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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 1.141 2010/09/22 08:30:08 jmc Exp $
38 .Dd $Mdocdate: September 22 2010 $
39 .Dt SSH_CONFIG 5
40 .Os
41 .Sh NAME
42 .Nm ssh_config
43 .Nd OpenSSH SSH client configuration files
44 .Sh SYNOPSIS
45 .Nm ~/.ssh/config
46 .Nm /etc/ssh/ssh_config
47 .Sh DESCRIPTION
48 .Xr ssh 1
49 obtains configuration data from the following sources in
50 the following order:
51 .Pp
52 .Bl -enum -offset indent -compact
53 .It
54 command-line options
55 .It
56 user's configuration file
57 .Pq Pa ~/.ssh/config
58 .It
59 system-wide configuration file
60 .Pq Pa /etc/ssh/ssh_config
61 .El
62 .Pp
63 For each parameter, the first obtained value
64 will be used.
65 The configuration files contain sections separated by
66 .Dq Host
67 specifications, and that section is only applied for hosts that
68 match one of the patterns given in the specification.
69 The matched host name is the one given on the command line.
70 .Pp
71 Since the first obtained value for each parameter is used, more
72 host-specific declarations should be given near the beginning of the
73 file, and general defaults at the end.
74 .Pp
75 The configuration file has the following format:
76 .Pp
77 Empty lines and lines starting with
78 .Ql #
79 are comments.
80 Otherwise a line is of the format
81 .Dq keyword arguments .
82 Configuration options may be separated by whitespace or
83 optional whitespace and exactly one
84 .Ql = ;
85 the latter format is useful to avoid the need to quote whitespace
86 when specifying configuration options using the
87 .Nm ssh ,
88 .Nm scp ,
89 and
90 .Nm sftp
91 .Fl o
92 option.
93 Arguments may optionally be enclosed in double quotes
94 .Pq \&"
95 in order to represent arguments containing spaces.
96 .Pp
97 The possible
98 keywords and their meanings are as follows (note that
99 keywords are case-insensitive and arguments are case-sensitive):
100 .Bl -tag -width Ds
101 .It Cm Host
102 Restricts the following declarations (up to the next
103 .Cm Host
104 keyword) to be only for those hosts that match one of the patterns
105 given after the keyword.
106 If more than one pattern is provided, they should be separated by whitespace.
107 A single
108 .Ql *
109 as a pattern can be used to provide global
110 defaults for all hosts.
111 The host is the
112 .Ar hostname
113 argument given on the command line (i.e. the name is not converted to
114 a canonicalized host name before matching).
115 .Pp
116 See
117 .Sx PATTERNS
118 for more information on patterns.
119 .It Cm AddressFamily
120 Specifies which address family to use when connecting.
121 Valid arguments are
122 .Dq any ,
123 .Dq inet
124 (use IPv4 only), or
125 .Dq inet6
126 (use IPv6 only).
127 .It Cm BatchMode
128 If set to
129 .Dq yes ,
130 passphrase/password querying will be disabled.
131 This option is useful in scripts and other batch jobs where no user
132 is present to supply the password.
133 The argument must be
134 .Dq yes
135 or
136 .Dq no .
137 The default is
138 .Dq no .
139 .It Cm BindAddress
140 Use the specified address on the local machine as the source address of
141 the connection.
142 Only useful on systems with more than one address.
143 Note that this option does not work if
144 .Cm UsePrivilegedPort
145 is set to
146 .Dq yes .
147 .It Cm ChallengeResponseAuthentication
148 Specifies whether to use challenge-response authentication.
149 The argument to this keyword must be
150 .Dq yes
151 or
152 .Dq no .
153 The default is
154 .Dq yes .
155 .It Cm CheckHostIP
156 If this flag is set to
157 .Dq yes ,
158 .Xr ssh 1
159 will additionally check the host IP address in the
160 .Pa known_hosts
161 file.
162 This allows ssh to detect if a host key changed due to DNS spoofing.
163 If the option is set to
164 .Dq no ,
165 the check will not be executed.
166 The default is
167 .Dq yes .
168 .It Cm Cipher
169 Specifies the cipher to use for encrypting the session
170 in protocol version 1.
171 Currently,
172 .Dq blowfish ,
173 .Dq 3des ,
174 and
175 .Dq des
176 are supported.
177 .Ar des
178 is only supported in the
179 .Xr ssh 1
180 client for interoperability with legacy protocol 1 implementations
181 that do not support the
182 .Ar 3des
183 cipher.
184 Its use is strongly discouraged due to cryptographic weaknesses.
185 The default is
186 .Dq 3des .
187 .It Cm Ciphers
188 Specifies the ciphers allowed for protocol version 2
189 in order of preference.
