| blob | e191bf04ee7520ce565a83a21eab53f6962c6a9b |
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 1.273 2008/02/11 07:58:28 jmc Exp $
38 .Dd $Mdocdate: February 11 2008 $
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 1246AaCfgKkMNnqsTtVvXxY
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 Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI
320 credentials to the server.
321 .It Fl k
322 Disables forwarding (delegation) of GSSAPI credentials to the server.
323 .It Fl L Xo
324 .Sm off
325 .Oo Ar bind_address : Oc
326 .Ar port : host : hostport
327 .Sm on
328 .Xc
329 Specifies that the given port on the local (client) host is to be
330 forwarded to the given host and port on the remote side.
331 This works by allocating a socket to listen to
332 .Ar port
333 on the local side, optionally bound to the specified
334 .Ar bind_address .
335 Whenever a connection is made to this port, the
336 connection is forwarded over the secure channel, and a connection is
337 made to
338 .Ar host
339 port
340 .Ar hostport
341 from the remote machine.
342 Port forwardings can also be specified in the configuration file.
343 IPv6 addresses can be specified with an alternative syntax:
344 .Sm off
345 .Xo
346 .Op Ar bind_address No /
347 .Ar port No / Ar host No /
348 .Ar hostport
349 .Xc
350 .Sm on
351 or by enclosing the address in square brackets.
352 Only the superuser can forward privileged ports.
353 By default, the local port is bound in accordance with the
354 .Cm GatewayPorts
355 setting.
356 However, an explicit
357 .Ar bind_address
358 may be used to bind the connection to a specific address.
359 The
360 .Ar bind_address
361 of
362 .Dq localhost
363 indicates that the listening port be bound for local use only, while an
364 empty address or
365 .Sq *
366 indicates that the port should be available from all interfaces.
367 .It Fl l Ar login_name
368 Specifies the user to log in as on the remote machine.
369 This also may be specified on a per-host basis in the configuration file.
370 .It Fl M
371 Places the
372 .Nm
373 client into
374 .Dq master
375 mode for connection sharing.
376 Multiple
377 .Fl M
378 options places
379 .Nm
380 into
381 .Dq master
382 mode with confirmation required before slave connections are accepted.
383 Refer to the description of
384 .Cm ControlMaster
385 in
386 .Xr ssh_config 5
387 for details.
388 .It Fl m Ar mac_spec
389 Additionally, for protocol version 2 a comma-separated list of MAC
390 (message authentication code) algorithms can
391 be specified in order of preference.
392 See the
393 .Cm MACs
394 keyword for more information.
395 .It Fl N
396 Do not execute a remote command.
397 This is useful for just forwarding ports
398 (protocol version 2 only).
399 .It Fl n
400 Redirects stdin from
401 .Pa /dev/null
402 (actually, prevents reading from stdin).
403 This must be used when
404 .Nm
405 is run in the background.
406 A common trick is to use this to run X11 programs on a remote machine.
407 For example,
408 .Ic ssh -n shadows.cs.hut.fi emacs &
409 will start an emacs on shadows.cs.hut.fi, and the X11
410 connection will be automatically forwarded over an encrypted channel.
411 The
412 .Nm
413 program will be put in the background.
414 (This does not work if
415 .Nm
416 needs to ask for a password or passphrase; see also the
417 .Fl f
418 option.)
419 .It Fl O Ar ctl_cmd
420 Control an active connection multiplexing master process.
421 When the
422 .Fl O
423 option is specified, the
424 .Ar ctl_cmd
425 argument is interpreted and passed to the master process.
426 Valid commands are:
427 .Dq check
428 (check that the master process is running) and
429 .Dq exit
430 (request the master to exit).
431 .It Fl o Ar option
432 Can be used to give options in the format used in the configuration file.
433 This is useful for specifying options for which there is no separate
434 command-line flag.
435 For full details of the options listed below, and their possible values, see
436 .Xr ssh_config 5 .
