| blob | ff5457dff0d999085e8cc003f44eb9cfa985de23 |
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 1.68 2006/07/21 12:43:36 dtucker 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 .Xr sshd 8
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 Arguments may optionally be enclosed in double quotes
60 .Pq \&"
61 in order to represent arguments containing spaces.
62 .Pp
63 The possible
64 keywords and their meanings are as follows (note that
65 keywords are case-insensitive and arguments are case-sensitive):
66 .Bl -tag -width Ds
67 .It Cm AcceptEnv
68 Specifies what environment variables sent by the client will be copied into
69 the session's
70 .Xr environ 7 .
71 See
72 .Cm SendEnv
73 in
74 .Xr ssh_config 5
75 for how to configure the client.
76 Note that environment passing is only supported for protocol 2.
77 Variables are specified by name, which may contain the wildcard characters
78 .Ql *
79 and
80 .Ql \&? .
81 Multiple environment variables may be separated by whitespace or spread
82 across multiple
83 .Cm AcceptEnv
84 directives.
85 Be warned that some environment variables could be used to bypass restricted
86 user environments.
87 For this reason, care should be taken in the use of this directive.
88 The default is not to accept any environment variables.
89 .It Cm AddressFamily
90 Specifies which address family should be used by
91 .Xr sshd 8 .
92 Valid arguments are
93 .Dq any ,
94 .Dq inet
95 (use IPv4 only), or
96 .Dq inet6
97 (use IPv6 only).
98 The default is
99 .Dq any .
100 .It Cm AllowGroups
101 This keyword can be followed by a list of group name patterns, separated
102 by spaces.
103 If specified, login is allowed only for users whose primary
104 group or supplementary group list matches one of the patterns.
105 Only group names are valid; a numerical group ID is not recognized.
106 By default, login is allowed for all groups.
107 The allow/deny directives are processed in the following order:
108 .Cm DenyUsers ,
109 .Cm AllowUsers ,
110 .Cm DenyGroups ,
111 and finally
112 .Cm AllowGroups .
113 .Pp
114 See
115 .Sx PATTERNS
116 in
117 .Xr ssh_config 5
118 for more information on patterns.
119 .It Cm AllowTcpForwarding
120 Specifies whether TCP forwarding is permitted.
121 The default is
122 .Dq yes .
123 Note that disabling TCP forwarding does not improve security unless
124 users are also denied shell access, as they can always install their
125 own forwarders.
126 .It Cm AllowUsers
127 This keyword can be followed by a list of user name patterns, separated
128 by spaces.
129 If specified, login is allowed only for user names that
130 match one of the patterns.
131 Only user names are valid; a numerical user ID is not recognized.
132 By default, login is allowed for all users.
133 If the pattern takes the form USER@HOST then USER and HOST
134 are separately checked, restricting logins to particular
135 users from particular hosts.
136 The allow/deny directives are processed in the following order:
137 .Cm DenyUsers ,
138 .Cm AllowUsers ,
139 .Cm DenyGroups ,
140 and finally
141 .Cm AllowGroups .
142 .Pp
143 See
144 .Sx PATTERNS
145 in
146 .Xr ssh_config 5
147 for more information on patterns.
148 .It Cm AuthorizedKeysFile
149 Specifies the file that contains the public keys that can be used
150 for user authentication.
151 .Cm AuthorizedKeysFile
152 may contain tokens of the form %T which are substituted during connection
153 setup.
154 The following tokens are defined: %% is replaced by a literal '%',
155 %h is replaced by the home directory of the user being authenticated, and
156 %u is replaced by the username of that user.
157 After expansion,
158 .Cm AuthorizedKeysFile
159 is taken to be an absolute path or one relative to the user's home
160 directory.
161 The default is
162 .Dq .ssh/authorized_keys .
163 .It Cm Banner
164 In some jurisdictions, sending a warning message before authentication
165 may be relevant for getting legal protection.
166 The contents of the specified file are sent to the remote user before
167 authentication is allowed.
168 This option is only available for protocol version 2.
169 By default, no banner is displayed.
170 .It Cm ChallengeResponseAuthentication
171 Specifies whether challenge-response authentication is allowed.
172 All authentication styles from
173 .Xr login.conf 5
174 are supported.
