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 1.76 2006/01/20 11:21:45 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
53 .Bl -enum -offset indent -compact
57 user's configuration file
60 GSSAPI configuration file
61 .Pq Pa $HOME/.ssh/config.gssapi
63 Kerberos configuration file
64 .Pq Pa $HOME/.ssh/config.krb
66 system-wide configuration file
67 .Pq Pa /etc/ssh/ssh_config
70 For each parameter, the first obtained value
72 The configuration files contain sections separated by
74 specifications, and that section is only applied for hosts that
75 match one of the patterns given in the specification.
76 The matched host name is the one given on the command line.
78 Since the first obtained value for each parameter is used, more
79 host-specific declarations should be given near the beginning of the
80 file, and general defaults at the end.
82 The configuration file has the following format:
84 Empty lines and lines starting with
88 Otherwise a line is of the format
89 .Dq keyword arguments .
90 Configuration options may be separated by whitespace or
91 optional whitespace and exactly one
93 the latter format is useful to avoid the need to quote whitespace
94 when specifying configuration options using the
103 keywords and their meanings are as follows (note that
104 keywords are case-insensitive and arguments are case-sensitive):
107 Restricts the following declarations (up to the next
109 keyword) to be only for those hosts that match one of the patterns
110 given after the keyword.
114 can be used as wildcards in the
118 as a pattern can be used to provide global
119 defaults for all hosts.
122 argument given on the command line (i.e., the name is not converted to
123 a canonicalized host name before matching).
125 Specifies which address family to use when connecting.
135 passphrase/password querying will be disabled.
136 This option is useful in scripts and other batch jobs where no user
137 is present to supply the password.
145 Use the specified address on the local machine as the source address of
147 Only useful on systems with more than one address.
148 Note that this option does not work if
149 .Cm UsePrivilegedPort
152 .It Cm ChallengeResponseAuthentication
153 Specifies whether to use challenge response authentication.
154 The argument to this keyword must be
161 If this flag is set to
163 ssh will additionally check the host IP address in the
166 This allows ssh to detect if a host key changed due to DNS spoofing.
167 If the option is set to
169 the check will not be executed.
173 Specifies the cipher to use for encrypting the session
174 in protocol version 1.
182 is only supported in the
184 client for interoperability with legacy protocol 1 implementations
185 that do not support the
188 Its use is strongly discouraged due to cryptographic weaknesses.
192 Specifies the ciphers allowed for protocol version 2
193 in order of preference.
194 Multiple ciphers must be comma-separated.
195 The supported ciphers are
211 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
212 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
213 aes192-ctr,aes256-ctr''
215 .It Cm ClearAllForwardings
216 Specifies that all local, remote and dynamic port forwardings
217 specified in the configuration files or on the command line be
219 This option is primarily useful when used from the
221 command line to clear port forwardings set in
222 configuration files, and is automatically set by
233 Specifies whether to use compression.
240 .It Cm CompressionLevel
241 Specifies the compression level to use if compression is enabled.
242 The argument must be an integer from 1 (fast) to 9 (slow, best).
243 The default level is 6, which is good for most applications.
244 The meaning of the values is the same as in
246 Note that this option applies to protocol version 1 only.
247 .It Cm ConnectionAttempts
248 Specifies the number of tries (one per second) to make before exiting.
249 The argument must be an integer.
250 This may be useful in scripts if the connection sometimes fails.
252 .It Cm ConnectTimeout
253 Specifies the timeout (in seconds) used when connecting to the ssh
254 server, instead of using the default system TCP timeout.
255 This value is used only when the target is down or really unreachable,
256 not when it refuses the connection.
258 Enables the sharing of multiple sessions over a single network connection.
262 will listen for connections on a control socket specified using the
265 Additional sessions can connect to this socket using the same
272 These sessions will try to reuse the master instance's network connection
273 rather than initiating new ones, but will fall back to connecting normally
274 if the control socket does not exist, or is not listening.
280 to listen for control connections, but require confirmation using the
282 program before they are accepted (see
289 will continue without connecting to a master instance.
