| blob | 7d8f92aba9da30a92f819e02ba1fd5ba439b1ee5 |
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 1.295 2010/02/26 20:29:54 djm Exp $
38 .Dd $Mdocdate: February 26 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 .Op Fl I Ar pkcs11
58 .Bk -words
59 .Op Fl i Ar identity_file
60 .Ek
61 .Oo Fl L\ \&
62 .Sm off
63 .Oo Ar bind_address : Oc
64 .Ar port : host : hostport
65 .Sm on
66 .Oc
67 .Bk -words
68 .Op Fl l Ar login_name
69 .Ek
70 .Op Fl m Ar mac_spec
71 .Op Fl O Ar ctl_cmd
72 .Op Fl o Ar option
73 .Op Fl p Ar port
74 .Oo Fl R\ \&
75 .Sm off
76 .Oo Ar bind_address : Oc
77 .Ar port : host : hostport
78 .Sm on
79 .Oc
80 .Op Fl S Ar ctl_path
81 .Op Fl W Ar host : Ns Ar port
82 .Oo Fl w Ar local_tun Ns
83 .Op : Ns Ar remote_tun Oc
84 .Oo Ar user Ns @ Oc Ns Ar hostname
85 .Op Ar command
86 .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
136 .Ux Ns -domain
137 socket) can access the local agent through the forwarded connection.
138 An attacker cannot obtain key material from the agent,
139 however they can perform operations on the keys that enable them to
140 authenticate using the identities loaded into the agent.
141 .It Fl a
142 Disables forwarding of the authentication agent connection.
143 .It Fl b Ar bind_address
144 Use
145 .Ar bind_address
146 on the local machine as the source address
147 of the connection.
148 Only useful on systems with more than one address.
149 .It Fl C
150 Requests compression of all data (including stdin, stdout, stderr, and
151 data for forwarded X11 and TCP connections).
152 The compression algorithm is the same used by
153 .Xr gzip 1 ,
154 and the
155 .Dq level
156 can be controlled by the
157 .Cm CompressionLevel
158 option for protocol version 1.
159 Compression is desirable on modem lines and other
160 slow connections, but will only slow down things on fast networks.
161 The default value can be set on a host-by-host basis in the
162 configuration files; see the
163 .Cm Compression
164 option.
165 .It Fl c Ar cipher_spec
166 Selects the cipher specification for encrypting the session.
167 .Pp
168 Protocol version 1 allows specification of a single cipher.
169 The supported values are
170 .Dq 3des ,
171 .Dq blowfish ,
172 and
173 .Dq des .
174 .Ar 3des
175 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
176 It is believed to be secure.
177 .Ar blowfish
178 is a fast block cipher; it appears very secure and is much faster than
179 .Ar 3des .
180 .Ar des
181 is only supported in the
182 .Nm
183 client for interoperability with legacy protocol 1 implementations
184 that do not support the
185 .Ar 3des
186 cipher.
187 Its use is strongly discouraged due to cryptographic weaknesses.
188 The default is
189 .Dq 3des .
190 .Pp
191 For protocol version 2,
192 .Ar cipher_spec
193 is a comma-separated list of ciphers
194 listed in order of preference.
195 See the
196 .Cm Ciphers
197 keyword for more information.
198 .It Fl D Xo
199 .Sm off
200 .Oo Ar bind_address : Oc
201 .Ar port
202 .Sm on
203 .Xc
204 Specifies a local
205 .Dq dynamic
206 application-level port forwarding.
207 This works by allocating a socket to listen to
208 .Ar port
209 on the local side, optionally bound to the specified
210 .Ar bind_address .
211 Whenever a connection is made to this port, the
212 connection is forwarded over the secure channel, and the application
213 protocol is then used to determine where to connect to from the
214 remote machine.
215 Currently the SOCKS4 and SOCKS5 protocols are supported, and
216 .Nm
217 will act as a SOCKS server.
218 Only root can forward privileged ports.
219 Dynamic port forwardings can also be specified in the configuration file.
220 .Pp
221 IPv6 addresses can be specified with an alternative syntax:
222 .Sm off
223 .Xo
224 .Op Ar bind_address No /
225 .Ar port
226 .Xc
227 .Sm on
228 or by enclosing the address in square brackets.
229 Only the superuser can forward privileged ports.
230 By default, the local port is bound in accordance with the
231 .Cm GatewayPorts
232 setting.
233 However, an explicit
234 .Ar bind_address
235 may be used to bind the connection to a specific address.
236 The
237 .Ar bind_address
238 of
239 .Dq localhost
240 indicates that the listening port be bound for local use only, while an
241 empty address or
242 .Sq *
243 indicates that the port should be available from all interfaces.
244 .It Fl e Ar escape_char
245 Sets the escape character for sessions with a pty (default:
246 .Ql ~ ) .
247 The escape character is only recognized at the beginning of a line.
248 The escape character followed by a dot
249 .Pq Ql \&.
250 closes the connection;
251 followed by control-Z suspends the connection;
252 and followed by itself sends the escape character once.
253 Setting the character to
254 .Dq none
255 disables any escapes and makes the session fully transparent.
256 .It Fl F Ar configfile
257 Specifies an alternative per-user configuration file.
258 If a configuration file is given on the command line,
259 the system-wide configuration file
260 .Pq Pa /etc/ssh/ssh_config
261 will be ignored.
262 The default for the per-user configuration file is
263 .Pa ~/.ssh/config .
264 .It Fl f
265 Requests
266 .Nm
267 to go to background just before command execution.
268 This is useful if
269 .Nm
270 is going to ask for passwords or passphrases, but the user
271 wants it in the background.
272 This implies
273 .Fl n .
