1 .\" $OpenBSD: dhclient.conf.5,v 1.5 2004/11/01 23:10:18 henning Exp $
3 .\" Copyright (c) 1997 The Internet Software Consortium.
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7 .\" modification, are permitted provided that the following conditions
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15 .\" 3. Neither the name of The Internet Software Consortium nor the names
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19 .\" THIS SOFTWARE IS PROVIDED BY THE INTERNET SOFTWARE CONSORTIUM AND
20 .\" CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
21 .\" INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
22 .\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
23 .\" DISCLAIMED. IN NO EVENT SHALL THE INTERNET SOFTWARE CONSORTIUM OR
24 .\" CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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33 .\" This software has been written for the Internet Software Consortium
34 .\" by Ted Lemon <mellon@fugue.com> in cooperation with Vixie
35 .\" Enterprises. To learn more about the Internet Software Consortium,
36 .\" see ``http://www.isc.org/isc''. To learn more about Vixie
37 .\" Enterprises, see ``http://www.vix.com''.
46 .Nd DHCP client configuration file
50 file contains configuration information for
52 the Internet Software Consortium DHCP Client.
56 file is a free-form ASCII text file.
57 It is parsed by the recursive-descent parser built into
59 The file may contain extra tabs and newlines for formatting purposes.
60 Keywords in the file are case-insensitive.
61 Comments may be placed anywhere within the file (except within quotes).
62 Comments begin with the
64 character and end at the end of the line.
68 file can be used to configure the behaviour of the client in a wide variety
69 of ways: protocol timing, information requested from the server, information
70 required of the server, defaults to use if the server does not provide
71 certain information, values with which to override information provided by
72 the server, or values to prepend or append to information provided by the
74 The configuration file can also be preinitialized with addresses to
75 use on networks that do not have DHCP servers.
77 The timing behaviour of the client need not be configured by the user.
78 If no timing configuration is provided by the user, a fairly
79 reasonable timing behaviour will be used by default - one which
80 results in fairly timely updates without placing an inordinate load on
83 The following statements can be used to adjust the timing behaviour of
84 the DHCP client if required, however:
85 .Bl -tag -width indent
86 .It Ic timeout Ar time ;
89 statement determines the amount of time that must pass between the
90 time that the client begins to try to determine its address and the
91 time that it decides that it is not going to be able to contact a server.
92 By default, this timeout is sixty seconds.
93 After the timeout has passed, if there are any static leases defined in the
94 configuration file, or any leases remaining in the lease database that
95 have not yet expired, the client will loop through these leases
96 attempting to validate them, and if it finds one that appears to be
97 valid, it will use that lease's address.
98 If there are no valid static leases or unexpired leases in the lease database,
99 the client will restart the protocol after the defined retry interval.
100 .It Ic retry Ar time ;
103 statement determines the time that must pass after the client has
104 determined that there is no DHCP server present before it tries again
105 to contact a DHCP server.
106 By default, this is five minutes.
107 .It Ic select-timeout Ar time ;
108 It is possible (some might say desirable) for there to be more than
109 one DHCP server serving any given network.
110 In this case, it is possible that a client may be sent more than one offer
111 in response to its initial lease discovery message.
112 It may be that one of these offers is preferable to the other
113 (e.g., one offer may have the address the client previously used,
114 and the other may not).
118 is the time after the client sends its first lease discovery request
119 at which it stops waiting for offers from servers, assuming that it
120 has received at least one such offer.
121 If no offers have been received by the time the
123 has expired, the client will accept the first offer that arrives.
127 is zero seconds - that is, the client will take the first offer it sees.
128 .It Ic reboot Ar time ;
129 When the client is restarted, it first tries to reacquire the last
131 This is called the INIT-REBOOT state.
132 If it is still attached to the same network it was attached to when it last
133 ran, this is the quickest way to get started.
136 statement sets the time that must elapse after the client first tries
137 to reacquire its old address before it gives up and tries to discover
139 By default, the reboot timeout is ten seconds.
140 .It Ic backoff-cutoff Ar time ;
141 The client uses an exponential backoff algorithm with some randomness,
142 so that if many clients try to configure themselves at the same time,
143 they will not make their requests in lockstep.
