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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 1.89 2008/05/07 08:00:14 jmc 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. | |
180 | All authentication styles from | |
181 | .Xr login.conf 5 | |
182 | are supported. | |
183 | The default is | |
184 | .Dq yes . | |
185 | .It Cm ChrootDirectory | |
186 | Specifies a path to | |
187 | .Xr chroot 2 | |
188 | to after authentication. | |
189 | This path, and all its components, must be root-owned directories that are | |
190 | not writable by any other user or group. | |
191 | .Pp | |
192 | The path may contain the following tokens that are expanded at runtime once | |
193 | the connecting user has been authenticated: %% is replaced by a literal '%', | |
194 | %h is replaced by the home directory of the user being authenticated, and | |
195 | %u is replaced by the username of that user. | |
196 | .Pp | |
197 | The | |
198 | .Cm ChrootDirectory | |
199 | must contain the necessary files and directories to support the | |
200 | users' session. | |
201 | For an interactive session this requires at least a shell, typically | |
202 | .Xr sh 1 , | |
203 | and basic | |
204 | .Pa /dev | |
205 | nodes such as | |
206 | .Xr null 4 , | |
207 | .Xr zero 4 , | |
208 | .Xr stdin 4 , | |
209 | .Xr stdout 4 , | |
210 | .Xr stderr 4 , | |
211 | .Xr arandom 4 | |
212 | and | |
213 | .Xr tty 4 | |
214 | devices. | |
215 | For file transfer sessions using | |
216 | .Dq sftp , | |
217 | no additional configuration of the environment is necessary if the | |
218 | in-process sftp server is used (see | |
219 | .Cm Subsystem | |
220 | for details). | |
221 | .Pp | |
222 | The default is not to | |
223 | .Xr chroot 2 . | |
224 | .It Cm Ciphers | |
225 | Specifies the ciphers allowed for protocol version 2. | |
226 | Multiple ciphers must be comma-separated. | |
227 | The supported ciphers are | |
228 | .Dq 3des-cbc , | |
229 | .Dq aes128-cbc , | |
230 | .Dq aes192-cbc , | |
231 | .Dq aes256-cbc , | |
232 | .Dq aes128-ctr , | |
233 | .Dq aes192-ctr , | |
234 | .Dq aes256-ctr , | |
235 | .Dq arcfour128 , | |
236 | .Dq arcfour256 , | |
237 | .Dq arcfour , | |
238 | .Dq blowfish-cbc , | |
239 | and | |
240 | .Dq cast128-cbc . | |
241 | The default is: | |
242 | .Bd -literal -offset 3n | |
243 | aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, | |
244 | arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, | |
245 | aes192-ctr,aes256-ctr | |
246 | .Ed | |
247 | .It Cm ClientAliveCountMax | |
248 | Sets the number of client alive messages (see below) which may be | |
249 | sent without | |
250 | .Xr sshd 8 | |
251 | receiving any messages back from the client. | |
252 | If this threshold is reached while client alive messages are being sent, | |
253 | sshd will disconnect the client, terminating the session. | |
254 | It is important to note that the use of client alive messages is very | |
255 | different from | |
256 | .Cm TCPKeepAlive | |
257 | (below). | |
258 | The client alive messages are sent through the encrypted channel | |
259 | and therefore will not be spoofable. | |
260 | The TCP keepalive option enabled by | |
261 | .Cm TCPKeepAlive | |
262 | is spoofable. | |
263 | The client alive mechanism is valuable when the client or | |
264 | server depend on knowing when a connection has become inactive. | |
265 | .Pp | |
266 | The default value is 3. | |
267 | If | |
268 | .Cm ClientAliveInterval | |
269 | (see below) is set to 15, and | |
270 | .Cm ClientAliveCountMax | |
271 | is left at the default, unresponsive SSH clients | |
272 | will be disconnected after approximately 45 seconds. | |
273 | This option applies to protocol version 2 only. | |
274 | .It Cm ClientAliveInterval | |
275 | Sets a timeout interval in seconds after which if no data has been received | |
276 | from the client, | |
277 | .Xr sshd 8 | |
278 | will send a message through the encrypted | |
279 | channel to request a response from the client. | |
