| blob | 50bcae82f66104d6e82a1afd24d6859126721bf4 |
1 .\"
2 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4 .\" All rights reserved
5 .\"
6 .\" As far as I am concerned, the code I have written for this software
7 .\" can be used freely for any purpose. Any derived versions of this
8 .\" software must be clearly marked as such, and if the derived work is
9 .\" incompatible with the protocol description in the RFC file, it must be
10 .\" called by a name other than "ssh" or "Secure Shell".
11 .\"
12 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
15 .\"
16 .\" Redistribution and use in source and binary forms, with or without
17 .\" modification, are permitted provided that the following conditions
18 .\" are met:
19 .\" 1. Redistributions of source code must retain the above copyright
20 .\" notice, this list of conditions and the following disclaimer.
21 .\" 2. Redistributions in binary form must reproduce the above copyright
22 .\" notice, this list of conditions and the following disclaimer in the
23 .\" documentation and/or other materials provided with the distribution.
24 .\"
25 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
35 .\"
36 .\" $OpenBSD: ssh_config.5,v 1.146 2010/12/08 04:02:47 djm Exp $
37 .Dd $Mdocdate: December 8 2010 $
38 .Dt SSH_CONFIG 5
39 .Os
40 .Sh NAME
41 .Nm ssh_config
42 .Nd OpenSSH SSH client configuration files
43 .Sh SYNOPSIS
44 .Nm ~/.ssh/config
45 .Nm /etc/ssh/ssh_config
46 .Sh DESCRIPTION
47 .Xr ssh 1
48 obtains configuration data from the following sources in
49 the following order:
50 .Pp
51 .Bl -enum -offset indent -compact
52 .It
53 command-line options
54 .It
55 user's configuration file
56 .Pq Pa ~/.ssh/config
57 .It
58 system-wide configuration file
59 .Pq Pa /etc/ssh/ssh_config
60 .El
61 .Pp
62 For each parameter, the first obtained value
63 will be used.
64 The configuration files contain sections separated by
65 .Dq Host
66 specifications, and that section is only applied for hosts that
67 match one of the patterns given in the specification.
68 The matched host name is the one given on the command line.
69 .Pp
70 Since the first obtained value for each parameter is used, more
71 host-specific declarations should be given near the beginning of the
72 file, and general defaults at the end.
73 .Pp
74 The configuration file has the following format:
75 .Pp
76 Empty lines and lines starting with
77 .Ql #
78 are comments.
79 Otherwise a line is of the format
80 .Dq keyword arguments .
81 Configuration options may be separated by whitespace or
82 optional whitespace and exactly one
83 .Ql = ;
84 the latter format is useful to avoid the need to quote whitespace
85 when specifying configuration options using the
86 .Nm ssh ,
87 .Nm scp ,
88 and
89 .Nm sftp
90 .Fl o
91 option.
92 Arguments may optionally be enclosed in double quotes
93 .Pq \&"
94 in order to represent arguments containing spaces.
95 .Pp
96 The possible
97 keywords and their meanings are as follows (note that
98 keywords are case-insensitive and arguments are case-sensitive):
99 .Bl -tag -width Ds
100 .It Cm Host
101 Restricts the following declarations (up to the next
102 .Cm Host
103 keyword) to be only for those hosts that match one of the patterns
104 given after the keyword.
105 If more than one pattern is provided, they should be separated by whitespace.
106 A single
107 .Ql *
108 as a pattern can be used to provide global
109 defaults for all hosts.
110 The host is the
111 .Ar hostname
112 argument given on the command line (i.e. the name is not converted to
113 a canonicalized host name before matching).
114 .Pp
115 See
116 .Sx PATTERNS
117 for more information on patterns.
118 .It Cm AddressFamily
119 Specifies which address family to use when connecting.
120 Valid arguments are
121 .Dq any ,
122 .Dq inet
123 (use IPv4 only), or
124 .Dq inet6
125 (use IPv6 only).
126 .It Cm BatchMode
127 If set to
128 .Dq yes ,
129 passphrase/password querying will be disabled.
130 This option is useful in scripts and other batch jobs where no user
131 is present to supply the password.
132 The argument must be
133 .Dq yes
134 or
135 .Dq no .
136 The default is
137 .Dq no .
138 .It Cm BindAddress
139 Use the specified address on the local machine as the source address of
140 the connection.
141 Only useful on systems with more than one address.
142 Note that this option does not work if
143 .Cm UsePrivilegedPort
144 is set to
145 .Dq yes .
146 .It Cm ChallengeResponseAuthentication
147 Specifies whether to use challenge-response authentication.
148 The argument to this keyword must be
149 .Dq yes
150 or
151 .Dq no .
152 The default is
153 .Dq yes .
154 .It Cm CheckHostIP
155 If this flag is set to
156 .Dq yes ,
157 .Xr ssh 1
158 will additionally check the host IP address in the
159 .Pa known_hosts
160 file.
161 This allows ssh to detect if a host key changed due to DNS spoofing.
162 If the option is set to
163 .Dq no ,
164 the check will not be executed.
