| blob | 8ec5e6009f7bb159f6df1b815aa82656c464609d |
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 1.102 2009/02/22 23:59:25 djm Exp $
38 .Dd $Mdocdate: February 22 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 Specifically, in
181 .Dx ,
182 this controls the use of PAM (see
183 .Xr pam 3 )
184 for authentication.
185 Note that this affects the effectiveness of the
186 .Cm PasswordAuthentication
187 and
188 .Cm PermitRootLogin
189 variables.
190 The default is
191 .Dq yes .
192 .It Cm ChrootDirectory
193 Specifies a path to
194 .Xr chroot 2
195 to after authentication.
196 This path, and all its components, must be root-owned directories that are
197 not writable by any other user or group.
198 .Pp
199 The path may contain the following tokens that are expanded at runtime once
200 the connecting user has been authenticated: %% is replaced by a literal '%',
201 %h is replaced by the home directory of the user being authenticated, and
202 %u is replaced by the username of that user.
203 .Pp
204 The
205 .Cm ChrootDirectory
206 must contain the necessary files and directories to support the
207 users' session.
208 For an interactive session this requires at least a shell, typically
209 .Xr sh 1 ,
210 and basic
211 .Pa /dev
212 nodes such as
213 .Xr null 4 ,
214 .Xr zero 4 ,
215 .Xr stdin 4 ,
216 .Xr stdout 4 ,
217 .Xr stderr 4 ,
218 .Xr arandom 4
219 and
220 .Xr tty 4
221 devices.
222 For file transfer sessions using
223 .Dq sftp ,
224 no additional configuration of the environment is necessary if the
225 in-process sftp server is used (see
226 .Cm Subsystem
227 for details).
228 .Pp
229 The default is not to
230 .Xr chroot 2 .
231 .It Cm Ciphers
232 Specifies the ciphers allowed for protocol version 2.
233 Multiple ciphers must be comma-separated.
234 The supported ciphers are
235 .Dq 3des-cbc ,
236 .Dq aes128-cbc ,
237 .Dq aes192-cbc ,
238 .Dq aes256-cbc ,
239 .Dq aes128-ctr ,
240 .Dq aes192-ctr ,
241 .Dq aes256-ctr ,
242 .Dq arcfour128 ,
243 .Dq arcfour256 ,
244 .Dq arcfour ,
245 .Dq blowfish-cbc ,
246 and
247 .Dq cast128-cbc .
248 The default is:
249 .Bd -literal -offset 3n
250 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
251 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
252 aes256-cbc,arcfour
253 .Ed
254 .It Cm ClientAliveCountMax
255 Sets the number of client alive messages (see below) which may be
256 sent without
257 .Xr sshd 8
258 receiving any messages back from the client.
259 If this threshold is reached while client alive messages are being sent,
260 sshd will disconnect the client, terminating the session.
261 It is important to note that the use of client alive messages is very
262 different from
263 .Cm TCPKeepAlive
264 (below).
265 The client alive messages are sent through the encrypted channel
266 and therefore will not be spoofable.
267 The TCP keepalive option enabled by
268 .Cm TCPKeepAlive
269 is spoofable.
270 The client alive mechanism is valuable when the client or
271 server depend on knowing when a connection has become inactive.
272 .Pp
273 The default value is 3.
274 If
275 .Cm ClientAliveInterval
276 (see below) is set to 15, and
277 .Cm ClientAliveCountMax
278 is left at the default, unresponsive SSH clients
279 will be disconnected after approximately 45 seconds.
280 This option applies to protocol version 2 only.
281 .It Cm ClientAliveInterval
282 Sets a timeout interval in seconds after which if no data has been received
283 from the client,
284 .Xr sshd 8
285 will send a message through the encrypted
286 channel to request a response from the client.
287 The default
288 is 0, indicating that these messages will not be sent to the client.
289 This option applies to protocol version 2 only.
290 .It Cm Compression
291 Specifies whether compression is allowed, or delayed until
292 the user has authenticated successfully.
293 The argument must be
294 .Dq yes ,
295 .Dq delayed ,
296 or
297 .Dq no .
298 The default is
299 .Dq delayed .
300 .It Cm DenyGroups
301 This keyword can be followed by a list of group name patterns, separated
302 by spaces.
303 Login is disallowed for users whose primary group or supplementary
304 group list matches one of the patterns.
