1 .\" $OpenBSD: dhclient.conf.5,v 1.5 2004/11/01 23:10:18 henning Exp $
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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
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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.
205 In some cases, a client may receive option data from the server which
206 is not really appropriate for that client, or may not receive
207 information that it needs, and for which a useful default value exists.
208 It may also receive information which is useful, but which needs to be
209 supplemented with local information.
210 To handle these needs, several option modifiers are available.
211 .Bl -tag -width indent
213 .Ic default No { Op Ar option declaration
214 .Oo , Ar ... option declaration Oc }
216 If for some set of options the client should use the value supplied by
217 the server, but needs to use some default value if no value was supplied
218 by the server, these values can be defined in the
222 .Ic supersede No { Op Ar option declaration
223 .Oo , Ar ... option declaration Oc }
225 If for some set of options the client should always use its own value
226 rather than any value supplied by the server, these values can be defined
231 .Ic prepend No { Op Ar option declaration
232 .Oo , Ar ... option declaration Oc }
234 If for some set of options the client should use a value you supply,
235 and then use the values supplied by the server, if any,
236 these values can be defined in the
241 statement can only be used for options which allow more than one value to
243 This restriction is not enforced - if violated, the results are unpredictable.
245 .Ic append No { Op Ar option declaration
246 .Oo , Ar ... option declaration Oc }
248 If for some set of options the client should first use the values
249 supplied by the server, if any, and then use values you supply, these
250 values can be defined in the
255 statement can only be used for options which allow more than one value to
257 This restriction is not enforced - if you ignore it,
258 the behaviour will be unpredictable.
260 .Sh LEASE DECLARATIONS
261 The lease declaration:
263 .D1 Ic lease No { Ar lease-declaration Oo Ar ... lease-declaration Oc }
265 The DHCP client may decide after some period of time (see
266 .Sx PROTOCOL TIMING )
267 that it is not going to succeed in contacting a server.
268 At that time, it consults its own database of old leases and tests each one
269 that has not yet timed out by pinging the listed router for that lease to
270 see if that lease could work.
271 It is possible to define one or more
273 leases in the client configuration file for networks where there is no DHCP
274 or BOOTP service, so that the client can still automatically configure its
276 This is done with the
280 NOTE: the lease statement is also used in the
282 file in order to record leases that have been received from DHCP servers.
283 Some of the syntax for leases as described below is only needed in the
286 Such syntax is documented here for completeness.
288 A lease statement consists of the
290 keyword, followed by a left
291 curly brace, followed by one or more lease declaration statements,
292 followed by a right curly brace.
293 The following lease declarations are possible:
294 .Bl -tag -width indent
298 statement is used to indicate that the lease was acquired using the
299 BOOTP protocol rather than the DHCP protocol.
300 It is never necessary to specify this in the client configuration file.
301 The client uses this syntax in its lease database file.
302 .It Ic interface Qq Ar string ;
305 lease statement is used to indicate the interface on which the lease is valid.
306 If set, this lease will only be tried on a particular interface.
307 When the client receives a lease from a server, it always records the
308 interface number on which it received that lease.
309 If predefined leases are specified in the
311 file, the interface should also be specified, although this is not required.
312 .It Ic fixed-address Ar ip-address ;
315 statement is used to set the IP address of a particular lease.
316 This is required for all lease statements.
317 The IP address must be specified as a dotted quad (e.g.,
319 .It Ic filename Qq Ar string ;
322 statement specifies the name of the boot filename to use.
323 This is not used by the standard client configuration script, but is
324 included for completeness.
325 .It Ic server-name Qq Ar string ;
328 statement specifies the name of the boot server name to use.
329 This is also not used by the standard client configuration script.
330 .It Ic option Ar option-declaration ;
333 statement is used to specify the value of an option supplied by the server,
334 or, in the case of predefined leases declared in
336 the value that the user wishes the client configuration script to use if the
337 predefined lease is used.
338 .It Ic script Qq Ar script-name ;
341 statement is used to specify the pathname of the DHCP client configuration
343 This script is used by the DHCP client to set each interface's initial
344 configuration prior to requesting an address, to test the address once it
345 has been offered, and to set the interface's final configuration once a
346 lease has been acquired.
347 If no lease is acquired, the script is used to test predefined leases, if
348 any, and also called once if no valid lease can be identified.
349 For more information, see
350 .Xr dhclient.leases 5 .
351 .It Ic medium Qq Ar "media setup" ;
354 statement can be used on systems where network interfaces cannot
355 automatically determine the type of network to which they are connected.
358 string is a system-dependent parameter which is passed
359 to the DHCP client configuration script when initializing the interface.
364 systems, the argument is passed on the
367 when configuring the interface.
369 The DHCP client automatically declares this parameter if it used a
372 statement) when configuring the interface in order to obtain a lease.
373 This statement should be used in predefined leases only if the network
374 interface requires media type configuration.
375 .It Ic renew Ar date ;
376 .It Ic rebind Ar date ;
377 .It Ic expire Ar date ;
380 statement defines the time at which the DHCP client should begin trying to
381 contact its server to renew a lease that it is using.