190 Multiple ciphers must be comma-separated.
191 The supported ciphers are
192 .Dq 3des-cbc ,
193 .Dq aes128-cbc ,
194 .Dq aes192-cbc ,
195 .Dq aes256-cbc ,
196 .Dq aes128-ctr ,
197 .Dq aes192-ctr ,
198 .Dq aes256-ctr ,
199 .Dq arcfour128 ,
200 .Dq arcfour256 ,
201 .Dq arcfour ,
202 .Dq blowfish-cbc ,
203 and
204 .Dq cast128-cbc .
205 The default is:
206 .Bd -literal -offset 3n
207 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
208 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
209 aes256-cbc,arcfour
210 .Ed
211 .It Cm ClearAllForwardings
212 Specifies that all local, remote, and dynamic port forwardings
213 specified in the configuration files or on the command line be
214 cleared.
215 This option is primarily useful when used from the
216 .Xr ssh 1
217 command line to clear port forwardings set in
218 configuration files, and is automatically set by
219 .Xr scp 1
220 and
221 .Xr sftp 1 .
222 The argument must be
223 .Dq yes
224 or
225 .Dq no .
226 The default is
227 .Dq no .
228 .It Cm Compression
229 Specifies whether to use compression.
230 The argument must be
231 .Dq yes
232 or
233 .Dq no .
234 The default is
235 .Dq no .
236 .It Cm CompressionLevel
237 Specifies the compression level to use if compression is enabled.
238 The argument must be an integer from 1 (fast) to 9 (slow, best).
239 The default level is 6, which is good for most applications.
240 The meaning of the values is the same as in
241 .Xr gzip 1 .
242 Note that this option applies to protocol version 1 only.
243 .It Cm ConnectionAttempts
244 Specifies the number of tries (one per second) to make before exiting.
245 The argument must be an integer.
246 This may be useful in scripts if the connection sometimes fails.
247 The default is 1.
248 .It Cm ConnectTimeout
249 Specifies the timeout (in seconds) used when connecting to the
250 SSH server, instead of using the default system TCP timeout.
251 This value is used only when the target is down or really unreachable,
252 not when it refuses the connection.
253 .It Cm ControlMaster
254 Enables the sharing of multiple sessions over a single network connection.
255 When set to
256 .Dq yes ,
257 .Xr ssh 1
258 will listen for connections on a control socket specified using the
259 .Cm ControlPath
260 argument.
261 Additional sessions can connect to this socket using the same
262 .Cm ControlPath
263 with
264 .Cm ControlMaster
265 set to
266 .Dq no
267 (the default).
268 These sessions will try to reuse the master instance's network connection
269 rather than initiating new ones, but will fall back to connecting normally
270 if the control socket does not exist, or is not listening.
271 .Pp
272 Setting this to
273 .Dq ask
274 will cause ssh
275 to listen for control connections, but require confirmation using the
276 .Ev SSH_ASKPASS
277 program before they are accepted (see
278 .Xr ssh-add 1
279 for details).
280 If the
281 .Cm ControlPath
282 cannot be opened,
283 ssh will continue without connecting to a master instance.
284 .Pp
285 X11 and
286 .Xr ssh-agent 1
287 forwarding is supported over these multiplexed connections, however the
288 display and agent forwarded will be the one belonging to the master
289 connection i.e. it is not possible to forward multiple displays or agents.
290 .Pp
291 Two additional options allow for opportunistic multiplexing: try to use a
292 master connection but fall back to creating a new one if one does not already
293 exist.
294 These options are:
295 .Dq auto
296 and
297 .Dq autoask .
298 The latter requires confirmation like the
299 .Dq ask
300 option.
301 .It Cm ControlPath
302 Specify the path to the control socket used for connection sharing as described
303 in the
304 .Cm ControlMaster
305 section above or the string
306 .Dq none
307 to disable connection sharing.
308 In the path,
309 .Ql %l
310 will be substituted by the local host name,
311 .Ql %h
312 will be substituted by the target host name,
313 .Ql %p
314 the port, and
315 .Ql %r
316 by the remote login username.
317 It is recommended that any
318 .Cm ControlPath
319 used for opportunistic connection sharing include
320 at least %h, %p, and %r.
321 This ensures that shared connections are uniquely identified.
322 .It Cm ControlPersist
323 When used in conjunction with
324 .Cm ControlMaster ,
325 specifies that the master connection should remain open
326 in the background (waiting for future client connections)
327 after the initial client connection has been closed.
328 If set to
329 .Dq no ,
330 then the master connection will not be placed into the background,
331 and will close as soon as the initial client connection is closed.
332 If set to
333 .Dq yes ,
334 then the master connection will remain in the background indefinitely
335 (until killed or closed via a mechanism such as the
336 .Xr ssh 1
337 .Dq Fl O No exit
338 option).
