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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 1.106 2009/04/21 15:13:17 stevesk Exp $
38 .Dd $Mdocdate: April 21 2009 $
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 AllowAgentForwarding
99 Specifies whether
100 .Xr ssh-agent 1
101 forwarding is permitted.
102 The default is
103 .Dq yes .
104 Note that disabling agent forwarding does not improve security
105 unless users are also denied shell access, as they can always install
106 their own forwarders.
107 .It Cm AllowGroups
108 This keyword can be followed by a list of group name patterns, separated
109 by spaces.
110 If specified, login is allowed only for users whose primary
111 group or supplementary group list matches one of the patterns.
112 Only group names are valid; a numerical group ID is not recognized.
113 By default, login is allowed for all groups.
114 The allow/deny directives are processed in the following order:
115 .Cm DenyUsers ,
116 .Cm AllowUsers ,
117 .Cm DenyGroups ,
118 and finally
119 .Cm AllowGroups .
120 .Pp
121 See
122 .Sx PATTERNS
123 in
124 .Xr ssh_config 5
125 for more information on patterns.
126 .It Cm AllowTcpForwarding
127 Specifies whether TCP forwarding is permitted.
128 The default is
129 .Dq yes .
130 Note that disabling TCP forwarding does not improve security unless
131 users are also denied shell access, as they can always install their
132 own forwarders.
133 .It Cm AllowUsers
134 This keyword can be followed by a list of user name patterns, separated
135 by spaces.
136 If specified, login is allowed only for user names that
137 match one of the patterns.
138 Only user names are valid; a numerical user ID is not recognized.
139 By default, login is allowed for all users.
140 If the pattern takes the form USER@HOST then USER and HOST
141 are separately checked, restricting logins to particular
142 users from particular hosts.
143 The allow/deny directives are processed in the following order:
144 .Cm DenyUsers ,
145 .Cm AllowUsers ,
146 .Cm DenyGroups ,
147 and finally
148 .Cm AllowGroups .
149 .Pp
150 See
151 .Sx PATTERNS
152 in
153 .Xr ssh_config 5
154 for more information on patterns.
155 .It Cm AuthorizedKeysFile
156 Specifies the file that contains the public keys that can be used
157 for user authentication.
158 .Cm AuthorizedKeysFile
159 may contain tokens of the form %T which are substituted during connection
160 setup.
161 The following tokens are defined: %% is replaced by a literal '%',
162 %h is replaced by the home directory of the user being authenticated, and
163 %u is replaced by the username of that user.
164 After expansion,
165 .Cm AuthorizedKeysFile
166 is taken to be an absolute path or one relative to the user's home
167 directory.
168 The default is
169 .Dq .ssh/authorized_keys .
170 .It Cm Banner
171 The contents of the specified file are sent to the remote user before
172 authentication is allowed.
173 If the argument is
174 .Dq none
175 then no banner is displayed.
176 This option is only available for protocol version 2.
177 By default, no banner is displayed.
178 .It Cm ChallengeResponseAuthentication
179 Specifies whether challenge-response authentication is allowed.
180 All authentication styles from
181 .Xr login.conf 5
182 are supported.
183 The default is
184 .Dq yes .
185 .It Cm ChrootDirectory
186 Specifies a path to
187 .Xr chroot 2
188 to after authentication.
189 This path, and all its components, must be root-owned directories that are
190 not writable by any other user or group.
191 After the chroot,
192 .Xr sshd 8
193 changes the working directory to the user's home directory.
194 .Pp
195 The path may contain the following tokens that are expanded at runtime once
196 the connecting user has been authenticated: %% is replaced by a literal '%',
197 %h is replaced by the home directory of the user being authenticated, and
198 %u is replaced by the username of that user.
199 .Pp
200 The
201 .Cm ChrootDirectory
202 must contain the necessary files and directories to support the
203 user's session.
