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 1.97 2006/07/27 08:00:50 jmc Exp $
39 .Dd September 25, 1999
44 .Nd OpenSSH SSH client configuration files
46 .Bl -tag -width Ds -compact
48 .It Pa /etc/ssh/ssh_config
52 obtains configuration data from the following sources in
55 .Bl -enum -offset indent -compact
59 user's configuration file
62 system-wide configuration file
63 .Pq Pa /etc/ssh/ssh_config
66 For each parameter, the first obtained value
68 The configuration files contain sections separated by
70 specifications, and that section is only applied for hosts that
71 match one of the patterns given in the specification.
72 The matched host name is the one given on the command line.
74 Since the first obtained value for each parameter is used, more
75 host-specific declarations should be given near the beginning of the
76 file, and general defaults at the end.
78 The configuration file has the following format:
80 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
96 Arguments may optionally be enclosed in double quotes
98 in order to represent arguments containing spaces.
101 keywords and their meanings are as follows (note that
102 keywords are case-insensitive and arguments are case-sensitive):
105 Restricts the following declarations (up to the next
107 keyword) to be only for those hosts that match one of the patterns
108 given after the keyword.
111 as a pattern can be used to provide global
112 defaults for all hosts.
115 argument given on the command line (i.e. the name is not converted to
116 a canonicalized host name before matching).
120 for more information on patterns.
122 Specifies which address family to use when connecting.
132 passphrase/password querying will be disabled.
133 This option is useful in scripts and other batch jobs where no user
134 is present to supply the password.
142 Use the specified address on the local machine as the source address of
144 Only useful on systems with more than one address.
145 Note that this option does not work if
146 .Cm UsePrivilegedPort
149 .It Cm ChallengeResponseAuthentication
150 Specifies whether to use challenge-response authentication.
151 The argument to this keyword must be
158 If this flag is set to
161 will additionally check the host IP address in the
164 This allows ssh to detect if a host key changed due to DNS spoofing.
165 If the option is set to
167 the check will not be executed.
171 Specifies the cipher to use for encrypting the session
172 in protocol version 1.
180 is only supported in the
182 client for interoperability with legacy protocol 1 implementations
183 that do not support the
186 Its use is strongly discouraged due to cryptographic weaknesses.
190 Specifies the ciphers allowed for protocol version 2
191 in order of preference.
192 Multiple ciphers must be comma-separated.
193 The supported ciphers are
208 .Bd -literal -offset 3n
209 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
210 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
211 aes192-ctr,aes256-ctr
213 .It Cm ClearAllForwardings
214 Specifies that all local, remote, and dynamic port forwardings
215 specified in the configuration files or on the command line be
217 This option is primarily useful when used from the
219 command line to clear port forwardings set in
220 configuration files, and is automatically set by
231 Specifies whether to use compression.
238 .It Cm CompressionLevel
239 Specifies the compression level to use if compression is enabled.
240 The argument must be an integer from 1 (fast) to 9 (slow, best).
241 The default level is 6, which is good for most applications.
242 The meaning of the values is the same as in
244 Note that this option applies to protocol version 1 only.
245 .It Cm ConnectionAttempts
246 Specifies the number of tries (one per second) to make before exiting.
247 The argument must be an integer.
248 This may be useful in scripts if the connection sometimes fails.
250 .It Cm ConnectTimeout
251 Specifies the timeout (in seconds) used when connecting to the
252 SSH server, instead of using the default system TCP timeout.
253 This value is used only when the target is down or really unreachable,
254 not when it refuses the connection.
256 Enables the sharing of multiple sessions over a single network connection.
260 will listen for connections on a control socket specified using the
263 Additional sessions can connect to this socket using the same
270 These sessions will try to reuse the master instance's network connection
271 rather than initiating new ones, but will fall back to connecting normally
272 if the control socket does not exist, or is not listening.
277 to listen for control connections, but require confirmation using the
279 program before they are accepted (see
285 ssh will continue without connecting to a master instance.