175 The default is
176 .Dq yes .
177 .It Cm Ciphers
178 Specifies the ciphers allowed for protocol version 2.
179 Multiple ciphers must be comma-separated.
180 The supported ciphers are
181 .Dq 3des-cbc ,
182 .Dq aes128-cbc ,
183 .Dq aes192-cbc ,
184 .Dq aes256-cbc ,
185 .Dq aes128-ctr ,
186 .Dq aes192-ctr ,
187 .Dq aes256-ctr ,
188 .Dq arcfour128 ,
189 .Dq arcfour256 ,
190 .Dq arcfour ,
191 .Dq blowfish-cbc ,
192 and
193 .Dq cast128-cbc .
194 The default is:
195 .Bd -literal -offset 3n
196 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
197 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
198 aes192-ctr,aes256-ctr
199 .Ed
200 .It Cm ClientAliveCountMax
201 Sets the number of client alive messages (see below) which may be
202 sent without
203 .Xr sshd 8
204 receiving any messages back from the client.
205 If this threshold is reached while client alive messages are being sent,
206 sshd will disconnect the client, terminating the session.
207 It is important to note that the use of client alive messages is very
208 different from
209 .Cm TCPKeepAlive
210 (below).
211 The client alive messages are sent through the encrypted channel
212 and therefore will not be spoofable.
213 The TCP keepalive option enabled by
214 .Cm TCPKeepAlive
215 is spoofable.
216 The client alive mechanism is valuable when the client or
217 server depend on knowing when a connection has become inactive.
218 .Pp
219 The default value is 3.
220 If
221 .Cm ClientAliveInterval
222 (see below) is set to 15, and
223 .Cm ClientAliveCountMax
224 is left at the default, unresponsive SSH clients
225 will be disconnected after approximately 45 seconds.
226 This option applies to protocol version 2 only.
227 .It Cm ClientAliveInterval
228 Sets a timeout interval in seconds after which if no data has been received
229 from the client,
230 .Xr sshd 8
231 will send a message through the encrypted
232 channel to request a response from the client.
233 The default
234 is 0, indicating that these messages will not be sent to the client.
235 This option applies to protocol version 2 only.
236 .It Cm Compression
237 Specifies whether compression is allowed, or delayed until
238 the user has authenticated successfully.
239 The argument must be
240 .Dq yes ,
241 .Dq delayed ,
242 or
243 .Dq no .
244 The default is
245 .Dq delayed .
246 .It Cm DenyGroups
247 This keyword can be followed by a list of group name patterns, separated
248 by spaces.
249 Login is disallowed for users whose primary group or supplementary
250 group list matches one of the patterns.
251 Only group names are valid; a numerical group ID is not recognized.
252 By default, login is allowed for all groups.
253 The allow/deny directives are processed in the following order:
254 .Cm DenyUsers ,
255 .Cm AllowUsers ,
256 .Cm DenyGroups ,
257 and finally
258 .Cm AllowGroups .
259 .Pp
260 See
261 .Sx PATTERNS
262 in
263 .Xr ssh_config 5
264 for more information on patterns.
265 .It Cm DenyUsers
266 This keyword can be followed by a list of user name patterns, separated
267 by spaces.
268 Login is disallowed for user names that match one of the patterns.
269 Only user names are valid; a numerical user ID is not recognized.
270 By default, login is allowed for all users.
271 If the pattern takes the form USER@HOST then USER and HOST
272 are separately checked, restricting logins to particular
273 users from particular hosts.
274 The allow/deny directives are processed in the following order:
275 .Cm DenyUsers ,
276 .Cm AllowUsers ,
277 .Cm DenyGroups ,
278 and finally
279 .Cm AllowGroups .
280 .Pp
281 See
282 .Sx PATTERNS
283 in
284 .Xr ssh_config 5
285 for more information on patterns.
286 .It Cm ForceCommand
287 Forces the execution of the command specified by
288 .Cm ForceCommand ,
289 ignoring any command supplied by the client.
290 The command is invoked by using the user's login shell with the -c option.
291 This applies to shell, command, or subsystem execution.
292 It is most useful inside a
293 .Cm Match
294 block.
