summaryrefslogtreecommitdiff
path: root/ssh.1
blob: 28913232fdbeaabdcbd8d5aba3f0ee35f65faa38 (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
.\"  -*- 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: ssh.1,v 1.84 2001/02/04 15:10:44 markus Exp $
.Dd September 25, 1999
.Dt SSH 1
.Os
.Sh NAME
.Nm ssh
.Nd OpenSSH secure shell client (remote login program)
.Sh SYNOPSIS
.Nm ssh
.Op Fl l Ar login_name
.Op Ar hostname | user@hostname
.Op Ar command
.Pp
.Nm ssh
.Op Fl afgknqstvxACNPTX246
.Op Fl c Ar cipher_spec
.Op Fl e Ar escape_char
.Op Fl i Ar identity_file
.Op Fl l Ar login_name
.Op Fl o Ar option
.Op Fl p Ar port
.Oo Fl L Xo
.Sm off
.Ar port :
.Ar host :
.Ar hostport
.Sm on
.Xc
.Oc
.Oo Fl R Xo
.Sm off
.Ar port :
.Ar host :
.Ar hostport
.Sm on
.Xc
.Oc
.Op Ar hostname | user@hostname
.Op Ar command
.Sh DESCRIPTION
.Nm
(Secure Shell) is a program for logging into a remote machine and for
executing commands on a remote machine.
It is intended to replace
rlogin and rsh, and provide secure encrypted communications between
two untrusted hosts over an insecure network.
X11 connections and
arbitrary TCP/IP ports can also be forwarded over the secure channel.
.Pp
.Nm
connects and logs into the specified
.Ar hostname .
The user must prove
his/her identity to the remote machine using one of several methods
depending on the protocol version used:
.Pp
.Ss SSH protocol version 1
.Pp
First, 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, the user is immediately permitted to log in.
Second, if
.Pa \&.rhosts
or
.Pa \&.shosts
exists in the user's home directory on the
remote machine and contains a line containing the name of the client
machine and the name of the user on that machine, the user is
permitted to log in.
This form of authentication alone is normally not
allowed by the server because it is not secure.
.Pp
The second (and primary) authentication method is the
.Pa rhosts
or
.Pa hosts.equiv
method combined with RSA-based host authentication.
It means that if the login would be permitted by
.Pa $HOME/.rhosts ,
.Pa $HOME/.shosts ,
.Pa /etc/hosts.equiv ,
or
.Pa /etc/shosts.equiv ,
and if additionally the server can verify the client's
host key (see
.Pa /etc/ssh_known_hosts
and
.Pa $HOME/.ssh/known_hosts
in the
.Sx FILES
section), only then login is 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 $HOME/.rhosts ,
and the rlogin/rsh protocol in general, are inherently insecure and should be
disabled if security is desired.]
.Pp
As a third authentication method,
.Nm
supports RSA based authentication.
The scheme is based on public-key cryptography: there are cryptosystems
where encryption and decryption are done using separate keys, and it
is not possible to derive the decryption key from the encryption key.
RSA is one such system.
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.
The file
.Pa $HOME/.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 server checks if this key is permitted, and if
so, sends the user (actually the
.Nm
program running on behalf of the user) a challenge, a random number,
encrypted by the user's public key.
The challenge can only be
decrypted using the proper private key.
The user's client then decrypts the
challenge using the private key, proving that he/she knows the private
key but without disclosing it to the server.
.Pp
.Nm
implements the RSA authentication protocol automatically.
The user creates his/her RSA key pair by running
.Xr ssh-keygen 1 .
This stores the private key in
.Pa $HOME/.ssh/identity
and the public key in
.Pa $HOME/.ssh/identity.pub
in the user's home directory.
The user should then copy the
.Pa identity.pub
to
.Pa $HOME/.ssh/authorized_keys
in his/her home directory on the remote machine (the
.Pa authorized_keys
file corresponds to the conventional
.Pa $HOME/.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.
RSA authentication is much
more secure than rhosts authentication.
.Pp
The most convenient way to use RSA authentication may be with an
authentication agent.
See
.Xr ssh-agent 1
for more information.
.Pp
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
.Ss SSH protocol version 2
.Pp
When a user connects using the protocol version 2
different authentication methods are available:
At first, the client attempts to authenticate using the public key method.
