| blob | 3f815b8e716119cb9f80ee86a794edc46ddea44e |
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 1.302 2010/03/05 10:28:21 djm Exp $
38 .Dd $Mdocdate: March 5 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 .Nm
310 will also try to load certificate information from the filename obtained
311 by appending
312 .Pa -cert.pub
313 to identity filenames.
314 .It Fl K
315 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
316 credentials to the server.
317 .It Fl k
318 Disables forwarding (delegation) of GSSAPI credentials to the server.
319 .It Fl L Xo
320 .Sm off
321 .Oo Ar bind_address : Oc
322 .Ar port : host : hostport
323 .Sm on
324 .Xc
325 Specifies that the given port on the local (client) host is to be
326 forwarded to the given host and port on the remote side.
327 This works by allocating a socket to listen to
328 .Ar port
329 on the local side, optionally bound to the specified
330 .Ar bind_address .
331 Whenever a connection is made to this port, the
332 connection is forwarded over the secure channel, and a connection is
333 made to
334 .Ar host
335 port
336 .Ar hostport
337 from the remote machine.
338 Port forwardings can also be specified in the configuration file.
339 IPv6 addresses can be specified with an alternative syntax:
340 .Sm off
341 .Xo
342 .Op Ar bind_address No /
343 .Ar port No / Ar host No /
344 .Ar hostport
345 .Xc
346 .Sm on
347 or by enclosing the address in square brackets.
348 Only the superuser can forward privileged ports.
349 By default, the local port is bound in accordance with the
350 .Cm GatewayPorts
351 setting.
352 However, an explicit
353 .Ar bind_address
354 may be used to bind the connection to a specific address.
355 The
356 .Ar bind_address
357 of
358 .Dq localhost
359 indicates that the listening port be bound for local use only, while an
360 empty address or
361 .Sq *
362 indicates that the port should be available from all interfaces.
363 .It Fl l Ar login_name
364 Specifies the user to log in as on the remote machine.
365 This also may be specified on a per-host basis in the configuration file.
366 .It Fl M
367 Places the
368 .Nm
369 client into
370 .Dq master
371 mode for connection sharing.
372 Multiple
373 .Fl M
374 options places
375 .Nm
376 into
377 .Dq master
378 mode with confirmation required before slave connections are accepted.
379 Refer to the description of
380 .Cm ControlMaster
381 in
382 .Xr ssh_config 5
383 for details.
384 .It Fl m Ar mac_spec
385 Additionally, for protocol version 2 a comma-separated list of MAC
386 (message authentication code) algorithms can
387 be specified in order of preference.
388 See the
389 .Cm MACs
390 keyword for more information.
391 .It Fl N
392 Do not execute a remote command.
393 This is useful for just forwarding ports
394 (protocol version 2 only).
395 .It Fl n
396 Redirects stdin from
397 .Pa /dev/null
398 (actually, prevents reading from stdin).
399 This must be used when
400 .Nm
401 is run in the background.
402 A common trick is to use this to run X11 programs on a remote machine.
403 For example,
404 .Ic ssh -n shadows.cs.hut.fi emacs &
405 will start an emacs on shadows.cs.hut.fi, and the X11
406 connection will be automatically forwarded over an encrypted channel.
407 The
408 .Nm
409 program will be put in the background.
410 (This does not work if
411 .Nm
412 needs to ask for a password or passphrase; see also the
413 .Fl f
414 option.)
415 .It Fl O Ar ctl_cmd
416 Control an active connection multiplexing master process.
417 When the
418 .Fl O
419 option is specified, the
420 .Ar ctl_cmd
421 argument is interpreted and passed to the master process.
422 Valid commands are:
423 .Dq check
424 (check that the master process is running) and
425 .Dq exit
426 (request the master to exit).
427 .It Fl o Ar option
428 Can be used to give options in the format used in the configuration file.
429 This is useful for specifying options for which there is no separate
430 command-line flag.
431 For full details of the options listed below, and their possible values, see
432 .Xr ssh_config 5 .
