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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 | .\" | |
37 | .\" $OpenBSD: sshd_config.5,v 1.116 2010/01/09 23:04:13 dtucker Exp $ | |
38 | .Dd $Mdocdate$ | |
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 | .Nm /etc/ssh/sshd_config | |
46 | .Sh DESCRIPTION | |
47 | .Xr sshd 8 | |
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. | |
57 | Arguments may optionally be enclosed in double quotes | |
58 | .Pq \&" | |
59 | in order to represent arguments containing spaces. | |
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 | |
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 | |
76 | .Ql * | |
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. | |
87 | .It Cm AddressFamily | |
88 | Specifies which address family should be used by | |
89 | .Xr sshd 8 . | |
90 | Valid arguments are | |
91 | .Dq any , | |
92 | .Dq inet | |
93 | (use IPv4 only), or | |
94 | .Dq inet6 | |
95 | (use IPv6 only). | |
96 | The default is | |
97 | .Dq any . | |
98 | .It Cm AllowAgentForwarding | |
99 | Specifies whether | |
100 | .Xr ssh-agent 1 | |
101 | forwarding is permitted. | |
102 | The default is | |
103 | .Dq yes . | |
104 | Note that disabling agent forwarding does not improve security | |
105 | unless users are also denied shell access, as they can always install | |
106 | their own forwarders. | |
107 | .It Cm AllowGroups | |
108 | This keyword can be followed by a list of group name patterns, separated | |
109 | by spaces. | |
110 | If specified, login is allowed only for users whose primary | |
111 | group or supplementary group list matches one of the patterns. | |
112 | Only group names are valid; a numerical group ID is not recognized. | |
113 | By default, login is allowed for all groups. | |
114 | The allow/deny directives are processed in the following order: | |
115 | .Cm DenyUsers , | |
116 | .Cm AllowUsers , | |
117 | .Cm DenyGroups , | |
118 | and finally | |
119 | .Cm AllowGroups . | |
120 | .Pp | |
121 | See | |
122 | .Sx PATTERNS | |
123 | in | |
124 | .Xr ssh_config 5 | |
125 | for more information on patterns. | |
126 | .It Cm AllowTcpForwarding | |
127 | Specifies whether TCP forwarding is permitted. | |
128 | The default is | |
129 | .Dq yes . | |
130 | Note that disabling TCP forwarding does not improve security unless | |
131 | users are also denied shell access, as they can always install their | |
132 | own forwarders. | |
133 | .It Cm AllowUsers | |
134 | This keyword can be followed by a list of user name patterns, separated | |
135 | by spaces. | |
136 | If specified, login is allowed only for user names that | |
137 | match one of the patterns. | |
138 | Only user names are valid; a numerical user ID is not recognized. | |
139 | By default, login is allowed for all users. | |
140 | If the pattern takes the form USER@HOST then USER and HOST | |
141 | are separately checked, restricting logins to particular | |
142 | users from particular hosts. | |
143 | The allow/deny directives are processed in the following order: | |
144 | .Cm DenyUsers , | |
145 | .Cm AllowUsers , | |
146 | .Cm DenyGroups , | |
147 | and finally | |
148 | .Cm AllowGroups . | |
149 | .Pp | |
150 | See | |
151 | .Sx PATTERNS | |
152 | in | |
153 | .Xr ssh_config 5 | |
154 | for more information on patterns. | |
155 | .It Cm AuthorizedKeysFile | |
156 | Specifies the file that contains the public keys that can be used | |
157 | for user authentication. | |
158 | .Cm AuthorizedKeysFile | |
159 | may contain tokens of the form %T which are substituted during connection | |
160 | setup. | |
161 | The following tokens are defined: %% is replaced by a literal '%', | |
162 | %h is replaced by the home directory of the user being authenticated, and | |
163 | %u is replaced by the username of that user. | |
164 | After expansion, | |
165 | .Cm AuthorizedKeysFile | |
166 | is taken to be an absolute path or one relative to the user's home | |
167 | directory. | |
168 | The default is | |
169 | .Dq .ssh/authorized_keys . | |
170 | .It Cm Banner | |
171 | The contents of the specified file are sent to the remote user before | |
172 | authentication is allowed. | |
173 | If the argument is | |
174 | .Dq none | |
175 | then no banner is displayed. | |
176 | This option is only available for protocol version 2. | |
177 | By default, no banner is displayed. | |
178 | .It Cm ChallengeResponseAuthentication | |
179 | Specifies whether challenge-response authentication is allowed (e.g. via | |
180 | PAM or though authentication styles supported in | |
181 | .Xr login.conf 5 ) | |
182 | The default is | |
183 | .Dq yes . | |
184 | .It Cm ChrootDirectory | |
185 | Specifies the pathname of a directory to | |
