| blob | 588aed56e1625cdd4d1753edf5ef71ee250be588 |
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 (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 a path to
186 .Xr chroot 2
187 to after authentication.
188 This path, and all its components, 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 path 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 HostKey
415 Specifies a file containing a private host key
416 used by SSH.
417 The default is
418 .Pa /etc/ssh/ssh_host_key
419 for protocol version 1, and
420 .Pa /etc/ssh/ssh_host_rsa_key
421 and
422 .Pa /etc/ssh/ssh_host_dsa_key
423 for protocol version 2.
424 Note that
425 .Xr sshd 8
426 will refuse to use a file if it is group/world-accessible.
427 It is possible to have multiple host key files.
428 .Dq rsa1
429 keys are used for version 1 and
430 .Dq dsa
431 or
432 .Dq rsa
433 are used for version 2 of the SSH protocol.
434 .It Cm IgnoreRhosts
435 Specifies that
436 .Pa .rhosts
437 and
438 .Pa .shosts
439 files will not be used in
440 .Cm RhostsRSAAuthentication
441 or
442 .Cm HostbasedAuthentication .
443 .Pp
444 .Pa /etc/hosts.equiv
445 and
446 .Pa /etc/shosts.equiv
447 are still used.
448 The default is
449 .Dq yes .
450 .It Cm IgnoreUserKnownHosts
451 Specifies whether
452 .Xr sshd 8
453 should ignore the user's
454 .Pa ~/.ssh/known_hosts
455 during
456 .Cm RhostsRSAAuthentication
457 or
458 .Cm HostbasedAuthentication .
459 The default is
460 .Dq no .
461 .It Cm KerberosAuthentication
462 Specifies whether the password provided by the user for
463 .Cm PasswordAuthentication
464 will be validated through the Kerberos KDC.
465 To use this option, the server needs a
466 Kerberos servtab which allows the verification of the KDC's identity.
467 The default is
468 .Dq no .
469 .It Cm KerberosGetAFSToken
470 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
471 an AFS token before accessing the user's home directory.
472 The default is
473 .Dq no .
474 .It Cm KerberosOrLocalPasswd
475 If password authentication through Kerberos fails then
476 the password will be validated via any additional local mechanism
477 such as
478 .Pa /etc/passwd .
479 The default is
480 .Dq yes .
481 .It Cm KerberosTicketCleanup
482 Specifies whether to automatically destroy the user's ticket cache
483 file on logout.
484 The default is
485 .Dq yes .
486 .It Cm KeyRegenerationInterval
487 In protocol version 1, the ephemeral server key is automatically regenerated
488 after this many seconds (if it has been used).
489 The purpose of regeneration is to prevent
490 decrypting captured sessions by later breaking into the machine and
491 stealing the keys.
492 The key is never stored anywhere.
493 If the value is 0, the key is never regenerated.
494 The default is 3600 (seconds).
495 .It Cm ListenAddress
496 Specifies the local addresses
497 .Xr sshd 8
498 should listen on.
499 The following forms may be used:
500 .Pp
501 .Bl -item -offset indent -compact
502 .It
503 .Cm ListenAddress
504 .Sm off
505 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
506 .Sm on
507 .It
508 .Cm ListenAddress
509 .Sm off
510 .Ar host No | Ar IPv4_addr No : Ar port
511 .Sm on
512 .It
513 .Cm ListenAddress
514 .Sm off
515 .Oo
516 .Ar host No | Ar IPv6_addr Oc : Ar port
517 .Sm on
518 .El
519 .Pp
520 If
521 .Ar port
522 is not specified,
523 sshd will listen on the address and all prior
524 .Cm Port
525 options specified.
526 The default is to listen on all local addresses.
527 Multiple
528 .Cm ListenAddress
529 options are permitted.
530 Additionally, any
531 .Cm Port
532 options must precede this option for non-port qualified addresses.
533 .It Cm LoginGraceTime
534 The server disconnects after this time if the user has not
535 successfully logged in.
536 If the value is 0, there is no time limit.
537 The default is 120 seconds.
538 .It Cm LogLevel
539 Gives the verbosity level that is used when logging messages from
540 .Xr sshd 8 .
