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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 | .\" | |
32cfd177 | 37 | .\" $OpenBSD: sshd_config.5,v 1.53 2006/02/24 23:51:17 jmc 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. | |
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 |
f09ffbdb | 75 | .Ql * |
61a2c1da | 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. | |
31b41ceb | 86 | .It Cm AddressFamily |
87 | Specifies which address family should be used by | |
32cfd177 | 88 | .Xr sshd 8 . |
31b41ceb | 89 | Valid arguments are |
90 | .Dq any , | |
91 | .Dq inet | |
b74c3b8c | 92 | (use IPv4 only), or |
31b41ceb | 93 | .Dq inet6 |
94 | (use IPv6 only). | |
95 | The default is | |
96 | .Dq any . | |
588df31a | 97 | .It Cm AllowGroups |
98 | This keyword can be followed by a list of group name patterns, separated | |
99 | by spaces. | |
100 | If specified, login is allowed only for users whose primary | |
101 | group or supplementary group list matches one of the patterns. | |
588df31a | 102 | Only group names are valid; a numerical group ID is not recognized. |
103 | By default, login is allowed for all groups. | |
ac1ec4d8 | 104 | .Pp |
105 | See | |
106 | .Sx PATTERNS | |
107 | in | |
108 | .Xr ssh_config 5 | |
109 | for more information on patterns. | |
588df31a | 110 | .It Cm AllowTcpForwarding |
111 | Specifies whether TCP forwarding is permitted. | |
112 | The default is | |
113 | .Dq yes . | |
114 | Note that disabling TCP forwarding does not improve security unless | |
115 | users are also denied shell access, as they can always install their | |
116 | own forwarders. | |
588df31a | 117 | .It Cm AllowUsers |
118 | This keyword can be followed by a list of user name patterns, separated | |
119 | by spaces. | |
1be697b6 | 120 | If specified, login is allowed only for user names that |
588df31a | 121 | match one of the patterns. |
588df31a | 122 | Only user names are valid; a numerical user ID is not recognized. |
123 | By default, login is allowed for all users. | |
124 | If the pattern takes the form USER@HOST then USER and HOST | |
125 | are separately checked, restricting logins to particular | |
126 | users from particular hosts. | |
ac1ec4d8 | 127 | .Pp |
128 | See | |
129 | .Sx PATTERNS | |
130 | in | |
131 | .Xr ssh_config 5 | |
132 | for more information on patterns. | |
588df31a | 133 | .It Cm AuthorizedKeysFile |
134 | Specifies the file that contains the public keys that can be used | |
135 | for user authentication. | |
136 | .Cm AuthorizedKeysFile | |
137 | may contain tokens of the form %T which are substituted during connection | |
b74c3b8c | 138 | setup. |
3cbc677d | 139 | The following tokens are defined: %% is replaced by a literal '%', |
b74c3b8c | 140 | %h is replaced by the home directory of the user being authenticated, and |
588df31a | 141 | %u is replaced by the username of that user. |
142 | After expansion, | |
143 | .Cm AuthorizedKeysFile | |
144 | is taken to be an absolute path or one relative to the user's home | |
145 | directory. | |
146 | The default is | |
147 | .Dq .ssh/authorized_keys . | |
148 | .It Cm Banner | |
149 | In some jurisdictions, sending a warning message before authentication | |
150 | may be relevant for getting legal protection. | |
151 | The contents of the specified file are sent to the remote user before | |
152 | authentication is allowed. | |
153 | This option is only available for protocol version 2. | |
154 | By default, no banner is displayed. | |
588df31a | 155 | .It Cm ChallengeResponseAuthentication |
340a4caf | 156 | Specifies whether challenge-response authentication is allowed. |
588df31a | 157 | All authentication styles from |
158 | .Xr login.conf 5 | |
159 | are supported. | |
160 | The default is | |
161 | .Dq yes . | |
162 | .It Cm Ciphers | |
163 | Specifies the ciphers allowed for protocol version 2. | |
