summaryrefslogtreecommitdiff
path: root/sshd.8
blob: af0486ba323048b20878790f191cabe518a3bbc8 (plain)
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
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
.\"  -*- nroff -*-
.\"
.\" Author: Tatu Ylonen <ylo@cs.hut.fi>
.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, 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.8,v 1.173 2002/03/19 05:23:08 stevesk Exp $
.Dd September 25, 1999
.Dt SSHD 8
.Os
.Sh NAME
.Nm sshd
.Nd OpenSSH SSH daemon
.Sh SYNOPSIS
.Nm sshd
.Op Fl deiqtD46
.Op Fl b Ar bits
.Op Fl f Ar config_file
.Op Fl g Ar login_grace_time
.Op Fl h Ar host_key_file
.Op Fl k Ar key_gen_time
.Op Fl o Ar option
.Op Fl p Ar port
.Op Fl u Ar len
.Sh DESCRIPTION
.Nm
(SSH Daemon) is the daemon program for
.Xr ssh 1 .
Together these programs replace rlogin and rsh, and
provide secure encrypted communications between two untrusted hosts
over an insecure network.
The programs are intended to be as easy to
install and use as possible.
.Pp
.Nm
is the daemon that listens for connections from clients.
It is normally started at boot from
.Pa /etc/rc .
It forks a new
daemon for each incoming connection.
The forked daemons handle
key exchange, encryption, authentication, command execution,
and data exchange.
This implementation of
.Nm
supports both SSH protocol version 1 and 2 simultaneously.
.Nm
works as follows.
.Pp
.Ss SSH protocol version 1
.Pp
Each host has a host-specific RSA key
(normally 1024 bits) used to identify the host.
Additionally, when
the daemon starts, it generates a server RSA key (normally 768 bits).
This key is normally regenerated every hour if it has been used, and
is never stored on disk.
.Pp
Whenever a client connects the daemon responds with its public
host and server keys.
The client compares the
RSA host key against its own database to verify that it has not changed.
The client then generates a 256 bit random number.
It encrypts this
random number using both the host key and the server key, and sends
the encrypted number to the server.
Both sides then use this
random number as a session key which is used to encrypt all further
communications in the session.
The rest of the session is encrypted
using a conventional cipher, currently Blowfish or 3DES, with 3DES
being used by default.
The client selects the encryption algorithm
to use from those offered by the server.
.Pp
Next, the server and the client enter an authentication dialog.
The client tries to authenticate itself using
.Pa .rhosts
authentication,
.Pa .rhosts
authentication combined with RSA host
authentication, RSA challenge-response authentication, or password
based authentication.
.Pp
Rhosts authentication is normally disabled
because it is fundamentally insecure, but can be enabled in the server
configuration file if desired.
System security is not improved unless
.Xr rshd 8 ,
.Xr rlogind 8 ,
and
.Xr rexecd 8
are disabled (thus completely disabling
.Xr rlogin 1
and
.Xr rsh 1
into the machine).
.Pp
.Ss SSH protocol version 2
.Pp
Version 2 works similarly:
Each host has a host-specific key (RSA or DSA) used to identify the host.
However, when the daemon starts, it does not generate a server key.
Forward security is provided through a Diffie-Hellman key agreement.
This key agreement results in a shared session key.
.Pp
The rest of the session is encrypted using a symmetric cipher, currently
128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit AES.
The client selects the encryption algorithm
to use from those offered by the server.
Additionally, session integrity is provided
through a cryptographic message authentication code
(hmac-sha1 or hmac-md5).
.Pp
Protocol version 2 provides a public key based
user (PubkeyAuthentication) or
client host (HostbasedAuthentication) authentication method,
conventional password authentication and challenge response based methods.
.Pp
.Ss Command execution and data forwarding
.Pp
If the client successfully authenticates itself, a dialog for
preparing the session is entered.
At this time the client may request
things like allocating a pseudo-tty, forwarding X11 connections,
forwarding TCP/IP connections, or forwarding the authentication agent
connection over the secure channel.
.Pp
Finally, the client either requests a shell or execution of a command.