If this method fails password authentication is tried.
.Pp
The public key method is similar to RSA authentication described
in the previous section except that the DSA or RSA algorithm is used
instead.
The client uses his private key
.Pa $HOME/.ssh/id_dsa
to sign the session identifier and sends the result to the server.
The server checks whether the matching public key is listed in
.Pa $HOME/.ssh/authorized_keys2
and grants access if both the key is found and the signature is correct.
The session identifier is derived from a shared Diffie-Hellman value
and is only known to the client and the server.
.Pp
If public key authentication fails or is not available a password
can be sent encrypted to the remote host for proving the user's identity.
This protocol 2 implementation does not yet support Kerberos or
S/Key authentication.
.Pp
Protocol 2 provides additional mechanisms for confidentiality
(the traffic is encrypted using 3DES, Blowfish, CAST128 or Arcfour)
and integrity (hmac-sha1, hmac-md5).
Note that protocol 1 lacks a strong mechanism for ensuring the
integrity of the connection.
.Pp
.Ss Login session and remote execution
.Pp
When the user's identity has been accepted by the server, the server
either executes the given command, or logs into the machine and gives
the user a normal shell on the remote machine.
All communication with
the remote command or shell will be automatically encrypted.
.Pp
If a pseudo-terminal has been allocated (normal login session), the
user can disconnect with
.Ic ~. ,
and suspend
.Nm
with
.Ic ~^Z .
All forwarded connections can be listed with
.Ic ~#
and if
the session blocks waiting for forwarded X11 or TCP/IP
connections to terminate, it can be backgrounded with
.Ic ~&
(this should not be used while the user shell is active, as it can cause the
shell to hang).
All available escapes can be listed with
.Ic ~? .
.Pp
A single tilde character can be sent as
.Ic ~~
(or by following the tilde by a character other than those described above).
The escape character must always follow a newline to be interpreted as
special.
The escape character can be changed in configuration files
or on the command line.
.Pp
If no pseudo tty 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 exists and all X11 and TCP/IP connections have been closed.
The exit status of the remote program is returned as the exit status
of
.Nm ssh .
.Pp
.Ss X11 and TCP forwarding
.Pp
If 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 user is using an authentication agent, the connection to the agent
is automatically forwarded to the remote side unless disabled on
command line or in a configuration file.
.Pp
Forwarding of arbitrary TCP/IP connections over the secure channel can
be specified either on command line or in a configuration file.
One possible application of TCP/IP forwarding is a secure connection to an
electronic purse; another is going trough firewalls.
.Pp
.Ss Server authentication
.Pp
.Nm
automatically maintains and checks a database containing
identifications for all hosts it has ever been used with.
RSA host keys are stored in
.Pa $HOME/.ssh/known_hosts
and
host keys used in the protocol version 2 are stored in
.Pa $HOME/.ssh/known_hosts2
in the user's home directory.
Additionally, the files
.Pa /etc/ssh_known_hosts
and
.Pa /etc/ssh_known_hosts2
are 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 a
trojan horse from getting the user's password.
Another purpose of
this mechanism is to prevent man-in-the-middle attacks which could
otherwise be used to circumvent the encryption.
The
.Cm StrictHostKeyChecking
option (see below) can be used to prevent logins to machines whose
host key is not known or has changed.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl a
Disables forwarding of the authentication agent connection.
.It Fl A
Enables forwarding of the authentication agent connection.
This can also be specified on a per-host basis in a configuration file.
.It Fl c Ar blowfish|3des
Selects the cipher to use for encrypting the session.
.Ar 3des
is used by default.
It is believed to be secure.
.Ar 3des
(triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
It is presumably more secure than the
.Ar des
cipher which is no longer fully supported in
.Nm ssh .
.Ar blowfish
is a fast block cipher, it appears very secure and is much faster than
.Ar 3des .
.It Fl c Ar "3des-cbc,blowfish-cbc,arcfour,cast128-cbc"
Additionally, for protocol version 2 a comma-separated list of ciphers can
be specified in order of preference.
Protocol version 2 supports 3DES, Blowfish, and CAST128 in CBC mode
and Arcfour.
.It Fl e Ar ch|^ch|none
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.
