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884dc78b | 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 | .\" | |
12a403af | 37 | .\" $OpenBSD: ssh_config.5,v 1.29 2004/03/05 10:53:58 markus Exp $ |
884dc78b | 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 $HOME/.ssh/config | |
47 | .It Pa /etc/ssh/ssh_config | |
48 | .El | |
49 | .Sh DESCRIPTION | |
50 | .Nm ssh | |
51 | obtains configuration data from the following sources in | |
52 | the following order: | |
d03f4262 | 53 | .Bl -enum -offset indent -compact |
54 | .It | |
55 | command-line options | |
56 | .It | |
44a053a3 | 57 | user's configuration file |
d03f4262 | 58 | .Pq Pa $HOME/.ssh/config |
59 | .It | |
d6862c36 | 60 | GSSAPI configuration file |
d03f4262 | 61 | .Pq Pa $HOME/.ssh/config.gssapi |
62 | .It | |
d6862c36 | 63 | Kerberos configuration file |
d03f4262 | 64 | .Pq Pa $HOME/.ssh/config.krb |
65 | .It | |
d03f4262 | 66 | system-wide configuration file |
67 | .Pq Pa /etc/ssh/ssh_config | |
44a053a3 | 68 | .El |
884dc78b | 69 | .Pp |
70 | For each parameter, the first obtained value | |
71 | will be used. | |
72 | The configuration files contain sections bracketed by | |
73 | .Dq Host | |
74 | specifications, and that section is only applied for hosts that | |
75 | match one of the patterns given in the specification. | |
76 | The matched host name is the one given on the command line. | |
77 | .Pp | |
78 | Since the first obtained value for each parameter is used, more | |
79 | host-specific declarations should be given near the beginning of the | |
80 | file, and general defaults at the end. | |
81 | .Pp | |
82 | The configuration file has the following format: | |
83 | .Pp | |
84 | Empty lines and lines starting with | |
85 | .Ql # | |
86 | are comments. | |
87 | .Pp | |
88 | Otherwise a line is of the format | |
89 | .Dq keyword arguments . | |
90 | Configuration options may be separated by whitespace or | |
91 | optional whitespace and exactly one | |
92 | .Ql = ; | |
93 | the latter format is useful to avoid the need to quote whitespace | |
94 | when specifying configuration options using the | |
95 | .Nm ssh , | |
96 | .Nm scp | |
97 | and | |
98 | .Nm sftp | |
99 | .Fl o | |
100 | option. | |
101 | .Pp | |
102 | The possible | |
103 | keywords and their meanings are as follows (note that | |
104 | keywords are case-insensitive and arguments are case-sensitive): | |
105 | .Bl -tag -width Ds | |
106 | .It Cm Host | |
107 | Restricts the following declarations (up to the next | |
108 | .Cm Host | |
109 | keyword) to be only for those hosts that match one of the patterns | |
110 | given after the keyword. | |
111 | .Ql \&* | |
112 | and | |
7cac2b65 | 113 | .Ql \&? |
884dc78b | 114 | can be used as wildcards in the |
115 | patterns. | |
116 | A single | |
117 | .Ql \&* | |
118 | as a pattern can be used to provide global | |
119 | defaults for all hosts. | |
120 | The host is the | |
121 | .Ar hostname | |
122 | argument given on the command line (i.e., the name is not converted to | |
123 | a canonicalized host name before matching). | |
7cac2b65 | 124 | .It Cm AddressFamily |
125 | Specifies which address family to use when connecting. | |
126 | Valid arguments are | |
127 | .Dq any , | |
128 | .Dq inet | |
129 | (Use IPv4 only) or | |
130 | .Dq inet6 | |
131 | (Use IPv6 only.) | |
884dc78b | 132 | .It Cm BatchMode |
133 | If set to | |
134 | .Dq yes , | |
135 | passphrase/password querying will be disabled. | |
136 | This option is useful in scripts and other batch jobs where no user | |
137 | is present to supply the password. | |
138 | The argument must be | |
139 | .Dq yes | |
140 | or | |
141 | .Dq no . | |
142 | The default is | |
143 | .Dq no . | |
144 | .It Cm BindAddress | |
145 | Specify the interface to transmit from on machines with multiple | |
146 | interfaces or aliased addresses. | |
147 | Note that this option does not work if | |
148 | .Cm UsePrivilegedPort | |
149 | is set to | |
150 | .Dq yes . | |
151 | .It Cm ChallengeResponseAuthentication | |
152 | Specifies whether to use challenge response authentication. | |
153 | The argument to this keyword must be | |
