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 1.105 2007/10/29 07:48:19 jmc Exp $
38 .Dd $Mdocdate: December 2 2007 $
43 .Nd OpenSSH SSH client configuration files
46 .Nm /etc/ssh/ssh_config
49 obtains configuration data from the following sources in
52 .Bl -enum -offset indent -compact
56 user's configuration file
59 system-wide configuration file
60 .Pq Pa /etc/ssh/ssh_config
63 For each parameter, the first obtained value
65 The configuration files contain sections separated by
67 specifications, and that section is only applied for hosts that
68 match one of the patterns given in the specification.
69 The matched host name is the one given on the command line.
71 Since the first obtained value for each parameter is used, more
72 host-specific declarations should be given near the beginning of the
73 file, and general defaults at the end.
75 The configuration file has the following format:
77 Empty lines and lines starting with
80 Otherwise a line is of the format
81 .Dq keyword arguments .
82 Configuration options may be separated by whitespace or
83 optional whitespace and exactly one
85 the latter format is useful to avoid the need to quote whitespace
86 when specifying configuration options using the
93 Arguments may optionally be enclosed in double quotes
95 in order to represent arguments containing spaces.
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.
108 as a pattern can be used to provide global
109 defaults for all hosts.
112 argument given on the command line (i.e. the name is not converted to
113 a canonicalized host name before matching).
117 for more information on patterns.
119 Specifies which address family to use when connecting.
129 passphrase/password querying will be disabled.
130 This option is useful in scripts and other batch jobs where no user
131 is present to supply the password.
139 Use the specified address on the local machine as the source address of
141 Only useful on systems with more than one address.
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
158 will additionally check the host IP address in the
161 This allows ssh to detect if a host key changed due to DNS spoofing.
162 If the option is set to
164 the check will not be executed.
168 Specifies the cipher to use for encrypting the session
169 in protocol version 1.
177 is only supported in the
179 client for interoperability with legacy protocol 1 implementations
180 that do not support the
183 Its use is strongly discouraged due to cryptographic weaknesses.
187 Specifies the ciphers allowed for protocol version 2
188 in order of preference.
189 Multiple ciphers must be comma-separated.
190 The supported ciphers are
205 .Bd -literal -offset 3n
206 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
207 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
208 aes192-ctr,aes256-ctr
210 .It Cm ClearAllForwardings
211 Specifies that all local, remote, and dynamic port forwardings
212 specified in the configuration files or on the command line be
214 This option is primarily useful when used from the
216 command line to clear port forwardings set in
217 configuration files, and is automatically set by
228 Specifies whether to use compression.
235 .It Cm CompressionLevel
236 Specifies the compression level to use if compression is enabled.
237 The argument must be an integer from 1 (fast) to 9 (slow, best).
238 The default level is 6, which is good for most applications.
239 The meaning of the values is the same as in
241 Note that this option applies to protocol version 1 only.
242 .It Cm ConnectionAttempts
243 Specifies the number of tries (one per second) to make before exiting.
244 The argument must be an integer.
245 This may be useful in scripts if the connection sometimes fails.
247 .It Cm ConnectTimeout
248 Specifies the timeout (in seconds) used when connecting to the
249 SSH server, instead of using the default system TCP timeout.
250 This value is used only when the target is down or really unreachable,
251 not when it refuses the connection.
253 Enables the sharing of multiple sessions over a single network connection.
257 will listen for connections on a control socket specified using the
260 Additional sessions can connect to this socket using the same
267 These sessions will try to reuse the master instance's network connection
268 rather than initiating new ones, but will fall back to connecting normally
269 if the control socket does not exist, or is not listening.
274 to listen for control connections, but require confirmation using the
276 program before they are accepted (see
282 ssh will continue without connecting to a master instance.
286 forwarding is supported over these multiplexed connections, however the
287 display and agent forwarded will be the one belonging to the master
288 connection i.e. it is not possible to forward multiple displays or agents.
290 Two additional options allow for opportunistic multiplexing: try to use a
291 master connection but fall back to creating a new one if one does not already
297 The latter requires confirmation like the
301 Specify the path to the control socket used for connection sharing as described
304 section above or the string
306 to disable connection sharing.
309 will be substituted by the local host name,
311 will be substituted by the target host name,
315 by the remote login username.
