| blob | 82c2a30b0cd5ee29ed9a21b7bcf60e23d8d39054 |
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 1.120 2009/10/08 14:03:41 markus Exp $
38 .Dd $Mdocdate: October 8 2009 $
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 .Nm ~/.ssh/config
46 .Nm /etc/ssh/ssh_config
47 .Sh DESCRIPTION
48 .Xr ssh 1
49 obtains configuration data from the following sources in
50 the following order:
51 .Pp
52 .Bl -enum -offset indent -compact
53 .It
54 command-line options
55 .It
56 user's configuration file
57 .Pq Pa ~/.ssh/config
58 .It
59 system-wide configuration file
60 .Pq Pa /etc/ssh/ssh_config
61 .El
62 .Pp
63 For each parameter, the first obtained value
64 will be used.
65 The configuration files contain sections separated by
66 .Dq Host
67 specifications, and that section is only applied for hosts that
68 match one of the patterns given in the specification.
69 The matched host name is the one given on the command line.
70 .Pp
71 Since the first obtained value for each parameter is used, more
72 host-specific declarations should be given near the beginning of the
73 file, and general defaults at the end.
74 .Pp
75 The configuration file has the following format:
76 .Pp
77 Empty lines and lines starting with
78 .Ql #
79 are comments.
80 Otherwise a line is of the format
81 .Dq keyword arguments .
82 Configuration options may be separated by whitespace or
83 optional whitespace and exactly one
84 .Ql = ;
85 the latter format is useful to avoid the need to quote whitespace
86 when specifying configuration options using the
87 .Nm ssh ,
88 .Nm scp ,
89 and
90 .Nm sftp
91 .Fl o
92 option.
93 Arguments may optionally be enclosed in double quotes
94 .Pq \&"
95 in order to represent arguments containing spaces.
96 .Pp
97 The possible
98 keywords and their meanings are as follows (note that
99 keywords are case-insensitive and arguments are case-sensitive):
100 .Bl -tag -width Ds
101 .It Cm Host
102 Restricts the following declarations (up to the next
103 .Cm Host
104 keyword) to be only for those hosts that match one of the patterns
105 given after the keyword.
106 If more than one pattern is provided, they should be separated by whitespace.
107 A single
108 .Ql *
109 as a pattern can be used to provide global
110 defaults for all hosts.
111 The host is the
112 .Ar hostname
113 argument given on the command line (i.e. the name is not converted to
114 a canonicalized host name before matching).
115 .Pp
116 See
117 .Sx PATTERNS
118 for more information on patterns.
119 .It Cm AddressFamily
120 Specifies which address family to use when connecting.
121 Valid arguments are
122 .Dq any ,
123 .Dq inet
124 (use IPv4 only), or
125 .Dq inet6
126 (use IPv6 only).
127 .It Cm BatchMode
128 If set to
129 .Dq yes ,
130 passphrase/password querying will be disabled.
131 This option is useful in scripts and other batch jobs where no user
132 is present to supply the password.
133 The argument must be
134 .Dq yes
135 or
136 .Dq no .
137 The default is
138 .Dq no .
139 .It Cm BindAddress
140 Use the specified address on the local machine as the source address of
141 the connection.
142 Only useful on systems with more than one address.
143 Note that this option does not work if
144 .Cm UsePrivilegedPort
145 is set to
146 .Dq yes .
147 .It Cm ChallengeResponseAuthentication
148 Specifies whether to use challenge-response authentication.
149 The argument to this keyword must be
150 .Dq yes
151 or
152 .Dq no .
153 The default is
154 .Dq yes .
155 .It Cm CheckHostIP
156 If this flag is set to
157 .Dq yes ,
158 .Xr ssh 1
159 will additionally check the host IP address in the
160 .Pa known_hosts
161 file.
162 This allows ssh to detect if a host key changed due to DNS spoofing.
163 If the option is set to
164 .Dq no ,
165 the check will not be executed.
166 The default is
167 .Dq yes .
168 .It Cm Cipher
169 Specifies the cipher to use for encrypting the session
170 in protocol version 1.
171 Currently,
172 .Dq blowfish ,
173 .Dq 3des ,
174 and
175 .Dq des
176 are supported.
177 .Ar des
178 is only supported in the
179 .Xr ssh 1
180 client for interoperability with legacy protocol 1 implementations
181 that do not support the
182 .Ar 3des
183 cipher.
184 Its use is strongly discouraged due to cryptographic weaknesses.
185 The default is
186 .Dq 3des .
