| blob | 7d3130dd0e07d6f1dcb4dc4d9f0ed957ae76dafd |
1 .\" $NetBSD: ssh.1,v 1.4 2009/12/27 01:40:47 christos Exp $
2 .\" -*- nroff -*-
3 .\"
4 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
5 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
6 .\" All rights reserved
7 .\"
8 .\" As far as I am concerned, the code I have written for this software
9 .\" can be used freely for any purpose. Any derived versions of this
10 .\" software must be clearly marked as such, and if the derived work is
11 .\" incompatible with the protocol description in the RFC file, it must be
12 .\" called by a name other than "ssh" or "Secure Shell".
13 .\"
14 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
15 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
16 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
17 .\"
18 .\" Redistribution and use in source and binary forms, with or without
19 .\" modification, are permitted provided that the following conditions
20 .\" are met:
21 .\" 1. Redistributions of source code must retain the above copyright
22 .\" notice, this list of conditions and the following disclaimer.
23 .\" 2. Redistributions in binary form must reproduce the above copyright
24 .\" notice, this list of conditions and the following disclaimer in the
25 .\" documentation and/or other materials provided with the distribution.
26 .\"
27 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
28 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
29 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
30 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
31 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
32 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
33 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
34 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
35 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
36 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
37 .\"
38 .\" $OpenBSD: ssh.1,v 1.283 2009/03/19 15:15:09 jmc Exp $
39 .Dd March 19, 2009
40 .Dt SSH 1
41 .Os
42 .Sh NAME
43 .Nm ssh
44 .Nd OpenSSH SSH client (remote login program)
45 .Sh SYNOPSIS
46 .Nm ssh
47 .Op Fl 1246AaCfgKkMNnqsTtVvXxYy
48 .Op Fl b Ar bind_address
49 .Op Fl c Ar cipher_spec
50 .Oo Fl D\ \&
51 .Sm off
52 .Oo Ar bind_address : Oc
53 .Ar port
54 .Sm on
55 .Oc
56 .Op Fl e Ar escape_char
57 .Op Fl F Ar configfile
58 .Bk -words
59 .Op Fl i Ar identity_file
60 .Ek
61 .Oo Fl L\ \&
62 .Sm off
63 .Oo Ar bind_address : Oc
64 .Ar port : host : hostport
65 .Sm on
66 .Oc
67 .Bk -words
68 .Op Fl l Ar login_name
69 .Ek
70 .Op Fl m Ar mac_spec
71 .Op Fl O Ar ctl_cmd
72 .Op Fl o Ar option
73 .Op Fl p Ar port
74 .Oo Fl R\ \&
75 .Sm off
76 .Oo Ar bind_address : Oc
77 .Ar port : host : hostport
78 .Sm on
79 .Oc
80 .Op Fl S Ar ctl_path
81 .Bk -words
82 .Oo Fl w Ar local_tun Ns
83 .Op : Ns Ar remote_tun Oc
84 .Oo Ar user Ns @ Oc Ns Ar hostname
85 .Op Ar command
86 .Ek
87 .Sh DESCRIPTION
88 .Nm
89 (SSH client) is a program for logging into a remote machine and for
90 executing commands on a remote machine.
91 It is intended to replace rlogin and rsh,
92 and provide secure encrypted communications between
93 two untrusted hosts over an insecure network.
94 X11 connections and arbitrary TCP ports
95 can also be forwarded over the secure channel.
96 .Pp
97 .Nm
98 connects and logs into the specified
99 .Ar hostname
100 (with optional
101 .Ar user
102 name).
103 The user must prove
104 his/her identity to the remote machine using one of several methods
105 depending on the protocol version used (see below).
106 .Pp
107 If
108 .Ar command
109 is specified,
110 it is executed on the remote host instead of a login shell.
111 .Pp
112 The options are as follows:
113 .Bl -tag -width Ds
114 .It Fl 1
115 Forces
116 .Nm
117 to try protocol version 1 only.
118 .It Fl 2
119 Forces
120 .Nm
121 to try protocol version 2 only.
122 .It Fl 4
123 Forces
124 .Nm
125 to use IPv4 addresses only.
126 .It Fl 6
127 Forces
128 .Nm
129 to use IPv6 addresses only.
130 .It Fl A
131 Enables forwarding of the authentication agent connection.
132 This can also be specified on a per-host basis in a configuration file.
133 .Pp
134 Agent forwarding should be enabled with caution.
135 Users with the ability to bypass file permissions on the remote host
136 (for the agent's Unix-domain socket)
137 can access the local agent through the forwarded connection.
138 An attacker cannot obtain key material from the agent,
139 however they can perform operations on the keys that enable them to
140 authenticate using the identities loaded into the agent.
141 .It Fl a
142 Disables forwarding of the authentication agent connection.
143 .It Fl b Ar bind_address
144 Use
145 .Ar bind_address
146 on the local machine as the source address
147 of the connection.
148 Only useful on systems with more than one address.
149 .It Fl C
150 Requests compression of all data (including stdin, stdout, stderr, and
151 data for forwarded X11 and TCP connections).
152 The compression algorithm is the same used by
153 .Xr gzip 1 ,
154 and the
155 .Dq level
156 can be controlled by the
157 .Cm CompressionLevel
158 option for protocol version 1.
159 Compression is desirable on modem lines and other
160 slow connections, but will only slow down things on fast networks.
161 The default value can be set on a host-by-host basis in the
162 configuration files; see the
163 .Cm Compression
164 option.
165 .It Fl c Ar cipher_spec
166 Selects the cipher specification for encrypting the session.
167 .Pp
168 Protocol version 1 allows specification of a single cipher.
169 The supported values are
170 .Dq 3des ,
171 .Dq blowfish ,
172 and
173 .Dq des .
174 .Ar 3des
175 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
176 It is believed to be secure.
177 .Ar blowfish
178 is a fast block cipher; it appears very secure and is much faster than
179 .Ar 3des .
