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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 1.119 2009/02/22 23:50:57 djm Exp $
38 .Dd $Mdocdate: February 22 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 no .
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 The default is
734 .Dq 2,1 .
735 This means that ssh
736 tries version 2 and falls back to version 1
737 if version 2 is not available.
738 .It Cm ProxyCommand
739 Specifies the command to use to connect to the server.
740 The command
741 string extends to the end of the line, and is executed with
742 the user's shell.
743 In the command string,
744 .Ql %h
745 will be substituted by the host name to
746 connect and
747 .Ql %p
748 by the port.
749 The command can be basically anything,
750 and should read from its standard input and write to its standard output.
751 It should eventually connect an
752 .Xr sshd 8
753 server running on some machine, or execute
754 .Ic sshd -i
755 somewhere.
756 Host key management will be done using the
757 HostName of the host being connected (defaulting to the name typed by
758 the user).
759 Setting the command to
760 .Dq none
761 disables this option entirely.
762 Note that
763 .Cm CheckHostIP
764 is not available for connects with a proxy command.
765 .Pp
766 This directive is useful in conjunction with
767 .Xr nc 1
768 and its proxy support.
769 For example, the following directive would connect via an HTTP proxy at
770 192.0.2.0:
771 .Bd -literal -offset 3n
772 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
773 .Ed
774 .It Cm PubkeyAuthentication
775 Specifies whether to try public key authentication.
776 The argument to this keyword must be
777 .Dq yes
778 or
779 .Dq no .
780 The default is
781 .Dq yes .
782 This option applies to protocol version 2 only.
783 .It Cm RekeyLimit
784 Specifies the maximum amount of data that may be transmitted before the
785 session key is renegotiated.
786 The argument is the number of bytes, with an optional suffix of
787 .Sq K ,
788 .Sq M ,
789 or
790 .Sq G
791 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
792 The default is between
793 .Sq 1G
794 and
795 .Sq 4G ,
796 depending on the cipher.
797 This option applies to protocol version 2 only.
798 .It Cm RemoteForward
799 Specifies that a TCP port on the remote machine be forwarded over
800 the secure channel to the specified host and port from the local machine.
801 The first argument must be
802 .Sm off
803 .Oo Ar bind_address : Oc Ar port
804 .Sm on
805 and the second argument must be
806 .Ar host : Ns Ar hostport .
807 IPv6 addresses can be specified by enclosing addresses in square brackets
808 or by using an alternative syntax:
809 .Oo Ar bind_address Ns / Oc Ns Ar port
810 and
811 .Ar host Ns / Ns Ar hostport .
812 Multiple forwardings may be specified, and additional
813 forwardings can be given on the command line.
814 Privileged ports can be forwarded only when
815 logging in as root on the remote machine.
816 .Pp
817 If the
818 .Ar port
819 argument is
820 .Ql 0 ,
821 the listen port will be dynamically allocated on the server and reported
822 to the client at run time.
823 .Pp
824 If the
825 .Ar bind_address
826 is not specified, the default is to only bind to loopback addresses.
827 If the
828 .Ar bind_address
829 is
830 .Ql *
831 or an empty string, then the forwarding is requested to listen on all
832 interfaces.
833 Specifying a remote
834 .Ar bind_address
835 will only succeed if the server's
836 .Cm GatewayPorts
837 option is enabled (see
838 .Xr sshd_config 5 ) .
839 .It Cm RhostsRSAAuthentication
840 Specifies whether to try rhosts based authentication with RSA host
841 authentication.
842 The argument must be
843 .Dq yes
844 or
845 .Dq no .
846 The default is
847 .Dq no .
848 This option applies to protocol version 1 only and requires
849 .Xr ssh 1
850 to be setuid root.
851 .It Cm RSAAuthentication
852 Specifies whether to try RSA authentication.
853 The argument to this keyword must be
854 .Dq yes
855 or
856 .Dq no .
857 RSA authentication will only be
858 attempted if the identity file exists, or an authentication agent is
859 running.
860 The default is
861 .Dq yes .
862 Note that this option applies to protocol version 1 only.
863 .It Cm SendEnv
864 Specifies what variables from the local
865 .Xr environ 7
866 should be sent to the server.
867 Note that environment passing is only supported for protocol 2.
868 The server must also support it, and the server must be configured to
869 accept these environment variables.
870 Refer to
871 .Cm AcceptEnv
872 in
873 .Xr sshd_config 5
874 for how to configure the server.
875 Variables are specified by name, which may contain wildcard characters.
876 Multiple environment variables may be separated by whitespace or spread
877 across multiple
878 .Cm SendEnv
879 directives.