186 | .Xr chroot 2 | |
187 | to after authentication. | |
188 | All components of the pathname must be root-owned directories that are | |
189 | not writable by any other user or group. | |
190 | After the chroot, | |
191 | .Xr sshd 8 | |
192 | changes the working directory to the user's home directory. | |
193 | .Pp | |
194 | The pathname may contain the following tokens that are expanded at runtime once | |
195 | the connecting user has been authenticated: %% is replaced by a literal '%', | |
196 | %h is replaced by the home directory of the user being authenticated, and | |
197 | %u is replaced by the username of that user. | |
198 | .Pp | |
199 | The | |
200 | .Cm ChrootDirectory | |
201 | must contain the necessary files and directories to support the | |
202 | user's session. | |
203 | For an interactive session this requires at least a shell, typically | |
204 | .Xr sh 1 , | |
205 | and basic | |
206 | .Pa /dev | |
207 | nodes such as | |
208 | .Xr null 4 , | |
209 | .Xr zero 4 , | |
210 | .Xr stdin 4 , | |
211 | .Xr stdout 4 , | |
212 | .Xr stderr 4 , | |
213 | .Xr arandom 4 | |
214 | and | |
215 | .Xr tty 4 | |
216 | devices. | |
217 | For file transfer sessions using | |
218 | .Dq sftp , | |
219 | no additional configuration of the environment is necessary if the | |
220 | in-process sftp server is used, | |
221 | though sessions which use logging do require | |
222 | .Pa /dev/log | |
223 | inside the chroot directory (see | |
224 | .Xr sftp-server 8 | |
225 | for details). | |
226 | .Pp | |
227 | The default is not to | |
228 | .Xr chroot 2 . | |
229 | .It Cm Ciphers | |
230 | Specifies the ciphers allowed for protocol version 2. | |
231 | Multiple ciphers must be comma-separated. | |
232 | The supported ciphers are | |
233 | .Dq 3des-cbc , | |
234 | .Dq aes128-cbc , | |
235 | .Dq aes192-cbc , | |
236 | .Dq aes256-cbc , | |
237 | .Dq aes128-ctr , | |
238 | .Dq aes192-ctr , | |
239 | .Dq aes256-ctr , | |
240 | .Dq arcfour128 , | |
241 | .Dq arcfour256 , | |
242 | .Dq arcfour , | |
243 | .Dq blowfish-cbc , | |
244 | and | |
245 | .Dq cast128-cbc . | |
246 | The default is: | |
247 | .Bd -literal -offset 3n | |
248 | aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, | |
249 | aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, | |
250 | aes256-cbc,arcfour | |
251 | .Ed | |
252 | .It Cm ClientAliveCountMax | |
253 | Sets the number of client alive messages (see below) which may be | |
254 | sent without | |
255 | .Xr sshd 8 | |
256 | receiving any messages back from the client. | |
257 | If this threshold is reached while client alive messages are being sent, | |
258 | sshd will disconnect the client, terminating the session. | |
259 | It is important to note that the use of client alive messages is very | |
260 | different from | |
261 | .Cm TCPKeepAlive | |
262 | (below). | |
263 | The client alive messages are sent through the encrypted channel | |
264 | and therefore will not be spoofable. | |
265 | The TCP keepalive option enabled by | |
266 | .Cm TCPKeepAlive | |
267 | is spoofable. | |
268 | The client alive mechanism is valuable when the client or | |
269 | server depend on knowing when a connection has become inactive. | |
270 | .Pp | |
271 | The default value is 3. | |
272 | If | |
273 | .Cm ClientAliveInterval | |
274 | (see below) is set to 15, and | |
275 | .Cm ClientAliveCountMax | |
276 | is left at the default, unresponsive SSH clients | |
277 | will be disconnected after approximately 45 seconds. | |
278 | This option applies to protocol version 2 only. | |
279 | .It Cm ClientAliveInterval | |
280 | Sets a timeout interval in seconds after which if no data has been received | |
281 | from the client, | |
282 | .Xr sshd 8 | |
283 | will send a message through the encrypted | |
284 | channel to request a response from the client. | |
285 | The default | |
286 | is 0, indicating that these messages will not be sent to the client. | |
287 | This option applies to protocol version 2 only. | |
288 | .It Cm Compression | |
289 | Specifies whether compression is allowed, or delayed until | |
290 | the user has authenticated successfully. | |
291 | The argument must be | |
292 | .Dq yes , | |
293 | .Dq delayed , | |
294 | or | |
295 | .Dq no . | |
296 | The default is | |
297 | .Dq delayed . | |
298 | .It Cm DenyGroups | |
299 | This keyword can be followed by a list of group name patterns, separated | |
300 | by spaces. | |
301 | Login is disallowed for users whose primary group or supplementary | |
302 | group list matches one of the patterns. | |
303 | Only group names are valid; a numerical group ID is not recognized. | |
304 | By default, login is allowed for all groups. | |