180 .Ar des
181 is only supported in the
182 .Nm
183 client for interoperability with legacy protocol 1 implementations
184 that do not support the
185 .Ar 3des
186 cipher.
187 Its use is strongly discouraged due to cryptographic weaknesses.
188 The default is
189 .Dq 3des .
190 .Pp
191 For protocol version 2,
192 .Ar cipher_spec
193 is a comma-separated list of ciphers
194 listed in order of preference.
195 See the
196 .Cm Ciphers
197 keyword for more information.
198 .It Fl D Oo Ar bind_address : Oc Ns Ar port
199 Specifies a local
200 .Dq dynamic
201 application-level port forwarding.
202 This works by allocating a socket to listen to
203 .Ar port
204 on the local side, optionally bound to the specified
205 .Ar bind_address .
206 Whenever a connection is made to this port, the
207 connection is forwarded over the secure channel, and the application
208 protocol is then used to determine where to connect to from the
209 remote machine.
210 Currently the SOCKS4 and SOCKS5 protocols are supported, and
211 .Nm
212 will act as a SOCKS server.
213 Only root can forward privileged ports.
214 Dynamic port forwardings can also be specified in the configuration file.
215 .Pp
216 IPv6 addresses can be specified with an alternative syntax:
217 .Sm off
218 .Xo
219 .Op Ar bind_address No /
220 .Ar port
221 .Xc
222 .Sm on
223 or by enclosing the address in square brackets.
224 Only the superuser can forward privileged ports.
225 By default, the local port is bound in accordance with the
226 .Cm GatewayPorts
227 setting.
228 However, an explicit
229 .Ar bind_address
230 may be used to bind the connection to a specific address.
231 The
232 .Ar bind_address
233 of
234 .Dq localhost
235 indicates that the listening port be bound for local use only, while an
236 empty address or
237 .Sq *
238 indicates that the port should be available from all interfaces.
239 .It Fl e Ar escape_char
240 Sets the escape character for sessions with a pty (default:
241 .Ql ~ ) .
242 The escape character is only recognized at the beginning of a line.
243 The escape character followed by a dot
244 .Pq Ql \&.
245 closes the connection;
246 followed by control-Z suspends the connection;
247 and followed by itself sends the escape character once.
248 Setting the character to
249 .Dq none
250 disables any escapes and makes the session fully transparent.
251 .It Fl F Ar configfile
252 Specifies an alternative per-user configuration file.
253 If a configuration file is given on the command line,
254 the system-wide configuration file
255 .Pq Pa /etc/ssh/ssh_config
256 will be ignored.
257 The default for the per-user configuration file is
258 .Pa ~/.ssh/config .
259 .It Fl f
260 Requests
261 .Nm
262 to go to background just before command execution.
263 This is useful if
264 .Nm
265 is going to ask for passwords or passphrases, but the user
266 wants it in the background.
267 This implies
268 .Fl n .
269 The recommended way to start X11 programs at a remote site is with
270 something like
271 .Ic ssh -f host xterm .
272 .Pp
273 If the
274 .Cm ExitOnForwardFailure
275 configuration option is set to
276 .Dq yes ,
277 then a client started with
278 .Fl f
279 will wait for all remote port forwards to be successfully established
280 before placing itself in the background.
281 .It Fl g
282 Allows remote hosts to connect to local forwarded ports.
283 .It Fl I Ar smartcard_device
284 Specify the device
285 .Nm
286 should use to communicate with a smartcard used for storing the user's
287 private RSA key.
288 This option is only available if support for smartcard devices
289 is compiled in (default is no support).
290 .It Fl i Ar identity_file
291 Selects a file from which the identity (private key) for
292 RSA or DSA authentication is read.
293 The default is
294 .Pa ~/.ssh/identity
295 for protocol version 1, and
296 .Pa ~/.ssh/id_rsa
297 and
298 .Pa ~/.ssh/id_dsa
299 for protocol version 2.
300 Identity files may also be specified on
301 a per-host basis in the configuration file.
302 It is possible to have multiple
303 .Fl i
304 options (and multiple identities specified in
305 configuration files).
306 .It Fl K
307 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
308 credentials to the server.
309 .It Fl k
310 Disables forwarding (delegation) of GSSAPI credentials to the server.
311 .It Fl L Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
312 Specifies that the given port on the local (client) host is to be
313 forwarded to the given host and port on the remote side.
314 This works by allocating a socket to listen to
315 .Ar port
316 on the local side, optionally bound to the specified
317 .Ar bind_address .
318 Whenever a connection is made to this port, the
319 connection is forwarded over the secure channel, and a connection is
320 made to
321 .Ar host
322 port
323 .Ar hostport
324 from the remote machine.
325 Port forwardings can also be specified in the configuration file.
326 IPv6 addresses can be specified with an alternative syntax:
327 .Sm off
328 .Xo
329 .Op Ar bind_address No /
330 .Ar port No / Ar host No /
331 .Ar hostport
332 .Xc
333 .Sm on
334 or by enclosing the address in square brackets.
335 Only the superuser can forward privileged ports.
336 By default, the local port is bound in accordance with the
337 .Cm GatewayPorts
338 setting.
339 However, an explicit
340 .Ar bind_address
341 may be used to bind the connection to a specific address.
342 The
343 .Ar bind_address
344 of
345 .Dq localhost
346 indicates that the listening port be bound for local use only, while an
347 empty address or
348 .Sq *
349 indicates that the port should be available from all interfaces.
350 .It Fl l Ar login_name
351 Specifies the user to log in as on the remote machine.
352 This also may be specified on a per-host basis in the configuration file.
353 .It Fl M
354 Places the
355 .Nm
356 client into
357 .Dq master
358 mode for connection sharing.
359 Multiple
360 .Fl M
361 options places
362 .Nm
363 into
364 .Dq master
365 mode with confirmation required before slave connections are accepted.
366 Refer to the description of
367 .Cm ControlMaster
368 in
369 .Xr ssh_config 5
370 for details.
371 .It Fl m Ar mac_spec
372 Additionally, for protocol version 2 a comma-separated list of MAC
373 (message authentication code) algorithms can
374 be specified in order of preference.