339 If set to a time in seconds, or a time in any of the formats documented in
340 .Xr sshd_config 5 ,
341 then the backgrounded master connection will automatically terminate
342 after it has remained idle (with no client connections) for the
343 specified time.
344 .It Cm DynamicForward
345 Specifies that a TCP port on the local machine be forwarded
346 over the secure channel, and the application
347 protocol is then used to determine where to connect to from the
348 remote machine.
349 .Pp
350 The argument must be
351 .Sm off
352 .Oo Ar bind_address : Oc Ar port .
353 .Sm on
354 IPv6 addresses can be specified by enclosing addresses in square brackets.
355 By default, the local port is bound in accordance with the
356 .Cm GatewayPorts
357 setting.
358 However, an explicit
359 .Ar bind_address
360 may be used to bind the connection to a specific address.
361 The
362 .Ar bind_address
363 of
364 .Dq localhost
365 indicates that the listening port be bound for local use only, while an
366 empty address or
367 .Sq *
368 indicates that the port should be available from all interfaces.
369 .Pp
370 Currently the SOCKS4 and SOCKS5 protocols are supported, and
371 .Xr ssh 1
372 will act as a SOCKS server.
373 Multiple forwardings may be specified, and
374 additional forwardings can be given on the command line.
375 Only the superuser can forward privileged ports.
376 .It Cm EnableSSHKeysign
377 Setting this option to
378 .Dq yes
379 in the global client configuration file
380 .Pa /etc/ssh/ssh_config
381 enables the use of the helper program
382 .Xr ssh-keysign 8
383 during
384 .Cm HostbasedAuthentication .
385 The argument must be
386 .Dq yes
387 or
388 .Dq no .
389 The default is
390 .Dq no .
391 This option should be placed in the non-hostspecific section.
392 See
393 .Xr ssh-keysign 8
394 for more information.
395 .It Cm EscapeChar
396 Sets the escape character (default:
397 .Ql ~ ) .
398 The escape character can also
399 be set on the command line.
400 The argument should be a single character,
401 .Ql ^
402 followed by a letter, or
403 .Dq none
404 to disable the escape
405 character entirely (making the connection transparent for binary
406 data).
407 .It Cm ExitOnForwardFailure
408 Specifies whether
409 .Xr ssh 1
410 should terminate the connection if it cannot set up all requested
411 dynamic, tunnel, local, and remote port forwardings.
412 The argument must be
413 .Dq yes
414 or
415 .Dq no .
416 The default is
417 .Dq no .
418 .It Cm ForwardAgent
419 Specifies whether the connection to the authentication agent (if any)
420 will be forwarded to the remote machine.
421 The argument must be
422 .Dq yes
423 or
424 .Dq no .
425 The default is
426 .Dq no .
427 .Pp
428 Agent forwarding should be enabled with caution.
429 Users with the ability to bypass file permissions on the remote host
430 (for the agent's Unix-domain socket)
431 can access the local agent through the forwarded connection.
432 An attacker cannot obtain key material from the agent,
433 however they can perform operations on the keys that enable them to
434 authenticate using the identities loaded into the agent.
435 .It Cm ForwardX11
436 Specifies whether X11 connections will be automatically redirected
437 over the secure channel and
438 .Ev DISPLAY
439 set.
440 The argument must be
441 .Dq yes
442 or
443 .Dq no .
444 The default is
445 .Dq no .
446 .Pp
447 X11 forwarding should be enabled with caution.
448 Users with the ability to bypass file permissions on the remote host
449 (for the user's X11 authorization database)
450 can access the local X11 display through the forwarded connection.
451 An attacker may then be able to perform activities such as keystroke monitoring
452 if the
453 .Cm ForwardX11Trusted
454 option is also enabled.
455 .It Cm ForwardX11Timeout
456 Specify a timeout for untrusted X11 forwarding
457 using the format described in the
458 .Sx TIME FORMATS
459 section of
460 .Xr sshd_config 5 .
461 X11 connections received by
462 .Xr ssh 1
463 after this time will be refused.
464 The default is to disable untrusted X11 forwarding after twenty minutes has
465 elapsed.
466 .It Cm ForwardX11Trusted
467 If this option is set to
468 .Dq yes ,
469 remote X11 clients will have full access to the original X11 display.
470 .Pp
471 If this option is set to
472 .Dq no ,
473 remote X11 clients will be considered untrusted and prevented
474 from stealing or tampering with data belonging to trusted X11
475 clients.
476 Furthermore, the
477 .Xr xauth 1
478 token used for the session will be set to expire after 20 minutes.
479 Remote clients will be refused access after this time.
480 .Pp
481 The default is
482 .Dq no .
483 .Pp
484 See the X11 SECURITY extension specification for full details on
485 the restrictions imposed on untrusted clients.
486 .It Cm GatewayPorts
487 Specifies whether remote hosts are allowed to connect to local
488 forwarded ports.
