| blob | 8d291e61d4410cbebbd84fb3404d9e5ce9f33daf |
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: sshd_config.5,v 1.39 2005/03/01 10:09:52 djm Exp $
38 .Dd September 25, 1999
39 .Dt SSHD_CONFIG 5
40 .Os
41 .Sh NAME
42 .Nm sshd_config
43 .Nd OpenSSH SSH daemon configuration file
44 .Sh SYNOPSIS
45 .Bl -tag -width Ds -compact
46 .It Pa /etc/ssh/sshd_config
47 .El
48 .Sh DESCRIPTION
49 .Nm sshd
50 reads configuration data from
51 .Pa /etc/ssh/sshd_config
52 (or the file specified with
53 .Fl f
54 on the command line).
55 The file contains keyword-argument pairs, one per line.
56 Lines starting with
57 .Ql #
58 and empty lines are interpreted as comments.
59 .Pp
60 The possible
61 keywords and their meanings are as follows (note that
62 keywords are case-insensitive and arguments are case-sensitive):
63 .Bl -tag -width Ds
64 .It Cm AcceptEnv
65 Specifies what environment variables sent by the client will be copied into
66 the session's
67 .Xr environ 7 .
68 See
69 .Cm SendEnv
70 in
71 .Xr ssh_config 5
72 for how to configure the client.
73 Note that environment passing is only supported for protocol 2.
74 Variables are specified by name, which may contain the wildcard characters
75 .Ql \&*
76 and
77 .Ql \&? .
78 Multiple environment variables may be separated by whitespace or spread
79 across multiple
80 .Cm AcceptEnv
81 directives.
82 Be warned that some environment variables could be used to bypass restricted
83 user environments.
84 For this reason, care should be taken in the use of this directive.
85 The default is not to accept any environment variables.
86 .It Cm AddressFamily
87 Specifies which address family should be used by
88 .Nm sshd .
89 Valid arguments are
90 .Dq any ,
91 .Dq inet
92 (use IPv4 only) or
93 .Dq inet6
94 (use IPv6 only).
95 The default is
96 .Dq any .
97 .It Cm AllowGroups
98 This keyword can be followed by a list of group name patterns, separated
99 by spaces.
100 If specified, login is allowed only for users whose primary
101 group or supplementary group list matches one of the patterns.
102 .Ql \&*
103 and
104 .Ql \&?
105 can be used as
106 wildcards in the patterns.
107 Only group names are valid; a numerical group ID is not recognized.
108 By default, login is allowed for all groups.
109 .It Cm AllowTcpForwarding
110 Specifies whether TCP forwarding is permitted.
111 The default is
112 .Dq yes .
113 Note that disabling TCP forwarding does not improve security unless
114 users are also denied shell access, as they can always install their
115 own forwarders.
116 .It Cm AllowUsers
117 This keyword can be followed by a list of user name patterns, separated
118 by spaces.
119 If specified, login is allowed only for user names that
120 match one of the patterns.
121 .Ql \&*
122 and
123 .Ql \&?
124 can be used as
125 wildcards in the patterns.
126 Only user names are valid; a numerical user ID is not recognized.
127 By default, login is allowed for all users.
128 If the pattern takes the form USER@HOST then USER and HOST
129 are separately checked, restricting logins to particular
130 users from particular hosts.
131 .It Cm AuthorizedKeysFile
132 Specifies the file that contains the public keys that can be used
133 for user authentication.
134 .Cm AuthorizedKeysFile
135 may contain tokens of the form %T which are substituted during connection
136 set-up.
137 The following tokens are defined: %% is replaced by a literal '%',
138 %h is replaced by the home directory of the user being authenticated and
139 %u is replaced by the username of that user.
140 After expansion,
141 .Cm AuthorizedKeysFile
142 is taken to be an absolute path or one relative to the user's home
143 directory.
144 The default is
145 .Dq .ssh/authorized_keys .
146 .It Cm Banner
147 In some jurisdictions, sending a warning message before authentication
148 may be relevant for getting legal protection.
149 The contents of the specified file are sent to the remote user before
150 authentication is allowed.
151 This option is only available for protocol version 2.
152 By default, no banner is displayed.
153 .It Cm ChallengeResponseAuthentication
154 Specifies whether challenge response authentication is allowed.
155 All authentication styles from
156 .Xr login.conf 5
157 are supported.