165 The default is
166 .Dq yes .
167 .It Cm Cipher
168 Specifies the cipher to use for encrypting the session
169 in protocol version 1.
170 Currently,
171 .Dq blowfish ,
172 .Dq 3des ,
173 and
174 .Dq des
175 are supported.
176 .Ar des
177 is only supported in the
178 .Xr ssh 1
179 client for interoperability with legacy protocol 1 implementations
180 that do not support the
181 .Ar 3des
182 cipher.
183 Its use is strongly discouraged due to cryptographic weaknesses.
184 The default is
185 .Dq 3des .
186 .It Cm Ciphers
187 Specifies the ciphers allowed for protocol version 2
188 in order of preference.
189 Multiple ciphers must be comma-separated.
190 The supported ciphers are
191 .Dq 3des-cbc ,
192 .Dq aes128-cbc ,
193 .Dq aes192-cbc ,
194 .Dq aes256-cbc ,
195 .Dq aes128-ctr ,
196 .Dq aes192-ctr ,
197 .Dq aes256-ctr ,
198 .Dq arcfour128 ,
199 .Dq arcfour256 ,
200 .Dq arcfour ,
201 .Dq blowfish-cbc ,
202 and
203 .Dq cast128-cbc .
204 The default is:
205 .Bd -literal -offset 3n
206 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
207 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
208 aes256-cbc,arcfour
209 .Ed
210 .It Cm ClearAllForwardings
211 Specifies that all local, remote, and dynamic port forwardings
212 specified in the configuration files or on the command line be
213 cleared.
214 This option is primarily useful when used from the
215 .Xr ssh 1
216 command line to clear port forwardings set in
217 configuration files, and is automatically set by
218 .Xr scp 1
219 and
220 .Xr sftp 1 .
221 The argument must be
222 .Dq yes
223 or
224 .Dq no .
225 The default is
226 .Dq no .
227 .It Cm Compression
228 Specifies whether to use compression.
229 The argument must be
230 .Dq yes
231 or
232 .Dq no .
233 The default is
234 .Dq no .
235 .It Cm CompressionLevel
236 Specifies the compression level to use if compression is enabled.
237 The argument must be an integer from 1 (fast) to 9 (slow, best).
238 The default level is 6, which is good for most applications.
239 The meaning of the values is the same as in
240 .Xr gzip 1 .
241 Note that this option applies to protocol version 1 only.
242 .It Cm ConnectionAttempts
243 Specifies the number of tries (one per second) to make before exiting.
244 The argument must be an integer.
245 This may be useful in scripts if the connection sometimes fails.
246 The default is 1.
247 .It Cm ConnectTimeout
248 Specifies the timeout (in seconds) used when connecting to the
249 SSH server, instead of using the default system TCP timeout.
250 This value is used only when the target is down or really unreachable,
251 not when it refuses the connection.
252 .It Cm ControlMaster
253 Enables the sharing of multiple sessions over a single network connection.
254 When set to
255 .Dq yes ,
256 .Xr ssh 1
257 will listen for connections on a control socket specified using the
258 .Cm ControlPath
259 argument.
260 Additional sessions can connect to this socket using the same
261 .Cm ControlPath
262 with
263 .Cm ControlMaster
264 set to
265 .Dq no
266 (the default).
267 These sessions will try to reuse the master instance's network connection
268 rather than initiating new ones, but will fall back to connecting normally
269 if the control socket does not exist, or is not listening.
270 .Pp
271 Setting this to
272 .Dq ask
273 will cause ssh
274 to listen for control connections, but require confirmation using the
275 .Ev SSH_ASKPASS
276 program before they are accepted (see
277 .Xr ssh-add 1
278 for details).
279 If the
280 .Cm ControlPath
281 cannot be opened,
282 ssh will continue without connecting to a master instance.
283 .Pp
284 X11 and
285 .Xr ssh-agent 1
286 forwarding is supported over these multiplexed connections, however the
287 display and agent forwarded will be the one belonging to the master
288 connection i.e. it is not possible to forward multiple displays or agents.
289 .Pp
290 Two additional options allow for opportunistic multiplexing: try to use a
291 master connection but fall back to creating a new one if one does not already
292 exist.
293 These options are:
294 .Dq auto
295 and
296 .Dq autoask .
297 The latter requires confirmation like the
298 .Dq ask
299 option.
300 .It Cm ControlPath
301 Specify the path to the control socket used for connection sharing as described
302 in the
303 .Cm ControlMaster
304 section above or the string
305 .Dq none
306 to disable connection sharing.
307 In the path,
308 .Ql %l
309 will be substituted by the local host name,
310 .Ql %h
311 will be substituted by the target host name,
312 .Ql %p
313 the port, and
314 .Ql %r
315 by the remote login username.
316 It is recommended that any
317 .Cm ControlPath
318 used for opportunistic connection sharing include
319 at least %h, %p, and %r.
320 This ensures that shared connections are uniquely identified.
321 .It Cm ControlPersist
322 When used in conjunction with
323 .Cm ControlMaster ,
324 specifies that the master connection should remain open
325 in the background (waiting for future client connections)
326 after the initial client connection has been closed.
