| blob | 6e41bcd8baea6abd35745859d50469e8edfc0027 |
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 1.263 2006/07/11 18:50:48 markus Exp $
38 .Dd September 25, 1999
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 1246AaCfgkMNnqsTtVvXxY
47 .Op Fl b Ar bind_address
48 .Op Fl c Ar cipher_spec
49 .Oo Fl D\ \&
50 .Sm off
51 .Oo Ar bind_address : Oc
52 .Ar port
53 .Sm on
54 .Oc
55 .Op Fl e Ar escape_char
56 .Op Fl F Ar configfile
57 .Bk -words
58 .Op Fl i Ar identity_file
59 .Ek
60 .Oo Fl L\ \&
61 .Sm off
62 .Oo Ar bind_address : Oc
63 .Ar port : host : hostport
64 .Sm on
65 .Oc
66 .Bk -words
67 .Op Fl l Ar login_name
68 .Ek
69 .Op Fl m Ar mac_spec
70 .Op Fl O Ar ctl_cmd
71 .Op Fl o Ar option
72 .Op Fl p Ar port
73 .Oo Fl R\ \&
74 .Sm off
75 .Oo Ar bind_address : Oc
76 .Ar port : host : hostport
77 .Sm on
78 .Oc
79 .Op Fl S Ar ctl_path
80 .Bk -words
81 .Oo Fl w Ar local_tun Ns
82 .Op : Ns Ar remote_tun Oc
83 .Oo Ar user Ns @ Oc Ns Ar hostname
84 .Op Ar command
85 .Ek
86 .Sh DESCRIPTION
87 .Nm
88 (SSH client) is a program for logging into a remote machine and for
89 executing commands on a remote machine.
90 It is intended to replace rlogin and rsh,
91 and provide secure encrypted communications between
92 two untrusted hosts over an insecure network.
93 X11 connections and arbitrary TCP ports
94 can also be forwarded over the secure channel.
95 .Pp
96 .Nm
97 connects and logs into the specified
98 .Ar hostname
99 (with optional
100 .Ar user
101 name).
102 The user must prove
103 his/her identity to the remote machine using one of several methods
104 depending on the protocol version used (see below).
105 .Pp
106 If
107 .Ar command
108 is specified,
109 it is executed on the remote host instead of a login shell.
110 .Pp
111 The options are as follows:
112 .Bl -tag -width Ds
113 .It Fl 1
114 Forces
115 .Nm
116 to try protocol version 1 only.
117 .It Fl 2
118 Forces
119 .Nm
120 to try protocol version 2 only.
121 .It Fl 4
122 Forces
123 .Nm
124 to use IPv4 addresses only.
125 .It Fl 6
126 Forces
127 .Nm
128 to use IPv6 addresses only.
129 .It Fl A
130 Enables forwarding of the authentication agent connection.
131 This can also be specified on a per-host basis in a configuration file.
132 .Pp
133 Agent forwarding should be enabled with caution.
134 Users with the ability to bypass file permissions on the remote host
135 (for the agent's Unix-domain socket)
136 can access the local agent through the forwarded connection.
137 An attacker cannot obtain key material from the agent,
138 however they can perform operations on the keys that enable them to
139 authenticate using the identities loaded into the agent.
140 .It Fl a
141 Disables forwarding of the authentication agent connection.
142 .It Fl b Ar bind_address
143 Use
144 .Ar bind_address
145 on the local machine as the source address
146 of the connection.
147 Only useful on systems with more than one address.
148 .It Fl C
149 Requests compression of all data (including stdin, stdout, stderr, and
150 data for forwarded X11 and TCP connections).
151 The compression algorithm is the same used by
152 .Xr gzip 1 ,
153 and the
154 .Dq level
155 can be controlled by the
156 .Cm CompressionLevel
157 option for protocol version 1.
158 Compression is desirable on modem lines and other
159 slow connections, but will only slow down things on fast networks.
160 The default value can be set on a host-by-host basis in the
161 configuration files; see the
162 .Cm Compression
163 option.
164 .It Fl c Ar cipher_spec
165 Selects the cipher specification for encrypting the session.
166 .Pp
167 Protocol version 1 allows specification of a single cipher.
168 The supported values are
169 .Dq 3des ,
170 .Dq blowfish ,
171 and
172 .Dq des .
173 .Ar 3des
174 (triple-des) is an encrypt-decrypt-encrypt triple with three different keys.
175 It is believed to be secure.
176 .Ar blowfish
177 is a fast block cipher; it appears very secure and is much faster than
178 .Ar 3des .
179 .Ar des
180 is only supported in the
181 .Nm
182 client for interoperability with legacy protocol 1 implementations
183 that do not support the
184 .Ar 3des
185 cipher.
186 Its use is strongly discouraged due to cryptographic weaknesses.
187 The default is
188 .Dq 3des .
189 .Pp
190 For protocol version 2,
191 .Ar cipher_spec
192 is a comma-separated list of ciphers
193 listed in order of preference.
194 The supported ciphers are:
195 3des-cbc,
196 aes128-cbc,
197 aes192-cbc,
198 aes256-cbc,
199 aes128-ctr,
200 aes192-ctr,
201 aes256-ctr,
202 arcfour128,
203 arcfour256,
204 arcfour,
205 blowfish-cbc,
206 and
207 cast128-cbc.
