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 1.80 2006/02/24 10:25:14 jmc Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH client configuration files
45 .Bl -tag -width Ds -compact
47 .It Pa /etc/ssh/ssh_config
51 obtains configuration data from the following sources in
54 .Bl -enum -offset indent -compact
58 user's configuration file
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.
79 consists of zero or more non-whitespace characters,
81 (a wildcard that matches zero or more characters),
84 (a wildcard that matches exactly one character).
85 For example, to specify a set of declarations for any host in the
88 the following pattern could be used:
93 would match any host in the 192.168.0.[0-9] network range:
99 is a comma-separated list of patterns.
100 Patterns within pattern-lists may be negated
101 by preceding them with an exclamation mark
104 to allow a key to be used from anywhere within an organisation
108 the following entry (in authorized_keys) could be used:
110 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
112 The configuration file has the following format:
114 Empty lines and lines starting with
117 Otherwise a line is of the format
118 .Dq keyword arguments .
119 Configuration options may be separated by whitespace or
120 optional whitespace and exactly one
122 the latter format is useful to avoid the need to quote whitespace
123 when specifying configuration options using the
132 keywords and their meanings are as follows (note that
133 keywords are case-insensitive and arguments are case-sensitive):
136 Restricts the following declarations (up to the next
138 keyword) to be only for those hosts that match one of the patterns
139 given after the keyword.
142 as a pattern can be used to provide global
143 defaults for all hosts.
146 for more information on patterns.
149 argument given on the command line (i.e., the name is not converted to
150 a canonicalized host name before matching).
152 Specifies which address family to use when connecting.
162 passphrase/password querying will be disabled.
163 This option is useful in scripts and other batch jobs where no user
164 is present to supply the password.
172 Use the specified address on the local machine as the source address of
174 Only useful on systems with more than one address.
175 Note that this option does not work if
176 .Cm UsePrivilegedPort
179 .It Cm ChallengeResponseAuthentication
180 Specifies whether to use challenge response authentication.
181 The argument to this keyword must be
188 If this flag is set to
190 ssh will additionally check the host IP address in the
193 This allows ssh to detect if a host key changed due to DNS spoofing.
194 If the option is set to
196 the check will not be executed.
200 Specifies the cipher to use for encrypting the session
201 in protocol version 1.
209 is only supported in the
211 client for interoperability with legacy protocol 1 implementations
212 that do not support the
215 Its use is strongly discouraged due to cryptographic weaknesses.
219 Specifies the ciphers allowed for protocol version 2
220 in order of preference.
221 Multiple ciphers must be comma-separated.
222 The supported ciphers are
238 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
239 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
240 aes192-ctr,aes256-ctr''
242 .It Cm ClearAllForwardings
243 Specifies that all local, remote and dynamic port forwardings
244 specified in the configuration files or on the command line be
246 This option is primarily useful when used from the
248 command line to clear port forwardings set in
249 configuration files, and is automatically set by
260 Specifies whether to use compression.
267 .It Cm CompressionLevel
268 Specifies the compression level to use if compression is enabled.
269 The argument must be an integer from 1 (fast) to 9 (slow, best).
270 The default level is 6, which is good for most applications.
271 The meaning of the values is the same as in
273 Note that this option applies to protocol version 1 only.
274 .It Cm ConnectionAttempts
275 Specifies the number of tries (one per second) to make before exiting.
276 The argument must be an integer.
277 This may be useful in scripts if the connection sometimes fails.
279 .It Cm ConnectTimeout
280 Specifies the timeout (in seconds) used when connecting to the ssh
281 server, instead of using the default system TCP timeout.
282 This value is used only when the target is down or really unreachable,
283 not when it refuses the connection.
285 Enables the sharing of multiple sessions over a single network connection.
289 will listen for connections on a control socket specified using the
292 Additional sessions can connect to this socket using the same
299 These sessions will try to reuse the master instance's network connection
300 rather than initiating new ones, but will fall back to connecting normally
301 if the control socket does not exist, or is not listening.
