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1 .\" $NetBSD: sshd_config.5,v 1.1.1.2 2009/12/27 01:07:17 christos Exp $
2 .\" -*- nroff -*-
3 .\"
4 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
5 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
6 .\" All rights reserved
7 .\"
8 .\" As far as I am concerned, the code I have written for this software
9 .\" can be used freely for any purpose. Any derived versions of this
10 .\" software must be clearly marked as such, and if the derived work is
11 .\" incompatible with the protocol description in the RFC file, it must be
12 .\" called by a name other than "ssh" or "Secure Shell".
13 .\"
14 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
15 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
16 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
17 .\"
18 .\" Redistribution and use in source and binary forms, with or without
19 .\" modification, are permitted provided that the following conditions
20 .\" are met:
21 .\" 1. Redistributions of source code must retain the above copyright
22 .\" notice, this list of conditions and the following disclaimer.
23 .\" 2. Redistributions in binary form must reproduce the above copyright
24 .\" notice, this list of conditions and the following disclaimer in the
25 .\" documentation and/or other materials provided with the distribution.
26 .\"
27 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
28 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
29 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
30 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
31 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
32 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
33 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
34 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
35 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
36 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
37 .\"
38 .\" $OpenBSD: sshd_config.5,v 1.106 2009/04/21 15:13:17 stevesk Exp $
39 .Dd April 21 2009
40 .Dt SSHD_CONFIG 5
41 .Os
42 .Sh NAME
43 .Nm sshd_config
44 .Nd OpenSSH SSH daemon configuration file
45 .Sh SYNOPSIS
46 .Nm /etc/ssh/sshd_config
47 .Sh DESCRIPTION
48 .Xr sshd 8
49 reads configuration data from
50 .Pa /etc/ssh/sshd_config
51 (or the file specified with
52 .Fl f
53 on the command line).
54 The file contains keyword-argument pairs, one per line.
55 Lines starting with
56 .Ql #
57 and empty lines are interpreted as comments.
58 Arguments may optionally be enclosed in double quotes
59 .Pq \&"
60 in order to represent arguments containing spaces.
61 .Pp
62 The possible
63 keywords and their meanings are as follows (note that
64 keywords are case-insensitive and arguments are case-sensitive):
65 .Bl -tag -width Ds
66 .It Cm AcceptEnv
67 Specifies what environment variables sent by the client will be copied into
68 the session's
69 .Xr environ 7 .
70 See
71 .Cm SendEnv
72 in
73 .Xr ssh_config 5
74 for how to configure the client.
75 Note that environment passing is only supported for protocol 2.
76 Variables are specified by name, which may contain the wildcard characters
77 .Ql *
78 and
79 .Ql \&? .
80 Multiple environment variables may be separated by whitespace or spread
81 across multiple
82 .Cm AcceptEnv
83 directives.
84 Be warned that some environment variables could be used to bypass restricted
85 user environments.
86 For this reason, care should be taken in the use of this directive.
87 The default is not to accept any environment variables.
88 .It Cm AddressFamily
89 Specifies which address family should be used by
90 .Xr sshd 8 .
91 Valid arguments are
92 .Dq any ,
93 .Dq inet
94 (use IPv4 only), or
95 .Dq inet6
96 (use IPv6 only).
97 The default is
98 .Dq any .
99 .It Cm AllowAgentForwarding
100 Specifies whether
101 .Xr ssh-agent 1
102 forwarding is permitted.
103 The default is
104 .Dq yes .
105 Note that disabling agent forwarding does not improve security
106 unless users are also denied shell access, as they can always install
107 their own forwarders.
108 .It Cm AllowGroups
109 This keyword can be followed by a list of group name patterns, separated
110 by spaces.
111 If specified, login is allowed only for users whose primary
112 group or supplementary group list matches one of the patterns.
113 Only group names are valid; a numerical group ID is not recognized.
114 By default, login is allowed for all groups.
115 The allow/deny directives are processed in the following order:
116 .Cm DenyUsers ,
117 .Cm AllowUsers ,
118 .Cm DenyGroups ,
119 and finally
120 .Cm AllowGroups .
