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 1.36 2004/06/13 15:03:02 djm 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.
247 .It Cm DynamicForward
248 Specifies that a TCP/IP port on the local machine be forwarded
249 over the secure channel, and the application
250 protocol is then used to determine where to connect to from the
252 The argument must be a port number.
253 Currently the SOCKS4 and SOCKS5 protocols are supported, and
255 will act as a SOCKS server.
256 Multiple forwardings may be specified, and
257 additional forwardings can be given on the command line.
258 Only the superuser can forward privileged ports.
260 Enables the sharing of multiple sessions over a single network connection.
264 will listen for connections on a control socket specified using the
267 Additional sessions can connect to this socket using the same
274 These sessions will reuse the master instance's network connection rather
275 than initiating new ones.
277 Specify a the path to the control socket used for connection sharing.
281 .It Cm EnableSSHKeysign
282 Setting this option to
284 in the global client configuration file
285 .Pa /etc/ssh/ssh_config
286 enables the use of the helper program
289 .Cm HostbasedAuthentication .
296 This option should be placed in the non-hostspecific section.
299 for more information.
301 Sets the escape character (default:
303 The escape character can also
304 be set on the command line.
305 The argument should be a single character,
307 followed by a letter, or
309 to disable the escape
310 character entirely (making the connection transparent for binary
313 Specifies whether the connection to the authentication agent (if any)
314 will be forwarded to the remote machine.
322 Agent forwarding should be enabled with caution.
323 Users with the ability to bypass file permissions on the remote host
324 (for the agent's Unix-domain socket)
325 can access the local agent through the forwarded connection.
326 An attacker cannot obtain key material from the agent,
327 however they can perform operations on the keys that enable them to
328 authenticate using the identities loaded into the agent.
330 Specifies whether X11 connections will be automatically redirected
331 over the secure channel and
341 X11 forwarding should be enabled with caution.
342 Users with the ability to bypass file permissions on the remote host
343 (for the user's X11 authorization database)
344 can access the local X11 display through the forwarded connection.
345 An attacker may then be able to perform activities such as keystroke monitoring
347 .Cm ForwardX11Trusted
348 option is also enabled.
349 .It Cm ForwardX11Trusted
350 If this option is set to
352 then remote X11 clients will have full access to the original X11 display.
353 If this option is set to
355 then remote X11 clients will be considered untrusted and prevented
356 from stealing or tampering with data belonging to trusted X11
362 See the X11 SECURITY extension specification for full details on
363 the restrictions imposed on untrusted clients.
365 Specifies whether remote hosts are allowed to connect to local
369 binds local port forwardings to the loopback address.
370 This prevents other remote hosts from connecting to forwarded ports.
372 can be used to specify that
374 should bind local port forwardings to the wildcard address,
375 thus allowing remote hosts to connect to forwarded ports.
382 .It Cm GlobalKnownHostsFile
383 Specifies a file to use for the global
384 host key database instead of
385 .Pa /etc/ssh/ssh_known_hosts .
386 .It Cm GSSAPIAuthentication
387 Specifies whether user authentication based on GSSAPI is allowed.
390 Note that this option applies to protocol version 2 only.
391 .It Cm GSSAPIDelegateCredentials
392 Forward (delegate) credentials to the server.
395 Note that this option applies to protocol version 2 only.
396 .It Cm HostbasedAuthentication
397 Specifies whether to try rhosts based authentication with public key
405 This option applies to protocol version 2 only and
407 .Cm RhostsRSAAuthentication .
408 .It Cm HostKeyAlgorithms
409 Specifies the protocol version 2 host key algorithms
410 that the client wants to use in order of preference.
411 The default for this option is:
412 .Dq ssh-rsa,ssh-dss .
414 Specifies an alias that should be used instead of the
415 real host name when looking up or saving the host key
416 in the host key database files.
417 This option is useful for tunneling ssh connections
418 or for multiple servers running on a single host.
420 Specifies the real host name to log into.
421 This can be used to specify nicknames or abbreviations for hosts.
422 Default is the name given on the command line.
423 Numeric IP addresses are also permitted (both on the command line and in
427 Specifies a file from which the user's RSA or DSA authentication identity
430 .Pa $HOME/.ssh/identity
431 for protocol version 1, and
432 .Pa $HOME/.ssh/id_rsa
434 .Pa $HOME/.ssh/id_dsa
435 for protocol version 2.