437 .Pp
438 .Bl -tag -width Ds -offset indent -compact
439 .It AddressFamily
440 .It BatchMode
441 .It BindAddress
442 .It ChallengeResponseAuthentication
443 .It CheckHostIP
444 .It Cipher
445 .It Ciphers
446 .It ClearAllForwardings
447 .It Compression
448 .It CompressionLevel
449 .It ConnectionAttempts
450 .It ConnectTimeout
451 .It ControlMaster
452 .It ControlPath
453 .It DynamicForward
454 .It EscapeChar
455 .It ExitOnForwardFailure
456 .It ForwardAgent
457 .It ForwardX11
458 .It ForwardX11Trusted
459 .It GatewayPorts
460 .It GlobalKnownHostsFile
461 .It GSSAPIAuthentication
462 .It GSSAPIDelegateCredentials
463 .It HashKnownHosts
464 .It Host
465 .It HostbasedAuthentication
466 .It HostKeyAlgorithms
467 .It HostKeyAlias
468 .It HostName
469 .It IdentityFile
470 .It IdentitiesOnly
471 .It KbdInteractiveDevices
472 .It LocalCommand
473 .It LocalForward
474 .It LogLevel
475 .It MACs
476 .It NoHostAuthenticationForLocalhost
477 .It NumberOfPasswordPrompts
478 .It PasswordAuthentication
479 .It PermitLocalCommand
480 .It Port
481 .It PreferredAuthentications
482 .It Protocol
483 .It ProxyCommand
484 .It PubkeyAuthentication
485 .It RekeyLimit
486 .It RemoteForward
487 .It RhostsRSAAuthentication
488 .It RSAAuthentication
489 .It SendEnv
490 .It ServerAliveInterval
491 .It ServerAliveCountMax
492 .It SmartcardDevice
493 .It StrictHostKeyChecking
494 .It TCPKeepAlive
495 .It Tunnel
496 .It TunnelDevice
497 .It UsePrivilegedPort
498 .It User
499 .It UserKnownHostsFile
500 .It VerifyHostKeyDNS
501 .It XAuthLocation
502 .El
503 .It Fl p Ar port
504 Port to connect to on the remote host.
505 This can be specified on a
506 per-host basis in the configuration file.
507 .It Fl q
508 Quiet mode.
509 Causes most warning and diagnostic messages to be suppressed.
510 .It Fl R Xo
511 .Sm off
512 .Oo Ar bind_address : Oc
513 .Ar port : host : hostport
514 .Sm on
515 .Xc
516 Specifies that the given port on the remote (server) host is to be
517 forwarded to the given host and port on the local side.
518 This works by allocating a socket to listen to
519 .Ar port
520 on the remote side, and whenever a connection is made to this port, the
521 connection is forwarded over the secure channel, and a connection is
522 made to
523 .Ar host
524 port
525 .Ar hostport
526 from the local machine.
527 .Pp
528 Port forwardings can also be specified in the configuration file.
529 Privileged ports can be forwarded only when
530 logging in as root on the remote machine.
531 IPv6 addresses can be specified by enclosing the address in square braces or
532 using an alternative syntax:
533 .Sm off
534 .Xo
535 .Op Ar bind_address No /
536 .Ar host No / Ar port No /
537 .Ar hostport
538 .Xc .
539 .Sm on
540 .Pp
541 By default, the listening socket on the server will be bound to the loopback
542 interface only.
543 This may be overriden by specifying a
544 .Ar bind_address .
545 An empty
546 .Ar bind_address ,
547 or the address
548 .Ql * ,
549 indicates that the remote socket should listen on all interfaces.
550 Specifying a remote
551 .Ar bind_address
552 will only succeed if the server's
553 .Cm GatewayPorts
554 option is enabled (see
555 .Xr sshd_config 5 ) .
556 .It Fl S Ar ctl_path
557 Specifies the location of a control socket for connection sharing.
558 Refer to the description of
559 .Cm ControlPath
560 and
561 .Cm ControlMaster
562 in
563 .Xr ssh_config 5
564 for details.
565 .It Fl s
566 May be used to request invocation of a subsystem on the remote system.
567 Subsystems are a feature of the SSH2 protocol which facilitate the use
568 of SSH as a secure transport for other applications (eg.\&
569 .Xr sftp 1 ) .
570 The subsystem is specified as the remote command.
571 .It Fl T
572 Disable pseudo-tty allocation.
573 .It Fl t
574 Force pseudo-tty allocation.
575 This can be used to execute arbitrary
576 screen-based programs on a remote machine, which can be very useful,
577 e.g. when implementing menu services.
578 Multiple
579 .Fl t
580 options force tty allocation, even if
581 .Nm
582 has no local tty.
583 .It Fl V
584 Display the version number and exit.
585 .It Fl v
586 Verbose mode.
587 Causes
588 .Nm
589 to print debugging messages about its progress.
590 This is helpful in
591 debugging connection, authentication, and configuration problems.
592 Multiple
593 .Fl v
594 options increase the verbosity.
595 The maximum is 3.
596 .It Fl w Xo
597 .Ar local_tun Ns Op : Ns Ar remote_tun
598 .Xc
599 Requests
600 tunnel
601 device forwarding with the specified
602 .Xr tun 4
603 devices between the client
604 .Pq Ar local_tun
605 and the server
606 .Pq Ar remote_tun .
607 .Pp
608 The devices may be specified by numerical ID or the keyword
609 .Dq any ,
610 which uses the next available tunnel device.