121 .Pp
122 See
123 .Sx PATTERNS
124 in
125 .Xr ssh_config 5
126 for more information on patterns.
127 .It Cm AllowTcpForwarding
128 Specifies whether TCP forwarding is permitted.
129 The default is
130 .Dq yes .
131 Note that disabling TCP forwarding does not improve security unless
132 users are also denied shell access, as they can always install their
133 own forwarders.
134 .It Cm AllowUsers
135 This keyword can be followed by a list of user name patterns, separated
136 by spaces.
137 If specified, login is allowed only for user names that
138 match one of the patterns.
139 Only user names are valid; a numerical user ID is not recognized.
140 By default, login is allowed for all users.
141 If the pattern takes the form USER@HOST then USER and HOST
142 are separately checked, restricting logins to particular
143 users from particular hosts.
144 The allow/deny directives are processed in the following order:
145 .Cm DenyUsers ,
146 .Cm AllowUsers ,
147 .Cm DenyGroups ,
148 and finally
149 .Cm AllowGroups .
150 .Pp
151 See
152 .Sx PATTERNS
153 in
154 .Xr ssh_config 5
155 for more information on patterns.
156 .It Cm AuthorizedKeysFile
157 Specifies the file that contains the public keys that can be used
158 for user authentication.
159 .Cm AuthorizedKeysFile
160 may contain tokens of the form %T which are substituted during connection
161 setup.
162 The following tokens are defined: %% is replaced by a literal '%',
163 %h is replaced by the home directory of the user being authenticated, and
164 %u is replaced by the username of that user.
165 After expansion,
166 .Cm AuthorizedKeysFile
167 is taken to be an absolute path or one relative to the user's home
168 directory.
169 The default is
170 .Dq .ssh/authorized_keys .
171 .It Cm Banner
172 The contents of the specified file are sent to the remote user before
173 authentication is allowed.
174 If the argument is
175 .Dq none
176 then no banner is displayed.
177 This option is only available for protocol version 2.
178 By default, no banner is displayed.
179 .It Cm ChallengeResponseAuthentication
180 Specifies whether challenge-response authentication is allowed.
181 All authentication styles from
182 .Xr login.conf 5
183 are supported.
184 The default is
185 .Dq yes .
186 .It Cm ChrootDirectory
187 Specifies a path to
188 .Xr chroot 2
189 to after authentication.
190 This path, and all its components, must be root-owned directories that are
191 not writable by any other user or group.
192 After the chroot,
193 .Xr sshd 8
194 changes the working directory to the user's home directory.
195 .Pp
196 The path may contain the following tokens that are expanded at runtime once
197 the connecting user has been authenticated: %% is replaced by a literal '%',
198 %h is replaced by the home directory of the user being authenticated, and
199 %u is replaced by the username of that user.
200 .Pp
201 The
202 .Cm ChrootDirectory
203 must contain the necessary files and directories to support the
204 user's session.
205 For an interactive session this requires at least a shell, typically
206 .Xr sh 1 ,
207 and basic
208 .Pa /dev
209 nodes such as
210 .Xr null 4 ,
211 .Xr zero 4 ,
212 .Xr stdin 4 ,
213 .Xr stdout 4 ,
214 .Xr stderr 4 ,
215 .Xr arandom 4
216 and
217 .Xr tty 4
218 devices.
219 For file transfer sessions using
220 .Dq sftp ,
221 no additional configuration of the environment is necessary if the
222 in-process sftp server is used,
223 though sessions which use logging do require
224 .Pa /dev/log
225 inside the chroot directory (see
226 .Xr sftp-server 8
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/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 .It Cm PermitEmptyPasswords
663 When password authentication is allowed, it specifies whether the
664 server allows login to accounts with empty password strings.
665 The default is
666 .Dq no .
