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 1.40 2005/01/28 15:05:43 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 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.
470 .It Cm KbdInteractiveDevices
471 Specifies the list of methods to use in keyboard-interactive authentication.
472 Multiple method names must be comma-separated.
473 The default is to use the server specified list.
475 Specifies that a TCP/IP port on the local machine be forwarded over
476 the secure channel to the specified host and port from the remote machine.
477 The first argument must be a port number, and the second must be
479 IPv6 addresses can be specified with an alternative syntax:
481 Multiple forwardings may be specified, and additional
482 forwardings can be given on the command line.
483 Only the superuser can forward privileged ports.
485 Gives the verbosity level that is used when logging messages from
487 The possible values are:
488 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
490 DEBUG and DEBUG1 are equivalent.
491 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
493 Specifies the MAC (message authentication code) algorithms
494 in order of preference.
495 The MAC algorithm is used in protocol version 2
496 for data integrity protection.
497 Multiple algorithms must be comma-separated.
499 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
500 .It Cm NoHostAuthenticationForLocalhost
501 This option can be used if the home directory is shared across machines.
502 In this case localhost will refer to a different machine on each of
503 the machines and the user will get many warnings about changed host keys.
504 However, this option disables host authentication for localhost.
505 The argument to this keyword must be
509 The default is to check the host key for localhost.
510 .It Cm NumberOfPasswordPrompts
511 Specifies the number of password prompts before giving up.
512 The argument to this keyword must be an integer.
514 .It Cm PasswordAuthentication
515 Specifies whether to use password authentication.
516 The argument to this keyword must be
523 Specifies the port number to connect on the remote host.
525 .It Cm PreferredAuthentications
526 Specifies the order in which the client should try protocol 2
527 authentication methods.
528 This allows a client to prefer one method (e.g.
529 .Cm keyboard-interactive )
530 over another method (e.g.
532 The default for this option is:
533 .Dq hostbased,publickey,keyboard-interactive,password .
535 Specifies the protocol versions
537 should support in order of preference.
538 The possible values are
542 Multiple versions must be comma-separated.
547 tries version 2 and falls back to version 1
548 if version 2 is not available.
550 Specifies the command to use to connect to the server.
552 string extends to the end of the line, and is executed with
554 In the command string,
556 will be substituted by the host name to
560 The command can be basically anything,
561 and should read from its standard input and write to its standard output.
562 It should eventually connect an
564 server running on some machine, or execute
567 Host key management will be done using the
568 HostName of the host being connected (defaulting to the name typed by
570 Setting the command to
572 disables this option entirely.
575 is not available for connects with a proxy command.
577 .It Cm PubkeyAuthentication
578 Specifies whether to try public key authentication.
579 The argument to this keyword must be
585 This option applies to protocol version 2 only.
587 Specifies that a TCP/IP port on the remote machine be forwarded over
588 the secure channel to the specified host and port from the local machine.
589 The first argument must be a port number, and the second must be
591 IPv6 addresses can be specified with an alternative syntax:
593 Multiple forwardings may be specified, and additional
594 forwardings can be given on the command line.
595 Only the superuser can forward privileged ports.
596 .It Cm RhostsRSAAuthentication
597 Specifies whether to try rhosts based authentication with RSA host
605 This option applies to protocol version 1 only and requires
608 .It Cm RSAAuthentication
609 Specifies whether to try RSA authentication.
610 The argument to this keyword must be
614 RSA authentication will only be
615 attempted if the identity file exists, or an authentication agent is
619 Note that this option applies to protocol version 1 only.
621 Specifies what variables from the local
623 should be sent to the server.
624 Note that environment passing is only supported for protocol 2, the
625 server must also support it, and the server must be configured to
626 accept these environment variables.
631 for how to configure the server.
632 Variables are specified by name, which may contain the wildcard characters
636 Multiple environment variables may be separated by whitespace or spread
640 The default is not to send any environment variables.
641 .It Cm ServerAliveInterval
642 Sets a timeout interval in seconds after which if no data has been received
645 will send a message through the encrypted
646 channel to request a response from the server.
