| blob | 2bcaf22454263eb385e17da44f3c78c0ff0d048c |
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 1.70 2006/08/21 08:14:01 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 HostbasedUsesNameFromPacketOnly
339 Specifies whether or not the server will attempt to perform a reverse
340 name lookup when matching the name in the
341 .Pa ~/.shosts ,
342 .Pa ~/.rhosts ,
343 and
344 .Pa /etc/hosts.equiv
345 files during
346 .Cm HostbasedAuthentication .
347 A setting of
348 .Dq yes
349 means that
350 .Xr sshd 8
351 uses the name supplied by the client rather than
352 attempting to resolve the name from the TCP connection itself.
353 The default is
354 .Dq no .
355 .It Cm HostKey
356 Specifies a file containing a private host key
357 used by SSH.
358 The default is
359 .Pa /etc/ssh/ssh_host_key
360 for protocol version 1, and
361 .Pa /etc/ssh/ssh_host_rsa_key
362 and
363 .Pa /etc/ssh/ssh_host_dsa_key
364 for protocol version 2.
365 Note that
366 .Xr sshd 8
367 will refuse to use a file if it is group/world-accessible.
368 It is possible to have multiple host key files.
369 .Dq rsa1
370 keys are used for version 1 and
371 .Dq dsa
372 or
373 .Dq rsa
374 are used for version 2 of the SSH protocol.
375 .It Cm IgnoreRhosts
376 Specifies that
377 .Pa .rhosts
378 and
379 .Pa .shosts
380 files will not be used in
381 .Cm RhostsRSAAuthentication
382 or
383 .Cm HostbasedAuthentication .
384 .Pp
385 .Pa /etc/hosts.equiv
386 and
387 .Pa /etc/shosts.equiv
388 are still used.
389 The default is
390 .Dq yes .
391 .It Cm IgnoreUserKnownHosts
392 Specifies whether
393 .Xr sshd 8
394 should ignore the user's
395 .Pa ~/.ssh/known_hosts
396 during
397 .Cm RhostsRSAAuthentication
398 or
399 .Cm HostbasedAuthentication .
400 The default is
401 .Dq no .
402 .It Cm KerberosAuthentication
403 Specifies whether the password provided by the user for
404 .Cm PasswordAuthentication
405 will be validated through the Kerberos KDC.
406 To use this option, the server needs a
407 Kerberos servtab which allows the verification of the KDC's identity.
408 The default is
409 .Dq no .
410 .It Cm KerberosGetAFSToken
411 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
412 an AFS token before accessing the user's home directory.
413 The default is
414 .Dq no .
415 .It Cm KerberosOrLocalPasswd
416 If password authentication through Kerberos fails then
417 the password will be validated via any additional local mechanism
418 such as
419 .Pa /etc/passwd .
420 The default is
421 .Dq yes .
422 .It Cm KerberosTicketCleanup
423 Specifies whether to automatically destroy the user's ticket cache
424 file on logout.
425 The default is
426 .Dq yes .
427 .It Cm KeyRegenerationInterval
428 In protocol version 1, the ephemeral server key is automatically regenerated
429 after this many seconds (if it has been used).
430 The purpose of regeneration is to prevent
431 decrypting captured sessions by later breaking into the machine and
432 stealing the keys.
433 The key is never stored anywhere.
434 If the value is 0, the key is never regenerated.
435 The default is 3600 (seconds).
436 .It Cm ListenAddress
437 Specifies the local addresses
438 .Xr sshd 8
439 should listen on.
440 The following forms may be used:
441 .Pp
442 .Bl -item -offset indent -compact
443 .It
444 .Cm ListenAddress
445 .Sm off
446 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
447 .Sm on
448 .It
449 .Cm ListenAddress
450 .Sm off
451 .Ar host No | Ar IPv4_addr No : Ar port
452 .Sm on
453 .It
454 .Cm ListenAddress
455 .Sm off
456 .Oo
457 .Ar host No | Ar IPv6_addr Oc : Ar port
458 .Sm on
459 .El
460 .Pp
461 If
462 .Ar port
463 is not specified,
464 sshd will listen on the address and all prior
465 .Cm Port
466 options specified.
