| blob | 20c58934add92a979b93bdaddfe5a9cff64557b2 |
1 .\" -*- nroff -*-
2 .\"
3 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5 .\" All rights reserved
6 .\"
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".
12 .\"
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.
16 .\"
17 .\" Redistribution and use in source and binary forms, with or without
18 .\" modification, are permitted provided that the following conditions
19 .\" are met:
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.
25 .\"
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.
36 .\"
37 .\" $OpenBSD: ssh_config.5,v 1.97 2006/07/27 08:00:50 jmc Exp $
38 .Dd September 25, 1999
39 .Dt SSH_CONFIG 5
40 .Os
41 .Sh NAME
42 .Nm ssh_config
43 .Nd OpenSSH SSH client configuration files
44 .Sh SYNOPSIS
45 .Bl -tag -width Ds -compact
46 .It Pa ~/.ssh/config
47 .It Pa /etc/ssh/ssh_config
48 .El
49 .Sh DESCRIPTION
50 .Xr ssh 1
51 obtains configuration data from the following sources in
52 the following order:
53 .Pp
54 .Bl -enum -offset indent -compact
55 .It
56 command-line options
57 .It
58 user's configuration file
59 .Pq Pa ~/.ssh/config
60 .It
61 system-wide configuration file
62 .Pq Pa /etc/ssh/ssh_config
63 .El
64 .Pp
65 For each parameter, the first obtained value
66 will be used.
67 The configuration files contain sections separated by
68 .Dq Host
69 specifications, and that section is only applied for hosts that
70 match one of the patterns given in the specification.
71 The matched host name is the one given on the command line.
72 .Pp
73 Since the first obtained value for each parameter is used, more
74 host-specific declarations should be given near the beginning of the
75 file, and general defaults at the end.
76 .Pp
77 The configuration file has the following format:
78 .Pp
79 Empty lines and lines starting with
80 .Ql #
81 are comments.
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
86 .Ql = ;
87 the latter format is useful to avoid the need to quote whitespace
88 when specifying configuration options using the
89 .Nm ssh ,
90 .Nm scp ,
91 and
92 .Nm sftp
93 .Fl o
94 option.
95 Arguments may optionally be enclosed in double quotes
96 .Pq \&"
97 in order to represent arguments containing spaces.
98 .Pp
99 The possible
100 keywords and their meanings are as follows (note that
101 keywords are case-insensitive and arguments are case-sensitive):
102 .Bl -tag -width Ds
103 .It Cm Host
104 Restricts the following declarations (up to the next
105 .Cm Host
106 keyword) to be only for those hosts that match one of the patterns
107 given after the keyword.
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-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
209 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
210 aes192-ctr,aes256-ctr
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, 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 KbdInteractiveDevices
582 Specifies the list of methods to use in keyboard-interactive authentication.
583 Multiple method names must be comma-separated.
584 The default is to use the server specified list.
585 The methods available vary depending on what the server supports.
586 For an OpenSSH server,
587 it may be zero or more of:
588 .Dq bsdauth ,
589 .Dq pam ,
590 and
591 .Dq skey .
592 .It Cm LocalCommand
593 Specifies a command to execute on the local machine after successfully
594 connecting to the server.
595 The command string extends to the end of the line, and is executed with
596 .Pa /bin/sh .
597 This directive is ignored unless
598 .Cm PermitLocalCommand
599 has been enabled.
600 .It Cm LocalForward
601 Specifies that a TCP port on the local machine be forwarded over
602 the secure channel to the specified host and port from the remote machine.
603 The first argument must be
604 .Sm off
605 .Oo Ar bind_address : Oc Ar port
606 .Sm on
607 and the second argument must be
608 .Ar host : Ns Ar hostport .
609 IPv6 addresses can be specified by enclosing addresses in square brackets or
610 by using an alternative syntax:
611 .Oo Ar bind_address Ns / Oc Ns Ar port
612 and
613 .Ar host Ns / Ns Ar hostport .
