| blob | dd39bfafbc371a150b9510f01174fd28ec4824a6 |
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 1.142 2010/10/28 18:33:28 jmc Exp $
37 .Dd $Mdocdate: October 28 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 .It Cm HostKeyAlias
559 Specifies an alias that should be used instead of the
560 real host name when looking up or saving the host key
561 in the host key database files.
562 This option is useful for tunneling SSH connections
563 or for multiple servers running on a single host.
564 .It Cm HostName
565 Specifies the real host name to log into.
566 This can be used to specify nicknames or abbreviations for hosts.
567 If the hostname contains the character sequence
568 .Ql %h ,
569 then this will be replaced with the host name specified on the commandline
570 (this is useful for manipulating unqualified names).
571 The default is the name given on the command line.
572 Numeric IP addresses are also permitted (both on the command line and in
573 .Cm HostName
574 specifications).
575 .It Cm IdentitiesOnly
576 Specifies that
577 .Xr ssh 1
578 should only use the authentication identity files configured in the
579 .Nm
580 files,
581 even if
582 .Xr ssh-agent 1
583 offers more identities.
584 The argument to this keyword must be
585 .Dq yes
586 or
587 .Dq no .
588 This option is intended for situations where ssh-agent
589 offers many different identities.
590 The default is
591 .Dq no .
592 .It Cm IdentityFile
593 Specifies a file from which the user's DSA, ECDSA or DSA authentication
594 identity is read.
595 The default is
596 .Pa ~/.ssh/identity
597 for protocol version 1, and
598 .Pa ~/.ssh/id_dsa ,
599 .Pa ~/.ssh/id_ecdsa
600 and
601 .Pa ~/.ssh/id_rsa
602 for protocol version 2.
603 Additionally, any identities represented by the authentication agent
604 will be used for authentication.
605 .Xr ssh 1
606 will try to load certificate information from the filename obtained by
607 appending
608 .Pa -cert.pub
609 to the path of a specified
610 .Cm IdentityFile .
611 .Pp
612 The file name may use the tilde
613 syntax to refer to a user's home directory or one of the following
614 escape characters:
615 .Ql %d
616 (local user's home directory),
617 .Ql %u
618 (local user name),
619 .Ql %l
620 (local host name),
621 .Ql %h
622 (remote host name) or
623 .Ql %r
624 (remote user name).
625 .Pp
626 It is possible to have
627 multiple identity files specified in configuration files; all these
628 identities will be tried in sequence.
629 .It Cm KbdInteractiveAuthentication
630 Specifies whether to use keyboard-interactive authentication.
631 The argument to this keyword must be
632 .Dq yes
633 or
634 .Dq no .
635 The default is
636 .Dq yes .
637 .It Cm KbdInteractiveDevices
638 Specifies the list of methods to use in keyboard-interactive authentication.
639 Multiple method names must be comma-separated.
640 The default is to use the server specified list.
641 The methods available vary depending on what the server supports.
642 For an OpenSSH server,
643 it may be zero or more of:
644 .Dq bsdauth ,
645 .Dq pam ,
646 and
647 .Dq skey .
648 .It Cm KexAlgorithms
649 Specifies the available KEX (Key Exchange) algorithms.
650 Multiple algorithms must be comma-separated.
651 The default is:
652 .Bd -literal -offset indent
653 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
654 diffie-hellman-group-exchange-sha256,
655 diffie-hellman-group-exchange-sha1,
656 diffie-hellman-group14-sha1,
657 diffie-hellman-group1-sha1
658 .Ed
659 .It Cm LocalCommand
660 Specifies a command to execute on the local machine after successfully
661 connecting to the server.
662 The command string extends to the end of the line, and is executed with
663 the user's shell.
664 The following escape character substitutions will be performed:
665 .Ql %d
666 (local user's home directory),
667 .Ql %h
668 (remote host name),
669 .Ql %l
670 (local host name),
671 .Ql %n
672 (host name as provided on the command line),
673 .Ql %p
674 (remote port),
675 .Ql %r
676 (remote user name) or
677 .Ql %u
678 (local user name).
679 .Pp
680 The command is run synchronously and does not have access to the
681 session of the
682 .Xr ssh 1
683 that spawned it.
684 It should not be used for interactive commands.
685 .Pp
686 This directive is ignored unless
687 .Cm PermitLocalCommand
688 has been enabled.
