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f5799ae1 | 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 | .\" | |
d4487008 | 37 | .\" $OpenBSD: sshd_config.5,v 1.77 2007/06/08 07:48:09 jmc Exp $ |
38 | .Dd $Mdocdate: June 11 2007 $ | |
f5799ae1 | 39 | .Dt SSHD_CONFIG 5 |
40 | .Os | |
41 | .Sh NAME | |
42 | .Nm sshd_config | |
43 | .Nd OpenSSH SSH daemon configuration file | |
44 | .Sh SYNOPSIS | |
799ae497 | 45 | .Nm /etc/ssh/sshd_config |
f5799ae1 | 46 | .Sh DESCRIPTION |
9108f8d9 | 47 | .Xr sshd 8 |
f5799ae1 | 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. | |
9108f8d9 | 57 | Arguments may optionally be enclosed in double quotes |
58 | .Pq \&" | |
59 | in order to represent arguments containing spaces. | |
f5799ae1 | 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 | |
c9f39d2c | 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 | |
9108f8d9 | 76 | .Ql * |
c9f39d2c | 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. | |
996d5e62 | 87 | .It Cm AddressFamily |
88 | Specifies which address family should be used by | |
9108f8d9 | 89 | .Xr sshd 8 . |
996d5e62 | 90 | Valid arguments are |
91 | .Dq any , | |
92 | .Dq inet | |
9108f8d9 | 93 | (use IPv4 only), or |
996d5e62 | 94 | .Dq inet6 |
95 | (use IPv6 only). | |
96 | The default is | |
97 | .Dq any . | |
f5799ae1 | 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. | |
f5799ae1 | 103 | Only group names are valid; a numerical group ID is not recognized. |
104 | By default, login is allowed for all groups. | |
9108f8d9 | 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. | |
f5799ae1 | 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. | |
f5799ae1 | 124 | .It Cm AllowUsers |
125 | This keyword can be followed by a list of user name patterns, separated | |
126 | by spaces. | |
6a9b3198 | 127 | If specified, login is allowed only for user names that |
f5799ae1 | 128 | match one of the patterns. |
f5799ae1 | 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. | |
9108f8d9 | 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. | |
f5799ae1 | 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 | |
9108f8d9 | 151 | setup. |
0fff78ff | 152 | The following tokens are defined: %% is replaced by a literal '%', |
9108f8d9 | 153 | %h is replaced by the home directory of the user being authenticated, and |
f5799ae1 | 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 | |
162 | In some jurisdictions, sending a warning message before authentication | |
163 | may be relevant for getting legal protection. | |
164 | The contents of the specified file are sent to the remote user before | |
165 | authentication is allowed. | |
166 | This option is only available for protocol version 2. | |
167 | By default, no banner is displayed. | |
f5799ae1 | 168 | .It Cm ChallengeResponseAuthentication |
9108f8d9 | 169 | Specifies whether challenge-response authentication is allowed. |
f5799ae1 | 170 | All authentication styles from |
171 | .Xr login.conf 5 | |
172 | are supported. | |
173 | The default is | |
174 | .Dq yes . | |
175 | .It Cm Ciphers | |
176 | Specifies the ciphers allowed for protocol version 2. | |
177 | Multiple ciphers must be comma-separated. | |
c9f39d2c | 178 | The supported ciphers are |
179 | .Dq 3des-cbc , | |
180 | .Dq aes128-cbc , | |
181 | .Dq aes192-cbc , | |
182 | .Dq aes256-cbc , | |
183 | .Dq aes128-ctr , | |
184 | .Dq aes192-ctr , | |
185 | .Dq aes256-ctr , | |
665a873d | 186 | .Dq arcfour128 , |
187 | .Dq arcfour256 , | |
c9f39d2c | 188 | .Dq arcfour , |
189 | .Dq blowfish-cbc , | |
190 | and | |
191 | .Dq cast128-cbc . | |
9108f8d9 | 192 | The default is: |
193 | .Bd -literal -offset 3n | |
194 | aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, | |
195 | arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, | |
196 | aes192-ctr,aes256-ctr | |
f5799ae1 | 197 | .Ed |
f5799ae1 | 198 | .It Cm ClientAliveCountMax |
2c06c99b | 199 | Sets the number of client alive messages (see below) which may be |
f5799ae1 | 200 | sent without |
9108f8d9 | 201 | .Xr sshd 8 |
0fff78ff | 202 | receiving any messages back from the client. |
203 | If this threshold is reached while client alive messages are being sent, | |
9108f8d9 | 204 | sshd will disconnect the client, terminating the session. |
