| blob | 1ff2cce4d721a692abfb46daacfb198b51636220 |
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 1.290 2010/01/11 01:39:46 dtucker Exp $
38 .Dd $Mdocdate: January 11 2010 $
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 .Op Fl W Ar host : Ns Ar port
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 .Sh DESCRIPTION
86 .Nm
87 (SSH client) is a program for logging into a remote machine and for
88 executing commands on a remote machine.
89 It is intended to replace rlogin and rsh,
90 and provide secure encrypted communications between
91 two untrusted hosts over an insecure network.
92 X11 connections and arbitrary TCP ports
93 can also be forwarded over the secure channel.
94 .Pp
95 .Nm
96 connects and logs into the specified
97 .Ar hostname
98 (with optional
99 .Ar user
100 name).
101 The user must prove
102 his/her identity to the remote machine using one of several methods
103 depending on the protocol version used (see below).
104 .Pp
105 If
106 .Ar command
107 is specified,
108 it is executed on the remote host instead of a login shell.
109 .Pp
110 The options are as follows:
111 .Bl -tag -width Ds
112 .It Fl 1
113 Forces
114 .Nm
115 to try protocol version 1 only.
116 .It Fl 2
117 Forces
118 .Nm
119 to try protocol version 2 only.
120 .It Fl 4
121 Forces
122 .Nm
123 to use IPv4 addresses only.
124 .It Fl 6
125 Forces
126 .Nm
127 to use IPv6 addresses only.
128 .It Fl A
129 Enables forwarding of the authentication agent connection.
130 This can also be specified on a per-host basis in a configuration file.
131 .Pp
132 Agent forwarding should be enabled with caution.
133 Users with the ability to bypass file permissions on the remote host
134 (for the agent's
135 .Ux Ns -domain
136 socket) 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 Ar host : Ns Ar port
597 Requests that standard input and output on the client be forwarded to
598 .Ar host
599 on
600 .Ar port
601 over the secure channel.
602 Implies
603 .Fl N ,
604 .Fl T ,
605 .Cm ExitOnForwardFailure
606 and
607 .Cm ClearAllForwardings
608 and works with Protocol version 2 only.
609 .It Fl w Xo
610 .Ar local_tun Ns Op : Ns Ar remote_tun
611 .Xc
612 Requests
613 tunnel
614 device forwarding with the specified
615 .Xr tun 4
616 devices between the client
617 .Pq Ar local_tun
618 and the server
619 .Pq Ar remote_tun .
620 .Pp
621 The devices may be specified by numerical ID or the keyword
622 .Dq any ,
623 which uses the next available tunnel device.
624 If
625 .Ar remote_tun
626 is not specified, it defaults to
627 .Dq any .
628 See also the
629 .Cm Tunnel
630 and
631 .Cm TunnelDevice
632 directives in
633 .Xr ssh_config 5 .
634 If the
635 .Cm Tunnel
636 directive is unset, it is set to the default tunnel mode, which is
637 .Dq point-to-point .
638 .It Fl X
639 Enables X11 forwarding.
640 This can also be specified on a per-host basis in a configuration file.
641 .Pp
642 X11 forwarding should be enabled with caution.
643 Users with the ability to bypass file permissions on the remote host
644 (for the user's X authorization database)
645 can access the local X11 display through the forwarded connection.
646 An attacker may then be able to perform activities such as keystroke monitoring.
647 .Pp
648 For this reason, X11 forwarding is subjected to X11 SECURITY extension
649 restrictions by default.
650 Please refer to the
651 .Nm
652 .Fl Y
653 option and the
654 .Cm ForwardX11Trusted
655 directive in
656 .Xr ssh_config 5
657 for more information.
658 .It Fl x
659 Disables X11 forwarding.
660 .It Fl Y
661 Enables trusted X11 forwarding.
662 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
663 controls.
664 .It Fl y
665 Send log information using the
666 .Xr syslog 3
667 system module.