467 The default is to listen on all local addresses.
468 Multiple
469 .Cm ListenAddress
470 options are permitted.
471 Additionally, any
472 .Cm Port
473 options must precede this option for non-port qualified addresses.
474 .It Cm LoginGraceTime
475 The server disconnects after this time if the user has not
476 successfully logged in.
477 If the value is 0, there is no time limit.
478 The default is 120 seconds.
479 .It Cm LogLevel
480 Gives the verbosity level that is used when logging messages from
481 .Xr sshd 8 .
482 The possible values are:
483 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
484 The default is INFO.
485 DEBUG and DEBUG1 are equivalent.
486 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
487 Logging with a DEBUG level violates the privacy of users and is not recommended.
488 .It Cm MACs
489 Specifies the available MAC (message authentication code) algorithms.
490 The MAC algorithm is used in protocol version 2
491 for data integrity protection.
492 Multiple algorithms must be comma-separated.
493 The default is:
494 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
495 .It Cm Match
496 Introduces a conditional block.
497 If all of the criteria on the
498 .Cm Match
499 line are satisfied, the keywords on the following lines override those
500 set in the global section of the config file, until either another
501 .Cm Match
502 line or the end of the file.
503 The arguments to
504 .Cm Match
505 are one or more criteria-pattern pairs.
506 The available criteria are
507 .Cm User ,
508 .Cm Group ,
509 .Cm Host ,
510 and
511 .Cm Address .
512 Only a subset of keywords may be used on the lines following a
513 .Cm Match
514 keyword.
515 Available keywords are
516 .Cm AllowTcpForwarding ,
517 .Cm ForceCommand ,
518 .Cm GatewayPorts ,
519 .Cm PermitOpen ,
520 .Cm X11DisplayOffset ,
521 .Cm X11Forwarding ,
522 and
523 .Cm X11UseLocalHost .
524 .It Cm MaxAuthTries
525 Specifies the maximum number of authentication attempts permitted per
526 connection.
527 Once the number of failures reaches half this value,
528 additional failures are logged.
529 The default is 6.
530 .It Cm MaxStartups
531 Specifies the maximum number of concurrent unauthenticated connections to the
532 SSH daemon.
533 Additional connections will be dropped until authentication succeeds or the
534 .Cm LoginGraceTime
535 expires for a connection.
536 The default is 10.
537 .Pp
538 Alternatively, random early drop can be enabled by specifying
539 the three colon separated values
540 .Dq start:rate:full
541 (e.g. "10:30:60").
542 .Xr sshd 8
543 will refuse connection attempts with a probability of
544 .Dq rate/100
545 (30%)
546 if there are currently
547 .Dq start
548 (10)
549 unauthenticated connections.
550 The probability increases linearly and all connection attempts
551 are refused if the number of unauthenticated connections reaches
552 .Dq full
553 (60).
554 .It Cm PasswordAuthentication
555 Specifies whether password authentication is allowed.
556 The default is
557 .Dq yes .
558 .It Cm PermitEmptyPasswords
559 When password authentication is allowed, it specifies whether the
560 server allows login to accounts with empty password strings.
561 The default is
562 .Dq no .
563 .It Cm PermitOpen
564 Specifies the destinations to which TCP port forwarding is permitted.
565 The forwarding specification must be one of the following forms:
566 .Pp
567 .Bl -item -offset indent -compact
568 .It
569 .Cm PermitOpen
570 .Sm off
571 .Ar host : port
572 .Sm on
573 .It
574 .Cm PermitOpen
575 .Sm off
576 .Ar IPv4_addr : port
577 .Sm on
578 .It
579 .Cm PermitOpen
580 .Sm off
581 .Ar \&[ IPv6_addr \&] : port
582 .Sm on
583 .El
584 .Pp
585 Multiple forwards may be specified by separating them with whitespace.
586 An argument of
587 .Dq any
588 can be used to remove all restrictions and permit any forwarding requests.
