| blob | 170125a0e4434e01ad9c6b6663d1c9eb3b59368f |
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 1.366 2021/09/25 09:40:33 kn Exp $
37 .Dd $Mdocdate: September 25 2021 $
38 .Dt SSH_CONFIG 5
39 .Os
40 .Sh NAME
41 .Nm ssh_config
42 .Nd OpenSSH client configuration file
43 .Sh DESCRIPTION
44 .Xr ssh 1
45 obtains configuration data from the following sources in
46 the following order:
47 .Pp
48 .Bl -enum -offset indent -compact
49 .It
50 command-line options
51 .It
52 user's configuration file
53 .Pq Pa ~/.ssh/config
54 .It
55 system-wide configuration file
56 .Pq Pa /etc/ssh/ssh_config
57 .El
58 .Pp
59 For each parameter, the first obtained value
60 will be used.
61 The configuration files contain sections separated by
62 .Cm Host
63 specifications, and that section is only applied for hosts that
64 match one of the patterns given in the specification.
65 The matched host name is usually the one given on the command line
66 (see the
67 .Cm CanonicalizeHostname
68 option for exceptions).
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 file contains keyword-argument pairs, one per line.
75 Lines starting with
76 .Ql #
77 and empty lines are interpreted as comments.
78 Arguments may optionally be enclosed in double quotes
79 .Pq \&"
80 in order to represent arguments containing spaces.
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 .Pp
93 The possible
94 keywords and their meanings are as follows (note that
95 keywords are case-insensitive and arguments are case-sensitive):
96 .Bl -tag -width Ds
97 .It Cm Host
98 Restricts the following declarations (up to the next
99 .Cm Host
100 or
101 .Cm Match
102 keyword) to be only for those hosts that match one of the patterns
103 given after the keyword.
104 If more than one pattern is provided, they should be separated by whitespace.
105 A single
106 .Ql *
107 as a pattern can be used to provide global
108 defaults for all hosts.
109 The host is usually the
110 .Ar hostname
111 argument given on the command line
112 (see the
113 .Cm CanonicalizeHostname
114 keyword for exceptions).
115 .Pp
116 A pattern entry may be negated by prefixing it with an exclamation mark
117 .Pq Sq !\& .
118 If a negated entry is matched, then the
119 .Cm Host
120 entry is ignored, regardless of whether any other patterns on the line
121 match.
122 Negated matches are therefore useful to provide exceptions for wildcard
123 matches.
124 .Pp
125 See
126 .Sx PATTERNS
127 for more information on patterns.
128 .It Cm Match
129 Restricts the following declarations (up to the next
130 .Cm Host
131 or
132 .Cm Match
133 keyword) to be used only when the conditions following the
134 .Cm Match
135 keyword are satisfied.
136 Match conditions are specified using one or more criteria
137 or the single token
138 .Cm all
139 which always matches.
140 The available criteria keywords are:
141 .Cm canonical ,
142 .Cm final ,
143 .Cm exec ,
144 .Cm host ,
145 .Cm originalhost ,
146 .Cm user ,
147 and
148 .Cm localuser .
149 The
150 .Cm all
151 criteria must appear alone or immediately after
152 .Cm canonical
153 or
154 .Cm final .
155 Other criteria may be combined arbitrarily.
156 All criteria but
157 .Cm all ,
158 .Cm canonical ,
159 and
160 .Cm final
161 require an argument.
162 Criteria may be negated by prepending an exclamation mark
163 .Pq Sq !\& .
164 .Pp
165 The
166 .Cm canonical
167 keyword matches only when the configuration file is being re-parsed
168 after hostname canonicalization (see the
169 .Cm CanonicalizeHostname
170 option).
171 This may be useful to specify conditions that work with canonical host
172 names only.
173 .Pp
174 The
175 .Cm final
176 keyword requests that the configuration be re-parsed (regardless of whether
177 .Cm CanonicalizeHostname
178 is enabled), and matches only during this final pass.
179 If
180 .Cm CanonicalizeHostname
181 is enabled, then
182 .Cm canonical
183 and
184 .Cm final
185 match during the same pass.
186 .Pp
187 The
188 .Cm exec
189 keyword executes the specified command under the user's shell.
190 If the command returns a zero exit status then the condition is considered true.
191 Commands containing whitespace characters must be quoted.
192 Arguments to
193 .Cm exec
194 accept the tokens described in the
195 .Sx TOKENS
196 section.
197 .Pp
198 The other keywords' criteria must be single entries or comma-separated
199 lists and may use the wildcard and negation operators described in the
200 .Sx PATTERNS
201 section.
202 The criteria for the
203 .Cm host
204 keyword are matched against the target hostname, after any substitution
205 by the
206 .Cm Hostname
207 or
208 .Cm CanonicalizeHostname
209 options.
210 The
211 .Cm originalhost
212 keyword matches against the hostname as it was specified on the command-line.
213 The
214 .Cm user
215 keyword matches against the target username on the remote host.
216 The
217 .Cm localuser
218 keyword matches against the name of the local user running
219 .Xr ssh 1
220 (this keyword may be useful in system-wide
221 .Nm
222 files).
223 .It Cm AddKeysToAgent
224 Specifies whether keys should be automatically added to a running
225 .Xr ssh-agent 1 .
226 If this option is set to
227 .Cm yes
228 and a key is loaded from a file, the key and its passphrase are added to
229 the agent with the default lifetime, as if by
230 .Xr ssh-add 1 .
231 If this option is set to
232 .Cm ask ,
233 .Xr ssh 1
234 will require confirmation using the
235 .Ev SSH_ASKPASS
236 program before adding a key (see
237 .Xr ssh-add 1
238 for details).
239 If this option is set to
240 .Cm confirm ,
241 each use of the key must be confirmed, as if the
242 .Fl c
243 option was specified to
244 .Xr ssh-add 1 .
245 If this option is set to
246 .Cm no ,
247 no keys are added to the agent.
248 Alternately, this option may be specified as a time interval
249 using the format described in the
250 .Sx TIME FORMATS
251 section of
252 .Xr sshd_config 5
253 to specify the key's lifetime in
254 .Xr ssh-agent 1 ,
255 after which it will automatically be removed.
256 The argument must be
257 .Cm no
258 (the default),
259 .Cm yes ,
260 .Cm confirm
261 (optionally followed by a time interval),
262 .Cm ask
263 or a time interval.
264 .It Cm AddressFamily
265 Specifies which address family to use when connecting.
266 Valid arguments are
267 .Cm any
268 (the default),
269 .Cm inet
270 (use IPv4 only), or
271 .Cm inet6
272 (use IPv6 only).
273 .It Cm BatchMode
274 If set to
275 .Cm yes ,
276 user interaction such as password prompts and host key confirmation requests
277 will be disabled.
278 This option is useful in scripts and other batch jobs where no user
279 is present to interact with
280 .Xr ssh 1 .
281 The argument must be
282 .Cm yes
283 or
284 .Cm no
285 (the default).
286 .It Cm BindAddress
287 Use the specified address on the local machine as the source address of
288 the connection.
289 Only useful on systems with more than one address.
290 .It Cm BindInterface
291 Use the address of the specified interface on the local machine as the
292 source address of the connection.
293 .It Cm CanonicalDomains
294 When
295 .Cm CanonicalizeHostname
296 is enabled, this option specifies the list of domain suffixes in which to
297 search for the specified destination host.
298 .It Cm CanonicalizeFallbackLocal
299 Specifies whether to fail with an error when hostname canonicalization fails.
300 The default,
301 .Cm yes ,
302 will attempt to look up the unqualified hostname using the system resolver's
303 search rules.
304 A value of
305 .Cm no
306 will cause
307 .Xr ssh 1
308 to fail instantly if
309 .Cm CanonicalizeHostname
310 is enabled and the target hostname cannot be found in any of the domains
311 specified by
312 .Cm CanonicalDomains .
313 .It Cm CanonicalizeHostname
314 Controls whether explicit hostname canonicalization is performed.
315 The default,
316 .Cm no ,
317 is not to perform any name rewriting and let the system resolver handle all
318 hostname lookups.
319 If set to
320 .Cm yes
321 then, for connections that do not use a
322 .Cm ProxyCommand
323 or
324 .Cm ProxyJump ,
325 .Xr ssh 1
326 will attempt to canonicalize the hostname specified on the command line
327 using the
328 .Cm CanonicalDomains
329 suffixes and
330 .Cm CanonicalizePermittedCNAMEs
331 rules.
332 If
333 .Cm CanonicalizeHostname
334 is set to
335 .Cm always ,
336 then canonicalization is applied to proxied connections too.
337 .Pp
338 If this option is enabled, then the configuration files are processed
339 again using the new target name to pick up any new configuration in matching
340 .Cm Host
341 and
342 .Cm Match
343 stanzas.
344 A value of
345 .Cm none
346 disables the use of a
347 .Cm ProxyJump
348 host.
349 .It Cm CanonicalizeMaxDots
350 Specifies the maximum number of dot characters in a hostname before
351 canonicalization is disabled.
352 The default, 1,
353 allows a single dot (i.e. hostname.subdomain).
