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ff2d7a98 | 1 | .\" -*- nroff -*- |
2 | .\" | |
3 | .\" Author: Tatu Ylonen <ylo@cs.hut.fi> | |
4 | .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland | |
5 | .\" All rights reserved | |
6 | .\" | |
7 | .\" As far as I am concerned, the code I have written for this software | |
8 | .\" can be used freely for any purpose. Any derived versions of this | |
9 | .\" software must be clearly marked as such, and if the derived work is | |
10 | .\" incompatible with the protocol description in the RFC file, it must be | |
11 | .\" called by a name other than "ssh" or "Secure Shell". | |
12 | .\" | |
13 | .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. | |
14 | .\" Copyright (c) 1999 Aaron Campbell. All rights reserved. | |
15 | .\" Copyright (c) 1999 Theo de Raadt. All rights reserved. | |
16 | .\" | |
17 | .\" Redistribution and use in source and binary forms, with or without | |
18 | .\" modification, are permitted provided that the following conditions | |
19 | .\" are met: | |
20 | .\" 1. Redistributions of source code must retain the above copyright | |
21 | .\" notice, this list of conditions and the following disclaimer. | |
22 | .\" 2. Redistributions in binary form must reproduce the above copyright | |
23 | .\" notice, this list of conditions and the following disclaimer in the | |
24 | .\" documentation and/or other materials provided with the distribution. | |
25 | .\" | |
26 | .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR | |
27 | .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES | |
28 | .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. | |
29 | .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, | |
30 | .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT | |
31 | .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, | |
32 | .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY | |
33 | .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT | |
34 | .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF | |
35 | .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. | |
36 | .\" | |
2e437378 | 37 | .\" $OpenBSD: sshd_config.5,v 1.70 2006/08/21 08:14:01 dtucker Exp $ |
ff2d7a98 | 38 | .Dd September 25, 1999 |
39 | .Dt SSHD_CONFIG 5 | |
40 | .Os | |
41 | .Sh NAME | |
42 | .Nm sshd_config | |
43 | .Nd OpenSSH SSH daemon configuration file | |
44 | .Sh SYNOPSIS | |
45 | .Bl -tag -width Ds -compact | |
46 | .It Pa /etc/ssh/sshd_config | |
47 | .El | |
48 | .Sh DESCRIPTION | |
2e437378 | 49 | .Xr sshd 8 |
ff2d7a98 | 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. | |
2e437378 | 59 | Arguments may optionally be enclosed in double quotes |
60 | .Pq \&" | |
61 | in order to represent arguments containing spaces. | |
ff2d7a98 | 62 | .Pp |
63 | The possible | |
64 | keywords and their meanings are as follows (note that | |
65 | keywords are case-insensitive and arguments are case-sensitive): | |
66 | .Bl -tag -width Ds | |
1b56ff3d | 67 | .It Cm AcceptEnv |
68 | Specifies what environment variables sent by the client will be copied into | |
69 | the session's | |
70 | .Xr environ 7 . | |
71 | See | |
72 | .Cm SendEnv | |
73 | in | |
74 | .Xr ssh_config 5 | |
75 | for how to configure the client. | |
76 | Note that environment passing is only supported for protocol 2. | |
77 | Variables are specified by name, which may contain the wildcard characters | |
2e437378 | 78 | .Ql * |
1b56ff3d | 79 | and |
80 | .Ql \&? . | |
81 | Multiple environment variables may be separated by whitespace or spread | |
82 | across multiple | |
83 | .Cm AcceptEnv | |
84 | directives. | |
85 | Be warned that some environment variables could be used to bypass restricted | |
86 | user environments. | |
87 | For this reason, care should be taken in the use of this directive. | |
88 | The default is not to accept any environment variables. | |
34fee935 | 89 | .It Cm AddressFamily |
90 | Specifies which address family should be used by | |
2e437378 | 91 | .Xr sshd 8 . |
34fee935 | 92 | Valid arguments are |
93 | .Dq any , | |
94 | .Dq inet | |
2e437378 | 95 | (use IPv4 only), or |
34fee935 | 96 | .Dq inet6 |
97 | (use IPv6 only). | |
98 | The default is | |
99 | .Dq any . | |
ff2d7a98 | 100 | .It Cm AllowGroups |
101 | This keyword can be followed by a list of group name patterns, separated | |
102 | by spaces. | |
103 | If specified, login is allowed only for users whose primary | |
104 | group or supplementary group list matches one of the patterns. | |
ff2d7a98 | 105 | Only group names are valid; a numerical group ID is not recognized. |
106 | By default, login is allowed for all groups. | |
2e437378 | 107 | The allow/deny directives are processed in the following order: |
108 | .Cm DenyUsers , | |
109 | .Cm AllowUsers , | |
110 | .Cm DenyGroups , | |
111 | and finally | |
112 | .Cm AllowGroups . | |
113 | .Pp | |
114 | See | |
115 | .Sx PATTERNS | |
116 | in | |
117 | .Xr ssh_config 5 | |
118 | for more information on patterns. | |
ff2d7a98 | 119 | .It Cm AllowTcpForwarding |
