1dhclient.conf(5) File Formats Manual dhclient.conf(5)
2
6 dhclient.conf - DHCP client configuration file
7
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 by
13 the recursive-descent parser built into dhclient. The file may contain
14 extra tabs and newlines for formatting purposes. Keywords in the file
15 are case-insensitive. Comments may be placed anywhere within the file
16 (except within quotes). Comments begin with the # character and end at
17 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 If required the following statements can be used to adjust the timing
35 behaviour of the DHCPv4 client. The DHCPv6 protocol provides values to
36 use and they are not currently configurable.
37 The timeout statement
39 timeout time;
41 The timeout statement determines the amount of time that must pass
43 between the time that the client begins to try to determine its address
44 and the time that it decides that it's not going to be able to contact
45 a server. By default, this timeout is sixty seconds. After the time‐
46 out has passed, if there are any static leases defined in the configu‐
47 ration file, or any leases remaining in the lease database that have
48 not yet expired, the client will loop through these leases attempting
49 to validate them, and if it finds one that appears to be valid, it will
50 use that lease's address. If there are no valid static leases or unex‐
51 pired leases in the lease database, the client will restart the proto‐
52 col after the defined retry interval.
53 The retry statement
55 retry time;
57 The retry statement determines the time that must pass after the client
59 has determined that there is no DHCP server present before it tries
60 again to contact a DHCP server. By default, this is five minutes.
61 The select-timeout statement
63 select-timeout time;
65 It is possible (some might say desirable) for there to be more than one
67 DHCP server serving any given network. In this case, it is possible
68 that a client may be sent more than one offer in response to its ini‐
69 tial lease discovery message. It may be that one of these offers is
70 preferable to the other (e.g., one offer may have the address the
71 client previously used, and the other may not).
72 The select-timeout is the time after the client sends its first lease
74 discovery request at which it stops waiting for offers from servers,
75 assuming that it has received at least one such offer. If no offers
76 have been received by the time the select-timeout has expired, the
77 client will accept the first offer that arrives.
78 By default, the select-timeout is zero seconds - that is, the client
80 will take the first offer it sees.
81 The reboot statement
83 reboot time;
85 When the client is restarted, it first tries to reacquire the last
87 address it had. This is called the INIT-REBOOT state. If it is still
88 attached to the same network it was attached to when it last ran, this
89 is the quickest way to get started. The reboot statement sets the time
90 that must elapse after the client first tries to reacquire its old
91 address before it gives up and tries to discover a new address. By
92 default, the reboot timeout is ten seconds.
93 The backoff-cutoff statement
95 backoff-cutoff time;
97 The client uses an exponential backoff algorithm with some randomness,
99 so that if many clients try to configure themselves at the same time,
100 they will not make their requests in lockstep. The backoff-cutoff
101 statement determines the maximum amount of time that the client is
102 allowed to back off, the actual value will be evaluated randomly
103 between 1/2 to 1 1/2 times the time specified. It defaults to fifteen
104 seconds.
105 The initial-interval statement
107 initial-interval time;
109 The initial-interval statement sets the amount of time between the
111 first attempt to reach a server and the second attempt to reach a
112 server. Each time a message is sent, the interval between messages is
113 incremented by twice the current interval multiplied by a random number
114 between zero and one. If it is greater than the backoff-cutoff amount,
115 it is set to that amount. It defaults to ten seconds.
116 The initial-delay statement
118 initial-delay time;
120 initial-delay parameter sets the maximum time client can wait after
122 start before commencing first transmission. According to RFC2131 Sec‐
123 tion 4.4.1, client should wait a random time between startup and the
124 actual first transmission. Previous versions of ISC DHCP client used to
125 wait random time up to 5 seconds, but that was unwanted due to impact
126 on startup time. As such, new versions have the default initial delay
127 set to 0. To restore old behavior, please set initial-delay to 5.
128
130 In the DHCPv6 protocol the client will wait a small amount of time to
131 allow ADVERTISE messages from multiple servers to arrive. It will then
132 need to choose from all of the messages that may have arrived before
133 proceeding to making a request of the selected server.