689 .It Cm LocalForward
690 Specifies that a TCP port on the local machine be forwarded over
691 the secure channel to the specified host and port from the remote machine.
692 The first argument must be
693 .Sm off
694 .Oo Ar bind_address : Oc Ar port
695 .Sm on
696 and the second argument must be
697 .Ar host : Ns Ar hostport .
698 IPv6 addresses can be specified by enclosing addresses in square brackets.
699 Multiple forwardings may be specified, and additional forwardings can be
700 given on the command line.
701 Only the superuser can forward privileged ports.
702 By default, the local port is bound in accordance with the
703 .Cm GatewayPorts
704 setting.
705 However, an explicit
706 .Ar bind_address
707 may be used to bind the connection to a specific address.
708 The
709 .Ar bind_address
710 of
711 .Dq localhost
712 indicates that the listening port be bound for local use only, while an
713 empty address or
714 .Sq *
715 indicates that the port should be available from all interfaces.
716 .It Cm LogLevel
717 Gives the verbosity level that is used when logging messages from
718 .Xr ssh 1 .
719 The possible values are:
720 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
721 The default is INFO.
722 DEBUG and DEBUG1 are equivalent.
723 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
724 .It Cm MACs
725 Specifies the MAC (message authentication code) algorithms
726 in order of preference.
727 The MAC algorithm is used in protocol version 2
728 for data integrity protection.
729 Multiple algorithms must be comma-separated.
730 The default is:
731 .Bd -literal -offset indent
732 hmac-md5,hmac-sha1,umac-64@openssh.com,
733 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
734 .Ed
735 .It Cm NoHostAuthenticationForLocalhost
736 This option can be used if the home directory is shared across machines.
737 In this case localhost will refer to a different machine on each of
738 the machines and the user will get many warnings about changed host keys.
739 However, this option disables host authentication for localhost.
740 The argument to this keyword must be
741 .Dq yes
742 or
743 .Dq no .
744 The default is to check the host key for localhost.
745 .It Cm NumberOfPasswordPrompts
746 Specifies the number of password prompts before giving up.
747 The argument to this keyword must be an integer.
748 The default is 3.
749 .It Cm PasswordAuthentication
750 Specifies whether to use password authentication.
751 The argument to this keyword must be
752 .Dq yes
753 or
754 .Dq no .
755 The default is
756 .Dq yes .
757 .It Cm PermitLocalCommand
758 Allow local command execution via the
759 .Ic LocalCommand
760 option or using the
761 .Ic !\& Ns Ar command
762 escape sequence in
763 .Xr ssh 1 .
764 The argument must be
765 .Dq yes
766 or
767 .Dq no .
768 The default is
769 .Dq no .
770 .It Cm PKCS11Provider
771 Specifies which PKCS#11 provider to use.
772 The argument to this keyword is the PKCS#11 shared libary
773 .Xr ssh 1
774 should use to communicate with a PKCS#11 token providing the user's
775 private RSA key.
776 .It Cm Port
777 Specifies the port number to connect on the remote host.
778 The default is 22.
779 .It Cm PreferredAuthentications
780 Specifies the order in which the client should try protocol 2
781 authentication methods.
782 This allows a client to prefer one method (e.g.\&
783 .Cm keyboard-interactive )
784 over another method (e.g.\&
785 .Cm password ) .
786 The default is:
787 .Bd -literal -offset indent
788 gssapi-with-mic,hostbased,publickey,
789 keyboard-interactive,password
790 .Ed
791 .It Cm Protocol
792 Specifies the protocol versions
793 .Xr ssh 1
794 should support in order of preference.
795 The possible values are
796 .Sq 1
797 and
798 .Sq 2 .
799 Multiple versions must be comma-separated.
800 When this option is set to
801 .Dq 2,1
802 .Nm ssh
803 will try version 2 and fall back to version 1
804 if version 2 is not available.
805 The default is
806 .Sq 2 .
807 .It Cm ProxyCommand
808 Specifies the command to use to connect to the server.
809 The command
810 string extends to the end of the line, and is executed with
811 the user's shell.
812 In the command string, any occurrence of
813 .Ql %h
814 will be substituted by the host name to
815 connect,
816 .Ql %p
817 by the port, and
818 .Ql %r
819 by the remote user name.
