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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_config.5,v 1.85 2006/02/25 12:26:17 jmc Exp $ | |
38 | .Dd September 25, 1999 | |
39 | .Dt SSH_CONFIG 5 | |
40 | .Os | |
41 | .Sh NAME | |
42 | .Nm ssh_config | |
43 | .Nd OpenSSH SSH client configuration files | |
44 | .Sh SYNOPSIS | |
45 | .Bl -tag -width Ds -compact | |
46 | .It Pa ~/.ssh/config | |
47 | .It Pa /etc/ssh/ssh_config | |
48 | .El | |
49 | .Sh DESCRIPTION | |
50 | .Xr ssh 1 | |
51 | obtains configuration data from the following sources in | |
52 | the following order: | |
53 | .Pp | |
54 | .Bl -enum -offset indent -compact | |
55 | .It | |
56 | command-line options | |
57 | .It | |
58 | user's configuration file | |
59 | .Pq Pa ~/.ssh/config | |
60 | .It | |
61 | system-wide configuration file | |
62 | .Pq Pa /etc/ssh/ssh_config | |
63 | .El | |
64 | .Pp | |
65 | For each parameter, the first obtained value | |
66 | will be used. | |
67 | The configuration files contain sections separated by | |
68 | .Dq Host | |
69 | specifications, and that section is only applied for hosts that | |
70 | match one of the patterns given in the specification. | |
71 | The matched host name is the one given on the command line. | |
72 | .Pp | |
73 | Since the first obtained value for each parameter is used, more | |
74 | host-specific declarations should be given near the beginning of the | |
75 | file, and general defaults at the end. | |
76 | .Sh PATTERNS | |
77 | A | |
78 | .Em pattern | |
79 | consists of zero or more non-whitespace characters, | |
80 | .Sq * | |
81 | (a wildcard that matches zero or more characters), | |
82 | or | |
83 | .Sq ?\& | |
84 | (a wildcard that matches exactly one character). | |
85 | For example, to specify a set of declarations for any host in the | |
86 | .Dq .co.uk | |
87 | set of domains, | |
88 | the following pattern could be used: | |
89 | .Pp | |
90 | .Dl Host *.co.uk | |
91 | .Pp | |
92 | The following pattern | |
93 | would match any host in the 192.168.0.[0-9] network range: | |
94 | .Pp | |
95 | .Dl Host 192.168.0.? | |
96 | .Pp | |
97 | A | |
98 | .Em pattern-list | |
99 | is a comma-separated list of patterns. | |
100 | Patterns within pattern-lists may be negated | |
101 | by preceding them with an exclamation mark | |
102 | .Pq Sq !\& . | |
103 | For example, | |
104 | to allow a key to be used from anywhere within an organisation | |
105 | except from the | |
106 | .Dq dialup | |
107 | pool, | |
108 | the following entry (in authorized_keys) could be used: | |
109 | .Pp | |
110 | .Dl from=\&"!*.dialup.example.com,*.example.com\&" | |
111 | .Sh FILE FORMAT | |
112 | The configuration file has the following format: | |
113 | .Pp | |
114 | Empty lines and lines starting with | |
115 | .Ql # | |
116 | are comments. | |
117 | Otherwise a line is of the format | |
118 | .Dq keyword arguments . | |
119 | Configuration options may be separated by whitespace or | |
120 | optional whitespace and exactly one | |
121 | .Ql = ; | |
122 | the latter format is useful to avoid the need to quote whitespace | |
123 | when specifying configuration options using the | |
124 | .Nm ssh , | |
125 | .Nm scp | |
126 | and | |
127 | .Nm sftp | |
128 | .Fl o | |
129 | option. | |
130 | .Pp | |
131 | The possible | |
132 | keywords and their meanings are as follows (note that | |
133 | keywords are case-insensitive and arguments are case-sensitive): | |
134 | .Bl -tag -width Ds | |
135 | .It Cm Host | |
136 | Restricts the following declarations (up to the next | |
137 | .Cm Host | |
138 | keyword) to be only for those hosts that match one of the patterns | |
139 | given after the keyword. | |
140 | A single | |
141 | .Ql * | |
142 | as a pattern can be used to provide global | |
143 | defaults for all hosts. | |
144 | The host is the | |
145 | .Ar hostname | |
146 | argument given on the command line (i.e. the name is not converted to | |
147 | a canonicalized host name before matching). | |
148 | .Pp | |
149 | See | |
150 | .Sx PATTERNS | |
151 | for more information on patterns. | |
152 | .It Cm AddressFamily | |
153 | Specifies which address family to use when connecting. | |
154 | Valid arguments are | |
155 | .Dq any , | |
156 | .Dq inet | |
157 | (use IPv4 only), or | |
158 | .Dq inet6 | |
159 | (use IPv6 only). | |
160 | .It Cm BatchMode | |
161 | If set to | |
162 | .Dq yes , | |
163 | passphrase/password querying will be disabled. | |
164 | This option is useful in scripts and other batch jobs where no user | |
165 | is present to supply the password. | |
166 | The argument must be | |
167 | .Dq yes | |
168 | or | |
169 | .Dq no . | |
170 | The default is | |
171 | .Dq no . | |
172 | .It Cm BindAddress | |
173 | Use the specified address on the local machine as the source address of | |
174 | the connection. | |