305 | The allow/deny directives are processed in the following order: | |
306 | .Cm DenyUsers , | |
307 | .Cm AllowUsers , | |
308 | .Cm DenyGroups , | |
309 | and finally | |
310 | .Cm AllowGroups . | |
311 | .Pp | |
312 | See | |
313 | .Sx PATTERNS | |
314 | in | |
315 | .Xr ssh_config 5 | |
316 | for more information on patterns. | |
317 | .It Cm DenyUsers | |
318 | This keyword can be followed by a list of user name patterns, separated | |
319 | by spaces. | |
320 | Login is disallowed for user names that match one of the patterns. | |
321 | Only user names are valid; a numerical user ID is not recognized. | |
322 | By default, login is allowed for all users. | |
323 | If the pattern takes the form USER@HOST then USER and HOST | |
324 | are separately checked, restricting logins to particular | |
325 | users from particular hosts. | |
326 | The allow/deny directives are processed in the following order: | |
327 | .Cm DenyUsers , | |
328 | .Cm AllowUsers , | |
329 | .Cm DenyGroups , | |
330 | and finally | |
331 | .Cm AllowGroups . | |
332 | .Pp | |
333 | See | |
334 | .Sx PATTERNS | |
335 | in | |
336 | .Xr ssh_config 5 | |
337 | for more information on patterns. | |
338 | .It Cm ForceCommand | |
339 | Forces the execution of the command specified by | |
340 | .Cm ForceCommand , | |
341 | ignoring any command supplied by the client and | |
342 | .Pa ~/.ssh/rc | |
343 | if present. | |
344 | The command is invoked by using the user's login shell with the -c option. | |
345 | This applies to shell, command, or subsystem execution. | |
346 | It is most useful inside a | |
347 | .Cm Match | |
348 | block. | |
349 | The command originally supplied by the client is available in the | |
350 | .Ev SSH_ORIGINAL_COMMAND | |
351 | environment variable. | |
352 | Specifying a command of | |
353 | .Dq internal-sftp | |
354 | will force the use of an in-process sftp server that requires no support | |
355 | files when used with | |
356 | .Cm ChrootDirectory . | |
357 | .It Cm GatewayPorts | |
358 | Specifies whether remote hosts are allowed to connect to ports | |
359 | forwarded for the client. | |
360 | By default, | |
361 | .Xr sshd 8 | |
362 | binds remote port forwardings to the loopback address. | |
363 | This prevents other remote hosts from connecting to forwarded ports. | |
364 | .Cm GatewayPorts | |
365 | can be used to specify that sshd | |
366 | should allow remote port forwardings to bind to non-loopback addresses, thus | |
367 | allowing other hosts to connect. | |
368 | The argument may be | |
369 | .Dq no | |
370 | to force remote port forwardings to be available to the local host only, | |
371 | .Dq yes | |
372 | to force remote port forwardings to bind to the wildcard address, or | |
373 | .Dq clientspecified | |
374 | to allow the client to select the address to which the forwarding is bound. | |
375 | The default is | |
376 | .Dq no . | |
377 | .It Cm GSSAPIAuthentication | |
378 | Specifies whether user authentication based on GSSAPI is allowed. | |
379 | The default is | |
380 | .Dq no . | |
381 | Note that this option applies to protocol version 2 only. | |
382 | .It Cm GSSAPICleanupCredentials | |
383 | Specifies whether to automatically destroy the user's credentials cache | |
384 | on logout. | |
385 | The default is | |
386 | .Dq yes . | |
387 | Note that this option applies to protocol version 2 only. | |
388 | .It Cm HostbasedAuthentication | |
389 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
390 | with successful public key client host authentication is allowed | |
391 | (host-based authentication). | |
392 | This option is similar to | |
393 | .Cm RhostsRSAAuthentication | |
394 | and applies to protocol version 2 only. | |
395 | The default is | |
396 | .Dq no . | |
397 | .It Cm HostbasedUsesNameFromPacketOnly | |
398 | Specifies whether or not the server will attempt to perform a reverse | |
399 | name lookup when matching the name in the | |
400 | .Pa ~/.shosts , | |
401 | .Pa ~/.rhosts , | |
402 | and | |
403 | .Pa /etc/hosts.equiv | |
404 | files during | |
405 | .Cm HostbasedAuthentication . | |
406 | A setting of | |
407 | .Dq yes | |
408 | means that | |
409 | .Xr sshd 8 | |
410 | uses the name supplied by the client rather than | |
411 | attempting to resolve the name from the TCP connection itself. | |
412 | The default is | |
413 | .Dq no . | |
414 | .It Cm HostKey | |
415 | Specifies a file containing a private host key | |
416 | used by SSH. | |
417 | The default is | |
418 | .Pa /etc/ssh/ssh_host_key | |
419 | for protocol version 1, and | |
420 | .Pa /etc/ssh/ssh_host_rsa_key | |
421 | and | |
422 | .Pa /etc/ssh/ssh_host_dsa_key | |
423 | for protocol version 2. | |
424 | Note that | |