120 | Specifies whether TCP forwarding is permitted. | |
121 | The default is | |
122 | .Dq yes . | |
123 | Note that disabling TCP forwarding does not improve security unless | |
124 | users are also denied shell access, as they can always install their | |
125 | own forwarders. | |
ff2d7a98 | 126 | .It Cm AllowUsers |
127 | This keyword can be followed by a list of user name patterns, separated | |
128 | by spaces. | |
1c14df9e | 129 | If specified, login is allowed only for user names that |
ff2d7a98 | 130 | match one of the patterns. |
ff2d7a98 | 131 | Only user names are valid; a numerical user ID is not recognized. |
132 | By default, login is allowed for all users. | |
133 | If the pattern takes the form USER@HOST then USER and HOST | |
134 | are separately checked, restricting logins to particular | |
135 | users from particular hosts. | |
2e437378 | 136 | The allow/deny directives are processed in the following order: |
137 | .Cm DenyUsers , | |
138 | .Cm AllowUsers , | |
139 | .Cm DenyGroups , | |
140 | and finally | |
141 | .Cm AllowGroups . | |
142 | .Pp | |
143 | See | |
144 | .Sx PATTERNS | |
145 | in | |
146 | .Xr ssh_config 5 | |
147 | for more information on patterns. | |
ff2d7a98 | 148 | .It Cm AuthorizedKeysFile |
149 | Specifies the file that contains the public keys that can be used | |
150 | for user authentication. | |
151 | .Cm AuthorizedKeysFile | |
152 | may contain tokens of the form %T which are substituted during connection | |
2e437378 | 153 | setup. |
70791e56 | 154 | The following tokens are defined: %% is replaced by a literal '%', |
2e437378 | 155 | %h is replaced by the home directory of the user being authenticated, and |
ff2d7a98 | 156 | %u is replaced by the username of that user. |
157 | After expansion, | |
158 | .Cm AuthorizedKeysFile | |
159 | is taken to be an absolute path or one relative to the user's home | |
160 | directory. | |
161 | The default is | |
162 | .Dq .ssh/authorized_keys . | |
163 | .It Cm Banner | |
164 | In some jurisdictions, sending a warning message before authentication | |
165 | may be relevant for getting legal protection. | |
166 | The contents of the specified file are sent to the remote user before | |
167 | authentication is allowed. | |
168 | This option is only available for protocol version 2. | |
169 | By default, no banner is displayed. | |
ff2d7a98 | 170 | .It Cm ChallengeResponseAuthentication |
2e437378 | 171 | Specifies whether challenge-response authentication is allowed. |
ff2d7a98 | 172 | All authentication styles from |
173 | .Xr login.conf 5 | |
174 | are supported. | |
175 | The default is | |
176 | .Dq yes . | |
177 | .It Cm Ciphers | |
178 | Specifies the ciphers allowed for protocol version 2. | |
179 | Multiple ciphers must be comma-separated. | |
1b56ff3d | 180 | The supported ciphers are |
181 | .Dq 3des-cbc , | |
182 | .Dq aes128-cbc , | |
183 | .Dq aes192-cbc , | |
184 | .Dq aes256-cbc , | |
185 | .Dq aes128-ctr , | |
186 | .Dq aes192-ctr , | |
187 | .Dq aes256-ctr , | |
34fee935 | 188 | .Dq arcfour128 , |
189 | .Dq arcfour256 , | |
1b56ff3d | 190 | .Dq arcfour , |
191 | .Dq blowfish-cbc , | |
192 | and | |
193 | .Dq cast128-cbc . | |
2e437378 | 194 | The default is: |
195 | .Bd -literal -offset 3n | |
196 | aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, | |
197 | arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, | |
198 | aes192-ctr,aes256-ctr | |
ff2d7a98 | 199 | .Ed |
ff2d7a98 | 200 | .It Cm ClientAliveCountMax |
e00da40d | 201 | Sets the number of client alive messages (see below) which may be |
ff2d7a98 | 202 | sent without |
2e437378 | 203 | .Xr sshd 8 |
70791e56 | 204 | receiving any messages back from the client. |
205 | If this threshold is reached while client alive messages are being sent, | |
2e437378 | 206 | sshd will disconnect the client, terminating the session. |
70791e56 | 207 | It is important to note that the use of client alive messages is very |
208 | different from | |
416fd2a8 | 209 | .Cm TCPKeepAlive |
70791e56 | 210 | (below). |
211 | The client alive messages are sent through the encrypted channel | |
212 | and therefore will not be spoofable. | |
213 | The TCP keepalive option enabled by | |
416fd2a8 | 214 | .Cm TCPKeepAlive |
70791e56 | 215 | is spoofable. |
216 | The client alive mechanism is valuable when the client or | |
ff2d7a98 | 217 | server depend on knowing when a connection has become inactive. |
218 | .Pp | |
70791e56 | 219 | The default value is 3. |
220 | If | |
ff2d7a98 | 221 | .Cm ClientAliveInterval |
e00da40d | 222 | (see below) is set to 15, and |
ff2d7a98 | 223 | .Cm ClientAliveCountMax |
2e437378 | 224 | is left at the default, unresponsive SSH clients |
ff2d7a98 | 225 | will be disconnected after approximately 45 seconds. |
2e437378 | 226 | This option applies to protocol version 2 only. |
34fee935 | 227 | .It Cm ClientAliveInterval |
228 | Sets a timeout interval in seconds after which if no data has been received | |
229 | from the client, | |
2e437378 | 230 | .Xr sshd 8 |
34fee935 | 231 | will send a message through the encrypted |