611 If
612 .Ar remote_tun
613 is not specified, it defaults to
614 .Dq any .
615 See also the
616 .Cm Tunnel
617 and
618 .Cm TunnelDevice
619 directives in
620 .Xr ssh_config 5 .
621 If the
622 .Cm Tunnel
623 directive is unset, it is set to the default tunnel mode, which is
624 .Dq point-to-point .
625 .It Fl X
626 Enables X11 forwarding.
627 This can also be specified on a per-host basis in a configuration file.
628 .Pp
629 X11 forwarding should be enabled with caution.
630 Users with the ability to bypass file permissions on the remote host
631 (for the user's X authorization database)
632 can access the local X11 display through the forwarded connection.
633 An attacker may then be able to perform activities such as keystroke monitoring.
634 .Pp
635 For this reason, X11 forwarding is subjected to X11 SECURITY extension
636 restrictions by default.
637 Please refer to the
638 .Nm
639 .Fl Y
640 option and the
641 .Cm ForwardX11Trusted
642 directive in
643 .Xr ssh_config 5
644 for more information.
645 .It Fl x
646 Disables X11 forwarding.
647 .It Fl Y
648 Enables trusted X11 forwarding.
649 Trusted X11 forwardings are not subjected to the X11 SECURITY extension
650 controls.
651 .El
652 .Pp
653 .Nm
654 may additionally obtain configuration data from
655 a per-user configuration file and a system-wide configuration file.
656 The file format and configuration options are described in
657 .Xr ssh_config 5 .
658 .Pp
659 .Nm
660 exits with the exit status of the remote command or with 255
661 if an error occurred.
662 .Sh AUTHENTICATION
663 The OpenSSH SSH client supports SSH protocols 1 and 2.
664 Protocol 2 is the default, with
665 .Nm
666 falling back to protocol 1 if it detects protocol 2 is unsupported.
667 These settings may be altered using the
668 .Cm Protocol
669 option in
670 .Xr ssh_config 5 ,
671 or enforced using the
672 .Fl 1
673 and
674 .Fl 2
675 options (see above).
676 Both protocols support similar authentication methods,
677 but protocol 2 is preferred since
678 it provides additional mechanisms for confidentiality
679 (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour)
680 and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160).
681 Protocol 1 lacks a strong mechanism for ensuring the
682 integrity of the connection.
683 .Pp
684 The methods available for authentication are:
685 GSSAPI-based authentication,
686 host-based authentication,
687 public key authentication,
688 challenge-response authentication,
689 and password authentication.
690 Authentication methods are tried in the order specified above,
691 though protocol 2 has a configuration option to change the default order:
692 .Cm PreferredAuthentications .
693 .Pp
694 Host-based authentication works as follows:
695 If the machine the user logs in from is listed in
696 .Pa /etc/hosts.equiv
697 or
698 .Pa /etc/shosts.equiv
699 on the remote machine, and the user names are
700 the same on both sides, or if the files
701 .Pa ~/.rhosts
702 or
703 .Pa ~/.shosts
704 exist in the user's home directory on the
705 remote machine and contain a line containing the name of the client
706 machine and the name of the user on that machine, the user is
707 considered for login.
708 Additionally, the server
709 .Em must
710 be able to verify the client's
711 host key (see the description of
712 .Pa /etc/ssh/ssh_known_hosts
713 and
714 .Pa ~/.ssh/known_hosts ,
715 below)
716 for login to be permitted.
717 This authentication method closes security holes due to IP
718 spoofing, DNS spoofing, and routing spoofing.
719 [Note to the administrator:
720 .Pa /etc/hosts.equiv ,
721 .Pa ~/.rhosts ,
722 and the rlogin/rsh protocol in general, are inherently insecure and should be
723 disabled if security is desired.]
724 .Pp
725 Public key authentication works as follows:
726 The scheme is based on public-key cryptography,
727 using cryptosystems
728 where encryption and decryption are done using separate keys,
729 and it is unfeasible to derive the decryption key from the encryption key.
730 The idea is that each user creates a public/private
731 key pair for authentication purposes.
732 The server knows the public key, and only the user knows the private key.
733 .Nm
734 implements public key authentication protocol automatically,
735 using either the RSA or DSA algorithms.
736 Protocol 1 is restricted to using only RSA keys,
737 but protocol 2 may use either.
738 The
739 .Sx HISTORY
740 section of
741 .Xr ssl 8
742 contains a brief discussion of the two algorithms.
743 .Pp
744 The file
745 .Pa ~/.ssh/authorized_keys
746 lists the public keys that are permitted for logging in.
747 When the user logs in, the
748 .Nm
749 program tells the server which key pair it would like to use for
750 authentication.
751 The client proves that it has access to the private key
752 and the server checks that the corresponding public key
753 is authorized to accept the account.
