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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 | .\" | |
ba8cfba6 | 37 | .\" $OpenBSD: sshd_config.5,v 1.78 2007/08/23 03:22:16 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 . | |
176 | .It Cm Ciphers | |
177 | Specifies the ciphers allowed for protocol version 2. | |
178 | Multiple ciphers must be comma-separated. | |
3b9baa7b | 179 | The supported ciphers are |
180 | .Dq 3des-cbc , | |
181 | .Dq aes128-cbc , | |
182 | .Dq aes192-cbc , | |
183 | .Dq aes256-cbc , | |
184 | .Dq aes128-ctr , | |
185 | .Dq aes192-ctr , | |
186 | .Dq aes256-ctr , | |
74a66cc8 | 187 | .Dq arcfour128 , |
188 | .Dq arcfour256 , | |
3b9baa7b | 189 | .Dq arcfour , |
190 | .Dq blowfish-cbc , | |
191 | and | |
192 | .Dq cast128-cbc . | |
b74c3b8c | 193 | The default is: |
194 | .Bd -literal -offset 3n | |
195 | aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, | |
196 | arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, | |
197 | aes192-ctr,aes256-ctr | |
588df31a | 198 | .Ed |
588df31a | 199 | .It Cm ClientAliveCountMax |
79e46360 | 200 | Sets the number of client alive messages (see below) which may be |
588df31a | 201 | sent without |
b74c3b8c | 202 | .Xr sshd 8 |
3cbc677d | 203 | receiving any messages back from the client. |
204 | If this threshold is reached while client alive messages are being sent, | |
b74c3b8c | 205 | sshd will disconnect the client, terminating the session. |
3cbc677d | 206 | It is important to note that the use of client alive messages is very |
207 | different from | |
fd573618 | 208 | .Cm TCPKeepAlive |
3cbc677d | 209 | (below). |
210 | The client alive messages are sent through the encrypted channel | |
211 | and therefore will not be spoofable. | |
212 | The TCP keepalive option enabled by | |
fd573618 | 213 | .Cm TCPKeepAlive |
3cbc677d | 214 | is spoofable. |
215 | The client alive mechanism is valuable when the client or | |
588df31a | 216 | server depend on knowing when a connection has become inactive. |
217 | .Pp | |
3cbc677d | 218 | The default value is 3. |
219 | If | |
588df31a | 220 | .Cm ClientAliveInterval |
79e46360 | 221 | (see below) is set to 15, and |
588df31a | 222 | .Cm ClientAliveCountMax |
b74c3b8c | 223 | is left at the default, unresponsive SSH clients |
588df31a | 224 | will be disconnected after approximately 45 seconds. |
2d762582 | 225 | This option applies to protocol version 2 only. |
2264526c | 226 | .It Cm ClientAliveInterval |
227 | Sets a timeout interval in seconds after which if no data has been received | |
228 | from the client, | |
b74c3b8c | 229 | .Xr sshd 8 |
2264526c | 230 | will send a message through the encrypted |
231 | channel to request a response from the client. | |
232 | The default | |
233 | is 0, indicating that these messages will not be sent to the client. | |
234 | This option applies to protocol version 2 only. | |
588df31a | 235 | .It Cm Compression |
07200973 | 236 | Specifies whether compression is allowed, or delayed until |
237 | the user has authenticated successfully. | |
588df31a | 238 | The argument must be |
07200973 | 239 | .Dq yes , |
240 | .Dq delayed , | |
588df31a | 241 | or |
242 | .Dq no . | |
243 | The default is | |
07200973 | 244 | .Dq delayed . |
588df31a | 245 | .It Cm DenyGroups |
246 | This keyword can be followed by a list of group name patterns, separated | |
247 | by spaces. | |
248 | Login is disallowed for users whose primary group or supplementary | |
249 | group list matches one of the patterns. | |
588df31a | 250 | Only group names are valid; a numerical group ID is not recognized. |
251 | By default, login is allowed for all groups. | |
84c1b530 | 252 | The allow/deny directives are processed in the following order: |
253 | .Cm DenyUsers , | |
254 | .Cm AllowUsers , | |
255 | .Cm DenyGroups , | |
256 | and finally | |
257 | .Cm AllowGroups . | |
ac1ec4d8 | 258 | .Pp |
259 | See | |
260 | .Sx PATTERNS | |
261 | in | |
262 | .Xr ssh_config 5 | |
