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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 | .\" | |
4895f836 | 37 | .\" $OpenBSD: sshd_config.5,v 1.65 2006/07/18 08:22:23 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 | |
32cfd177 | 49 | .Xr sshd 8 |
588df31a | 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. | |
533b9133 | 59 | Arguments may optionally be enclosed in double quotes |
60 | .Pq \&" | |
61 | in order to represent arguments containing spaces. | |
588df31a | 62 | .Pp |
63 | The possible | |
64 | keywords and their meanings are as follows (note that | |
65 | keywords are case-insensitive and arguments are case-sensitive): | |
66 | .Bl -tag -width Ds | |
61a2c1da | 67 | .It Cm AcceptEnv |
68 | Specifies what environment variables sent by the client will be copied into | |
69 | the session's | |
70 | .Xr environ 7 . | |
71 | See | |
72 | .Cm SendEnv | |
73 | in | |
74 | .Xr ssh_config 5 | |
75 | for how to configure the client. | |
b8b9f2e6 | 76 | Note that environment passing is only supported for protocol 2. |
61a2c1da | 77 | Variables are specified by name, which may contain the wildcard characters |
f09ffbdb | 78 | .Ql * |
61a2c1da | 79 | and |
80 | .Ql \&? . | |
b8b9f2e6 | 81 | Multiple environment variables may be separated by whitespace or spread |
61a2c1da | 82 | across multiple |
83 | .Cm AcceptEnv | |
84 | directives. | |
b8b9f2e6 | 85 | Be warned that some environment variables could be used to bypass restricted |
61a2c1da | 86 | user environments. |
87 | For this reason, care should be taken in the use of this directive. | |
88 | The default is not to accept any environment variables. | |
31b41ceb | 89 | .It Cm AddressFamily |
90 | Specifies which address family should be used by | |
32cfd177 | 91 | .Xr sshd 8 . |
31b41ceb | 92 | Valid arguments are |
93 | .Dq any , | |
94 | .Dq inet | |
b74c3b8c | 95 | (use IPv4 only), or |
31b41ceb | 96 | .Dq inet6 |
97 | (use IPv6 only). | |
98 | The default is | |
99 | .Dq any . | |
588df31a | 100 | .It Cm AllowGroups |
101 | This keyword can be followed by a list of group name patterns, separated | |
102 | by spaces. | |
103 | If specified, login is allowed only for users whose primary | |
104 | group or supplementary group list matches one of the patterns. | |
588df31a | 105 | Only group names are valid; a numerical group ID is not recognized. |
106 | By default, login is allowed for all groups. | |
84c1b530 | 107 | The allow/deny directives are processed in the following order: |
108 | .Cm DenyUsers , | |
109 | .Cm AllowUsers , | |
110 | .Cm DenyGroups , | |
111 | and finally | |
112 | .Cm AllowGroups . | |
ac1ec4d8 | 113 | .Pp |
114 | See | |
115 | .Sx PATTERNS | |
116 | in | |
117 | .Xr ssh_config 5 | |
118 | for more information on patterns. | |
588df31a | 119 | .It Cm AllowTcpForwarding |
120 | Specifies whether TCP forwarding is permitted. | |
121 | The default is | |
122 | .Dq yes . | |
123 | Note that disabling TCP forwarding does not improve security unless | |
124 | users are also denied shell access, as they can always install their | |
125 | own forwarders. | |
588df31a | 126 | .It Cm AllowUsers |
127 | This keyword can be followed by a list of user name patterns, separated | |
128 | by spaces. | |
1be697b6 | 129 | If specified, login is allowed only for user names that |
588df31a | 130 | match one of the patterns. |
588df31a | 131 | Only user names are valid; a numerical user ID is not recognized. |
132 | By default, login is allowed for all users. | |
133 | If the pattern takes the form USER@HOST then USER and HOST | |
134 | are separately checked, restricting logins to particular | |
135 | users from particular hosts. | |
84c1b530 | 136 | The allow/deny directives are processed in the following order: |
137 | .Cm DenyUsers , | |
138 | .Cm AllowUsers , | |
139 | .Cm DenyGroups , | |
140 | and finally | |
141 | .Cm AllowGroups . | |
ac1ec4d8 | 142 | .Pp |
143 | See | |
144 | .Sx PATTERNS | |
145 | in | |
146 | .Xr ssh_config 5 | |
147 | for more information on patterns. | |
588df31a | 148 | .It Cm AuthorizedKeysFile |
149 | Specifies the file that contains the public keys that can be used | |
150 | for user authentication. | |
151 | .Cm AuthorizedKeysFile | |
152 | may contain tokens of the form %T which are substituted during connection | |
b74c3b8c | 153 | setup. |
3cbc677d | 154 | The following tokens are defined: %% is replaced by a literal '%', |