375 See the
376 .Cm MACs
377 keyword for more information.
378 .It Fl N
379 Do not execute a remote command.
380 This is useful for just forwarding ports
381 (protocol version 2 only).
382 .It Fl n
383 Redirects stdin from
384 .Pa /dev/null
385 (actually, prevents reading from stdin).
386 This must be used when
387 .Nm
388 is run in the background.
389 A common trick is to use this to run X11 programs on a remote machine.
390 For example,
391 .Ic ssh -n shadows.cs.hut.fi emacs &
392 will start an emacs on shadows.cs.hut.fi, and the X11
393 connection will be automatically forwarded over an encrypted channel.
394 The
395 .Nm
396 program will be put in the background.
397 (This does not work if
398 .Nm
399 needs to ask for a password or passphrase; see also the
400 .Fl f
401 option.)
402 .It Fl O Ar ctl_cmd
403 Control an active connection multiplexing master process.
404 When the
405 .Fl O
406 option is specified, the
407 .Ar ctl_cmd
408 argument is interpreted and passed to the master process.
409 Valid commands are:
410 .Dq check
411 (check that the master process is running) and
412 .Dq exit
413 (request the master to exit).
414 .It Fl o Ar option
415 Can be used to give options in the format used in the configuration file.
416 This is useful for specifying options for which there is no separate
417 command-line flag.
418 For full details of the options listed below, and their possible values, see
419 .Xr ssh_config 5 .
420 .Pp
421 .Bl -tag -width Ds -offset indent -compact
422 .It AddressFamily
423 .It BatchMode
424 .It BindAddress
425 .It ChallengeResponseAuthentication
426 .It CheckHostIP
427 .It Cipher
428 .It Ciphers
429 .It ClearAllForwardings
430 .It Compression
431 .It CompressionLevel
432 .It ConnectionAttempts
433 .It ConnectTimeout
434 .It ControlMaster
435 .It ControlPath
436 .It DynamicForward
437 .It EscapeChar
438 .It ExitOnForwardFailure
439 .It ForwardAgent
440 .It ForwardX11
441 .It ForwardX11Trusted
442 .It GatewayPorts
443 .It GlobalKnownHostsFile
444 .It GSSAPIAuthentication
445 .It GSSAPIDelegateCredentials
446 .It HashKnownHosts
447 .It Host
448 .It HostbasedAuthentication
449 .It HostKeyAlgorithms
450 .It HostKeyAlias
451 .It HostName
452 .It IdentityFile
453 .It IdentitiesOnly
454 .It KbdInteractiveDevices
455 .It LocalCommand
456 .It LocalForward
457 .It LogLevel
458 .It MACs
459 .It NoHostAuthenticationForLocalhost
460 .It NumberOfPasswordPrompts
461 .It PasswordAuthentication
462 .It PermitLocalCommand
463 .It Port
464 .It PreferredAuthentications
465 .It Protocol
466 .It ProxyCommand
467 .It PubkeyAuthentication
468 .It RekeyLimit
469 .It RemoteForward
470 .It RhostsRSAAuthentication
471 .It RSAAuthentication
472 .It SendEnv
473 .It ServerAliveInterval
474 .It ServerAliveCountMax
475 .It SmartcardDevice
476 .It StrictHostKeyChecking
477 .It TCPKeepAlive
478 .It Tunnel
479 .It TunnelDevice
480 .It UsePrivilegedPort
481 .It User
482 .It UserKnownHostsFile
483 .It VerifyHostKeyDNS
484 .It VisualHostKey
485 .It XAuthLocation
486 .El
487 .It Fl p Ar port
488 Port to connect to on the remote host.
489 This can be specified on a
490 per-host basis in the configuration file.
491 .It Fl q
492 Quiet mode.
493 Causes most warning and diagnostic messages to be suppressed.
494 .It Fl R Oo Ar bind_address : Oc Ns Ar port : Ns Ar host : Ns Ar hostport
495 Specifies that the given port on the remote (server) host is to be
496 forwarded to the given host and port on the local side.
497 This works by allocating a socket to listen to
498 .Ar port
499 on the remote side, and whenever a connection is made to this port, the
500 connection is forwarded over the secure channel, and a connection is
501 made to
502 .Ar host
503 port
504 .Ar hostport
505 from the local machine.
506 .Pp
507 Port forwardings can also be specified in the configuration file.
508 Privileged ports can be forwarded only when
509 logging in as root on the remote machine.
510 IPv6 addresses can be specified by enclosing the address in square braces or
511 using an alternative syntax:
512 .Sm off
513 .Xo
514 .Op Ar bind_address No /
515 .Ar host No / Ar port No /
516 .Ar hostport
517 .Xc .
518 .Sm on
519 .Pp
520 By default, the listening socket on the server will be bound to the loopback
521 interface only.
522 This may be overridden by specifying a
523 .Ar bind_address .
524 An empty
525 .Ar bind_address ,
526 or the address
527 .Ql * ,
528 indicates that the remote socket should listen on all interfaces.
529 Specifying a remote
530 .Ar bind_address
531 will only succeed if the server's
532 .Cm GatewayPorts
533 option is enabled (see
534 .Xr sshd_config 5 ) .
535 .Pp
536 If the
537 .Ar port
538 argument is
539 .Ql 0 ,
540 the listen port will be dynamically allocated on the server and reported
541 to the client at run time.
542 .It Fl S Ar ctl_path
543 Specifies the location of a control socket for connection sharing.
544 Refer to the description of
545 .Cm ControlPath
546 and
547 .Cm ControlMaster
548 in
549 .Xr ssh_config 5
550 for details.
551 .It Fl s
552 May be used to request invocation of a subsystem on the remote system.
553 Subsystems are a feature of the SSH2 protocol which facilitate the use
554 of SSH as a secure transport for other applications (eg.\&
555 .Xr sftp 1 ) .
