| blob | af12214456ceeafeb9669db60b8cfe6fccd25de4 |
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 1.74 2007/03/01 16:19:33 jmc 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 .Nm /etc/ssh/sshd_config
46 .Sh DESCRIPTION
47 .Xr sshd 8
48 reads configuration data from
49 .Pa /etc/ssh/sshd_config
50 (or the file specified with
51 .Fl f
52 on the command line).
53 The file contains keyword-argument pairs, one per line.
54 Lines starting with
55 .Ql #
56 and empty lines are interpreted as comments.
57 Arguments may optionally be enclosed in double quotes
58 .Pq \&"
59 in order to represent arguments containing spaces.
60 .Pp
61 The possible
62 keywords and their meanings are as follows (note that
63 keywords are case-insensitive and arguments are case-sensitive):
64 .Bl -tag -width Ds
65 .It Cm AcceptEnv
66 Specifies what environment variables sent by the client will be copied into
67 the session's
68 .Xr environ 7 .
69 See
70 .Cm SendEnv
71 in
72 .Xr ssh_config 5
73 for how to configure the client.
74 Note that environment passing is only supported for protocol 2.
75 Variables are specified by name, which may contain the wildcard characters
76 .Ql *
77 and
78 .Ql \&? .
79 Multiple environment variables may be separated by whitespace or spread
80 across multiple
81 .Cm AcceptEnv
82 directives.
83 Be warned that some environment variables could be used to bypass restricted
84 user environments.
85 For this reason, care should be taken in the use of this directive.
86 The default is not to accept any environment variables.
87 .It Cm AddressFamily
88 Specifies which address family should be used by
89 .Xr sshd 8 .
90 Valid arguments are
91 .Dq any ,
92 .Dq inet
93 (use IPv4 only), or
94 .Dq inet6
95 (use IPv6 only).
96 The default is
97 .Dq any .
98 .It Cm AllowGroups
99 This keyword can be followed by a list of group name patterns, separated
100 by spaces.
101 If specified, login is allowed only for users whose primary
102 group or supplementary group list matches one of the patterns.
103 Only group names are valid; a numerical group ID is not recognized.
104 By default, login is allowed for all groups.
105 The allow/deny directives are processed in the following order:
106 .Cm DenyUsers ,
107 .Cm AllowUsers ,
108 .Cm DenyGroups ,
109 and finally
110 .Cm AllowGroups .
111 .Pp
112 See
113 .Sx PATTERNS
114 in
115 .Xr ssh_config 5
116 for more information on patterns.
117 .It Cm AllowTcpForwarding
118 Specifies whether TCP forwarding is permitted.
119 The default is
120 .Dq yes .
121 Note that disabling TCP forwarding does not improve security unless
122 users are also denied shell access, as they can always install their
123 own forwarders.
124 .It Cm AllowUsers
125 This keyword can be followed by a list of user name patterns, separated
126 by spaces.
127 If specified, login is allowed only for user names that
128 match one of the patterns.
129 Only user names are valid; a numerical user ID is not recognized.
130 By default, login is allowed for all users.
131 If the pattern takes the form USER@HOST then USER and HOST
132 are separately checked, restricting logins to particular
133 users from particular hosts.
134 The allow/deny directives are processed in the following order:
135 .Cm DenyUsers ,
136 .Cm AllowUsers ,
137 .Cm DenyGroups ,
138 and finally
139 .Cm AllowGroups .
140 .Pp
141 See
142 .Sx PATTERNS
143 in
144 .Xr ssh_config 5
145 for more information on patterns.
146 .It Cm AuthorizedKeysFile
147 Specifies the file that contains the public keys that can be used
148 for user authentication.
149 .Cm AuthorizedKeysFile
150 may contain tokens of the form %T which are substituted during connection
151 setup.
152 The following tokens are defined: %% is replaced by a literal '%',
153 %h is replaced by the home directory of the user being authenticated, and
154 %u is replaced by the username of that user.
155 After expansion,
156 .Cm AuthorizedKeysFile
157 is taken to be an absolute path or one relative to the user's home
158 directory.
159 The default is
160 .Dq .ssh/authorized_keys .
161 .It Cm Banner
162 In some jurisdictions, sending a warning message before authentication
163 may be relevant for getting legal protection.
164 The contents of the specified file are sent to the remote user before
165 authentication is allowed.