187 .It Cm Ciphers
188 Specifies the ciphers allowed for protocol version 2
189 in order of preference.
190 Multiple ciphers must be comma-separated.
191 The supported ciphers are
192 .Dq 3des-cbc ,
193 .Dq aes128-cbc ,
194 .Dq aes192-cbc ,
195 .Dq aes256-cbc ,
196 .Dq aes128-ctr ,
197 .Dq aes192-ctr ,
198 .Dq aes256-ctr ,
199 .Dq arcfour128 ,
200 .Dq arcfour256 ,
201 .Dq arcfour ,
202 .Dq blowfish-cbc ,
203 and
204 .Dq cast128-cbc .
205 The default is:
206 .Bd -literal -offset 3n
207 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
208 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
209 aes256-cbc,arcfour
210 .Ed
211 .It Cm ClearAllForwardings
212 Specifies that all local, remote, and dynamic port forwardings
213 specified in the configuration files or on the command line be
214 cleared.
215 This option is primarily useful when used from the
216 .Xr ssh 1
217 command line to clear port forwardings set in
218 configuration files, and is automatically set by
219 .Xr scp 1
220 and
221 .Xr sftp 1 .
222 The argument must be
223 .Dq yes
224 or
225 .Dq no .
226 The default is
227 .Dq no .
228 .It Cm Compression
229 Specifies whether to use compression.
230 The argument must be
231 .Dq yes
232 or
233 .Dq no .
234 The default is
235 .Dq no .
236 .It Cm CompressionLevel
237 Specifies the compression level to use if compression is enabled.
238 The argument must be an integer from 1 (fast) to 9 (slow, best).
239 The default level is 6, which is good for most applications.
240 The meaning of the values is the same as in
241 .Xr gzip 1 .
242 Note that this option applies to protocol version 1 only.
243 .It Cm ConnectionAttempts
244 Specifies the number of tries (one per second) to make before exiting.
245 The argument must be an integer.
246 This may be useful in scripts if the connection sometimes fails.
247 The default is 1.
248 .It Cm ConnectTimeout
249 Specifies the timeout (in seconds) used when connecting to the
250 SSH server, instead of using the default system TCP timeout.
251 This value is used only when the target is down or really unreachable,
252 not when it refuses the connection.
253 .It Cm ControlMaster
254 Enables the sharing of multiple sessions over a single network connection.
255 When set to
256 .Dq yes ,
257 .Xr ssh 1
258 will listen for connections on a control socket specified using the
259 .Cm ControlPath
260 argument.
261 Additional sessions can connect to this socket using the same
262 .Cm ControlPath
263 with
264 .Cm ControlMaster
265 set to
266 .Dq no
267 (the default).
268 These sessions will try to reuse the master instance's network connection
269 rather than initiating new ones, but will fall back to connecting normally
270 if the control socket does not exist, or is not listening.
271 .Pp
272 Setting this to
273 .Dq ask
274 will cause ssh
275 to listen for control connections, but require confirmation using the
276 .Ev SSH_ASKPASS
277 program before they are accepted (see
278 .Xr ssh-add 1
279 for details).
280 If the
281 .Cm ControlPath
282 cannot be opened,
283 ssh will continue without connecting to a master instance.
284 .Pp
285 X11 and
286 .Xr ssh-agent 1
287 forwarding is supported over these multiplexed connections, however the
288 display and agent forwarded will be the one belonging to the master
289 connection i.e. it is not possible to forward multiple displays or agents.
290 .Pp
291 Two additional options allow for opportunistic multiplexing: try to use a
292 master connection but fall back to creating a new one if one does not already
293 exist.
294 These options are:
295 .Dq auto
296 and
297 .Dq autoask .
298 The latter requires confirmation like the
299 .Dq ask
300 option.
301 .It Cm ControlPath
302 Specify the path to the control socket used for connection sharing as described
303 in the
304 .Cm ControlMaster
305 section above or the string
306 .Dq none
307 to disable connection sharing.
308 In the path,
309 .Ql %l
310 will be substituted by the local host name,
311 .Ql %h
312 will be substituted by the target host name,
313 .Ql %p
314 the port, and
315 .Ql %r
316 by the remote login username.
317 It is recommended that any
318 .Cm ControlPath
319 used for opportunistic connection sharing include
320 at least %h, %p, and %r.
321 This ensures that shared connections are uniquely identified.
322 .It Cm DynamicForward
323 Specifies that a TCP port on the local machine be forwarded
324 over the secure channel, and the application
325 protocol is then used to determine where to connect to from the
326 remote machine.