305 Only group names are valid; a numerical group ID is not recognized.
306 By default, login is allowed for all groups.
307 The allow/deny directives are processed in the following order:
308 .Cm DenyUsers ,
309 .Cm AllowUsers ,
310 .Cm DenyGroups ,
311 and finally
312 .Cm AllowGroups .
313 .Pp
314 See
315 .Sx PATTERNS
316 in
317 .Xr ssh_config 5
318 for more information on patterns.
319 .It Cm DenyUsers
320 This keyword can be followed by a list of user name patterns, separated
321 by spaces.
322 Login is disallowed for user names that match one of the patterns.
323 Only user names are valid; a numerical user ID is not recognized.
324 By default, login is allowed for all users.
325 If the pattern takes the form USER@HOST then USER and HOST
326 are separately checked, restricting logins to particular
327 users from particular hosts.
328 The allow/deny directives are processed in the following order:
329 .Cm DenyUsers ,
330 .Cm AllowUsers ,
331 .Cm DenyGroups ,
332 and finally
333 .Cm AllowGroups .
334 .Pp
335 See
336 .Sx PATTERNS
337 in
338 .Xr ssh_config 5
339 for more information on patterns.
340 .It Cm ForceCommand
341 Forces the execution of the command specified by
342 .Cm ForceCommand ,
343 ignoring any command supplied by the client and
344 .Pa ~/.ssh/rc
345 if present.
346 The command is invoked by using the user's login shell with the -c option.
347 This applies to shell, command, or subsystem execution.
348 It is most useful inside a
349 .Cm Match
350 block.
351 The command originally supplied by the client is available in the
352 .Ev SSH_ORIGINAL_COMMAND
353 environment variable.
354 Specifying a command of
355 .Dq internal-sftp
356 will force the use of an in-process sftp server that requires no support
357 files when used with
358 .Cm ChrootDirectory .
359 .It Cm GatewayPorts
360 Specifies whether remote hosts are allowed to connect to ports
361 forwarded for the client.
362 By default,
363 .Xr sshd 8
364 binds remote port forwardings to the loopback address.
365 This prevents other remote hosts from connecting to forwarded ports.
366 .Cm GatewayPorts
367 can be used to specify that sshd
368 should allow remote port forwardings to bind to non-loopback addresses, thus
369 allowing other hosts to connect.
370 The argument may be
371 .Dq no
372 to force remote port forwardings to be available to the local host only,
373 .Dq yes
374 to force remote port forwardings to bind to the wildcard address, or
375 .Dq clientspecified
376 to allow the client to select the address to which the forwarding is bound.
377 The default is
378 .Dq no .
379 .It Cm GSSAPIAuthentication
380 Specifies whether user authentication based on GSSAPI is allowed.
381 The default is
382 .Dq no .
383 Note that this option applies to protocol version 2 only.
384 .It Cm GSSAPICleanupCredentials
385 Specifies whether to automatically destroy the user's credentials cache
386 on logout.
387 The default is
388 .Dq yes .
389 Note that this option applies to protocol version 2 only.
390 .It Cm HostbasedAuthentication
391 Specifies whether rhosts or /etc/hosts.equiv authentication together
392 with successful public key client host authentication is allowed
393 (host-based authentication).
394 This option is similar to
395 .Cm RhostsRSAAuthentication
396 and applies to protocol version 2 only.
397 The default is
398 .Dq no .
399 .It Cm HostbasedUsesNameFromPacketOnly
400 Specifies whether or not the server will attempt to perform a reverse
401 name lookup when matching the name in the
402 .Pa ~/.shosts ,
403 .Pa ~/.rhosts ,
404 and
405 .Pa /etc/hosts.equiv
406 files during
407 .Cm HostbasedAuthentication .
408 A setting of
409 .Dq yes
410 means that
411 .Xr sshd 8
412 uses the name supplied by the client rather than
413 attempting to resolve the name from the TCP connection itself.
414 The default is
415 .Dq no .
416 .It Cm HostKey
417 Specifies a file containing a private host key
418 used by SSH.
419 The default is
420 .Pa /etc/ssh/ssh_host_key
421 for protocol version 1, and
422 .Pa /etc/ssh/ssh_host_rsa_key
423 and
424 .Pa /etc/ssh/ssh_host_dsa_key
425 for protocol version 2.