146 statement determines the maximum amount of time that the client is
148 It defaults to two minutes.
149 .It Ic initial-interval Ar time ;
152 statement sets the amount of time between the first attempt to reach a
153 server and the second attempt to reach a server.
154 Each time a message is sent, the interval between messages is incremented by
155 twice the current interval multiplied by a random number between zero and one.
156 If it is greater than the
158 amount, it is set to that
160 It defaults to ten seconds.
162 .Sh LEASE REQUIREMENTS AND REQUESTS
163 The DHCP protocol allows the client to request that the server send it
164 specific information, and not send it other information that it is not
166 The protocol also allows the client to reject offers from servers if they
167 do not contain information the client needs, or if the information provided
170 There is a variety of data contained in offers that DHCP servers send
172 The data that can be specifically requested is what are called
174 DHCP Options are defined in
176 .Bl -tag -width indent
177 .It Ic request Oo Ar option Oc Oo , Ar ... option Oc ;
180 statement causes the client to request that any server responding to the
181 client send the client its values for the specified options.
182 Only the option names should be specified in the request statement - not
184 .It Ic require Oo Ar option Oc Oo , Ar ... option Oc ;
187 statement lists options that must be sent in order for an offer to be accepted.
188 Offers that do not contain all the listed options will be ignored.
189 .It Ic send No { Oo Ar option declaration Oc Oo , Ar ... option declaration Oc }
192 statement causes the client to send the specified options to the server with
193 the specified values.
194 These are full option declarations as described in
196 Options that are always sent in the DHCP protocol should not be specified
197 here, except that the client can specify a
199 option other than the default requested lease time, which is two hours.
200 The other obvious use for this statement is to send information to the server
201 that will allow it to differentiate between this client and other
202 clients or kinds of clients.
203 .It Ic ignore Oo Ar option Oc Oo , Ar ... option Oc ;
206 statement causes the client to disregard the specified options in any offer
207 received, as though the server had never sent them at all.
210 In some cases, a client may receive option data from the server which
211 is not really appropriate for that client, or may not receive
212 information that it needs, and for which a useful default value exists.
213 It may also receive information which is useful, but which needs to be
214 supplemented with local information.
215 To handle these needs, several option modifiers are available.
216 .Bl -tag -width indent
218 .Ic default No { Op Ar option declaration
219 .Oo , Ar ... option declaration Oc }
221 If for some set of options the client should use the value supplied by
222 the server, but needs to use some default value if no value was supplied
223 by the server, these values can be defined in the
227 .Ic supersede No { Op Ar option declaration
228 .Oo , Ar ... option declaration Oc }
230 If for some set of options the client should always use its own value
231 rather than any value supplied by the server, these values can be defined
236 Some options values have special meaning:
237 .Bl -tag -width indent
239 Any server-supplied interface MTU is ignored by the client if a
241 zero value is configured.
244 .Ic prepend No { Op Ar option declaration
245 .Oo , Ar ... option declaration Oc }
247 If for some set of options the client should use a value you supply,
248 and then use the values supplied by the server, if any,
249 these values can be defined in the
254 statement can only be used for options which allow more than one value to
256 This restriction is not enforced - if violated, the results are unpredictable.
258 .Ic append No { Op Ar option declaration
259 .Oo , Ar ... option declaration Oc }
261 If for some set of options the client should first use the values
262 supplied by the server, if any, and then use values you supply, these
263 values can be defined in the
268 statement can only be used for options which allow more than one value to
270 This restriction is not enforced - if you ignore it,
271 the behaviour will be unpredictable.
273 .Sh LEASE DECLARATIONS
274 The lease declaration:
276 .D1 Ic lease No { Ar lease-declaration Oo Ar ... lease-declaration Oc }
278 The DHCP client may decide after some period of time (see
279 .Sx PROTOCOL TIMING )
280 that it is not going to succeed in contacting a server.
281 At that time, it consults its own database of old leases and tests each one
282 that has not yet timed out by pinging the listed router for that lease to
283 see if that lease could work.