589 By default all port forwarding requests are permitted.
590 .It Cm PermitRootLogin
591 Specifies whether root can log in using
592 .Xr ssh 1 .
593 The argument must be
594 .Dq yes ,
595 .Dq without-password ,
596 .Dq forced-commands-only ,
597 or
598 .Dq no .
599 The default is
600 .Dq yes .
601 .Pp
602 If this option is set to
603 .Dq without-password ,
604 password authentication is disabled for root.
605 .Pp
606 If this option is set to
607 .Dq forced-commands-only ,
608 root login with public key authentication will be allowed,
609 but only if the
610 .Ar command
611 option has been specified
612 (which may be useful for taking remote backups even if root login is
613 normally not allowed).
614 All other authentication methods are disabled for root.
615 .Pp
616 If this option is set to
617 .Dq no ,
618 root is not allowed to log in.
619 .It Cm PermitTunnel
620 Specifies whether
621 .Xr tun 4
622 device forwarding is allowed.
623 The argument must be
624 .Dq yes ,
625 .Dq point-to-point
626 (layer 3),
627 .Dq ethernet
628 (layer 2), or
629 .Dq no .
630 Specifying
631 .Dq yes
632 permits both
633 .Dq point-to-point
634 and
635 .Dq ethernet .
636 The default is
637 .Dq no .
638 .It Cm PermitUserEnvironment
639 Specifies whether
640 .Pa ~/.ssh/environment
641 and
642 .Cm environment=
643 options in
644 .Pa ~/.ssh/authorized_keys
645 are processed by
646 .Xr sshd 8 .
647 The default is
648 .Dq no .
649 Enabling environment processing may enable users to bypass access
650 restrictions in some configurations using mechanisms such as
651 .Ev LD_PRELOAD .
652 .It Cm PidFile
653 Specifies the file that contains the process ID of the
654 SSH daemon.
655 The default is
656 .Pa /var/run/sshd.pid .
657 .It Cm Port
658 Specifies the port number that
659 .Xr sshd 8
660 listens on.
661 The default is 22.
662 Multiple options of this type are permitted.
663 See also
664 .Cm ListenAddress .
665 .It Cm PrintLastLog
666 Specifies whether
667 .Xr sshd 8
668 should print the date and time of the last user login when a user logs
669 in interactively.
670 The default is
671 .Dq yes .
672 .It Cm PrintMotd
673 Specifies whether
674 .Xr sshd 8
675 should print
676 .Pa /etc/motd
677 when a user logs in interactively.
678 (On some systems it is also printed by the shell,
679 .Pa /etc/profile ,
680 or equivalent.)
681 The default is
682 .Dq yes .
683 .It Cm Protocol
684 Specifies the protocol versions
685 .Xr sshd 8
686 supports.
687 The possible values are
688 .Sq 1
689 and
690 .Sq 2 .
691 Multiple versions must be comma-separated.
692 The default is
693 .Dq 2,1 .
694 Note that the order of the protocol list does not indicate preference,
695 because the client selects among multiple protocol versions offered
696 by the server.
697 Specifying
698 .Dq 2,1
699 is identical to
700 .Dq 1,2 .
701 .It Cm PubkeyAuthentication
702 Specifies whether public key authentication is allowed.
703 The default is
704 .Dq yes .
705 Note that this option applies to protocol version 2 only.
706 .It Cm RhostsRSAAuthentication
707 Specifies whether rhosts or /etc/hosts.equiv authentication together
708 with successful RSA host authentication is allowed.
709 The default is
710 .Dq no .
711 This option applies to protocol version 1 only.
712 .It Cm RSAAuthentication
713 Specifies whether pure RSA authentication is allowed.
714 The default is
715 .Dq yes .