541 The possible values are:
542 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
543 The default is INFO.
544 DEBUG and DEBUG1 are equivalent.
545 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
546 Logging with a DEBUG level violates the privacy of users and is not recommended.
547 .It Cm MACs
548 Specifies the available MAC (message authentication code) algorithms.
549 The MAC algorithm is used in protocol version 2
550 for data integrity protection.
551 Multiple algorithms must be comma-separated.
552 The default is:
553 .Bd -literal -offset indent
554 hmac-md5,hmac-sha1,umac-64@openssh.com,
555 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
556 .Ed
557 .It Cm Match
558 Introduces a conditional block.
559 If all of the criteria on the
560 .Cm Match
561 line are satisfied, the keywords on the following lines override those
562 set in the global section of the config file, until either another
563 .Cm Match
564 line or the end of the file.
565 .Pp
566 The arguments to
567 .Cm Match
568 are one or more criteria-pattern pairs.
569 The available criteria are
570 .Cm User ,
571 .Cm Group ,
572 .Cm Host ,
573 and
574 .Cm Address .
575 The match patterns may consist of single entries or comma-separated
576 lists and may use the wildcard and negation operators described in the
577 .Sx PATTERNS
578 section of
579 .Xr ssh_config 5 .
580 .Pp
581 The patterns in an
582 .Cm Address
583 criteria may additionally contain addresses to match in CIDR
584 address/masklen format, e.g.\&
585 .Dq 192.0.2.0/24
586 or
587 .Dq 3ffe:ffff::/32 .
588 Note that the mask length provided must be consistent with the address -
589 it is an error to specify a mask length that is too long for the address
590 or one with bits set in this host portion of the address.
591 For example,
592 .Dq 192.0.2.0/33
593 and
594 .Dq 192.0.2.0/8
595 respectively.
596 .Pp
597 Only a subset of keywords may be used on the lines following a
598 .Cm Match
599 keyword.
600 Available keywords are
601 .Cm AllowAgentForwarding ,
602 .Cm AllowTcpForwarding ,
603 .Cm Banner ,
604 .Cm ChrootDirectory ,
605 .Cm ForceCommand ,
606 .Cm GatewayPorts ,
607 .Cm GSSAPIAuthentication ,
608 .Cm HostbasedAuthentication ,
609 .Cm KbdInteractiveAuthentication ,
610 .Cm KerberosAuthentication ,
611 .Cm MaxAuthTries ,
612 .Cm MaxSessions ,
613 .Cm PasswordAuthentication ,
614 .Cm PermitEmptyPasswords ,
615 .Cm PermitOpen ,
616 .Cm PermitRootLogin ,
617 .Cm RhostsRSAAuthentication ,
618 .Cm RSAAuthentication ,
619 .Cm X11DisplayOffset ,
620 .Cm X11Forwarding
621 and
622 .Cm X11UseLocalHost .
623 .It Cm MaxAuthTries
624 Specifies the maximum number of authentication attempts permitted per
625 connection.
626 Once the number of failures reaches half this value,
627 additional failures are logged.
628 The default is 6.
629 .It Cm MaxSessions
630 Specifies the maximum number of open sessions permitted per network connection.
631 The default is 10.
632 .It Cm MaxStartups
633 Specifies the maximum number of concurrent unauthenticated connections to the
634 SSH daemon.
635 Additional connections will be dropped until authentication succeeds or the
636 .Cm LoginGraceTime
637 expires for a connection.
638 The default is 10.
639 .Pp
640 Alternatively, random early drop can be enabled by specifying
641 the three colon separated values
642 .Dq start:rate:full
643 (e.g. "10:30:60").
644 .Xr sshd 8
645 will refuse connection attempts with a probability of
646 .Dq rate/100
647 (30%)
648 if there are currently
649 .Dq start
650 (10)
651 unauthenticated connections.
652 The probability increases linearly and all connection attempts
653 are refused if the number of unauthenticated connections reaches
654 .Dq full
655 (60).
656 .It Cm PasswordAuthentication
657 Specifies whether password authentication is allowed.
658 The default is
659 .Dq yes .
660 .It Cm PermitEmptyPasswords
661 When password authentication is allowed, it specifies whether the
662 server allows login to accounts with empty password strings.
663 The default is
664 .Dq no .
