| blob | 048e8924e3bc508170e531cfadf936b366008795 |
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 1.44 2005/07/25 11:59:40 markus 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 arcfour128 ,
172 .Dq arcfour256 ,
173 .Dq arcfour ,
174 .Dq blowfish-cbc ,
175 and
176 .Dq cast128-cbc .
177 The default is
178 .Bd -literal
179 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
180 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
181 aes192-ctr,aes256-ctr''
182 .Ed
183 .It Cm ClientAliveCountMax
184 Sets the number of client alive messages (see above) which may be
185 sent without
186 .Nm sshd
187 receiving any messages back from the client.
188 If this threshold is reached while client alive messages are being sent,
189 .Nm sshd
190 will disconnect the client, terminating the session.
191 It is important to note that the use of client alive messages is very
192 different from
193 .Cm TCPKeepAlive
194 (below).
195 The client alive messages are sent through the encrypted channel
196 and therefore will not be spoofable.
197 The TCP keepalive option enabled by
198 .Cm TCPKeepAlive
199 is spoofable.
200 The client alive mechanism is valuable when the client or
201 server depend on knowing when a connection has become inactive.
202 .Pp
203 The default value is 3.
204 If
205 .Cm ClientAliveInterval
206 (above) is set to 15, and
207 .Cm ClientAliveCountMax
208 is left at the default, unresponsive ssh clients
209 will be disconnected after approximately 45 seconds.
210 .It Cm ClientAliveInterval
211 Sets a timeout interval in seconds after which if no data has been received
212 from the client,
213 .Nm sshd
214 will send a message through the encrypted
215 channel to request a response from the client.
216 The default
217 is 0, indicating that these messages will not be sent to the client.
218 This option applies to protocol version 2 only.
219 .It Cm Compression
220 Specifies whether compression is allowed, or delayed until
221 the user has authenticated successfully.
222 The argument must be
223 .Dq yes ,
224 .Dq delayed ,
225 or
226 .Dq no .
227 The default is
228 .Dq delayed .
229 .It Cm DenyGroups
230 This keyword can be followed by a list of group name patterns, separated
231 by spaces.
232 Login is disallowed for users whose primary group or supplementary
233 group list matches one of the patterns.
234 .Ql \&*
235 and
236 .Ql \&?
237 can be used as
238 wildcards in the patterns.
239 Only group names are valid; a numerical group ID is not recognized.
240 By default, login is allowed for all groups.
241 .It Cm DenyUsers
242 This keyword can be followed by a list of user name patterns, separated
243 by spaces.
244 Login is disallowed for user names that match one of the patterns.
245 .Ql \&*
246 and
247 .Ql \&?
248 can be used as wildcards in the patterns.
249 Only user names are valid; a numerical user ID is not recognized.
250 By default, login is allowed for all users.
251 If the pattern takes the form USER@HOST then USER and HOST
252 are separately checked, restricting logins to particular
253 users from particular hosts.
254 .It Cm GatewayPorts
255 Specifies whether remote hosts are allowed to connect to ports
256 forwarded for the client.
257 By default,
258 .Nm sshd
259 binds remote port forwardings to the loopback address.
260 This prevents other remote hosts from connecting to forwarded ports.
261 .Cm GatewayPorts
262 can be used to specify that
263 .Nm sshd
264 should allow remote port forwardings to bind to non-loopback addresses, thus
265 allowing other hosts to connect.
266 The argument may be
267 .Dq no
268 to force remote port forwardings to be available to the local host only,
269 .Dq yes
270 to force remote port forwardings to bind to the wildcard address, or
271 .Dq clientspecified
272 to allow the client to select the address to which the forwarding is bound.
273 The default is
274 .Dq no .
275 .It Cm GSSAPIAuthentication
276 Specifies whether user authentication based on GSSAPI is allowed.
277 The default is
278 .Dq no .
279 Note that this option applies to protocol version 2 only.
280 .It Cm GSSAPICleanupCredentials
281 Specifies whether to automatically destroy the user's credentials cache
282 on logout.
283 The default is
284 .Dq yes .
285 Note that this option applies to protocol version 2 only.
286 .It Cm HostbasedAuthentication
287 Specifies whether rhosts or /etc/hosts.equiv authentication together
288 with successful public key client host authentication is allowed
289 (hostbased authentication).