b74c3b8c | 155 | %h is replaced by the home directory of the user being authenticated, and |
588df31a | 156 | %u is replaced by the username of that user. |
157 | After expansion, | |
158 | .Cm AuthorizedKeysFile | |
159 | is taken to be an absolute path or one relative to the user's home | |
160 | directory. | |
161 | The default is | |
162 | .Dq .ssh/authorized_keys . | |
163 | .It Cm Banner | |
164 | In some jurisdictions, sending a warning message before authentication | |
165 | may be relevant for getting legal protection. | |
166 | The contents of the specified file are sent to the remote user before | |
167 | authentication is allowed. | |
168 | This option is only available for protocol version 2. | |
169 | By default, no banner is displayed. | |
588df31a | 170 | .It Cm ChallengeResponseAuthentication |
340a4caf | 171 | Specifies whether challenge-response authentication is allowed. |
588df31a | 172 | All authentication styles from |
173 | .Xr login.conf 5 | |
174 | are supported. | |
175 | The default is | |
176 | .Dq yes . | |
177 | .It Cm Ciphers | |
178 | Specifies the ciphers allowed for protocol version 2. | |
179 | Multiple ciphers must be comma-separated. | |
3b9baa7b | 180 | The supported ciphers are |
181 | .Dq 3des-cbc , | |
182 | .Dq aes128-cbc , | |
183 | .Dq aes192-cbc , | |
184 | .Dq aes256-cbc , | |
185 | .Dq aes128-ctr , | |
186 | .Dq aes192-ctr , | |
187 | .Dq aes256-ctr , | |
74a66cc8 | 188 | .Dq arcfour128 , |
189 | .Dq arcfour256 , | |
3b9baa7b | 190 | .Dq arcfour , |
191 | .Dq blowfish-cbc , | |
192 | and | |
193 | .Dq cast128-cbc . | |
b74c3b8c | 194 | The default is: |
195 | .Bd -literal -offset 3n | |
196 | aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, | |
197 | arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, | |
198 | aes192-ctr,aes256-ctr | |
588df31a | 199 | .Ed |
588df31a | 200 | .It Cm ClientAliveCountMax |
79e46360 | 201 | Sets the number of client alive messages (see below) which may be |
588df31a | 202 | sent without |
b74c3b8c | 203 | .Xr sshd 8 |
3cbc677d | 204 | receiving any messages back from the client. |
205 | If this threshold is reached while client alive messages are being sent, | |
b74c3b8c | 206 | sshd will disconnect the client, terminating the session. |
3cbc677d | 207 | It is important to note that the use of client alive messages is very |
208 | different from | |
fd573618 | 209 | .Cm TCPKeepAlive |
3cbc677d | 210 | (below). |
211 | The client alive messages are sent through the encrypted channel | |
212 | and therefore will not be spoofable. | |
213 | The TCP keepalive option enabled by | |
fd573618 | 214 | .Cm TCPKeepAlive |
3cbc677d | 215 | is spoofable. |
216 | The client alive mechanism is valuable when the client or | |
588df31a | 217 | server depend on knowing when a connection has become inactive. |
218 | .Pp | |
3cbc677d | 219 | The default value is 3. |
220 | If | |
588df31a | 221 | .Cm ClientAliveInterval |
79e46360 | 222 | (see below) is set to 15, and |
588df31a | 223 | .Cm ClientAliveCountMax |
b74c3b8c | 224 | is left at the default, unresponsive SSH clients |
588df31a | 225 | will be disconnected after approximately 45 seconds. |
2d762582 | 226 | This option applies to protocol version 2 only. |
2264526c | 227 | .It Cm ClientAliveInterval |
228 | Sets a timeout interval in seconds after which if no data has been received | |
229 | from the client, | |
b74c3b8c | 230 | .Xr sshd 8 |
2264526c | 231 | will send a message through the encrypted |
232 | channel to request a response from the client. | |
233 | The default | |
234 | is 0, indicating that these messages will not be sent to the client. | |
235 | This option applies to protocol version 2 only. | |
588df31a | 236 | .It Cm Compression |
07200973 | 237 | Specifies whether compression is allowed, or delayed until |
238 | the user has authenticated successfully. | |
588df31a | 239 | The argument must be |
07200973 | 240 | .Dq yes , |
241 | .Dq delayed , | |
588df31a | 242 | or |
243 | .Dq no . | |
244 | The default is | |
07200973 | 245 | .Dq delayed . |
588df31a | 246 | .It Cm DenyGroups |
247 | This keyword can be followed by a list of group name patterns, separated | |
248 | by spaces. | |
249 | Login is disallowed for users whose primary group or supplementary | |
250 | group list matches one of the patterns. | |
588df31a | 251 | Only group names are valid; a numerical group ID is not recognized. |