327 .Pp
328 The argument must be
329 .Sm off
330 .Oo Ar bind_address : Oc Ar port .
331 .Sm on
332 IPv6 addresses can be specified by enclosing addresses in square brackets or
333 by using an alternative syntax:
334 .Oo Ar bind_address Ns / Oc Ns Ar port .
335 By default, the local port is bound in accordance with the
336 .Cm GatewayPorts
337 setting.
338 However, an explicit
339 .Ar bind_address
340 may be used to bind the connection to a specific address.
341 The
342 .Ar bind_address
343 of
344 .Dq localhost
345 indicates that the listening port be bound for local use only, while an
346 empty address or
347 .Sq *
348 indicates that the port should be available from all interfaces.
349 .Pp
350 Currently the SOCKS4 and SOCKS5 protocols are supported, and
351 .Xr ssh 1
352 will act as a SOCKS server.
353 Multiple forwardings may be specified, and
354 additional forwardings can be given on the command line.
355 Only the superuser can forward privileged ports.
356 .It Cm EnableSSHKeysign
357 Setting this option to
358 .Dq yes
359 in the global client configuration file
360 .Pa /etc/ssh/ssh_config
361 enables the use of the helper program
362 .Xr ssh-keysign 8
363 during
364 .Cm HostbasedAuthentication .
365 The argument must be
366 .Dq yes
367 or
368 .Dq no .
369 The default is
370 .Dq no .
371 This option should be placed in the non-hostspecific section.
372 See
373 .Xr ssh-keysign 8
374 for more information.
375 .It Cm EscapeChar
376 Sets the escape character (default:
377 .Ql ~ ) .
378 The escape character can also
379 be set on the command line.
380 The argument should be a single character,
381 .Ql ^
382 followed by a letter, or
383 .Dq none
384 to disable the escape
385 character entirely (making the connection transparent for binary
386 data).
387 .It Cm ExitOnForwardFailure
388 Specifies whether
389 .Xr ssh 1
390 should terminate the connection if it cannot set up all requested
391 dynamic, tunnel, local, and remote port forwardings.
392 The argument must be
393 .Dq yes
394 or
395 .Dq no .
396 The default is
397 .Dq no .
398 .It Cm ForwardAgent
399 Specifies whether the connection to the authentication agent (if any)
400 will be forwarded to the remote machine.
401 The argument must be
402 .Dq yes
403 or
404 .Dq no .
405 The default is
406 .Dq no .
407 .Pp
408 Agent forwarding should be enabled with caution.
409 Users with the ability to bypass file permissions on the remote host
410 (for the agent's Unix-domain socket)
411 can access the local agent through the forwarded connection.
412 An attacker cannot obtain key material from the agent,
413 however they can perform operations on the keys that enable them to
414 authenticate using the identities loaded into the agent.
415 .It Cm ForwardX11
416 Specifies whether X11 connections will be automatically redirected
417 over the secure channel and
418 .Ev DISPLAY
419 set.
420 The argument must be
421 .Dq yes
422 or
423 .Dq no .
424 The default is
425 .Dq no .
426 .Pp
427 X11 forwarding should be enabled with caution.
428 Users with the ability to bypass file permissions on the remote host
429 (for the user's X11 authorization database)
430 can access the local X11 display through the forwarded connection.
431 An attacker may then be able to perform activities such as keystroke monitoring
432 if the
433 .Cm ForwardX11Trusted
434 option is also enabled.
435 .It Cm ForwardX11Trusted
436 If this option is set to
437 .Dq yes ,
438 remote X11 clients will have full access to the original X11 display.
439 .Pp
440 If this option is set to
441 .Dq no ,
442 remote X11 clients will be considered untrusted and prevented
443 from stealing or tampering with data belonging to trusted X11
444 clients.
445 Furthermore, the
446 .Xr xauth 1
447 token used for the session will be set to expire after 20 minutes.
448 Remote clients will be refused access after this time.
449 .Pp
450 The default is
451 .Dq no .
452 .Pp
453 See the X11 SECURITY extension specification for full details on
454 the restrictions imposed on untrusted clients.
455 .It Cm GatewayPorts
456 Specifies whether remote hosts are allowed to connect to local
457 forwarded ports.
458 By default,
459 .Xr ssh 1
460 binds local port forwardings to the loopback address.
