| blob | d0c02ca7c4c5fa8464e4f339827ce68967233220 |
1 .\"
2 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
3 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
4 .\" All rights reserved
5 .\"
6 .\" As far as I am concerned, the code I have written for this software
7 .\" can be used freely for any purpose. Any derived versions of this
8 .\" software must be clearly marked as such, and if the derived work is
9 .\" incompatible with the protocol description in the RFC file, it must be
10 .\" called by a name other than "ssh" or "Secure Shell".
11 .\"
12 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
13 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
14 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
15 .\"
16 .\" Redistribution and use in source and binary forms, with or without
17 .\" modification, are permitted provided that the following conditions
18 .\" are met:
19 .\" 1. Redistributions of source code must retain the above copyright
20 .\" notice, this list of conditions and the following disclaimer.
21 .\" 2. Redistributions in binary form must reproduce the above copyright
22 .\" notice, this list of conditions and the following disclaimer in the
23 .\" documentation and/or other materials provided with the distribution.
24 .\"
25 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
26 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
27 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
28 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
29 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
30 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
31 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
32 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
33 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
34 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
35 .\"
36 .\" $OpenBSD: sshd_config.5,v 1.128 2010/10/28 18:33:28 jmc Exp $
37 .Dd $Mdocdate: October 28 2010 $
38 .Dt SSHD_CONFIG 5
39 .Os
40 .Sh NAME
41 .Nm sshd_config
42 .Nd OpenSSH SSH daemon configuration file
43 .Sh SYNOPSIS
44 .Nm /etc/ssh/sshd_config
45 .Sh DESCRIPTION
46 .Xr sshd 8
47 reads configuration data from
48 .Pa /etc/ssh/sshd_config
49 (or the file specified with
50 .Fl f
51 on the command line).
52 The file contains keyword-argument pairs, one per line.
53 Lines starting with
54 .Ql #
55 and empty lines are interpreted as comments.
56 Arguments may optionally be enclosed in double quotes
57 .Pq \&"
58 in order to represent arguments containing spaces.
59 .Pp
60 The possible
61 keywords and their meanings are as follows (note that
62 keywords are case-insensitive and arguments are case-sensitive):
63 .Bl -tag -width Ds
64 .It Cm AcceptEnv
65 Specifies what environment variables sent by the client will be copied into
66 the session's
67 .Xr environ 7 .
68 See
69 .Cm SendEnv
70 in
71 .Xr ssh_config 5
72 for how to configure the client.
73 Note that environment passing is only supported for protocol 2.
74 Variables are specified by name, which may contain the wildcard characters
75 .Ql *
76 and
77 .Ql \&? .
78 Multiple environment variables may be separated by whitespace or spread
79 across multiple
80 .Cm AcceptEnv
81 directives.
82 Be warned that some environment variables could be used to bypass restricted
83 user environments.
84 For this reason, care should be taken in the use of this directive.
85 The default is not to accept any environment variables.
86 .It Cm AddressFamily
87 Specifies which address family should be used by
88 .Xr sshd 8 .
89 Valid arguments are
90 .Dq any ,
91 .Dq inet
92 (use IPv4 only), or
93 .Dq inet6
94 (use IPv6 only).
95 The default is
96 .Dq any .
97 .It Cm AllowAgentForwarding
98 Specifies whether
99 .Xr ssh-agent 1
100 forwarding is permitted.
101 The default is
102 .Dq yes .
103 Note that disabling agent forwarding does not improve security
104 unless users are also denied shell access, as they can always install
105 their own forwarders.
106 .It Cm AllowGroups
107 This keyword can be followed by a list of group name patterns, separated
108 by spaces.
109 If specified, login is allowed only for users whose primary
110 group or supplementary group list matches one of the patterns.
111 Only group names are valid; a numerical group ID is not recognized.
112 By default, login is allowed for all groups.
113 The allow/deny directives are processed in the following order:
114 .Cm DenyUsers ,
115 .Cm AllowUsers ,
116 .Cm DenyGroups ,
117 and finally
118 .Cm AllowGroups .
119 .Pp
120 See
121 .Sx PATTERNS
122 in
123 .Xr ssh_config 5
124 for more information on patterns.
125 .It Cm AllowTcpForwarding
126 Specifies whether TCP forwarding is permitted.
127 The default is
128 .Dq yes .
129 Note that disabling TCP forwarding does not improve security unless
130 users are also denied shell access, as they can always install their
131 own forwarders.
132 .It Cm AllowUsers
133 This keyword can be followed by a list of user name patterns, separated
134 by spaces.
135 If specified, login is allowed only for user names that
136 match one of the patterns.
137 Only user names are valid; a numerical user ID is not recognized.
138 By default, login is allowed for all users.
139 If the pattern takes the form USER@HOST then USER and HOST
140 are separately checked, restricting logins to particular
141 users from particular hosts.