208 The default is:
209 .Bd -literal -offset indent
210 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128,
211 arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr,
212 aes192-ctr,aes256-ctr
213 .Ed
214 .It Fl D Xo
215 .Sm off
216 .Oo Ar bind_address : Oc
217 .Ar port
218 .Sm on
219 .Xc
220 Specifies a local
221 .Dq dynamic
222 application-level port forwarding.
223 This works by allocating a socket to listen to
224 .Ar port
225 on the local side, optionally bound to the specified
226 .Ar bind_address .
227 Whenever a connection is made to this port, the
228 connection is forwarded over the secure channel, and the application
229 protocol is then used to determine where to connect to from the
230 remote machine.
231 Currently the SOCKS4 and SOCKS5 protocols are supported, and
232 .Nm
233 will act as a SOCKS server.
234 Only root can forward privileged ports.
235 Dynamic port forwardings can also be specified in the configuration file.
236 .Pp
237 IPv6 addresses can be specified with an alternative syntax:
238 .Sm off
239 .Xo
240 .Op Ar bind_address No /
241 .Ar port
242 .Xc
243 .Sm on
244 or by enclosing the address in square brackets.
245 Only the superuser can forward privileged ports.
246 By default, the local port is bound in accordance with the
247 .Cm GatewayPorts
248 setting.
249 However, an explicit
250 .Ar bind_address
251 may be used to bind the connection to a specific address.
252 The
253 .Ar bind_address
254 of
255 .Dq localhost
256 indicates that the listening port be bound for local use only, while an
257 empty address or
258 .Sq *
259 indicates that the port should be available from all interfaces.
260 .It Fl e Ar escape_char
261 Sets the escape character for sessions with a pty (default:
262 .Ql ~ ) .
263 The escape character is only recognized at the beginning of a line.
264 The escape character followed by a dot
265 .Pq Ql \&.
266 closes the connection;
267 followed by control-Z suspends the connection;
268 and followed by itself sends the escape character once.
269 Setting the character to
270 .Dq none
271 disables any escapes and makes the session fully transparent.
272 .It Fl F Ar configfile
273 Specifies an alternative per-user configuration file.
274 If a configuration file is given on the command line,
275 the system-wide configuration file
276 .Pq Pa /etc/ssh/ssh_config
277 will be ignored.
278 The default for the per-user configuration file is
279 .Pa ~/.ssh/config .
280 .It Fl f
281 Requests
282 .Nm
283 to go to background just before command execution.
284 This is useful if
285 .Nm
286 is going to ask for passwords or passphrases, but the user
287 wants it in the background.
288 This implies
289 .Fl n .
290 The recommended way to start X11 programs at a remote site is with
291 something like
292 .Ic ssh -f host xterm .
293 .It Fl g
294 Allows remote hosts to connect to local forwarded ports.
295 .It Fl I Ar smartcard_device
296 Specify the device
297 .Nm
298 should use to communicate with a smartcard used for storing the user's
299 private RSA key.
300 This option is only available if support for smartcard devices
301 is compiled in (default is no support).
302 .It Fl i Ar identity_file
303 Selects a file from which the identity (private key) for
304 RSA or DSA authentication is read.
305 The default is
306 .Pa ~/.ssh/identity
307 for protocol version 1, and
308 .Pa ~/.ssh/id_rsa
309 and
310 .Pa ~/.ssh/id_dsa
311 for protocol version 2.
312 Identity files may also be specified on
313 a per-host basis in the configuration file.
314 It is possible to have multiple
315 .Fl i
316 options (and multiple identities specified in
317 configuration files).
318 .It Fl k
319 Disables forwarding (delegation) of GSSAPI credentials to the server.
320 .It Fl L Xo
321 .Sm off
322 .Oo Ar bind_address : Oc
323 .Ar port : host : hostport
324 .Sm on
325 .Xc
326 Specifies that the given port on the local (client) host is to be
327 forwarded to the given host and port on the remote side.
328 This works by allocating a socket to listen to
329 .Ar port
330 on the local side, optionally bound to the specified
331 .Ar bind_address .
332 Whenever a connection is made to this port, the
333 connection is forwarded over the secure channel, and a connection is
334 made to
335 .Ar host
336 port
337 .Ar hostport
338 from the remote machine.
339 Port forwardings can also be specified in the configuration file.
340 IPv6 addresses can be specified with an alternative syntax:
341 .Sm off
342 .Xo
343 .Op Ar bind_address No /
344 .Ar port No / Ar host No /
345 .Ar hostport
346 .Xc
347 .Sm on
348 or by enclosing the address in square brackets.
349 Only the superuser can forward privileged ports.
350 By default, the local port is bound in accordance with the
351 .Cm GatewayPorts
352 setting.
353 However, an explicit
354 .Ar bind_address
355 may be used to bind the connection to a specific address.
356 The
357 .Ar bind_address
358 of
359 .Dq localhost
360 indicates that the listening port be bound for local use only, while an
361 empty address or
362 .Sq *
363 indicates that the port should be available from all interfaces.
364 .It Fl l Ar login_name
365 Specifies the user to log in as on the remote machine.
366 This also may be specified on a per-host basis in the configuration file.
367 .It Fl M
368 Places the
369 .Nm
370 client into
371 .Dq master
372 mode for connection sharing.
373 Multiple
374 .Fl M
375 options places
376 .Nm
377 into
378 .Dq master
379 mode with confirmation required before slave connections are accepted.