614 Multiple forwardings may be specified, and additional forwardings can be
615 given on the command line.
616 Only the superuser can forward privileged ports.
617 By default, the local port is bound in accordance with the
618 .Cm GatewayPorts
619 setting.
620 However, an explicit
621 .Ar bind_address
622 may be used to bind the connection to a specific address.
623 The
624 .Ar bind_address
625 of
626 .Dq localhost
627 indicates that the listening port be bound for local use only, while an
628 empty address or
629 .Sq *
630 indicates that the port should be available from all interfaces.
631 .It Cm LogLevel
632 Gives the verbosity level that is used when logging messages from
633 .Xr ssh 1 .
634 The possible values are:
635 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
636 The default is INFO.
637 DEBUG and DEBUG1 are equivalent.
638 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
639 .It Cm MACs
640 Specifies the MAC (message authentication code) algorithms
641 in order of preference.
642 The MAC algorithm is used in protocol version 2
643 for data integrity protection.
644 Multiple algorithms must be comma-separated.
645 The default is:
646 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
647 .It Cm NoHostAuthenticationForLocalhost
648 This option can be used if the home directory is shared across machines.
649 In this case localhost will refer to a different machine on each of
650 the machines and the user will get many warnings about changed host keys.
651 However, this option disables host authentication for localhost.
652 The argument to this keyword must be
653 .Dq yes
654 or
655 .Dq no .
656 The default is to check the host key for localhost.
657 .It Cm NumberOfPasswordPrompts
658 Specifies the number of password prompts before giving up.
659 The argument to this keyword must be an integer.
660 The default is 3.
661 .It Cm PasswordAuthentication
662 Specifies whether to use password authentication.
663 The argument to this keyword must be
664 .Dq yes
665 or
666 .Dq no .
667 The default is
668 .Dq yes .
669 .It Cm PermitLocalCommand
670 Allow local command execution via the
671 .Ic LocalCommand
672 option or using the
673 .Ic !\& Ns Ar command
674 escape sequence in
675 .Xr ssh 1 .
676 The argument must be
677 .Dq yes
678 or
679 .Dq no .
680 The default is
681 .Dq no .
682 .It Cm Port
683 Specifies the port number to connect on the remote host.
684 The default is 22.
685 .It Cm PreferredAuthentications
686 Specifies the order in which the client should try protocol 2
687 authentication methods.
688 This allows a client to prefer one method (e.g.\&
689 .Cm keyboard-interactive )
690 over another method (e.g.\&
691 .Cm password )
692 The default for this option is:
693 .Do gssapi-with-mic ,
694 hostbased,
695 publickey,
696 keyboard-interactive,
697 password
698 .Dc .
699 .It Cm Protocol
700 Specifies the protocol versions
701 .Xr ssh 1
702 should support in order of preference.
703 The possible values are
704 .Sq 1
705 and
706 .Sq 2 .
707 Multiple versions must be comma-separated.
708 The default is
709 .Dq 2,1 .
710 This means that ssh
711 tries version 2 and falls back to version 1
712 if version 2 is not available.
713 .It Cm ProxyCommand
714 Specifies the command to use to connect to the server.
715 The command
716 string extends to the end of the line, and is executed with
717 .Pa /bin/sh .
718 In the command string,
719 .Ql %h
720 will be substituted by the host name to
721 connect and
722 .Ql %p
723 by the port.
724 The command can be basically anything,
725 and should read from its standard input and write to its standard output.
726 It should eventually connect an
727 .Xr sshd 8
728 server running on some machine, or execute
729 .Ic sshd -i
730 somewhere.
731 Host key management will be done using the
732 HostName of the host being connected (defaulting to the name typed by
733 the user).
734 Setting the command to
735 .Dq none
736 disables this option entirely.
737 Note that
738 .Cm CheckHostIP
739 is not available for connects with a proxy command.
740 .Pp
741 This directive is useful in conjunction with
742 .Xr nc 1
743 and its proxy support.