433 .Pp
434 .Bl -tag -width Ds -offset indent -compact
435 .It AddressFamily
436 .It BatchMode
437 .It BindAddress
438 .It ChallengeResponseAuthentication
439 .It CheckHostIP
440 .It Cipher
441 .It Ciphers
442 .It ClearAllForwardings
443 .It Compression
444 .It CompressionLevel
445 .It ConnectionAttempts
446 .It ConnectTimeout
447 .It ControlMaster
448 .It ControlPath
449 .It DynamicForward
450 .It EscapeChar
451 .It ExitOnForwardFailure
452 .It ForwardAgent
453 .It ForwardX11
454 .It ForwardX11Trusted
455 .It GatewayPorts
456 .It GlobalKnownHostsFile
457 .It GSSAPIAuthentication
458 .It GSSAPIDelegateCredentials
459 .It HashKnownHosts
460 .It Host
461 .It HostbasedAuthentication
462 .It HostKeyAlgorithms
463 .It HostKeyAlias
464 .It HostName
465 .It IdentityFile
466 .It IdentitiesOnly
467 .It KbdInteractiveDevices
468 .It LocalCommand
469 .It LocalForward
470 .It LogLevel
471 .It MACs
472 .It NoHostAuthenticationForLocalhost
473 .It NumberOfPasswordPrompts
474 .It PasswordAuthentication
475 .It PermitLocalCommand
476 .It PKCS11Provider
477 .It Port
478 .It PreferredAuthentications
479 .It Protocol
480 .It ProxyCommand
481 .It PubkeyAuthentication
482 .It RekeyLimit
483 .It RemoteForward
484 .It RhostsRSAAuthentication
485 .It RSAAuthentication
486 .It SendEnv
487 .It ServerAliveInterval
488 .It ServerAliveCountMax
489 .It StrictHostKeyChecking
490 .It TCPKeepAlive
491 .It Tunnel
492 .It TunnelDevice
493 .It UsePrivilegedPort
494 .It User
495 .It UserKnownHostsFile
496 .It VerifyHostKeyDNS
497 .It VisualHostKey
498 .It XAuthLocation
499 .El
500 .It Fl p Ar port
501 Port to connect to on the remote host.
502 This can be specified on a
503 per-host basis in the configuration file.
504 .It Fl q
505 Quiet mode.
506 Causes most warning and diagnostic messages to be suppressed.
507 .It Fl R Xo
508 .Sm off
509 .Oo Ar bind_address : Oc
510 .Ar port : host : hostport
511 .Sm on
512 .Xc
513 Specifies that the given port on the remote (server) host is to be
514 forwarded to the given host and port on the local side.
515 This works by allocating a socket to listen to
516 .Ar port
517 on the remote side, and whenever a connection is made to this port, the
518 connection is forwarded over the secure channel, and a connection is
519 made to
520 .Ar host
521 port
522 .Ar hostport
523 from the local machine.
524 .Pp
525 Port forwardings can also be specified in the configuration file.
526 Privileged ports can be forwarded only when
527 logging in as root on the remote machine.
528 IPv6 addresses can be specified by enclosing the address in square braces or
529 using an alternative syntax:
530 .Sm off
531 .Xo
532 .Op Ar bind_address No /
533 .Ar host No / Ar port No /
534 .Ar hostport
535 .Xc .
536 .Sm on
537 .Pp
538 By default, the listening socket on the server will be bound to the loopback
539 interface only.
540 This may be overridden by specifying a
541 .Ar bind_address .
542 An empty
543 .Ar bind_address ,
544 or the address
545 .Ql * ,
546 indicates that the remote socket should listen on all interfaces.
547 Specifying a remote
548 .Ar bind_address
549 will only succeed if the server's
550 .Cm GatewayPorts
551 option is enabled (see
552 .Xr sshd_config 5 ) .
553 .Pp
554 If the
555 .Ar port
556 argument is
557 .Ql 0 ,
558 the listen port will be dynamically allocated on the server and reported
559 to the client at run time.
560 .It Fl S Ar ctl_path
561 Specifies the location of a control socket for connection sharing.
562 Refer to the description of
563 .Cm ControlPath
564 and
565 .Cm ControlMaster
566 in
567 .Xr ssh_config 5
568 for details.
569 .It Fl s
570 May be used to request invocation of a subsystem on the remote system.
571 Subsystems are a feature of the SSH2 protocol which facilitate the use
572 of SSH as a secure transport for other applications (eg.\&
573 .Xr sftp 1 ) .
574 The subsystem is specified as the remote command.
575 .It Fl T
576 Disable pseudo-tty allocation.
577 .It Fl t
578 Force pseudo-tty allocation.
579 This can be used to execute arbitrary
580 screen-based programs on a remote machine, which can be very useful,
581 e.g. when implementing menu services.
582 Multiple
583 .Fl t
584 options force tty allocation, even if
585 .Nm
586 has no local tty.
587 .It Fl V
588 Display the version number and exit.
589 .It Fl v
590 Verbose mode.
591 Causes
592 .Nm
593 to print debugging messages about its progress.
594 This is helpful in
595 debugging connection, authentication, and configuration problems.
596 Multiple
597 .Fl v
598 options increase the verbosity.
599 The maximum is 3.
600 .It Fl W Ar host : Ns Ar port
601 Requests that standard input and output on the client be forwarded to
602 .Ar host
603 on
604 .Ar port
605 over the secure channel.
606 Implies
607 .Fl N ,
608 .Fl T ,
609 .Cm ExitOnForwardFailure
610 and
611 .Cm ClearAllForwardings
612 and works with Protocol version 2 only.
613 .It Fl w Xo
614 .Ar local_tun Ns Op : Ns Ar remote_tun
615 .Xc
616 Requests
617 tunnel
618 device forwarding with the specified
619 .Xr tun 4
620 devices between the client
621 .Pq Ar local_tun
622 and the server
623 .Pq Ar remote_tun .
