1dhclient.conf(5) File Formats Manual dhclient.conf(5)
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6 dhclient.conf - DHCP client configuration file
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9 The dhclient.conf file contains configuration information for dhclient,
10 the Internet Systems Consortium DHCP Client.
11 The dhclient.conf file is a free-form ASCII text file. It is parsed
13 by the recursive-descent parser built into dhclient. The file may
14 contain extra tabs and newlines for formatting purposes. Keywords in
15 the file are case-insensitive. Comments may be placed anywhere within
16 the file (except within quotes). Comments begin with the # character
17 and end at the end of the line.
18 The dhclient.conf file can be used to configure the behaviour of the
20 client in a wide variety of ways: protocol timing, information
21 requested from the server, information required of the server, defaults
22 to use if the server does not provide certain information, values with
23 which to override information provided by the server, or values to
24 prepend or append to information provided by the server. The configu‐
25 ration file can also be preinitialized with addresses to use on net‐
26 works that don't have DHCP servers.
27
29 The timing behaviour of the client need not be configured by the user.
30 If no timing configuration is provided by the user, a fairly reasonable
31 timing behaviour will be used by default - one which results in fairly
32 timely updates without placing an inordinate load on the server.
33 The following statements can be used to adjust the timing behaviour of
35 the DHCP client if required, however:
36 The timeout statement
38 timeout time ;
40 The timeout statement determines the amount of time that must pass
42 between the time that the client begins to try to determine its address
43 and the time that it decides that it's not going to be able to contact
44 a server. By default, this timeout is sixty seconds. After the
45 timeout has passed, if there are any static leases defined in the con‐
46 figuration file, or any leases remaining in the lease database that
47 have not yet expired, the client will loop through these leases
48 attempting to validate them, and if it finds one that appears to be
49 valid, it will use that lease's address. If there are no valid static
50 leases or unexpired leases in the lease database, the client will
51 restart the protocol after the defined retry interval.
52 The retry statement
54 retry time;
56 The retry statement determines the time that must pass after the client
58 has determined that there is no DHCP server present before it tries
59 again to contact a DHCP server. By default, this is five minutes.
60 The select-timeout statement
62 select-timeout time;
64 It is possible (some might say desirable) for there to be more than one
66 DHCP server serving any given network. In this case, it is possible
67 that a client may be sent more than one offer in response to its ini‐
68 tial lease discovery message. It may be that one of these offers is
69 preferable to the other (e.g., one offer may have the address the
70 client previously used, and the other may not).
71 The select-timeout is the time after the client sends its first lease
73 discovery request at which it stops waiting for offers from servers,
74 assuming that it has received at least one such offer. If no offers
75 have been received by the time the select-timeout has expired, the
76 client will accept the first offer that arrives.
77 By default, the select-timeout is zero seconds - that is, the client
79 will take the first offer it sees.
80 The reboot statement
82 reboot time;
84 When the client is restarted, it first tries to reacquire the last
86 address it had. This is called the INIT-REBOOT state. If it is
87 still attached to the same network it was attached to when it last ran,
88 this is the quickest way to get started. The reboot statement sets
89 the time that must elapse after the client first tries to reacquire its
90 old address before it gives up and tries to discover a new address.
91 By default, the reboot timeout is ten seconds.
92 The backoff-cutoff statement
94 backoff-cutoff time;
96 The client uses an exponential backoff algorithm with some randomness,
98 so that if many clients try to configure themselves at the same time,
99 they will not make their requests in lockstep. The backoff-cutoff
100 statement determines the maximum amount of time that the client is
101 allowed to back off, the actual value will be evaluated randomly
102 between 1/2 to 1 1/2 times the time specified. It defaults to two
103 minutes.
104 The initial-interval statement
106 initial-interval time;
108 The initial-interval statement sets the amount of time between the
110 first attempt to reach a server and the second attempt to reach a
111 server. Each time a message is sent, the interval between messages is
112 incremented by twice the current interval multiplied by a random number
113 between zero and one. If it is greater than the backoff-cutoff amount,
114 it is set to that amount. It defaults to ten seconds.