The sides then enter session mode.
In this mode, either side may send
data at any time, and such data is forwarded to/from the shell or
command on the server side, and the user terminal in the client side.
.Pp
When the user program terminates and all forwarded X11 and other
connections have been closed, the server sends command exit status to
the client, and both sides exit.
.Pp
.Nm
can be configured using command-line options or a configuration
file.
Command-line options override values specified in the
configuration file.
.Pp
.Nm
rereads its configuration file when it receives a hangup signal,
.Dv SIGHUP ,
by executing itself with the name it was started as, i.e.,
.Pa /usr/sbin/sshd .
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl b Ar bits
Specifies the number of bits in the ephemeral protocol version 1
server key (default 768).
.It Fl d
Debug mode.
The server sends verbose debug output to the system
log, and does not put itself in the background.
The server also will not fork and will only process one connection.
This option is only intended for debugging for the server.
Multiple -d options increase the debugging level.
Maximum is 3.
.It Fl e
When this option is specified,
.Nm
will send the output to the standard error instead of the system log.
.It Fl f Ar configuration_file
Specifies the name of the configuration file.
The default is
.Pa /etc/ssh/sshd_config .
.Nm
refuses to start if there is no configuration file.
.It Fl g Ar login_grace_time
Gives the grace time for clients to authenticate themselves (default
600 seconds).
If the client fails to authenticate the user within
this many seconds, the server disconnects and exits.
A value of zero indicates no limit.
.It Fl h Ar host_key_file
Specifies a file from which a host key is read.
This option must be given if
.Nm
is not run as root (as the normal
host key files are normally not readable by anyone but root).
The default is
.Pa /etc/ssh/ssh_host_key
for protocol version 1, and
.Pa /etc/ssh/ssh_host_rsa_key
and
.Pa /etc/ssh/ssh_host_dsa_key
for protocol version 2.
It is possible to have multiple host key files for
the different protocol versions and host key algorithms.
.It Fl i
Specifies that
.Nm
is being run from inetd.
.Nm
is normally not run
from inetd because it needs to generate the server key before it can
respond to the client, and this may take tens of seconds.
Clients would have to wait too long if the key was regenerated every time.
However, with small key sizes (e.g., 512) using
.Nm
from inetd may
be feasible.
.It Fl k Ar key_gen_time
Specifies how often the ephemeral protocol version 1 server key is
regenerated (default 3600 seconds, or one hour).
The motivation for regenerating the key fairly
often is that the key is not stored anywhere, and after about an hour,
it becomes impossible to recover the key for decrypting intercepted
communications even if the machine is cracked into or physically
seized.
A value of zero indicates that the key will never be regenerated.
.It Fl o Ar option
Can be used to give options in the format used in the configuration file.
This is useful for specifying options for which there is no separate
command-line flag.
.It Fl p Ar port
Specifies the port on which the server listens for connections
(default 22).
Multiple port options are permitted.
Ports specified in the configuration file are ignored when a
command-line port is specified.
.It Fl q
Quiet mode.
Nothing is sent to the system log.
Normally the beginning,
authentication, and termination of each connection is logged.
.It Fl t
Test mode.
Only check the validity of the configuration file and sanity of the keys.
This is useful for updating
.Nm
reliably as configuration options may change.
.It Fl u Ar len
This option is used to specify the size of the field
in the
.Li utmp
structure that holds the remote host name.
If the resolved host name is longer than
.Ar len ,
the dotted decimal value will be used instead.
This allows hosts with very long host names that
overflow this field to still be uniquely identified.
Specifying
.Fl u0
indicates that only dotted decimal addresses
should be put into the
.Pa utmp
file.
.Fl u0
is also be used to prevent
.Nm
from making DNS requests unless the authentication
mechanism or configuration requires it.
Authentication mechanisms that may require DNS include
.Cm RhostsAuthentication ,
.Cm RhostsRSAAuthentication ,
.Cm HostbasedAuthentication
and using a
.Cm from="pattern-list"
option in a key file.
Configuration options that require DNS include using a
USER@HOST pattern in
.Cm AllowUsers
or
.Cm DenyUsers .