0fff78ff | 205 | It is important to note that the use of client alive messages is very |
206 | different from | |
cdd66111 | 207 | .Cm TCPKeepAlive |
0fff78ff | 208 | (below). |
209 | The client alive messages are sent through the encrypted channel | |
210 | and therefore will not be spoofable. | |
211 | The TCP keepalive option enabled by | |
cdd66111 | 212 | .Cm TCPKeepAlive |
0fff78ff | 213 | is spoofable. |
214 | The client alive mechanism is valuable when the client or | |
f5799ae1 | 215 | server depend on knowing when a connection has become inactive. |
216 | .Pp | |
0fff78ff | 217 | The default value is 3. |
218 | If | |
f5799ae1 | 219 | .Cm ClientAliveInterval |
2c06c99b | 220 | (see below) is set to 15, and |
f5799ae1 | 221 | .Cm ClientAliveCountMax |
9108f8d9 | 222 | is left at the default, unresponsive SSH clients |
f5799ae1 | 223 | will be disconnected after approximately 45 seconds. |
9108f8d9 | 224 | This option applies to protocol version 2 only. |
665a873d | 225 | .It Cm ClientAliveInterval |
226 | Sets a timeout interval in seconds after which if no data has been received | |
227 | from the client, | |
9108f8d9 | 228 | .Xr sshd 8 |
665a873d | 229 | will send a message through the encrypted |
230 | channel to request a response from the client. | |
231 | The default | |
232 | is 0, indicating that these messages will not be sent to the client. | |
233 | This option applies to protocol version 2 only. | |
f5799ae1 | 234 | .It Cm Compression |
665a873d | 235 | Specifies whether compression is allowed, or delayed until |
236 | the user has authenticated successfully. | |
f5799ae1 | 237 | The argument must be |
665a873d | 238 | .Dq yes , |
239 | .Dq delayed , | |
f5799ae1 | 240 | or |
241 | .Dq no . | |
242 | The default is | |
665a873d | 243 | .Dq delayed . |
f5799ae1 | 244 | .It Cm DenyGroups |
245 | This keyword can be followed by a list of group name patterns, separated | |
246 | by spaces. | |
247 | Login is disallowed for users whose primary group or supplementary | |
248 | group list matches one of the patterns. | |
f5799ae1 | 249 | Only group names are valid; a numerical group ID is not recognized. |
250 | By default, login is allowed for all groups. | |
9108f8d9 | 251 | The allow/deny directives are processed in the following order: |
252 | .Cm DenyUsers , | |
253 | .Cm AllowUsers , | |
254 | .Cm DenyGroups , | |
255 | and finally | |
256 | .Cm AllowGroups . | |
257 | .Pp | |
258 | See | |
259 | .Sx PATTERNS | |
260 | in | |
261 | .Xr ssh_config 5 | |
262 | for more information on patterns. | |
f5799ae1 | 263 | .It Cm DenyUsers |
264 | This keyword can be followed by a list of user name patterns, separated | |
265 | by spaces. | |
266 | Login is disallowed for user names that match one of the patterns. | |
f5799ae1 | 267 | Only user names are valid; a numerical user ID is not recognized. |
268 | By default, login is allowed for all users. | |
269 | If the pattern takes the form USER@HOST then USER and HOST | |
270 | are separately checked, restricting logins to particular | |
271 | users from particular hosts. | |
9108f8d9 | 272 | The allow/deny directives are processed in the following order: |
273 | .Cm DenyUsers , | |
274 | .Cm AllowUsers , | |
275 | .Cm DenyGroups , | |
276 | and finally | |
277 | .Cm AllowGroups . | |
278 | .Pp | |
279 | See | |
280 | .Sx PATTERNS | |
281 | in | |
282 | .Xr ssh_config 5 | |
283 | for more information on patterns. | |
284 | .It Cm ForceCommand | |
285 | Forces the execution of the command specified by | |
286 | .Cm ForceCommand , | |
287 | ignoring any command supplied by the client. | |
288 | The command is invoked by using the user's login shell with the -c option. | |
289 | This applies to shell, command, or subsystem execution. | |
290 | It is most useful inside a | |
291 | .Cm Match | |
292 | block. | |
293 | The command originally supplied by the client is available in the | |
294 | .Ev SSH_ORIGINAL_COMMAND | |
295 | environment variable. | |
f5799ae1 | 296 | .It Cm GatewayPorts |
297 | Specifies whether remote hosts are allowed to connect to ports | |
298 | forwarded for the client. | |
299 | By default, | |
9108f8d9 | 300 | .Xr sshd 8 |
6a9b3198 | 301 | binds remote port forwardings to the loopback address. |
302 | This prevents other remote hosts from connecting to forwarded ports. | |