624 .Pp
625 The devices may be specified by numerical ID or the keyword
626 .Dq any ,
627 which uses the next available tunnel device.
628 If
629 .Ar remote_tun
630 is not specified, it defaults to
631 .Dq any .
632 See also the
633 .Cm Tunnel
634 and
635 .Cm TunnelDevice
636 directives in
637 .Xr ssh_config 5 .
638 If the
639 .Cm Tunnel
640 directive is unset, it is set to the default tunnel mode, which is
641 .Dq point-to-point .
642 .It Fl X
643 Enables X11 forwarding.
644 This can also be specified on a per-host basis in a configuration file.
645 .Pp
646 X11 forwarding should be enabled with caution.
647 Users with the ability to bypass file permissions on the remote host
648 (for the user's X authorization database)
649 can access the local X11 display through the forwarded connection.
650 An attacker may then be able to perform activities such as keystroke monitoring.
651 .Pp
652 For this reason, X11 forwarding is subjected to X11 SECURITY extension
653 restrictions by default.
654 Please refer to the
655 .Nm
656 .Fl Y
657 option and the
658 .Cm ForwardX11Trusted
659 directive in
660 .Xr ssh_config 5
661 for more information.
662 .It Fl x
663 Disables X11 forwarding.
664 .It Fl Y
665 Enables trusted X11 forwarding.
666 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
667 controls.
668 .It Fl y
669 Send log information using the
670 .Xr syslog 3
671 system module.
672 By default this information is sent to stderr.
673 .El
674 .Pp
675 .Nm
676 may additionally obtain configuration data from
677 a per-user configuration file and a system-wide configuration file.
678 The file format and configuration options are described in
679 .Xr ssh_config 5 .
680 .Pp
681 .Nm
682 exits with the exit status of the remote command or with 255
683 if an error occurred.
684 .Sh AUTHENTICATION
685 The OpenSSH SSH client supports SSH protocols 1 and 2.
686 The default is to use protocol 2 only,
687 though this can be changed via the
688 .Cm Protocol
689 option in
690 .Xr ssh_config 5
691 or the
692 .Fl 1
693 and
694 .Fl 2
695 options (see above).
696 Both protocols support similar authentication methods,
697 but protocol 2 is the default since
698 it provides additional mechanisms for confidentiality
699 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
700 and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
701 Protocol 1 lacks a strong mechanism for ensuring the
702 integrity of the connection.
703 .Pp
704 The methods available for authentication are:
705 GSSAPI-based authentication,
706 host-based authentication,
707 public key authentication,
708 challenge-response authentication,
709 and password authentication.
710 Authentication methods are tried in the order specified above,
711 though protocol 2 has a configuration option to change the default order:
712 .Cm PreferredAuthentications .
713 .Pp
714 Host-based authentication works as follows:
715 If the machine the user logs in from is listed in
716 .Pa /etc/hosts.equiv
717 or
718 .Pa /etc/shosts.equiv
719 on the remote machine, and the user names are
720 the same on both sides, or if the files
721 .Pa ~/.rhosts
722 or
723 .Pa ~/.shosts
724 exist in the user's home directory on the
725 remote machine and contain a line containing the name of the client
726 machine and the name of the user on that machine, the user is
727 considered for login.
728 Additionally, the server
729 .Em must
730 be able to verify the client's
731 host key (see the description of
732 .Pa /etc/ssh/ssh_known_hosts
733 and
734 .Pa ~/.ssh/known_hosts ,
735 below)
736 for login to be permitted.
737 This authentication method closes security holes due to IP
738 spoofing, DNS spoofing, and routing spoofing.
739 [Note to the administrator:
740 .Pa /etc/hosts.equiv ,
741 .Pa ~/.rhosts ,
742 and the rlogin/rsh protocol in general, are inherently insecure and should be
743 disabled if security is desired.]
744 .Pp
745 Public key authentication works as follows:
746 The scheme is based on public-key cryptography,
747 using cryptosystems
748 where encryption and decryption are done using separate keys,
749 and it is unfeasible to derive the decryption key from the encryption key.
750 The idea is that each user creates a public/private
751 key pair for authentication purposes.
752 The server knows the public key, and only the user knows the private key.
753 .Nm
754 implements public key authentication protocol automatically,
755 using either the RSA or DSA algorithms.
756 Protocol 1 is restricted to using only RSA keys,
757 but protocol 2 may use either.
758 The
759 .Sx HISTORY
760 section of
761 .Xr ssl 8
762 contains a brief discussion of the two algorithms.
763 .Pp
764 The file
765 .Pa ~/.ssh/authorized_keys
766 lists the public keys that are permitted for logging in.
767 When the user logs in, the
768 .Nm
769 program tells the server which key pair it would like to use for
770 authentication.
771 The client proves that it has access to the private key
772 and the server checks that the corresponding public key
773 is authorized to accept the account.
