| blob | cb15800a26cce0b66664c19b00e4d241bd06eafb |
1 .\" $NetBSD$
2 .\" -*- nroff -*-
3 .\"
4 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
5 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
6 .\" All rights reserved
7 .\"
8 .\" As far as I am concerned, the code I have written for this software
9 .\" can be used freely for any purpose. Any derived versions of this
10 .\" software must be clearly marked as such, and if the derived work is
11 .\" incompatible with the protocol description in the RFC file, it must be
12 .\" called by a name other than "ssh" or "Secure Shell".
13 .\"
14 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
15 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
16 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
17 .\"
18 .\" Redistribution and use in source and binary forms, with or without
19 .\" modification, are permitted provided that the following conditions
20 .\" are met:
21 .\" 1. Redistributions of source code must retain the above copyright
22 .\" notice, this list of conditions and the following disclaimer.
23 .\" 2. Redistributions in binary form must reproduce the above copyright
24 .\" notice, this list of conditions and the following disclaimer in the
25 .\" documentation and/or other materials provided with the distribution.
26 .\"
27 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
28 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
29 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
30 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
31 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
32 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
33 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
34 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
35 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
36 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
37 .\"
38 .\" $OpenBSD: ssh_config.5,v 1.119 2009/02/22 23:50:57 djm Exp $
39 .Dd February 22 2009
40 .Dt SSH_CONFIG 5
41 .Os
42 .Sh NAME
43 .Nm ssh_config
44 .Nd OpenSSH SSH client configuration files
45 .Sh SYNOPSIS
46 .Nm ~/.ssh/config
47 .Nm /etc/ssh/ssh_config
48 .Sh DESCRIPTION
49 .Xr ssh 1
50 obtains configuration data from the following sources in
51 the following order:
52 .Pp
53 .Bl -enum -offset indent -compact
54 .It
55 command-line options
56 .It
57 user's configuration file
58 .Pq Pa ~/.ssh/config
59 .It
60 system-wide configuration file
61 .Pq Pa /etc/ssh/ssh_config
62 .El
63 .Pp
64 For each parameter, the first obtained value
65 will be used.
66 The configuration files contain sections separated by
67 .Dq Host
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.
71 .Pp
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.
75 .Pp
76 The configuration file has the following format:
77 .Pp
78 Empty lines and lines starting with
79 .Ql #
80 are comments.
81 Otherwise a line is of the format
82 .Dq keyword arguments .
83 Configuration options may be separated by whitespace or
84 optional whitespace and exactly one
85 .Ql = ;
86 the latter format is useful to avoid the need to quote whitespace
87 when specifying configuration options using the
88 .Nm ssh ,
89 .Nm scp ,
90 and
91 .Nm sftp
92 .Fl o
93 option.
94 Arguments may optionally be enclosed in double quotes
95 .Pq \&"
96 in order to represent arguments containing spaces.
97 .Pp
98 The possible
99 keywords and their meanings are as follows (note that
100 keywords are case-insensitive and arguments are case-sensitive):
101 .Bl -tag -width Ds
102 .It Cm Host
103 Restricts the following declarations (up to the next
104 .Cm Host
105 keyword) to be only for those hosts that match one of the patterns
106 given after the keyword.
107 If more than one pattern is provided, they should be separated by whitespace.
108 A single
109 .Ql *
110 as a pattern can be used to provide global
111 defaults for all hosts.
112 The host is the
113 .Ar hostname
114 argument given on the command line (i.e. the name is not converted to
115 a canonicalized host name before matching).
116 .Pp
117 See
118 .Sx PATTERNS
119 for more information on patterns.
120 .It Cm AddressFamily
121 Specifies which address family to use when connecting.
122 Valid arguments are
123 .Dq any ,
124 .Dq inet
125 (use IPv4 only), or
126 .Dq inet6
127 (use IPv6 only).
128 .It Cm BatchMode
129 If set to
130 .Dq yes ,
131 passphrase/password querying will be disabled.
132 This option is useful in scripts and other batch jobs where no user
133 is present to supply the password.
134 The argument must be
135 .Dq yes
136 or
137 .Dq no .
138 The default is
139 .Dq no .
140 .It Cm BindAddress
141 Use the specified address on the local machine as the source address of
142 the connection.
143 Only useful on systems with more than one address.
144 Note that this option does not work if
145 .Cm UsePrivilegedPort
146 is set to
147 .Dq yes .
148 .It Cm ChallengeResponseAuthentication
149 Specifies whether to use challenge-response authentication.
150 The argument to this keyword must be
151 .Dq yes
152 or
153 .Dq no .
