1dhcp-options(5) File Formats Manual dhcp-options(5)
2
6 dhcp-options - Dynamic Host Configuration Protocol options
7
9 The Dynamic Host Configuration protocol allows the client to receive
10 options from the DHCP server describing the network configuration and
11 various services that are available on the network. When configuring
12 dhcpd(8) or dhclient(8) , options must often be declared. The syntax
13 for declaring options, and the names and formats of the options that
14 can be declared, are documented here.
15
17 DHCP option statements always start with the option keyword, followed
18 by an option name, followed by option data. The option names and data
19 formats are described below. It is not necessary to exhaustively spec‐
20 ify all DHCP options - only those options which are needed by clients
21 must be specified.
22 Option data comes in a variety of formats, as defined below:
24 The ip-address data type can be entered either as an explicit IP
26 address (e.g., 239.254.197.10) or as a domain name (e.g., haa‐
27 gen.isc.org). When entering a domain name, be sure that that domain
28 name resolves to a single IP address.
29 The ip6-address data specifies an IPv6 address, like ::1 or
31 3ffe:bbbb:aaaa:aaaa::1.
32 The int32 data type specifies a signed 32-bit integer. The uint32 data
34 type specifies an unsigned 32-bit integer. The int16 and uint16 data
35 types specify signed and unsigned 16-bit integers. The int8 and uint8
36 data types specify signed and unsigned 8-bit integers. Unsigned 8-bit
37 integers are also sometimes referred to as octets.
38 The text data type specifies an NVT ASCII string, which must be
40 enclosed in double quotes - for example, to specify a root-path option,
41 the syntax would be
42 option root-path "10.0.1.4:/var/tmp/rootfs";
44 The domain-name data type specifies a domain name, which must not be
46 enclosed in double quotes. This data type is not used for any existing
47 DHCP options. The domain name is stored just as if it were a text
48 option.
49 The domain-list data type specifies a list of domain names, enclosed in
51 double quotes and separated by commas ("example.com", "foo.exam‐
52 ple.com").
53 The flag data type specifies a boolean value. Booleans can be either
55 true or false (or on or off, if that makes more sense to you).
56 The string data type specifies either an NVT ASCII string enclosed in
58 double quotes, or a series of octets specified in hexadecimal, sepa‐
59 rated by colons. For example:
60 option dhcp-client-identifier "CLIENT-FOO";
62 or
63 option dhcp-client-identifier 43:4c:49:45:54:2d:46:4f:4f;
64 The destination-descriptor describe the IP subnet number and subnet
66 mask of a particular destination using a compact encoding. This encod‐
67 ing consists of one octet describing the width of the subnet mask, fol‐
68 lowed by all the significant octets of the subnet number. The follow‐
69 ing table contains some examples of how various subnet number/mask com‐
70 binations can be encoded:
71 Subnet number Subnet mask Destination descriptor
73 0 0 0
74 10.0.0.0 255.0.0.0 8.10
75 10.0.0.0 255.255.255.0 24.10.0.0
76 10.17.0.0 255.255.0.0 16.10.17
77 10.27.129.0 255.255.255.0 24.10.27.129
78 10.229.0.128 255.255.255.128 25.10.229.0.128
79 10.198.122.47 255.255.255.255 32.10.198.122.47
80
82 Sometimes it's helpful to be able to set the value of a DHCP option
83 based on some value that the client has sent. To do this, you can use
84 expression evaluation. The dhcp-eval(5) manual page describes how to
85 write expressions. To assign the result of an evaluation to an option,
86 define the option as follows:
87 option my-option = expression ;
89 For example:
91 option hostname = binary-to-ascii (16, 8, "-",
93 substring (hardware, 1, 6));
94
96 The documentation for the various options mentioned below is taken from
97 the latest IETF draft document on DHCP options. Options not listed
98 below may not yet be implemented, but it is possible to use such
99 options by defining them in the configuration file. Please see the
100 DEFINING NEW OPTIONS heading later in this document for more informa‐
101 tion.
102 Some of the options documented here are automatically generated by the
104 DHCP server or by clients, and cannot be configured by the user. The
105 value of such an option can be used in the configuration file of the
106 receiving DHCP protocol agent (server or client), for example in condi‐
107 tional expressions. However, the value of the option cannot be used in
108 the configuration file of the sending agent, because the value is
109 determined only after the configuration file has been processed. In the
110 following documentation, such options will be shown as "not user con‐
111 figurable"
112 The standard options are:
114 option all-subnets-local flag;
116 This option specifies whether or not the client may assume that all
118 subnets of the IP network to which the client is connected use the
119 same MTU as the subnet of that network to which the client is
120 directly connected. A value of true indicates that all subnets share
121 the same MTU. A value of false means that the client should assume
122 that some subnets of the directly connected network may have smaller
123 MTUs.
124 option arp-cache-timeout uint32;
126 This option specifies the timeout in seconds for ARP cache entries.
128 option bcms-controller-address ip-address [, ip-address... ];
130 This option configures a list of IPv4 addresses for use as Broadcast
132 and Multicast Controller Servers ("BCMS").
133 option bcms-controller-names domain-list;
135 This option contains the domain names of local Broadcast and Multi‐
137 cast Controller Servers ("BCMS") controllers which the client may
138 use.
139 option bootfile-name text;
141 This option is used to identify a bootstrap file. If supported by
143 the client, it should have the same effect as the filename declara‐
144 tion. BOOTP clients are unlikely to support this option. Some DHCP
145 clients will support it, and others actually require it.
146 option boot-size uint16;
148 This option specifies the length in 512-octet blocks of the default
150 boot image for the client.
151 option broadcast-address ip-address;
153 This option specifies the broadcast address in use on the client's
155 subnet. Legal values for broadcast addresses are specified in sec‐
156 tion 3.2.1.3 of STD 3 (RFC1122).
157 option cookie-servers ip-address [, ip-address... ];
159 The cookie server option specifies a list of RFC 865 cookie servers
161 available to the client. Servers should be listed in order of pref‐
162 erence.
163 option default-ip-ttl uint8;
165 This option specifies the default time-to-live that the client should
167 use on outgoing datagrams.
168 option default-tcp-ttl uint8;
170 This option specifies the default TTL that the client should use when
172 sending TCP segments. The minimum value is 1.
173 option default-url string;
175 The format and meaning of this option is not described in any stan‐
177 dards document, but is claimed to be in use by Apple Computer. It is
178 not known what clients may reasonably do if supplied with this
179 option. Use at your own risk.
180 option dhcp-client-identifier string;
182 This option can be used to specify a DHCP client identifier in a host
184 declaration, so that dhcpd can find the host record by matching
185 against the client identifier.
186 Please be aware that some DHCP clients, when configured with client
188 identifiers that are ASCII text, will prepend a zero to the ASCII
189 text. So you may need to write:
190 option dhcp-client-identifier "\0foo";
192 rather than:
194 option dhcp-client-identifier "foo";
196 option dhcp-lease-time uint32;
198 This option is used in a client request (DHCPDISCOVER or DHCPREQUEST)
200 to allow the client to request a lease time for the IP address. In a
201 server reply (DHCPOFFER), a DHCP server uses this option to specify
202 the lease time it is willing to offer.
203 This option is not directly user configurable in the server; refer to
205 the max-lease-time and default-lease-time server options in
206 dhcpd.conf(5).
207 option dhcp-max-message-size uint16;
209 This option, when sent by the client, specifies the maximum size of
211 any response that the server sends to the client. When specified on
212 the server, if the client did not send a dhcp-max-message-size
213 option, the size specified on the server is used. This works for
214 BOOTP as well as DHCP responses.
215 option dhcp-message text;
217 This option is used by a DHCP server to provide an error message to a
219 DHCP client in a DHCPNAK message in the event of a failure. A client
220 may use this option in a DHCPDECLINE message to indicate why the
221 client declined the offered parameters.
