1 .\" $OpenBSD: src/sbin/dhclient/dhclient.conf.5,v 1.18 2011/03/26 23:17:50 krw Exp $
3 .\" Copyright (c) 1997 The Internet Software Consortium.
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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
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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''.
44 .Nd DHCP client configuration file
48 file contains configuration information for
50 the Internet Software Consortium DHCP Client.
54 file is a free-form ASCII text file.
55 It is parsed by the recursive-descent parser built into
57 The file may contain extra tabs and newlines for formatting purposes.
58 Keywords in the file are case-insensitive.
59 Comments may be placed anywhere within the file (except within quotes).
60 Comments begin with the
62 character and end at the end of the line.
66 file can be used to configure the behaviour of the client in a wide variety
67 of ways: protocol timing, information requested from the server, information
68 required of the server, defaults to use if the server does not provide
69 certain information, values with which to override information provided by
70 the server, or values to prepend or append to information provided by the
72 The configuration file can also be preinitialized with addresses to
73 use on networks that don't have DHCP servers.
75 The timing behaviour of the client need not be configured by the user.
76 If no timing configuration is provided by the user, a fairly
77 reasonable timing behaviour will be used by default \- one which
78 results in fairly timely updates without placing an inordinate load on
81 The following statements can be used to adjust the timing behaviour of
82 the DHCP client if required, however:
84 .It Ic timeout Ar time ;
87 statement determines the amount of time that must pass between the
88 time that the client begins to try to determine its address and the
89 time that it decides that it's not going to be able to contact a server.
90 By default, this timeout is sixty seconds.
91 After the timeout has passed, if there are any static leases defined in the
92 configuration file, or any leases remaining in the lease database that
93 have not yet expired, the client will loop through these leases
94 attempting to validate them, and if it finds one that appears to be
95 valid, it will use that lease's address.
96 If there are no valid static leases or unexpired leases in the lease database,
97 the client will restart the protocol after the defined retry interval.
98 .It Ic retry Ar time ;
101 statement determines the time that must pass after the client has
102 determined that there is no DHCP server present before it tries again
103 to contact a DHCP server.
104 By default, this is five minutes.
105 .It Ic select-timeout Ar time ;
106 It is possible (some might say desirable) for there to be more than
107 one DHCP server serving any given network.
108 In this case, it is possible that a client may be sent more than one offer
109 in response to its initial lease discovery message.
110 It may be that one of these offers is preferable to the other
111 (e.g., one offer may have the address the client previously used,
112 and the other may not).
116 is the time after the client sends its first lease discovery request
117 at which it stops waiting for offers from servers, assuming that it
118 has received at least one such offer.
119 If no offers have been received by the time the
121 has expired, the client will accept the first offer that arrives.
125 is zero seconds \- that is, the client will take the first offer it sees.
126 .It Ic reboot Ar time ;
127 When the client is restarted, it first tries to reacquire the last
129 This is called the INIT-REBOOT state.
130 If it is still attached to the same network it was attached to when it last
131 ran, this is the quickest way to get started.
134 statement sets the time that must elapse after the client first tries
135 to reacquire its old address before it gives up and tries to discover
137 By default, the reboot timeout is ten seconds.
138 .It Ic backoff-cutoff Ar time ;
139 The client uses an exponential backoff algorithm with some randomness,
140 so that if many clients try to configure themselves at the same time,
141 they will not make their requests in lockstep.
144 statement determines the maximum amount of time that the client is
146 It defaults to fifteen seconds.
147 .It Ic initial-interval Ar time ;
150 statement sets the amount of time between the first attempt to reach a
151 server and the second attempt to reach a server.
152 Each time a message is sent, the interval between messages is incremented by
153 twice the current interval multiplied by a random number between zero and one.
154 If it is greater than the backoff-cutoff amount, it is set to that
156 It defaults to three seconds.
157 .It Ic link-timeout Ar time ;
160 statement sets the amount of time to wait for an interface link before timing
162 The default value is ten seconds. The value zero requests that
163 dhclient not wait for a link state change before timing out.
