-
Notifications
You must be signed in to change notification settings - Fork 4
/
Copy pathsshd_config.5
1234 lines (1234 loc) · 34.6 KB
/
sshd_config.5
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
.\"
.\" Author: Tatu Ylonen <[email protected]>
.\" Copyright (c) 1995 Tatu Ylonen <[email protected]>, Espoo, Finland
.\" All rights reserved
.\"
.\" As far as I am concerned, the code I have written for this software
.\" can be used freely for any purpose. Any derived versions of this
.\" software must be clearly marked as such, and if the derived work is
.\" incompatible with the protocol description in the RFC file, it must be
.\" called by a name other than "ssh" or "Secure Shell".
.\"
.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
.\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
.\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.\" $OpenBSD: sshd_config.5,v 1.131 2010/12/08 04:02:47 djm Exp $
.Dd $Mdocdate: December 8 2010 $
.Dt SSHD_CONFIG 5
.Os
.Sh NAME
.Nm sshd_config
.Nd OpenSSH SSH daemon configuration file
.Sh SYNOPSIS
.Nm /etc/ssh/sshd_config
.Sh DESCRIPTION
.Xr sshd 8
reads configuration data from
.Pa /etc/ssh/sshd_config
(or the file specified with
.Fl f
on the command line).
The file contains keyword-argument pairs, one per line.
Lines starting with
.Ql #
and empty lines are interpreted as comments.
Arguments may optionally be enclosed in double quotes
.Pq \&"
in order to represent arguments containing spaces.
.Pp
The possible
keywords and their meanings are as follows (note that
keywords are case-insensitive and arguments are case-sensitive):
.Bl -tag -width Ds
.It Cm AcceptEnv
Specifies what environment variables sent by the client will be copied into
the session's
.Xr environ 7 .
See
.Cm SendEnv
in
.Xr ssh_config 5
for how to configure the client.
Note that environment passing is only supported for protocol 2.
Variables are specified by name, which may contain the wildcard characters
.Ql *
and
.Ql \&? .
Multiple environment variables may be separated by whitespace or spread
across multiple
.Cm AcceptEnv
directives.
Be warned that some environment variables could be used to bypass restricted
user environments.
For this reason, care should be taken in the use of this directive.
The default is not to accept any environment variables.
.It Cm AddressFamily
Specifies which address family should be used by
.Xr sshd 8 .
Valid arguments are
.Dq any ,
.Dq inet
(use IPv4 only), or
.Dq inet6
(use IPv6 only).
The default is
.Dq any .
.It Cm AllowAgentForwarding
Specifies whether
.Xr ssh-agent 1
forwarding is permitted.
The default is
.Dq yes .
Note that disabling agent forwarding does not improve security
unless users are also denied shell access, as they can always install
their own forwarders.
.It Cm AllowGroups
This keyword can be followed by a list of group name patterns, separated
by spaces.
If specified, login is allowed only for users whose primary
group or supplementary group list matches one of the patterns.
Only group names are valid; a numerical group ID is not recognized.
By default, login is allowed for all groups.
The allow/deny directives are processed in the following order:
.Cm DenyUsers ,
.Cm AllowUsers ,
.Cm DenyGroups ,
and finally
.Cm AllowGroups .
.Pp
See
.Sx PATTERNS
in
.Xr ssh_config 5
for more information on patterns.
.It Cm AllowTcpForwarding
Specifies whether TCP forwarding is permitted.
The default is
.Dq yes .
Note that disabling TCP forwarding does not improve security unless
users are also denied shell access, as they can always install their
own forwarders.
.It Cm AllowUsers
This keyword can be followed by a list of user name patterns, separated
by spaces.
If specified, login is allowed only for user names that
match one of the patterns.
Only user names are valid; a numerical user ID is not recognized.
By default, login is allowed for all users.
If the pattern takes the form USER@HOST then USER and HOST
are separately checked, restricting logins to particular
users from particular hosts.
The allow/deny directives are processed in the following order:
.Cm DenyUsers ,
.Cm AllowUsers ,
.Cm DenyGroups ,
and finally
.Cm AllowGroups .
