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1 | .\" -*- nroff -*- | |
2 | .\" | |
3 | .\" Author: Tatu Ylonen <ylo@cs.hut.fi> | |
4 | .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland | |
5 | .\" All rights reserved | |
6 | .\" | |
7 | .\" As far as I am concerned, the code I have written for this software | |
8 | .\" can be used freely for any purpose. Any derived versions of this | |
9 | .\" software must be clearly marked as such, and if the derived work is | |
10 | .\" incompatible with the protocol description in the RFC file, it must be | |
11 | .\" called by a name other than "ssh" or "Secure Shell". | |
12 | .\" | |
13 | .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. | |
14 | .\" Copyright (c) 1999 Aaron Campbell. All rights reserved. | |
15 | .\" Copyright (c) 1999 Theo de Raadt. All rights reserved. | |
16 | .\" | |
17 | .\" Redistribution and use in source and binary forms, with or without | |
18 | .\" modification, are permitted provided that the following conditions | |
19 | .\" are met: | |
20 | .\" 1. Redistributions of source code must retain the above copyright | |
21 | .\" notice, this list of conditions and the following disclaimer. | |
22 | .\" 2. Redistributions in binary form must reproduce the above copyright | |
23 | .\" notice, this list of conditions and the following disclaimer in the | |
24 | .\" documentation and/or other materials provided with the distribution. | |
25 | .\" | |
26 | .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR | |
27 | .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES | |
28 | .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. | |
29 | .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, | |
30 | .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT | |
31 | .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, | |
32 | .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY | |
33 | .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT | |
34 | .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF | |
35 | .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. | |
36 | .\" | |
37 | .\" $OpenBSD: ssh.1,v 1.290 2010/01/11 01:39:46 dtucker Exp $ | |
38 | .Dd $Mdocdate$ | |
39 | .Dt SSH 1 | |
40 | .Os | |
41 | .Sh NAME | |
42 | .Nm ssh | |
43 | .Nd OpenSSH SSH client (remote login program) | |
44 | .Sh SYNOPSIS | |
45 | .Nm ssh | |
46 | .Op Fl 1246AaCfgKkMNnqsTtVvXxYy | |
47 | .Op Fl b Ar bind_address | |
48 | .Op Fl c Ar cipher_spec | |
49 | .Oo Fl D\ \& | |
50 | .Sm off | |
51 | .Oo Ar bind_address : Oc | |
52 | .Ar port | |
53 | .Sm on | |
54 | .Oc | |
55 | .Op Fl e Ar escape_char | |
56 | .Op Fl F Ar configfile | |
57 | .Bk -words | |
58 | .Op Fl i Ar identity_file | |
59 | .Ek | |
60 | .Oo Fl L\ \& | |
61 | .Sm off | |
62 | .Oo Ar bind_address : Oc | |
63 | .Ar port : host : hostport | |
64 | .Sm on | |
65 | .Oc | |
66 | .Bk -words | |
67 | .Op Fl l Ar login_name | |
68 | .Ek | |
69 | .Op Fl m Ar mac_spec | |
70 | .Op Fl O Ar ctl_cmd | |
71 | .Op Fl o Ar option | |
72 | .Op Fl p Ar port | |
73 | .Oo Fl R\ \& | |
74 | .Sm off | |
75 | .Oo Ar bind_address : Oc | |
76 | .Ar port : host : hostport | |
77 | .Sm on | |
78 | .Oc | |
79 | .Op Fl S Ar ctl_path | |
80 | .Op Fl W Ar host : Ns Ar port | |
81 | .Oo Fl w Ar local_tun Ns | |
82 | .Op : Ns Ar remote_tun Oc | |
83 | .Oo Ar user Ns @ Oc Ns Ar hostname | |
84 | .Op Ar command | |
85 | .Sh DESCRIPTION | |
86 | .Nm | |
87 | (SSH client) is a program for logging into a remote machine and for | |
88 | executing commands on a remote machine. | |
89 | It is intended to replace rlogin and rsh, | |
90 | and provide secure encrypted communications between | |
91 | two untrusted hosts over an insecure network. | |
92 | X11 connections and arbitrary TCP ports | |
93 | can also be forwarded over the secure channel. | |
94 | .Pp | |
95 | .Nm | |
96 | connects and logs into the specified | |
97 | .Ar hostname | |
98 | (with optional | |
99 | .Ar user | |
100 | name). | |
101 | The user must prove | |
102 | his/her identity to the remote machine using one of several methods | |
103 | depending on the protocol version used (see below). | |
104 | .Pp | |
105 | If | |
106 | .Ar command | |
107 | is specified, | |
108 | it is executed on the remote host instead of a login shell. | |
109 | .Pp | |
110 | The options are as follows: | |
111 | .Bl -tag -width Ds | |
112 | .It Fl 1 | |
113 | Forces | |
114 | .Nm | |
115 | to try protocol version 1 only. | |
116 | .It Fl 2 | |
117 | Forces | |
118 | .Nm | |
119 | to try protocol version 2 only. | |
120 | .It Fl 4 | |
121 | Forces | |
122 | .Nm | |
123 | to use IPv4 addresses only. | |
124 | .It Fl 6 | |
125 | Forces | |
126 | .Nm | |
127 | to use IPv6 addresses only. | |
128 | .It Fl A | |
129 | Enables forwarding of the authentication agent connection. | |
130 | This can also be specified on a per-host basis in a configuration file. | |
131 | .Pp | |
132 | Agent forwarding should be enabled with caution. | |
133 | Users with the ability to bypass file permissions on the remote host | |
134 | (for the agent's | |
135 | .Ux Ns -domain | |
136 | socket) can access the local agent through the forwarded connection. | |
137 | An attacker cannot obtain key material from the agent, | |
138 | however they can perform operations on the keys that enable them to | |
139 | authenticate using the identities loaded into the agent. | |
140 | .It Fl a | |
141 | Disables forwarding of the authentication agent connection. | |
142 | .It Fl b Ar bind_address | |
143 | Use | |
144 | .Ar bind_address | |
145 | on the local machine as the source address | |
146 | of the connection. | |
147 | Only useful on systems with more than one address. | |
148 | .It Fl C | |
149 | Requests compression of all data (including stdin, stdout, stderr, and | |
150 | data for forwarded X11 and TCP connections). | |
151 | The compression algorithm is the same used by | |
152 | .Xr gzip 1 , | |
153 | and the | |
154 | .Dq level | |
155 | can be controlled by the | |
156 | .Cm CompressionLevel | |
157 | option for protocol version 1. | |
158 | Compression is desirable on modem lines and other | |
159 | slow connections, but will only slow down things on fast networks. | |
160 | The default value can be set on a host-by-host basis in the | |
161 | configuration files; see the | |
162 | .Cm Compression | |
163 | option. | |
164 | .It Fl c Ar cipher_spec | |
165 | Selects the cipher specification for encrypting the session. | |
166 | .Pp | |
167 | Protocol version 1 allows specification of a single cipher. | |
168 | The supported values are | |
169 | .Dq 3des , | |
170 | .Dq blowfish , | |
171 | and | |
172 | .Dq des . | |
173 | .Ar 3des | |
174 | (triple-des) is an encrypt-decrypt-encrypt triple with three different keys. | |
175 | It is believed to be secure. | |
176 | .Ar blowfish | |
177 | is a fast block cipher; it appears very secure and is much faster than | |
178 | .Ar 3des . | |
179 | .Ar des | |
180 | is only supported in the | |
181 | .Nm | |
