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 1.85 2006/02/25 12:26:17 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 argument given on the command line (i.e. the name is not converted to
147 a canonicalized host name before matching).
151 for more information on patterns.
153 Specifies which address family to use when connecting.
163 passphrase/password querying will be disabled.
164 This option is useful in scripts and other batch jobs where no user
165 is present to supply the password.
173 Use the specified address on the local machine as the source address of
175 Only useful on systems with more than one address.
176 Note that this option does not work if
177 .Cm UsePrivilegedPort
180 .It Cm ChallengeResponseAuthentication
181 Specifies whether to use challenge-response authentication.
182 The argument to this keyword must be
189 If this flag is set to
192 will additionally check the host IP address in the
195 This allows ssh to detect if a host key changed due to DNS spoofing.
196 If the option is set to
198 the check will not be executed.
202 Specifies the cipher to use for encrypting the session
203 in protocol version 1.
211 is only supported in the
213 client for interoperability with legacy protocol 1 implementations
214 that do not support the
217 Its use is strongly discouraged due to cryptographic weaknesses.
221 Specifies the ciphers allowed for protocol version 2
222 in order of preference.
223 Multiple ciphers must be comma-separated.
224 The supported ciphers are
239 .Bd -literal -offset 3n
240 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
241 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
242 aes192-ctr,aes256-ctr
244 .It Cm ClearAllForwardings
245 Specifies that all local, remote, and dynamic port forwardings
246 specified in the configuration files or on the command line be
248 This option is primarily useful when used from the
250 command line to clear port forwardings set in
251 configuration files, and is automatically set by
262 Specifies whether to use compression.
269 .It Cm CompressionLevel
270 Specifies the compression level to use if compression is enabled.
271 The argument must be an integer from 1 (fast) to 9 (slow, best).
272 The default level is 6, which is good for most applications.
273 The meaning of the values is the same as in
275 Note that this option applies to protocol version 1 only.
276 .It Cm ConnectionAttempts
277 Specifies the number of tries (one per second) to make before exiting.
278 The argument must be an integer.
279 This may be useful in scripts if the connection sometimes fails.
281 .It Cm ConnectTimeout
282 Specifies the timeout (in seconds) used when connecting to the
283 SSH server, instead of using the default system TCP timeout.
284 This value is used only when the target is down or really unreachable,
285 not when it refuses the connection.
287 Enables the sharing of multiple sessions over a single network connection.
291 will listen for connections on a control socket specified using the
294 Additional sessions can connect to this socket using the same
301 These sessions will try to reuse the master instance's network connection
302 rather than initiating new ones, but will fall back to connecting normally
303 if the control socket does not exist, or is not listening.
308 to listen for control connections, but require confirmation using the
310 program before they are accepted (see
316 ssh 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 remote X11 clients will have full access to the original X11 display.
462 If this option is set to
464 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 ssh
486 should bind local port forwardings to the wildcard address,
487 thus allowing remote hosts to connect to forwarded ports.
494 .It Cm GlobalKnownHostsFile
495 Specifies a file to use for the global
496 host key database instead of
497 .Pa /etc/ssh/ssh_known_hosts .
498 .It Cm GSSAPIAuthentication
499 Specifies whether user authentication based on GSSAPI is allowed.
502 Note that this option applies to protocol version 2 only.
503 .It Cm GSSAPIDelegateCredentials
504 Forward (delegate) credentials to the server.
507 Note that this option applies to protocol version 2 only.
508 .It Cm HashKnownHosts
511 should hash host names and addresses when they are added to
512 .Pa ~/.ssh/known_hosts .
513 These hashed names may be used normally by
517 but they do not reveal identifying information should the file's contents
521 Note that hashing of names and addresses will not be retrospectively applied
522 to existing known hosts files, but these may be manually hashed using
524 .It Cm HostbasedAuthentication
525 Specifies whether to try rhosts based authentication with public key
533 This option applies to protocol version 2 only and
535 .Cm RhostsRSAAuthentication .
