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ff2d7a98 | 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 | .\" | |
416fd2a8 | 37 | .\" $OpenBSD: sshd_config.5,v 1.28 2004/02/17 19:35:21 jmc Exp $ |
ff2d7a98 | 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 | |
ff2d7a98 | 64 | .It Cm AllowGroups |
65 | This keyword can be followed by a list of group name patterns, separated | |
66 | by spaces. | |
67 | If specified, login is allowed only for users whose primary | |
68 | group or supplementary group list matches one of the patterns. | |
69 | .Ql \&* | |
70 | and | |
70791e56 | 71 | .Ql \&? |
ff2d7a98 | 72 | can be used as |
73 | wildcards in the patterns. | |
74 | Only group names are valid; a numerical group ID is not recognized. | |
75 | By default, login is allowed for all groups. | |
76 | .Pp | |
77 | .It Cm AllowTcpForwarding | |
78 | Specifies whether TCP forwarding is permitted. | |
79 | The default is | |
80 | .Dq yes . | |
81 | Note that disabling TCP forwarding does not improve security unless | |
82 | users are also denied shell access, as they can always install their | |
83 | own forwarders. | |
84 | .Pp | |
85 | .It Cm AllowUsers | |
86 | This keyword can be followed by a list of user name patterns, separated | |
87 | by spaces. | |
1c14df9e | 88 | If specified, login is allowed only for user names that |
ff2d7a98 | 89 | match one of the patterns. |
90 | .Ql \&* | |
91 | and | |
70791e56 | 92 | .Ql \&? |
ff2d7a98 | 93 | can be used as |
94 | wildcards in the patterns. | |
95 | Only user names are valid; a numerical user ID is not recognized. | |
96 | By default, login is allowed for all users. | |
97 | If the pattern takes the form USER@HOST then USER and HOST | |
98 | are separately checked, restricting logins to particular | |
99 | users from particular hosts. | |
100 | .Pp | |
101 | .It Cm AuthorizedKeysFile | |
102 | Specifies the file that contains the public keys that can be used | |
103 | for user authentication. | |
104 | .Cm AuthorizedKeysFile | |
105 | may contain tokens of the form %T which are substituted during connection | |
70791e56 | 106 | set-up. |
107 | The following tokens are defined: %% is replaced by a literal '%', | |
ff2d7a98 | 108 | %h is replaced by the home directory of the user being authenticated and |
109 | %u is replaced by the username of that user. | |
110 | After expansion, | |
111 | .Cm AuthorizedKeysFile | |
112 | is taken to be an absolute path or one relative to the user's home | |
113 | directory. | |
114 | The default is | |
115 | .Dq .ssh/authorized_keys . | |
116 | .It Cm Banner | |
117 | In some jurisdictions, sending a warning message before authentication | |
118 | may be relevant for getting legal protection. | |
119 | The contents of the specified file are sent to the remote user before | |
120 | authentication is allowed. | |
121 | This option is only available for protocol version 2. | |
122 | By default, no banner is displayed. | |
123 | .Pp | |
124 | .It Cm ChallengeResponseAuthentication | |
125 | Specifies whether challenge response authentication is allowed. | |
126 | All authentication styles from | |
127 | .Xr login.conf 5 | |
128 | are supported. | |
129 | The default is | |
130 | .Dq yes . | |
131 | .It Cm Ciphers | |
132 | Specifies the ciphers allowed for protocol version 2. | |
133 | Multiple ciphers must be comma-separated. | |
134 | The default is | |
135 | .Pp | |
136 | .Bd -literal | |
137 | ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour, | |
70791e56 | 138 | aes192-cbc,aes256-cbc,aes128-ctr,aes192-ctr,aes256-ctr'' |
ff2d7a98 | 139 | .Ed |
140 | .It Cm ClientAliveInterval | |
141 | Sets a timeout interval in seconds after which if no data has been received | |
142 | from the client, | |
143 | .Nm sshd | |
144 | will send a message through the encrypted | |
145 | channel to request a response from the client. | |
146 | The default | |
147 | is 0, indicating that these messages will not be sent to the client. | |
