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884dc78b | 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 | .\" | |
e74dc197 | 37 | .\" $OpenBSD: sshd_config.5,v 1.84 2008/03/25 11:58:02 djm Exp $ |
38 | .Dd $Mdocdate: March 27 2008 $ | |
884dc78b | 39 | .Dt SSHD_CONFIG 5 |
40 | .Os | |
41 | .Sh NAME | |
42 | .Nm sshd_config | |
43 | .Nd OpenSSH SSH daemon configuration file | |
44 | .Sh SYNOPSIS | |
0b90ac93 | 45 | .Nm /etc/ssh/sshd_config |
884dc78b | 46 | .Sh DESCRIPTION |
30460aeb | 47 | .Xr sshd 8 |
884dc78b | 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. | |
30460aeb | 57 | Arguments may optionally be enclosed in double quotes |
58 | .Pq \&" | |
59 | in order to represent arguments containing spaces. | |
884dc78b | 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 | |
7e82606e | 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. | |
74 | Note that environment passing is only supported for protocol 2. | |
75 | Variables are specified by name, which may contain the wildcard characters | |
30460aeb | 76 | .Ql * |
7e82606e | 77 | and |
78 | .Ql \&? . | |
79 | Multiple environment variables may be separated by whitespace or spread | |
80 | across multiple | |
81 | .Cm AcceptEnv | |
82 | directives. | |
83 | Be warned that some environment variables could be used to bypass restricted | |
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. | |
dfddba3d | 87 | .It Cm AddressFamily |
88 | Specifies which address family should be used by | |
30460aeb | 89 | .Xr sshd 8 . |
dfddba3d | 90 | Valid arguments are |
91 | .Dq any , | |
92 | .Dq inet | |
30460aeb | 93 | (use IPv4 only), or |
dfddba3d | 94 | .Dq inet6 |
95 | (use IPv6 only). | |
96 | The default is | |
97 | .Dq any . | |
884dc78b | 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. | |
884dc78b | 103 | Only group names are valid; a numerical group ID is not recognized. |
104 | By default, login is allowed for all groups. | |
30460aeb | 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 . | |
111 | .Pp | |
112 | See | |
113 | .Sx PATTERNS | |
114 | in | |
115 | .Xr ssh_config 5 | |
116 | for more information on patterns. | |
884dc78b | 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. | |
884dc78b | 124 | .It Cm AllowUsers |
125 | This keyword can be followed by a list of user name patterns, separated | |
126 | by spaces. | |
bfe49944 | 127 | If specified, login is allowed only for user names that |
884dc78b | 128 | match one of the patterns. |
884dc78b | 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. | |
30460aeb | 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 . | |
140 | .Pp | |
141 | See | |
142 | .Sx PATTERNS | |
143 | in | |
144 | .Xr ssh_config 5 | |
145 | for more information on patterns. | |
884dc78b | 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 | |
30460aeb | 151 | setup. |
7cac2b65 | 152 | The following tokens are defined: %% is replaced by a literal '%', |
30460aeb | 153 | %h is replaced by the home directory of the user being authenticated, and |
884dc78b | 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 | |
884dc78b | 162 | The contents of the specified file are sent to the remote user before |
163 | authentication is allowed. | |
e74dc197 | 164 | If the argument is |
165 | .Dq none | |
166 | then no banner is displayed. | |
884dc78b | 167 | This option is only available for protocol version 2. |
168 | By default, no banner is displayed. | |
884dc78b | 169 | .It Cm ChallengeResponseAuthentication |
30460aeb | 170 | Specifies whether challenge-response authentication is allowed. |
884dc78b | 171 | All authentication styles from |
172 | .Xr login.conf 5 | |
173 | are supported. | |
174 | The default is | |
175 | .Dq yes . | |
e74dc197 | 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 . | |
884dc78b | 215 | .It Cm Ciphers |
216 | Specifies the ciphers allowed for protocol version 2. | |
217 | Multiple ciphers must be comma-separated. | |
7e82606e | 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 , | |
2ce0bfe4 | 226 | .Dq arcfour128 , |
227 | .Dq arcfour256 , | |
7e82606e | 228 | .Dq arcfour , |
229 | .Dq blowfish-cbc , | |
230 | and | |
231 | .Dq cast128-cbc . | |
30460aeb | 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 | |
884dc78b | 237 | .Ed |
884dc78b | 238 | .It Cm ClientAliveCountMax |
08822d99 | 239 | Sets the number of client alive messages (see below) which may be |
884dc78b | 240 | sent without |
30460aeb | 241 | .Xr sshd 8 |
7cac2b65 | 242 | receiving any messages back from the client. |
