| blob | d87f60246561597dcf065d5d9f3840e60c0d158f |
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 1.127 2010/09/22 05:01:30 djm Exp $
38 .Dd $Mdocdate: September 22 2010 $
39 .Dt SSHD_CONFIG 5
40 .Os
41 .Sh NAME
42 .Nm sshd_config
43 .Nd OpenSSH SSH daemon configuration file
44 .Sh SYNOPSIS
45 .Nm /etc/ssh/sshd_config
46 .Sh DESCRIPTION
47 .Xr sshd 8
48 reads configuration data from
49 .Pa /etc/ssh/sshd_config
50 (or the file specified with
51 .Fl f
52 on the command line).
53 The file contains keyword-argument pairs, one per line.
54 Lines starting with
55 .Ql #
56 and empty lines are interpreted as comments.
57 Arguments may optionally be enclosed in double quotes
58 .Pq \&"
59 in order to represent arguments containing spaces.
60 .Pp
61 The possible
62 keywords and their meanings are as follows (note that
63 keywords are case-insensitive and arguments are case-sensitive):
64 .Bl -tag -width Ds
65 .It Cm AcceptEnv
66 Specifies what environment variables sent by the client will be copied into
67 the session's
68 .Xr environ 7 .
69 See
70 .Cm SendEnv
71 in
72 .Xr ssh_config 5
73 for how to configure the client.
74 Note that environment passing is only supported for protocol 2.
75 Variables are specified by name, which may contain the wildcard characters
76 .Ql *
77 and
78 .Ql \&? .
79 Multiple environment variables may be separated by whitespace or spread
80 across multiple
81 .Cm AcceptEnv
82 directives.
83 Be warned that some environment variables could be used to bypass restricted
84 user environments.
85 For this reason, care should be taken in the use of this directive.
86 The default is not to accept any environment variables.
87 .It Cm AddressFamily
88 Specifies which address family should be used by
89 .Xr sshd 8 .
90 Valid arguments are
91 .Dq any ,
92 .Dq inet
93 (use IPv4 only), or
94 .Dq inet6
95 (use IPv6 only).
96 The default is
97 .Dq any .
98 .It Cm AllowAgentForwarding
99 Specifies whether
100 .Xr ssh-agent 1
101 forwarding is permitted.
102 The default is
103 .Dq yes .
104 Note that disabling agent forwarding does not improve security
105 unless users are also denied shell access, as they can always install
106 their own forwarders.
107 .It Cm AllowGroups
108 This keyword can be followed by a list of group name patterns, separated
109 by spaces.
110 If specified, login is allowed only for users whose primary
111 group or supplementary group list matches one of the patterns.
112 Only group names are valid; a numerical group ID is not recognized.
113 By default, login is allowed for all groups.
114 The allow/deny directives are processed in the following order:
115 .Cm DenyUsers ,
116 .Cm AllowUsers ,
117 .Cm DenyGroups ,
118 and finally
119 .Cm AllowGroups .
120 .Pp
121 See
122 .Sx PATTERNS
123 in
124 .Xr ssh_config 5
125 for more information on patterns.
126 .It Cm AllowTcpForwarding
127 Specifies whether TCP forwarding is permitted.
128 The default is
129 .Dq yes .
130 Note that disabling TCP forwarding does not improve security unless
131 users are also denied shell access, as they can always install their
132 own forwarders.
133 .It Cm AllowUsers
134 This keyword can be followed by a list of user name patterns, separated
135 by spaces.
136 If specified, login is allowed only for user names that
137 match one of the patterns.
138 Only user names are valid; a numerical user ID is not recognized.
139 By default, login is allowed for all users.
140 If the pattern takes the form USER@HOST then USER and HOST
141 are separately checked, restricting logins to particular
142 users from particular hosts.
143 The allow/deny directives are processed in the following order:
144 .Cm DenyUsers ,
145 .Cm AllowUsers ,
146 .Cm DenyGroups ,
147 and finally
148 .Cm AllowGroups .
149 .Pp
150 See
151 .Sx PATTERNS
152 in
153 .Xr ssh_config 5
154 for more information on patterns.
155 .It Cm AuthorizedKeysFile
156 Specifies the file that contains the public keys that can be used
157 for user authentication.
158 The format is described in the
159 .Sx AUTHORIZED_KEYS FILE FORMAT
160 section of
161 .Xr sshd 8 .
162 .Cm AuthorizedKeysFile
163 may contain tokens of the form %T which are substituted during connection
164 setup.
165 The following tokens are defined: %% is replaced by a literal '%',
166 %h is replaced by the home directory of the user being authenticated, and
167 %u is replaced by the username of that user.
168 After expansion,
169 .Cm AuthorizedKeysFile
170 is taken to be an absolute path or one relative to the user's home
171 directory.
172 The default is
173 .Dq .ssh/authorized_keys .