354 .It Cm CanonicalizePermittedCNAMEs
355 Specifies rules to determine whether CNAMEs should be followed when
356 canonicalizing hostnames.
357 The rules consist of one or more arguments of
358 .Ar source_domain_list : Ns Ar target_domain_list ,
359 where
360 .Ar source_domain_list
361 is a pattern-list of domains that may follow CNAMEs in canonicalization,
362 and
363 .Ar target_domain_list
364 is a pattern-list of domains that they may resolve to.
365 .Pp
366 For example,
367 .Qq *.a.example.com:*.b.example.com,*.c.example.com
368 will allow hostnames matching
369 .Qq *.a.example.com
370 to be canonicalized to names in the
371 .Qq *.b.example.com
372 or
373 .Qq *.c.example.com
374 domains.
375 .Pp
376 A single argument of
377 .Qq none
378 causes no CNAMEs to be considered for canonicalization.
379 This is the default behaviour.
380 .It Cm CASignatureAlgorithms
381 Specifies which algorithms are allowed for signing of certificates
382 by certificate authorities (CAs).
383 The default is:
384 .Bd -literal -offset indent
385 ssh-ed25519,ecdsa-sha2-nistp256,
386 ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
387 sk-ssh-ed25519@openssh.com,
388 sk-ecdsa-sha2-nistp256@openssh.com,
389 rsa-sha2-512,rsa-sha2-256
390 .Ed
391 .Pp
392 If the specified list begins with a
393 .Sq +
394 character, then the specified algorithms will be appended to the default set
395 instead of replacing them.
396 If the specified list begins with a
397 .Sq -
398 character, then the specified algorithms (including wildcards) will be removed
399 from the default set instead of replacing them.
400 .Pp
401 .Xr ssh 1
402 will not accept host certificates signed using algorithms other than those
403 specified.
404 .It Cm CertificateFile
405 Specifies a file from which the user's certificate is read.
406 A corresponding private key must be provided separately in order
407 to use this certificate either
408 from an
409 .Cm IdentityFile
410 directive or
411 .Fl i
412 flag to
413 .Xr ssh 1 ,
414 via
415 .Xr ssh-agent 1 ,
416 or via a
417 .Cm PKCS11Provider
418 or
419 .Cm SecurityKeyProvider .
420 .Pp
421 Arguments to
422 .Cm CertificateFile
423 may use the tilde syntax to refer to a user's home directory,
424 the tokens described in the
425 .Sx TOKENS
426 section and environment variables as described in the
427 .Sx ENVIRONMENT VARIABLES
428 section.
429 .Pp
430 It is possible to have multiple certificate files specified in
431 configuration files; these certificates will be tried in sequence.
432 Multiple
433 .Cm CertificateFile
434 directives will add to the list of certificates used for
435 authentication.
436 .It Cm CheckHostIP
437 If set to
438 .Cm yes
439 .Xr ssh 1
440 will additionally check the host IP address in the
441 .Pa known_hosts
442 file.
443 This allows it to detect if a host key changed due to DNS spoofing
444 and will add addresses of destination hosts to
445 .Pa ~/.ssh/known_hosts
446 in the process, regardless of the setting of
447 .Cm StrictHostKeyChecking .
448 If the option is set to
449 .Cm no
450 (the default),
451 the check will not be executed.
452 .It Cm Ciphers
453 Specifies the ciphers allowed and their order of preference.
454 Multiple ciphers must be comma-separated.
455 If the specified list begins with a
456 .Sq +
457 character, then the specified ciphers will be appended to the default set
458 instead of replacing them.
459 If the specified list begins with a
460 .Sq -
461 character, then the specified ciphers (including wildcards) will be removed
462 from the default set instead of replacing them.
463 If the specified list begins with a
464 .Sq ^
465 character, then the specified ciphers will be placed at the head of the
466 default set.
467 .Pp
468 The supported ciphers are:
469 .Bd -literal -offset indent
470 3des-cbc
471 aes128-cbc
472 aes192-cbc
473 aes256-cbc
474 aes128-ctr
475 aes192-ctr
476 aes256-ctr
477 aes128-gcm@openssh.com
478 aes256-gcm@openssh.com
479 chacha20-poly1305@openssh.com
480 .Ed
481 .Pp
482 The default is:
483 .Bd -literal -offset indent
484 chacha20-poly1305@openssh.com,
485 aes128-ctr,aes192-ctr,aes256-ctr,
486 aes128-gcm@openssh.com,aes256-gcm@openssh.com
487 .Ed
488 .Pp
489 The list of available ciphers may also be obtained using
490 .Qq ssh -Q cipher .
491 .It Cm ClearAllForwardings
492 Specifies that all local, remote, and dynamic port forwardings
493 specified in the configuration files or on the command line be
494 cleared.
495 This option is primarily useful when used from the
496 .Xr ssh 1
497 command line to clear port forwardings set in
498 configuration files, and is automatically set by
499 .Xr scp 1
500 and
501 .Xr sftp 1 .
502 The argument must be
503 .Cm yes
504 or
505 .Cm no
506 (the default).
507 .It Cm Compression
508 Specifies whether to use compression.
509 The argument must be
510 .Cm yes
511 or
512 .Cm no
513 (the default).
514 .It Cm ConnectionAttempts
515 Specifies the number of tries (one per second) to make before exiting.
516 The argument must be an integer.
517 This may be useful in scripts if the connection sometimes fails.
518 The default is 1.
519 .It Cm ConnectTimeout
520 Specifies the timeout (in seconds) used when connecting to the
521 SSH server, instead of using the default system TCP timeout.
522 This timeout is applied both to establishing the connection and to performing
523 the initial SSH protocol handshake and key exchange.
524 .It Cm ControlMaster
525 Enables the sharing of multiple sessions over a single network connection.
526 When set to
527 .Cm yes ,
528 .Xr ssh 1
529 will listen for connections on a control socket specified using the
530 .Cm ControlPath
531 argument.
532 Additional sessions can connect to this socket using the same
533 .Cm ControlPath
534 with
535 .Cm ControlMaster
536 set to
537 .Cm no
538 (the default).
539 These sessions will try to reuse the master instance's network connection
540 rather than initiating new ones, but will fall back to connecting normally
541 if the control socket does not exist, or is not listening.
542 .Pp
543 Setting this to
544 .Cm ask
545 will cause
546 .Xr ssh 1
547 to listen for control connections, but require confirmation using
548 .Xr ssh-askpass 1 .
549 If the
550 .Cm ControlPath
551 cannot be opened,
552 .Xr ssh 1
553 will continue without connecting to a master instance.
554 .Pp
555 X11 and
556 .Xr ssh-agent 1
557 forwarding is supported over these multiplexed connections, however the
558 display and agent forwarded will be the one belonging to the master
559 connection i.e. it is not possible to forward multiple displays or agents.
560 .Pp
561 Two additional options allow for opportunistic multiplexing: try to use a
562 master connection but fall back to creating a new one if one does not already
563 exist.
564 These options are:
565 .Cm auto
566 and
567 .Cm autoask .
568 The latter requires confirmation like the
569 .Cm ask
570 option.
571 .It Cm ControlPath
572 Specify the path to the control socket used for connection sharing as described
573 in the
574 .Cm ControlMaster
575 section above or the string
576 .Cm none
577 to disable connection sharing.
578 Arguments to
579 .Cm ControlPath
580 may use the tilde syntax to refer to a user's home directory,
581 the tokens described in the
582 .Sx TOKENS
583 section and environment variables as described in the
584 .Sx ENVIRONMENT VARIABLES
585 section.
586 It is recommended that any
587 .Cm ControlPath
588 used for opportunistic connection sharing include
589 at least %h, %p, and %r (or alternatively %C) and be placed in a directory
590 that is not writable by other users.
591 This ensures that shared connections are uniquely identified.
592 .It Cm ControlPersist
593 When used in conjunction with
594 .Cm ControlMaster ,
595 specifies that the master connection should remain open
596 in the background (waiting for future client connections)
597 after the initial client connection has been closed.
598 If set to
599 .Cm no
600 (the default),
601 then the master connection will not be placed into the background,
602 and will close as soon as the initial client connection is closed.
603 If set to
604 .Cm yes
605 or 0,
606 then the master connection will remain in the background indefinitely
607 (until killed or closed via a mechanism such as the
608 .Qq ssh -O exit ) .
609 If set to a time in seconds, or a time in any of the formats documented in
610 .Xr sshd_config 5 ,
611 then the backgrounded master connection will automatically terminate
612 after it has remained idle (with no client connections) for the
613 specified time.
614 .It Cm DynamicForward
615 Specifies that a TCP port on the local machine be forwarded
616 over the secure channel, and the application
617 protocol is then used to determine where to connect to from the
618 remote machine.
619 .Pp
620 The argument must be
621 .Sm off
622 .Oo Ar bind_address : Oc Ar port .
623 .Sm on
624 IPv6 addresses can be specified by enclosing addresses in square brackets.
625 By default, the local port is bound in accordance with the
626 .Cm GatewayPorts
627 setting.