142 The allow/deny directives are processed in the following order:
143 .Cm DenyUsers ,
144 .Cm AllowUsers ,
145 .Cm DenyGroups ,
146 and finally
147 .Cm AllowGroups .
148 .Pp
149 See
150 .Sx PATTERNS
151 in
152 .Xr ssh_config 5
153 for more information on patterns.
154 .It Cm AuthorizedKeysFile
155 Specifies the file that contains the public keys that can be used
156 for user authentication.
157 The format is described in the
158 .Sx AUTHORIZED_KEYS FILE FORMAT
159 section of
160 .Xr sshd 8 .
161 .Cm AuthorizedKeysFile
162 may contain tokens of the form %T which are substituted during connection
163 setup.
164 The following tokens are defined: %% is replaced by a literal '%',
165 %h is replaced by the home directory of the user being authenticated, and
166 %u is replaced by the username of that user.
167 After expansion,
168 .Cm AuthorizedKeysFile
169 is taken to be an absolute path or one relative to the user's home
170 directory.
171 The default is
172 .Dq .ssh/authorized_keys .
173 .It Cm AuthorizedPrincipalsFile
174 Specifies a file that lists principal names that are accepted for
175 certificate authentication.
176 When using certificates signed by a key listed in
177 .Cm TrustedUserCAKeys ,
178 this file lists names, one of which must appear in the certificate for it
179 to be accepted for authentication.
180 Names are listed one per line preceded by key options (as described
181 in
182 .Sx AUTHORIZED_KEYS FILE FORMAT
183 in
184 .Xr sshd 8 ) .
185 Empty lines and comments starting with
186 .Ql #
187 are ignored.
188 .Pp
189 .Cm AuthorizedPrincipalsFile
190 may contain tokens of the form %T which are substituted during connection
191 setup.
192 The following tokens are defined: %% is replaced by a literal '%',
193 %h is replaced by the home directory of the user being authenticated, and
194 %u is replaced by the username of that user.
195 After expansion,
196 .Cm AuthorizedPrincipalsFile
197 is taken to be an absolute path or one relative to the user's home
198 directory.
199 .Pp
200 The default is not to use a principals file \(en in this case, the username
201 of the user must appear in a certificate's principals list for it to be
202 accepted.
203 Note that
204 .Cm AuthorizedPrincipalsFile
205 is only used when authentication proceeds using a CA listed in
206 .Cm TrustedUserCAKeys
207 and is not consulted for certification authorities trusted via
208 .Pa ~/.ssh/authorized_keys ,
209 though the
210 .Cm principals=
211 key option offers a similar facility (see
212 .Xr sshd 8
213 for details).
214 .It Cm Banner
215 The contents of the specified file are sent to the remote user before
216 authentication is allowed.
217 If the argument is
218 .Dq none
219 then no banner is displayed.
220 This option is only available for protocol version 2.
221 By default, no banner is displayed.
222 .It Cm ChallengeResponseAuthentication
223 Specifies whether challenge-response authentication is allowed (e.g. via
224 PAM or though authentication styles supported in
225 .Xr login.conf 5 )
226 The default is
227 .Dq yes .
228 .It Cm ChrootDirectory
229 Specifies the pathname of a directory to
230 .Xr chroot 2
231 to after authentication.
232 All components of the pathname must be root-owned directories that are
233 not writable by any other user or group.
234 After the chroot,
235 .Xr sshd 8
236 changes the working directory to the user's home directory.
237 .Pp
238 The pathname may contain the following tokens that are expanded at runtime once
239 the connecting user has been authenticated: %% is replaced by a literal '%',
240 %h is replaced by the home directory of the user being authenticated, and
241 %u is replaced by the username of that user.
242 .Pp
243 The
244 .Cm ChrootDirectory
245 must contain the necessary files and directories to support the
246 user's session.
247 For an interactive session this requires at least a shell, typically
248 .Xr sh 1 ,
249 and basic
250 .Pa /dev
251 nodes such as
252 .Xr null 4 ,
253 .Xr zero 4 ,
254 .Xr stdin 4 ,
255 .Xr stdout 4 ,
256 .Xr stderr 4 ,
257 .Xr arandom 4
258 and
259 .Xr tty 4
260 devices.
261 For file transfer sessions using
262 .Dq sftp ,
263 no additional configuration of the environment is necessary if the
264 in-process sftp server is used,
265 though sessions which use logging do require
266 .Pa /dev/log
267 inside the chroot directory (see
268 .Xr sftp-server 8
269 for details).
270 .Pp
271 The default is not to
272 .Xr chroot 2 .
273 .It Cm Ciphers
274 Specifies the ciphers allowed for protocol version 2.
275 Multiple ciphers must be comma-separated.
276 The supported ciphers are
277 .Dq 3des-cbc ,
278 .Dq aes128-cbc ,
279 .Dq aes192-cbc ,
280 .Dq aes256-cbc ,
281 .Dq aes128-ctr ,
282 .Dq aes192-ctr ,
283 .Dq aes256-ctr ,
284 .Dq arcfour128 ,
285 .Dq arcfour256 ,
286 .Dq arcfour ,
287 .Dq blowfish-cbc ,
288 and
289 .Dq cast128-cbc .