204 For an interactive session this requires at least a shell, typically
205 .Xr sh 1 ,
206 and basic
207 .Pa /dev
208 nodes such as
209 .Xr null 4 ,
210 .Xr zero 4 ,
211 .Xr stdin 4 ,
212 .Xr stdout 4 ,
213 .Xr stderr 4 ,
214 .Xr arandom 4
215 and
216 .Xr tty 4
217 devices.
218 For file transfer sessions using
219 .Dq sftp ,
220 no additional configuration of the environment is necessary if the
221 in-process sftp server is used,
222 though sessions which use logging do require
223 .Pa /dev/log
224 inside the chroot directory (see
225 .Xr sftp-server 8
226 for details).
227 .Pp
228 The default is not to
229 .Xr chroot 2 .
230 .It Cm Ciphers
231 Specifies the ciphers allowed for protocol version 2.
232 Multiple ciphers must be comma-separated.
233 The supported ciphers are
234 .Dq 3des-cbc ,
235 .Dq aes128-cbc ,
236 .Dq aes192-cbc ,
237 .Dq aes256-cbc ,
238 .Dq aes128-ctr ,
239 .Dq aes192-ctr ,
240 .Dq aes256-ctr ,
241 .Dq arcfour128 ,
242 .Dq arcfour256 ,
243 .Dq arcfour ,
244 .Dq blowfish-cbc ,
245 and
246 .Dq cast128-cbc .
247 The default is:
248 .Bd -literal -offset 3n
249 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
250 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
251 aes256-cbc,arcfour
252 .Ed
253 .It Cm ClientAliveCountMax
254 Sets the number of client alive messages (see below) which may be
255 sent without
256 .Xr sshd 8
257 receiving any messages back from the client.
258 If this threshold is reached while client alive messages are being sent,
259 sshd will disconnect the client, terminating the session.
260 It is important to note that the use of client alive messages is very
261 different from
262 .Cm TCPKeepAlive
263 (below).
264 The client alive messages are sent through the encrypted channel
265 and therefore will not be spoofable.
266 The TCP keepalive option enabled by
267 .Cm TCPKeepAlive
268 is spoofable.
269 The client alive mechanism is valuable when the client or
270 server depend on knowing when a connection has become inactive.
271 .Pp
272 The default value is 3.
273 If
274 .Cm ClientAliveInterval
275 (see below) is set to 15, and
276 .Cm ClientAliveCountMax
277 is left at the default, unresponsive SSH clients
278 will be disconnected after approximately 45 seconds.
279 This option applies to protocol version 2 only.
280 .It Cm ClientAliveInterval
281 Sets a timeout interval in seconds after which if no data has been received
282 from the client,
283 .Xr sshd 8
284 will send a message through the encrypted
285 channel to request a response from the client.
286 The default
287 is 0, indicating that these messages will not be sent to the client.
288 This option applies to protocol version 2 only.
289 .It Cm Compression
290 Specifies whether compression is allowed, or delayed until
291 the user has authenticated successfully.
292 The argument must be
293 .Dq yes ,
294 .Dq delayed ,
295 or
296 .Dq no .
297 The default is
298 .Dq delayed .
299 .It Cm DenyGroups
300 This keyword can be followed by a list of group name patterns, separated
301 by spaces.
302 Login is disallowed for users whose primary group or supplementary
303 group list matches one of the patterns.
304 Only group names are valid; a numerical group ID is not recognized.
305 By default, login is allowed for all groups.
306 The allow/deny directives are processed in the following order:
307 .Cm DenyUsers ,
308 .Cm AllowUsers ,
309 .Cm DenyGroups ,
310 and finally
311 .Cm AllowGroups .
312 .Pp
313 See
314 .Sx PATTERNS
315 in
316 .Xr ssh_config 5
317 for more information on patterns.
318 .It Cm DenyUsers
319 This keyword can be followed by a list of user name patterns, separated
320 by spaces.
321 Login is disallowed for user names that match one of the patterns.
322 Only user names are valid; a numerical user ID is not recognized.
323 By default, login is allowed for all users.
324 If the pattern takes the form USER@HOST then USER and HOST
325 are separately checked, restricting logins to particular
326 users from particular hosts.