536 .It Cm HostKeyAlgorithms
537 Specifies the protocol version 2 host key algorithms
538 that the client wants to use in order of preference.
539 The default for this option is:
540 .Dq ssh-rsa,ssh-dss .
542 Specifies an alias that should be used instead of the
543 real host name when looking up or saving the host key
544 in the host key database files.
545 This option is useful for tunneling SSH connections
546 or for multiple servers running on a single host.
548 Specifies the real host name to log into.
549 This can be used to specify nicknames or abbreviations for hosts.
550 The default is the name given on the command line.
551 Numeric IP addresses are also permitted (both on the command line and in
554 .It Cm IdentitiesOnly
557 should only use the authentication identity files configured in the
562 offers more identities.
563 The argument to this keyword must be
567 This option is intended for situations where ssh-agent
568 offers many different identities.
572 Specifies a file from which the user's RSA or DSA authentication identity
576 for protocol version 1, and
580 for protocol version 2.
581 Additionally, any identities represented by the authentication agent
582 will be used for authentication.
583 The file name may use the tilde
584 syntax to refer to a user's home directory.
585 It is possible to have
586 multiple identity files specified in configuration files; all these
587 identities will be tried in sequence.
588 .It Cm KbdInteractiveDevices
589 Specifies the list of methods to use in keyboard-interactive authentication.
590 Multiple method names must be comma-separated.
591 The default is to use the server specified list.
592 The methods available vary depending on what the server supports.
593 For an OpenSSH server,
594 it may be zero or more of:
600 Specifies a command to execute on the local machine after successfully
601 connecting to the server.
602 The command string extends to the end of the line, and is executed with
604 This directive is ignored unless
605 .Cm PermitLocalCommand
608 Specifies that a TCP port on the local machine be forwarded over
609 the secure channel to the specified host and port from the remote machine.
610 The first argument must be
612 .Oo Ar bind_address : Oc Ar port
614 and the second argument must be
615 .Ar host : Ns Ar hostport .
616 IPv6 addresses can be specified by enclosing addresses in square brackets or
617 by using an alternative syntax:
618 .Oo Ar bind_address Ns / Oc Ns Ar port
620 .Ar host Ns / Ns Ar hostport .
621 Multiple forwardings may be specified, and additional forwardings can be
622 given on the command line.
623 Only the superuser can forward privileged ports.
624 By default, the local port is bound in accordance with the
629 may be used to bind the connection to a specific address.
634 indicates that the listening port be bound for local use only, while an
637 indicates that the port should be available from all interfaces.
639 Gives the verbosity level that is used when logging messages from
641 The possible values are:
642 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
644 DEBUG and DEBUG1 are equivalent.
645 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
647 Specifies the MAC (message authentication code) algorithms
648 in order of preference.
649 The MAC algorithm is used in protocol version 2
650 for data integrity protection.
651 Multiple algorithms must be comma-separated.
653 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
654 .It Cm NoHostAuthenticationForLocalhost
655 This option can be used if the home directory is shared across machines.
656 In this case localhost will refer to a different machine on each of
657 the machines and the user will get many warnings about changed host keys.
658 However, this option disables host authentication for localhost.
659 The argument to this keyword must be
663 The default is to check the host key for localhost.
664 .It Cm NumberOfPasswordPrompts
665 Specifies the number of password prompts before giving up.
666 The argument to this keyword must be an integer.
668 .It Cm PasswordAuthentication
669 Specifies whether to use password authentication.
670 The argument to this keyword must be
676 .It Cm PermitLocalCommand
677 Allow local command execution via the
680 .Ic !\& Ns Ar command
690 Specifies the port number to connect on the remote host.
692 .It Cm PreferredAuthentications
693 Specifies the order in which the client should try protocol 2
694 authentication methods.