628 However, an explicit
629 .Ar bind_address
630 may be used to bind the connection to a specific address.
631 The
632 .Ar bind_address
633 of
634 .Cm localhost
635 indicates that the listening port be bound for local use only, while an
636 empty address or
637 .Sq *
638 indicates that the port should be available from all interfaces.
639 .Pp
640 Currently the SOCKS4 and SOCKS5 protocols are supported, and
641 .Xr ssh 1
642 will act as a SOCKS server.
643 Multiple forwardings may be specified, and
644 additional forwardings can be given on the command line.
645 Only the superuser can forward privileged ports.
646 .It Cm EnableSSHKeysign
647 Setting this option to
648 .Cm yes
649 in the global client configuration file
650 .Pa /etc/ssh/ssh_config
651 enables the use of the helper program
652 .Xr ssh-keysign 8
653 during
654 .Cm HostbasedAuthentication .
655 The argument must be
656 .Cm yes
657 or
658 .Cm no
659 (the default).
660 This option should be placed in the non-hostspecific section.
661 See
662 .Xr ssh-keysign 8
663 for more information.
664 .It Cm EscapeChar
665 Sets the escape character (default:
666 .Ql ~ ) .
667 The escape character can also
668 be set on the command line.
669 The argument should be a single character,
670 .Ql ^
671 followed by a letter, or
672 .Cm none
673 to disable the escape
674 character entirely (making the connection transparent for binary
675 data).
676 .It Cm ExitOnForwardFailure
677 Specifies whether
678 .Xr ssh 1
679 should terminate the connection if it cannot set up all requested
680 dynamic, tunnel, local, and remote port forwardings, (e.g.\&
681 if either end is unable to bind and listen on a specified port).
682 Note that
683 .Cm ExitOnForwardFailure
684 does not apply to connections made over port forwardings and will not,
685 for example, cause
686 .Xr ssh 1
687 to exit if TCP connections to the ultimate forwarding destination fail.
688 The argument must be
689 .Cm yes
690 or
691 .Cm no
692 (the default).
693 .It Cm FingerprintHash
694 Specifies the hash algorithm used when displaying key fingerprints.
695 Valid options are:
696 .Cm md5
697 and
698 .Cm sha256
699 (the default).
700 .It Cm ForkAfterAuthentication
701 Requests
702 .Nm ssh
703 to go to background just before command execution.
704 This is useful if
705 .Nm ssh
706 is going to ask for passwords or passphrases, but the user
707 wants it in the background.
708 This implies the
709 .Cm StdinNull
710 configuration option being set to
711 .Dq yes .
712 The recommended way to start X11 programs at a remote site is with
713 something like
714 .Ic ssh -f host xterm ,
715 which is the same as
716 .Ic ssh host xterm
717 if the
718 .Cm ForkAfterAuthentication
719 configuration option is set to
720 .Dq yes .
721 .Pp
722 If the
723 .Cm ExitOnForwardFailure
724 configuration option is set to
725 .Dq yes ,
726 then a client started with the
727 .Cm ForkAfterAuthentication
728 configuration option being set to
729 .Dq yes
730 will wait for all remote port forwards to be successfully established
731 before placing itself in the background.
732 The argument to this keyword must be
733 .Cm yes
734 (same as the
735 .Fl f
736 option) or
737 .Cm no
738 (the default).
739 .It Cm ForwardAgent
740 Specifies whether the connection to the authentication agent (if any)
741 will be forwarded to the remote machine.
742 The argument may be
743 .Cm yes ,
744 .Cm no
745 (the default),
746 an explicit path to an agent socket or the name of an environment variable
747 (beginning with
748 .Sq $ )
749 in which to find the path.
750 .Pp
751 Agent forwarding should be enabled with caution.
752 Users with the ability to bypass file permissions on the remote host
753 (for the agent's Unix-domain socket)
754 can access the local agent through the forwarded connection.
755 An attacker cannot obtain key material from the agent,
756 however they can perform operations on the keys that enable them to
757 authenticate using the identities loaded into the agent.
758 .It Cm ForwardX11
759 Specifies whether X11 connections will be automatically redirected
760 over the secure channel and
761 .Ev DISPLAY
762 set.
763 The argument must be
764 .Cm yes
765 or
766 .Cm no
767 (the default).
768 .Pp
769 X11 forwarding should be enabled with caution.
770 Users with the ability to bypass file permissions on the remote host
771 (for the user's X11 authorization database)
772 can access the local X11 display through the forwarded connection.
773 An attacker may then be able to perform activities such as keystroke monitoring
774 if the
775 .Cm ForwardX11Trusted
776 option is also enabled.
777 .It Cm ForwardX11Timeout
778 Specify a timeout for untrusted X11 forwarding
779 using the format described in the
780 .Sx TIME FORMATS
781 section of
782 .Xr sshd_config 5 .
783 X11 connections received by
784 .Xr ssh 1
785 after this time will be refused.
786 Setting
787 .Cm ForwardX11Timeout
788 to zero will disable the timeout and permit X11 forwarding for the life
789 of the connection.
790 The default is to disable untrusted X11 forwarding after twenty minutes has
791 elapsed.
792 .It Cm ForwardX11Trusted
793 If this option is set to
794 .Cm yes ,
795 remote X11 clients will have full access to the original X11 display.
796 .Pp
797 If this option is set to
798 .Cm no
799 (the default),
800 remote X11 clients will be considered untrusted and prevented
801 from stealing or tampering with data belonging to trusted X11
802 clients.
803 Furthermore, the
804 .Xr xauth 1
805 token used for the session will be set to expire after 20 minutes.
806 Remote clients will be refused access after this time.
807 .Pp
808 See the X11 SECURITY extension specification for full details on
809 the restrictions imposed on untrusted clients.
810 .It Cm GatewayPorts
811 Specifies whether remote hosts are allowed to connect to local
812 forwarded ports.
813 By default,
814 .Xr ssh 1
815 binds local port forwardings to the loopback address.
816 This prevents other remote hosts from connecting to forwarded ports.
817 .Cm GatewayPorts
818 can be used to specify that ssh
819 should bind local port forwardings to the wildcard address,
820 thus allowing remote hosts to connect to forwarded ports.
821 The argument must be
822 .Cm yes
823 or
824 .Cm no
825 (the default).
826 .It Cm GlobalKnownHostsFile
827 Specifies one or more files to use for the global
828 host key database, separated by whitespace.
829 The default is
830 .Pa /etc/ssh/ssh_known_hosts ,
831 .Pa /etc/ssh/ssh_known_hosts2 .
832 .It Cm GSSAPIAuthentication
833 Specifies whether user authentication based on GSSAPI is allowed.
834 The default is
835 .Cm no .
836 .It Cm GSSAPIDelegateCredentials
837 Forward (delegate) credentials to the server.
838 The default is
839 .Cm no .
840 .It Cm HashKnownHosts
841 Indicates that
842 .Xr ssh 1
843 should hash host names and addresses when they are added to
844 .Pa ~/.ssh/known_hosts .
845 These hashed names may be used normally by
846 .Xr ssh 1
847 and
848 .Xr sshd 8 ,
849 but they do not visually reveal identifying information if the
850 file's contents are disclosed.
851 The default is
852 .Cm no .
853 Note that existing names and addresses in known hosts files
854 will not be converted automatically,
855 but may be manually hashed using
856 .Xr ssh-keygen 1 .
857 .It Cm HostbasedAcceptedAlgorithms
858 Specifies the signature algorithms that will be used for hostbased
859 authentication as a comma-separated list of patterns.
860 Alternately if the specified list begins with a
861 .Sq +
862 character, then the specified signature algorithms will be appended
863 to the default set instead of replacing them.
864 If the specified list begins with a
865 .Sq -
866 character, then the specified signature algorithms (including wildcards)
867 will be removed from the default set instead of replacing them.
868 If the specified list begins with a
869 .Sq ^
870 character, then the specified signature algorithms will be placed
871 at the head of the default set.
872 The default for this option is:
873 .Bd -literal -offset 3n
874 ssh-ed25519-cert-v01@openssh.com,
875 ecdsa-sha2-nistp256-cert-v01@openssh.com,
876 ecdsa-sha2-nistp384-cert-v01@openssh.com,
877 ecdsa-sha2-nistp521-cert-v01@openssh.com,
878 sk-ssh-ed25519-cert-v01@openssh.com,
879 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
880 rsa-sha2-512-cert-v01@openssh.com,
881 rsa-sha2-256-cert-v01@openssh.com,
882 ssh-ed25519,
883 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
884 sk-ssh-ed25519@openssh.com,
885 sk-ecdsa-sha2-nistp256@openssh.com,
886 rsa-sha2-512,rsa-sha2-256
887 .Ed
888 .Pp
889 The
890 .Fl Q
891 option of
892 .Xr ssh 1
893 may be used to list supported signature algorithms.
894 This was formerly named HostbasedKeyTypes.
895 .It Cm HostbasedAuthentication
896 Specifies whether to try rhosts based authentication with public key
897 authentication.
898 The argument must be
899 .Cm yes
900 or
901 .Cm no
902 (the default).