.Pp
See
.Sx PATTERNS
in
.Xr ssh_config 5
for more information on patterns.
.It Cm AuthorizedKeysFile
Specifies the file that contains the public keys that can be used
for user authentication.
The format is described in the
.Sx AUTHORIZED_KEYS FILE FORMAT
section of
.Xr sshd 8 .
.Cm AuthorizedKeysFile
may contain tokens of the form %T which are substituted during connection
setup.
The following tokens are defined: %% is replaced by a literal '%',
%h is replaced by the home directory of the user being authenticated, and
%u is replaced by the username of that user.
After expansion,
.Cm AuthorizedKeysFile
is taken to be an absolute path or one relative to the user's home
directory.
The default is
.Dq .ssh/authorized_keys .
.It Cm AuthorizedPrincipalsFile
Specifies a file that lists principal names that are accepted for
certificate authentication.
When using certificates signed by a key listed in
.Cm TrustedUserCAKeys ,
this file lists names, one of which must appear in the certificate for it
to be accepted for authentication.
Names are listed one per line preceded by key options (as described
in
.Sx AUTHORIZED_KEYS FILE FORMAT
in
.Xr sshd 8 ) .
Empty lines and comments starting with
.Ql #
are ignored.
.Pp
.Cm AuthorizedPrincipalsFile
may contain tokens of the form %T which are substituted during connection
setup.
The following tokens are defined: %% is replaced by a literal '%',
%h is replaced by the home directory of the user being authenticated, and
%u is replaced by the username of that user.
After expansion,
.Cm AuthorizedPrincipalsFile
is taken to be an absolute path or one relative to the user's home
directory.
.Pp
The default is not to use a principals file \(en in this case, the username
of the user must appear in a certificate's principals list for it to be
accepted.
Note that
.Cm AuthorizedPrincipalsFile
is only used when authentication proceeds using a CA listed in
.Cm TrustedUserCAKeys
and is not consulted for certification authorities trusted via
.Pa ~/.ssh/authorized_keys ,
though the
.Cm principals=
key option offers a similar facility (see
.Xr sshd 8
for details).
.It Cm Banner
The contents of the specified file are sent to the remote user before
authentication is allowed.
If the argument is
.Dq none
then no banner is displayed.
This option is only available for protocol version 2.
By default, no banner is displayed.
.It Cm ChallengeResponseAuthentication
Specifies whether challenge-response authentication is allowed (e.g. via
PAM or though authentication styles supported in
.Xr login.conf 5 )
The default is
.Dq yes .
.It Cm ChrootDirectory
Specifies the pathname of a directory to
.Xr chroot 2
to after authentication.
All components of the pathname must be root-owned directories that are
not writable by any other user or group.
After the chroot,
.Xr sshd 8
changes the working directory to the user's home directory.
.Pp
The pathname may contain the following tokens that are expanded at runtime once
the connecting user has been authenticated: %% is replaced by a literal '%',
%h is replaced by the home directory of the user being authenticated, and
%u is replaced by the username of that user.
.Pp
The
.Cm ChrootDirectory
must contain the necessary files and directories to support the
user's session.
For an interactive session this requires at least a shell, typically
.Xr sh 1 ,
and basic
.Pa /dev
nodes such as
.Xr null 4 ,
.Xr zero 4 ,
.Xr stdin 4 ,
.Xr stdout 4 ,
.Xr stderr 4 ,
.Xr arandom 4
and
.Xr tty 4
devices.
For file transfer sessions using
.Dq sftp ,
no additional configuration of the environment is necessary if the
in-process sftp server is used,
though sessions which use logging do require
.Pa /dev/log
inside the chroot directory (see
.Xr sftp-server 8
for details).
.Pp
The default is not to
.Xr chroot 2 .
.It Cm Ciphers
Specifies the ciphers allowed for protocol version 2.
Multiple ciphers must be comma-separated.
The supported ciphers are
.Dq 3des-cbc ,
.Dq aes128-cbc ,
.Dq aes192-cbc ,
.Dq aes256-cbc ,
.Dq aes128-ctr ,
.Dq aes192-ctr ,
.Dq aes256-ctr ,
.Dq arcfour128 ,
.Dq arcfour256 ,
.Dq arcfour ,
.Dq blowfish-cbc ,
and
.Dq cast128-cbc .