158 The default is
159 .Dq yes .
160 .It Cm Ciphers
161 Specifies the ciphers allowed for protocol version 2.
162 Multiple ciphers must be comma-separated.
163 The supported ciphers are
164 .Dq 3des-cbc ,
165 .Dq aes128-cbc ,
166 .Dq aes192-cbc ,
167 .Dq aes256-cbc ,
168 .Dq aes128-ctr ,
169 .Dq aes192-ctr ,
170 .Dq aes256-ctr ,
171 .Dq arcfour ,
172 .Dq blowfish-cbc ,
173 and
174 .Dq cast128-cbc .
175 The default is
176 .Bd -literal
177 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
178 aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr''
179 .Ed
180 .It Cm ClientAliveInterval
181 Sets a timeout interval in seconds after which if no data has been received
182 from the client,
183 .Nm sshd
184 will send a message through the encrypted
185 channel to request a response from the client.
186 The default
187 is 0, indicating that these messages will not be sent to the client.
188 This option applies to protocol version 2 only.
189 .It Cm ClientAliveCountMax
190 Sets the number of client alive messages (see above) which may be
191 sent without
192 .Nm sshd
193 receiving any messages back from the client.
194 If this threshold is reached while client alive messages are being sent,
195 .Nm sshd
196 will disconnect the client, terminating the session.
197 It is important to note that the use of client alive messages is very
198 different from
199 .Cm TCPKeepAlive
200 (below).
201 The client alive messages are sent through the encrypted channel
202 and therefore will not be spoofable.
203 The TCP keepalive option enabled by
204 .Cm TCPKeepAlive
205 is spoofable.
206 The client alive mechanism is valuable when the client or
207 server depend on knowing when a connection has become inactive.
208 .Pp
209 The default value is 3.
210 If
211 .Cm ClientAliveInterval
212 (above) is set to 15, and
213 .Cm ClientAliveCountMax
214 is left at the default, unresponsive ssh clients
215 will be disconnected after approximately 45 seconds.
216 .It Cm Compression
217 Specifies whether compression is allowed.
218 The argument must be
219 .Dq yes
220 or
221 .Dq no .
222 The default is
223 .Dq yes .
224 .It Cm DenyGroups
225 This keyword can be followed by a list of group name patterns, separated
226 by spaces.
227 Login is disallowed for users whose primary group or supplementary
228 group list matches one of the patterns.
229 .Ql \&*
230 and
231 .Ql \&?
232 can be used as
233 wildcards in the patterns.
234 Only group names are valid; a numerical group ID is not recognized.
235 By default, login is allowed for all groups.
236 .It Cm DenyUsers
237 This keyword can be followed by a list of user name patterns, separated
238 by spaces.
239 Login is disallowed for user names that match one of the patterns.
240 .Ql \&*
241 and
242 .Ql \&?
243 can be used as wildcards in the patterns.
244 Only user names are valid; a numerical user ID is not recognized.
245 By default, login is allowed for all users.
246 If the pattern takes the form USER@HOST then USER and HOST
247 are separately checked, restricting logins to particular
248 users from particular hosts.
249 .It Cm GatewayPorts
250 Specifies whether remote hosts are allowed to connect to ports
251 forwarded for the client.
252 By default,
253 .Nm sshd
254 binds remote port forwardings to the loopback address.
255 This prevents other remote hosts from connecting to forwarded ports.
256 .Cm GatewayPorts
257 can be used to specify that
258 .Nm sshd
259 should allow remote port forwardings to bind to non-loopback addresses, thus
260 allowing other hosts to connect.
261 The argument may be
262 .Dq no
263 to force remote port forwardings to be available to the local host only,
264 .Dq yes
265 to force remote port forwardings to bind to the wildcard address, or
266 .Dq clientspecified
267 to allow the client to select the address to which the forwarding is bound.
268 The default is
269 .Dq no .
270 .It Cm GSSAPIAuthentication
271 Specifies whether user authentication based on GSSAPI is allowed.
272 The default is
273 .Dq no .
274 Note that this option applies to protocol version 2 only.
275 .It Cm GSSAPICleanupCredentials
276 Specifies whether to automatically destroy the user's credentials cache
277 on logout.
278 The default is
279 .Dq yes .