461 This prevents other remote hosts from connecting to forwarded ports.
462 .Cm GatewayPorts
463 can be used to specify that ssh
464 should bind local port forwardings to the wildcard address,
465 thus allowing remote hosts to connect to forwarded ports.
466 The argument must be
467 .Dq yes
468 or
469 .Dq no .
470 The default is
471 .Dq no .
472 .It Cm GlobalKnownHostsFile
473 Specifies a file to use for the global
474 host key database instead of
475 .Pa /etc/ssh/ssh_known_hosts .
476 .It Cm GSSAPIAuthentication
477 Specifies whether user authentication based on GSSAPI is allowed.
478 The default is
479 .Dq no .
480 Note that this option applies to protocol version 2 only.
481 .It Cm GSSAPIDelegateCredentials
482 Forward (delegate) credentials to the server.
483 The default is
484 .Dq no .
485 Note that this option applies to protocol version 2 only.
486 .It Cm HashKnownHosts
487 Indicates that
488 .Xr ssh 1
489 should hash host names and addresses when they are added to
490 .Pa ~/.ssh/known_hosts .
491 These hashed names may be used normally by
492 .Xr ssh 1
493 and
494 .Xr sshd 8 ,
495 but they do not reveal identifying information should the file's contents
496 be disclosed.
497 The default is
498 .Dq no .
499 Note that existing names and addresses in known hosts files
500 will not be converted automatically,
501 but may be manually hashed using
502 .Xr ssh-keygen 1 .
503 .It Cm HostbasedAuthentication
504 Specifies whether to try rhosts based authentication with public key
505 authentication.
506 The argument must be
507 .Dq yes
508 or
509 .Dq no .
510 The default is
511 .Dq no .
512 This option applies to protocol version 2 only and
513 is similar to
514 .Cm RhostsRSAAuthentication .
515 .It Cm HostKeyAlgorithms
516 Specifies the protocol version 2 host key algorithms
517 that the client wants to use in order of preference.
518 The default for this option is:
519 .Dq ssh-rsa,ssh-dss .
520 .It Cm HostKeyAlias
521 Specifies an alias that should be used instead of the
522 real host name when looking up or saving the host key
523 in the host key database files.
524 This option is useful for tunneling SSH connections
525 or for multiple servers running on a single host.
526 .It Cm HostName
527 Specifies the real host name to log into.
528 This can be used to specify nicknames or abbreviations for hosts.
529 The default is the name given on the command line.
530 Numeric IP addresses are also permitted (both on the command line and in
531 .Cm HostName
532 specifications).
533 .It Cm IdentitiesOnly
534 Specifies that
535 .Xr ssh 1
536 should only use the authentication identity files configured in the
537 .Nm
538 files,
539 even if
540 .Xr ssh-agent 1
541 offers more identities.
542 The argument to this keyword must be
543 .Dq yes
544 or
545 .Dq no .
546 This option is intended for situations where ssh-agent
547 offers many different identities.
548 The default is
549 .Dq no .
550 .It Cm IdentityFile
551 Specifies a file from which the user's RSA or DSA authentication identity
552 is read.
553 The default is
554 .Pa ~/.ssh/identity
555 for protocol version 1, and
556 .Pa ~/.ssh/id_rsa
557 and
558 .Pa ~/.ssh/id_dsa
559 for protocol version 2.
560 Additionally, any identities represented by the authentication agent
561 will be used for authentication.
562 .Pp
563 The file name may use the tilde
564 syntax to refer to a user's home directory or one of the following
565 escape characters:
566 .Ql %d
567 (local user's home directory),
568 .Ql %u
569 (local user name),
570 .Ql %l
571 (local host name),
572 .Ql %h
573 (remote host name) or
574 .Ql %r
575 (remote user name).
576 .Pp
577 It is possible to have
578 multiple identity files specified in configuration files; all these
579 identities will be tried in sequence.
580 .It Cm KbdInteractiveAuthentication
581 Specifies whether to use keyboard-interactive authentication.
582 The argument to this keyword must be
583 .Dq yes
584 or
585 .Dq no .
586 The default is
587 .Dq yes .
588 .It Cm KbdInteractiveDevices
589 Specifies the list of methods to use in keyboard-interactive authentication.
590 Multiple method names must be comma-separated.
591 The default is to use the server specified list.
592 The methods available vary depending on what the server supports.
593 For an OpenSSH server,
594 it may be zero or more of:
595 .Dq bsdauth ,
596 .Dq pam ,
597 and
598 .Dq skey .