The default is:
.Bd -literal -offset 3n
aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
aes256-cbc,arcfour
.Ed
.It Cm ClientAliveCountMax
Sets the number of client alive messages (see below) which may be
sent without
.Xr sshd 8
receiving any messages back from the client.
If this threshold is reached while client alive messages are being sent,
sshd will disconnect the client, terminating the session.
It is important to note that the use of client alive messages is very
different from
.Cm TCPKeepAlive
(below).
The client alive messages are sent through the encrypted channel
and therefore will not be spoofable.
The TCP keepalive option enabled by
.Cm TCPKeepAlive
is spoofable.
The client alive mechanism is valuable when the client or
server depend on knowing when a connection has become inactive.
.Pp
The default value is 3.
If
.Cm ClientAliveInterval
(see below) is set to 15, and
.Cm ClientAliveCountMax
is left at the default, unresponsive SSH clients
will be disconnected after approximately 45 seconds.
This option applies to protocol version 2 only.
.It Cm ClientAliveInterval
Sets a timeout interval in seconds after which if no data has been received
from the client,
.Xr sshd 8
will send a message through the encrypted
channel to request a response from the client.
The default
is 0, indicating that these messages will not be sent to the client.
This option applies to protocol version 2 only.
.It Cm Compression
Specifies whether compression is allowed, or delayed until
the user has authenticated successfully.
The argument must be
.Dq yes ,
.Dq delayed ,
or
.Dq no .
The default is
.Dq delayed .
.It Cm DenyGroups
This keyword can be followed by a list of group name patterns, separated
by spaces.
Login is disallowed for users whose primary group or supplementary
group list matches one of the patterns.
Only group names are valid; a numerical group ID is not recognized.
By default, login is allowed for all groups.
The allow/deny directives are processed in the following order:
.Cm DenyUsers ,
.Cm AllowUsers ,
.Cm DenyGroups ,
and finally
.Cm AllowGroups .
.Pp
See
.Sx PATTERNS
in
.Xr ssh_config 5
for more information on patterns.
.It Cm DenyUsers
This keyword can be followed by a list of user name patterns, separated
by spaces.
Login is disallowed for user names that match one of the patterns.
Only user names are valid; a numerical user ID is not recognized.
By default, login is allowed for all users.
If the pattern takes the form USER@HOST then USER and HOST
are separately checked, restricting logins to particular
users from particular hosts.
The allow/deny directives are processed in the following order:
.Cm DenyUsers ,
.Cm AllowUsers ,
.Cm DenyGroups ,
and finally
.Cm AllowGroups .
.Pp
See
.Sx PATTERNS
in
.Xr ssh_config 5
for more information on patterns.
.It Cm ForceCommand
Forces the execution of the command specified by
.Cm ForceCommand ,
ignoring any command supplied by the client and
.Pa ~/.ssh/rc
if present.
The command is invoked by using the user's login shell with the -c option.
This applies to shell, command, or subsystem execution.
It is most useful inside a
.Cm Match
block.
The command originally supplied by the client is available in the
.Ev SSH_ORIGINAL_COMMAND
environment variable.
Specifying a command of
.Dq internal-sftp
will force the use of an in-process sftp server that requires no support
files when used with
.Cm ChrootDirectory .
.It Cm GatewayPorts
Specifies whether remote hosts are allowed to connect to ports
forwarded for the client.
By default,
.Xr sshd 8
binds remote port forwardings to the loopback address.
This prevents other remote hosts from connecting to forwarded ports.
.Cm GatewayPorts
can be used to specify that sshd
should allow remote port forwardings to bind to non-loopback addresses, thus
allowing other hosts to connect.
The argument may be
.Dq no
to force remote port forwardings to be available to the local host only,
.Dq yes
to force remote port forwardings to bind to the wildcard address, or
.Dq clientspecified
to allow the client to select the address to which the forwarding is bound.
The default is
.Dq no .