668 By default this information is sent to stderr.
669 .El
670 .Pp
671 .Nm
672 may additionally obtain configuration data from
673 a per-user configuration file and a system-wide configuration file.
674 The file format and configuration options are described in
675 .Xr ssh_config 5 .
676 .Pp
677 .Nm
678 exits with the exit status of the remote command or with 255
679 if an error occurred.
680 .Sh AUTHENTICATION
681 The OpenSSH SSH client supports SSH protocols 1 and 2.
682 The default is to use protocol 2 only,
683 though this can be changed via the
684 .Cm Protocol
685 option in
686 .Xr ssh_config 5
687 or the
688 .Fl 1
689 and
690 .Fl 2
691 options (see above).
692 Both protocols support similar authentication methods,
693 but protocol 2 is the default since
694 it provides additional mechanisms for confidentiality
695 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
696 and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
697 Protocol 1 lacks a strong mechanism for ensuring the
698 integrity of the connection.
699 .Pp
700 The methods available for authentication are:
701 GSSAPI-based authentication,
702 host-based authentication,
703 public key authentication,
704 challenge-response authentication,
705 and password authentication.
706 Authentication methods are tried in the order specified above,
707 though protocol 2 has a configuration option to change the default order:
708 .Cm PreferredAuthentications .
709 .Pp
710 Host-based authentication works as follows:
711 If the machine the user logs in from is listed in
712 .Pa /etc/hosts.equiv
713 or
714 .Pa /etc/shosts.equiv
715 on the remote machine, and the user names are
716 the same on both sides, or if the files
717 .Pa ~/.rhosts
718 or
719 .Pa ~/.shosts
720 exist in the user's home directory on the
721 remote machine and contain a line containing the name of the client
722 machine and the name of the user on that machine, the user is
723 considered for login.
724 Additionally, the server
725 .Em must
726 be able to verify the client's
727 host key (see the description of
728 .Pa /etc/ssh/ssh_known_hosts
729 and
730 .Pa ~/.ssh/known_hosts ,
731 below)
732 for login to be permitted.
733 This authentication method closes security holes due to IP
734 spoofing, DNS spoofing, and routing spoofing.
735 [Note to the administrator:
736 .Pa /etc/hosts.equiv ,
737 .Pa ~/.rhosts ,
738 and the rlogin/rsh protocol in general, are inherently insecure and should be
739 disabled if security is desired.]
740 .Pp
741 Public key authentication works as follows:
742 The scheme is based on public-key cryptography,
743 using cryptosystems
744 where encryption and decryption are done using separate keys,
745 and it is unfeasible to derive the decryption key from the encryption key.
746 The idea is that each user creates a public/private
747 key pair for authentication purposes.
748 The server knows the public key, and only the user knows the private key.
749 .Nm
750 implements public key authentication protocol automatically,
751 using either the RSA or DSA algorithms.
752 Protocol 1 is restricted to using only RSA keys,
753 but protocol 2 may use either.
754 The
755 .Sx HISTORY
756 section of
757 .Xr ssl 8
758 contains a brief discussion of the two algorithms.
759 .Pp
760 The file
761 .Pa ~/.ssh/authorized_keys
762 lists the public keys that are permitted for logging in.
763 When the user logs in, the
764 .Nm
765 program tells the server which key pair it would like to use for
766 authentication.
767 The client proves that it has access to the private key
768 and the server checks that the corresponding public key
769 is authorized to accept the account.
770 .Pp
771 The user creates his/her key pair by running
772 .Xr ssh-keygen 1 .
773 This stores the private key in
774 .Pa ~/.ssh/identity
775 (protocol 1),
776 .Pa ~/.ssh/id_dsa
777 (protocol 2 DSA),
778 or
779 .Pa ~/.ssh/id_rsa
780 (protocol 2 RSA)
781 and stores the public key in
782 .Pa ~/.ssh/identity.pub
783 (protocol 1),
784 .Pa ~/.ssh/id_dsa.pub
785 (protocol 2 DSA),
786 or
787 .Pa ~/.ssh/id_rsa.pub
788 (protocol 2 RSA)
789 in the user's home directory.
