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 1.63 2005/10/30 01:23:19 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 fowarded 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 target host name,
314 by the remote login username.
315 It is recommended that any
317 used for opportunistic connection sharing include
318 all three of these escape sequences.
319 This ensures that shared connections are uniquely identified.
320 .It Cm DynamicForward
321 Specifies that a TCP/IP port on the local machine be forwarded
322 over the secure channel, and the application
323 protocol is then used to determine where to connect to from the
328 .Oo Ar bind_address : Oc Ar port .
330 IPv6 addresses can be specified by enclosing addresses in square brackets or
331 by using an alternative syntax:
332 .Oo Ar bind_address Ns / Oc Ns Ar port .
333 By default, the local port is bound in accordance with the
338 may be used to bind the connection to a specific address.
343 indicates that the listening port be bound for local use only, while an
346 indicates that the port should be available from all interfaces.
348 Currently the SOCKS4 and SOCKS5 protocols are supported, and
350 will act as a SOCKS server.
351 Multiple forwardings may be specified, and
352 additional forwardings can be given on the command line.
353 Only the superuser can forward privileged ports.
354 .It Cm EnableSSHKeysign
355 Setting this option to
357 in the global client configuration file
358 .Pa /etc/ssh/ssh_config
359 enables the use of the helper program
362 .Cm HostbasedAuthentication .
369 This option should be placed in the non-hostspecific section.
372 for more information.
374 Sets the escape character (default:
376 The escape character can also
377 be set on the command line.
378 The argument should be a single character,
380 followed by a letter, or
382 to disable the escape
383 character entirely (making the connection transparent for binary
386 Specifies whether the connection to the authentication agent (if any)
387 will be forwarded to the remote machine.
395 Agent forwarding should be enabled with caution.
396 Users with the ability to bypass file permissions on the remote host
397 (for the agent's Unix-domain socket)
398 can access the local agent through the forwarded connection.
399 An attacker cannot obtain key material from the agent,
400 however they can perform operations on the keys that enable them to
401 authenticate using the identities loaded into the agent.
403 Specifies whether X11 connections will be automatically redirected
404 over the secure channel and
414 X11 forwarding should be enabled with caution.
415 Users with the ability to bypass file permissions on the remote host
416 (for the user's X11 authorization database)
417 can access the local X11 display through the forwarded connection.
418 An attacker may then be able to perform activities such as keystroke monitoring
420 .Cm ForwardX11Trusted
421 option is also enabled.
422 .It Cm ForwardX11Trusted
423 If this option is set to
425 then remote X11 clients will have full access to the original X11 display.
427 If this option is set to
429 then remote X11 clients will be considered untrusted and prevented
430 from stealing or tampering with data belonging to trusted X11
434 token used for the session will be set to expire after 20 minutes.
435 Remote clients will be refused access after this time.
440 See the X11 SECURITY extension specification for full details on
441 the restrictions imposed on untrusted clients.
443 Specifies whether remote hosts are allowed to connect to local
447 binds local port forwardings to the loopback address.
448 This prevents other remote hosts from connecting to forwarded ports.
450 can be used to specify that
452 should bind local port forwardings to the wildcard address,
453 thus allowing remote hosts to connect to forwarded ports.
460 .It Cm GlobalKnownHostsFile
461 Specifies a file to use for the global
462 host key database instead of
463 .Pa /etc/ssh/ssh_known_hosts .
464 .It Cm GSSAPIAuthentication
465 Specifies whether user authentication based on GSSAPI is allowed.
468 Note that this option applies to protocol version 2 only.
469 .It Cm GSSAPIDelegateCredentials
470 Forward (delegate) credentials to the server.
473 Note that this option applies to protocol version 2 only.
474 .It Cm HashKnownHosts
477 should hash host names and addresses when they are added to
478 .Pa ~/.ssh/known_hosts .
479 These hashed names may be used normally by
483 but they do not reveal identifying information should the file's contents
487 Note that hashing of names and addresses will not be retrospectively applied
488 to existing known hosts files, but these may be manually hashed using
490 .It Cm HostbasedAuthentication
491 Specifies whether to try rhosts based authentication with public key
499 This option applies to protocol version 2 only and
501 .Cm RhostsRSAAuthentication .
502 .It Cm HostKeyAlgorithms
503 Specifies the protocol version 2 host key algorithms
504 that the client wants to use in order of preference.
505 The default for this option is:
506 .Dq ssh-rsa,ssh-dss .
508 Specifies an alias that should be used instead of the
509 real host name when looking up or saving the host key
510 in the host key database files.
511 This option is useful for tunneling ssh connections
512 or for multiple servers running on a single host.
514 Specifies the real host name to log into.
515 This can be used to specify nicknames or abbreviations for hosts.
516 Default is the name given on the command line.
517 Numeric IP addresses are also permitted (both on the command line and in
521 Specifies a file from which the user's RSA or DSA authentication identity
525 for protocol version 1, and
529 for protocol version 2.
