482 will be substituted by the host name to
486 The command can be basically anything,
487 and should read from its standard input and write to its standard output.
488 It should eventually connect an
490 server running on some machine, or execute
493 Host key management will be done using the
494 HostName of the host being connected (defaulting to the name typed by
496 Setting the command to
498 disables this option entirely.
501 is not available for connects with a proxy command.
503 .It Cm PubkeyAuthentication
504 Specifies whether to try public key authentication.
505 The argument to this keyword must be
511 This option applies to protocol version 2 only.
513 Specifies that a TCP/IP port on the remote machine be forwarded over
514 the secure channel to the specified host and port from the local machine.
515 The first argument must be a port number, and the second must be
517 IPv6 addresses can be specified with an alternative syntax:
519 Multiple forwardings may be specified, and additional
520 forwardings can be given on the command line.
521 Only the superuser can forward privileged ports.
522 .It Cm RhostsAuthentication
523 Specifies whether to try rhosts based authentication.
525 declaration only affects the client side and has no effect whatsoever
527 Most servers do not permit RhostsAuthentication because it
529 .Cm RhostsRSAAuthentication ) .
530 The argument to this keyword must be
536 This option applies to protocol version 1 only and requires
538 to be setuid root and
539 .Cm UsePrivilegedPort
542 .It Cm RhostsRSAAuthentication
543 Specifies whether to try rhosts based authentication with RSA host
551 This option applies to protocol version 1 only and requires
554 .It Cm RSAAuthentication
555 Specifies whether to try RSA authentication.
556 The argument to this keyword must be
560 RSA authentication will only be
561 attempted if the identity file exists, or an authentication agent is
565 Note that this option applies to protocol version 1 only.
566 .It Cm SmartcardDevice
567 Specifies which smartcard device to use.
568 The argument to this keyword is the device
570 should use to communicate with a smartcard used for storing the user's
572 By default, no device is specified and smartcard support is not activated.
573 .It Cm StrictHostKeyChecking
574 If this flag is set to
577 will never automatically add host keys to the
578 .Pa $HOME/.ssh/known_hosts
579 file, and refuses to connect to hosts whose host key has changed.
580 This provides maximum protection against trojan horse attacks,
581 however, can be annoying when the
582 .Pa /etc/ssh/ssh_known_hosts
583 file is poorly maintained, or connections to new hosts are
585 This option forces the user to manually
587 If this flag is set to
590 will automatically add new host keys to the
591 user known hosts files.
592 If this flag is set to
595 will be added to the user known host files only after the user
596 has confirmed that is what they really want to do, and
598 will refuse to connect to hosts whose host key has changed.
600 known hosts will be verified automatically in all cases.
608 .It Cm UsePrivilegedPort
609 Specifies whether to use a privileged port for outgoing connections.
620 Note that this option must be set to
623 .Cm RhostsAuthentication
625 .Cm RhostsRSAAuthentication
626 authentications are needed with older servers.
628 Specifies the user to log in as.
629 This can be useful when a different user name is used on different machines.
630 This saves the trouble of
631 having to remember to give the user name on the command line.
632 .It Cm UserKnownHostsFile
633 Specifies a file to use for the user
634 host key database instead of
635 .Pa $HOME/.ssh/known_hosts .
636 .It Cm VerifyHostKeyDNS
637 Specifies whether to verify the remote key using DNS and SSHFP resource
642 Specifies the full pathname of the
646 .Pa /usr/X11R6/bin/xauth .
650 .It Pa $HOME/.ssh/config
651 This is the per-user configuration file.
652 The format of this file is described above.
653 This file is used by the
656 This file does not usually contain any sensitive information,
657 but the recommended permissions are read/write for the user, and not
658 accessible by others.
659 .It Pa /etc/ssh/ssh_config
660 Systemwide configuration file.
661 This file provides defaults for those
662 values that are not specified in the user's configuration file, and
663 for those users who do not have a configuration file.
664 This file must be world-readable.
667 OpenSSH is a derivative of the original and free
668 ssh 1.2.12 release by Tatu Ylonen.
669 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
670 Theo de Raadt and Dug Song
671 removed many bugs, re-added newer features and
673 Markus Friedl contributed the support for SSH
674 protocol versions 1.5 and 2.0.