327 The allow/deny directives are processed in the following order:
328 .Cm DenyUsers ,
329 .Cm AllowUsers ,
330 .Cm DenyGroups ,
331 and finally
332 .Cm AllowGroups .
333 .Pp
334 See
335 .Sx PATTERNS
336 in
337 .Xr ssh_config 5
338 for more information on patterns.
339 .It Cm ForceCommand
340 Forces the execution of the command specified by
341 .Cm ForceCommand ,
342 ignoring any command supplied by the client and
343 .Pa ~/.ssh/rc
344 if present.
345 The command is invoked by using the user's login shell with the -c option.
346 This applies to shell, command, or subsystem execution.
347 It is most useful inside a
348 .Cm Match
349 block.
350 The command originally supplied by the client is available in the
351 .Ev SSH_ORIGINAL_COMMAND
352 environment variable.
353 Specifying a command of
354 .Dq internal-sftp
355 will force the use of an in-process sftp server that requires no support
356 files when used with
357 .Cm ChrootDirectory .
358 .It Cm GatewayPorts
359 Specifies whether remote hosts are allowed to connect to ports
360 forwarded for the client.
361 By default,
362 .Xr sshd 8
363 binds remote port forwardings to the loopback address.
364 This prevents other remote hosts from connecting to forwarded ports.
365 .Cm GatewayPorts
366 can be used to specify that sshd
367 should allow remote port forwardings to bind to non-loopback addresses, thus
368 allowing other hosts to connect.
369 The argument may be
370 .Dq no
371 to force remote port forwardings to be available to the local host only,
372 .Dq yes
373 to force remote port forwardings to bind to the wildcard address, or
374 .Dq clientspecified
375 to allow the client to select the address to which the forwarding is bound.
376 The default is
377 .Dq no .
378 .It Cm GSSAPIAuthentication
379 Specifies whether user authentication based on GSSAPI is allowed.
380 The default is
381 .Dq no .
382 Note that this option applies to protocol version 2 only.
383 .It Cm GSSAPICleanupCredentials
384 Specifies whether to automatically destroy the user's credentials cache
385 on logout.
386 The default is
387 .Dq yes .
388 Note that this option applies to protocol version 2 only.
389 .It Cm HostbasedAuthentication
390 Specifies whether rhosts or /etc/hosts.equiv authentication together
391 with successful public key client host authentication is allowed
392 (host-based authentication).
393 This option is similar to
394 .Cm RhostsRSAAuthentication
395 and applies to protocol version 2 only.
396 The default is
397 .Dq no .
398 .It Cm HostbasedUsesNameFromPacketOnly
399 Specifies whether or not the server will attempt to perform a reverse
400 name lookup when matching the name in the
401 .Pa ~/.shosts ,
402 .Pa ~/.rhosts ,
403 and
404 .Pa /etc/hosts.equiv
405 files during
406 .Cm HostbasedAuthentication .
407 A setting of
408 .Dq yes
409 means that
410 .Xr sshd 8
411 uses the name supplied by the client rather than
412 attempting to resolve the name from the TCP connection itself.
413 The default is
414 .Dq no .
415 .It Cm HostKey
416 Specifies a file containing a private host key
417 used by SSH.
418 The default is
419 .Pa /etc/ssh/ssh_host_key
420 for protocol version 1, and
421 .Pa /etc/ssh/ssh_host_rsa_key
422 and
423 .Pa /etc/ssh/ssh_host_dsa_key
424 for protocol version 2.
425 Note that
426 .Xr sshd 8
427 will refuse to use a file if it is group/world-accessible.
428 It is possible to have multiple host key files.
429 .Dq rsa1
430 keys are used for version 1 and
431 .Dq dsa
432 or
433 .Dq rsa
434 are used for version 2 of the SSH protocol.
435 .It Cm IgnoreRhosts
436 Specifies that
437 .Pa .rhosts
438 and
439 .Pa .shosts
440 files will not be used in
441 .Cm RhostsRSAAuthentication
442 or
443 .Cm HostbasedAuthentication .
444 .Pp
445 .Pa /etc/hosts.equiv
446 and
447 .Pa /etc/shosts.equiv
448 are still used.