154 The default is
155 .Dq yes .
156 .It Cm CheckHostIP
157 If this flag is set to
158 .Dq yes ,
159 .Xr ssh 1
160 will additionally check the host IP address in the
161 .Pa known_hosts
162 file.
163 This allows ssh to detect if a host key changed due to DNS spoofing.
164 If the option is set to
165 .Dq no ,
166 the check will not be executed.
167 The default is
168 .Dq yes .
169 .It Cm Cipher
170 Specifies the cipher to use for encrypting the session
171 in protocol version 1.
172 Currently,
173 .Dq blowfish ,
174 .Dq 3des ,
175 and
176 .Dq des
177 are supported.
178 .Ar des
179 is only supported in the
180 .Xr ssh 1
181 client for interoperability with legacy protocol 1 implementations
182 that do not support the
183 .Ar 3des
184 cipher.
185 Its use is strongly discouraged due to cryptographic weaknesses.
186 The default is
187 .Dq 3des .
188 .It Cm Ciphers
189 Specifies the ciphers allowed for protocol version 2
190 in order of preference.
191 Multiple ciphers must be comma-separated.
192 The supported ciphers are
193 .Dq 3des-cbc ,
194 .Dq aes128-cbc ,
195 .Dq aes192-cbc ,
196 .Dq aes256-cbc ,
197 .Dq aes128-ctr ,
198 .Dq aes192-ctr ,
199 .Dq aes256-ctr ,
200 .Dq arcfour128 ,
201 .Dq arcfour256 ,
202 .Dq arcfour ,
203 .Dq blowfish-cbc ,
204 and
205 .Dq cast128-cbc .
206 The default is:
207 .Bd -literal -offset 3n
208 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
209 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
210 aes256-cbc,arcfour
211 .Ed
212 .It Cm ClearAllForwardings
213 Specifies that all local, remote, and dynamic port forwardings
214 specified in the configuration files or on the command line be
215 cleared.
216 This option is primarily useful when used from the
217 .Xr ssh 1
218 command line to clear port forwardings set in
219 configuration files, and is automatically set by
220 .Xr scp 1
221 and
222 .Xr sftp 1 .
223 The argument must be
224 .Dq yes
225 or
226 .Dq no .
227 The default is
228 .Dq no .
229 .It Cm Compression
230 Specifies whether to use compression.
231 The argument must be
232 .Dq yes
233 or
234 .Dq no .
235 The default is
236 .Dq no .
237 .It Cm CompressionLevel
238 Specifies the compression level to use if compression is enabled.
239 The argument must be an integer from 1 (fast) to 9 (slow, best).
240 The default level is 6, which is good for most applications.
241 The meaning of the values is the same as in
242 .Xr gzip 1 .
243 Note that this option applies to protocol version 1 only.
244 .It Cm ConnectionAttempts
245 Specifies the number of tries (one per second) to make before exiting.
246 The argument must be an integer.
247 This may be useful in scripts if the connection sometimes fails.
248 The default is 1.
249 .It Cm ConnectTimeout
250 Specifies the timeout (in seconds) used when connecting to the
251 SSH server, instead of using the default system TCP timeout.
252 This value is used only when the target is down or really unreachable,
253 not when it refuses the connection.
254 .It Cm ControlMaster
255 Enables the sharing of multiple sessions over a single network connection.
256 When set to
257 .Dq yes ,
258 .Xr ssh 1
259 will listen for connections on a control socket specified using the
260 .Cm ControlPath
261 argument.
262 Additional sessions can connect to this socket using the same
263 .Cm ControlPath
264 with
265 .Cm ControlMaster
266 set to
267 .Dq no
268 (the default).
269 These sessions will try to reuse the master instance's network connection
270 rather than initiating new ones, but will fall back to connecting normally
271 if the control socket does not exist, or is not listening.
272 .Pp
273 Setting this to
274 .Dq ask
275 will cause ssh
276 to listen for control connections, but require confirmation using the
277 .Ev SSH_ASKPASS
278 program before they are accepted (see
279 .Xr ssh-add 1
280 for details).
281 If the
282 .Cm ControlPath
283 cannot be opened,
284 ssh will continue without connecting to a master instance.
285 .Pp
286 X11 and
287 .Xr ssh-agent 1
288 forwarding is supported over these multiplexed connections, however the
289 display and agent forwarded will be the one belonging to the master
290 connection i.e. it is not possible to forward multiple displays or agents.
291 .Pp
292 Two additional options allow for opportunistic multiplexing: try to use a
293 master connection but fall back to creating a new one if one does not already
294 exist.
295 These options are:
296 .Dq auto
297 and
298 .Dq autoask .
