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 1.62 2005/09/19 11:37:34 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 reuse the master instance's network connection rather
267 than initiating new ones.
272 to listen for control connections, but require confirmation using the
274 program before they are accepted (see
281 will continue without connecting to a master instance.
285 forwarding is supported over these multiplexed connections, however the
286 display and agent fowarded will be the one belonging to the master
287 connection i.e. it is not possible to forward multiple displays or agents.
289 Two additional options allow for opportunistic multiplexing: try to use a
290 master connection but fall back to creating a new one if one does not already
296 The latter requires confirmation like the
300 Specify the path to the control socket used for connection sharing as described
303 section above or the string
305 to disable connection sharing.
308 will be substituted by the target host name,
312 by the remote login username.
313 It is recommended that any
315 used for opportunistic connection sharing include
316 all three of these escape sequences.
317 This ensures that shared connections are uniquely identified.
318 .It Cm DynamicForward
319 Specifies that a TCP/IP port on the local machine be forwarded
320 over the secure channel, and the application
321 protocol is then used to determine where to connect to from the
326 .Oo Ar bind_address : Oc Ar port .
328 IPv6 addresses can be specified by enclosing addresses in square brackets or
329 by using an alternative syntax:
330 .Oo Ar bind_address Ns / Oc Ns Ar port .
331 By default, the local port is bound in accordance with the
336 may be used to bind the connection to a specific address.
341 indicates that the listening port be bound for local use only, while an
344 indicates that the port should be available from all interfaces.
346 Currently the SOCKS4 and SOCKS5 protocols are supported, and
348 will act as a SOCKS server.
349 Multiple forwardings may be specified, and
350 additional forwardings can be given on the command line.
351 Only the superuser can forward privileged ports.
352 .It Cm EnableSSHKeysign
353 Setting this option to
355 in the global client configuration file
356 .Pa /etc/ssh/ssh_config
357 enables the use of the helper program
360 .Cm HostbasedAuthentication .
367 This option should be placed in the non-hostspecific section.
370 for more information.
372 Sets the escape character (default:
374 The escape character can also
375 be set on the command line.
376 The argument should be a single character,
378 followed by a letter, or
380 to disable the escape
381 character entirely (making the connection transparent for binary
384 Specifies whether the connection to the authentication agent (if any)
385 will be forwarded to the remote machine.
393 Agent forwarding should be enabled with caution.
394 Users with the ability to bypass file permissions on the remote host
395 (for the agent's Unix-domain socket)
396 can access the local agent through the forwarded connection.
397 An attacker cannot obtain key material from the agent,
398 however they can perform operations on the keys that enable them to
399 authenticate using the identities loaded into the agent.
401 Specifies whether X11 connections will be automatically redirected
402 over the secure channel and
412 X11 forwarding should be enabled with caution.
413 Users with the ability to bypass file permissions on the remote host
414 (for the user's X11 authorization database)
415 can access the local X11 display through the forwarded connection.
416 An attacker may then be able to perform activities such as keystroke monitoring
418 .Cm ForwardX11Trusted
419 option is also enabled.
420 .It Cm ForwardX11Trusted
421 If this option is set to
423 then remote X11 clients will have full access to the original X11 display.
425 If this option is set to
427 then remote X11 clients will be considered untrusted and prevented
428 from stealing or tampering with data belonging to trusted X11
432 token used for the session will be set to expire after 20 minutes.
433 Remote clients will be refused access after this time.
438 See the X11 SECURITY extension specification for full details on
439 the restrictions imposed on untrusted clients.
441 Specifies whether remote hosts are allowed to connect to local
445 binds local port forwardings to the loopback address.
446 This prevents other remote hosts from connecting to forwarded ports.
448 can be used to specify that
450 should bind local port forwardings to the wildcard address,
451 thus allowing remote hosts to connect to forwarded ports.
458 .It Cm GlobalKnownHostsFile
459 Specifies a file to use for the global
460 host key database instead of
461 .Pa /etc/ssh/ssh_known_hosts .
462 .It Cm GSSAPIAuthentication
463 Specifies whether user authentication based on GSSAPI is allowed.
466 Note that this option applies to protocol version 2 only.
467 .It Cm GSSAPIDelegateCredentials
468 Forward (delegate) credentials to the server.
471 Note that this option applies to protocol version 2 only.
472 .It Cm HashKnownHosts
475 should hash host names and addresses when they are added to
476 .Pa ~/.ssh/known_hosts .
477 These hashed names may be used normally by
481 but they do not reveal identifying information should the file's contents
485 Note that hashing of names and addresses will not be retrospectively applied
486 to existing known hosts files, but these may be manually hashed using
488 .It Cm HostbasedAuthentication
489 Specifies whether to try rhosts based authentication with public key
497 This option applies to protocol version 2 only and
499 .Cm RhostsRSAAuthentication .
500 .It Cm HostKeyAlgorithms
501 Specifies the protocol version 2 host key algorithms
502 that the client wants to use in order of preference.
503 The default for this option is:
504 .Dq ssh-rsa,ssh-dss .
