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588df31a | 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 | .\" | |
af4bd935 | 37 | .\" $OpenBSD: sshd_config.5,v 1.33 2004/05/23 23:59:53 dtucker Exp $ |
588df31a | 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 | |
61a2c1da | 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. | |
b8b9f2e6 | 73 | Note that environment passing is only supported for protocol 2. |
61a2c1da | 74 | Variables are specified by name, which may contain the wildcard characters |
75 | .Ql \&* | |
76 | and | |
77 | .Ql \&? . | |
b8b9f2e6 | 78 | Multiple environment variables may be separated by whitespace or spread |
61a2c1da | 79 | across multiple |
80 | .Cm AcceptEnv | |
81 | directives. | |
b8b9f2e6 | 82 | Be warned that some environment variables could be used to bypass restricted |
61a2c1da | 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. | |
588df31a | 86 | .It Cm AllowGroups |
87 | This keyword can be followed by a list of group name patterns, separated | |
88 | by spaces. | |
89 | If specified, login is allowed only for users whose primary | |
90 | group or supplementary group list matches one of the patterns. | |
91 | .Ql \&* | |
92 | and | |
9a26a6e2 | 93 | .Ql \&? |
588df31a | 94 | can be used as |
95 | wildcards in the patterns. | |
96 | Only group names are valid; a numerical group ID is not recognized. | |
97 | By default, login is allowed for all groups. | |
588df31a | 98 | .It Cm AllowTcpForwarding |
99 | Specifies whether TCP forwarding is permitted. | |
100 | The default is | |
101 | .Dq yes . | |
102 | Note that disabling TCP forwarding does not improve security unless | |
103 | users are also denied shell access, as they can always install their | |
104 | own forwarders. | |
588df31a | 105 | .It Cm AllowUsers |
106 | This keyword can be followed by a list of user name patterns, separated | |
107 | by spaces. | |
1be697b6 | 108 | If specified, login is allowed only for user names that |
588df31a | 109 | match one of the patterns. |
110 | .Ql \&* | |
111 | and | |
9a26a6e2 | 112 | .Ql \&? |
588df31a | 113 | can be used as |
114 | wildcards in the patterns. | |
115 | Only user names are valid; a numerical user ID is not recognized. | |
116 | By default, login is allowed for all users. | |
117 | If the pattern takes the form USER@HOST then USER and HOST | |
118 | are separately checked, restricting logins to particular | |
119 | users from particular hosts. | |
588df31a | 120 | .It Cm AuthorizedKeysFile |
121 | Specifies the file that contains the public keys that can be used | |
122 | for user authentication. | |
123 | .Cm AuthorizedKeysFile | |
124 | may contain tokens of the form %T which are substituted during connection | |
3cbc677d | 125 | set-up. |
126 | The following tokens are defined: %% is replaced by a literal '%', | |
588df31a | 127 | %h is replaced by the home directory of the user being authenticated and |
128 | %u is replaced by the username of that user. | |
129 | After expansion, | |
130 | .Cm AuthorizedKeysFile | |
131 | is taken to be an absolute path or one relative to the user's home | |
132 | directory. | |
133 | The default is | |
134 | .Dq .ssh/authorized_keys . | |
135 | .It Cm Banner | |
136 | In some jurisdictions, sending a warning message before authentication | |
137 | may be relevant for getting legal protection. | |
138 | The contents of the specified file are sent to the remote user before | |
139 | authentication is allowed. | |
140 | This option is only available for protocol version 2. | |
141 | By default, no banner is displayed. | |