299 The latter requires confirmation like the
300 .Dq ask
301 option.
302 .It Cm ControlPath
303 Specify the path to the control socket used for connection sharing as described
304 in the
305 .Cm ControlMaster
306 section above or the string
307 .Dq none
308 to disable connection sharing.
309 In the path,
310 .Ql %l
311 will be substituted by the local host name,
312 .Ql %h
313 will be substituted by the target host name,
314 .Ql %p
315 the port, and
316 .Ql %r
317 by the remote login username.
318 It is recommended that any
319 .Cm ControlPath
320 used for opportunistic connection sharing include
321 at least %h, %p, and %r.
322 This ensures that shared connections are uniquely identified.
323 .It Cm DynamicForward
324 Specifies that a TCP port on the local machine be forwarded
325 over the secure channel, and the application
326 protocol is then used to determine where to connect to from the
327 remote machine.
328 .Pp
329 The argument must be
330 .Sm off
331 .Oo Ar bind_address : Oc Ar port .
332 .Sm on
333 IPv6 addresses can be specified by enclosing addresses in square brackets or
334 by using an alternative syntax:
335 .Oo Ar bind_address Ns / Oc Ns Ar port .
336 By default, the local port is bound in accordance with the
337 .Cm GatewayPorts
338 setting.
339 However, an explicit
340 .Ar bind_address
341 may be used to bind the connection to a specific address.
342 The
343 .Ar bind_address
344 of
345 .Dq localhost
346 indicates that the listening port be bound for local use only, while an
347 empty address or
348 .Sq *
349 indicates that the port should be available from all interfaces.
350 .Pp
351 Currently the SOCKS4 and SOCKS5 protocols are supported, and
352 .Xr ssh 1
353 will act as a SOCKS server.
354 Multiple forwardings may be specified, and
355 additional forwardings can be given on the command line.
356 Only the superuser can forward privileged ports.
357 .It Cm EnableSSHKeysign
358 Setting this option to
359 .Dq yes
360 in the global client configuration file
361 .Pa /etc/ssh/ssh_config
362 enables the use of the helper program
363 .Xr ssh-keysign 8
364 during
365 .Cm HostbasedAuthentication .
366 The argument must be
367 .Dq yes
368 or
369 .Dq no .
370 The default is
371 .Dq no .
372 This option should be placed in the non-hostspecific section.
373 See
374 .Xr ssh-keysign 8
375 for more information.
376 .It Cm EscapeChar
377 Sets the escape character (default:
378 .Ql ~ ) .
379 The escape character can also
380 be set on the command line.
381 The argument should be a single character,
382 .Ql ^
383 followed by a letter, or
384 .Dq none
385 to disable the escape
386 character entirely (making the connection transparent for binary
387 data).
388 .It Cm ExitOnForwardFailure
389 Specifies whether
390 .Xr ssh 1
391 should terminate the connection if it cannot set up all requested
392 dynamic, tunnel, local, and remote port forwardings.
393 The argument must be
394 .Dq yes
395 or
396 .Dq no .
397 The default is
398 .Dq no .
399 .It Cm ForwardAgent
400 Specifies whether the connection to the authentication agent (if any)
401 will be forwarded to the remote machine.
402 The argument must be
403 .Dq yes
404 or
405 .Dq no .
406 The default is
407 .Dq no .
408 .Pp
409 Agent forwarding should be enabled with caution.
410 Users with the ability to bypass file permissions on the remote host
411 (for the agent's Unix-domain socket)
412 can access the local agent through the forwarded connection.
413 An attacker cannot obtain key material from the agent,
414 however they can perform operations on the keys that enable them to
415 authenticate using the identities loaded into the agent.
416 .It Cm ForwardX11
417 Specifies whether X11 connections will be automatically redirected
418 over the secure channel and
419 .Ev DISPLAY
420 set.
421 The argument must be
422 .Dq yes
423 or
424 .Dq no .
425 The default is
426 .Dq no .
427 .Pp
428 X11 forwarding should be enabled with caution.
429 Users with the ability to bypass file permissions on the remote host
430 (for the user's X11 authorization database)
431 can access the local X11 display through the forwarded connection.
432 An attacker may then be able to perform activities such as keystroke monitoring
433 if the
434 .Cm ForwardX11Trusted
435 option is also enabled.
436 .It Cm ForwardX11Trusted
437 If this option is set to
438 .Dq yes ,
439 remote X11 clients will have full access to the original X11 display.
440 .Pp
441 If this option is set to
442 .Dq no ,
443 remote X11 clients will be considered untrusted and prevented
444 from stealing or tampering with data belonging to trusted X11
445 clients.