263 | for more information on patterns. | |
588df31a | 264 | .It Cm DenyUsers |
265 | This keyword can be followed by a list of user name patterns, separated | |
266 | by spaces. | |
267 | Login is disallowed for user names that match one of the patterns. | |
588df31a | 268 | Only user names are valid; a numerical user ID is not recognized. |
269 | By default, login is allowed for all users. | |
270 | If the pattern takes the form USER@HOST then USER and HOST | |
271 | are separately checked, restricting logins to particular | |
272 | users from particular hosts. | |
84c1b530 | 273 | The allow/deny directives are processed in the following order: |
274 | .Cm DenyUsers , | |
275 | .Cm AllowUsers , | |
276 | .Cm DenyGroups , | |
277 | and finally | |
278 | .Cm AllowGroups . | |
ac1ec4d8 | 279 | .Pp |
280 | See | |
281 | .Sx PATTERNS | |
282 | in | |
283 | .Xr ssh_config 5 | |
284 | for more information on patterns. | |
e7259e8d | 285 | .It Cm ForceCommand |
286 | Forces the execution of the command specified by | |
287 | .Cm ForceCommand , | |
288 | ignoring any command supplied by the client. | |
289 | The command is invoked by using the user's login shell with the -c option. | |
290 | This applies to shell, command, or subsystem execution. | |
291 | It is most useful inside a | |
292 | .Cm Match | |
293 | block. | |
294 | The command originally supplied by the client is available in the | |
295 | .Ev SSH_ORIGINAL_COMMAND | |
296 | environment variable. | |
588df31a | 297 | .It Cm GatewayPorts |
298 | Specifies whether remote hosts are allowed to connect to ports | |
299 | forwarded for the client. | |
300 | By default, | |
b74c3b8c | 301 | .Xr sshd 8 |
a4e5acef | 302 | binds remote port forwardings to the loopback address. |
303 | This prevents other remote hosts from connecting to forwarded ports. | |
588df31a | 304 | .Cm GatewayPorts |
b74c3b8c | 305 | can be used to specify that sshd |
3867aa0a | 306 | should allow remote port forwardings to bind to non-loopback addresses, thus |
307 | allowing other hosts to connect. | |
308 | The argument may be | |
309 | .Dq no | |
310 | to force remote port forwardings to be available to the local host only, | |
588df31a | 311 | .Dq yes |
3867aa0a | 312 | to force remote port forwardings to bind to the wildcard address, or |
313 | .Dq clientspecified | |
314 | to allow the client to select the address to which the forwarding is bound. | |
588df31a | 315 | The default is |
316 | .Dq no . | |
7364bd04 | 317 | .It Cm GSSAPIAuthentication |
105b07db | 318 | Specifies whether user authentication based on GSSAPI is allowed. |
aff51935 | 319 | The default is |
7364bd04 | 320 | .Dq no . |
321 | Note that this option applies to protocol version 2 only. | |
322 | .It Cm GSSAPICleanupCredentials | |
323 | Specifies whether to automatically destroy the user's credentials cache | |
324 | on logout. | |
325 | The default is | |
326 | .Dq yes . | |
327 | Note that this option applies to protocol version 2 only. | |
588df31a | 328 | .It Cm HostbasedAuthentication |
329 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
330 | with successful public key client host authentication is allowed | |
340a4caf | 331 | (host-based authentication). |
588df31a | 332 | This option is similar to |
333 | .Cm RhostsRSAAuthentication | |
334 | and applies to protocol version 2 only. | |
335 | The default is | |
336 | .Dq no . | |
e9f2e744 | 337 | .It Cm HostbasedUsesNameFromPacketOnly |
338 | Specifies whether or not the server will attempt to perform a reverse | |
339 | name lookup when matching the name in the | |
340 | .Pa ~/.shosts , | |
341 | .Pa ~/.rhosts , | |
342 | and | |
343 | .Pa /etc/hosts.equiv | |
344 | files during | |
345 | .Cm HostbasedAuthentication . | |
346 | A setting of | |
347 | .Dq yes | |
348 | means that | |
349 | .Xr sshd 8 | |
350 | uses the name supplied by the client rather than | |
351 | attempting to resolve the name from the TCP connection itself. | |