790 The user should then copy the public key
791 to
792 .Pa ~/.ssh/authorized_keys
793 in his/her home directory on the remote machine.
794 The
795 .Pa authorized_keys
796 file corresponds to the conventional
797 .Pa ~/.rhosts
798 file, and has one key
799 per line, though the lines can be very long.
800 After this, the user can log in without giving the password.
801 .Pp
802 The most convenient way to use public key authentication may be with an
803 authentication agent.
804 See
805 .Xr ssh-agent 1
806 for more information.
807 .Pp
808 Challenge-response authentication works as follows:
809 The server sends an arbitrary
810 .Qq challenge
811 text, and prompts for a response.
812 Protocol 2 allows multiple challenges and responses;
813 protocol 1 is restricted to just one challenge/response.
814 Examples of challenge-response authentication include
815 BSD Authentication (see
816 .Xr login.conf 5 )
817 and PAM (some non-OpenBSD systems).
818 .Pp
819 Finally, if other authentication methods fail,
820 .Nm
821 prompts the user for a password.
822 The password is sent to the remote
823 host for checking; however, since all communications are encrypted,
824 the password cannot be seen by someone listening on the network.
825 .Pp
826 .Nm
827 automatically maintains and checks a database containing
828 identification for all hosts it has ever been used with.
829 Host keys are stored in
830 .Pa ~/.ssh/known_hosts
831 in the user's home directory.
832 Additionally, the file
833 .Pa /etc/ssh/ssh_known_hosts
834 is automatically checked for known hosts.
835 Any new hosts are automatically added to the user's file.
836 If a host's identification ever changes,
837 .Nm
838 warns about this and disables password authentication to prevent
839 server spoofing or man-in-the-middle attacks,
840 which could otherwise be used to circumvent the encryption.
841 The
842 .Cm StrictHostKeyChecking
843 option can be used to control logins to machines whose
844 host key is not known or has changed.
845 .Pp
846 When the user's identity has been accepted by the server, the server
847 either executes the given command, or logs into the machine and gives
848 the user a normal shell on the remote machine.
849 All communication with
850 the remote command or shell will be automatically encrypted.
851 .Pp
852 If a pseudo-terminal has been allocated (normal login session), the
853 user may use the escape characters noted below.
854 .Pp
855 If no pseudo-tty has been allocated,
856 the session is transparent and can be used to reliably transfer binary data.
857 On most systems, setting the escape character to
858 .Dq none
859 will also make the session transparent even if a tty is used.
860 .Pp
861 The session terminates when the command or shell on the remote
862 machine exits and all X11 and TCP connections have been closed.
863 .Sh ESCAPE CHARACTERS
864 When a pseudo-terminal has been requested,
865 .Nm
866 supports a number of functions through the use of an escape character.
867 .Pp
868 A single tilde character can be sent as
869 .Ic ~~
870 or by following the tilde by a character other than those described below.
871 The escape character must always follow a newline to be interpreted as
872 special.
873 The escape character can be changed in configuration files using the
874 .Cm EscapeChar
875 configuration directive or on the command line by the
876 .Fl e
877 option.
878 .Pp
879 The supported escapes (assuming the default
880 .Ql ~ )
881 are:
882 .Bl -tag -width Ds
883 .It Cm ~.
884 Disconnect.
885 .It Cm ~^Z
886 Background
887 .Nm .
888 .It Cm ~#
889 List forwarded connections.
890 .It Cm ~&
891 Background
892 .Nm
893 at logout when waiting for forwarded connection / X11 sessions to terminate.
894 .It Cm ~?
895 Display a list of escape characters.
896 .It Cm ~B
897 Send a BREAK to the remote system
898 (only useful for SSH protocol version 2 and if the peer supports it).
899 .It Cm ~C
900 Open command line.
901 Currently this allows the addition of port forwardings using the
902 .Fl L ,
903 .Fl R
904 and
905 .Fl D
906 options (see above).