290 This option is similar to
291 .Cm RhostsRSAAuthentication
292 and applies to protocol version 2 only.
293 The default is
294 .Dq no .
295 .It Cm HostKey
296 Specifies a file containing a private host key
297 used by SSH.
298 The default is
299 .Pa /etc/ssh/ssh_host_key
300 for protocol version 1, and
301 .Pa /etc/ssh/ssh_host_rsa_key
302 and
303 .Pa /etc/ssh/ssh_host_dsa_key
304 for protocol version 2.
305 Note that
306 .Nm sshd
307 will refuse to use a file if it is group/world-accessible.
308 It is possible to have multiple host key files.
309 .Dq rsa1
310 keys are used for version 1 and
311 .Dq dsa
312 or
313 .Dq rsa
314 are used for version 2 of the SSH protocol.
315 .It Cm IgnoreRhosts
316 Specifies that
317 .Pa .rhosts
318 and
319 .Pa .shosts
320 files will not be used in
321 .Cm RhostsRSAAuthentication
322 or
323 .Cm HostbasedAuthentication .
324 .Pp
325 .Pa /etc/hosts.equiv
326 and
327 .Pa /etc/shosts.equiv
328 are still used.
329 The default is
330 .Dq yes .
331 .It Cm IgnoreUserKnownHosts
332 Specifies whether
333 .Nm sshd
334 should ignore the user's
335 .Pa ~/.ssh/known_hosts
336 during
337 .Cm RhostsRSAAuthentication
338 or
339 .Cm HostbasedAuthentication .
340 The default is
341 .Dq no .
342 .It Cm KerberosAuthentication
343 Specifies whether the password provided by the user for
344 .Cm PasswordAuthentication
345 will be validated through the Kerberos KDC.
346 To use this option, the server needs a
347 Kerberos servtab which allows the verification of the KDC's identity.
348 Default is
349 .Dq no .
350 .It Cm KerberosGetAFSToken
351 If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire
352 an AFS token before accessing the user's home directory.
353 Default is
354 .Dq no .
355 .It Cm KerberosOrLocalPasswd
356 If set then if password authentication through Kerberos fails then
357 the password will be validated via any additional local mechanism
358 such as
359 .Pa /etc/passwd .
360 Default is
361 .Dq yes .
362 .It Cm KerberosTicketCleanup
363 Specifies whether to automatically destroy the user's ticket cache
364 file on logout.
365 Default is
366 .Dq yes .
367 .It Cm KeyRegenerationInterval
368 In protocol version 1, the ephemeral server key is automatically regenerated
369 after this many seconds (if it has been used).
370 The purpose of regeneration is to prevent
371 decrypting captured sessions by later breaking into the machine and
372 stealing the keys.
373 The key is never stored anywhere.
374 If the value is 0, the key is never regenerated.
375 The default is 3600 (seconds).
376 .It Cm ListenAddress
377 Specifies the local addresses
378 .Nm sshd
379 should listen on.
380 The following forms may be used:
381 .Pp
382 .Bl -item -offset indent -compact
383 .It
384 .Cm ListenAddress
385 .Sm off
386 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
387 .Sm on
388 .It
389 .Cm ListenAddress
390 .Sm off
391 .Ar host No | Ar IPv4_addr No : Ar port
392 .Sm on
393 .It
394 .Cm ListenAddress
395 .Sm off
396 .Oo
397 .Ar host No | Ar IPv6_addr Oc : Ar port
398 .Sm on
399 .El
400 .Pp
401 If
402 .Ar port
403 is not specified,
404 .Nm sshd
405 will listen on the address and all prior
406 .Cm Port
407 options specified.
408 The default is to listen on all local addresses.
409 Multiple
410 .Cm ListenAddress
411 options are permitted.
412 Additionally, any
413 .Cm Port
414 options must precede this option for non port qualified addresses.
415 .It Cm LoginGraceTime
416 The server disconnects after this time if the user has not
417 successfully logged in.
418 If the value is 0, there is no time limit.
419 The default is 120 seconds.
420 .It Cm LogLevel
421 Gives the verbosity level that is used when logging messages from
422 .Nm sshd .
423 The possible values are:
424 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3.