446 Furthermore, the
447 .Xr xauth 1
448 token used for the session will be set to expire after 20 minutes.
449 Remote clients will be refused access after this time.
450 .Pp
451 The default is
452 .Dq no .
453 .Pp
454 See the X11 SECURITY extension specification for full details on
455 the restrictions imposed on untrusted clients.
456 .It Cm GatewayPorts
457 Specifies whether remote hosts are allowed to connect to local
458 forwarded ports.
459 By default,
460 .Xr ssh 1
461 binds local port forwardings to the loopback address.
462 This prevents other remote hosts from connecting to forwarded ports.
463 .Cm GatewayPorts
464 can be used to specify that ssh
465 should bind local port forwardings to the wildcard address,
466 thus allowing remote hosts to connect to forwarded ports.
467 The argument must be
468 .Dq yes
469 or
470 .Dq no .
471 The default is
472 .Dq no .
473 .It Cm GlobalKnownHostsFile
474 Specifies a file to use for the global
475 host key database instead of
476 .Pa /etc/ssh/ssh_known_hosts .
477 .It Cm GSSAPIAuthentication
478 Specifies whether user authentication based on GSSAPI is allowed.
479 The default is
480 .Dq no .
481 Note that this option applies to protocol version 2 only.
482 .It Cm GSSAPIDelegateCredentials
483 Forward (delegate) credentials to the server.
484 The default is
485 .Dq no .
486 Note that this option applies to protocol version 2 only.
487 .It Cm HashKnownHosts
488 Indicates that
489 .Xr ssh 1
490 should hash host names and addresses when they are added to
491 .Pa ~/.ssh/known_hosts .
492 These hashed names may be used normally by
493 .Xr ssh 1
494 and
495 .Xr sshd 8 ,
496 but they do not reveal identifying information should the file's contents
497 be disclosed.
498 The default is
499 .Dq no .
500 Note that existing names and addresses in known hosts files
501 will not be converted automatically,
502 but may be manually hashed using
503 .Xr ssh-keygen 1 .
504 .It Cm HostbasedAuthentication
505 Specifies whether to try rhosts based authentication with public key
506 authentication.
507 The argument must be
508 .Dq yes
509 or
510 .Dq no .
511 The default is
512 .Dq no .
513 This option applies to protocol version 2 only and
514 is similar to
515 .Cm RhostsRSAAuthentication .
516 .It Cm HostKeyAlgorithms
517 Specifies the protocol version 2 host key algorithms
518 that the client wants to use in order of preference.
519 The default for this option is:
520 .Dq ssh-rsa,ssh-dss .
521 .It Cm HostKeyAlias
522 Specifies an alias that should be used instead of the
523 real host name when looking up or saving the host key
524 in the host key database files.
525 This option is useful for tunneling SSH connections
526 or for multiple servers running on a single host.
527 .It Cm HostName
528 Specifies the real host name to log into.
529 This can be used to specify nicknames or abbreviations for hosts.
530 The default is the name given on the command line.
531 Numeric IP addresses are also permitted (both on the command line and in
532 .Cm HostName
533 specifications).
534 .It Cm IdentitiesOnly
535 Specifies that
536 .Xr ssh 1
537 should only use the authentication identity files configured in the
538 .Nm
539 files,
540 even if
541 .Xr ssh-agent 1
542 offers more identities.
543 The argument to this keyword must be
544 .Dq yes
545 or
546 .Dq no .
547 This option is intended for situations where ssh-agent
548 offers many different identities.
549 The default is
550 .Dq no .
551 .It Cm IdentityFile
552 Specifies a file from which the user's RSA or DSA authentication identity
553 is read.
554 The default is
555 .Pa ~/.ssh/identity
556 for protocol version 1, and
557 .Pa ~/.ssh/id_rsa
558 and
559 .Pa ~/.ssh/id_dsa
560 for protocol version 2.
561 Additionally, any identities represented by the authentication agent
562 will be used for authentication.
563 .Pp
564 The file name may use the tilde
565 syntax to refer to a user's home directory or one of the following
566 escape characters:
567 .Ql %d
568 (local user's home directory),
569 .Ql %u
570 (local user name),
571 .Ql %l
572 (local host name),
573 .Ql %h
574 (remote host name) or
575 .Ql %r
576 (remote user name).
577 .Pp
578 It is possible to have
579 multiple identity files specified in configuration files; all these
580 identities will be tried in sequence.
581 .It Cm KbdInteractiveAuthentication
582 Specifies whether to use keyboard-interactive authentication.
583 The argument to this keyword must be
584 .Dq yes
585 or
586 .Dq no .