f5799ae1 | 303 | .Cm GatewayPorts |
9108f8d9 | 304 | can be used to specify that sshd |
996d5e62 | 305 | should allow remote port forwardings to bind to non-loopback addresses, thus |
306 | allowing other hosts to connect. | |
307 | The argument may be | |
308 | .Dq no | |
309 | to force remote port forwardings to be available to the local host only, | |
f5799ae1 | 310 | .Dq yes |
996d5e62 | 311 | to force remote port forwardings to bind to the wildcard address, or |
312 | .Dq clientspecified | |
313 | to allow the client to select the address to which the forwarding is bound. | |
f5799ae1 | 314 | The default is |
315 | .Dq no . | |
0fff78ff | 316 | .It Cm GSSAPIAuthentication |
317 | Specifies whether user authentication based on GSSAPI is allowed. | |
cdd66111 | 318 | The default is |
0fff78ff | 319 | .Dq no . |
320 | Note that this option applies to protocol version 2 only. | |
321 | .It Cm GSSAPICleanupCredentials | |
322 | Specifies whether to automatically destroy the user's credentials cache | |
323 | on logout. | |
324 | The default is | |
325 | .Dq yes . | |
326 | Note that this option applies to protocol version 2 only. | |
f5799ae1 | 327 | .It Cm HostbasedAuthentication |
328 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
329 | with successful public key client host authentication is allowed | |
9108f8d9 | 330 | (host-based authentication). |
f5799ae1 | 331 | This option is similar to |
332 | .Cm RhostsRSAAuthentication | |
333 | and applies to protocol version 2 only. | |
334 | The default is | |
335 | .Dq no . | |
9108f8d9 | 336 | .It Cm HostbasedUsesNameFromPacketOnly |
337 | Specifies whether or not the server will attempt to perform a reverse | |
338 | name lookup when matching the name in the | |
339 | .Pa ~/.shosts , | |
340 | .Pa ~/.rhosts , | |
341 | and | |
342 | .Pa /etc/hosts.equiv | |
343 | files during | |
344 | .Cm HostbasedAuthentication . | |
345 | A setting of | |
346 | .Dq yes | |
347 | means that | |
348 | .Xr sshd 8 | |
349 | uses the name supplied by the client rather than | |
350 | attempting to resolve the name from the TCP connection itself. | |
351 | The default is | |
352 | .Dq no . | |
f5799ae1 | 353 | .It Cm HostKey |
354 | Specifies a file containing a private host key | |
355 | used by SSH. | |
356 | The default is | |
357 | .Pa /etc/ssh/ssh_host_key | |
358 | for protocol version 1, and | |
359 | .Pa /etc/ssh/ssh_host_rsa_key | |
360 | and | |
361 | .Pa /etc/ssh/ssh_host_dsa_key | |
362 | for protocol version 2. | |
363 | Note that | |
9108f8d9 | 364 | .Xr sshd 8 |
f5799ae1 | 365 | will refuse to use a file if it is group/world-accessible. |
366 | It is possible to have multiple host key files. | |
367 | .Dq rsa1 | |
368 | keys are used for version 1 and | |
369 | .Dq dsa | |
370 | or | |
371 | .Dq rsa | |
372 | are used for version 2 of the SSH protocol. | |
373 | .It Cm IgnoreRhosts | |
374 | Specifies that | |
375 | .Pa .rhosts | |
376 | and | |
377 | .Pa .shosts | |
378 | files will not be used in | |
f5799ae1 | 379 | .Cm RhostsRSAAuthentication |
380 | or | |
381 | .Cm HostbasedAuthentication . | |
382 | .Pp | |
383 | .Pa /etc/hosts.equiv | |
384 | and | |
385 | .Pa /etc/shosts.equiv | |
386 | are still used. | |
387 | The default is | |
388 | .Dq yes . | |
389 | .It Cm IgnoreUserKnownHosts | |
390 | Specifies whether | |
9108f8d9 | 391 | .Xr sshd 8 |
f5799ae1 | 392 | should ignore the user's |
665a873d | 393 | .Pa ~/.ssh/known_hosts |
f5799ae1 | 394 | during |
395 | .Cm RhostsRSAAuthentication | |
396 | or | |
397 | .Cm HostbasedAuthentication . | |
398 | The default is | |
399 | .Dq no . | |
f5799ae1 | 400 | .It Cm KerberosAuthentication |
0fff78ff | 401 | Specifies whether the password provided by the user for |
f5799ae1 | 402 | .Cm PasswordAuthentication |
0fff78ff | 403 | will be validated through the Kerberos KDC. |
f5799ae1 | 404 | To use this option, the server needs a |
405 | Kerberos servtab which allows the verification of the KDC's identity. | |
9108f8d9 | 406 | The default is |
f5799ae1 | 407 | .Dq no . |
99be0775 | 408 | .It Cm KerberosGetAFSToken |
2c06c99b | 409 | If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire |
99be0775 | 410 | an AFS token before accessing the user's home directory. |
9108f8d9 | 411 | The default is |