599 .It Cm LocalCommand
600 Specifies a command to execute on the local machine after successfully
601 connecting to the server.
602 The command string extends to the end of the line, and is executed with
603 the user's shell.
604 The following escape character substitutions will be performed:
605 .Ql %d
606 (local user's home directory),
607 .Ql %h
608 (remote host name),
609 .Ql %l
610 (local host name),
611 .Ql %n
612 (host name as provided on the command line),
613 .Ql %p
614 (remote port),
615 .Ql %r
616 (remote user name) or
617 .Ql %u
618 (local user name).
619 This directive is ignored unless
620 .Cm PermitLocalCommand
621 has been enabled.
622 .It Cm LocalForward
623 Specifies that a TCP port on the local machine be forwarded over
624 the secure channel to the specified host and port from the remote machine.
625 The first argument must be
626 .Sm off
627 .Oo Ar bind_address : Oc Ar port
628 .Sm on
629 and the second argument must be
630 .Ar host : Ns Ar hostport .
631 IPv6 addresses can be specified by enclosing addresses in square brackets or
632 by using an alternative syntax:
633 .Oo Ar bind_address Ns / Oc Ns Ar port
634 and
635 .Ar host Ns / Ns Ar hostport .
636 Multiple forwardings may be specified, and additional forwardings can be
637 given on the command line.
638 Only the superuser can forward privileged ports.
639 By default, the local port is bound in accordance with the
640 .Cm GatewayPorts
641 setting.
642 However, an explicit
643 .Ar bind_address
644 may be used to bind the connection to a specific address.
645 The
646 .Ar bind_address
647 of
648 .Dq localhost
649 indicates that the listening port be bound for local use only, while an
650 empty address or
651 .Sq *
652 indicates that the port should be available from all interfaces.
653 .It Cm LogLevel
654 Gives the verbosity level that is used when logging messages from
655 .Xr ssh 1 .
656 The possible values are:
657 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
658 The default is INFO.
659 DEBUG and DEBUG1 are equivalent.
660 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
661 .It Cm MACs
662 Specifies the MAC (message authentication code) algorithms
663 in order of preference.
664 The MAC algorithm is used in protocol version 2
665 for data integrity protection.
666 Multiple algorithms must be comma-separated.
667 The default is:
668 .Bd -literal -offset indent
669 hmac-md5,hmac-sha1,umac-64@openssh.com,
670 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
671 .Ed
672 .It Cm NoHostAuthenticationForLocalhost
673 This option can be used if the home directory is shared across machines.
674 In this case localhost will refer to a different machine on each of
675 the machines and the user will get many warnings about changed host keys.
676 However, this option disables host authentication for localhost.
677 The argument to this keyword must be
678 .Dq yes
679 or
680 .Dq no .
681 The default is to check the host key for localhost.
682 .It Cm NumberOfPasswordPrompts
683 Specifies the number of password prompts before giving up.
684 The argument to this keyword must be an integer.
685 The default is 3.
686 .It Cm PasswordAuthentication
687 Specifies whether to use password authentication.
688 The argument to this keyword must be
689 .Dq yes
690 or
691 .Dq no .
692 The default is
693 .Dq yes .
694 .It Cm PermitLocalCommand
695 Allow local command execution via the
696 .Ic LocalCommand
697 option or using the
698 .Ic !\& Ns Ar command
699 escape sequence in
700 .Xr ssh 1 .
701 The argument must be
702 .Dq yes
703 or
704 .Dq no .
705 The default is
706 .Dq no .
707 .It Cm Port
708 Specifies the port number to connect on the remote host.
709 The default is 22.
710 .It Cm PreferredAuthentications
711 Specifies the order in which the client should try protocol 2
712 authentication methods.
713 This allows a client to prefer one method (e.g.\&
714 .Cm keyboard-interactive )
715 over another method (e.g.\&
716 .Cm password )
717 The default for this option is:
718 .Do gssapi-with-mic ,
719 hostbased,
720 publickey,
721 keyboard-interactive,
722 password
723 .Dc .
724 .It Cm Protocol
725 Specifies the protocol versions
726 .Xr ssh 1
727 should support in order of preference.
728 The possible values are
729 .Sq 1
730 and
731 .Sq 2 .
732 Multiple versions must be comma-separated.
733 When this option is set to
734 .Dq 2,1
735 .Nm ssh
736 will try version 2 and fall back to version 1
737 if version 2 is not available.
738 The default is
739 .Dq 2 .