280 Note that this option applies to protocol version 2 only.
281 .It Cm HostbasedAuthentication
282 Specifies whether rhosts or /etc/hosts.equiv authentication together
283 with successful public key client host authentication is allowed
284 (hostbased authentication).
285 This option is similar to
286 .Cm RhostsRSAAuthentication
287 and applies to protocol version 2 only.
288 The default is
289 .Dq no .
290 .It Cm HostKey
291 Specifies a file containing a private host key
292 used by SSH.
293 The default is
294 .Pa /etc/ssh/ssh_host_key
295 for protocol version 1, and
296 .Pa /etc/ssh/ssh_host_rsa_key
297 and
298 .Pa /etc/ssh/ssh_host_dsa_key
299 for protocol version 2.
300 Note that
301 .Nm sshd
302 will refuse to use a file if it is group/world-accessible.
303 It is possible to have multiple host key files.
304 .Dq rsa1
305 keys are used for version 1 and
306 .Dq dsa
307 or
308 .Dq rsa
309 are used for version 2 of the SSH protocol.
310 .It Cm IgnoreRhosts
311 Specifies that
312 .Pa .rhosts
313 and
314 .Pa .shosts
315 files will not be used in
316 .Cm RhostsRSAAuthentication
317 or
318 .Cm HostbasedAuthentication .
319 .Pp
320 .Pa /etc/hosts.equiv
321 and
322 .Pa /etc/shosts.equiv
323 are still used.
324 The default is
325 .Dq yes .
326 .It Cm IgnoreUserKnownHosts
327 Specifies whether
328 .Nm sshd
329 should ignore the user's
330 .Pa $HOME/.ssh/known_hosts
331 during
332 .Cm RhostsRSAAuthentication
333 or
334 .Cm HostbasedAuthentication .
335 The default is
336 .Dq no .
337 .It Cm KerberosAuthentication
338 Specifies whether the password provided by the user for
339 .Cm PasswordAuthentication
340 will be validated through the Kerberos KDC.
341 To use this option, the server needs a
342 Kerberos servtab which allows the verification of the KDC's identity.
343 Default is
344 .Dq no .
345 .It Cm KerberosGetAFSToken
346 If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire
347 an AFS token before accessing the user's home directory.
348 Default is
349 .Dq no .
350 .It Cm KerberosOrLocalPasswd
351 If set then if password authentication through Kerberos fails then
352 the password will be validated via any additional local mechanism
353 such as
354 .Pa /etc/passwd .
355 Default is
356 .Dq yes .
357 .It Cm KerberosTicketCleanup
358 Specifies whether to automatically destroy the user's ticket cache
359 file on logout.
360 Default is
361 .Dq yes .
362 .It Cm KeyRegenerationInterval
363 In protocol version 1, the ephemeral server key is automatically regenerated
364 after this many seconds (if it has been used).
365 The purpose of regeneration is to prevent
366 decrypting captured sessions by later breaking into the machine and
367 stealing the keys.
368 The key is never stored anywhere.
369 If the value is 0, the key is never regenerated.
370 The default is 3600 (seconds).
371 .It Cm ListenAddress
372 Specifies the local addresses
373 .Nm sshd
374 should listen on.
375 The following forms may be used:
376 .Pp
377 .Bl -item -offset indent -compact
378 .It
379 .Cm ListenAddress
380 .Sm off
381 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
382 .Sm on
383 .It
384 .Cm ListenAddress
385 .Sm off
386 .Ar host No | Ar IPv4_addr No : Ar port
387 .Sm on
388 .It
389 .Cm ListenAddress
390 .Sm off
391 .Oo
392 .Ar host No | Ar IPv6_addr Oc : Ar port
393 .Sm on
394 .El
395 .Pp
396 If
397 .Ar port
398 is not specified,
399 .Nm sshd
400 will listen on the address and all prior
401 .Cm Port
402 options specified.
403 The default is to listen on all local addresses.
404 Multiple
405 .Cm ListenAddress
406 options are permitted.
407 Additionally, any
408 .Cm Port
409 options must precede this option for non port qualified addresses.
410 .It Cm LoginGraceTime
411 The server disconnects after this time if the user has not
412 successfully logged in.
413 If the value is 0, there is no time limit.
414 The default is 120 seconds.
415 .It Cm LogLevel
416 Gives the verbosity level that is used when logging messages from
417 .Nm sshd .