99be0775 | 412 | .Dq no . |
f5799ae1 | 413 | .It Cm KerberosOrLocalPasswd |
9108f8d9 | 414 | If password authentication through Kerberos fails then |
f5799ae1 | 415 | the password will be validated via any additional local mechanism |
416 | such as | |
417 | .Pa /etc/passwd . | |
9108f8d9 | 418 | The default is |
f5799ae1 | 419 | .Dq yes . |
f5799ae1 | 420 | .It Cm KerberosTicketCleanup |
421 | Specifies whether to automatically destroy the user's ticket cache | |
422 | file on logout. | |
9108f8d9 | 423 | The default is |
f5799ae1 | 424 | .Dq yes . |
425 | .It Cm KeyRegenerationInterval | |
426 | In protocol version 1, the ephemeral server key is automatically regenerated | |
427 | after this many seconds (if it has been used). | |
428 | The purpose of regeneration is to prevent | |
429 | decrypting captured sessions by later breaking into the machine and | |
430 | stealing the keys. | |
431 | The key is never stored anywhere. | |
432 | If the value is 0, the key is never regenerated. | |
433 | The default is 3600 (seconds). | |
434 | .It Cm ListenAddress | |
435 | Specifies the local addresses | |
9108f8d9 | 436 | .Xr sshd 8 |
f5799ae1 | 437 | should listen on. |
438 | The following forms may be used: | |
439 | .Pp | |
440 | .Bl -item -offset indent -compact | |
441 | .It | |
442 | .Cm ListenAddress | |
443 | .Sm off | |
444 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr | |
445 | .Sm on | |
446 | .It | |
447 | .Cm ListenAddress | |
448 | .Sm off | |
449 | .Ar host No | Ar IPv4_addr No : Ar port | |
450 | .Sm on | |
451 | .It | |
452 | .Cm ListenAddress | |
453 | .Sm off | |
454 | .Oo | |
455 | .Ar host No | Ar IPv6_addr Oc : Ar port | |
456 | .Sm on | |
457 | .El | |
458 | .Pp | |
459 | If | |
460 | .Ar port | |
461 | is not specified, | |
9108f8d9 | 462 | sshd will listen on the address and all prior |
f5799ae1 | 463 | .Cm Port |
0fff78ff | 464 | options specified. |
465 | The default is to listen on all local addresses. | |
6a9b3198 | 466 | Multiple |
f5799ae1 | 467 | .Cm ListenAddress |
0fff78ff | 468 | options are permitted. |
469 | Additionally, any | |
f5799ae1 | 470 | .Cm Port |
9108f8d9 | 471 | options must precede this option for non-port qualified addresses. |
f5799ae1 | 472 | .It Cm LoginGraceTime |
473 | The server disconnects after this time if the user has not | |
474 | successfully logged in. | |
475 | If the value is 0, there is no time limit. | |
41b2f314 | 476 | The default is 120 seconds. |
f5799ae1 | 477 | .It Cm LogLevel |
478 | Gives the verbosity level that is used when logging messages from | |
9108f8d9 | 479 | .Xr sshd 8 . |
f5799ae1 | 480 | The possible values are: |
9108f8d9 | 481 | QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. |
6a9b3198 | 482 | The default is INFO. |
483 | DEBUG and DEBUG1 are equivalent. | |
484 | DEBUG2 and DEBUG3 each specify higher levels of debugging output. | |
485 | Logging with a DEBUG level violates the privacy of users and is not recommended. | |
f5799ae1 | 486 | .It Cm MACs |
487 | Specifies the available MAC (message authentication code) algorithms. | |
488 | The MAC algorithm is used in protocol version 2 | |
489 | for data integrity protection. | |
490 | Multiple algorithms must be comma-separated. | |
9108f8d9 | 491 | The default is: |
d4487008 | 492 | .Bd -literal -offset indent |
493 | hmac-md5,hmac-sha1,umac-64@openssh.com, | |
494 | hmac-ripemd160,hmac-sha1-96,hmac-md5-96 | |
495 | .Ed | |
9108f8d9 | 496 | .It Cm Match |
497 | Introduces a conditional block. | |
498 | If all of the criteria on the | |
499 | .Cm Match | |
500 | line are satisfied, the keywords on the following lines override those | |
501 | set in the global section of the config file, until either another | |
502 | .Cm Match | |
503 | line or the end of the file. | |
504 | The arguments to | |
505 | .Cm Match | |
506 | are one or more criteria-pattern pairs. | |
507 | The available criteria are | |
508 | .Cm User , | |
509 | .Cm Group , | |
510 | .Cm Host , | |
511 | and | |
512 | .Cm Address . | |
513 | Only a subset of keywords may be used on the lines following a | |
514 | .Cm Match | |
515 | keyword. | |
516 | Available keywords are | |
517 | .Cm AllowTcpForwarding , | |
799ae497 | 518 | .Cm Banner , |
9108f8d9 | 519 | .Cm ForceCommand , |
520 | .Cm GatewayPorts , | |