587 The default is
588 .Dq yes .
589 .It Cm KbdInteractiveDevices
590 Specifies the list of methods to use in keyboard-interactive authentication.
591 Multiple method names must be comma-separated.
592 The default is to use the server specified list.
593 The methods available vary depending on what the server supports.
594 For an OpenSSH server,
595 it may be zero or more of:
596 .Dq bsdauth ,
597 .Dq pam ,
598 and
599 .Dq skey .
600 .It Cm LocalCommand
601 Specifies a command to execute on the local machine after successfully
602 connecting to the server.
603 The command string extends to the end of the line, and is executed with
604 the user's shell.
605 The following escape character substitutions will be performed:
606 .Ql %d
607 (local user's home directory),
608 .Ql %h
609 (remote host name),
610 .Ql %l
611 (local host name),
612 .Ql %n
613 (host name as provided on the command line),
614 .Ql %p
615 (remote port),
616 .Ql %r
617 (remote user name) or
618 .Ql %u
619 (local user name).
620 This directive is ignored unless
621 .Cm PermitLocalCommand
622 has been enabled.
623 .It Cm LocalForward
624 Specifies that a TCP port on the local machine be forwarded over
625 the secure channel to the specified host and port from the remote machine.
626 The first argument must be
627 .Sm off
628 .Oo Ar bind_address : Oc Ar port
629 .Sm on
630 and the second argument must be
631 .Ar host : Ns Ar hostport .
632 IPv6 addresses can be specified by enclosing addresses in square brackets or
633 by using an alternative syntax:
634 .Oo Ar bind_address Ns / Oc Ns Ar port
635 and
636 .Ar host Ns / Ns Ar hostport .
637 Multiple forwardings may be specified, and additional forwardings can be
638 given on the command line.
639 Only the superuser can forward privileged ports.
640 By default, the local port is bound in accordance with the
641 .Cm GatewayPorts
642 setting.
643 However, an explicit
644 .Ar bind_address
645 may be used to bind the connection to a specific address.
646 The
647 .Ar bind_address
648 of
649 .Dq localhost
650 indicates that the listening port be bound for local use only, while an
651 empty address or
652 .Sq *
653 indicates that the port should be available from all interfaces.
654 .It Cm LogLevel
655 Gives the verbosity level that is used when logging messages from
656 .Xr ssh 1 .
657 The possible values are:
658 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
659 The default is INFO.
660 DEBUG and DEBUG1 are equivalent.
661 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
662 .It Cm MACs
663 Specifies the MAC (message authentication code) algorithms
664 in order of preference.
665 The MAC algorithm is used in protocol version 2
666 for data integrity protection.
667 Multiple algorithms must be comma-separated.
668 The default is:
669 .Bd -literal -offset indent
670 hmac-md5,hmac-sha1,umac-64@openssh.com,
671 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
672 .Ed
673 .It Cm NoHostAuthenticationForLocalhost
674 This option can be used if the home directory is shared across machines.
675 In this case localhost will refer to a different machine on each of
676 the machines and the user will get many warnings about changed host keys.
677 However, this option disables host authentication for localhost.
678 The argument to this keyword must be
679 .Dq yes
680 or
681 .Dq no .
682 The default is to check the host key for localhost.
683 .It Cm NumberOfPasswordPrompts
684 Specifies the number of password prompts before giving up.
685 The argument to this keyword must be an integer.
686 The default is 3.
687 .It Cm PasswordAuthentication
688 Specifies whether to use password authentication.
689 The argument to this keyword must be
690 .Dq yes
691 or
692 .Dq no .
693 The default is
694 .Dq yes .
695 .It Cm PermitLocalCommand
696 Allow local command execution via the
697 .Ic LocalCommand
698 option or using the
699 .Ic !\& Ns Ar command
700 escape sequence in
701 .Xr ssh 1 .
702 The argument must be
703 .Dq yes
704 or
705 .Dq no .
706 The default is
707 .Dq no .
708 .It Cm Port
709 Specifies the port number to connect on the remote host.
710 The default is 22.
711 .It Cm PreferredAuthentications
712 Specifies the order in which the client should try protocol 2
713 authentication methods.
714 This allows a client to prefer one method (e.g.\&
715 .Cm keyboard-interactive )
716 over another method (e.g.\&
717 .Cm password )
718 The default for this option is:
719 .Do gssapi-with-mic ,
720 hostbased,
721 publickey,
722 keyboard-interactive,
723 password
724 .Dc .
725 .It Cm Protocol
726 Specifies the protocol versions
727 .Xr ssh 1
728 should support in order of preference.