290 The default is:
291 .Bd -literal -offset 3n
292 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
293 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
294 aes256-cbc,arcfour
295 .Ed
296 .It Cm ClientAliveCountMax
297 Sets the number of client alive messages (see below) which may be
298 sent without
299 .Xr sshd 8
300 receiving any messages back from the client.
301 If this threshold is reached while client alive messages are being sent,
302 sshd will disconnect the client, terminating the session.
303 It is important to note that the use of client alive messages is very
304 different from
305 .Cm TCPKeepAlive
306 (below).
307 The client alive messages are sent through the encrypted channel
308 and therefore will not be spoofable.
309 The TCP keepalive option enabled by
310 .Cm TCPKeepAlive
311 is spoofable.
312 The client alive mechanism is valuable when the client or
313 server depend on knowing when a connection has become inactive.
314 .Pp
315 The default value is 3.
316 If
317 .Cm ClientAliveInterval
318 (see below) is set to 15, and
319 .Cm ClientAliveCountMax
320 is left at the default, unresponsive SSH clients
321 will be disconnected after approximately 45 seconds.
322 This option applies to protocol version 2 only.
323 .It Cm ClientAliveInterval
324 Sets a timeout interval in seconds after which if no data has been received
325 from the client,
326 .Xr sshd 8
327 will send a message through the encrypted
328 channel to request a response from the client.
329 The default
330 is 0, indicating that these messages will not be sent to the client.
331 This option applies to protocol version 2 only.
332 .It Cm Compression
333 Specifies whether compression is allowed, or delayed until
334 the user has authenticated successfully.
335 The argument must be
336 .Dq yes ,
337 .Dq delayed ,
338 or
339 .Dq no .
340 The default is
341 .Dq delayed .
342 .It Cm DenyGroups
343 This keyword can be followed by a list of group name patterns, separated
344 by spaces.
345 Login is disallowed for users whose primary group or supplementary
346 group list matches one of the patterns.
347 Only group names are valid; a numerical group ID is not recognized.
348 By default, login is allowed for all groups.
349 The allow/deny directives are processed in the following order:
350 .Cm DenyUsers ,
351 .Cm AllowUsers ,
352 .Cm DenyGroups ,
353 and finally
354 .Cm AllowGroups .
355 .Pp
356 See
357 .Sx PATTERNS
358 in
359 .Xr ssh_config 5
360 for more information on patterns.
361 .It Cm DenyUsers
362 This keyword can be followed by a list of user name patterns, separated
363 by spaces.
364 Login is disallowed for user names that match one of the patterns.
365 Only user names are valid; a numerical user ID is not recognized.
366 By default, login is allowed for all users.
367 If the pattern takes the form USER@HOST then USER and HOST
368 are separately checked, restricting logins to particular
369 users from particular hosts.
370 The allow/deny directives are processed in the following order:
371 .Cm DenyUsers ,
372 .Cm AllowUsers ,
373 .Cm DenyGroups ,
374 and finally
375 .Cm AllowGroups .
376 .Pp
377 See
378 .Sx PATTERNS
379 in
380 .Xr ssh_config 5
381 for more information on patterns.
382 .It Cm ForceCommand
383 Forces the execution of the command specified by
384 .Cm ForceCommand ,
385 ignoring any command supplied by the client and
386 .Pa ~/.ssh/rc
387 if present.
388 The command is invoked by using the user's login shell with the -c option.
389 This applies to shell, command, or subsystem execution.
390 It is most useful inside a
391 .Cm Match
392 block.
393 The command originally supplied by the client is available in the
394 .Ev SSH_ORIGINAL_COMMAND
395 environment variable.
396 Specifying a command of
397 .Dq internal-sftp
398 will force the use of an in-process sftp server that requires no support
399 files when used with
400 .Cm ChrootDirectory .
401 .It Cm GatewayPorts
402 Specifies whether remote hosts are allowed to connect to ports
403 forwarded for the client.
404 By default,
405 .Xr sshd 8
406 binds remote port forwardings to the loopback address.
407 This prevents other remote hosts from connecting to forwarded ports.
408 .Cm GatewayPorts
409 can be used to specify that sshd
410 should allow remote port forwardings to bind to non-loopback addresses, thus
411 allowing other hosts to connect.
412 The argument may be
413 .Dq no
414 to force remote port forwardings to be available to the local host only,
415 .Dq yes
416 to force remote port forwardings to bind to the wildcard address, or
417 .Dq clientspecified
418 to allow the client to select the address to which the forwarding is bound.
419 The default is
420 .Dq no .
421 .It Cm GSSAPIAuthentication
422 Specifies whether user authentication based on GSSAPI is allowed.
423 The default is
424 .Dq no .