327 If set to
328 .Dq no ,
329 then the master connection will not be placed into the background,
330 and will close as soon as the initial client connection is closed.
331 If set to
332 .Dq yes ,
333 then the master connection will remain in the background indefinitely
334 (until killed or closed via a mechanism such as the
335 .Xr ssh 1
336 .Dq Fl O No exit
337 option).
338 If set to a time in seconds, or a time in any of the formats documented in
339 .Xr sshd_config 5 ,
340 then the backgrounded master connection will automatically terminate
341 after it has remained idle (with no client connections) for the
342 specified time.
343 .It Cm DynamicForward
344 Specifies that a TCP port on the local machine be forwarded
345 over the secure channel, and the application
346 protocol is then used to determine where to connect to from the
347 remote machine.
348 .Pp
349 The argument must be
350 .Sm off
351 .Oo Ar bind_address : Oc Ar port .
352 .Sm on
353 IPv6 addresses can be specified by enclosing addresses in square brackets.
354 By default, the local port is bound in accordance with the
355 .Cm GatewayPorts
356 setting.
357 However, an explicit
358 .Ar bind_address
359 may be used to bind the connection to a specific address.
360 The
361 .Ar bind_address
362 of
363 .Dq localhost
364 indicates that the listening port be bound for local use only, while an
365 empty address or
366 .Sq *
367 indicates that the port should be available from all interfaces.
368 .Pp
369 Currently the SOCKS4 and SOCKS5 protocols are supported, and
370 .Xr ssh 1
371 will act as a SOCKS server.
372 Multiple forwardings may be specified, and
373 additional forwardings can be given on the command line.
374 Only the superuser can forward privileged ports.
375 .It Cm EnableSSHKeysign
376 Setting this option to
377 .Dq yes
378 in the global client configuration file
379 .Pa /etc/ssh/ssh_config
380 enables the use of the helper program
381 .Xr ssh-keysign 8
382 during
383 .Cm HostbasedAuthentication .
384 The argument must be
385 .Dq yes
386 or
387 .Dq no .
388 The default is
389 .Dq no .
390 This option should be placed in the non-hostspecific section.
391 See
392 .Xr ssh-keysign 8
393 for more information.
394 .It Cm EscapeChar
395 Sets the escape character (default:
396 .Ql ~ ) .
397 The escape character can also
398 be set on the command line.
399 The argument should be a single character,
400 .Ql ^
401 followed by a letter, or
402 .Dq none
403 to disable the escape
404 character entirely (making the connection transparent for binary
405 data).
406 .It Cm ExitOnForwardFailure
407 Specifies whether
408 .Xr ssh 1
409 should terminate the connection if it cannot set up all requested
410 dynamic, tunnel, local, and remote port forwardings.
411 The argument must be
412 .Dq yes
413 or
414 .Dq no .
415 The default is
416 .Dq no .
417 .It Cm ForwardAgent
418 Specifies whether the connection to the authentication agent (if any)
419 will be forwarded to the remote machine.
420 The argument must be
421 .Dq yes
422 or
423 .Dq no .
424 The default is
425 .Dq no .
426 .Pp
427 Agent forwarding should be enabled with caution.
428 Users with the ability to bypass file permissions on the remote host
429 (for the agent's Unix-domain socket)
430 can access the local agent through the forwarded connection.
431 An attacker cannot obtain key material from the agent,
432 however they can perform operations on the keys that enable them to
433 authenticate using the identities loaded into the agent.
434 .It Cm ForwardX11
435 Specifies whether X11 connections will be automatically redirected
436 over the secure channel and
437 .Ev DISPLAY
438 set.
439 The argument must be
440 .Dq yes
441 or
442 .Dq no .
443 The default is
444 .Dq no .
445 .Pp
446 X11 forwarding should be enabled with caution.
447 Users with the ability to bypass file permissions on the remote host
448 (for the user's X11 authorization database)
449 can access the local X11 display through the forwarded connection.
450 An attacker may then be able to perform activities such as keystroke monitoring
451 if the
452 .Cm ForwardX11Trusted
453 option is also enabled.
454 .It Cm ForwardX11Timeout
455 Specify a timeout for untrusted X11 forwarding
456 using the format described in the
457 .Sx TIME FORMATS
458 section of
459 .Xr sshd_config 5 .
460 X11 connections received by
461 .Xr ssh 1
462 after this time will be refused.
463 The default is to disable untrusted X11 forwarding after twenty minutes has
464 elapsed.
465 .It Cm ForwardX11Trusted
466 If this option is set to
467 .Dq yes ,
468 remote X11 clients will have full access to the original X11 display.
469 .Pp
470 If this option is set to
471 .Dq no ,
472 remote X11 clients will be considered untrusted and prevented
473 from stealing or tampering with data belonging to trusted X11
474 clients.
475 Furthermore, the
476 .Xr xauth 1
477 token used for the session will be set to expire after 20 minutes.
478 Remote clients will be refused access after this time.
479 .Pp
480 The default is
481 .Dq no .
