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 $
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.
77 The configuration file has the following format:
79 Empty lines and lines starting with
82 Otherwise a line is of the format
83 .Dq keyword arguments .
84 Configuration options may be separated by whitespace or
85 optional whitespace and exactly one
87 the latter format is useful to avoid the need to quote whitespace
88 when specifying configuration options using the
95 Arguments may optionally be enclosed in double quotes
97 in order to represent arguments containing spaces.
100 keywords and their meanings are as follows (note that
101 keywords are case-insensitive and arguments are case-sensitive):
104 Restricts the following declarations (up to the next
106 keyword) to be only for those hosts that match one of the patterns
107 given after the keyword.
110 as a pattern can be used to provide global
111 defaults for all hosts.
114 argument given on the command line (i.e. the name is not converted to
115 a canonicalized host name before matching).
119 for more information on patterns.
121 Specifies which address family to use when connecting.
131 passphrase/password querying will be disabled.
132 This option is useful in scripts and other batch jobs where no user
133 is present to supply the password.
141 Use the specified address on the local machine as the source address of
143 Only useful on systems with more than one address.
144 Note that this option does not work if
145 .Cm UsePrivilegedPort
148 .It Cm ChallengeResponseAuthentication
149 Specifies whether to use challenge-response authentication.
150 The argument to this keyword must be
157 If this flag is set to
160 will additionally check the host IP address in the
163 This allows ssh to detect if a host key changed due to DNS spoofing.
164 If the option is set to
166 the check will not be executed.
170 Specifies the cipher to use for encrypting the session
171 in protocol version 1.
179 is only supported in the
181 client for interoperability with legacy protocol 1 implementations
182 that do not support the
185 Its use is strongly discouraged due to cryptographic weaknesses.
189 Specifies the ciphers allowed for protocol version 2
190 in order of preference.
191 Multiple ciphers must be comma-separated.
192 The supported ciphers are
207 .Bd -literal -offset 3n
208 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
209 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
210 aes192-ctr,aes256-ctr
212 .It Cm ClearAllForwardings
213 Specifies that all local, remote, and dynamic port forwardings
214 specified in the configuration files or on the command line be
216 This option is primarily useful when used from the
218 command line to clear port forwardings set in
219 configuration files, and is automatically set by
230 Specifies whether to use compression.
237 .It Cm CompressionLevel
238 Specifies the compression level to use if compression is enabled.
239 The argument must be an integer from 1 (fast) to 9 (slow, best).
240 The default level is 6, which is good for most applications.
241 The meaning of the values is the same as in
243 Note that this option applies to protocol version 1 only.
244 .It Cm ConnectionAttempts
245 Specifies the number of tries (one per second) to make before exiting.
246 The argument must be an integer.
247 This may be useful in scripts if the connection sometimes fails.
249 .It Cm ConnectTimeout
250 Specifies the timeout (in seconds) used when connecting to the
251 SSH server, instead of using the default system TCP timeout.
252 This value is used only when the target is down or really unreachable,
253 not when it refuses the connection.
255 Enables the sharing of multiple sessions over a single network connection.
259 will listen for connections on a control socket specified using the
262 Additional sessions can connect to this socket using the same
269 These sessions will try to reuse the master instance's network connection
270 rather than initiating new ones, but will fall back to connecting normally
271 if the control socket does not exist, or is not listening.
276 to listen for control connections, but require confirmation using the
278 program before they are accepted (see
284 ssh will continue without connecting to a master instance.
288 forwarding is supported over these multiplexed connections, however the
289 display and agent forwarded will be the one belonging to the master
290 connection i.e. it is not possible to forward multiple displays or agents.
292 Two additional options allow for opportunistic multiplexing: try to use a
293 master connection but fall back to creating a new one if one does not already
299 The latter requires confirmation like the
303 Specify the path to the control socket used for connection sharing as described
306 section above or the string
308 to disable connection sharing.
311 will be substituted by the local host name,
313 will be substituted by the target host name,
317 by the remote login username.
318 It is recommended that any
320 used for opportunistic connection sharing include
321 at least %h, %p, and %r.
322 This ensures that shared connections are uniquely identified.
323 .It Cm DynamicForward
324 Specifies that a TCP port on the local machine be forwarded
325 over the secure channel, and the application
326 protocol is then used to determine where to connect to from the
331 .Oo Ar bind_address : Oc Ar port .