222 This option is not user configurable.
224 option dhcp-message-type uint8;
226 This option, sent by both client and server, specifies the type of
228 DHCP message contained in the DHCP packet. Possible values (taken
229 directly from RFC2132) are:
230 1 DHCPDISCOVER
232 2 DHCPOFFER
233 3 DHCPREQUEST
234 4 DHCPDECLINE
235 5 DHCPACK
236 6 DHCPNAK
237 7 DHCPRELEASE
238 8 DHCPINFORM
239 This option is not user configurable.
241 option dhcp-option-overload uint8;
243 This option is used to indicate that the DHCP ´sname´ or ´file´
245 fields are being overloaded by using them to carry DHCP options. A
246 DHCP server inserts this option if the returned parameters will
247 exceed the usual space allotted for options.
248 If this option is present, the client interprets the specified addi‐
250 tional fields after it concludes interpretation of the standard
251 option fields.
252 Legal values for this option are:
254 1 the ´file´ field is used to hold options
256 2 the ´sname´ field is used to hold options
257 3 both fields are used to hold options
258 This option is not user configurable.
260 option dhcp-parameter-request-list uint16 [, uint16... ];
262 This option, when sent by the client, specifies which options the
264 client wishes the server to return. Normally, in the ISC DHCP
265 client, this is done using the request statement. If this option is
266 not specified by the client, the DHCP server will normally return
267 every option that is valid in scope and that fits into the reply.
268 When this option is specified on the server, the server returns the
269 specified options. This can be used to force a client to take
270 options that it hasn't requested, and it can also be used to tailor
271 the response of the DHCP server for clients that may need a more lim‐
272 ited set of options than those the server would normally return.
273 option dhcp-rebinding-time uint32;
275 This option specifies the number of seconds from the time a client
277 gets an address until the client transitions to the REBINDING state.
278 This option is user configurable, but it will be ignored if the value
280 is greater than or equal to the lease time.
281 To make DHCPv4+DHCPv6 migration easier in the future, any value con‐
283 figured in this option is also used as a DHCPv6 "T1" (renew) time.
284 option dhcp-renewal-time uint32;
286 This option specifies the number of seconds from the time a client
288 gets an address until the client transitions to the RENEWING state.
289 This option is user configurable, but it will be ignored if the value
291 is greater than or equal to the rebinding time, or lease time.
292 To make DHCPv4+DHCPv6 migration easier in the future, any value con‐
294 figured in this option is also used as a DHCPv6 "T2" (rebind) time.
295 option dhcp-requested-address ip-address;
297 This option is used by the client in a DHCPDISCOVER to request that a
299 particular IP address be assigned.
300 This option is not user configurable.
302 option dhcp-server-identifier ip-address;
304 This option is used in DHCPOFFER and DHCPREQUEST messages, and may
306 optionally be included in the DHCPACK and DHCPNAK messages. DHCP
307 servers include this option in the DHCPOFFER in order to allow the
308 client to distinguish between lease offers. DHCP clients use the
309 contents of the ´server identifier´ field as the destination address
310 for any DHCP messages unicast to the DHCP server. DHCP clients also
311 indicate which of several lease offers is being accepted by including
312 this option in a DHCPREQUEST message.
313 The value of this option is the IP address of the server.
315 This option is not directly user configurable. See the server-identi‐
317 fier server option in dhcpd.conf(5).
318 option domain-name text;
320 This option specifies the domain name that client should use when
322 resolving hostnames via the Domain Name System.
323 option domain-name-servers ip-address [, ip-address... ];
325 The domain-name-servers option specifies a list of Domain Name System
327 (STD 13, RFC 1035) name servers available to the client. Servers
328 should be listed in order of preference.
329 option domain-search domain-list;
331 The domain-search option specifies a ´search list´ of Domain Names to
333 be used by the client to locate not-fully-qualified domain names.
334 The difference between this option and historic use of the domain-
335 name option for the same ends is that this option is encoded in
336 RFC1035 compressed labels on the wire. For example:
337 option domain-search "example.com", "sales.example.com",
339 "eng.example.com";
340 option extensions-path text;
342 This option specifies the name of a file containing additional
344 options to be interpreted according to the DHCP option format as
345 specified in RFC2132.
346 option finger-server ip-address [, ip-address... ];
348 The Finger server option specifies a list of Finger servers available
350 to the client. Servers should be listed in order of preference.
351 option font-servers ip-address [, ip-address... ];
353 This option specifies a list of X Window System Font servers avail‐
355 able to the client. Servers should be listed in order of preference.
356 option host-name string;
358 This option specifies the name of the client. The name may or may
360 not be qualified with the local domain name (it is preferable to use
361 the domain-name option to specify the domain name). See RFC 1035 for
362 character set restrictions. This option is only honored by dhclient-
363 script(8) if the hostname for the client machine is not set.
364 option ieee802-3-encapsulation flag;
366 This option specifies whether or not the client should use Ethernet
368 Version 2 (RFC 894) or IEEE 802.3 (RFC 1042) encapsulation if the
369 interface is an Ethernet. A value of false indicates that the client
370 should use RFC 894 encapsulation. A value of true means that the
371 client should use RFC 1042 encapsulation.
372 option ien116-name-servers ip-address [, ip-address... ];
374 The ien116-name-servers option specifies a list of IEN 116 name
376 servers available to the client. Servers should be listed in order
377 of preference.
378 option impress-servers ip-address [, ip-address... ];
380 The impress-server option specifies a list of Imagen Impress servers
382 available to the client. Servers should be listed in order of pref‐
383 erence.
384 option interface-mtu uint16;
386 This option specifies the MTU to use on this interface. The minimum
388 legal value for the MTU is 68.
389 option ip-forwarding flag;
391 This option specifies whether the client should configure its IP
393 layer for packet forwarding. A value of false means disable IP for‐
394 warding, and a value of true means enable IP forwarding.
395 option irc-server ip-address [, ip-address... ];
397 The IRC server option specifies a list of IRC servers available to
399 the client. Servers should be listed in order of preference.
400 option log-servers ip-address [, ip-address... ];
402 The log-server option specifies a list of MIT-LCS UDP log servers
404 available to the client. Servers should be listed in order of pref‐
405 erence.
406 option lpr-servers ip-address [, ip-address... ];
408 The LPR server option specifies a list of RFC 1179 line printer
410 servers available to the client. Servers should be listed in order
411 of preference.
412 option mask-supplier flag;
414 This option specifies whether or not the client should respond to
416 subnet mask requests using ICMP. A value of false indicates that the
417 client should not respond. A value of true means that the client
418 should respond.
419 option max-dgram-reassembly uint16;
421 This option specifies the maximum size datagram that the client
423 should be prepared to reassemble. The minimum legal value is 576.
424 option merit-dump text;
426 This option specifies the path-name of a file to which the client's
428 core image should be dumped in the event the client crashes. The
429 path is formatted as a character string consisting of characters from
430 the NVT ASCII character set.
431 option mobile-ip-home-agent ip-address [, ip-address... ];
433 This option specifies a list of IP addresses indicating mobile IP
435 home agents available to the client. Agents should be listed in
436 order of preference, although normally there will be only one such
437 agent.
438 option nds-context string;
440 The nds-context option specifies the name of the initial Netware
442 Directory Service for an NDS client.
443 option nds-servers ip-address [, ip-address... ];
445 The nds-servers option specifies a list of IP addresses of NDS
447 servers.
448 option nds-tree-name string;
450 The nds-tree-name option specifies NDS tree name that the NDS client
452 should use.
453 option netbios-dd-server ip-address [, ip-address... ];
455 The NetBIOS datagram distribution server (NBDD) option specifies a
457 list of RFC 1001/1002 NBDD servers listed in order of preference.