588df31a | 142 | .It Cm ChallengeResponseAuthentication |
143 | Specifies whether challenge response authentication is allowed. | |
144 | All authentication styles from | |
145 | .Xr login.conf 5 | |
146 | are supported. | |
147 | The default is | |
148 | .Dq yes . | |
149 | .It Cm Ciphers | |
150 | Specifies the ciphers allowed for protocol version 2. | |
151 | Multiple ciphers must be comma-separated. | |
152 | The default is | |
588df31a | 153 | .Bd -literal |
154 | ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour, | |
1891396b | 155 | aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr'' |
588df31a | 156 | .Ed |
157 | .It Cm ClientAliveInterval | |
158 | Sets a timeout interval in seconds after which if no data has been received | |
159 | from the client, | |
160 | .Nm sshd | |
161 | will send a message through the encrypted | |
162 | channel to request a response from the client. | |
163 | The default | |
164 | is 0, indicating that these messages will not be sent to the client. | |
165 | This option applies to protocol version 2 only. | |
166 | .It Cm ClientAliveCountMax | |
167 | Sets the number of client alive messages (see above) which may be | |
168 | sent without | |
169 | .Nm sshd | |
3cbc677d | 170 | receiving any messages back from the client. |
171 | If this threshold is reached while client alive messages are being sent, | |
588df31a | 172 | .Nm sshd |
3cbc677d | 173 | will disconnect the client, terminating the session. |
174 | It is important to note that the use of client alive messages is very | |
175 | different from | |
fd573618 | 176 | .Cm TCPKeepAlive |
3cbc677d | 177 | (below). |
178 | The client alive messages are sent through the encrypted channel | |
179 | and therefore will not be spoofable. | |
180 | The TCP keepalive option enabled by | |
fd573618 | 181 | .Cm TCPKeepAlive |
3cbc677d | 182 | is spoofable. |
183 | The client alive mechanism is valuable when the client or | |
588df31a | 184 | server depend on knowing when a connection has become inactive. |
185 | .Pp | |
3cbc677d | 186 | The default value is 3. |
187 | If | |
588df31a | 188 | .Cm ClientAliveInterval |
189 | (above) is set to 15, and | |
190 | .Cm ClientAliveCountMax | |
191 | is left at the default, unresponsive ssh clients | |
192 | will be disconnected after approximately 45 seconds. | |
193 | .It Cm Compression | |
194 | Specifies whether compression is allowed. | |
195 | The argument must be | |
196 | .Dq yes | |
197 | or | |
198 | .Dq no . | |
199 | The default is | |
200 | .Dq yes . | |
201 | .It Cm DenyGroups | |
202 | This keyword can be followed by a list of group name patterns, separated | |
203 | by spaces. | |
204 | Login is disallowed for users whose primary group or supplementary | |
205 | group list matches one of the patterns. | |
206 | .Ql \&* | |
207 | and | |
9a26a6e2 | 208 | .Ql \&? |
588df31a | 209 | can be used as |
210 | wildcards in the patterns. | |
211 | Only group names are valid; a numerical group ID is not recognized. | |
212 | By default, login is allowed for all groups. | |
588df31a | 213 | .It Cm DenyUsers |
214 | This keyword can be followed by a list of user name patterns, separated | |
215 | by spaces. | |
216 | Login is disallowed for user names that match one of the patterns. | |
217 | .Ql \&* | |
218 | and | |
9a26a6e2 | 219 | .Ql \&? |
588df31a | 220 | can be used as wildcards in the patterns. |
221 | Only user names are valid; a numerical user ID is not recognized. | |
222 | By default, login is allowed for all users. | |
223 | If the pattern takes the form USER@HOST then USER and HOST | |
224 | are separately checked, restricting logins to particular | |