754 .Pp
755 The user creates his/her key pair by running
756 .Xr ssh-keygen 1 .
757 This stores the private key in
758 .Pa ~/.ssh/identity
759 (protocol 1),
760 .Pa ~/.ssh/id_dsa
761 (protocol 2 DSA),
762 or
763 .Pa ~/.ssh/id_rsa
764 (protocol 2 RSA)
765 and stores the public key in
766 .Pa ~/.ssh/identity.pub
767 (protocol 1),
768 .Pa ~/.ssh/id_dsa.pub
769 (protocol 2 DSA),
770 or
771 .Pa ~/.ssh/id_rsa.pub
772 (protocol 2 RSA)
773 in the user's home directory.
774 The user should then copy the public key
775 to
776 .Pa ~/.ssh/authorized_keys
777 in his/her home directory on the remote machine.
778 The
779 .Pa authorized_keys
780 file corresponds to the conventional
781 .Pa ~/.rhosts
782 file, and has one key
783 per line, though the lines can be very long.
784 After this, the user can log in without giving the password.
785 .Pp
786 The most convenient way to use public key authentication may be with an
787 authentication agent.
788 See
789 .Xr ssh-agent 1
790 for more information.
791 .Pp
792 Challenge-response authentication works as follows:
793 The server sends an arbitrary
794 .Qq challenge
795 text, and prompts for a response.
796 Protocol 2 allows multiple challenges and responses;
797 protocol 1 is restricted to just one challenge/response.
798 Examples of challenge-response authentication include
799 BSD Authentication (see
800 .Xr login.conf 5 )
801 and PAM (some non-OpenBSD systems).
802 .Pp
803 Finally, if other authentication methods fail,
804 .Nm
805 prompts the user for a password.
806 The password is sent to the remote
807 host for checking; however, since all communications are encrypted,
808 the password cannot be seen by someone listening on the network.
809 .Pp
810 .Nm
811 automatically maintains and checks a database containing
812 identification for all hosts it has ever been used with.
813 Host keys are stored in
814 .Pa ~/.ssh/known_hosts
815 in the user's home directory.
816 Additionally, the file
817 .Pa /etc/ssh/ssh_known_hosts
818 is automatically checked for known hosts.
819 Any new hosts are automatically added to the user's file.
820 If a host's identification ever changes,
821 .Nm
822 warns about this and disables password authentication to prevent
823 server spoofing or man-in-the-middle attacks,
824 which could otherwise be used to circumvent the encryption.
825 The
826 .Cm StrictHostKeyChecking
827 option can be used to control logins to machines whose
828 host key is not known or has changed.
829 .Pp
830 When the user's identity has been accepted by the server, the server
831 either executes the given command, or logs into the machine and gives
832 the user a normal shell on the remote machine.
833 All communication with
834 the remote command or shell will be automatically encrypted.
835 .Pp
836 If a pseudo-terminal has been allocated (normal login session), the
837 user may use the escape characters noted below.
838 .Pp
839 If no pseudo-tty has been allocated,
840 the session is transparent and can be used to reliably transfer binary data.
841 On most systems, setting the escape character to
842 .Dq none
843 will also make the session transparent even if a tty is used.
844 .Pp
845 The session terminates when the command or shell on the remote
846 machine exits and all X11 and TCP connections have been closed.
847 .Sh ESCAPE CHARACTERS
848 When a pseudo-terminal has been requested,
849 .Nm
850 supports a number of functions through the use of an escape character.
851 .Pp
852 A single tilde character can be sent as
853 .Ic ~~
854 or by following the tilde by a character other than those described below.
855 The escape character must always follow a newline to be interpreted as
856 special.
857 The escape character can be changed in configuration files using the
858 .Cm EscapeChar
859 configuration directive or on the command line by the
860 .Fl e
861 option.
862 .Pp
863 The supported escapes (assuming the default
864 .Ql ~ )
865 are:
866 .Bl -tag -width Ds
867 .It Cm ~.
868 Disconnect.
869 .It Cm ~^Z
870 Background
871 .Nm .
872 .It Cm ~#
873 List forwarded connections.
874 .It Cm ~&
875 Background
876 .Nm
877 at logout when waiting for forwarded connection / X11 sessions to terminate.
878 .It Cm ~?
879 Display a list of escape characters.
880 .It Cm ~B
881 Send a BREAK to the remote system
882 (only useful for SSH protocol version 2 and if the peer supports it).
883 .It Cm ~C
884 Open command line.
885 Currently this allows the addition of port forwardings using the
886 .Fl L
887 and
888 .Fl R
889 options (see above).
890 It also allows the cancellation of existing remote port-forwardings
891 using
892 .Sm off
893 .Fl KR Oo Ar bind_address : Oc Ar port .
