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 1.77 2006/02/12 06:45:34 djm 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 system-wide configuration file
61 .Pq Pa /etc/ssh/ssh_config
64 For each parameter, the first obtained value
66 The configuration files contain sections separated by
68 specifications, and that section is only applied for hosts that
69 match one of the patterns given in the specification.
70 The matched host name is the one given on the command line.
72 Since the first obtained value for each parameter is used, more
73 host-specific declarations should be given near the beginning of the
74 file, and general defaults at the end.
76 The configuration file has the following format:
78 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
97 keywords and their meanings are as follows (note that
98 keywords are case-insensitive and arguments are case-sensitive):
101 Restricts the following declarations (up to the next
103 keyword) to be only for those hosts that match one of the patterns
104 given after the keyword.
108 can be used as wildcards in the
112 as a pattern can be used to provide global
113 defaults for all hosts.
116 argument given on the command line (i.e., the name is not converted to
117 a canonicalized host name before matching).
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
157 ssh will additionally check the host IP address in the
160 This allows ssh to detect if a host key changed due to DNS spoofing.
161 If the option is set to
163 the check will not be executed.
167 Specifies the cipher to use for encrypting the session
168 in protocol version 1.
176 is only supported in the
178 client for interoperability with legacy protocol 1 implementations
179 that do not support the
182 Its use is strongly discouraged due to cryptographic weaknesses.
186 Specifies the ciphers allowed for protocol version 2
187 in order of preference.
188 Multiple ciphers must be comma-separated.
189 The supported ciphers are
205 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
206 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
207 aes192-ctr,aes256-ctr''
209 .It Cm ClearAllForwardings
210 Specifies that all local, remote and dynamic port forwardings
211 specified in the configuration files or on the command line be
213 This option is primarily useful when used from the
215 command line to clear port forwardings set in
216 configuration files, and is automatically set by
227 Specifies whether to use compression.
234 .It Cm CompressionLevel
235 Specifies the compression level to use if compression is enabled.
236 The argument must be an integer from 1 (fast) to 9 (slow, best).
237 The default level is 6, which is good for most applications.
238 The meaning of the values is the same as in
240 Note that this option applies to protocol version 1 only.
241 .It Cm ConnectionAttempts
242 Specifies the number of tries (one per second) to make before exiting.
243 The argument must be an integer.
244 This may be useful in scripts if the connection sometimes fails.
246 .It Cm ConnectTimeout
247 Specifies the timeout (in seconds) used when connecting to the ssh
248 server, instead of using the default system TCP timeout.
249 This value is used only when the target is down or really unreachable,
250 not when it refuses the connection.
252 Enables the sharing of multiple sessions over a single network connection.
256 will listen for connections on a control socket specified using the
259 Additional sessions can connect to this socket using the same
266 These sessions will try to reuse the master instance's network connection
267 rather than initiating new ones, but will fall back to connecting normally
268 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
283 will continue without connecting to a master instance.
287 forwarding is supported over these multiplexed connections, however the
288 display and agent forwarded will be the one belonging to the master
289 connection i.e. it is not possible to forward multiple displays or agents.
291 Two additional options allow for opportunistic multiplexing: try to use a
292 master connection but fall back to creating a new one if one does not already
298 The latter requires confirmation like the
302 Specify the path to the control socket used for connection sharing as described
305 section above or the string
307 to disable connection sharing.
310 will be substituted by the local host name,
312 will be substituted by the target host name,
316 by the remote login username.
317 It is recommended that any
319 used for opportunistic connection sharing include
320 at least the last three of these escape sequences (%h, %p and %r).
321 This ensures that shared connections are uniquely identified.
322 .It Cm DynamicForward
323 Specifies that a TCP port on the local machine be forwarded
324 over the secure channel, and the application
325 protocol is then used to determine where to connect to from the
330 .Oo Ar bind_address : Oc Ar port .
332 IPv6 addresses can be specified by enclosing addresses in square brackets or
333 by using an alternative syntax:
334 .Oo Ar bind_address Ns / Oc Ns Ar port .
335 By default, the local port is bound in accordance with the
340 may be used to bind the connection to a specific address.
345 indicates that the listening port be bound for local use only, while an
348 indicates that the port should be available from all interfaces.
350 Currently the SOCKS4 and SOCKS5 protocols are supported, and
352 will act as a SOCKS server.
353 Multiple forwardings may be specified, and
354 additional forwardings can be given on the command line.
355 Only the superuser can forward privileged ports.
356 .It Cm EnableSSHKeysign
357 Setting this option to
359 in the global client configuration file
360 .Pa /etc/ssh/ssh_config
361 enables the use of the helper program
364 .Cm HostbasedAuthentication .