154 | .Dq yes | |
155 | or | |
156 | .Dq no . | |
157 | The default is | |
158 | .Dq yes . | |
159 | .It Cm CheckHostIP | |
160 | If this flag is set to | |
161 | .Dq yes , | |
162 | ssh will additionally check the host IP address in the | |
163 | .Pa known_hosts | |
164 | file. | |
165 | This allows ssh to detect if a host key changed due to DNS spoofing. | |
166 | If the option is set to | |
167 | .Dq no , | |
168 | the check will not be executed. | |
169 | The default is | |
170 | .Dq yes . | |
171 | .It Cm Cipher | |
172 | Specifies the cipher to use for encrypting the session | |
173 | in protocol version 1. | |
174 | Currently, | |
175 | .Dq blowfish , | |
176 | .Dq 3des , | |
177 | and | |
178 | .Dq des | |
179 | are supported. | |
180 | .Ar des | |
181 | is only supported in the | |
182 | .Nm ssh | |
183 | client for interoperability with legacy protocol 1 implementations | |
184 | that do not support the | |
185 | .Ar 3des | |
bfe49944 | 186 | cipher. |
187 | Its use is strongly discouraged due to cryptographic weaknesses. | |
884dc78b | 188 | The default is |
189 | .Dq 3des . | |
190 | .It Cm Ciphers | |
191 | Specifies the ciphers allowed for protocol version 2 | |
192 | in order of preference. | |
193 | Multiple ciphers must be comma-separated. | |
194 | The default is | |
884dc78b | 195 | .Bd -literal |
196 | ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour, | |
197 | aes192-cbc,aes256-cbc'' | |
198 | .Ed | |
199 | .It Cm ClearAllForwardings | |
200 | Specifies that all local, remote and dynamic port forwardings | |
201 | specified in the configuration files or on the command line be | |
bfe49944 | 202 | cleared. |
203 | This option is primarily useful when used from the | |
884dc78b | 204 | .Nm ssh |
205 | command line to clear port forwardings set in | |
206 | configuration files, and is automatically set by | |
207 | .Xr scp 1 | |
208 | and | |
209 | .Xr sftp 1 . | |
210 | The argument must be | |
211 | .Dq yes | |
212 | or | |
213 | .Dq no . | |
214 | The default is | |
215 | .Dq no . | |
216 | .It Cm Compression | |
217 | Specifies whether to use compression. | |
218 | The argument must be | |
219 | .Dq yes | |
220 | or | |
221 | .Dq no . | |
222 | The default is | |
223 | .Dq no . | |
224 | .It Cm CompressionLevel | |
225 | Specifies the compression level to use if compression is enabled. | |
226 | The argument must be an integer from 1 (fast) to 9 (slow, best). | |
227 | The default level is 6, which is good for most applications. | |
228 | The meaning of the values is the same as in | |
229 | .Xr gzip 1 . | |
230 | Note that this option applies to protocol version 1 only. | |
231 | .It Cm ConnectionAttempts | |
232 | Specifies the number of tries (one per second) to make before exiting. | |
233 | The argument must be an integer. | |
234 | This may be useful in scripts if the connection sometimes fails. | |
235 | The default is 1. | |
7cac2b65 | 236 | .It Cm ConnectTimeout |
237 | Specifies the timeout (in seconds) used when connecting to the ssh | |
238 | server, instead of using the default system TCP timeout. | |
239 | This value is used only when the target is down or really unreachable, | |
240 | not when it refuses the connection. | |
884dc78b | 241 | .It Cm DynamicForward |
242 | Specifies that a TCP/IP port on the local machine be forwarded | |
243 | over the secure channel, and the application | |
244 | protocol is then used to determine where to connect to from the | |
bfe49944 | 245 | remote machine. |
246 | The argument must be a port number. | |
7cac2b65 | 247 | Currently the SOCKS4 and SOCKS5 protocols are supported, and |
884dc78b | 248 | .Nm ssh |
7cac2b65 | 249 | will act as a SOCKS server. |
884dc78b | 250 | Multiple forwardings may be specified, and |
bfe49944 | 251 | additional forwardings can be given on the command line. |
252 | Only the superuser can forward privileged ports. | |
7cac2b65 | 253 | .It Cm EnableSSHKeysign |
254 | Setting this option to | |
255 | .Dq yes | |
256 | in the global client configuration file | |
257 | .Pa /etc/ssh/ssh_config | |
258 | enables the use of the helper program | |
259 | .Xr ssh-keysign 8 | |
