| blob | 6c6271ee4f74dc6a109b466ea513f224a4da7251 |
1 .\" -*- nroff -*-
2 .\"
3 .\" Author: Tatu Ylonen <ylo@cs.hut.fi>
4 .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
5 .\" All rights reserved
6 .\"
7 .\" As far as I am concerned, the code I have written for this software
8 .\" can be used freely for any purpose. Any derived versions of this
9 .\" software must be clearly marked as such, and if the derived work is
10 .\" incompatible with the protocol description in the RFC file, it must be
11 .\" called by a name other than "ssh" or "Secure Shell".
12 .\"
13 .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved.
14 .\" Copyright (c) 1999 Aaron Campbell. All rights reserved.
15 .\" Copyright (c) 1999 Theo de Raadt. All rights reserved.
16 .\"
17 .\" Redistribution and use in source and binary forms, with or without
18 .\" modification, are permitted provided that the following conditions
19 .\" are met:
20 .\" 1. Redistributions of source code must retain the above copyright
21 .\" notice, this list of conditions and the following disclaimer.
22 .\" 2. Redistributions in binary form must reproduce the above copyright
23 .\" notice, this list of conditions and the following disclaimer in the
24 .\" documentation and/or other materials provided with the distribution.
25 .\"
26 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
27 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
28 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
29 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
30 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
31 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
32 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
33 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
34 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
35 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
36 .\"
37 .\" $OpenBSD: ssh.1,v 1.283 2009/03/19 15:15:09 jmc Exp $
38 .Dd $Mdocdate: March 19 2009 $
39 .Dt SSH 1
40 .Os
41 .Sh NAME
42 .Nm ssh
43 .Nd OpenSSH SSH client (remote login program)
44 .Sh SYNOPSIS
45 .Nm ssh
46 .Op Fl 1246AaCfgKkMNnqsTtVvXxYy
47 .Op Fl b Ar bind_address
48 .Op Fl c Ar cipher_spec
49 .Oo Fl D\ \&
50 .Sm off
51 .Oo Ar bind_address : Oc
52 .Ar port
53 .Sm on
54 .Oc
55 .Op Fl e Ar escape_char
56 .Op Fl F Ar configfile
57 .Bk -words
58 .Op Fl i Ar identity_file
59 .Ek
60 .Oo Fl L\ \&
61 .Sm off
62 .Oo Ar bind_address : Oc
63 .Ar port : host : hostport
64 .Sm on
65 .Oc
66 .Bk -words
67 .Op Fl l Ar login_name
68 .Ek
69 .Op Fl m Ar mac_spec
70 .Op Fl O Ar ctl_cmd
71 .Op Fl o Ar option
72 .Op Fl p Ar port
73 .Oo Fl R\ \&
74 .Sm off
75 .Oo Ar bind_address : Oc
76 .Ar port : host : hostport
77 .Sm on
78 .Oc
79 .Op Fl S Ar ctl_path
80 .Bk -words
81 .Oo Fl w Ar local_tun Ns
82 .Op : Ns Ar remote_tun Oc
83 .Oo Ar user Ns @ Oc Ns Ar hostname
84 .Op Ar command
85 .Ek
86 .Sh DESCRIPTION
87 .Nm
88 (SSH client) is a program for logging into a remote machine and for
89 executing commands on a remote machine.
90 It is intended to replace rlogin and rsh,
91 and provide secure encrypted communications between
92 two untrusted hosts over an insecure network.
93 X11 connections and arbitrary TCP ports
94 can also be forwarded over the secure channel.
95 .Pp
96 .Nm
97 connects and logs into the specified
98 .Ar hostname
99 (with optional
100 .Ar user
101 name).
102 The user must prove
103 his/her identity to the remote machine using one of several methods
104 depending on the protocol version used (see below).
105 .Pp
106 If
107 .Ar command
108 is specified,
109 it is executed on the remote host instead of a login shell.
110 .Pp
111 The options are as follows:
112 .Bl -tag -width Ds
113 .It Fl 1
114 Forces
115 .Nm
116 to try protocol version 1 only.
117 .It Fl 2
118 Forces
119 .Nm
120 to try protocol version 2 only.
121 .It Fl 4
122 Forces
123 .Nm
124 to use IPv4 addresses only.
125 .It Fl 6
126 Forces
127 .Nm
128 to use IPv6 addresses only.
129 .It Fl A
130 Enables forwarding of the authentication agent connection.
131 This can also be specified on a per-host basis in a configuration file.
132 .Pp
133 Agent forwarding should be enabled with caution.
134 Users with the ability to bypass file permissions on the remote host
135 (for the agent's Unix-domain socket)
136 can access the local agent through the forwarded connection.
137 An attacker cannot obtain key material from the agent,
138 however they can perform operations on the keys that enable them to
139 authenticate using the identities loaded into the agent.
140 .It Fl a
141 Disables forwarding of the authentication agent connection.
142 .It Fl b Ar bind_address
143 Use
144 .Ar bind_address
145 on the local machine as the source address
146 of the connection.
147 Only useful on systems with more than one address.
148 .It Fl C
149 Requests compression of all data (including stdin, stdout, stderr, and
150 data for forwarded X11 and TCP connections).
151 The compression algorithm is the same used by
152 .Xr gzip 1 ,
153 and the
154 .Dq level
155 can be controlled by the
156 .Cm CompressionLevel
157 option for protocol version 1.
158 Compression is desirable on modem lines and other
159 slow connections, but will only slow down things on fast networks.
160 The default value can be set on a host-by-host basis in the
161 configuration files; see the
162 .Cm Compression
163 option.
164 .It Fl c Ar cipher_spec
165 Selects the cipher specification for encrypting the session.
166 .Pp
167 Protocol version 1 allows specification of a single cipher.
168 The supported values are
169 .Dq 3des ,
170 .Dq blowfish ,
171 and
172 .Dq des .
173 .Ar 3des
174 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
175 It is believed to be secure.
176 .Ar blowfish
177 is a fast block cipher; it appears very secure and is much faster than
178 .Ar 3des .
