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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 | .\" | |
db49deeb | 37 | .\" $OpenBSD: sshd_config.5,v 1.80 2008/02/08 23:24:07 djm Exp $ |
e473dcd1 | 38 | .Dd $Mdocdate$ |
588df31a | 39 | .Dt SSHD_CONFIG 5 |
40 | .Os | |
41 | .Sh NAME | |
42 | .Nm sshd_config | |
43 | .Nd OpenSSH SSH daemon configuration file | |
44 | .Sh SYNOPSIS | |
5d9a4204 | 45 | .Nm /etc/ssh/sshd_config |
588df31a | 46 | .Sh DESCRIPTION |
32cfd177 | 47 | .Xr sshd 8 |
588df31a | 48 | reads configuration data from |
49 | .Pa /etc/ssh/sshd_config | |
50 | (or the file specified with | |
51 | .Fl f | |
52 | on the command line). | |
53 | The file contains keyword-argument pairs, one per line. | |
54 | Lines starting with | |
55 | .Ql # | |
56 | and empty lines are interpreted as comments. | |
533b9133 | 57 | Arguments may optionally be enclosed in double quotes |
58 | .Pq \&" | |
59 | in order to represent arguments containing spaces. | |
588df31a | 60 | .Pp |
61 | The possible | |
62 | keywords and their meanings are as follows (note that | |
63 | keywords are case-insensitive and arguments are case-sensitive): | |
64 | .Bl -tag -width Ds | |
61a2c1da | 65 | .It Cm AcceptEnv |
66 | Specifies what environment variables sent by the client will be copied into | |
67 | the session's | |
68 | .Xr environ 7 . | |
69 | See | |
70 | .Cm SendEnv | |
71 | in | |
72 | .Xr ssh_config 5 | |
73 | for how to configure the client. | |
b8b9f2e6 | 74 | Note that environment passing is only supported for protocol 2. |
61a2c1da | 75 | Variables are specified by name, which may contain the wildcard characters |
f09ffbdb | 76 | .Ql * |
61a2c1da | 77 | and |
78 | .Ql \&? . | |
b8b9f2e6 | 79 | Multiple environment variables may be separated by whitespace or spread |
61a2c1da | 80 | across multiple |
81 | .Cm AcceptEnv | |
82 | directives. | |
b8b9f2e6 | 83 | Be warned that some environment variables could be used to bypass restricted |
61a2c1da | 84 | user environments. |
85 | For this reason, care should be taken in the use of this directive. | |
86 | The default is not to accept any environment variables. | |
31b41ceb | 87 | .It Cm AddressFamily |
88 | Specifies which address family should be used by | |
32cfd177 | 89 | .Xr sshd 8 . |
31b41ceb | 90 | Valid arguments are |
91 | .Dq any , | |
92 | .Dq inet | |
b74c3b8c | 93 | (use IPv4 only), or |
31b41ceb | 94 | .Dq inet6 |
95 | (use IPv6 only). | |
96 | The default is | |
97 | .Dq any . | |
588df31a | 98 | .It Cm AllowGroups |
99 | This keyword can be followed by a list of group name patterns, separated | |
100 | by spaces. | |
101 | If specified, login is allowed only for users whose primary | |
102 | group or supplementary group list matches one of the patterns. | |
588df31a | 103 | Only group names are valid; a numerical group ID is not recognized. |
104 | By default, login is allowed for all groups. | |
84c1b530 | 105 | The allow/deny directives are processed in the following order: |
106 | .Cm DenyUsers , | |
107 | .Cm AllowUsers , | |
108 | .Cm DenyGroups , | |
109 | and finally | |
110 | .Cm AllowGroups . | |
ac1ec4d8 | 111 | .Pp |
112 | See | |
113 | .Sx PATTERNS | |
114 | in | |
115 | .Xr ssh_config 5 | |
116 | for more information on patterns. | |
588df31a | 117 | .It Cm AllowTcpForwarding |
118 | Specifies whether TCP forwarding is permitted. | |
119 | The default is | |
120 | .Dq yes . | |
121 | Note that disabling TCP forwarding does not improve security unless | |
122 | users are also denied shell access, as they can always install their | |
123 | own forwarders. | |
588df31a | 124 | .It Cm AllowUsers |
125 | This keyword can be followed by a list of user name patterns, separated | |
126 | by spaces. | |
1be697b6 | 127 | If specified, login is allowed only for user names that |
588df31a | 128 | match one of the patterns. |
588df31a | 129 | Only user names are valid; a numerical user ID is not recognized. |
130 | By default, login is allowed for all users. | |
131 | If the pattern takes the form USER@HOST then USER and HOST | |
132 | are separately checked, restricting logins to particular | |
133 | users from particular hosts. | |
84c1b530 | 134 | The allow/deny directives are processed in the following order: |
135 | .Cm DenyUsers , | |
136 | .Cm AllowUsers , | |
137 | .Cm DenyGroups , | |
138 | and finally | |
139 | .Cm AllowGroups . | |
ac1ec4d8 | 140 | .Pp |
141 | See | |
142 | .Sx PATTERNS | |
143 | in | |
144 | .Xr ssh_config 5 | |
145 | for more information on patterns. | |
588df31a | 146 | .It Cm AuthorizedKeysFile |
147 | Specifies the file that contains the public keys that can be used | |
148 | for user authentication. | |
149 | .Cm AuthorizedKeysFile | |
150 | may contain tokens of the form %T which are substituted during connection | |
b74c3b8c | 151 | setup. |
3cbc677d | 152 | The following tokens are defined: %% is replaced by a literal '%', |