426 Note that
427 .Xr sshd 8
428 will refuse to use a file if it is group/world-accessible.
429 It is possible to have multiple host key files.
430 .Dq rsa1
431 keys are used for version 1 and
432 .Dq dsa
433 or
434 .Dq rsa
435 are used for version 2 of the SSH protocol.
436 .It Cm IgnoreRhosts
437 Specifies that
438 .Pa .rhosts
439 and
440 .Pa .shosts
441 files will not be used in
442 .Cm RhostsRSAAuthentication
443 or
444 .Cm HostbasedAuthentication .
445 .Pp
446 .Pa /etc/hosts.equiv
447 and
448 .Pa /etc/ssh/shosts.equiv
449 are still used.
450 The default is
451 .Dq yes .
452 .It Cm IgnoreUserKnownHosts
453 Specifies whether
454 .Xr sshd 8
455 should ignore the user's
456 .Pa ~/.ssh/known_hosts
457 during
458 .Cm RhostsRSAAuthentication
459 or
460 .Cm HostbasedAuthentication .
461 The default is
462 .Dq no .
463 .It Cm KerberosAuthentication
464 Specifies whether the password provided by the user for
465 .Cm PasswordAuthentication
466 will be validated through the Kerberos KDC.
467 To use this option, the server needs a
468 Kerberos servtab which allows the verification of the KDC's identity.
469 The default is
470 .Dq no .
471 .It Cm KerberosGetAFSToken
472 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
473 an AFS token before accessing the user's home directory.
474 The default is
475 .Dq no .
476 .It Cm KerberosOrLocalPasswd
477 If password authentication through Kerberos fails then
478 the password will be validated via any additional local mechanism
479 such as
480 .Pa /etc/passwd .
481 The default is
482 .Dq yes .
483 .It Cm KerberosTicketCleanup
484 Specifies whether to automatically destroy the user's ticket cache
485 file on logout.
486 The default is
487 .Dq yes .
488 .It Cm KeyRegenerationInterval
489 In protocol version 1, the ephemeral server key is automatically regenerated
490 after this many seconds (if it has been used).
491 The purpose of regeneration is to prevent
492 decrypting captured sessions by later breaking into the machine and
493 stealing the keys.
494 The key is never stored anywhere.
495 If the value is 0, the key is never regenerated.
496 The default is 3600 (seconds).
497 .It Cm ListenAddress
498 Specifies the local addresses
499 .Xr sshd 8
500 should listen on.
501 The following forms may be used:
502 .Pp
503 .Bl -item -offset indent -compact
504 .It
505 .Cm ListenAddress
506 .Sm off
507 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
508 .Sm on
509 .It
510 .Cm ListenAddress
511 .Sm off
512 .Ar host No | Ar IPv4_addr No : Ar port
513 .Sm on
514 .It
515 .Cm ListenAddress
516 .Sm off
517 .Oo
518 .Ar host No | Ar IPv6_addr Oc : Ar port
519 .Sm on
520 .El
521 .Pp
522 If
523 .Ar port
524 is not specified,
525 sshd will listen on the address and all prior
526 .Cm Port
527 options specified.
528 The default is to listen on all local addresses.
529 Multiple
530 .Cm ListenAddress
531 options are permitted.
532 Additionally, any
533 .Cm Port
534 options must precede this option for non-port qualified addresses.
535 .It Cm LoginGraceTime
536 The server disconnects after this time if the user has not
537 successfully logged in.
538 If the value is 0, there is no time limit.
539 The default is 120 seconds.
540 .It Cm LogLevel
541 Gives the verbosity level that is used when logging messages from
542 .Xr sshd 8 .
543 The possible values are:
544 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
545 The default is INFO.
546 DEBUG and DEBUG1 are equivalent.
547 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
548 Logging with a DEBUG level violates the privacy of users and is not recommended.
549 .It Cm MACs
550 Specifies the available MAC (message authentication code) algorithms.
551 The MAC algorithm is used in protocol version 2
552 for data integrity protection.
553 Multiple algorithms must be comma-separated.
554 The default is:
555 .Bd -literal -offset indent
556 hmac-md5,hmac-sha1,umac-64@openssh.com,
557 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
558 .Ed
559 .It Cm Match
560 Introduces a conditional block.
561 If all of the criteria on the
562 .Cm Match
563 line are satisfied, the keywords on the following lines override those
564 set in the global section of the config file, until either another
565 .Cm Match
566 line or the end of the file.
