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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: sshd_config.5,v 1.40 2005/03/18 17:05:00 jmc Exp $ | |
38 | .Dd September 25, 1999 | |
39 | .Dt SSHD_CONFIG 5 | |
40 | .Os | |
41 | .Sh NAME | |
42 | .Nm sshd_config | |
43 | .Nd OpenSSH SSH daemon configuration file | |
44 | .Sh SYNOPSIS | |
45 | .Bl -tag -width Ds -compact | |
46 | .It Pa /etc/ssh/sshd_config | |
47 | .El | |
48 | .Sh DESCRIPTION | |
49 | .Nm sshd | |
50 | reads configuration data from | |
51 | .Pa /etc/ssh/sshd_config | |
52 | (or the file specified with | |
53 | .Fl f | |
54 | on the command line). | |
55 | The file contains keyword-argument pairs, one per line. | |
56 | Lines starting with | |
57 | .Ql # | |
58 | and empty lines are interpreted as comments. | |
59 | .Pp | |
60 | The possible | |
61 | keywords and their meanings are as follows (note that | |
62 | keywords are case-insensitive and arguments are case-sensitive): | |
63 | .Bl -tag -width Ds | |
64 | .It Cm AcceptEnv | |
65 | Specifies what environment variables sent by the client will be copied into | |
66 | the session's | |
67 | .Xr environ 7 . | |
68 | See | |
69 | .Cm SendEnv | |
70 | in | |
71 | .Xr ssh_config 5 | |
72 | for how to configure the client. | |
73 | Note that environment passing is only supported for protocol 2. | |
74 | Variables are specified by name, which may contain the wildcard characters | |
75 | .Ql \&* | |
76 | and | |
77 | .Ql \&? . | |
78 | Multiple environment variables may be separated by whitespace or spread | |
79 | across multiple | |
80 | .Cm AcceptEnv | |
81 | directives. | |
82 | Be warned that some environment variables could be used to bypass restricted | |
83 | user environments. | |
84 | For this reason, care should be taken in the use of this directive. | |
85 | The default is not to accept any environment variables. | |
86 | .It Cm AddressFamily | |
87 | Specifies which address family should be used by | |
88 | .Nm sshd . | |
89 | Valid arguments are | |
90 | .Dq any , | |
91 | .Dq inet | |
92 | (use IPv4 only) or | |
93 | .Dq inet6 | |
94 | (use IPv6 only). | |
95 | The default is | |
96 | .Dq any . | |
97 | .It Cm AllowGroups | |
98 | This keyword can be followed by a list of group name patterns, separated | |
99 | by spaces. | |
100 | If specified, login is allowed only for users whose primary | |
101 | group or supplementary group list matches one of the patterns. | |
102 | .Ql \&* | |
103 | and | |
104 | .Ql \&? | |
105 | can be used as | |
106 | wildcards in the patterns. | |
107 | Only group names are valid; a numerical group ID is not recognized. | |
108 | By default, login is allowed for all groups. | |
109 | .It Cm AllowTcpForwarding | |
110 | Specifies whether TCP forwarding is permitted. | |
111 | The default is | |
112 | .Dq yes . | |
113 | Note that disabling TCP forwarding does not improve security unless | |
114 | users are also denied shell access, as they can always install their | |
115 | own forwarders. | |
116 | .It Cm AllowUsers | |
117 | This keyword can be followed by a list of user name patterns, separated | |
118 | by spaces. | |
119 | If specified, login is allowed only for user names that | |
120 | match one of the patterns. | |
121 | .Ql \&* | |
122 | and | |
123 | .Ql \&? | |
124 | can be used as | |
125 | wildcards in the patterns. | |
126 | Only user names are valid; a numerical user ID is not recognized. | |
127 | By default, login is allowed for all users. | |
128 | If the pattern takes the form USER@HOST then USER and HOST | |
129 | are separately checked, restricting logins to particular | |
130 | users from particular hosts. | |
131 | .It Cm AuthorizedKeysFile | |
132 | Specifies the file that contains the public keys that can be used | |
133 | for user authentication. | |