489 By default,
490 .Xr ssh 1
491 binds local port forwardings to the loopback address.
492 This prevents other remote hosts from connecting to forwarded ports.
493 .Cm GatewayPorts
494 can be used to specify that ssh
495 should bind local port forwardings to the wildcard address,
496 thus allowing remote hosts to connect to forwarded ports.
497 The argument must be
498 .Dq yes
499 or
500 .Dq no .
501 The default is
502 .Dq no .
503 .It Cm GlobalKnownHostsFile
504 Specifies a file to use for the global
505 host key database instead of
506 .Pa /etc/ssh/ssh_known_hosts .
507 .It Cm GSSAPIAuthentication
508 Specifies whether user authentication based on GSSAPI is allowed.
509 The default is
510 .Dq no .
511 Note that this option applies to protocol version 2 only.
512 .It Cm GSSAPIDelegateCredentials
513 Forward (delegate) credentials to the server.
514 The default is
515 .Dq no .
516 Note that this option applies to protocol version 2 only.
517 .It Cm HashKnownHosts
518 Indicates that
519 .Xr ssh 1
520 should hash host names and addresses when they are added to
521 .Pa ~/.ssh/known_hosts .
522 These hashed names may be used normally by
523 .Xr ssh 1
524 and
525 .Xr sshd 8 ,
526 but they do not reveal identifying information should the file's contents
527 be disclosed.
528 The default is
529 .Dq no .
530 Note that existing names and addresses in known hosts files
531 will not be converted automatically,
532 but may be manually hashed using
533 .Xr ssh-keygen 1 .
534 .It Cm HostbasedAuthentication
535 Specifies whether to try rhosts based authentication with public key
536 authentication.
537 The argument must be
538 .Dq yes
539 or
540 .Dq no .
541 The default is
542 .Dq no .
543 This option applies to protocol version 2 only and
544 is similar to
545 .Cm RhostsRSAAuthentication .
546 .It Cm HostKeyAlgorithms
547 Specifies the protocol version 2 host key algorithms
548 that the client wants to use in order of preference.
549 The default for this option is:
550 .Bd -literal -offset 3n
551 ecdsa-sha2-nistp256-cert-v01@openssh.com,
552 ecdsa-sha2-nistp384-cert-v01@openssh.com,
553 ecdsa-sha2-nistp521-cert-v01@openssh.com,
554 ssh-rsa-cert-v01@openssh.com,ssh-dss-cert-v01@openssh.com,
555 ssh-rsa-cert-v00@openssh.com,ssh-dss-cert-v00@openssh.com,
556 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
557 ssh-rsa,ssh-dss
558 .Ed
559 .It Cm HostKeyAlias
560 Specifies an alias that should be used instead of the
561 real host name when looking up or saving the host key
562 in the host key database files.
563 This option is useful for tunneling SSH connections
564 or for multiple servers running on a single host.
565 .It Cm HostName
566 Specifies the real host name to log into.
567 This can be used to specify nicknames or abbreviations for hosts.
568 If the hostname contains the character sequence
569 .Ql %h ,
570 then this will be replaced with the host name specified on the commandline
571 (this is useful for manipulating unqualified names).
572 The default is the name given on the command line.
573 Numeric IP addresses are also permitted (both on the command line and in
574 .Cm HostName
575 specifications).
576 .It Cm IdentitiesOnly
577 Specifies that
578 .Xr ssh 1
579 should only use the authentication identity files configured in the
580 .Nm
581 files,
582 even if
583 .Xr ssh-agent 1
584 offers more identities.
585 The argument to this keyword must be
586 .Dq yes
587 or
588 .Dq no .
589 This option is intended for situations where ssh-agent
590 offers many different identities.
591 The default is
592 .Dq no .
593 .It Cm IdentityFile
594 Specifies a file from which the user's DSA, ECDSA or DSA authentication
595 identity is read.
596 The default is
597 .Pa ~/.ssh/identity
598 for protocol version 1, and
599 .Pa ~/.ssh/id_dsa ,
600 .Pa ~/.ssh/id_ecdsa
601 and
602 .Pa ~/.ssh/id_rsa
603 for protocol version 2.
604 Additionally, any identities represented by the authentication agent
605 will be used for authentication.
606 .Xr ssh 1
607 will try to load certificate information from the filename obtained by
608 appending
609 .Pa -cert.pub
610 to the path of a specified
611 .Cm IdentityFile .
612 .Pp
613 The file name may use the tilde
614 syntax to refer to a user's home directory or one of the following
615 escape characters:
616 .Ql %d
617 (local user's home directory),
618 .Ql %u
619 (local user name),
620 .Ql %l
621 (local host name),
622 .Ql %h
623 (remote host name) or
624 .Ql %r
625 (remote user name).