b74c3b8c | 153 | %h is replaced by the home directory of the user being authenticated, and |
588df31a | 154 | %u is replaced by the username of that user. |
155 | After expansion, | |
156 | .Cm AuthorizedKeysFile | |
157 | is taken to be an absolute path or one relative to the user's home | |
158 | directory. | |
159 | The default is | |
160 | .Dq .ssh/authorized_keys . | |
161 | .It Cm Banner | |
588df31a | 162 | The contents of the specified file are sent to the remote user before |
163 | authentication is allowed. | |
ba8cfba6 | 164 | If the argument is |
165 | .Dq none | |
166 | then no banner is displayed. | |
588df31a | 167 | This option is only available for protocol version 2. |
168 | By default, no banner is displayed. | |
588df31a | 169 | .It Cm ChallengeResponseAuthentication |
340a4caf | 170 | Specifies whether challenge-response authentication is allowed. |
588df31a | 171 | All authentication styles from |
172 | .Xr login.conf 5 | |
173 | are supported. | |
174 | The default is | |
175 | .Dq yes . | |
db49deeb | 176 | .It Cm ChrootDirectory |
177 | Specifies a path to | |
178 | .Xr chroot 2 | |
179 | to after authentication. | |
180 | This path, and all its components, must be root-owned directories that are | |
181 | not writable by any other user or group. | |
182 | .Pp | |
183 | The path may contain the following tokens that are expanded at runtime once | |
184 | the connecting user has been authenticated: %% is replaced by a literal '%', | |
185 | %h is replaced by the home directory of the user being authenticated, and | |
186 | %u is replaced by the username of that user. | |
187 | .Pp | |
188 | The | |
189 | .Cm ChrootDirectory | |
190 | must contain the necessary files and directories to support the | |
191 | users' session. | |
192 | For an interactive session this requires at least a shell, typically | |
193 | .Xr sh 1 , | |
194 | and basic | |
195 | .Pa /dev | |
196 | nodes such as | |
197 | .Xr null 4 , | |
198 | .Xr zero 4 , | |
199 | .Xr stdin 4 , | |
200 | .Xr stdout 4 , | |
201 | .Xr stderr 4 , | |
202 | .Xr arandom 4 | |
203 | and | |
204 | .Xr tty 4 | |
205 | devices. | |
206 | For file transfer sessions using | |
207 | .Dq sftp , | |
208 | no additional configuration of the environment is necessary if the | |
209 | in-process sftp server is used (see | |
210 | .Cm Subsystem | |
211 | for details. | |
212 | .Pp | |
213 | The default is not to | |
214 | .Xr chroot 2 . | |
588df31a | 215 | .It Cm Ciphers |
216 | Specifies the ciphers allowed for protocol version 2. | |
217 | Multiple ciphers must be comma-separated. | |
3b9baa7b | 218 | The supported ciphers are |
219 | .Dq 3des-cbc , | |
220 | .Dq aes128-cbc , | |
221 | .Dq aes192-cbc , | |
222 | .Dq aes256-cbc , | |
223 | .Dq aes128-ctr , | |
224 | .Dq aes192-ctr , | |
225 | .Dq aes256-ctr , | |
74a66cc8 | 226 | .Dq arcfour128 , |
227 | .Dq arcfour256 , | |
3b9baa7b | 228 | .Dq arcfour , |
229 | .Dq blowfish-cbc , | |
230 | and | |
231 | .Dq cast128-cbc . | |
b74c3b8c | 232 | The default is: |
233 | .Bd -literal -offset 3n | |
234 | aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, | |
235 | arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, | |
236 | aes192-ctr,aes256-ctr | |
588df31a | 237 | .Ed |
588df31a | 238 | .It Cm ClientAliveCountMax |
79e46360 | 239 | Sets the number of client alive messages (see below) which may be |
588df31a | 240 | sent without |
b74c3b8c | 241 | .Xr sshd 8 |
3cbc677d | 242 | receiving any messages back from the client. |
243 | If this threshold is reached while client alive messages are being sent, | |
b74c3b8c | 244 | sshd will disconnect the client, terminating the session. |
3cbc677d | 245 | It is important to note that the use of client alive messages is very |
246 | different from | |
fd573618 | 247 | .Cm TCPKeepAlive |
3cbc677d | 248 | (below). |
249 | The client alive messages are sent through the encrypted channel | |
250 | and therefore will not be spoofable. | |
251 | The TCP keepalive option enabled by | |
fd573618 | 252 | .Cm TCPKeepAlive |
3cbc677d | 253 | is spoofable. |
254 | The client alive mechanism is valuable when the client or | |
588df31a | 255 | server depend on knowing when a connection has become inactive. |
256 | .Pp | |
3cbc677d | 257 | The default value is 3. |
258 | If | |
588df31a | 259 | .Cm ClientAliveInterval |
79e46360 | 260 | (see below) is set to 15, and |
588df31a | 261 | .Cm ClientAliveCountMax |
b74c3b8c | 262 | is left at the default, unresponsive SSH clients |
588df31a | 263 | will be disconnected after approximately 45 seconds. |
2d762582 | 264 | This option applies to protocol version 2 only. |
2264526c | 265 | .It Cm ClientAliveInterval |
266 | Sets a timeout interval in seconds after which if no data has been received | |
267 | from the client, | |
b74c3b8c | 268 | .Xr sshd 8 |
2264526c | 269 | will send a message through the encrypted |
270 | channel to request a response from the client. | |