225 | users from particular hosts. | |
226 | .It Cm GatewayPorts | |
227 | Specifies whether remote hosts are allowed to connect to ports | |
228 | forwarded for the client. | |
229 | By default, | |
230 | .Nm sshd | |
a4e5acef | 231 | binds remote port forwardings to the loopback address. |
232 | This prevents other remote hosts from connecting to forwarded ports. | |
588df31a | 233 | .Cm GatewayPorts |
234 | can be used to specify that | |
235 | .Nm sshd | |
236 | should bind remote port forwardings to the wildcard address, | |
237 | thus allowing remote hosts to connect to forwarded ports. | |
238 | The argument must be | |
239 | .Dq yes | |
240 | or | |
241 | .Dq no . | |
242 | The default is | |
243 | .Dq no . | |
7364bd04 | 244 | .It Cm GSSAPIAuthentication |
105b07db | 245 | Specifies whether user authentication based on GSSAPI is allowed. |
aff51935 | 246 | The default is |
7364bd04 | 247 | .Dq no . |
248 | Note that this option applies to protocol version 2 only. | |
249 | .It Cm GSSAPICleanupCredentials | |
250 | Specifies whether to automatically destroy the user's credentials cache | |
251 | on logout. | |
252 | The default is | |
253 | .Dq yes . | |
254 | Note that this option applies to protocol version 2 only. | |
588df31a | 255 | .It Cm HostbasedAuthentication |
256 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
257 | with successful public key client host authentication is allowed | |
258 | (hostbased authentication). | |
259 | This option is similar to | |
260 | .Cm RhostsRSAAuthentication | |
261 | and applies to protocol version 2 only. | |
262 | The default is | |
263 | .Dq no . | |
264 | .It Cm HostKey | |
265 | Specifies a file containing a private host key | |
266 | used by SSH. | |
267 | The default is | |
268 | .Pa /etc/ssh/ssh_host_key | |
269 | for protocol version 1, and | |
270 | .Pa /etc/ssh/ssh_host_rsa_key | |
271 | and | |
272 | .Pa /etc/ssh/ssh_host_dsa_key | |
273 | for protocol version 2. | |
274 | Note that | |
275 | .Nm sshd | |
276 | will refuse to use a file if it is group/world-accessible. | |
277 | It is possible to have multiple host key files. | |
278 | .Dq rsa1 | |
279 | keys are used for version 1 and | |
280 | .Dq dsa | |
281 | or | |
282 | .Dq rsa | |
283 | are used for version 2 of the SSH protocol. | |
284 | .It Cm IgnoreRhosts | |
285 | Specifies that | |
286 | .Pa .rhosts | |
287 | and | |
288 | .Pa .shosts | |
289 | files will not be used in | |
588df31a | 290 | .Cm RhostsRSAAuthentication |
291 | or | |
292 | .Cm HostbasedAuthentication . | |
293 | .Pp | |
294 | .Pa /etc/hosts.equiv | |
295 | and | |
296 | .Pa /etc/shosts.equiv | |
297 | are still used. | |
298 | The default is | |
299 | .Dq yes . | |
300 | .It Cm IgnoreUserKnownHosts | |
301 | Specifies whether | |
302 | .Nm sshd | |
303 | should ignore the user's | |
304 | .Pa $HOME/.ssh/known_hosts | |
305 | during | |
306 | .Cm RhostsRSAAuthentication | |
307 | or | |
308 | .Cm HostbasedAuthentication . | |
309 | The default is | |
310 | .Dq no . | |
588df31a | 311 | .It Cm KerberosAuthentication |
8f73f7bb | 312 | Specifies whether the password provided by the user for |
588df31a | 313 | .Cm PasswordAuthentication |
8f73f7bb | 314 | will be validated through the Kerberos KDC. |
588df31a | 315 | To use this option, the server needs a |
316 | Kerberos servtab which allows the verification of the KDC's identity. | |
317 | Default is | |
318 | .Dq no . | |
24f37810 | 319 | .It Cm KerberosGetAFSToken |
320 | If AFS is active and the user has a Kerberos 5 TGT, attempt to aquire | |
321 | an AFS token before accessing the user's home directory. | |