.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 .
.It Fl g
Allows remote hosts to connect to local forwarded ports.
.It Fl i Ar identity_file
Selects the file from which the identity (private key) for
RSA or DSA authentication is read.
Default is
.Pa $HOME/.ssh/identity
in the user's home directory.
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).
.It Fl k
Disables forwarding of Kerberos tickets and AFS tokens.
This may also be specified on a per-host basis in the configuration file.
.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.
.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.)
.It Fl N
Do not execute a remote command.
This is useful if you just want to forward ports
(protocol version 2 only).
.It Fl o Ar option
Can be used to give options in the format used in the config file.
This is useful for specifying options for which there is no separate
command-line flag.
The option has the same format as a line in the configuration file.
.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.
.It Fl P
Use a non-privileged port for outgoing connections.
This can be used if your firewall does
not permit connections from privileged ports.
Note that this option turns off
.Cm RhostsAuthentication
and
.Cm RhostsRSAAuthentication
for older servers.
.It Fl q
Quiet mode.
Causes all warning and diagnostic messages to be suppressed.
Only fatal errors are displayed.
.It Fl s
May be used to request invocation of a subsystem on the remote system. Subsystems are a feature of the SSH2 protocol which facilitate the use 
of SSH as a secure transport for other application (eg. sftp). The 
subsystem is specified as the remote command.
.It Fl t
Force pseudo-tty 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.
.It Fl T
Disable pseudo-tty allocation.
.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 increases the verbosity.
Maximum is 3.
.It Fl x
Disables X11 forwarding.
.It Fl X
Enables X11 forwarding.
This can also be specified on a per-host basis in a configuration file.
.It Fl C
Requests compression of all data (including stdin, stdout, stderr, and
data for forwarded X11 and TCP/IP connections).
The compression algorithm is the same used by
.Xr gzip 1 ,
and the
.Dq level
can be controlled by the
.Cm CompressionLevel
option (see below).
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 Compress
option below.
.It Fl L Ar port:host:hostport
Specifies that the given port on the local (client) host is to be
forwarded to the given host and port on the remote side.
This works by allocating a socket to listen to
.Ar port
on the local side, and whenever a connection is made to this port, the
connection is forwarded over the secure channel, and a connection is
made to
.Ar host
port
.Ar hostport
from the remote machine.
Port forwardings can also be specified in the configuration file.
Only root can forward privileged ports.
IPv6 addresses can be specified with an alternative syntax:
.Ar port/host/hostport
.It Fl R Ar port:host:hostport
Specifies that the given port on the remote (server) host is to be
forwarded to the given host and port on the local side.
This works by allocating a socket to listen to
.Ar port
on the remote side, and whenever a connection is made to this port, the
connection is forwarded over the secure channel, and a connection is
made to
.Ar host
port
.Ar hostport
from the local machine.
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.
.It Fl 2
Forces
.Nm
to try protocol version 2 only.
.It Fl 4
Forces
.Nm
to use IPv4 addresses only.
.It Fl 6
Forces
.Nm
to use IPv6 addresses only.
.El
.Pp
If
.Nm
is not invoked with one of the standard program names
.Pf ( Dq ssh ,
.Dq slogin ,
.Dq rsh ,
.Dq rlogin ,
or
.Dq remsh ) ,
it uses this name as its
.Ar hostname
argument.
This is consistent with traditional
.Xr rsh 1
behavior.
.Sh CONFIGURATION FILES
.Nm
obtains configuration data from the following sources (in this order):
command line options, user's configuration file
.Pq Pa $HOME/.ssh/config ,
and system-wide configuration file
.Pq Pa /etc/ssh_config .
For each parameter, the first obtained value
will be used.
The configuration files contain sections bracketed by
.Dq Host
specifications, and that section is only applied for hosts that
match one of the patterns given in the specification.
The matched host name is the one given on the command line.
.Pp
Since the first obtained value for each parameter is used, more
host-specific declarations should be given near the beginning of the
file, and general defaults at the end.
.Pp
The configuration file has the following format:
.Pp
Empty lines and lines starting with
.Ql #
are comments.
.Pp
Otherwise a line is of the format
.Dq keyword arguments .
