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 1.49 2005/03/16 11:10:38 jmc Exp $
38 .Dd September 25, 1999
43 .Nd OpenSSH SSH client configuration files
45 .Bl -tag -width Ds -compact
46 .It Pa $HOME/.ssh/config
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
58 .Pq Pa $HOME/.ssh/config
60 GSSAPI configuration file
61 .Pq Pa $HOME/.ssh/config.gssapi
63 Kerberos configuration file
64 .Pq Pa $HOME/.ssh/config.krb
66 system-wide configuration file
67 .Pq Pa /etc/ssh/ssh_config
70 For each parameter, the first obtained value
72 The configuration files contain sections separated by
74 specifications, and that section is only applied for hosts that
75 match one of the patterns given in the specification.
76 The matched host name is the one given on the command line.
78 Since the first obtained value for each parameter is used, more
79 host-specific declarations should be given near the beginning of the
80 file, and general defaults at the end.
82 The configuration file has the following format:
84 Empty lines and lines starting with
88 Otherwise a line is of the format
89 .Dq keyword arguments .
90 Configuration options may be separated by whitespace or
91 optional whitespace and exactly one
93 the latter format is useful to avoid the need to quote whitespace
94 when specifying configuration options using the
103 keywords and their meanings are as follows (note that
104 keywords are case-insensitive and arguments are case-sensitive):
107 Restricts the following declarations (up to the next
109 keyword) to be only for those hosts that match one of the patterns
110 given after the keyword.
114 can be used as wildcards in the
118 as a pattern can be used to provide global
119 defaults for all hosts.
122 argument given on the command line (i.e., the name is not converted to
123 a canonicalized host name before matching).
125 Specifies which address family to use when connecting.
135 passphrase/password querying will be disabled.
136 This option is useful in scripts and other batch jobs where no user
137 is present to supply the password.
145 Specify the interface to transmit from on machines with multiple
146 interfaces or aliased addresses.
147 Note that this option does not work if
148 .Cm UsePrivilegedPort
151 .It Cm ChallengeResponseAuthentication
152 Specifies whether to use challenge response authentication.
153 The argument to this keyword must be
160 If this flag is set to
162 ssh will additionally check the host IP address in the
165 This allows ssh to detect if a host key changed due to DNS spoofing.
166 If the option is set to
168 the check will not be executed.
172 Specifies the cipher to use for encrypting the session
173 in protocol version 1.
181 is only supported in the
183 client for interoperability with legacy protocol 1 implementations
184 that do not support the
187 Its use is strongly discouraged due to cryptographic weaknesses.
191 Specifies the ciphers allowed for protocol version 2
192 in order of preference.
193 Multiple ciphers must be comma-separated.
194 The supported ciphers are
208 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
209 aes192-cbc,aes256-cbc''
211 .It Cm ClearAllForwardings
212 Specifies that all local, remote and dynamic port forwardings
213 specified in the configuration files or on the command line be
215 This option is primarily useful when used from the
217 command line to clear port forwardings set in
218 configuration files, and is automatically set by
229 Specifies whether to use compression.
236 .It Cm CompressionLevel
237 Specifies the compression level to use if compression is enabled.
238 The argument must be an integer from 1 (fast) to 9 (slow, best).
239 The default level is 6, which is good for most applications.
240 The meaning of the values is the same as in
242 Note that this option applies to protocol version 1 only.
243 .It Cm ConnectionAttempts
244 Specifies the number of tries (one per second) to make before exiting.
245 The argument must be an integer.
246 This may be useful in scripts if the connection sometimes fails.
248 .It Cm ConnectTimeout
249 Specifies the timeout (in seconds) used when connecting to the ssh
250 server, instead of using the default system TCP timeout.
251 This value is used only when the target is down or really unreachable,
252 not when it refuses the connection.
254 Enables the sharing of multiple sessions over a single network connection.
258 will listen for connections on a control socket specified using the
261 Additional sessions can connect to this socket using the same
268 These sessions will reuse the master instance's network connection rather
269 than initiating new ones.
274 to listen for control connections, but require confirmation using the
276 program before they are accepted (see
280 Specify the path to the control socket used for connection sharing.
284 .It Cm DynamicForward
285 Specifies that a TCP/IP port on the local machine be forwarded
286 over the secure channel, and the application
287 protocol is then used to determine where to connect to from the
289 The argument must be a port number.
290 Currently the SOCKS4 and SOCKS5 protocols are supported, and
292 will act as a SOCKS server.
293 Multiple forwardings may be specified, and
294 additional forwardings can be given on the command line.