458 option netbios-name-servers ip-address [, ip-address...];
460 The NetBIOS name server (NBNS) option specifies a list of RFC
462 1001/1002 NBNS name servers listed in order of preference. NetBIOS
463 Name Service is currently more commonly referred to as WINS. WINS
464 servers can be specified using the netbios-name-servers option.
465 option netbios-node-type uint8;
467 The NetBIOS node type option allows NetBIOS over TCP/IP clients which
469 are configurable to be configured as described in RFC 1001/1002. The
470 value is specified as a single octet which identifies the client
471 type.
472 Possible node types are:
474 1 B-node: Broadcast - no WINS
476 2 P-node: Peer - WINS only
478 4 M-node: Mixed - broadcast, then WINS
480 8 H-node: Hybrid - WINS, then broadcast
482 option netbios-scope string;
484 The NetBIOS scope option specifies the NetBIOS over TCP/IP scope
486 parameter for the client as specified in RFC 1001/1002. See RFC1001,
487 RFC1002, and RFC1035 for character-set restrictions.
488 option netinfo-server-address ip-address [, ip-address... ];
490 The netinfo-server-address option has not been described in any RFC,
492 but has been allocated (and is claimed to be in use) by Apple Comput‐
493 ers. It's hard to say if the above is the correct format, or what
494 clients might be expected to do if values were configured. Use at
495 your own risk.
496 option netinfo-server-tag text;
498 The netinfo-server-tag option has not been described in any RFC, but
500 has been allocated (and is claimed to be in use) by Apple Computers.
501 It's hard to say if the above is the correct format, or what clients
502 might be expected to do if values were configured. Use at your own
503 risk.
504 option nis-domain text;
506 This option specifies the name of the client's NIS (Sun Network
508 Information Services) domain. The domain is formatted as a character
509 string consisting of characters from the NVT ASCII character set.
510 option nis-servers ip-address [, ip-address... ];
512 This option specifies a list of IP addresses indicating NIS servers
514 available to the client. Servers should be listed in order of pref‐
515 erence.
516 option nisplus-domain text;
518 This option specifies the name of the client's NIS+ domain. The
520 domain is formatted as a character string consisting of characters
521 from the NVT ASCII character set.
522 option nisplus-servers ip-address [, ip-address... ];
524 This option specifies a list of IP addresses indicating NIS+ servers
526 available to the client. Servers should be listed in order of pref‐
527 erence.
528 option nntp-server ip-address [, ip-address... ];
530 The NNTP server option specifies a list of NNTP servesr available to
532 the client. Servers should be listed in order of preference.
533 option non-local-source-routing flag;
535 This option specifies whether the client should configure its IP
537 layer to allow forwarding of datagrams with non-local source routes
538 (see Section 3.3.5 of [4] for a discussion of this topic). A value
539 of false means disallow forwarding of such datagrams, and a value of
540 true means allow forwarding.
541 option ntp-servers ip-address [, ip-address... ];
543 This option specifies a list of IP addresses indicating NTP (RFC
545 1035) servers available to the client. Servers should be listed in
546 order of preference.
547 option nwip-domain string;
549 The name of the NetWare/IP domain that a NetWare/IP client should
551 use.
552 option nwip-suboptions string;
554 A sequence of suboptions for NetWare/IP clients - see RFC2242 for
556 details. Normally this option is set by specifying specific Net‐
557 Ware/IP suboptions - see the NETWARE/IP SUBOPTIONS section for more
558 information.
559 option path-mtu-aging-timeout uint32;
561 This option specifies the timeout (in seconds) to use when aging Path
563 MTU values discovered by the mechanism defined in RFC 1191.
564 option path-mtu-plateau-table uint16 [, uint16... ];
566 This option specifies a table of MTU sizes to use when performing
568 Path MTU Discovery as defined in RFC 1191. The table is formatted as
569 a list of 16-bit unsigned integers, ordered from smallest to largest.
570 The minimum MTU value cannot be smaller than 68.
571 option perform-mask-discovery flag;
573 This option specifies whether or not the client should perform subnet
575 mask discovery using ICMP. A value of false indicates that the
576 client should not perform mask discovery. A value of true means that
577 the client should perform mask discovery.
578 option policy-filter ip-address ip-address
580 [, ip-address ip-address...];
581 This option specifies policy filters for non-local source routing.
583 The filters consist of a list of IP addresses and masks which specify
584 destination/mask pairs with which to filter incoming source routes.
585 Any source routed datagram whose next-hop address does not match one
587 of the filters should be discarded by the client.
588 See STD 3 (RFC1122) for further information.
590 option pop-server ip-address [, ip-address... ];
592 The POP3 server option specifies a list of POP3 servers available to
594 the client. Servers should be listed in order of preference.
595 option resource-location-servers ip-address
597 [, ip-address...];
598 This option specifies a list of RFC 887 Resource Location servers
600 available to the client. Servers should be listed in order of pref‐
601 erence.
602 option root-path text;
604 This option specifies the path-name that contains the client's root
606 disk. The path is formatted as a character string consisting of
607 characters from the NVT ASCII character set.
608 option router-discovery flag;
610 This option specifies whether or not the client should solicit
612 routers using the Router Discovery mechanism defined in RFC 1256. A
613 value of false indicates that the client should not perform router
614 discovery. A value of true means that the client should perform
615 router discovery.
616 option router-solicitation-address ip-address;
618 This option specifies the address to which the client should transmit
620 router solicitation requests.
621 option routers ip-address [, ip-address... ];
623 The routers option specifies a list of IP addresses for routers on
625 the client's subnet. Routers should be listed in order of prefer‐
626 ence.
627 option slp-directory-agent boolean ip-address [, ip-address... ];
629 This option specifies two things: the IP addresses of one or more
631 Service Location Protocol Directory Agents, and whether the use of
632 these addresses is mandatory. If the initial boolean value is true,
633 the SLP agent should just use the IP addresses given. If the value
634 is false, the SLP agent may additionally do active or passive multi‐
635 cast discovery of SLP agents (see RFC2165 for details).
636 Please note that in this option and the slp-service-scope option, the
638 term "SLP Agent" is being used to refer to a Service Location Proto‐
639 col agent running on a machine that is being configured using the
640 DHCP protocol.
641 Also, please be aware that some companies may refer to SLP as NDS.
643 If you have an NDS directory agent whose address you need to config‐
644 ure, the slp-directory-agent option should work.
645 option slp-service-scope boolean text;
647 The Service Location Protocol Service Scope Option specifies two
649 things: a list of service scopes for SLP, and whether the use of this
650 list is mandatory. If the initial boolean value is true, the SLP
651 agent should only use the list of scopes provided in this option;
652 otherwise, it may use its own static configuration in preference to
653 the list provided in this option.
654 The text string should be a comma-separated list of scopes that the
656 SLP agent should use. It may be omitted, in which case the SLP Agent
657 will use the aggregated list of scopes of all directory agents known
658 to the SLP agent.
659 option smtp-server ip-address [, ip-address... ];
661 The SMTP server option specifies a list of SMTP servers available to
663 the client. Servers should be listed in order of preference.
664 option static-routes ip-address ip-address
666 [, ip-address ip-address...];
667 This option specifies a list of static routes that the client should
669 install in its routing cache. If multiple routes to the same desti‐
670 nation are specified, they are listed in descending order of prior‐
671 ity.
672 The routes consist of a list of IP address pairs. The first address
674 is the destination address, and the second address is the router for
675 the destination.
676 The default route (0.0.0.0) is an illegal destination for a static
678 route. To specify the default route, use the routers option. Also,
679 please note that this option is not intended for classless IP routing
680 - it does not include a subnet mask. Since classless IP routing is
681 now the most widely deployed routing standard, this option is virtu‐
682 ally useless, and is not implemented by any of the popular DHCP
683 clients, for example the Microsoft DHCP client.