665 .It Cm PermitOpen
666 Specifies the destinations to which TCP port forwarding is permitted.
667 The forwarding specification must be one of the following forms:
668 .Pp
669 .Bl -item -offset indent -compact
670 .It
671 .Cm PermitOpen
672 .Sm off
673 .Ar host : port
674 .Sm on
675 .It
676 .Cm PermitOpen
677 .Sm off
678 .Ar IPv4_addr : port
679 .Sm on
680 .It
681 .Cm PermitOpen
682 .Sm off
683 .Ar \&[ IPv6_addr \&] : port
684 .Sm on
685 .El
686 .Pp
687 Multiple forwards may be specified by separating them with whitespace.
688 An argument of
689 .Dq any
690 can be used to remove all restrictions and permit any forwarding requests.
691 By default all port forwarding requests are permitted.
692 .It Cm PermitRootLogin
693 Specifies whether root can log in using
694 .Xr ssh 1 .
695 The argument must be
696 .Dq yes ,
697 .Dq without-password ,
698 .Dq forced-commands-only ,
699 or
700 .Dq no .
701 The default is
702 .Dq yes .
703 .Pp
704 If this option is set to
705 .Dq without-password ,
706 password authentication is disabled for root.
707 .Pp
708 If this option is set to
709 .Dq forced-commands-only ,
710 root login with public key authentication will be allowed,
711 but only if the
712 .Ar command
713 option has been specified
714 (which may be useful for taking remote backups even if root login is
715 normally not allowed).
716 All other authentication methods are disabled for root.
717 .Pp
718 If this option is set to
719 .Dq no ,
720 root is not allowed to log in.
721 .It Cm PermitTunnel
722 Specifies whether
723 .Xr tun 4
724 device forwarding is allowed.
725 The argument must be
726 .Dq yes ,
727 .Dq point-to-point
728 (layer 3),
729 .Dq ethernet
730 (layer 2), or
731 .Dq no .
732 Specifying
733 .Dq yes
734 permits both
735 .Dq point-to-point
736 and
737 .Dq ethernet .
738 The default is
739 .Dq no .
740 .It Cm PermitUserEnvironment
741 Specifies whether
742 .Pa ~/.ssh/environment
743 and
744 .Cm environment=
745 options in
746 .Pa ~/.ssh/authorized_keys
747 are processed by
748 .Xr sshd 8 .
749 The default is
750 .Dq no .
751 Enabling environment processing may enable users to bypass access
752 restrictions in some configurations using mechanisms such as
753 .Ev LD_PRELOAD .
754 .It Cm PidFile
755 Specifies the file that contains the process ID of the
756 SSH daemon.
757 The default is
758 .Pa /var/run/sshd.pid .
759 .It Cm Port
760 Specifies the port number that
761 .Xr sshd 8
762 listens on.
763 The default is 22.
764 Multiple options of this type are permitted.
765 See also
766 .Cm ListenAddress .
767 .It Cm PrintLastLog
768 Specifies whether
769 .Xr sshd 8
770 should print the date and time of the last user login when a user logs
771 in interactively.
772 The default is
773 .Dq yes .
774 .It Cm PrintMotd
775 Specifies whether
776 .Xr sshd 8
777 should print
778 .Pa /etc/motd
779 when a user logs in interactively.
780 (On some systems it is also printed by the shell,
781 .Pa /etc/profile ,
782 or equivalent.)
783 The default is
784 .Dq yes .
785 .It Cm Protocol
786 Specifies the protocol versions
787 .Xr sshd 8
788 supports.
789 The possible values are
790 .Sq 1
791 and
792 .Sq 2 .
793 Multiple versions must be comma-separated.
794 The default is
795 .Dq 2,1 .
796 Note that the order of the protocol list does not indicate preference,
797 because the client selects among multiple protocol versions offered
798 by the server.
799 Specifying
800 .Dq 2,1
801 is identical to
802 .Dq 1,2 .
803 .It Cm PubkeyAuthentication
804 Specifies whether public key authentication is allowed.
805 The default is
806 .Dq yes .
807 Note that this option applies to protocol version 2 only.
808 .It Cm RhostsRSAAuthentication
809 Specifies whether rhosts or /etc/hosts.equiv authentication together
810 with successful RSA host authentication is allowed.
811 The default is
812 .Dq no .
813 This option applies to protocol version 1 only.
814 .It Cm RSAAuthentication
815 Specifies whether pure RSA authentication is allowed.
816 The default is
817 .Dq yes .