384 statement defines the time at which the DHCP client should begin to try to
387 DHCP server in order to renew its lease.
390 statement defines the time at which the DHCP client must stop using a lease
391 if it has not been able to contact a server in order to renew it.
394 These declarations are automatically set in leases acquired by the
395 DHCP client, but must also be configured in predefined leases - a
396 predefined lease whose expiry time has passed will not be used by the
399 Dates are specified as follows:
400 .Bd -ragged -offset indent
403 .Ar <year> No / Ar <month> No / Ar <day>
404 .Ar <hour> : <minute> : <second>
408 The weekday is present to make it easy for a human to tell when a
409 lease expires - it is specified as a number from zero to six, with zero
411 When declaring a predefined lease, it can always be specified as zero.
412 The year is specified with the century, so it should generally be four
413 digits except for really long leases.
414 The month is specified as a number starting with 1 for January.
415 The day of the month is likewise specified starting with 1.
416 The hour is a number between 0 and 23,
417 the minute a number between 0 and 59,
418 and the second also a number between 0 and 59.
419 .Sh ALIAS DECLARATIONS
420 .Ic alias No { Ar declarations ... No }
422 Some DHCP clients running TCP/IP roaming protocols may require that in
423 addition to the lease they may acquire via DHCP, their interface also
424 be configured with a predefined IP alias so that they can have a
425 permanent IP address even while roaming.
426 The Internet Software Consortium DHCP client does not support roaming with
427 fixed addresses directly, but in order to facilitate such experimentation,
428 the DHCP client can be set up to configure an IP alias using the
434 declaration resembles a lease declaration, except that options other than
435 the subnet-mask option are ignored by the standard client configuration
436 script, and expiry times are ignored.
437 A typical alias declaration includes an interface declaration, a fixed-address
438 declaration for the IP alias address, and a subnet-mask option declaration.
439 A medium statement should never be included in an alias declaration.
440 .Sh OTHER DECLARATIONS
441 .Bl -tag -width indent
442 .It Ic reject Ar ip-address ;
445 statement causes the DHCP client to reject offers from servers who use
446 the specified address as a server identifier.
447 This can be used to avoid being configured by rogue or misconfigured DHCP
448 servers, although it should be a last resort - better to track down
449 the bad DHCP server and fix it.
450 .It Ic interface Qo Ar name Qc { Ar declarations ... No }
451 A client with more than one network interface may require different
452 behaviour depending on which interface is being configured.
453 All timing parameters and declarations other than lease and alias
454 declarations can be enclosed in an interface declaration, and those
455 parameters will then be used only for the interface that matches the
457 Interfaces for which there is no interface declaration will use the
458 parameters declared outside of any interface declaration,
459 or the default settings.
460 .It Ic media Qo Ar "media setup" Qc Oo , Qo Ar "media setup" Qc , Ar ... Oc ;
463 statement defines one or more media configuration parameters which may
464 be tried while attempting to acquire an IP address.
465 The DHCP client will cycle through each media setup string on the list,
466 configuring the interface using that setup and attempting to boot,
467 and then trying the next one.
468 This can be used for network interfaces which are not capable of sensing
469 the media type unaided - whichever media type succeeds in getting a request
470 to the server and hearing the reply is probably right (no guarantees).
472 The media setup is only used for the initial phase of address
473 acquisition (the DHCPDISCOVER and DHCPOFFER packets).
474 Once an address has been acquired, the DHCP client will record it in its
475 lease database and will record the media type used to acquire the address.
476 Whenever the client tries to renew the lease, it will use that same media type.
477 The lease must expire before the client will go back to cycling through media
481 The following configuration file is used on a laptop
482 which has an IP alias of
484 and has one interface,
487 Booting intervals have been shortened somewhat from the default, because
488 the client is known to spend most of its time on networks with little DHCP
490 The laptop does roam to multiple networks.
491 .Bd -literal -offset indent
497 reject 192.33.137.209;
500 send host-name "andare.fugue.com";
501 send dhcp-client-identifier 1:0:a0:24:ab:fb:9c;
502 send dhcp-lease-time 3600;
503 supersede domain-name "fugue.com rc.vix.com home.vix.com";
504 prepend domain-name-servers 127.0.0.1;
505 request subnet-mask, broadcast-address, time-offset, routers,
506 domain-name, domain-name-servers, host-name;
507 require subnet-mask, domain-name-servers;
508 script "/etc/dhclient-script";
509 media "media 10baseT/UTP", "media 10base2/BNC";
514 fixed-address 192.5.5.213;
515 option subnet-mask 255.255.255.255;
519 This is a very complicated
521 file - in general, yours should be much simpler.
522 In many cases, it is sufficient to just create an empty
524 file - the defaults are usually fine.
526 .Xr dhclient.leases 5 ,
532 .%R "RFC 2132, RFC 2131"
540 .An Ted Lemon Aq mellon@vix.com
541 under a contract with Vixie Labs.
543 The current implementation was reworked by
544 .An Henning Brauer Aq henning@openbsd.org .