684 NOTE to Fedora dhclient users:
686 dhclient-script interprets trailing 0 octets of the target as indi‐
687 cating the subnet class of the route, so for the following static-
688 routes value:
689 option static-routes 172.0.0.0 172.16.2.254,
690 192.168.0.0 192.168.2.254;
691 dhclient-script will create routes:
692 172/8 via 172.16.2.254 dev $interface
693 192.168/16 via 192.168.2.254 dev $interface
694 option classless-static-routes destination-descriptor ip-address
696 [, destination-descriptor ip-address...];
697 This option (see RFC3442) specifies a list of classless static routes
699 that the client should install in its routing cache.
700 This option can contain one or more static routes, each of which con‐
702 sists of a destination descriptor and the IP address of the router
703 that should be used to reach that destination.
704 Many clients may not implement the Classless Static Routes option.
706 DHCP server administrators should therefore configure their DHCP
707 servers to send both a Router option and a Classless Static Routes
708 option, and should specify the default router(s) both in the Router
709 option and in the Classless Static Routes option.
710 If the DHCP server returns both a Classless Static Routes option and
712 a Router option, the DHCP client ignores the Router option.
713 option streettalk-directory-assistance-server ip-address
715 [, ip-address...];
716 The StreetTalk Directory Assistance (STDA) server option specifies a
718 list of STDA servers available to the client. Servers should be
719 listed in order of preference.
720 option streettalk-server ip-address [, ip-address... ];
722 The StreetTalk server option specifies a list of StreetTalk servers
724 available to the client. Servers should be listed in order of pref‐
725 erence.
726 option subnet-mask ip-address;
728 The subnet mask option specifies the client's subnet mask as per RFC
730 950. If no subnet mask option is provided anywhere in scope, as a
731 last resort dhcpd will use the subnet mask from the subnet declara‐
732 tion for the network on which an address is being assigned. However,
733 any subnet-mask option declaration that is in scope for the address
734 being assigned will override the subnet mask specified in the subnet
735 declaration.
736 option subnet-selection string;
738 Sent by the client if an address is required in a subnet other than
740 the one that would normally be selected (based on the relaying
741 address of the connected subnet the request is obtained from). See
742 RFC3011. Note that the option number used by this server is 118; this
743 has not always been the defined number, and some clients may use a
744 different value. Use of this option should be regarded as slightly
745 experimental!
746 This option is not user configurable in the server.
748 option swap-server ip-address;
750 This specifies the IP address of the client's swap server.
752 option tcp-keepalive-garbage flag;
754 This option specifies whether or not the client should send TCP
756 keepalive messages with an octet of garbage for compatibility with
757 older implementations. A value of false indicates that a garbage
758 octet should not be sent. A value of true indicates that a garbage
759 octet should be sent.
760 option tcp-keepalive-interval uint32;
762 This option specifies the interval (in seconds) that the client TCP
764 should wait before sending a keepalive message on a TCP connection.
765 The time is specified as a 32-bit unsigned integer. A value of zero
766 indicates that the client should not generate keepalive messages on
767 connections unless specifically requested by an application.
768 option tftp-server-name text;
770 This option is used to identify a TFTP server and, if supported by
772 the client, should have the same effect as the server-name declara‐
773 tion. BOOTP clients are unlikely to support this option. Some DHCP
774 clients will support it, and others actually require it.
775 option time-offset int32;
777 The time-offset option specifies the offset of the client's subnet in
779 seconds from Coordinated Universal Time (UTC).
780 option time-servers ip-address [, ip-address... ];
782 The time-server option specifies a list of RFC 868 time servers
784 available to the client. Servers should be listed in order of pref‐
785 erence.
786 option trailer-encapsulation flag;
788 This option specifies whether or not the client should negotiate the
790 use of trailers (RFC 893 [14]) when using the ARP protocol. A value
791 of false indicates that the client should not attempt to use trail‐
792 ers. A value of true means that the client should attempt to use
793 trailers.
794 option uap-servers text;
796 This option specifies a list of URLs, each pointing to a user authen‐
798 tication service that is capable of processing authentication
799 requests encapsulated in the User Authentication Protocol (UAP). UAP
800 servers can accept either HTTP 1.1 or SSLv3 connections. If the list
801 includes a URL that does not contain a port component, the normal
802 default port is assumed (i.e., port 80 for http and port 443 for
803 https). If the list includes a URL that does not contain a path com‐
804 ponent, the path /uap is assumed. If more than one URL is specified
805 in this list, the URLs are separated by spaces.
806 option user-class string;
808 This option is used by some DHCP clients as a way for users to spec‐
810 ify identifying information to the client. This can be used in a
811 similar way to the vendor-class-identifier option, but the value of
812 the option is specified by the user, not the vendor. Most recent
813 DHCP clients have a way in the user interface to specify the value
814 for this identifier, usually as a text string.
815 option vendor-class-identifier string;
817 This option is used by some DHCP clients to identify the vendor type
819 and possibly the configuration of a DHCP client. The information is
820 a string of bytes whose contents are specific to the vendor and are
821 not specified in a standard. To see what vendor class identifier
822 clients are sending, you can write the following in your DHCP server
823 configuration file:
824 set vendor-string = option vendor-class-identifier;
826 This will result in all entries in the DHCP server lease database
828 file for clients that sent vendor-class-identifier options having a
829 set statement that looks something like this:
830 set vendor-string = "SUNW.Ultra-5_10";
832 The vendor-class-identifier option is normally used by the DHCP
834 server to determine the options that are returned in the vendor-
835 encapsulated-options option. Please see the VENDOR ENCAPSULATED
836 OPTIONS section later in this manual page for further information.
837 option vendor-encapsulated-options string;
839 The vendor-encapsulated-options option can contain either a single
841 vendor-specific value or one or more vendor-specific suboptions.
842 This option is not normally specified in the DHCP server configura‐
843 tion file - instead, a vendor class is defined for each vendor, ven‐
844 dor class suboptions are defined, values for those suboptions are
845 defined, and the DHCP server makes up a response on that basis.
846 Some default behaviours for well-known DHCP client vendors (cur‐
848 rently, the Microsoft Windows 2000 DHCP client) are configured auto‐
849 matically, but otherwise this must be configured manually - see the
850 VENDOR ENCAPSULATED OPTIONS section later in this manual page for
851 details.
852 option vivso string;
854 The vivso option can contain multiple separate options, one for each
856 32-bit Enterprise ID. Each Enterprise-ID discriminated option then
857 contains additional options whose format is defined by the vendor who
858 holds that ID. This option is usually not configured manually, but
859 rather is configured via intervening option definitions. Please also
860 see the VENDOR ENCAPSULATED OPTIONS section later in this manual page
861 for details.
862 option www-server ip-address [, ip-address... ];
864 The WWW server option specifies a list of WWW servers available to
866 the client. Servers should be listed in order of preference.
867 option x-display-manager ip-address [, ip-address... ];
869 This option specifies a list of systems that are running the X Window
871 System Display Manager and are available to the client. Addresses
872 should be listed in order of preference.
873
875 An IETF draft, draft-ietf-dhc-agent-options-11.txt, defines a series of
876 encapsulated options that a relay agent can add to a DHCP packet when
877 relaying it to the DHCP server. The server can then make address allo‐
878 cation decisions (or whatever other decisions it wants) based on these
879 options. The server also returns these options in any replies it sends
880 through the relay agent, so that the relay agent can use the informa‐
881 tion in these options for delivery or accounting purposes.
882 The current draft defines two options. To reference these options in
884 the dhcp server, specify the option space name, "agent", followed by a
885 period, followed by the option name. It is not normally useful to
886 define values for these options in the server, although it is permissi‐
887 ble. These options are not supported in the client.