667 .It Cm PermitOpen
668 Specifies the destinations to which TCP port forwarding is permitted.
669 The forwarding specification must be one of the following forms:
670 .Pp
671 .Bl -item -offset indent -compact
672 .It
673 .Cm PermitOpen
674 .Sm off
675 .Ar host : port
676 .Sm on
677 .It
678 .Cm PermitOpen
679 .Sm off
680 .Ar IPv4_addr : port
681 .Sm on
682 .It
683 .Cm PermitOpen
684 .Sm off
685 .Ar \&[ IPv6_addr \&] : port
686 .Sm on
687 .El
688 .Pp
689 Multiple forwards may be specified by separating them with whitespace.
690 An argument of
691 .Dq any
692 can be used to remove all restrictions and permit any forwarding requests.
693 By default all port forwarding requests are permitted.
694 .It Cm PermitRootLogin
695 Specifies whether root can log in using
696 .Xr ssh 1 .
697 The argument must be
698 .Dq yes ,
699 .Dq without-password ,
700 .Dq forced-commands-only ,
701 or
702 .Dq no .
703 The default is
704 .Dq no .
705 .Pp
706 If this option is set to
707 .Dq without-password ,
708 password authentication is disabled for root.
709 .Pp
710 If this option is set to
711 .Dq forced-commands-only ,
712 root login with public key authentication will be allowed,
713 but only if the
714 .Ar command
715 option has been specified
716 (which may be useful for taking remote backups even if root login is
717 normally not allowed).
718 All other authentication methods are disabled for root.
719 .Pp
720 If this option is set to
721 .Dq no ,
722 root is not allowed to log in.
723 .It Cm PermitTunnel
724 Specifies whether
725 .Xr tun 4
726 device forwarding is allowed.
727 The argument must be
728 .Dq yes ,
729 .Dq point-to-point
730 (layer 3),
731 .Dq ethernet
732 (layer 2), or
733 .Dq no .
734 Specifying
735 .Dq yes
736 permits both
737 .Dq point-to-point
738 and
739 .Dq ethernet .
740 The default is
741 .Dq no .
742 .It Cm PermitUserEnvironment
743 Specifies whether
744 .Pa ~/.ssh/environment
745 and
746 .Cm environment=
747 options in
748 .Pa ~/.ssh/authorized_keys
749 are processed by
750 .Xr sshd 8 .
751 The default is
752 .Dq no .
753 Enabling environment processing may enable users to bypass access
754 restrictions in some configurations using mechanisms such as
755 .Ev LD_PRELOAD .
756 .It Cm PidFile
757 Specifies the file that contains the process ID of the
758 SSH daemon.
759 The default is
760 .Pa /var/run/sshd.pid .
761 .It Cm Port
762 Specifies the port number that
763 .Xr sshd 8
764 listens on.
765 The default is 22.
766 Multiple options of this type are permitted.
767 See also
768 .Cm ListenAddress .
769 .It Cm PrintLastLog
770 Specifies whether
771 .Xr sshd 8
772 should print the date and time of the last user login when a user logs
773 in interactively.
774 The default is
775 .Dq yes .
776 .It Cm PrintMotd
777 Specifies whether
778 .Xr sshd 8
779 should print
780 .Pa /etc/motd
781 when a user logs in interactively.
782 (On some systems it is also printed by the shell,
783 .Pa /etc/profile ,
784 or equivalent.)
785 The default is
786 .Dq yes .
787 .It Cm Protocol
788 Specifies the protocol versions
789 .Xr sshd 8
790 supports.
791 The possible values are
792 .Sq 1
793 and
794 .Sq 2 .
795 Multiple versions must be comma-separated.
796 The default is
797 .Dq 2,1 .
798 Note that the order of the protocol list does not indicate preference,
799 because the client selects among multiple protocol versions offered
800 by the server.
801 Specifying
802 .Dq 2,1
803 is identical to
804 .Dq 1,2 .
805 .It Cm PubkeyAuthentication
806 Specifies whether public key authentication is allowed.
807 The default is
808 .Dq yes .
