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 1.15 2003/07/02 14:51:16 markus 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 and SOCKS5 protocols are supported, and
251 will act as a SOCKS 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.
255 .It Cm EnableSSHKeysign
256 Setting this option to
258 in the global client configuration file
259 .Pa /etc/ssh/ssh_config
260 enables the use of the helper program
263 .Cm HostbasedAuthentication .
272 for more information.
274 Sets the escape character (default:
276 The escape character can also
277 be set on the command line.
278 The argument should be a single character,
280 followed by a letter, or
282 to disable the escape
283 character entirely (making the connection transparent for binary
286 Specifies whether the connection to the authentication agent (if any)
287 will be forwarded to the remote machine.
295 Agent forwarding should be enabled with caution.
296 Users with the ability to bypass file permissions on the remote host
297 (for the agent's Unix-domain socket)
298 can access the local agent through the forwarded connection.
299 An attacker cannot obtain key material from the agent,
300 however they can perform operations on the keys that enable them to
301 authenticate using the identities loaded into the agent.
303 Specifies whether X11 connections will be automatically redirected
304 over the secure channel and
314 X11 forwarding should be enabled with caution.
315 Users with the ability to bypass file permissions on the remote host
316 (for the user's X authorization database)
317 can access the local X11 display through the forwarded connection.
318 An attacker may then be able to perform activities such as keystroke monitoring.
320 Specifies whether remote hosts are allowed to connect to local
324 binds local port forwardings to the loopback address.
325 This prevents other remote hosts from connecting to forwarded ports.
327 can be used to specify that
329 should bind local port forwardings to the wildcard address,
330 thus allowing remote hosts to connect to forwarded ports.
337 .It Cm GlobalKnownHostsFile
338 Specifies a file to use for the global
339 host key database instead of
340 .Pa /etc/ssh/ssh_known_hosts .
341 .It Cm HostbasedAuthentication
342 Specifies whether to try rhosts based authentication with public key
350 This option applies to protocol version 2 only and
352 .Cm RhostsRSAAuthentication .
353 .It Cm HostKeyAlgorithms
354 Specifies the protocol version 2 host key algorithms
355 that the client wants to use in order of preference.
356 The default for this option is:
357 .Dq ssh-rsa,ssh-dss .
359 Specifies an alias that should be used instead of the
360 real host name when looking up or saving the host key
361 in the host key database files.
362 This option is useful for tunneling ssh connections
363 or for multiple servers running on a single host.
365 Specifies the real host name to log into.
366 This can be used to specify nicknames or abbreviations for hosts.
367 Default is the name given on the command line.
368 Numeric IP addresses are also permitted (both on the command line and in
372 Specifies a file from which the user's RSA or DSA authentication identity
375 .Pa $HOME/.ssh/identity
376 for protocol version 1, and
377 .Pa $HOME/.ssh/id_rsa
379 .Pa $HOME/.ssh/id_dsa
380 for protocol version 2.
381 Additionally, any identities represented by the authentication agent
382 will be used for authentication.
383 The file name may use the tilde
384 syntax to refer to a user's home directory.
385 It is possible to have
386 multiple identity files specified in configuration files; all these
387 identities will be tried in sequence.
389 Specifies whether the system should send TCP keepalive messages to the
391 If they are sent, death of the connection or crash of one
392 of the machines will be properly noticed.
393 However, this means that
394 connections will die if the route is down temporarily, and some people
399 (to send keepalives), and the client will notice
400 if the network goes down or the remote host dies.
401 This is important in scripts, and many users want it too.
403 To disable keepalives, the value should be set to
405 .It Cm KerberosAuthentication
406 Specifies whether Kerberos authentication will be used.
407 The argument to this keyword must be
411 .It Cm KerberosTgtPassing
412 Specifies whether a Kerberos TGT will be forwarded to the server.
413 This will only work if the Kerberos server is actually an AFS kaserver.
414 The argument to this keyword must be
419 Specifies that a TCP/IP port on the local machine be forwarded over
420 the secure channel to the specified host and port from the remote machine.
421 The first argument must be a port number, and the second must be
423 IPv6 addresses can be specified with an alternative syntax:
425 Multiple forwardings may be specified, and additional
426 forwardings can be given on the command line.
427 Only the superuser can forward privileged ports.
429 Gives the verbosity level that is used when logging messages from
431 The possible values are:
432 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
434 DEBUG and DEBUG1 are equivalent.
435 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
437 Specifies the MAC (message authentication code) algorithms
438 in order of preference.
439 The MAC algorithm is used in protocol version 2
440 for data integrity protection.
441 Multiple algorithms must be comma-separated.
443 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
444 .It Cm NoHostAuthenticationForLocalhost
445 This option can be used if the home directory is shared across machines.
446 In this case localhost will refer to a different machine on each of
447 the machines and the user will get many warnings about changed host keys.
