summaryrefslogtreecommitdiffstats
path: root/ssh.1
blob: 7760c307596f6664a3eb86db0323a4f3ad06077a (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
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
.\"
.\" 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: ssh.1,v 1.399 2018/09/20 06:58:48 jmc Exp $
.Dd $Mdocdate: September 20 2018 $
.Dt SSH 1
.Os
.Sh NAME
.Nm ssh
.Nd OpenSSH SSH client (remote login program)
.Sh SYNOPSIS
.Nm ssh
.Op Fl 46AaCfGgKkMNnqsTtVvXxYy
.Op Fl B Ar bind_interface
.Op Fl b Ar bind_address
.Op Fl c Ar cipher_spec
.Op Fl D Oo Ar bind_address : Oc Ns Ar port
.Op Fl E Ar log_file
.Op Fl e Ar escape_char
.Op Fl F Ar configfile
.Op Fl I Ar pkcs11
.Op Fl i Ar identity_file
.Op Fl J Ar destination
.Op Fl L Ar address
.Op Fl l Ar login_name
.Op Fl m Ar mac_spec
.Op Fl O Ar ctl_cmd
.Op Fl o Ar option
.Op Fl p Ar port
.Op Fl Q Ar query_option
.Op Fl R Ar address
.Op Fl S Ar ctl_path
.Op Fl W Ar host : Ns Ar port
.Op Fl w Ar local_tun Ns Op : Ns Ar remote_tun
.Ar destination
.Op Ar command
.Sh DESCRIPTION
.Nm
(SSH client) is a program for logging into a remote machine and for
executing commands on a remote machine.
It is intended to provide secure encrypted communications between
two untrusted hosts over an insecure network.
X11 connections, arbitrary TCP ports and
.Ux Ns -domain
sockets can also be forwarded over the secure channel.
.Pp
.Nm
connects and logs into the specified
.Ar destination ,
which may be specified as either
.Sm off
.Oo user @ Oc hostname
.Sm on
or a URI of the form
.Sm off
.No ssh:// Oo user @ Oc hostname Op : port .
.Sm on
The user must prove
his/her identity to the remote machine using one of several methods
(see below).
.Pp
If a
.Ar command
is specified,
it is executed on the remote host instead of a login shell.
.Pp
The options are as follows:
.Pp
.Bl -tag -width Ds -compact
.It Fl 4
Forces
.Nm
to use IPv4 addresses only.
.Pp
.It Fl 6
Forces
.Nm
to use IPv6 addresses only.
.Pp
.It Fl A
Enables forwarding of the authentication agent connection.
This can also be specified on a per-host basis in a configuration file.
.Pp
Agent forwarding should be enabled with caution.
Users with the ability to bypass file permissions on the remote host
(for the agent's
.Ux Ns -domain
socket) can access the local agent through the forwarded connection.
An attacker cannot obtain key material from the agent,
however they can perform operations on the keys that enable them to
authenticate using the identities loaded into the agent.
.Pp
.It Fl a
Disables forwarding of the authentication agent connection.
.Pp
.It Fl B Ar bind_interface
Bind to the address of
.Ar bind_interface
before attempting to connect to the destination host.
This is only useful on systems with more than one address.
.Pp
.It Fl b Ar bind_address
Use
.Ar bind_address
on the local machine as the source address
of the connection.
Only useful on systems with more than one address.
.Pp
.It Fl C
Requests compression of all data (including stdin, stdout, stderr, and
data for forwarded X11, TCP and
.Ux Ns -domain
connections).
The compression algorithm is the same used by
.Xr gzip 1 .
Compression is desirable on modem lines and other
slow connections, but will only slow down things on fast networks.
The default value can be set on a host-by-host basis in the
configuration files; see the
.Cm Compression
option.
.Pp
.It Fl c Ar cipher_spec
Selects the cipher specification for encrypting the session.
.Ar cipher_spec
is a comma-separated list of ciphers
listed in order of preference.
See the
.Cm Ciphers
keyword in
.Xr ssh_config 5
for more information.
.Pp
.It Fl D Xo
.Sm off
.Oo Ar bind_address : Oc
.Ar port
.Sm on
.Xc
Specifies a local
.Dq dynamic
application-level port forwarding.
This works by allocating a socket to listen to
.Ar port
on the local side, optionally bound to the specified
.Ar bind_address .
Whenever a connection is made to this port, the
connection is forwarded over the secure channel, and the application
protocol is then used to determine where to connect to from the
remote machine.
Currently the SOCKS4 and SOCKS5 protocols are supported, and
.Nm
will act as a SOCKS server.
Only root can forward privileged ports.
Dynamic port forwardings can also be specified in the configuration file.
.Pp
IPv6 addresses can be specified by enclosing the address in square brackets.
Only the superuser can forward privileged ports.
By default, the local port is bound in accordance with the
.Cm GatewayPorts
setting.
However, an explicit
.Ar bind_address
may be used to bind the connection to a specific address.
The
.Ar bind_address
of
.Dq localhost
indicates that the listening port be bound for local use only, while an
empty address or
.Sq *
indicates that the port should be available from all interfaces.
.Pp
.It Fl E Ar log_file
Append debug logs to
.Ar log_file
instead of standard error.
.Pp
.It Fl e Ar escape_char
Sets the escape character for sessions with a pty (default:
.Ql ~ ) .
The escape character is only recognized at the beginning of a line.
The escape character followed by a dot
.Pq Ql \&.
closes the connection;
followed by control-Z suspends the connection;
and followed by itself sends the escape character once.
Setting the character to
.Dq none
disables any escapes and makes the session fully transparent.