174 .It Cm AuthorizedPrincipalsFile
175 Specifies a file that lists principal names that are accepted for
176 certificate authentication.
177 When using certificates signed by a key listed in
178 .Cm TrustedUserCAKeys ,
179 this file lists names, one of which must appear in the certificate for it
180 to be accepted for authentication.
181 Names are listed one per line preceded by key options (as described
182 in
183 .Sx AUTHORIZED_KEYS FILE FORMAT
184 in
185 .Xr sshd 8 ) .
186 Empty lines and comments starting with
187 .Ql #
188 are ignored.
189 .Pp
190 .Cm AuthorizedPrincipalsFile
191 may contain tokens of the form %T which are substituted during connection
192 setup.
193 The following tokens are defined: %% is replaced by a literal '%',
194 %h is replaced by the home directory of the user being authenticated, and
195 %u is replaced by the username of that user.
196 After expansion,
197 .Cm AuthorizedPrincipalsFile
198 is taken to be an absolute path or one relative to the user's home
199 directory.
200 .Pp
201 The default is not to use a principals file \(en in this case, the username
202 of the user must appear in a certificate's principals list for it to be
203 accepted.
204 Note that
205 .Cm AuthorizedPrincipalsFile
206 is only used when authentication proceeds using a CA listed in
207 .Cm TrustedUserCAKeys
208 and is not consulted for certification authorities trusted via
209 .Pa ~/.ssh/authorized_keys ,
210 though the
211 .Cm principals=
212 key option offers a similar facility (see
213 .Xr sshd 8
214 for details).
215 .It Cm Banner
216 The contents of the specified file are sent to the remote user before
217 authentication is allowed.
218 If the argument is
219 .Dq none
220 then no banner is displayed.
221 This option is only available for protocol version 2.
222 By default, no banner is displayed.
223 .It Cm ChallengeResponseAuthentication
224 Specifies whether challenge-response authentication is allowed (e.g. via
225 PAM or though authentication styles supported in
226 .Xr login.conf 5 )
227 The default is
228 .Dq yes .
229 .It Cm ChrootDirectory
230 Specifies the pathname of a directory to
231 .Xr chroot 2
232 to after authentication.
233 All components of the pathname must be root-owned directories that are
234 not writable by any other user or group.
235 After the chroot,
236 .Xr sshd 8
237 changes the working directory to the user's home directory.
238 .Pp
239 The pathname may contain the following tokens that are expanded at runtime once
240 the connecting user has been authenticated: %% is replaced by a literal '%',
241 %h is replaced by the home directory of the user being authenticated, and
242 %u is replaced by the username of that user.
243 .Pp
244 The
245 .Cm ChrootDirectory
246 must contain the necessary files and directories to support the
247 user's session.
248 For an interactive session this requires at least a shell, typically
249 .Xr sh 1 ,
250 and basic
251 .Pa /dev
252 nodes such as
253 .Xr null 4 ,
254 .Xr zero 4 ,
255 .Xr stdin 4 ,
256 .Xr stdout 4 ,
257 .Xr stderr 4 ,
258 .Xr arandom 4
259 and
260 .Xr tty 4
261 devices.
262 For file transfer sessions using
263 .Dq sftp ,
264 no additional configuration of the environment is necessary if the
265 in-process sftp server is used,
266 though sessions which use logging do require
267 .Pa /dev/log
268 inside the chroot directory (see
269 .Xr sftp-server 8
270 for details).
271 .Pp
272 The default is not to
273 .Xr chroot 2 .
274 .It Cm Ciphers
275 Specifies the ciphers allowed for protocol version 2.
276 Multiple ciphers must be comma-separated.
277 The supported ciphers are
278 .Dq 3des-cbc ,
279 .Dq aes128-cbc ,
280 .Dq aes192-cbc ,
281 .Dq aes256-cbc ,
282 .Dq aes128-ctr ,
283 .Dq aes192-ctr ,
284 .Dq aes256-ctr ,
285 .Dq arcfour128 ,
286 .Dq arcfour256 ,
287 .Dq arcfour ,
288 .Dq blowfish-cbc ,
289 and
290 .Dq cast128-cbc .
291 The default is:
292 .Bd -literal -offset 3n
293 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
294 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
295 aes256-cbc,arcfour
296 .Ed
297 .It Cm ClientAliveCountMax
298 Sets the number of client alive messages (see below) which may be
299 sent without
300 .Xr sshd 8
301 receiving any messages back from the client.
302 If this threshold is reached while client alive messages are being sent,
303 sshd will disconnect the client, terminating the session.
304 It is important to note that the use of client alive messages is very
305 different from
306 .Cm TCPKeepAlive
307 (below).
308 The client alive messages are sent through the encrypted channel
309 and therefore will not be spoofable.
310 The TCP keepalive option enabled by
311 .Cm TCPKeepAlive
312 is spoofable.
