| blob | 2f5410281184a00720d21602b49eefeefdf67bff |
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 1.120 2010/03/04 23:17:25 djm Exp $
38 .Dd $Mdocdate: March 4 2010 $
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 (e.g. via
180 PAM or though authentication styles supported in
181 .Xr login.conf 5 )
182 The default is
183 .Dq yes .
184 .It Cm ChrootDirectory
185 Specifies the pathname of a directory to
186 .Xr chroot 2
187 to after authentication.
188 All components of the pathname must be root-owned directories that are
189 not writable by any other user or group.
190 After the chroot,
191 .Xr sshd 8
192 changes the working directory to the user's home directory.
193 .Pp
194 The pathname may contain the following tokens that are expanded at runtime once
195 the connecting user has been authenticated: %% is replaced by a literal '%',
196 %h is replaced by the home directory of the user being authenticated, and
197 %u is replaced by the username of that user.
198 .Pp
199 The
200 .Cm ChrootDirectory
201 must contain the necessary files and directories to support the
202 user's session.
203 For an interactive session this requires at least a shell, typically
204 .Xr sh 1 ,
205 and basic
206 .Pa /dev
207 nodes such as
208 .Xr null 4 ,
209 .Xr zero 4 ,
210 .Xr stdin 4 ,
211 .Xr stdout 4 ,
212 .Xr stderr 4 ,
213 .Xr arandom 4
214 and
215 .Xr tty 4
216 devices.
217 For file transfer sessions using
218 .Dq sftp ,
219 no additional configuration of the environment is necessary if the
220 in-process sftp server is used,
221 though sessions which use logging do require
222 .Pa /dev/log
223 inside the chroot directory (see
224 .Xr sftp-server 8
225 for details).
226 .Pp
227 The default is not to
228 .Xr chroot 2 .
229 .It Cm Ciphers
230 Specifies the ciphers allowed for protocol version 2.
231 Multiple ciphers must be comma-separated.
232 The supported ciphers are
233 .Dq 3des-cbc ,
234 .Dq aes128-cbc ,
235 .Dq aes192-cbc ,
236 .Dq aes256-cbc ,
237 .Dq aes128-ctr ,
238 .Dq aes192-ctr ,
239 .Dq aes256-ctr ,
240 .Dq arcfour128 ,
241 .Dq arcfour256 ,
242 .Dq arcfour ,
243 .Dq blowfish-cbc ,
244 and
245 .Dq cast128-cbc .
246 The default is:
247 .Bd -literal -offset 3n
248 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
249 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
250 aes256-cbc,arcfour
251 .Ed
252 .It Cm ClientAliveCountMax
253 Sets the number of client alive messages (see below) which may be
254 sent without
255 .Xr sshd 8
256 receiving any messages back from the client.
257 If this threshold is reached while client alive messages are being sent,
258 sshd will disconnect the client, terminating the session.
259 It is important to note that the use of client alive messages is very
260 different from
261 .Cm TCPKeepAlive
262 (below).
263 The client alive messages are sent through the encrypted channel
264 and therefore will not be spoofable.
265 The TCP keepalive option enabled by
266 .Cm TCPKeepAlive
267 is spoofable.
268 The client alive mechanism is valuable when the client or
269 server depend on knowing when a connection has become inactive.
270 .Pp
271 The default value is 3.
272 If
273 .Cm ClientAliveInterval
274 (see below) is set to 15, and
275 .Cm ClientAliveCountMax
276 is left at the default, unresponsive SSH clients
277 will be disconnected after approximately 45 seconds.
278 This option applies to protocol version 2 only.
279 .It Cm ClientAliveInterval
280 Sets a timeout interval in seconds after which if no data has been received
281 from the client,
282 .Xr sshd 8
283 will send a message through the encrypted
284 channel to request a response from the client.
285 The default
286 is 0, indicating that these messages will not be sent to the client.