260 | during | |
261 | .Cm HostbasedAuthentication . | |
262 | The argument must be | |
263 | .Dq yes | |
264 | or | |
265 | .Dq no . | |
266 | The default is | |
267 | .Dq no . | |
540d72c3 | 268 | This option should be placed in the non-hostspecific section. |
7cac2b65 | 269 | See |
270 | .Xr ssh-keysign 8 | |
271 | for more information. | |
884dc78b | 272 | .It Cm EscapeChar |
273 | Sets the escape character (default: | |
274 | .Ql ~ ) . | |
275 | The escape character can also | |
276 | be set on the command line. | |
277 | The argument should be a single character, | |
278 | .Ql ^ | |
279 | followed by a letter, or | |
280 | .Dq none | |
281 | to disable the escape | |
282 | character entirely (making the connection transparent for binary | |
283 | data). | |
284 | .It Cm ForwardAgent | |
285 | Specifies whether the connection to the authentication agent (if any) | |
286 | will be forwarded to the remote machine. | |
287 | The argument must be | |
288 | .Dq yes | |
289 | or | |
290 | .Dq no . | |
291 | The default is | |
292 | .Dq no . | |
d03f4262 | 293 | .Pp |
bfe49944 | 294 | Agent forwarding should be enabled with caution. |
295 | Users with the ability to bypass file permissions on the remote host | |
296 | (for the agent's Unix-domain socket) | |
297 | can access the local agent through the forwarded connection. | |
298 | An attacker cannot obtain key material from the agent, | |
d03f4262 | 299 | however they can perform operations on the keys that enable them to |
300 | authenticate using the identities loaded into the agent. | |
884dc78b | 301 | .It Cm ForwardX11 |
302 | Specifies whether X11 connections will be automatically redirected | |
303 | over the secure channel and | |
304 | .Ev DISPLAY | |
305 | set. | |
306 | The argument must be | |
307 | .Dq yes | |
308 | or | |
309 | .Dq no . | |
310 | The default is | |
311 | .Dq no . | |
d03f4262 | 312 | .Pp |
bfe49944 | 313 | X11 forwarding should be enabled with caution. |
314 | Users with the ability to bypass file permissions on the remote host | |
540d72c3 | 315 | (for the user's X11 authorization database) |
bfe49944 | 316 | can access the local X11 display through the forwarded connection. |
540d72c3 | 317 | An attacker may then be able to perform activities such as keystroke monitoring |
318 | if the | |
319 | .Cm ForwardX11Trusted | |
320 | option is also enabled. | |
321 | .It Cm ForwardX11Trusted | |
322 | If the this option is set to | |
323 | .Dq yes | |
324 | then remote X11 clients will have full access to the original X11 display. | |
325 | If this option is set to | |
326 | .Dq no | |
327 | then remote X11 clients will be considered untrusted and prevented | |
328 | from stealing or tampering with data belonging to trusted X11 | |
329 | clients. | |
330 | .Pp | |
331 | The default is | |
332 | .Dq no . | |
333 | .Pp | |
334 | See the X11 SECURITY extension specification for full details on | |
335 | the restrictions imposed on untrusted clients. | |
884dc78b | 336 | .It Cm GatewayPorts |
337 | Specifies whether remote hosts are allowed to connect to local | |
338 | forwarded ports. | |
339 | By default, | |
340 | .Nm ssh | |
bfe49944 | 341 | binds local port forwardings to the loopback address. |
342 | This prevents other remote hosts from connecting to forwarded ports. | |
884dc78b | 343 | .Cm GatewayPorts |
344 | can be used to specify that | |
345 | .Nm ssh | |
346 | should bind local port forwardings to the wildcard address, | |
347 | thus allowing remote hosts to connect to forwarded ports. | |
348 | The argument must be | |
349 | .Dq yes | |
350 | or | |
351 | .Dq no . | |
352 | The default is | |
353 | .Dq no . | |
354 | .It Cm GlobalKnownHostsFile | |
355 | Specifies a file to use for the global | |
356 | host key database instead of | |
357 | .Pa /etc/ssh/ssh_known_hosts . | |
7cac2b65 | 358 | .It Cm GSSAPIAuthentication |
540d72c3 | 359 | Specifies whether user authentication based on GSSAPI is allowed. |
44a053a3 | 360 | The default is |
361 | .Dq yes . | |
85740ad2 | 362 | Note that this option applies to protocol version 2 only. |
7cac2b65 | 363 | .It Cm GSSAPIKeyExchange |