322 | Default is | |
323 | .Dq no . | |
588df31a | 324 | .It Cm KerberosOrLocalPasswd |
325 | If set then if password authentication through Kerberos fails then | |
326 | the password will be validated via any additional local mechanism | |
327 | such as | |
328 | .Pa /etc/passwd . | |
329 | Default is | |
330 | .Dq yes . | |
588df31a | 331 | .It Cm KerberosTicketCleanup |
332 | Specifies whether to automatically destroy the user's ticket cache | |
333 | file on logout. | |
334 | Default is | |
335 | .Dq yes . | |
336 | .It Cm KeyRegenerationInterval | |
337 | In protocol version 1, the ephemeral server key is automatically regenerated | |
338 | after this many seconds (if it has been used). | |
339 | The purpose of regeneration is to prevent | |
340 | decrypting captured sessions by later breaking into the machine and | |
341 | stealing the keys. | |
342 | The key is never stored anywhere. | |
343 | If the value is 0, the key is never regenerated. | |
344 | The default is 3600 (seconds). | |
345 | .It Cm ListenAddress | |
346 | Specifies the local addresses | |
347 | .Nm sshd | |
348 | should listen on. | |
349 | The following forms may be used: | |
350 | .Pp | |
351 | .Bl -item -offset indent -compact | |
352 | .It | |
353 | .Cm ListenAddress | |
354 | .Sm off | |
355 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr | |
356 | .Sm on | |
357 | .It | |
358 | .Cm ListenAddress | |
359 | .Sm off | |
360 | .Ar host No | Ar IPv4_addr No : Ar port | |
361 | .Sm on | |
362 | .It | |
363 | .Cm ListenAddress | |
364 | .Sm off | |
365 | .Oo | |
366 | .Ar host No | Ar IPv6_addr Oc : Ar port | |
367 | .Sm on | |
368 | .El | |
369 | .Pp | |
370 | If | |
371 | .Ar port | |
372 | is not specified, | |
373 | .Nm sshd | |
374 | will listen on the address and all prior | |
375 | .Cm Port | |
3cbc677d | 376 | options specified. |
377 | The default is to listen on all local addresses. | |
a4e5acef | 378 | Multiple |
588df31a | 379 | .Cm ListenAddress |
3cbc677d | 380 | options are permitted. |
381 | Additionally, any | |
588df31a | 382 | .Cm Port |
383 | options must precede this option for non port qualified addresses. | |
384 | .It Cm LoginGraceTime | |
385 | The server disconnects after this time if the user has not | |
386 | successfully logged in. | |
387 | If the value is 0, there is no time limit. | |
3445ca02 | 388 | The default is 120 seconds. |
588df31a | 389 | .It Cm LogLevel |
390 | Gives the verbosity level that is used when logging messages from | |
391 | .Nm sshd . | |
392 | The possible values are: | |
393 | QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3. | |
a4e5acef | 394 | The default is INFO. |
395 | DEBUG and DEBUG1 are equivalent. | |
396 | DEBUG2 and DEBUG3 each specify higher levels of debugging output. | |
397 | Logging with a DEBUG level violates the privacy of users and is not recommended. | |
588df31a | 398 | .It Cm MACs |
399 | Specifies the available MAC (message authentication code) algorithms. | |
400 | The MAC algorithm is used in protocol version 2 | |
401 | for data integrity protection. | |
402 | Multiple algorithms must be comma-separated. | |
403 | The default is | |
404 | .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . | |
af4bd935 | 405 | .It Cm MaxAuthTries |
406 | Specifies the maximum number of authentication attempts permitted per | |
407 | connection. Once the number of failures reaches half this value, additional | |
408 | failures are logged. The default is 6. | |
588df31a | 409 | .It Cm MaxStartups |
410 | Specifies the maximum number of concurrent unauthenticated connections to the | |
411 | .Nm sshd | |