134 | .Cm AuthorizedKeysFile | |
135 | may contain tokens of the form %T which are substituted during connection | |
136 | set-up. | |
137 | The following tokens are defined: %% is replaced by a literal '%', | |
138 | %h is replaced by the home directory of the user being authenticated and | |
139 | %u is replaced by the username of that user. | |
140 | After expansion, | |
141 | .Cm AuthorizedKeysFile | |
142 | is taken to be an absolute path or one relative to the user's home | |
143 | directory. | |
144 | The default is | |
145 | .Dq .ssh/authorized_keys . | |
146 | .It Cm Banner | |
147 | In some jurisdictions, sending a warning message before authentication | |
148 | may be relevant for getting legal protection. | |
149 | The contents of the specified file are sent to the remote user before | |
150 | authentication is allowed. | |
151 | This option is only available for protocol version 2. | |
152 | By default, no banner is displayed. | |
153 | .It Cm ChallengeResponseAuthentication | |
154 | Specifies whether challenge response authentication is allowed. | |
155 | All authentication styles from | |
156 | .Xr login.conf 5 | |
157 | are supported. | |
158 | The default is | |
159 | .Dq yes . | |
160 | .It Cm Ciphers | |
161 | Specifies the ciphers allowed for protocol version 2. | |
162 | Multiple ciphers must be comma-separated. | |
163 | The supported ciphers are | |
164 | .Dq 3des-cbc , | |
165 | .Dq aes128-cbc , | |
166 | .Dq aes192-cbc , | |
167 | .Dq aes256-cbc , | |
168 | .Dq aes128-ctr , | |
169 | .Dq aes192-ctr , | |
170 | .Dq aes256-ctr , | |
171 | .Dq arcfour , | |
172 | .Dq blowfish-cbc , | |
173 | and | |
174 | .Dq cast128-cbc . | |
175 | The default is | |
176 | .Bd -literal | |
177 | ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour, | |
178 | aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr'' | |
179 | .Ed | |
180 | .It Cm ClientAliveInterval | |
181 | Sets a timeout interval in seconds after which if no data has been received | |
182 | from the client, | |
183 | .Nm sshd | |
184 | will send a message through the encrypted | |
185 | channel to request a response from the client. | |
186 | The default | |
187 | is 0, indicating that these messages will not be sent to the client. | |
188 | This option applies to protocol version 2 only. | |
189 | .It Cm ClientAliveCountMax | |
190 | Sets the number of client alive messages (see above) which may be | |
191 | sent without | |
192 | .Nm sshd | |
193 | receiving any messages back from the client. | |
194 | If this threshold is reached while client alive messages are being sent, | |
195 | .Nm sshd | |
196 | will disconnect the client, terminating the session. | |
197 | It is important to note that the use of client alive messages is very | |
198 | different from | |
199 | .Cm TCPKeepAlive | |
200 | (below). | |
201 | The client alive messages are sent through the encrypted channel | |
202 | and therefore will not be spoofable. | |
203 | The TCP keepalive option enabled by | |
204 | .Cm TCPKeepAlive | |
205 | is spoofable. | |
206 | The client alive mechanism is valuable when the client or | |
207 | server depend on knowing when a connection has become inactive. | |
208 | .Pp | |
209 | The default value is 3. | |
210 | If | |
211 | .Cm ClientAliveInterval | |
212 | (above) is set to 15, and | |
213 | .Cm ClientAliveCountMax | |
214 | is left at the default, unresponsive ssh clients | |
215 | will be disconnected after approximately 45 seconds. | |
216 | .It Cm Compression | |
217 | Specifies whether compression is allowed. | |
218 | The argument must be | |
219 | .Dq yes | |
220 | or | |
221 | .Dq no . | |
222 | The default is | |
223 | .Dq yes . | |
224 | .It Cm DenyGroups | |
225 | This keyword can be followed by a list of group name patterns, separated | |
