1 .\" $OpenBSD: src/sbin/dhclient/dhclient.conf.5,v 1.21 2011/04/09 19:53:00 krw Exp $
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34 .\" by Ted Lemon <mellon@fugue.com> in cooperation with Vixie
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44 .Nd DHCP client configuration file
48 file contains configuration information for
53 file is a free-form ASCII text file.
54 It is parsed by the recursive-descent parser built into
56 The file may contain extra tabs and newlines for formatting purposes.
57 Keywords in the file are case-insensitive.
58 Comments may be placed anywhere within the file (except within quotes).
59 Comments begin with the
61 character and end at the end of the line.
65 file can be used to configure the behaviour of the client in a wide variety
66 of ways: protocol timing, information requested from the server, information
67 required of the server, defaults to use if the server does not provide
68 certain information, values with which to override information provided by
69 the server, or values to prepend or append to information provided by the
71 The configuration file can also be preinitialized with addresses to
72 use on networks that don't have DHCP servers.
74 The timing behaviour of the client need not be configured by the user.
75 If no timing configuration is provided by the user, a fairly
76 reasonable timing behaviour will be used by default \- one which
77 results in fairly timely updates without placing an inordinate load on
80 The following statements can be used to adjust the timing behaviour of
81 the DHCP client if required, however:
83 .It Ic timeout Ar time ;
86 statement determines the amount of time that must pass between the
87 time that the client begins to try to determine its address and the
88 time that it decides that it's not going to be able to contact a server.
89 By default, this timeout is sixty seconds.
90 After the timeout has passed, if there are any static leases defined in the
91 configuration file, or any leases remaining in the lease database that
92 have not yet expired, the client will loop through these leases
93 attempting to validate them, and if it finds one that appears to be
94 valid, it will use that lease's address.
95 If there are no valid static leases or unexpired leases in the lease database,
96 the client will restart the protocol after the defined retry interval.
97 .It Ic retry Ar time ;
100 statement determines the time that must pass after the client has
101 determined that there is no DHCP server present before it tries again
102 to contact a DHCP server.
103 By default, this is five minutes.
104 .It Ic select-timeout Ar time ;
105 It is possible (some might say desirable) for there to be more than
106 one DHCP server serving any given network.
107 In this case, it is possible that a client may be sent more than one offer
108 in response to its initial lease discovery message.
109 It may be that one of these offers is preferable to the other
110 (e.g., one offer may have the address the client previously used,
111 and the other may not).
115 is the time after the client sends its first lease discovery request
116 at which it stops waiting for offers from servers, assuming that it
117 has received at least one such offer.
118 If no offers have been received by the time the
120 has expired, the client will accept the first offer that arrives.
124 is zero seconds \- that is, the client will take the first offer it sees.
125 .It Ic reboot Ar time ;
126 When the client is restarted, it first tries to reacquire the last
128 This is called the INIT-REBOOT state.
129 If it is still attached to the same network it was attached to when it last
130 ran, this is the quickest way to get started.
133 statement sets the time that must elapse after the client first tries
134 to reacquire its old address before it gives up and tries to discover
136 By default, the reboot timeout is ten seconds.
137 .It Ic backoff-cutoff Ar time ;
138 The client uses an exponential backoff algorithm with some randomness,
139 so that if many clients try to configure themselves at the same time,
140 they will not make their requests in lockstep.
143 statement determines the maximum amount of time that the client is
145 It defaults to fifteen seconds.
146 .It Ic initial-interval Ar time ;
149 statement sets the amount of time between the first attempt to reach a
150 server and the second attempt to reach a server.
151 Each time a message is sent, the interval between messages is incremented by
152 twice the current interval multiplied by a random number between zero and one.
153 If it is greater than the backoff-cutoff amount, it is set to that
155 It defaults to three seconds.
156 .It Ic link-timeout Ar time ;
159 statement sets the amount of time to wait for an interface link before timing
161 The default value is ten seconds.
162 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
180 .It Ic request Op Ar option , ... ;
183 statement causes the client to request that any server responding to the
184 client send the client its values for the specified options.
185 Only option names should be specified in the request statement \- not
189 statement has any effect, overriding any previous
192 .It Ic require Op Ar option , ... ;
195 statement lists options that must be sent in order for an offer to be accepted.
196 Offers that do not contain all the listed options will be ignored.
197 Only option names should be specified in the require statement \- not
201 statement has any effect, overriding any previous
204 .It Ic send Ar option option-value ;
207 statement causes the client to send the specified option and value
209 Options that are always sent in the DHCP protocol should not be specified
211 One use for this statement is to send information to the server
212 that will allow it to differentiate between this client and other
213 clients or kinds of clients.
216 Options in the lease can be modified before being passed to the client
217 configuration script,
218 .Xr dhclient-script 8 .
220 The default client configuration script
221 processes only options 1 (subnet
222 mask), 3 (routers), 6 (domain name servers), 15 (domain-name).
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.
229 .It Ic default Ar option option-value ;
234 if no value is supplied by the server.
235 .It Ic supersede Ar option option-value ;
240 regardless of the value supplied by the server.
241 .It Ic prepend Ar option option-value ;
246 and then use the value supplied by the server.
248 can only be used for options which allow more than one value to be given.
249 The restriction is not enforced \- if violated, the results are unpredictable.
250 .It Ic append Ar option option-value ;
255 after first using the value supplied by the server.
257 can only be used for options which allow more than one value to be given.