449 The default is
450 .Dq yes .
451 .It Cm IgnoreUserKnownHosts
452 Specifies whether
453 .Xr sshd 8
454 should ignore the user's
455 .Pa ~/.ssh/known_hosts
456 during
457 .Cm RhostsRSAAuthentication
458 or
459 .Cm HostbasedAuthentication .
460 The default is
461 .Dq no .
462 .It Cm KerberosAuthentication
463 Specifies whether the password provided by the user for
464 .Cm PasswordAuthentication
465 will be validated through the Kerberos KDC.
466 To use this option, the server needs a
467 Kerberos servtab which allows the verification of the KDC's identity.
468 The default is
469 .Dq no .
470 .It Cm KerberosGetAFSToken
471 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
472 an AFS token before accessing the user's home directory.
473 The default is
474 .Dq no .
475 .It Cm KerberosOrLocalPasswd
476 If password authentication through Kerberos fails then
477 the password will be validated via any additional local mechanism
478 such as
479 .Pa /etc/passwd .
480 The default is
481 .Dq yes .
482 .It Cm KerberosTicketCleanup
483 Specifies whether to automatically destroy the user's ticket cache
484 file on logout.
485 The default is
486 .Dq yes .
487 .It Cm KeyRegenerationInterval
488 In protocol version 1, the ephemeral server key is automatically regenerated
489 after this many seconds (if it has been used).
490 The purpose of regeneration is to prevent
491 decrypting captured sessions by later breaking into the machine and
492 stealing the keys.
493 The key is never stored anywhere.
494 If the value is 0, the key is never regenerated.
495 The default is 3600 (seconds).
496 .It Cm ListenAddress
497 Specifies the local addresses
498 .Xr sshd 8
499 should listen on.
500 The following forms may be used:
501 .Pp
502 .Bl -item -offset indent -compact
503 .It
504 .Cm ListenAddress
505 .Sm off
506 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
507 .Sm on
508 .It
509 .Cm ListenAddress
510 .Sm off
511 .Ar host No | Ar IPv4_addr No : Ar port
512 .Sm on
513 .It
514 .Cm ListenAddress
515 .Sm off
516 .Oo
517 .Ar host No | Ar IPv6_addr Oc : Ar port
518 .Sm on
519 .El
520 .Pp
521 If
522 .Ar port
523 is not specified,
524 sshd will listen on the address and all prior
525 .Cm Port
526 options specified.
527 The default is to listen on all local addresses.
528 Multiple
529 .Cm ListenAddress
530 options are permitted.
531 Additionally, any
532 .Cm Port
533 options must precede this option for non-port qualified addresses.
534 .It Cm LoginGraceTime
535 The server disconnects after this time if the user has not
536 successfully logged in.
537 If the value is 0, there is no time limit.
538 The default is 120 seconds.
539 .It Cm LogLevel
540 Gives the verbosity level that is used when logging messages from
541 .Xr sshd 8 .
542 The possible values are:
543 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
544 The default is INFO.
545 DEBUG and DEBUG1 are equivalent.
546 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
547 Logging with a DEBUG level violates the privacy of users and is not recommended.
548 .It Cm MACs
549 Specifies the available MAC (message authentication code) algorithms.
550 The MAC algorithm is used in protocol version 2
551 for data integrity protection.
552 Multiple algorithms must be comma-separated.
553 The default is:
554 .Bd -literal -offset indent
555 hmac-md5,hmac-sha1,umac-64@openssh.com,
556 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
557 .Ed
558 .It Cm Match
559 Introduces a conditional block.
560 If all of the criteria on the
561 .Cm Match
562 line are satisfied, the keywords on the following lines override those
563 set in the global section of the config file, until either another
564 .Cm Match
565 line or the end of the file.
566 .Pp
567 The arguments to
568 .Cm Match
569 are one or more criteria-pattern pairs.
570 The available criteria are
571 .Cm User ,
572 .Cm Group ,
573 .Cm Host ,
574 and
575 .Cm Address .