232 | channel to request a response from the client. | |
233 | The default | |
234 | is 0, indicating that these messages will not be sent to the client. | |
235 | This option applies to protocol version 2 only. | |
ff2d7a98 | 236 | .It Cm Compression |
34fee935 | 237 | Specifies whether compression is allowed, or delayed until |
238 | the user has authenticated successfully. | |
ff2d7a98 | 239 | The argument must be |
34fee935 | 240 | .Dq yes , |
241 | .Dq delayed , | |
ff2d7a98 | 242 | or |
243 | .Dq no . | |
244 | The default is | |
34fee935 | 245 | .Dq delayed . |
ff2d7a98 | 246 | .It Cm DenyGroups |
247 | This keyword can be followed by a list of group name patterns, separated | |
248 | by spaces. | |
249 | Login is disallowed for users whose primary group or supplementary | |
250 | group list matches one of the patterns. | |
ff2d7a98 | 251 | Only group names are valid; a numerical group ID is not recognized. |
252 | By default, login is allowed for all groups. | |
2e437378 | 253 | The allow/deny directives are processed in the following order: |
254 | .Cm DenyUsers , | |
255 | .Cm AllowUsers , | |
256 | .Cm DenyGroups , | |
257 | and finally | |
258 | .Cm AllowGroups . | |
259 | .Pp | |
260 | See | |
261 | .Sx PATTERNS | |
262 | in | |
263 | .Xr ssh_config 5 | |
264 | for more information on patterns. | |
ff2d7a98 | 265 | .It Cm DenyUsers |
266 | This keyword can be followed by a list of user name patterns, separated | |
267 | by spaces. | |
268 | Login is disallowed for user names that match one of the patterns. | |
ff2d7a98 | 269 | Only user names are valid; a numerical user ID is not recognized. |
270 | By default, login is allowed for all users. | |
271 | If the pattern takes the form USER@HOST then USER and HOST | |
272 | are separately checked, restricting logins to particular | |
273 | users from particular hosts. | |
2e437378 | 274 | The allow/deny directives are processed in the following order: |
275 | .Cm DenyUsers , | |
276 | .Cm AllowUsers , | |
277 | .Cm DenyGroups , | |
278 | and finally | |
279 | .Cm AllowGroups . | |
280 | .Pp | |
281 | See | |
282 | .Sx PATTERNS | |
283 | in | |
284 | .Xr ssh_config 5 | |
285 | for more information on patterns. | |
286 | .It Cm ForceCommand | |
287 | Forces the execution of the command specified by | |
288 | .Cm ForceCommand , | |
289 | ignoring any command supplied by the client. | |
290 | The command is invoked by using the user's login shell with the -c option. | |
291 | This applies to shell, command, or subsystem execution. | |
292 | It is most useful inside a | |
293 | .Cm Match | |
294 | block. | |
295 | The command originally supplied by the client is available in the | |
296 | .Ev SSH_ORIGINAL_COMMAND | |
297 | environment variable. | |
ff2d7a98 | 298 | .It Cm GatewayPorts |
299 | Specifies whether remote hosts are allowed to connect to ports | |
300 | forwarded for the client. | |
301 | By default, | |
2e437378 | 302 | .Xr sshd 8 |
1c14df9e | 303 | binds remote port forwardings to the loopback address. |
304 | This prevents other remote hosts from connecting to forwarded ports. | |
ff2d7a98 | 305 | .Cm GatewayPorts |
2e437378 | 306 | can be used to specify that sshd |
34fee935 | 307 | should allow remote port forwardings to bind to non-loopback addresses, thus |
308 | allowing other hosts to connect. | |
309 | The argument may be | |
310 | .Dq no | |
311 | to force remote port forwardings to be available to the local host only, | |
ff2d7a98 | 312 | .Dq yes |
34fee935 | 313 | to force remote port forwardings to bind to the wildcard address, or |
314 | .Dq clientspecified | |
315 | to allow the client to select the address to which the forwarding is bound. | |
ff2d7a98 | 316 | The default is |
317 | .Dq no . | |
70791e56 | 318 | .It Cm GSSAPIAuthentication |
319 | Specifies whether user authentication based on GSSAPI is allowed. | |
416fd2a8 | 320 | The default is |
ff2d7a98 | 321 | .Dq yes . |
70791e56 | 322 | Note that this option applies to protocol version 2 only. |
70791e56 | 323 | .It Cm GSSAPIKeyExchange |
34fee935 | 324 | Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange |
325 | doesn't rely on ssh keys to verify host identity. | |
ff2d7a98 | 326 | The default is |
327 | .Dq yes . | |
70791e56 | 328 | Note that this option applies to protocol version 2 only. |
34fee935 | 329 | .It Cm GSSAPICleanupCredentials |
330 | Specifies whether to automatically destroy the user's credentials cache | |
331 | on logout. | |
ff2d7a98 | 332 | The default is |
333 | .Dq yes . | |
70791e56 | 334 | Note that this option applies to protocol version 2 only. |
2e437378 | 335 | .It Cm GSSAPIStrictAcceptorCheck |
336 | Determines whether to be strict about the identity of the GSSAPI acceptor | |
337 | a client authenticates against. If | |
338 | .Dq yes | |
339 | then the client must authenticate against the | |
340 | .Pa host | |
341 | service on the current hostname. If | |
342 | .Dq no | |
343 | then the client may authenticate against any service key stored in the | |
344 | machine's default store. This facility is provided to assist with operation | |