175 | Only useful on systems with more than one address. | |
176 | Note that this option does not work if | |
177 | .Cm UsePrivilegedPort | |
178 | is set to | |
179 | .Dq yes . | |
180 | .It Cm ChallengeResponseAuthentication | |
181 | Specifies whether to use challenge-response authentication. | |
182 | The argument to this keyword must be | |
183 | .Dq yes | |
184 | or | |
185 | .Dq no . | |
186 | The default is | |
187 | .Dq yes . | |
188 | .It Cm CheckHostIP | |
189 | If this flag is set to | |
190 | .Dq yes , | |
191 | .Xr ssh 1 | |
192 | will additionally check the host IP address in the | |
193 | .Pa known_hosts | |
194 | file. | |
195 | This allows ssh to detect if a host key changed due to DNS spoofing. | |
196 | If the option is set to | |
197 | .Dq no , | |
198 | the check will not be executed. | |
199 | The default is | |
200 | .Dq yes . | |
201 | .It Cm Cipher | |
202 | Specifies the cipher to use for encrypting the session | |
203 | in protocol version 1. | |
204 | Currently, | |
205 | .Dq blowfish , | |
206 | .Dq 3des , | |
207 | and | |
208 | .Dq des | |
209 | are supported. | |
210 | .Ar des | |
211 | is only supported in the | |
212 | .Xr ssh 1 | |
213 | client for interoperability with legacy protocol 1 implementations | |
214 | that do not support the | |
215 | .Ar 3des | |
216 | cipher. | |
217 | Its use is strongly discouraged due to cryptographic weaknesses. | |
218 | The default is | |
219 | .Dq 3des . | |
220 | .It Cm Ciphers | |
221 | Specifies the ciphers allowed for protocol version 2 | |
222 | in order of preference. | |
223 | Multiple ciphers must be comma-separated. | |
224 | The supported ciphers are | |
225 | .Dq 3des-cbc , | |
226 | .Dq aes128-cbc , | |
227 | .Dq aes192-cbc , | |
228 | .Dq aes256-cbc , | |
229 | .Dq aes128-ctr , | |
230 | .Dq aes192-ctr , | |
231 | .Dq aes256-ctr , | |
232 | .Dq arcfour128 , | |
233 | .Dq arcfour256 , | |
234 | .Dq arcfour , | |
235 | .Dq blowfish-cbc , | |
236 | and | |
237 | .Dq cast128-cbc . | |
238 | The default is: | |
239 | .Bd -literal -offset 3n | |
240 | aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, | |
241 | arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, | |
242 | aes192-ctr,aes256-ctr | |
243 | .Ed | |
244 | .It Cm ClearAllForwardings | |
245 | Specifies that all local, remote, and dynamic port forwardings | |
246 | specified in the configuration files or on the command line be | |
247 | cleared. | |
248 | This option is primarily useful when used from the | |
249 | .Xr ssh 1 | |
250 | command line to clear port forwardings set in | |
251 | configuration files, and is automatically set by | |
252 | .Xr scp 1 | |
253 | and | |
254 | .Xr sftp 1 . | |
255 | The argument must be | |
256 | .Dq yes | |
257 | or | |
258 | .Dq no . | |
259 | The default is | |
260 | .Dq no . | |
261 | .It Cm Compression | |
262 | Specifies whether to use compression. | |
263 | The argument must be | |
264 | .Dq yes | |
265 | or | |
266 | .Dq no . | |
267 | The default is | |
268 | .Dq no . | |
269 | .It Cm CompressionLevel | |
270 | Specifies the compression level to use if compression is enabled. | |
271 | The argument must be an integer from 1 (fast) to 9 (slow, best). | |
272 | The default level is 6, which is good for most applications. | |
273 | The meaning of the values is the same as in | |
274 | .Xr gzip 1 . | |
275 | Note that this option applies to protocol version 1 only. | |
276 | .It Cm ConnectionAttempts | |
277 | Specifies the number of tries (one per second) to make before exiting. | |
278 | The argument must be an integer. | |
279 | This may be useful in scripts if the connection sometimes fails. | |
280 | The default is 1. | |
281 | .It Cm ConnectTimeout | |
282 | Specifies the timeout (in seconds) used when connecting to the | |
283 | SSH server, instead of using the default system TCP timeout. | |
284 | This value is used only when the target is down or really unreachable, | |
285 | not when it refuses the connection. | |
286 | .It Cm ControlMaster | |
287 | Enables the sharing of multiple sessions over a single network connection. | |
288 | When set to | |
289 | .Dq yes , | |
290 | .Xr ssh 1 | |
291 | will listen for connections on a control socket specified using the | |
292 | .Cm ControlPath | |
293 | argument. | |
294 | Additional sessions can connect to this socket using the same | |
295 | .Cm ControlPath | |
296 | with | |
297 | .Cm ControlMaster | |
298 | set to | |
299 | .Dq no | |
300 | (the default). | |
301 | These sessions will try to reuse the master instance's network connection | |
302 | rather than initiating new ones, but will fall back to connecting normally | |
303 | if the control socket does not exist, or is not listening. | |
304 | .Pp | |
305 | Setting this to | |
306 | .Dq ask | |
307 | will cause ssh | |
308 | to listen for control connections, but require confirmation using the | |