894 .Sm on
895 .Ic !\& Ns Ar command
896 allows the user to execute a local command if the
897 .Ic PermitLocalCommand
898 option is enabled in
899 .Xr ssh_config 5 .
900 Basic help is available, using the
901 .Fl h
902 option.
903 .It Cm ~R
904 Request rekeying of the connection
905 (only useful for SSH protocol version 2 and if the peer supports it).
906 .El
907 .Sh TCP FORWARDING
908 Forwarding of arbitrary TCP connections over the secure channel can
909 be specified either on the command line or in a configuration file.
910 One possible application of TCP forwarding is a secure connection to a
911 mail server; another is going through firewalls.
912 .Pp
913 In the example below, we look at encrypting communication between
914 an IRC client and server, even though the IRC server does not directly
915 support encrypted communications.
916 This works as follows:
917 the user connects to the remote host using
918 .Nm ,
919 specifying a port to be used to forward connections
920 to the remote server.
921 After that it is possible to start the service which is to be encrypted
922 on the client machine,
923 connecting to the same local port,
924 and
925 .Nm
926 will encrypt and forward the connection.
927 .Pp
928 The following example tunnels an IRC session from client machine
929 .Dq 127.0.0.1
930 (localhost)
931 to remote server
932 .Dq server.example.com :
933 .Bd -literal -offset 4n
934 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
935 $ irc -c '#users' -p 1234 pinky 127.0.0.1
936 .Ed
937 .Pp
938 This tunnels a connection to IRC server
939 .Dq server.example.com ,
940 joining channel
941 .Dq #users ,
942 nickname
943 .Dq pinky ,
944 using port 1234.
945 It doesn't matter which port is used,
946 as long as it's greater than 1023
947 (remember, only root can open sockets on privileged ports)
948 and doesn't conflict with any ports already in use.
949 The connection is forwarded to port 6667 on the remote server,
950 since that's the standard port for IRC services.
951 .Pp
952 The
953 .Fl f
954 option backgrounds
955 .Nm
956 and the remote command
957 .Dq sleep 10
958 is specified to allow an amount of time
959 (10 seconds, in the example)
960 to start the service which is to be tunnelled.
961 If no connections are made within the time specified,
962 .Nm
963 will exit.
964 .Sh X11 FORWARDING
965 If the
966 .Cm ForwardX11
967 variable is set to
968 .Dq yes
969 (or see the description of the
970 .Fl X ,
971 .Fl x ,
972 and
973 .Fl Y
974 options above)
975 and the user is using X11 (the
976 .Ev DISPLAY
977 environment variable is set), the connection to the X11 display is
978 automatically forwarded to the remote side in such a way that any X11
979 programs started from the shell (or command) will go through the
980 encrypted channel, and the connection to the real X server will be made
981 from the local machine.
982 The user should not manually set
983 .Ev DISPLAY .
984 Forwarding of X11 connections can be
985 configured on the command line or in configuration files.
986 .Pp
987 The
988 .Ev DISPLAY
989 value set by
990 .Nm
991 will point to the server machine, but with a display number greater than zero.
992 This is normal, and happens because
993 .Nm
994 creates a
995 .Dq proxy
996 X server on the server machine for forwarding the
997 connections over the encrypted channel.
998 .Pp
999 .Nm
1000 will also automatically set up Xauthority data on the server machine.
1001 For this purpose, it will generate a random authorization cookie,
1002 store it in Xauthority on the server, and verify that any forwarded
1003 connections carry this cookie and replace it by the real cookie when
1004 the connection is opened.
1005 The real authentication cookie is never
1006 sent to the server machine (and no cookies are sent in the plain).
1007 .Pp
1008 If the
1009 .Cm ForwardAgent
1010 variable is set to
1011 .Dq yes
1012 (or see the description of the
1013 .Fl A
1014 and
1015 .Fl a
1016 options above) and
1017 the user is using an authentication agent, the connection to the agent
1018 is automatically forwarded to the remote side.
1019 .Sh VERIFYING HOST KEYS
1020 When connecting to a server for the first time,
1021 a fingerprint of the server's public key is presented to the user
1022 (unless the option
1023 .Cm StrictHostKeyChecking
1024 has been disabled).
1025 Fingerprints can be determined using
1026 .Xr ssh-keygen 1 :
1027 .Pp
1028 .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
1029 .Pp
1030 If the fingerprint is already known,
1031 it can be matched and verified,
1032 and the key can be accepted.
1033 If the fingerprint is unknown,
1034 an alternative method of verification is available:
1035 SSH fingerprints verified by DNS.
1036 An additional resource record (RR),
1037 SSHFP,
1038 is added to a zonefile
1039 and the connecting client is able to match the fingerprint
1040 with that of the key presented.