695 This allows a client to prefer one method (e.g.\&
696 .Cm keyboard-interactive )
697 over another method (e.g.\&
699 The default for this option is:
700 .Dq hostbased,publickey,keyboard-interactive,password .
702 Specifies the protocol versions
704 should support in order of preference.
705 The possible values are
709 Multiple versions must be comma-separated.
713 tries version 2 and falls back to version 1
714 if version 2 is not available.
716 Specifies the command to use to connect to the server.
718 string extends to the end of the line, and is executed with
720 In the command string,
722 will be substituted by the host name to
726 The command can be basically anything,
727 and should read from its standard input and write to its standard output.
728 It should eventually connect an
730 server running on some machine, or execute
733 Host key management will be done using the
734 HostName of the host being connected (defaulting to the name typed by
736 Setting the command to
738 disables this option entirely.
741 is not available for connects with a proxy command.
743 This directive is useful in conjunction with
745 and its proxy support.
746 For example, the following directive would connect via an HTTP proxy at
748 .Bd -literal -offset 3n
749 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
751 .It Cm PubkeyAuthentication
752 Specifies whether to try public key authentication.
753 The argument to this keyword must be
759 This option applies to protocol version 2 only.
761 Specifies the maximum amount of data that may be transmitted before the
762 session key is renegotiated.
763 The argument is the number of bytes, with an optional suffix of
768 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
769 The default is between
773 depending on the cipher.
774 This option applies to protocol version 2 only.
776 Specifies that a TCP port on the remote machine be forwarded over
777 the secure channel to the specified host and port from the local machine.
778 The first argument must be
780 .Oo Ar bind_address : Oc Ar port
782 and the second argument must be
783 .Ar host : Ns Ar hostport .
784 IPv6 addresses can be specified by enclosing addresses in square brackets
785 or by using an alternative syntax:
786 .Oo Ar bind_address Ns / Oc Ns Ar port
788 .Ar host Ns / Ns Ar hostport .
789 Multiple forwardings may be specified, and additional
790 forwardings can be given on the command line.
791 Only the superuser can forward privileged ports.
795 is not specified, the default is to only bind to loopback addresses.
800 or an empty string, then the forwarding is requested to listen on all
804 will only succeed if the server's
806 option is enabled (see
807 .Xr sshd_config 5 ) .
808 .It Cm RhostsRSAAuthentication
809 Specifies whether to try rhosts based authentication with RSA host
817 This option applies to protocol version 1 only and requires
820 .It Cm RSAAuthentication
821 Specifies whether to try RSA authentication.
822 The argument to this keyword must be
826 RSA authentication will only be
827 attempted if the identity file exists, or an authentication agent is
831 Note that this option applies to protocol version 1 only.
833 Specifies what variables from the local
835 should be sent to the server.
836 Note that environment passing is only supported for protocol 2.
837 The server must also support it, and the server must be configured to
838 accept these environment variables.
843 for how to configure the server.
844 Variables are specified by name, which may contain wildcard characters.
845 Multiple environment variables may be separated by whitespace or spread
849 The default is not to send any environment variables.
853 for more information on patterns.
854 .It Cm ServerAliveCountMax
855 Sets the number of server alive messages (see below) which may be
858 receiving any messages back from the server.
859 If this threshold is reached while server alive messages are being sent,
860 ssh will disconnect from the server, terminating the session.
861 It is important to note that the use of server alive messages is very
865 The server alive messages are sent through the encrypted channel
866 and therefore will not be spoofable.
867 The TCP keepalive option enabled by
870 The server alive mechanism is valuable when the client or
871 server depend on knowing when a connection has become inactive.
873 The default value is 3.
875 .Cm ServerAliveInterval
876 (see below) is set to 15 and
877 .Cm ServerAliveCountMax
878 is left at the default, if the server becomes unresponsive,
879 ssh will disconnect after approximately 45 seconds.