313 The client alive mechanism is valuable when the client or
314 server depend on knowing when a connection has become inactive.
315 .Pp
316 The default value is 3.
317 If
318 .Cm ClientAliveInterval
319 (see below) is set to 15, and
320 .Cm ClientAliveCountMax
321 is left at the default, unresponsive SSH clients
322 will be disconnected after approximately 45 seconds.
323 This option applies to protocol version 2 only.
324 .It Cm ClientAliveInterval
325 Sets a timeout interval in seconds after which if no data has been received
326 from the client,
327 .Xr sshd 8
328 will send a message through the encrypted
329 channel to request a response from the client.
330 The default
331 is 0, indicating that these messages will not be sent to the client.
332 This option applies to protocol version 2 only.
333 .It Cm Compression
334 Specifies whether compression is allowed, or delayed until
335 the user has authenticated successfully.
336 The argument must be
337 .Dq yes ,
338 .Dq delayed ,
339 or
340 .Dq no .
341 The default is
342 .Dq delayed .
343 .It Cm DenyGroups
344 This keyword can be followed by a list of group name patterns, separated
345 by spaces.
346 Login is disallowed for users whose primary group or supplementary
347 group list matches one of the patterns.
348 Only group names are valid; a numerical group ID is not recognized.
349 By default, login is allowed for all groups.
350 The allow/deny directives are processed in the following order:
351 .Cm DenyUsers ,
352 .Cm AllowUsers ,
353 .Cm DenyGroups ,
354 and finally
355 .Cm AllowGroups .
356 .Pp
357 See
358 .Sx PATTERNS
359 in
360 .Xr ssh_config 5
361 for more information on patterns.
362 .It Cm DenyUsers
363 This keyword can be followed by a list of user name patterns, separated
364 by spaces.
365 Login is disallowed for user names that match one of the patterns.
366 Only user names are valid; a numerical user ID is not recognized.
367 By default, login is allowed for all users.
368 If the pattern takes the form USER@HOST then USER and HOST
369 are separately checked, restricting logins to particular
370 users from particular hosts.
371 The allow/deny directives are processed in the following order:
372 .Cm DenyUsers ,
373 .Cm AllowUsers ,
374 .Cm DenyGroups ,
375 and finally
376 .Cm AllowGroups .
377 .Pp
378 See
379 .Sx PATTERNS
380 in
381 .Xr ssh_config 5
382 for more information on patterns.
383 .It Cm ForceCommand
384 Forces the execution of the command specified by
385 .Cm ForceCommand ,
386 ignoring any command supplied by the client and
387 .Pa ~/.ssh/rc
388 if present.
389 The command is invoked by using the user's login shell with the -c option.
390 This applies to shell, command, or subsystem execution.
391 It is most useful inside a
392 .Cm Match
393 block.
394 The command originally supplied by the client is available in the
395 .Ev SSH_ORIGINAL_COMMAND
396 environment variable.
397 Specifying a command of
398 .Dq internal-sftp
399 will force the use of an in-process sftp server that requires no support
400 files when used with
401 .Cm ChrootDirectory .
402 .It Cm GatewayPorts
403 Specifies whether remote hosts are allowed to connect to ports
404 forwarded for the client.
405 By default,
406 .Xr sshd 8
407 binds remote port forwardings to the loopback address.
408 This prevents other remote hosts from connecting to forwarded ports.
409 .Cm GatewayPorts
410 can be used to specify that sshd
411 should allow remote port forwardings to bind to non-loopback addresses, thus
412 allowing other hosts to connect.
413 The argument may be
414 .Dq no
415 to force remote port forwardings to be available to the local host only,
416 .Dq yes
417 to force remote port forwardings to bind to the wildcard address, or
418 .Dq clientspecified
419 to allow the client to select the address to which the forwarding is bound.
420 The default is
421 .Dq no .
422 .It Cm GSSAPIAuthentication
423 Specifies whether user authentication based on GSSAPI is allowed.
424 The default is
425 .Dq no .
426 Note that this option applies to protocol version 2 only.
427 .It Cm GSSAPICleanupCredentials
428 Specifies whether to automatically destroy the user's credentials cache
429 on logout.
430 The default is
431 .Dq yes .
432 Note that this option applies to protocol version 2 only.
433 .It Cm HostbasedAuthentication
434 Specifies whether rhosts or /etc/hosts.equiv authentication together
435 with successful public key client host authentication is allowed
436 (host-based authentication).
437 This option is similar to
438 .Cm RhostsRSAAuthentication
439 and applies to protocol version 2 only.
440 The default is
441 .Dq no .
442 .It Cm HostbasedUsesNameFromPacketOnly
443 Specifies whether or not the server will attempt to perform a reverse
444 name lookup when matching the name in the
445 .Pa ~/.shosts ,
446 .Pa ~/.rhosts ,
447 and
448 .Pa /etc/hosts.equiv
449 files during
450 .Cm HostbasedAuthentication .