166 This option is only available for protocol version 2.
167 By default, no banner is displayed.
168 .It Cm ChallengeResponseAuthentication
169 Specifies whether challenge-response authentication is allowed.
170 All authentication styles from
171 .Xr login.conf 5
172 are supported.
173 The default is
174 .Dq yes .
175 .It Cm Ciphers
176 Specifies the ciphers allowed for protocol version 2.
177 Multiple ciphers must be comma-separated.
178 The supported ciphers are
179 .Dq 3des-cbc ,
180 .Dq aes128-cbc ,
181 .Dq aes192-cbc ,
182 .Dq aes256-cbc ,
183 .Dq aes128-ctr ,
184 .Dq aes192-ctr ,
185 .Dq aes256-ctr ,
186 .Dq arcfour128 ,
187 .Dq arcfour256 ,
188 .Dq arcfour ,
189 .Dq blowfish-cbc ,
190 and
191 .Dq cast128-cbc .
192 The default is:
193 .Bd -literal -offset 3n
194 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
195 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
196 aes192-ctr,aes256-ctr
197 .Ed
198 .It Cm ClientAliveCountMax
199 Sets the number of client alive messages (see below) which may be
200 sent without
201 .Xr sshd 8
202 receiving any messages back from the client.
203 If this threshold is reached while client alive messages are being sent,
204 sshd will disconnect the client, terminating the session.
205 It is important to note that the use of client alive messages is very
206 different from
207 .Cm TCPKeepAlive
208 (below).
209 The client alive messages are sent through the encrypted channel
210 and therefore will not be spoofable.
211 The TCP keepalive option enabled by
212 .Cm TCPKeepAlive
213 is spoofable.
214 The client alive mechanism is valuable when the client or
215 server depend on knowing when a connection has become inactive.
216 .Pp
217 The default value is 3.
218 If
219 .Cm ClientAliveInterval
220 (see below) is set to 15, and
221 .Cm ClientAliveCountMax
222 is left at the default, unresponsive SSH clients
223 will be disconnected after approximately 45 seconds.
224 This option applies to protocol version 2 only.
225 .It Cm ClientAliveInterval
226 Sets a timeout interval in seconds after which if no data has been received
227 from the client,
228 .Xr sshd 8
229 will send a message through the encrypted
230 channel to request a response from the client.
231 The default
232 is 0, indicating that these messages will not be sent to the client.
233 This option applies to protocol version 2 only.
234 .It Cm Compression
235 Specifies whether compression is allowed, or delayed until
236 the user has authenticated successfully.
237 The argument must be
238 .Dq yes ,
239 .Dq delayed ,
240 or
241 .Dq no .
242 The default is
243 .Dq delayed .
244 .It Cm DenyGroups
245 This keyword can be followed by a list of group name patterns, separated
246 by spaces.
247 Login is disallowed for users whose primary group or supplementary
248 group list matches one of the patterns.
249 Only group names are valid; a numerical group ID is not recognized.
250 By default, login is allowed for all groups.
251 The allow/deny directives are processed in the following order:
252 .Cm DenyUsers ,
253 .Cm AllowUsers ,
254 .Cm DenyGroups ,
255 and finally
256 .Cm AllowGroups .
257 .Pp
258 See
259 .Sx PATTERNS
260 in
261 .Xr ssh_config 5
262 for more information on patterns.
263 .It Cm DenyUsers
264 This keyword can be followed by a list of user name patterns, separated
265 by spaces.
266 Login is disallowed for user names that match one of the patterns.
267 Only user names are valid; a numerical user ID is not recognized.
268 By default, login is allowed for all users.
269 If the pattern takes the form USER@HOST then USER and HOST
270 are separately checked, restricting logins to particular
271 users from particular hosts.
272 The allow/deny directives are processed in the following order:
273 .Cm DenyUsers ,
274 .Cm AllowUsers ,
275 .Cm DenyGroups ,
276 and finally
277 .Cm AllowGroups .
278 .Pp
279 See
280 .Sx PATTERNS
281 in
282 .Xr ssh_config 5
283 for more information on patterns.
284 .It Cm ForceCommand
285 Forces the execution of the command specified by
286 .Cm ForceCommand ,
287 ignoring any command supplied by the client.
288 The command is invoked by using the user's login shell with the -c option.
289 This applies to shell, command, or subsystem execution.
290 It is most useful inside a
291 .Cm Match
292 block.