740 .It Cm ProxyCommand
741 Specifies the command to use to connect to the server.
742 The command
743 string extends to the end of the line, and is executed with
744 the user's shell.
745 In the command string,
746 .Ql %h
747 will be substituted by the host name to
748 connect and
749 .Ql %p
750 by the port.
751 The command can be basically anything,
752 and should read from its standard input and write to its standard output.
753 It should eventually connect an
754 .Xr sshd 8
755 server running on some machine, or execute
756 .Ic sshd -i
757 somewhere.
758 Host key management will be done using the
759 HostName of the host being connected (defaulting to the name typed by
760 the user).
761 Setting the command to
762 .Dq none
763 disables this option entirely.
764 Note that
765 .Cm CheckHostIP
766 is not available for connects with a proxy command.
767 .Pp
768 This directive is useful in conjunction with
769 .Xr nc 1
770 and its proxy support.
771 For example, the following directive would connect via an HTTP proxy at
772 192.0.2.0:
773 .Bd -literal -offset 3n
774 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
775 .Ed
776 .It Cm PubkeyAuthentication
777 Specifies whether to try public key authentication.
778 The argument to this keyword must be
779 .Dq yes
780 or
781 .Dq no .
782 The default is
783 .Dq yes .
784 This option applies to protocol version 2 only.
785 .It Cm RekeyLimit
786 Specifies the maximum amount of data that may be transmitted before the
787 session key is renegotiated.
788 The argument is the number of bytes, with an optional suffix of
789 .Sq K ,
790 .Sq M ,
791 or
792 .Sq G
793 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
794 The default is between
795 .Sq 1G
796 and
797 .Sq 4G ,
798 depending on the cipher.
799 This option applies to protocol version 2 only.
800 .It Cm RemoteForward
801 Specifies that a TCP port on the remote machine be forwarded over
802 the secure channel to the specified host and port from the local machine.
803 The first argument must be
804 .Sm off
805 .Oo Ar bind_address : Oc Ar port
806 .Sm on
807 and the second argument must be
808 .Ar host : Ns Ar hostport .
809 IPv6 addresses can be specified by enclosing addresses in square brackets
810 or by using an alternative syntax:
811 .Oo Ar bind_address Ns / Oc Ns Ar port
812 and
813 .Ar host Ns / Ns Ar hostport .
814 Multiple forwardings may be specified, and additional
815 forwardings can be given on the command line.
816 Privileged ports can be forwarded only when
817 logging in as root on the remote machine.
818 .Pp
819 If the
820 .Ar port
821 argument is
822 .Ql 0 ,
823 the listen port will be dynamically allocated on the server and reported
824 to the client at run time.
825 .Pp
826 If the
827 .Ar bind_address
828 is not specified, the default is to only bind to loopback addresses.
829 If the
830 .Ar bind_address
831 is
832 .Ql *
833 or an empty string, then the forwarding is requested to listen on all
834 interfaces.
835 Specifying a remote
836 .Ar bind_address
837 will only succeed if the server's
838 .Cm GatewayPorts
839 option is enabled (see
840 .Xr sshd_config 5 ) .
841 .It Cm RhostsRSAAuthentication
842 Specifies whether to try rhosts based authentication with RSA host
843 authentication.
844 The argument must be
845 .Dq yes
846 or
847 .Dq no .
848 The default is
849 .Dq no .
850 This option applies to protocol version 1 only and requires
851 .Xr ssh 1
852 to be setuid root.
853 .It Cm RSAAuthentication
854 Specifies whether to try RSA authentication.
855 The argument to this keyword must be
856 .Dq yes
857 or
858 .Dq no .
859 RSA authentication will only be
860 attempted if the identity file exists, or an authentication agent is
861 running.
862 The default is
863 .Dq yes .
864 Note that this option applies to protocol version 1 only.
865 .It Cm SendEnv
866 Specifies what variables from the local
867 .Xr environ 7
868 should be sent to the server.
869 Note that environment passing is only supported for protocol 2.
870 The server must also support it, and the server must be configured to
871 accept these environment variables.
872 Refer to
873 .Cm AcceptEnv
874 in
875 .Xr sshd_config 5
876 for how to configure the server.
877 Variables are specified by name, which may contain wildcard characters.
878 Multiple environment variables may be separated by whitespace or spread
879 across multiple
880 .Cm SendEnv
881 directives.
882 The default is not to send any environment variables.
883 .Pp
884 See
885 .Sx PATTERNS
886 for more information on patterns.