556 The subsystem is specified as the remote command.
557 .It Fl T
558 Disable pseudo-tty allocation.
559 .It Fl t
560 Force pseudo-tty allocation.
561 This can be used to execute arbitrary
562 screen-based programs on a remote machine, which can be very useful,
563 e.g. when implementing menu services.
564 Multiple
565 .Fl t
566 options force tty allocation, even if
567 .Nm
568 has no local tty.
569 .It Fl V
570 Display the version number and exit.
571 .It Fl v
572 Verbose mode.
573 Causes
574 .Nm
575 to print debugging messages about its progress.
576 This is helpful in
577 debugging connection, authentication, and configuration problems.
578 Multiple
579 .Fl v
580 options increase the verbosity.
581 The maximum is 3.
582 .It Fl w Ar local_tun Ns Op : Ns Ar remote_tun
583 Requests
584 tunnel
585 device forwarding with the specified
586 .Xr tun 4
587 devices between the client
588 .Pq Ar local_tun
589 and the server
590 .Pq Ar remote_tun .
591 .Pp
592 The devices may be specified by numerical ID or the keyword
593 .Dq any ,
594 which uses the next available tunnel device.
595 If
596 .Ar remote_tun
597 is not specified, it defaults to
598 .Dq any .
599 See also the
600 .Cm Tunnel
601 and
602 .Cm TunnelDevice
603 directives in
604 .Xr ssh_config 5 .
605 If the
606 .Cm Tunnel
607 directive is unset, it is set to the default tunnel mode, which is
608 .Dq point-to-point .
609 .It Fl X
610 Enables X11 forwarding.
611 This can also be specified on a per-host basis in a configuration file.
612 .Pp
613 X11 forwarding should be enabled with caution.
614 Users with the ability to bypass file permissions on the remote host
615 (for the user's X authorization database)
616 can access the local X11 display through the forwarded connection.
617 An attacker may then be able to perform activities such as keystroke monitoring.
618 .Pp
619 For this reason, X11 forwarding is subjected to X11 SECURITY extension
620 restrictions by default.
621 Please refer to the
622 .Nm
623 .Fl Y
624 option and the
625 .Cm ForwardX11Trusted
626 directive in
627 .Xr ssh_config 5
628 for more information.
629 .It Fl x
630 Disables X11 forwarding.
631 .It Fl Y
632 Enables trusted X11 forwarding.
633 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
634 controls.
635 .It Fl y
636 Send log information using the
637 .Xr syslog 3
638 system module.
639 By default this information is sent to stderr.
640 .El
641 .Pp
642 .Nm
643 may additionally obtain configuration data from
644 a per-user configuration file and a system-wide configuration file.
645 The file format and configuration options are described in
646 .Xr ssh_config 5 .
647 .Pp
648 .Nm
649 exits with the exit status of the remote command or with 255
650 if an error occurred.
651 .Sh AUTHENTICATION
652 The OpenSSH SSH client supports SSH protocols 1 and 2.
653 Protocol 2 is the default, with
654 .Nm
655 falling back to protocol 1 if it detects protocol 2 is unsupported.
656 These settings may be altered using the
657 .Cm Protocol
658 option in
659 .Xr ssh_config 5 ,
660 or enforced using the
661 .Fl 1
662 and
663 .Fl 2
664 options (see above).
665 Both protocols support similar authentication methods,
666 but protocol 2 is preferred since
667 it provides additional mechanisms for confidentiality
668 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
669 and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
670 Protocol 1 lacks a strong mechanism for ensuring the
671 integrity of the connection.
672 .Pp
673 The methods available for authentication are:
674 GSSAPI-based authentication,
675 host-based authentication,
676 public key authentication,
677 challenge-response authentication,
678 and password authentication.
679 Authentication methods are tried in the order specified above,
680 though protocol 2 has a configuration option to change the default order:
681 .Cm PreferredAuthentications .
682 .Pp
683 Host-based authentication works as follows:
684 If the machine the user logs in from is listed in
685 .Pa /etc/hosts.equiv
686 or
687 .Pa /etc/shosts.equiv
688 on the remote machine, and the user names are
689 the same on both sides, or if the files
690 .Pa ~/.rhosts
691 or
692 .Pa ~/.shosts
693 exist in the user's home directory on the
694 remote machine and contain a line containing the name of the client
695 machine and the name of the user on that machine, the user is
696 considered for login.
697 Additionally, the server
698 .Em must
699 be able to verify the client's
700 host key (see the description of
701 .Pa /etc/ssh/ssh_known_hosts
702 and
703 .Pa ~/.ssh/known_hosts ,
704 below)
705 for login to be permitted.
706 This authentication method closes security holes due to IP
707 spoofing, DNS spoofing, and routing spoofing.
708 [Note to the administrator:
709 .Pa /etc/hosts.equiv ,
710 .Pa ~/.rhosts ,
711 and the rlogin/rsh protocol in general, are inherently insecure and should be
712 disabled if security is desired.]
713 .Pp
714 Public key authentication works as follows:
715 The scheme is based on public-key cryptography,
716 using cryptosystems
717 where encryption and decryption are done using separate keys,
718 and it is unfeasible to derive the decryption key from the encryption key.
719 The idea is that each user creates a public/private
720 key pair for authentication purposes.
721 The server knows the public key, and only the user knows the private key.
722 .Nm
723 implements public key authentication protocol automatically,
724 using either the RSA or DSA algorithms.
725 Protocol 1 is restricted to using only RSA keys,
726 but protocol 2 may use either.
727 The
728 .Sx HISTORY
729 section of
730 .Xr ssl 8
731 contains a brief discussion of the two algorithms.
732 .Pp
733 The file
734 .Pa ~/.ssh/authorized_keys
735 lists the public keys that are permitted for logging in.
736 When the user logs in, the
737 .Nm
738 program tells the server which key pair it would like to use for
739 authentication.
740 The client proves that it has access to the private key
741 and the server checks that the corresponding public key
742 is authorized to accept the account.
743 .Pp
744 The user creates his/her key pair by running
745 .Xr ssh-keygen 1 .