164 | Multiple ciphers must be comma-separated. | |
3b9baa7b | 165 | The supported ciphers are |
166 | .Dq 3des-cbc , | |
167 | .Dq aes128-cbc , | |
168 | .Dq aes192-cbc , | |
169 | .Dq aes256-cbc , | |
170 | .Dq aes128-ctr , | |
171 | .Dq aes192-ctr , | |
172 | .Dq aes256-ctr , | |
74a66cc8 | 173 | .Dq arcfour128 , |
174 | .Dq arcfour256 , | |
3b9baa7b | 175 | .Dq arcfour , |
176 | .Dq blowfish-cbc , | |
177 | and | |
178 | .Dq cast128-cbc . | |
b74c3b8c | 179 | The default is: |
180 | .Bd -literal -offset 3n | |
181 | aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, | |
182 | arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, | |
183 | aes192-ctr,aes256-ctr | |
588df31a | 184 | .Ed |
588df31a | 185 | .It Cm ClientAliveCountMax |
79e46360 | 186 | Sets the number of client alive messages (see below) which may be |
588df31a | 187 | sent without |
b74c3b8c | 188 | .Xr sshd 8 |
3cbc677d | 189 | receiving any messages back from the client. |
190 | If this threshold is reached while client alive messages are being sent, | |
b74c3b8c | 191 | sshd will disconnect the client, terminating the session. |
3cbc677d | 192 | It is important to note that the use of client alive messages is very |
193 | different from | |
fd573618 | 194 | .Cm TCPKeepAlive |
3cbc677d | 195 | (below). |
196 | The client alive messages are sent through the encrypted channel | |
197 | and therefore will not be spoofable. | |
198 | The TCP keepalive option enabled by | |
fd573618 | 199 | .Cm TCPKeepAlive |
3cbc677d | 200 | is spoofable. |
201 | The client alive mechanism is valuable when the client or | |
588df31a | 202 | server depend on knowing when a connection has become inactive. |
203 | .Pp | |
3cbc677d | 204 | The default value is 3. |
205 | If | |
588df31a | 206 | .Cm ClientAliveInterval |
79e46360 | 207 | (see below) is set to 15, and |
588df31a | 208 | .Cm ClientAliveCountMax |
b74c3b8c | 209 | is left at the default, unresponsive SSH clients |
588df31a | 210 | will be disconnected after approximately 45 seconds. |
2264526c | 211 | .It Cm ClientAliveInterval |
212 | Sets a timeout interval in seconds after which if no data has been received | |
213 | from the client, | |
b74c3b8c | 214 | .Xr sshd 8 |
2264526c | 215 | will send a message through the encrypted |
216 | channel to request a response from the client. | |
217 | The default | |
218 | is 0, indicating that these messages will not be sent to the client. | |
219 | This option applies to protocol version 2 only. | |
588df31a | 220 | .It Cm Compression |
07200973 | 221 | Specifies whether compression is allowed, or delayed until |
222 | the user has authenticated successfully. | |
588df31a | 223 | The argument must be |
07200973 | 224 | .Dq yes , |
225 | .Dq delayed , | |
588df31a | 226 | or |
227 | .Dq no . | |
228 | The default is | |
07200973 | 229 | .Dq delayed . |
588df31a | 230 | .It Cm DenyGroups |
231 | This keyword can be followed by a list of group name patterns, separated | |
232 | by spaces. | |
233 | Login is disallowed for users whose primary group or supplementary | |
234 | group list matches one of the patterns. | |
588df31a | 235 | Only group names are valid; a numerical group ID is not recognized. |
236 | By default, login is allowed for all groups. | |
ac1ec4d8 | 237 | .Pp |
238 | See | |
239 | .Sx PATTERNS | |
240 | in | |
241 | .Xr ssh_config 5 | |
242 | for more information on patterns. | |
588df31a | 243 | .It Cm DenyUsers |
244 | This keyword can be followed by a list of user name patterns, separated | |
245 | by spaces. | |
246 | Login is disallowed for user names that match one of the patterns. | |
588df31a | 247 | Only user names are valid; a numerical user ID is not recognized. |
248 | By default, login is allowed for all users. | |
249 | If the pattern takes the form USER@HOST then USER and HOST | |
250 | are separately checked, restricting logins to particular | |