451 A setting of
452 .Dq yes
453 means that
454 .Xr sshd 8
455 uses the name supplied by the client rather than
456 attempting to resolve the name from the TCP connection itself.
457 The default is
458 .Dq no .
459 .It Cm HostCertificate
460 Specifies a file containing a public host certificate.
461 The certificate's public key must match a private host key already specified
462 by
463 .Cm HostKey .
464 The default behaviour of
465 .Xr sshd 8
466 is not to load any certificates.
467 .It Cm HostKey
468 Specifies a file containing a private host key
469 used by SSH.
470 The default is
471 .Pa /etc/ssh/ssh_host_key
472 for protocol version 1, and
473 .Pa /etc/ssh/ssh_host_dsa_key ,
474 .Pa /etc/ssh/ssh_host_ecdsa_key
475 and
476 .Pa /etc/ssh/ssh_host_rsa_key
477 for protocol version 2.
478 Note that
479 .Xr sshd 8
480 will refuse to use a file if it is group/world-accessible.
481 It is possible to have multiple host key files.
482 .Dq rsa1
483 keys are used for version 1 and
484 .Dq dsa ,
485 .Dq ecdsa
486 or
487 .Dq rsa
488 are used for version 2 of the SSH protocol.
489 .It Cm IgnoreRhosts
490 Specifies that
491 .Pa .rhosts
492 and
493 .Pa .shosts
494 files will not be used in
495 .Cm RhostsRSAAuthentication
496 or
497 .Cm HostbasedAuthentication .
498 .Pp
499 .Pa /etc/hosts.equiv
500 and
501 .Pa /etc/shosts.equiv
502 are still used.
503 The default is
504 .Dq yes .
505 .It Cm IgnoreUserKnownHosts
506 Specifies whether
507 .Xr sshd 8
508 should ignore the user's
509 .Pa ~/.ssh/known_hosts
510 during
511 .Cm RhostsRSAAuthentication
512 or
513 .Cm HostbasedAuthentication .
514 The default is
515 .Dq no .
516 .It Cm KerberosAuthentication
517 Specifies whether the password provided by the user for
518 .Cm PasswordAuthentication
519 will be validated through the Kerberos KDC.
520 To use this option, the server needs a
521 Kerberos servtab which allows the verification of the KDC's identity.
522 The default is
523 .Dq no .
524 .It Cm KerberosGetAFSToken
525 If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
526 an AFS token before accessing the user's home directory.
527 The default is
528 .Dq no .
529 .It Cm KerberosOrLocalPasswd
530 If password authentication through Kerberos fails then
531 the password will be validated via any additional local mechanism
532 such as
533 .Pa /etc/passwd .
534 The default is
535 .Dq yes .
536 .It Cm KerberosTicketCleanup
537 Specifies whether to automatically destroy the user's ticket cache
538 file on logout.
539 The default is
540 .Dq yes .
541 .It Cm KexAlgorithms
542 Specifies the available KEX (Key Exchange) algorithms.
543 Multiple algorithms must be comma-separated.
544 The default is
545 .Dq ecdh-sha2-nistp256 ,
546 .Dq ecdh-sha2-nistp384 ,
547 .Dq ecdh-sha2-nistp521 ,
548 .Dq diffie-hellman-group-exchange-sha256 ,
549 .Dq diffie-hellman-group-exchange-sha1 ,
550 .Dq diffie-hellman-group14-sha1 ,
551 .Dq diffie-hellman-group1-sha1 .
552 .It Cm KeyRegenerationInterval
553 In protocol version 1, the ephemeral server key is automatically regenerated
554 after this many seconds (if it has been used).
555 The purpose of regeneration is to prevent
556 decrypting captured sessions by later breaking into the machine and
557 stealing the keys.
558 The key is never stored anywhere.
559 If the value is 0, the key is never regenerated.
560 The default is 3600 (seconds).
561 .It Cm ListenAddress
562 Specifies the local addresses
563 .Xr sshd 8
564 should listen on.
565 The following forms may be used:
566 .Pp
567 .Bl -item -offset indent -compact
568 .It
569 .Cm ListenAddress
570 .Sm off
571 .Ar host No | Ar IPv4_addr No | Ar IPv6_addr
572 .Sm on
573 .It
574 .Cm ListenAddress
575 .Sm off
576 .Ar host No | Ar IPv4_addr No : Ar port
577 .Sm on
578 .It
579 .Cm ListenAddress
580 .Sm off
581 .Oo
582 .Ar host No | Ar IPv6_addr Oc : Ar port
583 .Sm on
584 .El
585 .Pp
586 If
587 .Ar port
588 is not specified,
589 sshd will listen on the address and all prior
590 .Cm Port
591 options specified.