746 This stores the private key in
747 .Pa ~/.ssh/identity
748 (protocol 1),
749 .Pa ~/.ssh/id_dsa
750 (protocol 2 DSA),
751 or
752 .Pa ~/.ssh/id_rsa
753 (protocol 2 RSA)
754 and stores the public key in
755 .Pa ~/.ssh/identity.pub
756 (protocol 1),
757 .Pa ~/.ssh/id_dsa.pub
758 (protocol 2 DSA),
759 or
760 .Pa ~/.ssh/id_rsa.pub
761 (protocol 2 RSA)
762 in the user's home directory.
763 The user should then copy the public key
764 to
765 .Pa ~/.ssh/authorized_keys
766 in his/her home directory on the remote machine.
767 The
768 .Pa authorized_keys
769 file corresponds to the conventional
770 .Pa ~/.rhosts
771 file, and has one key
772 per line, though the lines can be very long.
773 After this, the user can log in without giving the password.
774 .Pp
775 The most convenient way to use public key authentication may be with an
776 authentication agent.
777 See
778 .Xr ssh-agent 1
779 for more information.
780 .Pp
781 Challenge-response authentication works as follows:
782 The server sends an arbitrary
783 .Qq challenge
784 text, and prompts for a response.
785 Protocol 2 allows multiple challenges and responses;
786 protocol 1 is restricted to just one challenge/response.
787 Examples of challenge-response authentication include
788 BSD Authentication (see
789 .Xr login.conf 5 )
790 and PAM (some non-OpenBSD systems).
791 .Pp
792 Finally, if other authentication methods fail,
793 .Nm
794 prompts the user for a password.
795 The password is sent to the remote
796 host for checking; however, since all communications are encrypted,
797 the password cannot be seen by someone listening on the network.
798 .Pp
799 .Nm
800 automatically maintains and checks a database containing
801 identification for all hosts it has ever been used with.
802 Host keys are stored in
803 .Pa ~/.ssh/known_hosts
804 in the user's home directory.
805 Additionally, the file
806 .Pa /etc/ssh/ssh_known_hosts
807 is automatically checked for known hosts.
808 Any new hosts are automatically added to the user's file.
809 If a host's identification ever changes,
810 .Nm
811 warns about this and disables password authentication to prevent
812 server spoofing or man-in-the-middle attacks,
813 which could otherwise be used to circumvent the encryption.
814 The
815 .Cm StrictHostKeyChecking
816 option can be used to control logins to machines whose
817 host key is not known or has changed.
818 .Pp
819 When the user's identity has been accepted by the server, the server
820 either executes the given command, or logs into the machine and gives
821 the user a normal shell on the remote machine.
822 All communication with
823 the remote command or shell will be automatically encrypted.
824 .Pp
825 If a pseudo-terminal has been allocated (normal login session), the
826 user may use the escape characters noted below.
827 .Pp
828 If no pseudo-tty has been allocated,
829 the session is transparent and can be used to reliably transfer binary data.
830 On most systems, setting the escape character to
831 .Dq none
832 will also make the session transparent even if a tty is used.
833 .Pp
834 The session terminates when the command or shell on the remote
835 machine exits and all X11 and TCP connections have been closed.
836 .Sh ESCAPE CHARACTERS
837 When a pseudo-terminal has been requested,
838 .Nm
839 supports a number of functions through the use of an escape character.
840 .Pp
841 A single tilde character can be sent as
842 .Ic ~~
843 or by following the tilde by a character other than those described below.
844 The escape character must always follow a newline to be interpreted as
845 special.
846 The escape character can be changed in configuration files using the
847 .Cm EscapeChar
848 configuration directive or on the command line by the
849 .Fl e
850 option.
851 .Pp
852 The supported escapes (assuming the default
853 .Ql ~ )
854 are:
855 .Bl -tag -width Ds
856 .It Cm ~.
857 Disconnect.
858 .It Cm ~^Z
859 Background
860 .Nm .
861 .It Cm ~#
862 List forwarded connections.
863 .It Cm ~&
864 Background
865 .Nm
866 at logout when waiting for forwarded connection / X11 sessions to terminate.
867 .It Cm ~?
868 Display a list of escape characters.
869 .It Cm ~B
870 Send a BREAK to the remote system
871 (only useful for SSH protocol version 2 and if the peer supports it).
872 .It Cm ~C
873 Open command line.
874 Currently this allows the addition of port forwardings using the
875 .Fl L ,
876 .Fl R
877 and
878 .Fl D
879 options (see above).
880 It also allows the cancellation of existing remote port-forwardings
881 using
882 .Sm off
883 .Fl KR Oo Ar bind_address : Oc Ar port .
884 .Sm on
885 .Ic !\& Ns Ar command
886 allows the user to execute a local command if the
887 .Ic PermitLocalCommand
888 option is enabled in
889 .Xr ssh_config 5 .
890 Basic help is available, using the
891 .Fl h
892 option.
893 .It Cm ~R
894 Request rekeying of the connection
895 (only useful for SSH protocol version 2 and if the peer supports it).
896 .El
897 .Sh TCP FORWARDING
898 Forwarding of arbitrary TCP connections over the secure channel can
899 be specified either on the command line or in a configuration file.
900 One possible application of TCP forwarding is a secure connection to a
901 mail server; another is going through firewalls.
902 .Pp
903 In the example below, we look at encrypting communication between
904 an IRC client and server, even though the IRC server does not directly
905 support encrypted communications.
906 This works as follows:
907 the user connects to the remote host using
908 .Nm ,
909 specifying a port to be used to forward connections
910 to the remote server.
911 After that it is possible to start the service which is to be encrypted
912 on the client machine,
913 connecting to the same local port,
914 and
915 .Nm
916 will encrypt and forward the connection.
917 .Pp
918 The following example tunnels an IRC session from client machine
919 .Dq 127.0.0.1
920 (localhost)
921 to remote server
922 .Dq server.example.com :
923 .Bd -literal -offset 4n
924 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
925 $ irc -c '#users' -p 1234 pinky 127.0.0.1
926 .Ed
927 .Pp
928 This tunnels a connection to IRC server
929 .Dq server.example.com ,
930 joining channel
931 .Dq #users ,
932 nickname
933 .Dq pinky ,
934 using port 1234.