380 Refer to the description of
381 .Cm ControlMaster
382 in
383 .Xr ssh_config 5
384 for details.
385 .It Fl m Ar mac_spec
386 Additionally, for protocol version 2 a comma-separated list of MAC
387 (message authentication code) algorithms can
388 be specified in order of preference.
389 See the
390 .Cm MACs
391 keyword for more information.
392 .It Fl N
393 Do not execute a remote command.
394 This is useful for just forwarding ports
395 (protocol version 2 only).
396 .It Fl n
397 Redirects stdin from
398 .Pa /dev/null
399 (actually, prevents reading from stdin).
400 This must be used when
401 .Nm
402 is run in the background.
403 A common trick is to use this to run X11 programs on a remote machine.
404 For example,
405 .Ic ssh -n shadows.cs.hut.fi emacs &
406 will start an emacs on shadows.cs.hut.fi, and the X11
407 connection will be automatically forwarded over an encrypted channel.
408 The
409 .Nm
410 program will be put in the background.
411 (This does not work if
412 .Nm
413 needs to ask for a password or passphrase; see also the
414 .Fl f
415 option.)
416 .It Fl O Ar ctl_cmd
417 Control an active connection multiplexing master process.
418 When the
419 .Fl O
420 option is specified, the
421 .Ar ctl_cmd
422 argument is interpreted and passed to the master process.
423 Valid commands are:
424 .Dq check
425 (check that the master process is running) and
426 .Dq exit
427 (request the master to exit).
428 .It Fl o Ar option
429 Can be used to give options in the format used in the configuration file.
430 This is useful for specifying options for which there is no separate
431 command-line flag.
432 For full details of the options listed below, and their possible values, see
433 .Xr ssh_config 5 .
434 .Pp
435 .Bl -tag -width Ds -offset indent -compact
436 .It AddressFamily
437 .It BatchMode
438 .It BindAddress
439 .It ChallengeResponseAuthentication
440 .It CheckHostIP
441 .It Cipher
442 .It Ciphers
443 .It ClearAllForwardings
444 .It Compression
445 .It CompressionLevel
446 .It ConnectionAttempts
447 .It ConnectTimeout
448 .It ControlMaster
449 .It ControlPath
450 .It DynamicForward
451 .It EscapeChar
452 .It ExitOnForwardFailure
453 .It ForwardAgent
454 .It ForwardX11
455 .It ForwardX11Trusted
456 .It GatewayPorts
457 .It GlobalKnownHostsFile
458 .It GSSAPIAuthentication
459 .It GSSAPIDelegateCredentials
460 .It HashKnownHosts
461 .It Host
462 .It HostbasedAuthentication
463 .It HostKeyAlgorithms
464 .It HostKeyAlias
465 .It HostName
466 .It IdentityFile
467 .It IdentitiesOnly
468 .It KbdInteractiveDevices
469 .It LocalCommand
470 .It LocalForward
471 .It LogLevel
472 .It MACs
473 .It NoHostAuthenticationForLocalhost
474 .It NumberOfPasswordPrompts
475 .It PasswordAuthentication
476 .It PermitLocalCommand
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 SmartcardDevice
490 .It StrictHostKeyChecking
491 .It TCPKeepAlive
492 .It Tunnel
493 .It TunnelDevice
494 .It UsePrivilegedPort
495 .It User
496 .It UserKnownHostsFile
497 .It VerifyHostKeyDNS
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 all 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 overriden 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 .It Fl S Ar ctl_path
554 Specifies the location of a control socket for connection sharing.
555 Refer to the description of
556 .Cm ControlPath
557 and
558 .Cm ControlMaster
559 in
560 .Xr ssh_config 5
561 for details.
562 .It Fl s
563 May be used to request invocation of a subsystem on the remote system.
564 Subsystems are a feature of the SSH2 protocol which facilitate the use
565 of SSH as a secure transport for other applications (eg.\&
566 .Xr sftp 1 ) .
567 The subsystem is specified as the remote command.
568 .It Fl T
569 Disable pseudo-tty allocation.
570 .It Fl t
571 Force pseudo-tty allocation.
572 This can be used to execute arbitrary
573 screen-based programs on a remote machine, which can be very useful,
574 e.g. when implementing menu services.
575 Multiple
576 .Fl t
577 options force tty allocation, even if
578 .Nm
579 has no local tty.
580 .It Fl V
581 Display the version number and exit.
582 .It Fl v
583 Verbose mode.
584 Causes
585 .Nm
586 to print debugging messages about its progress.
587 This is helpful in
588 debugging connection, authentication, and configuration problems.
589 Multiple
590 .Fl v
591 options increase the verbosity.
592 The maximum is 3.
593 .It Fl w Xo
594 .Ar local_tun Ns Op : Ns Ar remote_tun
595 .Xc
596 Requests
597 tunnel
598 device forwarding with the specified
599 .Xr tun 4
600 devices between the client
601 .Pq Ar local_tun
602 and the server
603 .Pq Ar remote_tun .
604 .Pp
605 The devices may be specified by numerical ID or the keyword
606 .Dq any ,
607 which uses the next available tunnel device.
608 If
609 .Ar remote_tun
610 is not specified, it defaults to
611 .Dq any .
612 See also the
613 .Cm Tunnel
614 and
615 .Cm TunnelDevice
616 directives in
617 .Xr ssh_config 5 .