243 | If this threshold is reached while client alive messages are being sent, | |
30460aeb | 244 | sshd will disconnect the client, terminating the session. |
7cac2b65 | 245 | It is important to note that the use of client alive messages is very |
246 | different from | |
540d72c3 | 247 | .Cm TCPKeepAlive |
7cac2b65 | 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 | |
540d72c3 | 252 | .Cm TCPKeepAlive |
7cac2b65 | 253 | is spoofable. |
254 | The client alive mechanism is valuable when the client or | |
884dc78b | 255 | server depend on knowing when a connection has become inactive. |
256 | .Pp | |
7cac2b65 | 257 | The default value is 3. |
258 | If | |
884dc78b | 259 | .Cm ClientAliveInterval |
08822d99 | 260 | (see below) is set to 15, and |
884dc78b | 261 | .Cm ClientAliveCountMax |
30460aeb | 262 | is left at the default, unresponsive SSH clients |
884dc78b | 263 | will be disconnected after approximately 45 seconds. |
30460aeb | 264 | This option applies to protocol version 2 only. |
2ce0bfe4 | 265 | .It Cm ClientAliveInterval |
266 | Sets a timeout interval in seconds after which if no data has been received | |
267 | from the client, | |
30460aeb | 268 | .Xr sshd 8 |
2ce0bfe4 | 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. | |
884dc78b | 274 | .It Cm Compression |
2ce0bfe4 | 275 | Specifies whether compression is allowed, or delayed until |
276 | the user has authenticated successfully. | |
884dc78b | 277 | The argument must be |
2ce0bfe4 | 278 | .Dq yes , |
279 | .Dq delayed , | |
884dc78b | 280 | or |
281 | .Dq no . | |
282 | The default is | |
2ce0bfe4 | 283 | .Dq delayed . |
884dc78b | 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. | |
884dc78b | 289 | Only group names are valid; a numerical group ID is not recognized. |
290 | By default, login is allowed for all groups. | |
30460aeb | 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 . | |
297 | .Pp | |
298 | See | |
299 | .Sx PATTERNS | |
300 | in | |
301 | .Xr ssh_config 5 | |
302 | for more information on patterns. | |
884dc78b | 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. | |
884dc78b | 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. | |
30460aeb | 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 . | |
318 | .Pp | |
319 | See | |
320 | .Sx PATTERNS | |
321 | in | |
322 | .Xr ssh_config 5 | |
323 | for more information on patterns. | |
324 | .It Cm ForceCommand | |
325 | Forces the execution of the command specified by | |
326 | .Cm ForceCommand , | |
e74dc197 | 327 | ignoring any command supplied by the client and |
328 | .Pa ~/.ssh/rc | |
329 | if present. | |
30460aeb | 330 | The command is invoked by using the user's login shell with the -c option. |
331 | This applies to shell, command, or subsystem execution. | |
332 | It is most useful inside a | |
333 | .Cm Match | |
334 | block. | |
335 | The command originally supplied by the client is available in the | |
336 | .Ev SSH_ORIGINAL_COMMAND | |
337 | environment variable. | |
e74dc197 | 338 | Specifying a command of |
339 | .Dq internal-sftp | |
340 | will force the use of an in-process sftp server that requires no support | |
341 | files when used with | |
342 | .Cm ChrootDirectory . | |
884dc78b | 343 | .It Cm GatewayPorts |
344 | Specifies whether remote hosts are allowed to connect to ports | |
345 | forwarded for the client. | |
346 | By default, | |
30460aeb | 347 | .Xr sshd 8 |
bfe49944 | 348 | binds remote port forwardings to the loopback address. |
349 | This prevents other remote hosts from connecting to forwarded ports. | |
884dc78b | 350 | .Cm GatewayPorts |
30460aeb | 351 | can be used to specify that sshd |
dfddba3d | 352 | should allow remote port forwardings to bind to non-loopback addresses, thus |
353 | allowing other hosts to connect. | |
354 | The argument may be | |
355 | .Dq no | |
356 | to force remote port forwardings to be available to the local host only, | |
884dc78b | 357 | .Dq yes |
dfddba3d | 358 | to force remote port forwardings to bind to the wildcard address, or |
359 | .Dq clientspecified | |
360 | to allow the client to select the address to which the forwarding is bound. | |
884dc78b | 361 | The default is |
362 | .Dq no . | |
7cac2b65 | 363 | .It Cm GSSAPIAuthentication |
364 | Specifies whether user authentication based on GSSAPI is allowed. | |
540d72c3 | 365 | The default is |
c5448518 | 366 | .Dq yes . |
7cac2b65 | 367 | Note that this option applies to protocol version 2 only. |
05ed7e1e | 368 | .It Cm GSSAPIDelegateCredentials |
369 | Specifies whether delegated credentials are stored in the user's environment. | |