271 | The default | |
272 | is 0, indicating that these messages will not be sent to the client. | |
273 | This option applies to protocol version 2 only. | |
588df31a | 274 | .It Cm Compression |
07200973 | 275 | Specifies whether compression is allowed, or delayed until |
276 | the user has authenticated successfully. | |
588df31a | 277 | The argument must be |
07200973 | 278 | .Dq yes , |
279 | .Dq delayed , | |
588df31a | 280 | or |
281 | .Dq no . | |
282 | The default is | |
07200973 | 283 | .Dq delayed . |
588df31a | 284 | .It Cm DenyGroups |
285 | This keyword can be followed by a list of group name patterns, separated | |
286 | by spaces. | |
287 | Login is disallowed for users whose primary group or supplementary | |
288 | group list matches one of the patterns. | |
588df31a | 289 | Only group names are valid; a numerical group ID is not recognized. |
290 | By default, login is allowed for all groups. | |
84c1b530 | 291 | The allow/deny directives are processed in the following order: |
292 | .Cm DenyUsers , | |
293 | .Cm AllowUsers , | |
294 | .Cm DenyGroups , | |
295 | and finally | |
296 | .Cm AllowGroups . | |
ac1ec4d8 | 297 | .Pp |
298 | See | |
299 | .Sx PATTERNS | |
300 | in | |
301 | .Xr ssh_config 5 | |
302 | for more information on patterns. | |
588df31a | 303 | .It Cm DenyUsers |
304 | This keyword can be followed by a list of user name patterns, separated | |
305 | by spaces. | |
306 | Login is disallowed for user names that match one of the patterns. | |
588df31a | 307 | Only user names are valid; a numerical user ID is not recognized. |
308 | By default, login is allowed for all users. | |
309 | If the pattern takes the form USER@HOST then USER and HOST | |
310 | are separately checked, restricting logins to particular | |
311 | users from particular hosts. | |
84c1b530 | 312 | The allow/deny directives are processed in the following order: |
313 | .Cm DenyUsers , | |
314 | .Cm AllowUsers , | |
315 | .Cm DenyGroups , | |
316 | and finally | |
317 | .Cm AllowGroups . | |
ac1ec4d8 | 318 | .Pp |
319 | See | |
320 | .Sx PATTERNS | |
321 | in | |
322 | .Xr ssh_config 5 | |
323 | for more information on patterns. | |
e7259e8d | 324 | .It Cm ForceCommand |
325 | Forces the execution of the command specified by | |
326 | .Cm ForceCommand , | |
327 | ignoring any command supplied by the client. | |
328 | The command is invoked by using the user's login shell with the -c option. | |
329 | This applies to shell, command, or subsystem execution. | |
330 | It is most useful inside a | |
331 | .Cm Match | |
332 | block. | |
333 | The command originally supplied by the client is available in the | |
334 | .Ev SSH_ORIGINAL_COMMAND | |
335 | environment variable. | |
588df31a | 336 | .It Cm GatewayPorts |
337 | Specifies whether remote hosts are allowed to connect to ports | |
338 | forwarded for the client. | |
339 | By default, | |
b74c3b8c | 340 | .Xr sshd 8 |
a4e5acef | 341 | binds remote port forwardings to the loopback address. |
342 | This prevents other remote hosts from connecting to forwarded ports. | |
588df31a | 343 | .Cm GatewayPorts |
b74c3b8c | 344 | can be used to specify that sshd |
3867aa0a | 345 | should allow remote port forwardings to bind to non-loopback addresses, thus |
346 | allowing other hosts to connect. | |
347 | The argument may be | |
348 | .Dq no | |
349 | to force remote port forwardings to be available to the local host only, | |
588df31a | 350 | .Dq yes |
3867aa0a | 351 | to force remote port forwardings to bind to the wildcard address, or |
352 | .Dq clientspecified | |
353 | to allow the client to select the address to which the forwarding is bound. | |
588df31a | 354 | The default is |
355 | .Dq no . | |
7364bd04 | 356 | .It Cm GSSAPIAuthentication |
105b07db | 357 | Specifies whether user authentication based on GSSAPI is allowed. |
aff51935 | 358 | The default is |
7364bd04 | 359 | .Dq no . |
360 | Note that this option applies to protocol version 2 only. | |
361 | .It Cm GSSAPICleanupCredentials | |
362 | Specifies whether to automatically destroy the user's credentials cache | |
363 | on logout. | |
364 | The default is | |
365 | .Dq yes . | |
366 | Note that this option applies to protocol version 2 only. | |
588df31a | 367 | .It Cm HostbasedAuthentication |
368 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
369 | with successful public key client host authentication is allowed | |
340a4caf | 370 | (host-based authentication). |
588df31a | 371 | This option is similar to |
372 | .Cm RhostsRSAAuthentication | |
373 | and applies to protocol version 2 only. | |
374 | The default is | |
375 | .Dq no . | |
e9f2e744 | 376 | .It Cm HostbasedUsesNameFromPacketOnly |
377 | Specifies whether or not the server will attempt to perform a reverse | |
378 | name lookup when matching the name in the | |
379 | .Pa ~/.shosts , | |
380 | .Pa ~/.rhosts , | |
381 | and | |
