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 1.38 2004/06/26 09:11:14 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.
188 The supported ciphers are
202 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
203 aes192-cbc,aes256-cbc''
205 .It Cm ClearAllForwardings
206 Specifies that all local, remote and dynamic port forwardings
207 specified in the configuration files or on the command line be
209 This option is primarily useful when used from the
211 command line to clear port forwardings set in
212 configuration files, and is automatically set by
223 Specifies whether to use compression.
230 .It Cm CompressionLevel
231 Specifies the compression level to use if compression is enabled.
232 The argument must be an integer from 1 (fast) to 9 (slow, best).
233 The default level is 6, which is good for most applications.
234 The meaning of the values is the same as in
236 Note that this option applies to protocol version 1 only.
237 .It Cm ConnectionAttempts
238 Specifies the number of tries (one per second) to make before exiting.
239 The argument must be an integer.
240 This may be useful in scripts if the connection sometimes fails.
242 .It Cm ConnectTimeout
243 Specifies the timeout (in seconds) used when connecting to the ssh
244 server, instead of using the default system TCP timeout.
245 This value is used only when the target is down or really unreachable,
246 not when it refuses the connection.
248 Enables the sharing of multiple sessions over a single network connection.
252 will listen for connections on a control socket specified using the
255 Additional sessions can connect to this socket using the same
262 These sessions will reuse the master instance's network connection rather
263 than initiating new ones.
268 to listen for control connections, but require confirmation using the
270 program before they are accepted (see
274 Specify the path to the control socket used for connection sharing.
278 .It Cm DynamicForward
279 Specifies that a TCP/IP port on the local machine be forwarded
280 over the secure channel, and the application
281 protocol is then used to determine where to connect to from the
283 The argument must be a port number.
284 Currently the SOCKS4 and SOCKS5 protocols are supported, and
286 will act as a SOCKS server.
287 Multiple forwardings may be specified, and
288 additional forwardings can be given on the command line.
289 Only the superuser can forward privileged ports.
290 .It Cm EnableSSHKeysign
291 Setting this option to
293 in the global client configuration file
294 .Pa /etc/ssh/ssh_config
295 enables the use of the helper program
298 .Cm HostbasedAuthentication .
305 This option should be placed in the non-hostspecific section.
308 for more information.
310 Sets the escape character (default:
312 The escape character can also
313 be set on the command line.
314 The argument should be a single character,
316 followed by a letter, or
318 to disable the escape
319 character entirely (making the connection transparent for binary
322 Specifies whether the connection to the authentication agent (if any)
323 will be forwarded to the remote machine.
331 Agent forwarding should be enabled with caution.
332 Users with the ability to bypass file permissions on the remote host
333 (for the agent's Unix-domain socket)
334 can access the local agent through the forwarded connection.
335 An attacker cannot obtain key material from the agent,
336 however they can perform operations on the keys that enable them to
337 authenticate using the identities loaded into the agent.
339 Specifies whether X11 connections will be automatically redirected
340 over the secure channel and
350 X11 forwarding should be enabled with caution.
351 Users with the ability to bypass file permissions on the remote host
352 (for the user's X11 authorization database)
353 can access the local X11 display through the forwarded connection.
354 An attacker may then be able to perform activities such as keystroke monitoring
356 .Cm ForwardX11Trusted
357 option is also enabled.
358 .It Cm ForwardX11Trusted
359 If this option is set to
361 then remote X11 clients will have full access to the original X11 display.
362 If this option is set to
364 then remote X11 clients will be considered untrusted and prevented
365 from stealing or tampering with data belonging to trusted X11
371 See the X11 SECURITY extension specification for full details on
372 the restrictions imposed on untrusted clients.
374 Specifies whether remote hosts are allowed to connect to local
378 binds local port forwardings to the loopback address.
379 This prevents other remote hosts from connecting to forwarded ports.
381 can be used to specify that
383 should bind local port forwardings to the wildcard address,
384 thus allowing remote hosts to connect to forwarded ports.
391 .It Cm GlobalKnownHostsFile
392 Specifies a file to use for the global
393 host key database instead of
394 .Pa /etc/ssh/ssh_known_hosts .
395 .It Cm GSSAPIAuthentication
396 Specifies whether user authentication based on GSSAPI is allowed.
399 Note that this option applies to protocol version 2 only.
400 .It Cm GSSAPIDelegateCredentials
401 Forward (delegate) credentials to the server.
404 Note that this option applies to protocol version 2 only.
405 .It Cm HostbasedAuthentication
406 Specifies whether to try rhosts based authentication with public key
414 This option applies to protocol version 2 only and
416 .Cm RhostsRSAAuthentication .