506 Specifies an alias that should be used instead of the
507 real host name when looking up or saving the host key
508 in the host key database files.
509 This option is useful for tunneling ssh connections
510 or for multiple servers running on a single host.
512 Specifies the real host name to log into.
513 This can be used to specify nicknames or abbreviations for hosts.
514 Default is the name given on the command line.
515 Numeric IP addresses are also permitted (both on the command line and in
519 Specifies a file from which the user's RSA or DSA authentication identity
523 for protocol version 1, and
527 for protocol version 2.
528 Additionally, any identities represented by the authentication agent
529 will be used for authentication.
530 The file name may use the tilde
531 syntax to refer to a user's home directory.
532 It is possible to have
533 multiple identity files specified in configuration files; all these
534 identities will be tried in sequence.
535 .It Cm IdentitiesOnly
538 should only use the authentication identity files configured in the
543 offers more identities.
544 The argument to this keyword must be
548 This option is intented for situations where
550 offers many different identities.
553 .It Cm KbdInteractiveDevices
554 Specifies the list of methods to use in keyboard-interactive authentication.
555 Multiple method names must be comma-separated.
556 The default is to use the server specified list.
558 Specifies that a TCP/IP port on the local machine be forwarded over
559 the secure channel to the specified host and port from the remote machine.
560 The first argument must be
562 .Oo Ar bind_address : Oc Ar port
564 and the second argument must be
565 .Ar host : Ns Ar hostport .
566 IPv6 addresses can be specified by enclosing addresses in square brackets or
567 by using an alternative syntax:
568 .Oo Ar bind_address Ns / Oc Ns Ar port
570 .Ar host Ns / Ns Ar hostport .
571 Multiple forwardings may be specified, and additional forwardings can be
572 given on the command line.
573 Only the superuser can forward privileged ports.
574 By default, the local port is bound in accordance with the
579 may be used to bind the connection to a specific address.
584 indicates that the listening port be bound for local use only, while an
587 indicates that the port should be available from all interfaces.
589 Gives the verbosity level that is used when logging messages from
591 The possible values are:
592 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
594 DEBUG and DEBUG1 are equivalent.
595 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
597 Specifies the MAC (message authentication code) algorithms
598 in order of preference.
599 The MAC algorithm is used in protocol version 2
600 for data integrity protection.
601 Multiple algorithms must be comma-separated.
603 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
604 .It Cm NoHostAuthenticationForLocalhost
605 This option can be used if the home directory is shared across machines.
606 In this case localhost will refer to a different machine on each of
607 the machines and the user will get many warnings about changed host keys.
608 However, this option disables host authentication for localhost.
609 The argument to this keyword must be
613 The default is to check the host key for localhost.
614 .It Cm NumberOfPasswordPrompts
615 Specifies the number of password prompts before giving up.
616 The argument to this keyword must be an integer.
618 .It Cm PasswordAuthentication
619 Specifies whether to use password authentication.
620 The argument to this keyword must be
627 Specifies the port number to connect on the remote host.
629 .It Cm PreferredAuthentications
630 Specifies the order in which the client should try protocol 2
631 authentication methods.
632 This allows a client to prefer one method (e.g.\&
633 .Cm keyboard-interactive )
634 over another method (e.g.\&
636 The default for this option is:
637 .Dq hostbased,publickey,keyboard-interactive,password .
639 Specifies the protocol versions
641 should support in order of preference.
642 The possible values are
646 Multiple versions must be comma-separated.
651 tries version 2 and falls back to version 1
652 if version 2 is not available.
654 Specifies the command to use to connect to the server.
656 string extends to the end of the line, and is executed with
658 In the command string,
660 will be substituted by the host name to
664 The command can be basically anything,
665 and should read from its standard input and write to its standard output.
666 It should eventually connect an
668 server running on some machine, or execute
671 Host key management will be done using the
672 HostName of the host being connected (defaulting to the name typed by
674 Setting the command to
676 disables this option entirely.
679 is not available for connects with a proxy command.
681 This directive is useful in conjunction with
683 and its proxy support.
684 For example, the following directive would connect via an HTTP proxy at
686 .Bd -literal -offset 3n
687 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
689 .It Cm PubkeyAuthentication
690 Specifies whether to try public key authentication.
691 The argument to this keyword must be
697 This option applies to protocol version 2 only.
699 Specifies that a TCP/IP port on the remote machine be forwarded over
700 the secure channel to the specified host and port from the local machine.
701 The first argument must be
703 .Oo Ar bind_address : Oc Ar port
705 and the second argument must be
706 .Ar host : Ns Ar hostport .
707 IPv6 addresses can be specified by enclosing addresses in square brackets
708 or by using an alternative syntax:
709 .Oo Ar bind_address Ns / Oc Ns Ar port
711 .Ar host Ns / Ns Ar hostport .
712 Multiple forwardings may be specified, and additional
713 forwardings can be given on the command line.
714 Only the superuser can forward privileged ports.
718 is not specified, the default is to only bind to loopback addresses.