345 | on multi homed machines. | |
346 | The default is | |
347 | .Dq yes . | |
348 | Note that this option applies only to protocol version 2 GSSAPI connections, | |
349 | and setting it to | |
350 | .Dq no | |
351 | may only work with recent Kerberos GSSAPI libraries. | |
352 | .It Cm GSSAPICredentialsPath | |
353 | If specified, the delegated GSSAPI credential is stored in the | |
354 | given path, overwriting any existing credentials. | |
355 | Paths can be specified with syntax similar to the AuthorizedKeysFile | |
356 | option (i.e., accepting %h and %u tokens). | |
357 | When using this option, | |
358 | setting 'GssapiCleanupCredentials no' is recommended, | |
359 | so logging out of one session | |
360 | doesn't remove the credentials in use by another session of | |
361 | the same user. | |
362 | Currently only implemented for the GSI mechanism. | |
9374f80e | 363 | .It Cm GSIAllowLimitedProxy |
364 | Specifies whether to accept limited proxy credentials for | |
365 | authentication. | |
366 | The default is | |
367 | .Dq no . | |
70791e56 | 368 | .It Cm HostbasedAuthentication |
369 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
370 | with successful public key client host authentication is allowed | |
2e437378 | 371 | (host-based authentication). |
70791e56 | 372 | This option is similar to |
373 | .Cm RhostsRSAAuthentication | |
374 | and applies to protocol version 2 only. | |
c2397a66 | 375 | The default is |
70791e56 | 376 | .Dq no . |
2e437378 | 377 | .It Cm HostbasedUsesNameFromPacketOnly |
378 | Specifies whether or not the server will attempt to perform a reverse | |
379 | name lookup when matching the name in the | |
380 | .Pa ~/.shosts , | |
381 | .Pa ~/.rhosts , | |
382 | and | |
383 | .Pa /etc/hosts.equiv | |
384 | files during | |
385 | .Cm HostbasedAuthentication . | |
386 | A setting of | |
387 | .Dq yes | |
388 | means that | |
389 | .Xr sshd 8 | |
390 | uses the name supplied by the client rather than | |
391 | attempting to resolve the name from the TCP connection itself. | |
392 | The default is | |
393 | .Dq no . | |
ff2d7a98 | 394 | .It Cm HostKey |
395 | Specifies a file containing a private host key | |
396 | used by SSH. | |
397 | The default is | |
398 | .Pa /etc/ssh/ssh_host_key | |
399 | for protocol version 1, and | |
400 | .Pa /etc/ssh/ssh_host_rsa_key | |
401 | and | |
402 | .Pa /etc/ssh/ssh_host_dsa_key | |
403 | for protocol version 2. | |
404 | Note that | |
2e437378 | 405 | .Xr sshd 8 |
ff2d7a98 | 406 | will refuse to use a file if it is group/world-accessible. |
407 | It is possible to have multiple host key files. | |
408 | .Dq rsa1 | |
409 | keys are used for version 1 and | |
410 | .Dq dsa | |
411 | or | |
412 | .Dq rsa | |
413 | are used for version 2 of the SSH protocol. | |
414 | .It Cm IgnoreRhosts | |
415 | Specifies that | |
416 | .Pa .rhosts | |
417 | and | |
418 | .Pa .shosts | |
419 | files will not be used in | |
ff2d7a98 | 420 | .Cm RhostsRSAAuthentication |
421 | or | |
422 | .Cm HostbasedAuthentication . | |
423 | .Pp | |
424 | .Pa /etc/hosts.equiv | |
425 | and | |
426 | .Pa /etc/shosts.equiv | |
427 | are still used. | |
428 | The default is | |
429 | .Dq yes . | |
430 | .It Cm IgnoreUserKnownHosts | |
431 | Specifies whether | |
2e437378 | 432 | .Xr sshd 8 |
ff2d7a98 | 433 | should ignore the user's |
34fee935 | 434 | .Pa ~/.ssh/known_hosts |
ff2d7a98 | 435 | during |
436 | .Cm RhostsRSAAuthentication | |
437 | or | |
438 | .Cm HostbasedAuthentication . | |
439 | The default is | |
440 | .Dq no . | |
ff2d7a98 | 441 | .It Cm KerberosAuthentication |
70791e56 | 442 | Specifies whether the password provided by the user for |
ff2d7a98 | 443 | .Cm PasswordAuthentication |
70791e56 | 444 | will be validated through the Kerberos KDC. |
ff2d7a98 | 445 | To use this option, the server needs a |
446 | Kerberos servtab which allows the verification of the KDC's identity. | |
2e437378 | 447 | The default is |
ff2d7a98 | 448 | .Dq no . |
2a304a95 | 449 | .It Cm KerberosGetAFSToken |
e00da40d | 450 | If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire |
2a304a95 | 451 | an AFS token before accessing the user's home directory. |
2e437378 | 452 | The default is |
2a304a95 | 453 | .Dq no . |
ff2d7a98 | 454 | .It Cm KerberosOrLocalPasswd |
2e437378 | 455 | If password authentication through Kerberos fails then |
ff2d7a98 | 456 | the password will be validated via any additional local mechanism |
457 | such as | |
458 | .Pa /etc/passwd . | |
2e437378 | 459 | The default is |
ff2d7a98 | 460 | .Dq yes . |
ff2d7a98 | 461 | .It Cm KerberosTicketCleanup |
462 | Specifies whether to automatically destroy the user's ticket cache | |
463 | file on logout. | |
2e437378 | 464 | The default is |
ff2d7a98 | 465 | .Dq yes . |
466 | .It Cm KeyRegenerationInterval | |
467 | In protocol version 1, the ephemeral server key is automatically regenerated | |
468 | after this many seconds (if it has been used). | |
469 | The purpose of regeneration is to prevent | |