280 | The default | |
281 | is 0, indicating that these messages will not be sent to the client. | |
282 | This option applies to protocol version 2 only. | |
283 | .It Cm Compression | |
284 | Specifies whether compression is allowed, or delayed until | |
285 | the user has authenticated successfully. | |
286 | The argument must be | |
287 | .Dq yes , | |
288 | .Dq delayed , | |
289 | or | |
290 | .Dq no . | |
291 | The default is | |
292 | .Dq delayed . | |
293 | .It Cm DenyGroups | |
294 | This keyword can be followed by a list of group name patterns, separated | |
295 | by spaces. | |
296 | Login is disallowed for users whose primary group or supplementary | |
297 | group list matches one of the patterns. | |
298 | Only group names are valid; a numerical group ID is not recognized. | |
299 | By default, login is allowed for all groups. | |
300 | The allow/deny directives are processed in the following order: | |
301 | .Cm DenyUsers , | |
302 | .Cm AllowUsers , | |
303 | .Cm DenyGroups , | |
304 | and finally | |
305 | .Cm AllowGroups . | |
306 | .Pp | |
307 | See | |
308 | .Sx PATTERNS | |
309 | in | |
310 | .Xr ssh_config 5 | |
311 | for more information on patterns. | |
312 | .It Cm DenyUsers | |
313 | This keyword can be followed by a list of user name patterns, separated | |
314 | by spaces. | |
315 | Login is disallowed for user names that match one of the patterns. | |
316 | Only user names are valid; a numerical user ID is not recognized. | |
317 | By default, login is allowed for all users. | |
318 | If the pattern takes the form USER@HOST then USER and HOST | |
319 | are separately checked, restricting logins to particular | |
320 | users from particular hosts. | |
321 | The allow/deny directives are processed in the following order: | |
322 | .Cm DenyUsers , | |
323 | .Cm AllowUsers , | |
324 | .Cm DenyGroups , | |
325 | and finally | |
326 | .Cm AllowGroups . | |
327 | .Pp | |
328 | See | |
329 | .Sx PATTERNS | |
330 | in | |
331 | .Xr ssh_config 5 | |
332 | for more information on patterns. | |
333 | .It Cm ForceCommand | |
334 | Forces the execution of the command specified by | |
335 | .Cm ForceCommand , | |
336 | ignoring any command supplied by the client and | |
337 | .Pa ~/.ssh/rc | |
338 | if present. | |
339 | The command is invoked by using the user's login shell with the -c option. | |
340 | This applies to shell, command, or subsystem execution. | |
341 | It is most useful inside a | |
342 | .Cm Match | |
343 | block. | |
344 | The command originally supplied by the client is available in the | |
345 | .Ev SSH_ORIGINAL_COMMAND | |
346 | environment variable. | |
347 | Specifying a command of | |
348 | .Dq internal-sftp | |
349 | will force the use of an in-process sftp server that requires no support | |
350 | files when used with | |
351 | .Cm ChrootDirectory . | |
352 | .It Cm GatewayPorts | |
353 | Specifies whether remote hosts are allowed to connect to ports | |
354 | forwarded for the client. | |
355 | By default, | |
356 | .Xr sshd 8 | |
357 | binds remote port forwardings to the loopback address. | |
358 | This prevents other remote hosts from connecting to forwarded ports. | |
359 | .Cm GatewayPorts | |
360 | can be used to specify that sshd | |
361 | should allow remote port forwardings to bind to non-loopback addresses, thus | |
362 | allowing other hosts to connect. | |
363 | The argument may be | |
364 | .Dq no | |
365 | to force remote port forwardings to be available to the local host only, | |
366 | .Dq yes | |
367 | to force remote port forwardings to bind to the wildcard address, or | |
368 | .Dq clientspecified | |
369 | to allow the client to select the address to which the forwarding is bound. | |
370 | The default is | |
371 | .Dq no . | |
372 | .It Cm GSSAPIAuthentication | |
373 | Specifies whether user authentication based on GSSAPI is allowed. | |
374 | The default is | |
375 | .Dq no . | |
376 | Note that this option applies to protocol version 2 only. | |