425 Note that this option applies to protocol version 2 only.
426 .It Cm GSSAPICleanupCredentials
427 Specifies whether to automatically destroy the user's credentials cache
428 on logout.
429 The default is
430 .Dq yes .
431 Note that this option applies to protocol version 2 only.
432 .It Cm HostbasedAuthentication
433 Specifies whether rhosts or /etc/hosts.equiv authentication together
434 with successful public key client host authentication is allowed
435 (host-based authentication).
436 This option is similar to
437 .Cm RhostsRSAAuthentication
438 and applies to protocol version 2 only.
439 The default is
440 .Dq no .
441 .It Cm HostbasedUsesNameFromPacketOnly
442 Specifies whether or not the server will attempt to perform a reverse
443 name lookup when matching the name in the
444 .Pa ~/.shosts ,
445 .Pa ~/.rhosts ,
446 and
447 .Pa /etc/hosts.equiv
448 files during
449 .Cm HostbasedAuthentication .
450 A setting of
451 .Dq yes
452 means that
453 .Xr sshd 8
454 uses the name supplied by the client rather than
455 attempting to resolve the name from the TCP connection itself.
456 The default is
457 .Dq no .
458 .It Cm HostCertificate
459 Specifies a file containing a public host certificate.
460 The certificate's public key must match a private host key already specified
461 by
462 .Cm HostKey .
463 The default behaviour of
464 .Xr sshd 8
465 is not to load any certificates.
466 .It Cm HostKey
467 Specifies a file containing a private host key
468 used by SSH.
469 The default is
470 .Pa /etc/ssh/ssh_host_key
471 for protocol version 1, and
472 .Pa /etc/ssh/ssh_host_dsa_key ,
473 .Pa /etc/ssh/ssh_host_ecdsa_key
474 and
475 .Pa /etc/ssh/ssh_host_rsa_key
476 for protocol version 2.
477 Note that
478 .Xr sshd 8
479 will refuse to use a file if it is group/world-accessible.
480 It is possible to have multiple host key files.
481 .Dq rsa1
482 keys are used for version 1 and
483 .Dq dsa ,
484 .Dq ecdsa
485 or
486 .Dq rsa
487 are used for version 2 of the SSH protocol.
488 .It Cm IgnoreRhosts
489 Specifies that
490 .Pa .rhosts
491 and
492 .Pa .shosts
493 files will not be used in
494 .Cm RhostsRSAAuthentication
495 or
496 .Cm HostbasedAuthentication .
497 .Pp
498 .Pa /etc/hosts.equiv
499 and
500 .Pa /etc/shosts.equiv
501 are still used.
502 The default is
503 .Dq yes .
504 .It Cm IgnoreUserKnownHosts
505 Specifies whether
506 .Xr sshd 8
507 should ignore the user's
508 .Pa ~/.ssh/known_hosts
509 during
510 .Cm RhostsRSAAuthentication
511 or
512 .Cm HostbasedAuthentication .
513 The default is
514 .Dq no .
515 .It Cm KerberosAuthentication
516 Specifies whether the password provided by the user for
517 .Cm PasswordAuthentication
518 will be validated through the Kerberos KDC.
519 To use this option, the server needs a
520 Kerberos servtab which allows the verification of the KDC's identity.
521 The default is
522 .Dq no .
523 .It Cm KerberosGetAFSToken
524 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
525 an AFS token before accessing the user's home directory.
526 The default is
527 .Dq no .
528 .It Cm KerberosOrLocalPasswd
529 If password authentication through Kerberos fails then
530 the password will be validated via any additional local mechanism
531 such as
532 .Pa /etc/passwd .
533 The default is
534 .Dq yes .
535 .It Cm KerberosTicketCleanup
536 Specifies whether to automatically destroy the user's ticket cache
537 file on logout.
538 The default is
539 .Dq yes .
540 .It Cm KexAlgorithms
541 Specifies the available KEX (Key Exchange) algorithms.
542 Multiple algorithms must be comma-separated.
543 The default is
544 .Dq ecdh-sha2-nistp256 ,
545 .Dq ecdh-sha2-nistp384 ,
546 .Dq ecdh-sha2-nistp521 ,
547 .Dq diffie-hellman-group-exchange-sha256 ,
548 .Dq diffie-hellman-group-exchange-sha1 ,
549 .Dq diffie-hellman-group14-sha1 ,
550 .Dq diffie-hellman-group1-sha1 .
551 .It Cm KeyRegenerationInterval
552 In protocol version 1, the ephemeral server key is automatically regenerated
553 after this many seconds (if it has been used).
554 The purpose of regeneration is to prevent
555 decrypting captured sessions by later breaking into the machine and
556 stealing the keys.
557 The key is never stored anywhere.
558 If the value is 0, the key is never regenerated.
559 The default is 3600 (seconds).
560 .It Cm ListenAddress
561 Specifies the local addresses
562 .Xr sshd 8
563 should listen on.