907 It also allows the cancellation of existing remote port-forwardings
908 using
909 .Sm off
910 .Fl KR Oo Ar bind_address : Oc Ar port .
911 .Sm on
912 .Ic !\& Ns Ar command
913 allows the user to execute a local command if the
914 .Ic PermitLocalCommand
915 option is enabled in
916 .Xr ssh_config 5 .
917 Basic help is available, using the
918 .Fl h
919 option.
920 .It Cm ~R
921 Request rekeying of the connection
922 (only useful for SSH protocol version 2 and if the peer supports it).
923 .El
924 .Sh TCP FORWARDING
925 Forwarding of arbitrary TCP connections over the secure channel can
926 be specified either on the command line or in a configuration file.
927 One possible application of TCP forwarding is a secure connection to a
928 mail server; another is going through firewalls.
929 .Pp
930 In the example below, we look at encrypting communication between
931 an IRC client and server, even though the IRC server does not directly
932 support encrypted communications.
933 This works as follows:
934 the user connects to the remote host using
935 .Nm ,
936 specifying a port to be used to forward connections
937 to the remote server.
938 After that it is possible to start the service which is to be encrypted
939 on the client machine,
940 connecting to the same local port,
941 and
942 .Nm
943 will encrypt and forward the connection.
944 .Pp
945 The following example tunnels an IRC session from client machine
946 .Dq 127.0.0.1
947 (localhost)
948 to remote server
949 .Dq server.example.com :
950 .Bd -literal -offset 4n
951 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
952 $ irc -c '#users' -p 1234 pinky 127.0.0.1
953 .Ed
954 .Pp
955 This tunnels a connection to IRC server
956 .Dq server.example.com ,
957 joining channel
958 .Dq #users ,
959 nickname
960 .Dq pinky ,
961 using port 1234.
962 It doesn't matter which port is used,
963 as long as it's greater than 1023
964 (remember, only root can open sockets on privileged ports)
965 and doesn't conflict with any ports already in use.
966 The connection is forwarded to port 6667 on the remote server,
967 since that's the standard port for IRC services.
968 .Pp
969 The
970 .Fl f
971 option backgrounds
972 .Nm
973 and the remote command
974 .Dq sleep 10
975 is specified to allow an amount of time
976 (10 seconds, in the example)
977 to start the service which is to be tunnelled.
978 If no connections are made within the time specified,
979 .Nm
980 will exit.
981 .Sh X11 FORWARDING
982 If the
983 .Cm ForwardX11
984 variable is set to
985 .Dq yes
986 (or see the description of the
987 .Fl X ,
988 .Fl x ,
989 and
990 .Fl Y
991 options above)
992 and the user is using X11 (the
993 .Ev DISPLAY
994 environment variable is set), the connection to the X11 display is
995 automatically forwarded to the remote side in such a way that any X11
996 programs started from the shell (or command) will go through the
997 encrypted channel, and the connection to the real X server will be made
998 from the local machine.
999 The user should not manually set
1000 .Ev DISPLAY .
1001 Forwarding of X11 connections can be
1002 configured on the command line or in configuration files.
1003 .Pp
1004 The
1005 .Ev DISPLAY
1006 value set by
1007 .Nm
1008 will point to the server machine, but with a display number greater than zero.
1009 This is normal, and happens because
1010 .Nm
1011 creates a
1012 .Dq proxy
1013 X server on the server machine for forwarding the
1014 connections over the encrypted channel.
1015 .Pp
1016 .Nm
1017 will also automatically set up Xauthority data on the server machine.
1018 For this purpose, it will generate a random authorization cookie,
1019 store it in Xauthority on the server, and verify that any forwarded
1020 connections carry this cookie and replace it by the real cookie when
1021 the connection is opened.
1022 The real authentication cookie is never
1023 sent to the server machine (and no cookies are sent in the plain).