418 The possible values are:
419 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
420 The default is INFO.
421 DEBUG and DEBUG1 are equivalent.
422 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
423 Logging with a DEBUG level violates the privacy of users and is not recommended.
424 .It Cm MACs
425 Specifies the available MAC (message authentication code) algorithms.
426 The MAC algorithm is used in protocol version 2
427 for data integrity protection.
428 Multiple algorithms must be comma-separated.
429 The default is
430 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
431 .It Cm MaxAuthTries
432 Specifies the maximum number of authentication attempts permitted per
433 connection.
434 Once the number of failures reaches half this value,
435 additional failures are logged.
436 The default is 6.
437 .It Cm MaxStartups
438 Specifies the maximum number of concurrent unauthenticated connections to the
439 .Nm sshd
440 daemon.
441 Additional connections will be dropped until authentication succeeds or the
442 .Cm LoginGraceTime
443 expires for a connection.
444 The default is 10.
445 .Pp
446 Alternatively, random early drop can be enabled by specifying
447 the three colon separated values
448 .Dq start:rate:full
449 (e.g., "10:30:60").
450 .Nm sshd
451 will refuse connection attempts with a probability of
452 .Dq rate/100
453 (30%)
454 if there are currently
455 .Dq start
456 (10)
457 unauthenticated connections.
458 The probability increases linearly and all connection attempts
459 are refused if the number of unauthenticated connections reaches
460 .Dq full
461 (60).
462 .It Cm PasswordAuthentication
463 Specifies whether password authentication is allowed.
464 The default is
465 .Dq yes .
466 .It Cm PermitEmptyPasswords
467 When password authentication is allowed, it specifies whether the
468 server allows login to accounts with empty password strings.
469 The default is
470 .Dq no .
471 .It Cm PermitRootLogin
472 Specifies whether root can log in using
473 .Xr ssh 1 .
474 The argument must be
475 .Dq yes ,
476 .Dq without-password ,
477 .Dq forced-commands-only
478 or
479 .Dq no .
480 The default is
481 .Dq yes .
482 .Pp
483 If this option is set to
484 .Dq without-password
485 password authentication is disabled for root.
486 .Pp
487 If this option is set to
488 .Dq forced-commands-only
489 root login with public key authentication will be allowed,
490 but only if the
491 .Ar command
492 option has been specified
493 (which may be useful for taking remote backups even if root login is
494 normally not allowed).
495 All other authentication methods are disabled for root.
496 .Pp
497 If this option is set to
498 .Dq no
499 root is not allowed to log in.
500 .It Cm PermitUserEnvironment
501 Specifies whether
502 .Pa ~/.ssh/environment
503 and
504 .Cm environment=
505 options in
506 .Pa ~/.ssh/authorized_keys
507 are processed by
508 .Nm sshd .
509 The default is
510 .Dq no .
511 Enabling environment processing may enable users to bypass access
512 restrictions in some configurations using mechanisms such as
513 .Ev LD_PRELOAD .
514 .It Cm PidFile
515 Specifies the file that contains the process ID of the
516 .Nm sshd
517 daemon.
518 The default is
519 .Pa /var/run/sshd.pid .
520 .It Cm Port
521 Specifies the port number that
522 .Nm sshd
523 listens on.
524 The default is 22.
525 Multiple options of this type are permitted.
526 See also
527 .Cm ListenAddress .
528 .It Cm PrintLastLog
529 Specifies whether
530 .Nm sshd
531 should print the date and time of the last user login when a user logs
532 in interactively.
533 The default is
534 .Dq yes .
535 .It Cm PrintMotd
536 Specifies whether
537 .Nm sshd
538 should print
539 .Pa /etc/motd
540 when a user logs in interactively.
541 (On some systems it is also printed by the shell,
542 .Pa /etc/profile ,
543 or equivalent.)
544 The default is
545 .Dq yes .
546 .It Cm Protocol
547 Specifies the protocol versions
548 .Nm sshd
549 supports.
550 The possible values are
551 .Dq 1
552 and
553 .Dq 2 .
554 Multiple versions must be comma-separated.
555 The default is
556 .Dq 2,1 .
557 Note that the order of the protocol list does not indicate preference,
558 because the client selects among multiple protocol versions offered
559 by the server.