251 | users from particular hosts. | |
ac1ec4d8 | 252 | .Pp |
253 | See | |
254 | .Sx PATTERNS | |
255 | in | |
256 | .Xr ssh_config 5 | |
257 | for more information on patterns. | |
588df31a | 258 | .It Cm GatewayPorts |
259 | Specifies whether remote hosts are allowed to connect to ports | |
260 | forwarded for the client. | |
261 | By default, | |
b74c3b8c | 262 | .Xr sshd 8 |
a4e5acef | 263 | binds remote port forwardings to the loopback address. |
264 | This prevents other remote hosts from connecting to forwarded ports. | |
588df31a | 265 | .Cm GatewayPorts |
b74c3b8c | 266 | can be used to specify that sshd |
3867aa0a | 267 | should allow remote port forwardings to bind to non-loopback addresses, thus |
268 | allowing other hosts to connect. | |
269 | The argument may be | |
270 | .Dq no | |
271 | to force remote port forwardings to be available to the local host only, | |
588df31a | 272 | .Dq yes |
3867aa0a | 273 | to force remote port forwardings to bind to the wildcard address, or |
274 | .Dq clientspecified | |
275 | to allow the client to select the address to which the forwarding is bound. | |
588df31a | 276 | The default is |
277 | .Dq no . | |
7364bd04 | 278 | .It Cm GSSAPIAuthentication |
105b07db | 279 | Specifies whether user authentication based on GSSAPI is allowed. |
aff51935 | 280 | The default is |
7364bd04 | 281 | .Dq no . |
282 | Note that this option applies to protocol version 2 only. | |
283 | .It Cm GSSAPICleanupCredentials | |
284 | Specifies whether to automatically destroy the user's credentials cache | |
285 | on logout. | |
286 | The default is | |
287 | .Dq yes . | |
288 | Note that this option applies to protocol version 2 only. | |
588df31a | 289 | .It Cm HostbasedAuthentication |
290 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
291 | with successful public key client host authentication is allowed | |
340a4caf | 292 | (host-based authentication). |
588df31a | 293 | This option is similar to |
294 | .Cm RhostsRSAAuthentication | |
295 | and applies to protocol version 2 only. | |
296 | The default is | |
297 | .Dq no . | |
298 | .It Cm HostKey | |
299 | Specifies a file containing a private host key | |
300 | used by SSH. | |
301 | The default is | |
302 | .Pa /etc/ssh/ssh_host_key | |
303 | for protocol version 1, and | |
304 | .Pa /etc/ssh/ssh_host_rsa_key | |
305 | and | |
306 | .Pa /etc/ssh/ssh_host_dsa_key | |
307 | for protocol version 2. | |
308 | Note that | |
b74c3b8c | 309 | .Xr sshd 8 |
588df31a | 310 | will refuse to use a file if it is group/world-accessible. |
311 | It is possible to have multiple host key files. | |
312 | .Dq rsa1 | |
313 | keys are used for version 1 and | |
314 | .Dq dsa | |
315 | or | |
316 | .Dq rsa | |
317 | are used for version 2 of the SSH protocol. | |
318 | .It Cm IgnoreRhosts | |
319 | Specifies that | |
320 | .Pa .rhosts | |
321 | and | |
322 | .Pa .shosts | |
323 | files will not be used in | |
588df31a | 324 | .Cm RhostsRSAAuthentication |
325 | or | |
326 | .Cm HostbasedAuthentication . | |
327 | .Pp | |
328 | .Pa /etc/hosts.equiv | |
329 | and | |
330 | .Pa /etc/shosts.equiv | |
331 | are still used. | |
332 | The default is | |
333 | .Dq yes . | |
334 | .It Cm IgnoreUserKnownHosts | |
335 | Specifies whether | |
b74c3b8c | 336 | .Xr sshd 8 |
588df31a | 337 | should ignore the user's |
140e3e97 | 338 | .Pa ~/.ssh/known_hosts |
588df31a | 339 | during |
340 | .Cm RhostsRSAAuthentication | |
341 | or | |
342 | .Cm HostbasedAuthentication . | |
343 | The default is | |
344 | .Dq no . | |
588df31a | 345 | .It Cm KerberosAuthentication |
8f73f7bb | 346 | Specifies whether the password provided by the user for |
588df31a | 347 | .Cm PasswordAuthentication |
8f73f7bb | 348 | will be validated through the Kerberos KDC. |
588df31a | 349 | To use this option, the server needs a |