412 | daemon. | |
413 | Additional connections will be dropped until authentication succeeds or the | |
414 | .Cm LoginGraceTime | |
415 | expires for a connection. | |
416 | The default is 10. | |
417 | .Pp | |
418 | Alternatively, random early drop can be enabled by specifying | |
419 | the three colon separated values | |
420 | .Dq start:rate:full | |
421 | (e.g., "10:30:60"). | |
422 | .Nm sshd | |
423 | will refuse connection attempts with a probability of | |
424 | .Dq rate/100 | |
425 | (30%) | |
426 | if there are currently | |
427 | .Dq start | |
428 | (10) | |
429 | unauthenticated connections. | |
430 | The probability increases linearly and all connection attempts | |
431 | are refused if the number of unauthenticated connections reaches | |
432 | .Dq full | |
433 | (60). | |
434 | .It Cm PasswordAuthentication | |
435 | Specifies whether password authentication is allowed. | |
436 | The default is | |
437 | .Dq yes . | |
438 | .It Cm PermitEmptyPasswords | |
439 | When password authentication is allowed, it specifies whether the | |
440 | server allows login to accounts with empty password strings. | |
441 | The default is | |
442 | .Dq no . | |
443 | .It Cm PermitRootLogin | |
444 | Specifies whether root can login using | |
445 | .Xr ssh 1 . | |
446 | The argument must be | |
447 | .Dq yes , | |
448 | .Dq without-password , | |
449 | .Dq forced-commands-only | |
450 | or | |
451 | .Dq no . | |
452 | The default is | |
453 | .Dq yes . | |
454 | .Pp | |
455 | If this option is set to | |
456 | .Dq without-password | |
d1d10baa | 457 | password authentication is disabled for root. Note that other authentication |
458 | methods (e.g., keyboard-interactive/PAM) may still allow root to login using | |
459 | a password. | |
588df31a | 460 | .Pp |
461 | If this option is set to | |
462 | .Dq forced-commands-only | |
463 | root login with public key authentication will be allowed, | |
464 | but only if the | |
465 | .Ar command | |
466 | option has been specified | |
467 | (which may be useful for taking remote backups even if root login is | |
3cbc677d | 468 | normally not allowed). |
469 | All other authentication methods are disabled for root. | |
588df31a | 470 | .Pp |
471 | If this option is set to | |
472 | .Dq no | |
473 | root is not allowed to login. | |
f00bab84 | 474 | .It Cm PermitUserEnvironment |
475 | Specifies whether | |
476 | .Pa ~/.ssh/environment | |
35453849 | 477 | and |
f00bab84 | 478 | .Cm environment= |
479 | options in | |
480 | .Pa ~/.ssh/authorized_keys | |
35453849 | 481 | are processed by |
482 | .Nm sshd . | |
f00bab84 | 483 | The default is |
484 | .Dq no . | |
35453849 | 485 | Enabling environment processing may enable users to bypass access |
486 | restrictions in some configurations using mechanisms such as | |
487 | .Ev LD_PRELOAD . | |
588df31a | 488 | .It Cm PidFile |
baa08b92 | 489 | Specifies the file that contains the process ID of the |
588df31a | 490 | .Nm sshd |
491 | daemon. | |
492 | The default is | |
493 | .Pa /var/run/sshd.pid . | |
494 | .It Cm Port | |
495 | Specifies the port number that | |
496 | .Nm sshd | |
497 | listens on. | |
498 | The default is 22. | |
499 | Multiple options of this type are permitted. | |
500 | See also | |
501 | .Cm ListenAddress . | |
502 | .It Cm PrintLastLog | |
503 | Specifies whether | |
504 | .Nm sshd | |
505 | should print the date and time when the user last logged in. | |
506 | The default is | |
507 | .Dq yes . | |
508 | .It Cm PrintMotd | |
509 | Specifies whether | |
510 | .Nm sshd | |
511 | should print | |
512 | .Pa /etc/motd | |
513 | when a user logs in interactively. | |