.Pp
.It Fl F Ar configfile
Specifies an alternative per-user configuration file.
If a configuration file is given on the command line,
the system-wide configuration file
.Pq Pa /etc/ssh/ssh_config
will be ignored.
The default for the per-user configuration file is
.Pa ~/.ssh/config .
.Pp
.It Fl f
Requests
.Nm
to go to background just before command execution.
This is useful if
.Nm
is going to ask for passwords or passphrases, but the user
wants it in the background.
This implies
.Fl n .
The recommended way to start X11 programs at a remote site is with
something like
.Ic ssh -f host xterm .
.Pp
If the
.Cm ExitOnForwardFailure
configuration option is set to
.Dq yes ,
then a client started with
.Fl f
will wait for all remote port forwards to be successfully established
before placing itself in the background.
.Pp
.It Fl G
Causes
.Nm
to print its configuration after evaluating
.Cm Host
and
.Cm Match
blocks and exit.
.Pp
.It Fl g
Allows remote hosts to connect to local forwarded ports.
If used on a multiplexed connection, then this option must be specified
on the master process.
.Pp
.It Fl I Ar pkcs11
Specify the PKCS#11 shared library
.Nm
should use to communicate with a PKCS#11 token providing the user's
private RSA key.
.Pp
.It Fl i Ar identity_file
Selects a file from which the identity (private key) for
public key authentication is read.
The default is
.Pa ~/.ssh/id_dsa ,
.Pa ~/.ssh/id_ecdsa ,
.Pa ~/.ssh/id_ed25519
and
.Pa ~/.ssh/id_rsa .
Identity files may also be specified on
a per-host basis in the configuration file.
It is possible to have multiple
.Fl i
options (and multiple identities specified in
configuration files).
If no certificates have been explicitly specified by the
.Cm CertificateFile
directive,
.Nm
will also try to load certificate information from the filename obtained
by appending
.Pa -cert.pub
to identity filenames.
.Pp
.It Fl J Ar destination
Connect to the target host by first making a
.Nm
connection to the jump host described by
.Ar destination
and then establishing a TCP forwarding to the ultimate destination from
there.
Multiple jump hops may be specified separated by comma characters.
This is a shortcut to specify a
.Cm ProxyJump
configuration directive.
.Pp
.It Fl K
Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
credentials to the server.
.Pp
.It Fl k
Disables forwarding (delegation) of GSSAPI credentials to the server.
.Pp
.It Fl L Xo
.Sm off
.Oo Ar bind_address : Oc
.Ar port : host : hostport
.Sm on
.Xc
.It Fl L Xo
.Sm off
.Oo Ar bind_address : Oc
.Ar port : remote_socket
.Sm on
.Xc
.It Fl L Xo
.Sm off
.Ar local_socket : host : hostport
.Sm on
.Xc
.It Fl L Xo
.Sm off
.Ar local_socket : remote_socket
.Sm on
.Xc
Specifies that connections to the given TCP port or Unix socket on the local
(client) host are to be forwarded to the given host and port, or Unix socket,
on the remote side.
This works by allocating a socket to listen to either a TCP
.Ar port
on the local side, optionally bound to the specified
.Ar bind_address ,
or to a Unix socket.
Whenever a connection is made to the local port or socket, the
connection is forwarded over the secure channel, and a connection is
made to either
.Ar host
port
.Ar hostport ,
or the Unix socket
.Ar remote_socket ,
from the remote machine.
.Pp
Port forwardings can also be specified in the configuration file.
Only the superuser can forward privileged ports.
IPv6 addresses can be specified by enclosing the address in square brackets.
.Pp
By default, the local port is bound in accordance with the
.Cm GatewayPorts
setting.
However, an explicit
.Ar bind_address
may be used to bind the connection to a specific address.
The
.Ar bind_address
of
.Dq localhost
indicates that the listening port be bound for local use only, while an
empty address or
.Sq *
indicates that the port should be available from all interfaces.
.Pp
.It Fl l Ar login_name
Specifies the user to log in as on the remote machine.
This also may be specified on a per-host basis in the configuration file.
.Pp
.It Fl M
Places the
.Nm
client into
.Dq master
mode for connection sharing.
Multiple
.Fl M
options places
.Nm
into
.Dq master
mode but with confirmation required using
.Xr ssh-askpass 1
before each operation that changes the multiplexing state
(e.g. opening a new session).
Refer to the description of
.Cm ControlMaster
in
.Xr ssh_config 5
for details.
.Pp
.It Fl m Ar mac_spec
A comma-separated list of MAC (message authentication code) algorithms,
specified in order of preference.
See the
.Cm MACs
keyword for more information.
.Pp
.It Fl N
Do not execute a remote command.
This is useful for just forwarding ports.
.Pp
.It Fl n
Redirects stdin from
.Pa /dev/null
(actually, prevents reading from stdin).
This must be used when
.Nm
is run in the background.
A common trick is to use this to run X11 programs on a remote machine.
For example,
.Ic ssh -n shadows.cs.hut.fi emacs &
will start an emacs on shadows.cs.hut.fi, and the X11
connection will be automatically forwarded over an encrypted channel.
The
.Nm
program will be put in the background.
(This does not work if
.Nm
needs to ask for a password or passphrase; see also the
.Fl f
option.)
.Pp
.It Fl O Ar ctl_cmd
Control an active connection multiplexing master process.
When the
.Fl O
option is specified, the
.Ar ctl_cmd
argument is interpreted and passed to the master process.
Valid commands are:
.Dq check
(check that the master process is running),
.Dq forward
(request forwardings without command execution),
.Dq cancel
(cancel forwardings),
.Dq exit
(request the master to exit), and
.Dq stop
(request the master to stop accepting further multiplexing requests).