333 IPv6 addresses can be specified by enclosing addresses in square brackets or
334 by using an alternative syntax:
335 .Oo Ar bind_address Ns / Oc Ns Ar port .
336 By default, the local port is bound in accordance with the
341 may be used to bind the connection to a specific address.
346 indicates that the listening port be bound for local use only, while an
349 indicates that the port should be available from all interfaces.
351 Currently the SOCKS4 and SOCKS5 protocols are supported, and
353 will act as a SOCKS server.
354 Multiple forwardings may be specified, and
355 additional forwardings can be given on the command line.
356 Only the superuser can forward privileged ports.
357 .It Cm EnableSSHKeysign
358 Setting this option to
360 in the global client configuration file
361 .Pa /etc/ssh/ssh_config
362 enables the use of the helper program
365 .Cm HostbasedAuthentication .
372 This option should be placed in the non-hostspecific section.
375 for more information.
377 Sets the escape character (default:
379 The escape character can also
380 be set on the command line.
381 The argument should be a single character,
383 followed by a letter, or
385 to disable the escape
386 character entirely (making the connection transparent for binary
388 .It Cm ExitOnForwardFailure
391 should terminate the connection if it cannot set up all requested
392 dynamic, local, and remote port forwardings.
400 Specifies whether the connection to the authentication agent (if any)
401 will be forwarded to the remote machine.
409 Agent forwarding should be enabled with caution.
410 Users with the ability to bypass file permissions on the remote host
411 (for the agent's Unix-domain socket)
412 can access the local agent through the forwarded connection.
413 An attacker cannot obtain key material from the agent,
414 however they can perform operations on the keys that enable them to
415 authenticate using the identities loaded into the agent.
417 Specifies whether X11 connections will be automatically redirected
418 over the secure channel and
428 X11 forwarding should be enabled with caution.
429 Users with the ability to bypass file permissions on the remote host
430 (for the user's X11 authorization database)
431 can access the local X11 display through the forwarded connection.
432 An attacker may then be able to perform activities such as keystroke monitoring
434 .Cm ForwardX11Trusted
435 option is also enabled.
436 .It Cm ForwardX11Trusted
437 If this option is set to
439 remote X11 clients will have full access to the original X11 display.
441 If this option is set to
443 remote X11 clients will be considered untrusted and prevented
444 from stealing or tampering with data belonging to trusted X11
448 token used for the session will be set to expire after 20 minutes.
449 Remote clients will be refused access after this time.
454 See the X11 SECURITY extension specification for full details on
455 the restrictions imposed on untrusted clients.
457 Specifies whether remote hosts are allowed to connect to local
461 binds local port forwardings to the loopback address.
462 This prevents other remote hosts from connecting to forwarded ports.
464 can be used to specify that ssh
465 should bind local port forwardings to the wildcard address,
466 thus allowing remote hosts to connect to forwarded ports.
473 .It Cm GlobalKnownHostsFile
474 Specifies a file to use for the global
475 host key database instead of
476 .Pa /etc/ssh/ssh_known_hosts .
477 .It Cm GSSAPIAuthentication
478 Specifies whether user authentication based on GSSAPI is allowed.
481 Note that this option applies to protocol version 2 only.
482 .It Cm GSSAPIKeyExchange
483 Specifies whether key exchange based on GSSAPI may be used. When using
484 GSSAPI key exchange the server need not have a host key.
487 Note that this option applies to protocol version 2 only.
488 .It Cm GSSAPIDelegateCredentials
489 Forward (delegate) credentials to the server.
492 Note that this option applies to protocol version 2 only.
493 .It Cm GSSAPITrustDns
496 to indicate that the DNS is trusted to securely canonicalize
497 the name of the host being connected to. If
499 the hostname entered on the
500 command line will be passed untouched to the GSSAPI library.
503 This option only applies to protocol version 2 connections using GSSAPI.
504 .It Cm HashKnownHosts
507 should hash host names and addresses when they are added to
508 .Pa ~/.ssh/known_hosts .