567 .Pp
568 The arguments to
569 .Cm Match
570 are one or more criteria-pattern pairs.
571 The available criteria are
572 .Cm User ,
573 .Cm Group ,
574 .Cm Host ,
575 and
576 .Cm Address .
577 The match patterns may consist of single entries or comma-separated
578 lists and may use the wildcard and negation operators described in the
579 .Sx PATTERNS
580 section of
581 .Xr ssh_config 5 .
582 .Pp
583 The patterns in an
584 .Cm Address
585 criteria may additionally contain addresses to match in CIDR
586 address/masklen format, e.g.\&
587 .Dq 192.0.2.0/24
588 or
589 .Dq 3ffe:ffff::/32 .
590 Note that the mask length provided must be consistent with the address -
591 it is an error to specify a mask length that is too long for the address
592 or one with bits set in this host portion of the address.
593 For example,
594 .Dq 192.0.2.0/33
595 and
596 .Dq 192.0.2.0/8
597 respectively.
598 .Pp
599 Only a subset of keywords may be used on the lines following a
600 .Cm Match
601 keyword.
602 Available keywords are
603 .Cm AllowAgentForwarding ,
604 .Cm AllowTcpForwarding ,
605 .Cm Banner ,
606 .Cm ChrootDirectory ,
607 .Cm ForceCommand ,
608 .Cm GatewayPorts ,
609 .Cm GSSAPIAuthentication ,
610 .Cm HostbasedAuthentication ,
611 .Cm KbdInteractiveAuthentication ,
612 .Cm KerberosAuthentication ,
613 .Cm MaxAuthTries ,
614 .Cm MaxSessions ,
615 .Cm PasswordAuthentication ,
616 .Cm PermitEmptyPasswords ,
617 .Cm PermitOpen ,
618 .Cm PermitRootLogin ,
619 .Cm RhostsRSAAuthentication ,
620 .Cm RSAAuthentication ,
621 .Cm X11DisplayOffset ,
622 .Cm X11Forwarding
623 and
624 .Cm X11UseLocalHost .
625 .It Cm MaxAuthTries
626 Specifies the maximum number of authentication attempts permitted per
627 connection.
628 Once the number of failures reaches half this value,
629 additional failures are logged.
630 The default is 6.
631 .It Cm MaxSessions
632 Specifies the maximum number of open sessions permitted per network connection.
633 The default is 10.
634 .It Cm MaxStartups
635 Specifies the maximum number of concurrent unauthenticated connections to the
636 SSH daemon.
637 Additional connections will be dropped until authentication succeeds or the
638 .Cm LoginGraceTime
639 expires for a connection.
640 The default is 10.
641 .Pp
642 Alternatively, random early drop can be enabled by specifying
643 the three colon separated values
644 .Dq start:rate:full
645 (e.g. "10:30:60").
646 .Xr sshd 8
647 will refuse connection attempts with a probability of
648 .Dq rate/100
649 (30%)
650 if there are currently
651 .Dq start
652 (10)
653 unauthenticated connections.
654 The probability increases linearly and all connection attempts
655 are refused if the number of unauthenticated connections reaches
656 .Dq full
657 (60).
658 .It Cm PasswordAuthentication
659 Specifies whether password authentication is allowed.
660 The default is
661 .Dq yes .
662 Note that if
663 .Cm ChallengeResponseAuthentication
664 is
665 .Dq yes ,
666 .Cm UsePAM
667 is
668 .Dq yes ,
669 and the PAM authentication policy for
670 .Nm sshd
671 includes
672 .Xr pam_unix 8 ,
673 password authentication will be allowed through the challenge-response
674 mechanism regardless of the value of
675 .Cm PasswordAuthentication .
676 .It Cm PermitEmptyPasswords
677 When password authentication is allowed, it specifies whether the
678 server allows login to accounts with empty password strings.
679 The default is
680 .Dq no .
681 .It Cm PermitOpen
682 Specifies the destinations to which TCP port forwarding is permitted.
683 The forwarding specification must be one of the following forms:
684 .Pp
685 .Bl -item -offset indent -compact
686 .It
687 .Cm PermitOpen
688 .Sm off
689 .Ar host : port
690 .Sm on
691 .It
692 .Cm PermitOpen
693 .Sm off
694 .Ar IPv4_addr : port
695 .Sm on
696 .It
697 .Cm PermitOpen
698 .Sm off
699 .Ar \&[ IPv6_addr \&] : port
700 .Sm on
701 .El
702 .Pp
703 Multiple forwards may be specified by separating them with whitespace.
704 An argument of
705 .Dq any
706 can be used to remove all restrictions and permit any forwarding requests.