1024 .Pp
1025 If the
1026 .Cm ForwardAgent
1027 variable is set to
1028 .Dq yes
1029 (or see the description of the
1030 .Fl A
1031 and
1032 .Fl a
1033 options above) and
1034 the user is using an authentication agent, the connection to the agent
1035 is automatically forwarded to the remote side.
1036 .Sh VERIFYING HOST KEYS
1037 When connecting to a server for the first time,
1038 a fingerprint of the server's public key is presented to the user
1039 (unless the option
1040 .Cm StrictHostKeyChecking
1041 has been disabled).
1042 Fingerprints can be determined using
1043 .Xr ssh-keygen 1 :
1044 .Pp
1045 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1046 .Pp
1047 If the fingerprint is already known, it can be matched
1048 and the key can be accepted or rejected.
1049 Because of the difficulty of comparing host keys
1050 just by looking at hex strings,
1051 there is also support to compare host keys visually,
1052 using
1053 .Em random art .
1054 By setting the
1055 .Cm VisualHostKey
1056 option to
1057 .Dq yes ,
1058 a small ASCII graphic gets displayed on every login to a server, no matter
1059 if the session itself is interactive or not.
1060 By learning the pattern a known server produces, a user can easily
1061 find out that the host key has changed when a completely different pattern
1062 is displayed.
1063 Because these patterns are not unambiguous however, a pattern that looks
1064 similar to the pattern remembered only gives a good probability that the
1065 host key is the same, not guaranteed proof.
1066 .Pp
1067 To get a listing of the fingerprints along with their random art for
1068 all known hosts, the following command line can be used:
1069 .Pp
1070 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1071 .Pp
1072 If the fingerprint is unknown,
1073 an alternative method of verification is available:
1074 SSH fingerprints verified by DNS.
1075 An additional resource record (RR),
1076 SSHFP,
1077 is added to a zonefile
1078 and the connecting client is able to match the fingerprint
1079 with that of the key presented.
1080 .Pp
1081 In this example, we are connecting a client to a server,
1082 .Dq host.example.com .
1083 The SSHFP resource records should first be added to the zonefile for
1084 host.example.com:
1085 .Bd -literal -offset indent
1086 $ ssh-keygen -r host.example.com.
1087 .Ed
1088 .Pp
1089 The output lines will have to be added to the zonefile.
1090 To check that the zone is answering fingerprint queries:
1091 .Pp
1092 .Dl $ dig -t SSHFP host.example.com
1093 .Pp
1094 Finally the client connects:
1095 .Bd -literal -offset indent
1096 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1097 [...]
1098 Matching host key fingerprint found in DNS.
1099 Are you sure you want to continue connecting (yes/no)?
1100 .Ed
1101 .Pp
1102 See the
1103 .Cm VerifyHostKeyDNS
1104 option in
1105 .Xr ssh_config 5
1106 for more information.
1107 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1108 .Nm
1109 contains support for Virtual Private Network (VPN) tunnelling
1110 using the
1111 .Xr tun 4
1112 network pseudo-device,
1113 allowing two networks to be joined securely.
1114 The
1115 .Xr sshd_config 5
1116 configuration option
1117 .Cm PermitTunnel
1118 controls whether the server supports this,
1119 and at what level (layer 2 or 3 traffic).
1120 .Pp
1121 The following example would connect client network 10.0.50.0/24
1122 with remote network 10.0.99.0/24 using a point-to-point connection
1123 from 10.1.1.1 to 10.1.1.2,
1124 provided that the SSH server running on the gateway to the remote network,
1125 at 192.168.1.15, allows it.
1126 .Pp
1127 On the client:
1128 .Bd -literal -offset indent
1129 # ssh -f -w 0:1 192.168.1.15 true
1130 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1131 # route add 10.0.99.0/24 10.1.1.2
1132 .Ed
1133 .Pp
1134 On the server:
1135 .Bd -literal -offset indent
1136 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1137 # route add 10.0.50.0/24 10.1.1.1
1138 .Ed
1139 .Pp
1140 Client access may be more finely tuned via the
1141 .Pa /root/.ssh/authorized_keys
1142 file (see below) and the
1143 .Cm PermitRootLogin
1144 server option.