820 The command can be basically anything,
821 and should read from its standard input and write to its standard output.
822 It should eventually connect an
823 .Xr sshd 8
824 server running on some machine, or execute
825 .Ic sshd -i
826 somewhere.
827 Host key management will be done using the
828 HostName of the host being connected (defaulting to the name typed by
829 the user).
830 Setting the command to
831 .Dq none
832 disables this option entirely.
833 Note that
834 .Cm CheckHostIP
835 is not available for connects with a proxy command.
836 .Pp
837 This directive is useful in conjunction with
838 .Xr nc 1
839 and its proxy support.
840 For example, the following directive would connect via an HTTP proxy at
841 192.0.2.0:
842 .Bd -literal -offset 3n
843 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
844 .Ed
845 .It Cm PubkeyAuthentication
846 Specifies whether to try public key authentication.
847 The argument to this keyword must be
848 .Dq yes
849 or
850 .Dq no .
851 The default is
852 .Dq yes .
853 This option applies to protocol version 2 only.
854 .It Cm RekeyLimit
855 Specifies the maximum amount of data that may be transmitted before the
856 session key is renegotiated.
857 The argument is the number of bytes, with an optional suffix of
858 .Sq K ,
859 .Sq M ,
860 or
861 .Sq G
862 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
863 The default is between
864 .Sq 1G
865 and
866 .Sq 4G ,
867 depending on the cipher.
868 This option applies to protocol version 2 only.
869 .It Cm RemoteForward
870 Specifies that a TCP port on the remote machine be forwarded over
871 the secure channel to the specified host and port from the local machine.
872 The first argument must be
873 .Sm off
874 .Oo Ar bind_address : Oc Ar port
875 .Sm on
876 and the second argument must be
877 .Ar host : Ns Ar hostport .
878 IPv6 addresses can be specified by enclosing addresses in square brackets.
879 Multiple forwardings may be specified, and additional
880 forwardings can be given on the command line.
881 Privileged ports can be forwarded only when
882 logging in as root on the remote machine.
883 .Pp
884 If the
885 .Ar port
886 argument is
887 .Ql 0 ,
888 the listen port will be dynamically allocated on the server and reported
889 to the client at run time.
890 .Pp
891 If the
892 .Ar bind_address
893 is not specified, the default is to only bind to loopback addresses.
894 If the
895 .Ar bind_address
896 is
897 .Ql *
898 or an empty string, then the forwarding is requested to listen on all
899 interfaces.
900 Specifying a remote
901 .Ar bind_address
902 will only succeed if the server's
903 .Cm GatewayPorts
904 option is enabled (see
905 .Xr sshd_config 5 ) .
906 .It Cm RhostsRSAAuthentication
907 Specifies whether to try rhosts based authentication with RSA host
908 authentication.
909 The argument must be
910 .Dq yes
911 or
912 .Dq no .
913 The default is
914 .Dq no .
915 This option applies to protocol version 1 only and requires
916 .Xr ssh 1
917 to be setuid root.
918 .It Cm RSAAuthentication
919 Specifies whether to try RSA authentication.
920 The argument to this keyword must be
921 .Dq yes
922 or
923 .Dq no .
924 RSA authentication will only be
925 attempted if the identity file exists, or an authentication agent is
926 running.
927 The default is
928 .Dq yes .
929 Note that this option applies to protocol version 1 only.
930 .It Cm SendEnv
931 Specifies what variables from the local
932 .Xr environ 7
933 should be sent to the server.
934 Note that environment passing is only supported for protocol 2.
935 The server must also support it, and the server must be configured to
936 accept these environment variables.
937 Refer to
938 .Cm AcceptEnv
939 in
940 .Xr sshd_config 5
941 for how to configure the server.
942 Variables are specified by name, which may contain wildcard characters.
943 Multiple environment variables may be separated by whitespace or spread
944 across multiple
945 .Cm SendEnv
946 directives.
947 The default is not to send any environment variables.
948 .Pp
949 See
950 .Sx PATTERNS
951 for more information on patterns.
952 .It Cm ServerAliveCountMax
953 Sets the number of server alive messages (see below) which may be
954 sent without
955 .Xr ssh 1
956 receiving any messages back from the server.
957 If this threshold is reached while server alive messages are being sent,
958 ssh will disconnect from the server, terminating the session.