1041 .Pp
1042 In this example, we are connecting a client to a server,
1043 .Dq host.example.com .
1044 The SSHFP resource records should first be added to the zonefile for
1045 host.example.com:
1046 .Bd -literal -offset indent
1047 $ ssh-keygen -r host.example.com.
1048 .Ed
1049 .Pp
1050 The output lines will have to be added to the zonefile.
1051 To check that the zone is answering fingerprint queries:
1052 .Pp
1053 .Dl $ dig -t SSHFP host.example.com
1054 .Pp
1055 Finally the client connects:
1056 .Bd -literal -offset indent
1057 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
1058 [...]
1059 Matching host key fingerprint found in DNS.
1060 Are you sure you want to continue connecting (yes/no)?
1061 .Ed
1062 .Pp
1063 See the
1064 .Cm VerifyHostKeyDNS
1065 option in
1066 .Xr ssh_config 5
1067 for more information.
1068 .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS
1069 .Nm
1070 contains support for Virtual Private Network (VPN) tunnelling
1071 using the
1072 .Xr tun 4
1073 network pseudo-device,
1074 allowing two networks to be joined securely.
1075 The
1076 .Xr sshd_config 5
1077 configuration option
1078 .Cm PermitTunnel
1079 controls whether the server supports this,
1080 and at what level (layer 2 or 3 traffic).
1081 .Pp
1082 The following example would connect client network 10.0.50.0/24
1083 with remote network 10.0.99.0/24 using a point-to-point connection
1084 from 10.1.1.1 to 10.1.1.2,
1085 provided that the SSH server running on the gateway to the remote network,
1086 at 192.168.1.15, allows it.
1087 .Pp
1088 On the client:
1089 .Bd -literal -offset indent
1090 # ssh -f -w 0:1 192.168.1.15 true
1091 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
1092 # route add 10.0.99.0/24 10.1.1.2
1093 .Ed
1094 .Pp
1095 On the server:
1096 .Bd -literal -offset indent
1097 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
1098 # route add 10.0.50.0/24 10.1.1.1
1099 .Ed
1100 .Pp
1101 Client access may be more finely tuned via the
1102 .Pa /root/.ssh/authorized_keys
1103 file (see below) and the
1104 .Cm PermitRootLogin
1105 server option.
1106 The following entry would permit connections on
1107 .Xr tun 4
1108 device 1 from user
1109 .Dq jane
1110 and on tun device 2 from user
1111 .Dq john ,
1112 if
1113 .Cm PermitRootLogin
1114 is set to
1115 .Dq forced-commands-only :
1116 .Bd -literal -offset 2n
1117 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
1118 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
1119 .Ed
1120 .Pp
1121 Since an SSH-based setup entails a fair amount of overhead,
1122 it may be more suited to temporary setups,
1123 such as for wireless VPNs.
1124 More permanent VPNs are better provided by tools such as
1125 .Xr ipsecctl 8
1126 and
1127 .Xr isakmpd 8 .
1128 .Sh ENVIRONMENT
1129 .Nm
1130 will normally set the following environment variables:
1131 .Bl -tag -width "SSH_ORIGINAL_COMMAND"
1132 .It Ev DISPLAY
1133 The
1134 .Ev DISPLAY
1135 variable indicates the location of the X11 server.
1136 It is automatically set by
1137 .Nm
1138 to point to a value of the form
1139 .Dq hostname:n ,
1140 where
1141 .Dq hostname
1142 indicates the host where the shell runs, and
1143 .Sq n
1144 is an integer \*(Ge 1.
1145 .Nm
1146 uses this special value to forward X11 connections over the secure
1147 channel.
1148 The user should normally not set
1149 .Ev DISPLAY
1150 explicitly, as that
1151 will render the X11 connection insecure (and will require the user to
1152 manually copy any required authorization cookies).
1153 .It Ev HOME
1154 Set to the path of the user's home directory.
1155 .It Ev LOGNAME
1156 Synonym for
1157 .Ev USER ;
1158 set for compatibility with systems that use this variable.
1159 .It Ev MAIL
1160 Set to the path of the user's mailbox.
1161 .It Ev PATH
1162 Set to the default
1163 .Ev PATH ,
1164 as specified when compiling
1165 .Nm .
1166 .It Ev SSH_ASKPASS
1167 If
1168 .Nm
1169 needs a passphrase, it will read the passphrase from the current
1170 terminal if it was run from a terminal.
1171 If
1172 .Nm
1173 does not have a terminal associated with it but
1174 .Ev DISPLAY
1175 and
1176 .Ev SSH_ASKPASS
1177 are set, it will execute the program specified by
1178 .Ev SSH_ASKPASS
1179 and open an X11 window to read the passphrase.