.Pp
.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.
For full details of the options listed below, and their possible values, see
.Xr ssh_config 5 .
.Pp
.Bl -tag -width Ds -offset indent -compact
.It AddKeysToAgent
.It AddressFamily
.It BatchMode
.It BindAddress
.It CanonicalDomains
.It CanonicalizeFallbackLocal
.It CanonicalizeHostname
.It CanonicalizeMaxDots
.It CanonicalizePermittedCNAMEs
.It CASignatureAlgorithms
.It CertificateFile
.It ChallengeResponseAuthentication
.It CheckHostIP
.It Ciphers
.It ClearAllForwardings
.It Compression
.It ConnectionAttempts
.It ConnectTimeout
.It ControlMaster
.It ControlPath
.It ControlPersist
.It DynamicForward
.It EscapeChar
.It ExitOnForwardFailure
.It FingerprintHash
.It ForwardAgent
.It ForwardX11
.It ForwardX11Timeout
.It ForwardX11Trusted
.It GatewayPorts
.It GlobalKnownHostsFile
.It GSSAPIAuthentication
.It GSSAPIDelegateCredentials
.It HashKnownHosts
.It Host
.It HostbasedAuthentication
.It HostbasedKeyTypes
.It HostKeyAlgorithms
.It HostKeyAlias
.It HostName
.It IdentitiesOnly
.It IdentityAgent
.It IdentityFile
.It IPQoS
.It KbdInteractiveAuthentication
.It KbdInteractiveDevices
.It KexAlgorithms
.It LocalCommand
.It LocalForward
.It LogLevel
.It MACs
.It Match
.It NoHostAuthenticationForLocalhost
.It NumberOfPasswordPrompts
.It PasswordAuthentication
.It PermitLocalCommand
.It PKCS11Provider
.It Port
.It PreferredAuthentications
.It ProxyCommand
.It ProxyJump
.It ProxyUseFdpass
.It PubkeyAcceptedKeyTypes
.It PubkeyAuthentication
.It RekeyLimit
.It RemoteCommand
.It RemoteForward
.It RequestTTY
.It SendEnv
.It ServerAliveInterval
.It ServerAliveCountMax
.It SetEnv
.It StreamLocalBindMask
.It StreamLocalBindUnlink
.It StrictHostKeyChecking
.It TCPKeepAlive
.It Tunnel
.It TunnelDevice
.It UpdateHostKeys
.It User
.It UserKnownHostsFile
.It VerifyHostKeyDNS
.It VisualHostKey
.It XAuthLocation
.El
.Pp
.It Fl p Ar port
Port to connect to on the remote host.
This can be specified on a
per-host basis in the configuration file.
.Pp
.It Fl Q Ar query_option
Queries
.Nm
for the algorithms supported for the specified version 2.
The available features are:
.Ar cipher
(supported symmetric ciphers),
.Ar cipher-auth
(supported symmetric ciphers that support authenticated encryption),
.Ar help
(supported query terms for use with the
.Fl Q
flag),
.Ar mac
(supported message integrity codes),
.Ar kex
(key exchange algorithms),
.Ar key
(key types),
.Ar key-cert
(certificate key types),
.Ar key-plain
(non-certificate key types),
.Ar protocol-version
(supported SSH protocol versions), and
.Ar sig
(supported signature algorithms).
.Pp
.It Fl q
Quiet mode.
Causes most warning and diagnostic messages to be suppressed.
.Pp
.It Fl R Xo
.Sm off
.Oo Ar bind_address : Oc
.Ar port : host : hostport
.Sm on
.Xc
.It Fl R Xo
.Sm off
.Oo Ar bind_address : Oc
.Ar port : local_socket
.Sm on
.Xc
.It Fl R Xo
.Sm off
.Ar remote_socket : host : hostport
.Sm on
.Xc
.It Fl R Xo
.Sm off
.Ar remote_socket : local_socket
.Sm on
.Xc
.It Fl R Xo
.Sm off
.Oo Ar bind_address : Oc
.Ar port
.Sm on
.Xc
Specifies that connections to the given TCP port or Unix socket on the remote
(server) host are to be forwarded to the local side.
.Pp
This works by allocating a socket to listen to either a TCP
.Ar port
or to a Unix socket on the remote side.
Whenever a connection is made to this port or Unix socket, the
connection is forwarded over the secure channel, and a connection
is made from the local machine to either an explicit destination specified by
.Ar host
port
.Ar hostport ,
or
.Ar local_socket ,
or, if no explicit destination was specified,
.Nm
will act as a SOCKS 4/5 proxy and forward connections to the destinations
requested by the remote SOCKS client.
.Pp
Port forwardings can also be specified in the configuration file.
Privileged ports can be forwarded only when
logging in as root on the remote machine.
IPv6 addresses can be specified by enclosing the address in square brackets.
.Pp
By default, TCP listening sockets on the server will be bound to the loopback
interface only.
This may be overridden by specifying a
.Ar bind_address .
An empty
.Ar bind_address ,
or the address
.Ql * ,
indicates that the remote socket should listen on all interfaces.
Specifying a remote
.Ar bind_address
will only succeed if the server's
.Cm GatewayPorts
option is enabled (see
.Xr sshd_config 5 ) .
.Pp
If the
.Ar port
argument is
.Ql 0 ,
the listen port will be dynamically allocated on the server and reported
to the client at run time.
When used together with
.Ic -O forward
the allocated port will be printed to the standard output.