134 The first selection criteria is the set of options and addresses in the
136 message. Messages that don't include an option specified as required
137 will be given a score of 0 and not used. If the -R option is given on
138 the command line then messages that don't include the correct number of
139 bindings (IA-NA, IA-TA or IA-PD) will be discarded.
140 The next criteria is the preference value from the message. With the
142 highest preference value being used even if leases with better
143 addresses or options are available.
144 Finally the lease is scored and the lease with the highest score is
146 selected. A lease's score is based on the number of bindings, number
147 of addresses and number of options it contains:
148 bindings * X + addresses * Y + options
149 By default X = 10000 and Y = 100, this will cause the client to select
150 a lease with more bindings over a lease with less bindings but more
151 addresses. The weightings were changed as part of implementing RFC
152 7550. Previously they were X = 50 and Y = 100 meaning more addresses
153 were preferred over more bindings. If you wish to continue using the
154 old style you may do so by editing the file includes/site.h and uncom‐
155 menting the define for USE_ORIGINAL_CLIENT_LEASE_WEIGHTS.
156
158 The DHCP protocol allows the client to request that the server send it
159 specific information, and not send it other information that it is not
160 prepared to accept. The protocol also allows the client to reject
161 offers from servers if they don't contain information the client needs,
162 or if the information provided is not satisfactory.
163 There is a variety of data contained in offers that DHCP servers send
165 to DHCP clients. The data that can be specifically requested is what
166 are called DHCP Options. DHCP Options are defined in
167 dhcp-options(5).
168 The request statement
170 [ also ] request [ [ option-space . ] option ] [, ... ];
172 The request statement causes the client to request that any server
174 responding to the client send the client its values for the specified
175 options. Only the option names should be specified in the request
176 statement - not option parameters. By default, the DHCPv4 client
177 requests the subnet-mask, broadcast-address, time-offset, routers,
178 domain-search, domain-name, domain-name-servers, host-name, nis-domain,
179 nis-servers, ntp-servers and interface-mtu options while the DHCPv6
180 client requests the dhcp6 name-servers and domain-search options. Note
181 that if you enter a ´request´ statement, you over-ride these defaults
182 and these options will not be requested.
183 In some cases, it may be desirable to send no parameter request list at
185 all. To do this, simply write the request statement but specify no
186 parameters:
187 request;
189 In most cases, it is desirable to simply add one option to the request
191 list which is of interest to the client in question. In this case, it
192 is best to ´also request´ the additional options:
193 also request domain-search, dhcp6.sip-servers-addresses;
195 The require statement
197 [ also ] require [ [ option-space . ] option ] [, ... ];
199 The require statement lists options that must be sent in order for an
201 offer to be accepted. Offers that do not contain all the listed
202 options will be ignored. There is no default require list.
203 require name-servers;
205 interface eth0 {
207 also require domain-search;
208 }
209 NOTE: For ISC DHCP release 4.3.5 and earlier, dhclient running in -6
211 mode does not discard offers as described above. This has been cor‐
212 rected as of release 4.3.6 but must be enabled at compile time (see
213 ENFORCE_DHCPV6_CLIENT_REQUIRE in includes/site.h).
214 The send statement
216 send [ option declaration ] ;
218 The send statement causes the client to send the specified option to
220 the server with the specified value. This is a full option declaration
221 as described in dhcp-options(5). Options that are always sent in the
222 DHCP protocol should not be specified here, except that the client can
223 specify a requested dhcp-lease-time option other than the default
224 requested lease time, which is two hours. The other obvious use for
225 this statement is to send information to the server that will allow it
226 to differentiate between this client and other clients or kinds of
227 clients.
228
230 The client now has some very limited support for doing DNS updates when
231 a lease is acquired. This is prototypical, and probably doesn't do
232 what you want. It also only works if you happen to have control over
233 your DNS server, which isn't very likely.
234 Note that everything in this section is true whether you are using
236 DHCPv4 or DHCPv6. The exact same syntax is used for both.
237 To make it work, you have to declare a key and zone as in the DHCP
239 server (see dhcpd.conf(5) for details). You also need to configure the
240 fqdn option on the client, as follows:
241 send fqdn.fqdn "grosse.example.com.";
243 send fqdn.encoded on;
244 send fqdn.server-update off;
245 also request fqdn, dhcp6.fqdn;
246 The fqdn.fqdn option MUST be a fully-qualified domain name. You MUST
248 define a zone statement for the zone to be updated. The fqdn.encoded
249 option may need to be set to on or off, depending on the DHCP server
250 you are using.