371 This option should be placed in the non-hostspecific section.
374 for more information.
376 Sets the escape character (default:
378 The escape character can also
379 be set on the command line.
380 The argument should be a single character,
382 followed by a letter, or
384 to disable the escape
385 character entirely (making the connection transparent for binary
388 Specifies whether the connection to the authentication agent (if any)
389 will be forwarded to the remote machine.
397 Agent forwarding should be enabled with caution.
398 Users with the ability to bypass file permissions on the remote host
399 (for the agent's Unix-domain socket)
400 can access the local agent through the forwarded connection.
401 An attacker cannot obtain key material from the agent,
402 however they can perform operations on the keys that enable them to
403 authenticate using the identities loaded into the agent.
405 Specifies whether X11 connections will be automatically redirected
406 over the secure channel and
416 X11 forwarding should be enabled with caution.
417 Users with the ability to bypass file permissions on the remote host
418 (for the user's X11 authorization database)
419 can access the local X11 display through the forwarded connection.
420 An attacker may then be able to perform activities such as keystroke monitoring
422 .Cm ForwardX11Trusted
423 option is also enabled.
424 .It Cm ForwardX11Trusted
425 If this option is set to
427 then remote X11 clients will have full access to the original X11 display.
429 If this option is set to
431 then remote X11 clients will be considered untrusted and prevented
432 from stealing or tampering with data belonging to trusted X11
436 token used for the session will be set to expire after 20 minutes.
437 Remote clients will be refused access after this time.
442 See the X11 SECURITY extension specification for full details on
443 the restrictions imposed on untrusted clients.
445 Specifies whether remote hosts are allowed to connect to local
449 binds local port forwardings to the loopback address.
450 This prevents other remote hosts from connecting to forwarded ports.
452 can be used to specify that
454 should bind local port forwardings to the wildcard address,
455 thus allowing remote hosts to connect to forwarded ports.
462 .It Cm GlobalKnownHostsFile
463 Specifies a file to use for the global
464 host key database instead of
465 .Pa /etc/ssh/ssh_known_hosts .
466 .It Cm GSSAPIAuthentication
467 Specifies whether user authentication based on GSSAPI is allowed.
470 Note that this option applies to protocol version 2 only.
471 .It Cm GSSAPIDelegateCredentials
472 Forward (delegate) credentials to the server.
475 Note that this option applies to protocol version 2 only.
476 .It Cm HashKnownHosts
479 should hash host names and addresses when they are added to
480 .Pa ~/.ssh/known_hosts .
481 These hashed names may be used normally by
485 but they do not reveal identifying information should the file's contents
489 Note that hashing of names and addresses will not be retrospectively applied
490 to existing known hosts files, but these may be manually hashed using
492 .It Cm HostbasedAuthentication
493 Specifies whether to try rhosts based authentication with public key
501 This option applies to protocol version 2 only and
503 .Cm RhostsRSAAuthentication .
504 .It Cm HostKeyAlgorithms
505 Specifies the protocol version 2 host key algorithms
506 that the client wants to use in order of preference.
507 The default for this option is:
508 .Dq ssh-rsa,ssh-dss .
510 Specifies an alias that should be used instead of the
511 real host name when looking up or saving the host key
512 in the host key database files.
513 This option is useful for tunneling ssh connections
514 or for multiple servers running on a single host.
516 Specifies the real host name to log into.
517 This can be used to specify nicknames or abbreviations for hosts.
518 Default is the name given on the command line.
519 Numeric IP addresses are also permitted (both on the command line and in
522 .It Cm IdentitiesOnly
525 should only use the authentication identity files configured in the
530 offers more identities.
531 The argument to this keyword must be
535 This option is intended for situations where
537 offers many different identities.
541 Specifies a file from which the user's RSA or DSA authentication identity
545 for protocol version 1, and
549 for protocol version 2.
550 Additionally, any identities represented by the authentication agent
551 will be used for authentication.
552 The file name may use the tilde
553 syntax to refer to a user's home directory.
554 It is possible to have
555 multiple identity files specified in configuration files; all these
556 identities will be tried in sequence.
557 .It Cm KbdInteractiveDevices
558 Specifies the list of methods to use in keyboard-interactive authentication.
559 Multiple method names must be comma-separated.
560 The default is to use the server specified list.
562 Specifies a command to execute on the local machine after successfully
563 connecting to the server.
564 The command string extends to the end of the line, and is executed with
566 This directive is ignored unless
567 .Cm PermitLocalCommand
570 Specifies that a TCP port on the local machine be forwarded over
571 the secure channel to the specified host and port from the remote machine.
572 The first argument must be
574 .Oo Ar bind_address : Oc Ar port
576 and the second argument must be
577 .Ar host : Ns Ar hostport .