289 forwarding is supported over these multiplexed connections, however the
290 display and agent forwarded will be the one belonging to the master
291 connection i.e. it is not possible to forward multiple displays or agents.
293 Two additional options allow for opportunistic multiplexing: try to use a
294 master connection but fall back to creating a new one if one does not already
300 The latter requires confirmation like the
304 Specify the path to the control socket used for connection sharing as described
307 section above or the string
309 to disable connection sharing.
312 will be substituted by the local host name,
314 will be substituted by the target host name,
318 by the remote login username.
319 It is recommended that any
321 used for opportunistic connection sharing include
322 at least %h, %p, and %r.
323 This ensures that shared connections are uniquely identified.
324 .It Cm DynamicForward
325 Specifies that a TCP port on the local machine be forwarded
326 over the secure channel, and the application
327 protocol is then used to determine where to connect to from the
332 .Oo Ar bind_address : Oc Ar port .
334 IPv6 addresses can be specified by enclosing addresses in square brackets or
335 by using an alternative syntax:
336 .Oo Ar bind_address Ns / Oc Ns Ar port .
337 By default, the local port is bound in accordance with the
342 may be used to bind the connection to a specific address.
347 indicates that the listening port be bound for local use only, while an
350 indicates that the port should be available from all interfaces.
352 Currently the SOCKS4 and SOCKS5 protocols are supported, and
354 will act as a SOCKS server.
355 Multiple forwardings may be specified, and
356 additional forwardings can be given on the command line.
357 Only the superuser can forward privileged ports.
358 .It Cm EnableSSHKeysign
359 Setting this option to
361 in the global client configuration file
362 .Pa /etc/ssh/ssh_config
363 enables the use of the helper program
366 .Cm HostbasedAuthentication .
373 This option should be placed in the non-hostspecific section.
376 for more information.
378 Sets the escape character (default:
380 The escape character can also
381 be set on the command line.
382 The argument should be a single character,
384 followed by a letter, or
386 to disable the escape
387 character entirely (making the connection transparent for binary
389 .It Cm ExitOnForwardFailure
392 should terminate the connection if it cannot set up all requested
393 dynamic, local, and remote port forwardings.
401 Specifies whether the connection to the authentication agent (if any)
402 will be forwarded to the remote machine.
410 Agent forwarding should be enabled with caution.
411 Users with the ability to bypass file permissions on the remote host
412 (for the agent's Unix-domain socket)
413 can access the local agent through the forwarded connection.
414 An attacker cannot obtain key material from the agent,
415 however they can perform operations on the keys that enable them to
416 authenticate using the identities loaded into the agent.
418 Specifies whether X11 connections will be automatically redirected
419 over the secure channel and
429 X11 forwarding should be enabled with caution.
430 Users with the ability to bypass file permissions on the remote host
431 (for the user's X11 authorization database)
432 can access the local X11 display through the forwarded connection.
433 An attacker may then be able to perform activities such as keystroke monitoring
435 .Cm ForwardX11Trusted
436 option is also enabled.
437 .It Cm ForwardX11Trusted
438 If this option is set to
440 remote X11 clients will have full access to the original X11 display.
442 If this option is set to
444 remote X11 clients will be considered untrusted and prevented
445 from stealing or tampering with data belonging to trusted X11
449 token used for the session will be set to expire after 20 minutes.
450 Remote clients will be refused access after this time.
455 See the X11 SECURITY extension specification for full details on
456 the restrictions imposed on untrusted clients.
458 Specifies whether remote hosts are allowed to connect to local
462 binds local port forwardings to the loopback address.
463 This prevents other remote hosts from connecting to forwarded ports.
465 can be used to specify that ssh
466 should bind local port forwardings to the wildcard address,
467 thus allowing remote hosts to connect to forwarded ports.
474 .It Cm GlobalKnownHostsFile
475 Specifies a file to use for the global
476 host key database instead of
477 .Pa /etc/ssh/ssh_known_hosts .
