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 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 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 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
520 .It Cm IdentitiesOnly
523 should only use the authentication identity files configured in the
528 offers more identities.
529 The argument to this keyword must be
533 This option is intended for situations where
535 offers many different identities.
539 Specifies a file from which the user's RSA or DSA authentication identity
543 for protocol version 1, and
547 for protocol version 2.
548 Additionally, any identities represented by the authentication agent
549 will be used for authentication.
550 The file name may use the tilde
551 syntax to refer to a user's home directory.
552 It is possible to have
553 multiple identity files specified in configuration files; all these
554 identities will be tried in sequence.
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 a command to execute on the local machine after successfully
561 connecting to the server.
562 The command string extends to the end of the line, and is executed with
564 This directive is ignored unless
565 .Cm PermitLocalCommand
568 Specifies that a TCP port on the local machine be forwarded over
569 the secure channel to the specified host and port from the remote machine.
570 The first argument must be
572 .Oo Ar bind_address : Oc Ar port
574 and the second argument must be
575 .Ar host : Ns Ar hostport .
576 IPv6 addresses can be specified by enclosing addresses in square brackets or
577 by using an alternative syntax:
578 .Oo Ar bind_address Ns / Oc Ns Ar port
580 .Ar host Ns / Ns Ar hostport .
581 Multiple forwardings may be specified, and additional forwardings can be
582 given on the command line.
583 Only the superuser can forward privileged ports.
584 By default, the local port is bound in accordance with the
589 may be used to bind the connection to a specific address.
594 indicates that the listening port be bound for local use only, while an
597 indicates that the port should be available from all interfaces.
599 Gives the verbosity level that is used when logging messages from
601 The possible values are:
602 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
604 DEBUG and DEBUG1 are equivalent.
605 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
607 Specifies the MAC (message authentication code) algorithms
608 in order of preference.
609 The MAC algorithm is used in protocol version 2
610 for data integrity protection.
611 Multiple algorithms must be comma-separated.
613 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
614 .It Cm NoHostAuthenticationForLocalhost
615 This option can be used if the home directory is shared across machines.
616 In this case localhost will refer to a different machine on each of
617 the machines and the user will get many warnings about changed host keys.
618 However, this option disables host authentication for localhost.
619 The argument to this keyword must be
623 The default is to check the host key for localhost.
624 .It Cm NumberOfPasswordPrompts
625 Specifies the number of password prompts before giving up.
626 The argument to this keyword must be an integer.
628 .It Cm PasswordAuthentication
629 Specifies whether to use password authentication.
630 The argument to this keyword must be
636 .It Cm PermitLocalCommand
637 Allow local command execution via the
640 .Ic !\& Ns Ar command
650 Specifies the port number to connect on the remote host.
652 .It Cm PreferredAuthentications
653 Specifies the order in which the client should try protocol 2
654 authentication methods.
655 This allows a client to prefer one method (e.g.\&
656 .Cm keyboard-interactive )
657 over another method (e.g.\&
659 The default for this option is:
660 .Dq hostbased,publickey,keyboard-interactive,password .
662 Specifies the protocol versions
664 should support in order of preference.
665 The possible values are
669 Multiple versions must be comma-separated.
674 tries version 2 and falls back to version 1
675 if version 2 is not available.
677 Specifies the command to use to connect to the server.
679 string extends to the end of the line, and is executed with
681 In the command string,
683 will be substituted by the host name to
687 The command can be basically anything,
688 and should read from its standard input and write to its standard output.
689 It should eventually connect an
691 server running on some machine, or execute
694 Host key management will be done using the
695 HostName of the host being connected (defaulting to the name typed by
697 Setting the command to
699 disables this option entirely.
702 is not available for connects with a proxy command.
704 This directive is useful in conjunction with
706 and its proxy support.
707 For example, the following directive would connect via an HTTP proxy at
709 .Bd -literal -offset 3n
710 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
712 .It Cm PubkeyAuthentication
713 Specifies whether to try public key authentication.
714 The argument to this keyword must be
720 This option applies to protocol version 2 only.
722 Specifies the maximum amount of data that may be transmitted before the
723 session key is renegotiated.
724 The argument is the number of bytes, with an optional suffix of
729 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
730 The default is between
734 depending on the cipher.
735 This option applies to protocol version 2 only.
737 Specifies that a TCP port on the remote machine be forwarded over
738 the secure channel to the specified host and port from the local machine.
739 The first argument must be
741 .Oo Ar bind_address : Oc Ar port
743 and the second argument must be
744 .Ar host : Ns Ar hostport .
745 IPv6 addresses can be specified by enclosing addresses in square brackets
746 or by using an alternative syntax:
747 .Oo Ar bind_address Ns / Oc Ns Ar port
749 .Ar host Ns / Ns Ar hostport .
750 Multiple forwardings may be specified, and additional
751 forwardings can be given on the command line.
752 Only the superuser can forward privileged ports.
756 is not specified, the default is to only bind to loopback addresses.
761 or an empty string, then the forwarding is requested to listen on all
765 will only succeed if the server's
767 option is enabled (see
768 .Xr sshd_config 5 ) .