377 | .It Cm GSSAPICleanupCredentials | |
378 | Specifies whether to automatically destroy the user's credentials cache | |
379 | on logout. | |
380 | The default is | |
381 | .Dq yes . | |
382 | Note that this option applies to protocol version 2 only. | |
383 | .It Cm HostbasedAuthentication | |
384 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
385 | with successful public key client host authentication is allowed | |
386 | (host-based authentication). | |
387 | This option is similar to | |
388 | .Cm RhostsRSAAuthentication | |
389 | and applies to protocol version 2 only. | |
390 | The default is | |
391 | .Dq no . | |
392 | .It Cm HostbasedUsesNameFromPacketOnly | |
393 | Specifies whether or not the server will attempt to perform a reverse | |
394 | name lookup when matching the name in the | |
395 | .Pa ~/.shosts , | |
396 | .Pa ~/.rhosts , | |
397 | and | |
398 | .Pa /etc/hosts.equiv | |
399 | files during | |
400 | .Cm HostbasedAuthentication . | |
401 | A setting of | |
402 | .Dq yes | |
403 | means that | |
404 | .Xr sshd 8 | |
405 | uses the name supplied by the client rather than | |
406 | attempting to resolve the name from the TCP connection itself. | |
407 | The default is | |
408 | .Dq no . | |
409 | .It Cm HostKey | |
410 | Specifies a file containing a private host key | |
411 | used by SSH. | |
412 | The default is | |
413 | .Pa /etc/ssh/ssh_host_key | |
414 | for protocol version 1, and | |
415 | .Pa /etc/ssh/ssh_host_rsa_key | |
416 | and | |
417 | .Pa /etc/ssh/ssh_host_dsa_key | |
418 | for protocol version 2. | |
419 | Note that | |
420 | .Xr sshd 8 | |
421 | will refuse to use a file if it is group/world-accessible. | |
422 | It is possible to have multiple host key files. | |
423 | .Dq rsa1 | |
424 | keys are used for version 1 and | |
425 | .Dq dsa | |
426 | or | |
427 | .Dq rsa | |
428 | are used for version 2 of the SSH protocol. | |
429 | .It Cm IgnoreRhosts | |
430 | Specifies that | |
431 | .Pa .rhosts | |
432 | and | |
433 | .Pa .shosts | |
434 | files will not be used in | |
435 | .Cm RhostsRSAAuthentication | |
436 | or | |
437 | .Cm HostbasedAuthentication . | |
438 | .Pp | |
439 | .Pa /etc/hosts.equiv | |
440 | and | |
441 | .Pa /etc/shosts.equiv | |
442 | are still used. | |
443 | The default is | |
444 | .Dq yes . | |
445 | .It Cm IgnoreUserKnownHosts | |
446 | Specifies whether | |
447 | .Xr sshd 8 | |
448 | should ignore the user's | |
449 | .Pa ~/.ssh/known_hosts | |
450 | during | |
451 | .Cm RhostsRSAAuthentication | |
452 | or | |
453 | .Cm HostbasedAuthentication . | |
454 | The default is | |
455 | .Dq no . | |
456 | .It Cm KerberosAuthentication | |
457 | Specifies whether the password provided by the user for | |
458 | .Cm PasswordAuthentication | |
459 | will be validated through the Kerberos KDC. | |
460 | To use this option, the server needs a | |
461 | Kerberos servtab which allows the verification of the KDC's identity. | |
462 | The default is | |
463 | .Dq no . | |
464 | .It Cm KerberosGetAFSToken | |
465 | If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire | |
466 | an AFS token before accessing the user's home directory. | |
467 | The default is | |
468 | .Dq no . | |
469 | .It Cm KerberosOrLocalPasswd | |
470 | If password authentication through Kerberos fails then | |
471 | the password will be validated via any additional local mechanism | |
472 | such as | |
473 | .Pa /etc/passwd . | |
474 | The default is | |
475 | .Dq yes . | |
476 | .It Cm KerberosTicketCleanup | |
477 | Specifies whether to automatically destroy the user's ticket cache | |
478 | file on logout. | |
479 | The default is | |
480 | .Dq yes . | |
481 | .It Cm KeyRegenerationInterval | |
482 | In protocol version 1, the ephemeral server key is automatically regenerated | |
483 | after this many seconds (if it has been used). | |
484 | The purpose of regeneration is to prevent | |
485 | decrypting captured sessions by later breaking into the machine and | |
