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 1.49 2005/03/16 11:10:38 jmc Exp $
39 .Dd September 25, 1999
44 .Nd OpenSSH SSH client configuration files
46 .Bl -tag -width Ds -compact
47 .It Pa $HOME/.ssh/config
48 .It Pa /etc/ssh/ssh_config
52 obtains configuration data from the following sources in
54 .Bl -enum -offset indent -compact
58 user's configuration file
59 .Pq Pa $HOME/.ssh/config
61 system-wide configuration file
62 .Pq Pa /etc/ssh/ssh_config
65 For each parameter, the first obtained value
67 The configuration files contain sections separated by
69 specifications, and that section is only applied for hosts that
70 match one of the patterns given in the specification.
71 The matched host name is the one given on the command line.
73 Since the first obtained value for each parameter is used, more
74 host-specific declarations should be given near the beginning of the
75 file, and general defaults at the end.
77 The configuration file has the following format:
79 Empty lines and lines starting with
83 Otherwise a line is of the format
84 .Dq keyword arguments .
85 Configuration options may be separated by whitespace or
86 optional whitespace and exactly one
88 the latter format is useful to avoid the need to quote whitespace
89 when specifying configuration options using the
98 keywords and their meanings are as follows (note that
99 keywords are case-insensitive and arguments are case-sensitive):
102 Restricts the following declarations (up to the next
104 keyword) to be only for those hosts that match one of the patterns
105 given after the keyword.
109 can be used as wildcards in the
113 as a pattern can be used to provide global
114 defaults for all hosts.
117 argument given on the command line (i.e., the name is not converted to
118 a canonicalized host name before matching).
120 Specifies which address family to use when connecting.
130 passphrase/password querying will be disabled.
131 This option is useful in scripts and other batch jobs where no user
132 is present to supply the password.
140 Specify the interface to transmit from on machines with multiple
141 interfaces or aliased addresses.
142 Note that this option does not work if
143 .Cm UsePrivilegedPort
146 .It Cm ChallengeResponseAuthentication
147 Specifies whether to use challenge response authentication.
148 The argument to this keyword must be
155 If this flag is set to
157 ssh will additionally check the host IP address in the
160 This allows ssh to detect if a host key changed due to DNS spoofing.
161 If the option is set to
163 the check will not be executed.
167 Specifies the cipher to use for encrypting the session
168 in protocol version 1.
176 is only supported in the
178 client for interoperability with legacy protocol 1 implementations
179 that do not support the
182 Its use is strongly discouraged due to cryptographic weaknesses.
186 Specifies the ciphers allowed for protocol version 2
187 in order of preference.
188 Multiple ciphers must be comma-separated.
189 The supported ciphers are
203 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
204 aes192-cbc,aes256-cbc''
206 .It Cm ClearAllForwardings
207 Specifies that all local, remote and dynamic port forwardings
208 specified in the configuration files or on the command line be
210 This option is primarily useful when used from the
212 command line to clear port forwardings set in
213 configuration files, and is automatically set by
224 Specifies whether to use compression.
231 .It Cm CompressionLevel
232 Specifies the compression level to use if compression is enabled.
233 The argument must be an integer from 1 (fast) to 9 (slow, best).
234 The default level is 6, which is good for most applications.
235 The meaning of the values is the same as in
237 Note that this option applies to protocol version 1 only.
238 .It Cm ConnectionAttempts
239 Specifies the number of tries (one per second) to make before exiting.
240 The argument must be an integer.
241 This may be useful in scripts if the connection sometimes fails.
243 .It Cm ConnectTimeout
244 Specifies the timeout (in seconds) used when connecting to the ssh
245 server, instead of using the default system TCP timeout.
246 This value is used only when the target is down or really unreachable,
247 not when it refuses the connection.
249 Enables the sharing of multiple sessions over a single network connection.
253 will listen for connections on a control socket specified using the
256 Additional sessions can connect to this socket using the same
263 These sessions will reuse the master instance's network connection rather
264 than initiating new ones.
269 to listen for control connections, but require confirmation using the
271 program before they are accepted (see
275 Specify the path to the control socket used for connection sharing.
279 .It Cm DynamicForward
280 Specifies that a TCP/IP port on the local machine be forwarded
281 over the secure channel, and the application
282 protocol is then used to determine where to connect to from the
284 The argument must be a port number.
285 Currently the SOCKS4 and SOCKS5 protocols are supported, and
287 will act as a SOCKS server.
288 Multiple forwardings may be specified, and
289 additional forwardings can be given on the command line.