888 option agent.circuit-id string;
890 The circuit-id suboption encodes an agent-local identifier of the
892 circuit from which a DHCP client-to-server packet was received. It
893 is intended for use by agents in relaying DHCP responses back to the
894 proper circuit. The format of this option is currently defined to be
895 vendor-dependent, and will probably remain that way, although the
896 current draft allows for for the possibility of standardizing the
897 format in the future.
898 option agent.remote-id string;
900 The remote-id suboption encodes information about the remote host end
902 of a circuit. Examples of what it might contain include caller ID
903 information, username information, remote ATM address, cable modem
904 ID, and similar things. In principal, the meaning is not well-speci‐
905 fied, and it should generally be assumed to be an opaque object that
906 is administratively guaranteed to be unique to a particular remote
907 end of a circuit.
908 option agent.DOCSIS-device-class uint32;
910 The DOCSIS-device-class suboption is intended to convey information
912 about the host endpoint, hardware, and software, that either the host
913 operating system or the DHCP server may not otherwise be aware of
914 (but the relay is able to distinguish). This is implemented as a
915 32-bit field (4 octets), each bit representing a flag describing the
916 host in one of these ways. So far, only bit zero (being the least
917 significant bit) is defined in RFC3256. If this bit is set to one,
918 the host is considered a CPE Controlled Cable Modem (CCCM). All
919 other bits are reserved.
920 option agent.link-selection ip-address;
922 The link-selection suboption is provided by relay agents to inform
924 servers what subnet the client is actually attached to. This is use‐
925 ful in those cases where the giaddr (where responses must be sent to
926 the relay agent) is not on the same subnet as the client. When this
927 option is present in a packet from a relay agent, the DHCP server
928 will use its contents to find a subnet declared in configuration, and
929 from here take one step further backwards to any shared-network the
930 subnet may be defined within...the client may be given any address
931 within that shared network, as normally appropriate.
932
934 The Client FQDN option, currently defined in the Internet Draft draft-
935 ietf-dhc-fqdn-option-00.txt is not a standard yet, but is in suffi‐
936 ciently wide use already that we have implemented it. Due to the com‐
937 plexity of the option format, we have implemented it as a suboption
938 space rather than a single option. In general this option should not
939 be configured by the user - instead it should be used as part of an
940 automatic DNS update system.
941 option fqdn.no-client-update flag;
943 When the client sends this, if it is true, it means the client will
945 not attempt to update its A record. When sent by the server to the
946 client, it means that the client should not update its own A record.
947 option fqdn.server-update flag;
949 When the client sends this to the server, it is requesting that the
951 server update its A record. When sent by the server, it means that
952 the server has updated (or is about to update) the client's A record.
953 option fqdn.encoded flag;
955 If true, this indicates that the domain name included in the option
957 is encoded in DNS wire format, rather than as plain ASCII text. The
958 client normally sets this to false if it doesn't support DNS wire
959 format in the FQDN option. The server should always send back the
960 same value that the client sent. When this value is set on the con‐
961 figuration side, it controls the format in which the fqdn.fqdn subop‐
962 tion is encoded.
963 option fqdn.rcode1 flag;
965 option fqdn.rcode2 flag;
967 These options specify the result of the updates of the A and PTR
969 records, respectively, and are only sent by the DHCP server to the
970 DHCP client. The values of these fields are those defined in the DNS
971 protocol specification.
972 option fqdn.fqdn text;
974 Specifies the domain name that the client wishes to use. This can be
976 a fully-qualified domain name, or a single label. If there is no
977 trailing ´.´ character in the name, it is not fully-qualified, and
978 the server will generally update that name in some locally-defined
979 domain.
980 option fqdn.hostname --never set--;
982 This option should never be set, but it can be read back using the
984 option and config-option operators in an expression, in which case it
985 returns the first label in the fqdn.fqdn suboption - for example, if
986 the value of fqdn.fqdn is "foo.example.com.", then fqdn.hostname will
987 be "foo".
988 option fqdn.domainname --never set--;
990 This option should never be set, but it can be read back using the
992 option and config-option operators in an expression, in which case it
993 returns all labels after the first label in the fqdn.fqdn suboption -
994 for example, if the value of fqdn.fqdn is "foo.example.com.", then
995 fqdn.hostname will be "example.com.". If this suboption value is not
996 set, it means that an unqualified name was sent in the fqdn option,
997 or that no fqdn option was sent at all.
998 If you wish to use any of these suboptions, we strongly recommend that
1000 you refer to the Client FQDN option draft (or standard, when it becomes
1001 a standard) - the documentation here is sketchy and incomplete in com‐
1002 parison, and is just intended for reference by people who already
1003 understand the Client FQDN option specification.
1004
1006 RFC2242 defines a set of encapsulated options for Novell NetWare/IP
1007 clients. To use these options in the dhcp server, specify the option
1008 space name, "nwip", followed by a period, followed by the option name.
1009 The following options can be specified:
1010 option nwip.nsq-broadcast flag;
1012 If true, the client should use the NetWare Nearest Server Query to
1014 locate a NetWare/IP server. The behaviour of the Novell client if
1015 this suboption is false, or is not present, is not specified.
1016 option nwip.preferred-dss ip-address [, ip-address... ];
1018 This suboption specifies a list of up to five IP addresses, each of
1020 which should be the IP address of a NetWare Domain SAP/RIP server
1021 (DSS).
1022 option nwip.nearest-nwip-server ip-address
1024 [, ip-address...];
1025 This suboption specifies a list of up to five IP addresses, each of
1027 which should be the IP address of a Nearest NetWare IP server.
1028 option nwip.autoretries uint8;
1030 Specifies the number of times that a NetWare/IP client should attempt
1032 to communicate with a given DSS server at startup.
1033 option nwip.autoretry-secs uint8;
1035 Specifies the number of seconds that a Netware/IP client should wait
1037 between retries when attempting to establish communications with a
1038 DSS server at startup.
1039 option nwip.nwip-1-1 uint8;
1041 If true, the NetWare/IP client should support NetWare/IP version 1.1
1043 compatibility. This is only needed if the client will be contacting
1044 Netware/IP version 1.1 servers.
1045 option nwip.primary-dss ip-address;
1047 Specifies the IP address of the Primary Domain SAP/RIP Service server
1049 (DSS) for this NetWare/IP domain. The NetWare/IP administration
1050 utility uses this value as Primary DSS server when configuring a sec‐
1051 ondary DSS server.
1052
1054 DHCPv6 options differ from DHCPv4 options partially due to using 16-bit
1055 code and length tags, but semantically zero-length options are legal in
1056 DHCPv6, and multiple options are treated differently. Whereas in
1057 DHCPv4 multiple options would be concatenated to form one option, in
1058 DHCPv6 they are expected to be individual instantiations. Understand‐
1059 ably, many options are not "allowed" to have multiple instances in a
1060 packet - normally these are options which are digested by the DHCP pro‐
1061 tocol software, and not by users or applications.
1062 option dhcp6.client-id string;
1064 This option specifies the client's DUID identifier. DUIDs are simi‐
1066 lar but different from DHCPv4 client identifiers - there are docu‐
1067 mented duid types:
1068 duid-llt
1070 duid-en
1072 duid-ll
1074 This value should not be configured, but rather is provided by
1076 clients and treated as an opaque identifier key blob by servers.
1077 option dhcp6.server-id string;
1079 This option specifies the server's DUID identifier. One may use this
1081 option to configure an opaque binary blob for your server's identi‐
1082 fier.
1083 option dhcp6.ia-na string;
1085 The Identity Association for Non-temporary Addresses (ia-na) carries
1087 assigned addresses that are not temporary addresses for use by the
1088 DHCPv6 client. This option is produced by the DHCPv6 server soft‐
1089 ware, and should not be configured.
1090 option dhcp6.ia-ta string;
1092 The Identity Association for Temporary Addresses (ia-ta) carries tem‐
1094 porary addresses, which may change upon every renewal. There is no
1095 support for this in the current DHCPv6 software.