295 Only the superuser can forward privileged ports.
296 .It Cm EnableSSHKeysign
297 Setting this option to
299 in the global client configuration file
300 .Pa /etc/ssh/ssh_config
301 enables the use of the helper program
304 .Cm HostbasedAuthentication .
311 This option should be placed in the non-hostspecific section.
314 for more information.
316 Sets the escape character (default:
318 The escape character can also
319 be set on the command line.
320 The argument should be a single character,
322 followed by a letter, or
324 to disable the escape
325 character entirely (making the connection transparent for binary
328 Specifies whether the connection to the authentication agent (if any)
329 will be forwarded to the remote machine.
337 Agent forwarding should be enabled with caution.
338 Users with the ability to bypass file permissions on the remote host
339 (for the agent's Unix-domain socket)
340 can access the local agent through the forwarded connection.
341 An attacker cannot obtain key material from the agent,
342 however they can perform operations on the keys that enable them to
343 authenticate using the identities loaded into the agent.
345 Specifies whether X11 connections will be automatically redirected
346 over the secure channel and
356 X11 forwarding should be enabled with caution.
357 Users with the ability to bypass file permissions on the remote host
358 (for the user's X11 authorization database)
359 can access the local X11 display through the forwarded connection.
360 An attacker may then be able to perform activities such as keystroke monitoring
362 .Cm ForwardX11Trusted
363 option is also enabled.
364 .It Cm ForwardX11Trusted
365 If this option is set to
367 then remote X11 clients will have full access to the original X11 display.
369 If this option is set to
371 then remote X11 clients will be considered untrusted and prevented
372 from stealing or tampering with data belonging to trusted X11
376 token used for the session will be set to expire after 20 minutes.
377 Remote clients will be refused access after this time.
382 See the X11 SECURITY extension specification for full details on
383 the restrictions imposed on untrusted clients.
385 Specifies whether remote hosts are allowed to connect to local
389 binds local port forwardings to the loopback address.
390 This prevents other remote hosts from connecting to forwarded ports.
392 can be used to specify that
394 should bind local port forwardings to the wildcard address,
395 thus allowing remote hosts to connect to forwarded ports.
402 .It Cm GlobalKnownHostsFile
403 Specifies a file to use for the global
404 host key database instead of
405 .Pa /etc/ssh/ssh_known_hosts .
406 .It Cm GSSAPIAuthentication
407 Specifies whether user authentication based on GSSAPI is allowed.
410 Note that this option applies to protocol version 2 only.
411 .It Cm GSSAPIKeyExchange
412 Specifies whether key exchange based on GSSAPI may be used. When using
413 GSSAPI key exchange the server need not have a host key.
416 Note that this option applies to protocol version 2 only.
417 .It Cm GSSAPIDelegateCredentials
418 Forward (delegate) credentials to the server.
421 Note that this option applies to protocol version 2 only.
422 .It Cm HashKnownHosts
425 should hash host names and addresses when they are added to
426 .Pa $HOME/.ssh/known_hosts .
427 These hashed names may be used normally by
431 but they do not reveal identifying information should the file's contents
435 Note that hashing of names and addresses will not be retrospectively applied
436 to existing known hosts files, but these may be manually hashed using
438 .It Cm HostbasedAuthentication
439 Specifies whether to try rhosts based authentication with public key
447 This option applies to protocol version 2 only and
449 .Cm RhostsRSAAuthentication .
450 .It Cm HostKeyAlgorithms
451 Specifies the protocol version 2 host key algorithms
452 that the client wants to use in order of preference.
453 The default for this option is:
454 .Dq ssh-rsa,ssh-dss .
456 Specifies an alias that should be used instead of the
457 real host name when looking up or saving the host key
458 in the host key database files.
459 This option is useful for tunneling ssh connections
460 or for multiple servers running on a single host.
462 Specifies the real host name to log into.
463 This can be used to specify nicknames or abbreviations for hosts.
464 Default is the name given on the command line.
465 Numeric IP addresses are also permitted (both on the command line and in
469 Specifies a file from which the user's RSA or DSA authentication identity
472 .Pa $HOME/.ssh/identity
473 for protocol version 1, and
474 .Pa $HOME/.ssh/id_rsa
476 .Pa $HOME/.ssh/id_dsa
477 for protocol version 2.
478 Additionally, any identities represented by the authentication agent
479 will be used for authentication.
480 The file name may use the tilde
481 syntax to refer to a user's home directory.
482 It is possible to have
483 multiple identity files specified in configuration files; all these
484 identities will be tried in sequence.
485 .It Cm IdentitiesOnly
488 should only use the authentication identity files configured in the
493 offers more identities.