274 The recommended way to start X11 programs at a remote site is with
275 something like
276 .Ic ssh -f host xterm .
277 .Pp
278 If the
279 .Cm ExitOnForwardFailure
280 configuration option is set to
281 .Dq yes ,
282 then a client started with
283 .Fl f
284 will wait for all remote port forwards to be successfully established
285 before placing itself in the background.
286 .It Fl g
287 Allows remote hosts to connect to local forwarded ports.
288 .It Fl I Ar pkcs11
289 Specify the PKCS#11 shared library
290 .Nm
291 should use to communicate with a PKCS#11 token providing the user's
292 private RSA key.
293 .It Fl i Ar identity_file
294 Selects a file from which the identity (private key) for
295 RSA or DSA authentication is read.
296 The default is
297 .Pa ~/.ssh/identity
298 for protocol version 1, and
299 .Pa ~/.ssh/id_rsa
300 and
301 .Pa ~/.ssh/id_dsa
302 for protocol version 2.
303 Identity files may also be specified on
304 a per-host basis in the configuration file.
305 It is possible to have multiple
306 .Fl i
307 options (and multiple identities specified in
308 configuration files).
309 .It Fl K
310 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
311 credentials to the server.
312 .It Fl k
313 Disables forwarding (delegation) of GSSAPI credentials to the server.
314 .It Fl L Xo
315 .Sm off
316 .Oo Ar bind_address : Oc
317 .Ar port : host : hostport
318 .Sm on
319 .Xc
320 Specifies that the given port on the local (client) host is to be
321 forwarded to the given host and port on the remote side.
322 This works by allocating a socket to listen to
323 .Ar port
324 on the local side, optionally bound to the specified
325 .Ar bind_address .
326 Whenever a connection is made to this port, the
327 connection is forwarded over the secure channel, and a connection is
328 made to
329 .Ar host
330 port
331 .Ar hostport
332 from the remote machine.
333 Port forwardings can also be specified in the configuration file.
334 IPv6 addresses can be specified with an alternative syntax:
335 .Sm off
336 .Xo
337 .Op Ar bind_address No /
338 .Ar port No / Ar host No /
339 .Ar hostport
340 .Xc
341 .Sm on
342 or by enclosing the address in square brackets.
343 Only the superuser can forward privileged ports.
344 By default, the local port is bound in accordance with the
345 .Cm GatewayPorts
346 setting.
347 However, an explicit
348 .Ar bind_address
349 may be used to bind the connection to a specific address.
350 The
351 .Ar bind_address
352 of
353 .Dq localhost
354 indicates that the listening port be bound for local use only, while an
355 empty address or
356 .Sq *
357 indicates that the port should be available from all interfaces.
358 .It Fl l Ar login_name
359 Specifies the user to log in as on the remote machine.
360 This also may be specified on a per-host basis in the configuration file.
361 .It Fl M
362 Places the
363 .Nm
364 client into
365 .Dq master
366 mode for connection sharing.
367 Multiple
368 .Fl M
369 options places
370 .Nm
371 into
372 .Dq master
373 mode with confirmation required before slave connections are accepted.
374 Refer to the description of
375 .Cm ControlMaster
376 in
377 .Xr ssh_config 5
378 for details.
379 .It Fl m Ar mac_spec
380 Additionally, for protocol version 2 a comma-separated list of MAC
381 (message authentication code) algorithms can
382 be specified in order of preference.
383 See the
384 .Cm MACs
385 keyword for more information.
386 .It Fl N
387 Do not execute a remote command.
388 This is useful for just forwarding ports
389 (protocol version 2 only).
390 .It Fl n
391 Redirects stdin from
392 .Pa /dev/null
393 (actually, prevents reading from stdin).
394 This must be used when
395 .Nm
396 is run in the background.
397 A common trick is to use this to run X11 programs on a remote machine.
398 For example,
399 .Ic ssh -n shadows.cs.hut.fi emacs &
400 will start an emacs on shadows.cs.hut.fi, and the X11
401 connection will be automatically forwarded over an encrypted channel.
402 The
403 .Nm
404 program will be put in the background.
405 (This does not work if
406 .Nm
407 needs to ask for a password or passphrase; see also the
408 .Fl f
409 option.)
410 .It Fl O Ar ctl_cmd
411 Control an active connection multiplexing master process.
412 When the
413 .Fl O
414 option is specified, the
415 .Ar ctl_cmd
416 argument is interpreted and passed to the master process.
417 Valid commands are:
418 .Dq check
419 (check that the master process is running) and
420 .Dq exit
421 (request the master to exit).
422 .It Fl o Ar option
423 Can be used to give options in the format used in the configuration file.
424 This is useful for specifying options for which there is no separate
425 command-line flag.
426 For full details of the options listed below, and their possible values, see
427 .Xr ssh_config 5 .