309 | .Ev SSH_ASKPASS | |
310 | program before they are accepted (see | |
311 | .Xr ssh-add 1 | |
312 | for details). | |
313 | If the | |
314 | .Cm ControlPath | |
315 | cannot be opened, | |
316 | ssh will continue without connecting to a master instance. | |
317 | .Pp | |
318 | X11 and | |
319 | .Xr ssh-agent 1 | |
320 | forwarding is supported over these multiplexed connections, however the | |
321 | display and agent forwarded will be the one belonging to the master | |
322 | connection i.e. it is not possible to forward multiple displays or agents. | |
323 | .Pp | |
324 | Two additional options allow for opportunistic multiplexing: try to use a | |
325 | master connection but fall back to creating a new one if one does not already | |
326 | exist. | |
327 | These options are: | |
328 | .Dq auto | |
329 | and | |
330 | .Dq autoask . | |
331 | The latter requires confirmation like the | |
332 | .Dq ask | |
333 | option. | |
334 | .It Cm ControlPath | |
335 | Specify the path to the control socket used for connection sharing as described | |
336 | in the | |
337 | .Cm ControlMaster | |
338 | section above or the string | |
339 | .Dq none | |
340 | to disable connection sharing. | |
341 | In the path, | |
342 | .Ql %l | |
343 | will be substituted by the local host name, | |
344 | .Ql %h | |
345 | will be substituted by the target host name, | |
346 | .Ql %p | |
347 | the port, and | |
348 | .Ql %r | |
349 | by the remote login username. | |
350 | It is recommended that any | |
351 | .Cm ControlPath | |
352 | used for opportunistic connection sharing include | |
353 | at least %h, %p, and %r. | |
354 | This ensures that shared connections are uniquely identified. | |
355 | .It Cm DynamicForward | |
356 | Specifies that a TCP port on the local machine be forwarded | |
357 | over the secure channel, and the application | |
358 | protocol is then used to determine where to connect to from the | |
359 | remote machine. | |
360 | .Pp | |
361 | The argument must be | |
362 | .Sm off | |
363 | .Oo Ar bind_address : Oc Ar port . | |
364 | .Sm on | |
365 | IPv6 addresses can be specified by enclosing addresses in square brackets or | |
366 | by using an alternative syntax: | |
367 | .Oo Ar bind_address Ns / Oc Ns Ar port . | |
368 | By default, the local port is bound in accordance with the | |
369 | .Cm GatewayPorts | |
370 | setting. | |
371 | However, an explicit | |
372 | .Ar bind_address | |
373 | may be used to bind the connection to a specific address. | |
374 | The | |
375 | .Ar bind_address | |
376 | of | |
377 | .Dq localhost | |
378 | indicates that the listening port be bound for local use only, while an | |
379 | empty address or | |
380 | .Sq * | |
381 | indicates that the port should be available from all interfaces. | |
382 | .Pp | |
383 | Currently the SOCKS4 and SOCKS5 protocols are supported, and | |
384 | .Xr ssh 1 | |
385 | will act as a SOCKS server. | |
386 | Multiple forwardings may be specified, and | |
387 | additional forwardings can be given on the command line. | |
388 | Only the superuser can forward privileged ports. | |
389 | .It Cm EnableSSHKeysign | |
390 | Setting this option to | |
391 | .Dq yes | |
392 | in the global client configuration file | |
393 | .Pa /etc/ssh/ssh_config | |
394 | enables the use of the helper program | |
395 | .Xr ssh-keysign 8 | |
396 | during | |
397 | .Cm HostbasedAuthentication . | |
398 | The argument must be | |
399 | .Dq yes | |
400 | or | |
401 | .Dq no . | |
402 | The default is | |
403 | .Dq no . | |
404 | This option should be placed in the non-hostspecific section. | |
405 | See | |
406 | .Xr ssh-keysign 8 | |
407 | for more information. | |
408 | .It Cm EscapeChar | |
409 | Sets the escape character (default: | |
410 | .Ql ~ ) . | |
411 | The escape character can also | |
412 | be set on the command line. | |
413 | The argument should be a single character, | |
414 | .Ql ^ | |
415 | followed by a letter, or | |
416 | .Dq none | |
417 | to disable the escape | |
418 | character entirely (making the connection transparent for binary | |
419 | data). | |
420 | .It Cm ForwardAgent | |
421 | Specifies whether the connection to the authentication agent (if any) | |
422 | will be forwarded to the remote machine. | |
423 | The argument must be | |
424 | .Dq yes | |
425 | or | |
426 | .Dq no . | |
427 | The default is | |
428 | .Dq no . | |
429 | .Pp | |
430 | Agent forwarding should be enabled with caution. | |
431 | Users with the ability to bypass file permissions on the remote host | |
432 | (for the agent's Unix-domain socket) | |
433 | can access the local agent through the forwarded connection. | |
434 | An attacker cannot obtain key material from the agent, | |
435 | however they can perform operations on the keys that enable them to | |
436 | authenticate using the identities loaded into the agent. | |