509 These hashed names may be used normally by
513 but they do not reveal identifying information should the file's contents
517 Note that existing names and addresses in known hosts files
518 will not be converted automatically,
519 but may be manually hashed using
521 .It Cm HostbasedAuthentication
522 Specifies whether to try rhosts based authentication with public key
530 This option applies to protocol version 2 only and
532 .Cm RhostsRSAAuthentication .
533 .It Cm HostKeyAlgorithms
534 Specifies the protocol version 2 host key algorithms
535 that the client wants to use in order of preference.
536 The default for this option is:
537 .Dq ssh-rsa,ssh-dss .
539 Specifies an alias that should be used instead of the
540 real host name when looking up or saving the host key
541 in the host key database files.
542 This option is useful for tunneling SSH connections
543 or for multiple servers running on a single host.
545 Specifies the real host name to log into.
546 This can be used to specify nicknames or abbreviations for hosts.
547 The default is the name given on the command line.
548 Numeric IP addresses are also permitted (both on the command line and in
551 .It Cm IdentitiesOnly
554 should only use the authentication identity files configured in the
559 offers more identities.
560 The argument to this keyword must be
564 This option is intended for situations where ssh-agent
565 offers many different identities.
569 Specifies a file from which the user's RSA or DSA authentication identity
573 for protocol version 1, and
577 for protocol version 2.
578 Additionally, any identities represented by the authentication agent
579 will be used for authentication.
581 The file name may use the tilde
582 syntax to refer to a user's home directory or one of the following
585 (local user's home directory),
591 (remote host name) or
595 It is possible to have
596 multiple identity files specified in configuration files; all these
597 identities will be tried in sequence.
598 .It Cm KbdInteractiveDevices
599 Specifies the list of methods to use in keyboard-interactive authentication.
600 Multiple method names must be comma-separated.
601 The default is to use the server specified list.
602 The methods available vary depending on what the server supports.
603 For an OpenSSH server,
604 it may be zero or more of:
610 Specifies a command to execute on the local machine after successfully
611 connecting to the server.
612 The command string extends to the end of the line, and is executed with
614 This directive is ignored unless
615 .Cm PermitLocalCommand
618 Specifies that a TCP port on the local machine be forwarded over
619 the secure channel to the specified host and port from the remote machine.
620 The first argument must be
622 .Oo Ar bind_address : Oc Ar port
624 and the second argument must be
625 .Ar host : Ns Ar hostport .
626 IPv6 addresses can be specified by enclosing addresses in square brackets or
627 by using an alternative syntax:
628 .Oo Ar bind_address Ns / Oc Ns Ar port
630 .Ar host Ns / Ns Ar hostport .
631 Multiple forwardings may be specified, and additional forwardings can be
632 given on the command line.
633 Only the superuser can forward privileged ports.
634 By default, the local port is bound in accordance with the
639 may be used to bind the connection to a specific address.
644 indicates that the listening port be bound for local use only, while an
647 indicates that the port should be available from all interfaces.
649 Gives the verbosity level that is used when logging messages from
651 The possible values are:
652 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
654 DEBUG and DEBUG1 are equivalent.
655 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
657 Specifies the MAC (message authentication code) algorithms
658 in order of preference.
659 The MAC algorithm is used in protocol version 2
660 for data integrity protection.
661 Multiple algorithms must be comma-separated.
663 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
664 .It Cm NoHostAuthenticationForLocalhost
665 This option can be used if the home directory is shared across machines.
666 In this case localhost will refer to a different machine on each of
667 the machines and the user will get many warnings about changed host keys.
668 However, this option disables host authentication for localhost.
669 The argument to this keyword must be
673 The default is to check the host key for localhost.
674 .It Cm NumberOfPasswordPrompts
675 Specifies the number of password prompts before giving up.
676 The argument to this keyword must be an integer.
678 .It Cm PasswordAuthentication
679 Specifies whether to use password authentication.
680 The argument to this keyword must be
686 .It Cm PermitLocalCommand
687 Allow local command execution via the
690 .Ic !\& Ns Ar command
700 Specifies the port number to connect on the remote host.
702 .It Cm PreferredAuthentications
703 Specifies the order in which the client should try protocol 2
704 authentication methods.
705 This allows a client to prefer one method (e.g.\&
706 .Cm keyboard-interactive )
707 over another method (e.g.\&
709 The default for this option is:
710 .Do gssapi-with-mic ,
713 keyboard-interactive,
717 Specifies the protocol versions
719 should support in order of preference.