774 .Pp
775 The user creates his/her key pair by running
776 .Xr ssh-keygen 1 .
777 This stores the private key in
778 .Pa ~/.ssh/identity
779 (protocol 1),
780 .Pa ~/.ssh/id_dsa
781 (protocol 2 DSA),
782 or
783 .Pa ~/.ssh/id_rsa
784 (protocol 2 RSA)
785 and stores the public key in
786 .Pa ~/.ssh/identity.pub
787 (protocol 1),
788 .Pa ~/.ssh/id_dsa.pub
789 (protocol 2 DSA),
790 or
791 .Pa ~/.ssh/id_rsa.pub
792 (protocol 2 RSA)
793 in the user's home directory.
794 The user should then copy the public key
795 to
796 .Pa ~/.ssh/authorized_keys
797 in his/her home directory on the remote machine.
798 The
799 .Pa authorized_keys
800 file corresponds to the conventional
801 .Pa ~/.rhosts
802 file, and has one key
803 per line, though the lines can be very long.
804 After this, the user can log in without giving the password.
805 .Pp
806 A variation on public key authentication
807 is available in the form of certificate authentication:
808 instead of a set of public/private keys,
809 signed certificates are used.
810 This has the advantage that a single trusted certification authority
811 can be used in place of many public/private keys.
812 See the
813 .Sx CERTIFICATES
814 section of
815 .Xr ssh-keygen 1
816 for more information.
817 .Pp
818 The most convenient way to use public key or certificate authentication
819 may be with an authentication agent.
820 See
821 .Xr ssh-agent 1
822 for more information.
823 .Pp
824 Challenge-response authentication works as follows:
825 The server sends an arbitrary
826 .Qq challenge
827 text, and prompts for a response.
828 Protocol 2 allows multiple challenges and responses;
829 protocol 1 is restricted to just one challenge/response.
830 Examples of challenge-response authentication include
831 BSD Authentication (see
832 .Xr login.conf 5 )
833 and PAM (some non-OpenBSD systems).
834 .Pp
835 Finally, if other authentication methods fail,
836 .Nm
837 prompts the user for a password.
838 The password is sent to the remote
839 host for checking; however, since all communications are encrypted,
840 the password cannot be seen by someone listening on the network.
841 .Pp
842 .Nm
843 automatically maintains and checks a database containing
844 identification for all hosts it has ever been used with.
845 Host keys are stored in
846 .Pa ~/.ssh/known_hosts
847 in the user's home directory.
848 Additionally, the file
849 .Pa /etc/ssh/ssh_known_hosts
850 is automatically checked for known hosts.
851 Any new hosts are automatically added to the user's file.
852 If a host's identification ever changes,
853 .Nm
854 warns about this and disables password authentication to prevent
855 server spoofing or man-in-the-middle attacks,
856 which could otherwise be used to circumvent the encryption.
857 The
858 .Cm StrictHostKeyChecking
859 option can be used to control logins to machines whose
860 host key is not known or has changed.
861 .Pp
862 When the user's identity has been accepted by the server, the server
863 either executes the given command, or logs into the machine and gives
864 the user a normal shell on the remote machine.
865 All communication with
866 the remote command or shell will be automatically encrypted.
867 .Pp
868 If a pseudo-terminal has been allocated (normal login session), the
869 user may use the escape characters noted below.
870 .Pp
871 If no pseudo-tty has been allocated,
872 the session is transparent and can be used to reliably transfer binary data.
873 On most systems, setting the escape character to
874 .Dq none
875 will also make the session transparent even if a tty is used.
876 .Pp
877 The session terminates when the command or shell on the remote
878 machine exits and all X11 and TCP connections have been closed.
879 .Sh ESCAPE CHARACTERS
880 When a pseudo-terminal has been requested,
881 .Nm
882 supports a number of functions through the use of an escape character.
883 .Pp
884 A single tilde character can be sent as
885 .Ic ~~
886 or by following the tilde by a character other than those described below.
887 The escape character must always follow a newline to be interpreted as
888 special.
889 The escape character can be changed in configuration files using the
890 .Cm EscapeChar
891 configuration directive or on the command line by the
892 .Fl e
893 option.
894 .Pp
895 The supported escapes (assuming the default
896 .Ql ~ )
897 are:
898 .Bl -tag -width Ds
899 .It Cm ~.
900 Disconnect.
901 .It Cm ~^Z
902 Background
903 .Nm .
904 .It Cm ~#
905 List forwarded connections.
906 .It Cm ~&
907 Background
908 .Nm
909 at logout when waiting for forwarded connection / X11 sessions to terminate.
910 .It Cm ~?
911 Display a list of escape characters.
912 .It Cm ~B
913 Send a BREAK to the remote system
914 (only useful for SSH protocol version 2 and if the peer supports it).
915 .It Cm ~C
916 Open command line.
917 Currently this allows the addition of port forwardings using the
918 .Fl L ,
919 .Fl R
920 and
921 .Fl D
922 options (see above).