179 .Ar des
180 is only supported in the
181 .Nm
182 client for interoperability with legacy protocol 1 implementations
183 that do not support the
184 .Ar 3des
185 cipher.
186 Its use is strongly discouraged due to cryptographic weaknesses.
187 The default is
188 .Dq 3des .
189 .Pp
190 For protocol version 2,
191 .Ar cipher_spec
192 is a comma-separated list of ciphers
193 listed in order of preference.
194 See the
195 .Cm Ciphers
196 keyword for more information.
197 .It Fl D Xo
198 .Sm off
199 .Oo Ar bind_address : Oc
200 .Ar port
201 .Sm on
202 .Xc
203 Specifies a local
204 .Dq dynamic
205 application-level port forwarding.
206 This works by allocating a socket to listen to
207 .Ar port
208 on the local side, optionally bound to the specified
209 .Ar bind_address .
210 Whenever a connection is made to this port, the
211 connection is forwarded over the secure channel, and the application
212 protocol is then used to determine where to connect to from the
213 remote machine.
214 Currently the SOCKS4 and SOCKS5 protocols are supported, and
215 .Nm
216 will act as a SOCKS server.
217 Only root can forward privileged ports.
218 Dynamic port forwardings can also be specified in the configuration file.
219 .Pp
220 IPv6 addresses can be specified with an alternative syntax:
221 .Sm off
222 .Xo
223 .Op Ar bind_address No /
224 .Ar port
225 .Xc
226 .Sm on
227 or by enclosing the address in square brackets.
228 Only the superuser can forward privileged ports.
229 By default, the local port is bound in accordance with the
230 .Cm GatewayPorts
231 setting.
232 However, an explicit
233 .Ar bind_address
234 may be used to bind the connection to a specific address.
235 The
236 .Ar bind_address
237 of
238 .Dq localhost
239 indicates that the listening port be bound for local use only, while an
240 empty address or
241 .Sq *
242 indicates that the port should be available from all interfaces.
243 .It Fl e Ar escape_char
244 Sets the escape character for sessions with a pty (default:
245 .Ql ~ ) .
246 The escape character is only recognized at the beginning of a line.
247 The escape character followed by a dot
248 .Pq Ql \&.
249 closes the connection;
250 followed by control-Z suspends the connection;
251 and followed by itself sends the escape character once.
252 Setting the character to
253 .Dq none
254 disables any escapes and makes the session fully transparent.
255 .It Fl F Ar configfile
256 Specifies an alternative per-user configuration file.
257 If a configuration file is given on the command line,
258 the system-wide configuration file
259 .Pq Pa /etc/ssh/ssh_config
260 will be ignored.
261 The default for the per-user configuration file is
262 .Pa ~/.ssh/config .
263 .It Fl f
264 Requests
265 .Nm
266 to go to background just before command execution.
267 This is useful if
268 .Nm
269 is going to ask for passwords or passphrases, but the user
270 wants it in the background.
271 This implies
272 .Fl n .
273 The recommended way to start X11 programs at a remote site is with
274 something like
275 .Ic ssh -f host xterm .
276 .Pp
277 If the
278 .Cm ExitOnForwardFailure
279 configuration option is set to
280 .Dq yes ,
281 then a client started with
282 .Fl f
283 will wait for all remote port forwards to be successfully established
284 before placing itself in the background.
285 .It Fl g
286 Allows remote hosts to connect to local forwarded ports.
287 .It Fl I Ar smartcard_device
288 Specify the device
289 .Nm
290 should use to communicate with a smartcard used for storing the user's
291 private RSA key.
292 This option is only available if support for smartcard devices
293 is compiled in (default is no support).
294 .It Fl i Ar identity_file
295 Selects a file from which the identity (private key) for
296 RSA or DSA authentication is read.
297 The default is
298 .Pa ~/.ssh/identity
299 for protocol version 1, and
300 .Pa ~/.ssh/id_rsa
301 and
302 .Pa ~/.ssh/id_dsa
303 for protocol version 2.
304 Identity files may also be specified on
305 a per-host basis in the configuration file.
306 It is possible to have multiple
307 .Fl i
308 options (and multiple identities specified in
309 configuration files).
310 .It Fl K
311 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
312 credentials to the server.
313 .It Fl k
314 Disables forwarding (delegation) of GSSAPI credentials to the server.
315 .It Fl L Xo
316 .Sm off
317 .Oo Ar bind_address : Oc
318 .Ar port : host : hostport
319 .Sm on
320 .Xc
321 Specifies that the given port on the local (client) host is to be
322 forwarded to the given host and port on the remote side.
323 This works by allocating a socket to listen to
324 .Ar port
325 on the local side, optionally bound to the specified
326 .Ar bind_address .
327 Whenever a connection is made to this port, the
328 connection is forwarded over the secure channel, and a connection is
329 made to
330 .Ar host
331 port
332 .Ar hostport
333 from the remote machine.
334 Port forwardings can also be specified in the configuration file.
335 IPv6 addresses can be specified with an alternative syntax:
336 .Sm off
337 .Xo
338 .Op Ar bind_address No /
339 .Ar port No / Ar host No /
340 .Ar hostport
341 .Xc
342 .Sm on
343 or by enclosing the address in square brackets.
344 Only the superuser can forward privileged ports.
345 By default, the local port is bound in accordance with the
346 .Cm GatewayPorts
347 setting.
348 However, an explicit
349 .Ar bind_address
350 may be used to bind the connection to a specific address.
351 The
352 .Ar bind_address
353 of
354 .Dq localhost
355 indicates that the listening port be bound for local use only, while an
356 empty address or
357 .Sq *
358 indicates that the port should be available from all interfaces.
359 .It Fl l Ar login_name
360 Specifies the user to log in as on the remote machine.
361 This also may be specified on a per-host basis in the configuration file.
362 .It Fl M
363 Places the
364 .Nm
365 client into
366 .Dq master
367 mode for connection sharing.
368 Multiple
369 .Fl M
370 options places
371 .Nm
372 into
373 .Dq master
374 mode with confirmation required before slave connections are accepted.