818 This option applies to protocol version 1 only.
819 .It Cm ServerKeyBits
820 Defines the number of bits in the ephemeral protocol version 1 server key.
821 The minimum value is 512, and the default is 1024.
822 .It Cm StrictModes
823 Specifies whether
824 .Xr sshd 8
825 should check file modes and ownership of the
826 user's files and home directory before accepting login.
827 This is normally desirable because novices sometimes accidentally leave their
828 directory or files world-writable.
829 The default is
830 .Dq yes .
831 .It Cm Subsystem
832 Configures an external subsystem (e.g. file transfer daemon).
833 Arguments should be a subsystem name and a command (with optional arguments)
834 to execute upon subsystem request.
835 .Pp
836 The command
837 .Xr sftp-server 8
838 implements the
839 .Dq sftp
840 file transfer subsystem.
841 .Pp
842 Alternately the name
843 .Dq internal-sftp
844 implements an in-process
845 .Dq sftp
846 server.
847 This may simplify configurations using
848 .Cm ChrootDirectory
849 to force a different filesystem root on clients.
850 .Pp
851 By default no subsystems are defined.
852 Note that this option applies to protocol version 2 only.
853 .It Cm SyslogFacility
854 Gives the facility code that is used when logging messages from
855 .Xr sshd 8 .
856 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
857 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
858 The default is AUTH.
859 .It Cm TCPKeepAlive
860 Specifies whether the system should send TCP keepalive messages to the
861 other side.
862 If they are sent, death of the connection or crash of one
863 of the machines will be properly noticed.
864 However, this means that
865 connections will die if the route is down temporarily, and some people
866 find it annoying.
867 On the other hand, if TCP keepalives are not sent,
868 sessions may hang indefinitely on the server, leaving
869 .Dq ghost
870 users and consuming server resources.
871 .Pp
872 The default is
873 .Dq yes
874 (to send TCP keepalive messages), and the server will notice
875 if the network goes down or the client host crashes.
876 This avoids infinitely hanging sessions.
877 .Pp
878 To disable TCP keepalive messages, the value should be set to
879 .Dq no .
880 .It Cm UseDNS
881 Specifies whether
882 .Xr sshd 8
883 should look up the remote host name and check that
884 the resolved host name for the remote IP address maps back to the
885 very same IP address.
886 The default is
887 .Dq yes .
888 .It Cm UseLogin
889 Specifies whether
890 .Xr login 1
891 is used for interactive login sessions.
892 The default is
893 .Dq no .
894 Note that
895 .Xr login 1
896 is never used for remote command execution.
897 Note also, that if this is enabled,
898 .Cm X11Forwarding
899 will be disabled because
900 .Xr login 1
901 does not know how to handle
902 .Xr xauth 1
903 cookies.
904 If
905 .Cm UsePrivilegeSeparation
906 is specified, it will be disabled after authentication.
907 .It Cm UsePAM
908 Enables the Pluggable Authentication Module interface.
909 If set to
910 .Dq yes
911 this will enable PAM authentication using
912 .Cm ChallengeResponseAuthentication
913 and
914 .Cm PasswordAuthentication
915 in addition to PAM account and session module processing for all
916 authentication types.
917 .Pp
918 Because PAM challenge-response authentication usually serves an equivalent
919 role to password authentication, you should disable either
920 .Cm PasswordAuthentication
921 or
922 .Cm ChallengeResponseAuthentication.
923 .Pp
924 If
925 .Cm UsePAM
926 is enabled, you will not be able to run
927 .Xr sshd 8
928 as a non-root user.
929 The default is
930 .Dq no .
931 .It Cm UsePrivilegeSeparation
932 Specifies whether
933 .Xr sshd 8
934 separates privileges by creating an unprivileged child process
935 to deal with incoming network traffic.
936 After successful authentication, another process will be created that has
937 the privilege of the authenticated user.
938 The goal of privilege separation is to prevent privilege
939 escalation by containing any corruption within the unprivileged processes.
940 The default is
941 .Dq yes .