.It Cm GSSAPIAuthentication
Specifies whether user authentication based on GSSAPI is allowed.
The default is
.Dq no .
Note that this option applies to protocol version 2 only.
.It Cm GSSAPIKeyExchange
Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange
doesn't rely on ssh keys to verify host identity.
The default is
.Dq no .
Note that this option applies to protocol version 2 only.
.It Cm GSSAPICleanupCredentials
Specifies whether to automatically destroy the user's credentials cache
on logout.
The default is
.Dq yes .
Note that this option applies to protocol version 2 only.
.It Cm GSSAPIStrictAcceptorCheck
Determines whether to be strict about the identity of the GSSAPI acceptor
a client authenticates against. If
.Dq yes
then the client must authenticate against the
.Pa host
service on the current hostname. If
.Dq no
then the client may authenticate against any service key stored in the
machine's default store. This facility is provided to assist with operation
on multi homed machines.
The default is
.Dq yes .
Note that this option applies only to protocol version 2 GSSAPI connections,
and setting it to
.Dq no
may only work with recent Kerberos GSSAPI libraries.
.It Cm GSSAPIStoreCredentialsOnRekey
Controls whether the user's GSSAPI credentials should be updated following a
successful connection rekeying. This option can be used to accepted renewed
or updated credentials from a compatible client. The default is
.Dq no .
.It Cm HostbasedAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication together
with successful public key client host authentication is allowed
(host-based authentication).
This option is similar to
.Cm RhostsRSAAuthentication
and applies to protocol version 2 only.
The default is
.Dq no .
.It Cm HostbasedUsesNameFromPacketOnly
Specifies whether or not the server will attempt to perform a reverse
name lookup when matching the name in the
.Pa ~/.shosts ,
.Pa ~/.rhosts ,
and
.Pa /etc/hosts.equiv
files during
.Cm HostbasedAuthentication .
A setting of
.Dq yes
means that
.Xr sshd 8
uses the name supplied by the client rather than
attempting to resolve the name from the TCP connection itself.
The default is
.Dq no .
.It Cm HostCertificate
Specifies a file containing a public host certificate.
The certificate's public key must match a private host key already specified
by
.Cm HostKey .
The default behaviour of
.Xr sshd 8
is not to load any certificates.
.It Cm HostKey
Specifies a file containing a private host key
used by SSH.
The default is
.Pa /etc/ssh/ssh_host_key
for protocol version 1, and
.Pa /etc/ssh/ssh_host_dsa_key ,
.Pa /etc/ssh/ssh_host_ecdsa_key
and
.Pa /etc/ssh/ssh_host_rsa_key
for protocol version 2.
Note that
.Xr sshd 8
will refuse to use a file if it is group/world-accessible.
It is possible to have multiple host key files.
.Dq rsa1
keys are used for version 1 and
.Dq dsa ,
.Dq ecdsa
or
.Dq rsa
are used for version 2 of the SSH protocol.
.It Cm IgnoreRhosts
Specifies that
.Pa .rhosts
and
.Pa .shosts
files will not be used in
.Cm RhostsRSAAuthentication
or
.Cm HostbasedAuthentication .
.Pp
.Pa /etc/hosts.equiv
and
.Pa /etc/shosts.equiv
are still used.
The default is
.Dq yes .
.It Cm IgnoreUserKnownHosts
Specifies whether
.Xr sshd 8
should ignore the user's
.Pa ~/.ssh/known_hosts
during
.Cm RhostsRSAAuthentication
or
.Cm HostbasedAuthentication .
The default is
.Dq no .
.It Cm IPQoS
Specifies the IPv4 type-of-service or DSCP class for the connection.
Accepted values are
.Dq af11 ,
.Dq af12 ,
.Dq af13 ,
.Dq af14 ,
.Dq af22 ,
.Dq af23 ,
.Dq af31 ,
.Dq af32 ,
.Dq af33 ,
.Dq af41 ,
.Dq af42 ,
.Dq af43 ,
.Dq cs0 ,
.Dq cs1 ,
.Dq cs2 ,
.Dq cs3 ,
.Dq cs4 ,
.Dq cs5 ,
.Dq cs6 ,
.Dq cs7 ,
.Dq ef ,
.Dq lowdelay ,
.Dq throughput ,
.Dq reliability ,
or a numeric value.