7a056ed1 | 364 | Specifies whether key exchange based on GSSAPI may be used. When using |
365 | GSSAPI key exchange the server need not have a host key. | |
366 | The default is | |
367 | .Dq yes . | |
85740ad2 | 368 | Note that this option applies to protocol version 2 only. |
7cac2b65 | 369 | .It Cm GSSAPIDelegateCredentials |
85740ad2 | 370 | Forward (delegate) credentials to the server. |
44a053a3 | 371 | The default is |
372 | .Dq yes . | |
85740ad2 | 373 | Note that this option applies to protocol version 2 only. |
884dc78b | 374 | .It Cm HostbasedAuthentication |
375 | Specifies whether to try rhosts based authentication with public key | |
376 | authentication. | |
377 | The argument must be | |
378 | .Dq yes | |
379 | or | |
380 | .Dq no . | |
381 | The default is | |
382 | .Dq no . | |
383 | This option applies to protocol version 2 only and | |
384 | is similar to | |
385 | .Cm RhostsRSAAuthentication . | |
386 | .It Cm HostKeyAlgorithms | |
387 | Specifies the protocol version 2 host key algorithms | |
388 | that the client wants to use in order of preference. | |
389 | The default for this option is: | |
390 | .Dq ssh-rsa,ssh-dss . | |
391 | .It Cm HostKeyAlias | |
392 | Specifies an alias that should be used instead of the | |
393 | real host name when looking up or saving the host key | |
394 | in the host key database files. | |
395 | This option is useful for tunneling ssh connections | |
396 | or for multiple servers running on a single host. | |
397 | .It Cm HostName | |
398 | Specifies the real host name to log into. | |
399 | This can be used to specify nicknames or abbreviations for hosts. | |
400 | Default is the name given on the command line. | |
401 | Numeric IP addresses are also permitted (both on the command line and in | |
402 | .Cm HostName | |
403 | specifications). | |
404 | .It Cm IdentityFile | |
405 | Specifies a file from which the user's RSA or DSA authentication identity | |
7cac2b65 | 406 | is read. |
407 | The default is | |
884dc78b | 408 | .Pa $HOME/.ssh/identity |
409 | for protocol version 1, and | |
410 | .Pa $HOME/.ssh/id_rsa | |
411 | and | |
412 | .Pa $HOME/.ssh/id_dsa | |
413 | for protocol version 2. | |
414 | Additionally, any identities represented by the authentication agent | |
415 | will be used for authentication. | |
416 | The file name may use the tilde | |
417 | syntax to refer to a user's home directory. | |
418 | It is possible to have | |
419 | multiple identity files specified in configuration files; all these | |
420 | identities will be tried in sequence. | |
12a403af | 421 | .It Cm IdentitiesOnly |
422 | Specifies that | |
423 | .Nm ssh | |
424 | should only use the authentication identity files configured in the | |
425 | .Nm | |
426 | files, | |
427 | even if the | |
428 | .Nm ssh-agent | |
429 | offers more identities. | |
430 | The argument to this keyword must be | |
431 | .Dq yes | |
432 | or | |
433 | .Dq no . | |
434 | This option is intented for situations where | |
435 | .Nm ssh-agent | |
436 | offers many different identities. | |
437 | The default is | |
438 | .Dq no . | |
884dc78b | 439 | .It Cm LocalForward |
440 | Specifies that a TCP/IP port on the local machine be forwarded over | |
441 | the secure channel to the specified host and port from the remote machine. | |
442 | The first argument must be a port number, and the second must be | |
443 | .Ar host:port . | |
444 | IPv6 addresses can be specified with an alternative syntax: | |
445 | .Ar host/port . | |
446 | Multiple forwardings may be specified, and additional | |
447 | forwardings can be given on the command line. | |
448 | Only the superuser can forward privileged ports. | |
449 | .It Cm LogLevel | |
450 | Gives the verbosity level that is used when logging messages from | |
451 | .Nm ssh . | |
452 | The possible values are: | |
453 | QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3. | |
bfe49944 | 454 | The default is INFO. |
455 | DEBUG and DEBUG1 are equivalent. | |
456 | DEBUG2 and DEBUG3 each specify higher levels of verbose output. | |
884dc78b | 457 | .It Cm MACs |
458 | Specifies the MAC (message authentication code) algorithms | |