382 | .Pa /etc/hosts.equiv | |
383 | files during | |
384 | .Cm HostbasedAuthentication . | |
385 | A setting of | |
386 | .Dq yes | |
387 | means that | |
388 | .Xr sshd 8 | |
389 | uses the name supplied by the client rather than | |
390 | attempting to resolve the name from the TCP connection itself. | |
391 | The default is | |
392 | .Dq no . | |
588df31a | 393 | .It Cm HostKey |
394 | Specifies a file containing a private host key | |
395 | used by SSH. | |
396 | The default is | |
397 | .Pa /etc/ssh/ssh_host_key | |
398 | for protocol version 1, and | |
399 | .Pa /etc/ssh/ssh_host_rsa_key | |
400 | and | |
401 | .Pa /etc/ssh/ssh_host_dsa_key | |
402 | for protocol version 2. | |
403 | Note that | |
b74c3b8c | 404 | .Xr sshd 8 |
588df31a | 405 | will refuse to use a file if it is group/world-accessible. |
406 | It is possible to have multiple host key files. | |
407 | .Dq rsa1 | |
408 | keys are used for version 1 and | |
409 | .Dq dsa | |
410 | or | |
411 | .Dq rsa | |
412 | are used for version 2 of the SSH protocol. | |
413 | .It Cm IgnoreRhosts | |
414 | Specifies that | |
415 | .Pa .rhosts | |
416 | and | |
417 | .Pa .shosts | |
418 | files will not be used in | |
588df31a | 419 | .Cm RhostsRSAAuthentication |
420 | or | |
421 | .Cm HostbasedAuthentication . | |
422 | .Pp | |
423 | .Pa /etc/hosts.equiv | |
424 | and | |
425 | .Pa /etc/shosts.equiv | |
426 | are still used. | |
427 | The default is | |
428 | .Dq yes . | |
429 | .It Cm IgnoreUserKnownHosts | |
430 | Specifies whether | |
b74c3b8c | 431 | .Xr sshd 8 |
588df31a | 432 | should ignore the user's |
140e3e97 | 433 | .Pa ~/.ssh/known_hosts |
588df31a | 434 | during |
435 | .Cm RhostsRSAAuthentication | |
436 | or | |
437 | .Cm HostbasedAuthentication . | |
438 | The default is | |
439 | .Dq no . | |
588df31a | 440 | .It Cm KerberosAuthentication |
8f73f7bb | 441 | Specifies whether the password provided by the user for |
588df31a | 442 | .Cm PasswordAuthentication |
8f73f7bb | 443 | will be validated through the Kerberos KDC. |
588df31a | 444 | To use this option, the server needs a |
445 | Kerberos servtab which allows the verification of the KDC's identity. | |
b74c3b8c | 446 | The default is |
588df31a | 447 | .Dq no . |
24f37810 | 448 | .It Cm KerberosGetAFSToken |
0d3d1077 | 449 | If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire |
24f37810 | 450 | an AFS token before accessing the user's home directory. |
b74c3b8c | 451 | The default is |
24f37810 | 452 | .Dq no . |
588df31a | 453 | .It Cm KerberosOrLocalPasswd |
b74c3b8c | 454 | If password authentication through Kerberos fails then |
588df31a | 455 | the password will be validated via any additional local mechanism |
456 | such as | |
457 | .Pa /etc/passwd . | |
b74c3b8c | 458 | The default is |
588df31a | 459 | .Dq yes . |
588df31a | 460 | .It Cm KerberosTicketCleanup |
461 | Specifies whether to automatically destroy the user's ticket cache | |
462 | file on logout. | |
b74c3b8c | 463 | The default is |
588df31a | 464 | .Dq yes . |
465 | .It Cm KeyRegenerationInterval | |
466 | In protocol version 1, the ephemeral server key is automatically regenerated | |
467 | after this many seconds (if it has been used). | |
468 | The purpose of regeneration is to prevent | |
469 | decrypting captured sessions by later breaking into the machine and | |
470 | stealing the keys. | |
471 | The key is never stored anywhere. | |
472 | If the value is 0, the key is never regenerated. | |
473 | The default is 3600 (seconds). | |
474 | .It Cm ListenAddress | |
475 | Specifies the local addresses | |
b74c3b8c | 476 | .Xr sshd 8 |
588df31a | 477 | should listen on. |
478 | The following forms may be used: | |
479 | .Pp | |
480 | .Bl -item -offset indent -compact | |
481 | .It | |
482 | .Cm ListenAddress | |
483 | .Sm off | |
484 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr | |
485 | .Sm on | |
486 | .It | |
487 | .Cm ListenAddress | |
488 | .Sm off | |
489 | .Ar host No | Ar IPv4_addr No : Ar port | |
490 | .Sm on | |
491 | .It | |
492 | .Cm ListenAddress | |
493 | .Sm off | |
494 | .Oo | |
495 | .Ar host No | Ar IPv6_addr Oc : Ar port | |
496 | .Sm on | |
497 | .El | |
498 | .Pp | |
499 | If | |
500 | .Ar port | |
501 | is not specified, | |
b74c3b8c | 502 | sshd will listen on the address and all prior |
588df31a | 503 | .Cm Port |
3cbc677d | 504 | options specified. |
505 | The default is to listen on all local addresses. | |
a4e5acef | 506 | Multiple |
588df31a | 507 | .Cm ListenAddress |
3cbc677d | 508 | options are permitted. |
509 | Additionally, any | |
588df31a | 510 | .Cm Port |
b74c3b8c | 511 | options must precede this option for non-port qualified addresses. |
588df31a | 512 | .It Cm LoginGraceTime |
513 | The server disconnects after this time if the user has not | |
514 | successfully logged in. | |
515 | If the value is 0, there is no time limit. | |
3445ca02 | 516 | The default is 120 seconds. |