576 The match patterns may consist of single entries or comma-separated
577 lists and may use the wildcard and negation operators described in the
578 .Sx PATTERNS
579 section of
580 .Xr ssh_config 5 .
581 .Pp
582 The patterns in an
583 .Cm Address
584 criteria may additionally contain addresses to match in CIDR
585 address/masklen format, e.g.\&
586 .Dq 192.0.2.0/24
587 or
588 .Dq 3ffe:ffff::/32 .
589 Note that the mask length provided must be consistent with the address -
590 it is an error to specify a mask length that is too long for the address
591 or one with bits set in this host portion of the address.
592 For example,
593 .Dq 192.0.2.0/33
594 and
595 .Dq 192.0.2.0/8
596 respectively.
597 .Pp
598 Only a subset of keywords may be used on the lines following a
599 .Cm Match
600 keyword.
601 Available keywords are
602 .Cm AllowAgentForwarding ,
603 .Cm AllowTcpForwarding ,
604 .Cm Banner ,
605 .Cm ChrootDirectory ,
606 .Cm ForceCommand ,
607 .Cm GatewayPorts ,
608 .Cm GSSAPIAuthentication ,
609 .Cm HostbasedAuthentication ,
610 .Cm KbdInteractiveAuthentication ,
611 .Cm KerberosAuthentication ,
612 .Cm MaxAuthTries ,
613 .Cm MaxSessions ,
614 .Cm PasswordAuthentication ,
615 .Cm PermitEmptyPasswords ,
616 .Cm PermitOpen ,
617 .Cm PermitRootLogin ,
618 .Cm RhostsRSAAuthentication ,
619 .Cm RSAAuthentication ,
620 .Cm X11DisplayOffset ,
621 .Cm X11Forwarding
622 and
623 .Cm X11UseLocalHost .
624 .It Cm MaxAuthTries
625 Specifies the maximum number of authentication attempts permitted per
626 connection.
627 Once the number of failures reaches half this value,
628 additional failures are logged.
629 The default is 6.
630 .It Cm MaxSessions
631 Specifies the maximum number of open sessions permitted per network connection.
632 The default is 10.
633 .It Cm MaxStartups
634 Specifies the maximum number of concurrent unauthenticated connections to the
635 SSH daemon.
636 Additional connections will be dropped until authentication succeeds or the
637 .Cm LoginGraceTime
638 expires for a connection.
639 The default is 10.
640 .Pp
641 Alternatively, random early drop can be enabled by specifying
642 the three colon separated values
643 .Dq start:rate:full
644 (e.g. "10:30:60").
645 .Xr sshd 8
646 will refuse connection attempts with a probability of
647 .Dq rate/100
648 (30%)
649 if there are currently
650 .Dq start
651 (10)
652 unauthenticated connections.
653 The probability increases linearly and all connection attempts
654 are refused if the number of unauthenticated connections reaches
655 .Dq full
656 (60).
657 .It Cm PasswordAuthentication
658 Specifies whether password authentication is allowed.
659 The default is
660 .Dq yes .
661 .It Cm PermitEmptyPasswords
662 When password authentication is allowed, it specifies whether the
663 server allows login to accounts with empty password strings.
664 The default is
665 .Dq no .
666 .It Cm PermitOpen
667 Specifies the destinations to which TCP port forwarding is permitted.
668 The forwarding specification must be one of the following forms:
669 .Pp
670 .Bl -item -offset indent -compact
671 .It
672 .Cm PermitOpen
673 .Sm off
674 .Ar host : port
675 .Sm on
676 .It
677 .Cm PermitOpen
678 .Sm off
679 .Ar IPv4_addr : port
680 .Sm on
681 .It
682 .Cm PermitOpen
683 .Sm off
684 .Ar \&[ IPv6_addr \&] : port
685 .Sm on
686 .El
687 .Pp
688 Multiple forwards may be specified by separating them with whitespace.
689 An argument of
690 .Dq any
691 can be used to remove all restrictions and permit any forwarding requests.