744 For example, the following directive would connect via an HTTP proxy at
745 192.0.2.0:
746 .Bd -literal -offset 3n
747 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
748 .Ed
749 .It Cm PubkeyAuthentication
750 Specifies whether to try public key authentication.
751 The argument to this keyword must be
752 .Dq yes
753 or
754 .Dq no .
755 The default is
756 .Dq yes .
757 This option applies to protocol version 2 only.
758 .It Cm RekeyLimit
759 Specifies the maximum amount of data that may be transmitted before the
760 session key is renegotiated.
761 The argument is the number of bytes, with an optional suffix of
762 .Sq K ,
763 .Sq M ,
764 or
765 .Sq G
766 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
767 The default is between
768 .Sq 1G
769 and
770 .Sq 4G ,
771 depending on the cipher.
772 This option applies to protocol version 2 only.
773 .It Cm RemoteForward
774 Specifies that a TCP port on the remote machine be forwarded over
775 the secure channel to the specified host and port from the local machine.
776 The first argument must be
777 .Sm off
778 .Oo Ar bind_address : Oc Ar port
779 .Sm on
780 and the second argument must be
781 .Ar host : Ns Ar hostport .
782 IPv6 addresses can be specified by enclosing addresses in square brackets
783 or by using an alternative syntax:
784 .Oo Ar bind_address Ns / Oc Ns Ar port
785 and
786 .Ar host Ns / Ns Ar hostport .
787 Multiple forwardings may be specified, and additional
788 forwardings can be given on the command line.
789 Only the superuser can forward privileged ports.
790 .Pp
791 If the
792 .Ar bind_address
793 is not specified, the default is to only bind to loopback addresses.
794 If the
795 .Ar bind_address
796 is
797 .Ql *
798 or an empty string, then the forwarding is requested to listen on all
799 interfaces.
800 Specifying a remote
801 .Ar bind_address
802 will only succeed if the server's
803 .Cm GatewayPorts
804 option is enabled (see
805 .Xr sshd_config 5 ) .
806 .It Cm RhostsRSAAuthentication
807 Specifies whether to try rhosts based authentication with RSA host
808 authentication.
809 The argument must be
810 .Dq yes
811 or
812 .Dq no .
813 The default is
814 .Dq no .
815 This option applies to protocol version 1 only and requires
816 .Xr ssh 1
817 to be setuid root.
818 .It Cm RSAAuthentication
819 Specifies whether to try RSA authentication.
820 The argument to this keyword must be
821 .Dq yes
822 or
823 .Dq no .
824 RSA authentication will only be
825 attempted if the identity file exists, or an authentication agent is
826 running.
827 The default is
828 .Dq yes .
829 Note that this option applies to protocol version 1 only.
830 .It Cm SendEnv
831 Specifies what variables from the local
832 .Xr environ 7
833 should be sent to the server.
834 Note that environment passing is only supported for protocol 2.
835 The server must also support it, and the server must be configured to
836 accept these environment variables.
837 Refer to
838 .Cm AcceptEnv
839 in
840 .Xr sshd_config 5
841 for how to configure the server.
842 Variables are specified by name, which may contain wildcard characters.
843 Multiple environment variables may be separated by whitespace or spread
844 across multiple
845 .Cm SendEnv
846 directives.
847 The default is not to send any environment variables.
848 .Pp
849 See
850 .Sx PATTERNS
851 for more information on patterns.
852 .It Cm ServerAliveCountMax
853 Sets the number of server alive messages (see below) which may be
854 sent without
855 .Xr ssh 1
856 receiving any messages back from the server.
857 If this threshold is reached while server alive messages are being sent,
858 ssh will disconnect from the server, terminating the session.
859 It is important to note that the use of server alive messages is very
860 different from
861 .Cm TCPKeepAlive
862 (below).
863 The server alive messages are sent through the encrypted channel
864 and therefore will not be spoofable.
865 The TCP keepalive option enabled by
866 .Cm TCPKeepAlive
867 is spoofable.