287 This option applies to protocol version 2 only.
288 .It Cm Compression
289 Specifies whether compression is allowed, or delayed until
290 the user has authenticated successfully.
291 The argument must be
292 .Dq yes ,
293 .Dq delayed ,
294 or
295 .Dq no .
296 The default is
297 .Dq delayed .
298 .It Cm DenyGroups
299 This keyword can be followed by a list of group name patterns, separated
300 by spaces.
301 Login is disallowed for users whose primary group or supplementary
302 group list matches one of the patterns.
303 Only group names are valid; a numerical group ID is not recognized.
304 By default, login is allowed for all groups.
305 The allow/deny directives are processed in the following order:
306 .Cm DenyUsers ,
307 .Cm AllowUsers ,
308 .Cm DenyGroups ,
309 and finally
310 .Cm AllowGroups .
311 .Pp
312 See
313 .Sx PATTERNS
314 in
315 .Xr ssh_config 5
316 for more information on patterns.
317 .It Cm DenyUsers
318 This keyword can be followed by a list of user name patterns, separated
319 by spaces.
320 Login is disallowed for user names that match one of the patterns.
321 Only user names are valid; a numerical user ID is not recognized.
322 By default, login is allowed for all users.
323 If the pattern takes the form USER@HOST then USER and HOST
324 are separately checked, restricting logins to particular
325 users from particular hosts.
326 The allow/deny directives are processed in the following order:
327 .Cm DenyUsers ,
328 .Cm AllowUsers ,
329 .Cm DenyGroups ,
330 and finally
331 .Cm AllowGroups .
332 .Pp
333 See
334 .Sx PATTERNS
335 in
336 .Xr ssh_config 5
337 for more information on patterns.
338 .It Cm ForceCommand
339 Forces the execution of the command specified by
340 .Cm ForceCommand ,
341 ignoring any command supplied by the client and
342 .Pa ~/.ssh/rc
343 if present.
344 The command is invoked by using the user's login shell with the -c option.
345 This applies to shell, command, or subsystem execution.
346 It is most useful inside a
347 .Cm Match
348 block.
349 The command originally supplied by the client is available in the
350 .Ev SSH_ORIGINAL_COMMAND
351 environment variable.
352 Specifying a command of
353 .Dq internal-sftp
354 will force the use of an in-process sftp server that requires no support
355 files when used with
356 .Cm ChrootDirectory .
357 .It Cm GatewayPorts
358 Specifies whether remote hosts are allowed to connect to ports
359 forwarded for the client.
360 By default,
361 .Xr sshd 8
362 binds remote port forwardings to the loopback address.
363 This prevents other remote hosts from connecting to forwarded ports.
364 .Cm GatewayPorts
365 can be used to specify that sshd
366 should allow remote port forwardings to bind to non-loopback addresses, thus
367 allowing other hosts to connect.
368 The argument may be
369 .Dq no
370 to force remote port forwardings to be available to the local host only,
371 .Dq yes
372 to force remote port forwardings to bind to the wildcard address, or
373 .Dq clientspecified
374 to allow the client to select the address to which the forwarding is bound.
375 The default is
376 .Dq no .
377 .It Cm GSSAPIAuthentication
378 Specifies whether user authentication based on GSSAPI is allowed.
379 The default is
380 .Dq no .
381 Note that this option applies to protocol version 2 only.
382 .It Cm GSSAPICleanupCredentials
383 Specifies whether to automatically destroy the user's credentials cache
384 on logout.
385 The default is
386 .Dq yes .
387 Note that this option applies to protocol version 2 only.
388 .It Cm HostbasedAuthentication
389 Specifies whether rhosts or /etc/hosts.equiv authentication together
390 with successful public key client host authentication is allowed
391 (host-based authentication).
392 This option is similar to
393 .Cm RhostsRSAAuthentication
394 and applies to protocol version 2 only.