370 | The default is | |
371 | .Dq yes . | |
c5448518 | 372 | .It Cm GSSAPIKeyExchange |
fe4ad273 | 373 | Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange |
374 | doesn't rely on ssh keys to verify host identity. | |
44a053a3 | 375 | The default is |
376 | .Dq yes . | |
c5448518 | 377 | Note that this option applies to protocol version 2 only. |
fe4ad273 | 378 | .It Cm GSSAPICleanupCredentials |
379 | Specifies whether to automatically destroy the user's credentials cache | |
380 | on logout. | |
44a053a3 | 381 | The default is |
382 | .Dq yes . | |
c5448518 | 383 | Note that this option applies to protocol version 2 only. |
f713db99 | 384 | .It Cm GSSAPIStrictAcceptorCheck |
385 | Determines whether to be strict about the identity of the GSSAPI acceptor | |
386 | a client authenticates against. If | |
387 | .Dq yes | |
388 | then the client must authenticate against the | |
389 | .Pa host | |
390 | service on the current hostname. If | |
391 | .Dq no | |
392 | then the client may authenticate against any service key stored in the | |
393 | machine's default store. This facility is provided to assist with operation | |
394 | on multi homed machines. | |
395 | The default is | |
396 | .Dq yes . | |
397 | Note that this option applies only to protocol version 2 GSSAPI connections, | |
398 | and setting it to | |
399 | .Dq no | |
400 | may only work with recent Kerberos GSSAPI libraries. | |
ae82558b | 401 | .It Cm GSSAPICredentialsPath |
402 | If specified, the delegated GSSAPI credential is stored in the | |
403 | given path, overwriting any existing credentials. | |
404 | Paths can be specified with syntax similar to the AuthorizedKeysFile | |
405 | option (i.e., accepting %h and %u tokens). | |
406 | When using this option, | |
407 | setting 'GssapiCleanupCredentials no' is recommended, | |
408 | so logging out of one session | |
409 | doesn't remove the credentials in use by another session of | |
410 | the same user. | |
411 | Currently only implemented for the GSI mechanism. | |
826a9049 | 412 | .It Cm GSIAllowLimitedProxy |
413 | Specifies whether to accept limited proxy credentials for | |
414 | authentication. | |
415 | The default is | |
416 | .Dq no . | |
c5448518 | 417 | .It Cm HostbasedAuthentication |
418 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
419 | with successful public key client host authentication is allowed | |
30460aeb | 420 | (host-based authentication). |
c5448518 | 421 | This option is similar to |
422 | .Cm RhostsRSAAuthentication | |
423 | and applies to protocol version 2 only. | |
52b36949 | 424 | The default is |
c5448518 | 425 | .Dq no . |
30460aeb | 426 | .It Cm HostbasedUsesNameFromPacketOnly |
427 | Specifies whether or not the server will attempt to perform a reverse | |
428 | name lookup when matching the name in the | |
429 | .Pa ~/.shosts , | |
430 | .Pa ~/.rhosts , | |
431 | and | |
432 | .Pa /etc/hosts.equiv | |
433 | files during | |
434 | .Cm HostbasedAuthentication . | |
435 | A setting of | |
436 | .Dq yes | |
437 | means that | |
438 | .Xr sshd 8 | |
439 | uses the name supplied by the client rather than | |
440 | attempting to resolve the name from the TCP connection itself. | |
441 | The default is | |
442 | .Dq no . | |
884dc78b | 443 | .It Cm HostKey |
444 | Specifies a file containing a private host key | |
445 | used by SSH. | |
446 | The default is | |
447 | .Pa /etc/ssh/ssh_host_key | |
448 | for protocol version 1, and | |
449 | .Pa /etc/ssh/ssh_host_rsa_key | |
450 | and | |
451 | .Pa /etc/ssh/ssh_host_dsa_key | |
452 | for protocol version 2. | |
453 | Note that | |
30460aeb | 454 | .Xr sshd 8 |
884dc78b | 455 | will refuse to use a file if it is group/world-accessible. |
456 | It is possible to have multiple host key files. | |
457 | .Dq rsa1 | |
458 | keys are used for version 1 and | |
459 | .Dq dsa | |
460 | or | |
461 | .Dq rsa | |
462 | are used for version 2 of the SSH protocol. | |
463 | .It Cm IgnoreRhosts | |
464 | Specifies that | |
465 | .Pa .rhosts | |
466 | and | |
467 | .Pa .shosts | |
468 | files will not be used in | |
884dc78b | 469 | .Cm RhostsRSAAuthentication |
470 | or | |
471 | .Cm HostbasedAuthentication . | |
472 | .Pp | |
473 | .Pa /etc/hosts.equiv | |
474 | and | |
475 | .Pa /etc/shosts.equiv | |
476 | are still used. | |
477 | The default is | |
478 | .Dq yes . | |
479 | .It Cm IgnoreUserKnownHosts | |
480 | Specifies whether | |
30460aeb | 481 | .Xr sshd 8 |
884dc78b | 482 | should ignore the user's |
2ce0bfe4 | 483 | .Pa ~/.ssh/known_hosts |