251 The do-forward-updates statement
253 do-forward-updates [ flag ] ;
255 If you want to do DNS updates in the DHCP client script (see dhclient-
257 script(8)) rather than having the DHCP client do the update directly
258 (for example, if you want to use SIG(0) authentication, which is not
259 supported directly by the DHCP client, you can instruct the client not
260 to do the update using the do-forward-updates statement. Flag should
261 be true if you want the DHCP client to do the update, and false if you
262 don't want the DHCP client to do the update. By default, the DHCP
263 client will do the DNS update.
264
266 In some cases, a client may receive option data from the server which
267 is not really appropriate for that client, or may not receive informa‐
268 tion that it needs, and for which a useful default value exists. It
269 may also receive information which is useful, but which needs to be
270 supplemented with local information. To handle these needs, several
271 option modifiers are available.
272 The default statement
274 default [ option declaration ] ;
276 If for some option the client should use the value supplied by the
278 server, but needs to use some default value if no value was supplied by
279 the server, these values can be defined in the default statement.
280 The supersede statement
282 supersede [ option declaration ] ;
284 If for some option the client should always use a locally-configured
286 value or values rather than whatever is supplied by the server, these
287 values can be defined in the supersede statement.
288 The prepend statement
290 prepend [ option declaration ] ;
292 If for some set of options the client should use a value you supply,
294 and then use the values supplied by the server, if any, these values
295 can be defined in the prepend statement. The prepend statement can
296 only be used for options which allow more than one value to be given.
297 This restriction is not enforced - if you ignore it, the behaviour will
298 be unpredictable.
299 The append statement
301 append [ option declaration ] ;
303 If for some set of options the client should first use the values sup‐
305 plied by the server, if any, and then use values you supply, these val‐
306 ues can be defined in the append statement. The append statement can
307 only be used for options which allow more than one value to be given.
308 This restriction is not enforced - if you ignore it, the behaviour will
309 be unpredictable.
310
312 The lease declaration
313 lease { lease-declaration [ ... lease-declaration ] }
315 The DHCP client may decide after some period of time (see PROTOCOL TIM‐
317 ING) that it is not going to succeed in contacting a server. At that
318 time, it consults its own database of old leases and tests each one
319 that has not yet timed out by pinging the listed router for that lease
320 to see if that lease could work. It is possible to define one or more
321 fixed leases in the client configuration file for networks where there
322 is no DHCP or BOOTP service, so that the client can still automatically
323 configure its address. This is done with the lease statement.
324 NOTE: the lease statement is also used in the dhclient.leases file in
326 order to record leases that have been received from DHCP servers. Some
327 of the syntax for leases as described below is only needed in the
328 dhclient.leases file. Such syntax is documented here for completeness.
329 A lease statement consists of the lease keyword, followed by a left
331 curly brace, followed by one or more lease declaration statements, fol‐
332 lowed by a right curly brace. The following lease declarations are
333 possible:
334 bootp;
336 The bootp statement is used to indicate that the lease was acquired
338 using the BOOTP protocol rather than the DHCP protocol. It is never
339 necessary to specify this in the client configuration file. The client
340 uses this syntax in its lease database file.
341 interface "string";
343 The interface lease statement is used to indicate the interface on
345 which the lease is valid. If set, this lease will only be tried on a
346 particular interface. When the client receives a lease from a server,
347 it always records the interface number on which it received that lease.
348 If predefined leases are specified in the dhclient.conf file, the
349 interface should also be specified, although this is not required.
350 fixed-address ip-address;
352 The fixed-address statement is used to set the ip address of a particu‐
354 lar lease. This is required for all lease statements. The IP address
355 must be specified as a dotted quad (e.g., 12.34.56.78).
356 filename "string";
358 The filename statement specifies the name of the boot filename to use.
360 This is not used by the standard client configuration script, but is
361 included for completeness.
362 server-name "string";
364 The server-name statement specifies the name of the boot server name to
366 use. This is also not used by the standard client configuration
367 script.