350 | Kerberos servtab which allows the verification of the KDC's identity. | |
b74c3b8c | 351 | The default is |
588df31a | 352 | .Dq no . |
24f37810 | 353 | .It Cm KerberosGetAFSToken |
0d3d1077 | 354 | If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire |
24f37810 | 355 | an AFS token before accessing the user's home directory. |
b74c3b8c | 356 | The default is |
24f37810 | 357 | .Dq no . |
588df31a | 358 | .It Cm KerberosOrLocalPasswd |
b74c3b8c | 359 | If password authentication through Kerberos fails then |
588df31a | 360 | the password will be validated via any additional local mechanism |
361 | such as | |
362 | .Pa /etc/passwd . | |
b74c3b8c | 363 | The default is |
588df31a | 364 | .Dq yes . |
588df31a | 365 | .It Cm KerberosTicketCleanup |
366 | Specifies whether to automatically destroy the user's ticket cache | |
367 | file on logout. | |
b74c3b8c | 368 | The default is |
588df31a | 369 | .Dq yes . |
370 | .It Cm KeyRegenerationInterval | |
371 | In protocol version 1, the ephemeral server key is automatically regenerated | |
372 | after this many seconds (if it has been used). | |
373 | The purpose of regeneration is to prevent | |
374 | decrypting captured sessions by later breaking into the machine and | |
375 | stealing the keys. | |
376 | The key is never stored anywhere. | |
377 | If the value is 0, the key is never regenerated. | |
378 | The default is 3600 (seconds). | |
379 | .It Cm ListenAddress | |
380 | Specifies the local addresses | |
b74c3b8c | 381 | .Xr sshd 8 |
588df31a | 382 | should listen on. |
383 | The following forms may be used: | |
384 | .Pp | |
385 | .Bl -item -offset indent -compact | |
386 | .It | |
387 | .Cm ListenAddress | |
388 | .Sm off | |
389 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr | |
390 | .Sm on | |
391 | .It | |
392 | .Cm ListenAddress | |
393 | .Sm off | |
394 | .Ar host No | Ar IPv4_addr No : Ar port | |
395 | .Sm on | |
396 | .It | |
397 | .Cm ListenAddress | |
398 | .Sm off | |
399 | .Oo | |
400 | .Ar host No | Ar IPv6_addr Oc : Ar port | |
401 | .Sm on | |
402 | .El | |
403 | .Pp | |
404 | If | |
405 | .Ar port | |
406 | is not specified, | |
b74c3b8c | 407 | sshd will listen on the address and all prior |
588df31a | 408 | .Cm Port |
3cbc677d | 409 | options specified. |
410 | The default is to listen on all local addresses. | |
a4e5acef | 411 | Multiple |
588df31a | 412 | .Cm ListenAddress |
3cbc677d | 413 | options are permitted. |
414 | Additionally, any | |
588df31a | 415 | .Cm Port |
b74c3b8c | 416 | options must precede this option for non-port qualified addresses. |
588df31a | 417 | .It Cm LoginGraceTime |
418 | The server disconnects after this time if the user has not | |
419 | successfully logged in. | |
420 | If the value is 0, there is no time limit. | |
3445ca02 | 421 | The default is 120 seconds. |
588df31a | 422 | .It Cm LogLevel |
423 | Gives the verbosity level that is used when logging messages from | |
32cfd177 | 424 | .Xr sshd 8 . |
588df31a | 425 | The possible values are: |
b74c3b8c | 426 | QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. |
a4e5acef | 427 | The default is INFO. |
428 | DEBUG and DEBUG1 are equivalent. | |
429 | DEBUG2 and DEBUG3 each specify higher levels of debugging output. | |
430 | Logging with a DEBUG level violates the privacy of users and is not recommended. | |
588df31a | 431 | .It Cm MACs |
432 | Specifies the available MAC (message authentication code) algorithms. | |
433 | The MAC algorithm is used in protocol version 2 | |
434 | for data integrity protection. | |
435 | Multiple algorithms must be comma-separated. | |
b74c3b8c | 436 | The default is: |
588df31a | 437 | .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . |
af4bd935 | 438 | .It Cm MaxAuthTries |
439 | Specifies the maximum number of authentication attempts permitted per | |
8fca654b | 440 | connection. |
441 | Once the number of failures reaches half this value, | |