514 | (On some systems it is also printed by the shell, | |
515 | .Pa /etc/profile , | |
516 | or equivalent.) | |
517 | The default is | |
518 | .Dq yes . | |
519 | .It Cm Protocol | |
520 | Specifies the protocol versions | |
521 | .Nm sshd | |
94ad46d1 | 522 | supports. |
588df31a | 523 | The possible values are |
524 | .Dq 1 | |
525 | and | |
526 | .Dq 2 . | |
527 | Multiple versions must be comma-separated. | |
528 | The default is | |
529 | .Dq 2,1 . | |
94ad46d1 | 530 | Note that the order of the protocol list does not indicate preference, |
531 | because the client selects among multiple protocol versions offered | |
532 | by the server. | |
533 | Specifying | |
534 | .Dq 2,1 | |
535 | is identical to | |
536 | .Dq 1,2 . | |
588df31a | 537 | .It Cm PubkeyAuthentication |
538 | Specifies whether public key authentication is allowed. | |
539 | The default is | |
540 | .Dq yes . | |
541 | Note that this option applies to protocol version 2 only. | |
588df31a | 542 | .It Cm RhostsRSAAuthentication |
543 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
544 | with successful RSA host authentication is allowed. | |
545 | The default is | |
546 | .Dq no . | |
547 | This option applies to protocol version 1 only. | |
548 | .It Cm RSAAuthentication | |
549 | Specifies whether pure RSA authentication is allowed. | |
550 | The default is | |
551 | .Dq yes . | |
552 | This option applies to protocol version 1 only. | |
553 | .It Cm ServerKeyBits | |
554 | Defines the number of bits in the ephemeral protocol version 1 server key. | |
555 | The minimum value is 512, and the default is 768. | |
556 | .It Cm StrictModes | |
557 | Specifies whether | |
558 | .Nm sshd | |
559 | should check file modes and ownership of the | |
560 | user's files and home directory before accepting login. | |
561 | This is normally desirable because novices sometimes accidentally leave their | |
562 | directory or files world-writable. | |
563 | The default is | |
564 | .Dq yes . | |
565 | .It Cm Subsystem | |
566 | Configures an external subsystem (e.g., file transfer daemon). | |
567 | Arguments should be a subsystem name and a command to execute upon subsystem | |
568 | request. | |
569 | The command | |
570 | .Xr sftp-server 8 | |
571 | implements the | |
572 | .Dq sftp | |
573 | file transfer subsystem. | |
574 | By default no subsystems are defined. | |
575 | Note that this option applies to protocol version 2 only. | |
576 | .It Cm SyslogFacility | |
577 | Gives the facility code that is used when logging messages from | |
578 | .Nm sshd . | |
579 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, | |
580 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. | |
581 | The default is AUTH. | |
fd573618 | 582 | .It Cm TCPKeepAlive |
583 | Specifies whether the system should send TCP keepalive messages to the | |
584 | other side. | |
585 | If they are sent, death of the connection or crash of one | |
586 | of the machines will be properly noticed. | |
587 | However, this means that | |
588 | connections will die if the route is down temporarily, and some people | |
589 | find it annoying. | |
590 | On the other hand, if TCP keepalives are not sent, | |
591 | sessions may hang indefinitely on the server, leaving | |
592 | .Dq ghost | |
593 | users and consuming server resources. | |
594 | .Pp | |
595 | The default is | |
596 | .Dq yes | |
597 | (to send TCP keepalive messages), and the server will notice | |
598 | if the network goes down or the client host crashes. | |
599 | This avoids infinitely hanging sessions. | |
600 | .Pp | |