226 | by spaces. | |
227 | Login is disallowed for users whose primary group or supplementary | |
228 | group list matches one of the patterns. | |
229 | .Ql \&* | |
230 | and | |
231 | .Ql \&? | |
232 | can be used as | |
233 | wildcards in the patterns. | |
234 | Only group names are valid; a numerical group ID is not recognized. | |
235 | By default, login is allowed for all groups. | |
236 | .It Cm DenyUsers | |
237 | This keyword can be followed by a list of user name patterns, separated | |
238 | by spaces. | |
239 | Login is disallowed for user names that match one of the patterns. | |
240 | .Ql \&* | |
241 | and | |
242 | .Ql \&? | |
243 | can be used as wildcards in the patterns. | |
244 | Only user names are valid; a numerical user ID is not recognized. | |
245 | By default, login is allowed for all users. | |
246 | If the pattern takes the form USER@HOST then USER and HOST | |
247 | are separately checked, restricting logins to particular | |
248 | users from particular hosts. | |
249 | .It Cm GatewayPorts | |
250 | Specifies whether remote hosts are allowed to connect to ports | |
251 | forwarded for the client. | |
252 | By default, | |
253 | .Nm sshd | |
254 | binds remote port forwardings to the loopback address. | |
255 | This prevents other remote hosts from connecting to forwarded ports. | |
256 | .Cm GatewayPorts | |
257 | can be used to specify that | |
258 | .Nm sshd | |
259 | should allow remote port forwardings to bind to non-loopback addresses, thus | |
260 | allowing other hosts to connect. | |
261 | The argument may be | |
262 | .Dq no | |
263 | to force remote port forwardings to be available to the local host only, | |
264 | .Dq yes | |
265 | to force remote port forwardings to bind to the wildcard address, or | |
266 | .Dq clientspecified | |
267 | to allow the client to select the address to which the forwarding is bound. | |
268 | The default is | |
269 | .Dq no . | |
270 | .It Cm GSSAPIAuthentication | |
271 | Specifies whether user authentication based on GSSAPI is allowed. | |
272 | The default is | |
273 | .Dq yes . | |
274 | Note that this option applies to protocol version 2 only. | |
275 | .It Cm GSSAPICleanupCredentials | |
276 | Specifies whether to automatically destroy the user's credentials cache | |
277 | on logout. | |
278 | The default is | |
279 | .Dq yes . | |
280 | Note that this option applies to protocol version 2 only. | |
281 | .It Cm GSSAPIKeyExchange | |
282 | Specifies whether key exchange based on GSSAPI may be used. When using | |
283 | GSSAPI key exchange the server need not have a host key. | |
284 | The default is | |
285 | .Dq yes . | |
286 | Note that this option applies to protocol version 2 only. | |
287 | .It Cm GSSAPIUseSessionCredCache | |
288 | Specifies whether a unique credentials cache name should be generated per | |
289 | session for storing delegated credentials. | |
290 | The default is | |
291 | .Dq yes . | |
292 | Note that this option applies to protocol version 2 only. | |
293 | .It Cm HostbasedAuthentication | |
294 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
295 | with successful public key client host authentication is allowed | |
296 | (hostbased authentication). | |
297 | This option is similar to | |
298 | .Cm RhostsRSAAuthentication | |
299 | and applies to protocol version 2 only. | |
300 | The default is | |
301 | .Dq no . | |
302 | .It Cm HostKey | |
303 | Specifies a file containing a private host key | |
304 | used by SSH. | |
305 | The default is | |
306 | .Pa /etc/ssh/ssh_host_key | |
307 | for protocol version 1, and | |
308 | .Pa /etc/ssh/ssh_host_rsa_key | |
309 | and | |
310 | .Pa /etc/ssh/ssh_host_dsa_key | |
311 | for protocol version 2. | |
312 | Note that | |
313 | .Nm sshd | |
314 | will refuse to use a file if it is group/world-accessible. | |