290 Only the superuser can forward privileged ports.
291 .It Cm EnableSSHKeysign
292 Setting this option to
294 in the global client configuration file
295 .Pa /etc/ssh/ssh_config
296 enables the use of the helper program
299 .Cm HostbasedAuthentication .
306 This option should be placed in the non-hostspecific section.
309 for more information.
311 Sets the escape character (default:
313 The escape character can also
314 be set on the command line.
315 The argument should be a single character,
317 followed by a letter, or
319 to disable the escape
320 character entirely (making the connection transparent for binary
323 Specifies whether the connection to the authentication agent (if any)
324 will be forwarded to the remote machine.
332 Agent forwarding should be enabled with caution.
333 Users with the ability to bypass file permissions on the remote host
334 (for the agent's Unix-domain socket)
335 can access the local agent through the forwarded connection.
336 An attacker cannot obtain key material from the agent,
337 however they can perform operations on the keys that enable them to
338 authenticate using the identities loaded into the agent.
340 Specifies whether X11 connections will be automatically redirected
341 over the secure channel and
351 X11 forwarding should be enabled with caution.
352 Users with the ability to bypass file permissions on the remote host
353 (for the user's X11 authorization database)
354 can access the local X11 display through the forwarded connection.
355 An attacker may then be able to perform activities such as keystroke monitoring
357 .Cm ForwardX11Trusted
358 option is also enabled.
359 .It Cm ForwardX11Trusted
360 If this option is set to
362 then remote X11 clients will have full access to the original X11 display.
364 If this option is set to
366 then remote X11 clients will be considered untrusted and prevented
367 from stealing or tampering with data belonging to trusted X11
371 token used for the session will be set to expire after 20 minutes.
372 Remote clients will be refused access after this time.
377 See the X11 SECURITY extension specification for full details on
378 the restrictions imposed on untrusted clients.
380 Specifies whether remote hosts are allowed to connect to local
384 binds local port forwardings to the loopback address.
385 This prevents other remote hosts from connecting to forwarded ports.
387 can be used to specify that
389 should bind local port forwardings to the wildcard address,
390 thus allowing remote hosts to connect to forwarded ports.
397 .It Cm GlobalKnownHostsFile
398 Specifies a file to use for the global
399 host key database instead of
400 .Pa /etc/ssh/ssh_known_hosts .
401 .It Cm GSSAPIAuthentication
402 Specifies whether user authentication based on GSSAPI is allowed.
405 Note that this option applies to protocol version 2 only.
406 .It Cm GSSAPIDelegateCredentials
407 Forward (delegate) credentials to the server.
410 Note that this option applies to protocol version 2 only.
411 .It Cm HashKnownHosts
414 should hash host names and addresses when they are added to
415 .Pa $HOME/.ssh/known_hosts .
416 These hashed names may be used normally by
420 but they do not reveal identifying information should the file's contents
424 Note that hashing of names and addresses will not be retrospectively applied
425 to existing known hosts files, but these may be manually hashed using
427 .It Cm HostbasedAuthentication
428 Specifies whether to try rhosts based authentication with public key
436 This option applies to protocol version 2 only and
438 .Cm RhostsRSAAuthentication .
439 .It Cm HostKeyAlgorithms
440 Specifies the protocol version 2 host key algorithms
441 that the client wants to use in order of preference.
442 The default for this option is:
443 .Dq ssh-rsa,ssh-dss .
445 Specifies an alias that should be used instead of the
446 real host name when looking up or saving the host key
447 in the host key database files.
448 This option is useful for tunneling ssh connections
449 or for multiple servers running on a single host.
451 Specifies the real host name to log into.
452 This can be used to specify nicknames or abbreviations for hosts.
453 Default is the name given on the command line.
454 Numeric IP addresses are also permitted (both on the command line and in
458 Specifies a file from which the user's RSA or DSA authentication identity
461 .Pa $HOME/.ssh/identity
462 for protocol version 1, and
463 .Pa $HOME/.ssh/id_rsa
465 .Pa $HOME/.ssh/id_dsa
466 for protocol version 2.
467 Additionally, any identities represented by the authentication agent
468 will be used for authentication.
469 The file name may use the tilde
470 syntax to refer to a user's home directory.
471 It is possible to have
472 multiple identity files specified in configuration files; all these
473 identities will be tried in sequence.
474 .It Cm IdentitiesOnly
477 should only use the authentication identity files configured in the
482 offers more identities.
483 The argument to this keyword must be
487 This option is intented for situations where
489 offers many different identities.