470 | decrypting captured sessions by later breaking into the machine and | |
471 | stealing the keys. | |
472 | The key is never stored anywhere. | |
473 | If the value is 0, the key is never regenerated. | |
474 | The default is 3600 (seconds). | |
475 | .It Cm ListenAddress | |
476 | Specifies the local addresses | |
2e437378 | 477 | .Xr sshd 8 |
ff2d7a98 | 478 | should listen on. |
479 | The following forms may be used: | |
480 | .Pp | |
481 | .Bl -item -offset indent -compact | |
482 | .It | |
483 | .Cm ListenAddress | |
484 | .Sm off | |
485 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr | |
486 | .Sm on | |
487 | .It | |
488 | .Cm ListenAddress | |
489 | .Sm off | |
490 | .Ar host No | Ar IPv4_addr No : Ar port | |
491 | .Sm on | |
492 | .It | |
493 | .Cm ListenAddress | |
494 | .Sm off | |
495 | .Oo | |
496 | .Ar host No | Ar IPv6_addr Oc : Ar port | |
497 | .Sm on | |
498 | .El | |
499 | .Pp | |
500 | If | |
501 | .Ar port | |
502 | is not specified, | |
2e437378 | 503 | sshd will listen on the address and all prior |
ff2d7a98 | 504 | .Cm Port |
70791e56 | 505 | options specified. |
506 | The default is to listen on all local addresses. | |
1c14df9e | 507 | Multiple |
ff2d7a98 | 508 | .Cm ListenAddress |
70791e56 | 509 | options are permitted. |
510 | Additionally, any | |
ff2d7a98 | 511 | .Cm Port |
2e437378 | 512 | options must precede this option for non-port qualified addresses. |
ff2d7a98 | 513 | .It Cm LoginGraceTime |
514 | The server disconnects after this time if the user has not | |
515 | successfully logged in. | |
516 | If the value is 0, there is no time limit. | |
e54b3d7c | 517 | The default is 120 seconds. |
ff2d7a98 | 518 | .It Cm LogLevel |
519 | Gives the verbosity level that is used when logging messages from | |
2e437378 | 520 | .Xr sshd 8 . |
ff2d7a98 | 521 | The possible values are: |
2e437378 | 522 | QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. |
1c14df9e | 523 | The default is INFO. |
524 | DEBUG and DEBUG1 are equivalent. | |
525 | DEBUG2 and DEBUG3 each specify higher levels of debugging output. | |
526 | Logging with a DEBUG level violates the privacy of users and is not recommended. | |
ff2d7a98 | 527 | .It Cm MACs |
528 | Specifies the available MAC (message authentication code) algorithms. | |
529 | The MAC algorithm is used in protocol version 2 | |
530 | for data integrity protection. | |
531 | Multiple algorithms must be comma-separated. | |
2e437378 | 532 | The default is: |
ff2d7a98 | 533 | .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . |
2e437378 | 534 | .It Cm Match |
535 | Introduces a conditional block. | |
536 | If all of the criteria on the | |
537 | .Cm Match | |
538 | line are satisfied, the keywords on the following lines override those | |
539 | set in the global section of the config file, until either another | |
540 | .Cm Match | |
541 | line or the end of the file. | |
542 | The arguments to | |
543 | .Cm Match | |
544 | are one or more criteria-pattern pairs. | |
545 | The available criteria are | |
546 | .Cm User , | |
547 | .Cm Group , | |
548 | .Cm Host , | |
549 | and | |
550 | .Cm Address . | |
551 | Only a subset of keywords may be used on the lines following a | |
552 | .Cm Match | |
553 | keyword. | |
554 | Available keywords are | |
555 | .Cm AllowTcpForwarding , | |
556 | .Cm ForceCommand , | |
557 | .Cm GatewayPorts , | |
558 | .Cm PermitOpen , | |
559 | .Cm X11DisplayOffset , | |
560 | .Cm X11Forwarding , | |
561 | and | |
562 | .Cm X11UseLocalHost . | |
1b56ff3d | 563 | .It Cm MaxAuthTries |
564 | Specifies the maximum number of authentication attempts permitted per | |
565 | connection. | |
566 | Once the number of failures reaches half this value, | |
567 | additional failures are logged. | |
568 | The default is 6. | |
ff2d7a98 | 569 | .It Cm MaxStartups |
570 | Specifies the maximum number of concurrent unauthenticated connections to the | |
2e437378 | 571 | SSH daemon. |
ff2d7a98 | 572 | Additional connections will be dropped until authentication succeeds or the |
573 | .Cm LoginGraceTime | |
574 | expires for a connection. | |
575 | The default is 10. | |
576 | .Pp | |
577 | Alternatively, random early drop can be enabled by specifying | |
578 | the three colon separated values | |
579 | .Dq start:rate:full | |
2e437378 | 580 | (e.g. "10:30:60"). |
581 | .Xr sshd 8 | |
ff2d7a98 | 582 | will refuse connection attempts with a probability of |
583 | .Dq rate/100 | |
584 | (30%) | |
585 | if there are currently | |
586 | .Dq start | |
587 | (10) | |
588 | unauthenticated connections. | |
589 | The probability increases linearly and all connection attempts | |
590 | are refused if the number of unauthenticated connections reaches | |
591 | .Dq full | |
592 | (60). | |
ff2d7a98 | 593 | .It Cm PasswordAuthentication |
594 | Specifies whether password authentication is allowed. | |
595 | The default is | |
596 | .Dq yes . | |
597 | .It Cm PermitEmptyPasswords | |
598 | When password authentication is allowed, it specifies whether the | |
599 | server allows login to accounts with empty password strings. | |
