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 1.29 2004/03/05 10:53:58 markus 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 bracketed 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.
196 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
197 aes192-cbc,aes256-cbc''
199 .It Cm ClearAllForwardings
200 Specifies that all local, remote and dynamic port forwardings
201 specified in the configuration files or on the command line be
203 This option is primarily useful when used from the
205 command line to clear port forwardings set in
206 configuration files, and is automatically set by
217 Specifies whether to use compression.
224 .It Cm CompressionLevel
225 Specifies the compression level to use if compression is enabled.
226 The argument must be an integer from 1 (fast) to 9 (slow, best).
227 The default level is 6, which is good for most applications.
228 The meaning of the values is the same as in
230 Note that this option applies to protocol version 1 only.
231 .It Cm ConnectionAttempts
232 Specifies the number of tries (one per second) to make before exiting.
233 The argument must be an integer.
234 This may be useful in scripts if the connection sometimes fails.
236 .It Cm ConnectTimeout
237 Specifies the timeout (in seconds) used when connecting to the ssh
238 server, instead of using the default system TCP timeout.
239 This value is used only when the target is down or really unreachable,
240 not when it refuses the connection.
241 .It Cm DynamicForward
242 Specifies that a TCP/IP port on the local machine be forwarded
243 over the secure channel, and the application
244 protocol is then used to determine where to connect to from the
246 The argument must be a port number.
247 Currently the SOCKS4 and SOCKS5 protocols are supported, and
249 will act as a SOCKS server.
250 Multiple forwardings may be specified, and
251 additional forwardings can be given on the command line.
252 Only the superuser can forward privileged ports.
253 .It Cm EnableSSHKeysign
254 Setting this option to
256 in the global client configuration file
257 .Pa /etc/ssh/ssh_config
258 enables the use of the helper program
261 .Cm HostbasedAuthentication .
268 This option should be placed in the non-hostspecific section.
271 for more information.
273 Sets the escape character (default:
275 The escape character can also
276 be set on the command line.
277 The argument should be a single character,
279 followed by a letter, or
281 to disable the escape
282 character entirely (making the connection transparent for binary
285 Specifies whether the connection to the authentication agent (if any)
286 will be forwarded to the remote machine.
294 Agent forwarding should be enabled with caution.
295 Users with the ability to bypass file permissions on the remote host
296 (for the agent's Unix-domain socket)
297 can access the local agent through the forwarded connection.
298 An attacker cannot obtain key material from the agent,
299 however they can perform operations on the keys that enable them to
300 authenticate using the identities loaded into the agent.
302 Specifies whether X11 connections will be automatically redirected
303 over the secure channel and
313 X11 forwarding should be enabled with caution.
314 Users with the ability to bypass file permissions on the remote host
315 (for the user's X11 authorization database)
316 can access the local X11 display through the forwarded connection.
317 An attacker may then be able to perform activities such as keystroke monitoring
319 .Cm ForwardX11Trusted
320 option is also enabled.
321 .It Cm ForwardX11Trusted
322 If the this option is set to
324 then remote X11 clients will have full access to the original X11 display.
325 If this option is set to
327 then remote X11 clients will be considered untrusted and prevented
328 from stealing or tampering with data belonging to trusted X11
334 See the X11 SECURITY extension specification for full details on
335 the restrictions imposed on untrusted clients.
337 Specifies whether remote hosts are allowed to connect to local
341 binds local port forwardings to the loopback address.
342 This prevents other remote hosts from connecting to forwarded ports.
344 can be used to specify that
346 should bind local port forwardings to the wildcard address,
347 thus allowing remote hosts to connect to forwarded ports.
354 .It Cm GlobalKnownHostsFile
355 Specifies a file to use for the global
356 host key database instead of
357 .Pa /etc/ssh/ssh_known_hosts .
358 .It Cm GSSAPIAuthentication
359 Specifies whether user authentication based on GSSAPI is allowed.
362 Note that this option applies to protocol version 2 only.
363 .It Cm GSSAPIKeyExchange
364 Specifies whether key exchange based on GSSAPI may be used. When using
365 GSSAPI key exchange the server need not have a host key.
368 Note that this option applies to protocol version 2 only.
369 .It Cm GSSAPIDelegateCredentials
370 Forward (delegate) credentials to the server.
373 Note that this option applies to protocol version 2 only.
374 .It Cm HostbasedAuthentication
375 Specifies whether to try rhosts based authentication with public key
383 This option applies to protocol version 2 only and
385 .Cm RhostsRSAAuthentication .