The possible
keywords and their meanings are as follows (note that the
configuration files are case-sensitive):
.Bl -tag -width Ds
.It Cm Host
Restricts the following declarations (up to the next
.Cm Host
keyword) to be only for those hosts that match one of the patterns
given after the keyword.
.Ql \&*
and
.Ql ?
can be used as wildcards in the
patterns.
A single
.Ql \&*
as a pattern can be used to provide global
defaults for all hosts.
The host is the
.Ar hostname
argument given on the command line (i.e., the name is not converted to
a canonicalized host name before matching).
.It Cm AFSTokenPassing
Specifies whether to pass AFS tokens to remote host.
The argument to this keyword must be
.Dq yes
or
.Dq no .
.It Cm BatchMode
If set to
.Dq yes ,
passphrase/password querying will be disabled.
This option is useful in scripts and other batch jobs where you have no
user to supply the password.
The argument must be
.Dq yes
or
.Dq no .
.It Cm CheckHostIP
If this flag is set to
.Dq yes ,
ssh will additionally check the host ip address in the
.Pa known_hosts
file.
This allows ssh to detect if a host key changed due to DNS spoofing.
If the option is set to
.Dq no ,
the check will not be executed.
.It Cm Cipher
Specifies the cipher to use for encrypting the session
in protocol version 1.
Currently,
.Dq blowfish 
and
.Dq 3des
are supported.
The default is
.Dq 3des .
.It Cm Ciphers
Specifies the ciphers allowed for protocol version 2
in order of preference.
Multiple ciphers must be comma-separated.
The default is
.Dq 3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes128-cbc .
.It Cm Compression
Specifies whether to use compression.
The argument must be
.Dq yes
or
.Dq no .
.It Cm CompressionLevel
Specifies the compression level to use if compression is enable.
The argument must be an integer from 1 (fast) to 9 (slow, best).
The default level is 6, which is good for most applications.
The meaning of the values is the same as in
.Xr gzip 1 .
.It Cm ConnectionAttempts
Specifies the number of tries (one per second) to make before falling
back to rsh or exiting.
The argument must be an integer.
This may be useful in scripts if the connection sometimes fails.
.It Cm PubkeyAuthentication
Specifies whether to try public key authentication.
The argument to this keyword must be
.Dq yes
or
.Dq no .
Note that this option applies to protocol version 2 only.
.It Cm EscapeChar
Sets the escape character (default:
.Ql ~ ) .
The escape character can also
be set on the command line.
The argument should be a single character,
.Ql ^
followed by a letter, or
.Dq none
to disable the escape
character entirely (making the connection transparent for binary
data).
.It Cm FallBackToRsh
Specifies that if connecting via
.Nm
fails due to a connection refused error (there is no
.Xr sshd 8
listening on the remote host),
.Xr rsh 1
should automatically be used instead (after a suitable warning about
the session being unencrypted).
The argument must be
.Dq yes
or
.Dq no .
.It Cm ForwardAgent
Specifies whether the connection to the authentication agent (if any)
will be forwarded to the remote machine.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.It Cm ForwardX11
Specifies whether X11 connections will be automatically redirected
over the secure channel and
.Ev DISPLAY
set.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.It Cm GatewayPorts
Specifies whether remote hosts are allowed to connect to local
forwarded ports.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.It Cm GlobalKnownHostsFile
Specifies a file to use instead of
.Pa /etc/ssh_known_hosts .
.It Cm HostKeyAlias
Specifies an alias that should be used instead of the
real host name when looking up or saving the host key
in the known_hosts files.
This option is useful for tunneling ssh connections
or if you have multiple servers running on a single host.
.It Cm HostName
Specifies the real host name to log into.
This can be used to specify nicknames or abbreviations for hosts.
Default is the name given on the command line.
Numeric IP addresses are also permitted (both on the command line and in
.Cm HostName
specifications).
.It Cm IdentityFile
Specifies the file from which the user's RSA authentication identity
is read (default
.Pa $HOME/.ssh/identity
in the user's home directory).
Additionally, any identities represented by the authentication agent
will be used for authentication.
The file name may use the tilde
syntax to refer to a user's home directory.
It is possible to have
multiple identity files specified in configuration files; all these
identities will be tried in sequence.