884dc78b | 484 | during |
485 | .Cm RhostsRSAAuthentication | |
486 | or | |
487 | .Cm HostbasedAuthentication . | |
488 | The default is | |
489 | .Dq no . | |
884dc78b | 490 | .It Cm KerberosAuthentication |
7cac2b65 | 491 | Specifies whether the password provided by the user for |
884dc78b | 492 | .Cm PasswordAuthentication |
7cac2b65 | 493 | will be validated through the Kerberos KDC. |
884dc78b | 494 | To use this option, the server needs a |
495 | Kerberos servtab which allows the verification of the KDC's identity. | |
30460aeb | 496 | The default is |
884dc78b | 497 | .Dq no . |
12a403af | 498 | .It Cm KerberosGetAFSToken |
08822d99 | 499 | If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire |
12a403af | 500 | an AFS token before accessing the user's home directory. |
30460aeb | 501 | The default is |
12a403af | 502 | .Dq no . |
884dc78b | 503 | .It Cm KerberosOrLocalPasswd |
30460aeb | 504 | If password authentication through Kerberos fails then |
884dc78b | 505 | the password will be validated via any additional local mechanism |
506 | such as | |
507 | .Pa /etc/passwd . | |
30460aeb | 508 | The default is |
884dc78b | 509 | .Dq yes . |
884dc78b | 510 | .It Cm KerberosTicketCleanup |
511 | Specifies whether to automatically destroy the user's ticket cache | |
512 | file on logout. | |
30460aeb | 513 | The default is |
884dc78b | 514 | .Dq yes . |
515 | .It Cm KeyRegenerationInterval | |
516 | In protocol version 1, the ephemeral server key is automatically regenerated | |
517 | after this many seconds (if it has been used). | |
518 | The purpose of regeneration is to prevent | |
519 | decrypting captured sessions by later breaking into the machine and | |
520 | stealing the keys. | |
521 | The key is never stored anywhere. | |
522 | If the value is 0, the key is never regenerated. | |
523 | The default is 3600 (seconds). | |
524 | .It Cm ListenAddress | |
525 | Specifies the local addresses | |
30460aeb | 526 | .Xr sshd 8 |
884dc78b | 527 | should listen on. |
528 | The following forms may be used: | |
529 | .Pp | |
530 | .Bl -item -offset indent -compact | |
531 | .It | |
532 | .Cm ListenAddress | |
533 | .Sm off | |
534 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr | |
535 | .Sm on | |
536 | .It | |
537 | .Cm ListenAddress | |
538 | .Sm off | |
539 | .Ar host No | Ar IPv4_addr No : Ar port | |
540 | .Sm on | |
541 | .It | |
542 | .Cm ListenAddress | |
543 | .Sm off | |
544 | .Oo | |
545 | .Ar host No | Ar IPv6_addr Oc : Ar port | |
546 | .Sm on | |
547 | .El | |
548 | .Pp | |
549 | If | |
550 | .Ar port | |
551 | is not specified, | |
30460aeb | 552 | sshd will listen on the address and all prior |
884dc78b | 553 | .Cm Port |
7cac2b65 | 554 | options specified. |
555 | The default is to listen on all local addresses. | |
bfe49944 | 556 | Multiple |
884dc78b | 557 | .Cm ListenAddress |
7cac2b65 | 558 | options are permitted. |
559 | Additionally, any | |
884dc78b | 560 | .Cm Port |
30460aeb | 561 | options must precede this option for non-port qualified addresses. |
884dc78b | 562 | .It Cm LoginGraceTime |
563 | The server disconnects after this time if the user has not | |
564 | successfully logged in. | |
565 | If the value is 0, there is no time limit. | |
d03f4262 | 566 | The default is 120 seconds. |
884dc78b | 567 | .It Cm LogLevel |
568 | Gives the verbosity level that is used when logging messages from | |
30460aeb | 569 | .Xr sshd 8 . |
884dc78b | 570 | The possible values are: |
30460aeb | 571 | QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. |
bfe49944 | 572 | The default is INFO. |
573 | DEBUG and DEBUG1 are equivalent. | |
574 | DEBUG2 and DEBUG3 each specify higher levels of debugging output. | |
575 | Logging with a DEBUG level violates the privacy of users and is not recommended. | |
884dc78b | 576 | .It Cm MACs |
577 | Specifies the available MAC (message authentication code) algorithms. | |
578 | The MAC algorithm is used in protocol version 2 | |
579 | for data integrity protection. | |
580 | Multiple algorithms must be comma-separated. | |
30460aeb | 581 | The default is: |
fa0f0f45 | 582 | .Bd -literal -offset indent |
583 | hmac-md5,hmac-sha1,umac-64@openssh.com, | |
584 | hmac-ripemd160,hmac-sha1-96,hmac-md5-96 | |
585 | .Ed | |
30460aeb | 586 | .It Cm Match |
587 | Introduces a conditional block. | |
588 | If all of the criteria on the | |
589 | .Cm Match | |
590 | line are satisfied, the keywords on the following lines override those | |
591 | set in the global section of the config file, until either another | |