.It Fl D
When this option is specified
.Nm
will not detach and does not become a daemon.
This allows easy monitoring of
.Nm sshd .
.It Fl 4
Forces
.Nm
to use IPv4 addresses only.
.It Fl 6
Forces
.Nm
to use IPv6 addresses only.
.El
.Sh CONFIGURATION FILE
.Nm
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.
.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 AFSTokenPassing
Specifies whether an AFS token may be forwarded to the server.
Default is
.Dq yes .
.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.
.Ql \&*
and
.Ql ?
can be used as
wildcards in the patterns.
Only group names are valid; a numerical group ID is not recognized.
By default, login is allowed for all groups.
.Pp
.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.
.Pp
.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 users names that
match one of the patterns.
.Ql \&*
and
.Ql ?
can be used as
wildcards in 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.
.Pp
.It Cm AuthorizedKeysFile
Specifies the file that contains the public keys that can be used
for user authentication.
.Cm AuthorizedKeysFile
may contain tokens of the form %T which are substituted during connection
set-up. 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 Banner
In some jurisdictions, sending a warning message before authentication
may be relevant for getting legal protection.
The contents of the specified file are sent to the remote user before
authentication is allowed.
This option is only available for protocol version 2.
By default, no banner is displayed.
.Pp
.It Cm ChallengeResponseAuthentication
Specifies whether challenge response authentication is allowed.
All authentication styles from
.Xr login.conf 5
are supported.
The default is
.Dq yes .
.It Cm Ciphers
Specifies the ciphers allowed for protocol version 2.
Multiple ciphers must be comma-separated.
The default is
.Pp
.Bd -literal
  ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
    aes192-cbc,aes256-cbc''
.Ed
.It Cm ClientAliveInterval
Sets a timeout interval in seconds after which if no data has been received
from the client,
.Nm
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 ClientAliveCountMax
Sets the number of client alive messages (see above) which may be
sent without
.Nm
receiving any messages back from the client. If this threshold is
reached while client alive messages are being sent,
.Nm
will disconnect the client, terminating the session. It is important
to note that the use of client alive messages is very different from
.Cm KeepAlive
(below). The client alive messages are sent through the
encrypted channel and therefore will not be spoofable. The TCP keepalive
option enabled by
.Cm KeepAlive
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
(above) is set to 15, and
.Cm ClientAliveCountMax
is left at the default, unresponsive ssh clients
will be disconnected after approximately 45 seconds.
.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.
.Ql \&*
and
.Ql ?
can be used as
wildcards in the patterns.
Only group names are valid; a numerical group ID is not recognized.
By default, login is allowed for all groups.
.Pp
.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.
.Ql \&*
and
.Ql ?
can be used as wildcards in 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.
.It Cm GatewayPorts
Specifies whether remote hosts are allowed to connect to ports
forwarded for the client.
By default,
.Nm
binds remote port forwardings to the loopback addresss.  This
prevents other remote hosts from connecting to forwarded ports.
.Cm GatewayPorts
can be used to specify that
.Nm
should bind remote port forwardings to the wildcard address,
thus allowing remote hosts to connect to forwarded ports.
The argument must be
.Dq yes
or
.Dq no .
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
(hostbased authentication).
This option is similar to
.Cm RhostsRSAAuthentication
and applies to protocol version 2 only.
The default is
.Dq no .
.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_rsa_key
and
.Pa /etc/ssh/ssh_host_dsa_key
for protocol version 2.
Note that
.Nm
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
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 RhostsAuthentication ,
.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
.Nm
should ignore the user's
.Pa $HOME/.ssh/known_hosts
during
.Cm RhostsRSAAuthentication
or
.Cm HostbasedAuthentication .
The default is
.Dq no .
.It Cm KeepAlive
Specifies whether the system should send TCP keepalive messages to the
other side.
If they are sent, death of the connection or crash of one
of the machines will be properly noticed.
However, this means that
connections will die if the route is down temporarily, and some people
find it annoying.
On the other hand, if keepalives are not sent,
sessions may hang indefinitely on the server, leaving
.Dq ghost
users and consuming server resources.