352 | The default is | |
353 | .Dq no . | |
588df31a | 354 | .It Cm HostKey |
355 | Specifies a file containing a private host key | |
356 | used by SSH. | |
357 | The default is | |
358 | .Pa /etc/ssh/ssh_host_key | |
359 | for protocol version 1, and | |
360 | .Pa /etc/ssh/ssh_host_rsa_key | |
361 | and | |
362 | .Pa /etc/ssh/ssh_host_dsa_key | |
363 | for protocol version 2. | |
364 | Note that | |
b74c3b8c | 365 | .Xr sshd 8 |
588df31a | 366 | will refuse to use a file if it is group/world-accessible. |
367 | It is possible to have multiple host key files. | |
368 | .Dq rsa1 | |
369 | keys are used for version 1 and | |
370 | .Dq dsa | |
371 | or | |
372 | .Dq rsa | |
373 | are used for version 2 of the SSH protocol. | |
374 | .It Cm IgnoreRhosts | |
375 | Specifies that | |
376 | .Pa .rhosts | |
377 | and | |
378 | .Pa .shosts | |
379 | files will not be used in | |
588df31a | 380 | .Cm RhostsRSAAuthentication |
381 | or | |
382 | .Cm HostbasedAuthentication . | |
383 | .Pp | |
384 | .Pa /etc/hosts.equiv | |
385 | and | |
386 | .Pa /etc/shosts.equiv | |
387 | are still used. | |
388 | The default is | |
389 | .Dq yes . | |
390 | .It Cm IgnoreUserKnownHosts | |
391 | Specifies whether | |
b74c3b8c | 392 | .Xr sshd 8 |
588df31a | 393 | should ignore the user's |
140e3e97 | 394 | .Pa ~/.ssh/known_hosts |
588df31a | 395 | during |
396 | .Cm RhostsRSAAuthentication | |
397 | or | |
398 | .Cm HostbasedAuthentication . | |
399 | The default is | |
400 | .Dq no . | |
588df31a | 401 | .It Cm KerberosAuthentication |
8f73f7bb | 402 | Specifies whether the password provided by the user for |
588df31a | 403 | .Cm PasswordAuthentication |
8f73f7bb | 404 | will be validated through the Kerberos KDC. |
588df31a | 405 | To use this option, the server needs a |
406 | Kerberos servtab which allows the verification of the KDC's identity. | |
b74c3b8c | 407 | The default is |
588df31a | 408 | .Dq no . |
24f37810 | 409 | .It Cm KerberosGetAFSToken |
0d3d1077 | 410 | If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire |
24f37810 | 411 | an AFS token before accessing the user's home directory. |
b74c3b8c | 412 | The default is |
24f37810 | 413 | .Dq no . |
588df31a | 414 | .It Cm KerberosOrLocalPasswd |
b74c3b8c | 415 | If password authentication through Kerberos fails then |
588df31a | 416 | the password will be validated via any additional local mechanism |
417 | such as | |
418 | .Pa /etc/passwd . | |
b74c3b8c | 419 | The default is |
588df31a | 420 | .Dq yes . |
588df31a | 421 | .It Cm KerberosTicketCleanup |
422 | Specifies whether to automatically destroy the user's ticket cache | |
423 | file on logout. | |
b74c3b8c | 424 | The default is |
588df31a | 425 | .Dq yes . |
426 | .It Cm KeyRegenerationInterval | |
427 | In protocol version 1, the ephemeral server key is automatically regenerated | |
428 | after this many seconds (if it has been used). | |
429 | The purpose of regeneration is to prevent | |
430 | decrypting captured sessions by later breaking into the machine and | |
431 | stealing the keys. | |
432 | The key is never stored anywhere. | |
433 | If the value is 0, the key is never regenerated. | |
434 | The default is 3600 (seconds). | |
435 | .It Cm ListenAddress | |
436 | Specifies the local addresses | |
b74c3b8c | 437 | .Xr sshd 8 |
588df31a | 438 | should listen on. |
439 | The following forms may be used: | |
440 | .Pp | |
441 | .Bl -item -offset indent -compact | |
442 | .It | |
443 | .Cm ListenAddress | |
444 | .Sm off | |
445 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr | |
446 | .Sm on | |
447 | .It | |
448 | .Cm ListenAddress | |
449 | .Sm off | |
450 | .Ar host No | Ar IPv4_addr No : Ar port | |
451 | .Sm on | |
452 | .It | |
453 | .Cm ListenAddress | |
454 | .Sm off | |
455 | .Oo | |
456 | .Ar host No | Ar IPv6_addr Oc : Ar port | |
457 | .Sm on | |
458 | .El | |
459 | .Pp | |
460 | If | |
461 | .Ar port | |
462 | is not specified, | |
b74c3b8c | 463 | sshd will listen on the address and all prior |