375 Refer to the description of
376 .Cm ControlMaster
377 in
378 .Xr ssh_config 5
379 for details.
380 .It Fl m Ar mac_spec
381 Additionally, for protocol version 2 a comma-separated list of MAC
382 (message authentication code) algorithms can
383 be specified in order of preference.
384 See the
385 .Cm MACs
386 keyword for more information.
387 .It Fl N
388 Do not execute a remote command.
389 This is useful for just forwarding ports
390 (protocol version 2 only).
391 .It Fl n
392 Redirects stdin from
393 .Pa /dev/null
394 (actually, prevents reading from stdin).
395 This must be used when
396 .Nm
397 is run in the background.
398 A common trick is to use this to run X11 programs on a remote machine.
399 For example,
400 .Ic ssh -n shadows.cs.hut.fi emacs &
401 will start an emacs on shadows.cs.hut.fi, and the X11
402 connection will be automatically forwarded over an encrypted channel.
403 The
404 .Nm
405 program will be put in the background.
406 (This does not work if
407 .Nm
408 needs to ask for a password or passphrase; see also the
409 .Fl f
410 option.)
411 .It Fl O Ar ctl_cmd
412 Control an active connection multiplexing master process.
413 When the
414 .Fl O
415 option is specified, the
416 .Ar ctl_cmd
417 argument is interpreted and passed to the master process.
418 Valid commands are:
419 .Dq check
420 (check that the master process is running) and
421 .Dq exit
422 (request the master to exit).
423 .It Fl o Ar option
424 Can be used to give options in the format used in the configuration file.
425 This is useful for specifying options for which there is no separate
426 command-line flag.
427 For full details of the options listed below, and their possible values, see
428 .Xr ssh_config 5 .
429 .Pp
430 .Bl -tag -width Ds -offset indent -compact
431 .It AddressFamily
432 .It BatchMode
433 .It BindAddress
434 .It ChallengeResponseAuthentication
435 .It CheckHostIP
436 .It Cipher
437 .It Ciphers
438 .It ClearAllForwardings
439 .It Compression
440 .It CompressionLevel
441 .It ConnectionAttempts
442 .It ConnectTimeout
443 .It ControlMaster
444 .It ControlPath
445 .It DynamicForward
446 .It EscapeChar
447 .It ExitOnForwardFailure
448 .It ForwardAgent
449 .It ForwardX11
450 .It ForwardX11Trusted
451 .It GatewayPorts
452 .It GlobalKnownHostsFile
453 .It GSSAPIAuthentication
454 .It GSSAPIDelegateCredentials
455 .It HashKnownHosts
456 .It Host
457 .It HostbasedAuthentication
458 .It HostKeyAlgorithms
459 .It HostKeyAlias
460 .It HostName
461 .It IdentityFile
462 .It IdentitiesOnly
463 .It KbdInteractiveDevices
464 .It LocalCommand
465 .It LocalForward
466 .It LogLevel
467 .It MACs
468 .It NoHostAuthenticationForLocalhost
469 .It NumberOfPasswordPrompts
470 .It PasswordAuthentication
471 .It PermitLocalCommand
472 .It Port
473 .It PreferredAuthentications
474 .It Protocol
475 .It ProxyCommand
476 .It PubkeyAuthentication
477 .It RekeyLimit
478 .It RemoteForward
479 .It RhostsRSAAuthentication
480 .It RSAAuthentication
481 .It SendEnv
482 .It ServerAliveInterval
483 .It ServerAliveCountMax
484 .It SmartcardDevice
485 .It StrictHostKeyChecking
486 .It TCPKeepAlive
487 .It Tunnel
488 .It TunnelDevice
489 .It UsePrivilegedPort
490 .It User
491 .It UserKnownHostsFile
492 .It VerifyHostKeyDNS
493 .It VisualHostKey
494 .It XAuthLocation
495 .El
496 .It Fl p Ar port
497 Port to connect to on the remote host.
498 This can be specified on a
499 per-host basis in the configuration file.
500 .It Fl q
501 Quiet mode.
502 Causes most warning and diagnostic messages to be suppressed.
503 .It Fl R Xo
504 .Sm off
505 .Oo Ar bind_address : Oc
506 .Ar port : host : hostport
507 .Sm on
508 .Xc
509 Specifies that the given port on the remote (server) host is to be
510 forwarded to the given host and port on the local side.
511 This works by allocating a socket to listen to
512 .Ar port
513 on the remote side, and whenever a connection is made to this port, the
514 connection is forwarded over the secure channel, and a connection is
515 made to
516 .Ar host
517 port
518 .Ar hostport
519 from the local machine.
520 .Pp
521 Port forwardings can also be specified in the configuration file.
522 Privileged ports can be forwarded only when
523 logging in as root on the remote machine.
524 IPv6 addresses can be specified by enclosing the address in square braces or
525 using an alternative syntax:
526 .Sm off
527 .Xo
528 .Op Ar bind_address No /
529 .Ar host No / Ar port No /
530 .Ar hostport
531 .Xc .
532 .Sm on
533 .Pp
534 By default, the listening socket on the server will be bound to the loopback
535 interface only.
536 This may be overridden by specifying a
537 .Ar bind_address .
538 An empty
539 .Ar bind_address ,
540 or the address
541 .Ql * ,
542 indicates that the remote socket should listen on all interfaces.
543 Specifying a remote
544 .Ar bind_address
545 will only succeed if the server's
546 .Cm GatewayPorts
547 option is enabled (see
548 .Xr sshd_config 5 ) .
549 .Pp
550 If the
551 .Ar port
552 argument is
553 .Ql 0 ,
554 the listen port will be dynamically allocated on the server and reported
555 to the client at run time.
556 .It Fl S Ar ctl_path
557 Specifies the location of a control socket for connection sharing.
558 Refer to the description of
559 .Cm ControlPath
560 and
561 .Cm ControlMaster
562 in
563 .Xr ssh_config 5
564 for details.
565 .It Fl s
566 May be used to request invocation of a subsystem on the remote system.
567 Subsystems are a feature of the SSH2 protocol which facilitate the use
568 of SSH as a secure transport for other applications (eg.\&
569 .Xr sftp 1 ) .