530 Additionally, any identities represented by the authentication agent
531 will be used for authentication.
532 The file name may use the tilde
533 syntax to refer to a user's home directory.
534 It is possible to have
535 multiple identity files specified in configuration files; all these
536 identities will be tried in sequence.
537 .It Cm IdentitiesOnly
540 should only use the authentication identity files configured in the
545 offers more identities.
546 The argument to this keyword must be
550 This option is intented for situations where
552 offers many different identities.
555 .It Cm KbdInteractiveDevices
556 Specifies the list of methods to use in keyboard-interactive authentication.
557 Multiple method names must be comma-separated.
558 The default is to use the server specified list.
560 Specifies that a TCP/IP port on the local machine be forwarded over
561 the secure channel to the specified host and port from the remote machine.
562 The first argument must be
564 .Oo Ar bind_address : Oc Ar port
566 and the second argument must be
567 .Ar host : Ns Ar hostport .
568 IPv6 addresses can be specified by enclosing addresses in square brackets or
569 by using an alternative syntax:
570 .Oo Ar bind_address Ns / Oc Ns Ar port
572 .Ar host Ns / Ns Ar hostport .
573 Multiple forwardings may be specified, and additional forwardings can be
574 given on the command line.
575 Only the superuser can forward privileged ports.
576 By default, the local port is bound in accordance with the
581 may be used to bind the connection to a specific address.
586 indicates that the listening port be bound for local use only, while an
589 indicates that the port should be available from all interfaces.
591 Gives the verbosity level that is used when logging messages from
593 The possible values are:
594 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
596 DEBUG and DEBUG1 are equivalent.
597 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
599 Specifies the MAC (message authentication code) algorithms
600 in order of preference.
601 The MAC algorithm is used in protocol version 2
602 for data integrity protection.
603 Multiple algorithms must be comma-separated.
605 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
606 .It Cm NoHostAuthenticationForLocalhost
607 This option can be used if the home directory is shared across machines.
608 In this case localhost will refer to a different machine on each of
609 the machines and the user will get many warnings about changed host keys.
610 However, this option disables host authentication for localhost.
611 The argument to this keyword must be
615 The default is to check the host key for localhost.
616 .It Cm NumberOfPasswordPrompts
617 Specifies the number of password prompts before giving up.
618 The argument to this keyword must be an integer.
620 .It Cm PasswordAuthentication
621 Specifies whether to use password authentication.
622 The argument to this keyword must be
629 Specifies the port number to connect on the remote host.
631 .It Cm PreferredAuthentications
632 Specifies the order in which the client should try protocol 2
633 authentication methods.
634 This allows a client to prefer one method (e.g.\&
635 .Cm keyboard-interactive )
636 over another method (e.g.\&
638 The default for this option is:
639 .Dq hostbased,publickey,keyboard-interactive,password .
641 Specifies the protocol versions
643 should support in order of preference.
644 The possible values are
648 Multiple versions must be comma-separated.
653 tries version 2 and falls back to version 1
654 if version 2 is not available.
656 Specifies the command to use to connect to the server.
658 string extends to the end of the line, and is executed with
660 In the command string,
662 will be substituted by the host name to
666 The command can be basically anything,
667 and should read from its standard input and write to its standard output.
668 It should eventually connect an
670 server running on some machine, or execute
673 Host key management will be done using the
674 HostName of the host being connected (defaulting to the name typed by
676 Setting the command to
678 disables this option entirely.
681 is not available for connects with a proxy command.
683 This directive is useful in conjunction with
685 and its proxy support.
686 For example, the following directive would connect via an HTTP proxy at
688 .Bd -literal -offset 3n
689 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
691 .It Cm PubkeyAuthentication
692 Specifies whether to try public key authentication.
693 The argument to this keyword must be
699 This option applies to protocol version 2 only.
701 Specifies that a TCP/IP port on the remote machine be forwarded over
702 the secure channel to the specified host and port from the local machine.
703 The first argument must be
705 .Oo Ar bind_address : Oc Ar port
707 and the second argument must be
708 .Ar host : Ns Ar hostport .
709 IPv6 addresses can be specified by enclosing addresses in square brackets
710 or by using an alternative syntax:
711 .Oo Ar bind_address Ns / Oc Ns Ar port
713 .Ar host Ns / Ns Ar hostport .
714 Multiple forwardings may be specified, and additional
715 forwardings can be given on the command line.
716 Only the superuser can forward privileged ports.
720 is not specified, the default is to only bind to loopback addresses.
725 or an empty string, then the forwarding is requested to listen on all
729 will only succeed if the server's
731 option is enabled (see
732 .Xr sshd_config 5 ) .