560 Specifying
561 .Dq 2,1
562 is identical to
563 .Dq 1,2 .
564 .It Cm PubkeyAuthentication
565 Specifies whether public key authentication is allowed.
566 The default is
567 .Dq yes .
568 Note that this option applies to protocol version 2 only.
569 .It Cm RhostsRSAAuthentication
570 Specifies whether rhosts or /etc/hosts.equiv authentication together
571 with successful RSA host authentication is allowed.
572 The default is
573 .Dq no .
574 This option applies to protocol version 1 only.
575 .It Cm RSAAuthentication
576 Specifies whether pure RSA authentication is allowed.
577 The default is
578 .Dq yes .
579 This option applies to protocol version 1 only.
580 .It Cm ServerKeyBits
581 Defines the number of bits in the ephemeral protocol version 1 server key.
582 The minimum value is 512, and the default is 768.
583 .It Cm StrictModes
584 Specifies whether
585 .Nm sshd
586 should check file modes and ownership of the
587 user's files and home directory before accepting login.
588 This is normally desirable because novices sometimes accidentally leave their
589 directory or files world-writable.
590 The default is
591 .Dq yes .
592 .It Cm Subsystem
593 Configures an external subsystem (e.g., file transfer daemon).
594 Arguments should be a subsystem name and a command to execute upon subsystem
595 request.
596 The command
597 .Xr sftp-server 8
598 implements the
599 .Dq sftp
600 file transfer subsystem.
601 By default no subsystems are defined.
602 Note that this option applies to protocol version 2 only.
603 .It Cm SyslogFacility
604 Gives the facility code that is used when logging messages from
605 .Nm sshd .
606 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
607 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
608 The default is AUTH.
609 .It Cm TCPKeepAlive
610 Specifies whether the system should send TCP keepalive messages to the
611 other side.
612 If they are sent, death of the connection or crash of one
613 of the machines will be properly noticed.
614 However, this means that
615 connections will die if the route is down temporarily, and some people
616 find it annoying.
617 On the other hand, if TCP keepalives are not sent,
618 sessions may hang indefinitely on the server, leaving
619 .Dq ghost
620 users and consuming server resources.
621 .Pp
622 The default is
623 .Dq yes
624 (to send TCP keepalive messages), and the server will notice
625 if the network goes down or the client host crashes.
626 This avoids infinitely hanging sessions.
627 .Pp
628 To disable TCP keepalive messages, the value should be set to
629 .Dq no .
630 .It Cm UseDNS
631 Specifies whether
632 .Nm sshd
633 should lookup the remote host name and check that
634 the resolved host name for the remote IP address maps back to the
635 very same IP address.
636 The default is
637 .Dq yes .
638 .It Cm UseLogin
639 Specifies whether
640 .Xr login 1
641 is used for interactive login sessions.
642 The default is
643 .Dq no .
644 Note that
645 .Xr login 1
646 is never used for remote command execution.
647 Note also, that if this is enabled,
648 .Cm X11Forwarding
649 will be disabled because
650 .Xr login 1
651 does not know how to handle
652 .Xr xauth 1
653 cookies.
654 If
655 .Cm UsePrivilegeSeparation
656 is specified, it will be disabled after authentication.
657 .It Cm UsePAM
658 Enables the Pluggable Authentication Module interface.
659 If set to
660 .Dq yes
661 this will enable PAM authentication using
662 .Cm ChallengeResponseAuthentication
663 and PAM account and session module processing for all authentication types.
664 .Pp
665 Because PAM challenge-response authentication usually serves an equivalent
666 role to password authentication, you should disable either
667 .Cm PasswordAuthentication
668 or
669 .Cm ChallengeResponseAuthentication.
670 .Pp
671 If
672 .Cm UsePAM
673 is enabled, you will not be able to run
674 .Xr sshd 8
675 as a non-root user.
676 The default is
677 .Dq no .
678 .It Cm UsePrivilegeSeparation
679 Specifies whether
680 .Nm sshd
681 separates privileges by creating an unprivileged child process
682 to deal with incoming network traffic.
683 After successful authentication, another process will be created that has
684 the privilege of the authenticated user.
685 The goal of privilege separation is to prevent privilege
686 escalation by containing any corruption within the unprivileged processes.
687 The default is
688 .Dq yes .