293 forwarding is supported over these multiplexed connections, however the
294 display and agent forwarded will be the one belonging to the master
295 connection i.e. it is not possible to forward multiple displays or agents.
297 Two additional options allow for opportunistic multiplexing: try to use a
298 master connection but fall back to creating a new one if one does not already
304 The latter requires confirmation like the
308 Specify the path to the control socket used for connection sharing as described
311 section above or the string
313 to disable connection sharing.
316 will be substituted by the target host name,
320 by the remote login username.
321 It is recommended that any
323 used for opportunistic connection sharing include
324 all three of these escape sequences.
325 This ensures that shared connections are uniquely identified.
326 .It Cm DynamicForward
327 Specifies that a TCP port on the local machine be forwarded
328 over the secure channel, and the application
329 protocol is then used to determine where to connect to from the
334 .Oo Ar bind_address : Oc Ar port .
336 IPv6 addresses can be specified by enclosing addresses in square brackets or
337 by using an alternative syntax:
338 .Oo Ar bind_address Ns / Oc Ns Ar port .
339 By default, the local port is bound in accordance with the
344 may be used to bind the connection to a specific address.
349 indicates that the listening port be bound for local use only, while an
352 indicates that the port should be available from all interfaces.
354 Currently the SOCKS4 and SOCKS5 protocols are supported, and
356 will act as a SOCKS server.
357 Multiple forwardings may be specified, and
358 additional forwardings can be given on the command line.
359 Only the superuser can forward privileged ports.
360 .It Cm EnableSSHKeysign
361 Setting this option to
363 in the global client configuration file
364 .Pa /etc/ssh/ssh_config
365 enables the use of the helper program
368 .Cm HostbasedAuthentication .
375 This option should be placed in the non-hostspecific section.
378 for more information.
380 Sets the escape character (default:
382 The escape character can also
383 be set on the command line.
384 The argument should be a single character,
386 followed by a letter, or
388 to disable the escape
389 character entirely (making the connection transparent for binary
392 Specifies whether the connection to the authentication agent (if any)
393 will be forwarded to the remote machine.
401 Agent forwarding should be enabled with caution.
402 Users with the ability to bypass file permissions on the remote host
403 (for the agent's Unix-domain socket)
404 can access the local agent through the forwarded connection.
405 An attacker cannot obtain key material from the agent,
406 however they can perform operations on the keys that enable them to
407 authenticate using the identities loaded into the agent.
409 Specifies whether X11 connections will be automatically redirected
410 over the secure channel and
420 X11 forwarding should be enabled with caution.
421 Users with the ability to bypass file permissions on the remote host
422 (for the user's X11 authorization database)
423 can access the local X11 display through the forwarded connection.
424 An attacker may then be able to perform activities such as keystroke monitoring
426 .Cm ForwardX11Trusted
427 option is also enabled.
428 .It Cm ForwardX11Trusted
429 If this option is set to
431 then remote X11 clients will have full access to the original X11 display.
433 If this option is set to
435 then remote X11 clients will be considered untrusted and prevented
436 from stealing or tampering with data belonging to trusted X11
440 token used for the session will be set to expire after 20 minutes.
441 Remote clients will be refused access after this time.
446 See the X11 SECURITY extension specification for full details on
447 the restrictions imposed on untrusted clients.
449 Specifies whether remote hosts are allowed to connect to local
453 binds local port forwardings to the loopback address.
454 This prevents other remote hosts from connecting to forwarded ports.
456 can be used to specify that
458 should bind local port forwardings to the wildcard address,
459 thus allowing remote hosts to connect to forwarded ports.
466 .It Cm GlobalKnownHostsFile
467 Specifies a file to use for the global
468 host key database instead of
469 .Pa /etc/ssh/ssh_known_hosts .