588df31a | 464 | .Cm Port |
3cbc677d | 465 | options specified. |
466 | The default is to listen on all local addresses. | |
a4e5acef | 467 | Multiple |
588df31a | 468 | .Cm ListenAddress |
3cbc677d | 469 | options are permitted. |
470 | Additionally, any | |
588df31a | 471 | .Cm Port |
b74c3b8c | 472 | options must precede this option for non-port qualified addresses. |
588df31a | 473 | .It Cm LoginGraceTime |
474 | The server disconnects after this time if the user has not | |
475 | successfully logged in. | |
476 | If the value is 0, there is no time limit. | |
3445ca02 | 477 | The default is 120 seconds. |
588df31a | 478 | .It Cm LogLevel |
479 | Gives the verbosity level that is used when logging messages from | |
32cfd177 | 480 | .Xr sshd 8 . |
588df31a | 481 | The possible values are: |
b74c3b8c | 482 | QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. |
a4e5acef | 483 | The default is INFO. |
484 | DEBUG and DEBUG1 are equivalent. | |
485 | DEBUG2 and DEBUG3 each specify higher levels of debugging output. | |
486 | Logging with a DEBUG level violates the privacy of users and is not recommended. | |
588df31a | 487 | .It Cm MACs |
488 | Specifies the available MAC (message authentication code) algorithms. | |
489 | The MAC algorithm is used in protocol version 2 | |
490 | for data integrity protection. | |
491 | Multiple algorithms must be comma-separated. | |
b74c3b8c | 492 | The default is: |
33d2ae0f | 493 | .Bd -literal -offset indent |
494 | hmac-md5,hmac-sha1,umac-64@openssh.com, | |
495 | hmac-ripemd160,hmac-sha1-96,hmac-md5-96 | |
496 | .Ed | |
d231781a | 497 | .It Cm Match |
14e980ef | 498 | Introduces a conditional block. |
4895f836 | 499 | If all of the criteria on the |
d231781a | 500 | .Cm Match |
4895f836 | 501 | line are satisfied, the keywords on the following lines override those |
502 | set in the global section of the config file, until either another | |
d231781a | 503 | .Cm Match |
4895f836 | 504 | line or the end of the file. |
14e980ef | 505 | The arguments to |
d231781a | 506 | .Cm Match |
4895f836 | 507 | are one or more criteria-pattern pairs. |
d231781a | 508 | The available criteria are |
509 | .Cm User , | |
fa47fe3c | 510 | .Cm Group , |
d231781a | 511 | .Cm Host , |
512 | and | |
513 | .Cm Address . | |
514 | Only a subset of keywords may be used on the lines following a | |
515 | .Cm Match | |
516 | keyword. | |
517 | Available keywords are | |
2fefbadf | 518 | .Cm AllowTcpForwarding , |
03bcbf84 | 519 | .Cm Banner , |
e7259e8d | 520 | .Cm ForceCommand , |
2fefbadf | 521 | .Cm GatewayPorts , |
03bcbf84 | 522 | .Cm GSSApiAuthentication , |
121c4a34 | 523 | .Cm KbdInteractiveAuthentication , |
a863b75f | 524 | .Cm KerberosAuthentication , |
03bcbf84 | 525 | .Cm PasswordAuthentication , |
691712e0 | 526 | .Cm PermitOpen , |
03bcbf84 | 527 | .Cm RhostsRSAAuthentication , |
528 | .Cm RSAAuthentication , | |
691712e0 | 529 | .Cm X11DisplayOffset , |
530 | .Cm X11Forwarding , | |
d231781a | 531 | and |
691712e0 | 532 | .Cm X11UseLocalHost . |
af4bd935 | 533 | .It Cm MaxAuthTries |
534 | Specifies the maximum number of authentication attempts permitted per | |
8fca654b | 535 | connection. |
536 | Once the number of failures reaches half this value, | |
537 | additional failures are logged. | |
538 | The default is 6. | |
588df31a | 539 | .It Cm MaxStartups |
540 | Specifies the maximum number of concurrent unauthenticated connections to the | |
b74c3b8c | 541 | SSH daemon. |
588df31a | 542 | Additional connections will be dropped until authentication succeeds or the |
543 | .Cm LoginGraceTime | |
544 | expires for a connection. | |
545 | The default is 10. | |
546 | .Pp | |
547 | Alternatively, random early drop can be enabled by specifying | |
548 | the three colon separated values | |
549 | .Dq start:rate:full | |
f09ffbdb | 550 | (e.g. "10:30:60"). |
32cfd177 | 551 | .Xr sshd 8 |
588df31a | 552 | will refuse connection attempts with a probability of |
553 | .Dq rate/100 | |