459 | in order of preference. | |
460 | The MAC algorithm is used in protocol version 2 | |
461 | for data integrity protection. | |
462 | Multiple algorithms must be comma-separated. | |
463 | The default is | |
464 | .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . | |
465 | .It Cm NoHostAuthenticationForLocalhost | |
466 | This option can be used if the home directory is shared across machines. | |
467 | In this case localhost will refer to a different machine on each of | |
468 | the machines and the user will get many warnings about changed host keys. | |
469 | However, this option disables host authentication for localhost. | |
470 | The argument to this keyword must be | |
471 | .Dq yes | |
472 | or | |
473 | .Dq no . | |
474 | The default is to check the host key for localhost. | |
475 | .It Cm NumberOfPasswordPrompts | |
476 | Specifies the number of password prompts before giving up. | |
477 | The argument to this keyword must be an integer. | |
478 | Default is 3. | |
479 | .It Cm PasswordAuthentication | |
480 | Specifies whether to use password authentication. | |
481 | The argument to this keyword must be | |
482 | .Dq yes | |
483 | or | |
484 | .Dq no . | |
485 | The default is | |
486 | .Dq yes . | |
487 | .It Cm Port | |
488 | Specifies the port number to connect on the remote host. | |
489 | Default is 22. | |
490 | .It Cm PreferredAuthentications | |
491 | Specifies the order in which the client should try protocol 2 | |
7cac2b65 | 492 | authentication methods. |
493 | This allows a client to prefer one method (e.g. | |
884dc78b | 494 | .Cm keyboard-interactive ) |
495 | over another method (e.g. | |
496 | .Cm password ) | |
497 | The default for this option is: | |
540d72c3 | 498 | .Dq hostbased,external-keyx,gssapi-with-mic,gssapi,publickey,keyboard-interactive,password . |
884dc78b | 499 | .It Cm Protocol |
500 | Specifies the protocol versions | |
501 | .Nm ssh | |
502 | should support in order of preference. | |
503 | The possible values are | |
504 | .Dq 1 | |
505 | and | |
506 | .Dq 2 . | |
507 | Multiple versions must be comma-separated. | |
508 | The default is | |
509 | .Dq 2,1 . | |
510 | This means that | |
511 | .Nm ssh | |
512 | tries version 2 and falls back to version 1 | |
513 | if version 2 is not available. | |
514 | .It Cm ProxyCommand | |
515 | Specifies the command to use to connect to the server. | |
516 | The command | |
517 | string extends to the end of the line, and is executed with | |
518 | .Pa /bin/sh . | |
519 | In the command string, | |
520 | .Ql %h | |
521 | will be substituted by the host name to | |
522 | connect and | |
523 | .Ql %p | |
524 | by the port. | |
525 | The command can be basically anything, | |
526 | and should read from its standard input and write to its standard output. | |
527 | It should eventually connect an | |
528 | .Xr sshd 8 | |
529 | server running on some machine, or execute | |
530 | .Ic sshd -i | |
531 | somewhere. | |
532 | Host key management will be done using the | |
533 | HostName of the host being connected (defaulting to the name typed by | |
534 | the user). | |
bfe49944 | 535 | Setting the command to |
536 | .Dq none | |
537 | disables this option entirely. | |
884dc78b | 538 | Note that |
539 | .Cm CheckHostIP | |
540 | is not available for connects with a proxy command. | |
541 | .Pp | |
542 | .It Cm PubkeyAuthentication | |
543 | Specifies whether to try public key authentication. | |
544 | The argument to this keyword must be | |
545 | .Dq yes | |
546 | or | |
547 | .Dq no . | |
548 | The default is | |
549 | .Dq yes . | |
550 | This option applies to protocol version 2 only. | |
551 | .It Cm RemoteForward | |
552 | Specifies that a TCP/IP port on the remote machine be forwarded over | |
553 | the secure channel to the specified host and port from the local machine. | |
554 | The first argument must be a port number, and the second must be | |
555 | .Ar host:port . | |
556 | IPv6 addresses can be specified with an alternative syntax: | |
557 | .Ar host/port . | |
558 | Multiple forwardings may be specified, and additional | |
559 | forwardings can be given on the command line. | |