799ae497 | 521 | .Cm GSSApiAuthentication , |
522 | .Cm KbdInteractiveAuthentication , | |
523 | .Cm KerberosAuthentication , | |
524 | .Cm PasswordAuthentication , | |
9108f8d9 | 525 | .Cm PermitOpen , |
799ae497 | 526 | .Cm RhostsRSAAuthentication , |
527 | .Cm RSAAuthentication , | |
9108f8d9 | 528 | .Cm X11DisplayOffset , |
529 | .Cm X11Forwarding , | |
530 | and | |
531 | .Cm X11UseLocalHost . | |
c9f39d2c | 532 | .It Cm MaxAuthTries |
533 | Specifies the maximum number of authentication attempts permitted per | |
534 | connection. | |
535 | Once the number of failures reaches half this value, | |
536 | additional failures are logged. | |
537 | The default is 6. | |
f5799ae1 | 538 | .It Cm MaxStartups |
539 | Specifies the maximum number of concurrent unauthenticated connections to the | |
9108f8d9 | 540 | SSH daemon. |
f5799ae1 | 541 | Additional connections will be dropped until authentication succeeds or the |
542 | .Cm LoginGraceTime | |
543 | expires for a connection. | |
544 | The default is 10. | |
545 | .Pp | |
546 | Alternatively, random early drop can be enabled by specifying | |
547 | the three colon separated values | |
548 | .Dq start:rate:full | |
9108f8d9 | 549 | (e.g. "10:30:60"). |
550 | .Xr sshd 8 | |
f5799ae1 | 551 | will refuse connection attempts with a probability of |
552 | .Dq rate/100 | |
553 | (30%) | |
554 | if there are currently | |
555 | .Dq start | |
556 | (10) | |
557 | unauthenticated connections. | |
558 | The probability increases linearly and all connection attempts | |
559 | are refused if the number of unauthenticated connections reaches | |
560 | .Dq full | |
561 | (60). | |
562 | .It Cm PasswordAuthentication | |
563 | Specifies whether password authentication is allowed. | |
564 | The default is | |
565 | .Dq yes . | |
566 | .It Cm PermitEmptyPasswords | |
567 | When password authentication is allowed, it specifies whether the | |
568 | server allows login to accounts with empty password strings. | |
569 | The default is | |
570 | .Dq no . | |
9108f8d9 | 571 | .It Cm PermitOpen |
572 | Specifies the destinations to which TCP port forwarding is permitted. | |
573 | The forwarding specification must be one of the following forms: | |
574 | .Pp | |
575 | .Bl -item -offset indent -compact | |
576 | .It | |
577 | .Cm PermitOpen | |
578 | .Sm off | |
579 | .Ar host : port | |
580 | .Sm on | |
581 | .It | |
582 | .Cm PermitOpen | |
583 | .Sm off | |
584 | .Ar IPv4_addr : port | |
585 | .Sm on | |
586 | .It | |
587 | .Cm PermitOpen | |
588 | .Sm off | |
589 | .Ar \&[ IPv6_addr \&] : port | |
590 | .Sm on | |
591 | .El | |
592 | .Pp | |
593 | Multiple forwards may be specified by separating them with whitespace. | |
594 | An argument of | |
595 | .Dq any | |
596 | can be used to remove all restrictions and permit any forwarding requests. | |
597 | By default all port forwarding requests are permitted. | |
f5799ae1 | 598 | .It Cm PermitRootLogin |
996d5e62 | 599 | Specifies whether root can log in using |
f5799ae1 | 600 | .Xr ssh 1 . |
601 | The argument must be | |
602 | .Dq yes , | |
603 | .Dq without-password , | |
9108f8d9 | 604 | .Dq forced-commands-only , |
f5799ae1 | 605 | or |
606 | .Dq no . | |
607 | The default is | |
608 | .Dq yes . | |
609 | .Pp | |
610 | If this option is set to | |
9108f8d9 | 611 | .Dq without-password , |
996d5e62 | 612 | password authentication is disabled for root. |
f5799ae1 | 613 | .Pp |
614 | If this option is set to | |
9108f8d9 | 615 | .Dq forced-commands-only , |
f5799ae1 | 616 | root login with public key authentication will be allowed, |
617 | but only if the | |
618 | .Ar command | |
619 | option has been specified | |
620 | (which may be useful for taking remote backups even if root login is | |
0fff78ff | 621 | normally not allowed). |
622 | All other authentication methods are disabled for root. | |
f5799ae1 | 623 | .Pp |
624 | If this option is set to | |
9108f8d9 | 625 | .Dq no , |
996d5e62 | 626 | root is not allowed to log in. |
2c06c99b | 627 | .It Cm PermitTunnel |
628 | Specifies whether | |
629 | .Xr tun 4 | |
630 | device forwarding is allowed. | |
631 | The argument must be | |
632 | .Dq yes , | |
9108f8d9 | 633 | .Dq point-to-point |
634 | (layer 3), | |
2c06c99b | 635 | .Dq ethernet |
9108f8d9 | 636 | (layer 2), or |