692 By default all port forwarding requests are permitted.
693 .It Cm PermitRootLogin
694 Specifies whether root can log in using
695 .Xr ssh 1 .
696 The argument must be
697 .Dq yes ,
698 .Dq without-password ,
699 .Dq forced-commands-only ,
700 or
701 .Dq no .
702 The default is
703 .Dq yes .
704 .Pp
705 If this option is set to
706 .Dq without-password ,
707 password authentication is disabled for root.
708 .Pp
709 If this option is set to
710 .Dq forced-commands-only ,
711 root login with public key authentication will be allowed,
712 but only if the
713 .Ar command
714 option has been specified
715 (which may be useful for taking remote backups even if root login is
716 normally not allowed).
717 All other authentication methods are disabled for root.
718 .Pp
719 If this option is set to
720 .Dq no ,
721 root is not allowed to log in.
722 .It Cm PermitTunnel
723 Specifies whether
724 .Xr tun 4
725 device forwarding is allowed.
726 The argument must be
727 .Dq yes ,
728 .Dq point-to-point
729 (layer 3),
730 .Dq ethernet
731 (layer 2), or
732 .Dq no .
733 Specifying
734 .Dq yes
735 permits both
736 .Dq point-to-point
737 and
738 .Dq ethernet .
739 The default is
740 .Dq no .
741 .It Cm PermitUserEnvironment
742 Specifies whether
743 .Pa ~/.ssh/environment
744 and
745 .Cm environment=
746 options in
747 .Pa ~/.ssh/authorized_keys
748 are processed by
749 .Xr sshd 8 .
750 The default is
751 .Dq no .
752 Enabling environment processing may enable users to bypass access
753 restrictions in some configurations using mechanisms such as
754 .Ev LD_PRELOAD .
755 .It Cm PidFile
756 Specifies the file that contains the process ID of the
757 SSH daemon.
758 The default is
759 .Pa /var/run/sshd.pid .
760 .It Cm Port
761 Specifies the port number that
762 .Xr sshd 8
763 listens on.
764 The default is 22.
765 Multiple options of this type are permitted.
766 See also
767 .Cm ListenAddress .
768 .It Cm PrintLastLog
769 Specifies whether
770 .Xr sshd 8
771 should print the date and time of the last user login when a user logs
772 in interactively.
773 The default is
774 .Dq yes .
775 .It Cm PrintMotd
776 Specifies whether
777 .Xr sshd 8
778 should print
779 .Pa /etc/motd
780 when a user logs in interactively.
781 (On some systems it is also printed by the shell,
782 .Pa /etc/profile ,
783 or equivalent.)
784 The default is
785 .Dq yes .
786 .It Cm Protocol
787 Specifies the protocol versions
788 .Xr sshd 8
789 supports.
790 The possible values are
791 .Sq 1
792 and
793 .Sq 2 .
794 Multiple versions must be comma-separated.
795 The default is
796 .Dq 2,1 .
797 Note that the order of the protocol list does not indicate preference,
798 because the client selects among multiple protocol versions offered
799 by the server.
800 Specifying
801 .Dq 2,1
802 is identical to
803 .Dq 1,2 .
804 .It Cm PubkeyAuthentication
805 Specifies whether public key authentication is allowed.
806 The default is
807 .Dq yes .
808 Note that this option applies to protocol version 2 only.
809 .It Cm RhostsRSAAuthentication
810 Specifies whether rhosts or /etc/hosts.equiv authentication together
811 with successful RSA host authentication is allowed.
812 The default is
813 .Dq no .
814 This option applies to protocol version 1 only.
815 .It Cm RSAAuthentication
816 Specifies whether pure RSA authentication is allowed.
817 The default is
818 .Dq yes .
819 This option applies to protocol version 1 only.
820 .It Cm ServerKeyBits
821 Defines the number of bits in the ephemeral protocol version 1 server key.
822 The minimum value is 512, and the default is 1024.
823 .It Cm StrictModes
824 Specifies whether
825 .Xr sshd 8
826 should check file modes and ownership of the
827 user's files and home directory before accepting login.