395 The default is
396 .Dq no .
397 .It Cm HostbasedUsesNameFromPacketOnly
398 Specifies whether or not the server will attempt to perform a reverse
399 name lookup when matching the name in the
400 .Pa ~/.shosts ,
401 .Pa ~/.rhosts ,
402 and
403 .Pa /etc/hosts.equiv
404 files during
405 .Cm HostbasedAuthentication .
406 A setting of
407 .Dq yes
408 means that
409 .Xr sshd 8
410 uses the name supplied by the client rather than
411 attempting to resolve the name from the TCP connection itself.
412 The default is
413 .Dq no .
414 .It Cm HostCertificate
415 Specifies a file containing a public host certificate.
416 The certificate's public key must match a private host key already specified
417 by
418 .Cm HostKey .
419 The default behaviour of
420 .Xr sshd 8
421 is not to load any certificates.
422 .It Cm HostKey
423 Specifies a file containing a private host key
424 used by SSH.
425 The default is
426 .Pa /etc/ssh/ssh_host_key
427 for protocol version 1, and
428 .Pa /etc/ssh/ssh_host_rsa_key
429 and
430 .Pa /etc/ssh/ssh_host_dsa_key
431 for protocol version 2.
432 Note that
433 .Xr sshd 8
434 will refuse to use a file if it is group/world-accessible.
435 It is possible to have multiple host key files.
436 .Dq rsa1
437 keys are used for version 1 and
438 .Dq dsa
439 or
440 .Dq rsa
441 are used for version 2 of the SSH protocol.
442 .It Cm IgnoreRhosts
443 Specifies that
444 .Pa .rhosts
445 and
446 .Pa .shosts
447 files will not be used in
448 .Cm RhostsRSAAuthentication
449 or
450 .Cm HostbasedAuthentication .
451 .Pp
452 .Pa /etc/hosts.equiv
453 and
454 .Pa /etc/shosts.equiv
455 are still used.
456 The default is
457 .Dq yes .
458 .It Cm IgnoreUserKnownHosts
459 Specifies whether
460 .Xr sshd 8
461 should ignore the user's
462 .Pa ~/.ssh/known_hosts
463 during
464 .Cm RhostsRSAAuthentication
465 or
466 .Cm HostbasedAuthentication .
467 The default is
468 .Dq no .
469 .It Cm KerberosAuthentication
470 Specifies whether the password provided by the user for
471 .Cm PasswordAuthentication
472 will be validated through the Kerberos KDC.
473 To use this option, the server needs a
474 Kerberos servtab which allows the verification of the KDC's identity.
475 The default is
476 .Dq no .
477 .It Cm KerberosGetAFSToken
478 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
479 an AFS token before accessing the user's home directory.
480 The default is
481 .Dq no .
482 .It Cm KerberosOrLocalPasswd
483 If password authentication through Kerberos fails then
484 the password will be validated via any additional local mechanism
485 such as
486 .Pa /etc/passwd .
487 The default is
488 .Dq yes .
489 .It Cm KerberosTicketCleanup
490 Specifies whether to automatically destroy the user's ticket cache
491 file on logout.
492 The default is
493 .Dq yes .
494 .It Cm KeyRegenerationInterval
495 In protocol version 1, the ephemeral server key is automatically regenerated
496 after this many seconds (if it has been used).
497 The purpose of regeneration is to prevent
498 decrypting captured sessions by later breaking into the machine and
499 stealing the keys.
500 The key is never stored anywhere.
501 If the value is 0, the key is never regenerated.
502 The default is 3600 (seconds).
503 .It Cm ListenAddress
504 Specifies the local addresses
505 .Xr sshd 8
506 should listen on.