This option may take one or two arguments, separated by whitespace.
If one argument is specified, it is used as the packet class unconditionally.
If two values are specified, the first is automatically selected for
interactive sessions and the second for non-interactive sessions.
The default is
.Dq lowdelay
for interactive sessions and
.Dq throughput
for non-interactive sessions.
.It Cm KerberosAuthentication
Specifies whether the password provided by the user for
.Cm PasswordAuthentication
will be validated through the Kerberos KDC.
To use this option, the server needs a
Kerberos servtab which allows the verification of the KDC's identity.
The default is
.Dq no .
.It Cm KerberosGetAFSToken
If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire
an AFS token before accessing the user's home directory.
The default is
.Dq no .
.It Cm KerberosOrLocalPasswd
If password authentication through Kerberos fails then
the password will be validated via any additional local mechanism
such as
.Pa /etc/passwd .
The default is
.Dq yes .
.It Cm KerberosTicketCleanup
Specifies whether to automatically destroy the user's ticket cache
file on logout.
The default is
.Dq yes .
.It Cm KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms.
Multiple algorithms must be comma-separated.
The default is
.Dq ecdh-sha2-nistp256 ,
.Dq ecdh-sha2-nistp384 ,
.Dq ecdh-sha2-nistp521 ,
.Dq diffie-hellman-group-exchange-sha256 ,
.Dq diffie-hellman-group-exchange-sha1 ,
.Dq diffie-hellman-group14-sha1 ,
.Dq diffie-hellman-group1-sha1 .
.It Cm KeyRegenerationInterval
In protocol version 1, the ephemeral server key is automatically regenerated
after this many seconds (if it has been used).
The purpose of regeneration is to prevent
decrypting captured sessions by later breaking into the machine and
stealing the keys.
The key is never stored anywhere.
If the value is 0, the key is never regenerated.
The default is 3600 (seconds).
.It Cm ListenAddress
Specifies the local addresses
.Xr sshd 8
should listen on.
The following forms may be used:
.Pp
.Bl -item -offset indent -compact
.It
.Cm ListenAddress
.Sm off
.Ar host No | Ar IPv4_addr No | Ar IPv6_addr
.Sm on
.It
.Cm ListenAddress
.Sm off
.Ar host No | Ar IPv4_addr No : Ar port
.Sm on
.It
.Cm ListenAddress
.Sm off
.Oo
.Ar host No | Ar IPv6_addr Oc : Ar port
.Sm on
.El
.Pp
If
.Ar port
is not specified,
sshd will listen on the address and all prior
.Cm Port
options specified.
The default is to listen on all local addresses.
Multiple
.Cm ListenAddress
options are permitted.
Additionally, any
.Cm Port
options must precede this option for non-port qualified addresses.
.It Cm LoginGraceTime
The server disconnects after this time if the user has not
successfully logged in.
If the value is 0, there is no time limit.
The default is 120 seconds.
.It Cm LogLevel
Gives the verbosity level that is used when logging messages from
.Xr sshd 8 .
The possible values are:
QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3.
The default is INFO.
DEBUG and DEBUG1 are equivalent.
DEBUG2 and DEBUG3 each specify higher levels of debugging output.
Logging with a DEBUG level violates the privacy of users and is not recommended.
.It Cm MACs
Specifies the available MAC (message authentication code) algorithms.
The MAC algorithm is used in protocol version 2
for data integrity protection.
Multiple algorithms must be comma-separated.
The default is:
.Bd -literal -offset indent
hmac-md5,hmac-sha1,[email protected],
hmac-ripemd160,hmac-sha1-96,hmac-md5-96
.Ed
.It Cm Match
Introduces a conditional block.
If all of the criteria on the
.Cm Match
line are satisfied, the keywords on the following lines override those
set in the global section of the config file, until either another
.Cm Match
line or the end of the file.
.Pp
The arguments to
.Cm Match
are one or more criteria-pattern pairs.