442 | additional failures are logged. | |
443 | The default is 6. | |
588df31a | 444 | .It Cm MaxStartups |
445 | Specifies the maximum number of concurrent unauthenticated connections to the | |
b74c3b8c | 446 | SSH daemon. |
588df31a | 447 | Additional connections will be dropped until authentication succeeds or the |
448 | .Cm LoginGraceTime | |
449 | expires for a connection. | |
450 | The default is 10. | |
451 | .Pp | |
452 | Alternatively, random early drop can be enabled by specifying | |
453 | the three colon separated values | |
454 | .Dq start:rate:full | |
f09ffbdb | 455 | (e.g. "10:30:60"). |
32cfd177 | 456 | .Xr sshd 8 |
588df31a | 457 | will refuse connection attempts with a probability of |
458 | .Dq rate/100 | |
459 | (30%) | |
460 | if there are currently | |
461 | .Dq start | |
462 | (10) | |
463 | unauthenticated connections. | |
464 | The probability increases linearly and all connection attempts | |
465 | are refused if the number of unauthenticated connections reaches | |
466 | .Dq full | |
467 | (60). | |
468 | .It Cm PasswordAuthentication | |
469 | Specifies whether password authentication is allowed. | |
470 | The default is | |
471 | .Dq yes . | |
472 | .It Cm PermitEmptyPasswords | |
473 | When password authentication is allowed, it specifies whether the | |
474 | server allows login to accounts with empty password strings. | |
475 | The default is | |
476 | .Dq no . | |
477 | .It Cm PermitRootLogin | |
667e4135 | 478 | Specifies whether root can log in using |
588df31a | 479 | .Xr ssh 1 . |
480 | The argument must be | |
481 | .Dq yes , | |
482 | .Dq without-password , | |
b74c3b8c | 483 | .Dq forced-commands-only , |
588df31a | 484 | or |
485 | .Dq no . | |
486 | The default is | |
487 | .Dq yes . | |
488 | .Pp | |
489 | If this option is set to | |
b74c3b8c | 490 | .Dq without-password , |
8a4c4ee4 | 491 | password authentication is disabled for root. |
588df31a | 492 | .Pp |
493 | If this option is set to | |
b74c3b8c | 494 | .Dq forced-commands-only , |
588df31a | 495 | root login with public key authentication will be allowed, |
496 | but only if the | |
497 | .Ar command | |
498 | option has been specified | |
499 | (which may be useful for taking remote backups even if root login is | |
3cbc677d | 500 | normally not allowed). |
501 | All other authentication methods are disabled for root. | |
588df31a | 502 | .Pp |
503 | If this option is set to | |
b74c3b8c | 504 | .Dq no , |
667e4135 | 505 | root is not allowed to log in. |
d20f3c9e | 506 | .It Cm PermitTunnel |
507 | Specifies whether | |
508 | .Xr tun 4 | |
509 | device forwarding is allowed. | |
a4f24bf8 | 510 | The argument must be |
511 | .Dq yes , | |
512 | .Dq point-to-point , | |
b74c3b8c | 513 | .Dq ethernet , |
a4f24bf8 | 514 | or |
515 | .Dq no . | |
d20f3c9e | 516 | The default is |
517 | .Dq no . | |
f00bab84 | 518 | .It Cm PermitUserEnvironment |
519 | Specifies whether | |
520 | .Pa ~/.ssh/environment | |
35453849 | 521 | and |
f00bab84 | 522 | .Cm environment= |
523 | options in | |
524 | .Pa ~/.ssh/authorized_keys | |
35453849 | 525 | are processed by |
b74c3b8c | 526 | .Xr sshd 8 . |
f00bab84 | 527 | The default is |
528 | .Dq no . | |
35453849 | 529 | Enabling environment processing may enable users to bypass access |
530 | restrictions in some configurations using mechanisms such as | |
531 | .Ev LD_PRELOAD . | |
588df31a | 532 | .It Cm PidFile |
baa08b92 | 533 | Specifies the file that contains the process ID of the |
32cfd177 | 534 | SSH daemon. |
588df31a | 535 | The default is |
536 | .Pa /var/run/sshd.pid . | |
537 | .It Cm Port | |
538 | Specifies the port number that | |
b74c3b8c | 539 | .Xr sshd 8 |
588df31a | 540 | listens on. |
541 | The default is 22. | |
542 | Multiple options of this type are permitted. | |
543 | See also | |
544 | .Cm ListenAddress . | |
545 | .It Cm PrintLastLog | |