769 .It Cm RhostsRSAAuthentication
770 Specifies whether to try rhosts based authentication with RSA host
778 This option applies to protocol version 1 only and requires
781 .It Cm RSAAuthentication
782 Specifies whether to try RSA authentication.
783 The argument to this keyword must be
787 RSA authentication will only be
788 attempted if the identity file exists, or an authentication agent is
792 Note that this option applies to protocol version 1 only.
794 Specifies what variables from the local
796 should be sent to the server.
797 Note that environment passing is only supported for protocol 2, the
798 server must also support it, and the server must be configured to
799 accept these environment variables.
804 for how to configure the server.
805 Variables are specified by name, which may contain the wildcard characters
809 Multiple environment variables may be separated by whitespace or spread
813 The default is not to send any environment variables.
814 .It Cm ServerAliveCountMax
815 Sets the number of server alive messages (see below) which may be
818 receiving any messages back from the server.
819 If this threshold is reached while server alive messages are being sent,
821 will disconnect from the server, terminating the session.
822 It is important to note that the use of server alive messages is very
826 The server alive messages are sent through the encrypted channel
827 and therefore will not be spoofable.
828 The TCP keepalive option enabled by
831 The server alive mechanism is valuable when the client or
832 server depend on knowing when a connection has become inactive.
834 The default value is 3.
836 .Cm ServerAliveInterval
837 (see below) is set to 15, and
838 .Cm ServerAliveCountMax
839 is left at the default, if the server becomes unresponsive ssh
840 will disconnect after approximately 45 seconds.
841 .It Cm ServerAliveInterval
842 Sets a timeout interval in seconds after which if no data has been received
845 will send a message through the encrypted
846 channel to request a response from the server.
848 is 0, indicating that these messages will not be sent to the server.
849 This option applies to protocol version 2 only.
850 .It Cm SmartcardDevice
851 Specifies which smartcard device to use.
852 The argument to this keyword is the device
854 should use to communicate with a smartcard used for storing the user's
856 By default, no device is specified and smartcard support is not activated.
857 .It Cm StrictHostKeyChecking
858 If this flag is set to
861 will never automatically add host keys to the
862 .Pa ~/.ssh/known_hosts
863 file, and refuses to connect to hosts whose host key has changed.
864 This provides maximum protection against trojan horse attacks,
865 however, can be annoying when the
866 .Pa /etc/ssh/ssh_known_hosts
867 file is poorly maintained, or connections to new hosts are
869 This option forces the user to manually
871 If this flag is set to
874 will automatically add new host keys to the
875 user known hosts files.
876 If this flag is set to
879 will be added to the user known host files only after the user
880 has confirmed that is what they really want to do, and
882 will refuse to connect to hosts whose host key has changed.
884 known hosts will be verified automatically in all cases.
893 Specifies whether the system should send TCP keepalive messages to the
895 If they are sent, death of the connection or crash of one
896 of the machines will be properly noticed.
897 However, this means that
898 connections will die if the route is down temporarily, and some people
903 (to send TCP keepalive messages), and the client will notice
904 if the network goes down or the remote host dies.
905 This is important in scripts, and many users want it too.
907 To disable TCP keepalive messages, the value should be set to
912 device forwarding between the client and the server.
913 This option also allows requesting layer 2 (ethernet)
914 instead of layer 3 (point-to-point) tunneling from the server.
926 device on the client.
927 Without this option, the next available device will be used.
928 .It Cm UsePrivilegedPort
929 Specifies whether to use a privileged port for outgoing connections.
940 Note that this option must be set to
943 .Cm RhostsRSAAuthentication
946 Specifies the user to log in as.
947 This can be useful when a different user name is used on different machines.
948 This saves the trouble of
949 having to remember to give the user name on the command line.
950 .It Cm UserKnownHostsFile
951 Specifies a file to use for the user
952 host key database instead of
953 .Pa ~/.ssh/known_hosts .
954 .It Cm VerifyHostKeyDNS
955 Specifies whether to verify the remote key using DNS and SSHFP resource
957 If this option is set to
959 the client will implicitly trust keys that match a secure fingerprint
961 Insecure fingerprints will be handled as if this option was set to
963 If this option is set to
965 information on fingerprint match will be displayed, but the user will still
966 need to confirm new host keys according to the
967 .Cm StrictHostKeyChecking
976 Note that this option applies to protocol version 2 only.
978 Specifies the full pathname of the
982 .Pa /usr/X11R6/bin/xauth .
987 This is the per-user configuration file.
988 The format of this file is described above.
989 This file is used by the
992 Because of the potential for abuse, this file must have strict permissions:
993 read/write for the user, and not accessible by others.
994 .It Pa /etc/ssh/ssh_config
995 Systemwide configuration file.
996 This file provides defaults for those
997 values that are not specified in the user's configuration file, and
998 for those users who do not have a configuration file.
999 This file must be world-readable.
1004 OpenSSH is a derivative of the original and free
1005 ssh 1.2.12 release by Tatu Ylonen.
1006 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1007 Theo de Raadt and Dug Song
1008 removed many bugs, re-added newer features and
1010 Markus Friedl contributed the support for SSH
1011 protocol versions 1.5 and 2.0.