295 The command originally supplied by the client is available in the
296 .Ev SSH_ORIGINAL_COMMAND
297 environment variable.
298 .It Cm GatewayPorts
299 Specifies whether remote hosts are allowed to connect to ports
300 forwarded for the client.
301 By default,
302 .Xr sshd 8
303 binds remote port forwardings to the loopback address.
304 This prevents other remote hosts from connecting to forwarded ports.
305 .Cm GatewayPorts
306 can be used to specify that sshd
307 should allow remote port forwardings to bind to non-loopback addresses, thus
308 allowing other hosts to connect.
309 The argument may be
310 .Dq no
311 to force remote port forwardings to be available to the local host only,
312 .Dq yes
313 to force remote port forwardings to bind to the wildcard address, or
314 .Dq clientspecified
315 to allow the client to select the address to which the forwarding is bound.
316 The default is
317 .Dq no .
318 .It Cm GSSAPIAuthentication
319 Specifies whether user authentication based on GSSAPI is allowed.
320 The default is
321 .Dq no .
322 Note that this option applies to protocol version 2 only.
323 .It Cm GSSAPICleanupCredentials
324 Specifies whether to automatically destroy the user's credentials cache
325 on logout.
326 The default is
327 .Dq yes .
328 Note that this option applies to protocol version 2 only.
329 .It Cm HostbasedAuthentication
330 Specifies whether rhosts or /etc/hosts.equiv authentication together
331 with successful public key client host authentication is allowed
332 (host-based authentication).
333 This option is similar to
334 .Cm RhostsRSAAuthentication
335 and applies to protocol version 2 only.
336 The default is
337 .Dq no .
338 .It Cm HostKey
339 Specifies a file containing a private host key
340 used by SSH.
341 The default is
342 .Pa /etc/ssh/ssh_host_key
343 for protocol version 1, and
344 .Pa /etc/ssh/ssh_host_rsa_key
345 and
346 .Pa /etc/ssh/ssh_host_dsa_key
347 for protocol version 2.
348 Note that
349 .Xr sshd 8
350 will refuse to use a file if it is group/world-accessible.
351 It is possible to have multiple host key files.
352 .Dq rsa1
353 keys are used for version 1 and
354 .Dq dsa
355 or
356 .Dq rsa
357 are used for version 2 of the SSH protocol.
358 .It Cm IgnoreRhosts
359 Specifies that
360 .Pa .rhosts
361 and
362 .Pa .shosts
363 files will not be used in
364 .Cm RhostsRSAAuthentication
365 or
366 .Cm HostbasedAuthentication .
367 .Pp
368 .Pa /etc/hosts.equiv
369 and
370 .Pa /etc/shosts.equiv
371 are still used.
372 The default is
373 .Dq yes .
374 .It Cm IgnoreUserKnownHosts
375 Specifies whether
376 .Xr sshd 8
377 should ignore the user's
378 .Pa ~/.ssh/known_hosts
379 during
380 .Cm RhostsRSAAuthentication
381 or
382 .Cm HostbasedAuthentication .
383 The default is
384 .Dq no .
385 .It Cm KerberosAuthentication
386 Specifies whether the password provided by the user for
387 .Cm PasswordAuthentication
388 will be validated through the Kerberos KDC.
389 To use this option, the server needs a
390 Kerberos servtab which allows the verification of the KDC's identity.
391 The default is
392 .Dq no .
393 .It Cm KerberosGetAFSToken
394 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
395 an AFS token before accessing the user's home directory.
396 The default is
397 .Dq no .
398 .It Cm KerberosOrLocalPasswd
399 If password authentication through Kerberos fails then
400 the password will be validated via any additional local mechanism
401 such as
402 .Pa /etc/passwd .
403 The default is
404 .Dq yes .
405 .It Cm KerberosTicketCleanup
406 Specifies whether to automatically destroy the user's ticket cache
407 file on logout.
408 The default is
409 .Dq yes .
410 .It Cm KeyRegenerationInterval
411 In protocol version 1, the ephemeral server key is automatically regenerated
412 after this many seconds (if it has been used).