386 .It Cm HostKeyAlgorithms
387 Specifies the protocol version 2 host key algorithms
388 that the client wants to use in order of preference.
389 The default for this option is:
390 .Dq ssh-rsa,ssh-dss .
392 Specifies an alias that should be used instead of the
393 real host name when looking up or saving the host key
394 in the host key database files.
395 This option is useful for tunneling ssh connections
396 or for multiple servers running on a single host.
398 Specifies the real host name to log into.
399 This can be used to specify nicknames or abbreviations for hosts.
400 Default is the name given on the command line.
401 Numeric IP addresses are also permitted (both on the command line and in
405 Specifies a file from which the user's RSA or DSA authentication identity
408 .Pa $HOME/.ssh/identity
409 for protocol version 1, and
410 .Pa $HOME/.ssh/id_rsa
412 .Pa $HOME/.ssh/id_dsa
413 for protocol version 2.
414 Additionally, any identities represented by the authentication agent
415 will be used for authentication.
416 The file name may use the tilde
417 syntax to refer to a user's home directory.
418 It is possible to have
419 multiple identity files specified in configuration files; all these
420 identities will be tried in sequence.
421 .It Cm IdentitiesOnly
424 should only use the authentication identity files configured in the
429 offers more identities.
430 The argument to this keyword must be
434 This option is intented for situations where
436 offers many different identities.
440 Specifies that a TCP/IP port on the local machine be forwarded over
441 the secure channel to the specified host and port from the remote machine.
442 The first argument must be a port number, and the second must be
444 IPv6 addresses can be specified with an alternative syntax:
446 Multiple forwardings may be specified, and additional
447 forwardings can be given on the command line.
448 Only the superuser can forward privileged ports.
450 Gives the verbosity level that is used when logging messages from
452 The possible values are:
453 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
455 DEBUG and DEBUG1 are equivalent.
456 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
458 Specifies the MAC (message authentication code) algorithms
459 in order of preference.
460 The MAC algorithm is used in protocol version 2
461 for data integrity protection.
462 Multiple algorithms must be comma-separated.
464 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
465 .It Cm NoHostAuthenticationForLocalhost
466 This option can be used if the home directory is shared across machines.
467 In this case localhost will refer to a different machine on each of
468 the machines and the user will get many warnings about changed host keys.
469 However, this option disables host authentication for localhost.
470 The argument to this keyword must be
474 The default is to check the host key for localhost.
475 .It Cm NumberOfPasswordPrompts
476 Specifies the number of password prompts before giving up.
477 The argument to this keyword must be an integer.
479 .It Cm PasswordAuthentication
480 Specifies whether to use password authentication.
481 The argument to this keyword must be
488 Specifies the port number to connect on the remote host.
490 .It Cm PreferredAuthentications
491 Specifies the order in which the client should try protocol 2
492 authentication methods.
493 This allows a client to prefer one method (e.g.
494 .Cm keyboard-interactive )
495 over another method (e.g.
497 The default for this option is:
498 .Dq hostbased,external-keyx,gssapi-with-mic,gssapi,publickey,keyboard-interactive,password .
500 Specifies the protocol versions
502 should support in order of preference.
503 The possible values are
507 Multiple versions must be comma-separated.
512 tries version 2 and falls back to version 1
513 if version 2 is not available.
515 Specifies the command to use to connect to the server.
517 string extends to the end of the line, and is executed with
519 In the command string,
521 will be substituted by the host name to
525 The command can be basically anything,
526 and should read from its standard input and write to its standard output.
527 It should eventually connect an
529 server running on some machine, or execute
532 Host key management will be done using the
533 HostName of the host being connected (defaulting to the name typed by
535 Setting the command to
537 disables this option entirely.
540 is not available for connects with a proxy command.
542 .It Cm PubkeyAuthentication
543 Specifies whether to try public key authentication.
544 The argument to this keyword must be
550 This option applies to protocol version 2 only.
552 Specifies that a TCP/IP port on the remote machine be forwarded over
553 the secure channel to the specified host and port from the local machine.
554 The first argument must be a port number, and the second must be
556 IPv6 addresses can be specified with an alternative syntax:
558 Multiple forwardings may be specified, and additional
559 forwardings can be given on the command line.