482 .Pp
483 See the X11 SECURITY extension specification for full details on
484 the restrictions imposed on untrusted clients.
485 .It Cm GatewayPorts
486 Specifies whether remote hosts are allowed to connect to local
487 forwarded ports.
488 By default,
489 .Xr ssh 1
490 binds local port forwardings to the loopback address.
491 This prevents other remote hosts from connecting to forwarded ports.
492 .Cm GatewayPorts
493 can be used to specify that ssh
494 should bind local port forwardings to the wildcard address,
495 thus allowing remote hosts to connect to forwarded ports.
496 The argument must be
497 .Dq yes
498 or
499 .Dq no .
500 The default is
501 .Dq no .
502 .It Cm GlobalKnownHostsFile
503 Specifies a file to use for the global
504 host key database instead of
505 .Pa /etc/ssh/ssh_known_hosts .
506 .It Cm GSSAPIAuthentication
507 Specifies whether user authentication based on GSSAPI is allowed.
508 The default is
509 .Dq no .
510 Note that this option applies to protocol version 2 only.
511 .It Cm GSSAPIDelegateCredentials
512 Forward (delegate) credentials to the server.
513 The default is
514 .Dq no .
515 Note that this option applies to protocol version 2 only.
516 .It Cm HashKnownHosts
517 Indicates that
518 .Xr ssh 1
519 should hash host names and addresses when they are added to
520 .Pa ~/.ssh/known_hosts .
521 These hashed names may be used normally by
522 .Xr ssh 1
523 and
524 .Xr sshd 8 ,
525 but they do not reveal identifying information should the file's contents
526 be disclosed.
527 The default is
528 .Dq no .
529 Note that existing names and addresses in known hosts files
530 will not be converted automatically,
531 but may be manually hashed using
532 .Xr ssh-keygen 1 .
533 .It Cm HostbasedAuthentication
534 Specifies whether to try rhosts based authentication with public key
535 authentication.
536 The argument must be
537 .Dq yes
538 or
539 .Dq no .
540 The default is
541 .Dq no .
542 This option applies to protocol version 2 only and
543 is similar to
544 .Cm RhostsRSAAuthentication .
545 .It Cm HostKeyAlgorithms
546 Specifies the protocol version 2 host key algorithms
547 that the client wants to use in order of preference.
548 The default for this option is:
549 .Bd -literal -offset 3n
550 ecdsa-sha2-nistp256-cert-v01@openssh.com,
551 ecdsa-sha2-nistp384-cert-v01@openssh.com,
552 ecdsa-sha2-nistp521-cert-v01@openssh.com,
553 ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
554 ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
555 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
556 ssh-rsa,ssh-dss
557 .Ed
558 .Pp
559 If hostkeys are known for the destination host then this default is modified
560 to prefer their algorithms.
561 .It Cm HostKeyAlias
562 Specifies an alias that should be used instead of the
563 real host name when looking up or saving the host key
564 in the host key database files.
565 This option is useful for tunneling SSH connections
566 or for multiple servers running on a single host.
567 .It Cm HostName
568 Specifies the real host name to log into.
569 This can be used to specify nicknames or abbreviations for hosts.
570 If the hostname contains the character sequence
571 .Ql %h ,
572 then this will be replaced with the host name specified on the commandline
573 (this is useful for manipulating unqualified names).
574 The default is the name given on the command line.
575 Numeric IP addresses are also permitted (both on the command line and in
576 .Cm HostName
577 specifications).
578 .It Cm IdentitiesOnly
579 Specifies that
580 .Xr ssh 1
581 should only use the authentication identity files configured in the
582 .Nm
583 files,
584 even if
585 .Xr ssh-agent 1
586 offers more identities.
587 The argument to this keyword must be
588 .Dq yes
589 or
590 .Dq no .
591 This option is intended for situations where ssh-agent
592 offers many different identities.
593 The default is
594 .Dq no .
595 .It Cm IdentityFile
596 Specifies a file from which the user's DSA, ECDSA or DSA authentication
597 identity is read.
598 The default is
599 .Pa ~/.ssh/identity
600 for protocol version 1, and
601 .Pa ~/.ssh/id_dsa ,
602 .Pa ~/.ssh/id_ecdsa
603 and
604 .Pa ~/.ssh/id_rsa
605 for protocol version 2.
606 Additionally, any identities represented by the authentication agent
607 will be used for authentication.
608 .Xr ssh 1
609 will try to load certificate information from the filename obtained by
610 appending
611 .Pa -cert.pub
612 to the path of a specified
613 .Cm IdentityFile .
614 .Pp
615 The file name may use the tilde
616 syntax to refer to a user's home directory or one of the following
617 escape characters:
618 .Ql %d
619 (local user's home directory),
620 .Ql %u
621 (local user name),
622 .Ql %l
623 (local host name),
624 .Ql %h
625 (remote host name) or
626 .Ql %r
627 (remote user name).
628 .Pp
629 It is possible to have
630 multiple identity files specified in configuration files; all these
631 identities will be tried in sequence.