368 option option-declaration;
370 The option statement is used to specify the value of an option supplied
372 by the server, or, in the case of predefined leases declared in
373 dhclient.conf, the value that the user wishes the client configuration
374 script to use if the predefined lease is used.
375 script "script-name";
377 The script statement is used to specify the pathname of the dhcp client
379 configuration script. This script is used by the dhcp client to set
380 each interface's initial configuration prior to requesting an address,
381 to test the address once it has been offered, and to set the inter‐
382 face's final configuration once a lease has been acquired. If no lease
383 is acquired, the script is used to test predefined leases, if any, and
384 also called once if no valid lease can be identified. For more infor‐
385 mation, see dhclient-script(8).
386 vendor option space "name";
388 The vendor option space statement is used to specify which option space
390 should be used for decoding the vendor-encapsulate-options option if
391 one is received. The dhcp-vendor-identifier can be used to request a
392 specific class of vendor options from the server. See dhcp-options(5)
393 for details.
394 medium "media setup";
396 The medium statement can be used on systems where network interfaces
398 cannot automatically determine the type of network to which they are
399 connected. The media setup string is a system-dependent parameter
400 which is passed to the dhcp client configuration script when initializ‐
401 ing the interface. On Unix and Unix-like systems, the argument is
402 passed on the ifconfig command line when configuring the interface.
403 The dhcp client automatically declares this parameter if it uses a
405 media type (see the media statement) when configuring the interface in
406 order to obtain a lease. This statement should be used in predefined
407 leases only if the network interface requires media type configuration.
408 renew date;
410 rebind date;
412 expire date;
414 The renew statement defines the time at which the dhcp client should
416 begin trying to contact its server to renew a lease that it is using.
417 The rebind statement defines the time at which the dhcp client should
418 begin to try to contact any dhcp server in order to renew its lease.
419 The expire statement defines the time at which the dhcp client must
420 stop using a lease if it has not been able to contact a server in order
421 to renew it.
422 These declarations are automatically set in leases acquired by the DHCP
424 client, but must also be configured in predefined leases - a predefined
425 lease whose expiry time has passed will not be used by the DHCP client.
426 Dates are specified in one of two ways. The software will output times
428 in these two formats depending on if the db-time-format configuration
429 parameter has been set to default or local.
430 If it is set to default, then date values appear as follows:
432 <weekday> <year>/<month>/<day> <hour>:<minute>:<second>
434 The weekday is present to make it easy for a human to tell when a lease
436 expires - it's specified as a number from zero to six, with zero being
437 Sunday. When declaring a predefined lease, it can always be specified
438 as zero. The year is specified with the century, so it should gener‐
439 ally be four digits except for really long leases. The month is speci‐
440 fied as a number starting with 1 for January. The day of the month is
441 likewise specified starting with 1. The hour is a number between 0 and
442 23, the minute a number between 0 and 59, and the second also a number
443 between 0 and 59.
444 If the db-time-format configuration was set to local, then the date
446 values appear as follows:
447 epoch <seconds-since-epoch>; # <day-name> <month-name> <day-number>
449 <hours>:<minutes>:<seconds> <year>
450 The seconds-since-epoch is as according to the system's local clock
452 (often referred to as "unix time"). The # symbol supplies a comment
453 that describes what actual time this is as according to the system's
454 configured timezone, at the time the value was written. It is provided
455 only for human inspection, the epoch time is the only recommended value
456 for machine inspection.
457 Note that when defining a static lease, one may use either time format
459 one wishes, and need not include the comment or values after it.
460 If the time is infinite in duration, then the date is never instead of
462 an actual date.
463
465 alias { declarations ... }
466 Some DHCP clients running TCP/IP roaming protocols may require that in
468 addition to the lease they may acquire via DHCP, their interface also
469 be configured with a predefined IP alias so that they can have a perma‐
470 nent IP address even while roaming. The Internet Systems Consortium
471 DHCP client doesn't support roaming with fixed addresses directly, but
472 in order to facilitate such experimentation, the dhcp client can be set
473 up to configure an IP alias using the alias declaration.