560 | Only the superuser can forward privileged ports. | |
884dc78b | 561 | .It Cm RhostsRSAAuthentication |
562 | Specifies whether to try rhosts based authentication with RSA host | |
563 | authentication. | |
564 | The argument must be | |
565 | .Dq yes | |
566 | or | |
567 | .Dq no . | |
568 | The default is | |
569 | .Dq no . | |
570 | This option applies to protocol version 1 only and requires | |
571 | .Nm ssh | |
572 | to be setuid root. | |
573 | .It Cm RSAAuthentication | |
574 | Specifies whether to try RSA authentication. | |
575 | The argument to this keyword must be | |
576 | .Dq yes | |
577 | or | |
578 | .Dq no . | |
579 | RSA authentication will only be | |
580 | attempted if the identity file exists, or an authentication agent is | |
581 | running. | |
582 | The default is | |
583 | .Dq yes . | |
584 | Note that this option applies to protocol version 1 only. | |
540d72c3 | 585 | .It Cm ServerAliveInterval |
586 | Sets a timeout interval in seconds after which if no data has been received | |
587 | from the server, | |
588 | .Nm ssh | |
589 | will send a message through the encrypted | |
590 | channel to request a response from the server. | |
591 | The default | |
592 | is 0, indicating that these messages will not be sent to the server. | |
593 | This option applies to protocol version 2 only. | |
594 | .It Cm ServerAliveCountMax | |
595 | Sets the number of server alive messages (see above) which may be | |
596 | sent without | |
597 | .Nm ssh | |
598 | receiving any messages back from the server. | |
599 | If this threshold is reached while server alive messages are being sent, | |
600 | .Nm ssh | |
601 | will disconnect from the server, terminating the session. | |
602 | It is important to note that the use of server alive messages is very | |
603 | different from | |
604 | .Cm TCPKeepAlive | |
605 | (below). | |
606 | The server alive messages are sent through the encrypted channel | |
607 | and therefore will not be spoofable. | |
608 | The TCP keepalive option enabled by | |
609 | .Cm TCPKeepAlive | |
610 | is spoofable. | |
611 | The server alive mechanism is valuable when the client or | |
612 | server depend on knowing when a connection has become inactive. | |
613 | .Pp | |
614 | The default value is 3. | |
615 | If, for example, | |
616 | .Cm ServerAliveInterval | |
617 | (above) is set to 15, and | |
618 | .Cm ServerAliveCountMax | |
619 | is left at the default, if the server becomes unresponsive ssh | |
620 | will disconnect after approximately 45 seconds. | |
884dc78b | 621 | .It Cm SmartcardDevice |
7cac2b65 | 622 | Specifies which smartcard device to use. |
623 | The argument to this keyword is the device | |
884dc78b | 624 | .Nm ssh |
625 | should use to communicate with a smartcard used for storing the user's | |
7cac2b65 | 626 | private RSA key. |
627 | By default, no device is specified and smartcard support is not activated. | |
884dc78b | 628 | .It Cm StrictHostKeyChecking |
629 | If this flag is set to | |
630 | .Dq yes , | |
631 | .Nm ssh | |
632 | will never automatically add host keys to the | |
633 | .Pa $HOME/.ssh/known_hosts | |
634 | file, and refuses to connect to hosts whose host key has changed. | |
635 | This provides maximum protection against trojan horse attacks, | |
636 | however, can be annoying when the | |
637 | .Pa /etc/ssh/ssh_known_hosts | |
638 | file is poorly maintained, or connections to new hosts are | |
639 | frequently made. | |
640 | This option forces the user to manually | |
641 | add all new hosts. | |
642 | If this flag is set to | |
643 | .Dq no , | |
644 | .Nm ssh | |
645 | will automatically add new host keys to the | |
646 | user known hosts files. | |
647 | If this flag is set to | |
648 | .Dq ask , | |
649 | new host keys | |
650 | will be added to the user known host files only after the user | |
651 | has confirmed that is what they really want to do, and | |
652 | .Nm ssh | |
653 | will refuse to connect to hosts whose host key has changed. | |
654 | The host keys of | |
655 | known hosts will be verified automatically in all cases. | |
656 | The argument must be | |
657 | .Dq yes , | |