592 The default is to listen on all local addresses.
593 Multiple
594 .Cm ListenAddress
595 options are permitted.
596 Additionally, any
597 .Cm Port
598 options must precede this option for non-port qualified addresses.
599 .It Cm LoginGraceTime
600 The server disconnects after this time if the user has not
601 successfully logged in.
602 If the value is 0, there is no time limit.
603 The default is 120 seconds.
604 .It Cm LogLevel
605 Gives the verbosity level that is used when logging messages from
606 .Xr sshd 8 .
607 The possible values are:
608 QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
609 The default is INFO.
610 DEBUG and DEBUG1 are equivalent.
611 DEBUG2 and DEBUG3 each specify higher levels of debugging output.
612 Logging with a DEBUG level violates the privacy of users and is not recommended.
613 .It Cm MACs
614 Specifies the available MAC (message authentication code) algorithms.
615 The MAC algorithm is used in protocol version 2
616 for data integrity protection.
617 Multiple algorithms must be comma-separated.
618 The default is:
619 .Bd -literal -offset indent
620 hmac-md5,hmac-sha1,umac-64@openssh.com,
621 hmac-ripemd160,hmac-sha1-96,hmac-md5-96
622 .Ed
623 .It Cm Match
624 Introduces a conditional block.
625 If all of the criteria on the
626 .Cm Match
627 line are satisfied, the keywords on the following lines override those
628 set in the global section of the config file, until either another
629 .Cm Match
630 line or the end of the file.
631 .Pp
632 The arguments to
633 .Cm Match
634 are one or more criteria-pattern pairs.
635 The available criteria are
636 .Cm User ,
637 .Cm Group ,
638 .Cm Host ,
639 and
640 .Cm Address .
641 The match patterns may consist of single entries or comma-separated
642 lists and may use the wildcard and negation operators described in the
643 .Sx PATTERNS
644 section of
645 .Xr ssh_config 5 .
646 .Pp
647 The patterns in an
648 .Cm Address
649 criteria may additionally contain addresses to match in CIDR
650 address/masklen format, e.g.\&
651 .Dq 192.0.2.0/24
652 or
653 .Dq 3ffe:ffff::/32 .
654 Note that the mask length provided must be consistent with the address -
655 it is an error to specify a mask length that is too long for the address
656 or one with bits set in this host portion of the address.
657 For example,
658 .Dq 192.0.2.0/33
659 and
660 .Dq 192.0.2.0/8
661 respectively.
662 .Pp
663 Only a subset of keywords may be used on the lines following a
664 .Cm Match
665 keyword.
666 Available keywords are
667 .Cm AllowAgentForwarding ,
668 .Cm AllowTcpForwarding ,
669 .Cm AuthorizedKeysFile ,
670 .Cm AuthorizedPrincipalsFile ,
671 .Cm Banner ,
672 .Cm ChrootDirectory ,
673 .Cm ForceCommand ,
674 .Cm GatewayPorts ,
675 .Cm GSSAPIAuthentication ,
676 .Cm HostbasedAuthentication ,
677 .Cm HostbasedUsesNameFromPacketOnly ,
678 .Cm KbdInteractiveAuthentication ,
679 .Cm KerberosAuthentication ,
680 .Cm MaxAuthTries ,
681 .Cm MaxSessions ,
682 .Cm PasswordAuthentication ,
683 .Cm PermitEmptyPasswords ,
684 .Cm PermitOpen ,
685 .Cm PermitRootLogin ,
686 .Cm PermitTunnel ,
687 .Cm PubkeyAuthentication ,
688 .Cm RhostsRSAAuthentication ,
689 .Cm RSAAuthentication ,
690 .Cm X11DisplayOffset ,
691 .Cm X11Forwarding
692 and
693 .Cm X11UseLocalHost .
694 .It Cm MaxAuthTries
695 Specifies the maximum number of authentication attempts permitted per
696 connection.
697 Once the number of failures reaches half this value,
698 additional failures are logged.
699 The default is 6.
700 .It Cm MaxSessions
701 Specifies the maximum number of open sessions permitted per network connection.
702 The default is 10.
703 .It Cm MaxStartups
704 Specifies the maximum number of concurrent unauthenticated connections to the
705 SSH daemon.
706 Additional connections will be dropped until authentication succeeds or the
707 .Cm LoginGraceTime
708 expires for a connection.
709 The default is 10.
710 .Pp
711 Alternatively, random early drop can be enabled by specifying
712 the three colon separated values
713 .Dq start:rate:full
714 (e.g. "10:30:60").
715 .Xr sshd 8
716 will refuse connection attempts with a probability of
717 .Dq rate/100
718 (30%)
719 if there are currently
720 .Dq start
721 (10)
722 unauthenticated connections.
723 The probability increases linearly and all connection attempts
724 are refused if the number of unauthenticated connections reaches
725 .Dq full
726 (60).