316 It is recommended that any
318 used for opportunistic connection sharing include
319 at least %h, %p, and %r.
320 This ensures that shared connections are uniquely identified.
321 .It Cm DynamicForward
322 Specifies that a TCP port on the local machine be forwarded
323 over the secure channel, and the application
324 protocol is then used to determine where to connect to from the
329 .Oo Ar bind_address : Oc Ar port .
331 IPv6 addresses can be specified by enclosing addresses in square brackets or
332 by using an alternative syntax:
333 .Oo Ar bind_address Ns / Oc Ns Ar port .
334 By default, the local port is bound in accordance with the
339 may be used to bind the connection to a specific address.
344 indicates that the listening port be bound for local use only, while an
347 indicates that the port should be available from all interfaces.
349 Currently the SOCKS4 and SOCKS5 protocols are supported, and
351 will act as a SOCKS server.
352 Multiple forwardings may be specified, and
353 additional forwardings can be given on the command line.
354 Only the superuser can forward privileged ports.
355 .It Cm EnableSSHKeysign
356 Setting this option to
358 in the global client configuration file
359 .Pa /etc/ssh/ssh_config
360 enables the use of the helper program
363 .Cm HostbasedAuthentication .
370 This option should be placed in the non-hostspecific section.
373 for more information.
375 Sets the escape character (default:
377 The escape character can also
378 be set on the command line.
379 The argument should be a single character,
381 followed by a letter, or
383 to disable the escape
384 character entirely (making the connection transparent for binary
386 .It Cm ExitOnForwardFailure
389 should terminate the connection if it cannot set up all requested
390 dynamic, tunnel, local, and remote port forwardings.
398 Specifies whether the connection to the authentication agent (if any)
399 will be forwarded to the remote machine.
407 Agent forwarding should be enabled with caution.
408 Users with the ability to bypass file permissions on the remote host
409 (for the agent's Unix-domain socket)
410 can access the local agent through the forwarded connection.
411 An attacker cannot obtain key material from the agent,
412 however they can perform operations on the keys that enable them to
413 authenticate using the identities loaded into the agent.
415 Specifies whether X11 connections will be automatically redirected
416 over the secure channel and
426 X11 forwarding should be enabled with caution.
427 Users with the ability to bypass file permissions on the remote host
428 (for the user's X11 authorization database)
429 can access the local X11 display through the forwarded connection.
430 An attacker may then be able to perform activities such as keystroke monitoring
432 .Cm ForwardX11Trusted
433 option is also enabled.
434 .It Cm ForwardX11Trusted
435 If this option is set to
437 remote X11 clients will have full access to the original X11 display.
439 If this option is set to
441 remote X11 clients will be considered untrusted and prevented
442 from stealing or tampering with data belonging to trusted X11
446 token used for the session will be set to expire after 20 minutes.
447 Remote clients will be refused access after this time.
452 See the X11 SECURITY extension specification for full details on
453 the restrictions imposed on untrusted clients.
455 Specifies whether remote hosts are allowed to connect to local
459 binds local port forwardings to the loopback address.
460 This prevents other remote hosts from connecting to forwarded ports.
462 can be used to specify that ssh
463 should bind local port forwardings to the wildcard address,
464 thus allowing remote hosts to connect to forwarded ports.
471 .It Cm GlobalKnownHostsFile
472 Specifies a file to use for the global
473 host key database instead of
474 .Pa /etc/ssh/ssh_known_hosts .
475 .It Cm GSSAPIAuthentication
476 Specifies whether user authentication based on GSSAPI is allowed.
479 Note that this option applies to protocol version 2 only.
480 .It Cm GSSAPIDelegateCredentials
481 Forward (delegate) credentials to the server.
484 Note that this option applies to protocol version 2 only.
485 .It Cm HashKnownHosts
488 should hash host names and addresses when they are added to
489 .Pa ~/.ssh/known_hosts .
490 These hashed names may be used normally by
494 but they do not reveal identifying information should the file's contents
498 Note that existing names and addresses in known hosts files
499 will not be converted automatically,
500 but may be manually hashed using
502 .It Cm HostbasedAuthentication
503 Specifies whether to try rhosts based authentication with public key
511 This option applies to protocol version 2 only and
513 .Cm RhostsRSAAuthentication .