428 .Pp
429 .Bl -tag -width Ds -offset indent -compact
430 .It AddressFamily
431 .It BatchMode
432 .It BindAddress
433 .It ChallengeResponseAuthentication
434 .It CheckHostIP
435 .It Cipher
436 .It Ciphers
437 .It ClearAllForwardings
438 .It Compression
439 .It CompressionLevel
440 .It ConnectionAttempts
441 .It ConnectTimeout
442 .It ControlMaster
443 .It ControlPath
444 .It DynamicForward
445 .It EscapeChar
446 .It ExitOnForwardFailure
447 .It ForwardAgent
448 .It ForwardX11
449 .It ForwardX11Trusted
450 .It GatewayPorts
451 .It GlobalKnownHostsFile
452 .It GSSAPIAuthentication
453 .It GSSAPIDelegateCredentials
454 .It HashKnownHosts
455 .It Host
456 .It HostbasedAuthentication
457 .It HostKeyAlgorithms
458 .It HostKeyAlias
459 .It HostName
460 .It IdentityFile
461 .It IdentitiesOnly
462 .It KbdInteractiveDevices
463 .It LocalCommand
464 .It LocalForward
465 .It LogLevel
466 .It MACs
467 .It NoHostAuthenticationForLocalhost
468 .It NumberOfPasswordPrompts
469 .It PasswordAuthentication
470 .It PermitLocalCommand
471 .It PKCS11Provider
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 StrictHostKeyChecking
485 .It TCPKeepAlive
486 .It Tunnel
487 .It TunnelDevice
488 .It UsePrivilegedPort
489 .It User
490 .It UserKnownHostsFile
491 .It VerifyHostKeyDNS
492 .It VisualHostKey
493 .It XAuthLocation
494 .El
495 .It Fl p Ar port
496 Port to connect to on the remote host.
497 This can be specified on a
498 per-host basis in the configuration file.
499 .It Fl q
500 Quiet mode.
501 Causes most warning and diagnostic messages to be suppressed.
502 .It Fl R Xo
503 .Sm off
504 .Oo Ar bind_address : Oc
505 .Ar port : host : hostport
506 .Sm on
507 .Xc
508 Specifies that the given port on the remote (server) host is to be
509 forwarded to the given host and port on the local side.
510 This works by allocating a socket to listen to
511 .Ar port
512 on the remote side, and whenever a connection is made to this port, the
513 connection is forwarded over the secure channel, and a connection is
514 made to
515 .Ar host
516 port
517 .Ar hostport
518 from the local machine.
519 .Pp
520 Port forwardings can also be specified in the configuration file.
521 Privileged ports can be forwarded only when
522 logging in as root on the remote machine.
523 IPv6 addresses can be specified by enclosing the address in square braces or
524 using an alternative syntax:
525 .Sm off
526 .Xo
527 .Op Ar bind_address No /
528 .Ar host No / Ar port No /
529 .Ar hostport
530 .Xc .
531 .Sm on
532 .Pp
533 By default, the listening socket on the server will be bound to the loopback
534 interface only.
535 This may be overridden by specifying a
536 .Ar bind_address .
537 An empty
538 .Ar bind_address ,
539 or the address
540 .Ql * ,
541 indicates that the remote socket should listen on all interfaces.
542 Specifying a remote
543 .Ar bind_address
544 will only succeed if the server's
545 .Cm GatewayPorts
546 option is enabled (see
547 .Xr sshd_config 5 ) .
548 .Pp
549 If the
550 .Ar port
551 argument is
552 .Ql 0 ,
553 the listen port will be dynamically allocated on the server and reported
554 to the client at run time.
555 .It Fl S Ar ctl_path
556 Specifies the location of a control socket for connection sharing.
557 Refer to the description of
558 .Cm ControlPath
559 and
560 .Cm ControlMaster
561 in
562 .Xr ssh_config 5
563 for details.
564 .It Fl s
565 May be used to request invocation of a subsystem on the remote system.
566 Subsystems are a feature of the SSH2 protocol which facilitate the use
567 of SSH as a secure transport for other applications (eg.\&
568 .Xr sftp 1 ) .
569 The subsystem is specified as the remote command.
570 .It Fl T
571 Disable pseudo-tty allocation.
572 .It Fl t
573 Force pseudo-tty allocation.
574 This can be used to execute arbitrary
575 screen-based programs on a remote machine, which can be very useful,
576 e.g. when implementing menu services.
577 Multiple
578 .Fl t
579 options force tty allocation, even if
580 .Nm
581 has no local tty.
582 .It Fl V
583 Display the version number and exit.
584 .It Fl v
585 Verbose mode.
586 Causes
587 .Nm
588 to print debugging messages about its progress.
589 This is helpful in
590 debugging connection, authentication, and configuration problems.
591 Multiple
592 .Fl v
593 options increase the verbosity.
594 The maximum is 3.
595 .It Fl W Ar host : Ns Ar port
596 Requests that standard input and output on the client be forwarded to
597 .Ar host
598 on
599 .Ar port
600 over the secure channel.
601 Implies
602 .Fl N ,
603 .Fl T ,
604 .Cm ExitOnForwardFailure
605 and
606 .Cm ClearAllForwardings
607 and works with Protocol version 2 only.
608 .It Fl w Xo
609 .Ar local_tun Ns Op : Ns Ar remote_tun
610 .Xc
611 Requests
612 tunnel
613 device forwarding with the specified
614 .Xr tun 4
615 devices between the client
616 .Pq Ar local_tun
617 and the server
618 .Pq Ar remote_tun .
619 .Pp
620 The devices may be specified by numerical ID or the keyword
621 .Dq any ,
622 which uses the next available tunnel device.
623 If
624 .Ar remote_tun
625 is not specified, it defaults to
626 .Dq any .
627 See also the
628 .Cm Tunnel
629 and
630 .Cm TunnelDevice
631 directives in
632 .Xr ssh_config 5 .
633 If the
634 .Cm Tunnel
635 directive is unset, it is set to the default tunnel mode, which is
636 .Dq point-to-point .
637 .It Fl X
638 Enables X11 forwarding.
639 This can also be specified on a per-host basis in a configuration file.
640 .Pp
641 X11 forwarding should be enabled with caution.
642 Users with the ability to bypass file permissions on the remote host
643 (for the user's X authorization database)
644 can access the local X11 display through the forwarded connection.
645 An attacker may then be able to perform activities such as keystroke monitoring.
646 .Pp
647 For this reason, X11 forwarding is subjected to X11 SECURITY extension
648 restrictions by default.
649 Please refer to the
650 .Nm
651 .Fl Y
652 option and the
653 .Cm ForwardX11Trusted
654 directive in
655 .Xr ssh_config 5
656 for more information.