425 | .Xr sshd 8 | |
426 | will refuse to use a file if it is group/world-accessible. | |
427 | It is possible to have multiple host key files. | |
428 | .Dq rsa1 | |
429 | keys are used for version 1 and | |
430 | .Dq dsa | |
431 | or | |
432 | .Dq rsa | |
433 | are used for version 2 of the SSH protocol. | |
434 | .It Cm IgnoreRhosts | |
435 | Specifies that | |
436 | .Pa .rhosts | |
437 | and | |
438 | .Pa .shosts | |
439 | files will not be used in | |
440 | .Cm RhostsRSAAuthentication | |
441 | or | |
442 | .Cm HostbasedAuthentication . | |
443 | .Pp | |
444 | .Pa /etc/hosts.equiv | |
445 | and | |
446 | .Pa /etc/shosts.equiv | |
447 | are still used. | |
448 | The default is | |
449 | .Dq yes . | |
450 | .It Cm IgnoreUserKnownHosts | |
451 | Specifies whether | |
452 | .Xr sshd 8 | |
453 | should ignore the user's | |
454 | .Pa ~/.ssh/known_hosts | |
455 | during | |
456 | .Cm RhostsRSAAuthentication | |
457 | or | |
458 | .Cm HostbasedAuthentication . | |
459 | The default is | |
460 | .Dq no . | |
461 | .It Cm KerberosAuthentication | |
462 | Specifies whether the password provided by the user for | |
463 | .Cm PasswordAuthentication | |
464 | will be validated through the Kerberos KDC. | |
465 | To use this option, the server needs a | |
466 | Kerberos servtab which allows the verification of the KDC's identity. | |
467 | The default is | |
468 | .Dq no . | |
469 | .It Cm KerberosGetAFSToken | |
470 | If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire | |
471 | an AFS token before accessing the user's home directory. | |
472 | The default is | |
473 | .Dq no . | |
474 | .It Cm KerberosOrLocalPasswd | |
475 | If password authentication through Kerberos fails then | |
476 | the password will be validated via any additional local mechanism | |
477 | such as | |
478 | .Pa /etc/passwd . | |
479 | The default is | |
480 | .Dq yes . | |
481 | .It Cm KerberosTicketCleanup | |
482 | Specifies whether to automatically destroy the user's ticket cache | |
483 | file on logout. | |
484 | The default is | |
485 | .Dq yes . | |
486 | .It Cm KeyRegenerationInterval | |
487 | In protocol version 1, the ephemeral server key is automatically regenerated | |
488 | after this many seconds (if it has been used). | |
489 | The purpose of regeneration is to prevent | |
490 | decrypting captured sessions by later breaking into the machine and | |
491 | stealing the keys. | |
492 | The key is never stored anywhere. | |
493 | If the value is 0, the key is never regenerated. | |
494 | The default is 3600 (seconds). | |
495 | .It Cm ListenAddress | |
496 | Specifies the local addresses | |
497 | .Xr sshd 8 | |
498 | should listen on. | |
499 | The following forms may be used: | |
500 | .Pp | |
501 | .Bl -item -offset indent -compact | |
502 | .It | |
503 | .Cm ListenAddress | |
504 | .Sm off | |
505 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr | |
506 | .Sm on | |
507 | .It | |
508 | .Cm ListenAddress | |
509 | .Sm off | |
510 | .Ar host No | Ar IPv4_addr No : Ar port | |
511 | .Sm on | |
512 | .It | |
513 | .Cm ListenAddress | |
514 | .Sm off | |
515 | .Oo | |
516 | .Ar host No | Ar IPv6_addr Oc : Ar port | |
517 | .Sm on | |
518 | .El | |
519 | .Pp | |
520 | If | |
521 | .Ar port | |
522 | is not specified, | |
523 | sshd will listen on the address and all prior | |
524 | .Cm Port | |
525 | options specified. | |
526 | The default is to listen on all local addresses. | |
527 | Multiple | |
528 | .Cm ListenAddress | |
529 | options are permitted. | |
530 | Additionally, any | |
531 | .Cm Port | |
532 | options must precede this option for non-port qualified addresses. | |
533 | .It Cm LoginGraceTime | |
534 | The server disconnects after this time if the user has not | |
535 | successfully logged in. | |
536 | If the value is 0, there is no time limit. | |
537 | The default is 120 seconds. | |
538 | .It Cm LogLevel | |
539 | Gives the verbosity level that is used when logging messages from | |
540 | .Xr sshd 8 . | |
541 | The possible values are: | |
542 | QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. | |
543 | The default is INFO. | |
544 | DEBUG and DEBUG1 are equivalent. | |
545 | DEBUG2 and DEBUG3 each specify higher levels of debugging output. | |
546 | Logging with a DEBUG level violates the privacy of users and is not recommended. | |
547 | .It Cm MACs | |
548 | Specifies the available MAC (message authentication code) algorithms. | |
549 | The MAC algorithm is used in protocol version 2 | |
550 | for data integrity protection. | |
551 | Multiple algorithms must be comma-separated. | |
552 | The default is: | |
553 | .Bd -literal -offset indent | |
554 | hmac-md5,hmac-sha1,umac-64@openssh.com, | |