959 It is important to note that the use of server alive messages is very
960 different from
961 .Cm TCPKeepAlive
962 (below).
963 The server alive messages are sent through the encrypted channel
964 and therefore will not be spoofable.
965 The TCP keepalive option enabled by
966 .Cm TCPKeepAlive
967 is spoofable.
968 The server alive mechanism is valuable when the client or
969 server depend on knowing when a connection has become inactive.
970 .Pp
971 The default value is 3.
972 If, for example,
973 .Cm ServerAliveInterval
974 (see below) is set to 15 and
975 .Cm ServerAliveCountMax
976 is left at the default, if the server becomes unresponsive,
977 ssh will disconnect after approximately 45 seconds.
978 This option applies to protocol version 2 only.
979 .It Cm ServerAliveInterval
980 Sets a timeout interval in seconds after which if no data has been received
981 from the server,
982 .Xr ssh 1
983 will send a message through the encrypted
984 channel to request a response from the server.
985 The default
986 is 0, indicating that these messages will not be sent to the server.
987 This option applies to protocol version 2 only.
988 .It Cm StrictHostKeyChecking
989 If this flag is set to
990 .Dq yes ,
991 .Xr ssh 1
992 will never automatically add host keys to the
993 .Pa ~/.ssh/known_hosts
994 file, and refuses to connect to hosts whose host key has changed.
995 This provides maximum protection against trojan horse attacks,
996 though it can be annoying when the
997 .Pa /etc/ssh/ssh_known_hosts
998 file is poorly maintained or when connections to new hosts are
999 frequently made.
1000 This option forces the user to manually
1001 add all new hosts.
1002 If this flag is set to
1003 .Dq no ,
1004 ssh will automatically add new host keys to the
1005 user known hosts files.
1006 If this flag is set to
1007 .Dq ask ,
1008 new host keys
1009 will be added to the user known host files only after the user
1010 has confirmed that is what they really want to do, and
1011 ssh will refuse to connect to hosts whose host key has changed.
1012 The host keys of
1013 known hosts will be verified automatically in all cases.
1014 The argument must be
1015 .Dq yes ,
1016 .Dq no ,
1017 or
1018 .Dq ask .
1019 The default is
1020 .Dq ask .
1021 .It Cm TCPKeepAlive
1022 Specifies whether the system should send TCP keepalive messages to the
1023 other side.
1024 If they are sent, death of the connection or crash of one
1025 of the machines will be properly noticed.
1026 However, this means that
1027 connections will die if the route is down temporarily, and some people
1028 find it annoying.
1029 .Pp
1030 The default is
1031 .Dq yes
1032 (to send TCP keepalive messages), and the client will notice
1033 if the network goes down or the remote host dies.
1034 This is important in scripts, and many users want it too.
1035 .Pp
1036 To disable TCP keepalive messages, the value should be set to
1037 .Dq no .
1038 .It Cm Tunnel
1039 Request
1040 .Xr tun 4
1041 device forwarding between the client and the server.
1042 The argument must be
1043 .Dq yes ,
1044 .Dq point-to-point
1045 (layer 3),
1046 .Dq ethernet
1047 (layer 2),
1048 or
1049 .Dq no .
1050 Specifying
1051 .Dq yes
1052 requests the default tunnel mode, which is
1053 .Dq point-to-point .
1054 The default is
1055 .Dq no .
1056 .It Cm TunnelDevice
1057 Specifies the
1058 .Xr tun 4
1059 devices to open on the client
1060 .Pq Ar local_tun
1061 and the server
1062 .Pq Ar remote_tun .
1063 .Pp
1064 The argument must be
1065 .Sm off
1066 .Ar local_tun Op : Ar remote_tun .
1067 .Sm on
1068 The devices may be specified by numerical ID or the keyword
1069 .Dq any ,
1070 which uses the next available tunnel device.
1071 If
1072 .Ar remote_tun
1073 is not specified, it defaults to
1074 .Dq any .
1075 The default is
1076 .Dq any:any .
1077 .It Cm UsePrivilegedPort
1078 Specifies whether to use a privileged port for outgoing connections.
1079 The argument must be
1080 .Dq yes
1081 or
1082 .Dq no .
1083 The default is
1084 .Dq no .
1085 If set to
1086 .Dq yes ,
1087 .Xr ssh 1
1088 must be setuid root.