165 .Sh LEASE REQUIREMENTS AND REQUESTS
166 The DHCP protocol allows the client to request that the server send it
167 specific information, and not send it other information that it is not
169 The protocol also allows the client to reject offers from servers if they
170 don't contain information the client needs, or if the information provided
173 There is a variety of data contained in offers that DHCP servers send
175 The data that can be specifically requested is what are called
177 DHCP Options are defined in
181 .Ic request Op Ar option
182 .Oo , Ar ... option Oc ;
186 statement causes the client to request that any server responding to the
187 client send the client its values for the specified options.
188 Only the option names should be specified in the request statement \- not
191 .Ic require Op Ar option
192 .Oo , Ar ... option Oc ;
196 statement lists options that must be sent in order for an offer to be accepted.
197 Offers that do not contain all the listed options will be ignored.
199 .Ic send No { Op Ar option declaration
200 .Oo , Ar ... option declaration Oc }
204 statement causes the client to send the specified options to the server with
205 the specified values.
206 These are full option declarations as described in
208 Options that are always sent in the DHCP protocol should not be specified
210 One use for this statement is to send information to the server
211 that will allow it to differentiate between this client and other
212 clients or kinds of clients.
215 Options in the lease can be modified before being passed to the client
216 configuration script,
217 .Xr dhclient-script 8 .
219 The default client configuration script
220 processes only options 1 (subnet
221 mask), 3 (routers), 6 (domain name servers), 15 (domain-name), and 33
223 Use of option modifiers on other options will have no effect unless
224 .Xr dhclient-script 8
225 the client configuration script is modified.
227 Several option modifiers are available.
230 .Ic default No { Op Ar option declaration
231 .Oo , Ar ... option declaration Oc }
233 If for some set of options the client should use the value supplied by
234 the server, but needs to use some default value if no value was supplied
235 by the server, these values can be defined in the
239 .Ic supersede No { Op Ar option declaration
240 .Oo , Ar ... option declaration Oc }
242 If for some set of options the client should always use its own value
243 rather than any value supplied by the server, these values can be defined
248 .Ic prepend No { Op Ar option declaration
249 .Oo , Ar ... option declaration Oc }
251 If for some set of options the client should use a value you supply,
252 and then use the values supplied by the server, if any,
253 these values can be defined in the
258 statement can only be used for options which allow more than one value to
260 This restriction is not enforced \- if violated, the results are unpredictable.
262 .Ic append No { Op Ar option declaration
263 .Oo , Ar ... option declaration Oc }
265 If for some set of options the client should first use the values
266 supplied by the server, if any, and then use values you supply, these
267 values can be defined in the
272 statement can only be used for options which allow more than one value to
274 This restriction is not enforced \- if you ignore it,
275 the behaviour will be unpredictable.
277 .Sh LEASE DECLARATIONS
278 The lease declaration:
280 .D1 Ic lease No { Ar lease-declaration Oo Ar ... lease-declaration Oc }
282 The DHCP client may decide after some period of time (see
283 .Sx PROTOCOL TIMING )
284 that it is not going to succeed in contacting a server.
285 At that time, it consults its own database of old leases and tests each one
286 that has not yet timed out by pinging the listed router for that lease to
287 see if that lease could work.
288 It is possible to define one or more
290 leases in the client configuration file for networks where there is no DHCP
291 or BOOTP service, so that the client can still automatically configure its
293 This is done with the
297 NOTE: the lease statement is also used in the
299 file in order to record leases that have been received from DHCP servers.
300 Some of the syntax for leases as described below is only needed in the
303 Such syntax is documented here for completeness.
305 A lease statement consists of the lease keyword, followed by a left
306 curly brace, followed by one or more lease declaration statements,
307 followed by a right curly brace.
