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bf740959 | 1 | .\" -*- nroff -*- |
2 | .\" | |
bf740959 | 3 | .\" Author: Tatu Ylonen <ylo@cs.hut.fi> |
bf740959 | 4 | .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland |
5 | .\" All rights reserved | |
6 | .\" | |
bcbf86ec | 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 | .\" | |
f3c7c613 | 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. | |
bcbf86ec | 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. | |
bf740959 | 25 | .\" |
bcbf86ec | 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. | |
bf740959 | 36 | .\" |
0dba6d86 | 37 | .\" $OpenBSD: ssh.1,v 1.284 2009/10/08 20:42:12 jmc Exp $ |
7b3999b8 | 38 | .Dd $Mdocdate$ |
bf740959 | 39 | .Dt SSH 1 |
40 | .Os | |
41 | .Sh NAME | |
42 | .Nm ssh | |
2c86906e | 43 | .Nd OpenSSH SSH client (remote login program) |
bf740959 | 44 | .Sh SYNOPSIS |
45 | .Nm ssh | |
bf793210 | 46 | .Op Fl 1246AaCfgKkMNnqsTtVvXxYy |
3435f5a6 | 47 | .Op Fl b Ar bind_address |
d0c832f3 | 48 | .Op Fl c Ar cipher_spec |
5ddc5eb4 | 49 | .Oo Fl D\ \& |
50 | .Sm off | |
51 | .Oo Ar bind_address : Oc | |
52 | .Ar port | |
53 | .Sm on | |
54 | .Oc | |
bf740959 | 55 | .Op Fl e Ar escape_char |
e591b98a | 56 | .Op Fl F Ar configfile |
37c406a8 | 57 | .Bk -words |
5f4a0c58 | 58 | .Op Fl i Ar identity_file |
37c406a8 | 59 | .Ek |
6c7e3b94 | 60 | .Oo Fl L\ \& |
bf740959 | 61 | .Sm off |
3867aa0a | 62 | .Oo Ar bind_address : Oc |
6c7e3b94 | 63 | .Ar port : host : hostport |
bf740959 | 64 | .Sm on |
bf740959 | 65 | .Oc |
37c406a8 | 66 | .Bk -words |
5f4a0c58 | 67 | .Op Fl l Ar login_name |
37c406a8 | 68 | .Ek |
5f4a0c58 | 69 | .Op Fl m Ar mac_spec |
f8c6db83 | 70 | .Op Fl O Ar ctl_cmd |
5f4a0c58 | 71 | .Op Fl o Ar option |
5f4a0c58 | 72 | .Op Fl p Ar port |
6c7e3b94 | 73 | .Oo Fl R\ \& |
bf740959 | 74 | .Sm off |
3867aa0a | 75 | .Oo Ar bind_address : Oc |
6c7e3b94 | 76 | .Ar port : host : hostport |
bf740959 | 77 | .Sm on |
bf740959 | 78 | .Oc |
f8c6db83 | 79 | .Op Fl S Ar ctl_path |
985bb789 | 80 | .Bk -words |
06fa4ac1 | 81 | .Oo Fl w Ar local_tun Ns |
82 | .Op : Ns Ar remote_tun Oc | |
5f4a0c58 | 83 | .Oo Ar user Ns @ Oc Ns Ar hostname |
bf740959 | 84 | .Op Ar command |
985bb789 | 85 | .Ek |
f54651ce | 86 | .Sh DESCRIPTION |
bf740959 | 87 | .Nm |
2c86906e | 88 | (SSH client) is a program for logging into a remote machine and for |
610cd5c6 | 89 | executing commands on a remote machine. |
5f4a0c58 | 90 | It is intended to replace rlogin and rsh, |
91 | and provide secure encrypted communications between | |
610cd5c6 | 92 | two untrusted hosts over an insecure network. |
5d4e571c | 93 | X11 connections and arbitrary TCP ports |
5f4a0c58 | 94 | can also be forwarded over the secure channel. |
bf740959 | 95 | .Pp |
96 | .Nm | |
f54651ce | 97 | connects and logs into the specified |
5f4a0c58 | 98 | .Ar hostname |
99 | (with optional | |
100 | .Ar user | |
101 | name). | |
bf740959 | 102 | The user must prove |
1d1ffb87 | 103 | his/her identity to the remote machine using one of several methods |
a55c1733 | 104 | depending on the protocol version used (see below). |
1d1ffb87 | 105 | .Pp |
5f4a0c58 | 106 | If |
107 | .Ar command | |
108 | is specified, | |
e426efa9 | 109 | it is executed on the remote host instead of a login shell. |
bf740959 | 110 | .Pp |
442c8b33 | 111 | The options are as follows: |
112 | .Bl -tag -width Ds | |
113 | .It Fl 1 | |
114 | Forces | |
bf740959 | 115 | .Nm |
442c8b33 | 116 | to try protocol version 1 only. |
117 | .It Fl 2 | |
118 | Forces | |
bf740959 | 119 | .Nm |
442c8b33 | 120 | to try protocol version 2 only. |
121 | .It Fl 4 | |
122 | Forces | |
bf740959 | 123 | .Nm |
442c8b33 | 124 | to use IPv4 addresses only. |
125 | .It Fl 6 | |
126 | Forces | |
bf740959 | 127 | .Nm |
442c8b33 | 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. | |
bf740959 | 132 | .Pp |
442c8b33 | 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 | |
5d4e571c | 150 | data for forwarded X11 and TCP connections). |
442c8b33 | 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. | |
bf740959 | 166 | .Pp |
442c8b33 | 167 | Protocol version 1 allows specification of a single cipher. |
168 | The supported values are | |
169 | .Dq 3des , | |
0fe62d3d | 170 | .Dq blowfish , |
442c8b33 | 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 | |
bf740959 | 181 | .Nm |
442c8b33 | 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 . | |
da89cf4d | 189 | .Pp |
025fc42e | 190 | For protocol version 2, |
442c8b33 | 191 | .Ar cipher_spec |
192 | is a comma-separated list of ciphers | |
193 | listed in order of preference. | |
8fe25329 | 194 | See the |
195 | .Cm Ciphers | |
196 | keyword for more information. | |
442c8b33 | 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 | |
da89cf4d | 215 | .Nm |
442c8b33 | 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. | |
5ddc5eb4 | 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. | |
86131206 | 243 | .It Fl e Ar escape_char |
bf740959 | 244 | Sets the escape character for sessions with a pty (default: |
245 | .Ql ~ ) . | |
610cd5c6 | 246 | The escape character is only recognized at the beginning of a line. |
247 | The escape character followed by a dot | |
bf740959 | 248 | .Pq Ql \&. |
5f4a0c58 | 249 | closes the connection; |
250 | followed by control-Z suspends the connection; | |
251 | and followed by itself sends the escape character once. | |
610cd5c6 | 252 | Setting the character to |
bf740959 | 253 | .Dq none |
254 | disables any escapes and makes the session fully transparent. | |
5f4a0c58 | 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 | |
140e3e97 | 262 | .Pa ~/.ssh/config . |
bf740959 | 263 | .It Fl f |
264 | Requests | |
265 | .Nm | |
610cd5c6 | 266 | to go to background just before command execution. |
267 | This is useful if | |
bf740959 | 268 | .Nm |
269 | is going to ask for passwords or passphrases, but the user | |
610cd5c6 | 270 | wants it in the background. |
f54651ce | 271 | This implies |
bf740959 | 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 . | |
6c777090 | 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. | |
7b2ea3a1 | 285 | .It Fl g |