507 The following forms may be used:
508 .Pp
509 .Bl -item -offset indent -compact
510 .It
511 .Cm ListenAddress
512 .Sm off
513 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
514 .Sm on
515 .It
516 .Cm ListenAddress
517 .Sm off
518 .Ar host No | Ar IPv4_addr No : Ar port
519 .Sm on
520 .It
521 .Cm ListenAddress
522 .Sm off
523 .Oo
524 .Ar host No | Ar IPv6_addr Oc : Ar port
525 .Sm on
526 .El
527 .Pp
528 If
529 .Ar port
530 is not specified,
531 sshd will listen on the address and all prior
532 .Cm Port
533 options specified.
534 The default is to listen on all local addresses.
535 Multiple
536 .Cm ListenAddress
537 options are permitted.
538 Additionally, any
539 .Cm Port
540 options must precede this option for non-port qualified addresses.
541 .It Cm LoginGraceTime
542 The server disconnects after this time if the user has not
543 successfully logged in.
544 If the value is 0, there is no time limit.
545 The default is 120 seconds.
546 .It Cm LogLevel
547 Gives the verbosity level that is used when logging messages from
548 .Xr sshd 8 .
549 The possible values are:
550 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
551 The default is INFO.
552 DEBUG and DEBUG1 are equivalent.
553 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
554 Logging with a DEBUG level violates the privacy of users and is not recommended.
555 .It Cm MACs
556 Specifies the available MAC (message authentication code) algorithms.
557 The MAC algorithm is used in protocol version 2
558 for data integrity protection.
559 Multiple algorithms must be comma-separated.
560 The default is:
561 .Bd -literal -offset indent
562 hmac-md5,hmac-sha1,umac-64@openssh.com,
563 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
564 .Ed
565 .It Cm Match
566 Introduces a conditional block.
567 If all of the criteria on the
568 .Cm Match
569 line are satisfied, the keywords on the following lines override those
570 set in the global section of the config file, until either another
571 .Cm Match
572 line or the end of the file.
573 .Pp
574 The arguments to
575 .Cm Match
576 are one or more criteria-pattern pairs.
577 The available criteria are
578 .Cm User ,
579 .Cm Group ,
580 .Cm Host ,
581 and
582 .Cm Address .
583 The match patterns may consist of single entries or comma-separated
584 lists and may use the wildcard and negation operators described in the
585 .Sx PATTERNS
586 section of
587 .Xr ssh_config 5 .
588 .Pp
589 The patterns in an
590 .Cm Address
591 criteria may additionally contain addresses to match in CIDR
592 address/masklen format, e.g.\&
593 .Dq 192.0.2.0/24
594 or
595 .Dq 3ffe:ffff::/32 .
596 Note that the mask length provided must be consistent with the address -
597 it is an error to specify a mask length that is too long for the address
598 or one with bits set in this host portion of the address.
599 For example,
600 .Dq 192.0.2.0/33
601 and
602 .Dq 192.0.2.0/8
603 respectively.
604 .Pp
605 Only a subset of keywords may be used on the lines following a
606 .Cm Match
607 keyword.
608 Available keywords are
609 .Cm AllowAgentForwarding ,
610 .Cm AllowTcpForwarding ,
611 .Cm Banner ,
612 .Cm ChrootDirectory ,
613 .Cm ForceCommand ,
614 .Cm GatewayPorts ,
615 .Cm GSSAPIAuthentication ,
616 .Cm HostbasedAuthentication ,
617 .Cm KbdInteractiveAuthentication ,
618 .Cm KerberosAuthentication ,
619 .Cm MaxAuthTries ,
620 .Cm MaxSessions ,
621 .Cm PasswordAuthentication ,
622 .Cm PermitEmptyPasswords ,
623 .Cm PermitOpen ,
624 .Cm PermitRootLogin ,
625 .Cm PubkeyAuthentication ,
626 .Cm RhostsRSAAuthentication ,
627 .Cm RSAAuthentication ,
628 .Cm X11DisplayOffset ,
629 .Cm X11Forwarding
630 and
631 .Cm X11UseLocalHost .