903 .It Cm HostKeyAlgorithms
904 Specifies the host key signature algorithms
905 that the client wants to use in order of preference.
906 Alternately if the specified list begins with a
907 .Sq +
908 character, then the specified signature algorithms will be appended to
909 the default set instead of replacing them.
910 If the specified list begins with a
911 .Sq -
912 character, then the specified signature algorithms (including wildcards)
913 will be removed from the default set instead of replacing them.
914 If the specified list begins with a
915 .Sq ^
916 character, then the specified signature algorithms will be placed
917 at the head of the default set.
918 The default for this option is:
919 .Bd -literal -offset 3n
920 ssh-ed25519-cert-v01@openssh.com,
921 ecdsa-sha2-nistp256-cert-v01@openssh.com,
922 ecdsa-sha2-nistp384-cert-v01@openssh.com,
923 ecdsa-sha2-nistp521-cert-v01@openssh.com,
924 sk-ssh-ed25519-cert-v01@openssh.com,
925 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
926 rsa-sha2-512-cert-v01@openssh.com,
927 rsa-sha2-256-cert-v01@openssh.com,
928 ssh-ed25519,
929 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
930 sk-ecdsa-sha2-nistp256@openssh.com,
931 sk-ssh-ed25519@openssh.com,
932 rsa-sha2-512,rsa-sha2-256
933 .Ed
934 .Pp
935 If hostkeys are known for the destination host then this default is modified
936 to prefer their algorithms.
937 .Pp
938 The list of available signature algorithms may also be obtained using
939 .Qq ssh -Q HostKeyAlgorithms .
940 .It Cm HostKeyAlias
941 Specifies an alias that should be used instead of the
942 real host name when looking up or saving the host key
943 in the host key database files and when validating host certificates.
944 This option is useful for tunneling SSH connections
945 or for multiple servers running on a single host.
946 .It Cm Hostname
947 Specifies the real host name to log into.
948 This can be used to specify nicknames or abbreviations for hosts.
949 Arguments to
950 .Cm Hostname
951 accept the tokens described in the
952 .Sx TOKENS
953 section.
954 Numeric IP addresses are also permitted (both on the command line and in
955 .Cm Hostname
956 specifications).
957 The default is the name given on the command line.
958 .It Cm IdentitiesOnly
959 Specifies that
960 .Xr ssh 1
961 should only use the configured authentication identity and certificate files
962 (either the default files, or those explicitly configured in the
963 .Nm
964 files
965 or passed on the
966 .Xr ssh 1
967 command-line),
968 even if
969 .Xr ssh-agent 1
970 or a
971 .Cm PKCS11Provider
972 or
973 .Cm SecurityKeyProvider
974 offers more identities.
975 The argument to this keyword must be
976 .Cm yes
977 or
978 .Cm no
979 (the default).
980 This option is intended for situations where ssh-agent
981 offers many different identities.
982 .It Cm IdentityAgent
983 Specifies the
984 .Ux Ns -domain
985 socket used to communicate with the authentication agent.
986 .Pp
987 This option overrides the
988 .Ev SSH_AUTH_SOCK
989 environment variable and can be used to select a specific agent.
990 Setting the socket name to
991 .Cm none
992 disables the use of an authentication agent.
993 If the string
994 .Qq SSH_AUTH_SOCK
995 is specified, the location of the socket will be read from the
996 .Ev SSH_AUTH_SOCK
997 environment variable.
998 Otherwise if the specified value begins with a
999 .Sq $
1000 character, then it will be treated as an environment variable containing
1001 the location of the socket.
1002 .Pp
1003 Arguments to
1004 .Cm IdentityAgent
1005 may use the tilde syntax to refer to a user's home directory,
1006 the tokens described in the
1007 .Sx TOKENS
1008 section and environment variables as described in the
1009 .Sx ENVIRONMENT VARIABLES
1010 section.
1011 .It Cm IdentityFile
1012 Specifies a file from which the user's DSA, ECDSA, authenticator-hosted ECDSA,
1013 Ed25519, authenticator-hosted Ed25519 or RSA authentication identity is read.
1014 The default is
1015 .Pa ~/.ssh/id_dsa ,
1016 .Pa ~/.ssh/id_ecdsa ,
1017 .Pa ~/.ssh/id_ecdsa_sk ,
1018 .Pa ~/.ssh/id_ed25519 ,
1019 .Pa ~/.ssh/id_ed25519_sk
1020 and
1021 .Pa ~/.ssh/id_rsa .
1022 Additionally, any identities represented by the authentication agent
1023 will be used for authentication unless
1024 .Cm IdentitiesOnly
1025 is set.
1026 If no certificates have been explicitly specified by
1027 .Cm CertificateFile ,
1028 .Xr ssh 1
1029 will try to load certificate information from the filename obtained by
1030 appending
1031 .Pa -cert.pub
1032 to the path of a specified
1033 .Cm IdentityFile .
1034 .Pp
1035 Arguments to
1036 .Cm IdentityFile
1037 may use the tilde syntax to refer to a user's home directory
1038 or the tokens described in the
1039 .Sx TOKENS
1040 section.
1041 .Pp
1042 It is possible to have
1043 multiple identity files specified in configuration files; all these
1044 identities will be tried in sequence.
1045 Multiple
1046 .Cm IdentityFile
1047 directives will add to the list of identities tried (this behaviour
1048 differs from that of other configuration directives).
1049 .Pp
1050 .Cm IdentityFile
1051 may be used in conjunction with
1052 .Cm IdentitiesOnly
1053 to select which identities in an agent are offered during authentication.
1054 .Cm IdentityFile
1055 may also be used in conjunction with
1056 .Cm CertificateFile
1057 in order to provide any certificate also needed for authentication with
1058 the identity.
1059 .It Cm IgnoreUnknown
1060 Specifies a pattern-list of unknown options to be ignored if they are
1061 encountered in configuration parsing.
1062 This may be used to suppress errors if
1063 .Nm
1064 contains options that are unrecognised by
1065 .Xr ssh 1 .
1066 It is recommended that
1067 .Cm IgnoreUnknown
1068 be listed early in the configuration file as it will not be applied
1069 to unknown options that appear before it.
1070 .It Cm Include
1071 Include the specified configuration file(s).
1072 Multiple pathnames may be specified and each pathname may contain
1073 .Xr glob 7
1074 wildcards and, for user configurations, shell-like
1075 .Sq ~
1076 references to user home directories.
1077 Wildcards will be expanded and processed in lexical order.
1078 Files without absolute paths are assumed to be in
1079 .Pa ~/.ssh
1080 if included in a user configuration file or
1081 .Pa /etc/ssh
1082 if included from the system configuration file.
1083 .Cm Include
1084 directive may appear inside a
1085 .Cm Match
1086 or
1087 .Cm Host
1088 block
1089 to perform conditional inclusion.
1090 .It Cm IPQoS
1091 Specifies the IPv4 type-of-service or DSCP class for connections.
1092 Accepted values are
1093 .Cm af11 ,
1094 .Cm af12 ,
1095 .Cm af13 ,
1096 .Cm af21 ,
1097 .Cm af22 ,
1098 .Cm af23 ,
1099 .Cm af31 ,
1100 .Cm af32 ,
1101 .Cm af33 ,
1102 .Cm af41 ,
1103 .Cm af42 ,
1104 .Cm af43 ,
1105 .Cm cs0 ,
1106 .Cm cs1 ,
1107 .Cm cs2 ,
1108 .Cm cs3 ,
1109 .Cm cs4 ,
1110 .Cm cs5 ,
1111 .Cm cs6 ,
1112 .Cm cs7 ,
1113 .Cm ef ,
1114 .Cm le ,
1115 .Cm lowdelay ,
1116 .Cm throughput ,
1117 .Cm reliability ,
1118 a numeric value, or
1119 .Cm none
1120 to use the operating system default.
1121 This option may take one or two arguments, separated by whitespace.
1122 If one argument is specified, it is used as the packet class unconditionally.
1123 If two values are specified, the first is automatically selected for
1124 interactive sessions and the second for non-interactive sessions.
1125 The default is
1126 .Cm af21
1127 (Low-Latency Data)
1128 for interactive sessions and
1129 .Cm cs1
1130 (Lower Effort)
1131 for non-interactive sessions.
1132 .It Cm KbdInteractiveAuthentication
1133 Specifies whether to use keyboard-interactive authentication.
1134 The argument to this keyword must be
1135 .Cm yes
1136 (the default)
1137 or
1138 .Cm no .
1139 .Cm ChallengeResponseAuthentication
1140 is a deprecated alias for this.
1141 .It Cm KbdInteractiveDevices
1142 Specifies the list of methods to use in keyboard-interactive authentication.
1143 Multiple method names must be comma-separated.
1144 The default is to use the server specified list.
1145 The methods available vary depending on what the server supports.
1146 For an OpenSSH server,
1147 it may be zero or more of:
1148 .Cm bsdauth
1149 and
1150 .Cm pam .
1151 .It Cm KexAlgorithms
1152 Specifies the available KEX (Key Exchange) algorithms.
1153 Multiple algorithms must be comma-separated.
1154 If the specified list begins with a
1155 .Sq +
1156 character, then the specified algorithms will be appended to the default set
1157 instead of replacing them.