1096 option dhcp6.ia-addr string;
1098 The Identity Association Address option is encapsulated inside ia-na
1100 or ia-ta options in order to represent addresses associated with
1101 those IA's. These options are manufactured by the software, so
1102 should not be configured.
1103 option dhcp6.oro uint16 [ , uint16, ... ];
1105 The Option Request Option ("ORO") is the DHCPv6 equivalent of the
1107 parameter-request-list. Clients supply this option to ask servers to
1108 reply with options relevant to their needs and use. This option must
1109 not be directly configured, the request syntax in dhclient.conf (5)
1110 should be used instead.
1111 option dhcp6.preference uint8;
1113 The preference option informs a DHCPv6 client which server is ´pre‐
1115 ferred´ for use on a given subnet. This preference is only applied
1116 during the initial stages of configuration - once a client is bound
1117 to an IA, it will remain bound to that IA until it is no longer valid
1118 or has expired. This value may be configured on the server, and is
1119 digested by the client software.
1120 option dhcp6.elapsed-time uint16;
1122 The elapsed-time option is constructed by the DHCPv6 client software,
1124 and is potentially consumed by intermediaries. This option should
1125 not be configured.
1126 option dhcp6.relay-msg string;
1128 The relay-msg option is constructed by intervening DHCPv6 relay agent
1130 software. This option is entirely used by protocol software, and is
1131 not meant for user configuration.
1132 option dhcp6.unicast ip6-address;
1134 The unicast option is provided by DHCPv6 servers which are willing
1136 (or prefer) to receive Renew packets from their clients by exchanging
1137 UDP unicasts with them. Normally, DHCPv6 clients will multicast
1138 their Renew messages. This may be configured on the server, and
1139 should be configured as an address the server is ready to reply to.
1140 option dhcp6.status-code status-code [ string ] ;
1142 The status-code option is provided by DHCPv6 servers to inform
1144 clients of error conditions during protocol communication. This
1145 option is manufactured and digested by protocol software, and should
1146 not be configured.
1147 option dhcp6.rapid-commit ;
1149 The rapid-commit option is a zero-length option that clients use to
1151 indicate their desire to enter into rapid-commit with the server.
1152 option dhcp6.vendor-opts string;
1154 The vendor-opts option is actually an encapsulated sub-option space,
1156 in which each Vendor-specific Information Option (VSIO) is identified
1157 by a 32-bit Enterprise-ID number. The encapsulated option spaces
1158 within these options are defined by the vendors.
1159 To make use of this option, the best way is to examine the section
1161 titled VENDOR ENCAPSULATED OPTIONS below, in particular the bits
1162 about the "vsio" option space.
1163 option dhcp6.interface-id string;
1165 The interface-id option is manufactured by relay agents, and may be
1167 used to guide configuration differentiating clients by the interface
1168 they are remotely attached to. It does not make sense to configure a
1169 value for this option, but it may make sense to inspect its contents.
1170 option dhcp6.reconf-msg dhcpv6-message;
1172 The reconf-msg option is manufactured by servers, and sent to clients
1174 in Reconfigure messages to inform them of what message the client
1175 should Reconfigure using. There is no support for DHCPv6 Reconfigure
1176 extensions, and this option is documented informationally only.
1177 option dhcp6.reconf-accept ;
1179 The reconf-accept option is included by DHCPv6 clients that support
1181 the Reconfigure extentions, advertising that they will respond if the
1182 server were to ask them to Reconfigure. There is no support for
1183 DHCPv6 Reconfigure extensions, and this option is documented informa‐
1184 tionally only.
1185 option dhcp6.sip-servers-names domain-list;
1187 The sip-servers-names option allows SIP clients to locate a local SIP
1189 server that is to be used for all outbound SIP requests, a so-
1190 called"outbound proxy server." If you wish to use manually entered
1191 IPv6 addresses instead, please see the sip-servers-addresses option
1192 below.
1193 option dhcp6.sip-servers-addresses ip6-address [, ip6-address ... ] ;
1195 The sip-servers-addresses option allows SIP clients to locate a local
1197 SIP server that is to be used for all outbound SIP requests, a so-
1198 called "outbound proxy servers." If you wish to use domain names
1199 rather than IPv6 addresses, please see the sip-servers-names option
1200 above.
1201 option dhcp6.name-servers ip6-address [, ip6-address ... ] ;
1203 The name-servers option instructs clients about locally available
1205 recursive DNS servers. It is easiest to describe this as the "name‐
1206 server" line in /etc/resolv.conf.
1207 option dhcp6.domain-search domain-list;
1209 The domain-search option specifies the client's domain search path to
1211 be applied to recursive DNS queries. It is easiest to describe this
1212 as the "search" line in /etc/resolv.conf.
1213 option dhcp6.ia-pd string;
1215 The ia-pd option is manufactured by clients and servers to create a
1217 Prefix Delegation binding - to delegate an IPv6 prefix to the client.
1218 It is not directly edited in dhcpd.conf(5) or dhclient.conf(5), but
1219 rather is manufactured and consumed by the software.
1220 option dhcp6.ia-prefix string;
1222 The ia-prefix option is placed inside ia-pd options in order to iden‐
1224 tify the prefix(es) allocated to the client. It is not directly
1225 edited in dhcpd.conf(5) or dhclient.conf(5), but rather is manufac‐
1226 tured and consumed by the software.
1227 option dhcp6.nis-servers ip6-address [, ip6-address ... ] ;
1229 The nis-servers option identifies, in order, NIS servers available to
1231 the client.
1232 option dhcp6.nisp-servers ip6-address [, ip6-address ... ] ;
1234 The nisp-servers option identifies, in order, NIS+ servers available
1236 to the client.
1237 option nis-domain-name domain-list;
1239 The nis-domain-name option specifies the NIS domain name the client
1241 is expected to use, and is related to the nis-servers option.
1242 option dhcp6.nis-domain-name domain-name;
1244 The dhcp6.nis-domain-name option specfies NIS domain name the client
1246 is expected to use, and is related to dhcp6.nis-servers option.
1247 option nisp-domain-name domain-list;
1249 The nisp-domain-name option specifies the NIS+ domain name the client
1251 is expected to use, and is related to the nisp-servers option.
1252 option dhcp6.nisp-domain-name domain-name;
1254 The dhcp6.nis-domain-name option specfies NIS+ domain name the client
1256 is expected to use, and is related to dhcp6.nisp-servers option.
1257 option dhcp6.sntp-servers ip6-address [, ip6-address ... ] ;
1259 The sntp-servers option specifies a list of local SNTP servers avail‐
1261 able for the client to synchronize their clocks.
1262 option dhcp6.info-refresh-time uint32;
1264 The info-refresh-time option gives DHCPv6 clients using Information-
1266 request messages a hint as to how long they should between refreshing
1267 the information they were given. Note that this option will only be
1268 delivered to the client, and be likely to affect the client's behav‐
1269 iour, if the client requested the option.
1270 option dhcp6.bcms-server-d domain-list;
1272 The bcms-server-d option contains the domain names of local BCMS
1274 (Broadcast and Multicast Control Services) controllers which the
1275 client may use.
1276 option dhcp6.bcms-server-a ip6-address [, ip6-address ... ] ;
1278 The bcms-server-a option contains the IPv6 addresses of local BCMS
1280 (Broadcast and Multicast Control Services) controllers which the
1281 client may use.
1282 option dhcp6.remote-id string;
1284 The remote-id option is constructed by relay agents, to inform the
1286 server of details pertaining to what the relay knows about the client
1287 (such as what port it is attached to, and so forth). The contents of
1288 this option have some vendor-specific structure (similar to VSIO),
1289 but we have chosen to treat this option as an opaque field.