632 .It Cm MaxAuthTries
633 Specifies the maximum number of authentication attempts permitted per
634 connection.
635 Once the number of failures reaches half this value,
636 additional failures are logged.
637 The default is 6.
638 .It Cm MaxSessions
639 Specifies the maximum number of open sessions permitted per network connection.
640 The default is 10.
641 .It Cm MaxStartups
642 Specifies the maximum number of concurrent unauthenticated connections to the
643 SSH daemon.
644 Additional connections will be dropped until authentication succeeds or the
645 .Cm LoginGraceTime
646 expires for a connection.
647 The default is 10.
648 .Pp
649 Alternatively, random early drop can be enabled by specifying
650 the three colon separated values
651 .Dq start:rate:full
652 (e.g. "10:30:60").
653 .Xr sshd 8
654 will refuse connection attempts with a probability of
655 .Dq rate/100
656 (30%)
657 if there are currently
658 .Dq start
659 (10)
660 unauthenticated connections.
661 The probability increases linearly and all connection attempts
662 are refused if the number of unauthenticated connections reaches
663 .Dq full
664 (60).
665 .It Cm PasswordAuthentication
666 Specifies whether password authentication is allowed.
667 The default is
668 .Dq yes .
669 .It Cm PermitEmptyPasswords
670 When password authentication is allowed, it specifies whether the
671 server allows login to accounts with empty password strings.
672 The default is
673 .Dq no .
674 .It Cm PermitOpen
675 Specifies the destinations to which TCP port forwarding is permitted.
676 The forwarding specification must be one of the following forms:
677 .Pp
678 .Bl -item -offset indent -compact
679 .It
680 .Cm PermitOpen
681 .Sm off
682 .Ar host : port
683 .Sm on
684 .It
685 .Cm PermitOpen
686 .Sm off
687 .Ar IPv4_addr : port
688 .Sm on
689 .It
690 .Cm PermitOpen
691 .Sm off
692 .Ar \&[ IPv6_addr \&] : port
693 .Sm on
694 .El
695 .Pp
696 Multiple forwards may be specified by separating them with whitespace.
697 An argument of
698 .Dq any
699 can be used to remove all restrictions and permit any forwarding requests.
700 By default all port forwarding requests are permitted.
701 .It Cm PermitRootLogin
702 Specifies whether root can log in using
703 .Xr ssh 1 .
704 The argument must be
705 .Dq yes ,
706 .Dq without-password ,
707 .Dq forced-commands-only ,
708 or
709 .Dq no .
710 The default is
711 .Dq yes .
712 .Pp
713 If this option is set to
714 .Dq without-password ,
715 password authentication is disabled for root.
716 .Pp
717 If this option is set to
718 .Dq forced-commands-only ,
719 root login with public key authentication will be allowed,
720 but only if the
721 .Ar command
722 option has been specified
723 (which may be useful for taking remote backups even if root login is
724 normally not allowed).
725 All other authentication methods are disabled for root.
726 .Pp
727 If this option is set to
728 .Dq no ,
729 root is not allowed to log in.
730 .It Cm PermitTunnel
731 Specifies whether
732 .Xr tun 4
733 device forwarding is allowed.
734 The argument must be
735 .Dq yes ,
736 .Dq point-to-point
737 (layer 3),
738 .Dq ethernet
739 (layer 2), or
740 .Dq no .
741 Specifying
742 .Dq yes
743 permits both
744 .Dq point-to-point
745 and
746 .Dq ethernet .
747 The default is
748 .Dq no .
749 .It Cm PermitUserEnvironment
750 Specifies whether
751 .Pa ~/.ssh/environment
752 and
753 .Cm environment=
754 options in
755 .Pa ~/.ssh/authorized_keys
756 are processed by
757 .Xr sshd 8 .
758 The default is
759 .Dq no .