828 This is normally desirable because novices sometimes accidentally leave their
829 directory or files world-writable.
830 The default is
831 .Dq yes .
832 .It Cm Subsystem
833 Configures an external subsystem (e.g. file transfer daemon).
834 Arguments should be a subsystem name and a command (with optional arguments)
835 to execute upon subsystem request.
836 .Pp
837 The command
838 .Xr sftp-server 8
839 implements the
840 .Dq sftp
841 file transfer subsystem.
842 .Pp
843 Alternately the name
844 .Dq internal-sftp
845 implements an in-process
846 .Dq sftp
847 server.
848 This may simplify configurations using
849 .Cm ChrootDirectory
850 to force a different filesystem root on clients.
851 .Pp
852 By default no subsystems are defined.
853 Note that this option applies to protocol version 2 only.
854 .It Cm SyslogFacility
855 Gives the facility code that is used when logging messages from
856 .Xr sshd 8 .
857 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
858 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
859 The default is AUTH.
860 .It Cm TCPKeepAlive
861 Specifies whether the system should send TCP keepalive messages to the
862 other side.
863 If they are sent, death of the connection or crash of one
864 of the machines will be properly noticed.
865 However, this means that
866 connections will die if the route is down temporarily, and some people
867 find it annoying.
868 On the other hand, if TCP keepalives are not sent,
869 sessions may hang indefinitely on the server, leaving
870 .Dq ghost
871 users and consuming server resources.
872 .Pp
873 The default is
874 .Dq yes
875 (to send TCP keepalive messages), and the server will notice
876 if the network goes down or the client host crashes.
877 This avoids infinitely hanging sessions.
878 .Pp
879 To disable TCP keepalive messages, the value should be set to
880 .Dq no .
881 .It Cm UseDNS
882 Specifies whether
883 .Xr sshd 8
884 should look up the remote host name and check that
885 the resolved host name for the remote IP address maps back to the
886 very same IP address.
887 The default is
888 .Dq yes .
889 .It Cm UseLogin
890 Specifies whether
891 .Xr login 1
892 is used for interactive login sessions.
893 The default is
894 .Dq no .
895 Note that
896 .Xr login 1
897 is never used for remote command execution.
898 Note also, that if this is enabled,
899 .Cm X11Forwarding
900 will be disabled because
901 .Xr login 1
902 does not know how to handle
903 .Xr xauth 1
904 cookies.
905 If
906 .Cm UsePrivilegeSeparation
907 is specified, it will be disabled after authentication.
908 .It Cm UsePAM
909 Enables the Pluggable Authentication Module interface.
910 If set to
911 .Dq yes
912 this will enable PAM authentication using
913 .Cm ChallengeResponseAuthentication
914 and
915 .Cm PasswordAuthentication
916 in addition to PAM account and session module processing for all
917 authentication types.
918 .Pp
919 Because PAM challenge-response authentication usually serves an equivalent
920 role to password authentication, you should disable either
921 .Cm PasswordAuthentication
922 or
923 .Cm ChallengeResponseAuthentication.
924 .Pp
925 If
926 .Cm UsePAM
927 is enabled, you will not be able to run
928 .Xr sshd 8
929 as a non-root user.
930 The default is
931 .Dq no .
932 .It Cm UsePrivilegeSeparation
933 Specifies whether
934 .Xr sshd 8
935 separates privileges by creating an unprivileged child process
936 to deal with incoming network traffic.
937 After successful authentication, another process will be created that has
938 the privilege of the authenticated user.
939 The goal of privilege separation is to prevent privilege
940 escalation by containing any corruption within the unprivileged processes.
941 The default is
942 .Dq yes .
943 .It Cm X11DisplayOffset
944 Specifies the first display number available for
945 .Xr sshd 8 Ns 's
946 X11 forwarding.
947 This prevents sshd from interfering with real X11 servers.
948 The default is 10.
949 .It Cm X11Forwarding
950 Specifies whether X11 forwarding is permitted.
951 The argument must be
952 .Dq yes
953 or
954 .Dq no .
955 The default is
956 .Dq no .