437 | .It Cm ForwardX11 | |
438 | Specifies whether X11 connections will be automatically redirected | |
439 | over the secure channel and | |
440 | .Ev DISPLAY | |
441 | set. | |
442 | The argument must be | |
443 | .Dq yes | |
444 | or | |
445 | .Dq no . | |
446 | The default is | |
447 | .Dq no . | |
448 | .Pp | |
449 | X11 forwarding should be enabled with caution. | |
450 | Users with the ability to bypass file permissions on the remote host | |
451 | (for the user's X11 authorization database) | |
452 | can access the local X11 display through the forwarded connection. | |
453 | An attacker may then be able to perform activities such as keystroke monitoring | |
454 | if the | |
455 | .Cm ForwardX11Trusted | |
456 | option is also enabled. | |
457 | .It Cm ForwardX11Trusted | |
458 | If this option is set to | |
459 | .Dq yes , | |
460 | remote X11 clients will have full access to the original X11 display. | |
461 | .Pp | |
462 | If this option is set to | |
463 | .Dq no , | |
464 | remote X11 clients will be considered untrusted and prevented | |
465 | from stealing or tampering with data belonging to trusted X11 | |
466 | clients. | |
467 | Furthermore, the | |
468 | .Xr xauth 1 | |
469 | token used for the session will be set to expire after 20 minutes. | |
470 | Remote clients will be refused access after this time. | |
471 | .Pp | |
472 | The default is | |
473 | .Dq no . | |
474 | .Pp | |
475 | See the X11 SECURITY extension specification for full details on | |
476 | the restrictions imposed on untrusted clients. | |
477 | .It Cm GatewayPorts | |
478 | Specifies whether remote hosts are allowed to connect to local | |
479 | forwarded ports. | |
480 | By default, | |
481 | .Xr ssh 1 | |
482 | binds local port forwardings to the loopback address. | |
483 | This prevents other remote hosts from connecting to forwarded ports. | |
484 | .Cm GatewayPorts | |
485 | can be used to specify that ssh | |
486 | should bind local port forwardings to the wildcard address, | |
487 | thus allowing remote hosts to connect to forwarded ports. | |
488 | The argument must be | |
489 | .Dq yes | |
490 | or | |
491 | .Dq no . | |
492 | The default is | |
493 | .Dq no . | |
494 | .It Cm GlobalKnownHostsFile | |
495 | Specifies a file to use for the global | |
496 | host key database instead of | |
497 | .Pa /etc/ssh/ssh_known_hosts . | |
498 | .It Cm GSSAPIAuthentication | |
499 | Specifies whether user authentication based on GSSAPI is allowed. | |
500 | The default is | |
501 | .Dq no . | |
502 | Note that this option applies to protocol version 2 only. | |
503 | .It Cm GSSAPIDelegateCredentials | |
504 | Forward (delegate) credentials to the server. | |
505 | The default is | |
506 | .Dq no . | |
507 | Note that this option applies to protocol version 2 only. | |
508 | .It Cm HashKnownHosts | |
509 | Indicates that | |
510 | .Xr ssh 1 | |
511 | should hash host names and addresses when they are added to | |
512 | .Pa ~/.ssh/known_hosts . | |
513 | These hashed names may be used normally by | |
514 | .Xr ssh 1 | |
515 | and | |
516 | .Xr sshd 8 , | |
517 | but they do not reveal identifying information should the file's contents | |
518 | be disclosed. | |
519 | The default is | |
520 | .Dq no . | |
521 | Note that hashing of names and addresses will not be retrospectively applied | |
522 | to existing known hosts files, but these may be manually hashed using | |
523 | .Xr ssh-keygen 1 . | |
524 | .It Cm HostbasedAuthentication | |
525 | Specifies whether to try rhosts based authentication with public key | |
526 | authentication. | |
527 | The argument must be | |
528 | .Dq yes | |
529 | or | |
530 | .Dq no . | |
531 | The default is | |
532 | .Dq no . | |
533 | This option applies to protocol version 2 only and | |
534 | is similar to | |
535 | .Cm RhostsRSAAuthentication . | |
536 | .It Cm HostKeyAlgorithms | |
537 | Specifies the protocol version 2 host key algorithms | |
538 | that the client wants to use in order of preference. | |
539 | The default for this option is: | |
540 | .Dq ssh-rsa,ssh-dss . | |
541 | .It Cm HostKeyAlias | |
542 | Specifies an alias that should be used instead of the | |
543 | real host name when looking up or saving the host key | |
544 | in the host key database files. | |
545 | This option is useful for tunneling SSH connections | |
546 | or for multiple servers running on a single host. | |
547 | .It Cm HostName | |
548 | Specifies the real host name to log into. | |
549 | This can be used to specify nicknames or abbreviations for hosts. | |
550 | The default is the name given on the command line. | |
551 | Numeric IP addresses are also permitted (both on the command line and in | |
552 | .Cm HostName | |
553 | specifications). | |
554 | .It Cm IdentitiesOnly | |
555 | Specifies that | |
556 | .Xr ssh 1 | |