115
117 The DHCP protocol allows the client to request that the server send it
118 specific information, and not send it other information that it is not
119 prepared to accept. The protocol also allows the client to reject
120 offers from servers if they don't contain information the client needs,
121 or if the information provided is not satisfactory.
122 There is a variety of data contained in offers that DHCP servers send
124 to DHCP clients. The data that can be specifically requested is what
125 are called DHCP Options. DHCP Options are defined in
126 dhcp-options(5).
127 The request statement
129 request [ option ] [, ... option ];
131 The request statement causes the client to request that any server
133 responding to the client send the client its values for the specified
134 options. Only the option names should be specified in the request
135 statement - not option parameters. By default, the DHCP server
136 requests the subnet-mask, broadcast-address, time-offset, routers,
137 domain-name, domain-name-servers, host-name, nis-domain, nis-servers,
138 and ntp-servers options.
139 In some cases, it may be desirable to send no parameter request list at
141 all. To do this, simply write the request statement but specify no
142 parameters:
143 request;
145 The require statement
147 require [ option ] [, ... option ];
149 The require statement lists options that must be sent in order for an
151 offer to be accepted. Offers that do not contain all the listed
152 options will be ignored.
153 The send statement
155 send { [ option declaration ] [, ... option declaration ]}
157 The send statement causes the client to send the specified options to
159 the server with the specified values. These are full option declara‐
160 tions as described in dhcp-options(5). Options that are always sent in
161 the DHCP protocol should not be specified here, except that the client
162 can specify a requested-lease-time option other than the default
163 requested lease time, which is two hours. The other obvious use for
164 this statement is to send information to the server that will allow it
165 to differentiate between this client and other clients or kinds of
166 clients.
167
169 The client now has some very limited support for doing DNS updates when
170 a lease is acquired. This is prototypical, and probably doesn't do
171 what you want. It also only works if you happen to have control over
172 your DNS server, which isn't very likely.
173 To make it work, you have to declare a key and zone as in the DHCP
175 server (see dhcpd.conf(5) for details). You also need to configure
176 the fqdn option on the client, as follows:
177 send fqdn.fqdn "grosse.fugue.com.";
179 send fqdn.encoded on;
180 send fqdn.server-update off;
181 The fqdn.fqdn option MUST be a fully-qualified domain name. You MUST
183 define a zone statement for the zone to be updated. The fqdn.encoded
184 option may need to be set to on or off, depending on the DHCP server
185 you are using.
186 The do-forward-updates statement
188 do-forward-updates [ flag ] ;
190 If you want to do DNS updates in the DHCP client script (see dhclient-
192 script(8)) rather than having the DHCP client do the update directly
193 (for example, if you want to use SIG(0) authentication, which is not
194 supported directly by the DHCP client, you can instruct the client not
195 to do the update using the do-forward-updates statement. Flag should
196 be true if you want the DHCP client to do the update, and false if you
197 don't want the DHCP client to do the update. By default, the DHCP
198 client will do the DNS update.
199
201 In some cases, a client may receive option data from the server which
202 is not really appropriate for that client, or may not receive informa‐
203 tion that it needs, and for which a useful default value exists. It
204 may also receive information which is useful, but which needs to be
205 supplemented with local information. To handle these needs, several
206 option modifiers are available.
207 The default statement
209 default [ option declaration ] ;
211 If for some option the client should use the value supplied by the
213 server, but needs to use some default value if no value was supplied by
214 the server, these values can be defined in the default statement.
215 The supersede statement
217 supersede [ option declaration ] ;
219 If for some option the client should always use a locally-configured
221 value or values rather than whatever is supplied by the server, these
222 values can be defined in the supersede statement.