546 | Specifies whether | |
b74c3b8c | 547 | .Xr sshd 8 |
329a8666 | 548 | should print the date and time of the last user login when a user logs |
549 | in interactively. | |
588df31a | 550 | The default is |
551 | .Dq yes . | |
552 | .It Cm PrintMotd | |
553 | Specifies whether | |
b74c3b8c | 554 | .Xr sshd 8 |
588df31a | 555 | should print |
556 | .Pa /etc/motd | |
557 | when a user logs in interactively. | |
558 | (On some systems it is also printed by the shell, | |
559 | .Pa /etc/profile , | |
560 | or equivalent.) | |
561 | The default is | |
562 | .Dq yes . | |
563 | .It Cm Protocol | |
564 | Specifies the protocol versions | |
b74c3b8c | 565 | .Xr sshd 8 |
94ad46d1 | 566 | supports. |
588df31a | 567 | The possible values are |
b74c3b8c | 568 | .Sq 1 |
588df31a | 569 | and |
b74c3b8c | 570 | .Sq 2 . |
588df31a | 571 | Multiple versions must be comma-separated. |
572 | The default is | |
573 | .Dq 2,1 . | |
94ad46d1 | 574 | Note that the order of the protocol list does not indicate preference, |
575 | because the client selects among multiple protocol versions offered | |
576 | by the server. | |
577 | Specifying | |
578 | .Dq 2,1 | |
579 | is identical to | |
580 | .Dq 1,2 . | |
588df31a | 581 | .It Cm PubkeyAuthentication |
582 | Specifies whether public key authentication is allowed. | |
583 | The default is | |
584 | .Dq yes . | |
585 | Note that this option applies to protocol version 2 only. | |
588df31a | 586 | .It Cm RhostsRSAAuthentication |
587 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
588 | with successful RSA host authentication is allowed. | |
589 | The default is | |
590 | .Dq no . | |
591 | This option applies to protocol version 1 only. | |
592 | .It Cm RSAAuthentication | |
593 | Specifies whether pure RSA authentication is allowed. | |
594 | The default is | |
595 | .Dq yes . | |
596 | This option applies to protocol version 1 only. | |
597 | .It Cm ServerKeyBits | |
598 | Defines the number of bits in the ephemeral protocol version 1 server key. | |
599 | The minimum value is 512, and the default is 768. | |
600 | .It Cm StrictModes | |
601 | Specifies whether | |
b74c3b8c | 602 | .Xr sshd 8 |
588df31a | 603 | should check file modes and ownership of the |
604 | user's files and home directory before accepting login. | |
605 | This is normally desirable because novices sometimes accidentally leave their | |
606 | directory or files world-writable. | |
607 | The default is | |
608 | .Dq yes . | |
609 | .It Cm Subsystem | |
f09ffbdb | 610 | Configures an external subsystem (e.g. file transfer daemon). |
588df31a | 611 | Arguments should be a subsystem name and a command to execute upon subsystem |
612 | request. | |
613 | The command | |
614 | .Xr sftp-server 8 | |
615 | implements the | |
616 | .Dq sftp | |
617 | file transfer subsystem. | |
618 | By default no subsystems are defined. | |
619 | Note that this option applies to protocol version 2 only. | |
620 | .It Cm SyslogFacility | |
621 | Gives the facility code that is used when logging messages from | |
32cfd177 | 622 | .Xr sshd 8 . |
588df31a | 623 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, |
624 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. | |
625 | The default is AUTH. | |
fd573618 | 626 | .It Cm TCPKeepAlive |
627 | Specifies whether the system should send TCP keepalive messages to the | |
628 | other side. | |
629 | If they are sent, death of the connection or crash of one | |
630 | of the machines will be properly noticed. | |
631 | However, this means that | |
632 | connections will die if the route is down temporarily, and some people | |
633 | find it annoying. | |
634 | On the other hand, if TCP keepalives are not sent, | |
635 | sessions may hang indefinitely on the server, leaving | |
636 | .Dq ghost | |
637 | users and consuming server resources. | |
638 | .Pp | |
639 | The default is | |