478 .It Cm GSSAPIAuthentication
479 Specifies whether user authentication based on GSSAPI is allowed.
482 Note that this option applies to protocol version 2 only.
483 .It Cm GSSAPIDelegateCredentials
484 Forward (delegate) credentials to the server.
487 Note that this option applies to protocol version 2 only.
488 .It Cm HashKnownHosts
491 should hash host names and addresses when they are added to
492 .Pa ~/.ssh/known_hosts .
493 These hashed names may be used normally by
497 but they do not reveal identifying information should the file's contents
501 Note that existing names and addresses in known hosts files
502 will not be converted automatically,
503 but may be manually hashed using
505 .It Cm HostbasedAuthentication
506 Specifies whether to try rhosts based authentication with public key
514 This option applies to protocol version 2 only and
516 .Cm RhostsRSAAuthentication .
517 .It Cm HostKeyAlgorithms
518 Specifies the protocol version 2 host key algorithms
519 that the client wants to use in order of preference.
520 The default for this option is:
521 .Dq ssh-rsa,ssh-dss .
523 Specifies an alias that should be used instead of the
524 real host name when looking up or saving the host key
525 in the host key database files.
526 This option is useful for tunneling SSH connections
527 or for multiple servers running on a single host.
529 Specifies the real host name to log into.
530 This can be used to specify nicknames or abbreviations for hosts.
531 The default is the name given on the command line.
532 Numeric IP addresses are also permitted (both on the command line and in
535 .It Cm IdentitiesOnly
538 should only use the authentication identity files configured in the
543 offers more identities.
544 The argument to this keyword must be
548 This option is intended for situations where ssh-agent
549 offers many different identities.
553 Specifies a file from which the user's RSA or DSA authentication identity
557 for protocol version 1, and
561 for protocol version 2.
562 Additionally, any identities represented by the authentication agent
563 will be used for authentication.
565 The file name may use the tilde
566 syntax to refer to a user's home directory or one of the following
569 (local user's home directory),
575 (remote host name) or
579 It is possible to have
580 multiple identity files specified in configuration files; all these
581 identities will be tried in sequence.
582 .It Cm KbdInteractiveDevices
583 Specifies the list of methods to use in keyboard-interactive authentication.
584 Multiple method names must be comma-separated.
585 The default is to use the server specified list.
586 The methods available vary depending on what the server supports.
587 For an OpenSSH server,
588 it may be zero or more of:
594 Specifies a command to execute on the local machine after successfully
595 connecting to the server.
596 The command string extends to the end of the line, and is executed with
598 This directive is ignored unless
599 .Cm PermitLocalCommand
602 Specifies that a TCP port on the local machine be forwarded over
603 the secure channel to the specified host and port from the remote machine.
604 The first argument must be
606 .Oo Ar bind_address : Oc Ar port
608 and the second argument must be
609 .Ar host : Ns Ar hostport .
610 IPv6 addresses can be specified by enclosing addresses in square brackets or
611 by using an alternative syntax:
612 .Oo Ar bind_address Ns / Oc Ns Ar port
614 .Ar host Ns / Ns Ar hostport .
615 Multiple forwardings may be specified, and additional forwardings can be
616 given on the command line.
617 Only the superuser can forward privileged ports.
618 By default, the local port is bound in accordance with the
623 may be used to bind the connection to a specific address.
628 indicates that the listening port be bound for local use only, while an
631 indicates that the port should be available from all interfaces.
633 Gives the verbosity level that is used when logging messages from
635 The possible values are:
636 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
638 DEBUG and DEBUG1 are equivalent.
639 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
641 Specifies the MAC (message authentication code) algorithms
642 in order of preference.
643 The MAC algorithm is used in protocol version 2
644 for data integrity protection.
645 Multiple algorithms must be comma-separated.
647 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
648 .It Cm NoHostAuthenticationForLocalhost
649 This option can be used if the home directory is shared across machines.
650 In this case localhost will refer to a different machine on each of
651 the machines and the user will get many warnings about changed host keys.