626 .Pp
627 It is possible to have
628 multiple identity files specified in configuration files; all these
629 identities will be tried in sequence.
630 .It Cm KbdInteractiveAuthentication
631 Specifies whether to use keyboard-interactive authentication.
632 The argument to this keyword must be
633 .Dq yes
634 or
635 .Dq no .
636 The default is
637 .Dq yes .
638 .It Cm KbdInteractiveDevices
639 Specifies the list of methods to use in keyboard-interactive authentication.
640 Multiple method names must be comma-separated.
641 The default is to use the server specified list.
642 The methods available vary depending on what the server supports.
643 For an OpenSSH server,
644 it may be zero or more of:
645 .Dq bsdauth ,
646 .Dq pam ,
647 and
648 .Dq skey .
649 .It Cm KexAlgorithms
650 Specifies the available KEX (Key Exchange) algorithms.
651 Multiple algorithms must be comma-separated.
652 The default is:
653 .Bd -literal -offset indent
654 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
655 diffie-hellman-group-exchange-sha256,
656 diffie-hellman-group-exchange-sha1,
657 diffie-hellman-group14-sha1,
658 diffie-hellman-group1-sha1
659 .Ed
660 .It Cm LocalCommand
661 Specifies a command to execute on the local machine after successfully
662 connecting to the server.
663 The command string extends to the end of the line, and is executed with
664 the user's shell.
665 The following escape character substitutions will be performed:
666 .Ql %d
667 (local user's home directory),
668 .Ql %h
669 (remote host name),
670 .Ql %l
671 (local host name),
672 .Ql %n
673 (host name as provided on the command line),
674 .Ql %p
675 (remote port),
676 .Ql %r
677 (remote user name) or
678 .Ql %u
679 (local user name).
680 .Pp
681 The command is run synchronously and does not have access to the
682 session of the
683 .Xr ssh 1
684 that spawned it.
685 It should not be used for interactive commands.
686 .Pp
687 This directive is ignored unless
688 .Cm PermitLocalCommand
689 has been enabled.
690 .It Cm LocalForward
691 Specifies that a TCP port on the local machine be forwarded over
692 the secure channel to the specified host and port from the remote machine.
693 The first argument must be
694 .Sm off
695 .Oo Ar bind_address : Oc Ar port
696 .Sm on
697 and the second argument must be
698 .Ar host : Ns Ar hostport .
699 IPv6 addresses can be specified by enclosing addresses in square brackets.
700 Multiple forwardings may be specified, and additional forwardings can be
701 given on the command line.
702 Only the superuser can forward privileged ports.
703 By default, the local port is bound in accordance with the
704 .Cm GatewayPorts
705 setting.
706 However, an explicit
707 .Ar bind_address
708 may be used to bind the connection to a specific address.
709 The
710 .Ar bind_address
711 of
712 .Dq localhost
713 indicates that the listening port be bound for local use only, while an
714 empty address or
715 .Sq *
716 indicates that the port should be available from all interfaces.
717 .It Cm LogLevel
718 Gives the verbosity level that is used when logging messages from
719 .Xr ssh 1 .
720 The possible values are:
721 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
722 The default is INFO.
723 DEBUG and DEBUG1 are equivalent.
724 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
725 .It Cm MACs
726 Specifies the MAC (message authentication code) algorithms
727 in order of preference.
728 The MAC algorithm is used in protocol version 2
729 for data integrity protection.
730 Multiple algorithms must be comma-separated.
731 The default is:
732 .Bd -literal -offset indent
733 hmac-md5,hmac-sha1,umac-64@openssh.com,
734 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
735 .Ed
736 .It Cm NoHostAuthenticationForLocalhost
737 This option can be used if the home directory is shared across machines.
738 In this case localhost will refer to a different machine on each of
739 the machines and the user will get many warnings about changed host keys.
740 However, this option disables host authentication for localhost.
741 The argument to this keyword must be
742 .Dq yes
743 or
744 .Dq no .
745 The default is to check the host key for localhost.
746 .It Cm NumberOfPasswordPrompts
747 Specifies the number of password prompts before giving up.
748 The argument to this keyword must be an integer.
749 The default is 3.
750 .It Cm PasswordAuthentication
751 Specifies whether to use password authentication.
752 The argument to this keyword must be
753 .Dq yes
754 or
755 .Dq no .
756 The default is
757 .Dq yes .
758 .It Cm PermitLocalCommand
759 Allow local command execution via the
760 .Ic LocalCommand
761 option or using the
762 .Ic !\& Ns Ar command
763 escape sequence in
764 .Xr ssh 1 .
765 The argument must be
766 .Dq yes
767 or
768 .Dq no .
769 The default is
770 .Dq no .