592 | .Cm Match | |
593 | line or the end of the file. | |
594 | The arguments to | |
595 | .Cm Match | |
596 | are one or more criteria-pattern pairs. | |
597 | The available criteria are | |
598 | .Cm User , | |
599 | .Cm Group , | |
600 | .Cm Host , | |
601 | and | |
602 | .Cm Address . | |
603 | Only a subset of keywords may be used on the lines following a | |
604 | .Cm Match | |
605 | keyword. | |
606 | Available keywords are | |
607 | .Cm AllowTcpForwarding , | |
0b90ac93 | 608 | .Cm Banner , |
30460aeb | 609 | .Cm ForceCommand , |
610 | .Cm GatewayPorts , | |
0b90ac93 | 611 | .Cm GSSApiAuthentication , |
612 | .Cm KbdInteractiveAuthentication , | |
613 | .Cm KerberosAuthentication , | |
614 | .Cm PasswordAuthentication , | |
30460aeb | 615 | .Cm PermitOpen , |
e74dc197 | 616 | .Cm PermitRootLogin , |
0b90ac93 | 617 | .Cm RhostsRSAAuthentication , |
618 | .Cm RSAAuthentication , | |
30460aeb | 619 | .Cm X11DisplayOffset , |
620 | .Cm X11Forwarding , | |
621 | and | |
622 | .Cm X11UseLocalHost . | |
7e82606e | 623 | .It Cm MaxAuthTries |
624 | Specifies the maximum number of authentication attempts permitted per | |
625 | connection. | |
626 | Once the number of failures reaches half this value, | |
627 | additional failures are logged. | |
628 | The default is 6. | |
884dc78b | 629 | .It Cm MaxStartups |
630 | Specifies the maximum number of concurrent unauthenticated connections to the | |
30460aeb | 631 | SSH daemon. |
884dc78b | 632 | Additional connections will be dropped until authentication succeeds or the |
633 | .Cm LoginGraceTime | |
634 | expires for a connection. | |
635 | The default is 10. | |
636 | .Pp | |
637 | Alternatively, random early drop can be enabled by specifying | |
638 | the three colon separated values | |
639 | .Dq start:rate:full | |
30460aeb | 640 | (e.g. "10:30:60"). |
641 | .Xr sshd 8 | |
884dc78b | 642 | will refuse connection attempts with a probability of |
643 | .Dq rate/100 | |
644 | (30%) | |
645 | if there are currently | |
646 | .Dq start | |
647 | (10) | |
648 | unauthenticated connections. | |
649 | The probability increases linearly and all connection attempts | |
650 | are refused if the number of unauthenticated connections reaches | |
651 | .Dq full | |
652 | (60). | |
653 | .It Cm PasswordAuthentication | |
654 | Specifies whether password authentication is allowed. | |
655 | The default is | |
656 | .Dq yes . | |
657 | .It Cm PermitEmptyPasswords | |
658 | When password authentication is allowed, it specifies whether the | |
659 | server allows login to accounts with empty password strings. | |
660 | The default is | |
661 | .Dq no . | |
30460aeb | 662 | .It Cm PermitOpen |
663 | Specifies the destinations to which TCP port forwarding is permitted. | |
664 | The forwarding specification must be one of the following forms: | |
665 | .Pp | |
666 | .Bl -item -offset indent -compact | |
667 | .It | |
668 | .Cm PermitOpen | |
669 | .Sm off | |
670 | .Ar host : port | |
671 | .Sm on | |
672 | .It | |
673 | .Cm PermitOpen | |
674 | .Sm off | |
675 | .Ar IPv4_addr : port | |
676 | .Sm on | |
677 | .It | |
678 | .Cm PermitOpen | |
679 | .Sm off | |
680 | .Ar \&[ IPv6_addr \&] : port | |
681 | .Sm on | |
682 | .El | |
683 | .Pp | |
684 | Multiple forwards may be specified by separating them with whitespace. | |
685 | An argument of | |
686 | .Dq any | |
687 | can be used to remove all restrictions and permit any forwarding requests. | |
688 | By default all port forwarding requests are permitted. | |
884dc78b | 689 | .It Cm PermitRootLogin |
dfddba3d | 690 | Specifies whether root can log in using |
884dc78b | 691 | .Xr ssh 1 . |
692 | The argument must be | |
693 | .Dq yes , | |
694 | .Dq without-password , | |
30460aeb | 695 | .Dq forced-commands-only , |
884dc78b | 696 | or |
697 | .Dq no . | |
698 | The default is | |
699 | .Dq yes . | |
700 | .Pp | |
701 | If this option is set to | |
30460aeb | 702 | .Dq without-password , |
dfddba3d | 703 | password authentication is disabled for root. |
884dc78b | 704 | .Pp |
705 | If this option is set to | |
30460aeb | 706 | .Dq forced-commands-only , |
884dc78b | 707 | root login with public key authentication will be allowed, |
708 | but only if the | |
709 | .Ar command | |
710 | option has been specified | |
711 | (which may be useful for taking remote backups even if root login is | |
7cac2b65 | 712 | normally not allowed). |
713 | All other authentication methods are disabled for root. | |
884dc78b | 714 | .Pp |
715 | If this option is set to | |
30460aeb | 716 | .Dq no , |