720 The possible values are
724 Multiple versions must be comma-separated.
728 tries version 2 and falls back to version 1
729 if version 2 is not available.
731 Specifies the command to use to connect to the server.
733 string extends to the end of the line, and is executed with
735 In the command string,
737 will be substituted by the host name to
741 The command can be basically anything,
742 and should read from its standard input and write to its standard output.
743 It should eventually connect an
745 server running on some machine, or execute
748 Host key management will be done using the
749 HostName of the host being connected (defaulting to the name typed by
751 Setting the command to
753 disables this option entirely.
756 is not available for connects with a proxy command.
758 This directive is useful in conjunction with
760 and its proxy support.
761 For example, the following directive would connect via an HTTP proxy at
763 .Bd -literal -offset 3n
764 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
766 .It Cm PubkeyAuthentication
767 Specifies whether to try public key authentication.
768 The argument to this keyword must be
774 This option applies to protocol version 2 only.
776 Specifies the maximum amount of data that may be transmitted before the
777 session key is renegotiated.
778 The argument is the number of bytes, with an optional suffix of
783 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
784 The default is between
788 depending on the cipher.
789 This option applies to protocol version 2 only.
791 Specifies that a TCP port on the remote machine be forwarded over
792 the secure channel to the specified host and port from the local machine.
793 The first argument must be
795 .Oo Ar bind_address : Oc Ar port
797 and the second argument must be
798 .Ar host : Ns Ar hostport .
799 IPv6 addresses can be specified by enclosing addresses in square brackets
800 or by using an alternative syntax:
801 .Oo Ar bind_address Ns / Oc Ns Ar port
803 .Ar host Ns / Ns Ar hostport .
804 Multiple forwardings may be specified, and additional
805 forwardings can be given on the command line.
806 Only the superuser can forward privileged ports.
810 is not specified, the default is to only bind to loopback addresses.
815 or an empty string, then the forwarding is requested to listen on all
819 will only succeed if the server's
821 option is enabled (see
822 .Xr sshd_config 5 ) .
823 .It Cm RhostsRSAAuthentication
824 Specifies whether to try rhosts based authentication with RSA host
832 This option applies to protocol version 1 only and requires
835 .It Cm RSAAuthentication
836 Specifies whether to try RSA authentication.
837 The argument to this keyword must be
841 RSA authentication will only be
842 attempted if the identity file exists, or an authentication agent is
846 Note that this option applies to protocol version 1 only.
848 Specifies what variables from the local
850 should be sent to the server.
851 Note that environment passing is only supported for protocol 2.
852 The server must also support it, and the server must be configured to
853 accept these environment variables.
858 for how to configure the server.
859 Variables are specified by name, which may contain wildcard characters.
860 Multiple environment variables may be separated by whitespace or spread
864 The default is not to send any environment variables.
868 for more information on patterns.
869 .It Cm ServerAliveCountMax
870 Sets the number of server alive messages (see below) which may be
873 receiving any messages back from the server.
874 If this threshold is reached while server alive messages are being sent,
875 ssh will disconnect from the server, terminating the session.
876 It is important to note that the use of server alive messages is very
880 The server alive messages are sent through the encrypted channel
881 and therefore will not be spoofable.
882 The TCP keepalive option enabled by
885 The server alive mechanism is valuable when the client or
886 server depend on knowing when a connection has become inactive.
888 The default value is 3.
890 .Cm ServerAliveInterval
891 (see below) is set to 15 and
892 .Cm ServerAliveCountMax
893 is left at the default, if the server becomes unresponsive,
894 ssh will disconnect after approximately 45 seconds.
895 This option applies to protocol version 2 only.
896 .It Cm ServerAliveInterval
897 Sets a timeout interval in seconds after which if no data has been received
900 will send a message through the encrypted
901 channel to request a response from the server.
903 is 0, indicating that these messages will not be sent to the server.
904 This option applies to protocol version 2 only.
905 .It Cm SmartcardDevice
906 Specifies which smartcard device to use.
907 The argument to this keyword is the device
909 should use to communicate with a smartcard used for storing the user's
911 By default, no device is specified and smartcard support is not activated.
912 .It Cm StrictHostKeyChecking
913 If this flag is set to
916 will never automatically add host keys to the
917 .Pa ~/.ssh/known_hosts
918 file, and refuses to connect to hosts whose host key has changed.