1145 The following entry would permit connections on
1146 .Xr tun 4
1147 device 1 from user
1148 .Dq jane
1149 and on tun device 2 from user
1150 .Dq john ,
1151 if
1152 .Cm PermitRootLogin
1153 is set to
1154 .Dq forced-commands-only :
1155 .Bd -literal -offset 2n
1156 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1157 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1158 .Ed
1159 .Pp
1160 Since an SSH-based setup entails a fair amount of overhead,
1161 it may be more suited to temporary setups,
1162 such as for wireless VPNs.
1163 More permanent VPNs are better provided by tools such as
1164 .Xr ipsecctl 8
1165 and
1166 .Xr isakmpd 8 .
1167 .Sh ENVIRONMENT
1168 .Nm
1169 will normally set the following environment variables:
1170 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1171 .It Ev DISPLAY
1172 The
1173 .Ev DISPLAY
1174 variable indicates the location of the X11 server.
1175 It is automatically set by
1176 .Nm
1177 to point to a value of the form
1178 .Dq hostname:n ,
1179 where
1180 .Dq hostname
1181 indicates the host where the shell runs, and
1182 .Sq n
1183 is an integer \*(Ge 1.
1184 .Nm
1185 uses this special value to forward X11 connections over the secure
1186 channel.
1187 The user should normally not set
1188 .Ev DISPLAY
1189 explicitly, as that
1190 will render the X11 connection insecure (and will require the user to
1191 manually copy any required authorization cookies).
1192 .It Ev HOME
1193 Set to the path of the user's home directory.
1194 .It Ev LOGNAME
1195 Synonym for
1196 .Ev USER ;
1197 set for compatibility with systems that use this variable.
1198 .It Ev MAIL
1199 Set to the path of the user's mailbox.
1200 .It Ev PATH
1201 Set to the default
1202 .Ev PATH ,
1203 as specified when compiling
1204 .Nm .
1205 .It Ev SSH_ASKPASS
1206 If
1207 .Nm
1208 needs a passphrase, it will read the passphrase from the current
1209 terminal if it was run from a terminal.
1210 If
1211 .Nm
1212 does not have a terminal associated with it but
1213 .Ev DISPLAY
1214 and
1215 .Ev SSH_ASKPASS
1216 are set, it will execute the program specified by
1217 .Ev SSH_ASKPASS
1218 and open an X11 window to read the passphrase.
1219 This is particularly useful when calling
1220 .Nm
1221 from a
1222 .Pa .xsession
1223 or related script.
1224 (Note that on some machines it
1225 may be necessary to redirect the input from
1226 .Pa /dev/null
1227 to make this work.)
1228 .It Ev SSH_AUTH_SOCK
1229 Identifies the path of a
1230 .Ux Ns -domain
1231 socket used to communicate with the agent.
1232 .It Ev SSH_CONNECTION
1233 Identifies the client and server ends of the connection.
1234 The variable contains
1235 four space-separated values: client IP address, client port number,
1236 server IP address, and server port number.
1237 .It Ev SSH_ORIGINAL_COMMAND
1238 This variable contains the original command line if a forced command
1239 is executed.
1240 It can be used to extract the original arguments.
1241 .It Ev SSH_TTY
1242 This is set to the name of the tty (path to the device) associated
1243 with the current shell or command.
1244 If the current session has no tty,
1245 this variable is not set.
1246 .It Ev TZ
1247 This variable is set to indicate the present time zone if it
1248 was set when the daemon was started (i.e. the daemon passes the value
1249 on to new connections).
1250 .It Ev USER
1251 Set to the name of the user logging in.
1252 .El
1253 .Pp
1254 Additionally,
1255 .Nm
1256 reads
1257 .Pa ~/.ssh/environment ,
1258 and adds lines of the format
1259 .Dq VARNAME=value
1260 to the environment if the file exists and users are allowed to
1261 change their environment.