307 to listen for control connections, but require confirmation using the
309 program before they are accepted (see
316 will continue without connecting to a master instance.
320 forwarding is supported over these multiplexed connections, however the
321 display and agent forwarded will be the one belonging to the master
322 connection i.e. it is not possible to forward multiple displays or agents.
324 Two additional options allow for opportunistic multiplexing: try to use a
325 master connection but fall back to creating a new one if one does not already
331 The latter requires confirmation like the
335 Specify the path to the control socket used for connection sharing as described
338 section above or the string
340 to disable connection sharing.
343 will be substituted by the local host name,
345 will be substituted by the target host name,
349 by the remote login username.
350 It is recommended that any
352 used for opportunistic connection sharing include
353 at least %h, %p, and %r.
354 This ensures that shared connections are uniquely identified.
355 .It Cm DynamicForward
356 Specifies that a TCP port on the local machine be forwarded
357 over the secure channel, and the application
358 protocol is then used to determine where to connect to from the
363 .Oo Ar bind_address : Oc Ar port .
365 IPv6 addresses can be specified by enclosing addresses in square brackets or
366 by using an alternative syntax:
367 .Oo Ar bind_address Ns / Oc Ns Ar port .
368 By default, the local port is bound in accordance with the
373 may be used to bind the connection to a specific address.
378 indicates that the listening port be bound for local use only, while an
381 indicates that the port should be available from all interfaces.
383 Currently the SOCKS4 and SOCKS5 protocols are supported, and
385 will act as a SOCKS server.
386 Multiple forwardings may be specified, and
387 additional forwardings can be given on the command line.
388 Only the superuser can forward privileged ports.
389 .It Cm EnableSSHKeysign
390 Setting this option to
392 in the global client configuration file
393 .Pa /etc/ssh/ssh_config
394 enables the use of the helper program
397 .Cm HostbasedAuthentication .
404 This option should be placed in the non-hostspecific section.
407 for more information.
409 Sets the escape character (default:
411 The escape character can also
412 be set on the command line.
413 The argument should be a single character,
415 followed by a letter, or
417 to disable the escape
418 character entirely (making the connection transparent for binary
421 Specifies whether the connection to the authentication agent (if any)
422 will be forwarded to the remote machine.
430 Agent forwarding should be enabled with caution.
431 Users with the ability to bypass file permissions on the remote host
432 (for the agent's Unix-domain socket)
433 can access the local agent through the forwarded connection.
434 An attacker cannot obtain key material from the agent,
435 however they can perform operations on the keys that enable them to
436 authenticate using the identities loaded into the agent.
438 Specifies whether X11 connections will be automatically redirected
439 over the secure channel and
449 X11 forwarding should be enabled with caution.
450 Users with the ability to bypass file permissions on the remote host
451 (for the user's X11 authorization database)
452 can access the local X11 display through the forwarded connection.
453 An attacker may then be able to perform activities such as keystroke monitoring
455 .Cm ForwardX11Trusted
456 option is also enabled.
457 .It Cm ForwardX11Trusted
458 If this option is set to
460 then remote X11 clients will have full access to the original X11 display.
462 If this option is set to
464 then remote X11 clients will be considered untrusted and prevented
465 from stealing or tampering with data belonging to trusted X11
469 token used for the session will be set to expire after 20 minutes.
470 Remote clients will be refused access after this time.
475 See the X11 SECURITY extension specification for full details on
476 the restrictions imposed on untrusted clients.
478 Specifies whether remote hosts are allowed to connect to local
482 binds local port forwardings to the loopback address.
483 This prevents other remote hosts from connecting to forwarded ports.
485 can be used to specify that
487 should bind local port forwardings to the wildcard address,
488 thus allowing remote hosts to connect to forwarded ports.
495 .It Cm GlobalKnownHostsFile
496 Specifies a file to use for the global
497 host key database instead of
498 .Pa /etc/ssh/ssh_known_hosts .