919 This provides maximum protection against trojan horse attacks,
920 though it can be annoying when the
921 .Pa /etc/ssh/ssh_known_hosts
922 file is poorly maintained or when connections to new hosts are
924 This option forces the user to manually
926 If this flag is set to
928 ssh will automatically add new host keys to the
929 user known hosts files.
930 If this flag is set to
933 will be added to the user known host files only after the user
934 has confirmed that is what they really want to do, and
935 ssh will refuse to connect to hosts whose host key has changed.
937 known hosts will be verified automatically in all cases.
946 Specifies whether the system should send TCP keepalive messages to the
948 If they are sent, death of the connection or crash of one
949 of the machines will be properly noticed.
950 However, this means that
951 connections will die if the route is down temporarily, and some people
956 (to send TCP keepalive messages), and the client will notice
957 if the network goes down or the remote host dies.
958 This is important in scripts, and many users want it too.
960 To disable TCP keepalive messages, the value should be set to
965 device forwarding between the client and the server.
976 requests the default tunnel mode, which is
983 devices to open on the client
990 .Ar local_tun Op : Ar remote_tun .
992 The devices may be specified by numerical ID or the keyword
994 which uses the next available tunnel device.
997 is not specified, it defaults to
1001 .It Cm UsePrivilegedPort
1002 Specifies whether to use a privileged port for outgoing connections.
1003 The argument must be
1012 must be setuid root.
1013 Note that this option must be set to
1016 .Cm RhostsRSAAuthentication
1019 Specifies the user to log in as.
1020 This can be useful when a different user name is used on different machines.
1021 This saves the trouble of
1022 having to remember to give the user name on the command line.
1023 .It Cm UserKnownHostsFile
1024 Specifies a file to use for the user
1025 host key database instead of
1026 .Pa ~/.ssh/known_hosts .
1027 .It Cm VerifyHostKeyDNS
1028 Specifies whether to verify the remote key using DNS and SSHFP resource
1030 If this option is set to
1032 the client will implicitly trust keys that match a secure fingerprint
1034 Insecure fingerprints will be handled as if this option was set to
1036 If this option is set to
1038 information on fingerprint match will be displayed, but the user will still
1039 need to confirm new host keys according to the
1040 .Cm StrictHostKeyChecking
1042 The argument must be
1049 Note that this option applies to protocol version 2 only.
1052 .Sx VERIFYING HOST KEYS
1055 .It Cm XAuthLocation
1056 Specifies the full pathname of the
1060 .Pa /usr/X11R6/bin/xauth .
1065 consists of zero or more non-whitespace characters,
1067 (a wildcard that matches zero or more characters),
1070 (a wildcard that matches exactly one character).
1071 For example, to specify a set of declarations for any host in the
1074 the following pattern could be used:
1078 The following pattern
1079 would match any host in the 192.168.0.[0-9] network range:
1081 .Dl Host 192.168.0.?
1085 is a comma-separated list of patterns.
1086 Patterns within pattern-lists may be negated
1087 by preceding them with an exclamation mark
1090 to allow a key to be used from anywhere within an organisation
1094 the following entry (in authorized_keys) could be used:
1096 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1099 .It Pa ~/.ssh/config
1100 This is the per-user configuration file.
1101 The format of this file is described above.
1102 This file is used by the SSH client.
1103 Because of the potential for abuse, this file must have strict permissions:
1104 read/write for the user, and not accessible by others.
1105 .It Pa /etc/ssh/ssh_config
1106 Systemwide configuration file.
1107 This file provides defaults for those
1108 values that are not specified in the user's configuration file, and
1109 for those users who do not have a configuration file.
1110 This file must be world-readable.
1115 OpenSSH is a derivative of the original and free
1116 ssh 1.2.12 release by Tatu Ylonen.
1117 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1118 Theo de Raadt and Dug Song
1119 removed many bugs, re-added newer features and
1121 Markus Friedl contributed the support for SSH
1122 protocol versions 1.5 and 2.0.
andersk / gssapi-openssh.git / blob