727 .It Cm PasswordAuthentication
728 Specifies whether password authentication is allowed.
729 The default is
730 .Dq yes .
731 .It Cm PermitEmptyPasswords
732 When password authentication is allowed, it specifies whether the
733 server allows login to accounts with empty password strings.
734 The default is
735 .Dq no .
736 .It Cm PermitOpen
737 Specifies the destinations to which TCP port forwarding is permitted.
738 The forwarding specification must be one of the following forms:
739 .Pp
740 .Bl -item -offset indent -compact
741 .It
742 .Cm PermitOpen
743 .Sm off
744 .Ar host : port
745 .Sm on
746 .It
747 .Cm PermitOpen
748 .Sm off
749 .Ar IPv4_addr : port
750 .Sm on
751 .It
752 .Cm PermitOpen
753 .Sm off
754 .Ar \&[ IPv6_addr \&] : port
755 .Sm on
756 .El
757 .Pp
758 Multiple forwards may be specified by separating them with whitespace.
759 An argument of
760 .Dq any
761 can be used to remove all restrictions and permit any forwarding requests.
762 By default all port forwarding requests are permitted.
763 .It Cm PermitRootLogin
764 Specifies whether root can log in using
765 .Xr ssh 1 .
766 The argument must be
767 .Dq yes ,
768 .Dq without-password ,
769 .Dq forced-commands-only ,
770 or
771 .Dq no .
772 The default is
773 .Dq yes .
774 .Pp
775 If this option is set to
776 .Dq without-password ,
777 password authentication is disabled for root.
778 .Pp
779 If this option is set to
780 .Dq forced-commands-only ,
781 root login with public key authentication will be allowed,
782 but only if the
783 .Ar command
784 option has been specified
785 (which may be useful for taking remote backups even if root login is
786 normally not allowed).
787 All other authentication methods are disabled for root.
788 .Pp
789 If this option is set to
790 .Dq no ,
791 root is not allowed to log in.
792 .It Cm PermitTunnel
793 Specifies whether
794 .Xr tun 4
795 device forwarding is allowed.
796 The argument must be
797 .Dq yes ,
798 .Dq point-to-point
799 (layer 3),
800 .Dq ethernet
801 (layer 2), or
802 .Dq no .
803 Specifying
804 .Dq yes
805 permits both
806 .Dq point-to-point
807 and
808 .Dq ethernet .
809 The default is
810 .Dq no .
811 .It Cm PermitUserEnvironment
812 Specifies whether
813 .Pa ~/.ssh/environment
814 and
815 .Cm environment=
816 options in
817 .Pa ~/.ssh/authorized_keys
818 are processed by
819 .Xr sshd 8 .
820 The default is
821 .Dq no .
822 Enabling environment processing may enable users to bypass access
823 restrictions in some configurations using mechanisms such as
824 .Ev LD_PRELOAD .
825 .It Cm PidFile
826 Specifies the file that contains the process ID of the
827 SSH daemon.
828 The default is
829 .Pa /var/run/sshd.pid .
830 .It Cm Port
831 Specifies the port number that
832 .Xr sshd 8
833 listens on.
834 The default is 22.
835 Multiple options of this type are permitted.
836 See also
837 .Cm ListenAddress .
838 .It Cm PrintLastLog
839 Specifies whether
840 .Xr sshd 8
841 should print the date and time of the last user login when a user logs
842 in interactively.
843 The default is
844 .Dq yes .
845 .It Cm PrintMotd
846 Specifies whether
847 .Xr sshd 8
848 should print
849 .Pa /etc/motd
850 when a user logs in interactively.
851 (On some systems it is also printed by the shell,
852 .Pa /etc/profile ,
853 or equivalent.)
854 The default is
855 .Dq yes .
856 .It Cm Protocol
857 Specifies the protocol versions
858 .Xr sshd 8
859 supports.
860 The possible values are
861 .Sq 1
862 and
863 .Sq 2 .
864 Multiple versions must be comma-separated.
865 The default is
866 .Sq 2 .
867 Note that the order of the protocol list does not indicate preference,
868 because the client selects among multiple protocol versions offered
869 by the server.
870 Specifying
871 .Dq 2,1
872 is identical to
873 .Dq 1,2 .
874 .It Cm PubkeyAuthentication
875 Specifies whether public key authentication is allowed.
876 The default is
877 .Dq yes .
878 Note that this option applies to protocol version 2 only.
879 .It Cm RevokedKeys
880 Specifies a list of revoked public keys.
881 Keys listed in this file will be refused for public key authentication.
882 Note that if this file is not readable, then public key authentication will
883 be refused for all users.
884 .It Cm RhostsRSAAuthentication
885 Specifies whether rhosts or /etc/hosts.equiv authentication together
886 with successful RSA host authentication is allowed.
887 The default is
888 .Dq no .