923 It also allows the cancellation of existing remote port-forwardings
924 using
925 .Sm off
926 .Fl KR Oo Ar bind_address : Oc Ar port .
927 .Sm on
928 .Ic !\& Ns Ar command
929 allows the user to execute a local command if the
930 .Ic PermitLocalCommand
931 option is enabled in
932 .Xr ssh_config 5 .
933 Basic help is available, using the
934 .Fl h
935 option.
936 .It Cm ~R
937 Request rekeying of the connection
938 (only useful for SSH protocol version 2 and if the peer supports it).
939 .El
940 .Sh TCP FORWARDING
941 Forwarding of arbitrary TCP connections over the secure channel can
942 be specified either on the command line or in a configuration file.
943 One possible application of TCP forwarding is a secure connection to a
944 mail server; another is going through firewalls.
945 .Pp
946 In the example below, we look at encrypting communication between
947 an IRC client and server, even though the IRC server does not directly
948 support encrypted communications.
949 This works as follows:
950 the user connects to the remote host using
951 .Nm ,
952 specifying a port to be used to forward connections
953 to the remote server.
954 After that it is possible to start the service which is to be encrypted
955 on the client machine,
956 connecting to the same local port,
957 and
958 .Nm
959 will encrypt and forward the connection.
960 .Pp
961 The following example tunnels an IRC session from client machine
962 .Dq 127.0.0.1
963 (localhost)
964 to remote server
965 .Dq server.example.com :
966 .Bd -literal -offset 4n
967 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
968 $ irc -c '#users' -p 1234 pinky 127.0.0.1
969 .Ed
970 .Pp
971 This tunnels a connection to IRC server
972 .Dq server.example.com ,
973 joining channel
974 .Dq #users ,
975 nickname
976 .Dq pinky ,
977 using port 1234.
978 It doesn't matter which port is used,
979 as long as it's greater than 1023
980 (remember, only root can open sockets on privileged ports)
981 and doesn't conflict with any ports already in use.
982 The connection is forwarded to port 6667 on the remote server,
983 since that's the standard port for IRC services.
984 .Pp
985 The
986 .Fl f
987 option backgrounds
988 .Nm
989 and the remote command
990 .Dq sleep 10
991 is specified to allow an amount of time
992 (10 seconds, in the example)
993 to start the service which is to be tunnelled.
994 If no connections are made within the time specified,
995 .Nm
996 will exit.
997 .Sh X11 FORWARDING
998 If the
999 .Cm ForwardX11
1000 variable is set to
1001 .Dq yes
1002 (or see the description of the
1003 .Fl X ,
1004 .Fl x ,
1005 and
1006 .Fl Y
1007 options above)
1008 and the user is using X11 (the
1009 .Ev DISPLAY
1010 environment variable is set), the connection to the X11 display is
1011 automatically forwarded to the remote side in such a way that any X11
1012 programs started from the shell (or command) will go through the
1013 encrypted channel, and the connection to the real X server will be made
1014 from the local machine.
1015 The user should not manually set
1016 .Ev DISPLAY .
1017 Forwarding of X11 connections can be
1018 configured on the command line or in configuration files.
1019 .Pp
1020 The
1021 .Ev DISPLAY
1022 value set by
1023 .Nm
1024 will point to the server machine, but with a display number greater than zero.
1025 This is normal, and happens because
1026 .Nm
1027 creates a
1028 .Dq proxy
1029 X server on the server machine for forwarding the
1030 connections over the encrypted channel.
1031 .Pp
1032 .Nm
1033 will also automatically set up Xauthority data on the server machine.
1034 For this purpose, it will generate a random authorization cookie,
1035 store it in Xauthority on the server, and verify that any forwarded
1036 connections carry this cookie and replace it by the real cookie when
1037 the connection is opened.
1038 The real authentication cookie is never
1039 sent to the server machine (and no cookies are sent in the plain).
1040 .Pp
1041 If the
1042 .Cm ForwardAgent
1043 variable is set to
1044 .Dq yes
1045 (or see the description of the
1046 .Fl A
1047 and
1048 .Fl a
1049 options above) and
1050 the user is using an authentication agent, the connection to the agent
1051 is automatically forwarded to the remote side.
1052 .Sh VERIFYING HOST KEYS
1053 When connecting to a server for the first time,
1054 a fingerprint of the server's public key is presented to the user
1055 (unless the option
1056 .Cm StrictHostKeyChecking
1057 has been disabled).
1058 Fingerprints can be determined using
1059 .Xr ssh-keygen 1 :
1060 .Pp
1061 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1062 .Pp
1063 If the fingerprint is already known, it can be matched
1064 and the key can be accepted or rejected.
1065 Because of the difficulty of comparing host keys
1066 just by looking at hex strings,
1067 there is also support to compare host keys visually,
1068 using
1069 .Em random art .
1070 By setting the
1071 .Cm VisualHostKey
1072 option to
1073 .Dq yes ,
1074 a small ASCII graphic gets displayed on every login to a server, no matter
1075 if the session itself is interactive or not.