564 The following forms may be used:
565 .Pp
566 .Bl -item -offset indent -compact
567 .It
568 .Cm ListenAddress
569 .Sm off
570 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
571 .Sm on
572 .It
573 .Cm ListenAddress
574 .Sm off
575 .Ar host No | Ar IPv4_addr No : Ar port
576 .Sm on
577 .It
578 .Cm ListenAddress
579 .Sm off
580 .Oo
581 .Ar host No | Ar IPv6_addr Oc : Ar port
582 .Sm on
583 .El
584 .Pp
585 If
586 .Ar port
587 is not specified,
588 sshd will listen on the address and all prior
589 .Cm Port
590 options specified.
591 The default is to listen on all local addresses.
592 Multiple
593 .Cm ListenAddress
594 options are permitted.
595 Additionally, any
596 .Cm Port
597 options must precede this option for non-port qualified addresses.
598 .It Cm LoginGraceTime
599 The server disconnects after this time if the user has not
600 successfully logged in.
601 If the value is 0, there is no time limit.
602 The default is 120 seconds.
603 .It Cm LogLevel
604 Gives the verbosity level that is used when logging messages from
605 .Xr sshd 8 .
606 The possible values are:
607 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
608 The default is INFO.
609 DEBUG and DEBUG1 are equivalent.
610 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
611 Logging with a DEBUG level violates the privacy of users and is not recommended.
612 .It Cm MACs
613 Specifies the available MAC (message authentication code) algorithms.
614 The MAC algorithm is used in protocol version 2
615 for data integrity protection.
616 Multiple algorithms must be comma-separated.
617 The default is:
618 .Bd -literal -offset indent
619 hmac-md5,hmac-sha1,umac-64@openssh.com,
620 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
621 .Ed
622 .It Cm Match
623 Introduces a conditional block.
624 If all of the criteria on the
625 .Cm Match
626 line are satisfied, the keywords on the following lines override those
627 set in the global section of the config file, until either another
628 .Cm Match
629 line or the end of the file.
630 .Pp
631 The arguments to
632 .Cm Match
633 are one or more criteria-pattern pairs.
634 The available criteria are
635 .Cm User ,
636 .Cm Group ,
637 .Cm Host ,
638 and
639 .Cm Address .
640 The match patterns may consist of single entries or comma-separated
641 lists and may use the wildcard and negation operators described in the
642 .Sx PATTERNS
643 section of
644 .Xr ssh_config 5 .
645 .Pp
646 The patterns in an
647 .Cm Address
648 criteria may additionally contain addresses to match in CIDR
649 address/masklen format, e.g.\&
650 .Dq 192.0.2.0/24
651 or
652 .Dq 3ffe:ffff::/32 .
653 Note that the mask length provided must be consistent with the address -
654 it is an error to specify a mask length that is too long for the address
655 or one with bits set in this host portion of the address.
656 For example,
657 .Dq 192.0.2.0/33
658 and
659 .Dq 192.0.2.0/8
660 respectively.
661 .Pp
662 Only a subset of keywords may be used on the lines following a
663 .Cm Match
664 keyword.
665 Available keywords are
666 .Cm AllowAgentForwarding ,
667 .Cm AllowTcpForwarding ,
668 .Cm AuthorizedKeysFile ,
669 .Cm AuthorizedPrincipalsFile ,
670 .Cm Banner ,
671 .Cm ChrootDirectory ,
672 .Cm ForceCommand ,
673 .Cm GatewayPorts ,
674 .Cm GSSAPIAuthentication ,
675 .Cm HostbasedAuthentication ,
676 .Cm HostbasedUsesNameFromPacketOnly ,
677 .Cm KbdInteractiveAuthentication ,
678 .Cm KerberosAuthentication ,
679 .Cm MaxAuthTries ,
680 .Cm MaxSessions ,
681 .Cm PasswordAuthentication ,
682 .Cm PermitEmptyPasswords ,
683 .Cm PermitOpen ,
684 .Cm PermitRootLogin ,
685 .Cm PermitTunnel ,
686 .Cm PubkeyAuthentication ,
687 .Cm RhostsRSAAuthentication ,
688 .Cm RSAAuthentication ,
689 .Cm X11DisplayOffset ,
690 .Cm X11Forwarding
691 and
692 .Cm X11UseLocalHost .
693 .It Cm MaxAuthTries
694 Specifies the maximum number of authentication attempts permitted per
695 connection.
696 Once the number of failures reaches half this value,
697 additional failures are logged.
698 The default is 6.
699 .It Cm MaxSessions
700 Specifies the maximum number of open sessions permitted per network connection.
701 The default is 10.
702 .It Cm MaxStartups
703 Specifies the maximum number of concurrent unauthenticated connections to the
704 SSH daemon.
705 Additional connections will be dropped until authentication succeeds or the
706 .Cm LoginGraceTime
707 expires for a connection.
708 The default is 10.
709 .Pp
710 Alternatively, random early drop can be enabled by specifying
711 the three colon separated values
712 .Dq start:rate:full
713 (e.g. "10:30:60").