252 | By default, login is allowed for all groups. | |
84c1b530 | 253 | The allow/deny directives are processed in the following order: |
254 | .Cm DenyUsers , | |
255 | .Cm AllowUsers , | |
256 | .Cm DenyGroups , | |
257 | and finally | |
258 | .Cm AllowGroups . | |
ac1ec4d8 | 259 | .Pp |
260 | See | |
261 | .Sx PATTERNS | |
262 | in | |
263 | .Xr ssh_config 5 | |
264 | for more information on patterns. | |
588df31a | 265 | .It Cm DenyUsers |
266 | This keyword can be followed by a list of user name patterns, separated | |
267 | by spaces. | |
268 | Login is disallowed for user names that match one of the patterns. | |
588df31a | 269 | Only user names are valid; a numerical user ID is not recognized. |
270 | By default, login is allowed for all users. | |
271 | If the pattern takes the form USER@HOST then USER and HOST | |
272 | are separately checked, restricting logins to particular | |
273 | users from particular hosts. | |
84c1b530 | 274 | The allow/deny directives are processed in the following order: |
275 | .Cm DenyUsers , | |
276 | .Cm AllowUsers , | |
277 | .Cm DenyGroups , | |
278 | and finally | |
279 | .Cm AllowGroups . | |
ac1ec4d8 | 280 | .Pp |
281 | See | |
282 | .Sx PATTERNS | |
283 | in | |
284 | .Xr ssh_config 5 | |
285 | for more information on patterns. | |
588df31a | 286 | .It Cm GatewayPorts |
287 | Specifies whether remote hosts are allowed to connect to ports | |
288 | forwarded for the client. | |
289 | By default, | |
b74c3b8c | 290 | .Xr sshd 8 |
a4e5acef | 291 | binds remote port forwardings to the loopback address. |
292 | This prevents other remote hosts from connecting to forwarded ports. | |
588df31a | 293 | .Cm GatewayPorts |
b74c3b8c | 294 | can be used to specify that sshd |
3867aa0a | 295 | should allow remote port forwardings to bind to non-loopback addresses, thus |
296 | allowing other hosts to connect. | |
297 | The argument may be | |
298 | .Dq no | |
299 | to force remote port forwardings to be available to the local host only, | |
588df31a | 300 | .Dq yes |
3867aa0a | 301 | to force remote port forwardings to bind to the wildcard address, or |
302 | .Dq clientspecified | |
303 | to allow the client to select the address to which the forwarding is bound. | |
588df31a | 304 | The default is |
305 | .Dq no . | |
7364bd04 | 306 | .It Cm GSSAPIAuthentication |
105b07db | 307 | Specifies whether user authentication based on GSSAPI is allowed. |
aff51935 | 308 | The default is |
7364bd04 | 309 | .Dq no . |
310 | Note that this option applies to protocol version 2 only. | |
311 | .It Cm GSSAPICleanupCredentials | |
312 | Specifies whether to automatically destroy the user's credentials cache | |
313 | on logout. | |
314 | The default is | |
315 | .Dq yes . | |
316 | Note that this option applies to protocol version 2 only. | |
588df31a | 317 | .It Cm HostbasedAuthentication |
318 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
319 | with successful public key client host authentication is allowed | |
340a4caf | 320 | (host-based authentication). |
588df31a | 321 | This option is similar to |
322 | .Cm RhostsRSAAuthentication | |
323 | and applies to protocol version 2 only. | |
324 | The default is | |
325 | .Dq no . | |
326 | .It Cm HostKey | |
327 | Specifies a file containing a private host key | |
328 | used by SSH. | |
329 | The default is | |
330 | .Pa /etc/ssh/ssh_host_key | |
331 | for protocol version 1, and | |
332 | .Pa /etc/ssh/ssh_host_rsa_key | |
333 | and | |
334 | .Pa /etc/ssh/ssh_host_dsa_key | |
335 | for protocol version 2. | |
336 | Note that | |
b74c3b8c | 337 | .Xr sshd 8 |
588df31a | 338 | will refuse to use a file if it is group/world-accessible. |
339 | It is possible to have multiple host key files. | |
340 | .Dq rsa1 | |
341 | keys are used for version 1 and | |
342 | .Dq dsa | |
343 | or | |
344 | .Dq rsa | |
345 | are used for version 2 of the SSH protocol. | |
346 | .It Cm IgnoreRhosts | |
347 | Specifies that | |
348 | .Pa .rhosts | |
349 | and | |
350 | .Pa .shosts | |
351 | files will not be used in | |
588df31a | 352 | .Cm RhostsRSAAuthentication |
353 | or | |
354 | .Cm HostbasedAuthentication . | |
355 | .Pp | |
356 | .Pa /etc/hosts.equiv | |
357 | and | |
358 | .Pa /etc/shosts.equiv | |
359 | are still used. | |
360 | The default is | |
361 | .Dq yes . | |
362 | .It Cm IgnoreUserKnownHosts | |
363 | Specifies whether | |