723 or an empty string, then the forwarding is requested to listen on all
727 will only succeed if the server's
729 option is enabled (see
730 .Xr sshd_config 5 ) .
731 .It Cm RhostsRSAAuthentication
732 Specifies whether to try rhosts based authentication with RSA host
740 This option applies to protocol version 1 only and requires
743 .It Cm RSAAuthentication
744 Specifies whether to try RSA authentication.
745 The argument to this keyword must be
749 RSA authentication will only be
750 attempted if the identity file exists, or an authentication agent is
754 Note that this option applies to protocol version 1 only.
756 Specifies what variables from the local
758 should be sent to the server.
759 Note that environment passing is only supported for protocol 2, the
760 server must also support it, and the server must be configured to
761 accept these environment variables.
766 for how to configure the server.
767 Variables are specified by name, which may contain the wildcard characters
771 Multiple environment variables may be separated by whitespace or spread
775 The default is not to send any environment variables.
776 .It Cm ServerAliveInterval
777 Sets a timeout interval in seconds after which if no data has been received
780 will send a message through the encrypted
781 channel to request a response from the server.
783 is 0, indicating that these messages will not be sent to the server.
784 This option applies to protocol version 2 only.
785 .It Cm ServerAliveCountMax
786 Sets the number of server alive messages (see above) which may be
789 receiving any messages back from the server.
790 If this threshold is reached while server alive messages are being sent,
792 will disconnect from the server, terminating the session.
793 It is important to note that the use of server alive messages is very
797 The server alive messages are sent through the encrypted channel
798 and therefore will not be spoofable.
799 The TCP keepalive option enabled by
802 The server alive mechanism is valuable when the client or
803 server depend on knowing when a connection has become inactive.
805 The default value is 3.
807 .Cm ServerAliveInterval
808 (above) is set to 15, and
809 .Cm ServerAliveCountMax
810 is left at the default, if the server becomes unresponsive ssh
811 will disconnect after approximately 45 seconds.
812 .It Cm SmartcardDevice
813 Specifies which smartcard device to use.
814 The argument to this keyword is the device
816 should use to communicate with a smartcard used for storing the user's
818 By default, no device is specified and smartcard support is not activated.
819 .It Cm StrictHostKeyChecking
820 If this flag is set to
823 will never automatically add host keys to the
824 .Pa ~/.ssh/known_hosts
825 file, and refuses to connect to hosts whose host key has changed.
826 This provides maximum protection against trojan horse attacks,
827 however, can be annoying when the
828 .Pa /etc/ssh/ssh_known_hosts
829 file is poorly maintained, or connections to new hosts are
831 This option forces the user to manually
833 If this flag is set to
836 will automatically add new host keys to the
837 user known hosts files.
838 If this flag is set to
841 will be added to the user known host files only after the user
842 has confirmed that is what they really want to do, and
844 will refuse to connect to hosts whose host key has changed.
846 known hosts will be verified automatically in all cases.
855 Specifies whether the system should send TCP keepalive messages to the
857 If they are sent, death of the connection or crash of one
858 of the machines will be properly noticed.
859 However, this means that
860 connections will die if the route is down temporarily, and some people
865 (to send TCP keepalive messages), and the client will notice
866 if the network goes down or the remote host dies.
867 This is important in scripts, and many users want it too.
869 To disable TCP keepalive messages, the value should be set to
871 .It Cm UsePrivilegedPort
872 Specifies whether to use a privileged port for outgoing connections.
883 Note that this option must be set to
886 .Cm RhostsRSAAuthentication
889 Specifies the user to log in as.
890 This can be useful when a different user name is used on different machines.
891 This saves the trouble of
892 having to remember to give the user name on the command line.
893 .It Cm UserKnownHostsFile
894 Specifies a file to use for the user
895 host key database instead of
896 .Pa ~/.ssh/known_hosts .
897 .It Cm VerifyHostKeyDNS
898 Specifies whether to verify the remote key using DNS and SSHFP resource
900 If this option is set to
902 the client will implicitly trust keys that match a secure fingerprint
904 Insecure fingerprints will be handled as if this option was set to
906 If this option is set to
908 information on fingerprint match will be displayed, but the user will still
909 need to confirm new host keys according to the
910 .Cm StrictHostKeyChecking
919 Note that this option applies to protocol version 2 only.
921 Specifies the full pathname of the
925 .Pa /usr/X11R6/bin/xauth .
930 This is the per-user configuration file.
931 The format of this file is described above.
932 This file is used by the
935 Because of the potential for abuse, this file must have strict permissions:
936 read/write for the user, and not accessible by others.
937 .It Pa /etc/ssh/ssh_config
938 Systemwide configuration file.
939 This file provides defaults for those
940 values that are not specified in the user's configuration file, and
941 for those users who do not have a configuration file.
942 This file must be world-readable.
947 OpenSSH is a derivative of the original and free
948 ssh 1.2.12 release by Tatu Ylonen.
949 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
950 Theo de Raadt and Dug Song
951 removed many bugs, re-added newer features and
953 Markus Friedl contributed the support for SSH
954 protocol versions 1.5 and 2.0.