588df31a | 517 | .It Cm LogLevel |
518 | Gives the verbosity level that is used when logging messages from | |
32cfd177 | 519 | .Xr sshd 8 . |
588df31a | 520 | The possible values are: |
b74c3b8c | 521 | QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. |
a4e5acef | 522 | The default is INFO. |
523 | DEBUG and DEBUG1 are equivalent. | |
524 | DEBUG2 and DEBUG3 each specify higher levels of debugging output. | |
525 | Logging with a DEBUG level violates the privacy of users and is not recommended. | |
588df31a | 526 | .It Cm MACs |
527 | Specifies the available MAC (message authentication code) algorithms. | |
528 | The MAC algorithm is used in protocol version 2 | |
529 | for data integrity protection. | |
530 | Multiple algorithms must be comma-separated. | |
b74c3b8c | 531 | The default is: |
33d2ae0f | 532 | .Bd -literal -offset indent |
533 | hmac-md5,hmac-sha1,umac-64@openssh.com, | |
534 | hmac-ripemd160,hmac-sha1-96,hmac-md5-96 | |
535 | .Ed | |
d231781a | 536 | .It Cm Match |
14e980ef | 537 | Introduces a conditional block. |
4895f836 | 538 | If all of the criteria on the |
d231781a | 539 | .Cm Match |
4895f836 | 540 | line are satisfied, the keywords on the following lines override those |
541 | set in the global section of the config file, until either another | |
d231781a | 542 | .Cm Match |
4895f836 | 543 | line or the end of the file. |
14e980ef | 544 | The arguments to |
d231781a | 545 | .Cm Match |
4895f836 | 546 | are one or more criteria-pattern pairs. |
d231781a | 547 | The available criteria are |
548 | .Cm User , | |
fa47fe3c | 549 | .Cm Group , |
d231781a | 550 | .Cm Host , |
551 | and | |
552 | .Cm Address . | |
553 | Only a subset of keywords may be used on the lines following a | |
554 | .Cm Match | |
555 | keyword. | |
556 | Available keywords are | |
2fefbadf | 557 | .Cm AllowTcpForwarding , |
03bcbf84 | 558 | .Cm Banner , |
e7259e8d | 559 | .Cm ForceCommand , |
2fefbadf | 560 | .Cm GatewayPorts , |
03bcbf84 | 561 | .Cm GSSApiAuthentication , |
121c4a34 | 562 | .Cm KbdInteractiveAuthentication , |
a863b75f | 563 | .Cm KerberosAuthentication , |
03bcbf84 | 564 | .Cm PasswordAuthentication , |
691712e0 | 565 | .Cm PermitOpen , |
2ef741a3 | 566 | .Cm PermitRootLogin , |
03bcbf84 | 567 | .Cm RhostsRSAAuthentication , |
568 | .Cm RSAAuthentication , | |
691712e0 | 569 | .Cm X11DisplayOffset , |
570 | .Cm X11Forwarding , | |
d231781a | 571 | and |
691712e0 | 572 | .Cm X11UseLocalHost . |
af4bd935 | 573 | .It Cm MaxAuthTries |
574 | Specifies the maximum number of authentication attempts permitted per | |
8fca654b | 575 | connection. |
576 | Once the number of failures reaches half this value, | |
577 | additional failures are logged. | |
578 | The default is 6. | |
588df31a | 579 | .It Cm MaxStartups |
580 | Specifies the maximum number of concurrent unauthenticated connections to the | |
b74c3b8c | 581 | SSH daemon. |
588df31a | 582 | Additional connections will be dropped until authentication succeeds or the |
583 | .Cm LoginGraceTime | |
584 | expires for a connection. | |
585 | The default is 10. | |
586 | .Pp | |
587 | Alternatively, random early drop can be enabled by specifying | |
588 | the three colon separated values | |
589 | .Dq start:rate:full | |
f09ffbdb | 590 | (e.g. "10:30:60"). |
32cfd177 | 591 | .Xr sshd 8 |
588df31a | 592 | will refuse connection attempts with a probability of |
593 | .Dq rate/100 | |
594 | (30%) | |
595 | if there are currently | |
596 | .Dq start | |
597 | (10) | |
598 | unauthenticated connections. | |
599 | The probability increases linearly and all connection attempts | |
600 | are refused if the number of unauthenticated connections reaches | |
601 | .Dq full | |
602 | (60). | |
603 | .It Cm PasswordAuthentication | |
604 | Specifies whether password authentication is allowed. | |
605 | The default is | |
606 | .Dq yes . | |
607 | .It Cm PermitEmptyPasswords | |
608 | When password authentication is allowed, it specifies whether the | |
609 | server allows login to accounts with empty password strings. | |
610 | The default is | |
611 | .Dq no . | |
2fefbadf | 612 | .It Cm PermitOpen |
613 | Specifies the destinations to which TCP port forwarding is permitted. | |
614 | The forwarding specification must be one of the following forms: | |
615 | .Pp | |
616 | .Bl -item -offset indent -compact | |
617 | .It | |
618 | .Cm PermitOpen | |
619 | .Sm off | |
620 | .Ar host : port | |
621 | .Sm on | |
622 | .It | |
623 | .Cm PermitOpen | |
624 | .Sm off | |
625 | .Ar IPv4_addr : port | |
626 | .Sm on | |
627 | .It | |
628 | .Cm PermitOpen | |
629 | .Sm off | |
630 | .Ar \&[ IPv6_addr \&] : port | |
631 | .Sm on | |
632 | .El | |
633 | .Pp | |
ea46e550 | 634 | Multiple forwards may be specified by separating them with whitespace. |
2fefbadf | 635 | An argument of |
636 | .Dq any | |
637 | can be used to remove all restrictions and permit any forwarding requests. | |