470 .It Cm GSSAPIAuthentication
471 Specifies whether user authentication based on GSSAPI is allowed.
474 Note that this option applies to protocol version 2 only.
475 .It Cm GSSAPIKeyExchange
476 Specifies whether key exchange based on GSSAPI may be used. When using
477 GSSAPI key exchange the server need not have a host key.
480 Note that this option applies to protocol version 2 only.
481 .It Cm GSSAPIDelegateCredentials
482 Forward (delegate) credentials to the server.
485 Note that this option applies to protocol version 2 only.
486 .It Cm GSSAPITrustDns
488 .Dq yes to indicate that the DNS is trusted to securely canonicalize
489 the name of the host being connected to. If
490 .Dq no, the hostname entered on the
491 command line will be passed untouched to the GSSAPI library.
494 This option only applies to protocol version 2 connections using GSSAPI
496 .It Cm HashKnownHosts
499 should hash host names and addresses when they are added to
500 .Pa ~/.ssh/known_hosts .
501 These hashed names may be used normally by
505 but they do not reveal identifying information should the file's contents
509 Note that hashing of names and addresses will not be retrospectively applied
510 to existing known hosts files, but these may be manually hashed using
512 .It Cm HostbasedAuthentication
513 Specifies whether to try rhosts based authentication with public key
521 This option applies to protocol version 2 only and
523 .Cm RhostsRSAAuthentication .
524 .It Cm HostKeyAlgorithms
525 Specifies the protocol version 2 host key algorithms
526 that the client wants to use in order of preference.
527 The default for this option is:
528 .Dq ssh-rsa,ssh-dss .
530 Specifies an alias that should be used instead of the
531 real host name when looking up or saving the host key
532 in the host key database files.
533 This option is useful for tunneling ssh connections
534 or for multiple servers running on a single host.
536 Specifies the real host name to log into.
537 This can be used to specify nicknames or abbreviations for hosts.
538 Default is the name given on the command line.
539 Numeric IP addresses are also permitted (both on the command line and in
542 .It Cm IdentitiesOnly
545 should only use the authentication identity files configured in the
550 offers more identities.
551 The argument to this keyword must be
555 This option is intended for situations where
557 offers many different identities.
561 Specifies a file from which the user's RSA or DSA authentication identity
565 for protocol version 1, and
569 for protocol version 2.
570 Additionally, any identities represented by the authentication agent
571 will be used for authentication.
572 The file name may use the tilde
573 syntax to refer to a user's home directory.
574 It is possible to have
575 multiple identity files specified in configuration files; all these
576 identities will be tried in sequence.
577 .It Cm KbdInteractiveDevices
578 Specifies the list of methods to use in keyboard-interactive authentication.
579 Multiple method names must be comma-separated.
580 The default is to use the server specified list.
582 Specifies a command to execute on the local machine after successfully
583 connecting to the server.
584 The command string extends to the end of the line, and is executed with
586 This directive is ignored unless
587 .Cm PermitLocalCommand
590 Specifies that a TCP port on the local machine be forwarded over
591 the secure channel to the specified host and port from the remote machine.
592 The first argument must be
594 .Oo Ar bind_address : Oc Ar port
596 and the second argument must be
597 .Ar host : Ns Ar hostport .
598 IPv6 addresses can be specified by enclosing addresses in square brackets or
599 by using an alternative syntax:
600 .Oo Ar bind_address Ns / Oc Ns Ar port
602 .Ar host Ns / Ns Ar hostport .
603 Multiple forwardings may be specified, and additional forwardings can be
604 given on the command line.
605 Only the superuser can forward privileged ports.
606 By default, the local port is bound in accordance with the
611 may be used to bind the connection to a specific address.
616 indicates that the listening port be bound for local use only, while an
619 indicates that the port should be available from all interfaces.
621 Gives the verbosity level that is used when logging messages from
623 The possible values are:
624 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
626 DEBUG and DEBUG1 are equivalent.
627 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
629 Specifies the MAC (message authentication code) algorithms
630 in order of preference.
631 The MAC algorithm is used in protocol version 2
632 for data integrity protection.
633 Multiple algorithms must be comma-separated.
635 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
636 .It Cm NoHostAuthenticationForLocalhost
637 This option can be used if the home directory is shared across machines.
638 In this case localhost will refer to a different machine on each of
639 the machines and the user will get many warnings about changed host keys.