601 | To disable TCP keepalive messages, the value should be set to | |
602 | .Dq no . | |
c5a7d788 | 603 | .It Cm UseDNS |
604 | Specifies whether | |
605 | .Nm sshd | |
606 | should lookup the remote host name and check that | |
607 | the resolved host name for the remote IP address maps back to the | |
608 | very same IP address. | |
609 | The default is | |
610 | .Dq yes . | |
588df31a | 611 | .It Cm UseLogin |
612 | Specifies whether | |
613 | .Xr login 1 | |
614 | is used for interactive login sessions. | |
615 | The default is | |
616 | .Dq no . | |
617 | Note that | |
618 | .Xr login 1 | |
619 | is never used for remote command execution. | |
620 | Note also, that if this is enabled, | |
621 | .Cm X11Forwarding | |
622 | will be disabled because | |
623 | .Xr login 1 | |
624 | does not know how to handle | |
625 | .Xr xauth 1 | |
a4e5acef | 626 | cookies. |
627 | If | |
588df31a | 628 | .Cm UsePrivilegeSeparation |
629 | is specified, it will be disabled after authentication. | |
72c35df7 | 630 | .It Cm UsePAM |
d5c67850 | 631 | Enables the Pluggable Authentication Module interface. |
632 | If set to | |
633 | .Dq yes | |
634 | this will enable PAM authentication using | |
635 | .Cm ChallengeResponseAuthentication | |
636 | and PAM account and session module processing for all authentication types. | |
637 | .Pp | |
638 | Because PAM challenge-response authentication usually serves an equivalent | |
639 | role to password authentication, you should disable either | |
640 | .Cm PasswordAuthentication | |
641 | or | |
642 | .Cm ChallengeResponseAuthentication. | |
643 | .Pp | |
644 | If | |
645 | .Cm UsePAM | |
646 | is enabled, you will not be able to run | |
647 | .Xr sshd 8 | |
648 | as a non-root user. | |
649 | The default is | |
a83a3125 | 650 | .Dq no . |
588df31a | 651 | .It Cm UsePrivilegeSeparation |
652 | Specifies whether | |
653 | .Nm sshd | |
654 | separates privileges by creating an unprivileged child process | |
a4e5acef | 655 | to deal with incoming network traffic. |
656 | After successful authentication, another process will be created that has | |
657 | the privilege of the authenticated user. | |
658 | The goal of privilege separation is to prevent privilege | |
588df31a | 659 | escalation by containing any corruption within the unprivileged processes. |
660 | The default is | |
661 | .Dq yes . | |
588df31a | 662 | .It Cm X11DisplayOffset |
663 | Specifies the first display number available for | |
664 | .Nm sshd Ns 's | |
665 | X11 forwarding. | |
666 | This prevents | |
667 | .Nm sshd | |
668 | from interfering with real X11 servers. | |
669 | The default is 10. | |
670 | .It Cm X11Forwarding | |
671 | Specifies whether X11 forwarding is permitted. | |
e6fe1bab | 672 | The argument must be |
673 | .Dq yes | |
674 | or | |
675 | .Dq no . | |
588df31a | 676 | The default is |
677 | .Dq no . | |
e6fe1bab | 678 | .Pp |
679 | When X11 forwarding is enabled, there may be additional exposure to | |
680 | the server and to client displays if the | |
681 | .Nm sshd | |
682 | proxy display is configured to listen on the wildcard address (see | |
683 | .Cm X11UseLocalhost | |
684 | below), however this is not the default. | |
685 | Additionally, the authentication spoofing and authentication data | |
686 | verification and substitution occur on the client side. | |
687 | The security risk of using X11 forwarding is that the client's X11 | |
688 | display server may be exposed to attack when the ssh client requests | |
689 | forwarding (see the warnings for | |
690 | .Cm ForwardX11 | |
691 | in | |
be193d89 | 692 | .Xr ssh_config 5 ) . |
e6fe1bab | 693 | A system administrator may have a stance in which they want to |