425 The default is INFO.
426 DEBUG and DEBUG1 are equivalent.
427 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
428 Logging with a DEBUG level violates the privacy of users and is not recommended.
429 .It Cm MACs
430 Specifies the available MAC (message authentication code) algorithms.
431 The MAC algorithm is used in protocol version 2
432 for data integrity protection.
433 Multiple algorithms must be comma-separated.
434 The default is
435 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
436 .It Cm MaxAuthTries
437 Specifies the maximum number of authentication attempts permitted per
438 connection.
439 Once the number of failures reaches half this value,
440 additional failures are logged.
441 The default is 6.
442 .It Cm MaxStartups
443 Specifies the maximum number of concurrent unauthenticated connections to the
444 .Nm sshd
445 daemon.
446 Additional connections will be dropped until authentication succeeds or the
447 .Cm LoginGraceTime
448 expires for a connection.
449 The default is 10.
450 .Pp
451 Alternatively, random early drop can be enabled by specifying
452 the three colon separated values
453 .Dq start:rate:full
454 (e.g., "10:30:60").
455 .Nm sshd
456 will refuse connection attempts with a probability of
457 .Dq rate/100
458 (30%)
459 if there are currently
460 .Dq start
461 (10)
462 unauthenticated connections.
463 The probability increases linearly and all connection attempts
464 are refused if the number of unauthenticated connections reaches
465 .Dq full
466 (60).
467 .It Cm PasswordAuthentication
468 Specifies whether password authentication is allowed.
469 The default is
470 .Dq yes .
471 .It Cm PermitEmptyPasswords
472 When password authentication is allowed, it specifies whether the
473 server allows login to accounts with empty password strings.
474 The default is
475 .Dq no .
476 .It Cm PermitRootLogin
477 Specifies whether root can log in using
478 .Xr ssh 1 .
479 The argument must be
480 .Dq yes ,
481 .Dq without-password ,
482 .Dq forced-commands-only
483 or
484 .Dq no .
485 The default is
486 .Dq yes .
487 .Pp
488 If this option is set to
489 .Dq without-password
490 password authentication is disabled for root.
491 .Pp
492 If this option is set to
493 .Dq forced-commands-only
494 root login with public key authentication will be allowed,
495 but only if the
496 .Ar command
497 option has been specified
498 (which may be useful for taking remote backups even if root login is
499 normally not allowed).
500 All other authentication methods are disabled for root.
501 .Pp
502 If this option is set to
503 .Dq no
504 root is not allowed to log in.
505 .It Cm PermitUserEnvironment
506 Specifies whether
507 .Pa ~/.ssh/environment
508 and
509 .Cm environment=
510 options in
511 .Pa ~/.ssh/authorized_keys
512 are processed by
513 .Nm sshd .
514 The default is
515 .Dq no .
516 Enabling environment processing may enable users to bypass access
517 restrictions in some configurations using mechanisms such as
518 .Ev LD_PRELOAD .
519 .It Cm PidFile
520 Specifies the file that contains the process ID of the
521 .Nm sshd
522 daemon.
523 The default is
524 .Pa /var/run/sshd.pid .
525 .It Cm Port
526 Specifies the port number that
527 .Nm sshd
528 listens on.
529 The default is 22.
530 Multiple options of this type are permitted.
531 See also
532 .Cm ListenAddress .
533 .It Cm PrintLastLog
534 Specifies whether
535 .Nm sshd
536 should print the date and time of the last user login when a user logs
537 in interactively.
538 The default is
539 .Dq yes .
540 .It Cm PrintMotd
541 Specifies whether
542 .Nm sshd
543 should print
544 .Pa /etc/motd
545 when a user logs in interactively.
546 (On some systems it is also printed by the shell,
547 .Pa /etc/profile ,
548 or equivalent.)
549 The default is
550 .Dq yes .
551 .It Cm Protocol
552 Specifies the protocol versions
553 .Nm sshd
554 supports.
555 The possible values are
556 .Dq 1
557 and
558 .Dq 2 .
559 Multiple versions must be comma-separated.
560 The default is
561 .Dq 2,1 .
562 Note that the order of the protocol list does not indicate preference,
563 because the client selects among multiple protocol versions offered
564 by the server.
565 Specifying
566 .Dq 2,1
567 is identical to
568 .Dq 1,2 .