554 | (30%) | |
555 | if there are currently | |
556 | .Dq start | |
557 | (10) | |
558 | unauthenticated connections. | |
559 | The probability increases linearly and all connection attempts | |
560 | are refused if the number of unauthenticated connections reaches | |
561 | .Dq full | |
562 | (60). | |
563 | .It Cm PasswordAuthentication | |
564 | Specifies whether password authentication is allowed. | |
565 | The default is | |
566 | .Dq yes . | |
567 | .It Cm PermitEmptyPasswords | |
568 | When password authentication is allowed, it specifies whether the | |
569 | server allows login to accounts with empty password strings. | |
570 | The default is | |
571 | .Dq no . | |
2fefbadf | 572 | .It Cm PermitOpen |
573 | Specifies the destinations to which TCP port forwarding is permitted. | |
574 | The forwarding specification must be one of the following forms: | |
575 | .Pp | |
576 | .Bl -item -offset indent -compact | |
577 | .It | |
578 | .Cm PermitOpen | |
579 | .Sm off | |
580 | .Ar host : port | |
581 | .Sm on | |
582 | .It | |
583 | .Cm PermitOpen | |
584 | .Sm off | |
585 | .Ar IPv4_addr : port | |
586 | .Sm on | |
587 | .It | |
588 | .Cm PermitOpen | |
589 | .Sm off | |
590 | .Ar \&[ IPv6_addr \&] : port | |
591 | .Sm on | |
592 | .El | |
593 | .Pp | |
ea46e550 | 594 | Multiple forwards may be specified by separating them with whitespace. |
2fefbadf | 595 | An argument of |
596 | .Dq any | |
597 | can be used to remove all restrictions and permit any forwarding requests. | |
f22506ff | 598 | By default all port forwarding requests are permitted. |
588df31a | 599 | .It Cm PermitRootLogin |
667e4135 | 600 | Specifies whether root can log in using |
588df31a | 601 | .Xr ssh 1 . |
602 | The argument must be | |
603 | .Dq yes , | |
604 | .Dq without-password , | |
b74c3b8c | 605 | .Dq forced-commands-only , |
588df31a | 606 | or |
607 | .Dq no . | |
608 | The default is | |
609 | .Dq yes . | |
610 | .Pp | |
611 | If this option is set to | |
b74c3b8c | 612 | .Dq without-password , |
8a4c4ee4 | 613 | password authentication is disabled for root. |
588df31a | 614 | .Pp |
615 | If this option is set to | |
b74c3b8c | 616 | .Dq forced-commands-only , |
588df31a | 617 | root login with public key authentication will be allowed, |
618 | but only if the | |
619 | .Ar command | |
620 | option has been specified | |
621 | (which may be useful for taking remote backups even if root login is | |
3cbc677d | 622 | normally not allowed). |
623 | All other authentication methods are disabled for root. | |
588df31a | 624 | .Pp |
625 | If this option is set to | |
b74c3b8c | 626 | .Dq no , |
667e4135 | 627 | root is not allowed to log in. |
d20f3c9e | 628 | .It Cm PermitTunnel |
629 | Specifies whether | |
630 | .Xr tun 4 | |
631 | device forwarding is allowed. | |
a4f24bf8 | 632 | The argument must be |
633 | .Dq yes , | |
06fa4ac1 | 634 | .Dq point-to-point |
635 | (layer 3), | |
636 | .Dq ethernet | |
637 | (layer 2), or | |
a4f24bf8 | 638 | .Dq no . |
06fa4ac1 | 639 | Specifying |
640 | .Dq yes | |
641 | permits both | |
642 | .Dq point-to-point | |
643 | and | |
644 | .Dq ethernet . | |
d20f3c9e | 645 | The default is |
646 | .Dq no . | |
f00bab84 | 647 | .It Cm PermitUserEnvironment |
648 | Specifies whether | |
649 | .Pa ~/.ssh/environment | |
35453849 | 650 | and |
f00bab84 | 651 | .Cm environment= |
652 | options in | |
653 | .Pa ~/.ssh/authorized_keys | |
35453849 | 654 | are processed by |
b74c3b8c | 655 | .Xr sshd 8 . |
f00bab84 | 656 | The default is |
657 | .Dq no . | |
35453849 | 658 | Enabling environment processing may enable users to bypass access |
659 | restrictions in some configurations using mechanisms such as | |
660 | .Ev LD_PRELOAD . | |
588df31a | 661 | .It Cm PidFile |
baa08b92 | 662 | Specifies the file that contains the process ID of the |
32cfd177 | 663 | SSH daemon. |
588df31a | 664 | The default is |
665 | .Pa /var/run/sshd.pid . | |
666 | .It Cm Port | |
667 | Specifies the port number that | |