600 | The default is | |
601 | .Dq no . | |
2e437378 | 602 | .It Cm PermitOpen |
603 | Specifies the destinations to which TCP port forwarding is permitted. | |
604 | The forwarding specification must be one of the following forms: | |
605 | .Pp | |
606 | .Bl -item -offset indent -compact | |
607 | .It | |
608 | .Cm PermitOpen | |
609 | .Sm off | |
610 | .Ar host : port | |
611 | .Sm on | |
612 | .It | |
613 | .Cm PermitOpen | |
614 | .Sm off | |
615 | .Ar IPv4_addr : port | |
616 | .Sm on | |
617 | .It | |
618 | .Cm PermitOpen | |
619 | .Sm off | |
620 | .Ar \&[ IPv6_addr \&] : port | |
621 | .Sm on | |
622 | .El | |
623 | .Pp | |
624 | Multiple forwards may be specified by separating them with whitespace. | |
625 | An argument of | |
626 | .Dq any | |
627 | can be used to remove all restrictions and permit any forwarding requests. | |
628 | By default all port forwarding requests are permitted. | |
ff2d7a98 | 629 | .It Cm PermitRootLogin |
34fee935 | 630 | Specifies whether root can log in using |
ff2d7a98 | 631 | .Xr ssh 1 . |
632 | The argument must be | |
633 | .Dq yes , | |
634 | .Dq without-password , | |
2e437378 | 635 | .Dq forced-commands-only , |
ff2d7a98 | 636 | or |
637 | .Dq no . | |
638 | The default is | |
639 | .Dq yes . | |
640 | .Pp | |
641 | If this option is set to | |
2e437378 | 642 | .Dq without-password , |
34fee935 | 643 | password authentication is disabled for root. |
ff2d7a98 | 644 | .Pp |
645 | If this option is set to | |
2e437378 | 646 | .Dq forced-commands-only , |
ff2d7a98 | 647 | root login with public key authentication will be allowed, |
648 | but only if the | |
649 | .Ar command | |
650 | option has been specified | |
651 | (which may be useful for taking remote backups even if root login is | |
70791e56 | 652 | normally not allowed). |
653 | All other authentication methods are disabled for root. | |
ff2d7a98 | 654 | .Pp |
655 | If this option is set to | |
2e437378 | 656 | .Dq no , |
34fee935 | 657 | root is not allowed to log in. |
e00da40d | 658 | .It Cm PermitTunnel |
659 | Specifies whether | |
660 | .Xr tun 4 | |
661 | device forwarding is allowed. | |
662 | The argument must be | |
663 | .Dq yes , | |
2e437378 | 664 | .Dq point-to-point |
665 | (layer 3), | |
e00da40d | 666 | .Dq ethernet |
2e437378 | 667 | (layer 2), or |
e00da40d | 668 | .Dq no . |
2e437378 | 669 | Specifying |
670 | .Dq yes | |
671 | permits both | |
672 | .Dq point-to-point | |
673 | and | |
674 | .Dq ethernet . | |
e00da40d | 675 | The default is |
676 | .Dq no . | |
e54b3d7c | 677 | .It Cm PermitUserEnvironment |
678 | Specifies whether | |
679 | .Pa ~/.ssh/environment | |
680 | and | |
681 | .Cm environment= | |
682 | options in | |
683 | .Pa ~/.ssh/authorized_keys | |
684 | are processed by | |
2e437378 | 685 | .Xr sshd 8 . |
e54b3d7c | 686 | The default is |
687 | .Dq no . | |
688 | Enabling environment processing may enable users to bypass access | |
689 | restrictions in some configurations using mechanisms such as | |
690 | .Ev LD_PRELOAD . | |
ff2d7a98 | 691 | .It Cm PidFile |
692 | Specifies the file that contains the process ID of the | |
2e437378 | 693 | SSH daemon. |
ff2d7a98 | 694 | The default is |
695 | .Pa /var/run/sshd.pid . | |
696 | .It Cm Port | |
697 | Specifies the port number that | |
2e437378 | 698 | .Xr sshd 8 |
ff2d7a98 | 699 | listens on. |
700 | The default is 22. | |
701 | Multiple options of this type are permitted. | |
702 | See also | |
703 | .Cm ListenAddress . | |
704 | .It Cm PrintLastLog | |
705 | Specifies whether | |
2e437378 | 706 | .Xr sshd 8 |
34fee935 | 707 | should print the date and time of the last user login when a user logs |
708 | in interactively. | |
ff2d7a98 | 709 | The default is |
710 | .Dq yes . | |
711 | .It Cm PrintMotd | |
712 | Specifies whether | |
2e437378 | 713 | .Xr sshd 8 |
ff2d7a98 | 714 | should print |
715 | .Pa /etc/motd | |
716 | when a user logs in interactively. | |
717 | (On some systems it is also printed by the shell, | |
718 | .Pa /etc/profile , | |
719 | or equivalent.) | |
720 | The default is | |
721 | .Dq yes . | |
722 | .It Cm Protocol | |
723 | Specifies the protocol versions | |
2e437378 | 724 | .Xr sshd 8 |
e54b3d7c | 725 | supports. |
ff2d7a98 | 726 | The possible values are |
2e437378 | 727 | .Sq 1 |
ff2d7a98 | 728 | and |
2e437378 | 729 | .Sq 2 . |
ff2d7a98 | 730 | Multiple versions must be comma-separated. |
731 | The default is | |
732 | .Dq 2,1 . | |
e54b3d7c | 733 | Note that the order of the protocol list does not indicate preference, |
734 | because the client selects among multiple protocol versions offered | |
735 | by the server. | |
736 | Specifying | |
737 | .Dq 2,1 | |
738 | is identical to | |
739 | .Dq 1,2 . | |
ff2d7a98 | 740 | .It Cm PubkeyAuthentication |
741 | Specifies whether public key authentication is allowed. | |
742 | The default is | |
743 | .Dq yes . | |
744 | Note that this option applies to protocol version 2 only. | |
ff2d7a98 | 745 | .It Cm RhostsRSAAuthentication |
746 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