935 It doesn't matter which port is used,
936 as long as it's greater than 1023
937 (remember, only root can open sockets on privileged ports)
938 and doesn't conflict with any ports already in use.
939 The connection is forwarded to port 6667 on the remote server,
940 since that's the standard port for IRC services.
941 .Pp
942 The
943 .Fl f
944 option backgrounds
945 .Nm
946 and the remote command
947 .Dq sleep 10
948 is specified to allow an amount of time
949 (10 seconds, in the example)
950 to start the service which is to be tunnelled.
951 If no connections are made within the time specified,
952 .Nm
953 will exit.
954 .Sh X11 FORWARDING
955 If the
956 .Cm ForwardX11
957 variable is set to
958 .Dq yes
959 (or see the description of the
960 .Fl X ,
961 .Fl x ,
962 and
963 .Fl Y
964 options above)
965 and the user is using X11 (the
966 .Ev DISPLAY
967 environment variable is set), the connection to the X11 display is
968 automatically forwarded to the remote side in such a way that any X11
969 programs started from the shell (or command) will go through the
970 encrypted channel, and the connection to the real X server will be made
971 from the local machine.
972 The user should not manually set
973 .Ev DISPLAY .
974 Forwarding of X11 connections can be
975 configured on the command line or in configuration files.
976 .Pp
977 The
978 .Ev DISPLAY
979 value set by
980 .Nm
981 will point to the server machine, but with a display number greater than zero.
982 This is normal, and happens because
983 .Nm
984 creates a
985 .Dq proxy
986 X server on the server machine for forwarding the
987 connections over the encrypted channel.
988 .Pp
989 .Nm
990 will also automatically set up Xauthority data on the server machine.
991 For this purpose, it will generate a random authorization cookie,
992 store it in Xauthority on the server, and verify that any forwarded
993 connections carry this cookie and replace it by the real cookie when
994 the connection is opened.
995 The real authentication cookie is never
996 sent to the server machine (and no cookies are sent in the plain).
997 .Pp
998 If the
999 .Cm ForwardAgent
1000 variable is set to
1001 .Dq yes
1002 (or see the description of the
1003 .Fl A
1004 and
1005 .Fl a
1006 options above) and
1007 the user is using an authentication agent, the connection to the agent
1008 is automatically forwarded to the remote side.
1009 .Sh VERIFYING HOST KEYS
1010 When connecting to a server for the first time,
1011 a fingerprint of the server's public key is presented to the user
1012 (unless the option
1013 .Cm StrictHostKeyChecking
1014 has been disabled).
1015 Fingerprints can be determined using
1016 .Xr ssh-keygen 1 :
1017 .Pp
1018 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1019 .Pp
1020 If the fingerprint is already known, it can be matched
1021 and the key can be accepted or rejected.
1022 Because of the difficulty of comparing host keys
1023 just by looking at hex strings,
1024 there is also support to compare host keys visually,
1025 using
1026 .Em random art .
1027 By setting the
1028 .Cm VisualHostKey
1029 option to
1030 .Dq yes ,
1031 a small ASCII graphic gets displayed on every login to a server, no matter
1032 if the session itself is interactive or not.
1033 By learning the pattern a known server produces, a user can easily
1034 find out that the host key has changed when a completely different pattern
1035 is displayed.
1036 Because these patterns are not unambiguous however, a pattern that looks
1037 similar to the pattern remembered only gives a good probability that the
1038 host key is the same, not guaranteed proof.
1039 .Pp
1040 To get a listing of the fingerprints along with their random art for
1041 all known hosts, the following command line can be used:
1042 .Pp
1043 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1044 .Pp
1045 If the fingerprint is unknown,
1046 an alternative method of verification is available:
1047 SSH fingerprints verified by DNS.
1048 An additional resource record (RR),
1049 SSHFP,
1050 is added to a zonefile
1051 and the connecting client is able to match the fingerprint
1052 with that of the key presented.
1053 .Pp
1054 In this example, we are connecting a client to a server,
1055 .Dq host.example.com .
1056 The SSHFP resource records should first be added to the zonefile for
1057 host.example.com:
1058 .Bd -literal -offset indent
1059 $ ssh-keygen -r host.example.com.
1060 .Ed
1061 .Pp
1062 The output lines will have to be added to the zonefile.
1063 To check that the zone is answering fingerprint queries:
1064 .Pp
1065 .Dl $ dig -t SSHFP host.example.com
1066 .Pp
1067 Finally the client connects:
1068 .Bd -literal -offset indent
1069 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1070 [...]
1071 Matching host key fingerprint found in DNS.
1072 Are you sure you want to continue connecting (yes/no)?
1073 .Ed
1074 .Pp
1075 See the
1076 .Cm VerifyHostKeyDNS
1077 option in
1078 .Xr ssh_config 5
1079 for more information.
1080 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1081 .Nm
1082 contains support for Virtual Private Network (VPN) tunnelling
1083 using the
1084 .Xr tun 4
1085 network pseudo-device,
1086 allowing two networks to be joined securely.
1087 The
1088 .Xr sshd_config 5
1089 configuration option
1090 .Cm PermitTunnel
1091 controls whether the server supports this,
1092 and at what level (layer 2 or 3 traffic).
1093 .Pp
1094 The following example would connect client network 10.0.50.0/24
1095 with remote network 10.0.99.0/24 using a point-to-point connection
1096 from 10.1.1.1 to 10.1.1.2,
1097 provided that the SSH server running on the gateway to the remote network,
1098 at 192.168.1.15, allows it.