413 The purpose of regeneration is to prevent
414 decrypting captured sessions by later breaking into the machine and
415 stealing the keys.
416 The key is never stored anywhere.
417 If the value is 0, the key is never regenerated.
418 The default is 3600 (seconds).
419 .It Cm ListenAddress
420 Specifies the local addresses
421 .Xr sshd 8
422 should listen on.
423 The following forms may be used:
424 .Pp
425 .Bl -item -offset indent -compact
426 .It
427 .Cm ListenAddress
428 .Sm off
429 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
430 .Sm on
431 .It
432 .Cm ListenAddress
433 .Sm off
434 .Ar host No | Ar IPv4_addr No : Ar port
435 .Sm on
436 .It
437 .Cm ListenAddress
438 .Sm off
439 .Oo
440 .Ar host No | Ar IPv6_addr Oc : Ar port
441 .Sm on
442 .El
443 .Pp
444 If
445 .Ar port
446 is not specified,
447 sshd will listen on the address and all prior
448 .Cm Port
449 options specified.
450 The default is to listen on all local addresses.
451 Multiple
452 .Cm ListenAddress
453 options are permitted.
454 Additionally, any
455 .Cm Port
456 options must precede this option for non-port qualified addresses.
457 .It Cm LoginGraceTime
458 The server disconnects after this time if the user has not
459 successfully logged in.
460 If the value is 0, there is no time limit.
461 The default is 120 seconds.
462 .It Cm LogLevel
463 Gives the verbosity level that is used when logging messages from
464 .Xr sshd 8 .
465 The possible values are:
466 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
467 The default is INFO.
468 DEBUG and DEBUG1 are equivalent.
469 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
470 Logging with a DEBUG level violates the privacy of users and is not recommended.
471 .It Cm MACs
472 Specifies the available MAC (message authentication code) algorithms.
473 The MAC algorithm is used in protocol version 2
474 for data integrity protection.
475 Multiple algorithms must be comma-separated.
476 The default is:
477 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
478 .It Cm Match
479 Introduces a conditional block.
480 If all of the criteria on the
481 .Cm Match
482 line are satisfied, the keywords on the following lines override those
483 set in the global section of the config file, until either another
484 .Cm Match
485 line or the end of the file.
486 The arguments to
487 .Cm Match
488 are one or more criteria-pattern pairs.
489 The available criteria are
490 .Cm User ,
491 .Cm Host ,
492 and
493 .Cm Address .
494 Only a subset of keywords may be used on the lines following a
495 .Cm Match
496 keyword.
497 Available keywords are
498 .Cm AllowTcpForwarding ,
499 .Cm ForceCommand ,
500 .Cm GatewayPorts ,
501 .Cm PermitOpen ,
502 .Cm X11DisplayOffset ,
503 .Cm X11Forwarding ,
504 and
505 .Cm X11UseLocalHost .
506 .It Cm MaxAuthTries
507 Specifies the maximum number of authentication attempts permitted per
508 connection.
509 Once the number of failures reaches half this value,
510 additional failures are logged.
511 The default is 6.
512 .It Cm MaxStartups
513 Specifies the maximum number of concurrent unauthenticated connections to the
514 SSH daemon.
515 Additional connections will be dropped until authentication succeeds or the
516 .Cm LoginGraceTime
517 expires for a connection.
518 The default is 10.
519 .Pp
520 Alternatively, random early drop can be enabled by specifying
521 the three colon separated values
522 .Dq start:rate:full
523 (e.g. "10:30:60").
524 .Xr sshd 8
525 will refuse connection attempts with a probability of
526 .Dq rate/100
527 (30%)
528 if there are currently
529 .Dq start
530 (10)
531 unauthenticated connections.
532 The probability increases linearly and all connection attempts
533 are refused if the number of unauthenticated connections reaches
534 .Dq full
535 (60).
536 .It Cm PasswordAuthentication
537 Specifies whether password authentication is allowed.
538 The default is
539 .Dq yes .
540 .It Cm PermitEmptyPasswords
541 When password authentication is allowed, it specifies whether the
542 server allows login to accounts with empty password strings.
543 The default is
544 .Dq no .