223 The prepend statement
225 prepend [ option declaration ] ;
227 If for some set of options the client should use a value you supply,
229 and then use the values supplied by the server, if any, these values
230 can be defined in the prepend statement. The prepend statement can
231 only be used for options which allow more than one value to be given.
232 This restriction is not enforced - if you ignore it, the behaviour will
233 be unpredictable.
234 The append statement
236 append [ option declaration ] ;
238 If for some set of options the client should first use the values sup‐
240 plied by the server, if any, and then use values you supply, these val‐
241 ues can be defined in the append statement. The append statement can
242 only be used for options which allow more than one value to be given.
243 This restriction is not enforced - if you ignore it, the behaviour will
244 be unpredictable.
245
247 The lease declaration
248 lease { lease-declaration [ ... lease-declaration ] }
250 The DHCP client may decide after some period of time (see PROTOCOL TIM‐
252 ING) that it is not going to succeed in contacting a server. At that
253 time, it consults its own database of old leases and tests each one
254 that has not yet timed out by pinging the listed router for that lease
255 to see if that lease could work. It is possible to define one or more
256 fixed leases in the client configuration file for networks where there
257 is no DHCP or BOOTP service, so that the client can still automatically
258 configure its address. This is done with the lease statement.
259 NOTE: the lease statement is also used in the dhclient.leases file in
261 order to record leases that have been received from DHCP servers. Some
262 of the syntax for leases as described below is only needed in the
263 dhclient.leases file. Such syntax is documented here for complete‐
264 ness.
265 A lease statement consists of the lease keyword, followed by a left
267 curly brace, followed by one or more lease declaration statements, fol‐
268 lowed by a right curly brace. The following lease declarations are
269 possible:
270 bootp;
272 The bootp statement is used to indicate that the lease was acquired
274 using the BOOTP protocol rather than the DHCP protocol. It is never
275 necessary to specify this in the client configuration file. The
276 client uses this syntax in its lease database file.
277 interface "string";
279 The interface lease statement is used to indicate the interface on
281 which the lease is valid. If set, this lease will only be tried on a
282 particular interface. When the client receives a lease from a server,
283 it always records the interface number on which it received that lease.
284 If predefined leases are specified in the dhclient.conf file, the
285 interface should also be specified, although this is not required.
286 fixed-address ip-address;
288 The fixed-address statement is used to set the ip address of a particu‐
290 lar lease. This is required for all lease statements. The IP
291 address must be specified as a dotted quad (e.g., 12.34.56.78).
292 filename "string";
294 The filename statement specifies the name of the boot filename to use.
296 This is not used by the standard client configuration script, but is
297 included for completeness.
298 server-name "string";
300 The server-name statement specifies the name of the boot server name to
302 use. This is also not used by the standard client configuration
303 script.
304 option option-declaration;
306 The option statement is used to specify the value of an option supplied
308 by the server, or, in the case of predefined leases declared in
309 dhclient.conf, the value that the user wishes the client configuration
310 script to use if the predefined lease is used.
311 script "script-name";
313 The script statement is used to specify the pathname of the dhcp client
315 configuration script. This script is used by the dhcp client to set
316 each interface's initial configuration prior to requesting an address,
317 to test the address once it has been offered, and to set the inter‐
318 face's final configuration once a lease has been acquired. If no
319 lease is acquired, the script is used to test predefined leases, if
320 any, and also called once if no valid lease can be identified. For
321 more information, see dhclient-script(8).
322 vendor option space "name";
324 The vendor option space statement is used to specify which option space
326 should be used for decoding the vendor-encapsulate-options option if
327 one is received. The dhcp-vendor-identifier can be used to request a
328 specific class of vendor options from the server. See dhcp-options(5)
329 for details.
330 medium "media setup";
332 The medium statement can be used on systems where network interfaces
334 cannot automatically determine the type of network to which they are
335 connected. The media setup string is a system-dependent parameter
336 which is passed to the dhcp client configuration script when initializ‐
337 ing the interface. On Unix and Unix-like systems, the argument is
338 passed on the ifconfig command line when configuring the interface.