286 | Allows remote hosts to connect to local forwarded ports. | |
5f4a0c58 | 287 | .It Fl I Ar smartcard_device |
86131206 | 288 | Specify the device |
5f4a0c58 | 289 | .Nm |
290 | should use to communicate with a smartcard used for storing the user's | |
291 | private RSA key. | |
86131206 | 292 | This option is only available if support for smartcard devices |
293 | is compiled in (default is no support). | |
bf740959 | 294 | .It Fl i Ar identity_file |
cf5a07a8 | 295 | Selects a file from which the identity (private key) for |
fa08c86b | 296 | RSA or DSA authentication is read. |
cf5a07a8 | 297 | The default is |
140e3e97 | 298 | .Pa ~/.ssh/identity |
cf5a07a8 | 299 | for protocol version 1, and |
140e3e97 | 300 | .Pa ~/.ssh/id_rsa |
cf5a07a8 | 301 | and |
140e3e97 | 302 | .Pa ~/.ssh/id_dsa |
cf5a07a8 | 303 | for protocol version 2. |
610cd5c6 | 304 | Identity files may also be specified on |
305 | a per-host basis in the configuration file. | |
306 | It is possible to have multiple | |
bf740959 | 307 | .Fl i |
308 | options (and multiple identities specified in | |
309 | configuration files). | |
b2f4d5cc | 310 | .It Fl K |
311 | Enables GSSAPI-based authentication and forwarding (delegation) of GSSAPI | |
312 | credentials to the server. | |
bf740959 | 313 | .It Fl k |
f7926e97 | 314 | Disables forwarding (delegation) of GSSAPI credentials to the server. |
5f4a0c58 | 315 | .It Fl L Xo |
316 | .Sm off | |
3867aa0a | 317 | .Oo Ar bind_address : Oc |
5f4a0c58 | 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 | |
3867aa0a | 325 | on the local side, optionally bound to the specified |
326 | .Ar bind_address . | |
327 | Whenever a connection is made to this port, the | |
5f4a0c58 | 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. | |
5f4a0c58 | 335 | IPv6 addresses can be specified with an alternative syntax: |
336 | .Sm off | |
337 | .Xo | |
6d7a9e8f | 338 | .Op Ar bind_address No / |
5f4a0c58 | 339 | .Ar port No / Ar host No / |
3867aa0a | 340 | .Ar hostport |
5f4a0c58 | 341 | .Xc |
342 | .Sm on | |
3867aa0a | 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 | |
6c7e3b94 | 355 | indicates that the listening port be bound for local use only, while an |
356 | empty address or | |
357 | .Sq * | |
3867aa0a | 358 | indicates that the port should be available from all interfaces. |
bf740959 | 359 | .It Fl l Ar login_name |
610cd5c6 | 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. | |
5e96b616 | 362 | .It Fl M |
363 | Places the | |
364 | .Nm | |
365 | client into | |
366 | .Dq master | |
367 | mode for connection sharing. | |
f470cf48 | 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. | |
5e96b616 | 375 | Refer to the description of |
376 | .Cm ControlMaster | |
377 | in | |
378 | .Xr ssh_config 5 | |
379 | for details. | |
9affc5db | 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. | |
5f4a0c58 | 387 | .It Fl N |
388 | Do not execute a remote command. | |
389 | This is useful for just forwarding ports | |
390 | (protocol version 2 only). | |
bf740959 | 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 | |
610cd5c6 | 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, | |
bf740959 | 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.) | |
cf848a5e | 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). | |
bf740959 | 423 | .It Fl o Ar option |
38539909 | 424 | Can be used to give options in the format used in the configuration file. |
bf740959 | 425 | This is useful for specifying options for which there is no separate |
610cd5c6 | 426 | command-line flag. |
5f4a0c58 | 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 | |
20b267fb | 442 | .It ConnectTimeout |
5e96b616 | 443 | .It ControlMaster |
444 | .It ControlPath | |
5f4a0c58 | 445 | .It DynamicForward |
5f4a0c58 | 446 | .It EscapeChar |
42ea6f5e | 447 | .It ExitOnForwardFailure |
5f4a0c58 | 448 | .It ForwardAgent |
449 | .It ForwardX11 | |
d73a67d7 | 450 | .It ForwardX11Trusted |
5f4a0c58 | 451 | .It GatewayPorts |
452 | .It GlobalKnownHostsFile | |
453 | .It GSSAPIAuthentication | |
454 | .It GSSAPIDelegateCredentials | |
5c63c2ab | 455 | .It HashKnownHosts |
5f4a0c58 | 456 | .It Host |
457 | .It HostbasedAuthentication | |
458 | .It HostKeyAlgorithms | |
459 | .It HostKeyAlias | |
460 | .It HostName | |
461 | .It IdentityFile | |
3a065ed0 | 462 | .It IdentitiesOnly |
396070f8 | 463 | .It KbdInteractiveDevices |
d20f3c9e | 464 | .It LocalCommand |
5f4a0c58 | 465 | .It LocalForward |
466 | .It LogLevel | |
467 | .It MACs | |
468 | .It NoHostAuthenticationForLocalhost | |
469 | .It NumberOfPasswordPrompts | |
470 | .It PasswordAuthentication | |
d20f3c9e | 471 | .It PermitLocalCommand |
5f4a0c58 | 472 | .It Port |
473 | .It PreferredAuthentications | |
474 | .It Protocol | |
475 | .It ProxyCommand | |
476 | .It PubkeyAuthentication | |
db175906 | 477 | .It RekeyLimit |
5f4a0c58 | 478 | .It RemoteForward |
479 | .It RhostsRSAAuthentication | |
480 | .It RSAAuthentication | |
8e99a198 | 481 | .It SendEnv |
5d8d32a3 | 482 | .It ServerAliveInterval |
483 | .It ServerAliveCountMax | |
5f4a0c58 | 484 | .It SmartcardDevice |
485 | .It StrictHostKeyChecking | |
fd573618 | 486 | .It TCPKeepAlive |
d20f3c9e | 487 | .It Tunnel |
488 | .It TunnelDevice | |
5f4a0c58 | 489 | .It UsePrivilegedPort |
490 | .It User | |
491 | .It UserKnownHostsFile | |
492 | .It VerifyHostKeyDNS | |
2e8d3306 | 493 | .It VisualHostKey |
5f4a0c58 | 494 | .It XAuthLocation |
495 | .El | |
bf740959 | 496 | .It Fl p Ar port |
610cd5c6 | 497 | Port to connect to on the remote host. |
498 | This can be specified on a | |
bf740959 | 499 | per-host basis in the configuration file. |
bf740959 | 500 | .It Fl q |
610cd5c6 | 501 | Quiet mode. |
05841f5b | 502 | Causes most warning and diagnostic messages to be suppressed. |
5f4a0c58 | 503 | .It Fl R Xo |
504 | .Sm off | |
3867aa0a | 505 | .Oo Ar bind_address : Oc |
5f4a0c58 | 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. | |
3867aa0a | 520 | .Pp |
5f4a0c58 | 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. | |
3867aa0a | 524 | IPv6 addresses can be specified by enclosing the address in square braces or |
525 | using an alternative syntax: | |
3867aa0a | 526 | .Sm off |