dfddba3d | 717 | root is not allowed to log in. |
08822d99 | 718 | .It Cm PermitTunnel |
719 | Specifies whether | |
720 | .Xr tun 4 | |
721 | device forwarding is allowed. | |
722 | The argument must be | |
723 | .Dq yes , | |
30460aeb | 724 | .Dq point-to-point |
725 | (layer 3), | |
08822d99 | 726 | .Dq ethernet |
30460aeb | 727 | (layer 2), or |
08822d99 | 728 | .Dq no . |
30460aeb | 729 | Specifying |
730 | .Dq yes | |
731 | permits both | |
732 | .Dq point-to-point | |
733 | and | |
734 | .Dq ethernet . | |
08822d99 | 735 | The default is |
736 | .Dq no . | |
d03f4262 | 737 | .It Cm PermitUserEnvironment |
738 | Specifies whether | |
739 | .Pa ~/.ssh/environment | |
740 | and | |
741 | .Cm environment= | |
742 | options in | |
743 | .Pa ~/.ssh/authorized_keys | |
744 | are processed by | |
30460aeb | 745 | .Xr sshd 8 . |
d03f4262 | 746 | The default is |
747 | .Dq no . | |
748 | Enabling environment processing may enable users to bypass access | |
749 | restrictions in some configurations using mechanisms such as | |
750 | .Ev LD_PRELOAD . | |
884dc78b | 751 | .It Cm PidFile |
276b07a3 | 752 | Specifies the file that contains the process ID of the |
30460aeb | 753 | SSH daemon. |
884dc78b | 754 | The default is |
755 | .Pa /var/run/sshd.pid . | |
756 | .It Cm Port | |
757 | Specifies the port number that | |
30460aeb | 758 | .Xr sshd 8 |
884dc78b | 759 | listens on. |
760 | The default is 22. | |
761 | Multiple options of this type are permitted. | |
762 | See also | |
763 | .Cm ListenAddress . | |
764 | .It Cm PrintLastLog | |
765 | Specifies whether | |
30460aeb | 766 | .Xr sshd 8 |
dfddba3d | 767 | should print the date and time of the last user login when a user logs |
768 | in interactively. | |
884dc78b | 769 | The default is |
770 | .Dq yes . | |
771 | .It Cm PrintMotd | |
772 | Specifies whether | |
30460aeb | 773 | .Xr sshd 8 |
884dc78b | 774 | should print |
775 | .Pa /etc/motd | |
776 | when a user logs in interactively. | |
777 | (On some systems it is also printed by the shell, | |
778 | .Pa /etc/profile , | |
779 | or equivalent.) | |
780 | The default is | |
781 | .Dq yes . | |
782 | .It Cm Protocol | |
783 | Specifies the protocol versions | |
30460aeb | 784 | .Xr sshd 8 |
d03f4262 | 785 | supports. |
884dc78b | 786 | The possible values are |
30460aeb | 787 | .Sq 1 |
884dc78b | 788 | and |
30460aeb | 789 | .Sq 2 . |
884dc78b | 790 | Multiple versions must be comma-separated. |
791 | The default is | |
792 | .Dq 2,1 . | |
d03f4262 | 793 | Note that the order of the protocol list does not indicate preference, |
794 | because the client selects among multiple protocol versions offered | |
795 | by the server. | |
796 | Specifying | |
797 | .Dq 2,1 | |
798 | is identical to | |
799 | .Dq 1,2 . | |
884dc78b | 800 | .It Cm PubkeyAuthentication |
801 | Specifies whether public key authentication is allowed. | |
802 | The default is | |
803 | .Dq yes . | |
804 | Note that this option applies to protocol version 2 only. | |
884dc78b | 805 | .It Cm RhostsRSAAuthentication |
806 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
807 | with successful RSA host authentication is allowed. | |
808 | The default is | |
809 | .Dq no . | |
810 | This option applies to protocol version 1 only. | |
811 | .It Cm RSAAuthentication | |
812 | Specifies whether pure RSA authentication is allowed. | |
813 | The default is | |
814 | .Dq yes . | |
815 | This option applies to protocol version 1 only. | |
816 | .It Cm ServerKeyBits | |
817 | Defines the number of bits in the ephemeral protocol version 1 server key. | |
818 | The minimum value is 512, and the default is 768. | |
819 | .It Cm StrictModes | |
820 | Specifies whether | |
30460aeb | 821 | .Xr sshd 8 |
884dc78b | 822 | should check file modes and ownership of the |
823 | user's files and home directory before accepting login. | |
824 | This is normally desirable because novices sometimes accidentally leave their | |
825 | directory or files world-writable. | |
826 | The default is | |
827 | .Dq yes . | |
828 | .It Cm Subsystem | |
30460aeb | 829 | Configures an external subsystem (e.g. file transfer daemon). |
830 | Arguments should be a subsystem name and a command (with optional arguments) | |
831 | to execute upon subsystem request. | |
e74dc197 | 832 | .Pp |
884dc78b | 833 | The command |
834 | .Xr sftp-server 8 | |
835 | implements the | |
836 | .Dq sftp | |
837 | file transfer subsystem. | |
e74dc197 | 838 | .Pp |
839 | Alternately the name | |
840 | .Dq internal-sftp | |
841 | implements an in-process | |