514 .It Cm HostKeyAlgorithms
515 Specifies the protocol version 2 host key algorithms
516 that the client wants to use in order of preference.
517 The default for this option is:
518 .Dq ssh-rsa,ssh-dss .
520 Specifies an alias that should be used instead of the
521 real host name when looking up or saving the host key
522 in the host key database files.
523 This option is useful for tunneling SSH connections
524 or for multiple servers running on a single host.
526 Specifies the real host name to log into.
527 This can be used to specify nicknames or abbreviations for hosts.
528 The default is the name given on the command line.
529 Numeric IP addresses are also permitted (both on the command line and in
532 .It Cm IdentitiesOnly
535 should only use the authentication identity files configured in the
540 offers more identities.
541 The argument to this keyword must be
545 This option is intended for situations where ssh-agent
546 offers many different identities.
550 Specifies a file from which the user's RSA or DSA authentication identity
554 for protocol version 1, and
558 for protocol version 2.
559 Additionally, any identities represented by the authentication agent
560 will be used for authentication.
562 The file name may use the tilde
563 syntax to refer to a user's home directory or one of the following
566 (local user's home directory),
572 (remote host name) or
576 It is possible to have
577 multiple identity files specified in configuration files; all these
578 identities will be tried in sequence.
579 .It Cm KbdInteractiveAuthentication
580 Specifies whether to use keyboard-interactive authentication.
581 The argument to this keyword must be
587 .It Cm KbdInteractiveDevices
588 Specifies the list of methods to use in keyboard-interactive authentication.
589 Multiple method names must be comma-separated.
590 The default is to use the server specified list.
591 The methods available vary depending on what the server supports.
592 For an OpenSSH server,
593 it may be zero or more of:
599 Specifies a command to execute on the local machine after successfully
600 connecting to the server.
601 The command string extends to the end of the line, and is executed with
603 This directive is ignored unless
604 .Cm PermitLocalCommand
607 Specifies that a TCP port on the local machine be forwarded over
608 the secure channel to the specified host and port from the remote machine.
609 The first argument must be
611 .Oo Ar bind_address : Oc Ar port
613 and the second argument must be
614 .Ar host : Ns Ar hostport .
615 IPv6 addresses can be specified by enclosing addresses in square brackets or
616 by using an alternative syntax:
617 .Oo Ar bind_address Ns / Oc Ns Ar port
619 .Ar host Ns / Ns Ar hostport .
620 Multiple forwardings may be specified, and additional forwardings can be
621 given on the command line.
622 Only the superuser can forward privileged ports.
623 By default, the local port is bound in accordance with the
628 may be used to bind the connection to a specific address.
633 indicates that the listening port be bound for local use only, while an
636 indicates that the port should be available from all interfaces.
638 Gives the verbosity level that is used when logging messages from
640 The possible values are:
641 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
643 DEBUG and DEBUG1 are equivalent.
644 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
646 Specifies the MAC (message authentication code) algorithms
647 in order of preference.
648 The MAC algorithm is used in protocol version 2
649 for data integrity protection.
650 Multiple algorithms must be comma-separated.
652 .Bd -literal -offset indent
653 hmac-md5,hmac-sha1,umac-64@openssh.com,
654 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
656 .It Cm NoHostAuthenticationForLocalhost
657 This option can be used if the home directory is shared across machines.
658 In this case localhost will refer to a different machine on each of
659 the machines and the user will get many warnings about changed host keys.
660 However, this option disables host authentication for localhost.
661 The argument to this keyword must be
665 The default is to check the host key for localhost.
666 .It Cm NumberOfPasswordPrompts
667 Specifies the number of password prompts before giving up.
668 The argument to this keyword must be an integer.
670 .It Cm PasswordAuthentication
671 Specifies whether to use password authentication.
672 The argument to this keyword must be
678 .It Cm PermitLocalCommand
679 Allow local command execution via the
682 .Ic !\& Ns Ar command
692 Specifies the port number to connect on the remote host.
694 .It Cm PreferredAuthentications
695 Specifies the order in which the client should try protocol 2
696 authentication methods.
697 This allows a client to prefer one method (e.g.\&
698 .Cm keyboard-interactive )
699 over another method (e.g.\&
701 The default for this option is:
702 .Do gssapi-with-mic ,
705 keyboard-interactive,
709 Specifies the protocol versions
711 should support in order of preference.