.Pp
.It Fl S Ar ctl_path
Specifies the location of a control socket for connection sharing,
or the string
.Dq none
to disable connection sharing.
Refer to the description of
.Cm ControlPath
and
.Cm ControlMaster
in
.Xr ssh_config 5
for details.
.Pp
.It Fl s
May be used to request invocation of a subsystem on the remote system.
Subsystems facilitate the use of SSH
as a secure transport for other applications (e.g.\&
.Xr sftp 1 ) .
The subsystem is specified as the remote command.
.Pp
.It Fl T
Disable pseudo-terminal allocation.
.Pp
.It Fl t
Force pseudo-terminal allocation.
This can be used to execute arbitrary
screen-based programs on a remote machine, which can be very useful,
e.g. when implementing menu services.
Multiple
.Fl t
options force tty allocation, even if
.Nm
has no local tty.
.Pp
.It Fl V
Display the version number and exit.
.Pp
.It Fl v
Verbose mode.
Causes
.Nm
to print debugging messages about its progress.
This is helpful in
debugging connection, authentication, and configuration problems.
Multiple
.Fl v
options increase the verbosity.
The maximum is 3.
.Pp
.It Fl W Ar host : Ns Ar port
Requests that standard input and output on the client be forwarded to
.Ar host
on
.Ar port
over the secure channel.
Implies
.Fl N ,
.Fl T ,
.Cm ExitOnForwardFailure
and
.Cm ClearAllForwardings ,
though these can be overridden in the configuration file or using
.Fl o
command line options.
.Pp
.It Fl w Xo
.Ar local_tun Ns Op : Ns Ar remote_tun
.Xc
Requests
tunnel
device forwarding with the specified
.Xr tun 4
devices between the client
.Pq Ar local_tun
and the server
.Pq Ar remote_tun .
.Pp
The devices may be specified by numerical ID or the keyword
.Dq any ,
which uses the next available tunnel device.
If
.Ar remote_tun
is not specified, it defaults to
.Dq any .
See also the
.Cm Tunnel
and
.Cm TunnelDevice
directives in
.Xr ssh_config 5 .
.Pp
If the
.Cm Tunnel
directive is unset, it will be set to the default tunnel mode, which is
.Dq point-to-point .
If a different
.Cm Tunnel
forwarding mode it desired, then it should be specified before
.Fl w .
.Pp
.It Fl X
Enables X11 forwarding.
This can also be specified on a per-host basis in a configuration file.
.Pp
X11 forwarding should be enabled with caution.
Users with the ability to bypass file permissions on the remote host
(for the user's X authorization database)
can access the local X11 display through the forwarded connection.
An attacker may then be able to perform activities such as keystroke monitoring.
.Pp
For this reason, X11 forwarding is subjected to X11 SECURITY extension
restrictions by default.
Please refer to the
.Nm
.Fl Y
option and the
.Cm ForwardX11Trusted
directive in
.Xr ssh_config 5
for more information.
.Pp
.It Fl x
Disables X11 forwarding.
.Pp
.It Fl Y
Enables trusted X11 forwarding.
Trusted X11 forwardings are not subjected to the X11 SECURITY extension
controls.
.Pp
.It Fl y
Send log information using the
.Xr syslog 3
system module.
By default this information is sent to stderr.
.El
.Pp
.Nm
may additionally obtain configuration data from
a per-user configuration file and a system-wide configuration file.
The file format and configuration options are described in
.Xr ssh_config 5 .
.Sh AUTHENTICATION
The OpenSSH SSH client supports SSH protocol 2.
.Pp
The methods available for authentication are:
GSSAPI-based authentication,
host-based authentication,
public key authentication,
challenge-response authentication,
and password authentication.
Authentication methods are tried in the order specified above,
though
.Cm PreferredAuthentications
can be used to change the default order.
.Pp
Host-based authentication works as follows:
If the machine the user logs in from is listed in
.Pa /etc/hosts.equiv
or
.Pa /etc/shosts.equiv
on the remote machine, and the user names are
the same on both sides, or if the files
.Pa ~/.rhosts
or
.Pa ~/.shosts
exist in the user's home directory on the
remote machine and contain a line containing the name of the client
machine and the name of the user on that machine, the user is
considered for login.
Additionally, the server
.Em must
be able to verify the client's
host key (see the description of
.Pa /etc/ssh/ssh_known_hosts
and
.Pa ~/.ssh/known_hosts ,
below)
for login to be permitted.
This authentication method closes security holes due to IP
spoofing, DNS spoofing, and routing spoofing.
[Note to the administrator:
.Pa /etc/hosts.equiv ,
.Pa ~/.rhosts ,
and the rlogin/rsh protocol in general, are inherently insecure and should be
disabled if security is desired.]
.Pp
Public key authentication works as follows:
The scheme is based on public-key cryptography,
using cryptosystems
where encryption and decryption are done using separate keys,
and it is unfeasible to derive the decryption key from the encryption key.
The idea is that each user creates a public/private
key pair for authentication purposes.
The server knows the public key, and only the user knows the private key.
.Nm
implements public key authentication protocol automatically,
using one of the DSA, ECDSA, Ed25519 or RSA algorithms.
The HISTORY section of
.Xr ssl 8
contains a brief discussion of the DSA and RSA algorithms.
.Pp
The file
.Pa ~/.ssh/authorized_keys
lists the public keys that are permitted for logging in.
When the user logs in, the
.Nm
program tells the server which key pair it would like to use for
authentication.
The client proves that it has access to the private key
and the server checks that the corresponding public key
is authorized to accept the account.
.Pp
The server may inform the client of errors that prevented public key
authentication from succeeding after authentication completes using a
different method.