729 The possible values are
730 .Sq 1
731 and
732 .Sq 2 .
733 Multiple versions must be comma-separated.
734 The default is
735 .Dq 2,1 .
736 This means that ssh
737 tries version 2 and falls back to version 1
738 if version 2 is not available.
739 .It Cm ProxyCommand
740 Specifies the command to use to connect to the server.
741 The command
742 string extends to the end of the line, and is executed with
743 the user's shell.
744 In the command string,
745 .Ql %h
746 will be substituted by the host name to
747 connect and
748 .Ql %p
749 by the port.
750 The command can be basically anything,
751 and should read from its standard input and write to its standard output.
752 It should eventually connect an
753 .Xr sshd 8
754 server running on some machine, or execute
755 .Ic sshd -i
756 somewhere.
757 Host key management will be done using the
758 HostName of the host being connected (defaulting to the name typed by
759 the user).
760 Setting the command to
761 .Dq none
762 disables this option entirely.
763 Note that
764 .Cm CheckHostIP
765 is not available for connects with a proxy command.
766 .Pp
767 This directive is useful in conjunction with
768 .Xr nc 1
769 and its proxy support.
770 For example, the following directive would connect via an HTTP proxy at
771 192.0.2.0:
772 .Bd -literal -offset 3n
773 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
774 .Ed
775 .It Cm PubkeyAuthentication
776 Specifies whether to try public key authentication.
777 The argument to this keyword must be
778 .Dq yes
779 or
780 .Dq no .
781 The default is
782 .Dq yes .
783 This option applies to protocol version 2 only.
784 .It Cm RekeyLimit
785 Specifies the maximum amount of data that may be transmitted before the
786 session key is renegotiated.
787 The argument is the number of bytes, with an optional suffix of
788 .Sq K ,
789 .Sq M ,
790 or
791 .Sq G
792 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
793 The default is between
794 .Sq 1G
795 and
796 .Sq 4G ,
797 depending on the cipher.
798 This option applies to protocol version 2 only.
799 .It Cm RemoteForward
800 Specifies that a TCP port on the remote machine be forwarded over
801 the secure channel to the specified host and port from the local machine.
802 The first argument must be
803 .Sm off
804 .Oo Ar bind_address : Oc Ar port
805 .Sm on
806 and the second argument must be
807 .Ar host : Ns Ar hostport .
808 IPv6 addresses can be specified by enclosing addresses in square brackets
809 or by using an alternative syntax:
810 .Oo Ar bind_address Ns / Oc Ns Ar port
811 and
812 .Ar host Ns / Ns Ar hostport .
813 Multiple forwardings may be specified, and additional
814 forwardings can be given on the command line.
815 Privileged ports can be forwarded only when
816 logging in as root on the remote machine.
817 .Pp
818 If the
819 .Ar port
820 argument is
821 .Ql 0 ,
822 the listen port will be dynamically allocated on the server and reported
823 to the client at run time.
824 .Pp
825 If the
826 .Ar bind_address
827 is not specified, the default is to only bind to loopback addresses.
828 If the
829 .Ar bind_address
830 is
831 .Ql *
832 or an empty string, then the forwarding is requested to listen on all
833 interfaces.
834 Specifying a remote
835 .Ar bind_address
836 will only succeed if the server's
837 .Cm GatewayPorts
838 option is enabled (see
839 .Xr sshd_config 5 ) .
840 .It Cm RhostsRSAAuthentication
841 Specifies whether to try rhosts based authentication with RSA host
842 authentication.
843 The argument must be
844 .Dq yes
845 or
846 .Dq no .
847 The default is
848 .Dq no .
849 This option applies to protocol version 1 only and requires
850 .Xr ssh 1
851 to be setuid root.
852 .It Cm RSAAuthentication
853 Specifies whether to try RSA authentication.
854 The argument to this keyword must be
855 .Dq yes
856 or
857 .Dq no .
858 RSA authentication will only be
859 attempted if the identity file exists, or an authentication agent is
860 running.
861 The default is
862 .Dq yes .
863 Note that this option applies to protocol version 1 only.
864 .It Cm SendEnv
865 Specifies what variables from the local
866 .Xr environ 7
867 should be sent to the server.
868 Note that environment passing is only supported for protocol 2.
869 The server must also support it, and the server must be configured to
870 accept these environment variables.
871 Refer to
872 .Cm AcceptEnv
873 in
874 .Xr sshd_config 5
875 for how to configure the server.
876 Variables are specified by name, which may contain wildcard characters.
877 Multiple environment variables may be separated by whitespace or spread
878 across multiple
879 .Cm SendEnv
880 directives.