1099 .Pp
1100 On the client:
1101 .Bd -literal -offset indent
1102 # ssh -f -w 0:1 192.168.1.15 true
1103 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1104 # route add 10.0.99.0/24 10.1.1.2
1105 .Ed
1106 .Pp
1107 On the server:
1108 .Bd -literal -offset indent
1109 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1110 # route add 10.0.50.0/24 10.1.1.1
1111 .Ed
1112 .Pp
1113 Client access may be more finely tuned via the
1114 .Pa /root/.ssh/authorized_keys
1115 file (see below) and the
1116 .Cm PermitRootLogin
1117 server option.
1118 The following entry would permit connections on
1119 .Xr tun 4
1120 device 1 from user
1121 .Dq jane
1122 and on tun device 2 from user
1123 .Dq john ,
1124 if
1125 .Cm PermitRootLogin
1126 is set to
1127 .Dq forced-commands-only :
1128 .Bd -literal -offset 2n
1129 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1130 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1131 .Ed
1132 .Pp
1133 Since an SSH-based setup entails a fair amount of overhead,
1134 it may be more suited to temporary setups,
1135 such as for wireless VPNs.
1136 More permanent VPNs are better provided by tools such as
1137 .Xr ipsecctl 8
1138 and
1139 .Xr isakmpd 8 .
1140 .Sh ENVIRONMENT
1141 .Nm
1142 will normally set the following environment variables:
1143 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1144 .It Ev DISPLAY
1145 The
1146 .Ev DISPLAY
1147 variable indicates the location of the X11 server.
1148 It is automatically set by
1149 .Nm
1150 to point to a value of the form
1151 .Dq hostname:n ,
1152 where
1153 .Dq hostname
1154 indicates the host where the shell runs, and
1155 .Sq n
1156 is an integer \*(Ge 1.
1157 .Nm
1158 uses this special value to forward X11 connections over the secure
1159 channel.
1160 The user should normally not set
1161 .Ev DISPLAY
1162 explicitly, as that
1163 will render the X11 connection insecure (and will require the user to
1164 manually copy any required authorization cookies).
1165 .It Ev HOME
1166 Set to the path of the user's home directory.
1167 .It Ev LOGNAME
1168 Synonym for
1169 .Ev USER ;
1170 set for compatibility with systems that use this variable.
1171 .It Ev MAIL
1172 Set to the path of the user's mailbox.
1173 .It Ev PATH
1174 Set to the default
1175 .Ev PATH ,
1176 as specified when compiling
1177 .Nm .
1178 .It Ev SSH_ASKPASS
1179 If
1180 .Nm
1181 needs a passphrase, it will read the passphrase from the current
1182 terminal if it was run from a terminal.
1183 If
1184 .Nm
1185 does not have a terminal associated with it but
1186 .Ev DISPLAY
1187 and
1188 .Ev SSH_ASKPASS
1189 are set, it will execute the program specified by
1190 .Ev SSH_ASKPASS
1191 and open an X11 window to read the passphrase.
1192 This is particularly useful when calling
1193 .Nm
1194 from a
1195 .Pa .xsession
1196 or related script.
1197 (Note that on some machines it
1198 may be necessary to redirect the input from
1199 .Pa /dev/null
1200 to make this work.)
1201 .It Ev SSH_AUTH_SOCK
1202 Identifies the path of a
1203 .Ux Ns -domain
1204 socket used to communicate with the agent.
1205 .It Ev SSH_CONNECTION
1206 Identifies the client and server ends of the connection.
1207 The variable contains
1208 four space-separated values: client IP address, client port number,
1209 server IP address, and server port number.
1210 .It Ev SSH_ORIGINAL_COMMAND
1211 This variable contains the original command line if a forced command
1212 is executed.
1213 It can be used to extract the original arguments.
1214 .It Ev SSH_TTY
1215 This is set to the name of the tty (path to the device) associated
1216 with the current shell or command.
1217 If the current session has no tty,
1218 this variable is not set.
1219 .It Ev TZ
1220 This variable is set to indicate the present time zone if it
1221 was set when the daemon was started (i.e. the daemon passes the value
1222 on to new connections).
1223 .It Ev USER
1224 Set to the name of the user logging in.
1225 .El
1226 .Pp
1227 Additionally,
1228 .Nm
1229 reads
1230 .Pa ~/.ssh/environment ,
1231 and adds lines of the format
1232 .Dq VARNAME=value
1233 to the environment if the file exists and users are allowed to
1234 change their environment.
1235 For more information, see the
1236 .Cm PermitUserEnvironment
1237 option in
1238 .Xr sshd_config 5 .
1239 .Sh FILES
1240 .Bl -tag -width Ds -compact
1241 .It ~/.rhosts
1242 This file is used for host-based authentication (see above).
1243 On some machines this file may need to be
1244 world-readable if the user's home directory is on an NFS partition,
1245 because
1246 .Xr sshd 8
1247 reads it as root.
1248 Additionally, this file must be owned by the user,
1249 and must not have write permissions for anyone else.
1250 The recommended
1251 permission for most machines is read/write for the user, and not
1252 accessible by others.
1253 .Pp
1254 .It ~/.shosts
1255 This file is used in exactly the same way as
1256 .Pa .rhosts ,
1257 but allows host-based authentication without permitting login with
1258 rlogin/rsh.
1259 .Pp
1260 .It ~/.ssh/
1261 This directory is the default location for all user-specific configuration
1262 and authentication information.
1263 There is no general requirement to keep the entire contents of this directory
1264 secret, but the recommended permissions are read/write/execute for the user,
1265 and not accessible by others.
1266 .Pp
1267 .It ~/.ssh/authorized_keys
1268 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1269 The format of this file is described in the
1270 .Xr sshd 8
1271 manual page.
1272 This file is not highly sensitive, but the recommended
1273 permissions are read/write for the user, and not accessible by others.
1274 .Pp
1275 .It ~/.ssh/config
1276 This is the per-user configuration file.
1277 The file format and configuration options are described in
1278 .Xr ssh_config 5 .
1279 Because of the potential for abuse, this file must have strict permissions:
1280 read/write for the user, and not accessible by others.
1281 .Pp
1282 .It ~/.ssh/environment
1283 Contains additional definitions for environment variables; see
1284 .Sx ENVIRONMENT ,
1285 above.