570 The subsystem is specified as the remote command.
571 .It Fl T
572 Disable pseudo-tty allocation.
573 .It Fl t
574 Force pseudo-tty allocation.
575 This can be used to execute arbitrary
576 screen-based programs on a remote machine, which can be very useful,
577 e.g. when implementing menu services.
578 Multiple
579 .Fl t
580 options force tty allocation, even if
581 .Nm
582 has no local tty.
583 .It Fl V
584 Display the version number and exit.
585 .It Fl v
586 Verbose mode.
587 Causes
588 .Nm
589 to print debugging messages about its progress.
590 This is helpful in
591 debugging connection, authentication, and configuration problems.
592 Multiple
593 .Fl v
594 options increase the verbosity.
595 The maximum is 3.
596 .It Fl w Xo
597 .Ar local_tun Ns Op : Ns Ar remote_tun
598 .Xc
599 Requests
600 tunnel
601 device forwarding with the specified
602 .Xr tun 4
603 devices between the client
604 .Pq Ar local_tun
605 and the server
606 .Pq Ar remote_tun .
607 .Pp
608 The devices may be specified by numerical ID or the keyword
609 .Dq any ,
610 which uses the next available tunnel device.
611 If
612 .Ar remote_tun
613 is not specified, it defaults to
614 .Dq any .
615 See also the
616 .Cm Tunnel
617 and
618 .Cm TunnelDevice
619 directives in
620 .Xr ssh_config 5 .
621 If the
622 .Cm Tunnel
623 directive is unset, it is set to the default tunnel mode, which is
624 .Dq point-to-point .
625 .It Fl X
626 Enables X11 forwarding.
627 This can also be specified on a per-host basis in a configuration file.
628 .Pp
629 X11 forwarding should be enabled with caution.
630 Users with the ability to bypass file permissions on the remote host
631 (for the user's X authorization database)
632 can access the local X11 display through the forwarded connection.
633 An attacker may then be able to perform activities such as keystroke monitoring.
634 .Pp
635 For this reason, X11 forwarding is subjected to X11 SECURITY extension
636 restrictions by default.
637 Please refer to the
638 .Nm
639 .Fl Y
640 option and the
641 .Cm ForwardX11Trusted
642 directive in
643 .Xr ssh_config 5
644 for more information.
645 .It Fl x
646 Disables X11 forwarding.
647 .It Fl Y
648 Enables trusted X11 forwarding.
649 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
650 controls.
651 .It Fl y
652 Send log information using the
653 .Xr syslog 3
654 system module.
655 By default this information is sent to stderr.
656 .El
657 .Pp
658 .Nm
659 may additionally obtain configuration data from
660 a per-user configuration file and a system-wide configuration file.
661 The file format and configuration options are described in
662 .Xr ssh_config 5 .
663 .Pp
664 .Nm
665 exits with the exit status of the remote command or with 255
666 if an error occurred.
667 .Sh AUTHENTICATION
668 The OpenSSH SSH client supports SSH protocols 1 and 2.
669 Protocol 2 is the default, with
670 .Nm
671 falling back to protocol 1 if it detects protocol 2 is unsupported.
672 These settings may be altered using the
673 .Cm Protocol
674 option in
675 .Xr ssh_config 5 ,
676 or enforced using the
677 .Fl 1
678 and
679 .Fl 2
680 options (see above).
681 Both protocols support similar authentication methods,
682 but protocol 2 is preferred since
683 it provides additional mechanisms for confidentiality
684 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
685 and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
686 Protocol 1 lacks a strong mechanism for ensuring the
687 integrity of the connection.
688 .Pp
689 The methods available for authentication are:
690 GSSAPI-based authentication,
691 host-based authentication,
692 public key authentication,
693 challenge-response authentication,
694 and password authentication.
695 Authentication methods are tried in the order specified above,
696 though protocol 2 has a configuration option to change the default order:
697 .Cm PreferredAuthentications .
698 .Pp
699 Host-based authentication works as follows:
700 If the machine the user logs in from is listed in
701 .Pa /etc/hosts.equiv
702 or
703 .Pa /etc/shosts.equiv
704 on the remote machine, and the user names are
705 the same on both sides, or if the files
706 .Pa ~/.rhosts
707 or
708 .Pa ~/.shosts
709 exist in the user's home directory on the
710 remote machine and contain a line containing the name of the client
711 machine and the name of the user on that machine, the user is
712 considered for login.
713 Additionally, the server
714 .Em must
715 be able to verify the client's
716 host key (see the description of
717 .Pa /etc/ssh/ssh_known_hosts
718 and
719 .Pa ~/.ssh/known_hosts ,
720 below)
721 for login to be permitted.
722 This authentication method closes security holes due to IP
723 spoofing, DNS spoofing, and routing spoofing.
724 [Note to the administrator:
725 .Pa /etc/hosts.equiv ,
726 .Pa ~/.rhosts ,
727 and the rlogin/rsh protocol in general, are inherently insecure and should be
728 disabled if security is desired.]
729 .Pp
730 Public key authentication works as follows:
731 The scheme is based on public-key cryptography,
732 using cryptosystems
733 where encryption and decryption are done using separate keys,
734 and it is unfeasible to derive the decryption key from the encryption key.
735 The idea is that each user creates a public/private
736 key pair for authentication purposes.
737 The server knows the public key, and only the user knows the private key.
738 .Nm
739 implements public key authentication protocol automatically,
740 using either the RSA or DSA algorithms.
741 Protocol 1 is restricted to using only RSA keys,
742 but protocol 2 may use either.
743 The
744 .Sx HISTORY
745 section of
746 .Xr ssl 8
747 contains a brief discussion of the two algorithms.
748 .Pp
749 The file
750 .Pa ~/.ssh/authorized_keys
751 lists the public keys that are permitted for logging in.
752 When the user logs in, the
753 .Nm
754 program tells the server which key pair it would like to use for
755 authentication.