880 .It Cm ServerAliveInterval
881 Sets a timeout interval in seconds after which if no data has been received
884 will send a message through the encrypted
885 channel to request a response from the server.
887 is 0, indicating that these messages will not be sent to the server.
888 This option applies to protocol version 2 only.
889 .It Cm SmartcardDevice
890 Specifies which smartcard device to use.
891 The argument to this keyword is the device
893 should use to communicate with a smartcard used for storing the user's
895 By default, no device is specified and smartcard support is not activated.
896 .It Cm StrictHostKeyChecking
897 If this flag is set to
900 will never automatically add host keys to the
901 .Pa ~/.ssh/known_hosts
902 file, and refuses to connect to hosts whose host key has changed.
903 This provides maximum protection against trojan horse attacks,
904 though it can be annoying when the
905 .Pa /etc/ssh/ssh_known_hosts
906 file is poorly maintained or when connections to new hosts are
908 This option forces the user to manually
910 If this flag is set to
912 ssh will automatically add new host keys to the
913 user known hosts files.
914 If this flag is set to
917 will be added to the user known host files only after the user
918 has confirmed that is what they really want to do, and
919 ssh will refuse to connect to hosts whose host key has changed.
921 known hosts will be verified automatically in all cases.
930 Specifies whether the system should send TCP keepalive messages to the
932 If they are sent, death of the connection or crash of one
933 of the machines will be properly noticed.
934 However, this means that
935 connections will die if the route is down temporarily, and some people
940 (to send TCP keepalive messages), and the client will notice
941 if the network goes down or the remote host dies.
942 This is important in scripts, and many users want it too.
944 To disable TCP keepalive messages, the value should be set to
949 device forwarding between the client and the server.
950 This option also allows requesting layer 2 (ethernet)
951 instead of layer 3 (point-to-point) tunneling from the server.
963 device on the client.
964 Without this option, the next available device will be used.
965 .It Cm UsePrivilegedPort
966 Specifies whether to use a privileged port for outgoing connections.
977 Note that this option must be set to
980 .Cm RhostsRSAAuthentication
983 Specifies the user to log in as.
984 This can be useful when a different user name is used on different machines.
985 This saves the trouble of
986 having to remember to give the user name on the command line.
987 .It Cm UserKnownHostsFile
988 Specifies a file to use for the user
989 host key database instead of
990 .Pa ~/.ssh/known_hosts .
991 .It Cm VerifyHostKeyDNS
992 Specifies whether to verify the remote key using DNS and SSHFP resource
994 If this option is set to
996 the client will implicitly trust keys that match a secure fingerprint
998 Insecure fingerprints will be handled as if this option was set to
1000 If this option is set to
1002 information on fingerprint match will be displayed, but the user will still
1003 need to confirm new host keys according to the
1004 .Cm StrictHostKeyChecking
1006 The argument must be
1013 Note that this option applies to protocol version 2 only.
1016 .Sx VERIFYING HOST KEYS
1019 .It Cm XAuthLocation
1020 Specifies the full pathname of the
1024 .Pa /usr/X11R6/bin/xauth .
1028 .It Pa ~/.ssh/config
1029 This is the per-user configuration file.
1030 The format of this file is described above.
1031 This file is used by the SSH client.
1032 Because of the potential for abuse, this file must have strict permissions:
1033 read/write for the user, and not accessible by others.
1034 .It Pa /etc/ssh/ssh_config
1035 Systemwide configuration file.
1036 This file provides defaults for those
1037 values that are not specified in the user's configuration file, and
1038 for those users who do not have a configuration file.
1039 This file must be world-readable.
1044 OpenSSH is a derivative of the original and free
1045 ssh 1.2.12 release by Tatu Ylonen.
1046 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1047 Theo de Raadt and Dug Song
1048 removed many bugs, re-added newer features and
1050 Markus Friedl contributed the support for SSH
1051 protocol versions 1.5 and 2.0.
andersk / openssh.git / blob