b74c3b8c | 364 | .Xr sshd 8 |
588df31a | 365 | should ignore the user's |
140e3e97 | 366 | .Pa ~/.ssh/known_hosts |
588df31a | 367 | during |
368 | .Cm RhostsRSAAuthentication | |
369 | or | |
370 | .Cm HostbasedAuthentication . | |
371 | The default is | |
372 | .Dq no . | |
588df31a | 373 | .It Cm KerberosAuthentication |
8f73f7bb | 374 | Specifies whether the password provided by the user for |
588df31a | 375 | .Cm PasswordAuthentication |
8f73f7bb | 376 | will be validated through the Kerberos KDC. |
588df31a | 377 | To use this option, the server needs a |
378 | Kerberos servtab which allows the verification of the KDC's identity. | |
b74c3b8c | 379 | The default is |
588df31a | 380 | .Dq no . |
24f37810 | 381 | .It Cm KerberosGetAFSToken |
0d3d1077 | 382 | If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire |
24f37810 | 383 | an AFS token before accessing the user's home directory. |
b74c3b8c | 384 | The default is |
24f37810 | 385 | .Dq no . |
588df31a | 386 | .It Cm KerberosOrLocalPasswd |
b74c3b8c | 387 | If password authentication through Kerberos fails then |
588df31a | 388 | the password will be validated via any additional local mechanism |
389 | such as | |
390 | .Pa /etc/passwd . | |
b74c3b8c | 391 | The default is |
588df31a | 392 | .Dq yes . |
588df31a | 393 | .It Cm KerberosTicketCleanup |
394 | Specifies whether to automatically destroy the user's ticket cache | |
395 | file on logout. | |
b74c3b8c | 396 | The default is |
588df31a | 397 | .Dq yes . |
398 | .It Cm KeyRegenerationInterval | |
399 | In protocol version 1, the ephemeral server key is automatically regenerated | |
400 | after this many seconds (if it has been used). | |
401 | The purpose of regeneration is to prevent | |
402 | decrypting captured sessions by later breaking into the machine and | |
403 | stealing the keys. | |
404 | The key is never stored anywhere. | |
405 | If the value is 0, the key is never regenerated. | |
406 | The default is 3600 (seconds). | |
407 | .It Cm ListenAddress | |
408 | Specifies the local addresses | |
b74c3b8c | 409 | .Xr sshd 8 |
588df31a | 410 | should listen on. |
411 | The following forms may be used: | |
412 | .Pp | |
413 | .Bl -item -offset indent -compact | |
414 | .It | |
415 | .Cm ListenAddress | |
416 | .Sm off | |
417 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr | |
418 | .Sm on | |
419 | .It | |
420 | .Cm ListenAddress | |
421 | .Sm off | |
422 | .Ar host No | Ar IPv4_addr No : Ar port | |
423 | .Sm on | |
424 | .It | |
425 | .Cm ListenAddress | |
426 | .Sm off | |
427 | .Oo | |
428 | .Ar host No | Ar IPv6_addr Oc : Ar port | |
429 | .Sm on | |
430 | .El | |
431 | .Pp | |
432 | If | |
433 | .Ar port | |
434 | is not specified, | |
b74c3b8c | 435 | sshd will listen on the address and all prior |
588df31a | 436 | .Cm Port |
3cbc677d | 437 | options specified. |
438 | The default is to listen on all local addresses. | |
a4e5acef | 439 | Multiple |
588df31a | 440 | .Cm ListenAddress |
3cbc677d | 441 | options are permitted. |
442 | Additionally, any | |
588df31a | 443 | .Cm Port |
b74c3b8c | 444 | options must precede this option for non-port qualified addresses. |
588df31a | 445 | .It Cm LoginGraceTime |
446 | The server disconnects after this time if the user has not | |
447 | successfully logged in. | |
448 | If the value is 0, there is no time limit. | |
3445ca02 | 449 | The default is 120 seconds. |
588df31a | 450 | .It Cm LogLevel |
451 | Gives the verbosity level that is used when logging messages from | |
32cfd177 | 452 | .Xr sshd 8 . |
588df31a | 453 | The possible values are: |
b74c3b8c | 454 | QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. |
a4e5acef | 455 | The default is INFO. |
456 | DEBUG and DEBUG1 are equivalent. | |
457 | DEBUG2 and DEBUG3 each specify higher levels of debugging output. | |
458 | Logging with a DEBUG level violates the privacy of users and is not recommended. | |
588df31a | 459 | .It Cm MACs |
460 | Specifies the available MAC (message authentication code) algorithms. | |
461 | The MAC algorithm is used in protocol version 2 | |
462 | for data integrity protection. | |
463 | Multiple algorithms must be comma-separated. | |
b74c3b8c | 464 | The default is: |
588df31a | 465 | .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . |
d231781a | 466 | .It Cm Match |
14e980ef | 467 | Introduces a conditional block. |
4895f836 | 468 | If all of the criteria on the |