756 The client proves that it has access to the private key
757 and the server checks that the corresponding public key
758 is authorized to accept the account.
759 .Pp
760 The user creates his/her key pair by running
761 .Xr ssh-keygen 1 .
762 This stores the private key in
763 .Pa ~/.ssh/identity
764 (protocol 1),
765 .Pa ~/.ssh/id_dsa
766 (protocol 2 DSA),
767 or
768 .Pa ~/.ssh/id_rsa
769 (protocol 2 RSA)
770 and stores the public key in
771 .Pa ~/.ssh/identity.pub
772 (protocol 1),
773 .Pa ~/.ssh/id_dsa.pub
774 (protocol 2 DSA),
775 or
776 .Pa ~/.ssh/id_rsa.pub
777 (protocol 2 RSA)
778 in the user's home directory.
779 The user should then copy the public key
780 to
781 .Pa ~/.ssh/authorized_keys
782 in his/her home directory on the remote machine.
783 The
784 .Pa authorized_keys
785 file corresponds to the conventional
786 .Pa ~/.rhosts
787 file, and has one key
788 per line, though the lines can be very long.
789 After this, the user can log in without giving the password.
790 .Pp
791 The most convenient way to use public key authentication may be with an
792 authentication agent.
793 See
794 .Xr ssh-agent 1
795 for more information.
796 .Pp
797 Challenge-response authentication works as follows:
798 The server sends an arbitrary
799 .Qq challenge
800 text, and prompts for a response.
801 Protocol 2 allows multiple challenges and responses;
802 protocol 1 is restricted to just one challenge/response.
803 Examples of challenge-response authentication include
804 BSD Authentication (see
805 .Xr login.conf 5 )
806 and PAM (some non-OpenBSD systems).
807 .Pp
808 Finally, if other authentication methods fail,
809 .Nm
810 prompts the user for a password.
811 The password is sent to the remote
812 host for checking; however, since all communications are encrypted,
813 the password cannot be seen by someone listening on the network.
814 .Pp
815 .Nm
816 automatically maintains and checks a database containing
817 identification for all hosts it has ever been used with.
818 Host keys are stored in
819 .Pa ~/.ssh/known_hosts
820 in the user's home directory.
821 Additionally, the file
822 .Pa /etc/ssh/ssh_known_hosts
823 is automatically checked for known hosts.
824 Any new hosts are automatically added to the user's file.
825 If a host's identification ever changes,
826 .Nm
827 warns about this and disables password authentication to prevent
828 server spoofing or man-in-the-middle attacks,
829 which could otherwise be used to circumvent the encryption.
830 The
831 .Cm StrictHostKeyChecking
832 option can be used to control logins to machines whose
833 host key is not known or has changed.
834 .Pp
835 When the user's identity has been accepted by the server, the server
836 either executes the given command, or logs into the machine and gives
837 the user a normal shell on the remote machine.
838 All communication with
839 the remote command or shell will be automatically encrypted.
840 .Pp
841 If a pseudo-terminal has been allocated (normal login session), the
842 user may use the escape characters noted below.
843 .Pp
844 If no pseudo-tty has been allocated,
845 the session is transparent and can be used to reliably transfer binary data.
846 On most systems, setting the escape character to
847 .Dq none
848 will also make the session transparent even if a tty is used.
849 .Pp
850 The session terminates when the command or shell on the remote
851 machine exits and all X11 and TCP connections have been closed.
852 .Sh ESCAPE CHARACTERS
853 When a pseudo-terminal has been requested,
854 .Nm
855 supports a number of functions through the use of an escape character.
856 .Pp
857 A single tilde character can be sent as
858 .Ic ~~
859 or by following the tilde by a character other than those described below.
860 The escape character must always follow a newline to be interpreted as
861 special.
862 The escape character can be changed in configuration files using the
863 .Cm EscapeChar
864 configuration directive or on the command line by the
865 .Fl e
866 option.
867 .Pp
868 The supported escapes (assuming the default
869 .Ql ~ )
870 are:
871 .Bl -tag -width Ds
872 .It Cm ~.
873 Disconnect.
874 .It Cm ~^Z
875 Background
876 .Nm .
877 .It Cm ~#
878 List forwarded connections.
879 .It Cm ~&
880 Background
881 .Nm
882 at logout when waiting for forwarded connection / X11 sessions to terminate.
883 .It Cm ~?
884 Display a list of escape characters.
885 .It Cm ~B
886 Send a BREAK to the remote system
887 (only useful for SSH protocol version 2 and if the peer supports it).
888 .It Cm ~C
889 Open command line.
890 Currently this allows the addition of port forwardings using the
891 .Fl L ,
892 .Fl R
893 and
894 .Fl D
895 options (see above).
896 It also allows the cancellation of existing remote port-forwardings
897 using
898 .Sm off
899 .Fl KR Oo Ar bind_address : Oc Ar port .
900 .Sm on
901 .Ic !\& Ns Ar command
902 allows the user to execute a local command if the
903 .Ic PermitLocalCommand
904 option is enabled in
905 .Xr ssh_config 5 .
906 Basic help is available, using the
907 .Fl h
908 option.
909 .It Cm ~R
910 Request rekeying of the connection
911 (only useful for SSH protocol version 2 and if the peer supports it).
912 .El
913 .Sh TCP FORWARDING
914 Forwarding of arbitrary TCP connections over the secure channel can
915 be specified either on the command line or in a configuration file.
916 One possible application of TCP forwarding is a secure connection to a
917 mail server; another is going through firewalls.
918 .Pp
919 In the example below, we look at encrypting communication between
920 an IRC client and server, even though the IRC server does not directly
921 support encrypted communications.
922 This works as follows:
923 the user connects to the remote host using
924 .Nm ,
925 specifying a port to be used to forward connections
926 to the remote server.
927 After that it is possible to start the service which is to be encrypted
928 on the client machine,
929 connecting to the same local port,
930 and
931 .Nm
932 will encrypt and forward the connection.