545 .It Cm PermitOpen
546 Specifies the destinations to which TCP port forwarding is permitted.
547 The forwarding specification must be one of the following forms:
548 .Pp
549 .Bl -item -offset indent -compact
550 .It
551 .Cm PermitOpen
552 .Sm off
553 .Ar host : port
554 .Sm on
555 .It
556 .Cm PermitOpen
557 .Sm off
558 .Ar IPv4_addr : port
559 .Sm on
560 .It
561 .Cm PermitOpen
562 .Sm off
563 .Ar \&[ IPv6_addr \&] : port
564 .Sm on
565 .El
566 .Pp
567 Multiple forwards may be specified by separating them with whitespace.
568 An argument of
569 .Dq any
570 can be used to remove all restrictions and permit any forwarding requests.
571 By default all port forwarding requests are permitted.
572 .It Cm PermitRootLogin
573 Specifies whether root can log in using
574 .Xr ssh 1 .
575 The argument must be
576 .Dq yes ,
577 .Dq without-password ,
578 .Dq forced-commands-only ,
579 or
580 .Dq no .
581 The default is
582 .Dq yes .
583 .Pp
584 If this option is set to
585 .Dq without-password ,
586 password authentication is disabled for root.
587 .Pp
588 If this option is set to
589 .Dq forced-commands-only ,
590 root login with public key authentication will be allowed,
591 but only if the
592 .Ar command
593 option has been specified
594 (which may be useful for taking remote backups even if root login is
595 normally not allowed).
596 All other authentication methods are disabled for root.
597 .Pp
598 If this option is set to
599 .Dq no ,
600 root is not allowed to log in.
601 .It Cm PermitTunnel
602 Specifies whether
603 .Xr tun 4
604 device forwarding is allowed.
605 The argument must be
606 .Dq yes ,
607 .Dq point-to-point
608 (layer 3),
609 .Dq ethernet
610 (layer 2), or
611 .Dq no .
612 Specifying
613 .Dq yes
614 permits both
615 .Dq point-to-point
616 and
617 .Dq ethernet .
618 The default is
619 .Dq no .
620 .It Cm PermitUserEnvironment
621 Specifies whether
622 .Pa ~/.ssh/environment
623 and
624 .Cm environment=
625 options in
626 .Pa ~/.ssh/authorized_keys
627 are processed by
628 .Xr sshd 8 .
629 The default is
630 .Dq no .
631 Enabling environment processing may enable users to bypass access
632 restrictions in some configurations using mechanisms such as
633 .Ev LD_PRELOAD .
634 .It Cm PidFile
635 Specifies the file that contains the process ID of the
636 SSH daemon.
637 The default is
638 .Pa /var/run/sshd.pid .
639 .It Cm Port
640 Specifies the port number that
641 .Xr sshd 8
642 listens on.
643 The default is 22.
644 Multiple options of this type are permitted.
645 See also
646 .Cm ListenAddress .
647 .It Cm PrintLastLog
648 Specifies whether
649 .Xr sshd 8
650 should print the date and time of the last user login when a user logs
651 in interactively.
652 The default is
653 .Dq yes .
654 .It Cm PrintMotd
655 Specifies whether
656 .Xr sshd 8
657 should print
658 .Pa /etc/motd
659 when a user logs in interactively.
660 (On some systems it is also printed by the shell,
661 .Pa /etc/profile ,
662 or equivalent.)
663 The default is
664 .Dq yes .
665 .It Cm Protocol
666 Specifies the protocol versions
667 .Xr sshd 8
668 supports.
669 The possible values are
670 .Sq 1
671 and
672 .Sq 2 .
673 Multiple versions must be comma-separated.
674 The default is
675 .Dq 2,1 .
676 Note that the order of the protocol list does not indicate preference,
677 because the client selects among multiple protocol versions offered
678 by the server.
679 Specifying
680 .Dq 2,1
681 is identical to
682 .Dq 1,2 .
683 .It Cm PubkeyAuthentication
684 Specifies whether public key authentication is allowed.
685 The default is
686 .Dq yes .
687 Note that this option applies to protocol version 2 only.
688 .It Cm RhostsRSAAuthentication
689 Specifies whether rhosts or /etc/hosts.equiv authentication together
690 with successful RSA host authentication is allowed.