f22506ff | 638 | By default all port forwarding requests are permitted. |
588df31a | 639 | .It Cm PermitRootLogin |
667e4135 | 640 | Specifies whether root can log in using |
588df31a | 641 | .Xr ssh 1 . |
642 | The argument must be | |
643 | .Dq yes , | |
644 | .Dq without-password , | |
b74c3b8c | 645 | .Dq forced-commands-only , |
588df31a | 646 | or |
647 | .Dq no . | |
648 | The default is | |
649 | .Dq yes . | |
650 | .Pp | |
651 | If this option is set to | |
b74c3b8c | 652 | .Dq without-password , |
8a4c4ee4 | 653 | password authentication is disabled for root. |
588df31a | 654 | .Pp |
655 | If this option is set to | |
b74c3b8c | 656 | .Dq forced-commands-only , |
588df31a | 657 | root login with public key authentication will be allowed, |
658 | but only if the | |
659 | .Ar command | |
660 | option has been specified | |
661 | (which may be useful for taking remote backups even if root login is | |
3cbc677d | 662 | normally not allowed). |
663 | All other authentication methods are disabled for root. | |
588df31a | 664 | .Pp |
665 | If this option is set to | |
b74c3b8c | 666 | .Dq no , |
667e4135 | 667 | root is not allowed to log in. |
d20f3c9e | 668 | .It Cm PermitTunnel |
669 | Specifies whether | |
670 | .Xr tun 4 | |
671 | device forwarding is allowed. | |
a4f24bf8 | 672 | The argument must be |
673 | .Dq yes , | |
06fa4ac1 | 674 | .Dq point-to-point |
675 | (layer 3), | |
676 | .Dq ethernet | |
677 | (layer 2), or | |
a4f24bf8 | 678 | .Dq no . |
06fa4ac1 | 679 | Specifying |
680 | .Dq yes | |
681 | permits both | |
682 | .Dq point-to-point | |
683 | and | |
684 | .Dq ethernet . | |
d20f3c9e | 685 | The default is |
686 | .Dq no . | |
f00bab84 | 687 | .It Cm PermitUserEnvironment |
688 | Specifies whether | |
689 | .Pa ~/.ssh/environment | |
35453849 | 690 | and |
f00bab84 | 691 | .Cm environment= |
692 | options in | |
693 | .Pa ~/.ssh/authorized_keys | |
35453849 | 694 | are processed by |
b74c3b8c | 695 | .Xr sshd 8 . |
f00bab84 | 696 | The default is |
697 | .Dq no . | |
35453849 | 698 | Enabling environment processing may enable users to bypass access |
699 | restrictions in some configurations using mechanisms such as | |
700 | .Ev LD_PRELOAD . | |
588df31a | 701 | .It Cm PidFile |
baa08b92 | 702 | Specifies the file that contains the process ID of the |
32cfd177 | 703 | SSH daemon. |
588df31a | 704 | The default is |
705 | .Pa /var/run/sshd.pid . | |
706 | .It Cm Port | |
707 | Specifies the port number that | |
b74c3b8c | 708 | .Xr sshd 8 |
588df31a | 709 | listens on. |
710 | The default is 22. | |
711 | Multiple options of this type are permitted. | |
712 | See also | |
713 | .Cm ListenAddress . | |
714 | .It Cm PrintLastLog | |
715 | Specifies whether | |
b74c3b8c | 716 | .Xr sshd 8 |
329a8666 | 717 | should print the date and time of the last user login when a user logs |
718 | in interactively. | |
588df31a | 719 | The default is |
720 | .Dq yes . | |
721 | .It Cm PrintMotd | |
722 | Specifies whether | |
b74c3b8c | 723 | .Xr sshd 8 |
588df31a | 724 | should print |
725 | .Pa /etc/motd | |
726 | when a user logs in interactively. | |
727 | (On some systems it is also printed by the shell, | |
728 | .Pa /etc/profile , | |
729 | or equivalent.) | |
730 | The default is | |
731 | .Dq yes . | |
732 | .It Cm Protocol | |
733 | Specifies the protocol versions | |
b74c3b8c | 734 | .Xr sshd 8 |
94ad46d1 | 735 | supports. |
588df31a | 736 | The possible values are |
b74c3b8c | 737 | .Sq 1 |
588df31a | 738 | and |
b74c3b8c | 739 | .Sq 2 . |
588df31a | 740 | Multiple versions must be comma-separated. |
741 | The default is | |
742 | .Dq 2,1 . | |
94ad46d1 | 743 | Note that the order of the protocol list does not indicate preference, |
744 | because the client selects among multiple protocol versions offered | |
745 | by the server. | |
746 | Specifying | |
747 | .Dq 2,1 | |
748 | is identical to | |
749 | .Dq 1,2 . | |
588df31a | 750 | .It Cm PubkeyAuthentication |
751 | Specifies whether public key authentication is allowed. | |
752 | The default is | |
753 | .Dq yes . | |
754 | Note that this option applies to protocol version 2 only. | |
588df31a | 755 | .It Cm RhostsRSAAuthentication |
756 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
757 | with successful RSA host authentication is allowed. | |
758 | The default is | |
759 | .Dq no . | |
760 | This option applies to protocol version 1 only. | |
761 | .It Cm RSAAuthentication | |
762 | Specifies whether pure RSA authentication is allowed. | |
763 | The default is | |
764 | .Dq yes . | |
765 | This option applies to protocol version 1 only. | |
766 | .It Cm ServerKeyBits | |
767 | Defines the number of bits in the ephemeral protocol version 1 server key. | |
768 | The minimum value is 512, and the default is 768. | |
769 | .It Cm StrictModes | |
770 | Specifies whether | |
b74c3b8c | 771 | .Xr sshd 8 |
588df31a | 772 | should check file modes and ownership of the |