618 If the
619 .Cm Tunnel
620 directive is unset, it is set to the default tunnel mode, which is
621 .Dq point-to-point .
622 .It Fl X
623 Enables X11 forwarding.
624 This can also be specified on a per-host basis in a configuration file.
625 .Pp
626 X11 forwarding should be enabled with caution.
627 Users with the ability to bypass file permissions on the remote host
628 (for the user's X authorization database)
629 can access the local X11 display through the forwarded connection.
630 An attacker may then be able to perform activities such as keystroke monitoring.
631 .Pp
632 For this reason, X11 forwarding is subjected to X11 SECURITY extension
633 restrictions by default.
634 Please refer to the
635 .Nm
636 .Fl Y
637 option and the
638 .Cm ForwardX11Trusted
639 directive in
640 .Xr ssh_config 5
641 for more information.
642 .It Fl x
643 Disables X11 forwarding.
644 .It Fl Y
645 Enables trusted X11 forwarding.
646 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
647 controls.
648 .El
649 .Pp
650 .Nm
651 may additionally obtain configuration data from
652 a per-user configuration file and a system-wide configuration file.
653 The file format and configuration options are described in
654 .Xr ssh_config 5 .
655 .Pp
656 .Nm
657 exits with the exit status of the remote command or with 255
658 if an error occurred.
659 .Sh AUTHENTICATION
660 The OpenSSH SSH client supports SSH protocols 1 and 2.
661 Protocol 2 is the default, with
662 .Nm
663 falling back to protocol 1 if it detects protocol 2 is unsupported.
664 These settings may be altered using the
665 .Cm Protocol
666 option in
667 .Xr ssh_config 5 ,
668 or enforced using the
669 .Fl 1
670 and
671 .Fl 2
672 options (see above).
673 Both protocols support similar authentication methods,
674 but protocol 2 is preferred since
675 it provides additional mechanisms for confidentiality
676 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
677 and integrity (hmac-md5, hmac-sha1, hmac-ripemd160).
678 Protocol 1 lacks a strong mechanism for ensuring the
679 integrity of the connection.
680 .Pp
681 The methods available for authentication are:
682 GSSAPI-based authentication,
683 host-based authentication,
684 public key authentication,
685 challenge-response authentication,
686 and password authentication.
687 Authentication methods are tried in the order specified above,
688 though protocol 2 has a configuration option to change the default order:
689 .Cm PreferredAuthentications .
690 .Pp
691 Host-based authentication works as follows:
692 If the machine the user logs in from is listed in
693 .Pa /etc/hosts.equiv
694 or
695 .Pa /etc/shosts.equiv
696 on the remote machine, and the user names are
697 the same on both sides, or if the files
698 .Pa ~/.rhosts
699 or
700 .Pa ~/.shosts
701 exist in the user's home directory on the
702 remote machine and contain a line containing the name of the client
703 machine and the name of the user on that machine, the user is
704 considered for login.
705 Additionally, the server
706 .Em must
707 be able to verify the client's
708 host key (see the description of
709 .Pa /etc/ssh/ssh_known_hosts
710 and
711 .Pa ~/.ssh/known_hosts ,
712 below)
713 for login to be permitted.
714 This authentication method closes security holes due to IP
715 spoofing, DNS spoofing, and routing spoofing.
716 [Note to the administrator:
717 .Pa /etc/hosts.equiv ,
718 .Pa ~/.rhosts ,
719 and the rlogin/rsh protocol in general, are inherently insecure and should be
720 disabled if security is desired.]
721 .Pp
722 Public key authentication works as follows:
723 The scheme is based on public-key cryptography,
724 using cryptosystems
725 where encryption and decryption are done using separate keys,
726 and it is unfeasible to derive the decryption key from the encryption key.
727 The idea is that each user creates a public/private
728 key pair for authentication purposes.
729 The server knows the public key, and only the user knows the private key.
730 .Nm
731 implements public key authentication protocol automatically,
732 using either the RSA or DSA algorithms.
733 Protocol 1 is restricted to using only RSA keys,
734 but protocol 2 may use either.
735 The
736 .Sx HISTORY
737 section of
738 .Xr ssl 8
739 contains a brief discussion of the two algorithms.
740 .Pp
741 The file
742 .Pa ~/.ssh/authorized_keys
743 lists the public keys that are permitted for logging in.
744 When the user logs in, the
745 .Nm
746 program tells the server which key pair it would like to use for
747 authentication.
748 The client proves that it has access to the private key
749 and the server checks that the corresponding public key
750 is authorized to accept the account.
751 .Pp
752 The user creates his/her key pair by running
753 .Xr ssh-keygen 1 .
754 This stores the private key in
755 .Pa ~/.ssh/identity
756 (protocol 1),
757 .Pa ~/.ssh/id_dsa
758 (protocol 2 DSA),
759 or
760 .Pa ~/.ssh/id_rsa
761 (protocol 2 RSA)
762 and stores the public key in
763 .Pa ~/.ssh/identity.pub
764 (protocol 1),
765 .Pa ~/.ssh/id_dsa.pub
766 (protocol 2 DSA),
767 or
768 .Pa ~/.ssh/id_rsa.pub
769 (protocol 2 RSA)
770 in the user's home directory.
771 The user should then copy the public key
772 to
773 .Pa ~/.ssh/authorized_keys
774 in his/her home directory on the remote machine.
