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 re‐
21 quested 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 be‐
43 tween 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 ad‐
87 dress 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 ad‐
91 dress before it gives up and tries to discover a new address. By de‐
92 fault, 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 al‐
102 lowed to back off, the actual value will be evaluated randomly between
103 1/2 to 1 1/2 times the time specified. It defaults to fifteen seconds.
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 The initial-delay statement
117 initial-delay time;
119 initial-delay parameter sets the maximum time client can wait after
121 start before commencing first transmission. According to RFC2131 Sec‐
122 tion 4.4.1, client should wait a random time between startup and the
123 actual first transmission. Previous versions of ISC DHCP client used to
124 wait random time up to 5 seconds, but that was unwanted due to impact
125 on startup time. As such, new versions have the default initial delay
126 set to 0. To restore old behavior, please set initial-delay to 5.
127
129 In the DHCPv6 protocol the client will wait a small amount of time to
130 allow ADVERTISE messages from multiple servers to arrive. It will then
131 need to choose from all of the messages that may have arrived before
132 proceeding to making a request of the selected server.
133 The first selection criteria is the set of options and addresses in the
135 message. Messages that don't include an option specified as required
136 will be given a score of 0 and not used. If the -R option is given on
137 the command line then messages that don't include the correct number of
138 bindings (IA-NA, IA-TA or IA-PD) will be discarded.
139 The next criteria is the preference value from the message. With the
141 highest preference value being used even if leases with better ad‐
142 dresses or options are available.
143 Finally the lease is scored and the lease with the highest score is se‐
145 lected. A lease's score is based on the number of bindings, number of
146 addresses and number of options it contains:
147 bindings * X + addresses * Y + options
148 By default X = 10000 and Y = 100, this will cause the client to select
149 a lease with more bindings over a lease with less bindings but more ad‐
150 dresses. The weightings were changed as part of implementing RFC 7550.
151 Previously they were X = 50 and Y = 100 meaning more addresses were
152 preferred over more bindings. If you wish to continue using the old
153 style you may do so by editing the file includes/site.h and uncomment‐
154 ing the define for USE_ORIGINAL_CLIENT_LEASE_WEIGHTS.
155
157 The DHCP protocol allows the client to request that the server send it
158 specific information, and not send it other information that it is not
159 prepared to accept. The protocol also allows the client to reject of‐
160 fers from servers if they don't contain information the client needs,
161 or if the information provided is not satisfactory.
162 There is a variety of data contained in offers that DHCP servers send
164 to DHCP clients. The data that can be specifically requested is what
165 are called DHCP Options. DHCP Options are defined in
166 dhcp-options(5).
167 The request statement
169 [ also ] request [ [ option-space . ] option ] [, ... ];
171 The request statement causes the client to request that any server re‐
173 sponding to the client send the client its values for the specified op‐
174 tions. Only the option names should be specified in the request state‐
175 ment - not option parameters. By default, the DHCPv4 client requests
176 the subnet-mask, broadcast-address, time-offset, routers, domain-
177 search, domain-name, domain-name-servers, host-name, nis-domain, nis-
178 servers, ntp-servers and interface-mtu options while the DHCPv6 client
179 requests the dhcp6 name-servers and domain-search options. Note that
180 if you enter a ´request´ statement, you over-ride these defaults and
181 these options will not be requested.
182 In some cases, it may be desirable to send no parameter request list at
184 all. To do this, simply write the request statement but specify no pa‐
185 rameters:
186 request;
188 In most cases, it is desirable to simply add one option to the request
190 list which is of interest to the client in question. In this case, it
191 is best to ´also request´ the additional options:
192 also request domain-search, dhcp6.sip-servers-addresses;
194 The require statement
196 [ also ] require [ [ option-space . ] option ] [, ... ];
198 The require statement lists options that must be sent in order for an
200 offer to be accepted. Offers that do not contain all the listed op‐
201 tions will be ignored. There is no default require list.
202 require name-servers;
204 interface eth0 {
206 also require domain-search;
207 }
208 The send statement
210 send [ option declaration ] ;
212 The send statement causes the client to send the specified option to
214 the server with the specified value. This is a full option declaration
215 as described in dhcp-options(5). Options that are always sent in the
216 DHCP protocol should not be specified here, except that the client can
217 specify a requested dhcp-lease-time option other than the default re‐
218 quested lease time, which is two hours. The other obvious use for this
219 statement is to send information to the server that will allow it to
220 differentiate between this client and other clients or kinds of
221 clients.