148 | This option applies to protocol version 2 only. | |
149 | .It Cm ClientAliveCountMax | |
150 | Sets the number of client alive messages (see above) which may be | |
151 | sent without | |
152 | .Nm sshd | |
70791e56 | 153 | receiving any messages back from the client. |
154 | If this threshold is reached while client alive messages are being sent, | |
ff2d7a98 | 155 | .Nm sshd |
70791e56 | 156 | will disconnect the client, terminating the session. |
157 | It is important to note that the use of client alive messages is very | |
158 | different from | |
416fd2a8 | 159 | .Cm TCPKeepAlive |
70791e56 | 160 | (below). |
161 | The client alive messages are sent through the encrypted channel | |
162 | and therefore will not be spoofable. | |
163 | The TCP keepalive option enabled by | |
416fd2a8 | 164 | .Cm TCPKeepAlive |
70791e56 | 165 | is spoofable. |
166 | The client alive mechanism is valuable when the client or | |
ff2d7a98 | 167 | server depend on knowing when a connection has become inactive. |
168 | .Pp | |
70791e56 | 169 | The default value is 3. |
170 | If | |
ff2d7a98 | 171 | .Cm ClientAliveInterval |
172 | (above) is set to 15, and | |
173 | .Cm ClientAliveCountMax | |
174 | is left at the default, unresponsive ssh clients | |
175 | will be disconnected after approximately 45 seconds. | |
176 | .It Cm Compression | |
177 | Specifies whether compression is allowed. | |
178 | The argument must be | |
179 | .Dq yes | |
180 | or | |
181 | .Dq no . | |
182 | The default is | |
183 | .Dq yes . | |
184 | .It Cm DenyGroups | |
185 | This keyword can be followed by a list of group name patterns, separated | |
186 | by spaces. | |
187 | Login is disallowed for users whose primary group or supplementary | |
188 | group list matches one of the patterns. | |
189 | .Ql \&* | |
190 | and | |
70791e56 | 191 | .Ql \&? |
ff2d7a98 | 192 | can be used as |
193 | wildcards in the patterns. | |
194 | Only group names are valid; a numerical group ID is not recognized. | |
195 | By default, login is allowed for all groups. | |
196 | .Pp | |
197 | .It Cm DenyUsers | |
198 | This keyword can be followed by a list of user name patterns, separated | |
199 | by spaces. | |
200 | Login is disallowed for user names that match one of the patterns. | |
201 | .Ql \&* | |
202 | and | |
70791e56 | 203 | .Ql \&? |
ff2d7a98 | 204 | can be used as wildcards in the patterns. |
205 | Only user names are valid; a numerical user ID is not recognized. | |
206 | By default, login is allowed for all users. | |
207 | If the pattern takes the form USER@HOST then USER and HOST | |
208 | are separately checked, restricting logins to particular | |
209 | users from particular hosts. | |
210 | .It Cm GatewayPorts | |
211 | Specifies whether remote hosts are allowed to connect to ports | |
212 | forwarded for the client. | |
213 | By default, | |
214 | .Nm sshd | |
1c14df9e | 215 | binds remote port forwardings to the loopback address. |
216 | This prevents other remote hosts from connecting to forwarded ports. | |
ff2d7a98 | 217 | .Cm GatewayPorts |
218 | can be used to specify that | |
219 | .Nm sshd | |
220 | should bind remote port forwardings to the wildcard address, | |
221 | thus allowing remote hosts to connect to forwarded ports. | |
222 | The argument must be | |
223 | .Dq yes | |
224 | or | |
225 | .Dq no . | |
226 | The default is | |
227 | .Dq no . | |
70791e56 | 228 | .It Cm GSSAPIAuthentication |
229 | Specifies whether user authentication based on GSSAPI is allowed. | |
416fd2a8 | 230 | The default is |
ff2d7a98 | 231 | .Dq yes . |
70791e56 | 232 | Note that this option applies to protocol version 2 only. |
416fd2a8 | 233 | .It Cm GSSAPICleanupCredentials |
70791e56 | 234 | Specifies whether to automatically destroy the user's credentials cache |
235 | on logout. | |
236 | The default is | |
237 | .Dq yes . | |
238 | Note that this option applies to protocol version 2 only. | |
239 | .It Cm GSSAPIKeyExchange | |