868 The server alive mechanism is valuable when the client or
869 server depend on knowing when a connection has become inactive.
870 .Pp
871 The default value is 3.
872 If, for example,
873 .Cm ServerAliveInterval
874 (see below) is set to 15 and
875 .Cm ServerAliveCountMax
876 is left at the default, if the server becomes unresponsive,
877 ssh will disconnect after approximately 45 seconds.
878 This option applies to protocol version 2 only.
879 .It Cm ServerAliveInterval
880 Sets a timeout interval in seconds after which if no data has been received
881 from the server,
882 .Xr ssh 1
883 will send a message through the encrypted
884 channel to request a response from the server.
885 The default
886 is 0, indicating that these messages will not be sent to the server.
887 This option applies to protocol version 2 only.
888 .It Cm SmartcardDevice
889 Specifies which smartcard device to use.
890 The argument to this keyword is the device
891 .Xr ssh 1
892 should use to communicate with a smartcard used for storing the user's
893 private RSA key.
894 By default, no device is specified and smartcard support is not activated.
895 .It Cm StrictHostKeyChecking
896 If this flag is set to
897 .Dq yes ,
898 .Xr ssh 1
899 will never automatically add host keys to the
900 .Pa ~/.ssh/known_hosts
901 file, and refuses to connect to hosts whose host key has changed.
902 This provides maximum protection against trojan horse attacks,
903 though it can be annoying when the
904 .Pa /etc/ssh/ssh_known_hosts
905 file is poorly maintained or when connections to new hosts are
906 frequently made.
907 This option forces the user to manually
908 add all new hosts.
909 If this flag is set to
910 .Dq no ,
911 ssh will automatically add new host keys to the
912 user known hosts files.
913 If this flag is set to
914 .Dq ask ,
915 new host keys
916 will be added to the user known host files only after the user
917 has confirmed that is what they really want to do, and
918 ssh will refuse to connect to hosts whose host key has changed.
919 The host keys of
920 known hosts will be verified automatically in all cases.
921 The argument must be
922 .Dq yes ,
923 .Dq no ,
924 or
925 .Dq ask .
926 The default is
927 .Dq ask .
928 .It Cm TCPKeepAlive
929 Specifies whether the system should send TCP keepalive messages to the
930 other side.
931 If they are sent, death of the connection or crash of one
932 of the machines will be properly noticed.
933 However, this means that
934 connections will die if the route is down temporarily, and some people
935 find it annoying.
936 .Pp
937 The default is
938 .Dq yes
939 (to send TCP keepalive messages), and the client will notice
940 if the network goes down or the remote host dies.
941 This is important in scripts, and many users want it too.
942 .Pp
943 To disable TCP keepalive messages, the value should be set to
944 .Dq no .
945 .It Cm Tunnel
946 Request
947 .Xr tun 4
948 device forwarding between the client and the server.
949 The argument must be
950 .Dq yes ,
951 .Dq point-to-point
952 (layer 3),
953 .Dq ethernet
954 (layer 2),
955 or
956 .Dq no .
957 Specifying
958 .Dq yes
959 requests the default tunnel mode, which is
960 .Dq point-to-point .
961 The default is
962 .Dq no .
963 .It Cm TunnelDevice
964 Specifies the
965 .Xr tun 4
966 devices to open on the client
967 .Pq Ar local_tun
968 and the server
969 .Pq Ar remote_tun .
970 .Pp
971 The argument must be
972 .Sm off
973 .Ar local_tun Op : Ar remote_tun .
974 .Sm on
975 The devices may be specified by numerical ID or the keyword
976 .Dq any ,
977 which uses the next available tunnel device.
978 If
979 .Ar remote_tun
980 is not specified, it defaults to
981 .Dq any .
982 The default is
983 .Dq any:any .
984 .It Cm UsePrivilegedPort
985 Specifies whether to use a privileged port for outgoing connections.
986 The argument must be
987 .Dq yes
988 or
989 .Dq no .