The available criteria are
.Cm User ,
.Cm Group ,
.Cm Host ,
and
.Cm Address .
The match patterns may consist of single entries or comma-separated
lists and may use the wildcard and negation operators described in the
.Sx PATTERNS
section of
.Xr ssh_config 5 .
.Pp
The patterns in an
.Cm Address
criteria may additionally contain addresses to match in CIDR
address/masklen format, e.g.\&
.Dq 192.0.2.0/24
or
.Dq 3ffe:ffff::/32 .
Note that the mask length provided must be consistent with the address -
it is an error to specify a mask length that is too long for the address
or one with bits set in this host portion of the address.
For example,
.Dq 192.0.2.0/33
and
.Dq 192.0.2.0/8
respectively.
.Pp
Only a subset of keywords may be used on the lines following a
.Cm Match
keyword.
Available keywords are
.Cm AllowAgentForwarding ,
.Cm AllowTcpForwarding ,
.Cm AuthorizedKeysFile ,
.Cm AuthorizedPrincipalsFile ,
.Cm Banner ,
.Cm ChrootDirectory ,
.Cm ForceCommand ,
.Cm GatewayPorts ,
.Cm GSSAPIAuthentication ,
.Cm HostbasedAuthentication ,
.Cm HostbasedUsesNameFromPacketOnly ,
.Cm KbdInteractiveAuthentication ,
.Cm KerberosAuthentication ,
.Cm MaxAuthTries ,
.Cm MaxSessions ,
.Cm PasswordAuthentication ,
.Cm PermitEmptyPasswords ,
.Cm PermitOpen ,
.Cm PermitRootLogin ,
.Cm PermitTunnel ,
.Cm PubkeyAuthentication ,
.Cm RhostsRSAAuthentication ,
.Cm RSAAuthentication ,
.Cm X11DisplayOffset ,
.Cm X11Forwarding
and
.Cm X11UseLocalHost .
.It Cm MaxAuthTries
Specifies the maximum number of authentication attempts permitted per
connection.
Once the number of failures reaches half this value,
additional failures are logged.
The default is 6.
.It Cm MaxSessions
Specifies the maximum number of open sessions permitted per network connection.
The default is 10.
.It Cm MaxStartups
Specifies the maximum number of concurrent unauthenticated connections to the
SSH daemon.
Additional connections will be dropped until authentication succeeds or the
.Cm LoginGraceTime
expires for a connection.
The default is 10.
.Pp
Alternatively, random early drop can be enabled by specifying
the three colon separated values
.Dq start:rate:full
(e.g. "10:30:60").
.Xr sshd 8
will refuse connection attempts with a probability of
.Dq rate/100
(30%)
if there are currently
.Dq start
(10)
unauthenticated connections.
The probability increases linearly and all connection attempts
are refused if the number of unauthenticated connections reaches
.Dq full
(60).
.It Cm PasswordAuthentication
Specifies whether password authentication is allowed.
The default is
.Dq yes .
.It Cm PermitEmptyPasswords
When password authentication is allowed, it specifies whether the
server allows login to accounts with empty password strings.
The default is
.Dq no .
.It Cm PermitOpen
Specifies the destinations to which TCP port forwarding is permitted.
The forwarding specification must be one of the following forms:
.Pp
.Bl -item -offset indent -compact
.It
.Cm PermitOpen
.Sm off
.Ar host : port
.Sm on
.It
.Cm PermitOpen
.Sm off
.Ar IPv4_addr : port
.Sm on
.It
.Cm PermitOpen
.Sm off
.Ar \&[ IPv6_addr \&] : port
.Sm on
.El
.Pp
Multiple forwards may be specified by separating them with whitespace.
An argument of
.Dq any
can be used to remove all restrictions and permit any forwarding requests.
By default all port forwarding requests are permitted.
.It Cm PermitRootLogin
Specifies whether root can log in using
.Xr ssh 1 .
The argument must be
.Dq yes ,
.Dq without-password ,
.Dq forced-commands-only ,
or
.Dq no .
The default is
.Dq yes .
.Pp
If this option is set to
.Dq without-password ,
password authentication is disabled for root.