2c06c99b | 637 | .Dq no . |
9108f8d9 | 638 | Specifying |
639 | .Dq yes | |
640 | permits both | |
641 | .Dq point-to-point | |
642 | and | |
643 | .Dq ethernet . | |
2c06c99b | 644 | The default is |
645 | .Dq no . | |
41b2f314 | 646 | .It Cm PermitUserEnvironment |
647 | Specifies whether | |
648 | .Pa ~/.ssh/environment | |
649 | and | |
650 | .Cm environment= | |
651 | options in | |
652 | .Pa ~/.ssh/authorized_keys | |
653 | are processed by | |
9108f8d9 | 654 | .Xr sshd 8 . |
41b2f314 | 655 | The default is |
656 | .Dq no . | |
657 | Enabling environment processing may enable users to bypass access | |
658 | restrictions in some configurations using mechanisms such as | |
659 | .Ev LD_PRELOAD . | |
f5799ae1 | 660 | .It Cm PidFile |
680cee3b | 661 | Specifies the file that contains the process ID of the |
9108f8d9 | 662 | SSH daemon. |
f5799ae1 | 663 | The default is |
664 | .Pa /var/run/sshd.pid . | |
665 | .It Cm Port | |
666 | Specifies the port number that | |
9108f8d9 | 667 | .Xr sshd 8 |
f5799ae1 | 668 | listens on. |
669 | The default is 22. | |
670 | Multiple options of this type are permitted. | |
671 | See also | |
672 | .Cm ListenAddress . | |
673 | .It Cm PrintLastLog | |
674 | Specifies whether | |
9108f8d9 | 675 | .Xr sshd 8 |
996d5e62 | 676 | should print the date and time of the last user login when a user logs |
677 | in interactively. | |
f5799ae1 | 678 | The default is |
679 | .Dq yes . | |
680 | .It Cm PrintMotd | |
681 | Specifies whether | |
9108f8d9 | 682 | .Xr sshd 8 |
f5799ae1 | 683 | should print |
684 | .Pa /etc/motd | |
685 | when a user logs in interactively. | |
686 | (On some systems it is also printed by the shell, | |
687 | .Pa /etc/profile , | |
688 | or equivalent.) | |
689 | The default is | |
690 | .Dq yes . | |
691 | .It Cm Protocol | |
692 | Specifies the protocol versions | |
9108f8d9 | 693 | .Xr sshd 8 |
41b2f314 | 694 | supports. |
f5799ae1 | 695 | The possible values are |
9108f8d9 | 696 | .Sq 1 |
f5799ae1 | 697 | and |
9108f8d9 | 698 | .Sq 2 . |
f5799ae1 | 699 | Multiple versions must be comma-separated. |
700 | The default is | |
701 | .Dq 2,1 . | |
41b2f314 | 702 | Note that the order of the protocol list does not indicate preference, |
703 | because the client selects among multiple protocol versions offered | |
704 | by the server. | |
705 | Specifying | |
706 | .Dq 2,1 | |
707 | is identical to | |
708 | .Dq 1,2 . | |
f5799ae1 | 709 | .It Cm PubkeyAuthentication |
710 | Specifies whether public key authentication is allowed. | |
711 | The default is | |
712 | .Dq yes . | |
713 | Note that this option applies to protocol version 2 only. | |
f5799ae1 | 714 | .It Cm RhostsRSAAuthentication |
715 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
716 | with successful RSA host authentication is allowed. | |
717 | The default is | |
718 | .Dq no . | |
719 | This option applies to protocol version 1 only. | |
720 | .It Cm RSAAuthentication | |
721 | Specifies whether pure RSA authentication is allowed. | |
722 | The default is | |
723 | .Dq yes . | |
724 | This option applies to protocol version 1 only. | |
725 | .It Cm ServerKeyBits | |
726 | Defines the number of bits in the ephemeral protocol version 1 server key. | |
727 | The minimum value is 512, and the default is 768. | |
728 | .It Cm StrictModes | |
729 | Specifies whether | |
9108f8d9 | 730 | .Xr sshd 8 |
f5799ae1 | 731 | should check file modes and ownership of the |
732 | user's files and home directory before accepting login. | |
733 | This is normally desirable because novices sometimes accidentally leave their | |
734 | directory or files world-writable. | |
735 | The default is | |
736 | .Dq yes . | |
737 | .It Cm Subsystem | |
9108f8d9 | 738 | Configures an external subsystem (e.g. file transfer daemon). |
739 | Arguments should be a subsystem name and a command (with optional arguments) | |
740 | to execute upon subsystem request. | |
f5799ae1 | 741 | The command |
742 | .Xr sftp-server 8 | |
743 | implements the | |
744 | .Dq sftp | |
745 | file transfer subsystem. | |
746 | By default no subsystems are defined. | |
747 | Note that this option applies to protocol version 2 only. | |
748 | .It Cm SyslogFacility | |
749 | Gives the facility code that is used when logging messages from | |
9108f8d9 | 750 | .Xr sshd 8 . |