.Pp
The default is
.Dq yes
(to send keepalives), and the server will notice
if the network goes down or the client host crashes.
This avoids infinitely hanging sessions.
.Pp
To disable keepalives, the value should be set to
.Dq no .
.It Cm KerberosAuthentication
Specifies whether Kerberos authentication is allowed.
This can be in the form of a Kerberos ticket, or if
.Cm PasswordAuthentication
is yes, the password provided by the user 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.
Default is
.Dq yes .
.It Cm KerberosOrLocalPasswd
If set then if password authentication through Kerberos fails then
the password will be validated via any additional local mechanism
such as
.Pa /etc/passwd .
Default is
.Dq yes .
.It Cm KerberosTgtPassing
Specifies whether a Kerberos TGT may be forwarded to the server.
Default is
.Dq no ,
as this only works when the Kerberos KDC is actually an AFS kaserver.
.It Cm KerberosTicketCleanup
Specifies whether to automatically destroy the user's ticket cache
file on logout.
Default is
.Dq yes .
.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
.Nm
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,
.Nm
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 600 (seconds).
.It Cm LogLevel
Gives the verbosity level that is used when logging messages from
.Nm sshd .
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
.Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 .
.It Cm MaxStartups
Specifies the maximum number of concurrent unauthenticated connections to the
.Nm
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").
.Nm
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 PAMAuthenticationViaKbdInt
Specifies whether PAM challenge response authentication is allowed. This
allows the use of most PAM challenge response authentication modules, but 
it will allow password authentication regardless of whether 
.Cm PasswordAuthentication
is disabled.
The default is
.Dq no .
.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 PermitRootLogin
Specifies whether root can login 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 login.
.It Cm PidFile
Specifies the file that contains the process identifier of the
.Nm
daemon.
The default is
.Pa /var/run/sshd.pid .
.It Cm Port
Specifies the port number that
.Nm
listens on.
The default is 22.
Multiple options of this type are permitted.
See also
.Cm ListenAddress .
.It Cm PrintLastLog
Specifies whether
.Nm
should print the date and time when the user last logged in.
The default is
.Dq yes .
.It Cm PrintMotd
Specifies whether
.Nm
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
.Nm
should support.
The possible values are
.Dq 1
and
.Dq 2 .
Multiple versions must be comma-separated.
The default is
.Dq 2,1 .
.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 RhostsAuthentication
Specifies whether authentication using rhosts or /etc/hosts.equiv
files is sufficient.
Normally, this method should not be permitted because it is insecure.
.Cm RhostsRSAAuthentication
should be used
instead, because it performs RSA-based host authentication in addition
to normal rhosts or /etc/hosts.equiv authentication.
The default is
.Dq no .
This option applies to protocol version 1 only.
.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 768.
.It Cm StrictModes
Specifies whether
.Nm
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 .
.It Cm Subsystem
Configures an external subsystem (e.g., file transfer daemon).
Arguments should be a subsystem name and a command to execute upon subsystem
request.
The command
.Xr sftp-server 8
implements the
.Dq sftp
file transfer subsystem.
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
.Nm sshd .
The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2,
LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
The default is AUTH.
.It Cm UseLogin
Specifies whether
.Xr login 1
is used for interactive login sessions.
The default is
.Dq no .
Note that
.Xr login 1
is never used for remote command execution.
Note also, that if this is enabled,
.Cm X11Forwarding
will be disabled because
.Xr login 1
does not know how to handle
.Xr xauth 1
cookies.  If
.Cm UsePrivilegeSeparation
is specified, it will be disabled after authentication.
.It Cm UsePrivilegeSeparation
Specifies whether
.Nm
separated privileges by creating an unprivileged child process
to deal with incoming network traffic.  After successful authentication,
another process will be created that has the privilege of the authenticated
user.  The goal of privilege separation is to prevent privilege
escalation by containing any corruption within the unprivileged processes.
The default is
.Dq no .
.It Cm VerifyReverseMapping
Specifies whether
.Nm
should try to verify the remote host name and check that
the resolved host name for the remote IP address maps back to the
very same IP address.