436 Additionally, any identities represented by the authentication agent
437 will be used for authentication.
438 The file name may use the tilde
439 syntax to refer to a user's home directory.
440 It is possible to have
441 multiple identity files specified in configuration files; all these
442 identities will be tried in sequence.
443 .It Cm IdentitiesOnly
446 should only use the authentication identity files configured in the
451 offers more identities.
452 The argument to this keyword must be
456 This option is intented for situations where
458 offers many different identities.
462 Specifies that a TCP/IP port on the local machine be forwarded over
463 the secure channel to the specified host and port from the remote machine.
464 The first argument must be a port number, and the second must be
466 IPv6 addresses can be specified with an alternative syntax:
468 Multiple forwardings may be specified, and additional
469 forwardings can be given on the command line.
470 Only the superuser can forward privileged ports.
472 Gives the verbosity level that is used when logging messages from
474 The possible values are:
475 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
477 DEBUG and DEBUG1 are equivalent.
478 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
480 Specifies the MAC (message authentication code) algorithms
481 in order of preference.
482 The MAC algorithm is used in protocol version 2
483 for data integrity protection.
484 Multiple algorithms must be comma-separated.
486 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
487 .It Cm NoHostAuthenticationForLocalhost
488 This option can be used if the home directory is shared across machines.
489 In this case localhost will refer to a different machine on each of
490 the machines and the user will get many warnings about changed host keys.
491 However, this option disables host authentication for localhost.
492 The argument to this keyword must be
496 The default is to check the host key for localhost.
497 .It Cm NumberOfPasswordPrompts
498 Specifies the number of password prompts before giving up.
499 The argument to this keyword must be an integer.
501 .It Cm PasswordAuthentication
502 Specifies whether to use password authentication.
503 The argument to this keyword must be
510 Specifies the port number to connect on the remote host.
512 .It Cm PreferredAuthentications
513 Specifies the order in which the client should try protocol 2
514 authentication methods.
515 This allows a client to prefer one method (e.g.
516 .Cm keyboard-interactive )
517 over another method (e.g.
519 The default for this option is:
520 .Dq hostbased,publickey,keyboard-interactive,password .
522 Specifies the protocol versions
524 should support in order of preference.
525 The possible values are
529 Multiple versions must be comma-separated.
534 tries version 2 and falls back to version 1
535 if version 2 is not available.
537 Specifies the command to use to connect to the server.
539 string extends to the end of the line, and is executed with
541 In the command string,
543 will be substituted by the host name to
547 The command can be basically anything,
548 and should read from its standard input and write to its standard output.
549 It should eventually connect an
551 server running on some machine, or execute
554 Host key management will be done using the
555 HostName of the host being connected (defaulting to the name typed by
557 Setting the command to
559 disables this option entirely.
562 is not available for connects with a proxy command.
564 .It Cm PubkeyAuthentication
565 Specifies whether to try public key authentication.
566 The argument to this keyword must be
572 This option applies to protocol version 2 only.
574 Specifies that a TCP/IP port on the remote machine be forwarded over
575 the secure channel to the specified host and port from the local machine.
576 The first argument must be a port number, and the second must be
578 IPv6 addresses can be specified with an alternative syntax:
580 Multiple forwardings may be specified, and additional
581 forwardings can be given on the command line.
582 Only the superuser can forward privileged ports.
583 .It Cm RhostsRSAAuthentication
584 Specifies whether to try rhosts based authentication with RSA host
592 This option applies to protocol version 1 only and requires
595 .It Cm RSAAuthentication
596 Specifies whether to try RSA authentication.
597 The argument to this keyword must be
601 RSA authentication will only be
602 attempted if the identity file exists, or an authentication agent is
606 Note that this option applies to protocol version 1 only.
608 Specifies what variables from the local
610 should be sent to the server.
611 Note that environment passing is only supported for protocol 2, the
612 server must also support it, and the server must be configured to
613 accept these environment variables.
618 for how to configure the server.