747 | with successful RSA host authentication is allowed. | |
748 | The default is | |
749 | .Dq no . | |
750 | This option applies to protocol version 1 only. | |
751 | .It Cm RSAAuthentication | |
752 | Specifies whether pure RSA authentication is allowed. | |
753 | The default is | |
754 | .Dq yes . | |
755 | This option applies to protocol version 1 only. | |
756 | .It Cm ServerKeyBits | |
757 | Defines the number of bits in the ephemeral protocol version 1 server key. | |
758 | The minimum value is 512, and the default is 768. | |
759 | .It Cm StrictModes | |
760 | Specifies whether | |
2e437378 | 761 | .Xr sshd 8 |
ff2d7a98 | 762 | should check file modes and ownership of the |
763 | user's files and home directory before accepting login. | |
764 | This is normally desirable because novices sometimes accidentally leave their | |
765 | directory or files world-writable. | |
766 | The default is | |
767 | .Dq yes . | |
768 | .It Cm Subsystem | |
2e437378 | 769 | Configures an external subsystem (e.g. file transfer daemon). |
770 | Arguments should be a subsystem name and a command (with optional arguments) | |
771 | to execute upon subsystem request. | |
ff2d7a98 | 772 | The command |
773 | .Xr sftp-server 8 | |
774 | implements the | |
775 | .Dq sftp | |
776 | file transfer subsystem. | |
777 | By default no subsystems are defined. | |
778 | Note that this option applies to protocol version 2 only. | |
779 | .It Cm SyslogFacility | |
780 | Gives the facility code that is used when logging messages from | |
2e437378 | 781 | .Xr sshd 8 . |
ff2d7a98 | 782 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, |
783 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. | |
784 | The default is AUTH. | |
416fd2a8 | 785 | .It Cm TCPKeepAlive |
786 | Specifies whether the system should send TCP keepalive messages to the | |
787 | other side. | |
788 | If they are sent, death of the connection or crash of one | |
789 | of the machines will be properly noticed. | |
790 | However, this means that | |
791 | connections will die if the route is down temporarily, and some people | |
792 | find it annoying. | |
793 | On the other hand, if TCP keepalives are not sent, | |
794 | sessions may hang indefinitely on the server, leaving | |
795 | .Dq ghost | |
796 | users and consuming server resources. | |
797 | .Pp | |
798 | The default is | |
799 | .Dq yes | |
800 | (to send TCP keepalive messages), and the server will notice | |
801 | if the network goes down or the client host crashes. | |
802 | This avoids infinitely hanging sessions. | |
803 | .Pp | |
804 | To disable TCP keepalive messages, the value should be set to | |
805 | .Dq no . | |
70791e56 | 806 | .It Cm UseDNS |
807 | Specifies whether | |
2e437378 | 808 | .Xr sshd 8 |
34fee935 | 809 | should look up the remote host name and check that |
70791e56 | 810 | the resolved host name for the remote IP address maps back to the |
811 | very same IP address. | |
812 | The default is | |
813 | .Dq yes . | |
ff2d7a98 | 814 | .It Cm UseLogin |
815 | Specifies whether | |
816 | .Xr login 1 | |
817 | is used for interactive login sessions. | |
818 | The default is | |
819 | .Dq no . | |
820 | Note that | |
821 | .Xr login 1 | |
822 | is never used for remote command execution. | |
823 | Note also, that if this is enabled, | |
824 | .Cm X11Forwarding | |
825 | will be disabled because | |
826 | .Xr login 1 | |
827 | does not know how to handle | |
828 | .Xr xauth 1 | |
1c14df9e | 829 | cookies. |
830 | If | |
ff2d7a98 | 831 | .Cm UsePrivilegeSeparation |
832 | is specified, it will be disabled after authentication. | |
70791e56 | 833 | .It Cm UsePAM |
1b56ff3d | 834 | Enables the Pluggable Authentication Module interface. |
835 | If set to | |
836 | .Dq yes | |
837 | this will enable PAM authentication using | |
838 | .Cm ChallengeResponseAuthentication | |
2e437378 | 839 | and |
840 | .Cm PasswordAuthentication | |
841 | in addition to PAM account and session module processing for all | |
842 | authentication types. | |
1b56ff3d | 843 | .Pp |
844 | Because PAM challenge-response authentication usually serves an equivalent | |
845 | role to password authentication, you should disable either | |
846 | .Cm PasswordAuthentication | |
847 | or | |
848 | .Cm ChallengeResponseAuthentication. | |
849 | .Pp | |
850 | If | |
851 | .Cm UsePAM | |
852 | is enabled, you will not be able to run | |
853 | .Xr sshd 8 | |
854 | as a non-root user. | |
855 | The default is | |
416fd2a8 | 856 | .Dq no . |
ff2d7a98 | 857 | .It Cm UsePrivilegeSeparation |
858 | Specifies whether | |
2e437378 | 859 | .Xr sshd 8 |
ff2d7a98 | 860 | separates privileges by creating an unprivileged child process |
1c14df9e | 861 | to deal with incoming network traffic. |
862 | After successful authentication, another process will be created that has | |
863 | the privilege of the authenticated user. | |
864 | The goal of privilege separation is to prevent privilege | |
ff2d7a98 | 865 | escalation by containing any corruption within the unprivileged processes. |
866 | The default is | |
867 | .Dq yes . | |