716 This option applies to protocol version 1 only.
717 .It Cm ServerKeyBits
718 Defines the number of bits in the ephemeral protocol version 1 server key.
719 The minimum value is 512, and the default is 768.
720 .It Cm StrictModes
721 Specifies whether
722 .Xr sshd 8
723 should check file modes and ownership of the
724 user's files and home directory before accepting login.
725 This is normally desirable because novices sometimes accidentally leave their
726 directory or files world-writable.
727 The default is
728 .Dq yes .
729 .It Cm Subsystem
730 Configures an external subsystem (e.g. file transfer daemon).
731 Arguments should be a subsystem name and a command (with optional arguments)
732 to execute upon subsystem request.
733 The command
734 .Xr sftp-server 8
735 implements the
736 .Dq sftp
737 file transfer subsystem.
738 By default no subsystems are defined.
739 Note that this option applies to protocol version 2 only.
740 .It Cm SyslogFacility
741 Gives the facility code that is used when logging messages from
742 .Xr sshd 8 .
743 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
744 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
745 The default is AUTH.
746 .It Cm TCPKeepAlive
747 Specifies whether the system should send TCP keepalive messages to the
748 other side.
749 If they are sent, death of the connection or crash of one
750 of the machines will be properly noticed.
751 However, this means that
752 connections will die if the route is down temporarily, and some people
753 find it annoying.
754 On the other hand, if TCP keepalives are not sent,
755 sessions may hang indefinitely on the server, leaving
756 .Dq ghost
757 users and consuming server resources.
758 .Pp
759 The default is
760 .Dq yes
761 (to send TCP keepalive messages), and the server will notice
762 if the network goes down or the client host crashes.
763 This avoids infinitely hanging sessions.
764 .Pp
765 To disable TCP keepalive messages, the value should be set to
766 .Dq no .
767 .It Cm UseDNS
768 Specifies whether
769 .Xr sshd 8
770 should look up the remote host name and check that
771 the resolved host name for the remote IP address maps back to the
772 very same IP address.
773 The default is
774 .Dq yes .
775 .It Cm UseLogin
776 Specifies whether
777 .Xr login 1
778 is used for interactive login sessions.
779 The default is
780 .Dq no .
781 Note that
782 .Xr login 1
783 is never used for remote command execution.
784 Note also, that if this is enabled,
785 .Cm X11Forwarding
786 will be disabled because
787 .Xr login 1
788 does not know how to handle
789 .Xr xauth 1
790 cookies.
791 If
792 .Cm UsePrivilegeSeparation
793 is specified, it will be disabled after authentication.
794 .It Cm UsePAM
795 Enables the Pluggable Authentication Module interface.
796 If set to
797 .Dq yes
798 this will enable PAM authentication using
799 .Cm ChallengeResponseAuthentication
800 and
801 .Cm PasswordAuthentication
802 in addition to PAM account and session module processing for all
803 authentication types.
804 .Pp
805 Because PAM challenge-response authentication usually serves an equivalent
806 role to password authentication, you should disable either
807 .Cm PasswordAuthentication
808 or
809 .Cm ChallengeResponseAuthentication.
810 .Pp
811 If
812 .Cm UsePAM
813 is enabled, you will not be able to run
814 .Xr sshd 8
815 as a non-root user.
816 The default is
817 .Dq no .
818 .It Cm UsePrivilegeSeparation
819 Specifies whether
820 .Xr sshd 8
821 separates privileges by creating an unprivileged child process
822 to deal with incoming network traffic.
823 After successful authentication, another process will be created that has
824 the privilege of the authenticated user.
825 The goal of privilege separation is to prevent privilege
826 escalation by containing any corruption within the unprivileged processes.
827 The default is
828 .Dq yes .
829 .It Cm X11DisplayOffset
830 Specifies the first display number available for
831 .Xr sshd 8 Ns 's
832 X11 forwarding.
833 This prevents sshd from interfering with real X11 servers.
834 The default is 10.
835 .It Cm X11Forwarding
836 Specifies whether X11 forwarding is permitted.
837 The argument must be
838 .Dq yes
839 or
840 .Dq no .
