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 1.47 2005/03/07 23:41:54 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 separated 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.
363 If this option is set to
365 then remote X11 clients will be considered untrusted and prevented
366 from stealing or tampering with data belonging to trusted X11
370 token used for the session will be set to expire after 20 minutes.
371 Remote clients will be refused access after this time.
376 See the X11 SECURITY extension specification for full details on
377 the restrictions imposed on untrusted clients.
379 Specifies whether remote hosts are allowed to connect to local
383 binds local port forwardings to the loopback address.
384 This prevents other remote hosts from connecting to forwarded ports.
386 can be used to specify that
388 should bind local port forwardings to the wildcard address,
389 thus allowing remote hosts to connect to forwarded ports.
396 .It Cm GlobalKnownHostsFile
397 Specifies a file to use for the global
398 host key database instead of
399 .Pa /etc/ssh/ssh_known_hosts .
400 .It Cm GSSAPIAuthentication
401 Specifies whether user authentication based on GSSAPI is allowed.
404 Note that this option applies to protocol version 2 only.
405 .It Cm GSSAPIDelegateCredentials
406 Forward (delegate) credentials to the server.
409 Note that this option applies to protocol version 2 only.
410 .It Cm HashKnownHosts
413 should hash host names and addresses when they are added to
414 .Pa $HOME/.ssh/known_hosts .
415 These hashed names may be used normally by
419 but they do not reveal identifying information should the file's contents
423 Note that hashing of names and addresses will not be retrospectively applied
424 to existing known hosts files, but these may be manually hashed using
426 .It Cm HostbasedAuthentication
427 Specifies whether to try rhosts based authentication with public key
435 This option applies to protocol version 2 only and
437 .Cm RhostsRSAAuthentication .
438 .It Cm HostKeyAlgorithms
439 Specifies the protocol version 2 host key algorithms
440 that the client wants to use in order of preference.
441 The default for this option is:
442 .Dq ssh-rsa,ssh-dss .
444 Specifies an alias that should be used instead of the
445 real host name when looking up or saving the host key
446 in the host key database files.
447 This option is useful for tunneling ssh connections
448 or for multiple servers running on a single host.
450 Specifies the real host name to log into.
451 This can be used to specify nicknames or abbreviations for hosts.
452 Default is the name given on the command line.
453 Numeric IP addresses are also permitted (both on the command line and in
457 Specifies a file from which the user's RSA or DSA authentication identity
460 .Pa $HOME/.ssh/identity
461 for protocol version 1, and
462 .Pa $HOME/.ssh/id_rsa
464 .Pa $HOME/.ssh/id_dsa
465 for protocol version 2.
466 Additionally, any identities represented by the authentication agent
467 will be used for authentication.
468 The file name may use the tilde
469 syntax to refer to a user's home directory.
470 It is possible to have
471 multiple identity files specified in configuration files; all these
472 identities will be tried in sequence.
473 .It Cm IdentitiesOnly
476 should only use the authentication identity files configured in the
481 offers more identities.
482 The argument to this keyword must be
486 This option is intented for situations where
488 offers many different identities.
491 .It Cm KbdInteractiveDevices
492 Specifies the list of methods to use in keyboard-interactive authentication.
493 Multiple method names must be comma-separated.
494 The default is to use the server specified list.
496 Specifies that a TCP/IP port on the local machine be forwarded over
497 the secure channel to the specified host and port from the remote machine.
498 The first argument must be a port number, and the second must be
501 .Oo Ar bind_address : Oc
505 IPv6 addresses can be specified by enclosing addresses in square brackets or
506 by using an alternative syntax:
509 .Op Ar bind_address No /
510 .Ar host No / Ar port
513 Multiple forwardings may be specified, and additional forwardings can be
514 given on the command line.
515 Only the superuser can forward privileged ports.
516 By default, the local port is bound in accordance with the
521 may be used to bind the connection to a specific address.
526 indicates that the listening port be bound for local use only, while an
529 indicates that the port should be available from all interfaces.