640 However, this option disables host authentication for localhost.
641 The argument to this keyword must be
645 The default is to check the host key for localhost.
646 .It Cm NumberOfPasswordPrompts
647 Specifies the number of password prompts before giving up.
648 The argument to this keyword must be an integer.
650 .It Cm PasswordAuthentication
651 Specifies whether to use password authentication.
652 The argument to this keyword must be
658 .It Cm PermitLocalCommand
659 Allow local command execution via the
662 .Ic !\& Ns Ar command
672 Specifies the port number to connect on the remote host.
674 .It Cm PreferredAuthentications
675 Specifies the order in which the client should try protocol 2
676 authentication methods.
677 This allows a client to prefer one method (e.g.\&
678 .Cm keyboard-interactive )
679 over another method (e.g.\&
681 The default for this option is:
682 .Dq publickey,gssapi-keyex,external-keyx,gssapi-with-mic,gssapi,password,keyboard-interactive .
684 Specifies the protocol versions
686 should support in order of preference.
687 The possible values are
691 Multiple versions must be comma-separated.
696 tries version 2 and falls back to version 1
697 if version 2 is not available.
699 Specifies the command to use to connect to the server.
701 string extends to the end of the line, and is executed with
703 In the command string,
705 will be substituted by the host name to
709 The command can be basically anything,
710 and should read from its standard input and write to its standard output.
711 It should eventually connect an
713 server running on some machine, or execute
716 Host key management will be done using the
717 HostName of the host being connected (defaulting to the name typed by
719 Setting the command to
721 disables this option entirely.
724 is not available for connects with a proxy command.
726 This directive is useful in conjunction with
728 and its proxy support.
729 For example, the following directive would connect via an HTTP proxy at
731 .Bd -literal -offset 3n
732 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
734 .It Cm PubkeyAuthentication
735 Specifies whether to try public key authentication.
736 The argument to this keyword must be
742 This option applies to protocol version 2 only.
744 Specifies the maximum amount of data that may be transmitted before the
745 session key is renegotiated.
746 The argument is the number of bytes, with an optional suffix of
751 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
752 The default is between
756 depending on the cipher.
757 This option applies to protocol version 2 only.
759 Specifies that a TCP port on the remote machine be forwarded over
760 the secure channel to the specified host and port from the local machine.
761 The first argument must be
763 .Oo Ar bind_address : Oc Ar port
765 and the second argument must be
766 .Ar host : Ns Ar hostport .
767 IPv6 addresses can be specified by enclosing addresses in square brackets
768 or by using an alternative syntax:
769 .Oo Ar bind_address Ns / Oc Ns Ar port
771 .Ar host Ns / Ns Ar hostport .
772 Multiple forwardings may be specified, and additional
773 forwardings can be given on the command line.
774 Only the superuser can forward privileged ports.
778 is not specified, the default is to only bind to loopback addresses.
783 or an empty string, then the forwarding is requested to listen on all
787 will only succeed if the server's
789 option is enabled (see
790 .Xr sshd_config 5 ) .
791 .It Cm RhostsRSAAuthentication
792 Specifies whether to try rhosts based authentication with RSA host
800 This option applies to protocol version 1 only and requires
803 .It Cm RSAAuthentication
804 Specifies whether to try RSA authentication.
805 The argument to this keyword must be
809 RSA authentication will only be
810 attempted if the identity file exists, or an authentication agent is
814 Note that this option applies to protocol version 1 only.
816 Specifies what variables from the local
818 should be sent to the server.
819 Note that environment passing is only supported for protocol 2, the
820 server must also support it, and the server must be configured to
821 accept these environment variables.
826 for how to configure the server.
827 Variables are specified by name, which may contain the wildcard characters
831 Multiple environment variables may be separated by whitespace or spread
835 The default is not to send any environment variables.
836 .It Cm ServerAliveCountMax
837 Sets the number of server alive messages (see below) which may be
840 receiving any messages back from the server.
841 If this threshold is reached while server alive messages are being sent,
843 will disconnect from the server, terminating the session.
844 It is important to note that the use of server alive messages is very
848 The server alive messages are sent through the encrypted channel
849 and therefore will not be spoofable.