b74c3b8c | 668 | .Xr sshd 8 |
588df31a | 669 | listens on. |
670 | The default is 22. | |
671 | Multiple options of this type are permitted. | |
672 | See also | |
673 | .Cm ListenAddress . | |
674 | .It Cm PrintLastLog | |
675 | Specifies whether | |
b74c3b8c | 676 | .Xr sshd 8 |
329a8666 | 677 | should print the date and time of the last user login when a user logs |
678 | in interactively. | |
588df31a | 679 | The default is |
680 | .Dq yes . | |
681 | .It Cm PrintMotd | |
682 | Specifies whether | |
b74c3b8c | 683 | .Xr sshd 8 |
588df31a | 684 | should print |
685 | .Pa /etc/motd | |
686 | when a user logs in interactively. | |
687 | (On some systems it is also printed by the shell, | |
688 | .Pa /etc/profile , | |
689 | or equivalent.) | |
690 | The default is | |
691 | .Dq yes . | |
692 | .It Cm Protocol | |
693 | Specifies the protocol versions | |
b74c3b8c | 694 | .Xr sshd 8 |
94ad46d1 | 695 | supports. |
588df31a | 696 | The possible values are |
b74c3b8c | 697 | .Sq 1 |
588df31a | 698 | and |
b74c3b8c | 699 | .Sq 2 . |
588df31a | 700 | Multiple versions must be comma-separated. |
701 | The default is | |
702 | .Dq 2,1 . | |
94ad46d1 | 703 | Note that the order of the protocol list does not indicate preference, |
704 | because the client selects among multiple protocol versions offered | |
705 | by the server. | |
706 | Specifying | |
707 | .Dq 2,1 | |
708 | is identical to | |
709 | .Dq 1,2 . | |
588df31a | 710 | .It Cm PubkeyAuthentication |
711 | Specifies whether public key authentication is allowed. | |
712 | The default is | |
713 | .Dq yes . | |
714 | Note that this option applies to protocol version 2 only. | |
588df31a | 715 | .It Cm RhostsRSAAuthentication |
716 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
717 | with successful RSA host authentication is allowed. | |
718 | The default is | |
719 | .Dq no . | |
720 | This option applies to protocol version 1 only. | |
721 | .It Cm RSAAuthentication | |
722 | Specifies whether pure RSA authentication is allowed. | |
723 | The default is | |
724 | .Dq yes . | |
725 | This option applies to protocol version 1 only. | |
726 | .It Cm ServerKeyBits | |
727 | Defines the number of bits in the ephemeral protocol version 1 server key. | |
728 | The minimum value is 512, and the default is 768. | |
729 | .It Cm StrictModes | |
730 | Specifies whether | |
b74c3b8c | 731 | .Xr sshd 8 |
588df31a | 732 | should check file modes and ownership of the |
733 | user's files and home directory before accepting login. | |
734 | This is normally desirable because novices sometimes accidentally leave their | |
735 | directory or files world-writable. | |
736 | The default is | |
737 | .Dq yes . | |
738 | .It Cm Subsystem | |
f09ffbdb | 739 | Configures an external subsystem (e.g. file transfer daemon). |
d66ce1a1 | 740 | Arguments should be a subsystem name and a command (with optional arguments) |
741 | to execute upon subsystem request. | |
588df31a | 742 | The command |
743 | .Xr sftp-server 8 | |
744 | implements the | |
745 | .Dq sftp | |
746 | file transfer subsystem. | |
747 | By default no subsystems are defined. | |
748 | Note that this option applies to protocol version 2 only. | |
749 | .It Cm SyslogFacility | |
750 | Gives the facility code that is used when logging messages from | |
32cfd177 | 751 | .Xr sshd 8 . |
588df31a | 752 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, |
753 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. | |
754 | The default is AUTH. | |
fd573618 | 755 | .It Cm TCPKeepAlive |
756 | Specifies whether the system should send TCP keepalive messages to the | |
757 | other side. | |
758 | If they are sent, death of the connection or crash of one | |
759 | of the machines will be properly noticed. | |
760 | However, this means that | |
761 | connections will die if the route is down temporarily, and some people | |
762 | find it annoying. | |
763 | On the other hand, if TCP keepalives are not sent, | |