657 .It Fl x
658 Disables X11 forwarding.
659 .It Fl Y
660 Enables trusted X11 forwarding.
661 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
662 controls.
663 .It Fl y
664 Send log information using the
665 .Xr syslog 3
666 system module.
667 By default this information is sent to stderr.
668 .El
669 .Pp
670 .Nm
671 may additionally obtain configuration data from
672 a per-user configuration file and a system-wide configuration file.
673 The file format and configuration options are described in
674 .Xr ssh_config 5 .
675 .Pp
676 .Nm
677 exits with the exit status of the remote command or with 255
678 if an error occurred.
679 .Sh AUTHENTICATION
680 The OpenSSH SSH client supports SSH protocols 1 and 2.
681 The default is to use protocol 2 only,
682 though this can be changed via the
683 .Cm Protocol
684 option in
685 .Xr ssh_config 5
686 or the
687 .Fl 1
688 and
689 .Fl 2
690 options (see above).
691 Both protocols support similar authentication methods,
692 but protocol 2 is the default since
693 it provides additional mechanisms for confidentiality
694 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
695 and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
696 Protocol 1 lacks a strong mechanism for ensuring the
697 integrity of the connection.
698 .Pp
699 The methods available for authentication are:
700 GSSAPI-based authentication,
701 host-based authentication,
702 public key authentication,
703 challenge-response authentication,
704 and password authentication.
705 Authentication methods are tried in the order specified above,
706 though protocol 2 has a configuration option to change the default order:
707 .Cm PreferredAuthentications .
708 .Pp
709 Host-based authentication works as follows:
710 If the machine the user logs in from is listed in
711 .Pa /etc/hosts.equiv
712 or
713 .Pa /etc/shosts.equiv
714 on the remote machine, and the user names are
715 the same on both sides, or if the files
716 .Pa ~/.rhosts
717 or
718 .Pa ~/.shosts
719 exist in the user's home directory on the
720 remote machine and contain a line containing the name of the client
721 machine and the name of the user on that machine, the user is
722 considered for login.
723 Additionally, the server
724 .Em must
725 be able to verify the client's
726 host key (see the description of
727 .Pa /etc/ssh/ssh_known_hosts
728 and
729 .Pa ~/.ssh/known_hosts ,
730 below)
731 for login to be permitted.
732 This authentication method closes security holes due to IP
733 spoofing, DNS spoofing, and routing spoofing.
734 [Note to the administrator:
735 .Pa /etc/hosts.equiv ,
736 .Pa ~/.rhosts ,
737 and the rlogin/rsh protocol in general, are inherently insecure and should be
738 disabled if security is desired.]
739 .Pp
740 Public key authentication works as follows:
741 The scheme is based on public-key cryptography,
742 using cryptosystems
743 where encryption and decryption are done using separate keys,
744 and it is unfeasible to derive the decryption key from the encryption key.
745 The idea is that each user creates a public/private
746 key pair for authentication purposes.
747 The server knows the public key, and only the user knows the private key.
748 .Nm
749 implements public key authentication protocol automatically,
750 using either the RSA or DSA algorithms.
751 Protocol 1 is restricted to using only RSA keys,
752 but protocol 2 may use either.
753 The
754 .Sx HISTORY
755 section of
756 .Xr ssl 8
757 contains a brief discussion of the two algorithms.
758 .Pp
759 The file
760 .Pa ~/.ssh/authorized_keys
761 lists the public keys that are permitted for logging in.
762 When the user logs in, the
763 .Nm
764 program tells the server which key pair it would like to use for
765 authentication.
766 The client proves that it has access to the private key
767 and the server checks that the corresponding public key
768 is authorized to accept the account.
769 .Pp
770 The user creates his/her key pair by running
771 .Xr ssh-keygen 1 .
772 This stores the private key in
773 .Pa ~/.ssh/identity
774 (protocol 1),
775 .Pa ~/.ssh/id_dsa
776 (protocol 2 DSA),
777 or
778 .Pa ~/.ssh/id_rsa
779 (protocol 2 RSA)
780 and stores the public key in
781 .Pa ~/.ssh/identity.pub
782 (protocol 1),
783 .Pa ~/.ssh/id_dsa.pub
784 (protocol 2 DSA),
785 or
786 .Pa ~/.ssh/id_rsa.pub
787 (protocol 2 RSA)
788 in the user's home directory.
789 The user should then copy the public key
790 to
791 .Pa ~/.ssh/authorized_keys
792 in his/her home directory on the remote machine.
793 The
794 .Pa authorized_keys
795 file corresponds to the conventional
796 .Pa ~/.rhosts
797 file, and has one key
798 per line, though the lines can be very long.
799 After this, the user can log in without giving the password.
800 .Pp
801 The most convenient way to use public key authentication may be with an
802 authentication agent.
803 See
804 .Xr ssh-agent 1
805 for more information.
806 .Pp
807 Challenge-response authentication works as follows:
808 The server sends an arbitrary
809 .Qq challenge
810 text, and prompts for a response.
811 Protocol 2 allows multiple challenges and responses;
812 protocol 1 is restricted to just one challenge/response.
813 Examples of challenge-response authentication include
814 BSD Authentication (see
815 .Xr login.conf 5 )
816 and PAM (some non-OpenBSD systems).
817 .Pp
818 Finally, if other authentication methods fail,
819 .Nm
820 prompts the user for a password.
821 The password is sent to the remote
822 host for checking; however, since all communications are encrypted,
823 the password cannot be seen by someone listening on the network.
824 .Pp
825 .Nm
826 automatically maintains and checks a database containing
827 identification for all hosts it has ever been used with.