760 Enabling environment processing may enable users to bypass access
761 restrictions in some configurations using mechanisms such as
762 .Ev LD_PRELOAD .
763 .It Cm PidFile
764 Specifies the file that contains the process ID of the
765 SSH daemon.
766 The default is
767 .Pa /var/run/sshd.pid .
768 .It Cm Port
769 Specifies the port number that
770 .Xr sshd 8
771 listens on.
772 The default is 22.
773 Multiple options of this type are permitted.
774 See also
775 .Cm ListenAddress .
776 .It Cm PrintLastLog
777 Specifies whether
778 .Xr sshd 8
779 should print the date and time of the last user login when a user logs
780 in interactively.
781 The default is
782 .Dq yes .
783 .It Cm PrintMotd
784 Specifies whether
785 .Xr sshd 8
786 should print
787 .Pa /etc/motd
788 when a user logs in interactively.
789 (On some systems it is also printed by the shell,
790 .Pa /etc/profile ,
791 or equivalent.)
792 The default is
793 .Dq yes .
794 .It Cm Protocol
795 Specifies the protocol versions
796 .Xr sshd 8
797 supports.
798 The possible values are
799 .Sq 1
800 and
801 .Sq 2 .
802 Multiple versions must be comma-separated.
803 The default is
804 .Sq 2 .
805 Note that the order of the protocol list does not indicate preference,
806 because the client selects among multiple protocol versions offered
807 by the server.
808 Specifying
809 .Dq 2,1
810 is identical to
811 .Dq 1,2 .
812 .It Cm PubkeyAuthentication
813 Specifies whether public key authentication is allowed.
814 The default is
815 .Dq yes .
816 Note that this option applies to protocol version 2 only.
817 .It Cm RevokedKeys
818 Specifies a list of revoked public keys.
819 Keys listed in this file will be refused for public key authentication.
820 Note that if this file is not readable, then public key authentication will
821 be refused for all users.
822 .It Cm RhostsRSAAuthentication
823 Specifies whether rhosts or /etc/hosts.equiv authentication together
824 with successful RSA host authentication is allowed.
825 The default is
826 .Dq no .
827 This option applies to protocol version 1 only.
828 .It Cm RSAAuthentication
829 Specifies whether pure RSA authentication is allowed.
830 The default is
831 .Dq yes .
832 This option applies to protocol version 1 only.
833 .It Cm ServerKeyBits
834 Defines the number of bits in the ephemeral protocol version 1 server key.
835 The minimum value is 512, and the default is 1024.
836 .It Cm StrictModes
837 Specifies whether
838 .Xr sshd 8
839 should check file modes and ownership of the
840 user's files and home directory before accepting login.
841 This is normally desirable because novices sometimes accidentally leave their
842 directory or files world-writable.
843 The default is
844 .Dq yes .
845 Note that this does not apply to
846 .Cm ChrootDirectory ,
847 whose permissions and ownership are checked unconditionally.
848 .It Cm Subsystem
849 Configures an external subsystem (e.g. file transfer daemon).
850 Arguments should be a subsystem name and a command (with optional arguments)
851 to execute upon subsystem request.
852 .Pp
853 The command
854 .Xr sftp-server 8
855 implements the
856 .Dq sftp
857 file transfer subsystem.
858 .Pp
859 Alternately the name
860 .Dq internal-sftp
861 implements an in-process
862 .Dq sftp
863 server.
864 This may simplify configurations using
865 .Cm ChrootDirectory
866 to force a different filesystem root on clients.
867 .Pp
868 By default no subsystems are defined.
869 Note that this option applies to protocol version 2 only.
870 .It Cm SyslogFacility
871 Gives the facility code that is used when logging messages from
872 .Xr sshd 8 .
873 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
874 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
875 The default is AUTH.
876 .It Cm TCPKeepAlive
877 Specifies whether the system should send TCP keepalive messages to the
878 other side.