531 Gives the verbosity level that is used when logging messages from
533 The possible values are:
534 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
536 DEBUG and DEBUG1 are equivalent.
537 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
539 Specifies the MAC (message authentication code) algorithms
540 in order of preference.
541 The MAC algorithm is used in protocol version 2
542 for data integrity protection.
543 Multiple algorithms must be comma-separated.
545 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
546 .It Cm NoHostAuthenticationForLocalhost
547 This option can be used if the home directory is shared across machines.
548 In this case localhost will refer to a different machine on each of
549 the machines and the user will get many warnings about changed host keys.
550 However, this option disables host authentication for localhost.
551 The argument to this keyword must be
555 The default is to check the host key for localhost.
556 .It Cm NumberOfPasswordPrompts
557 Specifies the number of password prompts before giving up.
558 The argument to this keyword must be an integer.
560 .It Cm PasswordAuthentication
561 Specifies whether to use password authentication.
562 The argument to this keyword must be
569 Specifies the port number to connect on the remote host.
571 .It Cm PreferredAuthentications
572 Specifies the order in which the client should try protocol 2
573 authentication methods.
574 This allows a client to prefer one method (e.g.
575 .Cm keyboard-interactive )
576 over another method (e.g.
578 The default for this option is:
579 .Dq hostbased,publickey,keyboard-interactive,password .
581 Specifies the protocol versions
583 should support in order of preference.
584 The possible values are
588 Multiple versions must be comma-separated.
593 tries version 2 and falls back to version 1
594 if version 2 is not available.
596 Specifies the command to use to connect to the server.
598 string extends to the end of the line, and is executed with
600 In the command string,
602 will be substituted by the host name to
606 The command can be basically anything,
607 and should read from its standard input and write to its standard output.
608 It should eventually connect an
610 server running on some machine, or execute
613 Host key management will be done using the
614 HostName of the host being connected (defaulting to the name typed by
616 Setting the command to
618 disables this option entirely.
621 is not available for connects with a proxy command.
623 .It Cm PubkeyAuthentication
624 Specifies whether to try public key authentication.
625 The argument to this keyword must be
631 This option applies to protocol version 2 only.
633 Specifies that a TCP/IP port on the remote machine be forwarded over
634 the secure channel to the specified host and port from the local machine.
635 The first argument must be a port number, and the second must be
638 .Oo Ar bind_address : Oc
642 IPv6 addresses can be specified by enclosing any addresses in square brackets
643 or by using the alternative syntax:
646 .Op Ar bind_address No /
647 .Ar host No / Ar port
650 Multiple forwardings may be specified, and additional
651 forwardings can be given on the command line.
652 Only the superuser can forward privileged ports.
656 is not specified, the default is to only bind to loopback addresses.
661 or an empty string, then the forwarding is requested to listen on all
665 will only succeed if the server's
667 option is enabled (see
668 .Xr sshd_config 5 ) .
669 .It Cm RhostsRSAAuthentication
670 Specifies whether to try rhosts based authentication with RSA host
678 This option applies to protocol version 1 only and requires
681 .It Cm RSAAuthentication
682 Specifies whether to try RSA authentication.
683 The argument to this keyword must be
687 RSA authentication will only be
688 attempted if the identity file exists, or an authentication agent is
692 Note that this option applies to protocol version 1 only.
694 Specifies what variables from the local
696 should be sent to the server.
697 Note that environment passing is only supported for protocol 2, the
698 server must also support it, and the server must be configured to
699 accept these environment variables.
704 for how to configure the server.
705 Variables are specified by name, which may contain the wildcard characters
709 Multiple environment variables may be separated by whitespace or spread
713 The default is not to send any environment variables.
714 .It Cm ServerAliveInterval
715 Sets a timeout interval in seconds after which if no data has been received
718 will send a message through the encrypted
719 channel to request a response from the server.
721 is 0, indicating that these messages will not be sent to the server.