293 The command originally supplied by the client is available in the
294 .Ev SSH_ORIGINAL_COMMAND
295 environment variable.
296 .It Cm GatewayPorts
297 Specifies whether remote hosts are allowed to connect to ports
298 forwarded for the client.
299 By default,
300 .Xr sshd 8
301 binds remote port forwardings to the loopback address.
302 This prevents other remote hosts from connecting to forwarded ports.
303 .Cm GatewayPorts
304 can be used to specify that sshd
305 should allow remote port forwardings to bind to non-loopback addresses, thus
306 allowing other hosts to connect.
307 The argument may be
308 .Dq no
309 to force remote port forwardings to be available to the local host only,
310 .Dq yes
311 to force remote port forwardings to bind to the wildcard address, or
312 .Dq clientspecified
313 to allow the client to select the address to which the forwarding is bound.
314 The default is
315 .Dq no .
316 .It Cm GSSAPIAuthentication
317 Specifies whether user authentication based on GSSAPI is allowed.
318 The default is
319 .Dq no .
320 Note that this option applies to protocol version 2 only.
321 .It Cm GSSAPICleanupCredentials
322 Specifies whether to automatically destroy the user's credentials cache
323 on logout.
324 The default is
325 .Dq yes .
326 Note that this option applies to protocol version 2 only.
327 .It Cm HostbasedAuthentication
328 Specifies whether rhosts or /etc/hosts.equiv authentication together
329 with successful public key client host authentication is allowed
330 (host-based authentication).
331 This option is similar to
332 .Cm RhostsRSAAuthentication
333 and applies to protocol version 2 only.
334 The default is
335 .Dq no .
336 .It Cm HostbasedUsesNameFromPacketOnly
337 Specifies whether or not the server will attempt to perform a reverse
338 name lookup when matching the name in the
339 .Pa ~/.shosts ,
340 .Pa ~/.rhosts ,
341 and
342 .Pa /etc/hosts.equiv
343 files during
344 .Cm HostbasedAuthentication .
345 A setting of
346 .Dq yes
347 means that
348 .Xr sshd 8
349 uses the name supplied by the client rather than
350 attempting to resolve the name from the TCP connection itself.
351 The default is
352 .Dq no .
353 .It Cm HostKey
354 Specifies a file containing a private host key
355 used by SSH.
356 The default is
357 .Pa /etc/ssh/ssh_host_key
358 for protocol version 1, and
359 .Pa /etc/ssh/ssh_host_rsa_key
360 and
361 .Pa /etc/ssh/ssh_host_dsa_key
362 for protocol version 2.
363 Note that
364 .Xr sshd 8
365 will refuse to use a file if it is group/world-accessible.
366 It is possible to have multiple host key files.
367 .Dq rsa1
368 keys are used for version 1 and
369 .Dq dsa
370 or
371 .Dq rsa
372 are used for version 2 of the SSH protocol.
373 .It Cm IgnoreRhosts
374 Specifies that
375 .Pa .rhosts
376 and
377 .Pa .shosts
378 files will not be used in
379 .Cm RhostsRSAAuthentication
380 or
381 .Cm HostbasedAuthentication .
382 .Pp
383 .Pa /etc/hosts.equiv
384 and
385 .Pa /etc/shosts.equiv
386 are still used.
387 The default is
388 .Dq yes .
389 .It Cm IgnoreUserKnownHosts
390 Specifies whether
391 .Xr sshd 8
392 should ignore the user's
393 .Pa ~/.ssh/known_hosts
394 during
395 .Cm RhostsRSAAuthentication
396 or
397 .Cm HostbasedAuthentication .
398 The default is
399 .Dq no .
400 .It Cm KerberosAuthentication
401 Specifies whether the password provided by the user for
402 .Cm PasswordAuthentication
403 will be validated through the Kerberos KDC.
404 To use this option, the server needs a
405 Kerberos servtab which allows the verification of the KDC's identity.
406 The default is
407 .Dq no .
408 .It Cm KerberosGetAFSToken
409 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
410 an AFS token before accessing the user's home directory.
411 The default is
412 .Dq no .
413 .It Cm KerberosOrLocalPasswd
414 If password authentication through Kerberos fails then
415 the password will be validated via any additional local mechanism
416 such as
417 .Pa /etc/passwd .