879 If they are sent, death of the connection or crash of one
880 of the machines will be properly noticed.
881 However, this means that
882 connections will die if the route is down temporarily, and some people
883 find it annoying.
884 On the other hand, if TCP keepalives are not sent,
885 sessions may hang indefinitely on the server, leaving
886 .Dq ghost
887 users and consuming server resources.
888 .Pp
889 The default is
890 .Dq yes
891 (to send TCP keepalive messages), and the server will notice
892 if the network goes down or the client host crashes.
893 This avoids infinitely hanging sessions.
894 .Pp
895 To disable TCP keepalive messages, the value should be set to
896 .Dq no .
897 .It Cm TrustedUserCAKeys
898 Specifies a file containing public keys of certificate authorities that are
899 trusted to sign user certificates for authentication.
900 Keys are listed one per line; empty lines and comments starting with
901 .Ql #
902 are allowed.
903 If a certificate is presented for authentication and has its signing CA key
904 listed in this file, then it may be used for authentication for any user
905 listed in the certificate's principals list.
906 Note that certificates that lack a list of principals will not be permitted
907 for authentication using
908 .Cm TrustedUserCAKeys .
909 For more details on certificates, see the
910 .Sx CERTIFICATES
911 section in
912 .Xr ssh-keygen 1 .
913 .It Cm UseDNS
914 Specifies whether
915 .Xr sshd 8
916 should look up the remote host name and check that
917 the resolved host name for the remote IP address maps back to the
918 very same IP address.
919 The default is
920 .Dq yes .
921 .It Cm UseLogin
922 Specifies whether
923 .Xr login 1
924 is used for interactive login sessions.
925 The default is
926 .Dq no .
927 Note that
928 .Xr login 1
929 is never used for remote command execution.
930 Note also, that if this is enabled,
931 .Cm X11Forwarding
932 will be disabled because
933 .Xr login 1
934 does not know how to handle
935 .Xr xauth 1
936 cookies.
937 If
938 .Cm UsePrivilegeSeparation
939 is specified, it will be disabled after authentication.
940 .It Cm UsePAM
941 Enables the Pluggable Authentication Module interface.
942 If set to
943 .Dq yes
944 this will enable PAM authentication using
945 .Cm ChallengeResponseAuthentication
946 and
947 .Cm PasswordAuthentication
948 in addition to PAM account and session module processing for all
949 authentication types.
950 .Pp
951 Because PAM challenge-response authentication usually serves an equivalent
952 role to password authentication, you should disable either
953 .Cm PasswordAuthentication
954 or
955 .Cm ChallengeResponseAuthentication.
956 .Pp
957 If
958 .Cm UsePAM
959 is enabled, you will not be able to run
960 .Xr sshd 8
961 as a non-root user.
962 The default is
963 .Dq no .
964 .It Cm UsePrivilegeSeparation
965 Specifies whether
966 .Xr sshd 8
967 separates privileges by creating an unprivileged child process
968 to deal with incoming network traffic.
969 After successful authentication, another process will be created that has
970 the privilege of the authenticated user.
971 The goal of privilege separation is to prevent privilege
972 escalation by containing any corruption within the unprivileged processes.
973 The default is
974 .Dq yes .
975 .It Cm X11DisplayOffset
976 Specifies the first display number available for
977 .Xr sshd 8 Ns 's
978 X11 forwarding.
979 This prevents sshd from interfering with real X11 servers.
980 The default is 10.
981 .It Cm X11Forwarding
982 Specifies whether X11 forwarding is permitted.
983 The argument must be
984 .Dq yes
985 or
986 .Dq no .
987 The default is
988 .Dq no .
989 .Pp
990 When X11 forwarding is enabled, there may be additional exposure to
991 the server and to client displays if the
992 .Xr sshd 8
993 proxy display is configured to listen on the wildcard address (see
994 .Cm X11UseLocalhost
995 below), though this is not the default.