933 .Pp
934 The following example tunnels an IRC session from client machine
935 .Dq 127.0.0.1
936 (localhost)
937 to remote server
938 .Dq server.example.com :
939 .Bd -literal -offset 4n
940 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
941 $ irc -c '#users' -p 1234 pinky 127.0.0.1
942 .Ed
943 .Pp
944 This tunnels a connection to IRC server
945 .Dq server.example.com ,
946 joining channel
947 .Dq #users ,
948 nickname
949 .Dq pinky ,
950 using port 1234.
951 It doesn't matter which port is used,
952 as long as it's greater than 1023
953 (remember, only root can open sockets on privileged ports)
954 and doesn't conflict with any ports already in use.
955 The connection is forwarded to port 6667 on the remote server,
956 since that's the standard port for IRC services.
957 .Pp
958 The
959 .Fl f
960 option backgrounds
961 .Nm
962 and the remote command
963 .Dq sleep 10
964 is specified to allow an amount of time
965 (10 seconds, in the example)
966 to start the service which is to be tunnelled.
967 If no connections are made within the time specified,
968 .Nm
969 will exit.
970 .Sh X11 FORWARDING
971 If the
972 .Cm ForwardX11
973 variable is set to
974 .Dq yes
975 (or see the description of the
976 .Fl X ,
977 .Fl x ,
978 and
979 .Fl Y
980 options above)
981 and the user is using X11 (the
982 .Ev DISPLAY
983 environment variable is set), the connection to the X11 display is
984 automatically forwarded to the remote side in such a way that any X11
985 programs started from the shell (or command) will go through the
986 encrypted channel, and the connection to the real X server will be made
987 from the local machine.
988 The user should not manually set
989 .Ev DISPLAY .
990 Forwarding of X11 connections can be
991 configured on the command line or in configuration files.
992 .Pp
993 The
994 .Ev DISPLAY
995 value set by
996 .Nm
997 will point to the server machine, but with a display number greater than zero.
998 This is normal, and happens because
999 .Nm
1000 creates a
1001 .Dq proxy
1002 X server on the server machine for forwarding the
1003 connections over the encrypted channel.
1004 .Pp
1005 .Nm
1006 will also automatically set up Xauthority data on the server machine.
1007 For this purpose, it will generate a random authorization cookie,
1008 store it in Xauthority on the server, and verify that any forwarded
1009 connections carry this cookie and replace it by the real cookie when
1010 the connection is opened.
1011 The real authentication cookie is never
1012 sent to the server machine (and no cookies are sent in the plain).
1013 .Pp
1014 If the
1015 .Cm ForwardAgent
1016 variable is set to
1017 .Dq yes
1018 (or see the description of the
1019 .Fl A
1020 and
1021 .Fl a
1022 options above) and
1023 the user is using an authentication agent, the connection to the agent
1024 is automatically forwarded to the remote side.
1025 .Sh VERIFYING HOST KEYS
1026 When connecting to a server for the first time,
1027 a fingerprint of the server's public key is presented to the user
1028 (unless the option
1029 .Cm StrictHostKeyChecking
1030 has been disabled).
1031 Fingerprints can be determined using
1032 .Xr ssh-keygen 1 :
1033 .Pp
1034 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1035 .Pp
1036 If the fingerprint is already known, it can be matched
1037 and the key can be accepted or rejected.
1038 Because of the difficulty of comparing host keys
1039 just by looking at hex strings,
1040 there is also support to compare host keys visually,
1041 using
1042 .Em random art .
1043 By setting the
1044 .Cm VisualHostKey
1045 option to
1046 .Dq yes ,
1047 a small ASCII graphic gets displayed on every login to a server, no matter
1048 if the session itself is interactive or not.
1049 By learning the pattern a known server produces, a user can easily
1050 find out that the host key has changed when a completely different pattern
1051 is displayed.
1052 Because these patterns are not unambiguous however, a pattern that looks
1053 similar to the pattern remembered only gives a good probability that the
1054 host key is the same, not guaranteed proof.
1055 .Pp
1056 To get a listing of the fingerprints along with their random art for
1057 all known hosts, the following command line can be used:
1058 .Pp
1059 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1060 .Pp
1061 If the fingerprint is unknown,
1062 an alternative method of verification is available:
1063 SSH fingerprints verified by DNS.
1064 An additional resource record (RR),
1065 SSHFP,
1066 is added to a zonefile
1067 and the connecting client is able to match the fingerprint
1068 with that of the key presented.
1069 .Pp
1070 In this example, we are connecting a client to a server,
1071 .Dq host.example.com .
1072 The SSHFP resource records should first be added to the zonefile for
1073 host.example.com:
1074 .Bd -literal -offset indent
1075 $ ssh-keygen -r host.example.com.
1076 .Ed
1077 .Pp
1078 The output lines will have to be added to the zonefile.
1079 To check that the zone is answering fingerprint queries:
1080 .Pp
1081 .Dl $ dig -t SSHFP host.example.com
1082 .Pp
1083 Finally the client connects:
1084 .Bd -literal -offset indent
1085 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1086 [...]
1087 Matching host key fingerprint found in DNS.
1088 Are you sure you want to continue connecting (yes/no)?
1089 .Ed
1090 .Pp
1091 See the
1092 .Cm VerifyHostKeyDNS
1093 option in
1094 .Xr ssh_config 5
1095 for more information.
1096 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1097 .Nm
1098 contains support for Virtual Private Network (VPN) tunnelling
1099 using the
1100 .Xr tun 4
1101 network pseudo-device,
1102 allowing two networks to be joined securely.
1103 The
1104 .Xr sshd_config 5
1105 configuration option
1106 .Cm PermitTunnel
1107 controls whether the server supports this,
1108 and at what level (layer 2 or 3 traffic).
1109 .Pp
1110 The following example would connect client network 10.0.50.0/24
1111 with remote network 10.0.99.0/24 using a point-to-point connection
1112 from 10.1.1.1 to 10.1.1.2,
1113 provided that the SSH server running on the gateway to the remote network,
1114 at 192.168.1.15, allows it.