714 .Xr sshd 8
715 will refuse connection attempts with a probability of
716 .Dq rate/100
717 (30%)
718 if there are currently
719 .Dq start
720 (10)
721 unauthenticated connections.
722 The probability increases linearly and all connection attempts
723 are refused if the number of unauthenticated connections reaches
724 .Dq full
725 (60).
726 .It Cm PasswordAuthentication
727 Specifies whether password authentication is allowed.
728 The default is
729 .Dq yes .
730 .It Cm PermitEmptyPasswords
731 When password authentication is allowed, it specifies whether the
732 server allows login to accounts with empty password strings.
733 The default is
734 .Dq no .
735 .It Cm PermitOpen
736 Specifies the destinations to which TCP port forwarding is permitted.
737 The forwarding specification must be one of the following forms:
738 .Pp
739 .Bl -item -offset indent -compact
740 .It
741 .Cm PermitOpen
742 .Sm off
743 .Ar host : port
744 .Sm on
745 .It
746 .Cm PermitOpen
747 .Sm off
748 .Ar IPv4_addr : port
749 .Sm on
750 .It
751 .Cm PermitOpen
752 .Sm off
753 .Ar \&[ IPv6_addr \&] : port
754 .Sm on
755 .El
756 .Pp
757 Multiple forwards may be specified by separating them with whitespace.
758 An argument of
759 .Dq any
760 can be used to remove all restrictions and permit any forwarding requests.
761 By default all port forwarding requests are permitted.
762 .It Cm PermitRootLogin
763 Specifies whether root can log in using
764 .Xr ssh 1 .
765 The argument must be
766 .Dq yes ,
767 .Dq without-password ,
768 .Dq forced-commands-only ,
769 or
770 .Dq no .
771 The default is
772 .Dq yes .
773 .Pp
774 If this option is set to
775 .Dq without-password ,
776 password authentication is disabled for root.
777 .Pp
778 If this option is set to
779 .Dq forced-commands-only ,
780 root login with public key authentication will be allowed,
781 but only if the
782 .Ar command
783 option has been specified
784 (which may be useful for taking remote backups even if root login is
785 normally not allowed).
786 All other authentication methods are disabled for root.
787 .Pp
788 If this option is set to
789 .Dq no ,
790 root is not allowed to log in.
791 .It Cm PermitTunnel
792 Specifies whether
793 .Xr tun 4
794 device forwarding is allowed.
795 The argument must be
796 .Dq yes ,
797 .Dq point-to-point
798 (layer 3),
799 .Dq ethernet
800 (layer 2), or
801 .Dq no .
802 Specifying
803 .Dq yes
804 permits both
805 .Dq point-to-point
806 and
807 .Dq ethernet .
808 The default is
809 .Dq no .
810 .It Cm PermitUserEnvironment
811 Specifies whether
812 .Pa ~/.ssh/environment
813 and
814 .Cm environment=
815 options in
816 .Pa ~/.ssh/authorized_keys
817 are processed by
818 .Xr sshd 8 .
819 The default is
820 .Dq no .
821 Enabling environment processing may enable users to bypass access
822 restrictions in some configurations using mechanisms such as
823 .Ev LD_PRELOAD .
824 .It Cm PidFile
825 Specifies the file that contains the process ID of the
826 SSH daemon.
827 The default is
828 .Pa /var/run/sshd.pid .
829 .It Cm Port
830 Specifies the port number that
831 .Xr sshd 8
832 listens on.
833 The default is 22.
834 Multiple options of this type are permitted.
835 See also
836 .Cm ListenAddress .
837 .It Cm PrintLastLog
838 Specifies whether
839 .Xr sshd 8
840 should print the date and time of the last user login when a user logs
841 in interactively.
842 The default is
843 .Dq yes .
844 .It Cm PrintMotd
845 Specifies whether
846 .Xr sshd 8
847 should print
848 .Pa /etc/motd
849 when a user logs in interactively.
850 (On some systems it is also printed by the shell,
851 .Pa /etc/profile ,
852 or equivalent.)
853 The default is
854 .Dq yes .
855 .It Cm Protocol
856 Specifies the protocol versions
857 .Xr sshd 8
858 supports.
859 The possible values are
860 .Sq 1
861 and
862 .Sq 2 .
863 Multiple versions must be comma-separated.
864 The default is
865 .Sq 2 .
866 Note that the order of the protocol list does not indicate preference,
867 because the client selects among multiple protocol versions offered
868 by the server.
869 Specifying
870 .Dq 2,1
871 is identical to
872 .Dq 1,2 .
873 .It Cm PubkeyAuthentication
874 Specifies whether public key authentication is allowed.
875 The default is
876 .Dq yes .
877 Note that this option applies to protocol version 2 only.
878 .It Cm RevokedKeys
879 Specifies a list of revoked public keys.
880 Keys listed in this file will be refused for public key authentication.
881 Note that if this file is not readable, then public key authentication will
882 be refused for all users.