418 The default is
419 .Dq yes .
420 .It Cm KerberosTicketCleanup
421 Specifies whether to automatically destroy the user's ticket cache
422 file on logout.
423 The default is
424 .Dq yes .
425 .It Cm KeyRegenerationInterval
426 In protocol version 1, the ephemeral server key is automatically regenerated
427 after this many seconds (if it has been used).
428 The purpose of regeneration is to prevent
429 decrypting captured sessions by later breaking into the machine and
430 stealing the keys.
431 The key is never stored anywhere.
432 If the value is 0, the key is never regenerated.
433 The default is 3600 (seconds).
434 .It Cm ListenAddress
435 Specifies the local addresses
436 .Xr sshd 8
437 should listen on.
438 The following forms may be used:
439 .Pp
440 .Bl -item -offset indent -compact
441 .It
442 .Cm ListenAddress
443 .Sm off
444 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
445 .Sm on
446 .It
447 .Cm ListenAddress
448 .Sm off
449 .Ar host No | Ar IPv4_addr No : Ar port
450 .Sm on
451 .It
452 .Cm ListenAddress
453 .Sm off
454 .Oo
455 .Ar host No | Ar IPv6_addr Oc : Ar port
456 .Sm on
457 .El
458 .Pp
459 If
460 .Ar port
461 is not specified,
462 sshd will listen on the address and all prior
463 .Cm Port
464 options specified.
465 The default is to listen on all local addresses.
466 Multiple
467 .Cm ListenAddress
468 options are permitted.
469 Additionally, any
470 .Cm Port
471 options must precede this option for non-port qualified addresses.
472 .It Cm LoginGraceTime
473 The server disconnects after this time if the user has not
474 successfully logged in.
475 If the value is 0, there is no time limit.
476 The default is 120 seconds.
477 .It Cm LogLevel
478 Gives the verbosity level that is used when logging messages from
479 .Xr sshd 8 .
480 The possible values are:
481 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
482 The default is INFO.
483 DEBUG and DEBUG1 are equivalent.
484 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
485 Logging with a DEBUG level violates the privacy of users and is not recommended.
486 .It Cm MACs
487 Specifies the available MAC (message authentication code) algorithms.
488 The MAC algorithm is used in protocol version 2
489 for data integrity protection.
490 Multiple algorithms must be comma-separated.
491 The default is:
492 .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
493 .It Cm Match
494 Introduces a conditional block.
495 If all of the criteria on the
496 .Cm Match
497 line are satisfied, the keywords on the following lines override those
498 set in the global section of the config file, until either another
499 .Cm Match
500 line or the end of the file.
501 The arguments to
502 .Cm Match
503 are one or more criteria-pattern pairs.
504 The available criteria are
505 .Cm User ,
506 .Cm Group ,
507 .Cm Host ,
508 and
509 .Cm Address .
510 Only a subset of keywords may be used on the lines following a
511 .Cm Match
512 keyword.
513 Available keywords are
514 .Cm AllowTcpForwarding ,
515 .Cm Banner ,
516 .Cm ForceCommand ,
517 .Cm GatewayPorts ,
518 .Cm GSSApiAuthentication ,
519 .Cm KbdInteractiveAuthentication ,
520 .Cm KerberosAuthentication ,
521 .Cm PasswordAuthentication ,
522 .Cm PermitOpen ,
523 .Cm RhostsRSAAuthentication ,
524 .Cm RSAAuthentication ,
525 .Cm X11DisplayOffset ,
526 .Cm X11Forwarding ,
527 and
528 .Cm X11UseLocalHost .
529 .It Cm MaxAuthTries
530 Specifies the maximum number of authentication attempts permitted per
531 connection.
532 Once the number of failures reaches half this value,
533 additional failures are logged.
534 The default is 6.
535 .It Cm MaxStartups
536 Specifies the maximum number of concurrent unauthenticated connections to the
537 SSH daemon.
538 Additional connections will be dropped until authentication succeeds or the
539 .Cm LoginGraceTime
540 expires for a connection.
541 The default is 10.
542 .Pp
543 Alternatively, random early drop can be enabled by specifying
544 the three colon separated values
545 .Dq start:rate:full
546 (e.g. "10:30:60").
547 .Xr sshd 8
548 will refuse connection attempts with a probability of
549 .Dq rate/100
550 (30%)
551 if there are currently
552 .Dq start
553 (10)
554 unauthenticated connections.