1286 .Pp
1287 .It ~/.ssh/identity
1288 .It ~/.ssh/id_dsa
1289 .It ~/.ssh/id_rsa
1290 Contains the private key for authentication.
1291 These files
1292 contain sensitive data and should be readable by the user but not
1293 accessible by others (read/write/execute).
1294 .Nm
1295 will simply ignore a private key file if it is accessible by others.
1296 It is possible to specify a passphrase when
1297 generating the key which will be used to encrypt the
1298 sensitive part of this file using 3DES.
1299 .Pp
1300 .It ~/.ssh/identity.pub
1301 .It ~/.ssh/id_dsa.pub
1302 .It ~/.ssh/id_rsa.pub
1303 Contains the public key for authentication.
1304 These files are not
1305 sensitive and can (but need not) be readable by anyone.
1306 .Pp
1307 .It ~/.ssh/known_hosts
1308 Contains a list of host keys for all hosts the user has logged into
1309 that are not already in the systemwide list of known host keys.
1310 See
1311 .Xr sshd 8
1312 for further details of the format of this file.
1313 .Pp
1314 .It ~/.ssh/rc
1315 Commands in this file are executed by
1316 .Nm
1317 when the user logs in, just before the user's shell (or command) is
1318 started.
1319 See the
1320 .Xr sshd 8
1321 manual page for more information.
1322 .Pp
1323 .It /etc/hosts.equiv
1324 This file is for host-based authentication (see above).
1325 It should only be writable by root.
1326 .Pp
1327 .It /etc/shosts.equiv
1328 This file is used in exactly the same way as
1329 .Pa hosts.equiv ,
1330 but allows host-based authentication without permitting login with
1331 rlogin/rsh.
1332 .Pp
1333 .It Pa /etc/ssh/ssh_config
1334 Systemwide configuration file.
1335 The file format and configuration options are described in
1336 .Xr ssh_config 5 .
1337 .Pp
1338 .It /etc/ssh/ssh_host_key
1339 .It /etc/ssh/ssh_host_dsa_key
1340 .It /etc/ssh/ssh_host_rsa_key
1341 These three files contain the private parts of the host keys
1342 and are used for host-based authentication.
1343 If protocol version 1 is used,
1344 .Nm
1345 must be setuid root, since the host key is readable only by root.
1346 For protocol version 2,
1347 .Nm
1348 uses
1349 .Xr ssh-keysign 8
1350 to access the host keys,
1351 eliminating the requirement that
1352 .Nm
1353 be setuid root when host-based authentication is used.
1354 By default
1355 .Nm
1356 is not setuid root.
1357 .Pp
1358 .It /etc/ssh/ssh_known_hosts
1359 Systemwide list of known host keys.
1360 This file should be prepared by the
1361 system administrator to contain the public host keys of all machines in the
1362 organization.
1363 It should be world-readable.
1364 See
1365 .Xr sshd 8
1366 for further details of the format of this file.
1367 .Pp
1368 .It /etc/ssh/sshrc
1369 Commands in this file are executed by
1370 .Nm
1371 when the user logs in, just before the user's shell (or command) is started.
1372 See the
1373 .Xr sshd 8
1374 manual page for more information.
1375 .El
1376 .Sh SEE ALSO
1377 .Xr scp 1 ,
1378 .Xr sftp 1 ,
1379 .Xr ssh-add 1 ,
1380 .Xr ssh-agent 1 ,
1381 .Xr ssh-keygen 1 ,
1382 .Xr ssh-keyscan 1 ,
1383 .Xr tun 4 ,
1384 .Xr hosts.equiv 5 ,
1385 .Xr ssh_config 5 ,
1386 .Xr ssh-keysign 8 ,
1387 .Xr sshd 8
1388 .Rs
1389 .%R RFC 4250
1390 .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1391 .%D 2006
1392 .Re
1393 .Rs
1394 .%R RFC 4251
1395 .%T "The Secure Shell (SSH) Protocol Architecture"
1396 .%D 2006
1397 .Re
1398 .Rs
1399 .%R RFC 4252
1400 .%T "The Secure Shell (SSH) Authentication Protocol"
1401 .%D 2006
1402 .Re
1403 .Rs
1404 .%R RFC 4253
1405 .%T "The Secure Shell (SSH) Transport Layer Protocol"
1406 .%D 2006
1407 .Re
1408 .Rs
1409 .%R RFC 4254
1410 .%T "The Secure Shell (SSH) Connection Protocol"
1411 .%D 2006
1412 .Re
1413 .Rs
1414 .%R RFC 4255
1415 .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1416 .%D 2006
1417 .Re
1418 .Rs
1419 .%R RFC 4256
1420 .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1421 .%D 2006
1422 .Re
1423 .Rs
1424 .%R RFC 4335
1425 .%T "The Secure Shell (SSH) Session Channel Break Extension"
1426 .%D 2006
1427 .Re
1428 .Rs
1429 .%R RFC 4344
1430 .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1431 .%D 2006
1432 .Re
1433 .Rs
1434 .%R RFC 4345
1435 .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1436 .%D 2006
1437 .Re
1438 .Rs
1439 .%R RFC 4419
1440 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1441 .%D 2006
1442 .Re
1443 .Rs
1444 .%R RFC 4716
1445 .%T "The Secure Shell (SSH) Public Key File Format"
1446 .%D 2006
1447 .Re
1448 .Rs
1449 .%T "Hash Visualization: a New Technique to improve Real-World Security"
1450 .%A A. Perrig
1451 .%A D. Song
1452 .%D 1999
1453 .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1454 .Re
1455 .Sh AUTHORS
1456 OpenSSH is a derivative of the original and free
1457 ssh 1.2.12 release by Tatu Ylonen.
1458 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1459 Theo de Raadt and Dug Song
1460 removed many bugs, re-added newer features and
1461 created OpenSSH.
1462 Markus Friedl contributed the support for SSH
1463 protocol versions 1.5 and 2.0.