222
224 The client now has some very limited support for doing DNS updates when
225 a lease is acquired. This is prototypical, and probably doesn't do
226 what you want. It also only works if you happen to have control over
227 your DNS server, which isn't very likely.
228 Note that everything in this section is true whether you are using
230 DHCPv4 or DHCPv6. The exact same syntax is used for both.
231 To make it work, you have to declare a key and zone as in the DHCP
233 server (see dhcpd.conf(5) for details). You also need to configure the
234 fqdn option on the client, as follows:
235 send fqdn.fqdn "grosse.example.com.";
237 send fqdn.encoded on;
238 send fqdn.server-update off;
239 also request fqdn, dhcp6.fqdn;
240 The fqdn.fqdn option MUST be a fully-qualified domain name. You MUST
242 define a zone statement for the zone to be updated. The fqdn.encoded
243 option may need to be set to on or off, depending on the DHCP server
244 you are using.
245 The do-forward-updates statement
247 do-forward-updates [ flag ] ;
249 If you want to do DNS updates in the DHCP client script (see dhclient-
251 script(8)) rather than having the DHCP client do the update directly
252 (for example, if you want to use SIG(0) authentication, which is not
253 supported directly by the DHCP client, you can instruct the client not
254 to do the update using the do-forward-updates statement. Flag should
255 be true if you want the DHCP client to do the update, and false if you
256 don't want the DHCP client to do the update. By default, the DHCP
257 client will do the DNS update.
258
260 In some cases, a client may receive option data from the server which
261 is not really appropriate for that client, or may not receive informa‐
262 tion that it needs, and for which a useful default value exists. It
263 may also receive information which is useful, but which needs to be
264 supplemented with local information. To handle these needs, several
265 option modifiers are available.
266 The default statement
268 default [ option declaration ] ;
270 If for some option the client should use the value supplied by the
272 server, but needs to use some default value if no value was supplied by
273 the server, these values can be defined in the default statement.
274 The supersede statement
276 supersede [ option declaration ] ;
278 If for some option the client should always use a locally-configured
280 value or values rather than whatever is supplied by the server, these
281 values can be defined in the supersede statement.
282 The prepend statement
284 prepend [ option declaration ] ;
286 If for some set of options the client should use a value you supply,
288 and then use the values supplied by the server, if any, these values
289 can be defined in the prepend statement. The prepend statement can
290 only be used for options which allow more than one value to be given.
291 This restriction is not enforced - if you ignore it, the behaviour will
292 be unpredictable.
293 The append statement
295 append [ option declaration ] ;
297 If for some set of options the client should first use the values sup‐
299 plied by the server, if any, and then use values you supply, these val‐
300 ues can be defined in the append statement. The append statement can
301 only be used for options which allow more than one value to be given.
302 This restriction is not enforced - if you ignore it, the behaviour will
303 be unpredictable.
304
306 The lease declaration
307 lease { lease-declaration [ ... lease-declaration ] }
309 The DHCP client may decide after some period of time (see PROTOCOL TIM‐
311 ING) that it is not going to succeed in contacting a server. At that
312 time, it consults its own database of old leases and tests each one
313 that has not yet timed out by pinging the listed router for that lease
314 to see if that lease could work. It is possible to define one or more
315 fixed leases in the client configuration file for networks where there
316 is no DHCP or BOOTP service, so that the client can still automatically
317 configure its address. This is done with the lease statement.
318 NOTE: the lease statement is also used in the dhclient.leases file in
320 order to record leases that have been received from DHCP servers. Some
321 of the syntax for leases as described below is only needed in the
322 dhclient.leases file. Such syntax is documented here for completeness.
323 A lease statement consists of the lease keyword, followed by a left
325 curly brace, followed by one or more lease declaration statements, fol‐
326 lowed by a right curly brace. The following lease declarations are
327 possible:
328 bootp;
330 The bootp statement is used to indicate that the lease was acquired us‐
332 ing the BOOTP protocol rather than the DHCP protocol. It is never nec‐
333 essary to specify this in the client configuration file. The client
334 uses this syntax in its lease database file.
335 interface "string";
337 The interface lease statement is used to indicate the interface on
339 which the lease is valid. If set, this lease will only be tried on a
340 particular interface. When the client receives a lease from a server,
341 it always records the interface number on which it received that lease.
