1dhcpd-options(5) File Formats Manual dhcpd-options(5)
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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
20 specify all DHCP options - only those options which are needed by
21 clients 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
34 data type specifies an unsigned 32-bit integer. The int16 and uint16
35 data types specify signed and unsigned 16-bit integers. The int8 and
36 uint8 data types specify signed and unsigned 8-bit integers. Unsigned
37 8-bit 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 exist‐
47 ing 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
86 option, 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 not user configurable.
280 option dhcp-renewal-time uint32;
282 This option specifies the number of seconds from the time a client
284 gets an address until the client transitions to the RENEWING state.
285 This option is not user configurable.
287 option dhcp-requested-address ip-address;
289 This option is used by the client in a DHCPDISCOVER to request that a
291 particular IP address be assigned.
292 This option is not user configurable.
294 option dhcp-server-identifier ip-address;
296 This option is used in DHCPOFFER and DHCPREQUEST messages, and may
298 optionally be included in the DHCPACK and DHCPNAK messages. DHCP
299 servers include this option in the DHCPOFFER in order to allow the
300 client to distinguish between lease offers. DHCP clients use the
301 contents of the 'server identifier' field as the destination address
302 for any DHCP messages unicast to the DHCP server. DHCP clients also
303 indicate which of several lease offers is being accepted by including
304 this option in a DHCPREQUEST message.
305 The value of this option is the IP address of the server.
307 This option is not directly user configurable. See the server-identi‐
309 fier server option in dhcpd.conf(5).
310 option domain-name text;
312 This option specifies the domain name that client should use when
314 resolving hostnames via the Domain Name System.
315 option domain-name-servers ip-address [, ip-address... ];
317 The domain-name-servers option specifies a list of Domain Name System
319 (STD 13, RFC 1035) name servers available to the client. Servers
320 should be listed in order of preference.
321 option domain-search domain-list;
323 The domain-search option specifies a 'search list' of Domain Names to
325 be used by the client to locate not-fully-qualified domain names.
326 The difference between this option and historic use of the domain-
327 name option for the same ends is that this option is encoded in
328 RFC1035 compressed labels on the wire. For example:
329 option domain-search "example.com", "sales.example.com",
331 "eng.example.com";
332 option extensions-path text;
334 This option specifies the name of a file containing additional
336 options to be interpreted according to the DHCP option format as
337 specified in RFC2132.
338 option finger-server ip-address [, ip-address... ];
340 The Finger server option specifies a list of Finger servers available
342 to the client. Servers should be listed in order of preference.
343 option font-servers ip-address [, ip-address... ];
345 This option specifies a list of X Window System Font servers avail‐
347 able to the client. Servers should be listed in order of preference.
348 option host-name string;
350 This option specifies the name of the client. The name may or may
352 not be qualified with the local domain name (it is preferable to use
353 the domain-name option to specify the domain name). See RFC 1035 for
354 character set restrictions. This option is only honored by dhclient-
355 script(8) if the hostname for the client machine is not set.
356 option ieee802-3-encapsulation flag;
358 This option specifies whether or not the client should use Ethernet
360 Version 2 (RFC 894) or IEEE 802.3 (RFC 1042) encapsulation if the
361 interface is an Ethernet. A value of false indicates that the client
362 should use RFC 894 encapsulation. A value of true means that the
363 client should use RFC 1042 encapsulation.
364 option ien116-name-servers ip-address [, ip-address... ];
366 The ien116-name-servers option specifies a list of IEN 116 name
368 servers available to the client. Servers should be listed in order
369 of preference.
370 option impress-servers ip-address [, ip-address... ];
372 The impress-server option specifies a list of Imagen Impress servers
374 available to the client. Servers should be listed in order of pref‐
375 erence.
376 option interface-mtu uint16;
378 This option specifies the MTU to use on this interface. The minimum
380 legal value for the MTU is 68.
381 option ip-forwarding flag;
383 This option specifies whether the client should configure its IP
385 layer for packet forwarding. A value of false means disable IP for‐
386 warding, and a value of true means enable IP forwarding.
387 option irc-server ip-address [, ip-address... ];
389 The IRC server option specifies a list of IRC servers available to
391 the client. Servers should be listed in order of preference.
392 option log-servers ip-address [, ip-address... ];
394 The log-server option specifies a list of MIT-LCS UDP log servers
396 available to the client. Servers should be listed in order of pref‐
397 erence.