182 | client for interoperability with legacy protocol 1 implementations | |
183 | that do not support the | |
184 | .Ar 3des | |
185 | cipher. | |
186 | Its use is strongly discouraged due to cryptographic weaknesses. | |
187 | The default is | |
188 | .Dq 3des . | |
189 | .Pp | |
190 | For protocol version 2, | |
191 | .Ar cipher_spec | |
192 | is a comma-separated list of ciphers | |
193 | listed in order of preference. | |
194 | See the | |
195 | .Cm Ciphers | |
196 | keyword for more information. | |
197 | .It Fl D Xo | |
198 | .Sm off | |
199 | .Oo Ar bind_address : Oc | |
200 | .Ar port | |
201 | .Sm on | |
202 | .Xc | |
203 | Specifies a local | |
204 | .Dq dynamic | |
205 | application-level port forwarding. | |
206 | This works by allocating a socket to listen to | |
207 | .Ar port | |
208 | on the local side, optionally bound to the specified | |
209 | .Ar bind_address . | |
210 | Whenever a connection is made to this port, the | |
211 | connection is forwarded over the secure channel, and the application | |
212 | protocol is then used to determine where to connect to from the | |
213 | remote machine. | |
214 | Currently the SOCKS4 and SOCKS5 protocols are supported, and | |
215 | .Nm | |
216 | will act as a SOCKS server. | |
217 | Only root can forward privileged ports. | |
218 | Dynamic port forwardings can also be specified in the configuration file. | |
219 | .Pp | |
220 | IPv6 addresses can be specified with an alternative syntax: | |
221 | .Sm off | |
222 | .Xo | |
223 | .Op Ar bind_address No / | |
224 | .Ar port | |
225 | .Xc | |
226 | .Sm on | |
227 | or by enclosing the address in square brackets. | |
228 | Only the superuser can forward privileged ports. | |
229 | By default, the local port is bound in accordance with the | |
230 | .Cm GatewayPorts | |
231 | setting. | |
232 | However, an explicit | |
233 | .Ar bind_address | |
234 | may be used to bind the connection to a specific address. | |
235 | The | |
236 | .Ar bind_address | |
237 | of | |
238 | .Dq localhost | |
239 | indicates that the listening port be bound for local use only, while an | |
240 | empty address or | |
241 | .Sq * | |
242 | indicates that the port should be available from all interfaces. | |
243 | .It Fl e Ar escape_char | |
244 | Sets the escape character for sessions with a pty (default: | |
245 | .Ql ~ ) . | |
246 | The escape character is only recognized at the beginning of a line. | |
247 | The escape character followed by a dot | |
248 | .Pq Ql \&. | |
249 | closes the connection; | |
250 | followed by control-Z suspends the connection; | |
251 | and followed by itself sends the escape character once. | |
252 | Setting the character to | |
253 | .Dq none | |
254 | disables any escapes and makes the session fully transparent. | |
255 | .It Fl F Ar configfile | |
256 | Specifies an alternative per-user configuration file. | |
257 | If a configuration file is given on the command line, | |
258 | the system-wide configuration file | |
259 | .Pq Pa /etc/ssh/ssh_config | |
260 | will be ignored. | |
261 | The default for the per-user configuration file is | |
262 | .Pa ~/.ssh/config . | |
263 | .It Fl f | |
264 | Requests | |
265 | .Nm | |
266 | to go to background just before command execution. | |
267 | This is useful if | |
268 | .Nm | |
269 | is going to ask for passwords or passphrases, but the user | |
270 | wants it in the background. | |
271 | This implies | |
272 | .Fl n . | |
273 | The recommended way to start X11 programs at a remote site is with | |
274 | something like | |
275 | .Ic ssh -f host xterm . | |
276 | .Pp | |
277 | If the | |
278 | .Cm ExitOnForwardFailure | |
279 | configuration option is set to | |
280 | .Dq yes , | |
281 | then a client started with | |
282 | .Fl f | |
283 | will wait for all remote port forwards to be successfully established | |
284 | before placing itself in the background. | |
285 | .It Fl g | |
286 | Allows remote hosts to connect to local forwarded ports. | |
287 | .It Fl I Ar smartcard_device | |
288 | Specify the device | |
289 | .Nm | |
290 | should use to communicate with a smartcard used for storing the user's | |
291 | private RSA key. | |
292 | This option is only available if support for smartcard devices | |
293 | is compiled in (default is no support). | |
294 | .It Fl i Ar identity_file | |
295 | Selects a file from which the identity (private key) for | |
296 | RSA or DSA authentication is read. | |
297 | The default is | |
298 | .Pa ~/.ssh/identity | |
299 | for protocol version 1, and | |
300 | .Pa ~/.ssh/id_rsa | |
301 | and | |
302 | .Pa ~/.ssh/id_dsa | |
303 | for protocol version 2. | |
304 | Identity files may also be specified on | |
305 | a per-host basis in the configuration file. | |
306 | It is possible to have multiple | |
307 | .Fl i | |
308 | options (and multiple identities specified in | |
309 | configuration files). | |
310 | .It Fl K | |
311 | Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI | |
312 | credentials to the server. | |
313 | .It Fl k | |
314 | Disables forwarding (delegation) of GSSAPI credentials to the server. | |
315 | .It Fl L Xo | |
316 | .Sm off | |
317 | .Oo Ar bind_address : Oc | |
318 | .Ar port : host : hostport | |
319 | .Sm on | |
320 | .Xc | |
321 | Specifies that the given port on the local (client) host is to be | |
322 | forwarded to the given host and port on the remote side. | |
323 | This works by allocating a socket to listen to | |
324 | .Ar port | |
325 | on the local side, optionally bound to the specified | |
326 | .Ar bind_address . | |
327 | Whenever a connection is made to this port, the | |
328 | connection is forwarded over the secure channel, and a connection is | |
329 | made to | |
330 | .Ar host | |
331 | port | |
332 | .Ar hostport | |
333 | from the remote machine. | |
334 | Port forwardings can also be specified in the configuration file. | |
335 | IPv6 addresses can be specified with an alternative syntax: | |
336 | .Sm off | |
337 | .Xo | |
338 | .Op Ar bind_address No / | |
339 | .Ar port No / Ar host No / | |
340 | .Ar hostport | |
341 | .Xc | |
342 | .Sm on | |
343 | or by enclosing the address in square brackets. | |
344 | Only the superuser can forward privileged ports. | |
345 | By default, the local port is bound in accordance with the | |
346 | .Cm GatewayPorts | |
347 | setting. | |
348 | However, an explicit | |
349 | .Ar bind_address | |
350 | may be used to bind the connection to a specific address. | |
351 | The | |
352 | .Ar bind_address | |
353 | of | |
354 | .Dq localhost | |
355 | indicates that the listening port be bound for local use only, while an | |
356 | empty address or | |
357 | .Sq * | |
358 | indicates that the port should be available from all interfaces. | |
359 | .It Fl l Ar login_name | |
360 | Specifies the user to log in as on the remote machine. | |
361 | This also may be specified on a per-host basis in the configuration file. | |
362 | .It Fl M | |
363 | Places the | |
364 | .Nm | |
365 | client into | |
366 | .Dq master | |