555 | hmac-ripemd160,hmac-sha1-96,hmac-md5-96 | |
556 | .Ed | |
557 | .It Cm Match | |
558 | Introduces a conditional block. | |
559 | If all of the criteria on the | |
560 | .Cm Match | |
561 | line are satisfied, the keywords on the following lines override those | |
562 | set in the global section of the config file, until either another | |
563 | .Cm Match | |
564 | line or the end of the file. | |
565 | .Pp | |
566 | The arguments to | |
567 | .Cm Match | |
568 | are one or more criteria-pattern pairs. | |
569 | The available criteria are | |
570 | .Cm User , | |
571 | .Cm Group , | |
572 | .Cm Host , | |
573 | and | |
574 | .Cm Address . | |
575 | The match patterns may consist of single entries or comma-separated | |
576 | lists and may use the wildcard and negation operators described in the | |
577 | .Sx PATTERNS | |
578 | section of | |
579 | .Xr ssh_config 5 . | |
580 | .Pp | |
581 | The patterns in an | |
582 | .Cm Address | |
583 | criteria may additionally contain addresses to match in CIDR | |
584 | address/masklen format, e.g.\& | |
585 | .Dq 192.0.2.0/24 | |
586 | or | |
587 | .Dq 3ffe:ffff::/32 . | |
588 | Note that the mask length provided must be consistent with the address - | |
589 | it is an error to specify a mask length that is too long for the address | |
590 | or one with bits set in this host portion of the address. | |
591 | For example, | |
592 | .Dq 192.0.2.0/33 | |
593 | and | |
594 | .Dq 192.0.2.0/8 | |
595 | respectively. | |
596 | .Pp | |
597 | Only a subset of keywords may be used on the lines following a | |
598 | .Cm Match | |
599 | keyword. | |
600 | Available keywords are | |
601 | .Cm AllowAgentForwarding , | |
602 | .Cm AllowTcpForwarding , | |
603 | .Cm Banner , | |
604 | .Cm ChrootDirectory , | |
605 | .Cm ForceCommand , | |
606 | .Cm GatewayPorts , | |
607 | .Cm GSSAPIAuthentication , | |
608 | .Cm HostbasedAuthentication , | |
609 | .Cm KbdInteractiveAuthentication , | |
610 | .Cm KerberosAuthentication , | |
611 | .Cm MaxAuthTries , | |
612 | .Cm MaxSessions , | |
613 | .Cm PasswordAuthentication , | |
614 | .Cm PermitEmptyPasswords , | |
615 | .Cm PermitOpen , | |
616 | .Cm PermitRootLogin , | |
617 | .Cm PubkeyAuthentication , | |
618 | .Cm RhostsRSAAuthentication , | |
619 | .Cm RSAAuthentication , | |
620 | .Cm X11DisplayOffset , | |
621 | .Cm X11Forwarding | |
622 | and | |
623 | .Cm X11UseLocalHost . | |
624 | .It Cm MaxAuthTries | |
625 | Specifies the maximum number of authentication attempts permitted per | |
626 | connection. | |
627 | Once the number of failures reaches half this value, | |
628 | additional failures are logged. | |
629 | The default is 6. | |
630 | .It Cm MaxSessions | |
631 | Specifies the maximum number of open sessions permitted per network connection. | |
632 | The default is 10. | |
633 | .It Cm MaxStartups | |
634 | Specifies the maximum number of concurrent unauthenticated connections to the | |
635 | SSH daemon. | |
636 | Additional connections will be dropped until authentication succeeds or the | |
637 | .Cm LoginGraceTime | |
638 | expires for a connection. | |
639 | The default is 10. | |
640 | .Pp | |
641 | Alternatively, random early drop can be enabled by specifying | |
642 | the three colon separated values | |
643 | .Dq start:rate:full | |
644 | (e.g. "10:30:60"). | |
645 | .Xr sshd 8 | |
646 | will refuse connection attempts with a probability of | |
647 | .Dq rate/100 | |
648 | (30%) | |
649 | if there are currently | |
650 | .Dq start | |
651 | (10) | |
652 | unauthenticated connections. | |
653 | The probability increases linearly and all connection attempts | |
654 | are refused if the number of unauthenticated connections reaches | |
655 | .Dq full | |
656 | (60). | |
657 | .It Cm PasswordAuthentication | |
658 | Specifies whether password authentication is allowed. | |
659 | The default is | |
660 | .Dq yes . | |
661 | .It Cm PermitEmptyPasswords | |
662 | When password authentication is allowed, it specifies whether the | |
663 | server allows login to accounts with empty password strings. | |
664 | The default is | |
665 | .Dq no . | |
666 | .It Cm PermitOpen | |
667 | Specifies the destinations to which TCP port forwarding is permitted. | |
668 | The forwarding specification must be one of the following forms: | |
669 | .Pp | |
670 | .Bl -item -offset indent -compact | |
671 | .It | |
672 | .Cm PermitOpen | |
673 | .Sm off | |
674 | .Ar host : port | |
675 | .Sm on | |
676 | .It | |
677 | .Cm PermitOpen | |
678 | .Sm off | |
679 | .Ar IPv4_addr : port | |
680 | .Sm on | |
681 | .It | |
682 | .Cm PermitOpen | |
683 | .Sm off | |
684 | .Ar \&[ IPv6_addr \&] : port | |
685 | .Sm on | |
686 | .El | |
687 | .Pp | |