ff2d7a98 | 868 | .It Cm X11DisplayOffset |
869 | Specifies the first display number available for | |
2e437378 | 870 | .Xr sshd 8 Ns 's |
ff2d7a98 | 871 | X11 forwarding. |
2e437378 | 872 | This prevents sshd from interfering with real X11 servers. |
ff2d7a98 | 873 | The default is 10. |
874 | .It Cm X11Forwarding | |
875 | Specifies whether X11 forwarding is permitted. | |
e54b3d7c | 876 | The argument must be |
877 | .Dq yes | |
878 | or | |
879 | .Dq no . | |
ff2d7a98 | 880 | The default is |
881 | .Dq no . | |
e54b3d7c | 882 | .Pp |
883 | When X11 forwarding is enabled, there may be additional exposure to | |
884 | the server and to client displays if the | |
2e437378 | 885 | .Xr sshd 8 |
e54b3d7c | 886 | proxy display is configured to listen on the wildcard address (see |
887 | .Cm X11UseLocalhost | |
2e437378 | 888 | below), though this is not the default. |
e54b3d7c | 889 | Additionally, the authentication spoofing and authentication data |
890 | verification and substitution occur on the client side. | |
891 | The security risk of using X11 forwarding is that the client's X11 | |
2e437378 | 892 | display server may be exposed to attack when the SSH client requests |
e54b3d7c | 893 | forwarding (see the warnings for |
894 | .Cm ForwardX11 | |
895 | in | |
70791e56 | 896 | .Xr ssh_config 5 ) . |
e54b3d7c | 897 | A system administrator may have a stance in which they want to |
898 | protect clients that may expose themselves to attack by unwittingly | |
899 | requesting X11 forwarding, which can warrant a | |
900 | .Dq no | |
901 | setting. | |
902 | .Pp | |
903 | Note that disabling X11 forwarding does not prevent users from | |
904 | forwarding X11 traffic, as users can always install their own forwarders. | |
ff2d7a98 | 905 | X11 forwarding is automatically disabled if |
906 | .Cm UseLogin | |
907 | is enabled. | |
908 | .It Cm X11UseLocalhost | |
909 | Specifies whether | |
2e437378 | 910 | .Xr sshd 8 |
ff2d7a98 | 911 | should bind the X11 forwarding server to the loopback address or to |
1c14df9e | 912 | the wildcard address. |
913 | By default, | |
2e437378 | 914 | sshd binds the forwarding server to the loopback address and sets the |
ff2d7a98 | 915 | hostname part of the |
916 | .Ev DISPLAY | |
917 | environment variable to | |
918 | .Dq localhost . | |
e54b3d7c | 919 | This prevents remote hosts from connecting to the proxy display. |
ff2d7a98 | 920 | However, some older X11 clients may not function with this |
921 | configuration. | |
922 | .Cm X11UseLocalhost | |
923 | may be set to | |
924 | .Dq no | |
925 | to specify that the forwarding server should be bound to the wildcard | |
926 | address. | |
927 | The argument must be | |
928 | .Dq yes | |
929 | or | |
930 | .Dq no . | |
931 | The default is | |
932 | .Dq yes . | |
933 | .It Cm XAuthLocation | |
e54b3d7c | 934 | Specifies the full pathname of the |
ff2d7a98 | 935 | .Xr xauth 1 |
936 | program. | |
937 | The default is | |
938 | .Pa /usr/X11R6/bin/xauth . | |
939 | .El | |
2e437378 | 940 | .Sh TIME FORMATS |
941 | .Xr sshd 8 | |
ff2d7a98 | 942 | command-line arguments and configuration file options that specify time |
943 | may be expressed using a sequence of the form: | |
944 | .Sm off | |
e54b3d7c | 945 | .Ar time Op Ar qualifier , |
ff2d7a98 | 946 | .Sm on |
947 | where | |
948 | .Ar time | |
949 | is a positive integer value and | |
950 | .Ar qualifier | |
951 | is one of the following: | |
952 | .Pp | |
953 | .Bl -tag -width Ds -compact -offset indent | |
2e437378 | 954 | .It Aq Cm none |
ff2d7a98 | 955 | seconds |
956 | .It Cm s | Cm S | |
957 | seconds | |
958 | .It Cm m | Cm M | |
959 | minutes | |
960 | .It Cm h | Cm H | |
961 | hours | |
962 | .It Cm d | Cm D | |
963 | days | |
964 | .It Cm w | Cm W | |
965 | weeks | |
966 | .El | |
967 | .Pp | |
968 | Each member of the sequence is added together to calculate | |
969 | the total time value. | |
970 | .Pp | |
971 | Time format examples: | |
972 | .Pp | |
973 | .Bl -tag -width Ds -compact -offset indent | |
974 | .It 600 | |
975 | 600 seconds (10 minutes) | |
976 | .It 10m | |
977 | 10 minutes | |
978 | .It 1h30m | |
979 | 1 hour 30 minutes (90 minutes) | |
980 | .El | |
981 | .Sh FILES | |
982 | .Bl -tag -width Ds | |
983 | .It Pa /etc/ssh/sshd_config | |
984 | Contains configuration data for | |
2e437378 | 985 | .Xr sshd 8 . |
ff2d7a98 | 986 | This file should be writable by root only, but it is recommended |
987 | (though not necessary) that it be world-readable. | |
988 | .El | |
70791e56 | 989 | .Sh SEE ALSO |
990 | .Xr sshd 8 | |
ff2d7a98 | 991 | .Sh AUTHORS |
992 | OpenSSH is a derivative of the original and free | |
993 | ssh 1.2.12 release by Tatu Ylonen. | |
994 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
995 | Theo de Raadt and Dug Song | |
996 | removed many bugs, re-added newer features and | |
997 | created OpenSSH. | |
998 | Markus Friedl contributed the support for SSH | |
999 | protocol versions 1.5 and 2.0. | |
1000 | Niels Provos and Markus Friedl contributed support | |
1001 | for privilege separation. |