342 If predefined leases are specified in the dhclient.conf file, the in‐
343 terface should also be specified, although this is not required.
344 fixed-address ip-address;
346 The fixed-address statement is used to set the ip address of a particu‐
348 lar lease. This is required for all lease statements. The IP address
349 must be specified as a dotted quad (e.g., 12.34.56.78).
350 filename "string";
352 The filename statement specifies the name of the boot filename to use.
354 This is not used by the standard client configuration script, but is
355 included for completeness.
356 server-name "string";
358 The server-name statement specifies the name of the boot server name to
360 use. This is also not used by the standard client configuration
361 script.
362 option option-declaration;
364 The option statement is used to specify the value of an option supplied
366 by the server, or, in the case of predefined leases declared in
367 dhclient.conf, the value that the user wishes the client configuration
368 script to use if the predefined lease is used.
369 script "script-name";
371 The script statement is used to specify the pathname of the dhcp client
373 configuration script. This script is used by the dhcp client to set
374 each interface's initial configuration prior to requesting an address,
375 to test the address once it has been offered, and to set the inter‐
376 face's final configuration once a lease has been acquired. If no lease
377 is acquired, the script is used to test predefined leases, if any, and
378 also called once if no valid lease can be identified. For more infor‐
379 mation, see dhclient-script(8).
380 vendor option space "name";
382 The vendor option space statement is used to specify which option space
384 should be used for decoding the vendor-encapsulate-options option if
385 one is received. The dhcp-vendor-identifier can be used to request a
386 specific class of vendor options from the server. See dhcp-options(5)
387 for details.
388 medium "media setup";
390 The medium statement can be used on systems where network interfaces
392 cannot automatically determine the type of network to which they are
393 connected. The media setup string is a system-dependent parameter
394 which is passed to the dhcp client configuration script when initializ‐
395 ing the interface. On Unix and Unix-like systems, the argument is
396 passed on the ifconfig command line when configuring the interface.
397 The dhcp client automatically declares this parameter if it uses a me‐
399 dia type (see the media statement) when configuring the interface in
400 order to obtain a lease. This statement should be used in predefined
401 leases only if the network interface requires media type configuration.
402 renew date;
404 rebind date;
406 expire date;
408 The renew statement defines the time at which the dhcp client should
410 begin trying to contact its server to renew a lease that it is using.
411 The rebind statement defines the time at which the dhcp client should
412 begin to try to contact any dhcp server in order to renew its lease.
413 The expire statement defines the time at which the dhcp client must
414 stop using a lease if it has not been able to contact a server in order
415 to renew it.
416 These declarations are automatically set in leases acquired by the DHCP
418 client, but must also be configured in predefined leases - a predefined
419 lease whose expiry time has passed will not be used by the DHCP client.
420 Dates are specified in one of two ways. The software will output times
422 in these two formats depending on if the db-time-format configuration
423 parameter has been set to default or local.
424 If it is set to default, then date values appear as follows:
426 <weekday> <year>/<month>/<day> <hour>:<minute>:<second>
428 The weekday is present to make it easy for a human to tell when a lease
430 expires - it's specified as a number from zero to six, with zero being
431 Sunday. When declaring a predefined lease, it can always be specified
432 as zero. The year is specified with the century, so it should gener‐
433 ally be four digits except for really long leases. The month is speci‐
434 fied as a number starting with 1 for January. The day of the month is
435 likewise specified starting with 1. The hour is a number between 0 and
436 23, the minute a number between 0 and 59, and the second also a number
437 between 0 and 59.
438 If the db-time-format configuration was set to local, then the date
440 values appear as follows:
441 epoch <seconds-since-epoch>; # <day-name> <month-name> <day-number>
443 <hours>:<minutes>:<seconds> <year>
444 The seconds-since-epoch is as according to the system's local clock
446 (often referred to as "unix time"). The # symbol supplies a comment
447 that describes what actual time this is as according to the system's
448 configured timezone, at the time the value was written. It is provided
449 only for human inspection, the epoch time is the only recommended value
450 for machine inspection.
451 Note that when defining a static lease, one may use either time format
453 one wishes, and need not include the comment or values after it.
454 If the time is infinite in duration, then the date is never instead of
456 an actual date.