The default is
.Dq no .
.It Cm X11DisplayOffset
Specifies the first display number available for
.Nm sshd Ns 's
X11 forwarding.
This prevents
.Nm
from interfering with real X11 servers.
The default is 10.
.It Cm X11Forwarding
Specifies whether X11 forwarding is permitted.
The default is
.Dq no .
Note that disabling X11 forwarding does not improve security in any
way, as users can always install their own forwarders.
X11 forwarding is automatically disabled if
.Cm UseLogin
is enabled.
.It Cm X11UseLocalhost
Specifies whether
.Nm
should bind the X11 forwarding server to the loopback address or to
the wildcard address.  By default,
.Nm
binds the forwarding server to the loopback address and sets the
hostname part of the
.Ev DISPLAY
environment variable to
.Dq localhost .
This prevents remote hosts from connecting to the fake display.
However, some older X11 clients may not function with this
configuration.
.Cm X11UseLocalhost
may be set to
.Dq no
to specify that the forwarding server should be bound to the wildcard
address.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq yes .
.It Cm XAuthLocation
Specifies the location of the
.Xr xauth 1
program.
The default is
.Pa /usr/X11R6/bin/xauth .
.El
.Ss Time Formats
.Pp
.Nm
command-line arguments and configuration file options that specify time
may be expressed using a sequence of the form:
.Sm off
.Ar time Oo Ar qualifier Oc ,
.Sm on
where
.Ar time
is a positive integer value and
.Ar qualifier
is one of the following:
.Pp
.Bl -tag -width Ds -compact -offset indent
.It Cm <none>
seconds
.It Cm s | Cm S
seconds
.It Cm m | Cm M
minutes
.It Cm h | Cm H
hours
.It Cm d | Cm D
days
.It Cm w | Cm W
weeks
.El
.Pp
Each member of the sequence is added together to calculate
the total time value.
.Pp
Time format examples:
.Pp
.Bl -tag -width Ds -compact -offset indent
.It 600
600 seconds (10 minutes)
.It 10m
10 minutes
.It 1h30m
1 hour 30 minutes (90 minutes)
.El
.Sh LOGIN PROCESS
When a user successfully logs in,
.Nm
does the following:
.Bl -enum -offset indent
.It
If the login is on a tty, and no command has been specified,
prints last login time and
.Pa /etc/motd
(unless prevented in the configuration file or by
.Pa $HOME/.hushlogin ;
see the
.Sx FILES
section).
.It
If the login is on a tty, records login time.
.It
Checks
.Pa /etc/nologin ;
if it exists, prints contents and quits
(unless root).
.It
Changes to run with normal user privileges.
.It
Sets up basic environment.
.It
Reads
.Pa $HOME/.ssh/environment
if it exists.
.It
Changes to user's home directory.
.It
If
.Pa $HOME/.ssh/rc
exists, runs it; else if
.Pa /etc/ssh/sshrc
exists, runs
it; otherwise runs xauth.
The
.Dq rc
files are given the X11
authentication protocol and cookie in standard input.
.It
Runs user's shell or command.
.El
.Sh AUTHORIZED_KEYS FILE FORMAT
.Pa $HOME/.ssh/authorized_keys
is the default file that lists the public keys that are
permitted for RSA authentication in protocol version 1
and for public key authentication (PubkeyAuthentication)
in protocol version 2.
.Cm AuthorizedKeysFile
may be used to specify an alternative file.
.Pp
Each line of the file contains one
key (empty lines and lines starting with a
.Ql #
are ignored as
comments).
Each RSA public key consists of the following fields, separated by
spaces: options, bits, exponent, modulus, comment.
Each protocol version 2 public key consists of:
options, keytype, base64 encoded key, comment.
The options fields
are optional; its presence is determined by whether the line starts
with a number or not (the option field never starts with a number).
The bits, exponent, modulus and comment fields give the RSA key for
protocol version 1; the
comment field is not used for anything (but may be convenient for the
user to identify the key).
For protocol version 2 the keytype is
.Dq ssh-dss
or
.Dq ssh-rsa .