691 The default is
692 .Dq no .
693 This option applies to protocol version 1 only.
694 .It Cm RSAAuthentication
695 Specifies whether pure RSA authentication is allowed.
696 The default is
697 .Dq yes .
698 This option applies to protocol version 1 only.
699 .It Cm ServerKeyBits
700 Defines the number of bits in the ephemeral protocol version 1 server key.
701 The minimum value is 512, and the default is 768.
702 .It Cm StrictModes
703 Specifies whether
704 .Xr sshd 8
705 should check file modes and ownership of the
706 user's files and home directory before accepting login.
707 This is normally desirable because novices sometimes accidentally leave their
708 directory or files world-writable.
709 The default is
710 .Dq yes .
711 .It Cm Subsystem
712 Configures an external subsystem (e.g. file transfer daemon).
713 Arguments should be a subsystem name and a command (with optional arguments)
714 to execute upon subsystem request.
715 The command
716 .Xr sftp-server 8
717 implements the
718 .Dq sftp
719 file transfer subsystem.
720 By default no subsystems are defined.
721 Note that this option applies to protocol version 2 only.
722 .It Cm SyslogFacility
723 Gives the facility code that is used when logging messages from
724 .Xr sshd 8 .
725 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
726 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
727 The default is AUTH.
728 .It Cm TCPKeepAlive
729 Specifies whether the system should send TCP keepalive messages to the
730 other side.
731 If they are sent, death of the connection or crash of one
732 of the machines will be properly noticed.
733 However, this means that
734 connections will die if the route is down temporarily, and some people
735 find it annoying.
736 On the other hand, if TCP keepalives are not sent,
737 sessions may hang indefinitely on the server, leaving
738 .Dq ghost
739 users and consuming server resources.
740 .Pp
741 The default is
742 .Dq yes
743 (to send TCP keepalive messages), and the server will notice
744 if the network goes down or the client host crashes.
745 This avoids infinitely hanging sessions.
746 .Pp
747 To disable TCP keepalive messages, the value should be set to
748 .Dq no .
749 .It Cm UseDNS
750 Specifies whether
751 .Xr sshd 8
752 should look up the remote host name and check that
753 the resolved host name for the remote IP address maps back to the
754 very same IP address.
755 The default is
756 .Dq yes .
757 .It Cm UseLogin
758 Specifies whether
759 .Xr login 1
760 is used for interactive login sessions.
761 The default is
762 .Dq no .
763 Note that
764 .Xr login 1
765 is never used for remote command execution.
766 Note also, that if this is enabled,
767 .Cm X11Forwarding
768 will be disabled because
769 .Xr login 1
770 does not know how to handle
771 .Xr xauth 1
772 cookies.
773 If
774 .Cm UsePrivilegeSeparation
775 is specified, it will be disabled after authentication.
776 .It Cm UsePAM
777 Enables the Pluggable Authentication Module interface.
778 If set to
779 .Dq yes
780 this will enable PAM authentication using
781 .Cm ChallengeResponseAuthentication
782 and
783 .Cm PasswordAuthentication
784 in addition to PAM account and session module processing for all
785 authentication types.
786 .Pp
787 Because PAM challenge-response authentication usually serves an equivalent
788 role to password authentication, you should disable either
789 .Cm PasswordAuthentication
790 or
791 .Cm ChallengeResponseAuthentication.
792 .Pp
793 If
794 .Cm UsePAM
795 is enabled, you will not be able to run
796 .Xr sshd 8
797 as a non-root user.
798 The default is
799 .Dq no .
800 .It Cm UsePrivilegeSeparation
801 Specifies whether
802 .Xr sshd 8
803 separates privileges by creating an unprivileged child process
804 to deal with incoming network traffic.
805 After successful authentication, another process will be created that has
806 the privilege of the authenticated user.
807 The goal of privilege separation is to prevent privilege
808 escalation by containing any corruption within the unprivileged processes.
809 The default is
810 .Dq yes .