f5799ae1 | 751 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, |
752 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. | |
753 | The default is AUTH. | |
cdd66111 | 754 | .It Cm TCPKeepAlive |
755 | Specifies whether the system should send TCP keepalive messages to the | |
756 | other side. | |
757 | If they are sent, death of the connection or crash of one | |
758 | of the machines will be properly noticed. | |
759 | However, this means that | |
760 | connections will die if the route is down temporarily, and some people | |
761 | find it annoying. | |
762 | On the other hand, if TCP keepalives are not sent, | |
763 | sessions may hang indefinitely on the server, leaving | |
764 | .Dq ghost | |
765 | users and consuming server resources. | |
766 | .Pp | |
767 | The default is | |
768 | .Dq yes | |
769 | (to send TCP keepalive messages), and the server will notice | |
770 | if the network goes down or the client host crashes. | |
771 | This avoids infinitely hanging sessions. | |
772 | .Pp | |
773 | To disable TCP keepalive messages, the value should be set to | |
774 | .Dq no . | |
0fff78ff | 775 | .It Cm UseDNS |
776 | Specifies whether | |
9108f8d9 | 777 | .Xr sshd 8 |
dec6d9fe | 778 | should look up the remote host name and check that |
0fff78ff | 779 | the resolved host name for the remote IP address maps back to the |
780 | very same IP address. | |
781 | The default is | |
782 | .Dq yes . | |
f5799ae1 | 783 | .It Cm UseLogin |
784 | Specifies whether | |
785 | .Xr login 1 | |
786 | is used for interactive login sessions. | |
787 | The default is | |
788 | .Dq no . | |
789 | Note that | |
790 | .Xr login 1 | |
791 | is never used for remote command execution. | |
792 | Note also, that if this is enabled, | |
793 | .Cm X11Forwarding | |
794 | will be disabled because | |
795 | .Xr login 1 | |
796 | does not know how to handle | |
797 | .Xr xauth 1 | |
6a9b3198 | 798 | cookies. |
799 | If | |
f5799ae1 | 800 | .Cm UsePrivilegeSeparation |
801 | is specified, it will be disabled after authentication. | |
0fff78ff | 802 | .It Cm UsePAM |
c9f39d2c | 803 | Enables the Pluggable Authentication Module interface. |
804 | If set to | |
805 | .Dq yes | |
806 | this will enable PAM authentication using | |
807 | .Cm ChallengeResponseAuthentication | |
9108f8d9 | 808 | and |
809 | .Cm PasswordAuthentication | |
810 | in addition to PAM account and session module processing for all | |
811 | authentication types. | |
c9f39d2c | 812 | .Pp |
813 | Because PAM challenge-response authentication usually serves an equivalent | |
814 | role to password authentication, you should disable either | |
815 | .Cm PasswordAuthentication | |
816 | or | |
817 | .Cm ChallengeResponseAuthentication. | |
818 | .Pp | |
819 | If | |
820 | .Cm UsePAM | |
821 | is enabled, you will not be able to run | |
822 | .Xr sshd 8 | |
823 | as a non-root user. | |
824 | The default is | |
cdd66111 | 825 | .Dq no . |
f5799ae1 | 826 | .It Cm UsePrivilegeSeparation |
827 | Specifies whether | |
9108f8d9 | 828 | .Xr sshd 8 |
f5799ae1 | 829 | separates privileges by creating an unprivileged child process |
6a9b3198 | 830 | to deal with incoming network traffic. |
831 | After successful authentication, another process will be created that has | |
832 | the privilege of the authenticated user. | |
833 | The goal of privilege separation is to prevent privilege | |
f5799ae1 | 834 | escalation by containing any corruption within the unprivileged processes. |
835 | The default is | |
836 | .Dq yes . | |
f5799ae1 | 837 | .It Cm X11DisplayOffset |
838 | Specifies the first display number available for | |
9108f8d9 | 839 | .Xr sshd 8 Ns 's |
f5799ae1 | 840 | X11 forwarding. |
9108f8d9 | 841 | This prevents sshd from interfering with real X11 servers. |
f5799ae1 | 842 | The default is 10. |
843 | .It Cm X11Forwarding | |
844 | Specifies whether X11 forwarding is permitted. | |
41b2f314 | 845 | The argument must be |
846 | .Dq yes | |
847 | or | |
848 | .Dq no . | |
f5799ae1 | 849 | The default is |
850 | .Dq no . | |
41b2f314 | 851 | .Pp |
852 | When X11 forwarding is enabled, there may be additional exposure to | |
853 | the server and to client displays if the | |
9108f8d9 | 854 | .Xr sshd 8 |
41b2f314 | 855 | proxy display is configured to listen on the wildcard address (see |