850 The TCP keepalive option enabled by
853 The server alive mechanism is valuable when the client or
854 server depend on knowing when a connection has become inactive.
856 The default value is 3.
858 .Cm ServerAliveInterval
859 (see below) is set to 15, and
860 .Cm ServerAliveCountMax
861 is left at the default, if the server becomes unresponsive ssh
862 will disconnect after approximately 45 seconds.
863 .It Cm ServerAliveInterval
864 Sets a timeout interval in seconds after which if no data has been received
867 will send a message through the encrypted
868 channel to request a response from the server.
870 is 0, indicating that these messages will not be sent to the server.
871 This option applies to protocol version 2 only.
872 .It Cm SmartcardDevice
873 Specifies which smartcard device to use.
874 The argument to this keyword is the device
876 should use to communicate with a smartcard used for storing the user's
878 By default, no device is specified and smartcard support is not activated.
879 .It Cm StrictHostKeyChecking
880 If this flag is set to
883 will never automatically add host keys to the
884 .Pa ~/.ssh/known_hosts
885 file, and refuses to connect to hosts whose host key has changed.
886 This provides maximum protection against trojan horse attacks,
887 however, can be annoying when the
888 .Pa /etc/ssh/ssh_known_hosts
889 file is poorly maintained, or connections to new hosts are
891 This option forces the user to manually
893 If this flag is set to
896 will automatically add new host keys to the
897 user known hosts files.
898 If this flag is set to
901 will be added to the user known host files only after the user
902 has confirmed that is what they really want to do, and
904 will refuse to connect to hosts whose host key has changed.
906 known hosts will be verified automatically in all cases.
915 Specifies whether the system should send TCP keepalive messages to the
917 If they are sent, death of the connection or crash of one
918 of the machines will be properly noticed.
919 However, this means that
920 connections will die if the route is down temporarily, and some people
925 (to send TCP keepalive messages), and the client will notice
926 if the network goes down or the remote host dies.
927 This is important in scripts, and many users want it too.
929 To disable TCP keepalive messages, the value should be set to
934 device forwarding between the client and the server.
935 This option also allows requesting layer 2 (ethernet)
936 instead of layer 3 (point-to-point) tunneling from the server.
948 device on the client.
949 Without this option, the next available device will be used.
950 .It Cm UsePrivilegedPort
951 Specifies whether to use a privileged port for outgoing connections.
962 Note that this option must be set to
965 .Cm RhostsRSAAuthentication
968 Specifies the user to log in as.
969 This can be useful when a different user name is used on different machines.
970 This saves the trouble of
971 having to remember to give the user name on the command line.
972 .It Cm UserKnownHostsFile
973 Specifies a file to use for the user
974 host key database instead of
975 .Pa ~/.ssh/known_hosts .
976 .It Cm VerifyHostKeyDNS
977 Specifies whether to verify the remote key using DNS and SSHFP resource
979 If this option is set to
981 the client will implicitly trust keys that match a secure fingerprint
983 Insecure fingerprints will be handled as if this option was set to
985 If this option is set to
987 information on fingerprint match will be displayed, but the user will still
988 need to confirm new host keys according to the
989 .Cm StrictHostKeyChecking
998 Note that this option applies to protocol version 2 only.
1000 Specifies the full pathname of the
1004 .Pa /usr/X11R6/bin/xauth .
1008 .It Pa ~/.ssh/config
1009 This is the per-user configuration file.
1010 The format of this file is described above.
1011 This file is used by the
1014 Because of the potential for abuse, this file must have strict permissions:
1015 read/write for the user, and not accessible by others.
1016 .It Pa /etc/ssh/ssh_config
1017 Systemwide configuration file.
1018 This file provides defaults for those
1019 values that are not specified in the user's configuration file, and
1020 for those users who do not have a configuration file.
1021 This file must be world-readable.
1026 OpenSSH is a derivative of the original and free
1027 ssh 1.2.12 release by Tatu Ylonen.
1028 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1029 Theo de Raadt and Dug Song
1030 removed many bugs, re-added newer features and
1032 Markus Friedl contributed the support for SSH
1033 protocol versions 1.5 and 2.0.