1180 This is particularly useful when calling
1181 .Nm
1182 from a
1183 .Pa .xsession
1184 or related script.
1185 (Note that on some machines it
1186 may be necessary to redirect the input from
1187 .Pa /dev/null
1188 to make this work.)
1189 .It Ev SSH_AUTH_SOCK
1190 Identifies the path of a
1191 .Ux Ns -domain
1192 socket used to communicate with the agent.
1193 .It Ev SSH_CONNECTION
1194 Identifies the client and server ends of the connection.
1195 The variable contains
1196 four space-separated values: client IP address, client port number,
1197 server IP address, and server port number.
1198 .It Ev SSH_ORIGINAL_COMMAND
1199 This variable contains the original command line if a forced command
1200 is executed.
1201 It can be used to extract the original arguments.
1202 .It Ev SSH_TTY
1203 This is set to the name of the tty (path to the device) associated
1204 with the current shell or command.
1205 If the current session has no tty,
1206 this variable is not set.
1207 .It Ev TZ
1208 This variable is set to indicate the present time zone if it
1209 was set when the daemon was started (i.e. the daemon passes the value
1210 on to new connections).
1211 .It Ev USER
1212 Set to the name of the user logging in.
1213 .El
1214 .Pp
1215 Additionally,
1216 .Nm
1217 reads
1218 .Pa ~/.ssh/environment ,
1219 and adds lines of the format
1220 .Dq VARNAME=value
1221 to the environment if the file exists and users are allowed to
1222 change their environment.
1223 For more information, see the
1224 .Cm PermitUserEnvironment
1225 option in
1226 .Xr sshd_config 5 .
1227 .Sh FILES
1228 .Bl -tag -width Ds -compact
1229 .It ~/.rhosts
1230 This file is used for host-based authentication (see above).
1231 On some machines this file may need to be
1232 world-readable if the user's home directory is on an NFS partition,
1233 because
1234 .Xr sshd 8
1235 reads it as root.
1236 Additionally, this file must be owned by the user,
1237 and must not have write permissions for anyone else.
1238 The recommended
1239 permission for most machines is read/write for the user, and not
1240 accessible by others.
1241 .Pp
1242 .It ~/.shosts
1243 This file is used in exactly the same way as
1244 .Pa .rhosts ,
1245 but allows host-based authentication without permitting login with
1246 rlogin/rsh.
1247 .Pp
1248 .It ~/.ssh/
1249 This directory is the default location for all user-specific configuration
1250 and authentication information.
1251 There is no general requirement to keep the entire contents of this directory
1252 secret, but the recommended permissions are read/write/execute for the user,
1253 and not accessible by others.
1254 .Pp
1255 .It ~/.ssh/authorized_keys
1256 Lists the public keys (RSA/DSA) that can be used for logging in as this user.
1257 The format of this file is described in the
1258 .Xr sshd 8
1259 manual page.
1260 This file is not highly sensitive, but the recommended
1261 permissions are read/write for the user, and not accessible by others.
1262 .Pp
1263 .It ~/.ssh/config
1264 This is the per-user configuration file.
1265 The file format and configuration options are described in
1266 .Xr ssh_config 5 .
1267 Because of the potential for abuse, this file must have strict permissions:
1268 read/write for the user, and not accessible by others.
1269 .Pp
1270 .It ~/.ssh/environment
1271 Contains additional definitions for environment variables; see
1272 .Sx ENVIRONMENT ,
1273 above.
1274 .Pp
1275 .It ~/.ssh/identity
1276 .It ~/.ssh/id_dsa
1277 .It ~/.ssh/id_rsa
1278 Contains the private key for authentication.
1279 These files
1280 contain sensitive data and should be readable by the user but not
1281 accessible by others (read/write/execute).
1282 .Nm
1283 will simply ignore a private key file if it is accessible by others.
1284 It is possible to specify a passphrase when
1285 generating the key which will be used to encrypt the
1286 sensitive part of this file using 3DES.
1287 .Pp
1288 .It ~/.ssh/identity.pub
1289 .It ~/.ssh/id_dsa.pub
1290 .It ~/.ssh/id_rsa.pub
1291 Contains the public key for authentication.
1292 These files are not
1293 sensitive and can (but need not) be readable by anyone.
1294 .Pp
1295 .It ~/.ssh/known_hosts
1296 Contains a list of host keys for all hosts the user has logged into
1297 that are not already in the systemwide list of known host keys.
1298 See
1299 .Xr sshd 8
1300 for further details of the format of this file.
1301 .Pp
1302 .It ~/.ssh/rc
1303 Commands in this file are executed by
1304 .Nm
1305 when the user logs in, just before the user's shell (or command) is
1306 started.