688 | Multiple forwards may be specified by separating them with whitespace. | |
689 | An argument of | |
690 | .Dq any | |
691 | can be used to remove all restrictions and permit any forwarding requests. | |
692 | By default all port forwarding requests are permitted. | |
693 | .It Cm PermitRootLogin | |
694 | Specifies whether root can log in using | |
695 | .Xr ssh 1 . | |
696 | The argument must be | |
697 | .Dq yes , | |
698 | .Dq without-password , | |
699 | .Dq forced-commands-only , | |
700 | or | |
701 | .Dq no . | |
702 | The default is | |
703 | .Dq yes . | |
704 | .Pp | |
705 | If this option is set to | |
706 | .Dq without-password , | |
707 | password authentication is disabled for root. | |
708 | .Pp | |
709 | If this option is set to | |
710 | .Dq forced-commands-only , | |
711 | root login with public key authentication will be allowed, | |
712 | but only if the | |
713 | .Ar command | |
714 | option has been specified | |
715 | (which may be useful for taking remote backups even if root login is | |
716 | normally not allowed). | |
717 | All other authentication methods are disabled for root. | |
718 | .Pp | |
719 | If this option is set to | |
720 | .Dq no , | |
721 | root is not allowed to log in. | |
722 | .It Cm PermitTunnel | |
723 | Specifies whether | |
724 | .Xr tun 4 | |
725 | device forwarding is allowed. | |
726 | The argument must be | |
727 | .Dq yes , | |
728 | .Dq point-to-point | |
729 | (layer 3), | |
730 | .Dq ethernet | |
731 | (layer 2), or | |
732 | .Dq no . | |
733 | Specifying | |
734 | .Dq yes | |
735 | permits both | |
736 | .Dq point-to-point | |
737 | and | |
738 | .Dq ethernet . | |
739 | The default is | |
740 | .Dq no . | |
741 | .It Cm PermitUserEnvironment | |
742 | Specifies whether | |
743 | .Pa ~/.ssh/environment | |
744 | and | |
745 | .Cm environment= | |
746 | options in | |
747 | .Pa ~/.ssh/authorized_keys | |
748 | are processed by | |
749 | .Xr sshd 8 . | |
750 | The default is | |
751 | .Dq no . | |
752 | Enabling environment processing may enable users to bypass access | |
753 | restrictions in some configurations using mechanisms such as | |
754 | .Ev LD_PRELOAD . | |
755 | .It Cm PidFile | |
756 | Specifies the file that contains the process ID of the | |
757 | SSH daemon. | |
758 | The default is | |
759 | .Pa /var/run/sshd.pid . | |
760 | .It Cm Port | |
761 | Specifies the port number that | |
762 | .Xr sshd 8 | |
763 | listens on. | |
764 | The default is 22. | |
765 | Multiple options of this type are permitted. | |
766 | See also | |
767 | .Cm ListenAddress . | |
768 | .It Cm PrintLastLog | |
769 | Specifies whether | |
770 | .Xr sshd 8 | |
771 | should print the date and time of the last user login when a user logs | |
772 | in interactively. | |
773 | The default is | |
774 | .Dq yes . | |
775 | .It Cm PrintMotd | |
776 | Specifies whether | |
777 | .Xr sshd 8 | |
778 | should print | |
779 | .Pa /etc/motd | |
780 | when a user logs in interactively. | |
781 | (On some systems it is also printed by the shell, | |
782 | .Pa /etc/profile , | |
783 | or equivalent.) | |
784 | The default is | |
785 | .Dq yes . | |
786 | .It Cm Protocol | |
787 | Specifies the protocol versions | |
788 | .Xr sshd 8 | |
789 | supports. | |
790 | The possible values are | |
791 | .Sq 1 | |
792 | and | |
793 | .Sq 2 . | |
794 | Multiple versions must be comma-separated. | |
795 | The default is | |
796 | .Sq 2 . | |
797 | Note that the order of the protocol list does not indicate preference, | |
798 | because the client selects among multiple protocol versions offered | |
799 | by the server. | |
800 | Specifying | |
801 | .Dq 2,1 | |
802 | is identical to | |
803 | .Dq 1,2 . | |
804 | .It Cm PubkeyAuthentication | |
805 | Specifies whether public key authentication is allowed. | |
806 | The default is | |
807 | .Dq yes . | |
808 | Note that this option applies to protocol version 2 only. | |
809 | .It Cm RhostsRSAAuthentication | |
810 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
811 | with successful RSA host authentication is allowed. | |
812 | The default is | |
813 | .Dq no . | |
814 | This option applies to protocol version 1 only. | |
815 | .It Cm RSAAuthentication | |
816 | Specifies whether pure RSA authentication is allowed. | |
817 | The default is | |
818 | .Dq yes . | |
819 | This option applies to protocol version 1 only. | |
820 | .It Cm ServerKeyBits | |
821 | Defines the number of bits in the ephemeral protocol version 1 server key. | |
822 | The minimum value is 512, and the default is 1024. | |
823 | .It Cm StrictModes | |
824 | Specifies whether | |
825 | .Xr sshd 8 | |
826 | should check file modes and ownership of the | |
827 | user's files and home directory before accepting login. | |