d231781a | 469 | .Cm Match |
4895f836 | 470 | line are satisfied, the keywords on the following lines override those |
471 | set in the global section of the config file, until either another | |
d231781a | 472 | .Cm Match |
4895f836 | 473 | line or the end of the file. |
14e980ef | 474 | The arguments to |
d231781a | 475 | .Cm Match |
4895f836 | 476 | are one or more criteria-pattern pairs. |
d231781a | 477 | The available criteria are |
478 | .Cm User , | |
479 | .Cm Host , | |
480 | and | |
481 | .Cm Address . | |
482 | Only a subset of keywords may be used on the lines following a | |
483 | .Cm Match | |
484 | keyword. | |
485 | Available keywords are | |
2fefbadf | 486 | .Cm AllowTcpForwarding , |
487 | .Cm GatewayPorts , | |
d231781a | 488 | and |
2fefbadf | 489 | .Cm PermitOpen . |
af4bd935 | 490 | .It Cm MaxAuthTries |
491 | Specifies the maximum number of authentication attempts permitted per | |
8fca654b | 492 | connection. |
493 | Once the number of failures reaches half this value, | |
494 | additional failures are logged. | |
495 | The default is 6. | |
588df31a | 496 | .It Cm MaxStartups |
497 | Specifies the maximum number of concurrent unauthenticated connections to the | |
b74c3b8c | 498 | SSH daemon. |
588df31a | 499 | Additional connections will be dropped until authentication succeeds or the |
500 | .Cm LoginGraceTime | |
501 | expires for a connection. | |
502 | The default is 10. | |
503 | .Pp | |
504 | Alternatively, random early drop can be enabled by specifying | |
505 | the three colon separated values | |
506 | .Dq start:rate:full | |
f09ffbdb | 507 | (e.g. "10:30:60"). |
32cfd177 | 508 | .Xr sshd 8 |
588df31a | 509 | will refuse connection attempts with a probability of |
510 | .Dq rate/100 | |
511 | (30%) | |
512 | if there are currently | |
513 | .Dq start | |
514 | (10) | |
515 | unauthenticated connections. | |
516 | The probability increases linearly and all connection attempts | |
517 | are refused if the number of unauthenticated connections reaches | |
518 | .Dq full | |
519 | (60). | |
520 | .It Cm PasswordAuthentication | |
521 | Specifies whether password authentication is allowed. | |
522 | The default is | |
523 | .Dq yes . | |
524 | .It Cm PermitEmptyPasswords | |
525 | When password authentication is allowed, it specifies whether the | |
526 | server allows login to accounts with empty password strings. | |
527 | The default is | |
528 | .Dq no . | |
2fefbadf | 529 | .It Cm PermitOpen |
530 | Specifies the destinations to which TCP port forwarding is permitted. | |
531 | The forwarding specification must be one of the following forms: | |
532 | .Pp | |
533 | .Bl -item -offset indent -compact | |
534 | .It | |
535 | .Cm PermitOpen | |
536 | .Sm off | |
537 | .Ar host : port | |
538 | .Sm on | |
539 | .It | |
540 | .Cm PermitOpen | |
541 | .Sm off | |
542 | .Ar IPv4_addr : port | |
543 | .Sm on | |
544 | .It | |
545 | .Cm PermitOpen | |
546 | .Sm off | |
547 | .Ar \&[ IPv6_addr \&] : port | |
548 | .Sm on | |
549 | .El | |
550 | .Pp | |
551 | Multiple instances of | |
552 | .Cm PermitOpen | |
553 | are permitted. | |
554 | An argument of | |
555 | .Dq any | |
556 | can be used to remove all restrictions and permit any forwarding requests. | |
f22506ff | 557 | By default all port forwarding requests are permitted. |
588df31a | 558 | .It Cm PermitRootLogin |
667e4135 | 559 | Specifies whether root can log in using |
588df31a | 560 | .Xr ssh 1 . |
561 | The argument must be | |
562 | .Dq yes , | |
563 | .Dq without-password , | |
b74c3b8c | 564 | .Dq forced-commands-only , |
588df31a | 565 | or |
566 | .Dq no . | |
567 | The default is | |
568 | .Dq yes . | |
569 | .Pp | |
570 | If this option is set to | |
b74c3b8c | 571 | .Dq without-password , |
8a4c4ee4 | 572 | password authentication is disabled for root. |
588df31a | 573 | .Pp |
574 | If this option is set to | |
b74c3b8c | 575 | .Dq forced-commands-only , |
588df31a | 576 | root login with public key authentication will be allowed, |
577 | but only if the | |
578 | .Ar command | |
579 | option has been specified | |
580 | (which may be useful for taking remote backups even if root login is | |
3cbc677d | 581 | normally not allowed). |
582 | All other authentication methods are disabled for root. | |
588df31a | 583 | .Pp |
584 | If this option is set to | |
b74c3b8c | 585 | .Dq no , |
667e4135 | 586 | root is not allowed to log in. |