6c7e3b94 | 527 | .Xo |
6d7a9e8f | 528 | .Op Ar bind_address No / |
6c7e3b94 | 529 | .Ar host No / Ar port No / |
530 | .Ar hostport | |
3867aa0a | 531 | .Xc . |
6c7e3b94 | 532 | .Sm on |
3867aa0a | 533 | .Pp |
534 | By default, the listening socket on the server will be bound to the loopback | |
535 | interface only. | |
94087553 | 536 | This may be overridden by specifying a |
3867aa0a | 537 | .Ar bind_address . |
6c7e3b94 | 538 | An empty |
539 | .Ar bind_address , | |
3867aa0a | 540 | or the address |
6c7e3b94 | 541 | .Ql * , |
3867aa0a | 542 | indicates that the remote socket should listen on all interfaces. |
543 | Specifying a remote | |
544 | .Ar bind_address | |
6c7e3b94 | 545 | will only succeed if the server's |
546 | .Cm GatewayPorts | |
3867aa0a | 547 | option is enabled (see |
6c7e3b94 | 548 | .Xr sshd_config 5 ) . |
db9039d0 | 549 | .Pp |
550 | If the | |
551 | .Ar port | |
552 | argument is | |
1e709459 | 553 | .Ql 0 , |
db9039d0 | 554 | the listen port will be dynamically allocated on the server and reported |
555 | to the client at run time. | |
f8c6db83 | 556 | .It Fl S Ar ctl_path |
9affc5db | 557 | Specifies the location of a control socket for connection sharing. |
5e96b616 | 558 | Refer to the description of |
0e19494c | 559 | .Cm ControlPath |
560 | and | |
5e96b616 | 561 | .Cm ControlMaster |
562 | in | |
563 | .Xr ssh_config 5 | |
564 | for details. | |
ae810de7 | 565 | .It Fl s |
3cbc677d | 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 | |
5f4a0c58 | 568 | of SSH as a secure transport for other applications (eg.\& |
569 | .Xr sftp 1 ) . | |
3cbc677d | 570 | The subsystem is specified as the remote command. |
5f4a0c58 | 571 | .It Fl T |
572 | Disable pseudo-tty allocation. | |
bf740959 | 573 | .It Fl t |
610cd5c6 | 574 | Force pseudo-tty allocation. |
4fe2af09 | 575 | This can be used to execute arbitrary |
610cd5c6 | 576 | screen-based programs on a remote machine, which can be very useful, |
f09ffbdb | 577 | e.g. when implementing menu services. |
8abcdba4 | 578 | Multiple |
579 | .Fl t | |
580 | options force tty allocation, even if | |
581 | .Nm | |
582 | has no local tty. | |
5f4a0c58 | 583 | .It Fl V |
584 | Display the version number and exit. | |
bf740959 | 585 | .It Fl v |
610cd5c6 | 586 | Verbose mode. |
587 | Causes | |
bf740959 | 588 | .Nm |
442c8b33 | 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. | |
06fa4ac1 | 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 | |
442c8b33 | 602 | .Xr tun 4 |
06fa4ac1 | 603 | devices between the client |
604 | .Pq Ar local_tun | |
605 | and the server | |
606 | .Pq Ar remote_tun . | |
607 | .Pp | |
9bf41db3 | 608 | The devices may be specified by numerical ID or the keyword |
609 | .Dq any , | |
610 | which uses the next available tunnel device. | |
06fa4ac1 | 611 | If |
612 | .Ar remote_tun | |
613 | is not specified, it defaults to | |
614 | .Dq any . | |
9bf41db3 | 615 | See also the |
442c8b33 | 616 | .Cm Tunnel |
06fa4ac1 | 617 | and |
618 | .Cm TunnelDevice | |
619 | directives in | |
442c8b33 | 620 | .Xr ssh_config 5 . |
06fa4ac1 | 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 . | |
442c8b33 | 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. | |
bf793210 | 651 | .It Fl y |
652 | Send log information using the | |
653 | .Xr syslog 3 | |
654 | system module. | |
655 | By default this information is sent to stderr. | |
442c8b33 | 656 | .El |
e6c7c03e | 657 | .Pp |
658 | .Nm | |
659 | may additionally obtain configuration data from | |
660 | a per-user configuration file and a system-wide configuration file. | |
661 | The file format and configuration options are described in | |
662 | .Xr ssh_config 5 . | |
663 | .Pp | |
664 | .Nm | |
665 | exits with the exit status of the remote command or with 255 | |
666 | if an error occurred. | |
6b0117fd | 667 | .Sh AUTHENTICATION |
16ad260d | 668 | The OpenSSH SSH client supports SSH protocols 1 and 2. |
0dba6d86 | 669 | The default is to use protocol 2 only, |
670 | though this can be changed via the | |
6b0117fd | 671 | .Cm Protocol |
672 | option in | |
0dba6d86 | 673 | .Xr ssh_config 5 |
674 | or the | |
6b0117fd | 675 | .Fl 1 |
676 | and | |
677 | .Fl 2 | |
678 | options (see above). | |
679 | Both protocols support similar authentication methods, | |
0dba6d86 | 680 | but protocol 2 is the default since |
6b0117fd | 681 | it provides additional mechanisms for confidentiality |
682 | (the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) | |
f444d0f8 | 683 | and integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160). |
6b0117fd | 684 | Protocol 1 lacks a strong mechanism for ensuring the |
685 | integrity of the connection. | |
686 | .Pp | |
687 | The methods available for authentication are: | |
ed277f5e | 688 | GSSAPI-based authentication, |
6b0117fd | 689 | host-based authentication, |
690 | public key authentication, | |
691 | challenge-response authentication, | |
692 | and password authentication. | |
693 | Authentication methods are tried in the order specified above, | |
694 | though protocol 2 has a configuration option to change the default order: | |
695 | .Cm PreferredAuthentications . | |
696 | .Pp | |
697 | Host-based authentication works as follows: | |
442c8b33 | 698 | If the machine the user logs in from is listed in |
699 | .Pa /etc/hosts.equiv | |
700 | or | |
701 | .Pa /etc/shosts.equiv | |
702 | on the remote machine, and the user names are | |
703 | the same on both sides, or if the files | |
704 | .Pa ~/.rhosts | |
705 | or | |
706 | .Pa ~/.shosts | |
707 | exist in the user's home directory on the | |
708 | remote machine and contain a line containing the name of the client | |
709 | machine and the name of the user on that machine, the user is | |
6b0117fd | 710 | considered for login. |
711 | Additionally, the server | |
712 | .Em must | |
713 | be able to verify the client's | |
714 | host key (see the description of | |
442c8b33 | 715 | .Pa /etc/ssh/ssh_known_hosts |
716 | and | |
6b0117fd | 717 | .Pa ~/.ssh/known_hosts , |
718 | below) | |
719 | for login to be permitted. | |
442c8b33 | 720 | This authentication method closes security holes due to IP |