.It Cm KeepAlive
Specifies whether the system should send 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.
.Pp
The default is
.Dq yes
(to send keepalives), and the client will notice
if the network goes down or the remote host dies.
This is important in scripts, and many users want it too.
.Pp
To disable keepalives, the value should be set to
.Dq no
in both the server and the client configuration files.
.It Cm KerberosAuthentication
Specifies whether Kerberos authentication will be used.
The argument to this keyword must be
.Dq yes
or
.Dq no .
.It Cm KerberosTgtPassing
Specifies whether a Kerberos TGT will be forwarded to the server.
This will only work if the Kerberos server is actually an AFS kaserver.
The argument to this keyword must be
.Dq yes
or
.Dq no .
.It Cm LocalForward
Specifies that a TCP/IP port on the local machine be forwarded over
the secure channel to given host:port from the remote machine.
The first argument must be a port number, and the second must be
host:port.
Multiple forwardings may be specified, and additional
forwardings can be given on the command line.
Only the superuser can forward privileged ports.
.It Cm LogLevel
Gives the verbosity level that is used when logging messages from
.Nm ssh .
The possible values are:
QUIET, FATAL, ERROR, INFO, VERBOSE and DEBUG.
The default is INFO.
.It Cm NumberOfPasswordPrompts
Specifies the number of password prompts before giving up.
The argument to this keyword must be an integer.
Default is 3.
.It Cm PasswordAuthentication
Specifies whether to use password authentication.
The argument to this keyword must be
.Dq yes
or
.Dq no .
Note that this option applies to both protocol version 1 and 2.
.It Cm Port
Specifies the port number to connect on the remote host.
Default is 22.
.It Cm Protocol
Specifies the protocol versions
.Nm
should support in order of preference.
The possible values are
.Dq 1
and
.Dq 2 .
Multiple versions must be comma-separated.
The default is
.Dq 1,2 .
This means that
.Nm
tries version 1 and falls back to version 2
if version 1 is not available.
.It Cm ProxyCommand
Specifies the command to use to connect to the server.
The command
string extends to the end of the line, and is executed with
.Pa /bin/sh .
In the command string,
.Ql %h
will be substituted by the host name to
connect and
.Ql %p
by the port.
The command can be basically anything,
and should read from its standard input and write to its standard output.
It should eventually connect an
.Xr sshd 8
server running on some machine, or execute
.Ic sshd -i
somewhere.
Host key management will be done using the
HostName of the host being connected (defaulting to the name typed by
the user).
Note that
.Cm CheckHostIP
is not available for connects with a proxy command.
.Pp
.It Cm RemoteForward
Specifies that a TCP/IP port on the remote machine be forwarded over
the secure channel to given host:port from the local machine.
The first argument must be a port number, and the second must be
host:port.
Multiple forwardings may be specified, and additional
forwardings can be given on the command line.
Only the superuser can forward privileged ports.
.It Cm RhostsAuthentication
Specifies whether to try rhosts based authentication.
Note that this
declaration only affects the client side and has no effect whatsoever
on security.
Disabling rhosts authentication may reduce
authentication time on slow connections when rhosts authentication is
not used.
Most servers do not permit RhostsAuthentication because it
is not secure (see RhostsRSAAuthentication).
The argument to this keyword must be
.Dq yes
or
.Dq no .
.It Cm RhostsRSAAuthentication
Specifies whether to try rhosts based authentication with RSA host
authentication.
This is the primary authentication method for most sites.
The argument must be
.Dq yes
or
.Dq no .
.It Cm RSAAuthentication
Specifies whether to try RSA authentication.
The argument to this keyword must be
.Dq yes
or
.Dq no .
RSA authentication will only be
attempted if the identity file exists, or an authentication agent is
running.
Note that this option applies to protocol version 1 only.
.It Cm ChallengeResponseAuthentication
Specifies whether to use challenge response authentication.
Currently there is only support for
.Xr skey 1
authentication.
The argument to this keyword must be
.Dq yes
or
.Dq no .
The default is
.Dq no .
.It Cm StrictHostKeyChecking
If this flag is set to
.Dq yes ,
.Nm
will never automatically add host keys to the
.Pa $HOME/.ssh/known_hosts
and
.Pa $HOME/.ssh/known_hosts2
files, and refuses to connect to hosts whose host key has changed.