733 .It Cm RhostsRSAAuthentication
734 Specifies whether to try rhosts based authentication with RSA host
742 This option applies to protocol version 1 only and requires
745 .It Cm RSAAuthentication
746 Specifies whether to try RSA authentication.
747 The argument to this keyword must be
751 RSA authentication will only be
752 attempted if the identity file exists, or an authentication agent is
756 Note that this option applies to protocol version 1 only.
758 Specifies what variables from the local
760 should be sent to the server.
761 Note that environment passing is only supported for protocol 2, the
762 server must also support it, and the server must be configured to
763 accept these environment variables.
768 for how to configure the server.
769 Variables are specified by name, which may contain the wildcard characters
773 Multiple environment variables may be separated by whitespace or spread
777 The default is not to send any environment variables.
778 .It Cm ServerAliveInterval
779 Sets a timeout interval in seconds after which if no data has been received
782 will send a message through the encrypted
783 channel to request a response from the server.
785 is 0, indicating that these messages will not be sent to the server.
786 This option applies to protocol version 2 only.
787 .It Cm ServerAliveCountMax
788 Sets the number of server alive messages (see above) which may be
791 receiving any messages back from the server.
792 If this threshold is reached while server alive messages are being sent,
794 will disconnect from the server, terminating the session.
795 It is important to note that the use of server alive messages is very
799 The server alive messages are sent through the encrypted channel
800 and therefore will not be spoofable.
801 The TCP keepalive option enabled by
804 The server alive mechanism is valuable when the client or
805 server depend on knowing when a connection has become inactive.
807 The default value is 3.
809 .Cm ServerAliveInterval
810 (above) is set to 15, and
811 .Cm ServerAliveCountMax
812 is left at the default, if the server becomes unresponsive ssh
813 will disconnect after approximately 45 seconds.
814 .It Cm SmartcardDevice
815 Specifies which smartcard device to use.
816 The argument to this keyword is the device
818 should use to communicate with a smartcard used for storing the user's
820 By default, no device is specified and smartcard support is not activated.
821 .It Cm StrictHostKeyChecking
822 If this flag is set to
825 will never automatically add host keys to the
826 .Pa ~/.ssh/known_hosts
827 file, and refuses to connect to hosts whose host key has changed.
828 This provides maximum protection against trojan horse attacks,
829 however, can be annoying when the
830 .Pa /etc/ssh/ssh_known_hosts
831 file is poorly maintained, or connections to new hosts are
833 This option forces the user to manually
835 If this flag is set to
838 will automatically add new host keys to the
839 user known hosts files.
840 If this flag is set to
843 will be added to the user known host files only after the user
844 has confirmed that is what they really want to do, and
846 will refuse to connect to hosts whose host key has changed.
848 known hosts will be verified automatically in all cases.
857 Specifies whether the system should send TCP keepalive messages to the
859 If they are sent, death of the connection or crash of one
860 of the machines will be properly noticed.
861 However, this means that
862 connections will die if the route is down temporarily, and some people
867 (to send TCP keepalive messages), and the client will notice
868 if the network goes down or the remote host dies.
869 This is important in scripts, and many users want it too.
871 To disable TCP keepalive messages, the value should be set to
873 .It Cm UsePrivilegedPort
874 Specifies whether to use a privileged port for outgoing connections.
885 Note that this option must be set to
888 .Cm RhostsRSAAuthentication
891 Specifies the user to log in as.
892 This can be useful when a different user name is used on different machines.
893 This saves the trouble of
894 having to remember to give the user name on the command line.
895 .It Cm UserKnownHostsFile
896 Specifies a file to use for the user
897 host key database instead of
898 .Pa ~/.ssh/known_hosts .
899 .It Cm VerifyHostKeyDNS
900 Specifies whether to verify the remote key using DNS and SSHFP resource
902 If this option is set to
904 the client will implicitly trust keys that match a secure fingerprint
906 Insecure fingerprints will be handled as if this option was set to
908 If this option is set to
910 information on fingerprint match will be displayed, but the user will still
911 need to confirm new host keys according to the
912 .Cm StrictHostKeyChecking
921 Note that this option applies to protocol version 2 only.
923 Specifies the full pathname of the
927 .Pa /usr/X11R6/bin/xauth .
932 This is the per-user configuration file.
933 The format of this file is described above.
934 This file is used by the
937 Because of the potential for abuse, this file must have strict permissions:
938 read/write for the user, and not accessible by others.
939 .It Pa /etc/ssh/ssh_config
940 Systemwide configuration file.
941 This file provides defaults for those
942 values that are not specified in the user's configuration file, and
943 for those users who do not have a configuration file.
944 This file must be world-readable.
949 OpenSSH is a derivative of the original and free
950 ssh 1.2.12 release by Tatu Ylonen.
951 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
952 Theo de Raadt and Dug Song
953 removed many bugs, re-added newer features and
955 Markus Friedl contributed the support for SSH
956 protocol versions 1.5 and 2.0.