315 | It is possible to have multiple host key files. | |
316 | .Dq rsa1 | |
317 | keys are used for version 1 and | |
318 | .Dq dsa | |
319 | or | |
320 | .Dq rsa | |
321 | are used for version 2 of the SSH protocol. | |
322 | .It Cm IgnoreRhosts | |
323 | Specifies that | |
324 | .Pa .rhosts | |
325 | and | |
326 | .Pa .shosts | |
327 | files will not be used in | |
328 | .Cm RhostsRSAAuthentication | |
329 | or | |
330 | .Cm HostbasedAuthentication . | |
331 | .Pp | |
332 | .Pa /etc/hosts.equiv | |
333 | and | |
334 | .Pa /etc/shosts.equiv | |
335 | are still used. | |
336 | The default is | |
337 | .Dq yes . | |
338 | .It Cm IgnoreUserKnownHosts | |
339 | Specifies whether | |
340 | .Nm sshd | |
341 | should ignore the user's | |
342 | .Pa $HOME/.ssh/known_hosts | |
343 | during | |
344 | .Cm RhostsRSAAuthentication | |
345 | or | |
346 | .Cm HostbasedAuthentication . | |
347 | The default is | |
348 | .Dq no . | |
349 | .It Cm KerberosAuthentication | |
350 | Specifies whether the password provided by the user for | |
351 | .Cm PasswordAuthentication | |
352 | will be validated through the Kerberos KDC. | |
353 | To use this option, the server needs a | |
354 | Kerberos servtab which allows the verification of the KDC's identity. | |
355 | Default is | |
356 | .Dq no . | |
357 | .It Cm KerberosGetAFSToken | |
358 | If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire | |
359 | an AFS token before accessing the user's home directory. | |
360 | Default is | |
361 | .Dq no . | |
362 | .It Cm KerberosOrLocalPasswd | |
363 | If set then if password authentication through Kerberos fails then | |
364 | the password will be validated via any additional local mechanism | |
365 | such as | |
366 | .Pa /etc/passwd . | |
367 | Default is | |
368 | .Dq yes . | |
369 | .It Cm KerberosTicketCleanup | |
370 | Specifies whether to automatically destroy the user's ticket cache | |
371 | file on logout. | |
372 | Default is | |
373 | .Dq yes . | |
374 | .It Cm KeyRegenerationInterval | |
375 | In protocol version 1, the ephemeral server key is automatically regenerated | |
376 | after this many seconds (if it has been used). | |
377 | The purpose of regeneration is to prevent | |
378 | decrypting captured sessions by later breaking into the machine and | |
379 | stealing the keys. | |
380 | The key is never stored anywhere. | |
381 | If the value is 0, the key is never regenerated. | |
382 | The default is 3600 (seconds). | |
383 | .It Cm ListenAddress | |
384 | Specifies the local addresses | |
385 | .Nm sshd | |
386 | should listen on. | |
387 | The following forms may be used: | |
388 | .Pp | |
389 | .Bl -item -offset indent -compact | |
390 | .It | |
391 | .Cm ListenAddress | |
392 | .Sm off | |
393 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr | |
394 | .Sm on | |
395 | .It | |
396 | .Cm ListenAddress | |
397 | .Sm off | |
398 | .Ar host No | Ar IPv4_addr No : Ar port | |
399 | .Sm on | |
400 | .It | |
401 | .Cm ListenAddress | |
402 | .Sm off | |
403 | .Oo | |
404 | .Ar host No | Ar IPv6_addr Oc : Ar port | |
405 | .Sm on | |
406 | .El | |
407 | .Pp | |
408 | If | |
409 | .Ar port | |
410 | is not specified, | |
411 | .Nm sshd | |
412 | will listen on the address and all prior | |
413 | .Cm Port | |
414 | options specified. | |
415 | The default is to listen on all local addresses. | |
416 | Multiple | |
417 | .Cm ListenAddress | |
418 | options are permitted. | |
419 | Additionally, any | |
420 | .Cm Port | |
421 | options must precede this option for non port qualified addresses. | |
422 | .It Cm LoginGraceTime | |
423 | The server disconnects after this time if the user has not | |
424 | successfully logged in. | |
425 | If the value is 0, there is no time limit. | |
426 | The default is 120 seconds. | |