6b0117fd | 721 | spoofing, DNS spoofing, and routing spoofing. |
442c8b33 | 722 | [Note to the administrator: |
723 | .Pa /etc/hosts.equiv , | |
724 | .Pa ~/.rhosts , | |
725 | and the rlogin/rsh protocol in general, are inherently insecure and should be | |
726 | disabled if security is desired.] | |
727 | .Pp | |
6b0117fd | 728 | Public key authentication works as follows: |
729 | The scheme is based on public-key cryptography, | |
730 | using cryptosystems | |
731 | where encryption and decryption are done using separate keys, | |
732 | and it is unfeasible to derive the decryption key from the encryption key. | |
442c8b33 | 733 | The idea is that each user creates a public/private |
734 | key pair for authentication purposes. | |
735 | The server knows the public key, and only the user knows the private key. | |
6b0117fd | 736 | .Nm |
737 | implements public key authentication protocol automatically, | |
738 | using either the RSA or DSA algorithms. | |
739 | Protocol 1 is restricted to using only RSA keys, | |
740 | but protocol 2 may use either. | |
741 | The | |
742 | .Sx HISTORY | |
743 | section of | |
744 | .Xr ssl 8 | |
745 | contains a brief discussion of the two algorithms. | |
442c8b33 | 746 | .Pp |
747 | The file | |
748 | .Pa ~/.ssh/authorized_keys | |
749 | lists the public keys that are permitted for logging in. | |
750 | When the user logs in, the | |
751 | .Nm | |
752 | program tells the server which key pair it would like to use for | |
753 | authentication. | |
6b0117fd | 754 | The client proves that it has access to the private key |
755 | and the server checks that the corresponding public key | |
756 | is authorized to accept the account. | |
442c8b33 | 757 | .Pp |
6b0117fd | 758 | The user creates his/her key pair by running |
442c8b33 | 759 | .Xr ssh-keygen 1 . |
760 | This stores the private key in | |
761 | .Pa ~/.ssh/identity | |
6b0117fd | 762 | (protocol 1), |
763 | .Pa ~/.ssh/id_dsa | |
764 | (protocol 2 DSA), | |
765 | or | |
766 | .Pa ~/.ssh/id_rsa | |
767 | (protocol 2 RSA) | |
442c8b33 | 768 | and stores the public key in |
769 | .Pa ~/.ssh/identity.pub | |
6b0117fd | 770 | (protocol 1), |
771 | .Pa ~/.ssh/id_dsa.pub | |
772 | (protocol 2 DSA), | |
773 | or | |
774 | .Pa ~/.ssh/id_rsa.pub | |
775 | (protocol 2 RSA) | |
442c8b33 | 776 | in the user's home directory. |
6b0117fd | 777 | The user should then copy the public key |
442c8b33 | 778 | to |
779 | .Pa ~/.ssh/authorized_keys | |
6b0117fd | 780 | in his/her home directory on the remote machine. |
781 | The | |
442c8b33 | 782 | .Pa authorized_keys |
783 | file corresponds to the conventional | |
784 | .Pa ~/.rhosts | |
785 | file, and has one key | |
6b0117fd | 786 | per line, though the lines can be very long. |
442c8b33 | 787 | After this, the user can log in without giving the password. |
788 | .Pp | |
6b0117fd | 789 | The most convenient way to use public key authentication may be with an |
442c8b33 | 790 | authentication agent. |
791 | See | |
792 | .Xr ssh-agent 1 | |
793 | for more information. | |
794 | .Pp | |
6b0117fd | 795 | Challenge-response authentication works as follows: |
796 | The server sends an arbitrary | |
797 | .Qq challenge | |
798 | text, and prompts for a response. | |
799 | Protocol 2 allows multiple challenges and responses; | |
800 | protocol 1 is restricted to just one challenge/response. | |
801 | Examples of challenge-response authentication include | |
802 | BSD Authentication (see | |
803 | .Xr login.conf 5 ) | |
804 | and PAM (some non-OpenBSD systems). | |
805 | .Pp | |
806 | Finally, if other authentication methods fail, | |
442c8b33 | 807 | .Nm |
808 | prompts the user for a password. | |
809 | The password is sent to the remote | |
810 | host for checking; however, since all communications are encrypted, | |
811 | the password cannot be seen by someone listening on the network. | |
4b5e6c81 | 812 | .Pp |
813 | .Nm | |
814 | automatically maintains and checks a database containing | |
815 | identification for all hosts it has ever been used with. | |
816 | Host keys are stored in | |
817 | .Pa ~/.ssh/known_hosts | |
818 | in the user's home directory. | |
819 | Additionally, the file | |
820 | .Pa /etc/ssh/ssh_known_hosts | |
821 | is automatically checked for known hosts. | |
822 | Any new hosts are automatically added to the user's file. | |
823 | If a host's identification ever changes, | |
824 | .Nm | |
825 | warns about this and disables password authentication to prevent | |
826 | server spoofing or man-in-the-middle attacks, | |
827 | which could otherwise be used to circumvent the encryption. | |
828 | The | |
829 | .Cm StrictHostKeyChecking | |
830 | option can be used to control logins to machines whose | |
831 | host key is not known or has changed. | |
832 | .Pp | |
442c8b33 | 833 | When the user's identity has been accepted by the server, the server |
834 | either executes the given command, or logs into the machine and gives | |
835 | the user a normal shell on the remote machine. | |
836 | All communication with | |
837 | the remote command or shell will be automatically encrypted. | |
838 | .Pp | |
839 | If a pseudo-terminal has been allocated (normal login session), the | |
840 | user may use the escape characters noted below. | |
841 | .Pp | |
842 | If no pseudo-tty has been allocated, | |
843 | the session is transparent and can be used to reliably transfer binary data. | |
844 | On most systems, setting the escape character to | |
845 | .Dq none | |
846 | will also make the session transparent even if a tty is used. | |
847 | .Pp | |
848 | The session terminates when the command or shell on the remote | |
5d4e571c | 849 | machine exits and all X11 and TCP connections have been closed. |
5c5546be | 850 | .Sh ESCAPE CHARACTERS |
442c8b33 | 851 | When a pseudo-terminal has been requested, |
852 | .Nm | |
853 | supports a number of functions through the use of an escape character. | |
854 | .Pp | |
855 | A single tilde character can be sent as | |
856 | .Ic ~~ | |
857 | or by following the tilde by a character other than those described below. | |
858 | The escape character must always follow a newline to be interpreted as | |