339 The dhcp client automatically declares this parameter if it uses a
341 media type (see the media statement) when configuring the interface in
342 order to obtain a lease. This statement should be used in predefined
343 leases only if the network interface requires media type configuration.
344 renew date;
346 rebind date;
348 expire date;
350 The renew statement defines the time at which the dhcp client should
352 begin trying to contact its server to renew a lease that it is using.
353 The rebind statement defines the time at which the dhcp client should
354 begin to try to contact any dhcp server in order to renew its lease.
355 The expire statement defines the time at which the dhcp client must
356 stop using a lease if it has not been able to contact a server in order
357 to renew it.
358 These declarations are automatically set in leases acquired by the DHCP
360 client, but must also be configured in predefined leases - a predefined
361 lease whose expiry time has passed will not be used by the DHCP client.
362 Dates are specified as follows:
364 <weekday> <year>/<month>/<day> <hour>:<minute>:<second>
366 The weekday is present to make it easy for a human to tell when a lease
368 expires - it's specified as a number from zero to six, with zero being
369 Sunday. When declaring a predefined lease, it can always be specified
370 as zero. The year is specified with the century, so it should gener‐
371 ally be four digits except for really long leases. The month is speci‐
372 fied as a number starting with 1 for January. The day of the month is
373 likewise specified starting with 1. The hour is a number between 0 and
374 23, the minute a number between 0 and 59, and the second also a number
375 between 0 and 59.
376
378 alias { declarations ... }
379 Some DHCP clients running TCP/IP roaming protocols may require that in
381 addition to the lease they may acquire via DHCP, their interface also
382 be configured with a predefined IP alias so that they can have a perma‐
383 nent IP address even while roaming. The Internet Systems Consortium
384 DHCP client doesn't support roaming with fixed addresses directly, but
385 in order to facilitate such experimentation, the dhcp client can be set
386 up to configure an IP alias using the alias declaration.
387 The alias declaration resembles a lease declaration, except that
389 options other than the subnet-mask option are ignored by the standard
390 client configuration script, and expiry times are ignored. A typical
391 alias declaration includes an interface declaration, a fixed-address
392 declaration for the IP alias address, and a subnet-mask option declara‐
393 tion. A medium statement should never be included in an alias decla‐
394 ration.
395
397 reject ip-address;
398 The reject statement causes the DHCP client to reject offers from
400 servers who use the specified address as a server identifier. This
401 can be used to avoid being configured by rogue or misconfigured dhcp
402 servers, although it should be a last resort - better to track down the
403 bad DHCP server and fix it.
404 interface "name" { declarations ... }
406 A client with more than one network interface may require different be‐
408 haviour depending on which interface is being configured. All timing
409 parameters and declarations other than lease and alias declarations can
410 be enclosed in an interface declaration, and those parameters will then
411 be used only for the interface that matches the specified name.
412 Interfaces for which there is no interface declaration will use the
413 parameters declared outside of any interface declaration, or the
414 default settings.
415 Note well: ISC dhclient only maintains one list of interfaces, which is
417 either determined at startup from command line arguments, or otherwise
418 is autodetected. If you supplied the list of interfaces on the command
419 line, this configuration clause will add the named interface to the
420 list in such a way that will cause it to be configured by DHCP. Which
421 may not be the result you had intended. This is an undesirable side
422 effect that will be addressed in a future release.
423 pseudo "name" "real-name" { declarations ... }
425 Under some circumstances it can be useful to declare a pseudo-interface
427 and have the DHCP client acquire a configuration for that interface.
428 Each interface that the DHCP client is supporting normally has a DHCP
429 client state machine running on it to acquire and maintain its lease.