284 It is possible to define one or more
286 leases in the client configuration file for networks where there is no DHCP
287 or BOOTP service, so that the client can still automatically configure its
289 This is done with the
293 NOTE: the lease statement is also used in the
295 file in order to record leases that have been received from DHCP servers.
296 Some of the syntax for leases as described below is only needed in the
299 Such syntax is documented here for completeness.
301 A lease statement consists of the
303 keyword, followed by a left
304 curly brace, followed by one or more lease declaration statements,
305 followed by a right curly brace.
306 The following lease declarations are possible:
307 .Bl -tag -width indent
311 statement is used to indicate that the lease was acquired using the
312 BOOTP protocol rather than the DHCP protocol.
313 It is never necessary to specify this in the client configuration file.
314 The client uses this syntax in its lease database file.
315 .It Ic interface Qq Ar string ;
318 lease statement is used to indicate the interface on which the lease is valid.
319 If set, this lease will only be tried on a particular interface.
320 When the client receives a lease from a server, it always records the
321 interface number on which it received that lease.
322 If predefined leases are specified in the
324 file, the interface should also be specified, although this is not required.
325 .It Ic fixed-address Ar ip-address ;
328 statement is used to set the IP address of a particular lease.
329 This is required for all lease statements.
330 The IP address must be specified as a dotted quad (e.g.,
332 .It Ic filename Qq Ar string ;
335 statement specifies the name of the boot filename to use.
336 This is not used by the standard client configuration script, but is
337 included for completeness.
338 .It Ic server-name Qq Ar string ;
341 statement specifies the name of the boot server name to use.
342 This is also not used by the standard client configuration script.
343 .It Ic option Ar option-declaration ;
346 statement is used to specify the value of an option supplied by the server,
347 or, in the case of predefined leases declared in
349 the value that the user wishes the client configuration script to use if the
350 predefined lease is used.
351 .It Ic script Qq Ar script-name ;
354 statement is used to specify the pathname of the DHCP client configuration
356 This script is used by the DHCP client to set each interface's initial
357 configuration prior to requesting an address, to test the address once it
358 has been offered, and to set the interface's final configuration once a
359 lease has been acquired.
360 If no lease is acquired, the script is used to test predefined leases, if
361 any, and also called once if no valid lease can be identified.
362 For more information, see
363 .Xr dhclient.leases 5 .
364 .It Ic medium Qq Ar "media setup" ;
367 statement can be used on systems where network interfaces cannot
368 automatically determine the type of network to which they are connected.
371 string is a system-dependent parameter which is passed
372 to the DHCP client configuration script when initializing the interface.
377 systems, the argument is passed on the
380 when configuring the interface.
382 The DHCP client automatically declares this parameter if it used a
385 statement) when configuring the interface in order to obtain a lease.
386 This statement should be used in predefined leases only if the network
387 interface requires media type configuration.
388 .It Ic renew Ar date ;
389 .It Ic rebind Ar date ;
390 .It Ic expire Ar date ;
393 statement defines the time at which the DHCP client should begin trying to
394 contact its server to renew a lease that it is using.
397 statement defines the time at which the DHCP client should begin to try to
400 DHCP server in order to renew its lease.
403 statement defines the time at which the DHCP client must stop using a lease
404 if it has not been able to contact a server in order to renew it.
407 These declarations are automatically set in leases acquired by the
408 DHCP client, but must also be configured in predefined leases - a
409 predefined lease whose expiry time has passed will not be used by the
412 Dates are specified as follows:
413 .Bd -ragged -offset indent
416 .Ar <year> No / Ar <month> No / Ar <day>
417 .Ar <hour> : <minute> : <second>
421 The weekday is present to make it easy for a human to tell when a
422 lease expires - it is specified as a number from zero to six, with zero
424 When declaring a predefined lease, it can always be specified as zero.
425 The year is specified with the century, so it should generally be four
426 digits except for really long leases.
427 The month is specified as a number starting with 1 for January.
428 The day of the month is likewise specified starting with 1.
429 The hour is a number between 0 and 23,
430 the minute a number between 0 and 59,
431 and the second also a number between 0 and 59.