367 | mode for connection sharing. | |
368 | Multiple | |
369 | .Fl M | |
370 | options places | |
371 | .Nm | |
372 | into | |
373 | .Dq master | |
374 | mode with confirmation required before slave connections are accepted. | |
375 | Refer to the description of | |
376 | .Cm ControlMaster | |
377 | in | |
378 | .Xr ssh_config 5 | |
379 | for details. | |
380 | .It Fl m Ar mac_spec | |
381 | Additionally, for protocol version 2 a comma-separated list of MAC | |
382 | (message authentication code) algorithms can | |
383 | be specified in order of preference. | |
384 | See the | |
385 | .Cm MACs | |
386 | keyword for more information. | |
387 | .It Fl N | |
388 | Do not execute a remote command. | |
389 | This is useful for just forwarding ports | |
390 | (protocol version 2 only). | |
391 | .It Fl n | |
392 | Redirects stdin from | |
393 | .Pa /dev/null | |
394 | (actually, prevents reading from stdin). | |
395 | This must be used when | |
396 | .Nm | |
397 | is run in the background. | |
398 | A common trick is to use this to run X11 programs on a remote machine. | |
399 | For example, | |
400 | .Ic ssh -n shadows.cs.hut.fi emacs & | |
401 | will start an emacs on shadows.cs.hut.fi, and the X11 | |
402 | connection will be automatically forwarded over an encrypted channel. | |
403 | The | |
404 | .Nm | |
405 | program will be put in the background. | |
406 | (This does not work if | |
407 | .Nm | |
408 | needs to ask for a password or passphrase; see also the | |
409 | .Fl f | |
410 | option.) | |
411 | .It Fl O Ar ctl_cmd | |
412 | Control an active connection multiplexing master process. | |
413 | When the | |
414 | .Fl O | |
415 | option is specified, the | |
416 | .Ar ctl_cmd | |
417 | argument is interpreted and passed to the master process. | |
418 | Valid commands are: | |
419 | .Dq check | |
420 | (check that the master process is running) and | |
421 | .Dq exit | |
422 | (request the master to exit). | |
423 | .It Fl o Ar option | |
424 | Can be used to give options in the format used in the configuration file. | |
425 | This is useful for specifying options for which there is no separate | |
426 | command-line flag. | |
427 | For full details of the options listed below, and their possible values, see | |
428 | .Xr ssh_config 5 . | |
429 | .Pp | |
430 | .Bl -tag -width Ds -offset indent -compact | |
431 | .It AddressFamily | |
432 | .It BatchMode | |
433 | .It BindAddress | |
434 | .It ChallengeResponseAuthentication | |
435 | .It CheckHostIP | |
436 | .It Cipher | |
437 | .It Ciphers | |
438 | .It ClearAllForwardings | |
439 | .It Compression | |
440 | .It CompressionLevel | |
441 | .It ConnectionAttempts | |
442 | .It ConnectTimeout | |
443 | .It ControlMaster | |
444 | .It ControlPath | |
445 | .It DynamicForward | |
446 | .It EscapeChar | |
447 | .It ExitOnForwardFailure | |
448 | .It ForwardAgent | |
449 | .It ForwardX11 | |
450 | .It ForwardX11Trusted | |
451 | .It GatewayPorts | |
452 | .It GlobalKnownHostsFile | |
453 | .It GSSAPIAuthentication | |
454 | .It GSSAPIDelegateCredentials | |
455 | .It HashKnownHosts | |
456 | .It Host | |
457 | .It HostbasedAuthentication | |
458 | .It HostKeyAlgorithms | |
459 | .It HostKeyAlias | |
460 | .It HostName | |
461 | .It IdentityFile | |
462 | .It IdentitiesOnly | |
463 | .It KbdInteractiveDevices | |
464 | .It LocalCommand | |
465 | .It LocalForward | |
466 | .It LogLevel | |
467 | .It MACs | |
468 | .It NoHostAuthenticationForLocalhost | |
469 | .It NumberOfPasswordPrompts | |
470 | .It PasswordAuthentication | |
471 | .It PermitLocalCommand | |
472 | .It Port | |
473 | .It PreferredAuthentications | |
474 | .It Protocol | |
475 | .It ProxyCommand | |
476 | .It PubkeyAuthentication | |
477 | .It RekeyLimit | |
478 | .It RemoteForward | |
479 | .It RhostsRSAAuthentication | |
480 | .It RSAAuthentication | |
481 | .It SendEnv | |
482 | .It ServerAliveInterval | |
483 | .It ServerAliveCountMax | |
484 | .It SmartcardDevice | |
485 | .It StrictHostKeyChecking | |
486 | .It TCPKeepAlive | |
487 | .It Tunnel | |
488 | .It TunnelDevice | |
489 | .It UsePrivilegedPort | |
490 | .It User | |
491 | .It UserKnownHostsFile | |
492 | .It VerifyHostKeyDNS | |
493 | .It VisualHostKey | |
494 | .It XAuthLocation | |
495 | .El | |
496 | .It Fl p Ar port | |
497 | Port to connect to on the remote host. | |
498 | This can be specified on a | |
499 | per-host basis in the configuration file. | |
500 | .It Fl q | |
501 | Quiet mode. | |
502 | Causes most warning and diagnostic messages to be suppressed. | |
503 | .It Fl R Xo | |
504 | .Sm off | |
505 | .Oo Ar bind_address : Oc | |
506 | .Ar port : host : hostport | |
507 | .Sm on | |
508 | .Xc | |
509 | Specifies that the given port on the remote (server) host is to be | |
510 | forwarded to the given host and port on the local side. | |
511 | This works by allocating a socket to listen to | |
512 | .Ar port | |
513 | on the remote side, and whenever a connection is made to this port, the | |
514 | connection is forwarded over the secure channel, and a connection is | |
515 | made to | |
516 | .Ar host | |
517 | port | |
518 | .Ar hostport | |
519 | from the local machine. | |
520 | .Pp | |
521 | Port forwardings can also be specified in the configuration file. | |
522 | Privileged ports can be forwarded only when | |
523 | logging in as root on the remote machine. | |
524 | IPv6 addresses can be specified by enclosing the address in square braces or | |
525 | using an alternative syntax: | |
526 | .Sm off | |
527 | .Xo | |
528 | .Op Ar bind_address No / | |
529 | .Ar host No / Ar port No / | |
530 | .Ar hostport | |
531 | .Xc . | |
532 | .Sm on | |
533 | .Pp | |
534 | By default, the listening socket on the server will be bound to the loopback | |
535 | interface only. | |
536 | This may be overridden by specifying a | |
537 | .Ar bind_address . | |
538 | An empty | |
539 | .Ar bind_address , | |
540 | or the address | |
541 | .Ql * , | |
542 | indicates that the remote socket should listen on all interfaces. | |
543 | Specifying a remote | |
544 | .Ar bind_address | |
545 | will only succeed if the server's | |
546 | .Cm GatewayPorts | |
547 | option is enabled (see | |
548 | .Xr sshd_config 5 ) . | |
549 | .Pp | |
550 | If the | |
551 | .Ar port | |
552 | argument is | |
553 | .Ql 0 , | |
554 | the listen port will be dynamically allocated on the server and reported | |
555 | to the client at run time. | |
556 | .It Fl S Ar ctl_path | |
557 | Specifies the location of a control socket for connection sharing. | |
558 | Refer to the description of | |
559 | .Cm ControlPath | |
560 | and | |
561 | .Cm ControlMaster | |
562 | in | |
563 | .Xr ssh_config 5 | |
564 | for details. | |
565 | .It Fl s | |
566 | May be used to request invocation of a subsystem on the remote system. | |
567 | Subsystems are a feature of the SSH2 protocol which facilitate the use | |
568 | of SSH as a secure transport for other applications (eg.\& | |
569 | .Xr sftp 1 ) . | |
570 | The subsystem is specified as the remote command. | |
571 | .It Fl T | |