427 | .It Cm LogLevel | |
428 | Gives the verbosity level that is used when logging messages from | |
429 | .Nm sshd . | |
430 | The possible values are: | |
431 | QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3. | |
432 | The default is INFO. | |
433 | DEBUG and DEBUG1 are equivalent. | |
434 | DEBUG2 and DEBUG3 each specify higher levels of debugging output. | |
435 | Logging with a DEBUG level violates the privacy of users and is not recommended. | |
436 | .It Cm MACs | |
437 | Specifies the available MAC (message authentication code) algorithms. | |
438 | The MAC algorithm is used in protocol version 2 | |
439 | for data integrity protection. | |
440 | Multiple algorithms must be comma-separated. | |
441 | The default is | |
442 | .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . | |
443 | .It Cm MaxAuthTries | |
444 | Specifies the maximum number of authentication attempts permitted per | |
445 | connection. | |
446 | Once the number of failures reaches half this value, | |
447 | additional failures are logged. | |
448 | The default is 6. | |
449 | .It Cm MaxStartups | |
450 | Specifies the maximum number of concurrent unauthenticated connections to the | |
451 | .Nm sshd | |
452 | daemon. | |
453 | Additional connections will be dropped until authentication succeeds or the | |
454 | .Cm LoginGraceTime | |
455 | expires for a connection. | |
456 | The default is 10. | |
457 | .Pp | |
458 | Alternatively, random early drop can be enabled by specifying | |
459 | the three colon separated values | |
460 | .Dq start:rate:full | |
461 | (e.g., "10:30:60"). | |
462 | .Nm sshd | |
463 | will refuse connection attempts with a probability of | |
464 | .Dq rate/100 | |
465 | (30%) | |
466 | if there are currently | |
467 | .Dq start | |
468 | (10) | |
469 | unauthenticated connections. | |
470 | The probability increases linearly and all connection attempts | |
471 | are refused if the number of unauthenticated connections reaches | |
472 | .Dq full | |
473 | (60). | |
474 | .It Cm PasswordAuthentication | |
475 | Specifies whether password authentication is allowed. | |
476 | The default is | |
477 | .Dq yes . | |
478 | .It Cm PermitEmptyPasswords | |
479 | When password authentication is allowed, it specifies whether the | |
480 | server allows login to accounts with empty password strings. | |
481 | The default is | |
482 | .Dq no . | |
483 | .It Cm PermitRootLogin | |
484 | Specifies whether root can log in using | |
485 | .Xr ssh 1 . | |
486 | The argument must be | |
487 | .Dq yes , | |
488 | .Dq without-password , | |
489 | .Dq forced-commands-only | |
490 | or | |
491 | .Dq no . | |
492 | The default is | |
493 | .Dq yes . | |
494 | .Pp | |
495 | If this option is set to | |
496 | .Dq without-password | |
497 | password authentication is disabled for root. | |
498 | .Pp | |
499 | If this option is set to | |
500 | .Dq forced-commands-only | |
501 | root login with public key authentication will be allowed, | |
502 | but only if the | |
503 | .Ar command | |
504 | option has been specified | |
505 | (which may be useful for taking remote backups even if root login is | |
506 | normally not allowed). | |
507 | All other authentication methods are disabled for root. | |
508 | .Pp | |
509 | If this option is set to | |
510 | .Dq no | |
511 | root is not allowed to log in. | |
512 | .It Cm PermitUserEnvironment | |
513 | Specifies whether | |
514 | .Pa ~/.ssh/environment | |
515 | and | |
516 | .Cm environment= | |
517 | options in | |
518 | .Pa ~/.ssh/authorized_keys | |
519 | are processed by | |
520 | .Nm sshd . | |
521 | The default is | |
522 | .Dq no . | |
523 | Enabling environment processing may enable users to bypass access | |
524 | restrictions in some configurations using mechanisms such as | |