.Pp
Note that lines in this file are usually several hundred bytes long
(because of the size of the RSA key modulus).
You don't want to type them in; instead, copy the
.Pa identity.pub ,
.Pa id_dsa.pub
or the
.Pa id_rsa.pub
file and edit it.
.Pp
The options (if present) consist of comma-separated option
specifications.
No spaces are permitted, except within double quotes.
The following option specifications are supported (note
that option keywords are case-insensitive):
.Bl -tag -width Ds
.It Cm from="pattern-list"
Specifies that in addition to RSA authentication, the canonical name
of the remote host must be present in the comma-separated list of
patterns
.Pf ( Ql *
and
.Ql ?
serve as wildcards).
The list may also contain
patterns negated by prefixing them with
.Ql ! ;
if the canonical host name matches a negated pattern, the key is not accepted.
The purpose
of this option is to optionally increase security: RSA authentication
by itself does not trust the network or name servers or anything (but
the key); however, if somebody somehow steals the key, the key
permits an intruder to log in from anywhere in the world.
This additional option makes using a stolen key more difficult (name
servers and/or routers would have to be compromised in addition to
just the key).
.It Cm command="command"
Specifies that the command is executed whenever this key is used for
authentication.
The command supplied by the user (if any) is ignored.
The command is run on a pty if the client requests a pty;
otherwise it is run without a tty.
If a 8-bit clean channel is required,
one must not request a pty or should specify
.Cm no-pty .
A quote may be included in the command by quoting it with a backslash.
This option might be useful
to restrict certain RSA keys to perform just a specific operation.
An example might be a key that permits remote backups but nothing else.
Note that the client may specify TCP/IP and/or X11
forwarding unless they are explicitly prohibited.
Note that this option applies to shell, command or subsystem execution.
.It Cm environment="NAME=value"
Specifies that the string is to be added to the environment when
logging in using this key.
Environment variables set this way
override other default environment values.
Multiple options of this type are permitted.
This option is automatically disabled if
.Cm UseLogin
is enabled.
.It Cm no-port-forwarding
Forbids TCP/IP forwarding when this key is used for authentication.
Any port forward requests by the client will return an error.
This might be used, e.g., in connection with the
.Cm command
option.
.It Cm no-X11-forwarding
Forbids X11 forwarding when this key is used for authentication.
Any X11 forward requests by the client will return an error.
.It Cm no-agent-forwarding
Forbids authentication agent forwarding when this key is used for
authentication.
.It Cm no-pty
Prevents tty allocation (a request to allocate a pty will fail).
.It Cm permitopen="host:port"
Limit local
.Li ``ssh -L''
port forwarding such that it may only connect to the specified host and
port.
IPv6 addresses can be specified with an alternative syntax:
.Ar host/port .
Multiple
.Cm permitopen
options may be applied separated by commas. No pattern matching is
performed on the specified hostnames, they must be literal domains or
addresses.
.El
.Ss Examples
1024 33 12121.\|.\|.\|312314325 ylo@foo.bar
.Pp
from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula
.Pp
command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi
.Pp
permitopen="10.2.1.55:80",permitopen="10.2.1.56:25" 1024 33 23.\|.\|.\|2323
.Sh SSH_KNOWN_HOSTS FILE FORMAT
The
.Pa /etc/ssh/ssh_known_hosts ,
and
.Pa $HOME/.ssh/known_hosts
files contain host public keys for all known hosts.
The global file should
be prepared by the administrator (optional), and the per-user file is
maintained automatically: whenever the user connects from an unknown host
its key is added to the per-user file.
.Pp
Each line in these files contains the following fields: hostnames,
bits, exponent, modulus, comment.
The fields are separated by spaces.
.Pp
Hostnames is a comma-separated list of patterns ('*' and '?' act as
wildcards); each pattern in turn is matched against the canonical host
name (when authenticating a client) or against the user-supplied
name (when authenticating a server).
A pattern may also be preceded by
.Ql !
to indicate negation: if the host name matches a negated
pattern, it is not accepted (by that line) even if it matched another
pattern on the line.