This provides maximum protection against trojan horse attacks.
However, it can be somewhat annoying if you don't have good
.Pa /etc/ssh_known_hosts
and
.Pa /etc/ssh_known_hosts2
files installed and frequently
connect to new hosts.
This option forces the user to manually
add all new hosts.
If this flag is set to
.Dq no ,
.Nm
will automatically add new host keys to the
user known hosts files.
If this flag is set to
.Dq ask ,
new host keys
will be added to the user known host files only after the user
has confirmed that is what they really want to do, and
.Nm
will refuse to connect to hosts whose host key has changed.
The host keys of
known hosts will be verified automatically in all cases.
The argument must be
.Dq yes ,
.Dq no
or
.Dq ask .
The default is
.Dq ask .
.It Cm UsePrivilegedPort
Specifies whether to use a privileged port for outgoing connections.
The argument must be
.Dq yes
or
.Dq no .
The default is
.Dq yes .
Note that setting this option to
.Dq no
turns off
.Cm RhostsAuthentication
and
.Cm RhostsRSAAuthentication
for older servers.
.It Cm User
Specifies the user to log in as.
This can be useful if you have a different user name on different machines.
This saves the trouble of
having to remember to give the user name on the command line.
.It Cm UserKnownHostsFile
Specifies a file to use instead of
.Pa $HOME/.ssh/known_hosts .
.It Cm UseRsh
Specifies that rlogin/rsh should be used for this host.
It is possible that the host does not at all support the
.Nm
protocol.
This causes
.Nm
to immediately execute
.Xr rsh 1 .
All other options (except
.Cm HostName )
are ignored if this has been specified.
The argument must be
.Dq yes
or
.Dq no .
.It Cm XAuthLocation
Specifies the location of the
.Xr xauth 1
program.
The default is
.Pa /usr/X11R6/bin/xauth .
.El
.Sh ENVIRONMENT
.Nm
will normally set the following environment variables:
.Bl -tag -width Ds
.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 hostname indicates
the host where the shell runs, and n is an integer >= 1.
.Nm
uses this special value to forward X11 connections over the secure
channel.
The user should normally not set 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 point the user's mailbox.
.It Ev PATH
Set to the default
.Ev PATH ,
as specified when compiling
.Nm ssh .
.It Ev SSH_AUTH_SOCK
indicates the path of a unix-domain socket used to communicate with the
agent.
.It Ev SSH_CLIENT
Identifies the client end of the connection.
The variable contains
three space-separated values: client ip-address, client port number,
and server port number.
.It Ev SSH_ORIGINAL_COMMAND
The 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 TZ
The timezone variable is set to indicate the present timezone 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 $HOME/.ssh/environment ,
and adds lines of the format
.Dq VARNAME=value
to the environment.
.Sh FILES
.Bl -tag -width Ds
.It Pa $HOME/.ssh/known_hosts
Records host keys for all hosts the user has logged into (that are not
in
.Pa /etc/ssh_known_hosts ) .
See
.Xr sshd 8 .
.It Pa $HOME/.ssh/identity, $HOME/.ssh/id_dsa
Contains the RSA and the DSA authentication identity of the user.
These files
contain sensitive data and should be readable by the user but not
accessible by others (read/write/execute).
Note that
.Nm
ignores a private key file if it is accessible by others.
It is possible to specify a passphrase when
generating the key; the passphrase will be used to encrypt the
sensitive part of this file using 3DES.
.It Pa $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub
Contains the public key for authentication (public part of the
identity file in human-readable form).
The contents of the
.Pa $HOME/.ssh/identity.pub
file should be added to
.Pa $HOME/.ssh/authorized_keys
on all machines
where you wish to log in using RSA authentication.
The contents of the
.Pa $HOME/.ssh/id_dsa.pub
file should be added to
.Pa $HOME/.ssh/authorized_keys2
on all machines
where you wish to log in using DSA authentication.
These files are not
sensitive and can (but need not) be readable by anyone.
These files are
never used automatically and are not necessary; they are only provided for
the convenience of the user.
.It Pa $HOME/.ssh/config
This is the per-user configuration file.
The format of this file is described above.
This file is used by the
.Nm
client.