486 | stealing the keys. | |
487 | The key is never stored anywhere. | |
488 | If the value is 0, the key is never regenerated. | |
489 | The default is 3600 (seconds). | |
490 | .It Cm ListenAddress | |
491 | Specifies the local addresses | |
492 | .Xr sshd 8 | |
493 | should listen on. | |
494 | The following forms may be used: | |
495 | .Pp | |
496 | .Bl -item -offset indent -compact | |
497 | .It | |
498 | .Cm ListenAddress | |
499 | .Sm off | |
500 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr | |
501 | .Sm on | |
502 | .It | |
503 | .Cm ListenAddress | |
504 | .Sm off | |
505 | .Ar host No | Ar IPv4_addr No : Ar port | |
506 | .Sm on | |
507 | .It | |
508 | .Cm ListenAddress | |
509 | .Sm off | |
510 | .Oo | |
511 | .Ar host No | Ar IPv6_addr Oc : Ar port | |
512 | .Sm on | |
513 | .El | |
514 | .Pp | |
515 | If | |
516 | .Ar port | |
517 | is not specified, | |
518 | sshd will listen on the address and all prior | |
519 | .Cm Port | |
520 | options specified. | |
521 | The default is to listen on all local addresses. | |
522 | Multiple | |
523 | .Cm ListenAddress | |
524 | options are permitted. | |
525 | Additionally, any | |
526 | .Cm Port | |
527 | options must precede this option for non-port qualified addresses. | |
528 | .It Cm LoginGraceTime | |
529 | The server disconnects after this time if the user has not | |
530 | successfully logged in. | |
531 | If the value is 0, there is no time limit. | |
532 | The default is 120 seconds. | |
533 | .It Cm LogLevel | |
534 | Gives the verbosity level that is used when logging messages from | |
535 | .Xr sshd 8 . | |
536 | The possible values are: | |
537 | QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. | |
538 | The default is INFO. | |
539 | DEBUG and DEBUG1 are equivalent. | |
540 | DEBUG2 and DEBUG3 each specify higher levels of debugging output. | |
541 | Logging with a DEBUG level violates the privacy of users and is not recommended. | |
542 | .It Cm MACs | |
543 | Specifies the available MAC (message authentication code) algorithms. | |
544 | The MAC algorithm is used in protocol version 2 | |
545 | for data integrity protection. | |
546 | Multiple algorithms must be comma-separated. | |
547 | The default is: | |
548 | .Bd -literal -offset indent | |
549 | hmac-md5,hmac-sha1,umac-64@openssh.com, | |
550 | hmac-ripemd160,hmac-sha1-96,hmac-md5-96 | |
551 | .Ed | |
552 | .It Cm Match | |
553 | Introduces a conditional block. | |
554 | If all of the criteria on the | |
555 | .Cm Match | |
556 | line are satisfied, the keywords on the following lines override those | |
557 | set in the global section of the config file, until either another | |
558 | .Cm Match | |
559 | line or the end of the file. | |
560 | The arguments to | |
561 | .Cm Match | |
562 | are one or more criteria-pattern pairs. | |
563 | The available criteria are | |
564 | .Cm User , | |
565 | .Cm Group , | |
566 | .Cm Host , | |
567 | and | |
568 | .Cm Address . | |
569 | Only a subset of keywords may be used on the lines following a | |
570 | .Cm Match | |
571 | keyword. | |
572 | Available keywords are | |
573 | .Cm AllowTcpForwarding , | |
574 | .Cm Banner , | |
575 | .Cm ChrootDirectory , | |
576 | .Cm ForceCommand , | |
577 | .Cm GatewayPorts , | |
578 | .Cm GSSAPIAuthentication , | |
579 | .Cm HostbasedAuthentication , | |
580 | .Cm KbdInteractiveAuthentication , | |
581 | .Cm KerberosAuthentication , | |
582 | .Cm PasswordAuthentication , | |
583 | .Cm PermitOpen , | |
584 | .Cm PermitRootLogin , | |
585 | .Cm RhostsRSAAuthentication , | |
586 | .Cm RSAAuthentication , | |
587 | .Cm X11DisplayOffset , | |
588 | .Cm X11Forwarding , | |
589 | and | |
590 | .Cm X11UseLocalHost . | |
591 | .It Cm MaxAuthTries | |
592 | Specifies the maximum number of authentication attempts permitted per | |
593 | connection. | |
594 | Once the number of failures reaches half this value, | |
595 | additional failures are logged. | |
596 | The default is 6. | |