764 | sessions may hang indefinitely on the server, leaving | |
765 | .Dq ghost | |
766 | users and consuming server resources. | |
767 | .Pp | |
768 | The default is | |
769 | .Dq yes | |
770 | (to send TCP keepalive messages), and the server will notice | |
771 | if the network goes down or the client host crashes. | |
772 | This avoids infinitely hanging sessions. | |
773 | .Pp | |
774 | To disable TCP keepalive messages, the value should be set to | |
775 | .Dq no . | |
c5a7d788 | 776 | .It Cm UseDNS |
777 | Specifies whether | |
b74c3b8c | 778 | .Xr sshd 8 |
102c77c2 | 779 | should look up the remote host name and check that |
c5a7d788 | 780 | the resolved host name for the remote IP address maps back to the |
781 | very same IP address. | |
782 | The default is | |
783 | .Dq yes . | |
588df31a | 784 | .It Cm UseLogin |
785 | Specifies whether | |
786 | .Xr login 1 | |
787 | is used for interactive login sessions. | |
788 | The default is | |
789 | .Dq no . | |
790 | Note that | |
791 | .Xr login 1 | |
792 | is never used for remote command execution. | |
793 | Note also, that if this is enabled, | |
794 | .Cm X11Forwarding | |
795 | will be disabled because | |
796 | .Xr login 1 | |
797 | does not know how to handle | |
798 | .Xr xauth 1 | |
a4e5acef | 799 | cookies. |
800 | If | |
588df31a | 801 | .Cm UsePrivilegeSeparation |
802 | is specified, it will be disabled after authentication. | |
72c35df7 | 803 | .It Cm UsePAM |
d5c67850 | 804 | Enables the Pluggable Authentication Module interface. |
805 | If set to | |
806 | .Dq yes | |
807 | this will enable PAM authentication using | |
808 | .Cm ChallengeResponseAuthentication | |
05059810 | 809 | and |
810 | .Cm PasswordAuthentication | |
811 | in addition to PAM account and session module processing for all | |
812 | authentication types. | |
d5c67850 | 813 | .Pp |
814 | Because PAM challenge-response authentication usually serves an equivalent | |
815 | role to password authentication, you should disable either | |
816 | .Cm PasswordAuthentication | |
817 | or | |
818 | .Cm ChallengeResponseAuthentication. | |
819 | .Pp | |
820 | If | |
821 | .Cm UsePAM | |
822 | is enabled, you will not be able to run | |
823 | .Xr sshd 8 | |
824 | as a non-root user. | |
825 | The default is | |
a83a3125 | 826 | .Dq no . |
588df31a | 827 | .It Cm UsePrivilegeSeparation |
828 | Specifies whether | |
b74c3b8c | 829 | .Xr sshd 8 |
588df31a | 830 | separates privileges by creating an unprivileged child process |
a4e5acef | 831 | to deal with incoming network traffic. |
832 | After successful authentication, another process will be created that has | |
833 | the privilege of the authenticated user. | |
834 | The goal of privilege separation is to prevent privilege | |
588df31a | 835 | escalation by containing any corruption within the unprivileged processes. |
836 | The default is | |
837 | .Dq yes . | |
588df31a | 838 | .It Cm X11DisplayOffset |
839 | Specifies the first display number available for | |
b74c3b8c | 840 | .Xr sshd 8 Ns 's |
588df31a | 841 | X11 forwarding. |
b74c3b8c | 842 | This prevents sshd from interfering with real X11 servers. |
588df31a | 843 | The default is 10. |
844 | .It Cm X11Forwarding | |
845 | Specifies whether X11 forwarding is permitted. | |
e6fe1bab | 846 | The argument must be |
847 | .Dq yes | |
848 | or | |
849 | .Dq no . | |
588df31a | 850 | The default is |
851 | .Dq no . | |
e6fe1bab | 852 | .Pp |
853 | When X11 forwarding is enabled, there may be additional exposure to | |
854 | the server and to client displays if the | |
b74c3b8c | 855 | .Xr sshd 8 |
e6fe1bab | 856 | proxy display is configured to listen on the wildcard address (see |
857 | .Cm X11UseLocalhost | |
b74c3b8c | 858 | below), though this is not the default. |
e6fe1bab | 859 | Additionally, the authentication spoofing and authentication data |
860 | verification and substitution occur on the client side. | |
861 | The security risk of using X11 forwarding is that the client's X11 | |