712 The possible values are
716 Multiple versions must be comma-separated.
720 tries version 2 and falls back to version 1
721 if version 2 is not available.
723 Specifies the command to use to connect to the server.
725 string extends to the end of the line, and is executed with
727 In the command string,
729 will be substituted by the host name to
733 The command can be basically anything,
734 and should read from its standard input and write to its standard output.
735 It should eventually connect an
737 server running on some machine, or execute
740 Host key management will be done using the
741 HostName of the host being connected (defaulting to the name typed by
743 Setting the command to
745 disables this option entirely.
748 is not available for connects with a proxy command.
750 This directive is useful in conjunction with
752 and its proxy support.
753 For example, the following directive would connect via an HTTP proxy at
755 .Bd -literal -offset 3n
756 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
758 .It Cm PubkeyAuthentication
759 Specifies whether to try public key authentication.
760 The argument to this keyword must be
766 This option applies to protocol version 2 only.
768 Specifies the maximum amount of data that may be transmitted before the
769 session key is renegotiated.
770 The argument is the number of bytes, with an optional suffix of
775 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
776 The default is between
780 depending on the cipher.
781 This option applies to protocol version 2 only.
783 Specifies that a TCP port on the remote machine be forwarded over
784 the secure channel to the specified host and port from the local machine.
785 The first argument must be
787 .Oo Ar bind_address : Oc Ar port
789 and the second argument must be
790 .Ar host : Ns Ar hostport .
791 IPv6 addresses can be specified by enclosing addresses in square brackets
792 or by using an alternative syntax:
793 .Oo Ar bind_address Ns / Oc Ns Ar port
795 .Ar host Ns / Ns Ar hostport .
796 Multiple forwardings may be specified, and additional
797 forwardings can be given on the command line.
798 Only the superuser can forward privileged ports.
802 is not specified, the default is to only bind to loopback addresses.
807 or an empty string, then the forwarding is requested to listen on all
811 will only succeed if the server's
813 option is enabled (see
814 .Xr sshd_config 5 ) .
815 .It Cm RhostsRSAAuthentication
816 Specifies whether to try rhosts based authentication with RSA host
824 This option applies to protocol version 1 only and requires
827 .It Cm RSAAuthentication
828 Specifies whether to try RSA authentication.
829 The argument to this keyword must be
833 RSA authentication will only be
834 attempted if the identity file exists, or an authentication agent is
838 Note that this option applies to protocol version 1 only.
840 Specifies what variables from the local
842 should be sent to the server.
843 Note that environment passing is only supported for protocol 2.
844 The server must also support it, and the server must be configured to
845 accept these environment variables.
850 for how to configure the server.
851 Variables are specified by name, which may contain wildcard characters.
852 Multiple environment variables may be separated by whitespace or spread
856 The default is not to send any environment variables.
860 for more information on patterns.
861 .It Cm ServerAliveCountMax
862 Sets the number of server alive messages (see below) which may be
865 receiving any messages back from the server.
866 If this threshold is reached while server alive messages are being sent,
867 ssh will disconnect from the server, terminating the session.
868 It is important to note that the use of server alive messages is very
872 The server alive messages are sent through the encrypted channel
873 and therefore will not be spoofable.
874 The TCP keepalive option enabled by
877 The server alive mechanism is valuable when the client or
878 server depend on knowing when a connection has become inactive.
880 The default value is 3.
882 .Cm ServerAliveInterval
883 (see below) is set to 15 and
884 .Cm ServerAliveCountMax
885 is left at the default, if the server becomes unresponsive,
886 ssh will disconnect after approximately 45 seconds.
887 This option applies to protocol version 2 only.
888 .It Cm ServerAliveInterval
889 Sets a timeout interval in seconds after which if no data has been received
892 will send a message through the encrypted
893 channel to request a response from the server.
895 is 0, indicating that these messages will not be sent to the server.
896 This option applies to protocol version 2 only.
897 .It Cm SmartcardDevice
898 Specifies which smartcard device to use.
899 The argument to this keyword is the device
901 should use to communicate with a smartcard used for storing the user's
903 By default, no device is specified and smartcard support is not activated.
904 .It Cm StrictHostKeyChecking
905 If this flag is set to
908 will never automatically add host keys to the
909 .Pa ~/.ssh/known_hosts
910 file, and refuses to connect to hosts whose host key has changed.