652 However, this option disables host authentication for localhost.
653 The argument to this keyword must be
657 The default is to check the host key for localhost.
658 .It Cm NumberOfPasswordPrompts
659 Specifies the number of password prompts before giving up.
660 The argument to this keyword must be an integer.
662 .It Cm PasswordAuthentication
663 Specifies whether to use password authentication.
664 The argument to this keyword must be
670 .It Cm PermitLocalCommand
671 Allow local command execution via the
674 .Ic !\& Ns Ar command
684 Specifies the port number to connect on the remote host.
686 .It Cm PreferredAuthentications
687 Specifies the order in which the client should try protocol 2
688 authentication methods.
689 This allows a client to prefer one method (e.g.\&
690 .Cm keyboard-interactive )
691 over another method (e.g.\&
693 The default for this option is:
694 .Do gssapi-with-mic ,
697 keyboard-interactive,
701 Specifies the protocol versions
703 should support in order of preference.
704 The possible values are
708 Multiple versions must be comma-separated.
712 tries version 2 and falls back to version 1
713 if version 2 is not available.
715 Specifies the command to use to connect to the server.
717 string extends to the end of the line, and is executed with
719 In the command string,
721 will be substituted by the host name to
725 The command can be basically anything,
726 and should read from its standard input and write to its standard output.
727 It should eventually connect an
729 server running on some machine, or execute
732 Host key management will be done using the
733 HostName of the host being connected (defaulting to the name typed by
735 Setting the command to
737 disables this option entirely.
740 is not available for connects with a proxy command.
742 This directive is useful in conjunction with
744 and its proxy support.
745 For example, the following directive would connect via an HTTP proxy at
747 .Bd -literal -offset 3n
748 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
750 .It Cm PubkeyAuthentication
751 Specifies whether to try public key authentication.
752 The argument to this keyword must be
758 This option applies to protocol version 2 only.
760 Specifies the maximum amount of data that may be transmitted before the
761 session key is renegotiated.
762 The argument is the number of bytes, with an optional suffix of
767 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
768 The default is between
772 depending on the cipher.
773 This option applies to protocol version 2 only.
775 Specifies that a TCP port on the remote machine be forwarded over
776 the secure channel to the specified host and port from the local machine.
777 The first argument must be
779 .Oo Ar bind_address : Oc Ar port
781 and the second argument must be
782 .Ar host : Ns Ar hostport .
783 IPv6 addresses can be specified by enclosing addresses in square brackets
784 or by using an alternative syntax:
785 .Oo Ar bind_address Ns / Oc Ns Ar port
787 .Ar host Ns / Ns Ar hostport .
788 Multiple forwardings may be specified, and additional
789 forwardings can be given on the command line.
790 Only the superuser can forward privileged ports.
794 is not specified, the default is to only bind to loopback addresses.
799 or an empty string, then the forwarding is requested to listen on all
803 will only succeed if the server's
805 option is enabled (see
806 .Xr sshd_config 5 ) .
807 .It Cm RhostsRSAAuthentication
808 Specifies whether to try rhosts based authentication with RSA host
816 This option applies to protocol version 1 only and requires
819 .It Cm RSAAuthentication
820 Specifies whether to try RSA authentication.
821 The argument to this keyword must be
825 RSA authentication will only be
826 attempted if the identity file exists, or an authentication agent is
830 Note that this option applies to protocol version 1 only.
832 Specifies what variables from the local
834 should be sent to the server.
835 Note that environment passing is only supported for protocol 2.
836 The server must also support it, and the server must be configured to
837 accept these environment variables.
842 for how to configure the server.
843 Variables are specified by name, which may contain wildcard characters.
844 Multiple environment variables may be separated by whitespace or spread
848 The default is not to send any environment variables.
852 for more information on patterns.