883 .It Cm RhostsRSAAuthentication
884 Specifies whether rhosts or /etc/hosts.equiv authentication together
885 with successful RSA host authentication is allowed.
886 The default is
887 .Dq no .
888 This option applies to protocol version 1 only.
889 .It Cm RSAAuthentication
890 Specifies whether pure RSA authentication is allowed.
891 The default is
892 .Dq yes .
893 This option applies to protocol version 1 only.
894 .It Cm ServerKeyBits
895 Defines the number of bits in the ephemeral protocol version 1 server key.
896 The minimum value is 512, and the default is 1024.
897 .It Cm StrictModes
898 Specifies whether
899 .Xr sshd 8
900 should check file modes and ownership of the
901 user's files and home directory before accepting login.
902 This is normally desirable because novices sometimes accidentally leave their
903 directory or files world-writable.
904 The default is
905 .Dq yes .
906 Note that this does not apply to
907 .Cm ChrootDirectory ,
908 whose permissions and ownership are checked unconditionally.
909 .It Cm Subsystem
910 Configures an external subsystem (e.g. file transfer daemon).
911 Arguments should be a subsystem name and a command (with optional arguments)
912 to execute upon subsystem request.
913 .Pp
914 The command
915 .Xr sftp-server 8
916 implements the
917 .Dq sftp
918 file transfer subsystem.
919 .Pp
920 Alternately the name
921 .Dq internal-sftp
922 implements an in-process
923 .Dq sftp
924 server.
925 This may simplify configurations using
926 .Cm ChrootDirectory
927 to force a different filesystem root on clients.
928 .Pp
929 By default no subsystems are defined.
930 Note that this option applies to protocol version 2 only.
931 .It Cm SyslogFacility
932 Gives the facility code that is used when logging messages from
933 .Xr sshd 8 .
934 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
935 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
936 The default is AUTH.
937 .It Cm TCPKeepAlive
938 Specifies whether the system should send TCP keepalive messages to the
939 other side.
940 If they are sent, death of the connection or crash of one
941 of the machines will be properly noticed.
942 However, this means that
943 connections will die if the route is down temporarily, and some people
944 find it annoying.
945 On the other hand, if TCP keepalives are not sent,
946 sessions may hang indefinitely on the server, leaving
947 .Dq ghost
948 users and consuming server resources.
949 .Pp
950 The default is
951 .Dq yes
952 (to send TCP keepalive messages), and the server will notice
953 if the network goes down or the client host crashes.
954 This avoids infinitely hanging sessions.
955 .Pp
956 To disable TCP keepalive messages, the value should be set to
957 .Dq no .
958 .It Cm TrustedUserCAKeys
959 Specifies a file containing public keys of certificate authorities that are
960 trusted to sign user certificates for authentication.
961 Keys are listed one per line; empty lines and comments starting with
962 .Ql #
963 are allowed.
964 If a certificate is presented for authentication and has its signing CA key
965 listed in this file, then it may be used for authentication for any user
966 listed in the certificate's principals list.
967 Note that certificates that lack a list of principals will not be permitted
968 for authentication using
969 .Cm TrustedUserCAKeys .
970 For more details on certificates, see the
971 .Sx CERTIFICATES
972 section in
973 .Xr ssh-keygen 1 .
974 .It Cm UseDNS
975 Specifies whether
976 .Xr sshd 8
977 should look up the remote host name and check that
978 the resolved host name for the remote IP address maps back to the
979 very same IP address.
980 The default is
981 .Dq yes .
982 .It Cm UseLogin
983 Specifies whether
984 .Xr login 1
985 is used for interactive login sessions.
986 The default is
987 .Dq no .
988 Note that
989 .Xr login 1
990 is never used for remote command execution.
991 Note also, that if this is enabled,
992 .Cm X11Forwarding
993 will be disabled because
994 .Xr login 1
995 does not know how to handle
996 .Xr xauth 1
997 cookies.
998 If
999 .Cm UsePrivilegeSeparation
1000 is specified, it will be disabled after authentication.
1001 .It Cm UsePAM
1002 Enables the Pluggable Authentication Module interface.
1003 If set to
1004 .Dq yes
1005 this will enable PAM authentication using
1006 .Cm ChallengeResponseAuthentication
1007 and
1008 .Cm PasswordAuthentication
1009 in addition to PAM account and session module processing for all
1010 authentication types.
1011 .Pp
1012 Because PAM challenge-response authentication usually serves an equivalent
1013 role to password authentication, you should disable either
1014 .Cm PasswordAuthentication
1015 or
1016 .Cm ChallengeResponseAuthentication.
1017 .Pp
1018 If
1019 .Cm UsePAM
1020 is enabled, you will not be able to run
1021 .Xr sshd 8
1022 as a non-root user.
1023 The default is
1024 .Dq no .
1025 .It Cm UsePrivilegeSeparation
1026 Specifies whether
1027 .Xr sshd 8
1028 separates privileges by creating an unprivileged child process
1029 to deal with incoming network traffic.