771 .It Cm PKCS11Provider
772 Specifies which PKCS#11 provider to use.
773 The argument to this keyword is the PKCS#11 shared libary
774 .Xr ssh 1
775 should use to communicate with a PKCS#11 token providing the user's
776 private RSA key.
777 .It Cm Port
778 Specifies the port number to connect on the remote host.
779 The default is 22.
780 .It Cm PreferredAuthentications
781 Specifies the order in which the client should try protocol 2
782 authentication methods.
783 This allows a client to prefer one method (e.g.\&
784 .Cm keyboard-interactive )
785 over another method (e.g.\&
786 .Cm password ) .
787 The default is:
788 .Bd -literal -offset indent
789 gssapi-with-mic,hostbased,publickey,
790 keyboard-interactive,password
791 .Ed
792 .It Cm Protocol
793 Specifies the protocol versions
794 .Xr ssh 1
795 should support in order of preference.
796 The possible values are
797 .Sq 1
798 and
799 .Sq 2 .
800 Multiple versions must be comma-separated.
801 When this option is set to
802 .Dq 2,1
803 .Nm ssh
804 will try version 2 and fall back to version 1
805 if version 2 is not available.
806 The default is
807 .Sq 2 .
808 .It Cm ProxyCommand
809 Specifies the command to use to connect to the server.
810 The command
811 string extends to the end of the line, and is executed with
812 the user's shell.
813 In the command string, any occurrence of
814 .Ql %h
815 will be substituted by the host name to
816 connect,
817 .Ql %p
818 by the port, and
819 .Ql %r
820 by the remote user name.
821 The command can be basically anything,
822 and should read from its standard input and write to its standard output.
823 It should eventually connect an
824 .Xr sshd 8
825 server running on some machine, or execute
826 .Ic sshd -i
827 somewhere.
828 Host key management will be done using the
829 HostName of the host being connected (defaulting to the name typed by
830 the user).
831 Setting the command to
832 .Dq none
833 disables this option entirely.
834 Note that
835 .Cm CheckHostIP
836 is not available for connects with a proxy command.
837 .Pp
838 This directive is useful in conjunction with
839 .Xr nc 1
840 and its proxy support.
841 For example, the following directive would connect via an HTTP proxy at
842 192.0.2.0:
843 .Bd -literal -offset 3n
844 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
845 .Ed
846 .It Cm PubkeyAuthentication
847 Specifies whether to try public key authentication.
848 The argument to this keyword must be
849 .Dq yes
850 or
851 .Dq no .
852 The default is
853 .Dq yes .
854 This option applies to protocol version 2 only.
855 .It Cm RekeyLimit
856 Specifies the maximum amount of data that may be transmitted before the
857 session key is renegotiated.
858 The argument is the number of bytes, with an optional suffix of
859 .Sq K ,
860 .Sq M ,
861 or
862 .Sq G
863 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
864 The default is between
865 .Sq 1G
866 and
867 .Sq 4G ,
868 depending on the cipher.
869 This option applies to protocol version 2 only.
870 .It Cm RemoteForward
871 Specifies that a TCP port on the remote machine be forwarded over
872 the secure channel to the specified host and port from the local machine.
873 The first argument must be
874 .Sm off
875 .Oo Ar bind_address : Oc Ar port
876 .Sm on
877 and the second argument must be
878 .Ar host : Ns Ar hostport .
879 IPv6 addresses can be specified by enclosing addresses in square brackets.
880 Multiple forwardings may be specified, and additional
881 forwardings can be given on the command line.
882 Privileged ports can be forwarded only when
883 logging in as root on the remote machine.
884 .Pp
885 If the
886 .Ar port
887 argument is
888 .Ql 0 ,
889 the listen port will be dynamically allocated on the server and reported
890 to the client at run time.
891 .Pp
892 If the
893 .Ar bind_address
894 is not specified, the default is to only bind to loopback addresses.
895 If the
896 .Ar bind_address
897 is
898 .Ql *
899 or an empty string, then the forwarding is requested to listen on all
900 interfaces.
901 Specifying a remote
902 .Ar bind_address
903 will only succeed if the server's
904 .Cm GatewayPorts
905 option is enabled (see
906 .Xr sshd_config 5 ) .
907 .It Cm RhostsRSAAuthentication
908 Specifies whether to try rhosts based authentication with RSA host
909 authentication.
910 The argument must be
911 .Dq yes
912 or
913 .Dq no .
914 The default is
915 .Dq no .
916 This option applies to protocol version 1 only and requires
917 .Xr ssh 1
918 to be setuid root.
919 .It Cm RSAAuthentication
920 Specifies whether to try RSA authentication.
921 The argument to this keyword must be
922 .Dq yes
923 or
924 .Dq no .
925 RSA authentication will only be
926 attempted if the identity file exists, or an authentication agent is
927 running.
928 The default is
929 .Dq yes .