842 | .Dq sftp | |
843 | server. | |
844 | This may simplify configurations using | |
845 | .Cm ChrootDirectory | |
846 | to force a different filesystem root on clients. | |
847 | .Pp | |
884dc78b | 848 | By default no subsystems are defined. |
849 | Note that this option applies to protocol version 2 only. | |
850 | .It Cm SyslogFacility | |
851 | Gives the facility code that is used when logging messages from | |
30460aeb | 852 | .Xr sshd 8 . |
884dc78b | 853 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, |
854 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. | |
855 | The default is AUTH. | |
540d72c3 | 856 | .It Cm TCPKeepAlive |
857 | Specifies whether the system should send TCP keepalive messages to the | |
858 | other side. | |
859 | If they are sent, death of the connection or crash of one | |
860 | of the machines will be properly noticed. | |
861 | However, this means that | |
862 | connections will die if the route is down temporarily, and some people | |
863 | find it annoying. | |
864 | On the other hand, if TCP keepalives are not sent, | |
865 | sessions may hang indefinitely on the server, leaving | |
866 | .Dq ghost | |
867 | users and consuming server resources. | |
868 | .Pp | |
869 | The default is | |
870 | .Dq yes | |
871 | (to send TCP keepalive messages), and the server will notice | |
872 | if the network goes down or the client host crashes. | |
873 | This avoids infinitely hanging sessions. | |
874 | .Pp | |
875 | To disable TCP keepalive messages, the value should be set to | |
876 | .Dq no . | |
7cac2b65 | 877 | .It Cm UseDNS |
878 | Specifies whether | |
30460aeb | 879 | .Xr sshd 8 |
8b32eddc | 880 | should look up the remote host name and check that |
7cac2b65 | 881 | the resolved host name for the remote IP address maps back to the |
882 | very same IP address. | |
883 | The default is | |
884 | .Dq yes . | |
884dc78b | 885 | .It Cm UseLogin |
886 | Specifies whether | |
887 | .Xr login 1 | |
888 | is used for interactive login sessions. | |
889 | The default is | |
890 | .Dq no . | |
891 | Note that | |
892 | .Xr login 1 | |
893 | is never used for remote command execution. | |
894 | Note also, that if this is enabled, | |
895 | .Cm X11Forwarding | |
896 | will be disabled because | |
897 | .Xr login 1 | |
898 | does not know how to handle | |
899 | .Xr xauth 1 | |
bfe49944 | 900 | cookies. |
901 | If | |
884dc78b | 902 | .Cm UsePrivilegeSeparation |
903 | is specified, it will be disabled after authentication. | |
7cac2b65 | 904 | .It Cm UsePAM |
7e82606e | 905 | Enables the Pluggable Authentication Module interface. |
906 | If set to | |
907 | .Dq yes | |
908 | this will enable PAM authentication using | |
909 | .Cm ChallengeResponseAuthentication | |
30460aeb | 910 | and |
911 | .Cm PasswordAuthentication | |
912 | in addition to PAM account and session module processing for all | |
913 | authentication types. | |
7e82606e | 914 | .Pp |
915 | Because PAM challenge-response authentication usually serves an equivalent | |
916 | role to password authentication, you should disable either | |
917 | .Cm PasswordAuthentication | |
918 | or | |
919 | .Cm ChallengeResponseAuthentication. | |
920 | .Pp | |
921 | If | |
922 | .Cm UsePAM | |
923 | is enabled, you will not be able to run | |
924 | .Xr sshd 8 | |
925 | as a non-root user. | |
926 | The default is | |
540d72c3 | 927 | .Dq no . |
884dc78b | 928 | .It Cm UsePrivilegeSeparation |
929 | Specifies whether | |
30460aeb | 930 | .Xr sshd 8 |
884dc78b | 931 | separates privileges by creating an unprivileged child process |
bfe49944 | 932 | to deal with incoming network traffic. |
933 | After successful authentication, another process will be created that has | |
934 | the privilege of the authenticated user. | |
935 | The goal of privilege separation is to prevent privilege | |
884dc78b | 936 | escalation by containing any corruption within the unprivileged processes. |
937 | The default is | |
938 | .Dq yes . | |
884dc78b | 939 | .It Cm X11DisplayOffset |
940 | Specifies the first display number available for | |
30460aeb | 941 | .Xr sshd 8 Ns 's |
884dc78b | 942 | X11 forwarding. |
30460aeb | 943 | This prevents sshd from interfering with real X11 servers. |
884dc78b | 944 | The default is 10. |
945 | .It Cm X11Forwarding | |
946 | Specifies whether X11 forwarding is permitted. | |
d03f4262 | 947 | The argument must be |
948 | .Dq yes | |
949 | or | |
950 | .Dq no . | |
884dc78b | 951 | The default is |
952 | .Dq no . | |
d03f4262 | 953 | .Pp |