880 The default is not to send any environment variables.
881 .Pp
882 See
883 .Sx PATTERNS
884 for more information on patterns.
885 .It Cm ServerAliveCountMax
886 Sets the number of server alive messages (see below) which may be
887 sent without
888 .Xr ssh 1
889 receiving any messages back from the server.
890 If this threshold is reached while server alive messages are being sent,
891 ssh will disconnect from the server, terminating the session.
892 It is important to note that the use of server alive messages is very
893 different from
894 .Cm TCPKeepAlive
895 (below).
896 The server alive messages are sent through the encrypted channel
897 and therefore will not be spoofable.
898 The TCP keepalive option enabled by
899 .Cm TCPKeepAlive
900 is spoofable.
901 The server alive mechanism is valuable when the client or
902 server depend on knowing when a connection has become inactive.
903 .Pp
904 The default value is 3.
905 If, for example,
906 .Cm ServerAliveInterval
907 (see below) is set to 15 and
908 .Cm ServerAliveCountMax
909 is left at the default, if the server becomes unresponsive,
910 ssh will disconnect after approximately 45 seconds.
911 This option applies to protocol version 2 only.
912 .It Cm ServerAliveInterval
913 Sets a timeout interval in seconds after which if no data has been received
914 from the server,
915 .Xr ssh 1
916 will send a message through the encrypted
917 channel to request a response from the server.
918 The default
919 is 0, indicating that these messages will not be sent to the server.
920 This option applies to protocol version 2 only.
921 .It Cm SmartcardDevice
922 Specifies which smartcard device to use.
923 The argument to this keyword is the device
924 .Xr ssh 1
925 should use to communicate with a smartcard used for storing the user's
926 private RSA key.
927 By default, no device is specified and smartcard support is not activated.
928 .It Cm StrictHostKeyChecking
929 If this flag is set to
930 .Dq yes ,
931 .Xr ssh 1
932 will never automatically add host keys to the
933 .Pa ~/.ssh/known_hosts
934 file, and refuses to connect to hosts whose host key has changed.
935 This provides maximum protection against trojan horse attacks,
936 though it can be annoying when the
937 .Pa /etc/ssh/ssh_known_hosts
938 file is poorly maintained or when connections to new hosts are
939 frequently made.
940 This option forces the user to manually
941 add all new hosts.
942 If this flag is set to
943 .Dq no ,
944 ssh will automatically add new host keys to the
945 user known hosts files.
946 If this flag is set to
947 .Dq ask ,
948 new host keys
949 will be added to the user known host files only after the user
950 has confirmed that is what they really want to do, and
951 ssh will refuse to connect to hosts whose host key has changed.
952 The host keys of
953 known hosts will be verified automatically in all cases.
954 The argument must be
955 .Dq yes ,
956 .Dq no ,
957 or
958 .Dq ask .
959 The default is
960 .Dq ask .
961 .It Cm TCPKeepAlive
962 Specifies whether the system should send TCP keepalive messages to the
963 other side.
964 If they are sent, death of the connection or crash of one
965 of the machines will be properly noticed.
966 However, this means that
967 connections will die if the route is down temporarily, and some people
968 find it annoying.
969 .Pp
970 The default is
971 .Dq yes
972 (to send TCP keepalive messages), and the client will notice
973 if the network goes down or the remote host dies.
974 This is important in scripts, and many users want it too.
975 .Pp
976 To disable TCP keepalive messages, the value should be set to
977 .Dq no .
978 .It Cm Tunnel
979 Request
980 .Xr tun 4
981 device forwarding between the client and the server.
982 The argument must be
983 .Dq yes ,
984 .Dq point-to-point
985 (layer 3),
986 .Dq ethernet
987 (layer 2),
988 or
989 .Dq no .
990 Specifying
991 .Dq yes
992 requests the default tunnel mode, which is
993 .Dq point-to-point .
994 The default is
995 .Dq no .
996 .It Cm TunnelDevice
997 Specifies the
998 .Xr tun 4
999 devices to open on the client
1000 .Pq Ar local_tun
1001 and the server
1002 .Pq Ar remote_tun .
1003 .Pp
1004 The argument must be
1005 .Sm off
1006 .Ar local_tun Op : Ar remote_tun .
1007 .Sm on
1008 The devices may be specified by numerical ID or the keyword
1009 .Dq any ,
1010 which uses the next available tunnel device.
1011 If
1012 .Ar remote_tun
1013 is not specified, it defaults to
1014 .Dq any .
1015 The default is
1016 .Dq any:any .
1017 .It Cm UsePrivilegedPort
1018 Specifies whether to use a privileged port for outgoing connections.
1019 The argument must be
1020 .Dq yes
1021 or
1022 .Dq no .