.Pp
If this option is set to
.Dq forced-commands-only ,
root login with public key authentication will be allowed,
but only if the
.Ar command
option has been specified
(which may be useful for taking remote backups even if root login is
normally not allowed).
All other authentication methods are disabled for root.
.Pp
If this option is set to
.Dq no ,
root is not allowed to log in.
.It Cm PermitTunnel
Specifies whether
.Xr tun 4
device forwarding is allowed.
The argument must be
.Dq yes ,
.Dq point-to-point
(layer 3),
.Dq ethernet
(layer 2), or
.Dq no .
Specifying
.Dq yes
permits both
.Dq point-to-point
and
.Dq ethernet .
The default is
.Dq no .
.It Cm PermitUserEnvironment
Specifies whether
.Pa ~/.ssh/environment
and
.Cm environment=
options in
.Pa ~/.ssh/authorized_keys
are processed by
.Xr sshd 8 .
The default is
.Dq no .
Enabling environment processing may enable users to bypass access
restrictions in some configurations using mechanisms such as
.Ev LD_PRELOAD .
.It Cm PidFile
Specifies the file that contains the process ID of the
SSH daemon.
The default is
.Pa /var/run/sshd.pid .
.It Cm Port
Specifies the port number that
.Xr sshd 8
listens on.
The default is 22.
Multiple options of this type are permitted.
See also
.Cm ListenAddress .
.It Cm PrintLastLog
Specifies whether
.Xr sshd 8
should print the date and time of the last user login when a user logs
in interactively.
The default is
.Dq yes .
.It Cm PrintMotd
Specifies whether
.Xr sshd 8
should print
.Pa /etc/motd
when a user logs in interactively.
(On some systems it is also printed by the shell,
.Pa /etc/profile ,
or equivalent.)
The default is
.Dq yes .
.It Cm Protocol
Specifies the protocol versions
.Xr sshd 8
supports.
The possible values are
.Sq 1
and
.Sq 2 .
Multiple versions must be comma-separated.
The default is
.Sq 2 .
Note that the order of the protocol list does not indicate preference,
because the client selects among multiple protocol versions offered
by the server.
Specifying
.Dq 2,1
is identical to
.Dq 1,2 .
.It Cm PubkeyAuthentication
Specifies whether public key authentication is allowed.
The default is
.Dq yes .
Note that this option applies to protocol version 2 only.
.It Cm RevokedKeys
Specifies a list of revoked public keys.
Keys listed in this file will be refused for public key authentication.
Note that if this file is not readable, then public key authentication will
be refused for all users.
.It Cm RhostsRSAAuthentication
Specifies whether rhosts or /etc/hosts.equiv authentication together
with successful RSA host authentication is allowed.
The default is
.Dq no .
This option applies to protocol version 1 only.
.It Cm RSAAuthentication
Specifies whether pure RSA authentication is allowed.
The default is
.Dq yes .
This option applies to protocol version 1 only.
.It Cm ServerKeyBits
Defines the number of bits in the ephemeral protocol version 1 server key.
The minimum value is 512, and the default is 1024.
.It Cm StrictModes
Specifies whether
.Xr sshd 8
should check file modes and ownership of the
user's files and home directory before accepting login.
This is normally desirable because novices sometimes accidentally leave their
directory or files world-writable.
The default is
.Dq yes .
Note that this does not apply to
.Cm ChrootDirectory ,
whose permissions and ownership are checked unconditionally.
.It Cm Subsystem
Configures an external subsystem (e.g. file transfer daemon).
Arguments should be a subsystem name and a command (with optional arguments)
to execute upon subsystem request.
.Pp
The command
.Xr sftp-server 8
implements the
.Dq sftp
file transfer subsystem.
.Pp
Alternately the name
.Dq internal-sftp
implements an in-process
.Dq sftp
server.
This may simplify configurations using
.Cm ChrootDirectory
to force a different filesystem root on clients.
.Pp
By default no subsystems are defined.
Note that this option applies to protocol version 2 only.
.It Cm SyslogFacility
Gives the facility code that is used when logging messages from
.Xr sshd 8 .
The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.