457
459 alias { declarations ... }
460 Some DHCP clients running TCP/IP roaming protocols may require that in
462 addition to the lease they may acquire via DHCP, their interface also
463 be configured with a predefined IP alias so that they can have a perma‐
464 nent IP address even while roaming. The Internet Systems Consortium
465 DHCP client doesn't support roaming with fixed addresses directly, but
466 in order to facilitate such experimentation, the dhcp client can be set
467 up to configure an IP alias using the alias declaration.
468 The alias declaration resembles a lease declaration, except that op‐
470 tions other than the subnet-mask option are ignored by the standard
471 client configuration script, and expiry times are ignored. A typical
472 alias declaration includes an interface declaration, a fixed-address
473 declaration for the IP alias address, and a subnet-mask option declara‐
474 tion. A medium statement should never be included in an alias declara‐
475 tion.
476
478 db-time-format [ default | local ] ;
479 The db-time-format option determines which of two output methods are
481 used for printing times in leases files. The default format provides
482 day-and-time in UTC, whereas local uses a seconds-since-epoch to store
483 the time value, and helpfully places a local timezone time in a comment
484 on the same line. The formats are described in detail in this manpage,
485 within the LEASE DECLARATIONS section.
486 The lease-id-format parameter
488 lease-id-format format;
490 The format parameter must be either octal or hex. This parameter
492 governs the format used to write certain values to lease files. With
493 the default format, octal, values are written as quoted strings in
494 which non-printable characters are represented as octal escapes - a
495 backslash character followed by three octal digits. When the hex
496 format is specified, values are written as an unquoted series of
497 hexadecimal digit pairs, separated by colons.
498 Currently, the values written out based on lease-id-format are the
500 default-duid and the IAID value (DHCPv6 only). The client automati‐
501 cally reads the values in either format. Note that when the format
502 is octal, rather than as an octal string, IAID is output as hex if it
503 contains no printable characters or as a string if contains only
504 printable characters. This is done to maintain backward compatibil‐
505 ity.
506 reject cidr-ip-address [, ... cidr-ip-address ] ;
508 The reject statement causes the DHCP client to reject offers from
510 servers whose server identifier matches any of the specified hosts or
511 subnets. This can be used to avoid being configured by rogue or mis‐
512 configured dhcp servers, although it should be a last resort - better
513 to track down the bad DHCP server and fix it.
514 The cidr-ip-address configuration type is of the form ip-ad‐
516 dress[/prefixlen], where ip-address is a dotted quad IP address, and
517 prefixlen is the CIDR prefix length of the subnet, counting the num‐
518 ber of significant bits in the netmask starting from the leftmost
519 end. Example configuration syntax:
520 reject 192.168.0.0/16, 10.0.0.5;
522 The above example would cause offers from any server identifier in
524 the entire RFC 1918 "Class C" network 192.168.0.0/16, or the specific
525 single address 10.0.0.5, to be rejected.
526 interface "name" { declarations ... }
528 A client with more than one network interface may require different
530 behaviour depending on which interface is being configured. All tim‐
531 ing parameters and declarations other than lease and alias declara‐
532 tions can be enclosed in an interface declaration, and those parame‐
533 ters will then be used only for the interface that matches the speci‐
534 fied name. Interfaces for which there is no interface declaration
535 will use the parameters declared outside of any interface declara‐
536 tion, or the default settings.
537 Note well: ISC dhclient only maintains one list of interfaces, which
539 is either determined at startup from command line arguments, or oth‐
540 erwise is autodetected. If you supplied the list of interfaces on
541 the command line, this configuration clause will add the named inter‐
542 face to the list in such a way that will cause it to be configured by
543 DHCP. Which may not be the result you had intended. This is an un‐
544 desirable side effect that will be addressed in a future release.
545 pseudo "name" "real-name" { declarations ... }
547 Under some circumstances it can be useful to declare a pseudo-inter‐
549 face and have the DHCP client acquire a configuration for that inter‐
550 face. Each interface that the DHCP client is supporting normally has
551 a DHCP client state machine running on it to acquire and maintain its
552 lease. A pseudo-interface is just another state machine running on
553 the interface named real-name, with its own lease and its own state.
554 If you use this feature, you must provide a client identifier for
555 both the pseudo-interface and the actual interface, and the two iden‐
556 tifiers must be different. You must also provide a separate client
557 script for the pseudo-interface to do what you want with the IP ad‐
558 dress. For example:
559 interface "ep0" {
561 send dhcp-client-identifier "my-client-ep0";
562 }
563 pseudo "secondary" "ep0" {
564 send dhcp-client-identifier "my-client-ep0-secondary";
565 script "/etc/dhclient-secondary";
566 }
567 The client script for the pseudo-interface should not configure the
569 interface up or down - essentially, all it needs to handle are the
570 states where a lease has been acquired or renewed, and the states
571 where a lease has expired. See dhclient-script(8) for more informa‐
572 tion.