398 option lpr-servers ip-address [, ip-address... ];
400 The LPR server option specifies a list of RFC 1179 line printer
402 servers available to the client. Servers should be listed in order
403 of preference.
404 option mask-supplier flag;
406 This option specifies whether or not the client should respond to
408 subnet mask requests using ICMP. A value of false indicates that the
409 client should not respond. A value of true means that the client
410 should respond.
411 option max-dgram-reassembly uint16;
413 This option specifies the maximum size datagram that the client
415 should be prepared to reassemble. The minimum legal value is 576.
416 option merit-dump text;
418 This option specifies the path-name of a file to which the client's
420 core image should be dumped in the event the client crashes. The
421 path is formatted as a character string consisting of characters from
422 the NVT ASCII character set.
423 option mobile-ip-home-agent ip-address [, ip-address... ];
425 This option specifies a list of IP addresses indicating mobile IP
427 home agents available to the client. Agents should be listed in
428 order of preference, although normally there will be only one such
429 agent.
430 option nds-context string;
432 The nds-context option specifies the name of the initial Netware
434 Directory Service for an NDS client.
435 option nds-servers ip-address [, ip-address... ];
437 The nds-servers option specifies a list of IP addresses of NDS
439 servers.
440 option nds-tree-name string;
442 The nds-tree-name option specifies NDS tree name that the NDS client
444 should use.
445 option netbios-dd-server ip-address [, ip-address... ];
447 The NetBIOS datagram distribution server (NBDD) option specifies a
449 list of RFC 1001/1002 NBDD servers listed in order of preference.
450 option netbios-name-servers ip-address [, ip-address...];
452 The NetBIOS name server (NBNS) option specifies a list of RFC
454 1001/1002 NBNS name servers listed in order of preference. NetBIOS
455 Name Service is currently more commonly referred to as WINS. WINS
456 servers can be specified using the netbios-name-servers option.
457 option netbios-node-type uint8;
459 The NetBIOS node type option allows NetBIOS over TCP/IP clients which
461 are configurable to be configured as described in RFC 1001/1002. The
462 value is specified as a single octet which identifies the client
463 type.
464 Possible node types are:
466 1 B-node: Broadcast - no WINS
468 2 P-node: Peer - WINS only
470 4 M-node: Mixed - broadcast, then WINS
472 8 H-node: Hybrid - WINS, then broadcast
474 option netbios-scope string;
476 The NetBIOS scope option specifies the NetBIOS over TCP/IP scope
478 parameter for the client as specified in RFC 1001/1002. See RFC1001,
479 RFC1002, and RFC1035 for character-set restrictions.
480 option netinfo-server-address ip-address [, ip-address... ];
482 The netinfo-server-address option has not been described in any RFC,
484 but has been allocated (and is claimed to be in use) by Apple Comput‐
485 ers. It's hard to say if the above is the correct format, or what
486 clients might be expected to do if values were configured. Use at
487 your own risk.
488 option netinfo-server-tag text;
490 The netinfo-server-tag option has not been described in any RFC, but
492 has been allocated (and is claimed to be in use) by Apple Computers.
493 It's hard to say if the above is the correct format, or what clients
494 might be expected to do if values were configured. Use at your own
495 risk.
496 option nis-domain text;
498 This option specifies the name of the client's NIS (Sun Network
500 Information Services) domain. The domain is formatted as a character
501 string consisting of characters from the NVT ASCII character set.
502 option nis-servers ip-address [, ip-address... ];
504 This option specifies a list of IP addresses indicating NIS servers
506 available to the client. Servers should be listed in order of pref‐
507 erence.
508 option nisplus-domain text;
510 This option specifies the name of the client's NIS+ domain. The
512 domain is formatted as a character string consisting of characters
513 from the NVT ASCII character set.
514 option nisplus-servers ip-address [, ip-address... ];
516 This option specifies a list of IP addresses indicating NIS+ servers
518 available to the client. Servers should be listed in order of pref‐
519 erence.
520 option nntp-server ip-address [, ip-address... ];
522 The NNTP server option specifies a list of NNTP servesr available to
524 the client. Servers should be listed in order of preference.
525 option non-local-source-routing flag;
527 This option specifies whether the client should configure its IP
529 layer to allow forwarding of datagrams with non-local source routes
530 (see Section 3.3.5 of [4] for a discussion of this topic). A value
531 of false means disallow forwarding of such datagrams, and a value of
532 true means allow forwarding.