853 .It Cm ServerAliveCountMax
854 Sets the number of server alive messages (see below) which may be
857 receiving any messages back from the server.
858 If this threshold is reached while server alive messages are being sent,
859 ssh will disconnect from the server, terminating the session.
860 It is important to note that the use of server alive messages is very
864 The server alive messages are sent through the encrypted channel
865 and therefore will not be spoofable.
866 The TCP keepalive option enabled by
869 The server alive mechanism is valuable when the client or
870 server depend on knowing when a connection has become inactive.
872 The default value is 3.
874 .Cm ServerAliveInterval
875 (see below) is set to 15 and
876 .Cm ServerAliveCountMax
877 is left at the default, if the server becomes unresponsive,
878 ssh will disconnect after approximately 45 seconds.
879 This option applies to protocol version 2 only.
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.
960 requests the default tunnel mode, which is
967 devices to open on the client
974 .Ar local_tun Op : Ar remote_tun .
976 The devices may be specified by numerical ID or the keyword
978 which uses the next available tunnel device.
981 is not specified, it defaults to
985 .It Cm UsePrivilegedPort
986 Specifies whether to use a privileged port for outgoing connections.
997 Note that this option must be set to
1000 .Cm RhostsRSAAuthentication
1003 Specifies the user to log in as.
1004 This can be useful when a different user name is used on different machines.
1005 This saves the trouble of
1006 having to remember to give the user name on the command line.
1007 .It Cm UserKnownHostsFile
1008 Specifies a file to use for the user
1009 host key database instead of
1010 .Pa ~/.ssh/known_hosts .
1011 .It Cm VerifyHostKeyDNS
1012 Specifies whether to verify the remote key using DNS and SSHFP resource
1014 If this option is set to
1016 the client will implicitly trust keys that match a secure fingerprint
1018 Insecure fingerprints will be handled as if this option was set to
1020 If this option is set to
1022 information on fingerprint match will be displayed, but the user will still
1023 need to confirm new host keys according to the
1024 .Cm StrictHostKeyChecking
1026 The argument must be
1033 Note that this option applies to protocol version 2 only.
1036 .Sx VERIFYING HOST KEYS
1039 .It Cm VersionAddendum
1040 Specifies a string to append to the regular version string to identify
1041 OS- or site-specific modifications.
1043 .Dq FreeBSD-20061110 .
1044 .It Cm XAuthLocation
1045 Specifies the full pathname of the
1049 .Pa /usr/local/bin/xauth .
1054 consists of zero or more non-whitespace characters,
1056 (a wildcard that matches zero or more characters),
1059 (a wildcard that matches exactly one character).
1060 For example, to specify a set of declarations for any host in the
1063 the following pattern could be used:
1067 The following pattern
1068 would match any host in the 192.168.0.[0-9] network range:
1070 .Dl Host 192.168.0.?
1074 is a comma-separated list of patterns.
1075 Patterns within pattern-lists may be negated
1076 by preceding them with an exclamation mark
1079 to allow a key to be used from anywhere within an organisation
1083 the following entry (in authorized_keys) could be used:
1085 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1088 .It Pa ~/.ssh/config
1089 This is the per-user configuration file.
1090 The format of this file is described above.
1091 This file is used by the SSH client.
1092 Because of the potential for abuse, this file must have strict permissions:
1093 read/write for the user, and not accessible by others.
1094 .It Pa /etc/ssh/ssh_config
1095 Systemwide configuration file.
1096 This file provides defaults for those
1097 values that are not specified in the user's configuration file, and
1098 for those users who do not have a configuration file.
1099 This file must be world-readable.
1104 OpenSSH is a derivative of the original and free
1105 ssh 1.2.12 release by Tatu Ylonen.
1106 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1107 Theo de Raadt and Dug Song
1108 removed many bugs, re-added newer features and
1110 Markus Friedl contributed the support for SSH
1111 protocol versions 1.5 and 2.0.
projects / FreeBSD / FreeBSD.git / blob