d20f3c9e | 587 | .It Cm PermitTunnel |
588 | Specifies whether | |
589 | .Xr tun 4 | |
590 | device forwarding is allowed. | |
a4f24bf8 | 591 | The argument must be |
592 | .Dq yes , | |
06fa4ac1 | 593 | .Dq point-to-point |
594 | (layer 3), | |
595 | .Dq ethernet | |
596 | (layer 2), or | |
a4f24bf8 | 597 | .Dq no . |
06fa4ac1 | 598 | Specifying |
599 | .Dq yes | |
600 | permits both | |
601 | .Dq point-to-point | |
602 | and | |
603 | .Dq ethernet . | |
d20f3c9e | 604 | The default is |
605 | .Dq no . | |
f00bab84 | 606 | .It Cm PermitUserEnvironment |
607 | Specifies whether | |
608 | .Pa ~/.ssh/environment | |
35453849 | 609 | and |
f00bab84 | 610 | .Cm environment= |
611 | options in | |
612 | .Pa ~/.ssh/authorized_keys | |
35453849 | 613 | are processed by |
b74c3b8c | 614 | .Xr sshd 8 . |
f00bab84 | 615 | The default is |
616 | .Dq no . | |
35453849 | 617 | Enabling environment processing may enable users to bypass access |
618 | restrictions in some configurations using mechanisms such as | |
619 | .Ev LD_PRELOAD . | |
588df31a | 620 | .It Cm PidFile |
baa08b92 | 621 | Specifies the file that contains the process ID of the |
32cfd177 | 622 | SSH daemon. |
588df31a | 623 | The default is |
624 | .Pa /var/run/sshd.pid . | |
625 | .It Cm Port | |
626 | Specifies the port number that | |
b74c3b8c | 627 | .Xr sshd 8 |
588df31a | 628 | listens on. |
629 | The default is 22. | |
630 | Multiple options of this type are permitted. | |
631 | See also | |
632 | .Cm ListenAddress . | |
633 | .It Cm PrintLastLog | |
634 | Specifies whether | |
b74c3b8c | 635 | .Xr sshd 8 |
329a8666 | 636 | should print the date and time of the last user login when a user logs |
637 | in interactively. | |
588df31a | 638 | The default is |
639 | .Dq yes . | |
640 | .It Cm PrintMotd | |
641 | Specifies whether | |
b74c3b8c | 642 | .Xr sshd 8 |
588df31a | 643 | should print |
644 | .Pa /etc/motd | |
645 | when a user logs in interactively. | |
646 | (On some systems it is also printed by the shell, | |
647 | .Pa /etc/profile , | |
648 | or equivalent.) | |
649 | The default is | |
650 | .Dq yes . | |
651 | .It Cm Protocol | |
652 | Specifies the protocol versions | |
b74c3b8c | 653 | .Xr sshd 8 |
94ad46d1 | 654 | supports. |
588df31a | 655 | The possible values are |
b74c3b8c | 656 | .Sq 1 |
588df31a | 657 | and |
b74c3b8c | 658 | .Sq 2 . |
588df31a | 659 | Multiple versions must be comma-separated. |
660 | The default is | |
661 | .Dq 2,1 . | |
94ad46d1 | 662 | Note that the order of the protocol list does not indicate preference, |
663 | because the client selects among multiple protocol versions offered | |
664 | by the server. | |
665 | Specifying | |
666 | .Dq 2,1 | |
667 | is identical to | |
668 | .Dq 1,2 . | |
588df31a | 669 | .It Cm PubkeyAuthentication |
670 | Specifies whether public key authentication is allowed. | |
671 | The default is | |
672 | .Dq yes . | |
673 | Note that this option applies to protocol version 2 only. | |
588df31a | 674 | .It Cm RhostsRSAAuthentication |
675 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
676 | with successful RSA host authentication is allowed. | |
677 | The default is | |
678 | .Dq no . | |
679 | This option applies to protocol version 1 only. | |
680 | .It Cm RSAAuthentication | |
681 | Specifies whether pure RSA authentication is allowed. | |
682 | The default is | |
683 | .Dq yes . | |
684 | This option applies to protocol version 1 only. | |
685 | .It Cm ServerKeyBits | |
686 | Defines the number of bits in the ephemeral protocol version 1 server key. | |
687 | The minimum value is 512, and the default is 768. | |
688 | .It Cm StrictModes | |
689 | Specifies whether | |
b74c3b8c | 690 | .Xr sshd 8 |
588df31a | 691 | should check file modes and ownership of the |
692 | user's files and home directory before accepting login. | |
693 | This is normally desirable because novices sometimes accidentally leave their | |
694 | directory or files world-writable. | |
695 | The default is | |
696 | .Dq yes . | |
697 | .It Cm Subsystem | |
f09ffbdb | 698 | Configures an external subsystem (e.g. file transfer daemon). |
d66ce1a1 | 699 | Arguments should be a subsystem name and a command (with optional arguments) |
700 | to execute upon subsystem request. | |
588df31a | 701 | The command |
702 | .Xr sftp-server 8 | |
703 | implements the | |
704 | .Dq sftp | |
705 | file transfer subsystem. | |