619 Variables are specified by name, which may contain the wildcard characters
623 Multiple environment variables may be separated by whitespace or spread
627 The default is not to send any environment variables.
628 .It Cm ServerAliveInterval
629 Sets a timeout interval in seconds after which if no data has been received
632 will send a message through the encrypted
633 channel to request a response from the server.
635 is 0, indicating that these messages will not be sent to the server.
636 This option applies to protocol version 2 only.
637 .It Cm ServerAliveCountMax
638 Sets the number of server alive messages (see above) which may be
641 receiving any messages back from the server.
642 If this threshold is reached while server alive messages are being sent,
644 will disconnect from the server, terminating the session.
645 It is important to note that the use of server alive messages is very
649 The server alive messages are sent through the encrypted channel
650 and therefore will not be spoofable.
651 The TCP keepalive option enabled by
654 The server alive mechanism is valuable when the client or
655 server depend on knowing when a connection has become inactive.
657 The default value is 3.
659 .Cm ServerAliveInterval
660 (above) is set to 15, and
661 .Cm ServerAliveCountMax
662 is left at the default, if the server becomes unresponsive ssh
663 will disconnect after approximately 45 seconds.
664 .It Cm SmartcardDevice
665 Specifies which smartcard device to use.
666 The argument to this keyword is the device
668 should use to communicate with a smartcard used for storing the user's
670 By default, no device is specified and smartcard support is not activated.
671 .It Cm StrictHostKeyChecking
672 If this flag is set to
675 will never automatically add host keys to the
676 .Pa $HOME/.ssh/known_hosts
677 file, and refuses to connect to hosts whose host key has changed.
678 This provides maximum protection against trojan horse attacks,
679 however, can be annoying when the
680 .Pa /etc/ssh/ssh_known_hosts
681 file is poorly maintained, or connections to new hosts are
683 This option forces the user to manually
685 If this flag is set to
688 will automatically add new host keys to the
689 user known hosts files.
690 If this flag is set to
693 will be added to the user known host files only after the user
694 has confirmed that is what they really want to do, and
696 will refuse to connect to hosts whose host key has changed.
698 known hosts will be verified automatically in all cases.
707 Specifies whether the system should send TCP keepalive messages to the
709 If they are sent, death of the connection or crash of one
710 of the machines will be properly noticed.
711 However, this means that
712 connections will die if the route is down temporarily, and some people
717 (to send TCP keepalive messages), and the client will notice
718 if the network goes down or the remote host dies.
719 This is important in scripts, and many users want it too.
721 To disable TCP keepalive messages, the value should be set to
723 .It Cm UsePrivilegedPort
724 Specifies whether to use a privileged port for outgoing connections.
735 Note that this option must be set to
738 .Cm RhostsRSAAuthentication
741 Specifies the user to log in as.
742 This can be useful when a different user name is used on different machines.
743 This saves the trouble of
744 having to remember to give the user name on the command line.
745 .It Cm UserKnownHostsFile
746 Specifies a file to use for the user
747 host key database instead of
748 .Pa $HOME/.ssh/known_hosts .
749 .It Cm VerifyHostKeyDNS
750 Specifies whether to verify the remote key using DNS and SSHFP resource
752 If this option is set to
754 the client will implicitly trust keys that match a secure fingerprint
756 Insecure fingerprints will be handled as if this option was set to
758 If this option is set to
760 information on fingerprint match will be displayed, but the user will still
761 need to confirm new host keys according to the
762 .Cm StrictHostKeyChecking
771 Note that this option applies to protocol version 2 only.
773 Specifies the full pathname of the
777 .Pa /usr/X11R6/bin/xauth .
781 .It Pa $HOME/.ssh/config
782 This is the per-user configuration file.
783 The format of this file is described above.
784 This file is used by the
787 Because of the potential for abuse, this file must have strict permissions:
788 read/write for the user, and not accessible by others.
789 .It Pa /etc/ssh/ssh_config
790 Systemwide configuration file.
791 This file provides defaults for those
792 values that are not specified in the user's configuration file, and
793 for those users who do not have a configuration file.
794 This file must be world-readable.
799 OpenSSH is a derivative of the original and free
800 ssh 1.2.12 release by Tatu Ylonen.
801 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
802 Theo de Raadt and Dug Song
803 removed many bugs, re-added newer features and
805 Markus Friedl contributed the support for SSH
806 protocol versions 1.5 and 2.0.