557 | should only use the authentication identity files configured in the | |
558 | .Nm | |
559 | files, | |
560 | even if | |
561 | .Xr ssh-agent 1 | |
562 | offers more identities. | |
563 | The argument to this keyword must be | |
564 | .Dq yes | |
565 | or | |
566 | .Dq no . | |
567 | This option is intended for situations where ssh-agent | |
568 | offers many different identities. | |
569 | The default is | |
570 | .Dq no . | |
571 | .It Cm IdentityFile | |
572 | Specifies a file from which the user's RSA or DSA authentication identity | |
573 | is read. | |
574 | The default is | |
575 | .Pa ~/.ssh/identity | |
576 | for protocol version 1, and | |
577 | .Pa ~/.ssh/id_rsa | |
578 | and | |
579 | .Pa ~/.ssh/id_dsa | |
580 | for protocol version 2. | |
581 | Additionally, any identities represented by the authentication agent | |
582 | will be used for authentication. | |
583 | The file name may use the tilde | |
584 | syntax to refer to a user's home directory. | |
585 | It is possible to have | |
586 | multiple identity files specified in configuration files; all these | |
587 | identities will be tried in sequence. | |
588 | .It Cm KbdInteractiveDevices | |
589 | Specifies the list of methods to use in keyboard-interactive authentication. | |
590 | Multiple method names must be comma-separated. | |
591 | The default is to use the server specified list. | |
592 | The methods available vary depending on what the server supports. | |
593 | For an OpenSSH server, | |
594 | it may be zero or more of: | |
595 | .Dq bsdauth , | |
596 | .Dq pam , | |
597 | and | |
598 | .Dq skey . | |
599 | .It Cm LocalCommand | |
600 | Specifies a command to execute on the local machine after successfully | |
601 | connecting to the server. | |
602 | The command string extends to the end of the line, and is executed with | |
603 | .Pa /bin/sh . | |
604 | This directive is ignored unless | |
605 | .Cm PermitLocalCommand | |
606 | has been enabled. | |
607 | .It Cm LocalForward | |
608 | Specifies that a TCP port on the local machine be forwarded over | |
609 | the secure channel to the specified host and port from the remote machine. | |
610 | The first argument must be | |
611 | .Sm off | |
612 | .Oo Ar bind_address : Oc Ar port | |
613 | .Sm on | |
614 | and the second argument must be | |
615 | .Ar host : Ns Ar hostport . | |
616 | IPv6 addresses can be specified by enclosing addresses in square brackets or | |
617 | by using an alternative syntax: | |
618 | .Oo Ar bind_address Ns / Oc Ns Ar port | |
619 | and | |
620 | .Ar host Ns / Ns Ar hostport . | |
621 | Multiple forwardings may be specified, and additional forwardings can be | |
622 | given on the command line. | |
623 | Only the superuser can forward privileged ports. | |
624 | By default, the local port is bound in accordance with the | |
625 | .Cm GatewayPorts | |
626 | setting. | |
627 | However, an explicit | |
628 | .Ar bind_address | |
629 | may be used to bind the connection to a specific address. | |
630 | The | |
631 | .Ar bind_address | |
632 | of | |
633 | .Dq localhost | |
634 | indicates that the listening port be bound for local use only, while an | |
635 | empty address or | |
636 | .Sq * | |
637 | indicates that the port should be available from all interfaces. | |
638 | .It Cm LogLevel | |
639 | Gives the verbosity level that is used when logging messages from | |
640 | .Xr ssh 1 . | |
641 | The possible values are: | |
642 | QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. | |
643 | The default is INFO. | |
644 | DEBUG and DEBUG1 are equivalent. | |
645 | DEBUG2 and DEBUG3 each specify higher levels of verbose output. | |
646 | .It Cm MACs | |
647 | Specifies the MAC (message authentication code) algorithms | |
648 | in order of preference. | |
649 | The MAC algorithm is used in protocol version 2 | |
650 | for data integrity protection. | |
651 | Multiple algorithms must be comma-separated. | |
652 | The default is: | |
653 | .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . | |
654 | .It Cm NoHostAuthenticationForLocalhost | |
655 | This option can be used if the home directory is shared across machines. | |
656 | In this case localhost will refer to a different machine on each of | |
657 | the machines and the user will get many warnings about changed host keys. | |
658 | However, this option disables host authentication for localhost. | |
659 | The argument to this keyword must be | |
660 | .Dq yes | |
661 | or | |
662 | .Dq no . | |
663 | The default is to check the host key for localhost. | |
664 | .It Cm NumberOfPasswordPrompts | |
665 | Specifies the number of password prompts before giving up. | |
666 | The argument to this keyword must be an integer. | |
667 | The default is 3. | |
668 | .It Cm PasswordAuthentication | |
669 | Specifies whether to use password authentication. | |