ff2d7a98 | 240 | Specifies whether key exchange based on GSSAPI may be used. When using |
241 | GSSAPI key exchange the server need not have a host key. | |
242 | The default is | |
243 | .Dq yes . | |
70791e56 | 244 | Note that this option applies to protocol version 2 only. |
245 | .It Cm GSSAPIUseSessionCredCache | |
ff2d7a98 | 246 | Specifies whether a unique credentials cache name should be generated per |
247 | session for storing delegated credentials. | |
248 | The default is | |
249 | .Dq yes . | |
70791e56 | 250 | Note that this option applies to protocol version 2 only. |
251 | .It Cm HostbasedAuthentication | |
252 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
253 | with successful public key client host authentication is allowed | |
254 | (hostbased authentication). | |
255 | This option is similar to | |
256 | .Cm RhostsRSAAuthentication | |
257 | and applies to protocol version 2 only. | |
c2397a66 | 258 | The default is |
70791e56 | 259 | .Dq no . |
ff2d7a98 | 260 | .It Cm HostKey |
261 | Specifies a file containing a private host key | |
262 | used by SSH. | |
263 | The default is | |
264 | .Pa /etc/ssh/ssh_host_key | |
265 | for protocol version 1, and | |
266 | .Pa /etc/ssh/ssh_host_rsa_key | |
267 | and | |
268 | .Pa /etc/ssh/ssh_host_dsa_key | |
269 | for protocol version 2. | |
270 | Note that | |
271 | .Nm sshd | |
272 | will refuse to use a file if it is group/world-accessible. | |
273 | It is possible to have multiple host key files. | |
274 | .Dq rsa1 | |
275 | keys are used for version 1 and | |
276 | .Dq dsa | |
277 | or | |
278 | .Dq rsa | |
279 | are used for version 2 of the SSH protocol. | |
280 | .It Cm IgnoreRhosts | |
281 | Specifies that | |
282 | .Pa .rhosts | |
283 | and | |
284 | .Pa .shosts | |
285 | files will not be used in | |
ff2d7a98 | 286 | .Cm RhostsRSAAuthentication |
287 | or | |
288 | .Cm HostbasedAuthentication . | |
289 | .Pp | |
290 | .Pa /etc/hosts.equiv | |
291 | and | |
292 | .Pa /etc/shosts.equiv | |
293 | are still used. | |
294 | The default is | |
295 | .Dq yes . | |
296 | .It Cm IgnoreUserKnownHosts | |
297 | Specifies whether | |
298 | .Nm sshd | |
299 | should ignore the user's | |
300 | .Pa $HOME/.ssh/known_hosts | |
301 | during | |
302 | .Cm RhostsRSAAuthentication | |
303 | or | |
304 | .Cm HostbasedAuthentication . | |
305 | The default is | |
306 | .Dq no . | |
ff2d7a98 | 307 | .It Cm KerberosAuthentication |
70791e56 | 308 | Specifies whether the password provided by the user for |
ff2d7a98 | 309 | .Cm PasswordAuthentication |
70791e56 | 310 | will be validated through the Kerberos KDC. |
ff2d7a98 | 311 | To use this option, the server needs a |
312 | Kerberos servtab which allows the verification of the KDC's identity. | |
313 | Default is | |
314 | .Dq no . | |
315 | .It Cm KerberosOrLocalPasswd | |
316 | If set then if password authentication through Kerberos fails then | |
317 | the password will be validated via any additional local mechanism | |
318 | such as | |
319 | .Pa /etc/passwd . | |
320 | Default is | |
321 | .Dq yes . | |
ff2d7a98 | 322 | .It Cm KerberosTicketCleanup |
323 | Specifies whether to automatically destroy the user's ticket cache | |
324 | file on logout. | |
325 | Default is | |
326 | .Dq yes . | |
327 | .It Cm KeyRegenerationInterval | |
328 | In protocol version 1, the ephemeral server key is automatically regenerated | |
329 | after this many seconds (if it has been used). | |
330 | The purpose of regeneration is to prevent | |
331 | decrypting captured sessions by later breaking into the machine and | |
332 | stealing the keys. | |
333 | The key is never stored anywhere. | |
334 | If the value is 0, the key is never regenerated. | |
335 | The default is 3600 (seconds). | |
336 | .It Cm ListenAddress | |
337 | Specifies the local addresses | |
338 | .Nm sshd | |
339 | should listen on. | |
340 | The following forms may be used: | |
341 | .Pp | |
342 | .Bl -item -offset indent -compact | |
343 | .It | |
344 | .Cm ListenAddress | |
345 | .Sm off | |