990 The default is
991 .Dq no .
992 If set to
993 .Dq yes ,
994 .Xr ssh 1
995 must be setuid root.
996 Note that this option must be set to
997 .Dq yes
998 for
999 .Cm RhostsRSAAuthentication
1000 with older servers.
1001 .It Cm User
1002 Specifies the user to log in as.
1003 This can be useful when a different user name is used on different machines.
1004 This saves the trouble of
1005 having to remember to give the user name on the command line.
1006 .It Cm UserKnownHostsFile
1007 Specifies a file to use for the user
1008 host key database instead of
1009 .Pa ~/.ssh/known_hosts .
1010 .It Cm VerifyHostKeyDNS
1011 Specifies whether to verify the remote key using DNS and SSHFP resource
1012 records.
1013 If this option is set to
1014 .Dq yes ,
1015 the client will implicitly trust keys that match a secure fingerprint
1016 from DNS.
1017 Insecure fingerprints will be handled as if this option was set to
1018 .Dq ask .
1019 If this option is set to
1020 .Dq ask ,
1021 information on fingerprint match will be displayed, but the user will still
1022 need to confirm new host keys according to the
1023 .Cm StrictHostKeyChecking
1024 option.
1025 The argument must be
1026 .Dq yes ,
1027 .Dq no ,
1028 or
1029 .Dq ask .
1030 The default is
1031 .Dq no .
1032 Note that this option applies to protocol version 2 only.
1033 .Pp
1034 See also
1035 .Sx VERIFYING HOST KEYS
1036 in
1037 .Xr ssh 1 .
1038 .It Cm XAuthLocation
1039 Specifies the full pathname of the
1040 .Xr xauth 1
1041 program.
1042 The default is
1043 .Pa /usr/X11R6/bin/xauth .
1044 .El
1045 .Sh PATTERNS
1046 A
1047 .Em pattern
1048 consists of zero or more non-whitespace characters,
1049 .Sq *
1050 (a wildcard that matches zero or more characters),
1051 or
1052 .Sq ?\&
1053 (a wildcard that matches exactly one character).
1054 For example, to specify a set of declarations for any host in the
1055 .Dq .co.uk
1056 set of domains,
1057 the following pattern could be used:
1058 .Pp
1059 .Dl Host *.co.uk
1060 .Pp
1061 The following pattern
1062 would match any host in the 192.168.0.[0-9] network range:
1063 .Pp
1064 .Dl Host 192.168.0.?
1065 .Pp
1066 A
1067 .Em pattern-list
1068 is a comma-separated list of patterns.
1069 Patterns within pattern-lists may be negated
1070 by preceding them with an exclamation mark
1071 .Pq Sq !\& .
1072 For example,
1073 to allow a key to be used from anywhere within an organisation
1074 except from the
1075 .Dq dialup
1076 pool,
1077 the following entry (in authorized_keys) could be used:
1078 .Pp
1079 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1080 .Sh FILES
1081 .Bl -tag -width Ds
1082 .It Pa ~/.ssh/config
1083 This is the per-user configuration file.
1084 The format of this file is described above.
1085 This file is used by the SSH client.
1086 Because of the potential for abuse, this file must have strict permissions:
1087 read/write for the user, and not accessible by others.
1088 .It Pa /etc/ssh/ssh_config
1089 Systemwide configuration file.
1090 This file provides defaults for those
1091 values that are not specified in the user's configuration file, and
1092 for those users who do not have a configuration file.
1093 This file must be world-readable.
1094 .El
1095 .Sh SEE ALSO
1096 .Xr ssh 1
1097 .Sh AUTHORS
1098 OpenSSH is a derivative of the original and free
1099 ssh 1.2.12 release by Tatu Ylonen.
1100 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1101 Theo de Raadt and Dug Song
1102 removed many bugs, re-added newer features and
1103 created OpenSSH.
1104 Markus Friedl contributed the support for SSH
1105 protocol versions 1.5 and 2.0.