492 .It Cm KbdInteractiveDevices
493 Specifies the list of methods to use in keyboard-interactive authentication.
494 Multiple method names must be comma-separated.
495 The default is to use the server specified list.
497 Specifies that a TCP/IP port on the local machine be forwarded over
498 the secure channel to the specified host and port from the remote machine.
499 The first argument must be
501 .Oo Ar bind_address : Oc Ar port
503 and the second argument must be
504 .Ar host : Ns Ar hostport .
505 IPv6 addresses can be specified by enclosing addresses in square brackets or
506 by using an alternative syntax:
507 .Oo Ar bind_address Ns / Oc Ns Ar port
509 .Ar host Ns / Ns Ar hostport .
510 Multiple forwardings may be specified, and additional forwardings can be
511 given on the command line.
512 Only the superuser can forward privileged ports.
513 By default, the local port is bound in accordance with the
518 may be used to bind the connection to a specific address.
523 indicates that the listening port be bound for local use only, while an
526 indicates that the port should be available from all interfaces.
528 Gives the verbosity level that is used when logging messages from
530 The possible values are:
531 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
533 DEBUG and DEBUG1 are equivalent.
534 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
536 Specifies the MAC (message authentication code) algorithms
537 in order of preference.
538 The MAC algorithm is used in protocol version 2
539 for data integrity protection.
540 Multiple algorithms must be comma-separated.
542 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
543 .It Cm NoHostAuthenticationForLocalhost
544 This option can be used if the home directory is shared across machines.
545 In this case localhost will refer to a different machine on each of
546 the machines and the user will get many warnings about changed host keys.
547 However, this option disables host authentication for localhost.
548 The argument to this keyword must be
552 The default is to check the host key for localhost.
553 .It Cm NumberOfPasswordPrompts
554 Specifies the number of password prompts before giving up.
555 The argument to this keyword must be an integer.
557 .It Cm PasswordAuthentication
558 Specifies whether to use password authentication.
559 The argument to this keyword must be
566 Specifies the port number to connect on the remote host.
568 .It Cm PreferredAuthentications
569 Specifies the order in which the client should try protocol 2
570 authentication methods.
571 This allows a client to prefer one method (e.g.\&
572 .Cm keyboard-interactive )
573 over another method (e.g.\&
575 The default for this option is:
576 .Dq hostbased,publickey,keyboard-interactive,password .
578 Specifies the protocol versions
580 should support in order of preference.
581 The possible values are
585 Multiple versions must be comma-separated.
590 tries version 2 and falls back to version 1
591 if version 2 is not available.
593 Specifies the command to use to connect to the server.
595 string extends to the end of the line, and is executed with
597 In the command string,
599 will be substituted by the host name to
603 The command can be basically anything,
604 and should read from its standard input and write to its standard output.
605 It should eventually connect an
607 server running on some machine, or execute
610 Host key management will be done using the
611 HostName of the host being connected (defaulting to the name typed by
613 Setting the command to
615 disables this option entirely.
618 is not available for connects with a proxy command.
620 .It Cm PubkeyAuthentication
621 Specifies whether to try public key authentication.
622 The argument to this keyword must be
628 This option applies to protocol version 2 only.
630 Specifies that a TCP/IP port on the remote machine be forwarded over
631 the secure channel to the specified host and port from the local machine.
632 The first argument must be
634 .Oo Ar bind_address : Oc Ar port
636 and the second argument must be
637 .Ar host : Ns Ar hostport .
638 IPv6 addresses can be specified by enclosing addresses in square brackets
639 or by using an alternative syntax:
640 .Oo Ar bind_address Ns / Oc Ns Ar port
642 .Ar host Ns / Ns Ar hostport .
643 Multiple forwardings may be specified, and additional
644 forwardings can be given on the command line.
645 Only the superuser can forward privileged ports.
649 is not specified, the default is to only bind to loopback addresses.
654 or an empty string, then the forwarding is requested to listen on all
658 will only succeed if the server's
660 option is enabled (see
661 .Xr sshd_config 5 ) .
662 .It Cm RhostsRSAAuthentication
663 Specifies whether to try rhosts based authentication with RSA host
671 This option applies to protocol version 1 only and requires
674 .It Cm RSAAuthentication
675 Specifies whether to try RSA authentication.
676 The argument to this keyword must be
680 RSA authentication will only be
681 attempted if the identity file exists, or an authentication agent is
685 Note that this option applies to protocol version 1 only.
687 Specifies what variables from the local
689 should be sent to the server.
690 Note that environment passing is only supported for protocol 2, the
691 server must also support it, and the server must be configured to
692 accept these environment variables.
697 for how to configure the server.