499 .It Cm GSSAPIAuthentication
500 Specifies whether user authentication based on GSSAPI is allowed.
503 Note that this option applies to protocol version 2 only.
504 .It Cm GSSAPIDelegateCredentials
505 Forward (delegate) credentials to the server.
508 Note that this option applies to protocol version 2 only.
509 .It Cm HashKnownHosts
512 should hash host names and addresses when they are added to
513 .Pa ~/.ssh/known_hosts .
514 These hashed names may be used normally by
518 but they do not reveal identifying information should the file's contents
522 Note that hashing of names and addresses will not be retrospectively applied
523 to existing known hosts files, but these may be manually hashed using
525 .It Cm HostbasedAuthentication
526 Specifies whether to try rhosts based authentication with public key
534 This option applies to protocol version 2 only and
536 .Cm RhostsRSAAuthentication .
537 .It Cm HostKeyAlgorithms
538 Specifies the protocol version 2 host key algorithms
539 that the client wants to use in order of preference.
540 The default for this option is:
541 .Dq ssh-rsa,ssh-dss .
543 Specifies an alias that should be used instead of the
544 real host name when looking up or saving the host key
545 in the host key database files.
546 This option is useful for tunneling ssh connections
547 or for multiple servers running on a single host.
549 Specifies the real host name to log into.
550 This can be used to specify nicknames or abbreviations for hosts.
551 Default is the name given on the command line.
552 Numeric IP addresses are also permitted (both on the command line and in
555 .It Cm IdentitiesOnly
558 should only use the authentication identity files configured in the
563 offers more identities.
564 The argument to this keyword must be
568 This option is intended for situations where
570 offers many different identities.
574 Specifies a file from which the user's RSA or DSA authentication identity
578 for protocol version 1, and
582 for protocol version 2.
583 Additionally, any identities represented by the authentication agent
584 will be used for authentication.
585 The file name may use the tilde
586 syntax to refer to a user's home directory.
587 It is possible to have
588 multiple identity files specified in configuration files; all these
589 identities will be tried in sequence.
590 .It Cm KbdInteractiveDevices
591 Specifies the list of methods to use in keyboard-interactive authentication.
592 Multiple method names must be comma-separated.
593 The default is to use the server specified list.
595 Specifies a command to execute on the local machine after successfully
596 connecting to the server.
597 The command string extends to the end of the line, and is executed with
599 This directive is ignored unless
600 .Cm PermitLocalCommand
603 Specifies that a TCP port on the local machine be forwarded over
604 the secure channel to the specified host and port from the remote machine.
605 The first argument must be
607 .Oo Ar bind_address : Oc Ar port
609 and the second argument must be
610 .Ar host : Ns Ar hostport .
611 IPv6 addresses can be specified by enclosing addresses in square brackets or
612 by using an alternative syntax:
613 .Oo Ar bind_address Ns / Oc Ns Ar port
615 .Ar host Ns / Ns Ar hostport .
616 Multiple forwardings may be specified, and additional forwardings can be
617 given on the command line.
618 Only the superuser can forward privileged ports.
619 By default, the local port is bound in accordance with the
624 may be used to bind the connection to a specific address.
629 indicates that the listening port be bound for local use only, while an
632 indicates that the port should be available from all interfaces.
634 Gives the verbosity level that is used when logging messages from
636 The possible values are:
637 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
639 DEBUG and DEBUG1 are equivalent.
640 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
642 Specifies the MAC (message authentication code) algorithms
643 in order of preference.
644 The MAC algorithm is used in protocol version 2
645 for data integrity protection.
646 Multiple algorithms must be comma-separated.
648 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
649 .It Cm NoHostAuthenticationForLocalhost
650 This option can be used if the home directory is shared across machines.
651 In this case localhost will refer to a different machine on each of
652 the machines and the user will get many warnings about changed host keys.