773 | user's files and home directory before accepting login. | |
774 | This is normally desirable because novices sometimes accidentally leave their | |
775 | directory or files world-writable. | |
776 | The default is | |
777 | .Dq yes . | |
778 | .It Cm Subsystem | |
f09ffbdb | 779 | Configures an external subsystem (e.g. file transfer daemon). |
d66ce1a1 | 780 | Arguments should be a subsystem name and a command (with optional arguments) |
781 | to execute upon subsystem request. | |
db49deeb | 782 | .Pp |
588df31a | 783 | The command |
784 | .Xr sftp-server 8 | |
785 | implements the | |
786 | .Dq sftp | |
787 | file transfer subsystem. | |
db49deeb | 788 | .Pp |
789 | Alternately the name | |
790 | .Dq internal-sftp | |
791 | implements an in-process | |
792 | .Dq sftp | |
793 | server. | |
794 | This may simplify configurations using | |
795 | .Cm ChrootDirectory | |
796 | to force a different filesystem root on clients. | |
797 | .Pp | |
588df31a | 798 | By default no subsystems are defined. |
799 | Note that this option applies to protocol version 2 only. | |
800 | .It Cm SyslogFacility | |
801 | Gives the facility code that is used when logging messages from | |
32cfd177 | 802 | .Xr sshd 8 . |
588df31a | 803 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, |
804 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. | |
805 | The default is AUTH. | |
fd573618 | 806 | .It Cm TCPKeepAlive |
807 | Specifies whether the system should send TCP keepalive messages to the | |
808 | other side. | |
809 | If they are sent, death of the connection or crash of one | |
810 | of the machines will be properly noticed. | |
811 | However, this means that | |
812 | connections will die if the route is down temporarily, and some people | |
813 | find it annoying. | |
814 | On the other hand, if TCP keepalives are not sent, | |
815 | sessions may hang indefinitely on the server, leaving | |
816 | .Dq ghost | |
817 | users and consuming server resources. | |
818 | .Pp | |
819 | The default is | |
820 | .Dq yes | |
821 | (to send TCP keepalive messages), and the server will notice | |
822 | if the network goes down or the client host crashes. | |
823 | This avoids infinitely hanging sessions. | |
824 | .Pp | |
825 | To disable TCP keepalive messages, the value should be set to | |
826 | .Dq no . | |
c5a7d788 | 827 | .It Cm UseDNS |
828 | Specifies whether | |
b74c3b8c | 829 | .Xr sshd 8 |
102c77c2 | 830 | should look up the remote host name and check that |
c5a7d788 | 831 | the resolved host name for the remote IP address maps back to the |
832 | very same IP address. | |
833 | The default is | |
834 | .Dq yes . | |
588df31a | 835 | .It Cm UseLogin |
836 | Specifies whether | |
837 | .Xr login 1 | |
838 | is used for interactive login sessions. | |
839 | The default is | |
840 | .Dq no . | |
841 | Note that | |
842 | .Xr login 1 | |
843 | is never used for remote command execution. | |
844 | Note also, that if this is enabled, | |
845 | .Cm X11Forwarding | |
846 | will be disabled because | |
847 | .Xr login 1 | |
848 | does not know how to handle | |
849 | .Xr xauth 1 | |
a4e5acef | 850 | cookies. |
851 | If | |
588df31a | 852 | .Cm UsePrivilegeSeparation |
853 | is specified, it will be disabled after authentication. | |
72c35df7 | 854 | .It Cm UsePAM |
d5c67850 | 855 | Enables the Pluggable Authentication Module interface. |
856 | If set to | |
857 | .Dq yes | |
858 | this will enable PAM authentication using | |
859 | .Cm ChallengeResponseAuthentication | |
05059810 | 860 | and |
861 | .Cm PasswordAuthentication | |
862 | in addition to PAM account and session module processing for all | |
863 | authentication types. | |
d5c67850 | 864 | .Pp |
865 | Because PAM challenge-response authentication usually serves an equivalent | |
866 | role to password authentication, you should disable either | |
867 | .Cm PasswordAuthentication | |
868 | or | |
869 | .Cm ChallengeResponseAuthentication. | |
870 | .Pp | |
871 | If | |
872 | .Cm UsePAM | |
873 | is enabled, you will not be able to run | |
874 | .Xr sshd 8 | |
875 | as a non-root user. | |
876 | The default is | |
a83a3125 | 877 | .Dq no . |
588df31a | 878 | .It Cm UsePrivilegeSeparation |
879 | Specifies whether | |
b74c3b8c | 880 | .Xr sshd 8 |
588df31a | 881 | separates privileges by creating an unprivileged child process |
a4e5acef | 882 | to deal with incoming network traffic. |
883 | After successful authentication, another process will be created that has | |
884 | the privilege of the authenticated user. | |
885 | The goal of privilege separation is to prevent privilege | |
588df31a | 886 | escalation by containing any corruption within the unprivileged processes. |
887 | The default is | |
888 | .Dq yes . | |
588df31a | 889 | .It Cm X11DisplayOffset |
890 | Specifies the first display number available for | |
b74c3b8c | 891 | .Xr sshd 8 Ns 's |
588df31a | 892 | X11 forwarding. |