930 Note that this option applies to protocol version 1 only.
931 .It Cm SendEnv
932 Specifies what variables from the local
933 .Xr environ 7
934 should be sent to the server.
935 Note that environment passing is only supported for protocol 2.
936 The server must also support it, and the server must be configured to
937 accept these environment variables.
938 Refer to
939 .Cm AcceptEnv
940 in
941 .Xr sshd_config 5
942 for how to configure the server.
943 Variables are specified by name, which may contain wildcard characters.
944 Multiple environment variables may be separated by whitespace or spread
945 across multiple
946 .Cm SendEnv
947 directives.
948 The default is not to send any environment variables.
949 .Pp
950 See
951 .Sx PATTERNS
952 for more information on patterns.
953 .It Cm ServerAliveCountMax
954 Sets the number of server alive messages (see below) which may be
955 sent without
956 .Xr ssh 1
957 receiving any messages back from the server.
958 If this threshold is reached while server alive messages are being sent,
959 ssh will disconnect from the server, terminating the session.
960 It is important to note that the use of server alive messages is very
961 different from
962 .Cm TCPKeepAlive
963 (below).
964 The server alive messages are sent through the encrypted channel
965 and therefore will not be spoofable.
966 The TCP keepalive option enabled by
967 .Cm TCPKeepAlive
968 is spoofable.
969 The server alive mechanism is valuable when the client or
970 server depend on knowing when a connection has become inactive.
971 .Pp
972 The default value is 3.
973 If, for example,
974 .Cm ServerAliveInterval
975 (see below) is set to 15 and
976 .Cm ServerAliveCountMax
977 is left at the default, if the server becomes unresponsive,
978 ssh will disconnect after approximately 45 seconds.
979 This option applies to protocol version 2 only.
980 .It Cm ServerAliveInterval
981 Sets a timeout interval in seconds after which if no data has been received
982 from the server,
983 .Xr ssh 1
984 will send a message through the encrypted
985 channel to request a response from the server.
986 The default
987 is 0, indicating that these messages will not be sent to the server.
988 This option applies to protocol version 2 only.
989 .It Cm StrictHostKeyChecking
990 If this flag is set to
991 .Dq yes ,
992 .Xr ssh 1
993 will never automatically add host keys to the
994 .Pa ~/.ssh/known_hosts
995 file, and refuses to connect to hosts whose host key has changed.
996 This provides maximum protection against trojan horse attacks,
997 though it can be annoying when the
998 .Pa /etc/ssh/ssh_known_hosts
999 file is poorly maintained or when connections to new hosts are
1000 frequently made.
1001 This option forces the user to manually
1002 add all new hosts.
1003 If this flag is set to
1004 .Dq no ,
1005 ssh will automatically add new host keys to the
1006 user known hosts files.
1007 If this flag is set to
1008 .Dq ask ,
1009 new host keys
1010 will be added to the user known host files only after the user
1011 has confirmed that is what they really want to do, and
1012 ssh will refuse to connect to hosts whose host key has changed.
1013 The host keys of
1014 known hosts will be verified automatically in all cases.
1015 The argument must be
1016 .Dq yes ,
1017 .Dq no ,
1018 or
1019 .Dq ask .
1020 The default is
1021 .Dq ask .
1022 .It Cm TCPKeepAlive
1023 Specifies whether the system should send TCP keepalive messages to the
1024 other side.
1025 If they are sent, death of the connection or crash of one
1026 of the machines will be properly noticed.
1027 However, this means that
1028 connections will die if the route is down temporarily, and some people
1029 find it annoying.
1030 .Pp
1031 The default is
1032 .Dq yes
1033 (to send TCP keepalive messages), and the client will notice
1034 if the network goes down or the remote host dies.
1035 This is important in scripts, and many users want it too.
1036 .Pp
1037 To disable TCP keepalive messages, the value should be set to
1038 .Dq no .
1039 .It Cm Tunnel
1040 Request
1041 .Xr tun 4
1042 device forwarding between the client and the server.
1043 The argument must be
1044 .Dq yes ,
1045 .Dq point-to-point
1046 (layer 3),
1047 .Dq ethernet
1048 (layer 2),
1049 or
1050 .Dq no .
1051 Specifying
1052 .Dq yes
1053 requests the default tunnel mode, which is
1054 .Dq point-to-point .
1055 The default is
1056 .Dq no .
1057 .It Cm TunnelDevice
1058 Specifies the
1059 .Xr tun 4
1060 devices to open on the client
1061 .Pq Ar local_tun
1062 and the server
1063 .Pq Ar remote_tun .
1064 .Pp
1065 The argument must be
1066 .Sm off
1067 .Ar local_tun Op : Ar remote_tun .
1068 .Sm on
1069 The devices may be specified by numerical ID or the keyword
1070 .Dq any ,
1071 which uses the next available tunnel device.