889 This option applies to protocol version 1 only.
890 .It Cm RSAAuthentication
891 Specifies whether pure RSA authentication is allowed.
892 The default is
893 .Dq yes .
894 This option applies to protocol version 1 only.
895 .It Cm ServerKeyBits
896 Defines the number of bits in the ephemeral protocol version 1 server key.
897 The minimum value is 512, and the default is 1024.
898 .It Cm StrictModes
899 Specifies whether
900 .Xr sshd 8
901 should check file modes and ownership of the
902 user's files and home directory before accepting login.
903 This is normally desirable because novices sometimes accidentally leave their
904 directory or files world-writable.
905 The default is
906 .Dq yes .
907 Note that this does not apply to
908 .Cm ChrootDirectory ,
909 whose permissions and ownership are checked unconditionally.
910 .It Cm Subsystem
911 Configures an external subsystem (e.g. file transfer daemon).
912 Arguments should be a subsystem name and a command (with optional arguments)
913 to execute upon subsystem request.
914 .Pp
915 The command
916 .Xr sftp-server 8
917 implements the
918 .Dq sftp
919 file transfer subsystem.
920 .Pp
921 Alternately the name
922 .Dq internal-sftp
923 implements an in-process
924 .Dq sftp
925 server.
926 This may simplify configurations using
927 .Cm ChrootDirectory
928 to force a different filesystem root on clients.
929 .Pp
930 By default no subsystems are defined.
931 Note that this option applies to protocol version 2 only.
932 .It Cm SyslogFacility
933 Gives the facility code that is used when logging messages from
934 .Xr sshd 8 .
935 The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
936 LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
937 The default is AUTH.
938 .It Cm TCPKeepAlive
939 Specifies whether the system should send TCP keepalive messages to the
940 other side.
941 If they are sent, death of the connection or crash of one
942 of the machines will be properly noticed.
943 However, this means that
944 connections will die if the route is down temporarily, and some people
945 find it annoying.
946 On the other hand, if TCP keepalives are not sent,
947 sessions may hang indefinitely on the server, leaving
948 .Dq ghost
949 users and consuming server resources.
950 .Pp
951 The default is
952 .Dq yes
953 (to send TCP keepalive messages), and the server will notice
954 if the network goes down or the client host crashes.
955 This avoids infinitely hanging sessions.
956 .Pp
957 To disable TCP keepalive messages, the value should be set to
958 .Dq no .
959 .It Cm TrustedUserCAKeys
960 Specifies a file containing public keys of certificate authorities that are
961 trusted to sign user certificates for authentication.
962 Keys are listed one per line; empty lines and comments starting with
963 .Ql #
964 are allowed.
965 If a certificate is presented for authentication and has its signing CA key
966 listed in this file, then it may be used for authentication for any user
967 listed in the certificate's principals list.
968 Note that certificates that lack a list of principals will not be permitted
969 for authentication using
970 .Cm TrustedUserCAKeys .
971 For more details on certificates, see the
972 .Sx CERTIFICATES
973 section in
974 .Xr ssh-keygen 1 .
975 .It Cm UseDNS
976 Specifies whether
977 .Xr sshd 8
978 should look up the remote host name and check that
979 the resolved host name for the remote IP address maps back to the
980 very same IP address.
981 The default is
982 .Dq yes .
983 .It Cm UseLogin
984 Specifies whether
985 .Xr login 1
986 is used for interactive login sessions.
987 The default is
988 .Dq no .
989 Note that
990 .Xr login 1
991 is never used for remote command execution.
992 Note also, that if this is enabled,
993 .Cm X11Forwarding
994 will be disabled because
995 .Xr login 1
996 does not know how to handle
997 .Xr xauth 1
998 cookies.
999 If
1000 .Cm UsePrivilegeSeparation
1001 is specified, it will be disabled after authentication.
1002 .It Cm UsePAM
1003 Enables the Pluggable Authentication Module interface.
1004 If set to
1005 .Dq yes
1006 this will enable PAM authentication using
1007 .Cm ChallengeResponseAuthentication
1008 and
1009 .Cm PasswordAuthentication
1010 in addition to PAM account and session module processing for all
1011 authentication types.
1012 .Pp
1013 Because PAM challenge-response authentication usually serves an equivalent
1014 role to password authentication, you should disable either
1015 .Cm PasswordAuthentication
1016 or
1017 .Cm ChallengeResponseAuthentication.
1018 .Pp
1019 If
1020 .Cm UsePAM
1021 is enabled, you will not be able to run
1022 .Xr sshd 8
1023 as a non-root user.
1024 The default is
1025 .Dq no .
1026 .It Cm UsePrivilegeSeparation
1027 Specifies whether
1028 .Xr sshd 8
1029 separates privileges by creating an unprivileged child process
1030 to deal with incoming network traffic.