1158 If the specified list begins with a
1159 .Sq -
1160 character, then the specified algorithms (including wildcards) will be removed
1161 from the default set instead of replacing them.
1162 If the specified list begins with a
1163 .Sq ^
1164 character, then the specified algorithms will be placed at the head of the
1165 default set.
1166 The default is:
1167 .Bd -literal -offset indent
1168 curve25519-sha256,curve25519-sha256@libssh.org,
1169 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
1170 diffie-hellman-group-exchange-sha256,
1171 diffie-hellman-group16-sha512,
1172 diffie-hellman-group18-sha512,
1173 diffie-hellman-group14-sha256
1174 .Ed
1175 .Pp
1176 The list of available key exchange algorithms may also be obtained using
1177 .Qq ssh -Q kex .
1178 .It Cm KnownHostsCommand
1179 Specifies a command to use to obtain a list of host keys, in addition to
1180 those listed in
1181 .Cm UserKnownHostsFile
1182 and
1183 .Cm GlobalKnownHostsFile .
1184 This command is executed after the files have been read.
1185 It may write host key lines to standard output in identical format to the
1186 usual files (described in the
1187 .Sx VERIFYING HOST KEYS
1188 section in
1189 .Xr ssh 1 ) .
1190 Arguments to
1191 .Cm KnownHostsCommand
1192 accept the tokens described in the
1193 .Sx TOKENS
1194 section.
1195 The command may be invoked multiple times per connection: once when preparing
1196 the preference list of host key algorithms to use, again to obtain the
1197 host key for the requested host name and, if
1198 .Cm CheckHostIP
1199 is enabled, one more time to obtain the host key matching the server's
1200 address.
1201 If the command exits abnormally or returns a non-zero exit status then the
1202 connection is terminated.
1203 .It Cm LocalCommand
1204 Specifies a command to execute on the local machine after successfully
1205 connecting to the server.
1206 The command string extends to the end of the line, and is executed with
1207 the user's shell.
1208 Arguments to
1209 .Cm LocalCommand
1210 accept the tokens described in the
1211 .Sx TOKENS
1212 section.
1213 .Pp
1214 The command is run synchronously and does not have access to the
1215 session of the
1216 .Xr ssh 1
1217 that spawned it.
1218 It should not be used for interactive commands.
1219 .Pp
1220 This directive is ignored unless
1221 .Cm PermitLocalCommand
1222 has been enabled.
1223 .It Cm LocalForward
1224 Specifies that a TCP port on the local machine be forwarded over
1225 the secure channel to the specified host and port from the remote machine.
1226 The first argument specifies the listener and may be
1227 .Sm off
1228 .Oo Ar bind_address : Oc Ar port
1229 .Sm on
1230 or a Unix domain socket path.
1231 The second argument is the destination and may be
1232 .Ar host : Ns Ar hostport
1233 or a Unix domain socket path if the remote host supports it.
1234 .Pp
1235 IPv6 addresses can be specified by enclosing addresses in square brackets.
1236 Multiple forwardings may be specified, and additional forwardings can be
1237 given on the command line.
1238 Only the superuser can forward privileged ports.
1239 By default, the local port is bound in accordance with the
1240 .Cm GatewayPorts
1241 setting.
1242 However, an explicit
1243 .Ar bind_address
1244 may be used to bind the connection to a specific address.
1245 The
1246 .Ar bind_address
1247 of
1248 .Cm localhost
1249 indicates that the listening port be bound for local use only, while an
1250 empty address or
1251 .Sq *
1252 indicates that the port should be available from all interfaces.
1253 Unix domain socket paths may use the tokens described in the
1254 .Sx TOKENS
1255 section and environment variables as described in the
1256 .Sx ENVIRONMENT VARIABLES
1257 section.
1258 .It Cm LogLevel
1259 Gives the verbosity level that is used when logging messages from
1260 .Xr ssh 1 .
1261 The possible values are:
1262 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
1263 The default is INFO.
1264 DEBUG and DEBUG1 are equivalent.
1265 DEBUG2 and DEBUG3 each specify higher levels of verbose output.
1266 .It Cm LogVerbose
1267 Specify one or more overrides to LogLevel.
1268 An override consists of a pattern lists that matches the source file, function
1269 and line number to force detailed logging for.
1270 For example, an override pattern of:
1271 .Bd -literal -offset indent
1272 kex.c:*:1000,*:kex_exchange_identification():*,packet.c:*
1273 .Ed
1274 .Pp
1275 would enable detailed logging for line 1000 of
1276 .Pa kex.c ,
1277 everything in the
1278 .Fn kex_exchange_identification
1279 function, and all code in the
1280 .Pa packet.c
1281 file.
1282 This option is intended for debugging and no overrides are enabled by default.
1283 .It Cm MACs
1284 Specifies the MAC (message authentication code) algorithms
1285 in order of preference.
1286 The MAC algorithm is used for data integrity protection.
1287 Multiple algorithms must be comma-separated.
1288 If the specified list begins with a
1289 .Sq +
1290 character, then the specified algorithms will be appended to the default set
1291 instead of replacing them.
1292 If the specified list begins with a
1293 .Sq -
1294 character, then the specified algorithms (including wildcards) will be removed
1295 from the default set instead of replacing them.
1296 If the specified list begins with a
1297 .Sq ^
1298 character, then the specified algorithms will be placed at the head of the
1299 default set.
1300 .Pp
1301 The algorithms that contain
1302 .Qq -etm
1303 calculate the MAC after encryption (encrypt-then-mac).
1304 These are considered safer and their use recommended.
1305 .Pp
1306 The default is:
1307 .Bd -literal -offset indent
1308 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
1309 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
1310 hmac-sha1-etm@openssh.com,
1311 umac-64@openssh.com,umac-128@openssh.com,
1312 hmac-sha2-256,hmac-sha2-512,hmac-sha1
1313 .Ed
1314 .Pp
1315 The list of available MAC algorithms may also be obtained using
1316 .Qq ssh -Q mac .
1317 .It Cm NoHostAuthenticationForLocalhost
1318 Disable host authentication for localhost (loopback addresses).
1319 The argument to this keyword must be
1320 .Cm yes
1321 or
1322 .Cm no
1323 (the default).
1324 .It Cm NumberOfPasswordPrompts
1325 Specifies the number of password prompts before giving up.
1326 The argument to this keyword must be an integer.
1327 The default is 3.
1328 .It Cm PasswordAuthentication
1329 Specifies whether to use password authentication.
1330 The argument to this keyword must be
1331 .Cm yes
1332 (the default)
1333 or
1334 .Cm no .
1335 .It Cm PermitLocalCommand
1336 Allow local command execution via the
1337 .Ic LocalCommand
1338 option or using the
1339 .Ic !\& Ns Ar command
1340 escape sequence in
1341 .Xr ssh 1 .
1342 The argument must be
1343 .Cm yes
1344 or
1345 .Cm no
1346 (the default).
1347 .It Cm PermitRemoteOpen
1348 Specifies the destinations to which remote TCP port forwarding is permitted when
1349 .Cm RemoteForward
1350 is used as a SOCKS proxy.
1351 The forwarding specification must be one of the following forms:
1352 .Pp
1353 .Bl -item -offset indent -compact
1354 .It
1355 .Cm PermitRemoteOpen
1356 .Sm off
1357 .Ar host : port
1358 .Sm on
1359 .It
1360 .Cm PermitRemoteOpen
1361 .Sm off
1362 .Ar IPv4_addr : port
1363 .Sm on
1364 .It
1365 .Cm PermitRemoteOpen
1366 .Sm off
1367 .Ar \&[ IPv6_addr \&] : port
1368 .Sm on
1369 .El
1370 .Pp
1371 Multiple forwards may be specified by separating them with whitespace.
1372 An argument of
1373 .Cm any
1374 can be used to remove all restrictions and permit any forwarding requests.
1375 An argument of
1376 .Cm none
1377 can be used to prohibit all forwarding requests.
1378 The wildcard
1379 .Sq *
1380 can be used for host or port to allow all hosts or ports respectively.
1381 Otherwise, no pattern matching or address lookups are performed on supplied
1382 names.
1383 .It Cm PKCS11Provider
1384 Specifies which PKCS#11 provider to use or
1385 .Cm none
1386 to indicate that no provider should be used (the default).
1387 The argument to this keyword is a path to the PKCS#11 shared library
1388 .Xr ssh 1
1389 should use to communicate with a PKCS#11 token providing keys for user
1390 authentication.
1391 .It Cm Port
1392 Specifies the port number to connect on the remote host.
1393 The default is 22.
1394 .It Cm PreferredAuthentications
1395 Specifies the order in which the client should try authentication methods.
1396 This allows a client to prefer one method (e.g.\&
1397 .Cm keyboard-interactive )
1398 over another method (e.g.\&
1399 .Cm password ) .