859 | special. | |
860 | The escape character can be changed in configuration files using the | |
861 | .Cm EscapeChar | |
862 | configuration directive or on the command line by the | |
863 | .Fl e | |
864 | option. | |
865 | .Pp | |
866 | The supported escapes (assuming the default | |
867 | .Ql ~ ) | |
868 | are: | |
869 | .Bl -tag -width Ds | |
870 | .It Cm ~. | |
871 | Disconnect. | |
872 | .It Cm ~^Z | |
873 | Background | |
20892533 | 874 | .Nm . |
442c8b33 | 875 | .It Cm ~# |
876 | List forwarded connections. | |
877 | .It Cm ~& | |
878 | Background | |
879 | .Nm | |
880 | at logout when waiting for forwarded connection / X11 sessions to terminate. | |
881 | .It Cm ~? | |
882 | Display a list of escape characters. | |
883 | .It Cm ~B | |
884 | Send a BREAK to the remote system | |
885 | (only useful for SSH protocol version 2 and if the peer supports it). | |
886 | .It Cm ~C | |
887 | Open command line. | |
888 | Currently this allows the addition of port forwardings using the | |
10cf2ac3 | 889 | .Fl L , |
442c8b33 | 890 | .Fl R |
10cf2ac3 | 891 | and |
892 | .Fl D | |
e49f7abd | 893 | options (see above). |
442c8b33 | 894 | It also allows the cancellation of existing remote port-forwardings |
895 | using | |
33f13fa5 | 896 | .Sm off |
897 | .Fl KR Oo Ar bind_address : Oc Ar port . | |
898 | .Sm on | |
442c8b33 | 899 | .Ic !\& Ns Ar command |
900 | allows the user to execute a local command if the | |
901 | .Ic PermitLocalCommand | |
902 | option is enabled in | |
903 | .Xr ssh_config 5 . | |
904 | Basic help is available, using the | |
905 | .Fl h | |
906 | option. | |
907 | .It Cm ~R | |
908 | Request rekeying of the connection | |
909 | (only useful for SSH protocol version 2 and if the peer supports it). | |
910 | .El | |
dbb3bf96 | 911 | .Sh TCP FORWARDING |
912 | Forwarding of arbitrary TCP connections over the secure channel can | |
913 | be specified either on the command line or in a configuration file. | |
914 | One possible application of TCP forwarding is a secure connection to a | |
915 | mail server; another is going through firewalls. | |
916 | .Pp | |
917 | In the example below, we look at encrypting communication between | |
918 | an IRC client and server, even though the IRC server does not directly | |
919 | support encrypted communications. | |
920 | This works as follows: | |
921 | the user connects to the remote host using | |
922 | .Nm , | |
923 | specifying a port to be used to forward connections | |
924 | to the remote server. | |
925 | After that it is possible to start the service which is to be encrypted | |
926 | on the client machine, | |
927 | connecting to the same local port, | |
928 | and | |
929 | .Nm | |
930 | will encrypt and forward the connection. | |
931 | .Pp | |
932 | The following example tunnels an IRC session from client machine | |
933 | .Dq 127.0.0.1 | |
934 | (localhost) | |
935 | to remote server | |
936 | .Dq server.example.com : | |
937 | .Bd -literal -offset 4n | |
938 | $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 | |
939 | $ irc -c '#users' -p 1234 pinky 127.0.0.1 | |
940 | .Ed | |
941 | .Pp | |
942 | This tunnels a connection to IRC server | |
943 | .Dq server.example.com , | |
944 | joining channel | |
945 | .Dq #users , | |
946 | nickname | |
947 | .Dq pinky , | |
948 | using port 1234. | |
949 | It doesn't matter which port is used, | |
950 | as long as it's greater than 1023 | |
951 | (remember, only root can open sockets on privileged ports) | |
952 | and doesn't conflict with any ports already in use. | |
953 | The connection is forwarded to port 6667 on the remote server, | |
954 | since that's the standard port for IRC services. | |
955 | .Pp | |
956 | The | |
957 | .Fl f | |
958 | option backgrounds | |
959 | .Nm | |
960 | and the remote command | |
961 | .Dq sleep 10 | |
962 | is specified to allow an amount of time | |
963 | (10 seconds, in the example) | |
964 | to start the service which is to be tunnelled. | |
965 | If no connections are made within the time specified, | |
966 | .Nm | |
967 | will exit. | |
dbb3bf96 | 968 | .Sh X11 FORWARDING |
442c8b33 | 969 | If the |
970 | .Cm ForwardX11 | |
971 | variable is set to | |
972 | .Dq yes | |
973 | (or see the description of the | |
6cd6c442 | 974 | .Fl X , |
975 | .Fl x , | |
442c8b33 | 976 | and |
6cd6c442 | 977 | .Fl Y |
8770ef76 | 978 | options above) |
442c8b33 | 979 | and the user is using X11 (the |
980 | .Ev DISPLAY | |
981 | environment variable is set), the connection to the X11 display is | |
982 | automatically forwarded to the remote side in such a way that any X11 | |
983 | programs started from the shell (or command) will go through the | |
984 | encrypted channel, and the connection to the real X server will be made | |
985 | from the local machine. | |
986 | The user should not manually set | |
987 | .Ev DISPLAY . | |
988 | Forwarding of X11 connections can be | |
989 | configured on the command line or in configuration files. | |
990 | .Pp | |
991 | The | |
992 | .Ev DISPLAY | |
993 | value set by | |
994 | .Nm | |
995 | will point to the server machine, but with a display number greater than zero. | |
996 | This is normal, and happens because | |
997 | .Nm | |
998 | creates a | |
999 | .Dq proxy | |
1000 | X server on the server machine for forwarding the | |
1001 | connections over the encrypted channel. | |
1002 | .Pp | |
1003 | .Nm | |
1004 | will also automatically set up Xauthority data on the server machine. | |
1005 | For this purpose, it will generate a random authorization cookie, | |
1006 | store it in Xauthority on the server, and verify that any forwarded | |
1007 | connections carry this cookie and replace it by the real cookie when | |
1008 | the connection is opened. | |
1009 | The real authentication cookie is never | |
1010 | sent to the server machine (and no cookies are sent in the plain). | |
1011 | .Pp | |
1012 | If the | |
1013 | .Cm ForwardAgent | |
1014 | variable is set to | |
1015 | .Dq yes | |
1016 | (or see the description of the | |
1017 | .Fl A | |
1018 | and | |
1019 | .Fl a | |
8770ef76 | 1020 | options above) and |
442c8b33 | 1021 | the user is using an authentication agent, the connection to the agent |