569 .It Cm PubkeyAuthentication
570 Specifies whether public key authentication is allowed.
571 The default is
572 .Dq yes .
573 Note that this option applies to protocol version 2 only.
574 .It Cm RhostsRSAAuthentication
575 Specifies whether rhosts or /etc/hosts.equiv authentication together
576 with successful RSA host authentication is allowed.
577 The default is
578 .Dq no .
579 This option applies to protocol version 1 only.
580 .It Cm RSAAuthentication
581 Specifies whether pure RSA authentication is allowed.
582 The default is
583 .Dq yes .
584 This option applies to protocol version 1 only.
585 .It Cm ServerKeyBits
586 Defines the number of bits in the ephemeral protocol version 1 server key.
587 The minimum value is 512, and the default is 768.
588 .It Cm StrictModes
589 Specifies whether
590 .Nm sshd
591 should check file modes and ownership of the
592 user's files and home directory before accepting login.
593 This is normally desirable because novices sometimes accidentally leave their
594 directory or files world-writable.
595 The default is
596 .Dq yes .
597 .It Cm Subsystem
598 Configures an external subsystem (e.g., file transfer daemon).
599 Arguments should be a subsystem name and a command to execute upon subsystem
600 request.
601 The command
602 .Xr sftp-server 8
603 implements the
604 .Dq sftp
605 file transfer subsystem.
606 By default no subsystems are defined.
607 Note that this option applies to protocol version 2 only.
608 .It Cm SyslogFacility
609 Gives the facility code that is used when logging messages from
610 .Nm sshd .
611 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
612 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
613 The default is AUTH.
614 .It Cm TCPKeepAlive
615 Specifies whether the system should send TCP keepalive messages to the
616 other side.
617 If they are sent, death of the connection or crash of one
618 of the machines will be properly noticed.
619 However, this means that
620 connections will die if the route is down temporarily, and some people
621 find it annoying.
622 On the other hand, if TCP keepalives are not sent,
623 sessions may hang indefinitely on the server, leaving
624 .Dq ghost
625 users and consuming server resources.
626 .Pp
627 The default is
628 .Dq yes
629 (to send TCP keepalive messages), and the server will notice
630 if the network goes down or the client host crashes.
631 This avoids infinitely hanging sessions.
632 .Pp
633 To disable TCP keepalive messages, the value should be set to
634 .Dq no .
635 .It Cm UseDNS
636 Specifies whether
637 .Nm sshd
638 should look up the remote host name and check that
639 the resolved host name for the remote IP address maps back to the
640 very same IP address.
641 The default is
642 .Dq yes .
643 .It Cm UseLogin
644 Specifies whether
645 .Xr login 1
646 is used for interactive login sessions.
647 The default is
648 .Dq no .
649 Note that
650 .Xr login 1
651 is never used for remote command execution.
652 Note also, that if this is enabled,
653 .Cm X11Forwarding
654 will be disabled because
655 .Xr login 1
656 does not know how to handle
657 .Xr xauth 1
658 cookies.
659 If
660 .Cm UsePrivilegeSeparation
661 is specified, it will be disabled after authentication.
662 .It Cm UsePAM
663 Enables the Pluggable Authentication Module interface.
664 If set to
665 .Dq yes
666 this will enable PAM authentication using
667 .Cm ChallengeResponseAuthentication
668 and PAM account and session module processing for all authentication types.
669 .Pp
670 Because PAM challenge-response authentication usually serves an equivalent
671 role to password authentication, you should disable either
672 .Cm PasswordAuthentication
673 or
674 .Cm ChallengeResponseAuthentication.
675 .Pp
676 If
677 .Cm UsePAM
678 is enabled, you will not be able to run
679 .Xr sshd 8
680 as a non-root user.
681 The default is
682 .Dq no .
683 .It Cm UsePrivilegeSeparation
684 Specifies whether
685 .Nm sshd
686 separates privileges by creating an unprivileged child process
687 to deal with incoming network traffic.
688 After successful authentication, another process will be created that has
689 the privilege of the authenticated user.
690 The goal of privilege separation is to prevent privilege
691 escalation by containing any corruption within the unprivileged processes.
692 The default is
693 .Dq yes .