640 | .Dq yes | |
641 | (to send TCP keepalive messages), and the server will notice | |
642 | if the network goes down or the client host crashes. | |
643 | This avoids infinitely hanging sessions. | |
644 | .Pp | |
645 | To disable TCP keepalive messages, the value should be set to | |
646 | .Dq no . | |
c5a7d788 | 647 | .It Cm UseDNS |
648 | Specifies whether | |
b74c3b8c | 649 | .Xr sshd 8 |
102c77c2 | 650 | should look up the remote host name and check that |
c5a7d788 | 651 | the resolved host name for the remote IP address maps back to the |
652 | very same IP address. | |
653 | The default is | |
654 | .Dq yes . | |
588df31a | 655 | .It Cm UseLogin |
656 | Specifies whether | |
657 | .Xr login 1 | |
658 | is used for interactive login sessions. | |
659 | The default is | |
660 | .Dq no . | |
661 | Note that | |
662 | .Xr login 1 | |
663 | is never used for remote command execution. | |
664 | Note also, that if this is enabled, | |
665 | .Cm X11Forwarding | |
666 | will be disabled because | |
667 | .Xr login 1 | |
668 | does not know how to handle | |
669 | .Xr xauth 1 | |
a4e5acef | 670 | cookies. |
671 | If | |
588df31a | 672 | .Cm UsePrivilegeSeparation |
673 | is specified, it will be disabled after authentication. | |
72c35df7 | 674 | .It Cm UsePAM |
d5c67850 | 675 | Enables the Pluggable Authentication Module interface. |
676 | If set to | |
677 | .Dq yes | |
678 | this will enable PAM authentication using | |
679 | .Cm ChallengeResponseAuthentication | |
05059810 | 680 | and |
681 | .Cm PasswordAuthentication | |
682 | in addition to PAM account and session module processing for all | |
683 | authentication types. | |
d5c67850 | 684 | .Pp |
685 | Because PAM challenge-response authentication usually serves an equivalent | |
686 | role to password authentication, you should disable either | |
687 | .Cm PasswordAuthentication | |
688 | or | |
689 | .Cm ChallengeResponseAuthentication. | |
690 | .Pp | |
691 | If | |
692 | .Cm UsePAM | |
693 | is enabled, you will not be able to run | |
694 | .Xr sshd 8 | |
695 | as a non-root user. | |
696 | The default is | |
a83a3125 | 697 | .Dq no . |
588df31a | 698 | .It Cm UsePrivilegeSeparation |
699 | Specifies whether | |
b74c3b8c | 700 | .Xr sshd 8 |
588df31a | 701 | separates privileges by creating an unprivileged child process |
a4e5acef | 702 | to deal with incoming network traffic. |
703 | After successful authentication, another process will be created that has | |
704 | the privilege of the authenticated user. | |
705 | The goal of privilege separation is to prevent privilege | |
588df31a | 706 | escalation by containing any corruption within the unprivileged processes. |
707 | The default is | |
708 | .Dq yes . | |
588df31a | 709 | .It Cm X11DisplayOffset |
710 | Specifies the first display number available for | |
b74c3b8c | 711 | .Xr sshd 8 Ns 's |
588df31a | 712 | X11 forwarding. |
b74c3b8c | 713 | This prevents sshd from interfering with real X11 servers. |
588df31a | 714 | The default is 10. |
715 | .It Cm X11Forwarding | |
716 | Specifies whether X11 forwarding is permitted. | |
e6fe1bab | 717 | The argument must be |
718 | .Dq yes | |
719 | or | |
720 | .Dq no . | |
588df31a | 721 | The default is |
722 | .Dq no . | |
e6fe1bab | 723 | .Pp |
724 | When X11 forwarding is enabled, there may be additional exposure to | |
725 | the server and to client displays if the | |
b74c3b8c | 726 | .Xr sshd 8 |
e6fe1bab | 727 | proxy display is configured to listen on the wildcard address (see |
728 | .Cm X11UseLocalhost | |
b74c3b8c | 729 | below), though this is not the default. |
e6fe1bab | 730 | Additionally, the authentication spoofing and authentication data |
731 | verification and substitution occur on the client side. | |
732 | The security risk of using X11 forwarding is that the client's X11 | |
b74c3b8c | 733 | display server may be exposed to attack when the SSH client requests |