911 This provides maximum protection against trojan horse attacks,
912 though it can be annoying when the
913 .Pa /etc/ssh/ssh_known_hosts
914 file is poorly maintained or when connections to new hosts are
916 This option forces the user to manually
918 If this flag is set to
920 ssh will automatically add new host keys to the
921 user known hosts files.
922 If this flag is set to
925 will be added to the user known host files only after the user
926 has confirmed that is what they really want to do, and
927 ssh will refuse to connect to hosts whose host key has changed.
929 known hosts will be verified automatically in all cases.
938 Specifies whether the system should send TCP keepalive messages to the
940 If they are sent, death of the connection or crash of one
941 of the machines will be properly noticed.
942 However, this means that
943 connections will die if the route is down temporarily, and some people
948 (to send TCP keepalive messages), and the client will notice
949 if the network goes down or the remote host dies.
950 This is important in scripts, and many users want it too.
952 To disable TCP keepalive messages, the value should be set to
957 device forwarding between the client and the server.
968 requests the default tunnel mode, which is
975 devices to open on the client
982 .Ar local_tun Op : Ar remote_tun .
984 The devices may be specified by numerical ID or the keyword
986 which uses the next available tunnel device.
989 is not specified, it defaults to
993 .It Cm UsePrivilegedPort
994 Specifies whether to use a privileged port for outgoing connections.
1004 must be setuid root.
1005 Note that this option must be set to
1008 .Cm RhostsRSAAuthentication
1011 Specifies the user to log in as.
1012 This can be useful when a different user name is used on different machines.
1013 This saves the trouble of
1014 having to remember to give the user name on the command line.
1015 .It Cm UserKnownHostsFile
1016 Specifies a file to use for the user
1017 host key database instead of
1018 .Pa ~/.ssh/known_hosts .
1019 .It Cm VerifyHostKeyDNS
1020 Specifies whether to verify the remote key using DNS and SSHFP resource
1022 If this option is set to
1024 the client will implicitly trust keys that match a secure fingerprint
1026 Insecure fingerprints will be handled as if this option was set to
1028 If this option is set to
1030 information on fingerprint match will be displayed, but the user will still
1031 need to confirm new host keys according to the
1032 .Cm StrictHostKeyChecking
1034 The argument must be
1041 Note that this option applies to protocol version 2 only.
1044 .Sx VERIFYING HOST KEYS
1047 .It Cm VersionAddendum
1048 Specifies a string to append to the regular version string to identify
1049 OS- or site-specific modifications.
1051 .Dq DragonFly-20080406 .
1052 .It Cm XAuthLocation
1053 Specifies the full pathname of the
1057 .Pa /usr/X11R6/bin/xauth .
1062 consists of zero or more non-whitespace characters,
1064 (a wildcard that matches zero or more characters),
1067 (a wildcard that matches exactly one character).
1068 For example, to specify a set of declarations for any host in the
1071 the following pattern could be used:
1075 The following pattern
1076 would match any host in the 192.168.0.[0-9] network range:
1078 .Dl Host 192.168.0.?
1082 is a comma-separated list of patterns.
1083 Patterns within pattern-lists may be negated
1084 by preceding them with an exclamation mark
1087 to allow a key to be used from anywhere within an organisation
1091 the following entry (in authorized_keys) could be used:
1093 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1096 .It Pa ~/.ssh/config
1097 This is the per-user configuration file.
1098 The format of this file is described above.
1099 This file is used by the SSH client.
1100 Because of the potential for abuse, this file must have strict permissions:
1101 read/write for the user, and not accessible by others.
1102 .It Pa /etc/ssh/ssh_config
1103 Systemwide configuration file.
1104 This file provides defaults for those
1105 values that are not specified in the user's configuration file, and
1106 for those users who do not have a configuration file.
1107 This file must be world-readable.
1112 OpenSSH is a derivative of the original and free
1113 ssh 1.2.12 release by Tatu Ylonen.
1114 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1115 Theo de Raadt and Dug Song
1116 removed many bugs, re-added newer features and
1118 Markus Friedl contributed the support for SSH
1119 protocol versions 1.5 and 2.0.
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