572 | Disable pseudo-tty allocation. | |
573 | .It Fl t | |
574 | Force pseudo-tty allocation. | |
575 | This can be used to execute arbitrary | |
576 | screen-based programs on a remote machine, which can be very useful, | |
577 | e.g. when implementing menu services. | |
578 | Multiple | |
579 | .Fl t | |
580 | options force tty allocation, even if | |
581 | .Nm | |
582 | has no local tty. | |
583 | .It Fl V | |
584 | Display the version number and exit. | |
585 | .It Fl v | |
586 | Verbose mode. | |
587 | Causes | |
588 | .Nm | |
589 | to print debugging messages about its progress. | |
590 | This is helpful in | |
591 | debugging connection, authentication, and configuration problems. | |
592 | Multiple | |
593 | .Fl v | |
594 | options increase the verbosity. | |
595 | The maximum is 3. | |
596 | .It Fl W Ar host : Ns Ar port | |
597 | Requests that standard input and output on the client be forwarded to | |
598 | .Ar host | |
599 | on | |
600 | .Ar port | |
601 | over the secure channel. | |
602 | Implies | |
603 | .Fl N , | |
604 | .Fl T , | |
605 | .Cm ExitOnForwardFailure | |
606 | and | |
607 | .Cm ClearAllForwardings | |
608 | and works with Protocol version 2 only. | |
609 | .It Fl w Xo | |
610 | .Ar local_tun Ns Op : Ns Ar remote_tun | |
611 | .Xc | |
612 | Requests | |
613 | tunnel | |
614 | device forwarding with the specified | |
615 | .Xr tun 4 | |
616 | devices between the client | |
617 | .Pq Ar local_tun | |
618 | and the server | |
619 | .Pq Ar remote_tun . | |
620 | .Pp | |
621 | The devices may be specified by numerical ID or the keyword | |
622 | .Dq any , | |
623 | which uses the next available tunnel device. | |
624 | If | |
625 | .Ar remote_tun | |
626 | is not specified, it defaults to | |
627 | .Dq any . | |
628 | See also the | |
629 | .Cm Tunnel | |
630 | and | |
631 | .Cm TunnelDevice | |
632 | directives in | |
633 | .Xr ssh_config 5 . | |
634 | If the | |
635 | .Cm Tunnel | |
636 | directive is unset, it is set to the default tunnel mode, which is | |
637 | .Dq point-to-point . | |
638 | .It Fl X | |
639 | Enables X11 forwarding. | |
640 | This can also be specified on a per-host basis in a configuration file. | |
641 | .Pp | |
642 | X11 forwarding should be enabled with caution. | |
643 | Users with the ability to bypass file permissions on the remote host | |
644 | (for the user's X authorization database) | |
645 | can access the local X11 display through the forwarded connection. | |
646 | An attacker may then be able to perform activities such as keystroke monitoring. | |
647 | .Pp | |
648 | For this reason, X11 forwarding is subjected to X11 SECURITY extension | |
649 | restrictions by default. | |
650 | Please refer to the | |
651 | .Nm | |
652 | .Fl Y | |
653 | option and the | |
654 | .Cm ForwardX11Trusted | |
655 | directive in | |
656 | .Xr ssh_config 5 | |
657 | for more information. | |
658 | .It Fl x | |
659 | Disables X11 forwarding. | |
660 | .It Fl Y | |
661 | Enables trusted X11 forwarding. | |
662 | Trusted X11 forwardings are not subjected to the X11 SECURITY extension | |
663 | controls. | |
664 | .It Fl y | |
665 | Send log information using the | |
666 | .Xr syslog 3 | |
667 | system module. | |
668 | By default this information is sent to stderr. | |
669 | .El | |
670 | .Pp | |
671 | .Nm | |
672 | may additionally obtain configuration data from | |
673 | a per-user configuration file and a system-wide configuration file. | |
674 | The file format and configuration options are described in | |
675 | .Xr ssh_config 5 . | |
676 | .Pp | |
677 | .Nm | |
678 | exits with the exit status of the remote command or with 255 | |
679 | if an error occurred. | |
680 | .Sh AUTHENTICATION | |
681 | The OpenSSH SSH client supports SSH protocols 1 and 2. | |
682 | The default is to use protocol 2 only, | |
683 | though this can be changed via the | |
684 | .Cm Protocol | |
685 | option in | |
686 | .Xr ssh_config 5 | |
687 | or the | |
688 | .Fl 1 | |
689 | and | |
690 | .Fl 2 | |
691 | options (see above). | |
692 | Both protocols support similar authentication methods, | |
693 | but protocol 2 is the default since | |
694 | it provides additional mechanisms for confidentiality | |
695 | (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) | |
696 | and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160). | |
697 | Protocol 1 lacks a strong mechanism for ensuring the | |
698 | integrity of the connection. | |
699 | .Pp | |
700 | The methods available for authentication are: | |
701 | GSSAPI-based authentication, | |
702 | host-based authentication, | |
703 | public key authentication, | |
704 | challenge-response authentication, | |
705 | and password authentication. | |
706 | Authentication methods are tried in the order specified above, | |
707 | though protocol 2 has a configuration option to change the default order: | |
708 | .Cm PreferredAuthentications . | |
709 | .Pp | |
710 | Host-based authentication works as follows: | |
711 | If the machine the user logs in from is listed in | |
712 | .Pa /etc/hosts.equiv | |
713 | or | |
714 | .Pa /etc/shosts.equiv | |
715 | on the remote machine, and the user names are | |
716 | the same on both sides, or if the files | |
717 | .Pa ~/.rhosts | |
718 | or | |
719 | .Pa ~/.shosts | |
720 | exist in the user's home directory on the | |
721 | remote machine and contain a line containing the name of the client | |
722 | machine and the name of the user on that machine, the user is | |
723 | considered for login. | |
724 | Additionally, the server | |
725 | .Em must | |
726 | be able to verify the client's | |
727 | host key (see the description of | |
728 | .Pa /etc/ssh/ssh_known_hosts | |
729 | and | |
730 | .Pa ~/.ssh/known_hosts , | |
731 | below) | |
732 | for login to be permitted. | |
733 | This authentication method closes security holes due to IP | |
734 | spoofing, DNS spoofing, and routing spoofing. | |
735 | [Note to the administrator: | |
736 | .Pa /etc/hosts.equiv , | |
737 | .Pa ~/.rhosts , | |
738 | and the rlogin/rsh protocol in general, are inherently insecure and should be | |
739 | disabled if security is desired.] | |
740 | .Pp | |
741 | Public key authentication works as follows: | |
742 | The scheme is based on public-key cryptography, | |
743 | using cryptosystems | |
744 | where encryption and decryption are done using separate keys, | |
745 | and it is unfeasible to derive the decryption key from the encryption key. | |
746 | The idea is that each user creates a public/private | |
747 | key pair for authentication purposes. | |
748 | The server knows the public key, and only the user knows the private key. | |
749 | .Nm | |
750 | implements public key authentication protocol automatically, | |
751 | using either the RSA or DSA algorithms. | |
752 | Protocol 1 is restricted to using only RSA keys, | |
753 | but protocol 2 may use either. | |
754 | The | |
755 | .Sx HISTORY | |
756 | section of | |
757 | .Xr ssl 8 | |
758 | contains a brief discussion of the two algorithms. | |