1076 By learning the pattern a known server produces, a user can easily
1077 find out that the host key has changed when a completely different pattern
1078 is displayed.
1079 Because these patterns are not unambiguous however, a pattern that looks
1080 similar to the pattern remembered only gives a good probability that the
1081 host key is the same, not guaranteed proof.
1082 .Pp
1083 To get a listing of the fingerprints along with their random art for
1084 all known hosts, the following command line can be used:
1085 .Pp
1086 .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts
1087 .Pp
1088 If the fingerprint is unknown,
1089 an alternative method of verification is available:
1090 SSH fingerprints verified by DNS.
1091 An additional resource record (RR),
1092 SSHFP,
1093 is added to a zonefile
1094 and the connecting client is able to match the fingerprint
1095 with that of the key presented.
1096 .Pp
1097 In this example, we are connecting a client to a server,
1098 .Dq host.example.com .
1099 The SSHFP resource records should first be added to the zonefile for
1100 host.example.com:
1101 .Bd -literal -offset indent
1102 $ ssh-keygen -r host.example.com.
1103 .Ed
1104 .Pp
1105 The output lines will have to be added to the zonefile.
1106 To check that the zone is answering fingerprint queries:
1107 .Pp
1108 .Dl $ dig -t SSHFP host.example.com
1109 .Pp
1110 Finally the client connects:
1111 .Bd -literal -offset indent
1112 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1113 [...]
1114 Matching host key fingerprint found in DNS.
1115 Are you sure you want to continue connecting (yes/no)?
1116 .Ed
1117 .Pp
1118 See the
1119 .Cm VerifyHostKeyDNS
1120 option in
1121 .Xr ssh_config 5
1122 for more information.
1123 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1124 .Nm
1125 contains support for Virtual Private Network (VPN) tunnelling
1126 using the
1127 .Xr tun 4
1128 network pseudo-device,
1129 allowing two networks to be joined securely.
1130 The
1131 .Xr sshd_config 5
1132 configuration option
1133 .Cm PermitTunnel
1134 controls whether the server supports this,
1135 and at what level (layer 2 or 3 traffic).
1136 .Pp
1137 The following example would connect client network 10.0.50.0/24
1138 with remote network 10.0.99.0/24 using a point-to-point connection
1139 from 10.1.1.1 to 10.1.1.2,
1140 provided that the SSH server running on the gateway to the remote network,
1141 at 192.168.1.15, allows it.
1142 .Pp
1143 On the client:
1144 .Bd -literal -offset indent
1145 # ssh -f -w 0:1 192.168.1.15 true
1146 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1147 # route add 10.0.99.0/24 10.1.1.2
1148 .Ed
1149 .Pp
1150 On the server:
1151 .Bd -literal -offset indent
1152 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1153 # route add 10.0.50.0/24 10.1.1.1
1154 .Ed
1155 .Pp
1156 Client access may be more finely tuned via the
1157 .Pa /root/.ssh/authorized_keys
1158 file (see below) and the
1159 .Cm PermitRootLogin
1160 server option.
1161 The following entry would permit connections on
1162 .Xr tun 4
1163 device 1 from user
1164 .Dq jane
1165 and on tun device 2 from user
1166 .Dq john ,
1167 if
1168 .Cm PermitRootLogin
1169 is set to
1170 .Dq forced-commands-only :
1171 .Bd -literal -offset 2n
1172 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1173 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1174 .Ed
1175 .Pp
1176 Since an SSH-based setup entails a fair amount of overhead,
1177 it may be more suited to temporary setups,
1178 such as for wireless VPNs.
1179 More permanent VPNs are better provided by tools such as
1180 .Xr ipsecctl 8
1181 and
1182 .Xr isakmpd 8 .
1183 .Sh ENVIRONMENT
1184 .Nm
1185 will normally set the following environment variables:
1186 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1187 .It Ev DISPLAY
1188 The
1189 .Ev DISPLAY
1190 variable indicates the location of the X11 server.
1191 It is automatically set by
1192 .Nm
1193 to point to a value of the form
1194 .Dq hostname:n ,
1195 where
1196 .Dq hostname
1197 indicates the host where the shell runs, and
1198 .Sq n
1199 is an integer \*(Ge 1.
1200 .Nm
1201 uses this special value to forward X11 connections over the secure
1202 channel.
1203 The user should normally not set
1204 .Ev DISPLAY
1205 explicitly, as that
1206 will render the X11 connection insecure (and will require the user to
1207 manually copy any required authorization cookies).
1208 .It Ev HOME
1209 Set to the path of the user's home directory.
1210 .It Ev LOGNAME
1211 Synonym for
1212 .Ev USER ;
1213 set for compatibility with systems that use this variable.
1214 .It Ev MAIL
1215 Set to the path of the user's mailbox.
1216 .It Ev PATH
1217 Set to the default
1218 .Ev PATH ,
1219 as specified when compiling
1220 .Nm .