811 .It Cm X11DisplayOffset
812 Specifies the first display number available for
813 .Xr sshd 8 Ns 's
814 X11 forwarding.
815 This prevents sshd from interfering with real X11 servers.
816 The default is 10.
817 .It Cm X11Forwarding
818 Specifies whether X11 forwarding is permitted.
819 The argument must be
820 .Dq yes
821 or
822 .Dq no .
823 The default is
824 .Dq no .
825 .Pp
826 When X11 forwarding is enabled, there may be additional exposure to
827 the server and to client displays if the
828 .Xr sshd 8
829 proxy display is configured to listen on the wildcard address (see
830 .Cm X11UseLocalhost
831 below), though this is not the default.
832 Additionally, the authentication spoofing and authentication data
833 verification and substitution occur on the client side.
834 The security risk of using X11 forwarding is that the client's X11
835 display server may be exposed to attack when the SSH client requests
836 forwarding (see the warnings for
837 .Cm ForwardX11
838 in
839 .Xr ssh_config 5 ) .
840 A system administrator may have a stance in which they want to
841 protect clients that may expose themselves to attack by unwittingly
842 requesting X11 forwarding, which can warrant a
843 .Dq no
844 setting.
845 .Pp
846 Note that disabling X11 forwarding does not prevent users from
847 forwarding X11 traffic, as users can always install their own forwarders.
848 X11 forwarding is automatically disabled if
849 .Cm UseLogin
850 is enabled.
851 .It Cm X11UseLocalhost
852 Specifies whether
853 .Xr sshd 8
854 should bind the X11 forwarding server to the loopback address or to
855 the wildcard address.
856 By default,
857 sshd binds the forwarding server to the loopback address and sets the
858 hostname part of the
859 .Ev DISPLAY
860 environment variable to
861 .Dq localhost .
862 This prevents remote hosts from connecting to the proxy display.
863 However, some older X11 clients may not function with this
864 configuration.
865 .Cm X11UseLocalhost
866 may be set to
867 .Dq no
868 to specify that the forwarding server should be bound to the wildcard
869 address.
870 The argument must be
871 .Dq yes
872 or
873 .Dq no .
874 The default is
875 .Dq yes .
876 .It Cm XAuthLocation
877 Specifies the full pathname of the
878 .Xr xauth 1
879 program.
880 The default is
881 .Pa /usr/X11R6/bin/xauth .
882 .El
883 .Sh TIME FORMATS
884 .Xr sshd 8
885 command-line arguments and configuration file options that specify time
886 may be expressed using a sequence of the form:
887 .Sm off
888 .Ar time Op Ar qualifier ,
889 .Sm on
890 where
891 .Ar time
892 is a positive integer value and
893 .Ar qualifier
894 is one of the following:
895 .Pp
896 .Bl -tag -width Ds -compact -offset indent
897 .It Aq Cm none
898 seconds
899 .It Cm s | Cm S
900 seconds
901 .It Cm m | Cm M
902 minutes
903 .It Cm h | Cm H
904 hours
905 .It Cm d | Cm D
906 days
907 .It Cm w | Cm W
908 weeks
909 .El
910 .Pp
911 Each member of the sequence is added together to calculate
912 the total time value.
913 .Pp
914 Time format examples:
915 .Pp
916 .Bl -tag -width Ds -compact -offset indent
917 .It 600
918 600 seconds (10 minutes)
919 .It 10m
920 10 minutes
921 .It 1h30m
922 1 hour 30 minutes (90 minutes)
923 .El
924 .Sh FILES
925 .Bl -tag -width Ds
926 .It Pa /etc/ssh/sshd_config
927 Contains configuration data for
928 .Xr sshd 8 .
929 This file should be writable by root only, but it is recommended
930 (though not necessary) that it be world-readable.
931 .El
932 .Sh SEE ALSO
933 .Xr sshd 8
934 .Sh AUTHORS
935 OpenSSH is a derivative of the original and free
936 ssh 1.2.12 release by Tatu Ylonen.
937 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
938 Theo de Raadt and Dug Song
939 removed many bugs, re-added newer features and
940 created OpenSSH.
941 Markus Friedl contributed the support for SSH
942 protocol versions 1.5 and 2.0.
943 Niels Provos and Markus Friedl contributed support
944 for privilege separation.