494 The argument to this keyword must be
498 This option is intented for situations where
500 offers many different identities.
503 .It Cm KbdInteractiveDevices
504 Specifies the list of methods to use in keyboard-interactive authentication.
505 Multiple method names must be comma-separated.
506 The default is to use the server specified list.
508 Specifies that a TCP/IP port on the local machine be forwarded over
509 the secure channel to the specified host and port from the remote machine.
510 The first argument must be
512 .Oo Ar bind_address : Oc Ar port
514 and the second argument must be
515 .Ar host : Ns Ar hostport .
516 IPv6 addresses can be specified by enclosing addresses in square brackets or
517 by using an alternative syntax:
518 .Oo Ar bind_address Ns / Oc Ns Ar port
520 .Ar host Ns / Ns Ar hostport .
521 Multiple forwardings may be specified, and additional forwardings can be
522 given on the command line.
523 Only the superuser can forward privileged ports.
524 By default, the local port is bound in accordance with the
529 may be used to bind the connection to a specific address.
534 indicates that the listening port be bound for local use only, while an
537 indicates that the port should be available from all interfaces.
539 Gives the verbosity level that is used when logging messages from
541 The possible values are:
542 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
544 DEBUG and DEBUG1 are equivalent.
545 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
547 Specifies the MAC (message authentication code) algorithms
548 in order of preference.
549 The MAC algorithm is used in protocol version 2
550 for data integrity protection.
551 Multiple algorithms must be comma-separated.
553 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
554 .It Cm NoHostAuthenticationForLocalhost
555 This option can be used if the home directory is shared across machines.
556 In this case localhost will refer to a different machine on each of
557 the machines and the user will get many warnings about changed host keys.
558 However, this option disables host authentication for localhost.
559 The argument to this keyword must be
563 The default is to check the host key for localhost.
564 .It Cm NumberOfPasswordPrompts
565 Specifies the number of password prompts before giving up.
566 The argument to this keyword must be an integer.
568 .It Cm PasswordAuthentication
569 Specifies whether to use password authentication.
570 The argument to this keyword must be
577 Specifies the port number to connect on the remote host.
579 .It Cm PreferredAuthentications
580 Specifies the order in which the client should try protocol 2
581 authentication methods.
582 This allows a client to prefer one method (e.g.\&
583 .Cm keyboard-interactive )
584 over another method (e.g.\&
586 The default for this option is:
587 .Dq hostbased,external-keyx,gssapi-with-mic,gssapi,publickey,keyboard-interactive,password .
589 Specifies the protocol versions
591 should support in order of preference.
592 The possible values are
596 Multiple versions must be comma-separated.
601 tries version 2 and falls back to version 1
602 if version 2 is not available.
604 Specifies the command to use to connect to the server.
606 string extends to the end of the line, and is executed with
608 In the command string,
610 will be substituted by the host name to
614 The command can be basically anything,
615 and should read from its standard input and write to its standard output.
616 It should eventually connect an
618 server running on some machine, or execute
621 Host key management will be done using the
622 HostName of the host being connected (defaulting to the name typed by
624 Setting the command to
626 disables this option entirely.
629 is not available for connects with a proxy command.
631 .It Cm PubkeyAuthentication
632 Specifies whether to try public key authentication.
633 The argument to this keyword must be
639 This option applies to protocol version 2 only.
641 Specifies that a TCP/IP port on the remote machine be forwarded over
642 the secure channel to the specified host and port from the local machine.
643 The first argument must be
645 .Oo Ar bind_address : Oc Ar port
647 and the second argument must be
648 .Ar host : Ns Ar hostport .
649 IPv6 addresses can be specified by enclosing addresses in square brackets
650 or by using an alternative syntax:
651 .Oo Ar bind_address Ns / Oc Ns Ar port
653 .Ar host Ns / Ns Ar hostport .
654 Multiple forwardings may be specified, and additional
655 forwardings can be given on the command line.
656 Only the superuser can forward privileged ports.
660 is not specified, the default is to only bind to loopback addresses.
665 or an empty string, then the forwarding is requested to listen on all
669 will only succeed if the server's
671 option is enabled (see
672 .Xr sshd_config 5 ) .
673 .It Cm RhostsRSAAuthentication
674 Specifies whether to try rhosts based authentication with RSA host
682 This option applies to protocol version 1 only and requires
685 .It Cm RSAAuthentication
686 Specifies whether to try RSA authentication.
687 The argument to this keyword must be
691 RSA authentication will only be
692 attempted if the identity file exists, or an authentication agent is
696 Note that this option applies to protocol version 1 only.
698 Specifies what variables from the local
700 should be sent to the server.