578 IPv6 addresses can be specified by enclosing addresses in square brackets or
579 by using an alternative syntax:
580 .Oo Ar bind_address Ns / Oc Ns Ar port
582 .Ar host Ns / Ns Ar hostport .
583 Multiple forwardings may be specified, and additional forwardings can be
584 given on the command line.
585 Only the superuser can forward privileged ports.
586 By default, the local port is bound in accordance with the
591 may be used to bind the connection to a specific address.
596 indicates that the listening port be bound for local use only, while an
599 indicates that the port should be available from all interfaces.
601 Gives the verbosity level that is used when logging messages from
603 The possible values are:
604 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
606 DEBUG and DEBUG1 are equivalent.
607 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
609 Specifies the MAC (message authentication code) algorithms
610 in order of preference.
611 The MAC algorithm is used in protocol version 2
612 for data integrity protection.
613 Multiple algorithms must be comma-separated.
615 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
616 .It Cm NoHostAuthenticationForLocalhost
617 This option can be used if the home directory is shared across machines.
618 In this case localhost will refer to a different machine on each of
619 the machines and the user will get many warnings about changed host keys.
620 However, this option disables host authentication for localhost.
621 The argument to this keyword must be
625 The default is to check the host key for localhost.
626 .It Cm NumberOfPasswordPrompts
627 Specifies the number of password prompts before giving up.
628 The argument to this keyword must be an integer.
630 .It Cm PasswordAuthentication
631 Specifies whether to use password authentication.
632 The argument to this keyword must be
638 .It Cm PermitLocalCommand
639 Allow local command execution via the
642 .Ic !\& Ns Ar command
652 Specifies the port number to connect on the remote host.
654 .It Cm PreferredAuthentications
655 Specifies the order in which the client should try protocol 2
656 authentication methods.
657 This allows a client to prefer one method (e.g.\&
658 .Cm keyboard-interactive )
659 over another method (e.g.\&
661 The default for this option is:
662 .Dq hostbased,publickey,keyboard-interactive,password .
664 Specifies the protocol versions
666 should support in order of preference.
667 The possible values are
671 Multiple versions must be comma-separated.
676 tries version 2 and falls back to version 1
677 if version 2 is not available.
679 Specifies the command to use to connect to the server.
681 string extends to the end of the line, and is executed with
683 In the command string,
685 will be substituted by the host name to
689 The command can be basically anything,
690 and should read from its standard input and write to its standard output.
691 It should eventually connect an
693 server running on some machine, or execute
696 Host key management will be done using the
697 HostName of the host being connected (defaulting to the name typed by
699 Setting the command to
701 disables this option entirely.
704 is not available for connects with a proxy command.
706 This directive is useful in conjunction with
708 and its proxy support.
709 For example, the following directive would connect via an HTTP proxy at
711 .Bd -literal -offset 3n
712 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
714 .It Cm PubkeyAuthentication
715 Specifies whether to try public key authentication.
716 The argument to this keyword must be
722 This option applies to protocol version 2 only.
724 Specifies the maximum amount of data that may be transmitted before the
725 session key is renegotiated.
726 The argument is the number of bytes, with an optional suffix of
731 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
732 The default is between
736 depending on the cipher.
737 This option applies to protocol version 2 only.
739 Specifies that a TCP port on the remote machine be forwarded over
740 the secure channel to the specified host and port from the local machine.
741 The first argument must be
743 .Oo Ar bind_address : Oc Ar port
745 and the second argument must be
746 .Ar host : Ns Ar hostport .
747 IPv6 addresses can be specified by enclosing addresses in square brackets
748 or by using an alternative syntax:
749 .Oo Ar bind_address Ns / Oc Ns Ar port
751 .Ar host Ns / Ns Ar hostport .
752 Multiple forwardings may be specified, and additional
753 forwardings can be given on the command line.
754 Only the superuser can forward privileged ports.
758 is not specified, the default is to only bind to loopback addresses.
763 or an empty string, then the forwarding is requested to listen on all
767 will only succeed if the server's
769 option is enabled (see
770 .Xr sshd_config 5 ) .
771 .It Cm RhostsRSAAuthentication
772 Specifies whether to try rhosts based authentication with RSA host
780 This option applies to protocol version 1 only and requires
783 .It Cm RSAAuthentication
784 Specifies whether to try RSA authentication.
785 The argument to this keyword must be
789 RSA authentication will only be
790 attempted if the identity file exists, or an authentication agent is
794 Note that this option applies to protocol version 1 only.
796 Specifies what variables from the local
798 should be sent to the server.
799 Note that environment passing is only supported for protocol 2, the
800 server must also support it, and the server must be configured to
801 accept these environment variables.