775 The
776 .Pa authorized_keys
777 file corresponds to the conventional
778 .Pa ~/.rhosts
779 file, and has one key
780 per line, though the lines can be very long.
781 After this, the user can log in without giving the password.
782 .Pp
783 The most convenient way to use public key authentication may be with an
784 authentication agent.
785 See
786 .Xr ssh-agent 1
787 for more information.
788 .Pp
789 Challenge-response authentication works as follows:
790 The server sends an arbitrary
791 .Qq challenge
792 text, and prompts for a response.
793 Protocol 2 allows multiple challenges and responses;
794 protocol 1 is restricted to just one challenge/response.
795 Examples of challenge-response authentication include
796 BSD Authentication (see
797 .Xr login.conf 5 )
798 and PAM (some non-OpenBSD systems).
799 .Pp
800 Finally, if other authentication methods fail,
801 .Nm
802 prompts the user for a password.
803 The password is sent to the remote
804 host for checking; however, since all communications are encrypted,
805 the password cannot be seen by someone listening on the network.
806 .Pp
807 .Nm
808 automatically maintains and checks a database containing
809 identification for all hosts it has ever been used with.
810 Host keys are stored in
811 .Pa ~/.ssh/known_hosts
812 in the user's home directory.
813 Additionally, the file
814 .Pa /etc/ssh/ssh_known_hosts
815 is automatically checked for known hosts.
816 Any new hosts are automatically added to the user's file.
817 If a host's identification ever changes,
818 .Nm
819 warns about this and disables password authentication to prevent
820 server spoofing or man-in-the-middle attacks,
821 which could otherwise be used to circumvent the encryption.
822 The
823 .Cm StrictHostKeyChecking
824 option can be used to control logins to machines whose
825 host key is not known or has changed.
826 .Pp
827 When the user's identity has been accepted by the server, the server
828 either executes the given command, or logs into the machine and gives
829 the user a normal shell on the remote machine.
830 All communication with
831 the remote command or shell will be automatically encrypted.
832 .Pp
833 If a pseudo-terminal has been allocated (normal login session), the
834 user may use the escape characters noted below.
835 .Pp
836 If no pseudo-tty has been allocated,
837 the session is transparent and can be used to reliably transfer binary data.
838 On most systems, setting the escape character to
839 .Dq none
840 will also make the session transparent even if a tty is used.
841 .Pp
842 The session terminates when the command or shell on the remote
843 machine exits and all X11 and TCP connections have been closed.
844 .Sh ESCAPE CHARACTERS
845 When a pseudo-terminal has been requested,
846 .Nm
847 supports a number of functions through the use of an escape character.
848 .Pp
849 A single tilde character can be sent as
850 .Ic ~~
851 or by following the tilde by a character other than those described below.
852 The escape character must always follow a newline to be interpreted as
853 special.
854 The escape character can be changed in configuration files using the
855 .Cm EscapeChar
856 configuration directive or on the command line by the
857 .Fl e
858 option.
859 .Pp
860 The supported escapes (assuming the default
861 .Ql ~ )
862 are:
863 .Bl -tag -width Ds
864 .It Cm ~.
865 Disconnect.
866 .It Cm ~^Z
867 Background
868 .Nm .
869 .It Cm ~#
870 List forwarded connections.
871 .It Cm ~&
872 Background
873 .Nm
874 at logout when waiting for forwarded connection / X11 sessions to terminate.
875 .It Cm ~?
876 Display a list of escape characters.
877 .It Cm ~B
878 Send a BREAK to the remote system
879 (only useful for SSH protocol version 2 and if the peer supports it).
880 .It Cm ~C
881 Open command line.
882 Currently this allows the addition of port forwardings using the
883 .Fl L
884 and
885 .Fl R
886 options (see above).
887 It also allows the cancellation of existing remote port-forwardings
888 using
889 .Sm off
890 .Fl KR Oo Ar bind_address : Oc Ar port .
891 .Sm on
892 .Ic !\& Ns Ar command
893 allows the user to execute a local command if the
894 .Ic PermitLocalCommand
895 option is enabled in
896 .Xr ssh_config 5 .
897 Basic help is available, using the
898 .Fl h
899 option.
900 .It Cm ~R
901 Request rekeying of the connection
902 (only useful for SSH protocol version 2 and if the peer supports it).
903 .El
904 .Sh TCP FORWARDING
905 Forwarding of arbitrary TCP connections over the secure channel can
906 be specified either on the command line or in a configuration file.
907 One possible application of TCP forwarding is a secure connection to a
908 mail server; another is going through firewalls.
909 .Pp
910 In the example below, we look at encrypting communication between
911 an IRC client and server, even though the IRC server does not directly
912 support encrypted communications.
913 This works as follows:
914 the user connects to the remote host using
915 .Nm ,
916 specifying a port to be used to forward connections
917 to the remote server.
918 After that it is possible to start the service which is to be encrypted
919 on the client machine,
920 connecting to the same local port,
921 and
922 .Nm
923 will encrypt and forward the connection.
924 .Pp
925 The following example tunnels an IRC session from client machine
926 .Dq 127.0.0.1
927 (localhost)
928 to remote server
929 .Dq server.example.com :
930 .Bd -literal -offset 4n
931 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
932 $ irc -c '#users' -p 1234 pinky 127.0.0.1
933 .Ed
934 .Pp
935 This tunnels a connection to IRC server
936 .Dq server.example.com ,
937 joining channel
938 .Dq #users ,
939 nickname
940 .Dq pinky ,
941 using port 1234.