1072 If
1073 .Ar remote_tun
1074 is not specified, it defaults to
1075 .Dq any .
1076 The default is
1077 .Dq any:any .
1078 .It Cm UsePrivilegedPort
1079 Specifies whether to use a privileged port for outgoing connections.
1080 The argument must be
1081 .Dq yes
1082 or
1083 .Dq no .
1084 The default is
1085 .Dq no .
1086 If set to
1087 .Dq yes ,
1088 .Xr ssh 1
1089 must be setuid root.
1090 Note that this option must be set to
1091 .Dq yes
1092 for
1093 .Cm RhostsRSAAuthentication
1094 with older servers.
1095 .It Cm User
1096 Specifies the user to log in as.
1097 This can be useful when a different user name is used on different machines.
1098 This saves the trouble of
1099 having to remember to give the user name on the command line.
1100 .It Cm UserKnownHostsFile
1101 Specifies a file to use for the user
1102 host key database instead of
1103 .Pa ~/.ssh/known_hosts .
1104 .It Cm VerifyHostKeyDNS
1105 Specifies whether to verify the remote key using DNS and SSHFP resource
1106 records.
1107 If this option is set to
1108 .Dq yes ,
1109 the client will implicitly trust keys that match a secure fingerprint
1110 from DNS.
1111 Insecure fingerprints will be handled as if this option was set to
1112 .Dq ask .
1113 If this option is set to
1114 .Dq ask ,
1115 information on fingerprint match will be displayed, but the user will still
1116 need to confirm new host keys according to the
1117 .Cm StrictHostKeyChecking
1118 option.
1119 The argument must be
1120 .Dq yes ,
1121 .Dq no ,
1122 or
1123 .Dq ask .
1124 The default is
1125 .Dq no .
1126 Note that this option applies to protocol version 2 only.
1127 .Pp
1128 See also
1129 .Sx VERIFYING HOST KEYS
1130 in
1131 .Xr ssh 1 .
1132 .It Cm VisualHostKey
1133 If this flag is set to
1134 .Dq yes ,
1135 an ASCII art representation of the remote host key fingerprint is
1136 printed in addition to the hex fingerprint string at login and
1137 for unknown host keys.
1138 If this flag is set to
1139 .Dq no ,
1140 no fingerprint strings are printed at login and
1141 only the hex fingerprint string will be printed for unknown host keys.
1142 The default is
1143 .Dq no .
1144 .It Cm XAuthLocation
1145 Specifies the full pathname of the
1146 .Xr xauth 1
1147 program.
1148 The default is
1149 .Pa /usr/X11R6/bin/xauth .
1150 .El
1151 .Sh PATTERNS
1152 A
1153 .Em pattern
1154 consists of zero or more non-whitespace characters,
1155 .Sq *
1156 (a wildcard that matches zero or more characters),
1157 or
1158 .Sq ?\&
1159 (a wildcard that matches exactly one character).
1160 For example, to specify a set of declarations for any host in the
1161 .Dq .co.uk
1162 set of domains,
1163 the following pattern could be used:
1164 .Pp
1165 .Dl Host *.co.uk
1166 .Pp
1167 The following pattern
1168 would match any host in the 192.168.0.[0-9] network range:
1169 .Pp
1170 .Dl Host 192.168.0.?
1171 .Pp
1172 A
1173 .Em pattern-list
1174 is a comma-separated list of patterns.
1175 Patterns within pattern-lists may be negated
1176 by preceding them with an exclamation mark
1177 .Pq Sq !\& .
1178 For example,
1179 to allow a key to be used from anywhere within an organisation
1180 except from the
1181 .Dq dialup
1182 pool,
1183 the following entry (in authorized_keys) could be used:
1184 .Pp
1185 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
1186 .Sh FILES
1187 .Bl -tag -width Ds
1188 .It Pa ~/.ssh/config
1189 This is the per-user configuration file.
1190 The format of this file is described above.
1191 This file is used by the SSH client.
1192 Because of the potential for abuse, this file must have strict permissions:
1193 read/write for the user, and not accessible by others.
1194 .It Pa /etc/ssh/ssh_config
1195 Systemwide configuration file.
1196 This file provides defaults for those
1197 values that are not specified in the user's configuration file, and
1198 for those users who do not have a configuration file.
1199 This file must be world-readable.
1200 .El
1201 .Sh SEE ALSO
1202 .Xr ssh 1
1203 .Sh AUTHORS
1204 OpenSSH is a derivative of the original and free
1205 ssh 1.2.12 release by Tatu Ylonen.
1206 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1207 Theo de Raadt and Dug Song
1208 removed many bugs, re-added newer features and
1209 created OpenSSH.
1210 Markus Friedl contributed the support for SSH
1211 protocol versions 1.5 and 2.0.