474 The alias declaration resembles a lease declaration, except that
476 options other than the subnet-mask option are ignored by the standard
477 client configuration script, and expiry times are ignored. A typical
478 alias declaration includes an interface declaration, a fixed-address
479 declaration for the IP alias address, and a subnet-mask option declara‐
480 tion. A medium statement should never be included in an alias declara‐
481 tion.
482
484 db-time-format [ default | local ] ;
485 The db-time-format option determines which of two output methods are
487 used for printing times in leases files. The default format provides
488 day-and-time in UTC, whereas local uses a seconds-since-epoch to store
489 the time value, and helpfully places a local timezone time in a comment
490 on the same line. The formats are described in detail in this manpage,
491 within the LEASE DECLARATIONS section.
492 The lease-id-format parameter
494 lease-id-format format;
496 The format parameter must be either octal or hex. This parameter
498 governs the format used to write certain values to lease files. With
499 the default format, octal, values are written as quoted strings in
500 which non-printable characters are represented as octal escapes - a
501 backslash character followed by three octal digits. When the hex
502 format is specified, values are written as an unquoted series of
503 hexadecimal digit pairs, separated by colons.
504 Currently, the values written out based on lease-id-format are the
506 default-duid and the IAID value (DHCPv6 only). The client automati‐
507 cally reads the values in either format. Note that when the format
508 is octal, rather than as an octal string, IAID is output as hex if it
509 contains no printable characters or as a string if contains only
510 printable characters. This is done to maintain backward compatibil‐
511 ity.
512 reject cidr-ip-address [, ... cidr-ip-address ] ;
514 The reject statement causes the DHCP client to reject offers from
516 servers whose server identifier matches any of the specified hosts or
517 subnets. This can be used to avoid being configured by rogue or mis‐
518 configured dhcp servers, although it should be a last resort - better
519 to track down the bad DHCP server and fix it.
520 The cidr-ip-address configuration type is of the form ip-
522 address[/prefixlen], where ip-address is a dotted quad IP address,
523 and prefixlen is the CIDR prefix length of the subnet, counting the
524 number of significant bits in the netmask starting from the leftmost
525 end. Example configuration syntax:
526 reject 192.168.0.0/16, 10.0.0.5;
528 The above example would cause offers from any server identifier in
530 the entire RFC 1918 "Class C" network 192.168.0.0/16, or the specific
531 single address 10.0.0.5, to be rejected.
532 interface "name" { declarations ... }
534 A client with more than one network interface may require different
536 behaviour depending on which interface is being configured. All tim‐
537 ing parameters and declarations other than lease and alias declara‐
538 tions can be enclosed in an interface declaration, and those parame‐
539 ters will then be used only for the interface that matches the speci‐
540 fied name. Interfaces for which there is no interface declaration
541 will use the parameters declared outside of any interface declara‐
542 tion, or the default settings.
543 Note well: ISC dhclient only maintains one list of interfaces, which
545 is either determined at startup from command line arguments, or oth‐
546 erwise is autodetected. If you supplied the list of interfaces on
547 the command line, this configuration clause will add the named inter‐
548 face to the list in such a way that will cause it to be configured by
549 DHCP. Which may not be the result you had intended. This is an
550 undesirable side effect that will be addressed in a future release.
551 pseudo "name" "real-name" { declarations ... }
553 Under some circumstances it can be useful to declare a pseudo-inter‐
555 face and have the DHCP client acquire a configuration for that inter‐
556 face. Each interface that the DHCP client is supporting normally has
557 a DHCP client state machine running on it to acquire and maintain its
558 lease. A pseudo-interface is just another state machine running on
559 the interface named real-name, with its own lease and its own state.
560 If you use this feature, you must provide a client identifier for
561 both the pseudo-interface and the actual interface, and the two iden‐
562 tifiers must be different. You must also provide a separate client
563 script for the pseudo-interface to do what you want with the IP
564 address. For example:
565 interface "ep0" {
567 send dhcp-client-identifier "my-client-ep0";
568 }
569 pseudo "secondary" "ep0" {
570 send dhcp-client-identifier "my-client-ep0-secondary";
571 script "/etc/dhclient-secondary";
572 }
573 The client script for the pseudo-interface should not configure the
575 interface up or down - essentially, all it needs to handle are the
576 states where a lease has been acquired or renewed, and the states
577 where a lease has expired. See dhclient-script(8) for more informa‐
578 tion.