957 .Pp
958 When X11 forwarding is enabled, there may be additional exposure to
959 the server and to client displays if the
960 .Xr sshd 8
961 proxy display is configured to listen on the wildcard address (see
962 .Cm X11UseLocalhost
963 below), though this is not the default.
964 Additionally, the authentication spoofing and authentication data
965 verification and substitution occur on the client side.
966 The security risk of using X11 forwarding is that the client's X11
967 display server may be exposed to attack when the SSH client requests
968 forwarding (see the warnings for
969 .Cm ForwardX11
970 in
971 .Xr ssh_config 5 ) .
972 A system administrator may have a stance in which they want to
973 protect clients that may expose themselves to attack by unwittingly
974 requesting X11 forwarding, which can warrant a
975 .Dq no
976 setting.
977 .Pp
978 Note that disabling X11 forwarding does not prevent users from
979 forwarding X11 traffic, as users can always install their own forwarders.
980 X11 forwarding is automatically disabled if
981 .Cm UseLogin
982 is enabled.
983 .It Cm X11UseLocalhost
984 Specifies whether
985 .Xr sshd 8
986 should bind the X11 forwarding server to the loopback address or to
987 the wildcard address.
988 By default,
989 sshd binds the forwarding server to the loopback address and sets the
990 hostname part of the
991 .Ev DISPLAY
992 environment variable to
993 .Dq localhost .
994 This prevents remote hosts from connecting to the proxy display.
995 However, some older X11 clients may not function with this
996 configuration.
997 .Cm X11UseLocalhost
998 may be set to
999 .Dq no
1000 to specify that the forwarding server should be bound to the wildcard
1001 address.
1002 The argument must be
1003 .Dq yes
1004 or
1005 .Dq no .
1006 The default is
1007 .Dq yes .
1008 .It Cm XAuthLocation
1009 Specifies the full pathname of the
1010 .Xr xauth 1
1011 program.
1012 The default is
1013 .Pa /usr/X11R6/bin/xauth .
1014 .El
1015 .Sh TIME FORMATS
1016 .Xr sshd 8
1017 command-line arguments and configuration file options that specify time
1018 may be expressed using a sequence of the form:
1019 .Sm off
1020 .Ar time Op Ar qualifier ,
1021 .Sm on
1022 where
1023 .Ar time
1024 is a positive integer value and
1025 .Ar qualifier
1026 is one of the following:
1027 .Pp
1028 .Bl -tag -width Ds -compact -offset indent
1029 .It Aq Cm none
1030 seconds
1031 .It Cm s | Cm S
1032 seconds
1033 .It Cm m | Cm M
1034 minutes
1035 .It Cm h | Cm H
1036 hours
1037 .It Cm d | Cm D
1038 days
1039 .It Cm w | Cm W
1040 weeks
1041 .El
1042 .Pp
1043 Each member of the sequence is added together to calculate
1044 the total time value.
1045 .Pp
1046 Time format examples:
1047 .Pp
1048 .Bl -tag -width Ds -compact -offset indent
1049 .It 600
1050 600 seconds (10 minutes)
1051 .It 10m
1052 10 minutes
1053 .It 1h30m
1054 1 hour 30 minutes (90 minutes)
1055 .El
1056 .Sh FILES
1057 .Bl -tag -width Ds
1058 .It Pa /etc/ssh/sshd_config
1059 Contains configuration data for
1060 .Xr sshd 8 .
1061 This file should be writable by root only, but it is recommended
1062 (though not necessary) that it be world-readable.
1063 .El
1064 .Sh SEE ALSO
1065 .Xr sshd 8
1066 .Sh AUTHORS
1067 OpenSSH is a derivative of the original and free
1068 ssh 1.2.12 release by Tatu Ylonen.
1069 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1070 Theo de Raadt and Dug Song
1071 removed many bugs, re-added newer features and
1072 created OpenSSH.
1073 Markus Friedl contributed the support for SSH
1074 protocol versions 1.5 and 2.0.
1075 Niels Provos and Markus Friedl contributed support
1076 for privilege separation.