887 .It Cm ServerAliveCountMax
888 Sets the number of server alive messages (see below) which may be
889 sent without
890 .Xr ssh 1
891 receiving any messages back from the server.
892 If this threshold is reached while server alive messages are being sent,
893 ssh will disconnect from the server, terminating the session.
894 It is important to note that the use of server alive messages is very
895 different from
896 .Cm TCPKeepAlive
897 (below).
898 The server alive messages are sent through the encrypted channel
899 and therefore will not be spoofable.
900 The TCP keepalive option enabled by
901 .Cm TCPKeepAlive
902 is spoofable.
903 The server alive mechanism is valuable when the client or
904 server depend on knowing when a connection has become inactive.
905 .Pp
906 The default value is 3.
907 If, for example,
908 .Cm ServerAliveInterval
909 (see below) is set to 15 and
910 .Cm ServerAliveCountMax
911 is left at the default, if the server becomes unresponsive,
912 ssh will disconnect after approximately 45 seconds.
913 This option applies to protocol version 2 only.
914 .It Cm ServerAliveInterval
915 Sets a timeout interval in seconds after which if no data has been received
916 from the server,
917 .Xr ssh 1
918 will send a message through the encrypted
919 channel to request a response from the server.
920 The default
921 is 0, indicating that these messages will not be sent to the server.
922 This option applies to protocol version 2 only.
923 .It Cm SmartcardDevice
924 Specifies which smartcard device to use.
925 The argument to this keyword is the device
926 .Xr ssh 1
927 should use to communicate with a smartcard used for storing the user's
928 private RSA key.
929 By default, no device is specified and smartcard support is not activated.
930 .It Cm StrictHostKeyChecking
931 If this flag is set to
932 .Dq yes ,
933 .Xr ssh 1
934 will never automatically add host keys to the
935 .Pa ~/.ssh/known_hosts
936 file, and refuses to connect to hosts whose host key has changed.
937 This provides maximum protection against trojan horse attacks,
938 though it can be annoying when the
939 .Pa /etc/ssh/ssh_known_hosts
940 file is poorly maintained or when connections to new hosts are
941 frequently made.
942 This option forces the user to manually
943 add all new hosts.
944 If this flag is set to
945 .Dq no ,
946 ssh will automatically add new host keys to the
947 user known hosts files.
948 If this flag is set to
949 .Dq ask ,
950 new host keys
951 will be added to the user known host files only after the user
952 has confirmed that is what they really want to do, and
953 ssh will refuse to connect to hosts whose host key has changed.
954 The host keys of
955 known hosts will be verified automatically in all cases.
956 The argument must be
957 .Dq yes ,
958 .Dq no ,
959 or
960 .Dq ask .
961 The default is
962 .Dq ask .
963 .It Cm TCPKeepAlive
964 Specifies whether the system should send TCP keepalive messages to the
965 other side.
966 If they are sent, death of the connection or crash of one
967 of the machines will be properly noticed.
968 However, this means that
969 connections will die if the route is down temporarily, and some people
970 find it annoying.
971 .Pp
972 The default is
973 .Dq yes
974 (to send TCP keepalive messages), and the client will notice
975 if the network goes down or the remote host dies.
976 This is important in scripts, and many users want it too.
977 .Pp
978 To disable TCP keepalive messages, the value should be set to
979 .Dq no .
980 .It Cm Tunnel
981 Request
982 .Xr tun 4
983 device forwarding between the client and the server.
984 The argument must be
985 .Dq yes ,
986 .Dq point-to-point
987 (layer 3),
988 .Dq ethernet
989 (layer 2),
990 or
991 .Dq no .
992 Specifying
993 .Dq yes
994 requests the default tunnel mode, which is
995 .Dq point-to-point .
996 The default is
997 .Dq no .
998 .It Cm TunnelDevice
999 Specifies the
1000 .Xr tun 4
1001 devices to open on the client
1002 .Pq Ar local_tun
1003 and the server
1004 .Pq Ar remote_tun .
1005 .Pp
1006 The argument must be
1007 .Sm off
1008 .Ar local_tun Op : Ar remote_tun .
1009 .Sm on
1010 The devices may be specified by numerical ID or the keyword
1011 .Dq any ,
1012 which uses the next available tunnel device.
1013 If
1014 .Ar remote_tun
1015 is not specified, it defaults to
1016 .Dq any .
1017 The default is
1018 .Dq any:any .
1019 .It Cm UsePrivilegedPort
1020 Specifies whether to use a privileged port for outgoing connections.
1021 The argument must be
1022 .Dq yes
1023 or
1024 .Dq no .