707 By default all port forwarding requests are permitted.
708 .It Cm PermitRootLogin
709 Specifies whether root can log in using
710 .Xr ssh 1 .
711 The argument must be
712 .Dq yes ,
713 .Dq without-password ,
714 .Dq forced-commands-only ,
715 or
716 .Dq no .
717 The default is
718 .Dq no .
719 Note that if
720 .Cm ChallengeResponseAuthentication
721 is
722 .Dq yes ,
723 the root user may be allowed in with its password even if
724 .Cm PermitRootLogin is set to
725 .Dq without-password .
726 .Pp
727 If this option is set to
728 .Dq without-password ,
729 password authentication is disabled for root.
730 .Pp
731 If this option is set to
732 .Dq forced-commands-only ,
733 root login with public key authentication will be allowed,
734 but only if the
735 .Ar command
736 option has been specified
737 (which may be useful for taking remote backups even if root login is
738 normally not allowed).
739 All other authentication methods are disabled for root.
740 .Pp
741 If this option is set to
742 .Dq no ,
743 root is not allowed to log in.
744 .It Cm PermitTunnel
745 Specifies whether
746 .Xr tun 4
747 device forwarding is allowed.
748 The argument must be
749 .Dq yes ,
750 .Dq point-to-point
751 (layer 3),
752 .Dq ethernet
753 (layer 2), or
754 .Dq no .
755 Specifying
756 .Dq yes
757 permits both
758 .Dq point-to-point
759 and
760 .Dq ethernet .
761 The default is
762 .Dq no .
763 .It Cm PermitUserEnvironment
764 Specifies whether
765 .Pa ~/.ssh/environment
766 and
767 .Cm environment=
768 options in
769 .Pa ~/.ssh/authorized_keys
770 are processed by
771 .Xr sshd 8 .
772 The default is
773 .Dq no .
774 Enabling environment processing may enable users to bypass access
775 restrictions in some configurations using mechanisms such as
776 .Ev LD_PRELOAD .
777 .It Cm PidFile
778 Specifies the file that contains the process ID of the
779 SSH daemon.
780 The default is
781 .Pa /var/run/sshd.pid .
782 .It Cm Port
783 Specifies the port number that
784 .Xr sshd 8
785 listens on.
786 The default is 22.
787 Multiple options of this type are permitted.
788 See also
789 .Cm ListenAddress .
790 .It Cm PrintLastLog
791 Specifies whether
792 .Xr sshd 8
793 should print the date and time of the last user login when a user logs
794 in interactively.
795 The default is
796 .Dq yes .
797 .It Cm PrintMotd
798 Specifies whether
799 .Xr sshd 8
800 should print
801 .Pa /etc/motd
802 when a user logs in interactively.
803 (On some systems it is also printed by the shell,
804 .Pa /etc/profile ,
805 or equivalent.)
806 The default is
807 .Dq yes .
808 .It Cm Protocol
809 Specifies the protocol versions
810 .Xr sshd 8
811 supports.
812 The possible values are
813 .Sq 1
814 and
815 .Sq 2 .
816 Multiple versions must be comma-separated.
817 The default is
818 .Dq 2 .
819 Note that the order of the protocol list does not indicate preference,
820 because the client selects among multiple protocol versions offered
821 by the server.
822 Specifying
823 .Dq 2,1
824 is identical to
825 .Dq 1,2 .
826 .It Cm PubkeyAuthentication
827 Specifies whether public key authentication is allowed.
828 The default is
829 .Dq yes .
830 Note that this option applies to protocol version 2 only.
831 .It Cm RhostsRSAAuthentication
832 Specifies whether rhosts or
833 .Pa /etc/hosts.equiv
834 authentication together
835 with successful RSA host authentication is allowed.
836 The default is
837 .Dq no .
838 This option applies to protocol version 1 only.
839 .It Cm RSAAuthentication
840 Specifies whether pure RSA authentication is allowed.
841 The default is
842 .Dq yes .