942 It doesn't matter which port is used,
943 as long as it's greater than 1023
944 (remember, only root can open sockets on privileged ports)
945 and doesn't conflict with any ports already in use.
946 The connection is forwarded to port 6667 on the remote server,
947 since that's the standard port for IRC services.
948 .Pp
949 The
950 .Fl f
951 option backgrounds
952 .Nm
953 and the remote command
954 .Dq sleep 10
955 is specified to allow an amount of time
956 (10 seconds, in the example)
957 to start the service which is to be tunnelled.
958 If no connections are made within the time specified,
959 .Nm
960 will exit.
961 .Sh X11 FORWARDING
962 If the
963 .Cm ForwardX11
964 variable is set to
965 .Dq yes
966 (or see the description of the
967 .Fl X ,
968 .Fl x ,
969 and
970 .Fl Y
971 options above)
972 and the user is using X11 (the
973 .Ev DISPLAY
974 environment variable is set), the connection to the X11 display is
975 automatically forwarded to the remote side in such a way that any X11
976 programs started from the shell (or command) will go through the
977 encrypted channel, and the connection to the real X server will be made
978 from the local machine.
979 The user should not manually set
980 .Ev DISPLAY .
981 Forwarding of X11 connections can be
982 configured on the command line or in configuration files.
983 .Pp
984 The
985 .Ev DISPLAY
986 value set by
987 .Nm
988 will point to the server machine, but with a display number greater than zero.
989 This is normal, and happens because
990 .Nm
991 creates a
992 .Dq proxy
993 X server on the server machine for forwarding the
994 connections over the encrypted channel.
995 .Pp
996 .Nm
997 will also automatically set up Xauthority data on the server machine.
998 For this purpose, it will generate a random authorization cookie,
999 store it in Xauthority on the server, and verify that any forwarded
1000 connections carry this cookie and replace it by the real cookie when
1001 the connection is opened.
1002 The real authentication cookie is never
1003 sent to the server machine (and no cookies are sent in the plain).
1004 .Pp
1005 If the
1006 .Cm ForwardAgent
1007 variable is set to
1008 .Dq yes
1009 (or see the description of the
1010 .Fl A
1011 and
1012 .Fl a
1013 options above) and
1014 the user is using an authentication agent, the connection to the agent
1015 is automatically forwarded to the remote side.
1016 .Sh VERIFYING HOST KEYS
1017 When connecting to a server for the first time,
1018 a fingerprint of the server's public key is presented to the user
1019 (unless the option
1020 .Cm StrictHostKeyChecking
1021 has been disabled).
1022 Fingerprints can be determined using
1023 .Xr ssh-keygen 1 :
1024 .Pp
1025 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1026 .Pp
1027 If the fingerprint is already known,
1028 it can be matched and verified,
1029 and the key can be accepted.
1030 If the fingerprint is unknown,
1031 an alternative method of verification is available:
1032 SSH fingerprints verified by DNS.
1033 An additional resource record (RR),
1034 SSHFP,
1035 is added to a zonefile
1036 and the connecting client is able to match the fingerprint
1037 with that of the key presented.
1038 .Pp
1039 In this example, we are connecting a client to a server,
1040 .Dq host.example.com .
1041 The SSHFP resource records should first be added to the zonefile for
1042 host.example.com:
1043 .Bd -literal -offset indent
1044 $ ssh-keygen -r host.example.com.
1045 .Ed
1046 .Pp
1047 The output lines will have to be added to the zonefile.
1048 To check that the zone is answering fingerprint queries:
1049 .Pp
1050 .Dl $ dig -t SSHFP host.example.com
1051 .Pp
1052 Finally the client connects:
1053 .Bd -literal -offset indent
1054 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1055 [...]
1056 Matching host key fingerprint found in DNS.
1057 Are you sure you want to continue connecting (yes/no)?
1058 .Ed
1059 .Pp
1060 See the
1061 .Cm VerifyHostKeyDNS
1062 option in
1063 .Xr ssh_config 5
1064 for more information.
1065 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1066 .Nm
1067 contains support for Virtual Private Network (VPN) tunnelling
1068 using the
1069 .Xr tun 4
1070 network pseudo-device,
1071 allowing two networks to be joined securely.
1072 The
1073 .Xr sshd_config 5
1074 configuration option
1075 .Cm PermitTunnel
1076 controls whether the server supports this,
1077 and at what level (layer 2 or 3 traffic).
1078 .Pp
1079 The following example would connect client network 10.0.50.0/24
1080 with remote network 10.0.99.0/24, provided that the SSH server
1081 running on the gateway to the remote network,
1082 at 192.168.1.15, allows it:
1083 .Bd -literal -offset indent
1084 # ssh -f -w 0:1 192.168.1.15 true
1085 # ifconfig tun0 10.0.50.1 10.0.99.1 netmask 255.255.255.252
1086 .Ed
1087 .Pp
1088 Client access may be more finely tuned via the
1089 .Pa /root/.ssh/authorized_keys
1090 file (see below) and the
1091 .Cm PermitRootLogin
1092 server option.