632 .It Cm IPQoS
633 Specifies the IPv4 type-of-service or DSCP class for connections.
634 Accepted values are
635 .Dq af11 ,
636 .Dq af12 ,
637 .Dq af13 ,
638 .Dq af14 ,
639 .Dq af22 ,
640 .Dq af23 ,
641 .Dq af31 ,
642 .Dq af32 ,
643 .Dq af33 ,
644 .Dq af41 ,
645 .Dq af42 ,
646 .Dq af43 ,
647 .Dq cs0 ,
648 .Dq cs1 ,
649 .Dq cs2 ,
650 .Dq cs3 ,
651 .Dq cs4 ,
652 .Dq cs5 ,
653 .Dq cs6 ,
654 .Dq cs7 ,
655 .Dq ef ,
656 .Dq lowdelay ,
657 .Dq throughput ,
658 .Dq reliability ,
659 or a numeric value.
660 This option may take one or two arguments, separated by whitespace.
661 If one argument is specified, it is used as the packet class unconditionally.
662 If two values are specified, the first is automatically selected for
663 interactive sessions and the second for non-interactive sessions.
664 The default is
665 .Dq lowdelay
666 for interactive sessions and
667 .Dq throughput
668 for non-interactive sessions.
669 .It Cm KbdInteractiveAuthentication
670 Specifies whether to use keyboard-interactive authentication.
671 The argument to this keyword must be
672 .Dq yes
673 or
674 .Dq no .
675 The default is
676 .Dq yes .
677 .It Cm KbdInteractiveDevices
678 Specifies the list of methods to use in keyboard-interactive authentication.
679 Multiple method names must be comma-separated.
680 The default is to use the server specified list.
681 The methods available vary depending on what the server supports.
682 For an OpenSSH server,
683 it may be zero or more of:
684 .Dq bsdauth ,
685 .Dq pam ,
686 and
687 .Dq skey .
688 .It Cm KexAlgorithms
689 Specifies the available KEX (Key Exchange) algorithms.
690 Multiple algorithms must be comma-separated.
691 The default is:
692 .Bd -literal -offset indent
693 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
694 diffie-hellman-group-exchange-sha256,
695 diffie-hellman-group-exchange-sha1,
696 diffie-hellman-group14-sha1,
697 diffie-hellman-group1-sha1
698 .Ed
699 .It Cm LocalCommand
700 Specifies a command to execute on the local machine after successfully
701 connecting to the server.
702 The command string extends to the end of the line, and is executed with
703 the user's shell.
704 The following escape character substitutions will be performed:
705 .Ql %d
706 (local user's home directory),
707 .Ql %h
708 (remote host name),
709 .Ql %l
710 (local host name),
711 .Ql %n
712 (host name as provided on the command line),
713 .Ql %p
714 (remote port),
715 .Ql %r
716 (remote user name) or
717 .Ql %u
718 (local user name).
719 .Pp
720 The command is run synchronously and does not have access to the
721 session of the
722 .Xr ssh 1
723 that spawned it.
724 It should not be used for interactive commands.
725 .Pp
726 This directive is ignored unless
727 .Cm PermitLocalCommand
728 has been enabled.
729 .It Cm LocalForward
730 Specifies that a TCP port on the local machine be forwarded over
731 the secure channel to the specified host and port from the remote machine.
732 The first argument must be
733 .Sm off
734 .Oo Ar bind_address : Oc Ar port
735 .Sm on
736 and the second argument must be
737 .Ar host : Ns Ar hostport .
738 IPv6 addresses can be specified by enclosing addresses in square brackets.
739 Multiple forwardings may be specified, and additional forwardings can be
740 given on the command line.
741 Only the superuser can forward privileged ports.
742 By default, the local port is bound in accordance with the
743 .Cm GatewayPorts
744 setting.
745 However, an explicit
746 .Ar bind_address
747 may be used to bind the connection to a specific address.
748 The
749 .Ar bind_address
750 of
751 .Dq localhost
752 indicates that the listening port be bound for local use only, while an
753 empty address or
754 .Sq *
755 indicates that the port should be available from all interfaces.
756 .It Cm LogLevel
757 Gives the verbosity level that is used when logging messages from
758 .Xr ssh 1 .
759 The possible values are:
760 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
761 The default is INFO.
762 DEBUG and DEBUG1 are equivalent.
763 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
764 .It Cm MACs
765 Specifies the MAC (message authentication code) algorithms
766 in order of preference.
767 The MAC algorithm is used in protocol version 2
768 for data integrity protection.
769 Multiple algorithms must be comma-separated.
770 The default is:
771 .Bd -literal -offset indent
772 hmac-md5,hmac-sha1,umac-64@openssh.com,
773 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
774 .Ed
775 .It Cm NoHostAuthenticationForLocalhost
776 This option can be used if the home directory is shared across machines.
777 In this case localhost will refer to a different machine on each of
778 the machines and the user will get many warnings about changed host keys.
779 However, this option disables host authentication for localhost.
780 The argument to this keyword must be
781 .Dq yes
782 or
783 .Dq no .