.Pp
Bits, exponent, and modulus are taken directly from the RSA host key; they
can be obtained, e.g., from
.Pa /etc/ssh/ssh_host_key.pub .
The optional comment field continues to the end of the line, and is not used.
.Pp
Lines starting with
.Ql #
and empty lines are ignored as comments.
.Pp
When performing host authentication, authentication is accepted if any
matching line has the proper key.
It is thus permissible (but not
recommended) to have several lines or different host keys for the same
names.
This will inevitably happen when short forms of host names
from different domains are put in the file.
It is possible
that the files contain conflicting information; authentication is
accepted if valid information can be found from either file.
.Pp
Note that the lines in these files are typically hundreds of characters
long, and you definitely don't want to type in the host keys by hand.
Rather, generate them by a script
or by taking
.Pa /etc/ssh/ssh_host_key.pub
and adding the host names at the front.
.Ss Examples
.Bd -literal
closenet,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi
cvs.openbsd.org,199.185.137.3 ssh-rsa AAAA1234.....=
.Ed
.Sh FILES
.Bl -tag -width Ds
.It Pa /etc/ssh/sshd_config
Contains configuration data for
.Nm sshd .
This file should be writable by root only, but it is recommended
(though not necessary) that it be world-readable.
.It Pa /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_rsa_key
These three files contain the private parts of the host keys.
These files should only be owned by root, readable only by root, and not
accessible to others.
Note that
.Nm
does not start if this file is group/world-accessible.
.It Pa /etc/ssh/ssh_host_key.pub, /etc/ssh/ssh_host_dsa_key.pub, /etc/ssh/ssh_host_rsa_key.pub
These three files contain the public parts of the host keys.
These files should be world-readable but writable only by
root.
Their contents should match the respective private parts.
These files are not
really used for anything; they are provided for the convenience of
the user so their contents can be copied to known hosts files.
These files are created using
.Xr ssh-keygen 1 .
.It Pa /etc/moduli
Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange".
.It Pa /var/run/sshd.pid
Contains the process ID of the
.Nm
listening for connections (if there are several daemons running
concurrently for different ports, this contains the pid of the one
started last).
The content of this file is not sensitive; it can be world-readable.
.It Pa $HOME/.ssh/authorized_keys
Lists the public keys (RSA or DSA) that can be used to log into the user's account.
This file must be readable by root (which may on some machines imply
it being world-readable if the user's home directory resides on an NFS
volume).
It is recommended that it not be accessible by others.
The format of this file is described above.
Users will place the contents of their
.Pa identity.pub ,
.Pa id_dsa.pub
and/or
.Pa id_rsa.pub
files into this file, as described in
.Xr ssh-keygen 1 .
.It Pa "/etc/ssh/ssh_known_hosts" and "$HOME/.ssh/known_hosts"
These files are consulted when using rhosts with RSA host
authentication or protocol version 2 hostbased authentication
to check the public key of the host.
The key must be listed in one of these files to be accepted.
The client uses the same files
to verify that it is connecting to the correct remote host.
These files should be writable only by root/the owner.
.Pa /etc/ssh/ssh_known_hosts
should be world-readable, and
.Pa $HOME/.ssh/known_hosts
can but need not be world-readable.
.It Pa /etc/nologin
If this file exists,
.Nm
refuses to let anyone except root log in.
The contents of the file
are displayed to anyone trying to log in, and non-root connections are
refused.
The file should be world-readable.
.It Pa /etc/hosts.allow, /etc/hosts.deny
Access controls that should be enforced by tcp-wrappers are defined here.
Further details are described in
.Xr hosts_access 5 .
.It Pa $HOME/.rhosts
This file contains host-username pairs, separated by a space, one per
line.
The given user on the corresponding host is permitted to log in
without password.
The same file is used by rlogind and rshd.
The file must
be writable only by the user; it is recommended that it not be
accessible by others.
.Pp
If is also possible to use netgroups in the file.
Either host or user
name may be of the form +@groupname to specify all hosts or all users
in the group.
.It Pa $HOME/.shosts
For ssh,
this file is exactly the same as for
.Pa .rhosts .