881 The default is not to send any environment variables.
882 .Pp
883 See
884 .Sx PATTERNS
885 for more information on patterns.
886 .It Cm ServerAliveCountMax
887 Sets the number of server alive messages (see below) which may be
888 sent without
889 .Xr ssh 1
890 receiving any messages back from the server.
891 If this threshold is reached while server alive messages are being sent,
892 ssh will disconnect from the server, terminating the session.
893 It is important to note that the use of server alive messages is very
894 different from
895 .Cm TCPKeepAlive
896 (below).
897 The server alive messages are sent through the encrypted channel
898 and therefore will not be spoofable.
899 The TCP keepalive option enabled by
900 .Cm TCPKeepAlive
901 is spoofable.
902 The server alive mechanism is valuable when the client or
903 server depend on knowing when a connection has become inactive.
904 .Pp
905 The default value is 3.
906 If, for example,
907 .Cm ServerAliveInterval
908 (see below) is set to 15 and
909 .Cm ServerAliveCountMax
910 is left at the default, if the server becomes unresponsive,
911 ssh will disconnect after approximately 45 seconds.
912 This option applies to protocol version 2 only.
913 .It Cm ServerAliveInterval
914 Sets a timeout interval in seconds after which if no data has been received
915 from the server,
916 .Xr ssh 1
917 will send a message through the encrypted
918 channel to request a response from the server.
919 The default
920 is 0, indicating that these messages will not be sent to the server.
921 This option applies to protocol version 2 only.
922 .It Cm SmartcardDevice
923 Specifies which smartcard device to use.
924 The argument to this keyword is the device
925 .Xr ssh 1
926 should use to communicate with a smartcard used for storing the user's
927 private RSA key.
928 By default, no device is specified and smartcard support is not activated.
929 .It Cm StrictHostKeyChecking
930 If this flag is set to
931 .Dq yes ,
932 .Xr ssh 1
933 will never automatically add host keys to the
934 .Pa ~/.ssh/known_hosts
935 file, and refuses to connect to hosts whose host key has changed.
936 This provides maximum protection against trojan horse attacks,
937 though it can be annoying when the
938 .Pa /etc/ssh/ssh_known_hosts
939 file is poorly maintained or when connections to new hosts are
940 frequently made.
941 This option forces the user to manually
942 add all new hosts.
943 If this flag is set to
944 .Dq no ,
945 ssh will automatically add new host keys to the
946 user known hosts files.
947 If this flag is set to
948 .Dq ask ,
949 new host keys
950 will be added to the user known host files only after the user
951 has confirmed that is what they really want to do, and
952 ssh will refuse to connect to hosts whose host key has changed.
953 The host keys of
954 known hosts will be verified automatically in all cases.
955 The argument must be
956 .Dq yes ,
957 .Dq no ,
958 or
959 .Dq ask .
960 The default is
961 .Dq ask .
962 .It Cm TCPKeepAlive
963 Specifies whether the system should send TCP keepalive messages to the
964 other side.
965 If they are sent, death of the connection or crash of one
966 of the machines will be properly noticed.
967 However, this means that
968 connections will die if the route is down temporarily, and some people
969 find it annoying.
970 .Pp
971 The default is
972 .Dq yes
973 (to send TCP keepalive messages), and the client will notice
974 if the network goes down or the remote host dies.
975 This is important in scripts, and many users want it too.
976 .Pp
977 To disable TCP keepalive messages, the value should be set to
978 .Dq no .
979 .It Cm Tunnel
980 Request
981 .Xr tun 4
982 device forwarding between the client and the server.
983 The argument must be
984 .Dq yes ,
985 .Dq point-to-point
986 (layer 3),
987 .Dq ethernet
988 (layer 2),
989 or
990 .Dq no .
991 Specifying
992 .Dq yes
993 requests the default tunnel mode, which is
994 .Dq point-to-point .
995 The default is
996 .Dq no .
997 .It Cm TunnelDevice
998 Specifies the
999 .Xr tun 4
1000 devices to open on the client
1001 .Pq Ar local_tun
1002 and the server
1003 .Pq Ar remote_tun .
1004 .Pp
1005 The argument must be
1006 .Sm off
1007 .Ar local_tun Op : Ar remote_tun .
1008 .Sm on
1009 The devices may be specified by numerical ID or the keyword
1010 .Dq any ,
1011 which uses the next available tunnel device.
1012 If
1013 .Ar remote_tun
1014 is not specified, it defaults to
1015 .Dq any .
1016 The default is
1017 .Dq any:any .