722 This option applies to protocol version 2 only.
723 .It Cm ServerAliveCountMax
724 Sets the number of server alive messages (see above) which may be
727 receiving any messages back from the server.
728 If this threshold is reached while server alive messages are being sent,
730 will disconnect from the server, terminating the session.
731 It is important to note that the use of server alive messages is very
735 The server alive messages are sent through the encrypted channel
736 and therefore will not be spoofable.
737 The TCP keepalive option enabled by
740 The server alive mechanism is valuable when the client or
741 server depend on knowing when a connection has become inactive.
743 The default value is 3.
745 .Cm ServerAliveInterval
746 (above) is set to 15, and
747 .Cm ServerAliveCountMax
748 is left at the default, if the server becomes unresponsive ssh
749 will disconnect after approximately 45 seconds.
750 .It Cm SmartcardDevice
751 Specifies which smartcard device to use.
752 The argument to this keyword is the device
754 should use to communicate with a smartcard used for storing the user's
756 By default, no device is specified and smartcard support is not activated.
757 .It Cm StrictHostKeyChecking
758 If this flag is set to
761 will never automatically add host keys to the
762 .Pa $HOME/.ssh/known_hosts
763 file, and refuses to connect to hosts whose host key has changed.
764 This provides maximum protection against trojan horse attacks,
765 however, can be annoying when the
766 .Pa /etc/ssh/ssh_known_hosts
767 file is poorly maintained, or connections to new hosts are
769 This option forces the user to manually
771 If this flag is set to
774 will automatically add new host keys to the
775 user known hosts files.
776 If this flag is set to
779 will be added to the user known host files only after the user
780 has confirmed that is what they really want to do, and
782 will refuse to connect to hosts whose host key has changed.
784 known hosts will be verified automatically in all cases.
793 Specifies whether the system should send TCP keepalive messages to the
795 If they are sent, death of the connection or crash of one
796 of the machines will be properly noticed.
797 However, this means that
798 connections will die if the route is down temporarily, and some people
803 (to send TCP keepalive messages), and the client will notice
804 if the network goes down or the remote host dies.
805 This is important in scripts, and many users want it too.
807 To disable TCP keepalive messages, the value should be set to
809 .It Cm UsePrivilegedPort
810 Specifies whether to use a privileged port for outgoing connections.
821 Note that this option must be set to
824 .Cm RhostsRSAAuthentication
827 Specifies the user to log in as.
828 This can be useful when a different user name is used on different machines.
829 This saves the trouble of
830 having to remember to give the user name on the command line.
831 .It Cm UserKnownHostsFile
832 Specifies a file to use for the user
833 host key database instead of
834 .Pa $HOME/.ssh/known_hosts .
835 .It Cm VerifyHostKeyDNS
836 Specifies whether to verify the remote key using DNS and SSHFP resource
838 If this option is set to
840 the client will implicitly trust keys that match a secure fingerprint
842 Insecure fingerprints will be handled as if this option was set to
844 If this option is set to
846 information on fingerprint match will be displayed, but the user will still
847 need to confirm new host keys according to the
848 .Cm StrictHostKeyChecking
857 Note that this option applies to protocol version 2 only.
859 Specifies the full pathname of the
863 .Pa /usr/X11R6/bin/xauth .
867 .It Pa $HOME/.ssh/config
868 This is the per-user configuration file.
869 The format of this file is described above.
870 This file is used by the
873 Because of the potential for abuse, this file must have strict permissions:
874 read/write for the user, and not accessible by others.
875 .It Pa /etc/ssh/ssh_config
876 Systemwide configuration file.
877 This file provides defaults for those
878 values that are not specified in the user's configuration file, and
879 for those users who do not have a configuration file.
880 This file must be world-readable.
885 OpenSSH is a derivative of the original and free
886 ssh 1.2.12 release by Tatu Ylonen.
887 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
888 Theo de Raadt and Dug Song
889 removed many bugs, re-added newer features and
891 Markus Friedl contributed the support for SSH
892 protocol versions 1.5 and 2.0.