653 However, this option disables host authentication for localhost.
654 The argument to this keyword must be
658 The default is to check the host key for localhost.
659 .It Cm NumberOfPasswordPrompts
660 Specifies the number of password prompts before giving up.
661 The argument to this keyword must be an integer.
663 .It Cm PasswordAuthentication
664 Specifies whether to use password authentication.
665 The argument to this keyword must be
671 .It Cm PermitLocalCommand
672 Allow local command execution via the
675 .Ic !\& Ns Ar command
685 Specifies the port number to connect on the remote host.
687 .It Cm PreferredAuthentications
688 Specifies the order in which the client should try protocol 2
689 authentication methods.
690 This allows a client to prefer one method (e.g.\&
691 .Cm keyboard-interactive )
692 over another method (e.g.\&
694 The default for this option is:
695 .Dq hostbased,publickey,keyboard-interactive,password .
697 Specifies the protocol versions
699 should support in order of preference.
700 The possible values are
704 Multiple versions must be comma-separated.
709 tries version 2 and falls back to version 1
710 if version 2 is not available.
712 Specifies the command to use to connect to the server.
714 string extends to the end of the line, and is executed with
716 In the command string,
718 will be substituted by the host name to
722 The command can be basically anything,
723 and should read from its standard input and write to its standard output.
724 It should eventually connect an
726 server running on some machine, or execute
729 Host key management will be done using the
730 HostName of the host being connected (defaulting to the name typed by
732 Setting the command to
734 disables this option entirely.
737 is not available for connects with a proxy command.
739 This directive is useful in conjunction with
741 and its proxy support.
742 For example, the following directive would connect via an HTTP proxy at
744 .Bd -literal -offset 3n
745 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
747 .It Cm PubkeyAuthentication
748 Specifies whether to try public key authentication.
749 The argument to this keyword must be
755 This option applies to protocol version 2 only.
757 Specifies the maximum amount of data that may be transmitted before the
758 session key is renegotiated.
759 The argument is the number of bytes, with an optional suffix of
764 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
765 The default is between
769 depending on the cipher.
770 This option applies to protocol version 2 only.
772 Specifies that a TCP port on the remote machine be forwarded over
773 the secure channel to the specified host and port from the local machine.
774 The first argument must be
776 .Oo Ar bind_address : Oc Ar port
778 and the second argument must be
779 .Ar host : Ns Ar hostport .
780 IPv6 addresses can be specified by enclosing addresses in square brackets
781 or by using an alternative syntax:
782 .Oo Ar bind_address Ns / Oc Ns Ar port
784 .Ar host Ns / Ns Ar hostport .
785 Multiple forwardings may be specified, and additional
786 forwardings can be given on the command line.
787 Only the superuser can forward privileged ports.
791 is not specified, the default is to only bind to loopback addresses.
796 or an empty string, then the forwarding is requested to listen on all
800 will only succeed if the server's
802 option is enabled (see
803 .Xr sshd_config 5 ) .
804 .It Cm RhostsRSAAuthentication
805 Specifies whether to try rhosts based authentication with RSA host
813 This option applies to protocol version 1 only and requires
816 .It Cm RSAAuthentication
817 Specifies whether to try RSA authentication.
818 The argument to this keyword must be
822 RSA authentication will only be
823 attempted if the identity file exists, or an authentication agent is
827 Note that this option applies to protocol version 1 only.
829 Specifies what variables from the local
831 should be sent to the server.
832 Note that environment passing is only supported for protocol 2, the
833 server must also support it, and the server must be configured to
834 accept these environment variables.
839 for how to configure the server.
840 Variables are specified by name, which may contain wildcard characters.
843 for more information on patterns.
844 Multiple environment variables may be separated by whitespace or spread
848 The default is not to send any environment variables.
849 .It Cm ServerAliveCountMax
850 Sets the number of server alive messages (see below) which may be
853 receiving any messages back from the server.
854 If this threshold is reached while server alive messages are being sent,
856 will disconnect from the server, terminating the session.
857 It is important to note that the use of server alive messages is very
861 The server alive messages are sent through the encrypted channel
862 and therefore will not be spoofable.
863 The TCP keepalive option enabled by
866 The server alive mechanism is valuable when the client or
867 server depend on knowing when a connection has become inactive.