701 Note that environment passing is only supported for protocol 2, the
702 server must also support it, and the server must be configured to
703 accept these environment variables.
708 for how to configure the server.
709 Variables are specified by name, which may contain the wildcard characters
713 Multiple environment variables may be separated by whitespace or spread
717 The default is not to send any environment variables.
718 .It Cm ServerAliveInterval
719 Sets a timeout interval in seconds after which if no data has been received
722 will send a message through the encrypted
723 channel to request a response from the server.
725 is 0, indicating that these messages will not be sent to the server.
726 This option applies to protocol version 2 only.
727 .It Cm ServerAliveCountMax
728 Sets the number of server alive messages (see above) which may be
731 receiving any messages back from the server.
732 If this threshold is reached while server alive messages are being sent,
734 will disconnect from the server, terminating the session.
735 It is important to note that the use of server alive messages is very
739 The server alive messages are sent through the encrypted channel
740 and therefore will not be spoofable.
741 The TCP keepalive option enabled by
744 The server alive mechanism is valuable when the client or
745 server depend on knowing when a connection has become inactive.
747 The default value is 3.
749 .Cm ServerAliveInterval
750 (above) is set to 15, and
751 .Cm ServerAliveCountMax
752 is left at the default, if the server becomes unresponsive ssh
753 will disconnect after approximately 45 seconds.
754 .It Cm SmartcardDevice
755 Specifies which smartcard device to use.
756 The argument to this keyword is the device
758 should use to communicate with a smartcard used for storing the user's
760 By default, no device is specified and smartcard support is not activated.
761 .It Cm StrictHostKeyChecking
762 If this flag is set to
765 will never automatically add host keys to the
766 .Pa $HOME/.ssh/known_hosts
767 file, and refuses to connect to hosts whose host key has changed.
768 This provides maximum protection against trojan horse attacks,
769 however, can be annoying when the
770 .Pa /etc/ssh/ssh_known_hosts
771 file is poorly maintained, or connections to new hosts are
773 This option forces the user to manually
775 If this flag is set to
778 will automatically add new host keys to the
779 user known hosts files.
780 If this flag is set to
783 will be added to the user known host files only after the user
784 has confirmed that is what they really want to do, and
786 will refuse to connect to hosts whose host key has changed.
788 known hosts will be verified automatically in all cases.
797 Specifies whether the system should send TCP keepalive messages to the
799 If they are sent, death of the connection or crash of one
800 of the machines will be properly noticed.
801 However, this means that
802 connections will die if the route is down temporarily, and some people
807 (to send TCP keepalive messages), and the client will notice
808 if the network goes down or the remote host dies.
809 This is important in scripts, and many users want it too.
811 To disable TCP keepalive messages, the value should be set to
813 .It Cm UsePrivilegedPort
814 Specifies whether to use a privileged port for outgoing connections.
825 Note that this option must be set to
828 .Cm RhostsRSAAuthentication
831 Specifies the user to log in as.
832 This can be useful when a different user name is used on different machines.
833 This saves the trouble of
834 having to remember to give the user name on the command line.
835 .It Cm UserKnownHostsFile
836 Specifies a file to use for the user
837 host key database instead of
838 .Pa $HOME/.ssh/known_hosts .
839 .It Cm VerifyHostKeyDNS
840 Specifies whether to verify the remote key using DNS and SSHFP resource
842 If this option is set to
844 the client will implicitly trust keys that match a secure fingerprint
846 Insecure fingerprints will be handled as if this option was set to
848 If this option is set to
850 information on fingerprint match will be displayed, but the user will still
851 need to confirm new host keys according to the
852 .Cm StrictHostKeyChecking
861 Note that this option applies to protocol version 2 only.
863 Specifies the full pathname of the
867 .Pa /usr/X11R6/bin/xauth .
871 .It Pa $HOME/.ssh/config
872 This is the per-user configuration file.
873 The format of this file is described above.
874 This file is used by the
877 Because of the potential for abuse, this file must have strict permissions:
878 read/write for the user, and not accessible by others.
879 .It Pa /etc/ssh/ssh_config
880 Systemwide configuration file.
881 This file provides defaults for those
882 values that are not specified in the user's configuration file, and
883 for those users who do not have a configuration file.
884 This file must be world-readable.
889 OpenSSH is a derivative of the original and free
890 ssh 1.2.12 release by Tatu Ylonen.
891 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
892 Theo de Raadt and Dug Song
893 removed many bugs, re-added newer features and
895 Markus Friedl contributed the support for SSH
896 protocol versions 1.5 and 2.0.