809 Note that this option applies to protocol version 2 only.
810 .It Cm RhostsRSAAuthentication
811 Specifies whether rhosts or /etc/hosts.equiv authentication together
812 with successful RSA host authentication is allowed.
813 The default is
814 .Dq no .
815 This option applies to protocol version 1 only.
816 .It Cm RSAAuthentication
817 Specifies whether pure RSA authentication is allowed.
818 The default is
819 .Dq yes .
820 This option applies to protocol version 1 only.
821 .It Cm ServerKeyBits
822 Defines the number of bits in the ephemeral protocol version 1 server key.
823 The minimum value is 512, and the default is 1024.
824 .It Cm StrictModes
825 Specifies whether
826 .Xr sshd 8
827 should check file modes and ownership of the
828 user's files and home directory before accepting login.
829 This is normally desirable because novices sometimes accidentally leave their
830 directory or files world-writable.
831 The default is
832 .Dq yes .
833 .It Cm Subsystem
834 Configures an external subsystem (e.g. file transfer daemon).
835 Arguments should be a subsystem name and a command (with optional arguments)
836 to execute upon subsystem request.
837 .Pp
838 The command
839 .Xr sftp-server 8
840 implements the
841 .Dq sftp
842 file transfer subsystem.
843 .Pp
844 Alternately the name
845 .Dq internal-sftp
846 implements an in-process
847 .Dq sftp
848 server.
849 This may simplify configurations using
850 .Cm ChrootDirectory
851 to force a different filesystem root on clients.
852 .Pp
853 By default no subsystems are defined.
854 Note that this option applies to protocol version 2 only.
855 .It Cm SyslogFacility
856 Gives the facility code that is used when logging messages from
857 .Xr sshd 8 .
858 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
859 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
860 The default is AUTH.
861 .It Cm TCPKeepAlive
862 Specifies whether the system should send TCP keepalive messages to the
863 other side.
864 If they are sent, death of the connection or crash of one
865 of the machines will be properly noticed.
866 However, this means that
867 connections will die if the route is down temporarily, and some people
868 find it annoying.
869 On the other hand, if TCP keepalives are not sent,
870 sessions may hang indefinitely on the server, leaving
871 .Dq ghost
872 users and consuming server resources.
873 .Pp
874 The default is
875 .Dq yes
876 (to send TCP keepalive messages), and the server will notice
877 if the network goes down or the client host crashes.
878 This avoids infinitely hanging sessions.
879 .Pp
880 To disable TCP keepalive messages, the value should be set to
881 .Dq no .
882 .It Cm UseDNS
883 Specifies whether
884 .Xr sshd 8
885 should look up the remote host name and check that
886 the resolved host name for the remote IP address maps back to the
887 very same IP address.
888 The default is
889 .Dq yes .
890 .It Cm UseLogin
891 Specifies whether
892 .Xr login 1
893 is used for interactive login sessions.
894 The default is
895 .Dq no .
896 Note that
897 .Xr login 1
898 is never used for remote command execution.
899 Note also, that if this is enabled,
900 .Cm X11Forwarding
901 will be disabled because
902 .Xr login 1
903 does not know how to handle
904 .Xr xauth 1
905 cookies.
906 If
907 .Cm UsePrivilegeSeparation
908 is specified, it will be disabled after authentication.
909 .It Cm UsePrivilegeSeparation
910 Specifies whether
911 .Xr sshd 8
912 separates privileges by creating an unprivileged child process
913 to deal with incoming network traffic.
914 After successful authentication, another process will be created that has
915 the privilege of the authenticated user.
916 The goal of privilege separation is to prevent privilege
917 escalation by containing any corruption within the unprivileged processes.
918 The default is
919 .Dq yes .
920 .It Cm X11DisplayOffset
921 Specifies the first display number available for
922 .Xr sshd 8 Ns 's
923 X11 forwarding.
924 This prevents sshd from interfering with real X11 servers.
925 The default is 10.
926 .It Cm X11Forwarding
927 Specifies whether X11 forwarding is permitted.
928 The argument must be
929 .Dq yes
930 or
931 .Dq no .
932 The default is
933 .Dq no .