806 for how to configure the server.
807 Variables are specified by name, which may contain the wildcard characters
811 Multiple environment variables may be separated by whitespace or spread
815 The default is not to send any environment variables.
816 .It Cm ServerAliveCountMax
817 Sets the number of server alive messages (see below) which may be
820 receiving any messages back from the server.
821 If this threshold is reached while server alive messages are being sent,
823 will disconnect from the server, terminating the session.
824 It is important to note that the use of server alive messages is very
828 The server alive messages are sent through the encrypted channel
829 and therefore will not be spoofable.
830 The TCP keepalive option enabled by
833 The server alive mechanism is valuable when the client or
834 server depend on knowing when a connection has become inactive.
836 The default value is 3.
838 .Cm ServerAliveInterval
839 (see below) is set to 15, and
840 .Cm ServerAliveCountMax
841 is left at the default, if the server becomes unresponsive ssh
842 will disconnect after approximately 45 seconds.
843 .It Cm ServerAliveInterval
844 Sets a timeout interval in seconds after which if no data has been received
847 will send a message through the encrypted
848 channel to request a response from the server.
850 is 0, indicating that these messages will not be sent to the server.
851 This option applies to protocol version 2 only.
852 .It Cm SmartcardDevice
853 Specifies which smartcard device to use.
854 The argument to this keyword is the device
856 should use to communicate with a smartcard used for storing the user's
858 By default, no device is specified and smartcard support is not activated.
859 .It Cm StrictHostKeyChecking
860 If this flag is set to
863 will never automatically add host keys to the
864 .Pa ~/.ssh/known_hosts
865 file, and refuses to connect to hosts whose host key has changed.
866 This provides maximum protection against trojan horse attacks,
867 however, can be annoying when the
868 .Pa /etc/ssh/ssh_known_hosts
869 file is poorly maintained, or connections to new hosts are
871 This option forces the user to manually
873 If this flag is set to
876 will automatically add new host keys to the
877 user known hosts files.
878 If this flag is set to
881 will be added to the user known host files only after the user
882 has confirmed that is what they really want to do, and
884 will refuse to connect to hosts whose host key has changed.
886 known hosts will be verified automatically in all cases.
895 Specifies whether the system should send TCP keepalive messages to the
897 If they are sent, death of the connection or crash of one
898 of the machines will be properly noticed.
899 However, this means that
900 connections will die if the route is down temporarily, and some people
905 (to send TCP keepalive messages), and the client will notice
906 if the network goes down or the remote host dies.
907 This is important in scripts, and many users want it too.
909 To disable TCP keepalive messages, the value should be set to
914 device forwarding between the client and the server.
915 This option also allows requesting layer 2 (ethernet)
916 instead of layer 3 (point-to-point) tunneling from the server.
928 device on the client.
929 Without this option, the next available device will be used.
930 .It Cm UsePrivilegedPort
931 Specifies whether to use a privileged port for outgoing connections.
942 Note that this option must be set to
945 .Cm RhostsRSAAuthentication
948 Specifies the user to log in as.
949 This can be useful when a different user name is used on different machines.
950 This saves the trouble of
951 having to remember to give the user name on the command line.
952 .It Cm UserKnownHostsFile
953 Specifies a file to use for the user
954 host key database instead of
955 .Pa ~/.ssh/known_hosts .
956 .It Cm VerifyHostKeyDNS
957 Specifies whether to verify the remote key using DNS and SSHFP resource
959 If this option is set to
961 the client will implicitly trust keys that match a secure fingerprint
963 Insecure fingerprints will be handled as if this option was set to
965 If this option is set to
967 information on fingerprint match will be displayed, but the user will still
968 need to confirm new host keys according to the
969 .Cm StrictHostKeyChecking
978 Note that this option applies to protocol version 2 only.
980 Specifies the full pathname of the
984 .Pa /usr/X11R6/bin/xauth .
989 This is the per-user configuration file.
990 The format of this file is described above.
991 This file is used by the
994 Because of the potential for abuse, this file must have strict permissions:
995 read/write for the user, and not accessible by others.
996 .It Pa /etc/ssh/ssh_config
997 Systemwide configuration file.
998 This file provides defaults for those
999 values that are not specified in the user's configuration file, and
1000 for those users who do not have a configuration file.
1001 This file must be world-readable.
1006 OpenSSH is a derivative of the original and free
1007 ssh 1.2.12 release by Tatu Ylonen.
1008 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1009 Theo de Raadt and Dug Song
1010 removed many bugs, re-added newer features and
1012 Markus Friedl contributed the support for SSH
1013 protocol versions 1.5 and 2.0.
andersk / openssh.git / blob