560 Only the superuser can forward privileged ports.
561 .It Cm RhostsRSAAuthentication
562 Specifies whether to try rhosts based authentication with RSA host
570 This option applies to protocol version 1 only and requires
573 .It Cm RSAAuthentication
574 Specifies whether to try RSA authentication.
575 The argument to this keyword must be
579 RSA authentication will only be
580 attempted if the identity file exists, or an authentication agent is
584 Note that this option applies to protocol version 1 only.
585 .It Cm ServerAliveInterval
586 Sets a timeout interval in seconds after which if no data has been received
589 will send a message through the encrypted
590 channel to request a response from the server.
592 is 0, indicating that these messages will not be sent to the server.
593 This option applies to protocol version 2 only.
594 .It Cm ServerAliveCountMax
595 Sets the number of server alive messages (see above) which may be
598 receiving any messages back from the server.
599 If this threshold is reached while server alive messages are being sent,
601 will disconnect from the server, terminating the session.
602 It is important to note that the use of server alive messages is very
606 The server alive messages are sent through the encrypted channel
607 and therefore will not be spoofable.
608 The TCP keepalive option enabled by
611 The server alive mechanism is valuable when the client or
612 server depend on knowing when a connection has become inactive.
614 The default value is 3.
616 .Cm ServerAliveInterval
617 (above) is set to 15, and
618 .Cm ServerAliveCountMax
619 is left at the default, if the server becomes unresponsive ssh
620 will disconnect after approximately 45 seconds.
621 .It Cm SmartcardDevice
622 Specifies which smartcard device to use.
623 The argument to this keyword is the device
625 should use to communicate with a smartcard used for storing the user's
627 By default, no device is specified and smartcard support is not activated.
628 .It Cm StrictHostKeyChecking
629 If this flag is set to
632 will never automatically add host keys to the
633 .Pa $HOME/.ssh/known_hosts
634 file, and refuses to connect to hosts whose host key has changed.
635 This provides maximum protection against trojan horse attacks,
636 however, can be annoying when the
637 .Pa /etc/ssh/ssh_known_hosts
638 file is poorly maintained, or connections to new hosts are
640 This option forces the user to manually
642 If this flag is set to
645 will automatically add new host keys to the
646 user known hosts files.
647 If this flag is set to
650 will be added to the user known host files only after the user
651 has confirmed that is what they really want to do, and
653 will refuse to connect to hosts whose host key has changed.
655 known hosts will be verified automatically in all cases.
664 Specifies whether the system should send TCP keepalive messages to the
666 If they are sent, death of the connection or crash of one
667 of the machines will be properly noticed.
668 However, this means that
669 connections will die if the route is down temporarily, and some people
674 (to send TCP keepalive messages), and the client will notice
675 if the network goes down or the remote host dies.
676 This is important in scripts, and many users want it too.
678 To disable TCP keepalive messages, the value should be set to
680 .It Cm UsePrivilegedPort
681 Specifies whether to use a privileged port for outgoing connections.
692 Note that this option must be set to
695 .Cm RhostsRSAAuthentication
698 Specifies the user to log in as.
699 This can be useful when a different user name is used on different machines.
700 This saves the trouble of
701 having to remember to give the user name on the command line.
702 .It Cm UserKnownHostsFile
703 Specifies a file to use for the user
704 host key database instead of
705 .Pa $HOME/.ssh/known_hosts .
706 .It Cm VerifyHostKeyDNS
707 Specifies whether to verify the remote key using DNS and SSHFP resource
709 If this option is set to
711 the client will implicitly trust keys that match a secure fingerprint
713 Insecure fingerprints will be handled as if this option was set to
715 If this option is set to
717 information on fingerprint match will be displayed, but the user will still
718 need to confirm new host keys according to the
719 .Cm StrictHostKeyChecking
728 Note that this option applies to protocol version 2 only.
730 Specifies the full pathname of the
734 .Pa /usr/X11R6/bin/xauth .
738 .It Pa $HOME/.ssh/config
739 This is the per-user configuration file.
740 The format of this file is described above.
741 This file is used by the
744 This file does not usually contain any sensitive information,
745 but the recommended permissions are read/write for the user, and not
746 accessible by others.
747 .It Pa /etc/ssh/ssh_config
748 Systemwide configuration file.
749 This file provides defaults for those
750 values that are not specified in the user's configuration file, and
751 for those users who do not have a configuration file.
752 This file must be world-readable.
757 OpenSSH is a derivative of the original and free
758 ssh 1.2.12 release by Tatu Ylonen.
759 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
760 Theo de Raadt and Dug Song
761 removed many bugs, re-added newer features and
763 Markus Friedl contributed the support for SSH
764 protocol versions 1.5 and 2.0.