These may be viewed by increasing the
.Cm LogLevel
to
.Cm DEBUG
or higher (e.g. by using the
.Fl v
flag).
.Pp
The user creates his/her key pair by running
.Xr ssh-keygen 1 .
This stores the private key in
.Pa ~/.ssh/id_dsa
(DSA),
.Pa ~/.ssh/id_ecdsa
(ECDSA),
.Pa ~/.ssh/id_ed25519
(Ed25519),
or
.Pa ~/.ssh/id_rsa
(RSA)
and stores the public key in
.Pa ~/.ssh/id_dsa.pub
(DSA),
.Pa ~/.ssh/id_ecdsa.pub
(ECDSA),
.Pa ~/.ssh/id_ed25519.pub
(Ed25519),
or
.Pa ~/.ssh/id_rsa.pub
(RSA)
in the user's home directory.
The user should then copy the public key
to
.Pa ~/.ssh/authorized_keys
in his/her home directory on the remote machine.
The
.Pa authorized_keys
file corresponds to the conventional
.Pa ~/.rhosts
file, and has one key
per line, though the lines can be very long.
After this, the user can log in without giving the password.
.Pp
A variation on public key authentication
is available in the form of certificate authentication:
instead of a set of public/private keys,
signed certificates are used.
This has the advantage that a single trusted certification authority
can be used in place of many public/private keys.
See the CERTIFICATES section of
.Xr ssh-keygen 1
for more information.
.Pp
The most convenient way to use public key or certificate authentication
may be with an authentication agent.
See
.Xr ssh-agent 1
and (optionally) the
.Cm AddKeysToAgent
directive in
.Xr ssh_config 5
for more information.
.Pp
Challenge-response authentication works as follows:
The server sends an arbitrary
.Qq challenge
text, and prompts for a response.
Examples of challenge-response authentication include
.Bx
Authentication (see
.Xr login.conf 5 )
and PAM (some
.Pf non- Ox
systems).
.Pp
Finally, if other authentication methods fail,
.Nm
prompts the user for a password.
The password is sent to the remote
host for checking; however, since all communications are encrypted,
the password cannot be seen by someone listening on the network.
.Pp
.Nm
automatically maintains and checks a database containing
identification for all hosts it has ever been used with.
Host keys are stored in
.Pa ~/.ssh/known_hosts
in the user's home directory.
Additionally, the file
.Pa /etc/ssh/ssh_known_hosts
is automatically checked for known hosts.
Any new hosts are automatically added to the user's file.
If a host's identification ever changes,
.Nm
warns about this and disables password authentication to prevent
server spoofing or man-in-the-middle attacks,
which could otherwise be used to circumvent the encryption.
The
.Cm StrictHostKeyChecking
option can be used to control logins to machines whose
host key is not known or has changed.
.Pp
When the user's identity has been accepted by the server, the server
either executes the given command in a non-interactive session or,
if no command has been specified, logs into the machine and gives
the user a normal shell as an interactive session.
All communication with
the remote command or shell will be automatically encrypted.
.Pp
If an interactive session is requested
.Nm
by default will only request a pseudo-terminal (pty) for interactive
sessions when the client has one.
The flags
.Fl T
and
.Fl t
can be used to override this behaviour.
.Pp
If a pseudo-terminal has been allocated the
user may use the escape characters noted below.
.Pp
If no pseudo-terminal has been allocated,
the session is transparent and can be used to reliably transfer binary data.
On most systems, setting the escape character to
.Dq none
will also make the session transparent even if a tty is used.
.Pp
The session terminates when the command or shell on the remote
machine exits and all X11 and TCP connections have been closed.
.Sh ESCAPE CHARACTERS
When a pseudo-terminal has been requested,
.Nm
supports a number of functions through the use of an escape character.
.Pp
A single tilde character can be sent as
.Ic ~~
or by following the tilde by a character other than those described below.
The escape character must always follow a newline to be interpreted as
special.
The escape character can be changed in configuration files using the
.Cm EscapeChar
configuration directive or on the command line by the
.Fl e
option.
.Pp
The supported escapes (assuming the default
.Ql ~ )
are:
.Bl -tag -width Ds
.It Cm ~.
Disconnect.
.It Cm ~^Z
Background
.Nm .
.It Cm ~#
List forwarded connections.
.It Cm ~&
Background
.Nm
at logout when waiting for forwarded connection / X11 sessions to terminate.
.It Cm ~?
Display a list of escape characters.
.It Cm ~B
Send a BREAK to the remote system
(only useful if the peer supports it).
.It Cm ~C
Open command line.
Currently this allows the addition of port forwardings using the
.Fl L ,
.Fl R
and
.Fl D
options (see above).
It also allows the cancellation of existing port-forwardings
with
.Sm off
.Fl KL Oo Ar bind_address : Oc Ar port
.Sm on
for local,
.Sm off
.Fl KR Oo Ar bind_address : Oc Ar port
.Sm on
for remote and
.Sm off
.Fl KD Oo Ar bind_address : Oc Ar port
.Sm on
for dynamic port-forwardings.
.Ic !\& Ns Ar command
allows the user to execute a local command if the
.Ic PermitLocalCommand
option is enabled in
.Xr ssh_config 5 .
Basic help is available, using the
.Fl h
option.
.It Cm ~R
Request rekeying of the connection
(only useful if the peer supports it).
.It Cm ~V
Decrease the verbosity
.Pq Ic LogLevel
when errors are being written to stderr.
.It Cm ~v
Increase the verbosity
.Pq Ic LogLevel
when errors are being written to stderr.