1093 The following entry would permit connections on
1094 .Xr tun 4
1095 device 1 from user
1096 .Dq jane
1097 and on tun device 2 from user
1098 .Dq john ,
1099 if
1100 .Cm PermitRootLogin
1101 is set to
1102 .Dq forced-commands-only :
1103 .Bd -literal -offset 2n
1104 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1105 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1106 .Ed
1107 .Pp
1108 Since a SSH-based setup entails a fair amount of overhead,
1109 it may be more suited to temporary setups,
1110 such as for wireless VPNs.
1111 More permanent VPNs are better provided by tools such as
1112 .Xr ipsecctl 8
1113 and
1114 .Xr isakmpd 8 .
1115 .Sh ENVIRONMENT
1116 .Nm
1117 will normally set the following environment variables:
1118 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1119 .It Ev DISPLAY
1120 The
1121 .Ev DISPLAY
1122 variable indicates the location of the X11 server.
1123 It is automatically set by
1124 .Nm
1125 to point to a value of the form
1126 .Dq hostname:n ,
1127 where
1128 .Dq hostname
1129 indicates the host where the shell runs, and
1130 .Sq n
1131 is an integer \*(Ge 1.
1132 .Nm
1133 uses this special value to forward X11 connections over the secure
1134 channel.
1135 The user should normally not set
1136 .Ev DISPLAY
1137 explicitly, as that
1138 will render the X11 connection insecure (and will require the user to
1139 manually copy any required authorization cookies).
1140 .It Ev HOME
1141 Set to the path of the user's home directory.
1142 .It Ev LOGNAME
1143 Synonym for
1144 .Ev USER ;
1145 set for compatibility with systems that use this variable.
1146 .It Ev MAIL
1147 Set to the path of the user's mailbox.
1148 .It Ev PATH
1149 Set to the default
1150 .Ev PATH ,
1151 as specified when compiling
1152 .Nm .
1153 .It Ev SSH_ASKPASS
1154 If
1155 .Nm
1156 needs a passphrase, it will read the passphrase from the current
1157 terminal if it was run from a terminal.
1158 If
1159 .Nm
1160 does not have a terminal associated with it but
1161 .Ev DISPLAY
1162 and
1163 .Ev SSH_ASKPASS
1164 are set, it will execute the program specified by
1165 .Ev SSH_ASKPASS
1166 and open an X11 window to read the passphrase.
1167 This is particularly useful when calling
1168 .Nm
1169 from a
1170 .Pa .xsession
1171 or related script.
1172 (Note that on some machines it
1173 may be necessary to redirect the input from
1174 .Pa /dev/null
1175 to make this work.)
1176 .It Ev SSH_AUTH_SOCK
1177 Identifies the path of a
1178 .Ux Ns -domain
1179 socket used to communicate with the agent.
1180 .It Ev SSH_CONNECTION
1181 Identifies the client and server ends of the connection.
1182 The variable contains
1183 four space-separated values: client IP address, client port number,
1184 server IP address, and server port number.
1185 .It Ev SSH_ORIGINAL_COMMAND
1186 This variable contains the original command line if a forced command
1187 is executed.
1188 It can be used to extract the original arguments.
1189 .It Ev SSH_TTY
1190 This is set to the name of the tty (path to the device) associated
1191 with the current shell or command.
1192 If the current session has no tty,
1193 this variable is not set.
1194 .It Ev TZ
1195 This variable is set to indicate the present time zone if it
1196 was set when the daemon was started (i.e. the daemon passes the value
1197 on to new connections).
1198 .It Ev USER
1199 Set to the name of the user logging in.
1200 .El
1201 .Pp
1202 Additionally,
1203 .Nm
1204 reads
1205 .Pa ~/.ssh/environment ,
1206 and adds lines of the format
1207 .Dq VARNAME=value
1208 to the environment if the file exists and users are allowed to
1209 change their environment.
1210 For more information, see the
1211 .Cm PermitUserEnvironment
1212 option in
1213 .Xr sshd_config 5 .
1214 .Sh FILES
1215 .Bl -tag -width Ds -compact
1216 .It ~/.rhosts
1217 This file is used for host-based authentication (see above).
1218 On some machines this file may need to be
1219 world-readable if the user's home directory is on an NFS partition,
1220 because
1221 .Xr sshd 8
1222 reads it as root.
1223 Additionally, this file must be owned by the user,
1224 and must not have write permissions for anyone else.
1225 The recommended
1226 permission for most machines is read/write for the user, and not
1227 accessible by others.
1228 .Pp
1229 .It ~/.shosts
1230 This file is used in exactly the same way as
1231 .Pa .rhosts ,
1232 but allows host-based authentication without permitting login with
1233 rlogin/rsh.
1234 .Pp
1235 .It ~/.ssh/authorized_keys
1236 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1237 The format of this file is described in the
1238 .Xr sshd 8
1239 manual page.
1240 This file is not highly sensitive, but the recommended
1241 permissions are read/write for the user, and not accessible by others.
1242 .Pp
1243 .It ~/.ssh/config
1244 This is the per-user configuration file.
1245 The file format and configuration options are described in
1246 .Xr ssh_config 5 .
1247 Because of the potential for abuse, this file must have strict permissions:
1248 read/write for the user, and not accessible by others.
1249 .Pp
1250 .It ~/.ssh/environment
1251 Contains additional definitions for environment variables; see
1252 .Sx ENVIRONMENT ,
1253 above.
1254 .Pp
1255 .It ~/.ssh/identity
1256 .It ~/.ssh/id_dsa
1257 .It ~/.ssh/id_rsa
1258 Contains the private key for authentication.
1259 These files
1260 contain sensitive data and should be readable by the user but not
1261 accessible by others (read/write/execute).