942 .It Cm X11DisplayOffset
943 Specifies the first display number available for
944 .Xr sshd 8 Ns 's
945 X11 forwarding.
946 This prevents sshd from interfering with real X11 servers.
947 The default is 10.
948 .It Cm X11Forwarding
949 Specifies whether X11 forwarding is permitted.
950 The argument must be
951 .Dq yes
952 or
953 .Dq no .
954 The default is
955 .Dq no .
956 .Pp
957 When X11 forwarding is enabled, there may be additional exposure to
958 the server and to client displays if the
959 .Xr sshd 8
960 proxy display is configured to listen on the wildcard address (see
961 .Cm X11UseLocalhost
962 below), though this is not the default.
963 Additionally, the authentication spoofing and authentication data
964 verification and substitution occur on the client side.
965 The security risk of using X11 forwarding is that the client's X11
966 display server may be exposed to attack when the SSH client requests
967 forwarding (see the warnings for
968 .Cm ForwardX11
969 in
970 .Xr ssh_config 5 ) .
971 A system administrator may have a stance in which they want to
972 protect clients that may expose themselves to attack by unwittingly
973 requesting X11 forwarding, which can warrant a
974 .Dq no
975 setting.
976 .Pp
977 Note that disabling X11 forwarding does not prevent users from
978 forwarding X11 traffic, as users can always install their own forwarders.
979 X11 forwarding is automatically disabled if
980 .Cm UseLogin
981 is enabled.
982 .It Cm X11UseLocalhost
983 Specifies whether
984 .Xr sshd 8
985 should bind the X11 forwarding server to the loopback address or to
986 the wildcard address.
987 By default,
988 sshd binds the forwarding server to the loopback address and sets the
989 hostname part of the
990 .Ev DISPLAY
991 environment variable to
992 .Dq localhost .
993 This prevents remote hosts from connecting to the proxy display.
994 However, some older X11 clients may not function with this
995 configuration.
996 .Cm X11UseLocalhost
997 may be set to
998 .Dq no
999 to specify that the forwarding server should be bound to the wildcard
1000 address.
1001 The argument must be
1002 .Dq yes
1003 or
1004 .Dq no .
1005 The default is
1006 .Dq yes .
1007 .It Cm XAuthLocation
1008 Specifies the full pathname of the
1009 .Xr xauth 1
1010 program.
1011 The default is
1012 .Pa /usr/X11R6/bin/xauth .
1013 .El
1014 .Sh TIME FORMATS
1015 .Xr sshd 8
1016 command-line arguments and configuration file options that specify time
1017 may be expressed using a sequence of the form:
1018 .Sm off
1019 .Ar time Op Ar qualifier ,
1020 .Sm on
1021 where
1022 .Ar time
1023 is a positive integer value and
1024 .Ar qualifier
1025 is one of the following:
1026 .Pp
1027 .Bl -tag -width Ds -compact -offset indent
1028 .It Aq Cm none
1029 seconds
1030 .It Cm s | Cm S
1031 seconds
1032 .It Cm m | Cm M
1033 minutes
1034 .It Cm h | Cm H
1035 hours
1036 .It Cm d | Cm D
1037 days
1038 .It Cm w | Cm W
1039 weeks
1040 .El
1041 .Pp
1042 Each member of the sequence is added together to calculate
1043 the total time value.
1044 .Pp
1045 Time format examples:
1046 .Pp
1047 .Bl -tag -width Ds -compact -offset indent
1048 .It 600
1049 600 seconds (10 minutes)
1050 .It 10m
1051 10 minutes
1052 .It 1h30m
1053 1 hour 30 minutes (90 minutes)
1054 .El
1055 .Sh FILES
1056 .Bl -tag -width Ds
1057 .It Pa /etc/ssh/sshd_config
1058 Contains configuration data for
1059 .Xr sshd 8 .
1060 This file should be writable by root only, but it is recommended
1061 (though not necessary) that it be world-readable.
1062 .El
1063 .Sh SEE ALSO
1064 .Xr sshd 8
1065 .Sh AUTHORS
1066 OpenSSH is a derivative of the original and free
1067 ssh 1.2.12 release by Tatu Ylonen.
1068 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1069 Theo de Raadt and Dug Song
1070 removed many bugs, re-added newer features and
1071 created OpenSSH.
1072 Markus Friedl contributed the support for SSH
1073 protocol versions 1.5 and 2.0.
1074 Niels Provos and Markus Friedl contributed support
1075 for privilege separation.