828 Host keys are stored in
829 .Pa ~/.ssh/known_hosts
830 in the user's home directory.
831 Additionally, the file
832 .Pa /etc/ssh/ssh_known_hosts
833 is automatically checked for known hosts.
834 Any new hosts are automatically added to the user's file.
835 If a host's identification ever changes,
836 .Nm
837 warns about this and disables password authentication to prevent
838 server spoofing or man-in-the-middle attacks,
839 which could otherwise be used to circumvent the encryption.
840 The
841 .Cm StrictHostKeyChecking
842 option can be used to control logins to machines whose
843 host key is not known or has changed.
844 .Pp
845 When the user's identity has been accepted by the server, the server
846 either executes the given command, or logs into the machine and gives
847 the user a normal shell on the remote machine.
848 All communication with
849 the remote command or shell will be automatically encrypted.
850 .Pp
851 If a pseudo-terminal has been allocated (normal login session), the
852 user may use the escape characters noted below.
853 .Pp
854 If no pseudo-tty has been allocated,
855 the session is transparent and can be used to reliably transfer binary data.
856 On most systems, setting the escape character to
857 .Dq none
858 will also make the session transparent even if a tty is used.
859 .Pp
860 The session terminates when the command or shell on the remote
861 machine exits and all X11 and TCP connections have been closed.
862 .Sh ESCAPE CHARACTERS
863 When a pseudo-terminal has been requested,
864 .Nm
865 supports a number of functions through the use of an escape character.
866 .Pp
867 A single tilde character can be sent as
868 .Ic ~~
869 or by following the tilde by a character other than those described below.
870 The escape character must always follow a newline to be interpreted as
871 special.
872 The escape character can be changed in configuration files using the
873 .Cm EscapeChar
874 configuration directive or on the command line by the
875 .Fl e
876 option.
877 .Pp
878 The supported escapes (assuming the default
879 .Ql ~ )
880 are:
881 .Bl -tag -width Ds
882 .It Cm ~.
883 Disconnect.
884 .It Cm ~^Z
885 Background
886 .Nm .
887 .It Cm ~#
888 List forwarded connections.
889 .It Cm ~&
890 Background
891 .Nm
892 at logout when waiting for forwarded connection / X11 sessions to terminate.
893 .It Cm ~?
894 Display a list of escape characters.
895 .It Cm ~B
896 Send a BREAK to the remote system
897 (only useful for SSH protocol version 2 and if the peer supports it).
898 .It Cm ~C
899 Open command line.
900 Currently this allows the addition of port forwardings using the
901 .Fl L ,
902 .Fl R
903 and
904 .Fl D
905 options (see above).
906 It also allows the cancellation of existing remote port-forwardings
907 using
908 .Sm off
909 .Fl KR Oo Ar bind_address : Oc Ar port .
910 .Sm on
911 .Ic !\& Ns Ar command
912 allows the user to execute a local command if the
913 .Ic PermitLocalCommand
914 option is enabled in
915 .Xr ssh_config 5 .
916 Basic help is available, using the
917 .Fl h
918 option.
919 .It Cm ~R
920 Request rekeying of the connection
921 (only useful for SSH protocol version 2 and if the peer supports it).
922 .El
923 .Sh TCP FORWARDING
924 Forwarding of arbitrary TCP connections over the secure channel can
925 be specified either on the command line or in a configuration file.
926 One possible application of TCP forwarding is a secure connection to a
927 mail server; another is going through firewalls.
928 .Pp
929 In the example below, we look at encrypting communication between
930 an IRC client and server, even though the IRC server does not directly
931 support encrypted communications.
932 This works as follows:
933 the user connects to the remote host using
934 .Nm ,
935 specifying a port to be used to forward connections
936 to the remote server.
937 After that it is possible to start the service which is to be encrypted
938 on the client machine,
939 connecting to the same local port,
940 and
941 .Nm
942 will encrypt and forward the connection.
943 .Pp
944 The following example tunnels an IRC session from client machine
945 .Dq 127.0.0.1
946 (localhost)
947 to remote server
948 .Dq server.example.com :
949 .Bd -literal -offset 4n
950 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
951 $ irc -c '#users' -p 1234 pinky 127.0.0.1
952 .Ed
953 .Pp
954 This tunnels a connection to IRC server
955 .Dq server.example.com ,
956 joining channel
957 .Dq #users ,
958 nickname
959 .Dq pinky ,
960 using port 1234.
961 It doesn't matter which port is used,
962 as long as it's greater than 1023
963 (remember, only root can open sockets on privileged ports)
964 and doesn't conflict with any ports already in use.
965 The connection is forwarded to port 6667 on the remote server,
966 since that's the standard port for IRC services.
967 .Pp
968 The
969 .Fl f
970 option backgrounds
971 .Nm
972 and the remote command
973 .Dq sleep 10
974 is specified to allow an amount of time
975 (10 seconds, in the example)
976 to start the service which is to be tunnelled.
977 If no connections are made within the time specified,
978 .Nm
979 will exit.
980 .Sh X11 FORWARDING
981 If the
982 .Cm ForwardX11
983 variable is set to
984 .Dq yes
985 (or see the description of the
986 .Fl X ,
987 .Fl x ,
988 and
989 .Fl Y
990 options above)
991 and the user is using X11 (the
992 .Ev DISPLAY
993 environment variable is set), the connection to the X11 display is
994 automatically forwarded to the remote side in such a way that any X11
995 programs started from the shell (or command) will go through the
996 encrypted channel, and the connection to the real X server will be made
997 from the local machine.
998 The user should not manually set
999 .Ev DISPLAY .
1000 Forwarding of X11 connections can be
1001 configured on the command line or in configuration files.