525 | .Ev LD_PRELOAD . | |
526 | .It Cm PidFile | |
527 | Specifies the file that contains the process ID of the | |
528 | .Nm sshd | |
529 | daemon. | |
530 | The default is | |
531 | .Pa /var/run/sshd.pid . | |
532 | .It Cm Port | |
533 | Specifies the port number that | |
534 | .Nm sshd | |
535 | listens on. | |
536 | The default is 22. | |
537 | Multiple options of this type are permitted. | |
538 | See also | |
539 | .Cm ListenAddress . | |
540 | .It Cm PrintLastLog | |
541 | Specifies whether | |
542 | .Nm sshd | |
543 | should print the date and time of the last user login when a user logs | |
544 | in interactively. | |
545 | The default is | |
546 | .Dq yes . | |
547 | .It Cm PrintMotd | |
548 | Specifies whether | |
549 | .Nm sshd | |
550 | should print | |
551 | .Pa /etc/motd | |
552 | when a user logs in interactively. | |
553 | (On some systems it is also printed by the shell, | |
554 | .Pa /etc/profile , | |
555 | or equivalent.) | |
556 | The default is | |
557 | .Dq yes . | |
558 | .It Cm Protocol | |
559 | Specifies the protocol versions | |
560 | .Nm sshd | |
561 | supports. | |
562 | The possible values are | |
563 | .Dq 1 | |
564 | and | |
565 | .Dq 2 . | |
566 | Multiple versions must be comma-separated. | |
567 | The default is | |
568 | .Dq 2,1 . | |
569 | Note that the order of the protocol list does not indicate preference, | |
570 | because the client selects among multiple protocol versions offered | |
571 | by the server. | |
572 | Specifying | |
573 | .Dq 2,1 | |
574 | is identical to | |
575 | .Dq 1,2 . | |
576 | .It Cm PubkeyAuthentication | |
577 | Specifies whether public key authentication is allowed. | |
578 | The default is | |
579 | .Dq yes . | |
580 | Note that this option applies to protocol version 2 only. | |
581 | .It Cm RhostsRSAAuthentication | |
582 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
583 | with successful RSA host authentication is allowed. | |
584 | The default is | |
585 | .Dq no . | |
586 | This option applies to protocol version 1 only. | |
587 | .It Cm RSAAuthentication | |
588 | Specifies whether pure RSA authentication is allowed. | |
589 | The default is | |
590 | .Dq yes . | |
591 | This option applies to protocol version 1 only. | |
592 | .It Cm ServerKeyBits | |
593 | Defines the number of bits in the ephemeral protocol version 1 server key. | |
594 | The minimum value is 512, and the default is 768. | |
595 | .It Cm StrictModes | |
596 | Specifies whether | |
597 | .Nm sshd | |
598 | should check file modes and ownership of the | |
599 | user's files and home directory before accepting login. | |
600 | This is normally desirable because novices sometimes accidentally leave their | |
601 | directory or files world-writable. | |
602 | The default is | |
603 | .Dq yes . | |
604 | .It Cm Subsystem | |
605 | Configures an external subsystem (e.g., file transfer daemon). | |
606 | Arguments should be a subsystem name and a command to execute upon subsystem | |
607 | request. | |
608 | The command | |
609 | .Xr sftp-server 8 | |
610 | implements the | |
611 | .Dq sftp | |
612 | file transfer subsystem. | |
613 | By default no subsystems are defined. | |
614 | Note that this option applies to protocol version 2 only. | |
615 | .It Cm SyslogFacility | |
616 | Gives the facility code that is used when logging messages from | |
617 | .Nm sshd . | |
618 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, | |
619 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. | |
620 | The default is AUTH. | |
621 | .It Cm TCPKeepAlive | |
622 | Specifies whether the system should send TCP keepalive messages to the | |
623 | other side. | |
624 | If they are sent, death of the connection or crash of one | |
625 | of the machines will be properly noticed. | |
626 | However, this means that | |