841 The default is
842 .Dq no .
843 .Pp
844 When X11 forwarding is enabled, there may be additional exposure to
845 the server and to client displays if the
846 .Xr sshd 8
847 proxy display is configured to listen on the wildcard address (see
848 .Cm X11UseLocalhost
849 below), though this is not the default.
850 Additionally, the authentication spoofing and authentication data
851 verification and substitution occur on the client side.
852 The security risk of using X11 forwarding is that the client's X11
853 display server may be exposed to attack when the SSH client requests
854 forwarding (see the warnings for
855 .Cm ForwardX11
856 in
857 .Xr ssh_config 5 ) .
858 A system administrator may have a stance in which they want to
859 protect clients that may expose themselves to attack by unwittingly
860 requesting X11 forwarding, which can warrant a
861 .Dq no
862 setting.
863 .Pp
864 Note that disabling X11 forwarding does not prevent users from
865 forwarding X11 traffic, as users can always install their own forwarders.
866 X11 forwarding is automatically disabled if
867 .Cm UseLogin
868 is enabled.
869 .It Cm X11UseLocalhost
870 Specifies whether
871 .Xr sshd 8
872 should bind the X11 forwarding server to the loopback address or to
873 the wildcard address.
874 By default,
875 sshd binds the forwarding server to the loopback address and sets the
876 hostname part of the
877 .Ev DISPLAY
878 environment variable to
879 .Dq localhost .
880 This prevents remote hosts from connecting to the proxy display.
881 However, some older X11 clients may not function with this
882 configuration.
883 .Cm X11UseLocalhost
884 may be set to
885 .Dq no
886 to specify that the forwarding server should be bound to the wildcard
887 address.
888 The argument must be
889 .Dq yes
890 or
891 .Dq no .
892 The default is
893 .Dq yes .
894 .It Cm XAuthLocation
895 Specifies the full pathname of the
896 .Xr xauth 1
897 program.
898 The default is
899 .Pa /usr/X11R6/bin/xauth .
900 .El
901 .Sh TIME FORMATS
902 .Xr sshd 8
903 command-line arguments and configuration file options that specify time
904 may be expressed using a sequence of the form:
905 .Sm off
906 .Ar time Op Ar qualifier ,
907 .Sm on
908 where
909 .Ar time
910 is a positive integer value and
911 .Ar qualifier
912 is one of the following:
913 .Pp
914 .Bl -tag -width Ds -compact -offset indent
915 .It Aq Cm none
916 seconds
917 .It Cm s | Cm S
918 seconds
919 .It Cm m | Cm M
920 minutes
921 .It Cm h | Cm H
922 hours
923 .It Cm d | Cm D
924 days
925 .It Cm w | Cm W
926 weeks
927 .El
928 .Pp
929 Each member of the sequence is added together to calculate
930 the total time value.
931 .Pp
932 Time format examples:
933 .Pp
934 .Bl -tag -width Ds -compact -offset indent
935 .It 600
936 600 seconds (10 minutes)
937 .It 10m
938 10 minutes
939 .It 1h30m
940 1 hour 30 minutes (90 minutes)
941 .El
942 .Sh FILES
943 .Bl -tag -width Ds
944 .It Pa /etc/ssh/sshd_config
945 Contains configuration data for
946 .Xr sshd 8 .
947 This file should be writable by root only, but it is recommended
948 (though not necessary) that it be world-readable.
949 .El
950 .Sh SEE ALSO
951 .Xr sshd 8
952 .Sh AUTHORS
953 OpenSSH is a derivative of the original and free
954 ssh 1.2.12 release by Tatu Ylonen.
955 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
956 Theo de Raadt and Dug Song
957 removed many bugs, re-added newer features and
958 created OpenSSH.
959 Markus Friedl contributed the support for SSH
960 protocol versions 1.5 and 2.0.
961 Niels Provos and Markus Friedl contributed support
962 for privilege separation.