258 The restriction is not enforced \- if violated, the results are unpredictable.
260 .Sh LEASE DECLARATIONS
261 The lease declaration:
263 .D1 Ic lease No { Ar lease-declaration ; ... ; No }
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
281 .Pa /var/db/dhclient.leases. Ns Aq Ar IFNAME
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
284 .Pa /var/db/dhclient.leases. Ns Aq Ar IFNAME
286 Such syntax is documented here for completeness.
288 A lease statement consists of the lease keyword, followed by a left
289 curly brace, followed by one or more lease declaration statements,
290 followed by a right curly brace.
291 The following lease declarations are possible:
293 .Bl -tag -width Ds -compact
297 statement is used to indicate that the lease was acquired using the
298 BOOTP protocol rather than the DHCP protocol.
299 It is never necessary to specify this in the client configuration file.
300 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.
313 .It Ic fixed-address Ar ip-address ;
316 statement is used to set the IP address of a particular lease.
317 This is required for all lease statements.
318 The IP address must be specified as a dotted quad (e.g., 12.34.56.78).
320 .It Ic filename Qq Ar string ;
323 statement specifies the name of the boot filename to use.
324 This is not used by the standard client, but is included for completeness.
326 .It Ic server-name Qq Ar string ;
329 statement specifies the name of the boot server name to use.
330 This is not used by the standard client, but is included for completeness.
332 .It Ic option Ar option option-value ;
335 statement is used to specify the value of an option supplied by the server,
336 or, in the case of predefined leases declared in
338 the value that the user wishes the client to use if the
339 predefined lease is used.
341 .It Ic renew Ar date ;
342 .It Ic rebind Ar date ;
343 .It Ic expire Ar date ;
346 statement defines the time at which the DHCP client should begin trying to
347 contact its server to renew a lease that it is using.
350 statement defines the time at which the DHCP client should begin to try to
353 DHCP server in order to renew its lease.
356 statement defines the time at which the DHCP client must stop using a lease
357 if it has not been able to contact a server in order to renew it.
360 These declarations are automatically set in leases acquired by the
361 DHCP client, but must also be configured in predefined leases \- a
362 predefined lease whose expiry time has passed will not be used by the
365 Dates are specified as follows:
367 .D1 <weekday> <year>/<month>/<day> <hour>:<minute>:<second>
369 The weekday is present to make it easy for a human to tell when a
370 lease expires \- it's specified as a number from zero to six, with zero
372 When declaring a predefined lease, it can always be specified as zero.
373 The year is specified with the century, so it should generally be four
374 digits except for really long leases.
375 The month is specified as a number starting with 1 for January.
376 The day of the month is likewise specified starting with 1.
377 The hour is a number between 0 and 23,
378 the minute a number between 0 and 59,
379 and the second also a number between 0 and 59.
380 .Sh OTHER DECLARATIONS
382 .It Ic reject Ar ip-address ;
385 statement causes the DHCP client to reject offers from servers who use
386 the specified address as a server identifier.
387 This can be used to avoid being configured by rogue or misconfigured DHCP
388 servers, although it should be a last resort \- better to track down
389 the bad DHCP server and fix it.
390 .It Ic interface Qo Ar name Qc No { Ar declaration ; ... ; No }
391 A client with more than one network interface may require different
392 behaviour depending on which interface is being configured.
393 All timing parameters and declarations other than lease
394 declarations can be enclosed in an interface declaration, and those
395 parameters will then be used only for the interface that matches the
397 Interfaces for which there is no interface declaration will use the
398 parameters declared outside of any interface declaration,
399 or the default settings.
400 .It Ic script Ar \&"script-name\&" ;
403 statement is used to specify the pathname of the client configuration
405 This script is used by the DHCP client to set each interface's initial
406 configuration prior to requesting an address, to test the address once it
407 has been offered, and to set the interface's final configuration once a
408 lease has been acquired.
409 If no lease is acquired, the script is used to test predefined leases, if
410 any, and also called once if no valid lease can be identified.
411 For more information, see
412 .Xr dhclient.leases 5 .
415 The following configuration file is used on a laptop
416 which has one interface, ep0 (a 3Com 3C589C).
417 Booting intervals have been shortened somewhat from the default, because
418 the client is known to spend most of its time on networks with little DHCP
420 The laptop does roam to multiple networks.
421 .Bd -literal -offset indent
427 reject 192.33.137.209;
430 send host-name "andare.fugue.com";
431 send dhcp-client-identifier 1:0:a0:24:ab:fb:9c;
432 send dhcp-lease-time 3600;
433 supersede domain-name "fugue.com rc.vix.com home.vix.com";
434 prepend domain-name-servers 127.0.0.1;
435 request subnet-mask, broadcast-address, time-offset, routers,
436 domain-name, domain-name-servers, host-name;
437 require subnet-mask, domain-name-servers;
438 script "/etc/dhclient-script";
442 This is a very complicated
444 file \- in general, yours should be much simpler.
445 In many cases, it's sufficient to just create an empty
447 file \- the defaults are usually fine.
449 .Xr dhclient.leases 5 ,
451 .Xr dhcpd.conf 5 Pq Pa net/isc-dhcp42-server ,
453 .Xr dhclient-script 8 ,
454 .Xr dhcpd 8 Pq Pa net/isc-dhcp42-server
460 .%T Dynamic Host Configuration Protocol
468 .%T DHCP Options and BOOTP Vendor Extensions
474 .An Ted Lemon Aq Mt mellon@vix.com
475 under a contract with Vixie Labs.
477 The current implementation was reworked by
478 .An Henning Brauer Aq Mt henning@openbsd.org .