1262 For more information, see the
1263 .Cm PermitUserEnvironment
1264 option in
1265 .Xr sshd_config 5 .
1266 .Sh FILES
1267 .Bl -tag -width Ds -compact
1268 .It ~/.rhosts
1269 This file is used for host-based authentication (see above).
1270 On some machines this file may need to be
1271 world-readable if the user's home directory is on an NFS partition,
1272 because
1273 .Xr sshd 8
1274 reads it as root.
1275 Additionally, this file must be owned by the user,
1276 and must not have write permissions for anyone else.
1277 The recommended
1278 permission for most machines is read/write for the user, and not
1279 accessible by others.
1280 .Pp
1281 .It ~/.shosts
1282 This file is used in exactly the same way as
1283 .Pa .rhosts ,
1284 but allows host-based authentication without permitting login with
1285 rlogin/rsh.
1286 .Pp
1287 .It ~/.ssh/
1288 This directory is the default location for all user-specific configuration
1289 and authentication information.
1290 There is no general requirement to keep the entire contents of this directory
1291 secret, but the recommended permissions are read/write/execute for the user,
1292 and not accessible by others.
1293 .Pp
1294 .It ~/.ssh/authorized_keys
1295 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1296 The format of this file is described in the
1297 .Xr sshd 8
1298 manual page.
1299 This file is not highly sensitive, but the recommended
1300 permissions are read/write for the user, and not accessible by others.
1301 .Pp
1302 .It ~/.ssh/config
1303 This is the per-user configuration file.
1304 The file format and configuration options are described in
1305 .Xr ssh_config 5 .
1306 Because of the potential for abuse, this file must have strict permissions:
1307 read/write for the user, and not accessible by others.
1308 .Pp
1309 .It ~/.ssh/environment
1310 Contains additional definitions for environment variables; see
1311 .Sx ENVIRONMENT ,
1312 above.
1313 .Pp
1314 .It ~/.ssh/identity
1315 .It ~/.ssh/id_dsa
1316 .It ~/.ssh/id_rsa
1317 Contains the private key for authentication.
1318 These files
1319 contain sensitive data and should be readable by the user but not
1320 accessible by others (read/write/execute).
1321 .Nm
1322 will simply ignore a private key file if it is accessible by others.
1323 It is possible to specify a passphrase when
1324 generating the key which will be used to encrypt the
1325 sensitive part of this file using 3DES.
1326 .Pp
1327 .It ~/.ssh/identity.pub
1328 .It ~/.ssh/id_dsa.pub
1329 .It ~/.ssh/id_rsa.pub
1330 Contains the public key for authentication.
1331 These files are not
1332 sensitive and can (but need not) be readable by anyone.
1333 .Pp
1334 .It ~/.ssh/known_hosts
1335 Contains a list of host keys for all hosts the user has logged into
1336 that are not already in the systemwide list of known host keys.
1337 See
1338 .Xr sshd 8
1339 for further details of the format of this file.
1340 .Pp
1341 .It ~/.ssh/rc
1342 Commands in this file are executed by
1343 .Nm
1344 when the user logs in, just before the user's shell (or command) is
1345 started.
1346 See the
1347 .Xr sshd 8
1348 manual page for more information.
1349 .Pp
1350 .It /etc/hosts.equiv
1351 This file is for host-based authentication (see above).
1352 It should only be writable by root.
1353 .Pp
1354 .It /etc/shosts.equiv
1355 This file is used in exactly the same way as
1356 .Pa hosts.equiv ,
1357 but allows host-based authentication without permitting login with
1358 rlogin/rsh.
1359 .Pp
1360 .It Pa /etc/ssh/ssh_config
1361 Systemwide configuration file.
1362 The file format and configuration options are described in
1363 .Xr ssh_config 5 .