.El
.Sh TCP FORWARDING
Forwarding of arbitrary TCP connections over the secure channel can
be specified either on the command line or in a configuration file.
One possible application of TCP forwarding is a secure connection to a
mail server; another is going through firewalls.
.Pp
In the example below, we look at encrypting communication between
an IRC client and server, even though the IRC server does not directly
support encrypted communications.
This works as follows:
the user connects to the remote host using
.Nm ,
specifying a port to be used to forward connections
to the remote server.
After that it is possible to start the service which is to be encrypted
on the client machine,
connecting to the same local port,
and
.Nm
will encrypt and forward the connection.
.Pp
The following example tunnels an IRC session from client machine
.Dq 127.0.0.1
(localhost)
to remote server
.Dq server.example.com :
.Bd -literal -offset 4n
$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
$ irc -c '#users' -p 1234 pinky 127.0.0.1
.Ed
.Pp
This tunnels a connection to IRC server
.Dq server.example.com ,
joining channel
.Dq #users ,
nickname
.Dq pinky ,
using port 1234.
It doesn't matter which port is used,
as long as it's greater than 1023
(remember, only root can open sockets on privileged ports)
and doesn't conflict with any ports already in use.
The connection is forwarded to port 6667 on the remote server,
since that's the standard port for IRC services.
.Pp
The
.Fl f
option backgrounds
.Nm
and the remote command
.Dq sleep 10
is specified to allow an amount of time
(10 seconds, in the example)
to start the service which is to be tunnelled.
If no connections are made within the time specified,
.Nm
will exit.
.Sh X11 FORWARDING
If the
.Cm ForwardX11
variable is set to
.Dq yes
(or see the description of the
.Fl X ,
.Fl x ,
and
.Fl Y
options above)
and the user is using X11 (the
.Ev DISPLAY
environment variable is set), the connection to the X11 display is
automatically forwarded to the remote side in such a way that any X11
programs started from the shell (or command) will go through the
encrypted channel, and the connection to the real X server will be made
from the local machine.
The user should not manually set
.Ev DISPLAY .
Forwarding of X11 connections can be
configured on the command line or in configuration files.
.Pp
The
.Ev DISPLAY
value set by
.Nm
will point to the server machine, but with a display number greater than zero.
This is normal, and happens because
.Nm
creates a
.Dq proxy
X server on the server machine for forwarding the
connections over the encrypted channel.
.Pp
.Nm
will also automatically set up Xauthority data on the server machine.
For this purpose, it will generate a random authorization cookie,
store it in Xauthority on the server, and verify that any forwarded
connections carry this cookie and replace it by the real cookie when
the connection is opened.
The real authentication cookie is never
sent to the server machine (and no cookies are sent in the plain).
.Pp
If the
.Cm ForwardAgent
variable is set to
.Dq yes
(or see the description of the
.Fl A
and
.Fl a
options above) and
the user is using an authentication agent, the connection to the agent
is automatically forwarded to the remote side.
.Sh VERIFYING HOST KEYS
When connecting to a server for the first time,
a fingerprint of the server's public key is presented to the user
(unless the option
.Cm StrictHostKeyChecking
has been disabled).
Fingerprints can be determined using
.Xr ssh-keygen 1 :
.Pp
.Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
.Pp
If the fingerprint is already known, it can be matched
and the key can be accepted or rejected.
If only legacy (MD5) fingerprints for the server are available, the
.Xr ssh-keygen 1
.Fl E
option may be used to downgrade the fingerprint algorithm to match.
.Pp
Because of the difficulty of comparing host keys
just by looking at fingerprint strings,
there is also support to compare host keys visually,
using
.Em random art .
By setting the
.Cm VisualHostKey
option to
.Dq yes ,
a small ASCII graphic gets displayed on every login to a server, no matter
if the session itself is interactive or not.
By learning the pattern a known server produces, a user can easily
find out that the host key has changed when a completely different pattern
is displayed.
Because these patterns are not unambiguous however, a pattern that looks
similar to the pattern remembered only gives a good probability that the
host key is the same, not guaranteed proof.
.Pp
To get a listing of the fingerprints along with their random art for
all known hosts, the following command line can be used:
.Pp
.Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
.Pp
If the fingerprint is unknown,
an alternative method of verification is available:
SSH fingerprints verified by DNS.
An additional resource record (RR),
SSHFP,
is added to a zonefile
and the connecting client is able to match the fingerprint
with that of the key presented.
.Pp
In this example, we are connecting a client to a server,
.Dq host.example.com .
The SSHFP resource records should first be added to the zonefile for
host.example.com:
.Bd -literal -offset indent
$ ssh-keygen -r host.example.com.
.Ed
.Pp
The output lines will have to be added to the zonefile.
To check that the zone is answering fingerprint queries:
.Pp
.Dl $ dig -t SSHFP host.example.com
.Pp
Finally the client connects:
.Bd -literal -offset indent
$ ssh -o "VerifyHostKeyDNS ask" host.example.com
[...]
Matching host key fingerprint found in DNS.
Are you sure you want to continue connecting (yes/no)?
.Ed
.Pp
See the
.Cm VerifyHostKeyDNS
option in
.Xr ssh_config 5
for more information.
.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
.Nm
contains support for Virtual Private Network (VPN) tunnelling
using the
.Xr tun 4
network pseudo-device,
allowing two networks to be joined securely.
The
.Xr sshd_config 5
configuration option
.Cm PermitTunnel
controls whether the server supports this,
and at what level (layer 2 or 3 traffic).
.Pp
The following example would connect client network 10.0.50.0/24
with remote network 10.0.99.0/24 using a point-to-point connection
from 10.1.1.1 to 10.1.1.2,
provided that the SSH server running on the gateway to the remote network,
at 192.168.1.15, allows it.