555 The probability increases linearly and all connection attempts
556 are refused if the number of unauthenticated connections reaches
557 .Dq full
558 (60).
559 .It Cm PasswordAuthentication
560 Specifies whether password authentication is allowed.
561 The default is
562 .Dq yes .
563 .It Cm PermitEmptyPasswords
564 When password authentication is allowed, it specifies whether the
565 server allows login to accounts with empty password strings.
566 The default is
567 .Dq no .
568 .It Cm PermitOpen
569 Specifies the destinations to which TCP port forwarding is permitted.
570 The forwarding specification must be one of the following forms:
571 .Pp
572 .Bl -item -offset indent -compact
573 .It
574 .Cm PermitOpen
575 .Sm off
576 .Ar host : port
577 .Sm on
578 .It
579 .Cm PermitOpen
580 .Sm off
581 .Ar IPv4_addr : port
582 .Sm on
583 .It
584 .Cm PermitOpen
585 .Sm off
586 .Ar \&[ IPv6_addr \&] : port
587 .Sm on
588 .El
589 .Pp
590 Multiple forwards may be specified by separating them with whitespace.
591 An argument of
592 .Dq any
593 can be used to remove all restrictions and permit any forwarding requests.
594 By default all port forwarding requests are permitted.
595 .It Cm PermitRootLogin
596 Specifies whether root can log in using
597 .Xr ssh 1 .
598 The argument must be
599 .Dq yes ,
600 .Dq without-password ,
601 .Dq forced-commands-only ,
602 or
603 .Dq no .
604 The default is
605 .Dq yes .
606 .Pp
607 If this option is set to
608 .Dq without-password ,
609 password authentication is disabled for root.
610 .Pp
611 If this option is set to
612 .Dq forced-commands-only ,
613 root login with public key authentication will be allowed,
614 but only if the
615 .Ar command
616 option has been specified
617 (which may be useful for taking remote backups even if root login is
618 normally not allowed).
619 All other authentication methods are disabled for root.
620 .Pp
621 If this option is set to
622 .Dq no ,
623 root is not allowed to log in.
624 .It Cm PermitTunnel
625 Specifies whether
626 .Xr tun 4
627 device forwarding is allowed.
628 The argument must be
629 .Dq yes ,
630 .Dq point-to-point
631 (layer 3),
632 .Dq ethernet
633 (layer 2), or
634 .Dq no .
635 Specifying
636 .Dq yes
637 permits both
638 .Dq point-to-point
639 and
640 .Dq ethernet .
641 The default is
642 .Dq no .
643 .It Cm PermitUserEnvironment
644 Specifies whether
645 .Pa ~/.ssh/environment
646 and
647 .Cm environment=
648 options in
649 .Pa ~/.ssh/authorized_keys
650 are processed by
651 .Xr sshd 8 .
652 The default is
653 .Dq no .
654 Enabling environment processing may enable users to bypass access
655 restrictions in some configurations using mechanisms such as
656 .Ev LD_PRELOAD .
657 .It Cm PidFile
658 Specifies the file that contains the process ID of the
659 SSH daemon.
660 The default is
661 .Pa /var/run/sshd.pid .
662 .It Cm Port
663 Specifies the port number that
664 .Xr sshd 8
665 listens on.
666 The default is 22.
667 Multiple options of this type are permitted.
668 See also
669 .Cm ListenAddress .
670 .It Cm PrintLastLog
671 Specifies whether
672 .Xr sshd 8
673 should print the date and time of the last user login when a user logs
674 in interactively.
675 The default is
676 .Dq yes .
677 .It Cm PrintMotd
678 Specifies whether
679 .Xr sshd 8
680 should print
681 .Pa /etc/motd
682 when a user logs in interactively.
683 (On some systems it is also printed by the shell,
684 .Pa /etc/profile ,
685 or equivalent.)
686 The default is
687 .Dq yes .
688 .It Cm Protocol
689 Specifies the protocol versions
690 .Xr sshd 8
691 supports.
692 The possible values are
693 .Sq 1
694 and
695 .Sq 2 .
696 Multiple versions must be comma-separated.
697 The default is
698 .Dq 2,1 .
699 Note that the order of the protocol list does not indicate preference,
700 because the client selects among multiple protocol versions offered
701 by the server.