1307 See the
1308 .Xr sshd 8
1309 manual page for more information.
1310 .Pp
1311 .It /etc/hosts.equiv
1312 This file is for host-based authentication (see above).
1313 It should only be writable by root.
1314 .Pp
1315 .It /etc/shosts.equiv
1316 This file is used in exactly the same way as
1317 .Pa hosts.equiv ,
1318 but allows host-based authentication without permitting login with
1319 rlogin/rsh.
1320 .Pp
1321 .It Pa /etc/ssh/ssh_config
1322 Systemwide configuration file.
1323 The file format and configuration options are described in
1324 .Xr ssh_config 5 .
1325 .Pp
1326 .It /etc/ssh/ssh_host_key
1327 .It /etc/ssh/ssh_host_dsa_key
1328 .It /etc/ssh/ssh_host_rsa_key
1329 These three files contain the private parts of the host keys
1330 and are used for host-based authentication.
1331 If protocol version 1 is used,
1332 .Nm
1333 must be setuid root, since the host key is readable only by root.
1334 For protocol version 2,
1335 .Nm
1336 uses
1337 .Xr ssh-keysign 8
1338 to access the host keys,
1339 eliminating the requirement that
1340 .Nm
1341 be setuid root when host-based authentication is used.
1342 By default
1343 .Nm
1344 is not setuid root.
1345 .Pp
1346 .It /etc/ssh/ssh_known_hosts
1347 Systemwide list of known host keys.
1348 This file should be prepared by the
1349 system administrator to contain the public host keys of all machines in the
1350 organization.
1351 It should be world-readable.
1352 See
1353 .Xr sshd 8
1354 for further details of the format of this file.
1355 .Pp
1356 .It /etc/ssh/sshrc
1357 Commands in this file are executed by
1358 .Nm
1359 when the user logs in, just before the user's shell (or command) is started.
1360 See the
1361 .Xr sshd 8
1362 manual page for more information.
1363 .El
1364 .Sh SEE ALSO
1365 .Xr scp 1 ,
1366 .Xr sftp 1 ,
1367 .Xr ssh-add 1 ,
1368 .Xr ssh-agent 1 ,
1369 .Xr ssh-keygen 1 ,
1370 .Xr ssh-keyscan 1 ,
1371 .Xr tun 4 ,
1372 .Xr hosts.equiv 5 ,
1373 .Xr ssh_config 5 ,
1374 .Xr ssh-keysign 8 ,
1375 .Xr sshd 8
1376 .Rs
1377 .%R RFC 4250
1378 .%T "The Secure Shell (SSH) Protocol Assigned Numbers"
1379 .%D 2006
1380 .Re
1381 .Rs
1382 .%R RFC 4251
1383 .%T "The Secure Shell (SSH) Protocol Architecture"
1384 .%D 2006
1385 .Re
1386 .Rs
1387 .%R RFC 4252
1388 .%T "The Secure Shell (SSH) Authentication Protocol"
1389 .%D 2006
1390 .Re
1391 .Rs
1392 .%R RFC 4253
1393 .%T "The Secure Shell (SSH) Transport Layer Protocol"
1394 .%D 2006
1395 .Re
1396 .Rs
1397 .%R RFC 4254
1398 .%T "The Secure Shell (SSH) Connection Protocol"
1399 .%D 2006
1400 .Re
1401 .Rs
1402 .%R RFC 4255
1403 .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints"
1404 .%D 2006
1405 .Re
1406 .Rs
1407 .%R RFC 4256
1408 .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)"
1409 .%D 2006
1410 .Re
1411 .Rs
1412 .%R RFC 4335
1413 .%T "The Secure Shell (SSH) Session Channel Break Extension"
1414 .%D 2006
1415 .Re
1416 .Rs
1417 .%R RFC 4344
1418 .%T "The Secure Shell (SSH) Transport Layer Encryption Modes"
1419 .%D 2006
1420 .Re
1421 .Rs
1422 .%R RFC 4345
1423 .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol"
1424 .%D 2006
1425 .Re
1426 .Rs
1427 .%R RFC 4419
1428 .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol"
1429 .%D 2006
1430 .Re
1431 .Rs
1432 .%R RFC 4716
1433 .%T "The Secure Shell (SSH) Public Key File Format"
1434 .%D 2006
1435 .Re
1436 .Sh AUTHORS
1437 OpenSSH is a derivative of the original and free
1438 ssh 1.2.12 release by Tatu Ylonen.
1439 Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos,
1440 Theo de Raadt and Dug Song
1441 removed many bugs, re-added newer features and
1442 created OpenSSH.
1443 Markus Friedl contributed the support for SSH
1444 protocol versions 1.5 and 2.0.