597 | .It Cm MaxStartups | |
598 | Specifies the maximum number of concurrent unauthenticated connections to the | |
599 | SSH daemon. | |
600 | Additional connections will be dropped until authentication succeeds or the | |
601 | .Cm LoginGraceTime | |
602 | expires for a connection. | |
603 | The default is 10. | |
604 | .Pp | |
605 | Alternatively, random early drop can be enabled by specifying | |
606 | the three colon separated values | |
607 | .Dq start:rate:full | |
608 | (e.g. "10:30:60"). | |
609 | .Xr sshd 8 | |
610 | will refuse connection attempts with a probability of | |
611 | .Dq rate/100 | |
612 | (30%) | |
613 | if there are currently | |
614 | .Dq start | |
615 | (10) | |
616 | unauthenticated connections. | |
617 | The probability increases linearly and all connection attempts | |
618 | are refused if the number of unauthenticated connections reaches | |
619 | .Dq full | |
620 | (60). | |
621 | .It Cm PasswordAuthentication | |
622 | Specifies whether password authentication is allowed. | |
623 | The default is | |
624 | .Dq yes . | |
625 | .It Cm PermitEmptyPasswords | |
626 | When password authentication is allowed, it specifies whether the | |
627 | server allows login to accounts with empty password strings. | |
628 | The default is | |
629 | .Dq no . | |
630 | .It Cm PermitOpen | |
631 | Specifies the destinations to which TCP port forwarding is permitted. | |
632 | The forwarding specification must be one of the following forms: | |
633 | .Pp | |
634 | .Bl -item -offset indent -compact | |
635 | .It | |
636 | .Cm PermitOpen | |
637 | .Sm off | |
638 | .Ar host : port | |
639 | .Sm on | |
640 | .It | |
641 | .Cm PermitOpen | |
642 | .Sm off | |
643 | .Ar IPv4_addr : port | |
644 | .Sm on | |
645 | .It | |
646 | .Cm PermitOpen | |
647 | .Sm off | |
648 | .Ar \&[ IPv6_addr \&] : port | |
649 | .Sm on | |
650 | .El | |
651 | .Pp | |
652 | Multiple forwards may be specified by separating them with whitespace. | |
653 | An argument of | |
654 | .Dq any | |
655 | can be used to remove all restrictions and permit any forwarding requests. | |
656 | By default all port forwarding requests are permitted. | |
657 | .It Cm PermitRootLogin | |
658 | Specifies whether root can log in using | |
659 | .Xr ssh 1 . | |
660 | The argument must be | |
661 | .Dq yes , | |
662 | .Dq without-password , | |
663 | .Dq forced-commands-only , | |
664 | or | |
665 | .Dq no . | |
666 | The default is | |
667 | .Dq yes . | |
668 | .Pp | |
669 | If this option is set to | |
670 | .Dq without-password , | |
671 | password authentication is disabled for root. | |
672 | .Pp | |
673 | If this option is set to | |
674 | .Dq forced-commands-only , | |
675 | root login with public key authentication will be allowed, | |
676 | but only if the | |
677 | .Ar command | |
678 | option has been specified | |
679 | (which may be useful for taking remote backups even if root login is | |
680 | normally not allowed). | |
681 | All other authentication methods are disabled for root. | |
682 | .Pp | |
683 | If this option is set to | |
684 | .Dq no , | |
685 | root is not allowed to log in. | |
686 | .It Cm PermitTunnel | |
687 | Specifies whether | |
688 | .Xr tun 4 | |
689 | device forwarding is allowed. | |
690 | The argument must be | |
691 | .Dq yes , | |
692 | .Dq point-to-point | |
693 | (layer 3), | |
694 | .Dq ethernet | |
695 | (layer 2), or | |
696 | .Dq no . | |
697 | Specifying | |
698 | .Dq yes | |
699 | permits both | |
700 | .Dq point-to-point | |
701 | and | |
702 | .Dq ethernet . | |
703 | The default is | |
704 | .Dq no . | |
705 | .It Cm PermitUserEnvironment | |
706 | Specifies whether | |
707 | .Pa ~/.ssh/environment | |
708 | and | |
709 | .Cm environment= | |
710 | options in | |
711 | .Pa ~/.ssh/authorized_keys | |
712 | are processed by | |
713 | .Xr sshd 8 . | |
714 | The default is | |
715 | .Dq no . | |
716 | Enabling environment processing may enable users to bypass access | |