1002 .Pp
1003 The
1004 .Ev DISPLAY
1005 value set by
1006 .Nm
1007 will point to the server machine, but with a display number greater than zero.
1008 This is normal, and happens because
1009 .Nm
1010 creates a
1011 .Dq proxy
1012 X server on the server machine for forwarding the
1013 connections over the encrypted channel.
1014 .Pp
1015 .Nm
1016 will also automatically set up Xauthority data on the server machine.
1017 For this purpose, it will generate a random authorization cookie,
1018 store it in Xauthority on the server, and verify that any forwarded
1019 connections carry this cookie and replace it by the real cookie when
1020 the connection is opened.
1021 The real authentication cookie is never
1022 sent to the server machine (and no cookies are sent in the plain).
1023 .Pp
1024 If the
1025 .Cm ForwardAgent
1026 variable is set to
1027 .Dq yes
1028 (or see the description of the
1029 .Fl A
1030 and
1031 .Fl a
1032 options above) and
1033 the user is using an authentication agent, the connection to the agent
1034 is automatically forwarded to the remote side.
1035 .Sh VERIFYING HOST KEYS
1036 When connecting to a server for the first time,
1037 a fingerprint of the server's public key is presented to the user
1038 (unless the option
1039 .Cm StrictHostKeyChecking
1040 has been disabled).
1041 Fingerprints can be determined using
1042 .Xr ssh-keygen 1 :
1043 .Pp
1044 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1045 .Pp
1046 If the fingerprint is already known, it can be matched
1047 and the key can be accepted or rejected.
1048 Because of the difficulty of comparing host keys
1049 just by looking at hex strings,
1050 there is also support to compare host keys visually,
1051 using
1052 .Em random art .
1053 By setting the
1054 .Cm VisualHostKey
1055 option to
1056 .Dq yes ,
1057 a small ASCII graphic gets displayed on every login to a server, no matter
1058 if the session itself is interactive or not.
1059 By learning the pattern a known server produces, a user can easily
1060 find out that the host key has changed when a completely different pattern
1061 is displayed.
1062 Because these patterns are not unambiguous however, a pattern that looks
1063 similar to the pattern remembered only gives a good probability that the
1064 host key is the same, not guaranteed proof.
1065 .Pp
1066 To get a listing of the fingerprints along with their random art for
1067 all known hosts, the following command line can be used:
1068 .Pp
1069 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1070 .Pp
1071 If the fingerprint is unknown,
1072 an alternative method of verification is available:
1073 SSH fingerprints verified by DNS.
1074 An additional resource record (RR),
1075 SSHFP,
1076 is added to a zonefile
1077 and the connecting client is able to match the fingerprint
1078 with that of the key presented.
1079 .Pp
1080 In this example, we are connecting a client to a server,
1081 .Dq host.example.com .
1082 The SSHFP resource records should first be added to the zonefile for
1083 host.example.com:
1084 .Bd -literal -offset indent
1085 $ ssh-keygen -r host.example.com.
1086 .Ed
1087 .Pp
1088 The output lines will have to be added to the zonefile.
1089 To check that the zone is answering fingerprint queries:
1090 .Pp
1091 .Dl $ dig -t SSHFP host.example.com
1092 .Pp
1093 Finally the client connects:
1094 .Bd -literal -offset indent
1095 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1096 [...]
1097 Matching host key fingerprint found in DNS.
1098 Are you sure you want to continue connecting (yes/no)?
1099 .Ed
1100 .Pp
1101 See the
1102 .Cm VerifyHostKeyDNS
1103 option in
1104 .Xr ssh_config 5
1105 for more information.
1106 .Pp
1107 Host keys may also be presented as certificates signed by a trusted
1108 certification authority (CA).
1109 In this case, trust of the CA key alone is sufficient for the host key
1110 to be accepted.
1111 To specify a public key as a trusted CA key in a known hosts file,
1112 it should be added after a
1113 .Dq @cert-authority
1114 tag and a set of one or more domain-name wildcards separated by commas.
1115 For example:
1116 .Pp
1117 .Dl @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W...
1118 .Pp
1119 See the
1120 .Sx CERTIFICATES
1121 section of
1122 .Xr ssh-keygen 1
1123 for more details.
1124 .Pp
1125 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1126 .Nm
1127 contains support for Virtual Private Network (VPN) tunnelling
1128 using the
1129 .Xr tun 4
1130 network pseudo-device,
1131 allowing two networks to be joined securely.
1132 The
1133 .Xr sshd_config 5
1134 configuration option
1135 .Cm PermitTunnel
1136 controls whether the server supports this,
1137 and at what level (layer 2 or 3 traffic).
1138 .Pp
1139 The following example would connect client network 10.0.50.0/24
1140 with remote network 10.0.99.0/24 using a point-to-point connection
1141 from 10.1.1.1 to 10.1.1.2,
1142 provided that the SSH server running on the gateway to the remote network,
1143 at 192.168.1.15, allows it.
1144 .Pp
1145 On the client:
1146 .Bd -literal -offset indent
1147 # ssh -f -w 0:1 192.168.1.15 true
1148 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1149 # route add 10.0.99.0/24 10.1.1.2
1150 .Ed
1151 .Pp
1152 On the server:
1153 .Bd -literal -offset indent
1154 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1155 # route add 10.0.50.0/24 10.1.1.1
1156 .Ed
1157 .Pp
1158 Client access may be more finely tuned via the
1159 .Pa /root/.ssh/authorized_keys
1160 file (see below) and the
1161 .Cm PermitRootLogin
1162 server option.