759 | .Pp | |
760 | The file | |
761 | .Pa ~/.ssh/authorized_keys | |
762 | lists the public keys that are permitted for logging in. | |
763 | When the user logs in, the | |
764 | .Nm | |
765 | program tells the server which key pair it would like to use for | |
766 | authentication. | |
767 | The client proves that it has access to the private key | |
768 | and the server checks that the corresponding public key | |
769 | is authorized to accept the account. | |
770 | .Pp | |
771 | The user creates his/her key pair by running | |
772 | .Xr ssh-keygen 1 . | |
773 | This stores the private key in | |
774 | .Pa ~/.ssh/identity | |
775 | (protocol 1), | |
776 | .Pa ~/.ssh/id_dsa | |
777 | (protocol 2 DSA), | |
778 | or | |
779 | .Pa ~/.ssh/id_rsa | |
780 | (protocol 2 RSA) | |
781 | and stores the public key in | |
782 | .Pa ~/.ssh/identity.pub | |
783 | (protocol 1), | |
784 | .Pa ~/.ssh/id_dsa.pub | |
785 | (protocol 2 DSA), | |
786 | or | |
787 | .Pa ~/.ssh/id_rsa.pub | |
788 | (protocol 2 RSA) | |
789 | in the user's home directory. | |
790 | The user should then copy the public key | |
791 | to | |
792 | .Pa ~/.ssh/authorized_keys | |
793 | in his/her home directory on the remote machine. | |
794 | The | |
795 | .Pa authorized_keys | |
796 | file corresponds to the conventional | |
797 | .Pa ~/.rhosts | |
798 | file, and has one key | |
799 | per line, though the lines can be very long. | |
800 | After this, the user can log in without giving the password. | |
801 | .Pp | |
802 | The most convenient way to use public key authentication may be with an | |
803 | authentication agent. | |
804 | See | |
805 | .Xr ssh-agent 1 | |
806 | for more information. | |
807 | .Pp | |
808 | Challenge-response authentication works as follows: | |
809 | The server sends an arbitrary | |
810 | .Qq challenge | |
811 | text, and prompts for a response. | |
812 | Protocol 2 allows multiple challenges and responses; | |
813 | protocol 1 is restricted to just one challenge/response. | |
814 | Examples of challenge-response authentication include | |
815 | BSD Authentication (see | |
816 | .Xr login.conf 5 ) | |
817 | and PAM (some non-OpenBSD systems). | |
818 | .Pp | |
819 | Finally, if other authentication methods fail, | |
820 | .Nm | |
821 | prompts the user for a password. | |
822 | The password is sent to the remote | |
823 | host for checking; however, since all communications are encrypted, | |
824 | the password cannot be seen by someone listening on the network. | |
825 | .Pp | |
826 | .Nm | |
827 | automatically maintains and checks a database containing | |
828 | identification for all hosts it has ever been used with. | |
829 | Host keys are stored in | |
830 | .Pa ~/.ssh/known_hosts | |
831 | in the user's home directory. | |
832 | Additionally, the file | |
833 | .Pa /etc/ssh/ssh_known_hosts | |
834 | is automatically checked for known hosts. | |
835 | Any new hosts are automatically added to the user's file. | |
836 | If a host's identification ever changes, | |
837 | .Nm | |
838 | warns about this and disables password authentication to prevent | |
839 | server spoofing or man-in-the-middle attacks, | |
840 | which could otherwise be used to circumvent the encryption. | |
841 | The | |
842 | .Cm StrictHostKeyChecking | |
843 | option can be used to control logins to machines whose | |
844 | host key is not known or has changed. | |
845 | .Pp | |
846 | When the user's identity has been accepted by the server, the server | |
847 | either executes the given command, or logs into the machine and gives | |
848 | the user a normal shell on the remote machine. | |
849 | All communication with | |
850 | the remote command or shell will be automatically encrypted. | |
851 | .Pp | |
852 | If a pseudo-terminal has been allocated (normal login session), the | |
853 | user may use the escape characters noted below. | |
854 | .Pp | |
855 | If no pseudo-tty has been allocated, | |
856 | the session is transparent and can be used to reliably transfer binary data. | |
857 | On most systems, setting the escape character to | |
858 | .Dq none | |
859 | will also make the session transparent even if a tty is used. | |
860 | .Pp | |
861 | The session terminates when the command or shell on the remote | |
862 | machine exits and all X11 and TCP connections have been closed. | |
863 | .Sh ESCAPE CHARACTERS | |
864 | When a pseudo-terminal has been requested, | |
865 | .Nm | |
866 | supports a number of functions through the use of an escape character. | |
867 | .Pp | |
868 | A single tilde character can be sent as | |
869 | .Ic ~~ | |
870 | or by following the tilde by a character other than those described below. | |
871 | The escape character must always follow a newline to be interpreted as | |
872 | special. | |
873 | The escape character can be changed in configuration files using the | |
874 | .Cm EscapeChar | |
875 | configuration directive or on the command line by the | |
876 | .Fl e | |
877 | option. | |
878 | .Pp | |
879 | The supported escapes (assuming the default | |
880 | .Ql ~ ) | |
881 | are: | |
882 | .Bl -tag -width Ds | |
883 | .It Cm ~. | |
884 | Disconnect. | |
885 | .It Cm ~^Z | |
886 | Background | |
887 | .Nm . | |
888 | .It Cm ~# | |
889 | List forwarded connections. | |
890 | .It Cm ~& | |
891 | Background | |
892 | .Nm | |
893 | at logout when waiting for forwarded connection / X11 sessions to terminate. | |
894 | .It Cm ~? | |
895 | Display a list of escape characters. | |
896 | .It Cm ~B | |
897 | Send a BREAK to the remote system | |
898 | (only useful for SSH protocol version 2 and if the peer supports it). | |
899 | .It Cm ~C | |
900 | Open command line. | |
901 | Currently this allows the addition of port forwardings using the | |
902 | .Fl L , | |
903 | .Fl R | |
904 | and | |
905 | .Fl D | |
906 | options (see above). | |
907 | It also allows the cancellation of existing remote port-forwardings | |
908 | using | |
909 | .Sm off | |
910 | .Fl KR Oo Ar bind_address : Oc Ar port . | |
911 | .Sm on | |
912 | .Ic !\& Ns Ar command | |
913 | allows the user to execute a local command if the | |
914 | .Ic PermitLocalCommand | |
915 | option is enabled in | |
916 | .Xr ssh_config 5 . | |
917 | Basic help is available, using the | |
918 | .Fl h | |
919 | option. | |
920 | .It Cm ~R | |
921 | Request rekeying of the connection | |
922 | (only useful for SSH protocol version 2 and if the peer supports it). | |
923 | .El | |
924 | .Sh TCP FORWARDING | |
925 | Forwarding of arbitrary TCP connections over the secure channel can | |
926 | be specified either on the command line or in a configuration file. | |
927 | One possible application of TCP forwarding is a secure connection to a | |
928 | mail server; another is going through firewalls. | |
929 | .Pp | |
930 | In the example below, we look at encrypting communication between | |
931 | an IRC client and server, even though the IRC server does not directly | |
932 | support encrypted communications. | |
933 | This works as follows: | |