717 | restrictions in some configurations using mechanisms such as | |
718 | .Ev LD_PRELOAD . | |
719 | .It Cm PidFile | |
720 | Specifies the file that contains the process ID of the | |
721 | SSH daemon. | |
722 | The default is | |
723 | .Pa /var/run/sshd.pid . | |
724 | .It Cm Port | |
725 | Specifies the port number that | |
726 | .Xr sshd 8 | |
727 | listens on. | |
728 | The default is 22. | |
729 | Multiple options of this type are permitted. | |
730 | See also | |
731 | .Cm ListenAddress . | |
732 | .It Cm PrintLastLog | |
733 | Specifies whether | |
734 | .Xr sshd 8 | |
735 | should print the date and time of the last user login when a user logs | |
736 | in interactively. | |
737 | The default is | |
738 | .Dq yes . | |
739 | .It Cm PrintMotd | |
740 | Specifies whether | |
741 | .Xr sshd 8 | |
742 | should print | |
743 | .Pa /etc/motd | |
744 | when a user logs in interactively. | |
745 | (On some systems it is also printed by the shell, | |
746 | .Pa /etc/profile , | |
747 | or equivalent.) | |
748 | The default is | |
749 | .Dq yes . | |
750 | .It Cm Protocol | |
751 | Specifies the protocol versions | |
752 | .Xr sshd 8 | |
753 | supports. | |
754 | The possible values are | |
755 | .Sq 1 | |
756 | and | |
757 | .Sq 2 . | |
758 | Multiple versions must be comma-separated. | |
759 | The default is | |
760 | .Dq 2,1 . | |
761 | Note that the order of the protocol list does not indicate preference, | |
762 | because the client selects among multiple protocol versions offered | |
763 | by the server. | |
764 | Specifying | |
765 | .Dq 2,1 | |
766 | is identical to | |
767 | .Dq 1,2 . | |
768 | .It Cm PubkeyAuthentication | |
769 | Specifies whether public key authentication is allowed. | |
770 | The default is | |
771 | .Dq yes . | |
772 | Note that this option applies to protocol version 2 only. | |
773 | .It Cm RhostsRSAAuthentication | |
774 | Specifies whether rhosts or /etc/hosts.equiv authentication together | |
775 | with successful RSA host authentication is allowed. | |
776 | The default is | |
777 | .Dq no . | |
778 | This option applies to protocol version 1 only. | |
779 | .It Cm RSAAuthentication | |
780 | Specifies whether pure RSA authentication is allowed. | |
781 | The default is | |
782 | .Dq yes . | |
783 | This option applies to protocol version 1 only. | |
784 | .It Cm ServerKeyBits | |
785 | Defines the number of bits in the ephemeral protocol version 1 server key. | |
786 | The minimum value is 512, and the default is 768. | |
787 | .It Cm StrictModes | |
788 | Specifies whether | |
789 | .Xr sshd 8 | |
790 | should check file modes and ownership of the | |
791 | user's files and home directory before accepting login. | |
792 | This is normally desirable because novices sometimes accidentally leave their | |
793 | directory or files world-writable. | |
794 | The default is | |
795 | .Dq yes . | |
796 | .It Cm Subsystem | |
797 | Configures an external subsystem (e.g. file transfer daemon). | |
798 | Arguments should be a subsystem name and a command (with optional arguments) | |
799 | to execute upon subsystem request. | |
800 | .Pp | |
801 | The command | |
802 | .Xr sftp-server 8 | |
803 | implements the | |
804 | .Dq sftp | |
805 | file transfer subsystem. | |
806 | .Pp | |
807 | Alternately the name | |
808 | .Dq internal-sftp | |
809 | implements an in-process | |
810 | .Dq sftp | |
811 | server. | |
812 | This may simplify configurations using | |
813 | .Cm ChrootDirectory | |
814 | to force a different filesystem root on clients. | |
815 | .Pp | |
816 | By default no subsystems are defined. | |
817 | Note that this option applies to protocol version 2 only. | |
818 | .It Cm SyslogFacility | |
819 | Gives the facility code that is used when logging messages from | |
820 | .Xr sshd 8 . | |
821 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, | |
822 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. | |
823 | The default is AUTH. | |
824 | .It Cm TCPKeepAlive | |