702 Specifying
703 .Dq 2,1
704 is identical to
705 .Dq 1,2 .
706 .It Cm PubkeyAuthentication
707 Specifies whether public key authentication is allowed.
708 The default is
709 .Dq yes .
710 Note that this option applies to protocol version 2 only.
711 .It Cm RhostsRSAAuthentication
712 Specifies whether rhosts or /etc/hosts.equiv authentication together
713 with successful RSA host authentication is allowed.
714 The default is
715 .Dq no .
716 This option applies to protocol version 1 only.
717 .It Cm RSAAuthentication
718 Specifies whether pure RSA authentication is allowed.
719 The default is
720 .Dq yes .
721 This option applies to protocol version 1 only.
722 .It Cm ServerKeyBits
723 Defines the number of bits in the ephemeral protocol version 1 server key.
724 The minimum value is 512, and the default is 768.
725 .It Cm StrictModes
726 Specifies whether
727 .Xr sshd 8
728 should check file modes and ownership of the
729 user's files and home directory before accepting login.
730 This is normally desirable because novices sometimes accidentally leave their
731 directory or files world-writable.
732 The default is
733 .Dq yes .
734 .It Cm Subsystem
735 Configures an external subsystem (e.g. file transfer daemon).
736 Arguments should be a subsystem name and a command (with optional arguments)
737 to execute upon subsystem request.
738 The command
739 .Xr sftp-server 8
740 implements the
741 .Dq sftp
742 file transfer subsystem.
743 By default no subsystems are defined.
744 Note that this option applies to protocol version 2 only.
745 .It Cm SyslogFacility
746 Gives the facility code that is used when logging messages from
747 .Xr sshd 8 .
748 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
749 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
750 The default is AUTH.
751 .It Cm TCPKeepAlive
752 Specifies whether the system should send TCP keepalive messages to the
753 other side.
754 If they are sent, death of the connection or crash of one
755 of the machines will be properly noticed.
756 However, this means that
757 connections will die if the route is down temporarily, and some people
758 find it annoying.
759 On the other hand, if TCP keepalives are not sent,
760 sessions may hang indefinitely on the server, leaving
761 .Dq ghost
762 users and consuming server resources.
763 .Pp
764 The default is
765 .Dq yes
766 (to send TCP keepalive messages), and the server will notice
767 if the network goes down or the client host crashes.
768 This avoids infinitely hanging sessions.
769 .Pp
770 To disable TCP keepalive messages, the value should be set to
771 .Dq no .
772 .It Cm UseDNS
773 Specifies whether
774 .Xr sshd 8
775 should look up the remote host name and check that
776 the resolved host name for the remote IP address maps back to the
777 very same IP address.
778 The default is
779 .Dq yes .
780 .It Cm UseLogin
781 Specifies whether
782 .Xr login 1
783 is used for interactive login sessions.
784 The default is
785 .Dq no .
786 Note that
787 .Xr login 1
788 is never used for remote command execution.
789 Note also, that if this is enabled,
790 .Cm X11Forwarding
791 will be disabled because
792 .Xr login 1
793 does not know how to handle
794 .Xr xauth 1
795 cookies.
796 If
797 .Cm UsePrivilegeSeparation
798 is specified, it will be disabled after authentication.
799 .It Cm UsePAM
800 Enables the Pluggable Authentication Module interface.
801 If set to
802 .Dq yes
803 this will enable PAM authentication using
804 .Cm ChallengeResponseAuthentication
805 and
806 .Cm PasswordAuthentication
807 in addition to PAM account and session module processing for all
808 authentication types.
809 .Pp
810 Because PAM challenge-response authentication usually serves an equivalent
811 role to password authentication, you should disable either
812 .Cm PasswordAuthentication
813 or
814 .Cm ChallengeResponseAuthentication.
815 .Pp
816 If
817 .Cm UsePAM
818 is enabled, you will not be able to run
819 .Xr sshd 8
820 as a non-root user.
821 The default is
822 .Dq no .
823 .It Cm UsePrivilegeSeparation
824 Specifies whether
825 .Xr sshd 8
826 separates privileges by creating an unprivileged child process
827 to deal with incoming network traffic.
828 After successful authentication, another process will be created that has
829 the privilege of the authenticated user.
830 The goal of privilege separation is to prevent privilege
831 escalation by containing any corruption within the unprivileged processes.