1400 The default is:
1401 .Bd -literal -offset indent
1402 gssapi-with-mic,hostbased,publickey,
1403 keyboard-interactive,password
1404 .Ed
1405 .It Cm ProxyCommand
1406 Specifies the command to use to connect to the server.
1407 The command
1408 string extends to the end of the line, and is executed
1409 using the user's shell
1410 .Ql exec
1411 directive to avoid a lingering shell process.
1412 .Pp
1413 Arguments to
1414 .Cm ProxyCommand
1415 accept the tokens described in the
1416 .Sx TOKENS
1417 section.
1418 The command can be basically anything,
1419 and should read from its standard input and write to its standard output.
1420 It should eventually connect an
1421 .Xr sshd 8
1422 server running on some machine, or execute
1423 .Ic sshd -i
1424 somewhere.
1425 Host key management will be done using the
1426 .Cm Hostname
1427 of the host being connected (defaulting to the name typed by the user).
1428 Setting the command to
1429 .Cm none
1430 disables this option entirely.
1431 Note that
1432 .Cm CheckHostIP
1433 is not available for connects with a proxy command.
1434 .Pp
1435 This directive is useful in conjunction with
1436 .Xr nc 1
1437 and its proxy support.
1438 For example, the following directive would connect via an HTTP proxy at
1439 192.0.2.0:
1440 .Bd -literal -offset 3n
1441 ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p
1442 .Ed
1443 .It Cm ProxyJump
1444 Specifies one or more jump proxies as either
1445 .Xo
1446 .Sm off
1447 .Op Ar user No @
1448 .Ar host
1449 .Op : Ns Ar port
1450 .Sm on
1451 or an ssh URI
1452 .Xc .
1453 Multiple proxies may be separated by comma characters and will be visited
1454 sequentially.
1455 Setting this option will cause
1456 .Xr ssh 1
1457 to connect to the target host by first making a
1458 .Xr ssh 1
1459 connection to the specified
1460 .Cm ProxyJump
1461 host and then establishing a
1462 TCP forwarding to the ultimate target from there.
1463 Setting the host to
1464 .Cm none
1465 disables this option entirely.
1466 .Pp
1467 Note that this option will compete with the
1468 .Cm ProxyCommand
1469 option - whichever is specified first will prevent later instances of the
1470 other from taking effect.
1471 .Pp
1472 Note also that the configuration for the destination host (either supplied
1473 via the command-line or the configuration file) is not generally applied
1474 to jump hosts.
1475 .Pa ~/.ssh/config
1476 should be used if specific configuration is required for jump hosts.
1477 .It Cm ProxyUseFdpass
1478 Specifies that
1479 .Cm ProxyCommand
1480 will pass a connected file descriptor back to
1481 .Xr ssh 1
1482 instead of continuing to execute and pass data.
1483 The default is
1484 .Cm no .
1485 .It Cm PubkeyAcceptedAlgorithms
1486 Specifies the signature algorithms that will be used for public key
1487 authentication as a comma-separated list of patterns.
1488 If the specified list begins with a
1489 .Sq +
1490 character, then the algorithms after it will be appended to the default
1491 instead of replacing it.
1492 If the specified list begins with a
1493 .Sq -
1494 character, then the specified algorithms (including wildcards) will be removed
1495 from the default set instead of replacing them.
1496 If the specified list begins with a
1497 .Sq ^
1498 character, then the specified algorithms will be placed at the head of the
1499 default set.
1500 The default for this option is:
1501 .Bd -literal -offset 3n
1502 ssh-ed25519-cert-v01@openssh.com,
1503 ecdsa-sha2-nistp256-cert-v01@openssh.com,
1504 ecdsa-sha2-nistp384-cert-v01@openssh.com,
1505 ecdsa-sha2-nistp521-cert-v01@openssh.com,
1506 sk-ssh-ed25519-cert-v01@openssh.com,
1507 sk-ecdsa-sha2-nistp256-cert-v01@openssh.com,
1508 rsa-sha2-512-cert-v01@openssh.com,
1509 rsa-sha2-256-cert-v01@openssh.com,
1510 ssh-ed25519,
1511 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
1512 sk-ssh-ed25519@openssh.com,
1513 sk-ecdsa-sha2-nistp256@openssh.com,
1514 rsa-sha2-512,rsa-sha2-256
1515 .Ed
1516 .Pp
1517 The list of available signature algorithms may also be obtained using
1518 .Qq ssh -Q PubkeyAcceptedAlgorithms .
1519 .It Cm PubkeyAuthentication
1520 Specifies whether to try public key authentication.
1521 The argument to this keyword must be
1522 .Cm yes
1523 (the default)
1524 or
1525 .Cm no .
1526 .It Cm RekeyLimit
1527 Specifies the maximum amount of data that may be transmitted before the
1528 session key is renegotiated, optionally followed by a maximum amount of
1529 time that may pass before the session key is renegotiated.
1530 The first argument is specified in bytes and may have a suffix of
1531 .Sq K ,
1532 .Sq M ,
1533 or
1534 .Sq G
1535 to indicate Kilobytes, Megabytes, or Gigabytes, respectively.
1536 The default is between
1537 .Sq 1G
1538 and
1539 .Sq 4G ,
1540 depending on the cipher.
1541 The optional second value is specified in seconds and may use any of the
1542 units documented in the TIME FORMATS section of
1543 .Xr sshd_config 5 .
1544 The default value for
1545 .Cm RekeyLimit
1546 is
1547 .Cm default none ,
1548 which means that rekeying is performed after the cipher's default amount
1549 of data has been sent or received and no time based rekeying is done.
1550 .It Cm RemoteCommand
1551 Specifies a command to execute on the remote machine after successfully
1552 connecting to the server.
1553 The command string extends to the end of the line, and is executed with
1554 the user's shell.
1555 Arguments to
1556 .Cm RemoteCommand
1557 accept the tokens described in the
1558 .Sx TOKENS
1559 section.
1560 .It Cm RemoteForward
1561 Specifies that a TCP port on the remote machine be forwarded over
1562 the secure channel.
1563 The remote port may either be forwarded to a specified host and port
1564 from the local machine, or may act as a SOCKS 4/5 proxy that allows a remote
1565 client to connect to arbitrary destinations from the local machine.
1566 The first argument is the listening specification and may be
1567 .Sm off
1568 .Oo Ar bind_address : Oc Ar port
1569 .Sm on
1570 or, if the remote host supports it, a Unix domain socket path.
1571 If forwarding to a specific destination then the second argument must be
1572 .Ar host : Ns Ar hostport
1573 or a Unix domain socket path,
1574 otherwise if no destination argument is specified then the remote forwarding
1575 will be established as a SOCKS proxy.
1576 When acting as a SOCKS proxy the destination of the connection can be
1577 restricted by
1578 .Cm PermitRemoteOpen .
1579 .Pp
1580 IPv6 addresses can be specified by enclosing addresses in square brackets.
1581 Multiple forwardings may be specified, and additional
1582 forwardings can be given on the command line.
1583 Privileged ports can be forwarded only when
1584 logging in as root on the remote machine.
1585 Unix domain socket paths may use the tokens described in the
1586 .Sx TOKENS
1587 section and environment variables as described in the
1588 .Sx ENVIRONMENT VARIABLES
1589 section.
1590 .Pp
1591 If the
1592 .Ar port
1593 argument is 0,
1594 the listen port will be dynamically allocated on the server and reported
1595 to the client at run time.
1596 .Pp
1597 If the
1598 .Ar bind_address
1599 is not specified, the default is to only bind to loopback addresses.
1600 If the
1601 .Ar bind_address
1602 is
1603 .Ql *
1604 or an empty string, then the forwarding is requested to listen on all
1605 interfaces.
1606 Specifying a remote
1607 .Ar bind_address
1608 will only succeed if the server's
1609 .Cm GatewayPorts
1610 option is enabled (see
1611 .Xr sshd_config 5 ) .
1612 .It Cm RequestTTY
1613 Specifies whether to request a pseudo-tty for the session.
1614 The argument may be one of:
1615 .Cm no
1616 (never request a TTY),
1617 .Cm yes
1618 (always request a TTY when standard input is a TTY),
1619 .Cm force
1620 (always request a TTY) or
1621 .Cm auto
1622 (request a TTY when opening a login session).
1623 This option mirrors the
1624 .Fl t
1625 and
1626 .Fl T
1627 flags for
1628 .Xr ssh 1 .
1629 .It Cm RevokedHostKeys
1630 Specifies revoked host public keys.
1631 Keys listed in this file will be refused for host authentication.
1632 Note that if this file does not exist or is not readable,
1633 then host authentication will be refused for all hosts.
1634 Keys may be specified as a text file, listing one public key per line, or as
1635 an OpenSSH Key Revocation List (KRL) as generated by
1636 .Xr ssh-keygen 1 .
1637 For more information on KRLs, see the KEY REVOCATION LISTS section in
1638 .Xr ssh-keygen 1 .
1639 .It Cm SecurityKeyProvider
1640 Specifies a path to a library that will be used when loading any
1641 FIDO authenticator-hosted keys, overriding the default of using
1642 the built-in USB HID support.
1643 .Pp
1644 If the specified value begins with a
1645 .Sq $
1646 character, then it will be treated as an environment variable containing
1647 the path to the library.