825 | Specifies whether the system should send TCP keepalive messages to the | |
826 | other side. | |
827 | If they are sent, death of the connection or crash of one | |
828 | of the machines will be properly noticed. | |
829 | However, this means that | |
830 | connections will die if the route is down temporarily, and some people | |
831 | find it annoying. | |
832 | On the other hand, if TCP keepalives are not sent, | |
833 | sessions may hang indefinitely on the server, leaving | |
834 | .Dq ghost | |
835 | users and consuming server resources. | |
836 | .Pp | |
837 | The default is | |
838 | .Dq yes | |
839 | (to send TCP keepalive messages), and the server will notice | |
840 | if the network goes down or the client host crashes. | |
841 | This avoids infinitely hanging sessions. | |
842 | .Pp | |
843 | To disable TCP keepalive messages, the value should be set to | |
844 | .Dq no . | |
845 | .It Cm UseDNS | |
846 | Specifies whether | |
847 | .Xr sshd 8 | |
848 | should look up the remote host name and check that | |
849 | the resolved host name for the remote IP address maps back to the | |
850 | very same IP address. | |
851 | The default is | |
852 | .Dq yes . | |
853 | .It Cm UseLogin | |
854 | Specifies whether | |
855 | .Xr login 1 | |
856 | is used for interactive login sessions. | |
857 | The default is | |
858 | .Dq no . | |
859 | Note that | |
860 | .Xr login 1 | |
861 | is never used for remote command execution. | |
862 | Note also, that if this is enabled, | |
863 | .Cm X11Forwarding | |
864 | will be disabled because | |
865 | .Xr login 1 | |
866 | does not know how to handle | |
867 | .Xr xauth 1 | |
868 | cookies. | |
869 | If | |
870 | .Cm UsePrivilegeSeparation | |
871 | is specified, it will be disabled after authentication. | |
872 | .It Cm UsePAM | |
873 | Enables the Pluggable Authentication Module interface. | |
874 | If set to | |
875 | .Dq yes | |
876 | this will enable PAM authentication using | |
877 | .Cm ChallengeResponseAuthentication | |
878 | and | |
879 | .Cm PasswordAuthentication | |
880 | in addition to PAM account and session module processing for all | |
881 | authentication types. | |
882 | .Pp | |
883 | Because PAM challenge-response authentication usually serves an equivalent | |
884 | role to password authentication, you should disable either | |
885 | .Cm PasswordAuthentication | |
886 | or | |
887 | .Cm ChallengeResponseAuthentication. | |
888 | .Pp | |
889 | If | |
890 | .Cm UsePAM | |
891 | is enabled, you will not be able to run | |
892 | .Xr sshd 8 | |
893 | as a non-root user. | |
894 | The default is | |
895 | .Dq no . | |
896 | .It Cm UsePrivilegeSeparation | |
897 | Specifies whether | |
898 | .Xr sshd 8 | |
899 | separates privileges by creating an unprivileged child process | |
900 | to deal with incoming network traffic. | |
901 | After successful authentication, another process will be created that has | |
902 | the privilege of the authenticated user. | |
903 | The goal of privilege separation is to prevent privilege | |
904 | escalation by containing any corruption within the unprivileged processes. | |
905 | The default is | |
906 | .Dq yes . | |
907 | .It Cm X11DisplayOffset | |
908 | Specifies the first display number available for | |
909 | .Xr sshd 8 Ns 's | |
910 | X11 forwarding. | |
911 | This prevents sshd from interfering with real X11 servers. | |
912 | The default is 10. | |
913 | .It Cm X11Forwarding | |
914 | Specifies whether X11 forwarding is permitted. | |
915 | The argument must be | |
916 | .Dq yes | |
917 | or | |
918 | .Dq no . | |
919 | The default is | |
920 | .Dq no . | |
921 | .Pp | |
922 | When X11 forwarding is enabled, there may be additional exposure to | |
923 | the server and to client displays if the | |
924 | .Xr sshd 8 | |
925 | proxy display is configured to listen on the wildcard address (see | |
926 | .Cm X11UseLocalhost | |
927 | below), though this is not the default. | |