784 The default is to check the host key for localhost.
785 .It Cm NumberOfPasswordPrompts
786 Specifies the number of password prompts before giving up.
787 The argument to this keyword must be an integer.
788 The default is 3.
789 .It Cm PasswordAuthentication
790 Specifies whether to use password authentication.
791 The argument to this keyword must be
792 .Dq yes
793 or
794 .Dq no .
795 The default is
796 .Dq yes .
797 .It Cm PermitLocalCommand
798 Allow local command execution via the
799 .Ic LocalCommand
800 option or using the
801 .Ic !\& Ns Ar command
802 escape sequence in
803 .Xr ssh 1 .
804 The argument must be
805 .Dq yes
806 or
807 .Dq no .
808 The default is
809 .Dq no .
810 .It Cm PKCS11Provider
811 Specifies which PKCS#11 provider to use.
812 The argument to this keyword is the PKCS#11 shared library
813 .Xr ssh 1
814 should use to communicate with a PKCS#11 token providing the user's
815 private RSA key.
816 .It Cm Port
817 Specifies the port number to connect on the remote host.
818 The default is 22.
819 .It Cm PreferredAuthentications
820 Specifies the order in which the client should try protocol 2
821 authentication methods.
822 This allows a client to prefer one method (e.g.\&
823 .Cm keyboard-interactive )
824 over another method (e.g.\&
825 .Cm password ) .
826 The default is:
827 .Bd -literal -offset indent
828 gssapi-with-mic,hostbased,publickey,
829 keyboard-interactive,password
830 .Ed
831 .It Cm Protocol
832 Specifies the protocol versions
833 .Xr ssh 1
834 should support in order of preference.
835 The possible values are
836 .Sq 1
837 and
838 .Sq 2 .
839 Multiple versions must be comma-separated.
840 When this option is set to
841 .Dq 2,1
842 .Nm ssh
843 will try version 2 and fall back to version 1
844 if version 2 is not available.
845 The default is
846 .Sq 2 .
847 .It Cm ProxyCommand
848 Specifies the command to use to connect to the server.
849 The command
850 string extends to the end of the line, and is executed with
851 the user's shell.
852 In the command string, any occurrence of
853 .Ql %h
854 will be substituted by the host name to
855 connect,
856 .Ql %p
857 by the port, and
858 .Ql %r
859 by the remote user name.
860 The command can be basically anything,
861 and should read from its standard input and write to its standard output.
862 It should eventually connect an
863 .Xr sshd 8
864 server running on some machine, or execute
865 .Ic sshd -i
866 somewhere.
867 Host key management will be done using the
868 HostName of the host being connected (defaulting to the name typed by
869 the user).
870 Setting the command to
871 .Dq none
872 disables this option entirely.
873 Note that
874 .Cm CheckHostIP
875 is not available for connects with a proxy command.
876 .Pp
877 This directive is useful in conjunction with
878 .Xr nc 1
879 and its proxy support.
880 For example, the following directive would connect via an HTTP proxy at
881 192.0.2.0:
882 .Bd -literal -offset 3n
883 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
884 .Ed
885 .It Cm PubkeyAuthentication
886 Specifies whether to try public key authentication.
887 The argument to this keyword must be
888 .Dq yes
889 or
890 .Dq no .
891 The default is
892 .Dq yes .
893 This option applies to protocol version 2 only.
894 .It Cm RekeyLimit
895 Specifies the maximum amount of data that may be transmitted before the
896 session key is renegotiated.
897 The argument is the number of bytes, with an optional suffix of
898 .Sq K ,
899 .Sq M ,
900 or
901 .Sq G
902 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
903 The default is between
904 .Sq 1G
905 and
906 .Sq 4G ,
907 depending on the cipher.
908 This option applies to protocol version 2 only.
909 .It Cm RemoteForward
910 Specifies that a TCP port on the remote machine be forwarded over
911 the secure channel to the specified host and port from the local machine.
912 The first argument must be
913 .Sm off
914 .Oo Ar bind_address : Oc Ar port
915 .Sm on
916 and the second argument must be
917 .Ar host : Ns Ar hostport .
918 IPv6 addresses can be specified by enclosing addresses in square brackets.
919 Multiple forwardings may be specified, and additional
920 forwardings can be given on the command line.
921 Privileged ports can be forwarded only when
922 logging in as root on the remote machine.
923 .Pp
924 If the
925 .Ar port
926 argument is
927 .Ql 0 ,
928 the listen port will be dynamically allocated on the server and reported
929 to the client at run time.
930 .Pp
931 If the
932 .Ar bind_address
933 is not specified, the default is to only bind to loopback addresses.
934 If the
935 .Ar bind_address
936 is
937 .Ql *
938 or an empty string, then the forwarding is requested to listen on all
939 interfaces.
940 Specifying a remote
941 .Ar bind_address
942 will only succeed if the server's
943 .Cm GatewayPorts
944 option is enabled (see
945 .Xr sshd_config 5 ) .
946 .It Cm RhostsRSAAuthentication
947 Specifies whether to try rhosts based authentication with RSA host
948 authentication.
949 The argument must be
950 .Dq yes
951 or
952 .Dq no .