1221 .It Ev SSH_ASKPASS
1222 If
1223 .Nm
1224 needs a passphrase, it will read the passphrase from the current
1225 terminal if it was run from a terminal.
1226 If
1227 .Nm
1228 does not have a terminal associated with it but
1229 .Ev DISPLAY
1230 and
1231 .Ev SSH_ASKPASS
1232 are set, it will execute the program specified by
1233 .Ev SSH_ASKPASS
1234 and open an X11 window to read the passphrase.
1235 This is particularly useful when calling
1236 .Nm
1237 from a
1238 .Pa .xsession
1239 or related script.
1240 (Note that on some machines it
1241 may be necessary to redirect the input from
1242 .Pa /dev/null
1243 to make this work.)
1244 .It Ev SSH_AUTH_SOCK
1245 Identifies the path of a
1246 .Ux Ns -domain
1247 socket used to communicate with the agent.
1248 .It Ev SSH_CONNECTION
1249 Identifies the client and server ends of the connection.
1250 The variable contains
1251 four space-separated values: client IP address, client port number,
1252 server IP address, and server port number.
1253 .It Ev SSH_ORIGINAL_COMMAND
1254 This variable contains the original command line if a forced command
1255 is executed.
1256 It can be used to extract the original arguments.
1257 .It Ev SSH_TTY
1258 This is set to the name of the tty (path to the device) associated
1259 with the current shell or command.
1260 If the current session has no tty,
1261 this variable is not set.
1262 .It Ev TZ
1263 This variable is set to indicate the present time zone if it
1264 was set when the daemon was started (i.e. the daemon passes the value
1265 on to new connections).
1266 .It Ev USER
1267 Set to the name of the user logging in.
1268 .El
1269 .Pp
1270 Additionally,
1271 .Nm
1272 reads
1273 .Pa ~/.ssh/environment ,
1274 and adds lines of the format
1275 .Dq VARNAME=value
1276 to the environment if the file exists and users are allowed to
1277 change their environment.
1278 For more information, see the
1279 .Cm PermitUserEnvironment
1280 option in
1281 .Xr sshd_config 5 .
1282 .Sh FILES
1283 .Bl -tag -width Ds -compact
1284 .It ~/.rhosts
1285 This file is used for host-based authentication (see above).
1286 On some machines this file may need to be
1287 world-readable if the user's home directory is on an NFS partition,
1288 because
1289 .Xr sshd 8
1290 reads it as root.
1291 Additionally, this file must be owned by the user,
1292 and must not have write permissions for anyone else.
1293 The recommended
1294 permission for most machines is read/write for the user, and not
1295 accessible by others.
1296 .Pp
1297 .It ~/.shosts
1298 This file is used in exactly the same way as
1299 .Pa .rhosts ,
1300 but allows host-based authentication without permitting login with
1301 rlogin/rsh.
1302 .Pp
1303 .It ~/.ssh/
1304 This directory is the default location for all user-specific configuration
1305 and authentication information.
1306 There is no general requirement to keep the entire contents of this directory
1307 secret, but the recommended permissions are read/write/execute for the user,
1308 and not accessible by others.
1309 .Pp
1310 .It ~/.ssh/authorized_keys
1311 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1312 The format of this file is described in the
1313 .Xr sshd 8
1314 manual page.
1315 This file is not highly sensitive, but the recommended
1316 permissions are read/write for the user, and not accessible by others.
1317 .Pp
1318 .It ~/.ssh/config
1319 This is the per-user configuration file.
1320 The file format and configuration options are described in
1321 .Xr ssh_config 5 .
1322 Because of the potential for abuse, this file must have strict permissions:
1323 read/write for the user, and not accessible by others.
1324 .Pp
1325 .It ~/.ssh/environment
1326 Contains additional definitions for environment variables; see
1327 .Sx ENVIRONMENT ,
1328 above.
1329 .Pp
1330 .It ~/.ssh/identity
1331 .It ~/.ssh/id_dsa
1332 .It ~/.ssh/id_rsa
1333 Contains the private key for authentication.
1334 These files
1335 contain sensitive data and should be readable by the user but not
1336 accessible by others (read/write/execute).
1337 .Nm
1338 will simply ignore a private key file if it is accessible by others.
1339 It is possible to specify a passphrase when
1340 generating the key which will be used to encrypt the
1341 sensitive part of this file using 3DES.
1342 .Pp
1343 .It ~/.ssh/identity.pub
1344 .It ~/.ssh/id_dsa.pub
1345 .It ~/.ssh/id_rsa.pub
1346 Contains the public key for authentication.
1347 These files are not
1348 sensitive and can (but need not) be readable by anyone.
1349 .Pp
1350 .It ~/.ssh/known_hosts
1351 Contains a list of host keys for all hosts the user has logged into
1352 that are not already in the systemwide list of known host keys.
1353 See
1354 .Xr sshd 8
1355 for further details of the format of this file.