1018 .It Cm UsePrivilegedPort
1019 Specifies whether to use a privileged port for outgoing connections.
1020 The argument must be
1021 .Dq yes
1022 or
1023 .Dq no .
1024 The default is
1025 .Dq no .
1026 If set to
1027 .Dq yes ,
1028 .Xr ssh 1
1029 must be setuid root.
1030 Note that this option must be set to
1031 .Dq yes
1032 for
1033 .Cm RhostsRSAAuthentication
1034 with older servers.
1035 .It Cm User
1036 Specifies the user to log in as.
1037 This can be useful when a different user name is used on different machines.
1038 This saves the trouble of
1039 having to remember to give the user name on the command line.
1040 .It Cm UserKnownHostsFile
1041 Specifies a file to use for the user
1042 host key database instead of
1043 .Pa ~/.ssh/known_hosts .
1044 .It Cm VerifyHostKeyDNS
1045 Specifies whether to verify the remote key using DNS and SSHFP resource
1046 records.
1047 If this option is set to
1048 .Dq yes ,
1049 the client will implicitly trust keys that match a secure fingerprint
1050 from DNS.
1051 Insecure fingerprints will be handled as if this option was set to
1052 .Dq ask .
1053 If this option is set to
1054 .Dq ask ,
1055 information on fingerprint match will be displayed, but the user will still
1056 need to confirm new host keys according to the
1057 .Cm StrictHostKeyChecking
1058 option.
1059 The argument must be
1060 .Dq yes ,
1061 .Dq no ,
1062 or
1063 .Dq ask .
1064 The default is
1065 .Dq no .
1066 Note that this option applies to protocol version 2 only.
1067 .Pp
1068 See also
1069 .Sx VERIFYING HOST KEYS
1070 in
1071 .Xr ssh 1 .
1072 .It Cm VisualHostKey
1073 If this flag is set to
1074 .Dq yes ,
1075 an ASCII art representation of the remote host key fingerprint is
1076 printed in addition to the hex fingerprint string at login and
1077 for unknown host keys.
1078 If this flag is set to
1079 .Dq no ,
1080 no fingerprint strings are printed at login and
1081 only the hex fingerprint string will be printed for unknown host keys.
1082 The default is
1083 .Dq no .
1084 .It Cm XAuthLocation
1085 Specifies the full pathname of the
1086 .Xr xauth 1
1087 program.
1088 The default is
1089 .Pa /usr/X11R6/bin/xauth .
1090 .El
1091 .Sh PATTERNS
1092 A
1093 .Em pattern
1094 consists of zero or more non-whitespace characters,
1095 .Sq *
1096 (a wildcard that matches zero or more characters),
1097 or
1098 .Sq ?\&
1099 (a wildcard that matches exactly one character).
1100 For example, to specify a set of declarations for any host in the
1101 .Dq .co.uk
1102 set of domains,
1103 the following pattern could be used:
1104 .Pp
1105 .Dl Host *.co.uk
1106 .Pp
1107 The following pattern
1108 would match any host in the 192.168.0.[0-9] network range:
1109 .Pp
1110 .Dl Host 192.168.0.?
1111 .Pp
1112 A
1113 .Em pattern-list
1114 is a comma-separated list of patterns.
1115 Patterns within pattern-lists may be negated
1116 by preceding them with an exclamation mark
1117 .Pq Sq !\& .
1118 For example,
1119 to allow a key to be used from anywhere within an organisation
1120 except from the
1121 .Dq dialup
1122 pool,
1123 the following entry (in authorized_keys) could be used:
1124 .Pp
1125 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1126 .Sh FILES
1127 .Bl -tag -width Ds
1128 .It Pa ~/.ssh/config
1129 This is the per-user configuration file.
1130 The format of this file is described above.
1131 This file is used by the SSH client.
1132 Because of the potential for abuse, this file must have strict permissions:
1133 read/write for the user, and not accessible by others.
1134 .It Pa /etc/ssh/ssh_config
1135 Systemwide configuration file.
1136 This file provides defaults for those
1137 values that are not specified in the user's configuration file, and
1138 for those users who do not have a configuration file.
1139 This file must be world-readable.
1140 .El
1141 .Sh SEE ALSO
1142 .Xr ssh 1
1143 .Sh AUTHORS
1144 OpenSSH is a derivative of the original and free
1145 ssh 1.2.12 release by Tatu Ylonen.
1146 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1147 Theo de Raadt and Dug Song
1148 removed many bugs, re-added newer features and
1149 created OpenSSH.
1150 Markus Friedl contributed the support for SSH
1151 protocol versions 1.5 and 2.0.