417 .It Cm HostKeyAlgorithms
418 Specifies the protocol version 2 host key algorithms
419 that the client wants to use in order of preference.
420 The default for this option is:
421 .Dq ssh-rsa,ssh-dss .
423 Specifies an alias that should be used instead of the
424 real host name when looking up or saving the host key
425 in the host key database files.
426 This option is useful for tunneling ssh connections
427 or for multiple servers running on a single host.
429 Specifies the real host name to log into.
430 This can be used to specify nicknames or abbreviations for hosts.
431 Default is the name given on the command line.
432 Numeric IP addresses are also permitted (both on the command line and in
436 Specifies a file from which the user's RSA or DSA authentication identity
439 .Pa $HOME/.ssh/identity
440 for protocol version 1, and
441 .Pa $HOME/.ssh/id_rsa
443 .Pa $HOME/.ssh/id_dsa
444 for protocol version 2.
445 Additionally, any identities represented by the authentication agent
446 will be used for authentication.
447 The file name may use the tilde
448 syntax to refer to a user's home directory.
449 It is possible to have
450 multiple identity files specified in configuration files; all these
451 identities will be tried in sequence.
452 .It Cm IdentitiesOnly
455 should only use the authentication identity files configured in the
460 offers more identities.
461 The argument to this keyword must be
465 This option is intented for situations where
467 offers many different identities.
471 Specifies that a TCP/IP port on the local machine be forwarded over
472 the secure channel to the specified host and port from the remote machine.
473 The first argument must be a port number, and the second must be
475 IPv6 addresses can be specified with an alternative syntax:
477 Multiple forwardings may be specified, and additional
478 forwardings can be given on the command line.
479 Only the superuser can forward privileged ports.
481 Gives the verbosity level that is used when logging messages from
483 The possible values are:
484 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
486 DEBUG and DEBUG1 are equivalent.
487 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
489 Specifies the MAC (message authentication code) algorithms
490 in order of preference.
491 The MAC algorithm is used in protocol version 2
492 for data integrity protection.
493 Multiple algorithms must be comma-separated.
495 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
496 .It Cm NoHostAuthenticationForLocalhost
497 This option can be used if the home directory is shared across machines.
498 In this case localhost will refer to a different machine on each of
499 the machines and the user will get many warnings about changed host keys.
500 However, this option disables host authentication for localhost.
501 The argument to this keyword must be
505 The default is to check the host key for localhost.
506 .It Cm NumberOfPasswordPrompts
507 Specifies the number of password prompts before giving up.
508 The argument to this keyword must be an integer.
510 .It Cm PasswordAuthentication
511 Specifies whether to use password authentication.
512 The argument to this keyword must be
519 Specifies the port number to connect on the remote host.
521 .It Cm PreferredAuthentications
522 Specifies the order in which the client should try protocol 2
523 authentication methods.
524 This allows a client to prefer one method (e.g.
525 .Cm keyboard-interactive )
526 over another method (e.g.
528 The default for this option is:
529 .Dq hostbased,publickey,keyboard-interactive,password .
531 Specifies the protocol versions
533 should support in order of preference.
534 The possible values are
538 Multiple versions must be comma-separated.
543 tries version 2 and falls back to version 1
544 if version 2 is not available.
546 Specifies the command to use to connect to the server.
548 string extends to the end of the line, and is executed with
550 In the command string,
552 will be substituted by the host name to
556 The command can be basically anything,
557 and should read from its standard input and write to its standard output.
558 It should eventually connect an
560 server running on some machine, or execute
563 Host key management will be done using the
564 HostName of the host being connected (defaulting to the name typed by
566 Setting the command to
568 disables this option entirely.
571 is not available for connects with a proxy command.
573 .It Cm PubkeyAuthentication
574 Specifies whether to try public key authentication.
575 The argument to this keyword must be
581 This option applies to protocol version 2 only.
583 Specifies that a TCP/IP port on the remote machine be forwarded over
584 the secure channel to the specified host and port from the local machine.
585 The first argument must be a port number, and the second must be
587 IPv6 addresses can be specified with an alternative syntax:
589 Multiple forwardings may be specified, and additional
590 forwardings can be given on the command line.
591 Only the superuser can forward privileged ports.
592 .It Cm RhostsRSAAuthentication
593 Specifies whether to try rhosts based authentication with RSA host
601 This option applies to protocol version 1 only and requires
604 .It Cm RSAAuthentication
605 Specifies whether to try RSA authentication.
606 The argument to this keyword must be
610 RSA authentication will only be
611 attempted if the identity file exists, or an authentication agent is
615 Note that this option applies to protocol version 1 only.
617 Specifies what variables from the local
619 should be sent to the server.