573 media "media setup" [ , "media setup", ... ];
575 The media statement defines one or more media configuration parame‐
577 ters which may be tried while attempting to acquire an IP address.
578 The dhcp client will cycle through each media setup string on the
579 list, configuring the interface using that setup and attempting to
580 boot, and then trying the next one. This can be used for network in‐
581 terfaces which aren't capable of sensing the media type unaided -
582 whichever media type succeeds in getting a request to the server and
583 hearing the reply is probably right (no guarantees).
584 The media setup is only used for the initial phase of address acqui‐
586 sition (the DHCPDISCOVER and DHCPOFFER packets). Once an address has
587 been acquired, the dhcp client will record it in its lease database
588 and will record the media type used to acquire the address. Whenever
589 the client tries to renew the lease, it will use that same media
590 type. The lease must expire before the client will go back to cy‐
591 cling through media types.
592 hardware link-type mac-address;
594 The hardware statement defines the hardware MAC address to use for
596 this interface, for DHCP servers or relays to direct their replies.
597 dhclient will determine the interface's MAC address automatically, so
598 use of this parameter is not recommended. The link-type corresponds
599 to the interface's link layer type (example: ´ethernet´), while the
600 mac-address is a string of colon-separated hexadecimal values for
601 octets.
602 anycast-mac link-type mac-address;
604 The anycast-mac statement over-rides the all-ones broadcast MAC ad‐
606 dress dhclient will use when it is transmitting packets to the all-
607 ones limited broadcast IPv4 address. This configuration parameter is
608 useful to reduce the number of broadcast packets transmitted by DHCP
609 clients, but is only useful if you know the DHCP service(s) anycast
610 MAC address prior to configuring your client. The link-type and mac-
611 address parameters are configured in a similar manner to the hardware
612 statement.
613 bootp-broadcast-always;
615 The bootp-broadcast-always statement instructs dhclient to always set
617 the bootp broadcast flag in request packets, so that servers will al‐
618 ways broadcast replies. This is equivalent to supplying the dhclient
619 -B argument, and has the same effect as specifying 'always-broadcast'
620 in the server's dhcpd.conf. This option is provided as an extension
621 to enable dhclient to work on IBM s390 Linux guests.
622
624 The following configuration file was used on a laptop running NetBSD
625 1.3, though the domains have been modified. The laptop has an IP alias
626 of 192.5.5.213, and has one interface, ep0 (a 3com 3C589C). Booting
627 intervals have been shortened somewhat from the default, because the
628 client is known to spend most of its time on networks with little DHCP
629 activity. The laptop does roam to multiple networks.
630 timeout 60;
633 retry 60;
634 reboot 10;
635 select-timeout 5;
636 initial-interval 2;
637 reject 192.33.137.209;
638 interface "ep0" {
640 send host-name "andare.example.com";
641 hardware ethernet 00:a0:24:ab:fb:9c;
642 send dhcp-client-identifier 1:0:a0:24:ab:fb:9c;
643 send dhcp-lease-time 3600;
644 supersede domain-search "example.com", "rc.isc.org", "home.isc.org";
645 prepend domain-name-servers 127.0.0.1;
646 request subnet-mask, broadcast-address, time-offset, routers,
647 domain-name, domain-name-servers, host-name;
648 require subnet-mask, domain-name-servers;
649 script "/usr/sbin/dhclient-script";
650 media "media 10baseT/UTP", "media 10base2/BNC";
651 }
652 alias {
654 interface "ep0";
655 fixed-address 192.5.5.213;
656 option subnet-mask 255.255.255.255;
657 }
658 This is a very complicated dhclient.conf file - in general, yours
659 should be much simpler. In many cases, it's sufficient to just create
660 an empty dhclient.conf file - the defaults are usually fine.
661
663 dhcp-options(5), dhcp-eval(5), dhclient.leases(5), dhcpd(8),
664 dhcpd.conf(5), RFC2132, RFC2131.
665
667 dhclient(8) Information about Internet Systems Consortium can be found
668 at https://www.isc.org.
669 dhclient.conf(5)