828 | This is normally desirable because novices sometimes accidentally leave their | |
829 | directory or files world-writable. | |
830 | The default is | |
831 | .Dq yes . | |
832 | Note that this does not apply to | |
833 | .Cm ChrootDirectory , | |
834 | whose permissions and ownership are checked unconditionally. | |
835 | .It Cm Subsystem | |
836 | Configures an external subsystem (e.g. file transfer daemon). | |
837 | Arguments should be a subsystem name and a command (with optional arguments) | |
838 | to execute upon subsystem request. | |
839 | .Pp | |
840 | The command | |
841 | .Xr sftp-server 8 | |
842 | implements the | |
843 | .Dq sftp | |
844 | file transfer subsystem. | |
845 | .Pp | |
846 | Alternately the name | |
847 | .Dq internal-sftp | |
848 | implements an in-process | |
849 | .Dq sftp | |
850 | server. | |
851 | This may simplify configurations using | |
852 | .Cm ChrootDirectory | |
853 | to force a different filesystem root on clients. | |
854 | .Pp | |
855 | By default no subsystems are defined. | |
856 | Note that this option applies to protocol version 2 only. | |
857 | .It Cm SyslogFacility | |
858 | Gives the facility code that is used when logging messages from | |
859 | .Xr sshd 8 . | |
860 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, | |
861 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. | |
862 | The default is AUTH. | |
863 | .It Cm TCPKeepAlive | |
864 | Specifies whether the system should send TCP keepalive messages to the | |
865 | other side. | |
866 | If they are sent, death of the connection or crash of one | |
867 | of the machines will be properly noticed. | |
868 | However, this means that | |
869 | connections will die if the route is down temporarily, and some people | |
870 | find it annoying. | |
871 | On the other hand, if TCP keepalives are not sent, | |
872 | sessions may hang indefinitely on the server, leaving | |
873 | .Dq ghost | |
874 | users and consuming server resources. | |
875 | .Pp | |
876 | The default is | |
877 | .Dq yes | |
878 | (to send TCP keepalive messages), and the server will notice | |
879 | if the network goes down or the client host crashes. | |
880 | This avoids infinitely hanging sessions. | |
881 | .Pp | |
882 | To disable TCP keepalive messages, the value should be set to | |
883 | .Dq no . | |
884 | .It Cm UseDNS | |
885 | Specifies whether | |
886 | .Xr sshd 8 | |
887 | should look up the remote host name and check that | |
888 | the resolved host name for the remote IP address maps back to the | |
889 | very same IP address. | |
890 | The default is | |
891 | .Dq yes . | |
892 | .It Cm UseLogin | |
893 | Specifies whether | |
894 | .Xr login 1 | |
895 | is used for interactive login sessions. | |
896 | The default is | |
897 | .Dq no . | |
898 | Note that | |
899 | .Xr login 1 | |
900 | is never used for remote command execution. | |
901 | Note also, that if this is enabled, | |
902 | .Cm X11Forwarding | |
903 | will be disabled because | |
904 | .Xr login 1 | |
905 | does not know how to handle | |
906 | .Xr xauth 1 | |
907 | cookies. | |
908 | If | |
909 | .Cm UsePrivilegeSeparation | |
910 | is specified, it will be disabled after authentication. | |
911 | .It Cm UsePAM | |
912 | Enables the Pluggable Authentication Module interface. | |
913 | If set to | |
914 | .Dq yes | |
915 | this will enable PAM authentication using | |
916 | .Cm ChallengeResponseAuthentication | |
917 | and | |
918 | .Cm PasswordAuthentication | |
919 | in addition to PAM account and session module processing for all | |
920 | authentication types. | |
921 | .Pp | |
922 | Because PAM challenge-response authentication usually serves an equivalent | |
923 | role to password authentication, you should disable either | |
924 | .Cm PasswordAuthentication | |
925 | or | |
926 | .Cm ChallengeResponseAuthentication. | |
927 | .Pp | |
928 | If | |
929 | .Cm UsePAM | |
930 | is enabled, you will not be able to run | |
931 | .Xr sshd 8 | |
932 | as a non-root user. | |
933 | The default is | |
934 | .Dq no . | |
935 | .It Cm UsePrivilegeSeparation | |
936 | Specifies whether | |
937 | .Xr sshd 8 | |
938 | separates privileges by creating an unprivileged child process | |
939 | to deal with incoming network traffic. | |
940 | After successful authentication, another process will be created that has | |
941 | the privilege of the authenticated user. | |
942 | The goal of privilege separation is to prevent privilege | |
943 | escalation by containing any corruption within the unprivileged processes. | |
944 | The default is | |
945 | .Dq yes . | |
946 | .It Cm X11DisplayOffset | |
947 | Specifies the first display number available for | |