869 The default value is 3.
871 .Cm ServerAliveInterval
872 (see below) is set to 15, and
873 .Cm ServerAliveCountMax
874 is left at the default, if the server becomes unresponsive ssh
875 will disconnect after approximately 45 seconds.
876 .It Cm ServerAliveInterval
877 Sets a timeout interval in seconds after which if no data has been received
880 will send a message through the encrypted
881 channel to request a response from the server.
883 is 0, indicating that these messages will not be sent to the server.
884 This option applies to protocol version 2 only.
885 .It Cm SmartcardDevice
886 Specifies which smartcard device to use.
887 The argument to this keyword is the device
889 should use to communicate with a smartcard used for storing the user's
891 By default, no device is specified and smartcard support is not activated.
892 .It Cm StrictHostKeyChecking
893 If this flag is set to
896 will never automatically add host keys to the
897 .Pa ~/.ssh/known_hosts
898 file, and refuses to connect to hosts whose host key has changed.
899 This provides maximum protection against trojan horse attacks,
900 however, can be annoying when the
901 .Pa /etc/ssh/ssh_known_hosts
902 file is poorly maintained, or connections to new hosts are
904 This option forces the user to manually
906 If this flag is set to
909 will automatically add new host keys to the
910 user known hosts files.
911 If this flag is set to
914 will be added to the user known host files only after the user
915 has confirmed that is what they really want to do, and
917 will refuse to connect to hosts whose host key has changed.
919 known hosts will be verified automatically in all cases.
928 Specifies whether the system should send TCP keepalive messages to the
930 If they are sent, death of the connection or crash of one
931 of the machines will be properly noticed.
932 However, this means that
933 connections will die if the route is down temporarily, and some people
938 (to send TCP keepalive messages), and the client will notice
939 if the network goes down or the remote host dies.
940 This is important in scripts, and many users want it too.
942 To disable TCP keepalive messages, the value should be set to
947 device forwarding between the client and the server.
948 This option also allows requesting layer 2 (ethernet)
949 instead of layer 3 (point-to-point) tunneling from the server.
961 device on the client.
962 Without this option, the next available device will be used.
963 .It Cm UsePrivilegedPort
964 Specifies whether to use a privileged port for outgoing connections.
975 Note that this option must be set to
978 .Cm RhostsRSAAuthentication
981 Specifies the user to log in as.
982 This can be useful when a different user name is used on different machines.
983 This saves the trouble of
984 having to remember to give the user name on the command line.
985 .It Cm UserKnownHostsFile
986 Specifies a file to use for the user
987 host key database instead of
988 .Pa ~/.ssh/known_hosts .
989 .It Cm VerifyHostKeyDNS
990 Specifies whether to verify the remote key using DNS and SSHFP resource
992 If this option is set to
994 the client will implicitly trust keys that match a secure fingerprint
996 Insecure fingerprints will be handled as if this option was set to
998 If this option is set to
1000 information on fingerprint match will be displayed, but the user will still
1001 need to confirm new host keys according to the
1002 .Cm StrictHostKeyChecking
1004 The argument must be
1011 Note that this option applies to protocol version 2 only.
1012 .It Cm XAuthLocation
1013 Specifies the full pathname of the
1017 .Pa /usr/X11R6/bin/xauth .
1021 .It Pa ~/.ssh/config
1022 This is the per-user configuration file.
1023 The format of this file is described above.
1024 This file is used by the
1027 Because of the potential for abuse, this file must have strict permissions:
1028 read/write for the user, and not accessible by others.
1029 .It Pa /etc/ssh/ssh_config
1030 Systemwide configuration file.
1031 This file provides defaults for those
1032 values that are not specified in the user's configuration file, and
1033 for those users who do not have a configuration file.
1034 This file must be world-readable.
1039 OpenSSH is a derivative of the original and free
1040 ssh 1.2.12 release by Tatu Ylonen.
1041 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1042 Theo de Raadt and Dug Song
1043 removed many bugs, re-added newer features and
1045 Markus Friedl contributed the support for SSH
1046 protocol versions 1.5 and 2.0.