1364 .Pp
1365 .It /etc/ssh/ssh_host_key
1366 .It /etc/ssh/ssh_host_dsa_key
1367 .It /etc/ssh/ssh_host_rsa_key
1368 These three files contain the private parts of the host keys
1369 and are used for host-based authentication.
1370 If protocol version 1 is used,
1371 .Nm
1372 must be setuid root, since the host key is readable only by root.
1373 For protocol version 2,
1374 .Nm
1375 uses
1376 .Xr ssh-keysign 8
1377 to access the host keys,
1378 eliminating the requirement that
1379 .Nm
1380 be setuid root when host-based authentication is used.
1381 By default
1382 .Nm
1383 is not setuid root.
1384 .Pp
1385 .It /etc/ssh/ssh_known_hosts
1386 Systemwide list of known host keys.
1387 This file should be prepared by the
1388 system administrator to contain the public host keys of all machines in the
1389 organization.
1390 It should be world-readable.
1391 See
1392 .Xr sshd 8
1393 for further details of the format of this file.
1394 .Pp
1395 .It /etc/ssh/sshrc
1396 Commands in this file are executed by
1397 .Nm
1398 when the user logs in, just before the user's shell (or command) is started.
1399 See the
1400 .Xr sshd 8
1401 manual page for more information.
1402 .El
1403 .Sh SEE ALSO
1404 .Xr scp 1 ,
1405 .Xr sftp 1 ,
1406 .Xr ssh-add 1 ,
1407 .Xr ssh-agent 1 ,
1408 .Xr ssh-keygen 1 ,
1409 .Xr ssh-keyscan 1 ,
1410 .Xr tun 4 ,
1411 .Xr hosts.equiv 5 ,
1412 .Xr ssh_config 5 ,
1413 .Xr ssh-keysign 8 ,
1414 .Xr sshd 8
1415 .Rs
1416 .%R RFC 4250
1417 .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1418 .%D 2006
1419 .Re
1420 .Rs
1421 .%R RFC 4251
1422 .%T "The Secure Shell (SSH) Protocol Architecture"
1423 .%D 2006
1424 .Re
1425 .Rs
1426 .%R RFC 4252
1427 .%T "The Secure Shell (SSH) Authentication Protocol"
1428 .%D 2006
1429 .Re
1430 .Rs
1431 .%R RFC 4253
1432 .%T "The Secure Shell (SSH) Transport Layer Protocol"
1433 .%D 2006
1434 .Re
1435 .Rs
1436 .%R RFC 4254
1437 .%T "The Secure Shell (SSH) Connection Protocol"
1438 .%D 2006
1439 .Re
1440 .Rs
1441 .%R RFC 4255
1442 .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1443 .%D 2006
1444 .Re
1445 .Rs
1446 .%R RFC 4256
1447 .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1448 .%D 2006
1449 .Re
1450 .Rs
1451 .%R RFC 4335
1452 .%T "The Secure Shell (SSH) Session Channel Break Extension"
1453 .%D 2006
1454 .Re
1455 .Rs
1456 .%R RFC 4344
1457 .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1458 .%D 2006
1459 .Re
1460 .Rs
1461 .%R RFC 4345
1462 .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1463 .%D 2006
1464 .Re
1465 .Rs
1466 .%R RFC 4419
1467 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1468 .%D 2006
1469 .Re
1470 .Rs
1471 .%R RFC 4716
1472 .%T "The Secure Shell (SSH) Public Key File Format"
1473 .%D 2006
1474 .Re
1475 .Rs
1476 .%T "Hash Visualization: a New Technique to improve Real-World Security"
1477 .%A A. Perrig
1478 .%A D. Song
1479 .%D 1999
1480 .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1481 .Re
1482 .Sh AUTHORS
1483 OpenSSH is a derivative of the original and free
1484 ssh 1.2.12 release by Tatu Ylonen.
1485 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1486 Theo de Raadt and Dug Song
1487 removed many bugs, re-added newer features and
1488 created OpenSSH.
1489 Markus Friedl contributed the support for SSH
1490 protocol versions 1.5 and 2.0.