1023 The default is
1024 .Dq no .
1025 If set to
1026 .Dq yes ,
1027 .Xr ssh 1
1028 must be setuid root.
1029 Note that this option must be set to
1030 .Dq yes
1031 for
1032 .Cm RhostsRSAAuthentication
1033 with older servers.
1034 .It Cm User
1035 Specifies the user to log in as.
1036 This can be useful when a different user name is used on different machines.
1037 This saves the trouble of
1038 having to remember to give the user name on the command line.
1039 .It Cm UserKnownHostsFile
1040 Specifies a file to use for the user
1041 host key database instead of
1042 .Pa ~/.ssh/known_hosts .
1043 .It Cm VerifyHostKeyDNS
1044 Specifies whether to verify the remote key using DNS and SSHFP resource
1045 records.
1046 If this option is set to
1047 .Dq yes ,
1048 the client will implicitly trust keys that match a secure fingerprint
1049 from DNS.
1050 Insecure fingerprints will be handled as if this option was set to
1051 .Dq ask .
1052 If this option is set to
1053 .Dq ask ,
1054 information on fingerprint match will be displayed, but the user will still
1055 need to confirm new host keys according to the
1056 .Cm StrictHostKeyChecking
1057 option.
1058 The argument must be
1059 .Dq yes ,
1060 .Dq no ,
1061 or
1062 .Dq ask .
1063 The default is
1064 .Dq no .
1065 Note that this option applies to protocol version 2 only.
1066 .Pp
1067 See also
1068 .Sx VERIFYING HOST KEYS
1069 in
1070 .Xr ssh 1 .
1071 .It Cm VersionAddendum
1072 Specifies a string to append to the regular version string to identify
1073 OS- or site-specific modifications.
1074 The default is
1075 .Dq DragonFly-20090620 .
1076 .It Cm VisualHostKey
1077 If this flag is set to
1078 .Dq yes ,
1079 an ASCII art representation of the remote host key fingerprint is
1080 printed in addition to the hex fingerprint string at login and
1081 for unknown host keys.
1082 If this flag is set to
1083 .Dq no ,
1084 no fingerprint strings are printed at login and
1085 only the hex fingerprint string will be printed for unknown host keys.
1086 The default is
1087 .Dq no .
1088 .It Cm XAuthLocation
1089 Specifies the full pathname of the
1090 .Xr xauth 1
1091 program.
1092 The default is
1093 .Pa /usr/X11R6/bin/xauth .
1094 .El
1095 .Sh PATTERNS
1096 A
1097 .Em pattern
1098 consists of zero or more non-whitespace characters,
1099 .Sq *
1100 (a wildcard that matches zero or more characters),
1101 or
1102 .Sq ?\&
1103 (a wildcard that matches exactly one character).
1104 For example, to specify a set of declarations for any host in the
1105 .Dq .co.uk
1106 set of domains,
1107 the following pattern could be used:
1108 .Pp
1109 .Dl Host *.co.uk
1110 .Pp
1111 The following pattern
1112 would match any host in the 192.168.0.[0-9] network range:
1113 .Pp
1114 .Dl Host 192.168.0.?
1115 .Pp
1116 A
1117 .Em pattern-list
1118 is a comma-separated list of patterns.
1119 Patterns within pattern-lists may be negated
1120 by preceding them with an exclamation mark
1121 .Pq Sq !\& .
1122 For example,
1123 to allow a key to be used from anywhere within an organisation
1124 except from the
1125 .Dq dialup
1126 pool,
1127 the following entry (in authorized_keys) could be used:
1128 .Pp
1129 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1130 .Sh FILES
1131 .Bl -tag -width Ds
1132 .It Pa ~/.ssh/config
1133 This is the per-user configuration file.
1134 The format of this file is described above.
1135 This file is used by the SSH client.
1136 Because of the potential for abuse, this file must have strict permissions:
1137 read/write for the user, and not accessible by others.
1138 .It Pa /etc/ssh/ssh_config
1139 Systemwide configuration file.
1140 This file provides defaults for those
1141 values that are not specified in the user's configuration file, and
1142 for those users who do not have a configuration file.
1143 This file must be world-readable.
1144 .El
1145 .Sh SEE ALSO
1146 .Xr ssh 1
1147 .Sh AUTHORS
1148 OpenSSH is a derivative of the original and free
1149 ssh 1.2.12 release by Tatu Ylonen.
1150 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1151 Theo de Raadt and Dug Song
1152 removed many bugs, re-added newer features and
1153 created OpenSSH.
1154 Markus Friedl contributed the support for SSH
1155 protocol versions 1.5 and 2.0.