346 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr | |
347 | .Sm on | |
348 | .It | |
349 | .Cm ListenAddress | |
350 | .Sm off | |
351 | .Ar host No | Ar IPv4_addr No : Ar port | |
352 | .Sm on | |
353 | .It | |
354 | .Cm ListenAddress | |
355 | .Sm off | |
356 | .Oo | |
357 | .Ar host No | Ar IPv6_addr Oc : Ar port | |
358 | .Sm on | |
359 | .El | |
360 | .Pp | |
361 | If | |
362 | .Ar port | |
363 | is not specified, | |
364 | .Nm sshd | |
365 | will listen on the address and all prior | |
366 | .Cm Port | |
70791e56 | 367 | options specified. |
368 | The default is to listen on all local addresses. | |
1c14df9e | 369 | Multiple |
ff2d7a98 | 370 | .Cm ListenAddress |
70791e56 | 371 | options are permitted. |
372 | Additionally, any | |
ff2d7a98 | 373 | .Cm Port |
374 | options must precede this option for non port qualified addresses. | |
375 | .It Cm LoginGraceTime | |
376 | The server disconnects after this time if the user has not | |
377 | successfully logged in. | |
378 | If the value is 0, there is no time limit. | |
e54b3d7c | 379 | The default is 120 seconds. |
ff2d7a98 | 380 | .It Cm LogLevel |
381 | Gives the verbosity level that is used when logging messages from | |
382 | .Nm sshd . | |
383 | The possible values are: | |
384 | QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3. | |
1c14df9e | 385 | The default is INFO. |
386 | DEBUG and DEBUG1 are equivalent. | |
387 | DEBUG2 and DEBUG3 each specify higher levels of debugging output. | |
388 | Logging with a DEBUG level violates the privacy of users and is not recommended. | |
ff2d7a98 | 389 | .It Cm MACs |
390 | Specifies the available MAC (message authentication code) algorithms. | |
391 | The MAC algorithm is used in protocol version 2 | |
392 | for data integrity protection. | |
393 | Multiple algorithms must be comma-separated. | |
394 | The default is | |
395 | .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . | |
396 | .It Cm MaxStartups | |
397 | Specifies the maximum number of concurrent unauthenticated connections to the | |
398 | .Nm sshd | |
399 | daemon. | |
400 | Additional connections will be dropped until authentication succeeds or the | |
401 | .Cm LoginGraceTime | |
402 | expires for a connection. | |
403 | The default is 10. | |
404 | .Pp | |
405 | Alternatively, random early drop can be enabled by specifying | |
406 | the three colon separated values | |
407 | .Dq start:rate:full | |
408 | (e.g., "10:30:60"). | |
409 | .Nm sshd | |
410 | will refuse connection attempts with a probability of | |
411 | .Dq rate/100 | |
412 | (30%) | |
413 | if there are currently | |
414 | .Dq start | |
415 | (10) | |
416 | unauthenticated connections. | |
417 | The probability increases linearly and all connection attempts | |
418 | are refused if the number of unauthenticated connections reaches | |
419 | .Dq full | |
420 | (60). | |
ff2d7a98 | 421 | .It Cm PasswordAuthentication |
422 | Specifies whether password authentication is allowed. | |
423 | The default is | |
424 | .Dq yes . | |
425 | .It Cm PermitEmptyPasswords | |
426 | When password authentication is allowed, it specifies whether the | |
427 | server allows login to accounts with empty password strings. | |
428 | The default is | |
429 | .Dq no . | |
430 | .It Cm PermitRootLogin | |
431 | Specifies whether root can login using | |
432 | .Xr ssh 1 . | |
433 | The argument must be | |
434 | .Dq yes , | |
435 | .Dq without-password , | |
436 | .Dq forced-commands-only | |
437 | or | |
438 | .Dq no . | |
439 | The default is | |
440 | .Dq yes . | |
441 | .Pp | |
442 | If this option is set to | |
443 | .Dq without-password | |
444 | password authentication is disabled for root. | |
445 | .Pp | |
446 | If this option is set to | |
447 | .Dq forced-commands-only | |
448 | root login with public key authentication will be allowed, | |
449 | but only if the | |
450 | .Ar command | |
451 | option has been specified | |
452 | (which may be useful for taking remote backups even if root login is | |