694 | protect clients that may expose themselves to attack by unwittingly | |
695 | requesting X11 forwarding, which can warrant a | |
696 | .Dq no | |
697 | setting. | |
698 | .Pp | |
699 | Note that disabling X11 forwarding does not prevent users from | |
700 | forwarding X11 traffic, as users can always install their own forwarders. | |
588df31a | 701 | X11 forwarding is automatically disabled if |
702 | .Cm UseLogin | |
703 | is enabled. | |
704 | .It Cm X11UseLocalhost | |
705 | Specifies whether | |
706 | .Nm sshd | |
707 | should bind the X11 forwarding server to the loopback address or to | |
a4e5acef | 708 | the wildcard address. |
709 | By default, | |
588df31a | 710 | .Nm sshd |
711 | binds the forwarding server to the loopback address and sets the | |
712 | hostname part of the | |
713 | .Ev DISPLAY | |
714 | environment variable to | |
715 | .Dq localhost . | |
b3641662 | 716 | This prevents remote hosts from connecting to the proxy display. |
588df31a | 717 | However, some older X11 clients may not function with this |
718 | configuration. | |
719 | .Cm X11UseLocalhost | |
720 | may be set to | |
721 | .Dq no | |
722 | to specify that the forwarding server should be bound to the wildcard | |
723 | address. | |
724 | The argument must be | |
725 | .Dq yes | |
726 | or | |
727 | .Dq no . | |
728 | The default is | |
729 | .Dq yes . | |
730 | .It Cm XAuthLocation | |
57ff5eeb | 731 | Specifies the full pathname of the |
588df31a | 732 | .Xr xauth 1 |
733 | program. | |
734 | The default is | |
735 | .Pa /usr/X11R6/bin/xauth . | |
736 | .El | |
737 | .Ss Time Formats | |
588df31a | 738 | .Nm sshd |
739 | command-line arguments and configuration file options that specify time | |
740 | may be expressed using a sequence of the form: | |
741 | .Sm off | |
36535ee6 | 742 | .Ar time Op Ar qualifier , |
588df31a | 743 | .Sm on |
744 | where | |
745 | .Ar time | |
746 | is a positive integer value and | |
747 | .Ar qualifier | |
748 | is one of the following: | |
749 | .Pp | |
750 | .Bl -tag -width Ds -compact -offset indent | |
751 | .It Cm <none> | |
752 | seconds | |
753 | .It Cm s | Cm S | |
754 | seconds | |
755 | .It Cm m | Cm M | |
756 | minutes | |
757 | .It Cm h | Cm H | |
758 | hours | |
759 | .It Cm d | Cm D | |
760 | days | |
761 | .It Cm w | Cm W | |
762 | weeks | |
763 | .El | |
764 | .Pp | |
765 | Each member of the sequence is added together to calculate | |
766 | the total time value. | |
767 | .Pp | |
768 | Time format examples: | |
769 | .Pp | |
770 | .Bl -tag -width Ds -compact -offset indent | |
771 | .It 600 | |
772 | 600 seconds (10 minutes) | |
773 | .It 10m | |
774 | 10 minutes | |
775 | .It 1h30m | |
776 | 1 hour 30 minutes (90 minutes) | |
777 | .El | |
778 | .Sh FILES | |
779 | .Bl -tag -width Ds | |
780 | .It Pa /etc/ssh/sshd_config | |
781 | Contains configuration data for | |
782 | .Nm sshd . | |
783 | This file should be writable by root only, but it is recommended | |
784 | (though not necessary) that it be world-readable. | |
785 | .El | |
be193d89 | 786 | .Sh SEE ALSO |
787 | .Xr sshd 8 | |
588df31a | 788 | .Sh AUTHORS |
789 | OpenSSH is a derivative of the original and free | |
790 | ssh 1.2.12 release by Tatu Ylonen. | |
791 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
792 | Theo de Raadt and Dug Song | |
793 | removed many bugs, re-added newer features and | |
794 | created OpenSSH. | |
795 | Markus Friedl contributed the support for SSH | |
796 | protocol versions 1.5 and 2.0. | |
797 | Niels Provos and Markus Friedl contributed support | |
798 | for privilege separation. |