.Pp
On the client:
.Bd -literal -offset indent
# ssh -f -w 0:1 192.168.1.15 true
# ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
# route add 10.0.99.0/24 10.1.1.2
.Ed
.Pp
On the server:
.Bd -literal -offset indent
# ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
# route add 10.0.50.0/24 10.1.1.1
.Ed
.Pp
Client access may be more finely tuned via the
.Pa /root/.ssh/authorized_keys
file (see below) and the
.Cm PermitRootLogin
server option.
The following entry would permit connections on
.Xr tun 4
device 1 from user
.Dq jane
and on tun device 2 from user
.Dq john ,
if
.Cm PermitRootLogin
is set to
.Dq forced-commands-only :
.Bd -literal -offset 2n
tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
.Ed
.Pp
Since an SSH-based setup entails a fair amount of overhead,
it may be more suited to temporary setups,
such as for wireless VPNs.
More permanent VPNs are better provided by tools such as
.Xr ipsecctl 8
and
.Xr isakmpd 8 .
.Sh ENVIRONMENT
.Nm
will normally set the following environment variables:
.Bl -tag -width "SSH_ORIGINAL_COMMAND"
.It Ev DISPLAY
The
.Ev DISPLAY
variable indicates the location of the X11 server.
It is automatically set by
.Nm
to point to a value of the form
.Dq hostname:n ,
where
.Dq hostname
indicates the host where the shell runs, and
.Sq n
is an integer \*(Ge 1.
.Nm
uses this special value to forward X11 connections over the secure
channel.
The user should normally not set
.Ev DISPLAY
explicitly, as that
will render the X11 connection insecure (and will require the user to
manually copy any required authorization cookies).
.It Ev HOME
Set to the path of the user's home directory.
.It Ev LOGNAME
Synonym for
.Ev USER ;
set for compatibility with systems that use this variable.
.It Ev MAIL
Set to the path of the user's mailbox.
.It Ev PATH
Set to the default
.Ev PATH ,
as specified when compiling
.Nm .
.It Ev SSH_ASKPASS
If
.Nm
needs a passphrase, it will read the passphrase from the current
terminal if it was run from a terminal.
If
.Nm
does not have a terminal associated with it but
.Ev DISPLAY
and
.Ev SSH_ASKPASS
are set, it will execute the program specified by
.Ev SSH_ASKPASS
and open an X11 window to read the passphrase.
This is particularly useful when calling
.Nm
from a
.Pa .xsession
or related script.
(Note that on some machines it
may be necessary to redirect the input from
.Pa /dev/null
to make this work.)
.It Ev SSH_AUTH_SOCK
Identifies the path of a
.Ux Ns -domain
socket used to communicate with the agent.
.It Ev SSH_CONNECTION
Identifies the client and server ends of the connection.
The variable contains
four space-separated values: client IP address, client port number,
server IP address, and server port number.
.It Ev SSH_ORIGINAL_COMMAND
This variable contains the original command line if a forced command
is executed.
It can be used to extract the original arguments.
.It Ev SSH_TTY
This is set to the name of the tty (path to the device) associated
with the current shell or command.
If the current session has no tty,
this variable is not set.
.It Ev SSH_TUNNEL
Optionally set by
.Xr sshd 8
to contain the interface names assigned if tunnel forwarding was
requested by the client.
.It Ev SSH_USER_AUTH
Optionally set by
.Xr sshd 8 ,
this variable may contain a pathname to a file that lists the authentication
methods successfully used when the session was established, including any
public keys that were used.
.It Ev TZ
This variable is set to indicate the present time zone if it
was set when the daemon was started (i.e. the daemon passes the value
on to new connections).
.It Ev USER
Set to the name of the user logging in.
.El
.Pp
Additionally,
.Nm
reads
.Pa ~/.ssh/environment ,
and adds lines of the format
.Dq VARNAME=value
to the environment if the file exists and users are allowed to
change their environment.
For more information, see the
.Cm PermitUserEnvironment
option in
.Xr sshd_config 5 .
.Sh FILES
.Bl -tag -width Ds -compact
.It Pa ~/.rhosts
This file is used for host-based authentication (see above).
On some machines this file may need to be
world-readable if the user's home directory is on an NFS partition,
because
.Xr sshd 8
reads it as root.
Additionally, this file must be owned by the user,
and must not have write permissions for anyone else.
The recommended
permission for most machines is read/write for the user, and not
accessible by others.
.Pp
.It Pa ~/.shosts
This file is used in exactly the same way as
.Pa .rhosts ,
but allows host-based authentication without permitting login with
rlogin/rsh.
.Pp
.It Pa ~/.ssh/
This directory is the default location for all user-specific configuration
and authentication information.
There is no general requirement to keep the entire contents of this directory
secret, but the recommended permissions are read/write/execute for the user,
and not accessible by others.
.Pp
.It Pa ~/.ssh/authorized_keys
Lists the public keys (DSA, ECDSA, Ed25519, RSA)
that can be used for logging in as this user.
The format of this file is described in the
.Xr sshd 8
manual page.
This file is not highly sensitive, but the recommended
permissions are read/write for the user, and not accessible by others.
.Pp
.It Pa ~/.ssh/config
This is the per-user configuration file.
The file format and configuration options are described in
.Xr ssh_config 5 .
Because of the potential for abuse, this file must have strict permissions:
read/write for the user, and not writable by others.
.Pp
.It Pa ~/.ssh/environment
Contains additional definitions for environment variables; see
.Sx ENVIRONMENT ,
above.