1115 .Pp
1116 On the client:
1117 .Bd -literal -offset indent
1118 # ssh -f -w 0:1 192.168.1.15 true
1119 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1120 # route add 10.0.99.0/24 10.1.1.2
1121 .Ed
1122 .Pp
1123 On the server:
1124 .Bd -literal -offset indent
1125 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1126 # route add 10.0.50.0/24 10.1.1.1
1127 .Ed
1128 .Pp
1129 Client access may be more finely tuned via the
1130 .Pa /root/.ssh/authorized_keys
1131 file (see below) and the
1132 .Cm PermitRootLogin
1133 server option.
1134 The following entry would permit connections on
1135 .Xr tun 4
1136 device 1 from user
1137 .Dq jane
1138 and on tun device 2 from user
1139 .Dq john ,
1140 if
1141 .Cm PermitRootLogin
1142 is set to
1143 .Dq forced-commands-only :
1144 .Bd -literal -offset 2n
1145 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1146 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1147 .Ed
1148 .Pp
1149 Since an SSH-based setup entails a fair amount of overhead,
1150 it may be more suited to temporary setups,
1151 such as for wireless VPNs.
1152 More permanent VPNs are better provided by tools such as
1153 .Xr ipsecctl 8
1154 and
1155 .Xr isakmpd 8 .
1156 .Sh ENVIRONMENT
1157 .Nm
1158 will normally set the following environment variables:
1159 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1160 .It Ev DISPLAY
1161 The
1162 .Ev DISPLAY
1163 variable indicates the location of the X11 server.
1164 It is automatically set by
1165 .Nm
1166 to point to a value of the form
1167 .Dq hostname:n ,
1168 where
1169 .Dq hostname
1170 indicates the host where the shell runs, and
1171 .Sq n
1172 is an integer \*(Ge 1.
1173 .Nm
1174 uses this special value to forward X11 connections over the secure
1175 channel.
1176 The user should normally not set
1177 .Ev DISPLAY
1178 explicitly, as that
1179 will render the X11 connection insecure (and will require the user to
1180 manually copy any required authorization cookies).
1181 .It Ev HOME
1182 Set to the path of the user's home directory.
1183 .It Ev LOGNAME
1184 Synonym for
1185 .Ev USER ;
1186 set for compatibility with systems that use this variable.
1187 .It Ev MAIL
1188 Set to the path of the user's mailbox.
1189 .It Ev PATH
1190 Set to the default
1191 .Ev PATH ,
1192 as specified when compiling
1193 .Nm .
1194 .It Ev SSH_ASKPASS
1195 If
1196 .Nm
1197 needs a passphrase, it will read the passphrase from the current
1198 terminal if it was run from a terminal.
1199 If
1200 .Nm
1201 does not have a terminal associated with it but
1202 .Ev DISPLAY
1203 and
1204 .Ev SSH_ASKPASS
1205 are set, it will execute the program specified by
1206 .Ev SSH_ASKPASS
1207 and open an X11 window to read the passphrase.
1208 This is particularly useful when calling
1209 .Nm
1210 from a
1211 .Pa .xsession
1212 or related script.
1213 (Note that on some machines it
1214 may be necessary to redirect the input from
1215 .Pa /dev/null
1216 to make this work.)
1217 .It Ev SSH_AUTH_SOCK
1218 Identifies the path of a
1219 .Ux Ns -domain
1220 socket used to communicate with the agent.
1221 .It Ev SSH_CONNECTION
1222 Identifies the client and server ends of the connection.
1223 The variable contains
1224 four space-separated values: client IP address, client port number,
1225 server IP address, and server port number.
1226 .It Ev SSH_ORIGINAL_COMMAND
1227 This variable contains the original command line if a forced command
1228 is executed.
1229 It can be used to extract the original arguments.
1230 .It Ev SSH_TTY
1231 This is set to the name of the tty (path to the device) associated
1232 with the current shell or command.
1233 If the current session has no tty,
1234 this variable is not set.
1235 .It Ev TZ
1236 This variable is set to indicate the present time zone if it
1237 was set when the daemon was started (i.e. the daemon passes the value
1238 on to new connections).
1239 .It Ev USER
1240 Set to the name of the user logging in.
1241 .El
1242 .Pp
1243 Additionally,
1244 .Nm
1245 reads
1246 .Pa ~/.ssh/environment ,
1247 and adds lines of the format
1248 .Dq VARNAME=value
1249 to the environment if the file exists and users are allowed to
1250 change their environment.
1251 For more information, see the
1252 .Cm PermitUserEnvironment
1253 option in
1254 .Xr sshd_config 5 .
1255 .Sh FILES
1256 .Bl -tag -width Ds -compact
1257 .It ~/.rhosts
1258 This file is used for host-based authentication (see above).
1259 On some machines this file may need to be
1260 world-readable if the user's home directory is on an NFS partition,
1261 because
1262 .Xr sshd 8
1263 reads it as root.
1264 Additionally, this file must be owned by the user,
1265 and must not have write permissions for anyone else.
1266 The recommended
1267 permission for most machines is read/write for the user, and not
1268 accessible by others.
1269 .Pp
1270 .It ~/.shosts
1271 This file is used in exactly the same way as
1272 .Pa .rhosts ,
1273 but allows host-based authentication without permitting login with
1274 rlogin/rsh.
1275 .Pp
1276 .It ~/.ssh/
1277 This directory is the default location for all user-specific configuration
1278 and authentication information.
1279 There is no general requirement to keep the entire contents of this directory
1280 secret, but the recommended permissions are read/write/execute for the user,
1281 and not accessible by others.
1282 .Pp
1283 .It ~/.ssh/authorized_keys
1284 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1285 The format of this file is described in the
1286 .Xr sshd 8
1287 manual page.
1288 This file is not highly sensitive, but the recommended
1289 permissions are read/write for the user, and not accessible by others.
1290 .Pp
1291 .It ~/.ssh/config
1292 This is the per-user configuration file.
1293 The file format and configuration options are described in
1294 .Xr ssh_config 5 .
1295 Because of the potential for abuse, this file must have strict permissions:
1296 read/write for the user, and not accessible by others.