70791e56 | 453 | normally not allowed). |
454 | All other authentication methods are disabled for root. | |
ff2d7a98 | 455 | .Pp |
456 | If this option is set to | |
457 | .Dq no | |
458 | root is not allowed to login. | |
e54b3d7c | 459 | .It Cm PermitUserEnvironment |
460 | Specifies whether | |
461 | .Pa ~/.ssh/environment | |
462 | and | |
463 | .Cm environment= | |
464 | options in | |
465 | .Pa ~/.ssh/authorized_keys | |
466 | are processed by | |
467 | .Nm sshd . | |
468 | The default is | |
469 | .Dq no . | |
470 | Enabling environment processing may enable users to bypass access | |
471 | restrictions in some configurations using mechanisms such as | |
472 | .Ev LD_PRELOAD . | |
ff2d7a98 | 473 | .It Cm PidFile |
474 | Specifies the file that contains the process ID of the | |
475 | .Nm sshd | |
476 | daemon. | |
477 | The default is | |
478 | .Pa /var/run/sshd.pid . | |
479 | .It Cm Port | |
480 | Specifies the port number that | |
481 | .Nm sshd | |
482 | listens on. | |
483 | The default is 22. | |
484 | Multiple options of this type are permitted. | |
485 | See also | |
486 | .Cm ListenAddress . | |
487 | .It Cm PrintLastLog | |
488 | Specifies whether | |
489 | .Nm sshd | |
490 | should print the date and time when the user last logged in. | |
491 | The default is | |
492 | .Dq yes . | |
493 | .It Cm PrintMotd | |
494 | Specifies whether | |
495 | .Nm sshd | |
496 | should print | |
497 | .Pa /etc/motd | |
498 | when a user logs in interactively. | |
499 | (On some systems it is also printed by the shell, | |
500 | .Pa /etc/profile , | |
501 | or equivalent.) | |
502 | The default is | |
503 | .Dq yes . | |
504 | .It Cm Protocol | |
505 | Specifies the protocol versions | |
506 | .Nm sshd | |
e54b3d7c | 507 | supports. |
ff2d7a98 | 508 | The possible values are |
509 | .Dq 1 | |
510 | and | |
511 | .Dq 2 . | |
512 | Multiple versions must be comma-separated. | |
513 | The default is | |
514 | .Dq 2,1 . | |
e54b3d7c | 515 | Note that the order of the protocol list does not indicate preference, |
516 | because the client selects among multiple protocol versions offered | |
517 | by the server. | |
518 | Specifying | |
519 | .Dq 2,1 | |
520 | is identical to | |
521 | .Dq 1,2 . | |
ff2d7a98 | 522 | .It Cm PubkeyAuthentication |
523 | Specifies whether public key authentication is allowed. | |
524 | The default is | |
525 | .Dq yes . | |
526 | Note that this option applies to protocol version 2 only. | |
ff2d7a98 | 527 | .It Cm RhostsRSAAuthentication |
528 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
529 | with successful RSA host authentication is allowed. | |
530 | The default is | |
531 | .Dq no . | |
532 | This option applies to protocol version 1 only. | |
533 | .It Cm RSAAuthentication | |
534 | Specifies whether pure RSA authentication is allowed. | |
535 | The default is | |
536 | .Dq yes . | |
537 | This option applies to protocol version 1 only. | |
538 | .It Cm ServerKeyBits | |
539 | Defines the number of bits in the ephemeral protocol version 1 server key. | |
540 | The minimum value is 512, and the default is 768. | |
541 | .It Cm StrictModes | |
542 | Specifies whether | |
543 | .Nm sshd | |
544 | should check file modes and ownership of the | |
545 | user's files and home directory before accepting login. | |
546 | This is normally desirable because novices sometimes accidentally leave their | |
547 | directory or files world-writable. | |
548 | The default is | |
549 | .Dq yes . | |
550 | .It Cm Subsystem | |
551 | Configures an external subsystem (e.g., file transfer daemon). | |
552 | Arguments should be a subsystem name and a command to execute upon subsystem | |
553 | request. | |
554 | The command | |
555 | .Xr sftp-server 8 | |
556 | implements the | |
557 | .Dq sftp | |
558 | file transfer subsystem. | |
559 | By default no subsystems are defined. | |
560 | Note that this option applies to protocol version 2 only. | |
561 | .It Cm SyslogFacility | |