706 | By default no subsystems are defined. | |
707 | Note that this option applies to protocol version 2 only. | |
708 | .It Cm SyslogFacility | |
709 | Gives the facility code that is used when logging messages from | |
32cfd177 | 710 | .Xr sshd 8 . |
588df31a | 711 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, |
712 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. | |
713 | The default is AUTH. | |
fd573618 | 714 | .It Cm TCPKeepAlive |
715 | Specifies whether the system should send TCP keepalive messages to the | |
716 | other side. | |
717 | If they are sent, death of the connection or crash of one | |
718 | of the machines will be properly noticed. | |
719 | However, this means that | |
720 | connections will die if the route is down temporarily, and some people | |
721 | find it annoying. | |
722 | On the other hand, if TCP keepalives are not sent, | |
723 | sessions may hang indefinitely on the server, leaving | |
724 | .Dq ghost | |
725 | users and consuming server resources. | |
726 | .Pp | |
727 | The default is | |
728 | .Dq yes | |
729 | (to send TCP keepalive messages), and the server will notice | |
730 | if the network goes down or the client host crashes. | |
731 | This avoids infinitely hanging sessions. | |
732 | .Pp | |
733 | To disable TCP keepalive messages, the value should be set to | |
734 | .Dq no . | |
c5a7d788 | 735 | .It Cm UseDNS |
736 | Specifies whether | |
b74c3b8c | 737 | .Xr sshd 8 |
102c77c2 | 738 | should look up the remote host name and check that |
c5a7d788 | 739 | the resolved host name for the remote IP address maps back to the |
740 | very same IP address. | |
741 | The default is | |
742 | .Dq yes . | |
588df31a | 743 | .It Cm UseLogin |
744 | Specifies whether | |
745 | .Xr login 1 | |
746 | is used for interactive login sessions. | |
747 | The default is | |
748 | .Dq no . | |
749 | Note that | |
750 | .Xr login 1 | |
751 | is never used for remote command execution. | |
752 | Note also, that if this is enabled, | |
753 | .Cm X11Forwarding | |
754 | will be disabled because | |
755 | .Xr login 1 | |
756 | does not know how to handle | |
757 | .Xr xauth 1 | |
a4e5acef | 758 | cookies. |
759 | If | |
588df31a | 760 | .Cm UsePrivilegeSeparation |
761 | is specified, it will be disabled after authentication. | |
72c35df7 | 762 | .It Cm UsePAM |
d5c67850 | 763 | Enables the Pluggable Authentication Module interface. |
764 | If set to | |
765 | .Dq yes | |
766 | this will enable PAM authentication using | |
767 | .Cm ChallengeResponseAuthentication | |
05059810 | 768 | and |
769 | .Cm PasswordAuthentication | |
770 | in addition to PAM account and session module processing for all | |
771 | authentication types. | |
d5c67850 | 772 | .Pp |
773 | Because PAM challenge-response authentication usually serves an equivalent | |
774 | role to password authentication, you should disable either | |
775 | .Cm PasswordAuthentication | |
776 | or | |
777 | .Cm ChallengeResponseAuthentication. | |
778 | .Pp | |
779 | If | |
780 | .Cm UsePAM | |
781 | is enabled, you will not be able to run | |
782 | .Xr sshd 8 | |
783 | as a non-root user. | |
784 | The default is | |
a83a3125 | 785 | .Dq no . |
588df31a | 786 | .It Cm UsePrivilegeSeparation |
787 | Specifies whether | |
b74c3b8c | 788 | .Xr sshd 8 |
588df31a | 789 | separates privileges by creating an unprivileged child process |
a4e5acef | 790 | to deal with incoming network traffic. |
791 | After successful authentication, another process will be created that has | |
792 | the privilege of the authenticated user. | |
793 | The goal of privilege separation is to prevent privilege | |
588df31a | 794 | escalation by containing any corruption within the unprivileged processes. |
795 | The default is | |
796 | .Dq yes . | |
588df31a | 797 | .It Cm X11DisplayOffset |
798 | Specifies the first display number available for | |
b74c3b8c | 799 | .Xr sshd 8 Ns 's |
588df31a | 800 | X11 forwarding. |
b74c3b8c | 801 | This prevents sshd from interfering with real X11 servers. |
588df31a | 802 | The default is 10. |
803 | .It Cm X11Forwarding | |
804 | Specifies whether X11 forwarding is permitted. | |
e6fe1bab | 805 | The argument must be |
806 | .Dq yes | |
807 | or | |
808 | .Dq no . | |
588df31a | 809 | The default is |
810 | .Dq no . | |
e6fe1bab | 811 | .Pp |
812 | When X11 forwarding is enabled, there may be additional exposure to | |
813 | the server and to client displays if the | |