934 | the user connects to the remote host using | |
935 | .Nm , | |
936 | specifying a port to be used to forward connections | |
937 | to the remote server. | |
938 | After that it is possible to start the service which is to be encrypted | |
939 | on the client machine, | |
940 | connecting to the same local port, | |
941 | and | |
942 | .Nm | |
943 | will encrypt and forward the connection. | |
944 | .Pp | |
945 | The following example tunnels an IRC session from client machine | |
946 | .Dq 127.0.0.1 | |
947 | (localhost) | |
948 | to remote server | |
949 | .Dq server.example.com : | |
950 | .Bd -literal -offset 4n | |
951 | $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 | |
952 | $ irc -c '#users' -p 1234 pinky 127.0.0.1 | |
953 | .Ed | |
954 | .Pp | |
955 | This tunnels a connection to IRC server | |
956 | .Dq server.example.com , | |
957 | joining channel | |
958 | .Dq #users , | |
959 | nickname | |
960 | .Dq pinky , | |
961 | using port 1234. | |
962 | It doesn't matter which port is used, | |
963 | as long as it's greater than 1023 | |
964 | (remember, only root can open sockets on privileged ports) | |
965 | and doesn't conflict with any ports already in use. | |
966 | The connection is forwarded to port 6667 on the remote server, | |
967 | since that's the standard port for IRC services. | |
968 | .Pp | |
969 | The | |
970 | .Fl f | |
971 | option backgrounds | |
972 | .Nm | |
973 | and the remote command | |
974 | .Dq sleep 10 | |
975 | is specified to allow an amount of time | |
976 | (10 seconds, in the example) | |
977 | to start the service which is to be tunnelled. | |
978 | If no connections are made within the time specified, | |
979 | .Nm | |
980 | will exit. | |
981 | .Sh X11 FORWARDING | |
982 | If the | |
983 | .Cm ForwardX11 | |
984 | variable is set to | |
985 | .Dq yes | |
986 | (or see the description of the | |
987 | .Fl X , | |
988 | .Fl x , | |
989 | and | |
990 | .Fl Y | |
991 | options above) | |
992 | and the user is using X11 (the | |
993 | .Ev DISPLAY | |
994 | environment variable is set), the connection to the X11 display is | |
995 | automatically forwarded to the remote side in such a way that any X11 | |
996 | programs started from the shell (or command) will go through the | |
997 | encrypted channel, and the connection to the real X server will be made | |
998 | from the local machine. | |
999 | The user should not manually set | |
1000 | .Ev DISPLAY . | |
1001 | Forwarding of X11 connections can be | |
1002 | configured on the command line or in configuration files. | |
1003 | .Pp | |
1004 | The | |
1005 | .Ev DISPLAY | |
1006 | value set by | |
1007 | .Nm | |
1008 | will point to the server machine, but with a display number greater than zero. | |
1009 | This is normal, and happens because | |
1010 | .Nm | |
1011 | creates a | |
1012 | .Dq proxy | |
1013 | X server on the server machine for forwarding the | |
1014 | connections over the encrypted channel. | |
1015 | .Pp | |
1016 | .Nm | |
1017 | will also automatically set up Xauthority data on the server machine. | |
1018 | For this purpose, it will generate a random authorization cookie, | |
1019 | store it in Xauthority on the server, and verify that any forwarded | |
1020 | connections carry this cookie and replace it by the real cookie when | |
1021 | the connection is opened. | |
1022 | The real authentication cookie is never | |
1023 | sent to the server machine (and no cookies are sent in the plain). | |
1024 | .Pp | |
1025 | If the | |
1026 | .Cm ForwardAgent | |
1027 | variable is set to | |
1028 | .Dq yes | |
1029 | (or see the description of the | |
1030 | .Fl A | |
1031 | and | |
1032 | .Fl a | |
1033 | options above) and | |
1034 | the user is using an authentication agent, the connection to the agent | |
1035 | is automatically forwarded to the remote side. | |
1036 | .Sh VERIFYING HOST KEYS | |
1037 | When connecting to a server for the first time, | |
1038 | a fingerprint of the server's public key is presented to the user | |
1039 | (unless the option | |
1040 | .Cm StrictHostKeyChecking | |
1041 | has been disabled). | |
1042 | Fingerprints can be determined using | |
1043 | .Xr ssh-keygen 1 : | |
1044 | .Pp | |
1045 | .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key | |
1046 | .Pp | |
1047 | If the fingerprint is already known, it can be matched | |
1048 | and the key can be accepted or rejected. | |
1049 | Because of the difficulty of comparing host keys | |
1050 | just by looking at hex strings, | |
1051 | there is also support to compare host keys visually, | |
1052 | using | |
1053 | .Em random art . | |
1054 | By setting the | |
1055 | .Cm VisualHostKey | |
1056 | option to | |
1057 | .Dq yes , | |
1058 | a small ASCII graphic gets displayed on every login to a server, no matter | |
1059 | if the session itself is interactive or not. | |
1060 | By learning the pattern a known server produces, a user can easily | |
1061 | find out that the host key has changed when a completely different pattern | |
1062 | is displayed. | |
1063 | Because these patterns are not unambiguous however, a pattern that looks | |
1064 | similar to the pattern remembered only gives a good probability that the | |
1065 | host key is the same, not guaranteed proof. | |
1066 | .Pp | |
1067 | To get a listing of the fingerprints along with their random art for | |
1068 | all known hosts, the following command line can be used: | |
1069 | .Pp | |
1070 | .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts | |
1071 | .Pp | |
1072 | If the fingerprint is unknown, | |
1073 | an alternative method of verification is available: | |
1074 | SSH fingerprints verified by DNS. | |
1075 | An additional resource record (RR), | |
1076 | SSHFP, | |
1077 | is added to a zonefile | |
1078 | and the connecting client is able to match the fingerprint | |
1079 | with that of the key presented. | |
1080 | .Pp | |
1081 | In this example, we are connecting a client to a server, | |
1082 | .Dq host.example.com . | |
1083 | The SSHFP resource records should first be added to the zonefile for | |
1084 | host.example.com: | |
1085 | .Bd -literal -offset indent | |
1086 | $ ssh-keygen -r host.example.com. | |
1087 | .Ed | |
1088 | .Pp | |
1089 | The output lines will have to be added to the zonefile. | |
1090 | To check that the zone is answering fingerprint queries: | |
1091 | .Pp | |
1092 | .Dl $ dig -t SSHFP host.example.com | |
1093 | .Pp | |
1094 | Finally the client connects: | |
1095 | .Bd -literal -offset indent | |
1096 | $ ssh -o "VerifyHostKeyDNS ask" host.example.com | |
1097 | [...] | |
1098 | Matching host key fingerprint found in DNS. | |
1099 | Are you sure you want to continue connecting (yes/no)? | |
1100 | .Ed | |
1101 | .Pp | |
1102 | See the | |
1103 | .Cm VerifyHostKeyDNS | |
1104 | option in | |
1105 | .Xr ssh_config 5 | |
1106 | for more information. | |
1107 | .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS | |
1108 | .Nm | |
1109 | contains support for Virtual Private Network (VPN) tunnelling | |