However, this file is
not used by rlogin and rshd, so using this permits access using SSH only.
.It Pa /etc/hosts.equiv
This file is used during
.Pa .rhosts
authentication.
In the simplest form, this file contains host names, one per line.
Users on
those hosts are permitted to log in without a password, provided they
have the same user name on both machines.
The host name may also be
followed by a user name; such users are permitted to log in as
.Em any
user on this machine (except root).
Additionally, the syntax
.Dq +@group
can be used to specify netgroups.
Negated entries start with
.Ql \&- .
.Pp
If the client host/user is successfully matched in this file, login is
automatically permitted provided the client and server user names are the
same.
Additionally, successful RSA host authentication is normally required.
This file must be writable only by root; it is recommended
that it be world-readable.
.Pp
.Sy "Warning: It is almost never a good idea to use user names in"
.Pa hosts.equiv .
Beware that it really means that the named user(s) can log in as
.Em anybody ,
which includes bin, daemon, adm, and other accounts that own critical
binaries and directories.
Using a user name practically grants the user root access.
The only valid use for user names that I can think
of is in negative entries.
.Pp
Note that this warning also applies to rsh/rlogin.
.It Pa /etc/shosts.equiv
This is processed exactly as
.Pa /etc/hosts.equiv .
However, this file may be useful in environments that want to run both
rsh/rlogin and ssh.
.It Pa $HOME/.ssh/environment
This file is read into the environment at login (if it exists).
It can only contain empty lines, comment lines (that start with
.Ql # ) ,
and assignment lines of the form name=value.
The file should be writable
only by the user; it need not be readable by anyone else.
.It Pa $HOME/.ssh/rc
If this file exists, it is run with /bin/sh after reading the
environment files but before starting the user's shell or command.
If X11 spoofing is in use, this will receive the "proto cookie" pair in
standard input (and
.Ev DISPLAY
in environment).
This must call
.Xr xauth 1
in that case.
.Pp
The primary purpose of this file is to run any initialization routines
which may be needed before the user's home directory becomes
accessible; AFS is a particular example of such an environment.
.Pp
This file will probably contain some initialization code followed by
something similar to:
.Bd -literal
	if read proto cookie; then
		echo add $DISPLAY $proto $cookie | xauth -q -
	fi
.Ed
.Pp
If this file does not exist,
.Pa /etc/ssh/sshrc
is run, and if that
does not exist either, xauth is used to store the cookie.
.Pp
This file should be writable only by the user, and need not be
readable by anyone else.
.It Pa /etc/ssh/sshrc
Like
.Pa $HOME/.ssh/rc .
This can be used to specify
machine-specific login-time initializations globally.
This file should be writable only by root, and should be world-readable.
.El
.Sh AUTHORS
OpenSSH is a derivative of the original and free
ssh 1.2.12 release by Tatu Ylonen.
Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
Theo de Raadt and Dug Song
removed many bugs, re-added newer features and
created OpenSSH.
Markus Friedl contributed the support for SSH
protocol versions 1.5 and 2.0.
Niels Provos and Markus Friedl contributed support
for privilege separation.
.Sh SEE ALSO
.Xr scp 1 ,
.Xr sftp 1 ,
.Xr ssh 1 ,
.Xr ssh-add 1 ,
.Xr ssh-agent 1 ,
.Xr ssh-keygen 1 ,
.Xr login.conf 5 ,
.Xr moduli 5 ,
.Xr sftp-server 8
.Rs
.%A T. Ylonen
.%A T. Kivinen
.%A M. Saarinen
.%A T. Rinne
.%A S. Lehtinen
.%T "SSH Protocol Architecture"
.%N draft-ietf-secsh-architecture-09.txt
.%D July 2001
.%O work in progress material
.Re
.Rs
.%A M. Friedl
.%A N. Provos
.%A W. A. Simpson
.%T "Diffie-Hellman Group Exchange for the SSH Transport Layer Protocol"
.%N draft-ietf-secsh-dh-group-exchange-01.txt
.%D April 2001
.%O work in progress material
.Re