670 | The argument to this keyword must be | |
671 | .Dq yes | |
672 | or | |
673 | .Dq no . | |
674 | The default is | |
675 | .Dq yes . | |
676 | .It Cm PermitLocalCommand | |
677 | Allow local command execution via the | |
678 | .Ic LocalCommand | |
679 | option or using the | |
680 | .Ic !\& Ns Ar command | |
681 | escape sequence in | |
682 | .Xr ssh 1 . | |
683 | The argument must be | |
684 | .Dq yes | |
685 | or | |
686 | .Dq no . | |
687 | The default is | |
688 | .Dq no . | |
689 | .It Cm Port | |
690 | Specifies the port number to connect on the remote host. | |
691 | The default is 22. | |
692 | .It Cm PreferredAuthentications | |
693 | Specifies the order in which the client should try protocol 2 | |
694 | authentication methods. | |
695 | This allows a client to prefer one method (e.g.\& | |
696 | .Cm keyboard-interactive ) | |
697 | over another method (e.g.\& | |
698 | .Cm password ) | |
699 | The default for this option is: | |
700 | .Dq hostbased,publickey,keyboard-interactive,password . | |
701 | .It Cm Protocol | |
702 | Specifies the protocol versions | |
703 | .Xr ssh 1 | |
704 | should support in order of preference. | |
705 | The possible values are | |
706 | .Sq 1 | |
707 | and | |
708 | .Sq 2 . | |
709 | Multiple versions must be comma-separated. | |
710 | The default is | |
711 | .Dq 2,1 . | |
712 | This means that ssh | |
713 | tries version 2 and falls back to version 1 | |
714 | if version 2 is not available. | |
715 | .It Cm ProxyCommand | |
716 | Specifies the command to use to connect to the server. | |
717 | The command | |
718 | string extends to the end of the line, and is executed with | |
719 | .Pa /bin/sh . | |
720 | In the command string, | |
721 | .Ql %h | |
722 | will be substituted by the host name to | |
723 | connect and | |
724 | .Ql %p | |
725 | by the port. | |
726 | The command can be basically anything, | |
727 | and should read from its standard input and write to its standard output. | |
728 | It should eventually connect an | |
729 | .Xr sshd 8 | |
730 | server running on some machine, or execute | |
731 | .Ic sshd -i | |
732 | somewhere. | |
733 | Host key management will be done using the | |
734 | HostName of the host being connected (defaulting to the name typed by | |
735 | the user). | |
736 | Setting the command to | |
737 | .Dq none | |
738 | disables this option entirely. | |
739 | Note that | |
740 | .Cm CheckHostIP | |
741 | is not available for connects with a proxy command. | |
742 | .Pp | |
743 | This directive is useful in conjunction with | |
744 | .Xr nc 1 | |
745 | and its proxy support. | |
746 | For example, the following directive would connect via an HTTP proxy at | |
747 | 192.0.2.0: | |
748 | .Bd -literal -offset 3n | |
749 | ProxyCommand /usr/bin/nc -X connect -x 192.0.2.0:8080 %h %p | |
750 | .Ed | |
751 | .It Cm PubkeyAuthentication | |
752 | Specifies whether to try public key authentication. | |
753 | The argument to this keyword must be | |
754 | .Dq yes | |
755 | or | |
756 | .Dq no . | |
757 | The default is | |
758 | .Dq yes . | |
759 | This option applies to protocol version 2 only. | |
760 | .It Cm RekeyLimit | |
761 | Specifies the maximum amount of data that may be transmitted before the | |
762 | session key is renegotiated. | |
763 | The argument is the number of bytes, with an optional suffix of | |
764 | .Sq K , | |
765 | .Sq M , | |
766 | or | |
767 | .Sq G | |
768 | to indicate Kilobytes, Megabytes, or Gigabytes, respectively. | |
769 | The default is between | |
770 | .Sq 1G | |
771 | and | |
772 | .Sq 4G , | |
773 | depending on the cipher. | |
774 | This option applies to protocol version 2 only. | |
775 | .It Cm RemoteForward | |
776 | Specifies that a TCP port on the remote machine be forwarded over | |
777 | the secure channel to the specified host and port from the local machine. | |
778 | The first argument must be | |
779 | .Sm off | |
780 | .Oo Ar bind_address : Oc Ar port | |
781 | .Sm on | |
782 | and the second argument must be | |
783 | .Ar host : Ns Ar hostport . | |
784 | IPv6 addresses can be specified by enclosing addresses in square brackets | |
785 | or by using an alternative syntax: | |
786 | .Oo Ar bind_address Ns / Oc Ns Ar port | |
787 | and | |
788 | .Ar host Ns / Ns Ar hostport . | |
789 | Multiple forwardings may be specified, and additional | |
790 | forwardings can be given on the command line. | |
791 | Only the superuser can forward privileged ports. | |
792 | .Pp | |
793 | If the | |
794 | .Ar bind_address | |
795 | is not specified, the default is to only bind to loopback addresses. | |
796 | If the | |
797 | .Ar bind_address | |
798 | is | |
799 | .Ql * | |
800 | or an empty string, then the forwarding is requested to listen on all | |
801 | interfaces. | |
802 | Specifying a remote | |
803 | .Ar bind_address | |
804 | will only succeed if the server's | |