1290 option dhcp6.subscriber-id;
1292 The subscriber-id option is an opaque field provided by the relay
1294 agent, which provides additional information about the subscriber in
1295 question. The exact contents of this option depend upon the vendor
1296 and/or the operator's configuration of the remote device, and as such
1297 is an opaque field.
1298 option dhcp6.fqdn string;
1300 The fqdn option is normally constructed by the client or server, and
1302 negotiates the client's Fully Qualified Domain Name, as well as which
1303 party is responsible for Dynamic DNS Updates. See the section on the
1304 Client FQDN SubOptions for full details (the DHCPv4 and DHCPv6 FQDN
1305 options use the same "fqdn." encapsulated space, so are in all ways
1306 identical).
1307 option dhcp6.lq-query string;
1309 The lq-query option is used internally by for lease query.
1311 option dhcp6.client-data string;
1313 The client-data option is used internally by for lease query.
1315 option dhcp6.clt-time uint32;
1317 The clt-time option is used internally by for lease query.
1319 option dhcp6.lq-relay-data ip6-address string;
1321 The lq-relay-data option is used internally by for lease query.
1323 option dhcp6.lq-client-link ip6-address [, ip6-address ... ] ;
1325 The lq-client-link option is used internally by for lease query.
1327 option dhcp6.bootfile-url string ;
1329 The server sends this option to inform the client about a URL to a
1331 boot file. Used primarily for UEFI network booting, it contains an
1332 RFC3986 compliant URI which the client may use to boot an operating
1333 system. This option is defined in RFC5970
1334 option dhcp6.arch-type arch-id [, arch-id...] ;
1336 A client will send this option to a server so that the server may
1338 make decisions on what options and addresses to offer the requesting
1339 client. The option consists of a list of 16 bit unsigned values that
1340 represent the architecture of the requesting client. These values
1341 corespond to the values available to the dhcpv4 option architecture-
1342 type, as defined in RFC4578, section 2.1. This option is defined in
1343 RFC5970
1344 .RE
1345 option dhcp6.net-id uint8 uint8 uint8 ;
1347 A client will send this option to a server to inform it about the
1349 clients level of UNDI support. The option consists of 3 octets (a
1350 type, major and minor value). Specific meanings of these values
1351 are doumented in section 2.2 of RFC4578. This option is defined
1352 in RFC5970
1353
1355 The Internet Systems Consortium DHCP client and server provide the
1356 capability to define new options. Each DHCP option has a name, a code,
1357 and a structure. The name is used by you to refer to the option. The
1358 code is a number, used by the DHCP server and client to refer to an
1359 option. The structure describes what the contents of an option looks
1360 like.
1361 To define a new option, you need to choose a name for it that is not in
1363 use for some other option - for example, you can't use "host-name"
1364 because the DHCP protocol already defines a host-name option, which is
1365 documented earlier in this manual page. If an option name doesn't
1366 appear in this manual page, you can use it, but it's probably a good
1367 idea to put some kind of unique string at the beginning so you can be
1368 sure that future options don't take your name. For example, you might
1369 define an option, "local-host-name", feeling some confidence that no
1370 official DHCP option name will ever start with "local".
1371 Once you have chosen a name, you must choose a code. All codes between
1373 224 and 254 are reserved as ´site-local´ DHCP options, so you can pick
1374 any one of these for your site (not for your product/application). In
1375 RFC3942, site-local space was moved from starting at 128 to starting at
1376 224. In practice, some vendors have interpreted the protocol rather
1377 loosely and have used option code values greater than 128 themselves.
1378 There's no real way to avoid this problem, and it was thought to be
1379 unlikely to cause too much trouble in practice. If you come across a
1380 vendor-documented option code in either the new or old site-local spa‐
1381 ces, please contact your vendor and inform them about rfc3942.
1382 The structure of an option is simply the format in which the option
1384 data appears. The ISC DHCP server currently supports a few simple
1385 types, like integers, booleans, strings and IP addresses, and it also
1386 supports the ability to define arrays of single types or arrays of
1387 fixed sequences of types.
1388 New options are declared as follows:
1390 option new-name code new-code = definition ;
1392 The values of new-name and new-code should be the name you have chosen
1394 for the new option and the code you have chosen. The definition should
1395 be the definition of the structure of the option.
1396 The following simple option type definitions are supported:
1398 BOOLEAN
1400 option new-name code new-code = boolean ;
1402 An option of type boolean is a flag with a value of either on or off
1404 (or true or false). So an example use of the boolean type would be:
1405 option use-zephyr code 180 = boolean;
1407 option use-zephyr on;
1408 INTEGER
1410 option new-name code new-code = sign integer width ;
1412 The sign token should either be blank, unsigned or signed. The width
1414 can be either 8, 16 or 32, and refers to the number of bits in the
1415 integer. So for example, the following two lines show a definition of
1416 the sql-connection-max option and its use:
1417 option sql-connection-max code 192 = unsigned integer 16;
1419 option sql-connection-max 1536;
1420 IP-ADDRESS
1422 option new-name code new-code = ip-address ;
1424 An option whose structure is an IP address can be expressed either as a
1426 domain name or as a dotted quad. So the following is an example use of
1427 the ip-address type:
1428 option sql-server-address code 193 = ip-address;
1430 option sql-server-address sql.example.com;
1431 IP6-ADDRESS
1433 option new-name code new-code = ip6-address ;
1435 An option whose structure is an IPv6 address must be expressed as a
1437 valid IPv6 address. The following is an example use of the ip6-address
1438 type:
1439 option dhcp6.some-server code 1234 = array of ip6-address;
1441 option dhcp6.some-server 3ffe:bbbb:aaaa:aaaa::1, 3ffe:bbbb:aaaa:aaaa::2;
1442 TEXT
1445 option new-name code new-code = text ;
1447 An option whose type is text will encode an ASCII text string. For
1449 example:
1450 option sql-default-connection-name code 194 = text;
1452 option sql-default-connection-name "PRODZA";
1453 DATA STRING
1456 option new-name code new-code = string ;
1458 An option whose type is a data string is essentially just a collection
1460 of bytes, and can be specified either as quoted text, like the text
1461 type, or as a list of hexadecimal contents separated by colons whose
1462 values must be between 0 and FF. For example:
1463 option sql-identification-token code 195 = string;
1465 option sql-identification-token 17:23:19:a6:42:ea:99:7c:22;
1466 DOMAIN-LIST
1469 option new-name code new-code = domain-list [compressed] ;
1471 An option whose type is domain-list is an RFC1035 formatted (on the
1473 wire, "DNS Format") list of domain names, separated by root labels.
1474 The optional compressed keyword indicates if the option should be com‐
1475 pressed relative to the start of the option contents (not the packet
1476 contents).
1477 When in doubt, omit the compressed keyword. When the software recieves
1479 an option that is compressed and the compressed keyword is omitted, it
1480 will still decompress the option (relative to the option contents
1481 field). The keyword only controls whether or not transmitted packets
1482 are compressed.
1483 Note that when domain-list formatted options are output as environment
1485 variables to dhclient-script(8), the standard DNS -escape mechanism is
1486 used: they are decimal. This is appropriate for direct use in eg
1487 /etc/resolv.conf.
1488 ENCAPSULATION
1491 option new-name code new-code = encapsulate identifier ;
1493 An option whose type is encapsulate will encapsulate the contents of
1495 the option space specified in identifier. Examples of encapsulated
1496 options in the DHCP protocol as it currently exists include the vendor-
1497 encapsulated-options option, the netware-suboptions option and the
1498 relay-agent-information option.
1499 option space local;
1501 option local.demo code 1 = text;
1502 option local-encapsulation code 197 = encapsulate local;
1503 option local.demo "demo";
1504 ARRAYS
1507 Options can contain arrays of any of the above types except for the
1509 text and data string types, which aren't currently supported in arrays.