620 Note that environment passing is only supported for protocol 2, the
621 server must also support it, and the server must be configured to
622 accept these environment variables.
627 for how to configure the server.
628 Variables are specified by name, which may contain the wildcard characters
632 Multiple environment variables may be separated by whitespace or spread
636 The default is not to send any environment variables.
637 .It Cm ServerAliveInterval
638 Sets a timeout interval in seconds after which if no data has been received
641 will send a message through the encrypted
642 channel to request a response from the server.
644 is 0, indicating that these messages will not be sent to the server.
645 This option applies to protocol version 2 only.
646 .It Cm ServerAliveCountMax
647 Sets the number of server alive messages (see above) which may be
650 receiving any messages back from the server.
651 If this threshold is reached while server alive messages are being sent,
653 will disconnect from the server, terminating the session.
654 It is important to note that the use of server alive messages is very
658 The server alive messages are sent through the encrypted channel
659 and therefore will not be spoofable.
660 The TCP keepalive option enabled by
663 The server alive mechanism is valuable when the client or
664 server depend on knowing when a connection has become inactive.
666 The default value is 3.
668 .Cm ServerAliveInterval
669 (above) is set to 15, and
670 .Cm ServerAliveCountMax
671 is left at the default, if the server becomes unresponsive ssh
672 will disconnect after approximately 45 seconds.
673 .It Cm SmartcardDevice
674 Specifies which smartcard device to use.
675 The argument to this keyword is the device
677 should use to communicate with a smartcard used for storing the user's
679 By default, no device is specified and smartcard support is not activated.
680 .It Cm StrictHostKeyChecking
681 If this flag is set to
684 will never automatically add host keys to the
685 .Pa $HOME/.ssh/known_hosts
686 file, and refuses to connect to hosts whose host key has changed.
687 This provides maximum protection against trojan horse attacks,
688 however, can be annoying when the
689 .Pa /etc/ssh/ssh_known_hosts
690 file is poorly maintained, or connections to new hosts are
692 This option forces the user to manually
694 If this flag is set to
697 will automatically add new host keys to the
698 user known hosts files.
699 If this flag is set to
702 will be added to the user known host files only after the user
703 has confirmed that is what they really want to do, and
705 will refuse to connect to hosts whose host key has changed.
707 known hosts will be verified automatically in all cases.
716 Specifies whether the system should send TCP keepalive messages to the
718 If they are sent, death of the connection or crash of one
719 of the machines will be properly noticed.
720 However, this means that
721 connections will die if the route is down temporarily, and some people
726 (to send TCP keepalive messages), and the client will notice
727 if the network goes down or the remote host dies.
728 This is important in scripts, and many users want it too.
730 To disable TCP keepalive messages, the value should be set to
732 .It Cm UsePrivilegedPort
733 Specifies whether to use a privileged port for outgoing connections.
744 Note that this option must be set to
747 .Cm RhostsRSAAuthentication
750 Specifies the user to log in as.
751 This can be useful when a different user name is used on different machines.
752 This saves the trouble of
753 having to remember to give the user name on the command line.
754 .It Cm UserKnownHostsFile
755 Specifies a file to use for the user
756 host key database instead of
757 .Pa $HOME/.ssh/known_hosts .
758 .It Cm VerifyHostKeyDNS
759 Specifies whether to verify the remote key using DNS and SSHFP resource
761 If this option is set to
763 the client will implicitly trust keys that match a secure fingerprint
765 Insecure fingerprints will be handled as if this option was set to
767 If this option is set to
769 information on fingerprint match will be displayed, but the user will still
770 need to confirm new host keys according to the
771 .Cm StrictHostKeyChecking
780 Note that this option applies to protocol version 2 only.
782 Specifies the full pathname of the
786 .Pa /usr/X11R6/bin/xauth .
790 .It Pa $HOME/.ssh/config
791 This is the per-user configuration file.
792 The format of this file is described above.
793 This file is used by the
796 Because of the potential for abuse, this file must have strict permissions:
797 read/write for the user, and not accessible by others.
798 .It Pa /etc/ssh/ssh_config
799 Systemwide configuration file.
800 This file provides defaults for those
801 values that are not specified in the user's configuration file, and
802 for those users who do not have a configuration file.
803 This file must be world-readable.
808 OpenSSH is a derivative of the original and free
809 ssh 1.2.12 release by Tatu Ylonen.
810 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
811 Theo de Raadt and Dug Song
812 removed many bugs, re-added newer features and
814 Markus Friedl contributed the support for SSH
815 protocol versions 1.5 and 2.0.