1110 | using the | |
1111 | .Xr tun 4 | |
1112 | network pseudo-device, | |
1113 | allowing two networks to be joined securely. | |
1114 | The | |
1115 | .Xr sshd_config 5 | |
1116 | configuration option | |
1117 | .Cm PermitTunnel | |
1118 | controls whether the server supports this, | |
1119 | and at what level (layer 2 or 3 traffic). | |
1120 | .Pp | |
1121 | The following example would connect client network 10.0.50.0/24 | |
1122 | with remote network 10.0.99.0/24 using a point-to-point connection | |
1123 | from 10.1.1.1 to 10.1.1.2, | |
1124 | provided that the SSH server running on the gateway to the remote network, | |
1125 | at 192.168.1.15, allows it. | |
1126 | .Pp | |
1127 | On the client: | |
1128 | .Bd -literal -offset indent | |
1129 | # ssh -f -w 0:1 192.168.1.15 true | |
1130 | # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 | |
1131 | # route add 10.0.99.0/24 10.1.1.2 | |
1132 | .Ed | |
1133 | .Pp | |
1134 | On the server: | |
1135 | .Bd -literal -offset indent | |
1136 | # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 | |
1137 | # route add 10.0.50.0/24 10.1.1.1 | |
1138 | .Ed | |
1139 | .Pp | |
1140 | Client access may be more finely tuned via the | |
1141 | .Pa /root/.ssh/authorized_keys | |
1142 | file (see below) and the | |
1143 | .Cm PermitRootLogin | |
1144 | server option. | |
1145 | The following entry would permit connections on | |
1146 | .Xr tun 4 | |
1147 | device 1 from user | |
1148 | .Dq jane | |
1149 | and on tun device 2 from user | |
1150 | .Dq john , | |
1151 | if | |
1152 | .Cm PermitRootLogin | |
1153 | is set to | |
1154 | .Dq forced-commands-only : | |
1155 | .Bd -literal -offset 2n | |
1156 | tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane | |
1157 | tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john | |
1158 | .Ed | |
1159 | .Pp | |
1160 | Since an SSH-based setup entails a fair amount of overhead, | |
1161 | it may be more suited to temporary setups, | |
1162 | such as for wireless VPNs. | |
1163 | More permanent VPNs are better provided by tools such as | |
1164 | .Xr ipsecctl 8 | |
1165 | and | |
1166 | .Xr isakmpd 8 . | |
1167 | .Sh ENVIRONMENT | |
1168 | .Nm | |
1169 | will normally set the following environment variables: | |
1170 | .Bl -tag -width "SSH_ORIGINAL_COMMAND" | |
1171 | .It Ev DISPLAY | |
1172 | The | |
1173 | .Ev DISPLAY | |
1174 | variable indicates the location of the X11 server. | |
1175 | It is automatically set by | |
1176 | .Nm | |
1177 | to point to a value of the form | |
1178 | .Dq hostname:n , | |
1179 | where | |
1180 | .Dq hostname | |
1181 | indicates the host where the shell runs, and | |
1182 | .Sq n | |
1183 | is an integer \*(Ge 1. | |
1184 | .Nm | |
1185 | uses this special value to forward X11 connections over the secure | |
1186 | channel. | |
1187 | The user should normally not set | |
1188 | .Ev DISPLAY | |
1189 | explicitly, as that | |
1190 | will render the X11 connection insecure (and will require the user to | |
1191 | manually copy any required authorization cookies). | |
1192 | .It Ev HOME | |
1193 | Set to the path of the user's home directory. | |
1194 | .It Ev LOGNAME | |
1195 | Synonym for | |
1196 | .Ev USER ; | |
1197 | set for compatibility with systems that use this variable. | |
1198 | .It Ev MAIL | |
1199 | Set to the path of the user's mailbox. | |
1200 | .It Ev PATH | |
1201 | Set to the default | |
1202 | .Ev PATH , | |
1203 | as specified when compiling | |
1204 | .Nm . | |
1205 | .It Ev SSH_ASKPASS | |
1206 | If | |
1207 | .Nm | |
1208 | needs a passphrase, it will read the passphrase from the current | |
1209 | terminal if it was run from a terminal. | |
1210 | If | |
1211 | .Nm | |
1212 | does not have a terminal associated with it but | |
1213 | .Ev DISPLAY | |
1214 | and | |
1215 | .Ev SSH_ASKPASS | |
1216 | are set, it will execute the program specified by | |
1217 | .Ev SSH_ASKPASS | |
1218 | and open an X11 window to read the passphrase. | |
1219 | This is particularly useful when calling | |
1220 | .Nm | |
1221 | from a | |
1222 | .Pa .xsession | |
1223 | or related script. | |
1224 | (Note that on some machines it | |
1225 | may be necessary to redirect the input from | |
1226 | .Pa /dev/null | |
1227 | to make this work.) | |
1228 | .It Ev SSH_AUTH_SOCK | |
1229 | Identifies the path of a | |
1230 | .Ux Ns -domain | |
1231 | socket used to communicate with the agent. | |
1232 | .It Ev SSH_CONNECTION | |
1233 | Identifies the client and server ends of the connection. | |
1234 | The variable contains | |
1235 | four space-separated values: client IP address, client port number, | |
1236 | server IP address, and server port number. | |
1237 | .It Ev SSH_ORIGINAL_COMMAND | |
1238 | This variable contains the original command line if a forced command | |
1239 | is executed. | |
1240 | It can be used to extract the original arguments. | |
1241 | .It Ev SSH_TTY | |
1242 | This is set to the name of the tty (path to the device) associated | |
1243 | with the current shell or command. | |
1244 | If the current session has no tty, | |
1245 | this variable is not set. | |
1246 | .It Ev TZ | |
1247 | This variable is set to indicate the present time zone if it | |
1248 | was set when the daemon was started (i.e. the daemon passes the value | |
1249 | on to new connections). | |
1250 | .It Ev USER | |
1251 | Set to the name of the user logging in. | |
1252 | .El | |
1253 | .Pp | |
1254 | Additionally, | |
1255 | .Nm | |
1256 | reads | |
1257 | .Pa ~/.ssh/environment , | |
1258 | and adds lines of the format | |
1259 | .Dq VARNAME=value | |
1260 | to the environment if the file exists and users are allowed to | |
1261 | change their environment. | |
1262 | For more information, see the | |
1263 | .Cm PermitUserEnvironment | |
1264 | option in | |
1265 | .Xr sshd_config 5 . | |
1266 | .Sh FILES | |
1267 | .Bl -tag -width Ds -compact | |
1268 | .It ~/.rhosts | |
1269 | This file is used for host-based authentication (see above). | |
1270 | On some machines this file may need to be | |
1271 | world-readable if the user's home directory is on an NFS partition, | |
1272 | because | |
1273 | .Xr sshd 8 | |
1274 | reads it as root. | |
1275 | Additionally, this file must be owned by the user, | |
1276 | and must not have write permissions for anyone else. | |
1277 | The recommended | |
1278 | permission for most machines is read/write for the user, and not | |
1279 | accessible by others. | |
1280 | .Pp | |
1281 | .It ~/.shosts | |
1282 | This file is used in exactly the same way as | |
1283 | .Pa .rhosts , | |
1284 | but allows host-based authentication without permitting login with | |
1285 | rlogin/rsh. | |
1286 | .Pp | |
1287 | .It ~/.ssh/ | |
1288 | This directory is the default location for all user-specific configuration | |
1289 | and authentication information. | |
1290 | There is no general requirement to keep the entire contents of this directory | |
1291 | secret, but the recommended permissions are read/write/execute for the user, | |
1292 | and not accessible by others. | |