1030 After successful authentication, another process will be created that has
1031 the privilege of the authenticated user.
1032 The goal of privilege separation is to prevent privilege
1033 escalation by containing any corruption within the unprivileged processes.
1034 The default is
1035 .Dq yes .
1036 .It Cm X11DisplayOffset
1037 Specifies the first display number available for
1038 .Xr sshd 8 Ns 's
1039 X11 forwarding.
1040 This prevents sshd from interfering with real X11 servers.
1041 The default is 10.
1042 .It Cm X11Forwarding
1043 Specifies whether X11 forwarding is permitted.
1044 The argument must be
1045 .Dq yes
1046 or
1047 .Dq no .
1048 The default is
1049 .Dq no .
1050 .Pp
1051 When X11 forwarding is enabled, there may be additional exposure to
1052 the server and to client displays if the
1053 .Xr sshd 8
1054 proxy display is configured to listen on the wildcard address (see
1055 .Cm X11UseLocalhost
1056 below), though this is not the default.
1057 Additionally, the authentication spoofing and authentication data
1058 verification and substitution occur on the client side.
1059 The security risk of using X11 forwarding is that the client's X11
1060 display server may be exposed to attack when the SSH client requests
1061 forwarding (see the warnings for
1062 .Cm ForwardX11
1063 in
1064 .Xr ssh_config 5 ) .
1065 A system administrator may have a stance in which they want to
1066 protect clients that may expose themselves to attack by unwittingly
1067 requesting X11 forwarding, which can warrant a
1068 .Dq no
1069 setting.
1070 .Pp
1071 Note that disabling X11 forwarding does not prevent users from
1072 forwarding X11 traffic, as users can always install their own forwarders.
1073 X11 forwarding is automatically disabled if
1074 .Cm UseLogin
1075 is enabled.
1076 .It Cm X11UseLocalhost
1077 Specifies whether
1078 .Xr sshd 8
1079 should bind the X11 forwarding server to the loopback address or to
1080 the wildcard address.
1081 By default,
1082 sshd binds the forwarding server to the loopback address and sets the
1083 hostname part of the
1084 .Ev DISPLAY
1085 environment variable to
1086 .Dq localhost .
1087 This prevents remote hosts from connecting to the proxy display.
1088 However, some older X11 clients may not function with this
1089 configuration.
1090 .Cm X11UseLocalhost
1091 may be set to
1092 .Dq no
1093 to specify that the forwarding server should be bound to the wildcard
1094 address.
1095 The argument must be
1096 .Dq yes
1097 or
1098 .Dq no .
1099 The default is
1100 .Dq yes .
1101 .It Cm XAuthLocation
1102 Specifies the full pathname of the
1103 .Xr xauth 1
1104 program.
1105 The default is
1106 .Pa /usr/X11R6/bin/xauth .
1107 .El
1108 .Sh TIME FORMATS
1109 .Xr sshd 8
1110 command-line arguments and configuration file options that specify time
1111 may be expressed using a sequence of the form:
1112 .Sm off
1113 .Ar time Op Ar qualifier ,
1114 .Sm on
1115 where
1116 .Ar time
1117 is a positive integer value and
1118 .Ar qualifier
1119 is one of the following:
1120 .Pp
1121 .Bl -tag -width Ds -compact -offset indent
1122 .It Aq Cm none
1123 seconds
1124 .It Cm s | Cm S
1125 seconds
1126 .It Cm m | Cm M
1127 minutes
1128 .It Cm h | Cm H
1129 hours
1130 .It Cm d | Cm D
1131 days
1132 .It Cm w | Cm W
1133 weeks
1134 .El
1135 .Pp
1136 Each member of the sequence is added together to calculate
1137 the total time value.
1138 .Pp
1139 Time format examples:
1140 .Pp
1141 .Bl -tag -width Ds -compact -offset indent
1142 .It 600
1143 600 seconds (10 minutes)
1144 .It 10m
1145 10 minutes
1146 .It 1h30m
1147 1 hour 30 minutes (90 minutes)
1148 .El
1149 .Sh FILES
1150 .Bl -tag -width Ds
1151 .It Pa /etc/ssh/sshd_config
1152 Contains configuration data for
1153 .Xr sshd 8 .
1154 This file should be writable by root only, but it is recommended
1155 (though not necessary) that it be world-readable.
1156 .El
1157 .Sh SEE ALSO
1158 .Xr sshd 8
1159 .Sh AUTHORS
1160 OpenSSH is a derivative of the original and free
1161 ssh 1.2.12 release by Tatu Ylonen.
1162 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1163 Theo de Raadt and Dug Song
1164 removed many bugs, re-added newer features and
1165 created OpenSSH.
1166 Markus Friedl contributed the support for SSH
1167 protocol versions 1.5 and 2.0.
1168 Niels Provos and Markus Friedl contributed support
1169 for privilege separation.