1022 | is automatically forwarded to the remote side. | |
b661b7fb | 1023 | .Sh VERIFYING HOST KEYS |
1024 | When connecting to a server for the first time, | |
1025 | a fingerprint of the server's public key is presented to the user | |
1026 | (unless the option | |
1027 | .Cm StrictHostKeyChecking | |
1028 | has been disabled). | |
1029 | Fingerprints can be determined using | |
1030 | .Xr ssh-keygen 1 : | |
1031 | .Pp | |
1032 | .Dl $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key | |
1033 | .Pp | |
990ada29 | 1034 | If the fingerprint is already known, it can be matched |
1035 | and the key can be accepted or rejected. | |
1036 | Because of the difficulty of comparing host keys | |
1037 | just by looking at hex strings, | |
1038 | there is also support to compare host keys visually, | |
1039 | using | |
1040 | .Em random art . | |
1041 | By setting the | |
7b3999b8 | 1042 | .Cm VisualHostKey |
990ada29 | 1043 | option to |
7b3999b8 | 1044 | .Dq yes , |
990ada29 | 1045 | a small ASCII graphic gets displayed on every login to a server, no matter |
1046 | if the session itself is interactive or not. | |
1047 | By learning the pattern a known server produces, a user can easily | |
1048 | find out that the host key has changed when a completely different pattern | |
1049 | is displayed. | |
1050 | Because these patterns are not unambiguous however, a pattern that looks | |
1051 | similar to the pattern remembered only gives a good probability that the | |
1052 | host key is the same, not guaranteed proof. | |
1053 | .Pp | |
1054 | To get a listing of the fingerprints along with their random art for | |
1055 | all known hosts, the following command line can be used: | |
1056 | .Pp | |
1057 | .Dl $ ssh-keygen -lv -f ~/.ssh/known_hosts | |
1058 | .Pp | |
b661b7fb | 1059 | If the fingerprint is unknown, |
1060 | an alternative method of verification is available: | |
1061 | SSH fingerprints verified by DNS. | |
1062 | An additional resource record (RR), | |
1063 | SSHFP, | |
1064 | is added to a zonefile | |
1065 | and the connecting client is able to match the fingerprint | |
1066 | with that of the key presented. | |
1067 | .Pp | |
1068 | In this example, we are connecting a client to a server, | |
1069 | .Dq host.example.com . | |
1070 | The SSHFP resource records should first be added to the zonefile for | |
1071 | host.example.com: | |
1072 | .Bd -literal -offset indent | |
f0eea41f | 1073 | $ ssh-keygen -r host.example.com. |
b661b7fb | 1074 | .Ed |
1075 | .Pp | |
1076 | The output lines will have to be added to the zonefile. | |
1077 | To check that the zone is answering fingerprint queries: | |
1078 | .Pp | |
1079 | .Dl $ dig -t SSHFP host.example.com | |
1080 | .Pp | |
1081 | Finally the client connects: | |
1082 | .Bd -literal -offset indent | |
1083 | $ ssh -o "VerifyHostKeyDNS ask" host.example.com | |
1084 | [...] | |
1085 | Matching host key fingerprint found in DNS. | |
1086 | Are you sure you want to continue connecting (yes/no)? | |
1087 | .Ed | |
1088 | .Pp | |
1089 | See the | |
1090 | .Cm VerifyHostKeyDNS | |
1091 | option in | |
1092 | .Xr ssh_config 5 | |
1093 | for more information. | |
43a7d9e7 | 1094 | .Sh SSH-BASED VIRTUAL PRIVATE NETWORKS |
1095 | .Nm | |
1096 | contains support for Virtual Private Network (VPN) tunnelling | |
1097 | using the | |
1098 | .Xr tun 4 | |
1099 | network pseudo-device, | |
1100 | allowing two networks to be joined securely. | |
1101 | The | |
1102 | .Xr sshd_config 5 | |
1103 | configuration option | |
1104 | .Cm PermitTunnel | |
1105 | controls whether the server supports this, | |
1106 | and at what level (layer 2 or 3 traffic). | |
1107 | .Pp | |
1108 | The following example would connect client network 10.0.50.0/24 | |
b8e51e81 | 1109 | with remote network 10.0.99.0/24 using a point-to-point connection |
1110 | from 10.1.1.1 to 10.1.1.2, | |
1111 | provided that the SSH server running on the gateway to the remote network, | |
1112 | at 192.168.1.15, allows it. | |
1113 | .Pp | |
1114 | On the client: | |
43a7d9e7 | 1115 | .Bd -literal -offset indent |
1116 | # ssh -f -w 0:1 192.168.1.15 true | |
b8e51e81 | 1117 | # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 |
1118 | # route add 10.0.99.0/24 10.1.1.2 | |
1119 | .Ed | |
1120 | .Pp | |
1121 | On the server: | |
1122 | .Bd -literal -offset indent | |
1123 | # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 | |
1124 | # route add 10.0.50.0/24 10.1.1.1 | |
43a7d9e7 | 1125 | .Ed |
1126 | .Pp | |
1127 | Client access may be more finely tuned via the | |
1128 | .Pa /root/.ssh/authorized_keys | |
1129 | file (see below) and the | |
1130 | .Cm PermitRootLogin | |
1131 | server option. | |
260c414c | 1132 | The following entry would permit connections on |
43a7d9e7 | 1133 | .Xr tun 4 |
260c414c | 1134 | device 1 from user |
43a7d9e7 | 1135 | .Dq jane |
260c414c | 1136 | and on tun device 2 from user |
43a7d9e7 | 1137 | .Dq john , |
1138 | if | |
1139 | .Cm PermitRootLogin | |
1140 | is set to | |
1141 | .Dq forced-commands-only : | |
1142 | .Bd -literal -offset 2n | |
1143 | tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane | |
0ac58ab4 | 1144 | tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john |
43a7d9e7 | 1145 | .Ed |
1146 | .Pp | |
525251b0 | 1147 | Since an SSH-based setup entails a fair amount of overhead, |
43a7d9e7 | 1148 | it may be more suited to temporary setups, |
1149 | such as for wireless VPNs. | |
1150 | More permanent VPNs are better provided by tools such as | |
1151 | .Xr ipsecctl 8 | |
1152 | and | |
1153 | .Xr isakmpd 8 . | |
bf740959 | 1154 | .Sh ENVIRONMENT |
1155 | .Nm | |
1156 | will normally set the following environment variables: | |
0502727e | 1157 | .Bl -tag -width "SSH_ORIGINAL_COMMAND" |
bf740959 | 1158 | .It Ev DISPLAY |
1159 | The | |
1160 | .Ev DISPLAY | |
610cd5c6 | 1161 | variable indicates the location of the X11 server. |
f54651ce | 1162 | It is automatically set by |
bf740959 | 1163 | .Nm |
1164 | to point to a value of the form | |
b92605e1 | 1165 | .Dq hostname:n , |
1166 | where | |
1167 | .Dq hostname | |
1168 | indicates the host where the shell runs, and | |