b74c3b8c | 893 | This prevents sshd from interfering with real X11 servers. |
588df31a | 894 | The default is 10. |
895 | .It Cm X11Forwarding | |
896 | Specifies whether X11 forwarding is permitted. | |
e6fe1bab | 897 | The argument must be |
898 | .Dq yes | |
899 | or | |
900 | .Dq no . | |
588df31a | 901 | The default is |
902 | .Dq no . | |
e6fe1bab | 903 | .Pp |
904 | When X11 forwarding is enabled, there may be additional exposure to | |
905 | the server and to client displays if the | |
b74c3b8c | 906 | .Xr sshd 8 |
e6fe1bab | 907 | proxy display is configured to listen on the wildcard address (see |
908 | .Cm X11UseLocalhost | |
b74c3b8c | 909 | below), though this is not the default. |
e6fe1bab | 910 | Additionally, the authentication spoofing and authentication data |
911 | verification and substitution occur on the client side. | |
912 | The security risk of using X11 forwarding is that the client's X11 | |
b74c3b8c | 913 | display server may be exposed to attack when the SSH client requests |
e6fe1bab | 914 | forwarding (see the warnings for |
915 | .Cm ForwardX11 | |
916 | in | |
be193d89 | 917 | .Xr ssh_config 5 ) . |
e6fe1bab | 918 | A system administrator may have a stance in which they want to |
919 | protect clients that may expose themselves to attack by unwittingly | |
920 | requesting X11 forwarding, which can warrant a | |
921 | .Dq no | |
922 | setting. | |
923 | .Pp | |
924 | Note that disabling X11 forwarding does not prevent users from | |
925 | forwarding X11 traffic, as users can always install their own forwarders. | |
588df31a | 926 | X11 forwarding is automatically disabled if |
927 | .Cm UseLogin | |
928 | is enabled. | |
929 | .It Cm X11UseLocalhost | |
930 | Specifies whether | |
b74c3b8c | 931 | .Xr sshd 8 |
588df31a | 932 | should bind the X11 forwarding server to the loopback address or to |
a4e5acef | 933 | the wildcard address. |
934 | By default, | |
b74c3b8c | 935 | sshd binds the forwarding server to the loopback address and sets the |
588df31a | 936 | hostname part of the |
937 | .Ev DISPLAY | |
938 | environment variable to | |
939 | .Dq localhost . | |
b3641662 | 940 | This prevents remote hosts from connecting to the proxy display. |
588df31a | 941 | However, some older X11 clients may not function with this |
942 | configuration. | |
943 | .Cm X11UseLocalhost | |
944 | may be set to | |
945 | .Dq no | |
946 | to specify that the forwarding server should be bound to the wildcard | |
947 | address. | |
948 | The argument must be | |
949 | .Dq yes | |
950 | or | |
951 | .Dq no . | |
952 | The default is | |
953 | .Dq yes . | |
954 | .It Cm XAuthLocation | |
57ff5eeb | 955 | Specifies the full pathname of the |
588df31a | 956 | .Xr xauth 1 |
957 | program. | |
958 | The default is | |
959 | .Pa /usr/X11R6/bin/xauth . | |
960 | .El | |
ef1c6497 | 961 | .Sh TIME FORMATS |
32cfd177 | 962 | .Xr sshd 8 |
588df31a | 963 | command-line arguments and configuration file options that specify time |
964 | may be expressed using a sequence of the form: | |
965 | .Sm off | |
36535ee6 | 966 | .Ar time Op Ar qualifier , |
588df31a | 967 | .Sm on |
968 | where | |
969 | .Ar time | |
970 | is a positive integer value and | |
971 | .Ar qualifier | |
972 | is one of the following: | |
973 | .Pp | |
974 | .Bl -tag -width Ds -compact -offset indent | |
874d319b | 975 | .It Aq Cm none |
588df31a | 976 | seconds |
977 | .It Cm s | Cm S | |
978 | seconds | |
979 | .It Cm m | Cm M | |
980 | minutes | |
981 | .It Cm h | Cm H | |
982 | hours | |
983 | .It Cm d | Cm D | |
984 | days | |
985 | .It Cm w | Cm W | |
986 | weeks | |
987 | .El | |
988 | .Pp | |
989 | Each member of the sequence is added together to calculate | |
990 | the total time value. | |
991 | .Pp | |
992 | Time format examples: | |
993 | .Pp | |
994 | .Bl -tag -width Ds -compact -offset indent | |
995 | .It 600 | |
996 | 600 seconds (10 minutes) | |
997 | .It 10m | |
998 | 10 minutes | |
999 | .It 1h30m | |
1000 | 1 hour 30 minutes (90 minutes) | |
1001 | .El | |
1002 | .Sh FILES | |
1003 | .Bl -tag -width Ds | |
1004 | .It Pa /etc/ssh/sshd_config | |
1005 | Contains configuration data for | |
32cfd177 | 1006 | .Xr sshd 8 . |
588df31a | 1007 | This file should be writable by root only, but it is recommended |
1008 | (though not necessary) that it be world-readable. | |
1009 | .El | |
be193d89 | 1010 | .Sh SEE ALSO |
1011 | .Xr sshd 8 | |
588df31a | 1012 | .Sh AUTHORS |
1013 | OpenSSH is a derivative of the original and free | |
1014 | ssh 1.2.12 release by Tatu Ylonen. | |
1015 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
1016 | Theo de Raadt and Dug Song | |
1017 | removed many bugs, re-added newer features and | |
1018 | created OpenSSH. | |
1019 | Markus Friedl contributed the support for SSH | |
1020 | protocol versions 1.5 and 2.0. | |
1021 | Niels Provos and Markus Friedl contributed support | |
1022 | for privilege separation. |