562 | Gives the facility code that is used when logging messages from | |
563 | .Nm sshd . | |
564 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, | |
565 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. | |
566 | The default is AUTH. | |
416fd2a8 | 567 | .It Cm TCPKeepAlive |
568 | Specifies whether the system should send TCP keepalive messages to the | |
569 | other side. | |
570 | If they are sent, death of the connection or crash of one | |
571 | of the machines will be properly noticed. | |
572 | However, this means that | |
573 | connections will die if the route is down temporarily, and some people | |
574 | find it annoying. | |
575 | On the other hand, if TCP keepalives are not sent, | |
576 | sessions may hang indefinitely on the server, leaving | |
577 | .Dq ghost | |
578 | users and consuming server resources. | |
579 | .Pp | |
580 | The default is | |
581 | .Dq yes | |
582 | (to send TCP keepalive messages), and the server will notice | |
583 | if the network goes down or the client host crashes. | |
584 | This avoids infinitely hanging sessions. | |
585 | .Pp | |
586 | To disable TCP keepalive messages, the value should be set to | |
587 | .Dq no . | |
70791e56 | 588 | .It Cm UseDNS |
589 | Specifies whether | |
590 | .Nm sshd | |
591 | should lookup the remote host name and check that | |
592 | the resolved host name for the remote IP address maps back to the | |
593 | very same IP address. | |
594 | The default is | |
595 | .Dq yes . | |
ff2d7a98 | 596 | .It Cm UseLogin |
597 | Specifies whether | |
598 | .Xr login 1 | |
599 | is used for interactive login sessions. | |
600 | The default is | |
601 | .Dq no . | |
602 | Note that | |
603 | .Xr login 1 | |
604 | is never used for remote command execution. | |
605 | Note also, that if this is enabled, | |
606 | .Cm X11Forwarding | |
607 | will be disabled because | |
608 | .Xr login 1 | |
609 | does not know how to handle | |
610 | .Xr xauth 1 | |
1c14df9e | 611 | cookies. |
612 | If | |
ff2d7a98 | 613 | .Cm UsePrivilegeSeparation |
614 | is specified, it will be disabled after authentication. | |
70791e56 | 615 | .It Cm UsePAM |
416fd2a8 | 616 | Enables PAM authentication (via challenge-response) and session set up. |
617 | If you enable this, you should probably disable | |
70791e56 | 618 | .Cm PasswordAuthentication . |
416fd2a8 | 619 | If you enable |
70791e56 | 620 | .CM UsePAM |
416fd2a8 | 621 | then you will not be able to run sshd as a non-root user. The default is |
622 | .Dq no . | |
ff2d7a98 | 623 | .It Cm UsePrivilegeSeparation |
624 | Specifies whether | |
625 | .Nm sshd | |
626 | separates privileges by creating an unprivileged child process | |
1c14df9e | 627 | to deal with incoming network traffic. |
628 | After successful authentication, another process will be created that has | |
629 | the privilege of the authenticated user. | |
630 | The goal of privilege separation is to prevent privilege | |
ff2d7a98 | 631 | escalation by containing any corruption within the unprivileged processes. |
632 | The default is | |
633 | .Dq yes . | |
ff2d7a98 | 634 | .It Cm X11DisplayOffset |
635 | Specifies the first display number available for | |
636 | .Nm sshd Ns 's | |
637 | X11 forwarding. | |
638 | This prevents | |
639 | .Nm sshd | |
640 | from interfering with real X11 servers. | |
641 | The default is 10. | |
642 | .It Cm X11Forwarding | |
643 | Specifies whether X11 forwarding is permitted. | |
e54b3d7c | 644 | The argument must be |
645 | .Dq yes | |
646 | or | |
647 | .Dq no . | |
ff2d7a98 | 648 | The default is |
649 | .Dq no . | |
e54b3d7c | 650 | .Pp |
651 | When X11 forwarding is enabled, there may be additional exposure to | |
652 | the server and to client displays if the | |
653 | .Nm sshd | |
654 | proxy display is configured to listen on the wildcard address (see | |
655 | .Cm X11UseLocalhost | |
656 | below), however this is not the default. | |
657 | Additionally, the authentication spoofing and authentication data | |