b74c3b8c | 814 | .Xr sshd 8 |
e6fe1bab | 815 | proxy display is configured to listen on the wildcard address (see |
816 | .Cm X11UseLocalhost | |
b74c3b8c | 817 | below), though this is not the default. |
e6fe1bab | 818 | Additionally, the authentication spoofing and authentication data |
819 | verification and substitution occur on the client side. | |
820 | The security risk of using X11 forwarding is that the client's X11 | |
b74c3b8c | 821 | display server may be exposed to attack when the SSH client requests |
e6fe1bab | 822 | forwarding (see the warnings for |
823 | .Cm ForwardX11 | |
824 | in | |
be193d89 | 825 | .Xr ssh_config 5 ) . |
e6fe1bab | 826 | A system administrator may have a stance in which they want to |
827 | protect clients that may expose themselves to attack by unwittingly | |
828 | requesting X11 forwarding, which can warrant a | |
829 | .Dq no | |
830 | setting. | |
831 | .Pp | |
832 | Note that disabling X11 forwarding does not prevent users from | |
833 | forwarding X11 traffic, as users can always install their own forwarders. | |
588df31a | 834 | X11 forwarding is automatically disabled if |
835 | .Cm UseLogin | |
836 | is enabled. | |
837 | .It Cm X11UseLocalhost | |
838 | Specifies whether | |
b74c3b8c | 839 | .Xr sshd 8 |
588df31a | 840 | should bind the X11 forwarding server to the loopback address or to |
a4e5acef | 841 | the wildcard address. |
842 | By default, | |
b74c3b8c | 843 | sshd binds the forwarding server to the loopback address and sets the |
588df31a | 844 | hostname part of the |
845 | .Ev DISPLAY | |
846 | environment variable to | |
847 | .Dq localhost . | |
b3641662 | 848 | This prevents remote hosts from connecting to the proxy display. |
588df31a | 849 | However, some older X11 clients may not function with this |
850 | configuration. | |
851 | .Cm X11UseLocalhost | |
852 | may be set to | |
853 | .Dq no | |
854 | to specify that the forwarding server should be bound to the wildcard | |
855 | address. | |
856 | The argument must be | |
857 | .Dq yes | |
858 | or | |
859 | .Dq no . | |
860 | The default is | |
861 | .Dq yes . | |
862 | .It Cm XAuthLocation | |
57ff5eeb | 863 | Specifies the full pathname of the |
588df31a | 864 | .Xr xauth 1 |
865 | program. | |
866 | The default is | |
867 | .Pa /usr/X11R6/bin/xauth . | |
868 | .El | |
ef1c6497 | 869 | .Sh TIME FORMATS |
32cfd177 | 870 | .Xr sshd 8 |
588df31a | 871 | command-line arguments and configuration file options that specify time |
872 | may be expressed using a sequence of the form: | |
873 | .Sm off | |
36535ee6 | 874 | .Ar time Op Ar qualifier , |
588df31a | 875 | .Sm on |
876 | where | |
877 | .Ar time | |
878 | is a positive integer value and | |
879 | .Ar qualifier | |
880 | is one of the following: | |
881 | .Pp | |
882 | .Bl -tag -width Ds -compact -offset indent | |
874d319b | 883 | .It Aq Cm none |
588df31a | 884 | seconds |
885 | .It Cm s | Cm S | |
886 | seconds | |
887 | .It Cm m | Cm M | |
888 | minutes | |
889 | .It Cm h | Cm H | |
890 | hours | |
891 | .It Cm d | Cm D | |
892 | days | |
893 | .It Cm w | Cm W | |
894 | weeks | |
895 | .El | |
896 | .Pp | |
897 | Each member of the sequence is added together to calculate | |
898 | the total time value. | |
899 | .Pp | |
900 | Time format examples: | |
901 | .Pp | |
902 | .Bl -tag -width Ds -compact -offset indent | |
903 | .It 600 | |
904 | 600 seconds (10 minutes) | |
905 | .It 10m | |
906 | 10 minutes | |
907 | .It 1h30m | |
908 | 1 hour 30 minutes (90 minutes) | |
909 | .El | |
910 | .Sh FILES | |
911 | .Bl -tag -width Ds | |
912 | .It Pa /etc/ssh/sshd_config | |
913 | Contains configuration data for | |
32cfd177 | 914 | .Xr sshd 8 . |
588df31a | 915 | This file should be writable by root only, but it is recommended |
916 | (though not necessary) that it be world-readable. | |
917 | .El | |
be193d89 | 918 | .Sh SEE ALSO |
919 | .Xr sshd 8 | |
588df31a | 920 | .Sh AUTHORS |
921 | OpenSSH is a derivative of the original and free | |
922 | ssh 1.2.12 release by Tatu Ylonen. | |
923 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
924 | Theo de Raadt and Dug Song | |
925 | removed many bugs, re-added newer features and | |
926 | created OpenSSH. | |
927 | Markus Friedl contributed the support for SSH | |
928 | protocol versions 1.5 and 2.0. | |
929 | Niels Provos and Markus Friedl contributed support | |
930 | for privilege separation. |