b74c3b8c | 862 | display server may be exposed to attack when the SSH client requests |
e6fe1bab | 863 | forwarding (see the warnings for |
864 | .Cm ForwardX11 | |
865 | in | |
be193d89 | 866 | .Xr ssh_config 5 ) . |
e6fe1bab | 867 | A system administrator may have a stance in which they want to |
868 | protect clients that may expose themselves to attack by unwittingly | |
869 | requesting X11 forwarding, which can warrant a | |
870 | .Dq no | |
871 | setting. | |
872 | .Pp | |
873 | Note that disabling X11 forwarding does not prevent users from | |
874 | forwarding X11 traffic, as users can always install their own forwarders. | |
588df31a | 875 | X11 forwarding is automatically disabled if |
876 | .Cm UseLogin | |
877 | is enabled. | |
878 | .It Cm X11UseLocalhost | |
879 | Specifies whether | |
b74c3b8c | 880 | .Xr sshd 8 |
588df31a | 881 | should bind the X11 forwarding server to the loopback address or to |
a4e5acef | 882 | the wildcard address. |
883 | By default, | |
b74c3b8c | 884 | sshd binds the forwarding server to the loopback address and sets the |
588df31a | 885 | hostname part of the |
886 | .Ev DISPLAY | |
887 | environment variable to | |
888 | .Dq localhost . | |
b3641662 | 889 | This prevents remote hosts from connecting to the proxy display. |
588df31a | 890 | However, some older X11 clients may not function with this |
891 | configuration. | |
892 | .Cm X11UseLocalhost | |
893 | may be set to | |
894 | .Dq no | |
895 | to specify that the forwarding server should be bound to the wildcard | |
896 | address. | |
897 | The argument must be | |
898 | .Dq yes | |
899 | or | |
900 | .Dq no . | |
901 | The default is | |
902 | .Dq yes . | |
903 | .It Cm XAuthLocation | |
57ff5eeb | 904 | Specifies the full pathname of the |
588df31a | 905 | .Xr xauth 1 |
906 | program. | |
907 | The default is | |
908 | .Pa /usr/X11R6/bin/xauth . | |
909 | .El | |
ef1c6497 | 910 | .Sh TIME FORMATS |
32cfd177 | 911 | .Xr sshd 8 |
588df31a | 912 | command-line arguments and configuration file options that specify time |
913 | may be expressed using a sequence of the form: | |
914 | .Sm off | |
36535ee6 | 915 | .Ar time Op Ar qualifier , |
588df31a | 916 | .Sm on |
917 | where | |
918 | .Ar time | |
919 | is a positive integer value and | |
920 | .Ar qualifier | |
921 | is one of the following: | |
922 | .Pp | |
923 | .Bl -tag -width Ds -compact -offset indent | |
874d319b | 924 | .It Aq Cm none |
588df31a | 925 | seconds |
926 | .It Cm s | Cm S | |
927 | seconds | |
928 | .It Cm m | Cm M | |
929 | minutes | |
930 | .It Cm h | Cm H | |
931 | hours | |
932 | .It Cm d | Cm D | |
933 | days | |
934 | .It Cm w | Cm W | |
935 | weeks | |
936 | .El | |
937 | .Pp | |
938 | Each member of the sequence is added together to calculate | |
939 | the total time value. | |
940 | .Pp | |
941 | Time format examples: | |
942 | .Pp | |
943 | .Bl -tag -width Ds -compact -offset indent | |
944 | .It 600 | |
945 | 600 seconds (10 minutes) | |
946 | .It 10m | |
947 | 10 minutes | |
948 | .It 1h30m | |
949 | 1 hour 30 minutes (90 minutes) | |
950 | .El | |
951 | .Sh FILES | |
952 | .Bl -tag -width Ds | |
953 | .It Pa /etc/ssh/sshd_config | |
954 | Contains configuration data for | |
32cfd177 | 955 | .Xr sshd 8 . |
588df31a | 956 | This file should be writable by root only, but it is recommended |
957 | (though not necessary) that it be world-readable. | |
958 | .El | |
be193d89 | 959 | .Sh SEE ALSO |
960 | .Xr sshd 8 | |
588df31a | 961 | .Sh AUTHORS |
962 | OpenSSH is a derivative of the original and free | |
963 | ssh 1.2.12 release by Tatu Ylonen. | |
964 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
965 | Theo de Raadt and Dug Song | |
966 | removed many bugs, re-added newer features and | |
967 | created OpenSSH. | |
968 | Markus Friedl contributed the support for SSH | |
969 | protocol versions 1.5 and 2.0. | |
970 | Niels Provos and Markus Friedl contributed support | |
971 | for privilege separation. |