This file does not usually contain any sensitive information,
but the recommended permissions are read/write for the user, and not
accessible by others.
.It Pa $HOME/.ssh/authorized_keys
Lists the RSA keys that can be used for logging in as this user.
The format of this file is described in the
.Xr sshd 8
manual page.
In the simplest form the format is the same as the .pub
identity files (that is, each line contains the number of bits in
modulus, public exponent, modulus, and comment fields, separated by
spaces).
This file is not highly sensitive, but the recommended
permissions are read/write for the user, and not accessible by others.
.It Pa $HOME/.ssh/authorized_keys2
Lists the public keys (DSA/RSA) that can be used for logging in as this user.
This file is not highly sensitive, but the recommended
permissions are read/write for the user, and not accessible by others.
.It Pa /etc/ssh_known_hosts, /etc/ssh_known_hosts2
Systemwide list of known host keys.
.Pa /etc/ssh_known_hosts
contains RSA and
.Pa /etc/ssh_known_hosts2
contains DSA or RSA keys for protocol version 2.
These files should be prepared by the
system administrator to contain the public host keys of all machines in the
organization.
This file should be world-readable.
This file contains
public keys, one per line, in the following format (fields separated
by spaces): system name, number of bits in modulus, public exponent,
modulus, and optional comment field.
When different names are used
for the same machine, all such names should be listed, separated by
commas.
The format is described on the
.Xr sshd 8
manual page.
.Pp
The canonical system name (as returned by name servers) is used by
.Xr sshd 8
to verify the client host when logging in; other names are needed because
.Nm
does not convert the user-supplied name to a canonical name before
checking the key, because someone with access to the name servers
would then be able to fool host authentication.
.It Pa /etc/ssh_config
Systemwide configuration file.
This file provides defaults for those
values that are not specified in the user's configuration file, and
for those users who do not have a configuration file.
This file must be world-readable.
.It Pa $HOME/.rhosts
This file is used in
.Pa \&.rhosts
authentication to list the
host/user pairs that are permitted to log in.
(Note that this file is
also used by rlogin and rsh, which makes using this file insecure.)
Each line of the file contains a host name (in the canonical form
returned by name servers), and then a user name on that host,
separated by a space.
One some machines this file may need to be
world-readable if the user's home directory is on a 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
Note that by default
.Xr sshd 8
will be installed so that it requires successful RSA host
authentication before permitting \s+2.\s0rhosts authentication.
If your server machine does not have the client's host key in
.Pa /etc/ssh_known_hosts ,
you can store it in
.Pa $HOME/.ssh/known_hosts .
The easiest way to do this is to
connect back to the client from the server machine using ssh; this
will automatically add the host key to
.Pa $HOME/.ssh/known_hosts .
.It Pa $HOME/.shosts
This file is used exactly the same way as
.Pa \&.rhosts .
The purpose for
having this file is to be able to use rhosts authentication with
.Nm
without permitting login with
.Xr rlogin 1
or
.Xr rsh 1 .
.It Pa /etc/hosts.equiv
This file is used during
.Pa \&.rhosts authentication.
It contains
canonical hosts names, one per line (the full format is described on
the
.Xr sshd 8
manual page).
If the client host is found in this file, login is
automatically permitted provided client and server user names are the
same.
Additionally, successful RSA host authentication is normally
required.
This file should only be writable by root.
.It Pa /etc/shosts.equiv
This file is processed exactly as
.Pa /etc/hosts.equiv .
This file may be useful to permit logins using
.Nm
but not using rsh/rlogin.
.It Pa /etc/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.
.It Pa $HOME/.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.
.It Pa $HOME/.ssh/environment
Contains additional definitions for environment variables, see section
.Sx ENVIRONMENT
above.
.It Pa libcrypto.so.X.1
A version of this library which includes support for the RSA algorithm
is required for proper operation.
.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.
.Sh SEE ALSO
.Xr rlogin 1 ,
.Xr rsh 1 ,
.Xr scp 1 ,
.Xr sftp 1 ,
.Xr ssh-add 1 ,
.Xr ssh-agent 1 ,
.Xr ssh-keygen 1 ,
.Xr telnet 1 ,
.Xr sshd 8 ,
.Xr ssl 8