689 .It Cm X11DisplayOffset
690 Specifies the first display number available for
691 .Nm sshd Ns 's
692 X11 forwarding.
693 This prevents
694 .Nm sshd
695 from interfering with real X11 servers.
696 The default is 10.
697 .It Cm X11Forwarding
698 Specifies whether X11 forwarding is permitted.
699 The argument must be
700 .Dq yes
701 or
702 .Dq no .
703 The default is
704 .Dq no .
705 .Pp
706 When X11 forwarding is enabled, there may be additional exposure to
707 the server and to client displays if the
708 .Nm sshd
709 proxy display is configured to listen on the wildcard address (see
710 .Cm X11UseLocalhost
711 below), however this is not the default.
712 Additionally, the authentication spoofing and authentication data
713 verification and substitution occur on the client side.
714 The security risk of using X11 forwarding is that the client's X11
715 display server may be exposed to attack when the ssh client requests
716 forwarding (see the warnings for
717 .Cm ForwardX11
718 in
719 .Xr ssh_config 5 ) .
720 A system administrator may have a stance in which they want to
721 protect clients that may expose themselves to attack by unwittingly
722 requesting X11 forwarding, which can warrant a
723 .Dq no
724 setting.
725 .Pp
726 Note that disabling X11 forwarding does not prevent users from
727 forwarding X11 traffic, as users can always install their own forwarders.
728 X11 forwarding is automatically disabled if
729 .Cm UseLogin
730 is enabled.
731 .It Cm X11UseLocalhost
732 Specifies whether
733 .Nm sshd
734 should bind the X11 forwarding server to the loopback address or to
735 the wildcard address.
736 By default,
737 .Nm sshd
738 binds the forwarding server to the loopback address and sets the
739 hostname part of the
740 .Ev DISPLAY
741 environment variable to
742 .Dq localhost .
743 This prevents remote hosts from connecting to the proxy display.
744 However, some older X11 clients may not function with this
745 configuration.
746 .Cm X11UseLocalhost
747 may be set to
748 .Dq no
749 to specify that the forwarding server should be bound to the wildcard
750 address.
751 The argument must be
752 .Dq yes
753 or
754 .Dq no .
755 The default is
756 .Dq yes .
757 .It Cm XAuthLocation
758 Specifies the full pathname of the
759 .Xr xauth 1
760 program.
761 The default is
762 .Pa /usr/X11R6/bin/xauth .
763 .El
764 .Ss Time Formats
765 .Nm sshd
766 command-line arguments and configuration file options that specify time
767 may be expressed using a sequence of the form:
768 .Sm off
769 .Ar time Op Ar qualifier ,
770 .Sm on
771 where
772 .Ar time
773 is a positive integer value and
774 .Ar qualifier
775 is one of the following:
776 .Pp
777 .Bl -tag -width Ds -compact -offset indent
778 .It Cm <none>
779 seconds
780 .It Cm s | Cm S
781 seconds
782 .It Cm m | Cm M
783 minutes
784 .It Cm h | Cm H
785 hours
786 .It Cm d | Cm D
787 days
788 .It Cm w | Cm W
789 weeks
790 .El
791 .Pp
792 Each member of the sequence is added together to calculate
793 the total time value.
794 .Pp
795 Time format examples:
796 .Pp
797 .Bl -tag -width Ds -compact -offset indent
798 .It 600
799 600 seconds (10 minutes)
800 .It 10m
801 10 minutes
802 .It 1h30m
803 1 hour 30 minutes (90 minutes)
804 .El
805 .Sh FILES
806 .Bl -tag -width Ds
807 .It Pa /etc/ssh/sshd_config
808 Contains configuration data for
809 .Nm sshd .
810 This file should be writable by root only, but it is recommended
811 (though not necessary) that it be world-readable.
812 .El
813 .Sh SEE ALSO
814 .Xr sshd 8
815 .Sh AUTHORS
816 OpenSSH is a derivative of the original and free
817 ssh 1.2.12 release by Tatu Ylonen.
818 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
819 Theo de Raadt and Dug Song
820 removed many bugs, re-added newer features and
821 created OpenSSH.
822 Markus Friedl contributed the support for SSH
823 protocol versions 1.5 and 2.0.
824 Niels Provos and Markus Friedl contributed support
825 for privilege separation.