308 The following lease declarations are possible:
313 statement is used to indicate that the lease was acquired using the
314 BOOTP protocol rather than the DHCP protocol.
315 It is never necessary to specify this in the client configuration file.
316 The client uses this syntax in its lease database file.
317 .It Ic interface Ar \&"string\&" ;
320 lease statement is used to indicate the interface on which the lease is valid.
321 If set, this lease will only be tried on a particular interface.
322 When the client receives a lease from a server, it always records the
323 interface number on which it received that lease.
324 If predefined leases are specified in the
326 file, the interface should also be specified, although this is not required.
327 .It Ic fixed-address Ar ip-address ;
330 statement is used to set the IP address of a particular lease.
331 This is required for all lease statements.
332 The IP address must be specified as a dotted quad (e.g., 12.34.56.78).
333 .It Ic filename Ar \&"string\&" ;
336 statement specifies the name of the boot filename to use.
337 This is not used by the standard client configuration script, but is
338 included for completeness.
339 .It Ic server-name Ar \&"string\&" ;
342 statement specifies the name of the boot server name to use.
343 This is also not used by the standard client configuration script.
344 .It Ic option Ar option-declaration ;
347 statement is used to specify the value of an option supplied by the server,
348 or, in the case of predefined leases declared in
350 the value that the user wishes the client configuration script to use if the
351 predefined lease is used.
352 .It Ic medium Ar \&"media setup\&" ;
355 statement can be used on systems where network interfaces cannot
356 automatically determine the type of network to which they are connected.
357 The media setup string is a system-dependent parameter which is passed
358 to the client configuration script when initializing the interface.
361 and UNIX-like systems, the argument is passed on the ifconfig command line
362 when configuring the interface.
364 The DHCP client automatically declares this parameter if it used a
367 statement) when configuring the interface in order to obtain a lease.
368 This statement should be used in predefined leases only if the network
369 interface requires media type configuration.
370 .It Ic renew Ar date ;
371 .It Ic rebind Ar date ;
372 .It Ic expire Ar date ;
375 statement defines the time at which the DHCP client should begin trying to
376 contact its server to renew a lease that it is using.
379 statement defines the time at which the DHCP client should begin to try to
382 DHCP server in order to renew its lease.
385 statement defines the time at which the DHCP client must stop using a lease
386 if it has not been able to contact a server in order to renew it.
389 These declarations are automatically set in leases acquired by the
390 DHCP client, but must also be configured in predefined leases \- a
391 predefined lease whose expiry time has passed will not be used by the
394 Dates are specified as follows:
395 .Bd -ragged -offset indent
398 .Ar <year> No / Ar <month> No / Ar <day>
399 .Ar <hour> : <minute> : <second>
403 The weekday is present to make it easy for a human to tell when a
404 lease expires \- it's specified as a number from zero to six, with zero
406 When declaring a predefined lease, it can always be specified as zero.
407 The year is specified with the century, so it should generally be four
408 digits except for really long leases.
409 The month is specified as a number starting with 1 for January.
410 The day of the month is likewise specified starting with 1.
411 The hour is a number between 0 and 23,
412 the minute a number between 0 and 59,
413 and the second also a number between 0 and 59.
414 .Sh ALIAS DECLARATIONS
415 .Ic alias No { Ar declarations ... No }
417 Some DHCP clients running TCP/IP roaming protocols may require that in
418 addition to the lease they may acquire via DHCP, their interface also
419 be configured with a predefined IP alias so that they can have a
420 permanent IP address even while roaming.
421 The Internet Software Consortium DHCP client doesn't support roaming with
422 fixed addresses directly, but in order to facilitate such experimentation,
423 the DHCP client can be set up to configure an IP alias using the
429 declaration resembles a lease declaration, except that options other than
430 the subnet-mask option are ignored by the standard client configuration
431 script, and expiry times are ignored.
432 A typical alias declaration includes an interface declaration, a fixed-address
433 declaration for the IP alias address, and a subnet-mask option declaration.