694 .It Cm X11DisplayOffset
695 Specifies the first display number available for
696 .Nm sshd Ns 's
697 X11 forwarding.
698 This prevents
699 .Nm sshd
700 from interfering with real X11 servers.
701 The default is 10.
702 .It Cm X11Forwarding
703 Specifies whether X11 forwarding is permitted.
704 The argument must be
705 .Dq yes
706 or
707 .Dq no .
708 The default is
709 .Dq no .
710 .Pp
711 When X11 forwarding is enabled, there may be additional exposure to
712 the server and to client displays if the
713 .Nm sshd
714 proxy display is configured to listen on the wildcard address (see
715 .Cm X11UseLocalhost
716 below), however this is not the default.
717 Additionally, the authentication spoofing and authentication data
718 verification and substitution occur on the client side.
719 The security risk of using X11 forwarding is that the client's X11
720 display server may be exposed to attack when the ssh client requests
721 forwarding (see the warnings for
722 .Cm ForwardX11
723 in
724 .Xr ssh_config 5 ) .
725 A system administrator may have a stance in which they want to
726 protect clients that may expose themselves to attack by unwittingly
727 requesting X11 forwarding, which can warrant a
728 .Dq no
729 setting.
730 .Pp
731 Note that disabling X11 forwarding does not prevent users from
732 forwarding X11 traffic, as users can always install their own forwarders.
733 X11 forwarding is automatically disabled if
734 .Cm UseLogin
735 is enabled.
736 .It Cm X11UseLocalhost
737 Specifies whether
738 .Nm sshd
739 should bind the X11 forwarding server to the loopback address or to
740 the wildcard address.
741 By default,
742 .Nm sshd
743 binds the forwarding server to the loopback address and sets the
744 hostname part of the
745 .Ev DISPLAY
746 environment variable to
747 .Dq localhost .
748 This prevents remote hosts from connecting to the proxy display.
749 However, some older X11 clients may not function with this
750 configuration.
751 .Cm X11UseLocalhost
752 may be set to
753 .Dq no
754 to specify that the forwarding server should be bound to the wildcard
755 address.
756 The argument must be
757 .Dq yes
758 or
759 .Dq no .
760 The default is
761 .Dq yes .
762 .It Cm XAuthLocation
763 Specifies the full pathname of the
764 .Xr xauth 1
765 program.
766 The default is
767 .Pa /usr/X11R6/bin/xauth .
768 .El
769 .Ss Time Formats
770 .Nm sshd
771 command-line arguments and configuration file options that specify time
772 may be expressed using a sequence of the form:
773 .Sm off
774 .Ar time Op Ar qualifier ,
775 .Sm on
776 where
777 .Ar time
778 is a positive integer value and
779 .Ar qualifier
780 is one of the following:
781 .Pp
782 .Bl -tag -width Ds -compact -offset indent
783 .It Cm <none>
784 seconds
785 .It Cm s | Cm S
786 seconds
787 .It Cm m | Cm M
788 minutes
789 .It Cm h | Cm H
790 hours
791 .It Cm d | Cm D
792 days
793 .It Cm w | Cm W
794 weeks
795 .El
796 .Pp
797 Each member of the sequence is added together to calculate
798 the total time value.
799 .Pp
800 Time format examples:
801 .Pp
802 .Bl -tag -width Ds -compact -offset indent
803 .It 600
804 600 seconds (10 minutes)
805 .It 10m
806 10 minutes
807 .It 1h30m
808 1 hour 30 minutes (90 minutes)
809 .El
810 .Sh FILES
811 .Bl -tag -width Ds
812 .It Pa /etc/ssh/sshd_config
813 Contains configuration data for
814 .Nm sshd .
815 This file should be writable by root only, but it is recommended
816 (though not necessary) that it be world-readable.
817 .El
818 .Sh SEE ALSO
819 .Xr sshd 8
820 .Sh AUTHORS
821 OpenSSH is a derivative of the original and free
822 ssh 1.2.12 release by Tatu Ylonen.
823 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
824 Theo de Raadt and Dug Song
825 removed many bugs, re-added newer features and
826 created OpenSSH.
827 Markus Friedl contributed the support for SSH
828 protocol versions 1.5 and 2.0.
829 Niels Provos and Markus Friedl contributed support
830 for privilege separation.