430 A pseudo-interface is just another state machine running on the inter‐
431 face named real-name, with its own lease and its own state. If you
432 use this feature, you must provide a client identifier for both the
433 pseudo-interface and the actual interface, and the two identifiers must
434 be different. You must also provide a separate client script for the
435 pseudo-interface to do what you want with the IP address. For exam‐
436 ple:
437 interface "ep0" {
439 send dhcp-client-identifier "my-client-ep0";
440 }
441 pseudo "secondary" "ep0" {
442 send dhcp-client-identifier "my-client-ep0-secondary";
443 script "/etc/dhclient-secondary";
444 }
445 The client script for the pseudo-interface should not configure the
447 interface up or down - essentially, all it needs to handle are the
448 states where a lease has been acquired or renewed, and the states where
449 a lease has expired. See dhclient-script(8) for more information.
450 media "media setup" [ , "media setup", ... ];
452 The media statement defines one or more media configuration parameters
454 which may be tried while attempting to acquire an IP address. The
455 dhcp client will cycle through each media setup string on the list,
456 configuring the interface using that setup and attempting to boot, and
457 then trying the next one. This can be used for network interfaces
458 which aren't capable of sensing the media type unaided - whichever
459 media type succeeds in getting a request to the server and hearing the
460 reply is probably right (no guarantees).
461 The media setup is only used for the initial phase of address acquisi‐
463 tion (the DHCPDISCOVER and DHCPOFFER packets). Once an address has
464 been acquired, the dhcp client will record it in its lease database and
465 will record the media type used to acquire the address. Whenever the
466 client tries to renew the lease, it will use that same media type.
467 The lease must expire before the client will go back to cycling through
468 media types.
469 bootp-broadcast-always;
471 The bootp-broadcast-always statement instructs dhclient to always set
473 the bootp broadcast flag in request packets, so that servers will
474 always broadcast replies. This is equivalent to supplying the dhclient
475 -B argument, and has the same effect as specifying 'always-broadcast'
476 in the server's dhcpd.conf. This option is provided as a Red Hat
477 extension to enable dhclient to work on IBM zSeries z/OS Linux guests .
478
480 The following configuration file is used on a laptop running NetBSD
481 1.3. The laptop has an IP alias of 192.5.5.213, and has one inter‐
482 face, ep0 (a 3com 3C589C). Booting intervals have been shortened
483 somewhat from the default, because the client is known to spend most of
484 its time on networks with little DHCP activity. The laptop does roam
485 to multiple networks.
486 timeout 60;
489 retry 60;
490 reboot 10;
491 select-timeout 5;
492 initial-interval 2;
493 reject 192.33.137.209;
494 interface "ep0" {
496 send host-name "andare.fugue.com";
497 send dhcp-client-identifier 1:0:a0:24:ab:fb:9c;
498 send dhcp-lease-time 3600;
499 supersede domain-name "fugue.com rc.vix.com home.vix.com";
500 prepend domain-name-servers 127.0.0.1;
501 request subnet-mask, broadcast-address, time-offset, routers,
502 domain-name, domain-name-servers, host-name;
503 require subnet-mask, domain-name-servers;
504 script "/sbin/dhclient-script";
505 media "media 10baseT/UTP", "media 10base2/BNC";
506 }
507 alias {
509 interface "ep0";
510 fixed-address 192.5.5.213;
511 option subnet-mask 255.255.255.255;
512 }
513 This is a very complicated dhclient.conf file - in general, yours
514 should be much simpler. In many cases, it's sufficient to just create
515 an empty dhclient.conf file - the defaults are usually fine.
516
518 dhcp-options(5), dhclient.leases(5), dhcpd(8), dhcpd.conf(5), RFC2132,
519 RFC2131.
520
522 dhclient(8) was written by Ted Lemon under a contract with Vixie Labs.
523 Funding for this project was provided by Internet Systems Consortium.
524 Information about Internet Systems Consortium can be found at
525 http://www.isc.org.
526 dhclient.conf(5)