928 | Additionally, the authentication spoofing and authentication data | |
929 | verification and substitution occur on the client side. | |
930 | The security risk of using X11 forwarding is that the client's X11 | |
931 | display server may be exposed to attack when the SSH client requests | |
932 | forwarding (see the warnings for | |
933 | .Cm ForwardX11 | |
934 | in | |
935 | .Xr ssh_config 5 ) . | |
936 | A system administrator may have a stance in which they want to | |
937 | protect clients that may expose themselves to attack by unwittingly | |
938 | requesting X11 forwarding, which can warrant a | |
939 | .Dq no | |
940 | setting. | |
941 | .Pp | |
942 | Note that disabling X11 forwarding does not prevent users from | |
943 | forwarding X11 traffic, as users can always install their own forwarders. | |
944 | X11 forwarding is automatically disabled if | |
945 | .Cm UseLogin | |
946 | is enabled. | |
947 | .It Cm X11UseLocalhost | |
948 | Specifies whether | |
949 | .Xr sshd 8 | |
950 | should bind the X11 forwarding server to the loopback address or to | |
951 | the wildcard address. | |
952 | By default, | |
953 | sshd binds the forwarding server to the loopback address and sets the | |
954 | hostname part of the | |
955 | .Ev DISPLAY | |
956 | environment variable to | |
957 | .Dq localhost . | |
958 | This prevents remote hosts from connecting to the proxy display. | |
959 | However, some older X11 clients may not function with this | |
960 | configuration. | |
961 | .Cm X11UseLocalhost | |
962 | may be set to | |
963 | .Dq no | |
964 | to specify that the forwarding server should be bound to the wildcard | |
965 | address. | |
966 | The argument must be | |
967 | .Dq yes | |
968 | or | |
969 | .Dq no . | |
970 | The default is | |
971 | .Dq yes . | |
972 | .It Cm XAuthLocation | |
973 | Specifies the full pathname of the | |
974 | .Xr xauth 1 | |
975 | program. | |
976 | The default is | |
977 | .Pa /usr/X11R6/bin/xauth . | |
978 | .El | |
979 | .Sh TIME FORMATS | |
980 | .Xr sshd 8 | |
981 | command-line arguments and configuration file options that specify time | |
982 | may be expressed using a sequence of the form: | |
983 | .Sm off | |
984 | .Ar time Op Ar qualifier , | |
985 | .Sm on | |
986 | where | |
987 | .Ar time | |
988 | is a positive integer value and | |
989 | .Ar qualifier | |
990 | is one of the following: | |
991 | .Pp | |
992 | .Bl -tag -width Ds -compact -offset indent | |
993 | .It Aq Cm none | |
994 | seconds | |
995 | .It Cm s | Cm S | |
996 | seconds | |
997 | .It Cm m | Cm M | |
998 | minutes | |
999 | .It Cm h | Cm H | |
1000 | hours | |
1001 | .It Cm d | Cm D | |
1002 | days | |
1003 | .It Cm w | Cm W | |
1004 | weeks | |
1005 | .El | |
1006 | .Pp | |
1007 | Each member of the sequence is added together to calculate | |
1008 | the total time value. | |
1009 | .Pp | |
1010 | Time format examples: | |
1011 | .Pp | |
1012 | .Bl -tag -width Ds -compact -offset indent | |
1013 | .It 600 | |
1014 | 600 seconds (10 minutes) | |
1015 | .It 10m | |
1016 | 10 minutes | |
1017 | .It 1h30m | |
1018 | 1 hour 30 minutes (90 minutes) | |
1019 | .El | |
1020 | .Sh FILES | |
1021 | .Bl -tag -width Ds | |
1022 | .It Pa /etc/ssh/sshd_config | |
1023 | Contains configuration data for | |
1024 | .Xr sshd 8 . | |
1025 | This file should be writable by root only, but it is recommended | |
1026 | (though not necessary) that it be world-readable. | |
1027 | .El | |
1028 | .Sh SEE ALSO | |
1029 | .Xr sshd 8 | |
1030 | .Sh AUTHORS | |
1031 | OpenSSH is a derivative of the original and free | |
1032 | ssh 1.2.12 release by Tatu Ylonen. | |
1033 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, | |
1034 | Theo de Raadt and Dug Song | |
1035 | removed many bugs, re-added newer features and | |
1036 | created OpenSSH. | |
1037 | Markus Friedl contributed the support for SSH | |
1038 | protocol versions 1.5 and 2.0. | |
1039 | Niels Provos and Markus Friedl contributed support | |
1040 | for privilege separation. |