948 | .Xr sshd 8 Ns 's | |
949 | X11 forwarding. | |
950 | This prevents sshd from interfering with real X11 servers. | |
951 | The default is 10. | |
952 | .It Cm X11Forwarding | |
953 | Specifies whether X11 forwarding is permitted. | |
954 | The argument must be | |
955 | .Dq yes | |
956 | or | |
957 | .Dq no . | |
958 | The default is | |
959 | .Dq no . | |
960 | .Pp | |
961 | When X11 forwarding is enabled, there may be additional exposure to | |
962 | the server and to client displays if the | |
963 | .Xr sshd 8 | |
964 | proxy display is configured to listen on the wildcard address (see | |
965 | .Cm X11UseLocalhost | |
966 | below), though this is not the default. | |
967 | Additionally, the authentication spoofing and authentication data | |
968 | verification and substitution occur on the client side. | |
969 | The security risk of using X11 forwarding is that the client's X11 | |
970 | display server may be exposed to attack when the SSH client requests | |
971 | forwarding (see the warnings for | |
972 | .Cm ForwardX11 | |
973 | in | |
974 | .Xr ssh_config 5 ) . | |
975 | A system administrator may have a stance in which they want to | |
976 | protect clients that may expose themselves to attack by unwittingly | |
977 | requesting X11 forwarding, which can warrant a | |
978 | .Dq no | |
979 | setting. | |
980 | .Pp | |
981 | Note that disabling X11 forwarding does not prevent users from | |
982 | forwarding X11 traffic, as users can always install their own forwarders. | |
983 | X11 forwarding is automatically disabled if | |
984 | .Cm UseLogin | |
985 | is enabled. | |
986 | .It Cm X11UseLocalhost | |
987 | Specifies whether | |
988 | .Xr sshd 8 | |
989 | should bind the X11 forwarding server to the loopback address or to | |
990 | the wildcard address. | |
991 | By default, | |
992 | sshd binds the forwarding server to the loopback address and sets the | |
993 | hostname part of the | |
994 | .Ev DISPLAY | |
995 | environment variable to | |
996 | .Dq localhost . | |
997 | This prevents remote hosts from connecting to the proxy display. | |
998 | However, some older X11 clients may not function with this | |
999 | configuration. | |
1000 | .Cm X11UseLocalhost | |
1001 | may be set to | |
1002 | .Dq no | |
1003 | to specify that the forwarding server should be bound to the wildcard | |
1004 | address. | |
1005 | The argument must be | |
1006 | .Dq yes | |
1007 | or | |
1008 | .Dq no . | |
1009 | The default is | |
1010 | .Dq yes . | |
1011 | .It Cm XAuthLocation | |
1012 | Specifies the full pathname of the | |
1013 | .Xr xauth 1 | |
1014 | program. | |
1015 | The default is | |
1016 | .Pa /usr/X11R6/bin/xauth . | |
1017 | .El | |
1018 | .Sh TIME FORMATS | |
1019 | .Xr sshd 8 | |
1020 | command-line arguments and configuration file options that specify time | |
1021 | may be expressed using a sequence of the form: | |
1022 | .Sm off | |
1023 | .Ar time Op Ar qualifier , | |
1024 | .Sm on | |
1025 | where | |
1026 | .Ar time | |
1027 | is a positive integer value and | |
1028 | .Ar qualifier | |
1029 | is one of the following: | |
1030 | .Pp | |
1031 | .Bl -tag -width Ds -compact -offset indent | |
1032 | .It Aq Cm none | |
1033 | seconds | |
1034 | .It Cm s | Cm S | |
1035 | seconds | |
1036 | .It Cm m | Cm M | |
1037 | minutes | |
1038 | .It Cm h | Cm H | |
1039 | hours | |
1040 | .It Cm d | Cm D | |
1041 | days | |
1042 | .It Cm w | Cm W | |
1043 | weeks | |
1044 | .El | |
1045 | .Pp | |
1046 | Each member of the sequence is added together to calculate | |
1047 | the total time value. | |
1048 | .Pp | |
1049 | Time format examples: | |
1050 | .Pp | |
1051 | .Bl -tag -width Ds -compact -offset indent | |
1052 | .It 600 | |
1053 | 600 seconds (10 minutes) | |
1054 | .It 10m | |
1055 | 10 minutes | |
1056 | .It 1h30m | |
1057 | 1 hour 30 minutes (90 minutes) | |
1058 | .El | |
1059 | .Sh FILES | |
1060 | .Bl -tag -width Ds | |
1061 | .It Pa /etc/ssh/sshd_config | |
1062 | Contains configuration data for | |
1063 | .Xr sshd 8 . | |
1064 | This file should be writable by root only, but it is recommended | |
1065 | (though not necessary) that it be world-readable. | |
1066 | .El | |
1067 | .Sh SEE ALSO | |
1068 | .Xr sshd 8 | |
1069 | .Sh AUTHORS | |
1070 | OpenSSH is a derivative of the original and free | |
1071 | ssh 1.2.12 release by Tatu Ylonen. | |
1072 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
1073 | Theo de Raadt and Dug Song | |
1074 | removed many bugs, re-added newer features and | |
1075 | created OpenSSH. | |
1076 | Markus Friedl contributed the support for SSH | |
1077 | protocol versions 1.5 and 2.0. | |
1078 | Niels Provos and Markus Friedl contributed support | |
1079 | for privilege separation. |