.Pp
.It Pa ~/.ssh/id_dsa
.It Pa ~/.ssh/id_ecdsa
.It Pa ~/.ssh/id_ed25519
.It Pa ~/.ssh/id_rsa
Contains the private key for authentication.
These files
contain sensitive data and should be readable by the user but not
accessible by others (read/write/execute).
.Nm
will simply ignore a private key file if it is accessible by others.
It is possible to specify a passphrase when
generating the key which will be used to encrypt the
sensitive part of this file using AES-128.
.Pp
.It Pa ~/.ssh/id_dsa.pub
.It Pa ~/.ssh/id_ecdsa.pub
.It Pa ~/.ssh/id_ed25519.pub
.It Pa ~/.ssh/id_rsa.pub
Contains the public key for authentication.
These files are not
sensitive and can (but need not) be readable by anyone.
.Pp
.It Pa ~/.ssh/known_hosts
Contains a list of host keys for all hosts the user has logged into
that are not already in the systemwide list of known host keys.
See
.Xr sshd 8
for further details of the format of this file.
.Pp
.It Pa ~/.ssh/rc
Commands in this file are executed by
.Nm
when the user logs in, just before the user's shell (or command) is
started.
See the
.Xr sshd 8
manual page for more information.
.Pp
.It Pa /etc/hosts.equiv
This file is for host-based authentication (see above).
It should only be writable by root.
.Pp
.It Pa /etc/shosts.equiv
This file is used in exactly the same way as
.Pa hosts.equiv ,
but allows host-based authentication without permitting login with
rlogin/rsh.
.Pp
.It Pa /etc/ssh/ssh_config
Systemwide configuration file.
The file format and configuration options are described in
.Xr ssh_config 5 .
.Pp
.It Pa /etc/ssh/ssh_host_key
.It Pa /etc/ssh/ssh_host_dsa_key
.It Pa /etc/ssh/ssh_host_ecdsa_key
.It Pa /etc/ssh/ssh_host_ed25519_key
.It Pa /etc/ssh/ssh_host_rsa_key
These files contain the private parts of the host keys
and are used for host-based authentication.
.Pp
.It Pa /etc/ssh/ssh_known_hosts
Systemwide list of known host keys.
This file should be prepared by the
system administrator to contain the public host keys of all machines in the
organization.
It should be world-readable.
See
.Xr sshd 8
for further details of the format of this file.
.Pp
.It Pa /etc/ssh/sshrc
Commands in this file are executed by
.Nm
when the user logs in, just before the user's shell (or command) is started.
See the
.Xr sshd 8
manual page for more information.
.El
.Sh EXIT STATUS
.Nm
exits with the exit status of the remote command or with 255
if an error occurred.
.Sh SEE ALSO
.Xr scp 1 ,
.Xr sftp 1 ,
.Xr ssh-add 1 ,
.Xr ssh-agent 1 ,
.Xr ssh-keygen 1 ,
.Xr ssh-keyscan 1 ,
.Xr tun 4 ,
.Xr ssh_config 5 ,
.Xr ssh-keysign 8 ,
.Xr sshd 8
.Sh STANDARDS
.Rs
.%A S. Lehtinen
.%A C. Lonvick
.%D January 2006
.%R RFC 4250
.%T The Secure Shell (SSH) Protocol Assigned Numbers
.Re
.Pp
.Rs
.%A T. Ylonen
.%A C. Lonvick
.%D January 2006
.%R RFC 4251
.%T The Secure Shell (SSH) Protocol Architecture
.Re
.Pp
.Rs
.%A T. Ylonen
.%A C. Lonvick
.%D January 2006
.%R RFC 4252
.%T The Secure Shell (SSH) Authentication Protocol
.Re
.Pp
.Rs
.%A T. Ylonen
.%A C. Lonvick
.%D January 2006
.%R RFC 4253
.%T The Secure Shell (SSH) Transport Layer Protocol
.Re
.Pp
.Rs
.%A T. Ylonen
.%A C. Lonvick
.%D January 2006
.%R RFC 4254
.%T The Secure Shell (SSH) Connection Protocol
.Re
.Pp
.Rs
.%A J. Schlyter
.%A W. Griffin
.%D January 2006
.%R RFC 4255
.%T Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
.Re
.Pp
.Rs
.%A F. Cusack
.%A M. Forssen
.%D January 2006
.%R RFC 4256
.%T Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
.Re
.Pp
.Rs
.%A J. Galbraith
.%A P. Remaker
.%D January 2006
.%R RFC 4335
.%T The Secure Shell (SSH) Session Channel Break Extension
.Re
.Pp
.Rs
.%A M. Bellare
.%A T. Kohno
.%A C. Namprempre
.%D January 2006
.%R RFC 4344
.%T The Secure Shell (SSH) Transport Layer Encryption Modes
.Re
.Pp
.Rs
.%A B. Harris
.%D January 2006
.%R RFC 4345
.%T Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
.Re
.Pp
.Rs
.%A M. Friedl
.%A N. Provos
.%A W. Simpson
.%D March 2006
.%R RFC 4419
.%T Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol
.Re
.Pp
.Rs
.%A J. Galbraith
.%A R. Thayer
.%D November 2006
.%R RFC 4716
.%T The Secure Shell (SSH) Public Key File Format
.Re
.Pp
.Rs
.%A D. Stebila
.%A J. Green
.%D December 2009
.%R RFC 5656
.%T Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
.Re
.Pp
.Rs
.%A A. Perrig
.%A D. Song
.%D 1999
.%O International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)
.%T Hash Visualization: a New Technique to improve Real-World Security
.Re
.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.