843 This option applies to protocol version 1 only.
844 .It Cm ServerKeyBits
845 Defines the number of bits in the ephemeral protocol version 1 server key.
846 The minimum value is 512, and the default is 1024.
847 .It Cm StrictModes
848 Specifies whether
849 .Xr sshd 8
850 should check file modes and ownership of the
851 user's files and home directory before accepting login.
852 This is normally desirable because novices sometimes accidentally leave their
853 directory or files world-writable.
854 The default is
855 .Dq yes .
856 .It Cm Subsystem
857 Configures an external subsystem (e.g. file transfer daemon).
858 Arguments should be a subsystem name and a command (with optional arguments)
859 to execute upon subsystem request.
860 .Pp
861 The command
862 .Xr sftp-server 8
863 implements the
864 .Dq sftp
865 file transfer subsystem.
866 .Pp
867 Alternately the name
868 .Dq internal-sftp
869 implements an in-process
870 .Dq sftp
871 server.
872 This may simplify configurations using
873 .Cm ChrootDirectory
874 to force a different filesystem root on clients.
875 .Pp
876 By default no subsystems are defined.
877 Note that this option applies to protocol version 2 only.
878 .It Cm SyslogFacility
879 Gives the facility code that is used when logging messages from
880 .Xr sshd 8 .
881 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
882 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
883 The default is AUTH.
884 .It Cm TCPKeepAlive
885 Specifies whether the system should send TCP keepalive messages to the
886 other side.
887 If they are sent, death of the connection or crash of one
888 of the machines will be properly noticed.
889 However, this means that
890 connections will die if the route is down temporarily, and some people
891 find it annoying.
892 On the other hand, if TCP keepalives are not sent,
893 sessions may hang indefinitely on the server, leaving
894 .Dq ghost
895 users and consuming server resources.
896 .Pp
897 The default is
898 .Dq yes
899 (to send TCP keepalive messages), and the server will notice
900 if the network goes down or the client host crashes.
901 This avoids infinitely hanging sessions.
902 .Pp
903 To disable TCP keepalive messages, the value should be set to
904 .Dq no .
905 .It Cm UseDNS
906 Specifies whether
907 .Xr sshd 8
908 should look up the remote host name and check that
909 the resolved host name for the remote IP address maps back to the
910 very same IP address.
911 The default is
912 .Dq yes .
913 .It Cm UseLogin
914 Specifies whether
915 .Xr login 1
916 is used for interactive login sessions.
917 The default is
918 .Dq no .
919 Note that
920 .Xr login 1
921 is never used for remote command execution.
922 Note also, that if this is enabled,
923 .Cm X11Forwarding
924 will be disabled because
925 .Xr login 1
926 does not know how to handle
927 .Xr xauth 1
928 cookies.
929 If
930 .Cm UsePrivilegeSeparation
931 is specified, it will be disabled after authentication.
932 .It Cm UsePAM
933 Enables the Pluggable Authentication Module interface.
934 If set to
935 .Dq yes
936 this will enable PAM authentication using
937 .Cm ChallengeResponseAuthentication
938 and
939 .Cm PasswordAuthentication
940 in addition to PAM account and session module processing for all
941 authentication types.
942 .Pp
943 Because PAM challenge-response authentication usually serves an equivalent
944 role to password authentication, you should disable either
945 .Cm PasswordAuthentication
946 or
947 .Cm ChallengeResponseAuthentication.
948 .Pp
949 If
950 .Cm UsePAM
951 is enabled, you will not be able to run
952 .Xr sshd 8
953 as a non-root user.
954 The default is
955 .Dq no .
956 .It Cm UsePrivilegeSeparation
957 Specifies whether
958 .Xr sshd 8
959 separates privileges by creating an unprivileged child process
960 to deal with incoming network traffic.
961 After successful authentication, another process will be created that has
962 the privilege of the authenticated user.
963 The goal of privilege separation is to prevent privilege
964 escalation by containing any corruption within the unprivileged processes.
965 The default is
966 .Dq yes .
967 .It Cm VersionAddendum
968 Specifies a string to append to the regular version string to identify
969 OS- or site-specific modifications.
970 The default is
971 .Dq DragonFly-20090620 .
972 .It Cm X11DisplayOffset
973 Specifies the first display number available for
974 .Xr sshd 8 Ns 's
975 X11 forwarding.
976 This prevents sshd from interfering with real X11 servers.
977 The default is 10.
978 .It Cm X11Forwarding
979 Specifies whether X11 forwarding is permitted.
980 The argument must be
981 .Dq yes
982 or
983 .Dq no .