832 The default is
833 .Dq yes .
834 .It Cm X11DisplayOffset
835 Specifies the first display number available for
836 .Xr sshd 8 Ns 's
837 X11 forwarding.
838 This prevents sshd from interfering with real X11 servers.
839 The default is 10.
840 .It Cm X11Forwarding
841 Specifies whether X11 forwarding is permitted.
842 The argument must be
843 .Dq yes
844 or
845 .Dq no .
846 The default is
847 .Dq no .
848 .Pp
849 When X11 forwarding is enabled, there may be additional exposure to
850 the server and to client displays if the
851 .Xr sshd 8
852 proxy display is configured to listen on the wildcard address (see
853 .Cm X11UseLocalhost
854 below), though this is not the default.
855 Additionally, the authentication spoofing and authentication data
856 verification and substitution occur on the client side.
857 The security risk of using X11 forwarding is that the client's X11
858 display server may be exposed to attack when the SSH client requests
859 forwarding (see the warnings for
860 .Cm ForwardX11
861 in
862 .Xr ssh_config 5 ) .
863 A system administrator may have a stance in which they want to
864 protect clients that may expose themselves to attack by unwittingly
865 requesting X11 forwarding, which can warrant a
866 .Dq no
867 setting.
868 .Pp
869 Note that disabling X11 forwarding does not prevent users from
870 forwarding X11 traffic, as users can always install their own forwarders.
871 X11 forwarding is automatically disabled if
872 .Cm UseLogin
873 is enabled.
874 .It Cm X11UseLocalhost
875 Specifies whether
876 .Xr sshd 8
877 should bind the X11 forwarding server to the loopback address or to
878 the wildcard address.
879 By default,
880 sshd binds the forwarding server to the loopback address and sets the
881 hostname part of the
882 .Ev DISPLAY
883 environment variable to
884 .Dq localhost .
885 This prevents remote hosts from connecting to the proxy display.
886 However, some older X11 clients may not function with this
887 configuration.
888 .Cm X11UseLocalhost
889 may be set to
890 .Dq no
891 to specify that the forwarding server should be bound to the wildcard
892 address.
893 The argument must be
894 .Dq yes
895 or
896 .Dq no .
897 The default is
898 .Dq yes .
899 .It Cm XAuthLocation
900 Specifies the full pathname of the
901 .Xr xauth 1
902 program.
903 The default is
904 .Pa /usr/X11R6/bin/xauth .
905 .El
906 .Sh TIME FORMATS
907 .Xr sshd 8
908 command-line arguments and configuration file options that specify time
909 may be expressed using a sequence of the form:
910 .Sm off
911 .Ar time Op Ar qualifier ,
912 .Sm on
913 where
914 .Ar time
915 is a positive integer value and
916 .Ar qualifier
917 is one of the following:
918 .Pp
919 .Bl -tag -width Ds -compact -offset indent
920 .It Aq Cm none
921 seconds
922 .It Cm s | Cm S
923 seconds
924 .It Cm m | Cm M
925 minutes
926 .It Cm h | Cm H
927 hours
928 .It Cm d | Cm D
929 days
930 .It Cm w | Cm W
931 weeks
932 .El
933 .Pp
934 Each member of the sequence is added together to calculate
935 the total time value.
936 .Pp
937 Time format examples:
938 .Pp
939 .Bl -tag -width Ds -compact -offset indent
940 .It 600
941 600 seconds (10 minutes)
942 .It 10m
943 10 minutes
944 .It 1h30m
945 1 hour 30 minutes (90 minutes)
946 .El
947 .Sh FILES
948 .Bl -tag -width Ds
949 .It Pa /etc/ssh/sshd_config
950 Contains configuration data for
951 .Xr sshd 8 .
952 This file should be writable by root only, but it is recommended
953 (though not necessary) that it be world-readable.
954 .El
955 .Sh SEE ALSO
956 .Xr sshd 8
957 .Sh AUTHORS
958 OpenSSH is a derivative of the original and free
959 ssh 1.2.12 release by Tatu Ylonen.
960 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
961 Theo de Raadt and Dug Song
962 removed many bugs, re-added newer features and
963 created OpenSSH.
964 Markus Friedl contributed the support for SSH
965 protocol versions 1.5 and 2.0.
966 Niels Provos and Markus Friedl contributed support
967 for privilege separation.