698 Variables are specified by name, which may contain the wildcard characters
702 Multiple environment variables may be separated by whitespace or spread
706 The default is not to send any environment variables.
707 .It Cm ServerAliveInterval
708 Sets a timeout interval in seconds after which if no data has been received
711 will send a message through the encrypted
712 channel to request a response from the server.
714 is 0, indicating that these messages will not be sent to the server.
715 This option applies to protocol version 2 only.
716 .It Cm ServerAliveCountMax
717 Sets the number of server alive messages (see above) which may be
720 receiving any messages back from the server.
721 If this threshold is reached while server alive messages are being sent,
723 will disconnect from the server, terminating the session.
724 It is important to note that the use of server alive messages is very
728 The server alive messages are sent through the encrypted channel
729 and therefore will not be spoofable.
730 The TCP keepalive option enabled by
733 The server alive mechanism is valuable when the client or
734 server depend on knowing when a connection has become inactive.
736 The default value is 3.
738 .Cm ServerAliveInterval
739 (above) is set to 15, and
740 .Cm ServerAliveCountMax
741 is left at the default, if the server becomes unresponsive ssh
742 will disconnect after approximately 45 seconds.
743 .It Cm SmartcardDevice
744 Specifies which smartcard device to use.
745 The argument to this keyword is the device
747 should use to communicate with a smartcard used for storing the user's
749 By default, no device is specified and smartcard support is not activated.
750 .It Cm StrictHostKeyChecking
751 If this flag is set to
754 will never automatically add host keys to the
755 .Pa $HOME/.ssh/known_hosts
756 file, and refuses to connect to hosts whose host key has changed.
757 This provides maximum protection against trojan horse attacks,
758 however, can be annoying when the
759 .Pa /etc/ssh/ssh_known_hosts
760 file is poorly maintained, or connections to new hosts are
762 This option forces the user to manually
764 If this flag is set to
767 will automatically add new host keys to the
768 user known hosts files.
769 If this flag is set to
772 will be added to the user known host files only after the user
773 has confirmed that is what they really want to do, and
775 will refuse to connect to hosts whose host key has changed.
777 known hosts will be verified automatically in all cases.
786 Specifies whether the system should send TCP keepalive messages to the
788 If they are sent, death of the connection or crash of one
789 of the machines will be properly noticed.
790 However, this means that
791 connections will die if the route is down temporarily, and some people
796 (to send TCP keepalive messages), and the client will notice
797 if the network goes down or the remote host dies.
798 This is important in scripts, and many users want it too.
800 To disable TCP keepalive messages, the value should be set to
802 .It Cm UsePrivilegedPort
803 Specifies whether to use a privileged port for outgoing connections.
814 Note that this option must be set to
817 .Cm RhostsRSAAuthentication
820 Specifies the user to log in as.
821 This can be useful when a different user name is used on different machines.
822 This saves the trouble of
823 having to remember to give the user name on the command line.
824 .It Cm UserKnownHostsFile
825 Specifies a file to use for the user
826 host key database instead of
827 .Pa $HOME/.ssh/known_hosts .
828 .It Cm VerifyHostKeyDNS
829 Specifies whether to verify the remote key using DNS and SSHFP resource
831 If this option is set to
833 the client will implicitly trust keys that match a secure fingerprint
835 Insecure fingerprints will be handled as if this option was set to
837 If this option is set to
839 information on fingerprint match will be displayed, but the user will still
840 need to confirm new host keys according to the
841 .Cm StrictHostKeyChecking
850 Note that this option applies to protocol version 2 only.
851 .It Cm VersionAddendum
852 Specifies a string to append to the regular version string to identify
853 OS- or site-specific modifications.
855 .Dq FreeBSD-20050605 .
857 Specifies the full pathname of the
861 .Pa /usr/X11R6/bin/xauth .
865 .It Pa $HOME/.ssh/config
866 This is the per-user configuration file.
867 The format of this file is described above.
868 This file is used by the
871 Because of the potential for abuse, this file must have strict permissions:
872 read/write for the user, and not accessible by others.
873 .It Pa /etc/ssh/ssh_config
874 Systemwide configuration file.
875 This file provides defaults for those
876 values that are not specified in the user's configuration file, and
877 for those users who do not have a configuration file.
878 This file must be world-readable.
883 OpenSSH is a derivative of the original and free
884 ssh 1.2.12 release by Tatu Ylonen.
885 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
886 Theo de Raadt and Dug Song
887 removed many bugs, re-added newer features and
889 Markus Friedl contributed the support for SSH
890 protocol versions 1.5 and 2.0.