856 | .Cm X11UseLocalhost | |
9108f8d9 | 857 | below), though this is not the default. |
41b2f314 | 858 | Additionally, the authentication spoofing and authentication data |
859 | verification and substitution occur on the client side. | |
860 | The security risk of using X11 forwarding is that the client's X11 | |
9108f8d9 | 861 | display server may be exposed to attack when the SSH client requests |
41b2f314 | 862 | forwarding (see the warnings for |
863 | .Cm ForwardX11 | |
864 | in | |
0fff78ff | 865 | .Xr ssh_config 5 ) . |
41b2f314 | 866 | A system administrator may have a stance in which they want to |
867 | protect clients that may expose themselves to attack by unwittingly | |
868 | requesting X11 forwarding, which can warrant a | |
869 | .Dq no | |
870 | setting. | |
871 | .Pp | |
872 | Note that disabling X11 forwarding does not prevent users from | |
873 | forwarding X11 traffic, as users can always install their own forwarders. | |
f5799ae1 | 874 | X11 forwarding is automatically disabled if |
875 | .Cm UseLogin | |
876 | is enabled. | |
877 | .It Cm X11UseLocalhost | |
878 | Specifies whether | |
9108f8d9 | 879 | .Xr sshd 8 |
f5799ae1 | 880 | should bind the X11 forwarding server to the loopback address or to |
6a9b3198 | 881 | the wildcard address. |
882 | By default, | |
9108f8d9 | 883 | sshd binds the forwarding server to the loopback address and sets the |
f5799ae1 | 884 | hostname part of the |
885 | .Ev DISPLAY | |
886 | environment variable to | |
887 | .Dq localhost . | |
41b2f314 | 888 | This prevents remote hosts from connecting to the proxy display. |
f5799ae1 | 889 | However, some older X11 clients may not function with this |
890 | configuration. | |
891 | .Cm X11UseLocalhost | |
892 | may be set to | |
893 | .Dq no | |
894 | to specify that the forwarding server should be bound to the wildcard | |
895 | address. | |
896 | The argument must be | |
897 | .Dq yes | |
898 | or | |
899 | .Dq no . | |
900 | The default is | |
901 | .Dq yes . | |
902 | .It Cm XAuthLocation | |
41b2f314 | 903 | Specifies the full pathname of the |
f5799ae1 | 904 | .Xr xauth 1 |
905 | program. | |
906 | The default is | |
907 | .Pa /usr/X11R6/bin/xauth . | |
908 | .El | |
9108f8d9 | 909 | .Sh TIME FORMATS |
910 | .Xr sshd 8 | |
f5799ae1 | 911 | command-line arguments and configuration file options that specify time |
912 | may be expressed using a sequence of the form: | |
913 | .Sm off | |
41b2f314 | 914 | .Ar time Op Ar qualifier , |
f5799ae1 | 915 | .Sm on |
916 | where | |
917 | .Ar time | |
918 | is a positive integer value and | |
919 | .Ar qualifier | |
920 | is one of the following: | |
921 | .Pp | |
922 | .Bl -tag -width Ds -compact -offset indent | |
9108f8d9 | 923 | .It Aq Cm none |
f5799ae1 | 924 | seconds |
925 | .It Cm s | Cm S | |
926 | seconds | |
927 | .It Cm m | Cm M | |
928 | minutes | |
929 | .It Cm h | Cm H | |
930 | hours | |
931 | .It Cm d | Cm D | |
932 | days | |
933 | .It Cm w | Cm W | |
934 | weeks | |
935 | .El | |
936 | .Pp | |
937 | Each member of the sequence is added together to calculate | |
938 | the total time value. | |
939 | .Pp | |
940 | Time format examples: | |
941 | .Pp | |
942 | .Bl -tag -width Ds -compact -offset indent | |
943 | .It 600 | |
944 | 600 seconds (10 minutes) | |
945 | .It 10m | |
946 | 10 minutes | |
947 | .It 1h30m | |
948 | 1 hour 30 minutes (90 minutes) | |
949 | .El | |
950 | .Sh FILES | |
951 | .Bl -tag -width Ds | |
952 | .It Pa /etc/ssh/sshd_config | |
953 | Contains configuration data for | |
9108f8d9 | 954 | .Xr sshd 8 . |
f5799ae1 | 955 | This file should be writable by root only, but it is recommended |
956 | (though not necessary) that it be world-readable. | |
957 | .El | |
0fff78ff | 958 | .Sh SEE ALSO |
959 | .Xr sshd 8 | |
f5799ae1 | 960 | .Sh AUTHORS |
961 | OpenSSH is a derivative of the original and free | |
962 | ssh 1.2.12 release by Tatu Ylonen. | |
963 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
964 | Theo de Raadt and Dug Song | |
965 | removed many bugs, re-added newer features and | |
966 | created OpenSSH. | |
967 | Markus Friedl contributed the support for SSH | |
968 | protocol versions 1.5 and 2.0. | |
969 | Niels Provos and Markus Friedl contributed support | |
970 | for privilege separation. |