1169 | .Sq n | |
1170 | is an integer \*(Ge 1. | |
610cd5c6 | 1171 | .Nm |
1172 | uses this special value to forward X11 connections over the secure | |
1173 | channel. | |
da89cf4d | 1174 | The user should normally not set |
1175 | .Ev DISPLAY | |
1176 | explicitly, as that | |
bf740959 | 1177 | will render the X11 connection insecure (and will require the user to |
1178 | manually copy any required authorization cookies). | |
1179 | .It Ev HOME | |
1180 | Set to the path of the user's home directory. | |
1181 | .It Ev LOGNAME | |
1182 | Synonym for | |
1183 | .Ev USER ; | |
1184 | set for compatibility with systems that use this variable. | |
1185 | .It Ev MAIL | |
ae897d7c | 1186 | Set to the path of the user's mailbox. |
610cd5c6 | 1187 | .It Ev PATH |
bf740959 | 1188 | Set to the default |
1189 | .Ev PATH , | |
1190 | as specified when compiling | |
20892533 | 1191 | .Nm . |
3474b2b4 | 1192 | .It Ev SSH_ASKPASS |
1193 | If | |
1194 | .Nm | |
1195 | needs a passphrase, it will read the passphrase from the current | |
1196 | terminal if it was run from a terminal. | |
1197 | If | |
1198 | .Nm | |
1199 | does not have a terminal associated with it but | |
1200 | .Ev DISPLAY | |
1201 | and | |
1202 | .Ev SSH_ASKPASS | |
1203 | are set, it will execute the program specified by | |
1204 | .Ev SSH_ASKPASS | |
1205 | and open an X11 window to read the passphrase. | |
1206 | This is particularly useful when calling | |
1207 | .Nm | |
1208 | from a | |
caeffafb | 1209 | .Pa .xsession |
3474b2b4 | 1210 | or related script. |
1211 | (Note that on some machines it | |
1212 | may be necessary to redirect the input from | |
1213 | .Pa /dev/null | |
1214 | to make this work.) | |
bf740959 | 1215 | .It Ev SSH_AUTH_SOCK |
b92605e1 | 1216 | Identifies the path of a |
1217 | .Ux Ns -domain | |
1218 | socket used to communicate with the agent. | |
da0561eb | 1219 | .It Ev SSH_CONNECTION |
1220 | Identifies the client and server ends of the connection. | |
610cd5c6 | 1221 | The variable contains |
b92605e1 | 1222 | four space-separated values: client IP address, client port number, |
1223 | server IP address, and server port number. | |
8abcdba4 | 1224 | .It Ev SSH_ORIGINAL_COMMAND |
b92605e1 | 1225 | This variable contains the original command line if a forced command |
8abcdba4 | 1226 | is executed. |
1227 | It can be used to extract the original arguments. | |
bf740959 | 1228 | .It Ev SSH_TTY |
1229 | This is set to the name of the tty (path to the device) associated | |
610cd5c6 | 1230 | with the current shell or command. |
1231 | If the current session has no tty, | |
bf740959 | 1232 | this variable is not set. |
1233 | .It Ev TZ | |
04ac3e62 | 1234 | This variable is set to indicate the present time zone if it |
f09ffbdb | 1235 | was set when the daemon was started (i.e. the daemon passes the value |
bf740959 | 1236 | on to new connections). |
1237 | .It Ev USER | |
1238 | Set to the name of the user logging in. | |
1239 | .El | |
1240 | .Pp | |
f54651ce | 1241 | Additionally, |
bf740959 | 1242 | .Nm |
f54651ce | 1243 | reads |
140e3e97 | 1244 | .Pa ~/.ssh/environment , |
bf740959 | 1245 | and adds lines of the format |
1246 | .Dq VARNAME=value | |
b92605e1 | 1247 | to the environment if the file exists and users are allowed to |
6a342527 | 1248 | change their environment. |
5f4a0c58 | 1249 | For more information, see the |
6a342527 | 1250 | .Cm PermitUserEnvironment |
35453849 | 1251 | option in |
6a342527 | 1252 | .Xr sshd_config 5 . |
bf740959 | 1253 | .Sh FILES |
0624a70b | 1254 | .Bl -tag -width Ds -compact |
0624a70b | 1255 | .It ~/.rhosts |
f3119772 | 1256 | This file is used for host-based authentication (see above). |
e91c60f2 | 1257 | On some machines this file may need to be |
f3119772 | 1258 | world-readable if the user's home directory is on an NFS partition, |
bf740959 | 1259 | because |
1260 | .Xr sshd 8 | |
610cd5c6 | 1261 | reads it as root. |
1262 | Additionally, this file must be owned by the user, | |
1263 | and must not have write permissions for anyone else. | |
1264 | The recommended | |
bf740959 | 1265 | permission for most machines is read/write for the user, and not |
1266 | accessible by others. | |
1267 | .Pp | |
0624a70b | 1268 | .It ~/.shosts |
f3119772 | 1269 | This file is used in exactly the same way as |
1270 | .Pa .rhosts , | |
1271 | but allows host-based authentication without permitting login with | |
1272 | rlogin/rsh. | |
0624a70b | 1273 | .Pp |
7c36e880 | 1274 | .It ~/.ssh/ |
1275 | This directory is the default location for all user-specific configuration | |
1276 | and authentication information. | |
1277 | There is no general requirement to keep the entire contents of this directory | |
1278 | secret, but the recommended permissions are read/write/execute for the user, | |
1279 | and not accessible by others. | |
1280 | .Pp | |
f403d7b5 | 1281 | .It ~/.ssh/authorized_keys |
1282 | Lists the public keys (RSA/DSA) that can be used for logging in as this user. | |
1283 | The format of this file is described in the | |
1284 | .Xr sshd 8 | |
1285 | manual page. | |
f403d7b5 | 1286 | This file is not highly sensitive, but the recommended |
1287 | permissions are read/write for the user, and not accessible by others. | |
1288 | .Pp | |
1289 | .It ~/.ssh/config | |
1290 | This is the per-user configuration file. | |
1291 | The file format and configuration options are described in | |
1292 | .Xr ssh_config 5 . | |
1293 | Because of the potential for abuse, this file must have strict permissions: | |
1294 | read/write for the user, and not accessible by others. | |
1295 | .Pp | |
1296 | .It ~/.ssh/environment | |
c0907b37 | 1297 | Contains additional definitions for environment variables; see |
1298 | .Sx ENVIRONMENT , | |
f403d7b5 | 1299 | above. |
1300 | .Pp | |
1301 | .It ~/.ssh/identity | |
1302 | .It ~/.ssh/id_dsa | |
1303 | .It ~/.ssh/id_rsa | |
1304 | Contains the private key for authentication. | |
1305 | These files | |
1306 | contain sensitive data and should be readable by the user but not | |
1307 | accessible by others (read/write/execute). | |
1308 | .Nm | |
1309 | will simply ignore a private key file if it is accessible by others. | |