1648 .It Cm SendEnv
1649 Specifies what variables from the local
1650 .Xr environ 7
1651 should be sent to the server.
1652 The server must also support it, and the server must be configured to
1653 accept these environment variables.
1654 Note that the
1655 .Ev TERM
1656 environment variable is always sent whenever a
1657 pseudo-terminal is requested as it is required by the protocol.
1658 Refer to
1659 .Cm AcceptEnv
1660 in
1661 .Xr sshd_config 5
1662 for how to configure the server.
1663 Variables are specified by name, which may contain wildcard characters.
1664 Multiple environment variables may be separated by whitespace or spread
1665 across multiple
1666 .Cm SendEnv
1667 directives.
1668 .Pp
1669 See
1670 .Sx PATTERNS
1671 for more information on patterns.
1672 .Pp
1673 It is possible to clear previously set
1674 .Cm SendEnv
1675 variable names by prefixing patterns with
1676 .Pa - .
1677 The default is not to send any environment variables.
1678 .It Cm ServerAliveCountMax
1679 Sets the number of server alive messages (see below) which may be
1680 sent without
1681 .Xr ssh 1
1682 receiving any messages back from the server.
1683 If this threshold is reached while server alive messages are being sent,
1684 ssh will disconnect from the server, terminating the session.
1685 It is important to note that the use of server alive messages is very
1686 different from
1687 .Cm TCPKeepAlive
1688 (below).
1689 The server alive messages are sent through the encrypted channel
1690 and therefore will not be spoofable.
1691 The TCP keepalive option enabled by
1692 .Cm TCPKeepAlive
1693 is spoofable.
1694 The server alive mechanism is valuable when the client or
1695 server depend on knowing when a connection has become unresponsive.
1696 .Pp
1697 The default value is 3.
1698 If, for example,
1699 .Cm ServerAliveInterval
1700 (see below) is set to 15 and
1701 .Cm ServerAliveCountMax
1702 is left at the default, if the server becomes unresponsive,
1703 ssh will disconnect after approximately 45 seconds.
1704 .It Cm ServerAliveInterval
1705 Sets a timeout interval in seconds after which if no data has been received
1706 from the server,
1707 .Xr ssh 1
1708 will send a message through the encrypted
1709 channel to request a response from the server.
1710 The default
1711 is 0, indicating that these messages will not be sent to the server.
1712 .It Cm SessionType
1713 May be used to either request invocation of a subsystem on the remote system,
1714 or to prevent the execution of a remote command at all.
1715 The latter is useful for just forwarding ports.
1716 The argument to this keyword must be
1717 .Cm none
1718 (same as the
1719 .Fl N
1720 option),
1721 .Cm subsystem
1722 (same as the
1723 .Fl s
1724 option) or
1725 .Cm default
1726 (shell or command execution).
1727 .It Cm SetEnv
1728 Directly specify one or more environment variables and their contents to
1729 be sent to the server.
1730 Similarly to
1731 .Cm SendEnv ,
1732 with the exception of the
1733 .Ev TERM
1734 variable, the server must be prepared to accept the environment variable.
1735 .It Cm StdinNull
1736 Redirects stdin from
1737 .Pa /dev/null
1738 (actually, prevents reading from stdin).
1739 Either this or the equivalent
1740 .Fl n
1741 option must be used when
1742 .Nm ssh
1743 is run in the background.
1744 The argument to this keyword must be
1745 .Cm yes
1746 (same as the
1747 .Fl n
1748 option) or
1749 .Cm no
1750 (the default).
1751 .It Cm StreamLocalBindMask
1752 Sets the octal file creation mode mask
1753 .Pq umask
1754 used when creating a Unix-domain socket file for local or remote
1755 port forwarding.
1756 This option is only used for port forwarding to a Unix-domain socket file.
1757 .Pp
1758 The default value is 0177, which creates a Unix-domain socket file that is
1759 readable and writable only by the owner.
1760 Note that not all operating systems honor the file mode on Unix-domain
1761 socket files.
1762 .It Cm StreamLocalBindUnlink
1763 Specifies whether to remove an existing Unix-domain socket file for local
1764 or remote port forwarding before creating a new one.
1765 If the socket file already exists and
1766 .Cm StreamLocalBindUnlink
1767 is not enabled,
1768 .Nm ssh
1769 will be unable to forward the port to the Unix-domain socket file.
1770 This option is only used for port forwarding to a Unix-domain socket file.
1771 .Pp
1772 The argument must be
1773 .Cm yes
1774 or
1775 .Cm no
1776 (the default).
1777 .It Cm StrictHostKeyChecking
1778 If this flag is set to
1779 .Cm yes ,
1780 .Xr ssh 1
1781 will never automatically add host keys to the
1782 .Pa ~/.ssh/known_hosts
1783 file, and refuses to connect to hosts whose host key has changed.
1784 This provides maximum protection against man-in-the-middle (MITM) attacks,
1785 though it can be annoying when the
1786 .Pa /etc/ssh/ssh_known_hosts
1787 file is poorly maintained or when connections to new hosts are
1788 frequently made.
1789 This option forces the user to manually
1790 add all new hosts.
1791 .Pp
1792 If this flag is set to
1793 .Cm accept-new
1794 then ssh will automatically add new host keys to the user's
1795 .Pa known_hosts
1796 file, but will not permit connections to hosts with
1797 changed host keys.
1798 If this flag is set to
1799 .Cm no
1800 or
1801 .Cm off ,
1802 ssh will automatically add new host keys to the user known hosts files
1803 and allow connections to hosts with changed hostkeys to proceed,
1804 subject to some restrictions.
1805 If this flag is set to
1806 .Cm ask
1807 (the default),
1808 new host keys
1809 will be added to the user known host files only after the user
1810 has confirmed that is what they really want to do, and
1811 ssh will refuse to connect to hosts whose host key has changed.
1812 The host keys of
1813 known hosts will be verified automatically in all cases.
1814 .It Cm SyslogFacility
1815 Gives the facility code that is used when logging messages from
1816 .Xr ssh 1 .
1817 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
1818 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
1819 The default is USER.
1820 .It Cm TCPKeepAlive
1821 Specifies whether the system should send TCP keepalive messages to the
1822 other side.
1823 If they are sent, death of the connection or crash of one
1824 of the machines will be properly noticed.
1825 However, this means that
1826 connections will die if the route is down temporarily, and some people
1827 find it annoying.
1828 .Pp
1829 The default is
1830 .Cm yes
1831 (to send TCP keepalive messages), and the client will notice
1832 if the network goes down or the remote host dies.
1833 This is important in scripts, and many users want it too.
1834 .Pp
1835 To disable TCP keepalive messages, the value should be set to
1836 .Cm no .
1837 See also
1838 .Cm ServerAliveInterval
1839 for protocol-level keepalives.
1840 .It Cm Tunnel
1841 Request
1842 .Xr tun 4
1843 device forwarding between the client and the server.
1844 The argument must be
1845 .Cm yes ,
1846 .Cm point-to-point
1847 (layer 3),
1848 .Cm ethernet
1849 (layer 2),
1850 or
1851 .Cm no
1852 (the default).
1853 Specifying
1854 .Cm yes
1855 requests the default tunnel mode, which is
1856 .Cm point-to-point .
1857 .It Cm TunnelDevice
1858 Specifies the
1859 .Xr tun 4
1860 devices to open on the client
1861 .Pq Ar local_tun
1862 and the server
1863 .Pq Ar remote_tun .
1864 .Pp
1865 The argument must be
1866 .Sm off
1867 .Ar local_tun Op : Ar remote_tun .
1868 .Sm on
1869 The devices may be specified by numerical ID or the keyword
1870 .Cm any ,
1871 which uses the next available tunnel device.
1872 If
1873 .Ar remote_tun
1874 is not specified, it defaults to
1875 .Cm any .
1876 The default is
1877 .Cm any:any .
1878 .It Cm UpdateHostKeys
1879 Specifies whether
1880 .Xr ssh 1
1881 should accept notifications of additional hostkeys from the server sent
1882 after authentication has completed and add them to
1883 .Cm UserKnownHostsFile .
1884 The argument must be
1885 .Cm yes ,
1886 .Cm no
1887 or
1888 .Cm ask .
1889 This option allows learning alternate hostkeys for a server
1890 and supports graceful key rotation by allowing a server to send replacement
1891 public keys before old ones are removed.
1892 .Pp
1893 Additional hostkeys are only accepted if the key used to authenticate the
1894 host was already trusted or explicitly accepted by the user, the host was
1895 authenticated via
1896 .Cm UserKnownHostsFile
1897 (i.e. not
1898 .Cm GlobalKnownHostsFile )
1899 and the host was authenticated using a plain key and not a certificate.
1900 .Pp
1901 .Cm UpdateHostKeys
1902 is enabled by default if the user has not overridden the default
1903 .Cm UserKnownHostsFile
1904 setting and has not enabled
1905 .Cm VerifyHostKeyDNS ,
1906 otherwise
1907 .Cm UpdateHostKeys
1908 will be set to
1909 .Cm no .