432 .Sh ALIAS DECLARATIONS
433 .Ic alias No { Ar declarations ... No }
435 Some DHCP clients running TCP/IP roaming protocols may require that in
436 addition to the lease they may acquire via DHCP, their interface also
437 be configured with a predefined IP alias so that they can have a
438 permanent IP address even while roaming.
439 The Internet Software Consortium DHCP client does not support roaming with
440 fixed addresses directly, but in order to facilitate such experimentation,
441 the DHCP client can be set up to configure an IP alias using the
447 declaration resembles a lease declaration, except that options other than
448 the subnet-mask option are ignored by the standard client configuration
449 script, and expiry times are ignored.
450 A typical alias declaration includes an interface declaration, a fixed-address
451 declaration for the IP alias address, and a subnet-mask option declaration.
452 A medium statement should never be included in an alias declaration.
453 .Sh OTHER DECLARATIONS
454 .Bl -tag -width indent
455 .It Ic reject Ar ip-address ;
458 statement causes the DHCP client to reject offers from servers who use
459 the specified address as a server identifier.
460 This can be used to avoid being configured by rogue or misconfigured DHCP
461 servers, although it should be a last resort - better to track down
462 the bad DHCP server and fix it.
463 .It Ic interface Qo Ar name Qc { Ar declarations ... No }
464 A client with more than one network interface may require different
465 behaviour depending on which interface is being configured.
466 All timing parameters and declarations other than lease and alias
467 declarations can be enclosed in an interface declaration, and those
468 parameters will then be used only for the interface that matches the
470 Interfaces for which there is no interface declaration will use the
471 parameters declared outside of any interface declaration,
472 or the default settings.
473 .It Ic media Qo Ar "media setup" Qc Oo , Qo Ar "media setup" Qc , Ar ... Oc ;
476 statement defines one or more media configuration parameters which may
477 be tried while attempting to acquire an IP address.
478 The DHCP client will cycle through each media setup string on the list,
479 configuring the interface using that setup and attempting to boot,
480 and then trying the next one.
481 This can be used for network interfaces which are not capable of sensing
482 the media type unaided - whichever media type succeeds in getting a request
483 to the server and hearing the reply is probably right (no guarantees).
485 The media setup is only used for the initial phase of address
486 acquisition (the DHCPDISCOVER and DHCPOFFER packets).
487 Once an address has been acquired, the DHCP client will record it in its
488 lease database and will record the media type used to acquire the address.
489 Whenever the client tries to renew the lease, it will use that same media type.
490 The lease must expire before the client will go back to cycling through media
492 .It Ic vlan-pcp Ar code ;
495 statement sets the PCP (Priority Code Point) value for the VLAN header.
497 .Va net.link.vlan.mtag_pcp
498 sysctl to be set to 1.
501 The following configuration file is used on a laptop
502 which has an IP alias of
504 and has one interface,
507 Booting intervals have been shortened somewhat from the default, because
508 the client is known to spend most of its time on networks with little DHCP
510 The laptop does roam to multiple networks.
511 .Bd -literal -offset indent
517 reject 192.33.137.209;
520 send host-name "andare.fugue.com";
521 send dhcp-client-identifier 1:0:a0:24:ab:fb:9c;
522 send dhcp-lease-time 3600;
523 supersede domain-name "fugue.com rc.vix.com home.vix.com";
524 prepend domain-name-servers 127.0.0.1;
525 request subnet-mask, broadcast-address, time-offset, routers,
526 domain-name, domain-name-servers, host-name;
527 require subnet-mask, domain-name-servers;
528 script "/etc/dhclient-script";
529 media "media 10baseT/UTP", "media 10base2/BNC";
534 fixed-address 192.5.5.213;
535 option subnet-mask 255.255.255.255;
539 This is a very complicated
541 file - in general, yours should be much simpler.
542 In many cases, it is sufficient to just create an empty
544 file - the defaults are usually fine.
546 .Xr dhclient.leases 5 ,
552 .%R "RFC 2132, RFC 2131"
560 .An Ted Lemon Aq Mt mellon@vix.com
561 under a contract with Vixie Labs.
563 The current implementation was reworked by
564 .An Henning Brauer Aq Mt henning@openbsd.org .
projects / FreeBSD / FreeBSD.git / blob