1025 The default is
1026 .Dq no .
1027 If set to
1028 .Dq yes ,
1029 .Xr ssh 1
1030 must be setuid root.
1031 Note that this option must be set to
1032 .Dq yes
1033 for
1034 .Cm RhostsRSAAuthentication
1035 with older servers.
1036 .It Cm User
1037 Specifies the user to log in as.
1038 This can be useful when a different user name is used on different machines.
1039 This saves the trouble of
1040 having to remember to give the user name on the command line.
1041 .It Cm UserKnownHostsFile
1042 Specifies a file to use for the user
1043 host key database instead of
1044 .Pa ~/.ssh/known_hosts .
1045 .It Cm VerifyHostKeyDNS
1046 Specifies whether to verify the remote key using DNS and SSHFP resource
1047 records.
1048 If this option is set to
1049 .Dq yes ,
1050 the client will implicitly trust keys that match a secure fingerprint
1051 from DNS.
1052 Insecure fingerprints will be handled as if this option was set to
1053 .Dq ask .
1054 If this option is set to
1055 .Dq ask ,
1056 information on fingerprint match will be displayed, but the user will still
1057 need to confirm new host keys according to the
1058 .Cm StrictHostKeyChecking
1059 option.
1060 The argument must be
1061 .Dq yes ,
1062 .Dq no ,
1063 or
1064 .Dq ask .
1065 The default is
1066 .Dq no .
1067 Note that this option applies to protocol version 2 only.
1068 .Pp
1069 See also
1070 .Sx VERIFYING HOST KEYS
1071 in
1072 .Xr ssh 1 .
1073 .It Cm VisualHostKey
1074 If this flag is set to
1075 .Dq yes ,
1076 an ASCII art representation of the remote host key fingerprint is
1077 printed in addition to the hex fingerprint string at login and
1078 for unknown host keys.
1079 If this flag is set to
1080 .Dq no ,
1081 no fingerprint strings are printed at login and
1082 only the hex fingerprint string will be printed for unknown host keys.
1083 The default is
1084 .Dq no .
1085 .It Cm XAuthLocation
1086 Specifies the full pathname of the
1087 .Xr xauth 1
1088 program.
1089 The default is
1090 .Pa /usr/X11R6/bin/xauth .
1091 .El
1092 .Sh PATTERNS
1093 A
1094 .Em pattern
1095 consists of zero or more non-whitespace characters,
1096 .Sq *
1097 (a wildcard that matches zero or more characters),
1098 or
1099 .Sq ?\&
1100 (a wildcard that matches exactly one character).
1101 For example, to specify a set of declarations for any host in the
1102 .Dq .co.uk
1103 set of domains,
1104 the following pattern could be used:
1105 .Pp
1106 .Dl Host *.co.uk
1107 .Pp
1108 The following pattern
1109 would match any host in the 192.168.0.[0-9] network range:
1110 .Pp
1111 .Dl Host 192.168.0.?
1112 .Pp
1113 A
1114 .Em pattern-list
1115 is a comma-separated list of patterns.
1116 Patterns within pattern-lists may be negated
1117 by preceding them with an exclamation mark
1118 .Pq Sq !\& .
1119 For example,
1120 to allow a key to be used from anywhere within an organisation
1121 except from the
1122 .Dq dialup
1123 pool,
1124 the following entry (in authorized_keys) could be used:
1125 .Pp
1126 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1127 .Sh FILES
1128 .Bl -tag -width Ds
1129 .It Pa ~/.ssh/config
1130 This is the per-user configuration file.
1131 The format of this file is described above.
1132 This file is used by the SSH client.
1133 Because of the potential for abuse, this file must have strict permissions:
1134 read/write for the user, and not accessible by others.
1135 .It Pa /etc/ssh/ssh_config
1136 Systemwide configuration file.
1137 This file provides defaults for those
1138 values that are not specified in the user's configuration file, and
1139 for those users who do not have a configuration file.
1140 This file must be world-readable.
1141 .El
1142 .Sh SEE ALSO
1143 .Xr ssh 1
1144 .Sh AUTHORS
1145 OpenSSH is a derivative of the original and free
1146 ssh 1.2.12 release by Tatu Ylonen.
1147 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1148 Theo de Raadt and Dug Song
1149 removed many bugs, re-added newer features and
1150 created OpenSSH.
1151 Markus Friedl contributed the support for SSH
1152 protocol versions 1.5 and 2.0.