953 The default is
954 .Dq no .
955 This option applies to protocol version 1 only and requires
956 .Xr ssh 1
957 to be setuid root.
958 .It Cm RSAAuthentication
959 Specifies whether to try RSA authentication.
960 The argument to this keyword must be
961 .Dq yes
962 or
963 .Dq no .
964 RSA authentication will only be
965 attempted if the identity file exists, or an authentication agent is
966 running.
967 The default is
968 .Dq yes .
969 Note that this option applies to protocol version 1 only.
970 .It Cm SendEnv
971 Specifies what variables from the local
972 .Xr environ 7
973 should be sent to the server.
974 Note that environment passing is only supported for protocol 2.
975 The server must also support it, and the server must be configured to
976 accept these environment variables.
977 Refer to
978 .Cm AcceptEnv
979 in
980 .Xr sshd_config 5
981 for how to configure the server.
982 Variables are specified by name, which may contain wildcard characters.
983 Multiple environment variables may be separated by whitespace or spread
984 across multiple
985 .Cm SendEnv
986 directives.
987 The default is not to send any environment variables.
988 .Pp
989 See
990 .Sx PATTERNS
991 for more information on patterns.
992 .It Cm ServerAliveCountMax
993 Sets the number of server alive messages (see below) which may be
994 sent without
995 .Xr ssh 1
996 receiving any messages back from the server.
997 If this threshold is reached while server alive messages are being sent,
998 ssh will disconnect from the server, terminating the session.
999 It is important to note that the use of server alive messages is very
1000 different from
1001 .Cm TCPKeepAlive
1002 (below).
1003 The server alive messages are sent through the encrypted channel
1004 and therefore will not be spoofable.
1005 The TCP keepalive option enabled by
1006 .Cm TCPKeepAlive
1007 is spoofable.
1008 The server alive mechanism is valuable when the client or
1009 server depend on knowing when a connection has become inactive.
1010 .Pp
1011 The default value is 3.
1012 If, for example,
1013 .Cm ServerAliveInterval
1014 (see below) is set to 15 and
1015 .Cm ServerAliveCountMax
1016 is left at the default, if the server becomes unresponsive,
1017 ssh will disconnect after approximately 45 seconds.
1018 This option applies to protocol version 2 only.
1019 .It Cm ServerAliveInterval
1020 Sets a timeout interval in seconds after which if no data has been received
1021 from the server,
1022 .Xr ssh 1
1023 will send a message through the encrypted
1024 channel to request a response from the server.
1025 The default
1026 is 0, indicating that these messages will not be sent to the server.
1027 This option applies to protocol version 2 only.
1028 .It Cm StrictHostKeyChecking
1029 If this flag is set to
1030 .Dq yes ,
1031 .Xr ssh 1
1032 will never automatically add host keys to the
1033 .Pa ~/.ssh/known_hosts
1034 file, and refuses to connect to hosts whose host key has changed.
1035 This provides maximum protection against trojan horse attacks,
1036 though it can be annoying when the
1037 .Pa /etc/ssh/ssh_known_hosts
1038 file is poorly maintained or when connections to new hosts are
1039 frequently made.
1040 This option forces the user to manually
1041 add all new hosts.
1042 If this flag is set to
1043 .Dq no ,
1044 ssh will automatically add new host keys to the
1045 user known hosts files.
1046 If this flag is set to
1047 .Dq ask ,
1048 new host keys
1049 will be added to the user known host files only after the user
1050 has confirmed that is what they really want to do, and
1051 ssh will refuse to connect to hosts whose host key has changed.
1052 The host keys of
1053 known hosts will be verified automatically in all cases.
1054 The argument must be
1055 .Dq yes ,
1056 .Dq no ,
1057 or
1058 .Dq ask .
1059 The default is
1060 .Dq ask .
1061 .It Cm TCPKeepAlive
1062 Specifies whether the system should send TCP keepalive messages to the
1063 other side.
1064 If they are sent, death of the connection or crash of one
1065 of the machines will be properly noticed.
1066 However, this means that
1067 connections will die if the route is down temporarily, and some people
1068 find it annoying.
1069 .Pp
1070 The default is
1071 .Dq yes
1072 (to send TCP keepalive messages), and the client will notice
1073 if the network goes down or the remote host dies.
1074 This is important in scripts, and many users want it too.
1075 .Pp
1076 To disable TCP keepalive messages, the value should be set to
1077 .Dq no .
1078 .It Cm Tunnel
1079 Request
1080 .Xr tun 4
1081 device forwarding between the client and the server.
1082 The argument must be
1083 .Dq yes ,
1084 .Dq point-to-point
1085 (layer 3),
1086 .Dq ethernet
1087 (layer 2),
1088 or
1089 .Dq no .
1090 Specifying
1091 .Dq yes
1092 requests the default tunnel mode, which is
1093 .Dq point-to-point .
1094 The default is
1095 .Dq no .
1096 .It Cm TunnelDevice
1097 Specifies the
1098 .Xr tun 4
1099 devices to open on the client
1100 .Pq Ar local_tun
1101 and the server
1102 .Pq Ar remote_tun .