e6fe1bab | 734 | forwarding (see the warnings for |
735 | .Cm ForwardX11 | |
736 | in | |
be193d89 | 737 | .Xr ssh_config 5 ) . |
e6fe1bab | 738 | A system administrator may have a stance in which they want to |
739 | protect clients that may expose themselves to attack by unwittingly | |
740 | requesting X11 forwarding, which can warrant a | |
741 | .Dq no | |
742 | setting. | |
743 | .Pp | |
744 | Note that disabling X11 forwarding does not prevent users from | |
745 | forwarding X11 traffic, as users can always install their own forwarders. | |
588df31a | 746 | X11 forwarding is automatically disabled if |
747 | .Cm UseLogin | |
748 | is enabled. | |
749 | .It Cm X11UseLocalhost | |
750 | Specifies whether | |
b74c3b8c | 751 | .Xr sshd 8 |
588df31a | 752 | should bind the X11 forwarding server to the loopback address or to |
a4e5acef | 753 | the wildcard address. |
754 | By default, | |
b74c3b8c | 755 | sshd binds the forwarding server to the loopback address and sets the |
588df31a | 756 | hostname part of the |
757 | .Ev DISPLAY | |
758 | environment variable to | |
759 | .Dq localhost . | |
b3641662 | 760 | This prevents remote hosts from connecting to the proxy display. |
588df31a | 761 | However, some older X11 clients may not function with this |
762 | configuration. | |
763 | .Cm X11UseLocalhost | |
764 | may be set to | |
765 | .Dq no | |
766 | to specify that the forwarding server should be bound to the wildcard | |
767 | address. | |
768 | The argument must be | |
769 | .Dq yes | |
770 | or | |
771 | .Dq no . | |
772 | The default is | |
773 | .Dq yes . | |
774 | .It Cm XAuthLocation | |
57ff5eeb | 775 | Specifies the full pathname of the |
588df31a | 776 | .Xr xauth 1 |
777 | program. | |
778 | The default is | |
779 | .Pa /usr/X11R6/bin/xauth . | |
780 | .El | |
781 | .Ss Time Formats | |
32cfd177 | 782 | .Xr sshd 8 |
588df31a | 783 | command-line arguments and configuration file options that specify time |
784 | may be expressed using a sequence of the form: | |
785 | .Sm off | |
36535ee6 | 786 | .Ar time Op Ar qualifier , |
588df31a | 787 | .Sm on |
788 | where | |
789 | .Ar time | |
790 | is a positive integer value and | |
791 | .Ar qualifier | |
792 | is one of the following: | |
793 | .Pp | |
794 | .Bl -tag -width Ds -compact -offset indent | |
795 | .It Cm <none> | |
796 | seconds | |
797 | .It Cm s | Cm S | |
798 | seconds | |
799 | .It Cm m | Cm M | |
800 | minutes | |
801 | .It Cm h | Cm H | |
802 | hours | |
803 | .It Cm d | Cm D | |
804 | days | |
805 | .It Cm w | Cm W | |
806 | weeks | |
807 | .El | |
808 | .Pp | |
809 | Each member of the sequence is added together to calculate | |
810 | the total time value. | |
811 | .Pp | |
812 | Time format examples: | |
813 | .Pp | |
814 | .Bl -tag -width Ds -compact -offset indent | |
815 | .It 600 | |
816 | 600 seconds (10 minutes) | |
817 | .It 10m | |
818 | 10 minutes | |
819 | .It 1h30m | |
820 | 1 hour 30 minutes (90 minutes) | |
821 | .El | |
822 | .Sh FILES | |
823 | .Bl -tag -width Ds | |
824 | .It Pa /etc/ssh/sshd_config | |
825 | Contains configuration data for | |
32cfd177 | 826 | .Xr sshd 8 . |
588df31a | 827 | This file should be writable by root only, but it is recommended |
828 | (though not necessary) that it be world-readable. | |
829 | .El | |
be193d89 | 830 | .Sh SEE ALSO |
831 | .Xr sshd 8 | |
588df31a | 832 | .Sh AUTHORS |
833 | OpenSSH is a derivative of the original and free | |
834 | ssh 1.2.12 release by Tatu Ylonen. | |
835 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
836 | Theo de Raadt and Dug Song | |
837 | removed many bugs, re-added newer features and | |
838 | created OpenSSH. | |
839 | Markus Friedl contributed the support for SSH | |
840 | protocol versions 1.5 and 2.0. | |
841 | Niels Provos and Markus Friedl contributed support | |
842 | for privilege separation. |