1031 After successful authentication, another process will be created that has
1032 the privilege of the authenticated user.
1033 The goal of privilege separation is to prevent privilege
1034 escalation by containing any corruption within the unprivileged processes.
1035 The default is
1036 .Dq yes .
1037 .It Cm X11DisplayOffset
1038 Specifies the first display number available for
1039 .Xr sshd 8 Ns 's
1040 X11 forwarding.
1041 This prevents sshd from interfering with real X11 servers.
1042 The default is 10.
1043 .It Cm X11Forwarding
1044 Specifies whether X11 forwarding is permitted.
1045 The argument must be
1046 .Dq yes
1047 or
1048 .Dq no .
1049 The default is
1050 .Dq no .
1051 .Pp
1052 When X11 forwarding is enabled, there may be additional exposure to
1053 the server and to client displays if the
1054 .Xr sshd 8
1055 proxy display is configured to listen on the wildcard address (see
1056 .Cm X11UseLocalhost
1057 below), though this is not the default.
1058 Additionally, the authentication spoofing and authentication data
1059 verification and substitution occur on the client side.
1060 The security risk of using X11 forwarding is that the client's X11
1061 display server may be exposed to attack when the SSH client requests
1062 forwarding (see the warnings for
1063 .Cm ForwardX11
1064 in
1065 .Xr ssh_config 5 ) .
1066 A system administrator may have a stance in which they want to
1067 protect clients that may expose themselves to attack by unwittingly
1068 requesting X11 forwarding, which can warrant a
1069 .Dq no
1070 setting.
1071 .Pp
1072 Note that disabling X11 forwarding does not prevent users from
1073 forwarding X11 traffic, as users can always install their own forwarders.
1074 X11 forwarding is automatically disabled if
1075 .Cm UseLogin
1076 is enabled.
1077 .It Cm X11UseLocalhost
1078 Specifies whether
1079 .Xr sshd 8
1080 should bind the X11 forwarding server to the loopback address or to
1081 the wildcard address.
1082 By default,
1083 sshd binds the forwarding server to the loopback address and sets the
1084 hostname part of the
1085 .Ev DISPLAY
1086 environment variable to
1087 .Dq localhost .
1088 This prevents remote hosts from connecting to the proxy display.
1089 However, some older X11 clients may not function with this
1090 configuration.
1091 .Cm X11UseLocalhost
1092 may be set to
1093 .Dq no
1094 to specify that the forwarding server should be bound to the wildcard
1095 address.
1096 The argument must be
1097 .Dq yes
1098 or
1099 .Dq no .
1100 The default is
1101 .Dq yes .
1102 .It Cm XAuthLocation
1103 Specifies the full pathname of the
1104 .Xr xauth 1
1105 program.
1106 The default is
1107 .Pa /usr/X11R6/bin/xauth .
1108 .El
1109 .Sh TIME FORMATS
1110 .Xr sshd 8
1111 command-line arguments and configuration file options that specify time
1112 may be expressed using a sequence of the form:
1113 .Sm off
1114 .Ar time Op Ar qualifier ,
1115 .Sm on
1116 where
1117 .Ar time
1118 is a positive integer value and
1119 .Ar qualifier
1120 is one of the following:
1121 .Pp
1122 .Bl -tag -width Ds -compact -offset indent
1123 .It Aq Cm none
1124 seconds
1125 .It Cm s | Cm S
1126 seconds
1127 .It Cm m | Cm M
1128 minutes
1129 .It Cm h | Cm H
1130 hours
1131 .It Cm d | Cm D
1132 days
1133 .It Cm w | Cm W
1134 weeks
1135 .El
1136 .Pp
1137 Each member of the sequence is added together to calculate
1138 the total time value.
1139 .Pp
1140 Time format examples:
1141 .Pp
1142 .Bl -tag -width Ds -compact -offset indent
1143 .It 600
1144 600 seconds (10 minutes)
1145 .It 10m
1146 10 minutes
1147 .It 1h30m
1148 1 hour 30 minutes (90 minutes)
1149 .El
1150 .Sh FILES
1151 .Bl -tag -width Ds
1152 .It Pa /etc/ssh/sshd_config
1153 Contains configuration data for
1154 .Xr sshd 8 .
1155 This file should be writable by root only, but it is recommended
1156 (though not necessary) that it be world-readable.
1157 .El
1158 .Sh SEE ALSO
1159 .Xr sshd 8
1160 .Sh AUTHORS
1161 OpenSSH is a derivative of the original and free
1162 ssh 1.2.12 release by Tatu Ylonen.
1163 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1164 Theo de Raadt and Dug Song
1165 removed many bugs, re-added newer features and
1166 created OpenSSH.
1167 Markus Friedl contributed the support for SSH
1168 protocol versions 1.5 and 2.0.
1169 Niels Provos and Markus Friedl contributed support
1170 for privilege separation.