1262 .Nm
1263 will simply ignore a private key file if it is accessible by others.
1264 It is possible to specify a passphrase when
1265 generating the key which will be used to encrypt the
1266 sensitive part of this file using 3DES.
1267 .Pp
1268 .It ~/.ssh/identity.pub
1269 .It ~/.ssh/id_dsa.pub
1270 .It ~/.ssh/id_rsa.pub
1271 Contains the public key for authentication.
1272 These files are not
1273 sensitive and can (but need not) be readable by anyone.
1274 .Pp
1275 .It ~/.ssh/known_hosts
1276 Contains a list of host keys for all hosts the user has logged into
1277 that are not already in the systemwide list of known host keys.
1278 See
1279 .Xr sshd 8
1280 for further details of the format of this file.
1281 .Pp
1282 .It ~/.ssh/rc
1283 Commands in this file are executed by
1284 .Nm
1285 when the user logs in, just before the user's shell (or command) is
1286 started.
1287 See the
1288 .Xr sshd 8
1289 manual page for more information.
1290 .Pp
1291 .It /etc/hosts.equiv
1292 This file is for host-based authentication (see above).
1293 It should only be writable by root.
1294 .Pp
1295 .It /etc/shosts.equiv
1296 This file is used in exactly the same way as
1297 .Pa hosts.equiv ,
1298 but allows host-based authentication without permitting login with
1299 rlogin/rsh.
1300 .Pp
1301 .It Pa /etc/ssh/ssh_config
1302 Systemwide configuration file.
1303 The file format and configuration options are described in
1304 .Xr ssh_config 5 .
1305 .Pp
1306 .It /etc/ssh/ssh_host_key
1307 .It /etc/ssh/ssh_host_dsa_key
1308 .It /etc/ssh/ssh_host_rsa_key
1309 These three files contain the private parts of the host keys
1310 and are used for host-based authentication.
1311 If protocol version 1 is used,
1312 .Nm
1313 must be setuid root, since the host key is readable only by root.
1314 For protocol version 2,
1315 .Nm
1316 uses
1317 .Xr ssh-keysign 8
1318 to access the host keys,
1319 eliminating the requirement that
1320 .Nm
1321 be setuid root when host-based authentication is used.
1322 By default
1323 .Nm
1324 is not setuid root.
1325 .Pp
1326 .It /etc/ssh/ssh_known_hosts
1327 Systemwide list of known host keys.
1328 This file should be prepared by the
1329 system administrator to contain the public host keys of all machines in the
1330 organization.
1331 It should be world-readable.
1332 See
1333 .Xr sshd 8
1334 for further details of the format of this file.
1335 .Pp
1336 .It /etc/ssh/sshrc
1337 Commands in this file are executed by
1338 .Nm
1339 when the user logs in, just before the user's shell (or command) is started.
1340 See the
1341 .Xr sshd 8
1342 manual page for more information.
1343 .El
1344 .Sh SEE ALSO
1345 .Xr scp 1 ,
1346 .Xr sftp 1 ,
1347 .Xr ssh-add 1 ,
1348 .Xr ssh-agent 1 ,
1349 .Xr ssh-keygen 1 ,
1350 .Xr ssh-keyscan 1 ,
1351 .Xr tun 4 ,
1352 .Xr hosts.equiv 5 ,
1353 .Xr ssh_config 5 ,
1354 .Xr ssh-keysign 8 ,
1355 .Xr sshd 8
1356 .Rs
1357 .%R RFC 4250
1358 .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1359 .%D 2006
1360 .Re
1361 .Rs
1362 .%R RFC 4251
1363 .%T "The Secure Shell (SSH) Protocol Architecture"
1364 .%D 2006
1365 .Re
1366 .Rs
1367 .%R RFC 4252
1368 .%T "The Secure Shell (SSH) Authentication Protocol"
1369 .%D 2006
1370 .Re
1371 .Rs
1372 .%R RFC 4253
1373 .%T "The Secure Shell (SSH) Transport Layer Protocol"
1374 .%D 2006
1375 .Re
1376 .Rs
1377 .%R RFC 4254
1378 .%T "The Secure Shell (SSH) Connection Protocol"
1379 .%D 2006
1380 .Re
1381 .Rs
1382 .%R RFC 4255
1383 .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1384 .%D 2006
1385 .Re
1386 .Rs
1387 .%R RFC 4256
1388 .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1389 .%D 2006
1390 .Re
1391 .Rs
1392 .%R RFC 4335
1393 .%T "The Secure Shell (SSH) Session Channel Break Extension"
1394 .%D 2006
1395 .Re
1396 .Rs
1397 .%R RFC 4344
1398 .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1399 .%D 2006
1400 .Re
1401 .Rs
1402 .%R RFC 4345
1403 .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1404 .%D 2006
1405 .Re
1406 .Rs
1407 .%R RFC 4419
1408 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1409 .%D 2006
1410 .Re
1411 .Sh AUTHORS
1412 OpenSSH is a derivative of the original and free
1413 ssh 1.2.12 release by Tatu Ylonen.
1414 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1415 Theo de Raadt and Dug Song
1416 removed many bugs, re-added newer features and
1417 created OpenSSH.
1418 Markus Friedl contributed the support for SSH
1419 protocol versions 1.5 and 2.0.