448 However, this option disables host authentication for localhost.
449 The argument to this keyword must be
453 The default is to check the host key for localhost.
454 .It Cm NumberOfPasswordPrompts
455 Specifies the number of password prompts before giving up.
456 The argument to this keyword must be an integer.
458 .It Cm PasswordAuthentication
459 Specifies whether to use password authentication.
460 The argument to this keyword must be
467 Specifies the port number to connect on the remote host.
469 .It Cm PreferredAuthentications
470 Specifies the order in which the client should try protocol 2
471 authentication methods.
472 This allows a client to prefer one method (e.g.
473 .Cm keyboard-interactive )
474 over another method (e.g.
476 The default for this option is:
477 .Dq hostbased,publickey,keyboard-interactive,password .
479 Specifies the protocol versions
481 should support in order of preference.
482 The possible values are
486 Multiple versions must be comma-separated.
491 tries version 2 and falls back to version 1
492 if version 2 is not available.
494 Specifies the command to use to connect to the server.
496 string extends to the end of the line, and is executed with
498 In the command string,
500 will be substituted by the host name to
504 The command can be basically anything,
505 and should read from its standard input and write to its standard output.
506 It should eventually connect an
508 server running on some machine, or execute
511 Host key management will be done using the
512 HostName of the host being connected (defaulting to the name typed by
514 Setting the command to
516 disables this option entirely.
519 is not available for connects with a proxy command.
521 .It Cm PubkeyAuthentication
522 Specifies whether to try public key authentication.
523 The argument to this keyword must be
529 This option applies to protocol version 2 only.
531 Specifies that a TCP/IP port on the remote machine be forwarded over
532 the secure channel to the specified host and port from the local machine.
533 The first argument must be a port number, and the second must be
535 IPv6 addresses can be specified with an alternative syntax:
537 Multiple forwardings may be specified, and additional
538 forwardings can be given on the command line.
539 Only the superuser can forward privileged ports.
540 .It Cm RhostsAuthentication
541 Specifies whether to try rhosts based authentication.
543 declaration only affects the client side and has no effect whatsoever
545 Most servers do not permit RhostsAuthentication because it
547 .Cm RhostsRSAAuthentication ) .
548 The argument to this keyword must be
554 This option applies to protocol version 1 only and requires
556 to be setuid root and
557 .Cm UsePrivilegedPort
560 .It Cm RhostsRSAAuthentication
561 Specifies whether to try rhosts based authentication with RSA host
569 This option applies to protocol version 1 only and requires
572 .It Cm RSAAuthentication
573 Specifies whether to try RSA authentication.
574 The argument to this keyword must be
578 RSA authentication will only be
579 attempted if the identity file exists, or an authentication agent is
583 Note that this option applies to protocol version 1 only.
584 .It Cm SmartcardDevice
585 Specifies which smartcard device to use.
586 The argument to this keyword is the device
588 should use to communicate with a smartcard used for storing the user's
590 By default, no device is specified and smartcard support is not activated.
591 .It Cm StrictHostKeyChecking
592 If this flag is set to
595 will never automatically add host keys to the
596 .Pa $HOME/.ssh/known_hosts
597 file, and refuses to connect to hosts whose host key has changed.
598 This provides maximum protection against trojan horse attacks,
599 however, can be annoying when the
600 .Pa /etc/ssh/ssh_known_hosts
601 file is poorly maintained, or connections to new hosts are
603 This option forces the user to manually
605 If this flag is set to
608 will automatically add new host keys to the
609 user known hosts files.
610 If this flag is set to
613 will be added to the user known host files only after the user
614 has confirmed that is what they really want to do, and
616 will refuse to connect to hosts whose host key has changed.
618 known hosts will be verified automatically in all cases.
626 .It Cm UsePrivilegedPort
627 Specifies whether to use a privileged port for outgoing connections.
638 Note that this option must be set to
641 .Cm RhostsAuthentication
643 .Cm RhostsRSAAuthentication
644 authentications are needed with older servers.
646 Specifies the user to log in as.
647 This can be useful when a different user name is used on different machines.
648 This saves the trouble of
649 having to remember to give the user name on the command line.
650 .It Cm UserKnownHostsFile
651 Specifies a file to use for the user
652 host key database instead of
653 .Pa $HOME/.ssh/known_hosts .
654 .It Cm VerifyHostKeyDNS
655 Specifies whether to verify the remote key using DNS and SSHFP resource
659 Note that this option applies to protocol version 2 only.
661 Specifies the full pathname of the
665 .Pa /usr/X11R6/bin/xauth .
669 .It Pa $HOME/.ssh/config
670 This is the per-user configuration file.
671 The format of this file is described above.
672 This file is used by the
675 This file does not usually contain any sensitive information,
676 but the recommended permissions are read/write for the user, and not
677 accessible by others.
678 .It Pa /etc/ssh/ssh_config
679 Systemwide configuration file.
680 This file provides defaults for those
681 values that are not specified in the user's configuration file, and
682 for those users who do not have a configuration file.
683 This file must be world-readable.
688 OpenSSH is a derivative of the original and free
689 ssh 1.2.12 release by Tatu Ylonen.
690 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
691 Theo de Raadt and Dug Song
692 removed many bugs, re-added newer features and
694 Markus Friedl contributed the support for SSH
695 protocol versions 1.5 and 2.0.