1356 .Pp
1357 .It ~/.ssh/rc
1358 Commands in this file are executed by
1359 .Nm
1360 when the user logs in, just before the user's shell (or command) is
1361 started.
1362 See the
1363 .Xr sshd 8
1364 manual page for more information.
1365 .Pp
1366 .It /etc/hosts.equiv
1367 This file is for host-based authentication (see above).
1368 It should only be writable by root.
1369 .Pp
1370 .It /etc/shosts.equiv
1371 This file is used in exactly the same way as
1372 .Pa hosts.equiv ,
1373 but allows host-based authentication without permitting login with
1374 rlogin/rsh.
1375 .Pp
1376 .It Pa /etc/ssh/ssh_config
1377 Systemwide configuration file.
1378 The file format and configuration options are described in
1379 .Xr ssh_config 5 .
1380 .Pp
1381 .It /etc/ssh/ssh_host_key
1382 .It /etc/ssh/ssh_host_dsa_key
1383 .It /etc/ssh/ssh_host_rsa_key
1384 These three files contain the private parts of the host keys
1385 and are used for host-based authentication.
1386 If protocol version 1 is used,
1387 .Nm
1388 must be setuid root, since the host key is readable only by root.
1389 For protocol version 2,
1390 .Nm
1391 uses
1392 .Xr ssh-keysign 8
1393 to access the host keys,
1394 eliminating the requirement that
1395 .Nm
1396 be setuid root when host-based authentication is used.
1397 By default
1398 .Nm
1399 is not setuid root.
1400 .Pp
1401 .It /etc/ssh/ssh_known_hosts
1402 Systemwide list of known host keys.
1403 This file should be prepared by the
1404 system administrator to contain the public host keys of all machines in the
1405 organization.
1406 It should be world-readable.
1407 See
1408 .Xr sshd 8
1409 for further details of the format of this file.
1410 .Pp
1411 .It /etc/ssh/sshrc
1412 Commands in this file are executed by
1413 .Nm
1414 when the user logs in, just before the user's shell (or command) is started.
1415 See the
1416 .Xr sshd 8
1417 manual page for more information.
1418 .El
1419 .Sh SEE ALSO
1420 .Xr scp 1 ,
1421 .Xr sftp 1 ,
1422 .Xr ssh-add 1 ,
1423 .Xr ssh-agent 1 ,
1424 .Xr ssh-keygen 1 ,
1425 .Xr ssh-keyscan 1 ,
1426 .Xr tun 4 ,
1427 .Xr hosts.equiv 5 ,
1428 .Xr ssh_config 5 ,
1429 .Xr ssh-keysign 8 ,
1430 .Xr sshd 8
1431 .Rs
1432 .%R RFC 4250
1433 .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1434 .%D 2006
1435 .Re
1436 .Rs
1437 .%R RFC 4251
1438 .%T "The Secure Shell (SSH) Protocol Architecture"
1439 .%D 2006
1440 .Re
1441 .Rs
1442 .%R RFC 4252
1443 .%T "The Secure Shell (SSH) Authentication Protocol"
1444 .%D 2006
1445 .Re
1446 .Rs
1447 .%R RFC 4253
1448 .%T "The Secure Shell (SSH) Transport Layer Protocol"
1449 .%D 2006
1450 .Re
1451 .Rs
1452 .%R RFC 4254
1453 .%T "The Secure Shell (SSH) Connection Protocol"
1454 .%D 2006
1455 .Re
1456 .Rs
1457 .%R RFC 4255
1458 .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1459 .%D 2006
1460 .Re
1461 .Rs
1462 .%R RFC 4256
1463 .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1464 .%D 2006
1465 .Re
1466 .Rs
1467 .%R RFC 4335
1468 .%T "The Secure Shell (SSH) Session Channel Break Extension"
1469 .%D 2006
1470 .Re
1471 .Rs
1472 .%R RFC 4344
1473 .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1474 .%D 2006
1475 .Re
1476 .Rs
1477 .%R RFC 4345
1478 .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1479 .%D 2006
1480 .Re
1481 .Rs
1482 .%R RFC 4419
1483 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1484 .%D 2006
1485 .Re
1486 .Rs
1487 .%R RFC 4716
1488 .%T "The Secure Shell (SSH) Public Key File Format"
1489 .%D 2006
1490 .Re
1491 .Rs
1492 .%T "Hash Visualization: a New Technique to improve Real-World Security"
1493 .%A A. Perrig
1494 .%A D. Song
1495 .%D 1999
1496 .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)"
1497 .Re
1498 .Sh AUTHORS
1499 OpenSSH is a derivative of the original and free
1500 ssh 1.2.12 release by Tatu Ylonen.
1501 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1502 Theo de Raadt and Dug Song
1503 removed many bugs, re-added newer features and
1504 created OpenSSH.
1505 Markus Friedl contributed the support for SSH
1506 protocol versions 1.5 and 2.0.