996 Additionally, the authentication spoofing and authentication data
997 verification and substitution occur on the client side.
998 The security risk of using X11 forwarding is that the client's X11
999 display server may be exposed to attack when the SSH client requests
1000 forwarding (see the warnings for
1001 .Cm ForwardX11
1002 in
1003 .Xr ssh_config 5 ) .
1004 A system administrator may have a stance in which they want to
1005 protect clients that may expose themselves to attack by unwittingly
1006 requesting X11 forwarding, which can warrant a
1007 .Dq no
1008 setting.
1009 .Pp
1010 Note that disabling X11 forwarding does not prevent users from
1011 forwarding X11 traffic, as users can always install their own forwarders.
1012 X11 forwarding is automatically disabled if
1013 .Cm UseLogin
1014 is enabled.
1015 .It Cm X11UseLocalhost
1016 Specifies whether
1017 .Xr sshd 8
1018 should bind the X11 forwarding server to the loopback address or to
1019 the wildcard address.
1020 By default,
1021 sshd binds the forwarding server to the loopback address and sets the
1022 hostname part of the
1023 .Ev DISPLAY
1024 environment variable to
1025 .Dq localhost .
1026 This prevents remote hosts from connecting to the proxy display.
1027 However, some older X11 clients may not function with this
1028 configuration.
1029 .Cm X11UseLocalhost
1030 may be set to
1031 .Dq no
1032 to specify that the forwarding server should be bound to the wildcard
1033 address.
1034 The argument must be
1035 .Dq yes
1036 or
1037 .Dq no .
1038 The default is
1039 .Dq yes .
1040 .It Cm XAuthLocation
1041 Specifies the full pathname of the
1042 .Xr xauth 1
1043 program.
1044 The default is
1045 .Pa /usr/X11R6/bin/xauth .
1046 .El
1047 .Sh TIME FORMATS
1048 .Xr sshd 8
1049 command-line arguments and configuration file options that specify time
1050 may be expressed using a sequence of the form:
1051 .Sm off
1052 .Ar time Op Ar qualifier ,
1053 .Sm on
1054 where
1055 .Ar time
1056 is a positive integer value and
1057 .Ar qualifier
1058 is one of the following:
1059 .Pp
1060 .Bl -tag -width Ds -compact -offset indent
1061 .It Aq Cm none
1062 seconds
1063 .It Cm s | Cm S
1064 seconds
1065 .It Cm m | Cm M
1066 minutes
1067 .It Cm h | Cm H
1068 hours
1069 .It Cm d | Cm D
1070 days
1071 .It Cm w | Cm W
1072 weeks
1073 .El
1074 .Pp
1075 Each member of the sequence is added together to calculate
1076 the total time value.
1077 .Pp
1078 Time format examples:
1079 .Pp
1080 .Bl -tag -width Ds -compact -offset indent
1081 .It 600
1082 600 seconds (10 minutes)
1083 .It 10m
1084 10 minutes
1085 .It 1h30m
1086 1 hour 30 minutes (90 minutes)
1087 .El
1088 .Sh FILES
1089 .Bl -tag -width Ds
1090 .It Pa /etc/ssh/sshd_config
1091 Contains configuration data for
1092 .Xr sshd 8 .
1093 This file should be writable by root only, but it is recommended
1094 (though not necessary) that it be world-readable.
1095 .El
1096 .Sh SEE ALSO
1097 .Xr sshd 8
1098 .Sh AUTHORS
1099 OpenSSH is a derivative of the original and free
1100 ssh 1.2.12 release by Tatu Ylonen.
1101 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1102 Theo de Raadt and Dug Song
1103 removed many bugs, re-added newer features and
1104 created OpenSSH.
1105 Markus Friedl contributed the support for SSH
1106 protocol versions 1.5 and 2.0.
1107 Niels Provos and Markus Friedl contributed support
1108 for privilege separation.