1163 The following entry would permit connections on
1164 .Xr tun 4
1165 device 1 from user
1166 .Dq jane
1167 and on tun device 2 from user
1168 .Dq john ,
1169 if
1170 .Cm PermitRootLogin
1171 is set to
1172 .Dq forced-commands-only :
1173 .Bd -literal -offset 2n
1174 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1175 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1176 .Ed
1177 .Pp
1178 Since an SSH-based setup entails a fair amount of overhead,
1179 it may be more suited to temporary setups,
1180 such as for wireless VPNs.
1181 More permanent VPNs are better provided by tools such as
1182 .Xr ipsecctl 8
1183 and
1184 .Xr isakmpd 8 .
1185 .Sh ENVIRONMENT
1186 .Nm
1187 will normally set the following environment variables:
1188 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1189 .It Ev DISPLAY
1190 The
1191 .Ev DISPLAY
1192 variable indicates the location of the X11 server.
1193 It is automatically set by
1194 .Nm
1195 to point to a value of the form
1196 .Dq hostname:n ,
1197 where
1198 .Dq hostname
1199 indicates the host where the shell runs, and
1200 .Sq n
1201 is an integer \*(Ge 1.
1202 .Nm
1203 uses this special value to forward X11 connections over the secure
1204 channel.
1205 The user should normally not set
1206 .Ev DISPLAY
1207 explicitly, as that
1208 will render the X11 connection insecure (and will require the user to
1209 manually copy any required authorization cookies).
1210 .It Ev HOME
1211 Set to the path of the user's home directory.
1212 .It Ev LOGNAME
1213 Synonym for
1214 .Ev USER ;
1215 set for compatibility with systems that use this variable.
1216 .It Ev MAIL
1217 Set to the path of the user's mailbox.
1218 .It Ev PATH
1219 Set to the default
1220 .Ev PATH ,
1221 as specified when compiling
1222 .Nm .
1223 .It Ev SSH_ASKPASS
1224 If
1225 .Nm
1226 needs a passphrase, it will read the passphrase from the current
1227 terminal if it was run from a terminal.
1228 If
1229 .Nm
1230 does not have a terminal associated with it but
1231 .Ev DISPLAY
1232 and
1233 .Ev SSH_ASKPASS
1234 are set, it will execute the program specified by
1235 .Ev SSH_ASKPASS
1236 and open an X11 window to read the passphrase.
1237 This is particularly useful when calling
1238 .Nm
1239 from a
1240 .Pa .xsession
1241 or related script.
1242 (Note that on some machines it
1243 may be necessary to redirect the input from
1244 .Pa /dev/null
1245 to make this work.)
1246 .It Ev SSH_AUTH_SOCK
1247 Identifies the path of a
1248 .Ux Ns -domain
1249 socket used to communicate with the agent.
1250 .It Ev SSH_CONNECTION
1251 Identifies the client and server ends of the connection.
1252 The variable contains
1253 four space-separated values: client IP address, client port number,
1254 server IP address, and server port number.
1255 .It Ev SSH_ORIGINAL_COMMAND
1256 This variable contains the original command line if a forced command
1257 is executed.
1258 It can be used to extract the original arguments.
1259 .It Ev SSH_TTY
1260 This is set to the name of the tty (path to the device) associated
1261 with the current shell or command.
1262 If the current session has no tty,
1263 this variable is not set.
1264 .It Ev TZ
1265 This variable is set to indicate the present time zone if it
1266 was set when the daemon was started (i.e. the daemon passes the value
1267 on to new connections).
1268 .It Ev USER
1269 Set to the name of the user logging in.
1270 .El
1271 .Pp
1272 Additionally,
1273 .Nm
1274 reads
1275 .Pa ~/.ssh/environment ,
1276 and adds lines of the format
1277 .Dq VARNAME=value
1278 to the environment if the file exists and users are allowed to
1279 change their environment.
1280 For more information, see the
1281 .Cm PermitUserEnvironment
1282 option in
1283 .Xr sshd_config 5 .
1284 .Sh FILES
1285 .Bl -tag -width Ds -compact
1286 .It ~/.rhosts
1287 This file is used for host-based authentication (see above).
1288 On some machines this file may need to be
1289 world-readable if the user's home directory is on an NFS partition,
1290 because
1291 .Xr sshd 8
1292 reads it as root.
1293 Additionally, this file must be owned by the user,
1294 and must not have write permissions for anyone else.
1295 The recommended
1296 permission for most machines is read/write for the user, and not
1297 accessible by others.
1298 .Pp
1299 .It ~/.shosts
1300 This file is used in exactly the same way as
1301 .Pa .rhosts ,
1302 but allows host-based authentication without permitting login with
1303 rlogin/rsh.
1304 .Pp
1305 .It ~/.ssh/
1306 This directory is the default location for all user-specific configuration
1307 and authentication information.
1308 There is no general requirement to keep the entire contents of this directory
1309 secret, but the recommended permissions are read/write/execute for the user,
1310 and not accessible by others.
1311 .Pp
1312 .It ~/.ssh/authorized_keys
1313 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1314 The format of this file is described in the
1315 .Xr sshd 8
1316 manual page.
1317 This file is not highly sensitive, but the recommended
1318 permissions are read/write for the user, and not accessible by others.
1319 .Pp
1320 .It ~/.ssh/config
1321 This is the per-user configuration file.
1322 The file format and configuration options are described in
1323 .Xr ssh_config 5 .
1324 Because of the potential for abuse, this file must have strict permissions:
1325 read/write for the user, and not accessible by others.
1326 .Pp
1327 .It ~/.ssh/environment
1328 Contains additional definitions for environment variables; see
1329 .Sx ENVIRONMENT ,
1330 above.
1331 .Pp
1332 .It ~/.ssh/identity
1333 .It ~/.ssh/id_dsa
1334 .It ~/.ssh/id_rsa
1335 Contains the private key for authentication.
1336 These files
1337 contain sensitive data and should be readable by the user but not
1338 accessible by others (read/write/execute).