1293 | .Pp | |
1294 | .It ~/.ssh/authorized_keys | |
1295 | Lists the public keys (RSA/DSA) that can be used for logging in as this user. | |
1296 | The format of this file is described in the | |
1297 | .Xr sshd 8 | |
1298 | manual page. | |
1299 | This file is not highly sensitive, but the recommended | |
1300 | permissions are read/write for the user, and not accessible by others. | |
1301 | .Pp | |
1302 | .It ~/.ssh/config | |
1303 | This is the per-user configuration file. | |
1304 | The file format and configuration options are described in | |
1305 | .Xr ssh_config 5 . | |
1306 | Because of the potential for abuse, this file must have strict permissions: | |
1307 | read/write for the user, and not accessible by others. | |
1308 | .Pp | |
1309 | .It ~/.ssh/environment | |
1310 | Contains additional definitions for environment variables; see | |
1311 | .Sx ENVIRONMENT , | |
1312 | above. | |
1313 | .Pp | |
1314 | .It ~/.ssh/identity | |
1315 | .It ~/.ssh/id_dsa | |
1316 | .It ~/.ssh/id_rsa | |
1317 | Contains the private key for authentication. | |
1318 | These files | |
1319 | contain sensitive data and should be readable by the user but not | |
1320 | accessible by others (read/write/execute). | |
1321 | .Nm | |
1322 | will simply ignore a private key file if it is accessible by others. | |
1323 | It is possible to specify a passphrase when | |
1324 | generating the key which will be used to encrypt the | |
1325 | sensitive part of this file using 3DES. | |
1326 | .Pp | |
1327 | .It ~/.ssh/identity.pub | |
1328 | .It ~/.ssh/id_dsa.pub | |
1329 | .It ~/.ssh/id_rsa.pub | |
1330 | Contains the public key for authentication. | |
1331 | These files are not | |
1332 | sensitive and can (but need not) be readable by anyone. | |
1333 | .Pp | |
1334 | .It ~/.ssh/known_hosts | |
1335 | Contains a list of host keys for all hosts the user has logged into | |
1336 | that are not already in the systemwide list of known host keys. | |
1337 | See | |
1338 | .Xr sshd 8 | |
1339 | for further details of the format of this file. | |
1340 | .Pp | |
1341 | .It ~/.ssh/rc | |
1342 | Commands in this file are executed by | |
1343 | .Nm | |
1344 | when the user logs in, just before the user's shell (or command) is | |
1345 | started. | |
1346 | See the | |
1347 | .Xr sshd 8 | |
1348 | manual page for more information. | |
1349 | .Pp | |
1350 | .It /etc/hosts.equiv | |
1351 | This file is for host-based authentication (see above). | |
1352 | It should only be writable by root. | |
1353 | .Pp | |
1354 | .It /etc/shosts.equiv | |
1355 | This file is used in exactly the same way as | |
1356 | .Pa hosts.equiv , | |
1357 | but allows host-based authentication without permitting login with | |
1358 | rlogin/rsh. | |
1359 | .Pp | |
1360 | .It Pa /etc/ssh/ssh_config | |
1361 | Systemwide configuration file. | |
1362 | The file format and configuration options are described in | |
1363 | .Xr ssh_config 5 . | |
1364 | .Pp | |
1365 | .It /etc/ssh/ssh_host_key | |
1366 | .It /etc/ssh/ssh_host_dsa_key | |
1367 | .It /etc/ssh/ssh_host_rsa_key | |
1368 | These three files contain the private parts of the host keys | |
1369 | and are used for host-based authentication. | |
1370 | If protocol version 1 is used, | |
1371 | .Nm | |
1372 | must be setuid root, since the host key is readable only by root. | |
1373 | For protocol version 2, | |
1374 | .Nm | |
1375 | uses | |
1376 | .Xr ssh-keysign 8 | |
1377 | to access the host keys, | |
1378 | eliminating the requirement that | |
1379 | .Nm | |
1380 | be setuid root when host-based authentication is used. | |
1381 | By default | |
1382 | .Nm | |
1383 | is not setuid root. | |
1384 | .Pp | |
1385 | .It /etc/ssh/ssh_known_hosts | |
1386 | Systemwide list of known host keys. | |
1387 | This file should be prepared by the | |
1388 | system administrator to contain the public host keys of all machines in the | |
1389 | organization. | |
1390 | It should be world-readable. | |
1391 | See | |
1392 | .Xr sshd 8 | |
1393 | for further details of the format of this file. | |
1394 | .Pp | |
1395 | .It /etc/ssh/sshrc | |
1396 | Commands in this file are executed by | |
1397 | .Nm | |
1398 | when the user logs in, just before the user's shell (or command) is started. | |
1399 | See the | |
1400 | .Xr sshd 8 | |
1401 | manual page for more information. | |
1402 | .El | |
1403 | .Sh SEE ALSO | |
1404 | .Xr scp 1 , | |
1405 | .Xr sftp 1 , | |
1406 | .Xr ssh-add 1 , | |
1407 | .Xr ssh-agent 1 , | |
1408 | .Xr ssh-keygen 1 , | |
1409 | .Xr ssh-keyscan 1 , | |
1410 | .Xr tun 4 , | |
1411 | .Xr hosts.equiv 5 , | |
1412 | .Xr ssh_config 5 , | |
1413 | .Xr ssh-keysign 8 , | |
1414 | .Xr sshd 8 | |
1415 | .Rs | |
1416 | .%R RFC 4250 | |
1417 | .%T "The Secure Shell (SSH) Protocol Assigned Numbers" | |
1418 | .%D 2006 | |
1419 | .Re | |
1420 | .Rs | |
1421 | .%R RFC 4251 | |
1422 | .%T "The Secure Shell (SSH) Protocol Architecture" | |
1423 | .%D 2006 | |
1424 | .Re | |
1425 | .Rs | |
1426 | .%R RFC 4252 | |
1427 | .%T "The Secure Shell (SSH) Authentication Protocol" | |
1428 | .%D 2006 | |
1429 | .Re | |
1430 | .Rs | |
1431 | .%R RFC 4253 | |
1432 | .%T "The Secure Shell (SSH) Transport Layer Protocol" | |
1433 | .%D 2006 | |
1434 | .Re | |
1435 | .Rs | |
1436 | .%R RFC 4254 | |
1437 | .%T "The Secure Shell (SSH) Connection Protocol" | |
1438 | .%D 2006 | |
1439 | .Re | |
1440 | .Rs | |
1441 | .%R RFC 4255 | |
1442 | .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints" | |
1443 | .%D 2006 | |
1444 | .Re | |
1445 | .Rs | |
1446 | .%R RFC 4256 | |
1447 | .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)" | |
1448 | .%D 2006 | |
1449 | .Re | |
1450 | .Rs | |
1451 | .%R RFC 4335 | |
1452 | .%T "The Secure Shell (SSH) Session Channel Break Extension" | |
1453 | .%D 2006 | |
1454 | .Re | |
1455 | .Rs | |
1456 | .%R RFC 4344 | |
1457 | .%T "The Secure Shell (SSH) Transport Layer Encryption Modes" | |
1458 | .%D 2006 | |
1459 | .Re | |
1460 | .Rs | |
1461 | .%R RFC 4345 | |
1462 | .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol" | |
1463 | .%D 2006 | |
1464 | .Re | |
1465 | .Rs | |
1466 | .%R RFC 4419 | |
1467 | .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol" | |
1468 | .%D 2006 | |
1469 | .Re | |
1470 | .Rs | |
1471 | .%R RFC 4716 | |
1472 | .%T "The Secure Shell (SSH) Public Key File Format" | |
1473 | .%D 2006 | |
1474 | .Re | |
1475 | .Rs | |
1476 | .%T "Hash Visualization: a New Technique to improve Real-World Security" | |
1477 | .%A A. Perrig | |
1478 | .%A D. Song | |
1479 | .%D 1999 | |
1480 | .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)" | |
1481 | .Re | |
1482 | .Sh AUTHORS | |
1483 | OpenSSH is a derivative of the original and free | |
1484 | ssh 1.2.12 release by Tatu Ylonen. | |
1485 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
1486 | Theo de Raadt and Dug Song | |
1487 | removed many bugs, re-added newer features and | |
1488 | created OpenSSH. | |
1489 | Markus Friedl contributed the support for SSH | |
1490 | protocol versions 1.5 and 2.0. |