627 | connections will die if the route is down temporarily, and some people | |
628 | find it annoying. | |
629 | On the other hand, if TCP keepalives are not sent, | |
630 | sessions may hang indefinitely on the server, leaving | |
631 | .Dq ghost | |
632 | users and consuming server resources. | |
633 | .Pp | |
634 | The default is | |
635 | .Dq yes | |
636 | (to send TCP keepalive messages), and the server will notice | |
637 | if the network goes down or the client host crashes. | |
638 | This avoids infinitely hanging sessions. | |
639 | .Pp | |
640 | To disable TCP keepalive messages, the value should be set to | |
641 | .Dq no . | |
642 | .It Cm UseDNS | |
643 | Specifies whether | |
644 | .Nm sshd | |
645 | should look up the remote host name and check that | |
646 | the resolved host name for the remote IP address maps back to the | |
647 | very same IP address. | |
648 | The default is | |
649 | .Dq yes . | |
650 | .It Cm UseLogin | |
651 | Specifies whether | |
652 | .Xr login 1 | |
653 | is used for interactive login sessions. | |
654 | The default is | |
655 | .Dq no . | |
656 | Note that | |
657 | .Xr login 1 | |
658 | is never used for remote command execution. | |
659 | Note also, that if this is enabled, | |
660 | .Cm X11Forwarding | |
661 | will be disabled because | |
662 | .Xr login 1 | |
663 | does not know how to handle | |
664 | .Xr xauth 1 | |
665 | cookies. | |
666 | If | |
667 | .Cm UsePrivilegeSeparation | |
668 | is specified, it will be disabled after authentication. | |
669 | .It Cm UsePAM | |
670 | Enables the Pluggable Authentication Module interface. | |
671 | If set to | |
672 | .Dq yes | |
673 | this will enable PAM authentication using | |
674 | .Cm ChallengeResponseAuthentication | |
675 | and PAM account and session module processing for all authentication types. | |
676 | .Pp | |
677 | Because PAM challenge-response authentication usually serves an equivalent | |
678 | role to password authentication, you should disable either | |
679 | .Cm PasswordAuthentication | |
680 | or | |
681 | .Cm ChallengeResponseAuthentication. | |
682 | .Pp | |
683 | If | |
684 | .Cm UsePAM | |
685 | is enabled, you will not be able to run | |
686 | .Xr sshd 8 | |
687 | as a non-root user. | |
688 | The default is | |
689 | .Dq no . | |
690 | .It Cm UsePrivilegeSeparation | |
691 | Specifies whether | |
692 | .Nm sshd | |
693 | separates privileges by creating an unprivileged child process | |
694 | to deal with incoming network traffic. | |
695 | After successful authentication, another process will be created that has | |
696 | the privilege of the authenticated user. | |
697 | The goal of privilege separation is to prevent privilege | |
698 | escalation by containing any corruption within the unprivileged processes. | |
699 | The default is | |
700 | .Dq yes . | |
701 | .It Cm X11DisplayOffset | |
702 | Specifies the first display number available for | |
703 | .Nm sshd Ns 's | |
704 | X11 forwarding. | |
705 | This prevents | |
706 | .Nm sshd | |
707 | from interfering with real X11 servers. | |
708 | The default is 10. | |
709 | .It Cm X11Forwarding | |
710 | Specifies whether X11 forwarding is permitted. | |
711 | The argument must be | |
712 | .Dq yes | |
713 | or | |
714 | .Dq no . | |
715 | The default is | |
716 | .Dq no . | |
717 | .Pp | |
718 | When X11 forwarding is enabled, there may be additional exposure to | |
719 | the server and to client displays if the | |
720 | .Nm sshd | |
721 | proxy display is configured to listen on the wildcard address (see | |
722 | .Cm X11UseLocalhost | |
723 | below), however this is not the default. | |
724 | Additionally, the authentication spoofing and authentication data | |
725 | verification and substitution occur on the client side. | |