1339 .Nm
1340 will simply ignore a private key file if it is accessible by others.
1341 It is possible to specify a passphrase when
1342 generating the key which will be used to encrypt the
1343 sensitive part of this file using 3DES.
1344 .Pp
1345 .It ~/.ssh/identity.pub
1346 .It ~/.ssh/id_dsa.pub
1347 .It ~/.ssh/id_rsa.pub
1348 Contains the public key for authentication.
1349 These files are not
1350 sensitive and can (but need not) be readable by anyone.
1351 .Pp
1352 .It ~/.ssh/known_hosts
1353 Contains a list of host keys for all hosts the user has logged into
1354 that are not already in the systemwide list of known host keys.
1355 See
1356 .Xr sshd 8
1357 for further details of the format of this file.
1358 .Pp
1359 .It ~/.ssh/rc
1360 Commands in this file are executed by
1361 .Nm
1362 when the user logs in, just before the user's shell (or command) is
1363 started.
1364 See the
1365 .Xr sshd 8
1366 manual page for more information.
1367 .Pp
1368 .It /etc/hosts.equiv
1369 This file is for host-based authentication (see above).
1370 It should only be writable by root.
1371 .Pp
1372 .It /etc/shosts.equiv
1373 This file is used in exactly the same way as
1374 .Pa hosts.equiv ,
1375 but allows host-based authentication without permitting login with
1376 rlogin/rsh.
1377 .Pp
1378 .It Pa /etc/ssh/ssh_config
1379 Systemwide configuration file.
1380 The file format and configuration options are described in
1381 .Xr ssh_config 5 .
1382 .Pp
1383 .It /etc/ssh/ssh_host_key
1384 .It /etc/ssh/ssh_host_dsa_key
1385 .It /etc/ssh/ssh_host_rsa_key
1386 These three files contain the private parts of the host keys
1387 and are used for host-based authentication.
1388 If protocol version 1 is used,
1389 .Nm
1390 must be setuid root, since the host key is readable only by root.
1391 For protocol version 2,
1392 .Nm
1393 uses
1394 .Xr ssh-keysign 8
1395 to access the host keys,
1396 eliminating the requirement that
1397 .Nm
1398 be setuid root when host-based authentication is used.
1399 By default
1400 .Nm
1401 is not setuid root.
1402 .Pp
1403 .It /etc/ssh/ssh_known_hosts
1404 Systemwide list of known host keys.
1405 This file should be prepared by the
1406 system administrator to contain the public host keys of all machines in the
1407 organization.
1408 It should be world-readable.
1409 See
1410 .Xr sshd 8
1411 for further details of the format of this file.
1412 .Pp
1413 .It /etc/ssh/sshrc
1414 Commands in this file are executed by
1415 .Nm
1416 when the user logs in, just before the user's shell (or command) is started.
1417 See the
1418 .Xr sshd 8
1419 manual page for more information.
1420 .El
1421 .Sh SEE ALSO
1422 .Xr scp 1 ,
1423 .Xr sftp 1 ,
1424 .Xr ssh-add 1 ,
1425 .Xr ssh-agent 1 ,
1426 .Xr ssh-keygen 1 ,
1427 .Xr ssh-keyscan 1 ,
1428 .Xr tun 4 ,
1429 .Xr hosts.equiv 5 ,
1430 .Xr ssh_config 5 ,
1431 .Xr ssh-keysign 8 ,
1432 .Xr sshd 8
1433 .Rs
1434 .%R RFC 4250
1435 .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1436 .%D 2006
1437 .Re
1438 .Rs
1439 .%R RFC 4251
1440 .%T "The Secure Shell (SSH) Protocol Architecture"
1441 .%D 2006
1442 .Re
1443 .Rs
1444 .%R RFC 4252
1445 .%T "The Secure Shell (SSH) Authentication Protocol"
1446 .%D 2006
1447 .Re
1448 .Rs
1449 .%R RFC 4253
1450 .%T "The Secure Shell (SSH) Transport Layer Protocol"
1451 .%D 2006
1452 .Re
1453 .Rs
1454 .%R RFC 4254
1455 .%T "The Secure Shell (SSH) Connection Protocol"
1456 .%D 2006
1457 .Re
1458 .Rs
1459 .%R RFC 4255
1460 .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1461 .%D 2006
1462 .Re
1463 .Rs
1464 .%R RFC 4256
1465 .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1466 .%D 2006
1467 .Re
1468 .Rs
1469 .%R RFC 4335
1470 .%T "The Secure Shell (SSH) Session Channel Break Extension"
1471 .%D 2006
1472 .Re
1473 .Rs
1474 .%R RFC 4344
1475 .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1476 .%D 2006
1477 .Re
1478 .Rs
1479 .%R RFC 4345
1480 .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1481 .%D 2006
1482 .Re
1483 .Rs
1484 .%R RFC 4419
1485 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1486 .%D 2006
1487 .Re
1488 .Rs
1489 .%R RFC 4716
1490 .%T "The Secure Shell (SSH) Public Key File Format"
1491 .%D 2006
1492 .Re
1493 .Rs
1494 .%T "Hash Visualization: a New Technique to improve Real-World Security"
1495 .%A A. Perrig
1496 .%A D. Song
1497 .%D 1999
1498 .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1499 .Re
1500 .Sh AUTHORS
1501 OpenSSH is a derivative of the original and free
1502 ssh 1.2.12 release by Tatu Ylonen.
1503 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1504 Theo de Raadt and Dug Song
1505 removed many bugs, re-added newer features and
1506 created OpenSSH.
1507 Markus Friedl contributed the support for SSH
1508 protocol versions 1.5 and 2.0.