1910 .Pp
1911 If
1912 .Cm UpdateHostKeys
1913 is set to
1914 .Cm ask ,
1915 then the user is asked to confirm the modifications to the known_hosts file.
1916 Confirmation is currently incompatible with
1917 .Cm ControlPersist ,
1918 and will be disabled if it is enabled.
1919 .Pp
1920 Presently, only
1921 .Xr sshd 8
1922 from OpenSSH 6.8 and greater support the
1923 .Qq hostkeys@openssh.com
1924 protocol extension used to inform the client of all the server's hostkeys.
1925 .It Cm User
1926 Specifies the user to log in as.
1927 This can be useful when a different user name is used on different machines.
1928 This saves the trouble of
1929 having to remember to give the user name on the command line.
1930 .It Cm UserKnownHostsFile
1931 Specifies one or more files to use for the user
1932 host key database, separated by whitespace.
1933 Each filename may use tilde notation to refer to the user's home directory,
1934 the tokens described in the
1935 .Sx TOKENS
1936 section and environment variables as described in the
1937 .Sx ENVIRONMENT VARIABLES
1938 section.
1939 The default is
1940 .Pa ~/.ssh/known_hosts ,
1941 .Pa ~/.ssh/known_hosts2 .
1942 .It Cm VerifyHostKeyDNS
1943 Specifies whether to verify the remote key using DNS and SSHFP resource
1944 records.
1945 If this option is set to
1946 .Cm yes ,
1947 the client will implicitly trust keys that match a secure fingerprint
1948 from DNS.
1949 Insecure fingerprints will be handled as if this option was set to
1950 .Cm ask .
1951 If this option is set to
1952 .Cm ask ,
1953 information on fingerprint match will be displayed, but the user will still
1954 need to confirm new host keys according to the
1955 .Cm StrictHostKeyChecking
1956 option.
1957 The default is
1958 .Cm no .
1959 .Pp
1960 See also
1961 .Sx VERIFYING HOST KEYS
1962 in
1963 .Xr ssh 1 .
1964 .It Cm VisualHostKey
1965 If this flag is set to
1966 .Cm yes ,
1967 an ASCII art representation of the remote host key fingerprint is
1968 printed in addition to the fingerprint string at login and
1969 for unknown host keys.
1970 If this flag is set to
1971 .Cm no
1972 (the default),
1973 no fingerprint strings are printed at login and
1974 only the fingerprint string will be printed for unknown host keys.
1975 .It Cm XAuthLocation
1976 Specifies the full pathname of the
1977 .Xr xauth 1
1978 program.
1979 The default is
1980 .Pa /usr/X11R6/bin/xauth .
1981 .El
1982 .Sh PATTERNS
1983 A
1984 .Em pattern
1985 consists of zero or more non-whitespace characters,
1986 .Sq *
1987 (a wildcard that matches zero or more characters),
1988 or
1989 .Sq ?\&
1990 (a wildcard that matches exactly one character).
1991 For example, to specify a set of declarations for any host in the
1992 .Qq .co.uk
1993 set of domains,
1994 the following pattern could be used:
1995 .Pp
1996 .Dl Host *.co.uk
1997 .Pp
1998 The following pattern
1999 would match any host in the 192.168.0.[0-9] network range:
2000 .Pp
2001 .Dl Host 192.168.0.?
2002 .Pp
2003 A
2004 .Em pattern-list
2005 is a comma-separated list of patterns.
2006 Patterns within pattern-lists may be negated
2007 by preceding them with an exclamation mark
2008 .Pq Sq !\& .
2009 For example,
2010 to allow a key to be used from anywhere within an organization
2011 except from the
2012 .Qq dialup
2013 pool,
2014 the following entry (in authorized_keys) could be used:
2015 .Pp
2016 .Dl from=\&"!*.dialup.example.com,*.example.com\&"
2017 .Pp
2018 Note that a negated match will never produce a positive result by itself.
2019 For example, attempting to match
2020 .Qq host3
2021 against the following pattern-list will fail:
2022 .Pp
2023 .Dl from=\&"!host1,!host2\&"
2024 .Pp
2025 The solution here is to include a term that will yield a positive match,
2026 such as a wildcard:
2027 .Pp
2028 .Dl from=\&"!host1,!host2,*\&"
2029 .Sh TOKENS
2030 Arguments to some keywords can make use of tokens,
2031 which are expanded at runtime:
2032 .Pp
2033 .Bl -tag -width XXXX -offset indent -compact
2034 .It %%
2035 A literal
2036 .Sq % .
2037 .It \&%C
2038 Hash of %l%h%p%r.
2039 .It %d
2040 Local user's home directory.
2041 .It %f
2042 The fingerprint of the server's host key.
2043 .It %H
2044 The
2045 .Pa known_hosts
2046 hostname or address that is being searched for.
2047 .It %h
2048 The remote hostname.
2049 .It \%%I
2050 A string describing the reason for a
2051 .Cm KnownHostsCommand
2052 execution: either
2053 .Cm ADDRESS
2054 when looking up a host by address (only when
2055 .Cm CheckHostIP
2056 is enabled),
2057 .Cm HOSTNAME
2058 when searching by hostname, or
2059 .Cm ORDER
2060 when preparing the host key algorithm preference list to use for the
2061 destination host.
2062 .It %i
2063 The local user ID.
2064 .It %K
2065 The base64 encoded host key.
2066 .It %k
2067 The host key alias if specified, otherwise the original remote hostname given
2068 on the command line.
2069 .It %L
2070 The local hostname.
2071 .It %l
2072 The local hostname, including the domain name.
2073 .It %n
2074 The original remote hostname, as given on the command line.
2075 .It %p
2076 The remote port.
2077 .It %r
2078 The remote username.
2079 .It \&%T
2080 The local
2081 .Xr tun 4
2082 or
2083 .Xr tap 4
2084 network interface assigned if
2085 tunnel forwarding was requested, or
2086 .Qq NONE
2087 otherwise.
2088 .It %t
2089 The type of the server host key, e.g.
2090 .Cm ssh-ed25519 .
2091 .It %u
2092 The local username.
2093 .El
2094 .Pp
2095 .Cm CertificateFile ,
2096 .Cm ControlPath ,
2097 .Cm IdentityAgent ,
2098 .Cm IdentityFile ,
2099 .Cm KnownHostsCommand ,
2100 .Cm LocalForward ,
2101 .Cm Match exec ,
2102 .Cm RemoteCommand ,
2103 .Cm RemoteForward ,
2104 and
2105 .Cm UserKnownHostsFile
2106 accept the tokens %%, %C, %d, %h, %i, %k, %L, %l, %n, %p, %r, and %u.
2107 .Pp
2108 .Cm KnownHostsCommand
2109 additionally accepts the tokens %f, %H, %I, %K and %t.
2110 .Pp
2111 .Cm Hostname
2112 accepts the tokens %% and %h.
2113 .Pp
2114 .Cm LocalCommand
2115 accepts all tokens.
2116 .Pp
2117 .Cm ProxyCommand
2118 accepts the tokens %%, %h, %n, %p, and %r.
2119 .Sh ENVIRONMENT VARIABLES
2120 Arguments to some keywords can be expanded at runtime from environment
2121 variables on the client by enclosing them in
2122 .Ic ${} ,
2123 for example
2124 .Ic ${HOME}/.ssh
2125 would refer to the user's .ssh directory.
2126 If a specified environment variable does not exist then an error will be
2127 returned and the setting for that keyword will be ignored.
2128 .Pp
2129 The keywords
2130 .Cm CertificateFile ,
2131 .Cm ControlPath ,
2132 .Cm IdentityAgent ,
2133 .Cm IdentityFile ,
2134 .Cm KnownHostsCommand ,
2135 and
2136 .Cm UserKnownHostsFile
2137 support environment variables.
2138 The keywords
2139 .Cm LocalForward
2140 and
2141 .Cm RemoteForward
2142 support environment variables only for Unix domain socket paths.
2143 .Sh FILES
2144 .Bl -tag -width Ds
2145 .It Pa ~/.ssh/config
2146 This is the per-user configuration file.
2147 The format of this file is described above.
2148 This file is used by the SSH client.
2149 Because of the potential for abuse, this file must have strict permissions:
2150 read/write for the user, and not writable by others.
2151 .It Pa /etc/ssh/ssh_config
2152 Systemwide configuration file.
2153 This file provides defaults for those
2154 values that are not specified in the user's configuration file, and
2155 for those users who do not have a configuration file.
2156 This file must be world-readable.
2157 .El
2158 .Sh SEE ALSO
2159 .Xr ssh 1
2160 .Sh AUTHORS
2161 .An -nosplit
2162 OpenSSH is a derivative of the original and free
2163 ssh 1.2.12 release by
2164 .An Tatu Ylonen .
2165 .An Aaron Campbell , Bob Beck , Markus Friedl ,
2166 .An Niels Provos , Theo de Raadt
2167 and
2168 .An Dug Song
2169 removed many bugs, re-added newer features and
2170 created OpenSSH.
2171 .An Markus Friedl
2172 contributed the support for SSH protocol versions 1.5 and 2.0.