726 | The security risk of using X11 forwarding is that the client's X11 | |
727 | display server may be exposed to attack when the ssh client requests | |
728 | forwarding (see the warnings for | |
729 | .Cm ForwardX11 | |
730 | in | |
731 | .Xr ssh_config 5 ) . | |
732 | A system administrator may have a stance in which they want to | |
733 | protect clients that may expose themselves to attack by unwittingly | |
734 | requesting X11 forwarding, which can warrant a | |
735 | .Dq no | |
736 | setting. | |
737 | .Pp | |
738 | Note that disabling X11 forwarding does not prevent users from | |
739 | forwarding X11 traffic, as users can always install their own forwarders. | |
740 | X11 forwarding is automatically disabled if | |
741 | .Cm UseLogin | |
742 | is enabled. | |
743 | .It Cm X11UseLocalhost | |
744 | Specifies whether | |
745 | .Nm sshd | |
746 | should bind the X11 forwarding server to the loopback address or to | |
747 | the wildcard address. | |
748 | By default, | |
749 | .Nm sshd | |
750 | binds the forwarding server to the loopback address and sets the | |
751 | hostname part of the | |
752 | .Ev DISPLAY | |
753 | environment variable to | |
754 | .Dq localhost . | |
755 | This prevents remote hosts from connecting to the proxy display. | |
756 | However, some older X11 clients may not function with this | |
757 | configuration. | |
758 | .Cm X11UseLocalhost | |
759 | may be set to | |
760 | .Dq no | |
761 | to specify that the forwarding server should be bound to the wildcard | |
762 | address. | |
763 | The argument must be | |
764 | .Dq yes | |
765 | or | |
766 | .Dq no . | |
767 | The default is | |
768 | .Dq yes . | |
769 | .It Cm XAuthLocation | |
770 | Specifies the full pathname of the | |
771 | .Xr xauth 1 | |
772 | program. | |
773 | The default is | |
774 | .Pa /usr/X11R6/bin/xauth . | |
775 | .El | |
776 | .Ss Time Formats | |
777 | .Nm sshd | |
778 | command-line arguments and configuration file options that specify time | |
779 | may be expressed using a sequence of the form: | |
780 | .Sm off | |
781 | .Ar time Op Ar qualifier , | |
782 | .Sm on | |
783 | where | |
784 | .Ar time | |
785 | is a positive integer value and | |
786 | .Ar qualifier | |
787 | is one of the following: | |
788 | .Pp | |
789 | .Bl -tag -width Ds -compact -offset indent | |
790 | .It Cm <none> | |
791 | seconds | |
792 | .It Cm s | Cm S | |
793 | seconds | |
794 | .It Cm m | Cm M | |
795 | minutes | |
796 | .It Cm h | Cm H | |
797 | hours | |
798 | .It Cm d | Cm D | |
799 | days | |
800 | .It Cm w | Cm W | |
801 | weeks | |
802 | .El | |
803 | .Pp | |
804 | Each member of the sequence is added together to calculate | |
805 | the total time value. | |
806 | .Pp | |
807 | Time format examples: | |
808 | .Pp | |
809 | .Bl -tag -width Ds -compact -offset indent | |
810 | .It 600 | |
811 | 600 seconds (10 minutes) | |
812 | .It 10m | |
813 | 10 minutes | |
814 | .It 1h30m | |
815 | 1 hour 30 minutes (90 minutes) | |
816 | .El | |
817 | .Sh FILES | |
818 | .Bl -tag -width Ds | |
819 | .It Pa /etc/ssh/sshd_config | |
820 | Contains configuration data for | |
821 | .Nm sshd . | |
822 | This file should be writable by root only, but it is recommended | |
823 | (though not necessary) that it be world-readable. | |
824 | .El | |
825 | .Sh SEE ALSO | |
826 | .Xr sshd 8 | |
827 | .Sh AUTHORS | |
828 | OpenSSH is a derivative of the original and free | |
829 | ssh 1.2.12 release by Tatu Ylonen. | |
830 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
831 | Theo de Raadt and Dug Song | |
832 | removed many bugs, re-added newer features and | |
833 | created OpenSSH. | |
834 | Markus Friedl contributed the support for SSH | |
835 | protocol versions 1.5 and 2.0. | |
836 | Niels Provos and Markus Friedl contributed support | |
837 | for privilege separation. |