934 .Pp
935 When X11 forwarding is enabled, there may be additional exposure to
936 the server and to client displays if the
937 .Xr sshd 8
938 proxy display is configured to listen on the wildcard address (see
939 .Cm X11UseLocalhost
940 below), though this is not the default.
941 Additionally, the authentication spoofing and authentication data
942 verification and substitution occur on the client side.
943 The security risk of using X11 forwarding is that the client's X11
944 display server may be exposed to attack when the SSH client requests
945 forwarding (see the warnings for
946 .Cm ForwardX11
947 in
948 .Xr ssh_config 5 ) .
949 A system administrator may have a stance in which they want to
950 protect clients that may expose themselves to attack by unwittingly
951 requesting X11 forwarding, which can warrant a
952 .Dq no
953 setting.
954 .Pp
955 Note that disabling X11 forwarding does not prevent users from
956 forwarding X11 traffic, as users can always install their own forwarders.
957 X11 forwarding is automatically disabled if
958 .Cm UseLogin
959 is enabled.
960 .It Cm X11UseLocalhost
961 Specifies whether
962 .Xr sshd 8
963 should bind the X11 forwarding server to the loopback address or to
964 the wildcard address.
965 By default,
966 sshd binds the forwarding server to the loopback address and sets the
967 hostname part of the
968 .Ev DISPLAY
969 environment variable to
970 .Dq localhost .
971 This prevents remote hosts from connecting to the proxy display.
972 However, some older X11 clients may not function with this
973 configuration.
974 .Cm X11UseLocalhost
975 may be set to
976 .Dq no
977 to specify that the forwarding server should be bound to the wildcard
978 address.
979 The argument must be
980 .Dq yes
981 or
982 .Dq no .
983 The default is
984 .Dq yes .
985 .It Cm XAuthLocation
986 Specifies the full pathname of the
987 .Xr xauth 1
988 program.
989 The default is
990 .Pa /usr/X11R6/bin/xauth .
991 .El
992 .Sh TIME FORMATS
993 .Xr sshd 8
994 command-line arguments and configuration file options that specify time
995 may be expressed using a sequence of the form:
996 .Sm off
997 .Ar time Op Ar qualifier ,
998 .Sm on
999 where
1000 .Ar time
1001 is a positive integer value and
1002 .Ar qualifier
1003 is one of the following:
1004 .Pp
1005 .Bl -tag -width Ds -compact -offset indent
1006 .It Aq Cm none
1007 seconds
1008 .It Cm s | Cm S
1009 seconds
1010 .It Cm m | Cm M
1011 minutes
1012 .It Cm h | Cm H
1013 hours
1014 .It Cm d | Cm D
1015 days
1016 .It Cm w | Cm W
1017 weeks
1018 .El
1019 .Pp
1020 Each member of the sequence is added together to calculate
1021 the total time value.
1022 .Pp
1023 Time format examples:
1024 .Pp
1025 .Bl -tag -width Ds -compact -offset indent
1026 .It 600
1027 600 seconds (10 minutes)
1028 .It 10m
1029 10 minutes
1030 .It 1h30m
1031 1 hour 30 minutes (90 minutes)
1032 .El
1033 .Sh FILES
1034 .Bl -tag -width Ds
1035 .It Pa /etc/ssh/sshd_config
1036 Contains configuration data for
1037 .Xr sshd 8 .
1038 This file should be writable by root only, but it is recommended
1039 (though not necessary) that it be world-readable.
1040 .El
1041 .Sh SEE ALSO
1042 .Xr sshd 8
1043 .Sh AUTHORS
1044 OpenSSH is a derivative of the original and free
1045 ssh 1.2.12 release by Tatu Ylonen.
1046 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1047 Theo de Raadt and Dug Song
1048 removed many bugs, re-added newer features and
1049 created OpenSSH.
1050 Markus Friedl contributed the support for SSH
1051 protocol versions 1.5 and 2.0.
1052 Niels Provos and Markus Friedl contributed support
1053 for privilege separation.