805 | .Cm GatewayPorts | |
806 | option is enabled (see | |
807 | .Xr sshd_config 5 ) . | |
808 | .It Cm RhostsRSAAuthentication | |
809 | Specifies whether to try rhosts based authentication with RSA host | |
810 | authentication. | |
811 | The argument must be | |
812 | .Dq yes | |
813 | or | |
814 | .Dq no . | |
815 | The default is | |
816 | .Dq no . | |
817 | This option applies to protocol version 1 only and requires | |
818 | .Xr ssh 1 | |
819 | to be setuid root. | |
820 | .It Cm RSAAuthentication | |
821 | Specifies whether to try RSA authentication. | |
822 | The argument to this keyword must be | |
823 | .Dq yes | |
824 | or | |
825 | .Dq no . | |
826 | RSA authentication will only be | |
827 | attempted if the identity file exists, or an authentication agent is | |
828 | running. | |
829 | The default is | |
830 | .Dq yes . | |
831 | Note that this option applies to protocol version 1 only. | |
832 | .It Cm SendEnv | |
833 | Specifies what variables from the local | |
834 | .Xr environ 7 | |
835 | should be sent to the server. | |
836 | Note that environment passing is only supported for protocol 2. | |
837 | The server must also support it, and the server must be configured to | |
838 | accept these environment variables. | |
839 | Refer to | |
840 | .Cm AcceptEnv | |
841 | in | |
842 | .Xr sshd_config 5 | |
843 | for how to configure the server. | |
844 | Variables are specified by name, which may contain wildcard characters. | |
845 | Multiple environment variables may be separated by whitespace or spread | |
846 | across multiple | |
847 | .Cm SendEnv | |
848 | directives. | |
849 | The default is not to send any environment variables. | |
850 | .Pp | |
851 | See | |
852 | .Sx PATTERNS | |
853 | for more information on patterns. | |
854 | .It Cm ServerAliveCountMax | |
855 | Sets the number of server alive messages (see below) which may be | |
856 | sent without | |
857 | .Xr ssh 1 | |
858 | receiving any messages back from the server. | |
859 | If this threshold is reached while server alive messages are being sent, | |
860 | ssh will disconnect from the server, terminating the session. | |
861 | It is important to note that the use of server alive messages is very | |
862 | different from | |
863 | .Cm TCPKeepAlive | |
864 | (below). | |
865 | The server alive messages are sent through the encrypted channel | |
866 | and therefore will not be spoofable. | |
867 | The TCP keepalive option enabled by | |
868 | .Cm TCPKeepAlive | |
869 | is spoofable. | |
870 | The server alive mechanism is valuable when the client or | |
871 | server depend on knowing when a connection has become inactive. | |
872 | .Pp | |
873 | The default value is 3. | |
874 | If, for example, | |
875 | .Cm ServerAliveInterval | |
876 | (see below) is set to 15 and | |
877 | .Cm ServerAliveCountMax | |
878 | is left at the default, if the server becomes unresponsive, | |
879 | ssh will disconnect after approximately 45 seconds. | |
880 | .It Cm ServerAliveInterval | |
881 | Sets a timeout interval in seconds after which if no data has been received | |
882 | from the server, | |
883 | .Xr ssh 1 | |
884 | will send a message through the encrypted | |
885 | channel to request a response from the server. | |
886 | The default | |
887 | is 0, indicating that these messages will not be sent to the server. | |
888 | This option applies to protocol version 2 only. | |
889 | .It Cm SmartcardDevice | |
890 | Specifies which smartcard device to use. | |
891 | The argument to this keyword is the device | |
892 | .Xr ssh 1 | |
893 | should use to communicate with a smartcard used for storing the user's | |
894 | private RSA key. | |
895 | By default, no device is specified and smartcard support is not activated. | |
896 | .It Cm StrictHostKeyChecking | |
897 | If this flag is set to | |
898 | .Dq yes , | |
899 | .Xr ssh 1 | |
900 | will never automatically add host keys to the | |
901 | .Pa ~/.ssh/known_hosts | |
902 | file, and refuses to connect to hosts whose host key has changed. | |
903 | This provides maximum protection against trojan horse attacks, | |
904 | though it can be annoying when the | |
905 | .Pa /etc/ssh/ssh_known_hosts | |
906 | file is poorly maintained or when connections to new hosts are | |
907 | frequently made. | |
908 | This option forces the user to manually | |
909 | add all new hosts. | |
910 | If this flag is set to | |
911 | .Dq no , | |
912 | ssh will automatically add new host keys to the | |
913 | user known hosts files. | |
914 | If this flag is set to | |
915 | .Dq ask , | |
916 | new host keys | |
917 | will be added to the user known host files only after the user | |
918 | has confirmed that is what they really want to do, and | |
919 | ssh will refuse to connect to hosts whose host key has changed. | |
920 | The host keys of | |