434 A medium statement should never be included in an alias declaration.
435 .Sh OTHER DECLARATIONS
437 .It Ic reject Ar ip-address ;
440 statement causes the DHCP client to reject offers from servers who use
441 the specified address as a server identifier.
442 This can be used to avoid being configured by rogue or misconfigured DHCP
443 servers, although it should be a last resort \- better to track down
444 the bad DHCP server and fix it.
446 .Ic interface Ar \&"name\&" No { Ar declarations
449 A client with more than one network interface may require different
450 behaviour depending on which interface is being configured.
451 All timing parameters and declarations other than lease and alias
452 declarations can be enclosed in an interface declaration, and those
453 parameters will then be used only for the interface that matches the
455 Interfaces for which there is no interface declaration will use the
456 parameters declared outside of any interface declaration,
457 or the default settings.
459 .Ic media Ar \&"media setup\&"
460 .Oo , Ar \&"media setup\&" , ... Oc ;
464 statement defines one or more media configuration parameters which may
465 be tried while attempting to acquire an IP address.
466 The DHCP client will cycle through each media setup string on the list,
467 configuring the interface using that setup and attempting to boot,
468 and then trying the next one.
469 This can be used for network interfaces which aren't capable of sensing
470 the media type unaided \- whichever media type succeeds in getting a request
471 to the server and hearing the reply is probably right (no guarantees).
473 The media setup is only used for the initial phase of address
474 acquisition (the DHCPDISCOVER and DHCPOFFER packets).
475 Once an address has been acquired, the DHCP client will record it in its
476 lease database and will record the media type used to acquire the address.
477 Whenever the client tries to renew the lease, it will use that same media type.
478 The lease must expire before the client will go back to cycling through media
480 .It Ic script Ar \&"script-name\&" ;
483 statement is used to specify the pathname of the client configuration
485 This script is used by the DHCP client to set each interface's initial
486 configuration prior to requesting an address, to test the address once it
487 has been offered, and to set the interface's final configuration once a
488 lease has been acquired.
489 If no lease is acquired, the script is used to test predefined leases, if
490 any, and also called once if no valid lease can be identified.
491 For more information, see
492 .Xr dhclient.leases 5 .
495 The following configuration file is used on a laptop
496 which has an IP alias of 192.5.5.213, and has one interface,
498 Booting intervals have been shortened somewhat from the default, because
499 the client is known to spend most of its time on networks with little DHCP
501 The laptop does roam to multiple networks.
502 .Bd -literal -offset indent
508 reject 192.33.137.209;
511 send host-name "andare.fugue.com";
512 send dhcp-client-identifier 1:0:a0:24:ab:fb:9c;
513 send dhcp-lease-time 3600;
514 supersede domain-name "fugue.com rc.vix.com home.vix.com";
515 prepend domain-name-servers 127.0.0.1;
516 request subnet-mask, broadcast-address, time-offset, routers,
517 domain-name, domain-name-servers, host-name;
518 require subnet-mask, domain-name-servers;
519 script "/etc/dhclient-script";
520 media "media 10baseT/UTP", "media 10base2/BNC";
525 fixed-address 192.5.5.213;
526 option subnet-mask 255.255.255.255;
530 This is a very complicated
532 file \- in general, yours should be much simpler.
533 In many cases, it's sufficient to just create an empty
535 file \- the defaults are usually fine.
537 .Xr dhclient.leases 5 ,
538 .Xr dhclient-script 8 ,
540 .Xr dhcpd.conf 5 Pq Pa pkgsrc/net/isc-dhcpd4 ,
542 .Xr dhcpd 8 Pq Pa pkgsrc/net/isc-dhcpd4
549 .An Ted Lemon Aq mellon@vix.com
550 under a contract with Vixie Labs.
552 The current implementation was reworked by
553 .An Henning Brauer Aq henning@openbsd.org .
projects / dragonfly.git / blob