1089 Note that this option must be set to
1090 .Dq yes
1091 for
1092 .Cm RhostsRSAAuthentication
1093 with older servers.
1094 .It Cm User
1095 Specifies the user to log in as.
1096 This can be useful when a different user name is used on different machines.
1097 This saves the trouble of
1098 having to remember to give the user name on the command line.
1099 .It Cm UserKnownHostsFile
1100 Specifies a file to use for the user
1101 host key database instead of
1102 .Pa ~/.ssh/known_hosts .
1103 .It Cm VerifyHostKeyDNS
1104 Specifies whether to verify the remote key using DNS and SSHFP resource
1105 records.
1106 If this option is set to
1107 .Dq yes ,
1108 the client will implicitly trust keys that match a secure fingerprint
1109 from DNS.
1110 Insecure fingerprints will be handled as if this option was set to
1111 .Dq ask .
1112 If this option is set to
1113 .Dq ask ,
1114 information on fingerprint match will be displayed, but the user will still
1115 need to confirm new host keys according to the
1116 .Cm StrictHostKeyChecking
1117 option.
1118 The argument must be
1119 .Dq yes ,
1120 .Dq no ,
1121 or
1122 .Dq ask .
1123 The default is
1124 .Dq no .
1125 Note that this option applies to protocol version 2 only.
1126 .Pp
1127 See also
1128 .Sx VERIFYING HOST KEYS
1129 in
1130 .Xr ssh 1 .
1131 .It Cm VisualHostKey
1132 If this flag is set to
1133 .Dq yes ,
1134 an ASCII art representation of the remote host key fingerprint is
1135 printed in addition to the hex fingerprint string at login and
1136 for unknown host keys.
1137 If this flag is set to
1138 .Dq no ,
1139 no fingerprint strings are printed at login and
1140 only the hex fingerprint string will be printed for unknown host keys.
1141 The default is
1142 .Dq no .
1143 .It Cm XAuthLocation
1144 Specifies the full pathname of the
1145 .Xr xauth 1
1146 program.
1147 The default is
1148 .Pa /usr/X11R6/bin/xauth .
1149 .El
1150 .Sh PATTERNS
1151 A
1152 .Em pattern
1153 consists of zero or more non-whitespace characters,
1154 .Sq *
1155 (a wildcard that matches zero or more characters),
1156 or
1157 .Sq ?\&
1158 (a wildcard that matches exactly one character).
1159 For example, to specify a set of declarations for any host in the
1160 .Dq .co.uk
1161 set of domains,
1162 the following pattern could be used:
1163 .Pp
1164 .Dl Host *.co.uk
1165 .Pp
1166 The following pattern
1167 would match any host in the 192.168.0.[0-9] network range:
1168 .Pp
1169 .Dl Host 192.168.0.?
1170 .Pp
1171 A
1172 .Em pattern-list
1173 is a comma-separated list of patterns.
1174 Patterns within pattern-lists may be negated
1175 by preceding them with an exclamation mark
1176 .Pq Sq !\& .
1177 For example,
1178 to allow a key to be used from anywhere within an organisation
1179 except from the
1180 .Dq dialup
1181 pool,
1182 the following entry (in authorized_keys) could be used:
1183 .Pp
1184 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1185 .Sh FILES
1186 .Bl -tag -width Ds
1187 .It Pa ~/.ssh/config
1188 This is the per-user configuration file.
1189 The format of this file is described above.
1190 This file is used by the SSH client.
1191 Because of the potential for abuse, this file must have strict permissions:
1192 read/write for the user, and not accessible by others.
1193 .It Pa /etc/ssh/ssh_config
1194 Systemwide configuration file.
1195 This file provides defaults for those
1196 values that are not specified in the user's configuration file, and
1197 for those users who do not have a configuration file.
1198 This file must be world-readable.
1199 .El
1200 .Sh SEE ALSO
1201 .Xr ssh 1
1202 .Sh AUTHORS
1203 OpenSSH is a derivative of the original and free
1204 ssh 1.2.12 release by Tatu Ylonen.
1205 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1206 Theo de Raadt and Dug Song
1207 removed many bugs, re-added newer features and
1208 created OpenSSH.
1209 Markus Friedl contributed the support for SSH
1210 protocol versions 1.5 and 2.0.