658 | verification and substitution occur on the client side. | |
659 | The security risk of using X11 forwarding is that the client's X11 | |
660 | display server may be exposed to attack when the ssh client requests | |
661 | forwarding (see the warnings for | |
662 | .Cm ForwardX11 | |
663 | in | |
70791e56 | 664 | .Xr ssh_config 5 ) . |
e54b3d7c | 665 | A system administrator may have a stance in which they want to |
666 | protect clients that may expose themselves to attack by unwittingly | |
667 | requesting X11 forwarding, which can warrant a | |
668 | .Dq no | |
669 | setting. | |
670 | .Pp | |
671 | Note that disabling X11 forwarding does not prevent users from | |
672 | forwarding X11 traffic, as users can always install their own forwarders. | |
ff2d7a98 | 673 | X11 forwarding is automatically disabled if |
674 | .Cm UseLogin | |
675 | is enabled. | |
676 | .It Cm X11UseLocalhost | |
677 | Specifies whether | |
678 | .Nm sshd | |
679 | should bind the X11 forwarding server to the loopback address or to | |
1c14df9e | 680 | the wildcard address. |
681 | By default, | |
ff2d7a98 | 682 | .Nm sshd |
683 | binds the forwarding server to the loopback address and sets the | |
684 | hostname part of the | |
685 | .Ev DISPLAY | |
686 | environment variable to | |
687 | .Dq localhost . | |
e54b3d7c | 688 | This prevents remote hosts from connecting to the proxy display. |
ff2d7a98 | 689 | However, some older X11 clients may not function with this |
690 | configuration. | |
691 | .Cm X11UseLocalhost | |
692 | may be set to | |
693 | .Dq no | |
694 | to specify that the forwarding server should be bound to the wildcard | |
695 | address. | |
696 | The argument must be | |
697 | .Dq yes | |
698 | or | |
699 | .Dq no . | |
700 | The default is | |
701 | .Dq yes . | |
702 | .It Cm XAuthLocation | |
e54b3d7c | 703 | Specifies the full pathname of the |
ff2d7a98 | 704 | .Xr xauth 1 |
705 | program. | |
706 | The default is | |
707 | .Pa /usr/X11R6/bin/xauth . | |
708 | .El | |
709 | .Ss Time Formats | |
ff2d7a98 | 710 | .Nm sshd |
711 | command-line arguments and configuration file options that specify time | |
712 | may be expressed using a sequence of the form: | |
713 | .Sm off | |
e54b3d7c | 714 | .Ar time Op Ar qualifier , |
ff2d7a98 | 715 | .Sm on |
716 | where | |
717 | .Ar time | |
718 | is a positive integer value and | |
719 | .Ar qualifier | |
720 | is one of the following: | |
721 | .Pp | |
722 | .Bl -tag -width Ds -compact -offset indent | |
723 | .It Cm <none> | |
724 | seconds | |
725 | .It Cm s | Cm S | |
726 | seconds | |
727 | .It Cm m | Cm M | |
728 | minutes | |
729 | .It Cm h | Cm H | |
730 | hours | |
731 | .It Cm d | Cm D | |
732 | days | |
733 | .It Cm w | Cm W | |
734 | weeks | |
735 | .El | |
736 | .Pp | |
737 | Each member of the sequence is added together to calculate | |
738 | the total time value. | |
739 | .Pp | |
740 | Time format examples: | |
741 | .Pp | |
742 | .Bl -tag -width Ds -compact -offset indent | |
743 | .It 600 | |
744 | 600 seconds (10 minutes) | |
745 | .It 10m | |
746 | 10 minutes | |
747 | .It 1h30m | |
748 | 1 hour 30 minutes (90 minutes) | |
749 | .El | |
750 | .Sh FILES | |
751 | .Bl -tag -width Ds | |
752 | .It Pa /etc/ssh/sshd_config | |
753 | Contains configuration data for | |
754 | .Nm sshd . | |
755 | This file should be writable by root only, but it is recommended | |
756 | (though not necessary) that it be world-readable. | |
757 | .El | |
70791e56 | 758 | .Sh SEE ALSO |
759 | .Xr sshd 8 | |
ff2d7a98 | 760 | .Sh AUTHORS |
761 | OpenSSH is a derivative of the original and free | |
762 | ssh 1.2.12 release by Tatu Ylonen. | |
763 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
764 | Theo de Raadt and Dug Song | |
765 | removed many bugs, re-added newer features and | |
766 | created OpenSSH. | |
767 | Markus Friedl contributed the support for SSH | |
768 | protocol versions 1.5 and 2.0. | |
769 | Niels Provos and Markus Friedl contributed support | |
770 | for privilege separation. |