1103 .Pp
1104 The argument must be
1105 .Sm off
1106 .Ar local_tun Op : Ar remote_tun .
1107 .Sm on
1108 The devices may be specified by numerical ID or the keyword
1109 .Dq any ,
1110 which uses the next available tunnel device.
1111 If
1112 .Ar remote_tun
1113 is not specified, it defaults to
1114 .Dq any .
1115 The default is
1116 .Dq any:any .
1117 .It Cm UsePrivilegedPort
1118 Specifies whether to use a privileged port for outgoing connections.
1119 The argument must be
1120 .Dq yes
1121 or
1122 .Dq no .
1123 The default is
1124 .Dq no .
1125 If set to
1126 .Dq yes ,
1127 .Xr ssh 1
1128 must be setuid root.
1129 Note that this option must be set to
1130 .Dq yes
1131 for
1132 .Cm RhostsRSAAuthentication
1133 with older servers.
1134 .It Cm User
1135 Specifies the user to log in as.
1136 This can be useful when a different user name is used on different machines.
1137 This saves the trouble of
1138 having to remember to give the user name on the command line.
1139 .It Cm UserKnownHostsFile
1140 Specifies a file to use for the user
1141 host key database instead of
1142 .Pa ~/.ssh/known_hosts .
1143 .It Cm VerifyHostKeyDNS
1144 Specifies whether to verify the remote key using DNS and SSHFP resource
1145 records.
1146 If this option is set to
1147 .Dq yes ,
1148 the client will implicitly trust keys that match a secure fingerprint
1149 from DNS.
1150 Insecure fingerprints will be handled as if this option was set to
1151 .Dq ask .
1152 If this option is set to
1153 .Dq ask ,
1154 information on fingerprint match will be displayed, but the user will still
1155 need to confirm new host keys according to the
1156 .Cm StrictHostKeyChecking
1157 option.
1158 The argument must be
1159 .Dq yes ,
1160 .Dq no ,
1161 or
1162 .Dq ask .
1163 The default is
1164 .Dq no .
1165 Note that this option applies to protocol version 2 only.
1166 .Pp
1167 See also
1168 .Sx VERIFYING HOST KEYS
1169 in
1170 .Xr ssh 1 .
1171 .It Cm VisualHostKey
1172 If this flag is set to
1173 .Dq yes ,
1174 an ASCII art representation of the remote host key fingerprint is
1175 printed in addition to the hex fingerprint string at login and
1176 for unknown host keys.
1177 If this flag is set to
1178 .Dq no ,
1179 no fingerprint strings are printed at login and
1180 only the hex fingerprint string will be printed for unknown host keys.
1181 The default is
1182 .Dq no .
1183 .It Cm XAuthLocation
1184 Specifies the full pathname of the
1185 .Xr xauth 1
1186 program.
1187 The default is
1188 .Pa /usr/X11R6/bin/xauth .
1189 .El
1190 .Sh PATTERNS
1191 A
1192 .Em pattern
1193 consists of zero or more non-whitespace characters,
1194 .Sq *
1195 (a wildcard that matches zero or more characters),
1196 or
1197 .Sq ?\&
1198 (a wildcard that matches exactly one character).
1199 For example, to specify a set of declarations for any host in the
1200 .Dq .co.uk
1201 set of domains,
1202 the following pattern could be used:
1203 .Pp
1204 .Dl Host *.co.uk
1205 .Pp
1206 The following pattern
1207 would match any host in the 192.168.0.[0-9] network range:
1208 .Pp
1209 .Dl Host 192.168.0.?
1210 .Pp
1211 A
1212 .Em pattern-list
1213 is a comma-separated list of patterns.
1214 Patterns within pattern-lists may be negated
1215 by preceding them with an exclamation mark
1216 .Pq Sq !\& .
1217 For example,
1218 to allow a key to be used from anywhere within an organisation
1219 except from the
1220 .Dq dialup
1221 pool,
1222 the following entry (in authorized_keys) could be used:
1223 .Pp
1224 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1225 .Sh FILES
1226 .Bl -tag -width Ds
1227 .It Pa ~/.ssh/config
1228 This is the per-user configuration file.
1229 The format of this file is described above.
1230 This file is used by the SSH client.
1231 Because of the potential for abuse, this file must have strict permissions:
1232 read/write for the user, and not accessible by others.
1233 .It Pa /etc/ssh/ssh_config
1234 Systemwide configuration file.
1235 This file provides defaults for those
1236 values that are not specified in the user's configuration file, and
1237 for those users who do not have a configuration file.
1238 This file must be world-readable.
1239 .El
1240 .Sh SEE ALSO
1241 .Xr ssh 1
1242 .Sh AUTHORS
1243 OpenSSH is a derivative of the original and free
1244 ssh 1.2.12 release by Tatu Ylonen.
1245 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1246 Theo de Raadt and Dug Song
1247 removed many bugs, re-added newer features and
1248 created OpenSSH.
1249 Markus Friedl contributed the support for SSH
1250 protocol versions 1.5 and 2.0.