954 | When X11 forwarding is enabled, there may be additional exposure to | |
955 | the server and to client displays if the | |
30460aeb | 956 | .Xr sshd 8 |
d03f4262 | 957 | proxy display is configured to listen on the wildcard address (see |
958 | .Cm X11UseLocalhost | |
30460aeb | 959 | below), though this is not the default. |
d03f4262 | 960 | Additionally, the authentication spoofing and authentication data |
961 | verification and substitution occur on the client side. | |
962 | The security risk of using X11 forwarding is that the client's X11 | |
30460aeb | 963 | display server may be exposed to attack when the SSH client requests |
d03f4262 | 964 | forwarding (see the warnings for |
965 | .Cm ForwardX11 | |
966 | in | |
7cac2b65 | 967 | .Xr ssh_config 5 ) . |
d03f4262 | 968 | A system administrator may have a stance in which they want to |
969 | protect clients that may expose themselves to attack by unwittingly | |
970 | requesting X11 forwarding, which can warrant a | |
971 | .Dq no | |
972 | setting. | |
973 | .Pp | |
974 | Note that disabling X11 forwarding does not prevent users from | |
975 | forwarding X11 traffic, as users can always install their own forwarders. | |
884dc78b | 976 | X11 forwarding is automatically disabled if |
977 | .Cm UseLogin | |
978 | is enabled. | |
979 | .It Cm X11UseLocalhost | |
980 | Specifies whether | |
30460aeb | 981 | .Xr sshd 8 |
884dc78b | 982 | should bind the X11 forwarding server to the loopback address or to |
bfe49944 | 983 | the wildcard address. |
984 | By default, | |
30460aeb | 985 | sshd binds the forwarding server to the loopback address and sets the |
884dc78b | 986 | hostname part of the |
987 | .Ev DISPLAY | |
988 | environment variable to | |
989 | .Dq localhost . | |
d03f4262 | 990 | This prevents remote hosts from connecting to the proxy display. |
884dc78b | 991 | However, some older X11 clients may not function with this |
992 | configuration. | |
993 | .Cm X11UseLocalhost | |
994 | may be set to | |
995 | .Dq no | |
996 | to specify that the forwarding server should be bound to the wildcard | |
997 | address. | |
998 | The argument must be | |
999 | .Dq yes | |
1000 | or | |
1001 | .Dq no . | |
1002 | The default is | |
1003 | .Dq yes . | |
1004 | .It Cm XAuthLocation | |
d03f4262 | 1005 | Specifies the full pathname of the |
884dc78b | 1006 | .Xr xauth 1 |
1007 | program. | |
1008 | The default is | |
1009 | .Pa /usr/X11R6/bin/xauth . | |
1010 | .El | |
30460aeb | 1011 | .Sh TIME FORMATS |
1012 | .Xr sshd 8 | |
884dc78b | 1013 | command-line arguments and configuration file options that specify time |
1014 | may be expressed using a sequence of the form: | |
1015 | .Sm off | |
d03f4262 | 1016 | .Ar time Op Ar qualifier , |
884dc78b | 1017 | .Sm on |
1018 | where | |
1019 | .Ar time | |
1020 | is a positive integer value and | |
1021 | .Ar qualifier | |
1022 | is one of the following: | |
1023 | .Pp | |
1024 | .Bl -tag -width Ds -compact -offset indent | |
30460aeb | 1025 | .It Aq Cm none |
884dc78b | 1026 | seconds |
1027 | .It Cm s | Cm S | |
1028 | seconds | |
1029 | .It Cm m | Cm M | |
1030 | minutes | |
1031 | .It Cm h | Cm H | |
1032 | hours | |
1033 | .It Cm d | Cm D | |
1034 | days | |
1035 | .It Cm w | Cm W | |
1036 | weeks | |
1037 | .El | |
1038 | .Pp | |
1039 | Each member of the sequence is added together to calculate | |
1040 | the total time value. | |
1041 | .Pp | |
1042 | Time format examples: | |
1043 | .Pp | |
1044 | .Bl -tag -width Ds -compact -offset indent | |
1045 | .It 600 | |
1046 | 600 seconds (10 minutes) | |
1047 | .It 10m | |
1048 | 10 minutes | |
1049 | .It 1h30m | |
1050 | 1 hour 30 minutes (90 minutes) | |
1051 | .El | |
1052 | .Sh FILES | |
1053 | .Bl -tag -width Ds | |
1054 | .It Pa /etc/ssh/sshd_config | |
1055 | Contains configuration data for | |
30460aeb | 1056 | .Xr sshd 8 . |
884dc78b | 1057 | This file should be writable by root only, but it is recommended |
1058 | (though not necessary) that it be world-readable. | |
1059 | .El | |
7cac2b65 | 1060 | .Sh SEE ALSO |
1061 | .Xr sshd 8 | |
884dc78b | 1062 | .Sh AUTHORS |
1063 | OpenSSH is a derivative of the original and free | |
1064 | ssh 1.2.12 release by Tatu Ylonen. | |
1065 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
1066 | Theo de Raadt and Dug Song | |
1067 | removed many bugs, re-added newer features and | |
1068 | created OpenSSH. | |
1069 | Markus Friedl contributed the support for SSH | |
1070 | protocol versions 1.5 and 2.0. | |
1071 | Niels Provos and Markus Friedl contributed support | |
1072 | for privilege separation. |