658 | .Dq no | |
659 | or | |
660 | .Dq ask . | |
661 | The default is | |
662 | .Dq ask . | |
540d72c3 | 663 | .It Cm TCPKeepAlive |
664 | Specifies whether the system should send TCP keepalive messages to the | |
665 | other side. | |
666 | If they are sent, death of the connection or crash of one | |
667 | of the machines will be properly noticed. | |
668 | However, this means that | |
669 | connections will die if the route is down temporarily, and some people | |
670 | find it annoying. | |
671 | .Pp | |
672 | The default is | |
673 | .Dq yes | |
674 | (to send TCP keepalive messages), and the client will notice | |
675 | if the network goes down or the remote host dies. | |
676 | This is important in scripts, and many users want it too. | |
677 | .Pp | |
678 | To disable TCP keepalive messages, the value should be set to | |
679 | .Dq no . | |
884dc78b | 680 | .It Cm UsePrivilegedPort |
681 | Specifies whether to use a privileged port for outgoing connections. | |
682 | The argument must be | |
683 | .Dq yes | |
684 | or | |
685 | .Dq no . | |
686 | The default is | |
687 | .Dq no . | |
d03f4262 | 688 | If set to |
689 | .Dq yes | |
690 | .Nm ssh | |
691 | must be setuid root. | |
884dc78b | 692 | Note that this option must be set to |
693 | .Dq yes | |
7cac2b65 | 694 | for |
884dc78b | 695 | .Cm RhostsRSAAuthentication |
7cac2b65 | 696 | with older servers. |
884dc78b | 697 | .It Cm User |
698 | Specifies the user to log in as. | |
699 | This can be useful when a different user name is used on different machines. | |
700 | This saves the trouble of | |
701 | having to remember to give the user name on the command line. | |
702 | .It Cm UserKnownHostsFile | |
703 | Specifies a file to use for the user | |
704 | host key database instead of | |
705 | .Pa $HOME/.ssh/known_hosts . | |
7cac2b65 | 706 | .It Cm VerifyHostKeyDNS |
707 | Specifies whether to verify the remote key using DNS and SSHFP resource | |
708 | records. | |
540d72c3 | 709 | If this option is set to |
710 | .Dq yes , | |
711 | the client will implicitly trust keys that match a secure fingerprint | |
712 | from DNS. | |
713 | Insecure fingerprints will be handled as if this option was set to | |
714 | .Dq ask . | |
715 | If this option is set to | |
716 | .Dq ask , | |
717 | information on fingerprint match will be displayed, but the user will still | |
718 | need to confirm new host keys according to the | |
719 | .Cm StrictHostKeyChecking | |
720 | option. | |
721 | The argument must be | |
722 | .Dq yes , | |
723 | .Dq no | |
724 | or | |
725 | .Dq ask . | |
7cac2b65 | 726 | The default is |
727 | .Dq no . | |
728 | Note that this option applies to protocol version 2 only. | |
884dc78b | 729 | .It Cm XAuthLocation |
d03f4262 | 730 | Specifies the full pathname of the |
884dc78b | 731 | .Xr xauth 1 |
732 | program. | |
733 | The default is | |
734 | .Pa /usr/X11R6/bin/xauth . | |
735 | .El | |
736 | .Sh FILES | |
737 | .Bl -tag -width Ds | |
738 | .It Pa $HOME/.ssh/config | |
739 | This is the per-user configuration file. | |
740 | The format of this file is described above. | |
741 | This file is used by the | |
742 | .Nm ssh | |
743 | client. | |
744 | This file does not usually contain any sensitive information, | |
745 | but the recommended permissions are read/write for the user, and not | |
746 | accessible by others. | |
747 | .It Pa /etc/ssh/ssh_config | |
748 | Systemwide configuration file. | |
749 | This file provides defaults for those | |
750 | values that are not specified in the user's configuration file, and | |
751 | for those users who do not have a configuration file. | |
752 | This file must be world-readable. | |
753 | .El | |
7cac2b65 | 754 | .Sh SEE ALSO |
755 | .Xr ssh 1 | |
884dc78b | 756 | .Sh AUTHORS |
757 | OpenSSH is a derivative of the original and free | |
758 | ssh 1.2.12 release by Tatu Ylonen. | |
759 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
760 | Theo de Raadt and Dug Song | |
761 | removed many bugs, re-added newer features and | |
762 | created OpenSSH. | |
763 | Markus Friedl contributed the support for SSH | |
764 | protocol versions 1.5 and 2.0. |