1297 .Pp
1298 .It ~/.ssh/environment
1299 Contains additional definitions for environment variables; see
1300 .Sx ENVIRONMENT ,
1301 above.
1302 .Pp
1303 .It ~/.ssh/identity
1304 .It ~/.ssh/id_dsa
1305 .It ~/.ssh/id_rsa
1306 Contains the private key for authentication.
1307 These files
1308 contain sensitive data and should be readable by the user but not
1309 accessible by others (read/write/execute).
1310 .Nm
1311 will simply ignore a private key file if it is accessible by others.
1312 It is possible to specify a passphrase when
1313 generating the key which will be used to encrypt the
1314 sensitive part of this file using 3DES.
1315 .Pp
1316 .It ~/.ssh/identity.pub
1317 .It ~/.ssh/id_dsa.pub
1318 .It ~/.ssh/id_rsa.pub
1319 Contains the public key for authentication.
1320 These files are not
1321 sensitive and can (but need not) be readable by anyone.
1322 .Pp
1323 .It ~/.ssh/known_hosts
1324 Contains a list of host keys for all hosts the user has logged into
1325 that are not already in the systemwide list of known host keys.
1326 See
1327 .Xr sshd 8
1328 for further details of the format of this file.
1329 .Pp
1330 .It ~/.ssh/rc
1331 Commands in this file are executed by
1332 .Nm
1333 when the user logs in, just before the user's shell (or command) is
1334 started.
1335 See the
1336 .Xr sshd 8
1337 manual page for more information.
1338 .Pp
1339 .It /etc/hosts.equiv
1340 This file is for host-based authentication (see above).
1341 It should only be writable by root.
1342 .Pp
1343 .It /etc/shosts.equiv
1344 This file is used in exactly the same way as
1345 .Pa hosts.equiv ,
1346 but allows host-based authentication without permitting login with
1347 rlogin/rsh.
1348 .Pp
1349 .It Pa /etc/ssh/ssh_config
1350 Systemwide configuration file.
1351 The file format and configuration options are described in
1352 .Xr ssh_config 5 .
1353 .Pp
1354 .It /etc/ssh/ssh_host_key
1355 .It /etc/ssh/ssh_host_dsa_key
1356 .It /etc/ssh/ssh_host_rsa_key
1357 These three files contain the private parts of the host keys
1358 and are used for host-based authentication.
1359 If protocol version 1 is used,
1360 .Nm
1361 must be setuid root, since the host key is readable only by root.
1362 For protocol version 2,
1363 .Nm
1364 uses
1365 .Xr ssh-keysign 8
1366 to access the host keys,
1367 eliminating the requirement that
1368 .Nm
1369 be setuid root when host-based authentication is used.
1370 By default
1371 .Nm
1372 is not setuid root.
1373 .Pp
1374 .It /etc/ssh/ssh_known_hosts
1375 Systemwide list of known host keys.
1376 This file should be prepared by the
1377 system administrator to contain the public host keys of all machines in the
1378 organization.
1379 It should be world-readable.
1380 See
1381 .Xr sshd 8
1382 for further details of the format of this file.
1383 .Pp
1384 .It /etc/ssh/sshrc
1385 Commands in this file are executed by
1386 .Nm
1387 when the user logs in, just before the user's shell (or command) is started.
1388 See the
1389 .Xr sshd 8
1390 manual page for more information.
1391 .El
1392 .Sh SEE ALSO
1393 .Xr scp 1 ,
1394 .Xr sftp 1 ,
1395 .Xr ssh-add 1 ,
1396 .Xr ssh-agent 1 ,
1397 .Xr ssh-keygen 1 ,
1398 .Xr ssh-keyscan 1 ,
1399 .Xr tun 4 ,
1400 .Xr hosts.equiv 5 ,
1401 .Xr ssh_config 5 ,
1402 .Xr ssh-keysign 8 ,
1403 .Xr sshd 8
1404 .Rs
1405 .%R RFC 4250
1406 .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1407 .%D 2006
1408 .Re
1409 .Rs
1410 .%R RFC 4251
1411 .%T "The Secure Shell (SSH) Protocol Architecture"
1412 .%D 2006
1413 .Re
1414 .Rs
1415 .%R RFC 4252
1416 .%T "The Secure Shell (SSH) Authentication Protocol"
1417 .%D 2006
1418 .Re
1419 .Rs
1420 .%R RFC 4253
1421 .%T "The Secure Shell (SSH) Transport Layer Protocol"
1422 .%D 2006
1423 .Re
1424 .Rs
1425 .%R RFC 4254
1426 .%T "The Secure Shell (SSH) Connection Protocol"
1427 .%D 2006
1428 .Re
1429 .Rs
1430 .%R RFC 4255
1431 .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1432 .%D 2006
1433 .Re
1434 .Rs
1435 .%R RFC 4256
1436 .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1437 .%D 2006
1438 .Re
1439 .Rs
1440 .%R RFC 4335
1441 .%T "The Secure Shell (SSH) Session Channel Break Extension"
1442 .%D 2006
1443 .Re
1444 .Rs
1445 .%R RFC 4344
1446 .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1447 .%D 2006
1448 .Re
1449 .Rs
1450 .%R RFC 4345
1451 .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1452 .%D 2006
1453 .Re
1454 .Rs
1455 .%R RFC 4419
1456 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1457 .%D 2006
1458 .Re
1459 .Rs
1460 .%R RFC 4716
1461 .%T "The Secure Shell (SSH) Public Key File Format"
1462 .%D 2006
1463 .Re
1464 .Rs
1465 .%T "Hash Visualization: a New Technique to improve Real-World Security"
1466 .%A A. Perrig
1467 .%A D. Song
1468 .%D 1999
1469 .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1470 .Re
1471 .Sh AUTHORS
1472 OpenSSH is a derivative of the original and free
1473 ssh 1.2.12 release by Tatu Ylonen.
1474 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1475 Theo de Raadt and Dug Song
1476 removed many bugs, re-added newer features and
1477 created OpenSSH.
1478 Markus Friedl contributed the support for SSH
1479 protocol versions 1.5 and 2.0.