984 The default is
985 .Dq yes .
986 .Pp
987 When X11 forwarding is enabled, there may be additional exposure to
988 the server and to client displays if the
989 .Xr sshd 8
990 proxy display is configured to listen on the wildcard address (see
991 .Cm X11UseLocalhost
992 below), though this is not the default.
993 Additionally, the authentication spoofing and authentication data
994 verification and substitution occur on the client side.
995 The security risk of using X11 forwarding is that the client's X11
996 display server may be exposed to attack when the SSH client requests
997 forwarding (see the warnings for
998 .Cm ForwardX11
999 in
1000 .Xr ssh_config 5 ) .
1001 A system administrator may have a stance in which they want to
1002 protect clients that may expose themselves to attack by unwittingly
1003 requesting X11 forwarding, which can warrant a
1004 .Dq no
1005 setting.
1006 .Pp
1007 Note that disabling X11 forwarding does not prevent users from
1008 forwarding X11 traffic, as users can always install their own forwarders.
1009 X11 forwarding is automatically disabled if
1010 .Cm UseLogin
1011 is enabled.
1012 .It Cm X11UseLocalhost
1013 Specifies whether
1014 .Xr sshd 8
1015 should bind the X11 forwarding server to the loopback address or to
1016 the wildcard address.
1017 By default,
1018 sshd binds the forwarding server to the loopback address and sets the
1019 hostname part of the
1020 .Ev DISPLAY
1021 environment variable to
1022 .Dq localhost .
1023 This prevents remote hosts from connecting to the proxy display.
1024 However, some older X11 clients may not function with this
1025 configuration.
1026 .Cm X11UseLocalhost
1027 may be set to
1028 .Dq no
1029 to specify that the forwarding server should be bound to the wildcard
1030 address.
1031 The argument must be
1032 .Dq yes
1033 or
1034 .Dq no .
1035 The default is
1036 .Dq yes .
1037 .It Cm XAuthLocation
1038 Specifies the full pathname of the
1039 .Xr xauth 1
1040 program.
1041 The default is
1042 .Pa /usr/X11R6/bin/xauth .
1043 .El
1044 .Sh TIME FORMATS
1045 .Xr sshd 8
1046 command-line arguments and configuration file options that specify time
1047 may be expressed using a sequence of the form:
1048 .Sm off
1049 .Ar time Op Ar qualifier ,
1050 .Sm on
1051 where
1052 .Ar time
1053 is a positive integer value and
1054 .Ar qualifier
1055 is one of the following:
1056 .Pp
1057 .Bl -tag -width Ds -compact -offset indent
1058 .It Aq Cm none
1059 seconds
1060 .It Cm s | Cm S
1061 seconds
1062 .It Cm m | Cm M
1063 minutes
1064 .It Cm h | Cm H
1065 hours
1066 .It Cm d | Cm D
1067 days
1068 .It Cm w | Cm W
1069 weeks
1070 .El
1071 .Pp
1072 Each member of the sequence is added together to calculate
1073 the total time value.
1074 .Pp
1075 Time format examples:
1076 .Pp
1077 .Bl -tag -width Ds -compact -offset indent
1078 .It 600
1079 600 seconds (10 minutes)
1080 .It 10m
1081 10 minutes
1082 .It 1h30m
1083 1 hour 30 minutes (90 minutes)
1084 .El
1085 .Sh FILES
1086 .Bl -tag -width Ds
1087 .It Pa /etc/ssh/sshd_config
1088 Contains configuration data for
1089 .Xr sshd 8 .
1090 This file should be writable by root only, but it is recommended
1091 (though not necessary) that it be world-readable.
1092 .El
1093 .Sh SEE ALSO
1094 .Xr sshd 8
1095 .Sh AUTHORS
1096 OpenSSH is a derivative of the original and free
1097 ssh 1.2.12 release by Tatu Ylonen.
1098 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1099 Theo de Raadt and Dug Song
1100 removed many bugs, re-added newer features and
1101 created OpenSSH.
1102 Markus Friedl contributed the support for SSH
1103 protocol versions 1.5 and 2.0.
1104 Niels Provos and Markus Friedl contributed support
1105 for privilege separation.