1510 An example of an array definition is as follows:
1511 option kerberos-servers code 200 = array of ip-address;
1513 option kerberos-servers 10.20.10.1, 10.20.11.1;
1514 RECORDS
1516 Options can also contain data structures consisting of a sequence of
1518 data types, which is sometimes called a record type. For example:
1519 option contrived-001 code 201 = { boolean, integer 32, text };
1521 option contrived-001 on 1772 "contrivance";
1522 It's also possible to have options that are arrays of records, for
1524 example:
1525 option new-static-routes code 201 = array of {
1527 ip-address, ip-address, ip-address, integer 8 };
1528 option static-routes
1529 10.0.0.0 255.255.255.0 net-0-rtr.example.com 1,
1530 10.0.1.0 255.255.255.0 net-1-rtr.example.com 1,
1531 10.2.0.0 255.255.224.0 net-2-0-rtr.example.com 3;
1532
1535 The DHCP protocol defines the vendor-encapsulated-options option, which
1536 allows vendors to define their own options that will be sent encapsu‐
1537 lated in a standard DHCP option. It also defines the Vendor Identified
1538 Vendor Sub Options option ("VIVSO"), and the DHCPv6 protocol defines
1539 the Vendor-specific Information Option ("VSIO"). The format of all of
1540 these options is usually internally a string of options, similarly to
1541 other normal DHCP options. The VIVSO and VSIO options differ in that
1542 that they contain options that correspond to vendor Enterprise-ID num‐
1543 bers (assigned by IANA), which then contain options according to each
1544 Vendor's specifications. You will need to refer to your vendor's docu‐
1545 mentation in order to form options to their specification.
1546 The value of these options can be set in one of two ways. The first
1548 way is to simply specify the data directly, using a text string or a
1549 colon-separated list of hexadecimal values. For help in forming these
1550 strings, please refer to RFC2132 for the DHCPv4 Vendor Specific Infor‐
1551 mation Option, RFC3925 for the DHCPv4 Vendor Identified Vendor Sub
1552 Options, or RFC3315 for the DHCPv6 Vendor-specific Information Option.
1553 For example:
1554 option vendor-encapsulated-options
1556 2:4:
1557 AC:11:41:1:
1558 3:12:
1559 73:75:6e:64:68:63:70:2d:73:65:72:76:65:72:31:37:2d:31:
1560 4:12:
1561 2f:65:78:70:6f:72:74:2f:72:6f:6f:74:2f:69:38:36:70:63;
1562 option vivso
1563 00:00:09:bf:0E:
1564 01:0c:
1565 48:65:6c:6c:6f:20:77:6f:72:6c:64:21;
1566 option dhcp6.vendor-opts
1567 00:00:09:bf:
1568 00:01:00:0c:
1569 48:65:6c:6c:6f:20:77:6f:72:6c:64:21;
1570 The second way of setting the value of these options is to have the
1572 DHCP server generate a vendor-specific option buffer. To do this, you
1573 must do four things: define an option space, define some options in
1574 that option space, provide values for them, and specify that that
1575 option space should be used to generate the relevant option.
1576 To define a new option space in which vendor options can be stored, use
1578 the option space statement:
1579 option space name [ [ code width number ] [ length width number ] [
1581 hash size number ] ] ;
1582 Where the numbers following code width, length width, and hash size
1584 respectively identify the number of bytes used to describe option
1585 codes, option lengths, and the size in buckets of the hash tables to
1586 hold options in this space (most DHCPv4 option spaces use 1 byte codes
1587 and lengths, which is the default, whereas most DHCPv6 option spaces
1588 use 2 byte codes and lengths).
1589 The code and length widths are used in DHCP protocol - you must config‐
1591 ure these numbers to match the applicable option space you are config‐
1592 uring. They each default to 1. Valid values for code widths are 1, 2
1593 or 4. Valid values for length widths are 0, 1 or 2. Most DHCPv4
1594 option spaces use 1 byte codes and lengths, which is the default,
1595 whereas most DHCPv6 option spaces use 2 byte codes and lengths. A
1596 zero-byte length produces options similar to the DHCPv6 Vendor-specific
1597 Information Option - but not their contents!
1598 The hash size defaults depend upon the code width selected, and may be
1600 254 or 1009. Valid values range between 1 and 65535. Note that the
1601 higher you configure this value, the more memory will be used. It is
1602 considered good practice to configure a value that is slightly larger
1603 than the estimated number of options you plan to configure within the
1604 space. Previous versions of ISC DHCP (up to and including DHCP 3.0.*),
1605 this value was fixed at 9973.
1606 The name can then be used in option definitions, as described earlier
1608 in this document. For example:
1609 option space SUNW code width 1 length width 1 hash size 3;
1611 option SUNW.server-address code 2 = ip-address;
1612 option SUNW.server-name code 3 = text;
1613 option SUNW.root-path code 4 = text;
1614 option space ISC code width 1 length width 1 hash size 3;
1616 option ISC.sample code 1 = text;
1617 option vendor.ISC code 2495 = encapsulate vivso-sample;
1618 option vendor-class.ISC code 2495 = text;
1619 option ISC.sample "configuration text here";
1621 option vendor-class.ISC "vendor class here";
1622 option space docsis code width 2 length width 2 hash size 17;
1624 option docsis.tftp-servers code 32 = array of ip6-address;
1625 option docsis.cablelabs-configuration-file code 33 = text;
1626 option docsis.cablelabs-syslog-servers code 34 = array of ip6-address;
1627 option docsis.device-id code 36 = string;
1628 option docsis.time-servers code 37 = array of ip6-address;
1629 option docsis.time-offset code 38 = signed integer 32;
1630 option vsio.docsis code 4491 = encapsulate docsis;
1631 Once you have defined an option space and the format of some options,
1633 you can set up scopes that define values for those options, and you can
1634 say when to use them. For example, suppose you want to handle two dif‐
1635 ferent classes of clients. Using the option space definition shown in
1636 the previous example, you can send different option values to different
1637 clients based on the vendor-class-identifier option that the clients
1638 send, as follows:
1639 class "vendor-classes" {
1641 match option vendor-class-identifier;
1642 }
1643 subclass "vendor-classes" "SUNW.Ultra-5_10" {
1645 vendor-option-space SUNW;
1646 option SUNW.root-path "/export/root/sparc";
1647 }
1648 subclass "vendor-classes" "SUNW.i86pc" {
1650 vendor-option-space SUNW;
1651 option SUNW.root-path "/export/root/i86pc";
1652 }
1653 option SUNW.server-address 172.17.65.1;
1655 option SUNW.server-name "sundhcp-server17-1";
1656 option vivso-sample.sample "Hello world!";
1658 option docsis.tftp-servers ::1;
1660 As you can see in the preceding example, regular scoping rules apply,
1663 so you can define values that are global in the global scope, and only
1664 define values that are specific to a particular class in the local
1665 scope. The vendor-option-space declaration tells the DHCP server to
1666 use options in the SUNW option space to construct the DHCPv4 vendor-
1667 encapsulated-options option. This is a limitation of that option - the
1668 DHCPv4 VIVSO and the DHCPv6 VSIO options can have multiple vendor defi‐
1669 nitions all at once (even transmitted to the same client), so it is not
1670 necessary to configure this.
1671
1673 dhcpd.conf(5), dhcpd.leases(5), dhclient.conf(5), dhcp-eval(5),
1674 dhcpd(8), dhclient(8), RFC2132, RFC2131, RFC3046, RFC3315.
1675
1677 The Internet Systems Consortium DHCP Distribution was written by Ted
1678 Lemon under a contract with Vixie Labs. Funding for this project was
1679 provided through Internet Systems Consortium. Information about Inter‐
1680 net Systems Consortium can be found at https://www.isc.org.
1681 dhcp-options(5)