648 is 0, indicating that these messages will not be sent to the server.
649 This option applies to protocol version 2 only.
650 .It Cm ServerAliveCountMax
651 Sets the number of server alive messages (see above) which may be
654 receiving any messages back from the server.
655 If this threshold is reached while server alive messages are being sent,
657 will disconnect from the server, terminating the session.
658 It is important to note that the use of server alive messages is very
662 The server alive messages are sent through the encrypted channel
663 and therefore will not be spoofable.
664 The TCP keepalive option enabled by
667 The server alive mechanism is valuable when the client or
668 server depend on knowing when a connection has become inactive.
670 The default value is 3.
672 .Cm ServerAliveInterval
673 (above) is set to 15, and
674 .Cm ServerAliveCountMax
675 is left at the default, if the server becomes unresponsive ssh
676 will disconnect after approximately 45 seconds.
677 .It Cm SmartcardDevice
678 Specifies which smartcard device to use.
679 The argument to this keyword is the device
681 should use to communicate with a smartcard used for storing the user's
683 By default, no device is specified and smartcard support is not activated.
684 .It Cm StrictHostKeyChecking
685 If this flag is set to
688 will never automatically add host keys to the
689 .Pa $HOME/.ssh/known_hosts
690 file, and refuses to connect to hosts whose host key has changed.
691 This provides maximum protection against trojan horse attacks,
692 however, can be annoying when the
693 .Pa /etc/ssh/ssh_known_hosts
694 file is poorly maintained, or connections to new hosts are
696 This option forces the user to manually
698 If this flag is set to
701 will automatically add new host keys to the
702 user known hosts files.
703 If this flag is set to
706 will be added to the user known host files only after the user
707 has confirmed that is what they really want to do, and
709 will refuse to connect to hosts whose host key has changed.
711 known hosts will be verified automatically in all cases.
720 Specifies whether the system should send TCP keepalive messages to the
722 If they are sent, death of the connection or crash of one
723 of the machines will be properly noticed.
724 However, this means that
725 connections will die if the route is down temporarily, and some people
730 (to send TCP keepalive messages), and the client will notice
731 if the network goes down or the remote host dies.
732 This is important in scripts, and many users want it too.
734 To disable TCP keepalive messages, the value should be set to
736 .It Cm UsePrivilegedPort
737 Specifies whether to use a privileged port for outgoing connections.
748 Note that this option must be set to
751 .Cm RhostsRSAAuthentication
754 Specifies the user to log in as.
755 This can be useful when a different user name is used on different machines.
756 This saves the trouble of
757 having to remember to give the user name on the command line.
758 .It Cm UserKnownHostsFile
759 Specifies a file to use for the user
760 host key database instead of
761 .Pa $HOME/.ssh/known_hosts .
762 .It Cm VerifyHostKeyDNS
763 Specifies whether to verify the remote key using DNS and SSHFP resource
765 If this option is set to
767 the client will implicitly trust keys that match a secure fingerprint
769 Insecure fingerprints will be handled as if this option was set to
771 If this option is set to
773 information on fingerprint match will be displayed, but the user will still
774 need to confirm new host keys according to the
775 .Cm StrictHostKeyChecking
784 Note that this option applies to protocol version 2 only.
786 Specifies the full pathname of the
790 .Pa /usr/X11R6/bin/xauth .
794 .It Pa $HOME/.ssh/config
795 This is the per-user configuration file.
796 The format of this file is described above.
797 This file is used by the
800 Because of the potential for abuse, this file must have strict permissions:
801 read/write for the user, and not accessible by others.
802 .It Pa /etc/ssh/ssh_config
803 Systemwide configuration file.
804 This file provides defaults for those
805 values that are not specified in the user's configuration file, and
806 for those users who do not have a configuration file.
807 This file must be world-readable.
812 OpenSSH is a derivative of the original and free
813 ssh 1.2.12 release by Tatu Ylonen.
814 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
815 Theo de Raadt and Dug Song
816 removed many bugs, re-added newer features and
818 Markus Friedl contributed the support for SSH
819 protocol versions 1.5 and 2.0.