921 | known hosts will be verified automatically in all cases. | |
922 | The argument must be | |
923 | .Dq yes , | |
924 | .Dq no , | |
925 | or | |
926 | .Dq ask . | |
927 | The default is | |
928 | .Dq ask . | |
929 | .It Cm TCPKeepAlive | |
930 | Specifies whether the system should send TCP keepalive messages to the | |
931 | other side. | |
932 | If they are sent, death of the connection or crash of one | |
933 | of the machines will be properly noticed. | |
934 | However, this means that | |
935 | connections will die if the route is down temporarily, and some people | |
936 | find it annoying. | |
937 | .Pp | |
938 | The default is | |
939 | .Dq yes | |
940 | (to send TCP keepalive messages), and the client will notice | |
941 | if the network goes down or the remote host dies. | |
942 | This is important in scripts, and many users want it too. | |
943 | .Pp | |
944 | To disable TCP keepalive messages, the value should be set to | |
945 | .Dq no . | |
946 | .It Cm Tunnel | |
947 | Request starting | |
948 | .Xr tun 4 | |
949 | device forwarding between the client and the server. | |
950 | This option also allows requesting layer 2 (ethernet) | |
951 | instead of layer 3 (point-to-point) tunneling from the server. | |
952 | The argument must be | |
953 | .Dq yes , | |
954 | .Dq point-to-point , | |
955 | .Dq ethernet , | |
956 | or | |
957 | .Dq no . | |
958 | The default is | |
959 | .Dq no . | |
960 | .It Cm TunnelDevice | |
961 | Force a specified | |
962 | .Xr tun 4 | |
963 | device on the client. | |
964 | Without this option, the next available device will be used. | |
965 | .It Cm UsePrivilegedPort | |
966 | Specifies whether to use a privileged port for outgoing connections. | |
967 | The argument must be | |
968 | .Dq yes | |
969 | or | |
970 | .Dq no . | |
971 | The default is | |
972 | .Dq no . | |
973 | If set to | |
974 | .Dq yes , | |
975 | .Xr ssh 1 | |
976 | must be setuid root. | |
977 | Note that this option must be set to | |
978 | .Dq yes | |
979 | for | |
980 | .Cm RhostsRSAAuthentication | |
981 | with older servers. | |
982 | .It Cm User | |
983 | Specifies the user to log in as. | |
984 | This can be useful when a different user name is used on different machines. | |
985 | This saves the trouble of | |
986 | having to remember to give the user name on the command line. | |
987 | .It Cm UserKnownHostsFile | |
988 | Specifies a file to use for the user | |
989 | host key database instead of | |
990 | .Pa ~/.ssh/known_hosts . | |
991 | .It Cm VerifyHostKeyDNS | |
992 | Specifies whether to verify the remote key using DNS and SSHFP resource | |
993 | records. | |
994 | If this option is set to | |
995 | .Dq yes , | |
996 | the client will implicitly trust keys that match a secure fingerprint | |
997 | from DNS. | |
998 | Insecure fingerprints will be handled as if this option was set to | |
999 | .Dq ask . | |
1000 | If this option is set to | |
1001 | .Dq ask , | |
1002 | information on fingerprint match will be displayed, but the user will still | |
1003 | need to confirm new host keys according to the | |
1004 | .Cm StrictHostKeyChecking | |
1005 | option. | |
1006 | The argument must be | |
1007 | .Dq yes , | |
1008 | .Dq no , | |
1009 | or | |
1010 | .Dq ask . | |
1011 | The default is | |
1012 | .Dq no . | |
1013 | Note that this option applies to protocol version 2 only. | |
1014 | .Pp | |
1015 | See also | |
1016 | .Sx VERIFYING HOST KEYS | |
1017 | in | |
1018 | .Xr ssh 1 . | |
1019 | .It Cm XAuthLocation | |
1020 | Specifies the full pathname of the | |
1021 | .Xr xauth 1 | |
1022 | program. | |
1023 | The default is | |
1024 | .Pa /usr/X11R6/bin/xauth . | |
1025 | .El | |
1026 | .Sh FILES | |
1027 | .Bl -tag -width Ds | |
1028 | .It Pa ~/.ssh/config | |
1029 | This is the per-user configuration file. | |
1030 | The format of this file is described above. | |
1031 | This file is used by the SSH client. | |
1032 | Because of the potential for abuse, this file must have strict permissions: | |
1033 | read/write for the user, and not accessible by others. | |
1034 | .It Pa /etc/ssh/ssh_config | |
1035 | Systemwide configuration file. | |
1036 | This file provides defaults for those | |
1037 | values that are not specified in the user's configuration file, and | |
1038 | for those users who do not have a configuration file. | |
1039 | This file must be world-readable. | |
1040 | .El | |
1041 | .Sh SEE ALSO | |
1042 | .Xr ssh 1 | |
1043 | .Sh AUTHORS | |
1044 | OpenSSH is a derivative of the original and free | |
1045 | ssh 1.2.12 release by Tatu Ylonen. | |
1046 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
1047 | Theo de Raadt and Dug Song | |
1048 | removed many bugs, re-added newer features and | |
1049 | created OpenSSH. | |
1050 | Markus Friedl contributed the support for SSH | |
1051 | protocol versions 1.5 and 2.0. |