1310 | It is possible to specify a passphrase when | |
1311 | generating the key which will be used to encrypt the | |
1312 | sensitive part of this file using 3DES. | |
1313 | .Pp | |
1314 | .It ~/.ssh/identity.pub | |
1315 | .It ~/.ssh/id_dsa.pub | |
1316 | .It ~/.ssh/id_rsa.pub | |
1317 | Contains the public key for authentication. | |
1318 | These files are not | |
1319 | sensitive and can (but need not) be readable by anyone. | |
f403d7b5 | 1320 | .Pp |
1321 | .It ~/.ssh/known_hosts | |
aaa18db9 | 1322 | Contains a list of host keys for all hosts the user has logged into |
1323 | that are not already in the systemwide list of known host keys. | |
f403d7b5 | 1324 | See |
aaa18db9 | 1325 | .Xr sshd 8 |
1326 | for further details of the format of this file. | |
f403d7b5 | 1327 | .Pp |
1328 | .It ~/.ssh/rc | |
1329 | Commands in this file are executed by | |
1330 | .Nm | |
5d7b356f | 1331 | when the user logs in, just before the user's shell (or command) is |
f403d7b5 | 1332 | started. |
1333 | See the | |
1334 | .Xr sshd 8 | |
1335 | manual page for more information. | |
1336 | .Pp | |
0624a70b | 1337 | .It /etc/hosts.equiv |
f3119772 | 1338 | This file is for host-based authentication (see above). |
1339 | It should only be writable by root. | |
0624a70b | 1340 | .Pp |
1341 | .It /etc/shosts.equiv | |
f3119772 | 1342 | This file is used in exactly the same way as |
1343 | .Pa hosts.equiv , | |
1344 | but allows host-based authentication without permitting login with | |
1345 | rlogin/rsh. | |
0624a70b | 1346 | .Pp |
f403d7b5 | 1347 | .It Pa /etc/ssh/ssh_config |
1348 | Systemwide configuration file. | |
1349 | The file format and configuration options are described in | |
1350 | .Xr ssh_config 5 . | |
1351 | .Pp | |
1352 | .It /etc/ssh/ssh_host_key | |
1353 | .It /etc/ssh/ssh_host_dsa_key | |
1354 | .It /etc/ssh/ssh_host_rsa_key | |
1355 | These three files contain the private parts of the host keys | |
5d7b356f | 1356 | and are used for host-based authentication. |
1357 | If protocol version 1 is used, | |
bf740959 | 1358 | .Nm |
f403d7b5 | 1359 | must be setuid root, since the host key is readable only by root. |
1360 | For protocol version 2, | |
1361 | .Nm | |
1362 | uses | |
1363 | .Xr ssh-keysign 8 | |
5d7b356f | 1364 | to access the host keys, |
1365 | eliminating the requirement that | |
f403d7b5 | 1366 | .Nm |
5d7b356f | 1367 | be setuid root when host-based authentication is used. |
f403d7b5 | 1368 | By default |
1369 | .Nm | |
1370 | is not setuid root. | |
1371 | .Pp | |
1372 | .It /etc/ssh/ssh_known_hosts | |
1373 | Systemwide list of known host keys. | |
1374 | This file should be prepared by the | |
1375 | system administrator to contain the public host keys of all machines in the | |
1376 | organization. | |
aaa18db9 | 1377 | It should be world-readable. |
1378 | See | |
f403d7b5 | 1379 | .Xr sshd 8 |
aaa18db9 | 1380 | for further details of the format of this file. |
f403d7b5 | 1381 | .Pp |
1382 | .It /etc/ssh/sshrc | |
bf740959 | 1383 | Commands in this file are executed by |
1384 | .Nm | |
5d7b356f | 1385 | when the user logs in, just before the user's shell (or command) is started. |
f54651ce | 1386 | See the |
bf740959 | 1387 | .Xr sshd 8 |
1388 | manual page for more information. | |
b5e300c2 | 1389 | .El |
bf740959 | 1390 | .Sh SEE ALSO |
bf740959 | 1391 | .Xr scp 1 , |
61e96248 | 1392 | .Xr sftp 1 , |
bf740959 | 1393 | .Xr ssh-add 1 , |
1394 | .Xr ssh-agent 1 , | |
1395 | .Xr ssh-keygen 1 , | |
926f6a7a | 1396 | .Xr ssh-keyscan 1 , |
43a7d9e7 | 1397 | .Xr tun 4 , |
5f4a0c58 | 1398 | .Xr hosts.equiv 5 , |
1ae02182 | 1399 | .Xr ssh_config 5 , |
b2843ec6 | 1400 | .Xr ssh-keysign 8 , |
9afadca8 | 1401 | .Xr sshd 8 |
2cad6cef | 1402 | .Rs |
0e505e42 | 1403 | .%R RFC 4250 |
1404 | .%T "The Secure Shell (SSH) Protocol Assigned Numbers" | |
1405 | .%D 2006 | |
1406 | .Re | |
1407 | .Rs | |
1408 | .%R RFC 4251 | |
1409 | .%T "The Secure Shell (SSH) Protocol Architecture" | |
1410 | .%D 2006 | |
1411 | .Re | |
1412 | .Rs | |
1413 | .%R RFC 4252 | |
1414 | .%T "The Secure Shell (SSH) Authentication Protocol" | |
1415 | .%D 2006 | |
1416 | .Re | |
1417 | .Rs | |
1418 | .%R RFC 4253 | |
1419 | .%T "The Secure Shell (SSH) Transport Layer Protocol" | |
1420 | .%D 2006 | |
1421 | .Re | |
1422 | .Rs | |
1423 | .%R RFC 4254 | |
1424 | .%T "The Secure Shell (SSH) Connection Protocol" | |
1425 | .%D 2006 | |
1426 | .Re | |
1427 | .Rs | |
1428 | .%R RFC 4255 | |
1429 | .%T "Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints" | |
1430 | .%D 2006 | |
1431 | .Re | |
1432 | .Rs | |
1433 | .%R RFC 4256 | |
1434 | .%T "Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)" | |
1435 | .%D 2006 | |
1436 | .Re | |
1437 | .Rs | |
1438 | .%R RFC 4335 | |
1439 | .%T "The Secure Shell (SSH) Session Channel Break Extension" | |
1440 | .%D 2006 | |
1441 | .Re | |
1442 | .Rs | |
1443 | .%R RFC 4344 | |
1444 | .%T "The Secure Shell (SSH) Transport Layer Encryption Modes" | |
1445 | .%D 2006 | |
1446 | .Re | |
1447 | .Rs | |
1448 | .%R RFC 4345 | |
1449 | .%T "Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol" | |
1450 | .%D 2006 | |
2cad6cef | 1451 | .Re |
df938409 | 1452 | .Rs |
1453 | .%R RFC 4419 | |
1454 | .%T "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol" | |
1455 | .%D 2006 | |
1456 | .Re | |
4e3c0053 | 1457 | .Rs |
1458 | .%R RFC 4716 | |
1459 | .%T "The Secure Shell (SSH) Public Key File Format" | |
1460 | .%D 2006 | |
1461 | .Re | |
990ada29 | 1462 | .Rs |
1463 | .%T "Hash Visualization: a New Technique to improve Real-World Security" | |
1464 | .%A A. Perrig | |
1465 | .%A D. Song | |
1466 | .%D 1999 | |
1467 | .%O "International Workshop on Cryptographic Techniques and E-Commerce (CrypTEC '99)" | |
1468 | .Re | |
be193d89 | 1469 | .Sh AUTHORS |
1470 | OpenSSH is a derivative of the original and free | |
1471 | ssh 1.2.12 release by Tatu Ylonen. | |
1472 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
1473 | Theo de Raadt and Dug Song | |
1474 | removed many bugs, re-added newer features and | |
1475 | created OpenSSH. | |
1476 | Markus Friedl contributed the support for SSH | |
1477 | protocol versions 1.5 and 2.0. |