579 media "media setup" [ , "media setup", ... ];
581 The media statement defines one or more media configuration parame‐
583 ters which may be tried while attempting to acquire an IP address.
584 The dhcp client will cycle through each media setup string on the
585 list, configuring the interface using that setup and attempting to
586 boot, and then trying the next one. This can be used for network
587 interfaces which aren't capable of sensing the media type unaided -
588 whichever media type succeeds in getting a request to the server and
589 hearing the reply is probably right (no guarantees).
590 The media setup is only used for the initial phase of address acqui‐
592 sition (the DHCPDISCOVER and DHCPOFFER packets). Once an address has
593 been acquired, the dhcp client will record it in its lease database
594 and will record the media type used to acquire the address. Whenever
595 the client tries to renew the lease, it will use that same media
596 type. The lease must expire before the client will go back to
597 cycling through media types.
598 hardware link-type mac-address;
600 The hardware statement defines the hardware MAC address to use for
602 this interface, for DHCP servers or relays to direct their replies.
603 dhclient will determine the interface's MAC address automatically, so
604 use of this parameter is not recommended. The link-type corresponds
605 to the interface's link layer type (example: ´ethernet´), while the
606 mac-address is a string of colon-separated hexadecimal values for
607 octets.
608 anycast-mac link-type mac-address;
610 The anycast-mac statement over-rides the all-ones broadcast MAC
612 address dhclient will use when it is transmitting packets to the all-
613 ones limited broadcast IPv4 address. This configuration parameter is
614 useful to reduce the number of broadcast packets transmitted by DHCP
615 clients, but is only useful if you know the DHCP service(s) anycast
616 MAC address prior to configuring your client. The link-type and mac-
617 address parameters are configured in a similar manner to the hardware
618 statement.
619 bootp-broadcast-always;
621 The bootp-broadcast-always statement instructs dhclient to always set
623 the bootp broadcast flag in request packets, so that servers will
624 always broadcast replies. This is equivalent to supplying the
625 dhclient -B argument, and has the same effect as specifying 'always-
626 broadcast' in the server's dhcpd.conf. This option is provided as an
627 extension to enable dhclient to work on IBM s390 Linux guests.
628
630 The following configuration file was used on a laptop running NetBSD
631 1.3, though the domains have been modified. The laptop has an IP alias
632 of 192.5.5.213, and has one interface, ep0 (a 3com 3C589C). Booting
633 intervals have been shortened somewhat from the default, because the
634 client is known to spend most of its time on networks with little DHCP
635 activity. The laptop does roam to multiple networks.
636 timeout 60;
639 retry 60;
640 reboot 10;
641 select-timeout 5;
642 initial-interval 2;
643 reject 192.33.137.209;
644 interface "ep0" {
646 send host-name "andare.example.com";
647 hardware ethernet 00:a0:24:ab:fb:9c;
648 send dhcp-client-identifier 1:0:a0:24:ab:fb:9c;
649 send dhcp-lease-time 3600;
650 supersede domain-search "example.com", "rc.isc.org", "home.isc.org";
651 prepend domain-name-servers 127.0.0.1;
652 request subnet-mask, broadcast-address, time-offset, routers,
653 domain-name, domain-name-servers, host-name;
654 require subnet-mask, domain-name-servers;
655 script "/usr/sbin/dhclient-script";
656 media "media 10baseT/UTP", "media 10base2/BNC";
657 }
658 alias {
660 interface "ep0";
661 fixed-address 192.5.5.213;
662 option subnet-mask 255.255.255.255;
663 }
664 This is a very complicated dhclient.conf file - in general, yours
665 should be much simpler. In many cases, it's sufficient to just create
666 an empty dhclient.conf file - the defaults are usually fine.
667
669 dhcp-options(5), dhcp-eval(5), dhclient.leases(5), dhcpd(8),
670 dhcpd.conf(5), RFC2132, RFC2131.
671
673 dhclient(8) Information about Internet Systems Consortium can be found
674 at https://www.isc.org.
675 dhclient.conf(5)