533 option ntp-servers ip-address [, ip-address... ];
535 This option specifies a list of IP addresses indicating NTP (RFC
537 1035) servers available to the client. Servers should be listed in
538 order of preference.
539 option nwip-domain string;
541 The name of the NetWare/IP domain that a NetWare/IP client should
543 use.
544 option nwip-suboptions string;
546 A sequence of suboptions for NetWare/IP clients - see RFC2242 for
548 details. Normally this option is set by specifying specific Net‐
549 Ware/IP suboptions - see the NETWARE/IP SUBOPTIONS section for more
550 information.
551 option path-mtu-aging-timeout uint32;
553 This option specifies the timeout (in seconds) to use when aging Path
555 MTU values discovered by the mechanism defined in RFC 1191.
556 option path-mtu-plateau-table uint16 [, uint16... ];
558 This option specifies a table of MTU sizes to use when performing
560 Path MTU Discovery as defined in RFC 1191. The table is formatted as
561 a list of 16-bit unsigned integers, ordered from smallest to largest.
562 The minimum MTU value cannot be smaller than 68.
563 option perform-mask-discovery flag;
565 This option specifies whether or not the client should perform subnet
567 mask discovery using ICMP. A value of false indicates that the
568 client should not perform mask discovery. A value of true means that
569 the client should perform mask discovery.
570 option policy-filter ip-address ip-address
572 [, ip-address ip-address...];
573 This option specifies policy filters for non-local source routing.
575 The filters consist of a list of IP addresses and masks which specify
576 destination/mask pairs with which to filter incoming source routes.
577 Any source routed datagram whose next-hop address does not match one
579 of the filters should be discarded by the client.
580 See STD 3 (RFC1122) for further information.
582 option pop-server ip-address [, ip-address... ];
584 The POP3 server option specifies a list of POP3 servers available to
586 the client. Servers should be listed in order of preference.
587 option resource-location-servers ip-address
589 [, ip-address...];
590 This option specifies a list of RFC 887 Resource Location servers
592 available to the client. Servers should be listed in order of pref‐
593 erence.
594 option root-path text;
596 This option specifies the path-name that contains the client's root
598 disk. The path is formatted as a character string consisting of
599 characters from the NVT ASCII character set.
600 option router-discovery flag;
602 This option specifies whether or not the client should solicit
604 routers using the Router Discovery mechanism defined in RFC 1256. A
605 value of false indicates that the client should not perform router
606 discovery. A value of true means that the client should perform
607 router discovery.
608 option router-solicitation-address ip-address;
610 This option specifies the address to which the client should transmit
612 router solicitation requests.
613 option routers ip-address [, ip-address... ];
615 The routers option specifies a list of IP addresses for routers on
617 the client's subnet. Routers should be listed in order of prefer‐
618 ence.
619 option slp-directory-agent boolean ip-address [, ip-address... ];
621 This option specifies two things: the IP addresses of one or more
623 Service Location Protocol Directory Agents, and whether the use of
624 these addresses is mandatory. If the initial boolean value is true,
625 the SLP agent should just use the IP addresses given. If the value
626 is false, the SLP agent may additionally do active or passive multi‐
627 cast discovery of SLP agents (see RFC2165 for details).
628 Please note that in this option and the slp-service-scope option, the
630 term "SLP Agent" is being used to refer to a Service Location Proto‐
631 col agent running on a machine that is being configured using the
632 DHCP protocol.
633 Also, please be aware that some companies may refer to SLP as NDS.
635 If you have an NDS directory agent whose address you need to config‐
636 ure, the slp-directory-agent option should work.
637 option slp-service-scope boolean text;
639 The Service Location Protocol Service Scope Option specifies two
641 things: a list of service scopes for SLP, and whether the use of this
642 list is mandatory. If the initial boolean value is true, the SLP
643 agent should only use the list of scopes provided in this option;
644 otherwise, it may use its own static configuration in preference to
645 the list provided in this option.
646 The text string should be a comma-separated list of scopes that the
648 SLP agent should use. It may be omitted, in which case the SLP
649 Agent will use the aggregated list of scopes of all directory agents
650 known to the SLP agent.
651 option smtp-server ip-address [, ip-address... ];
653 The SMTP server option specifies a list of SMTP servers available to
655 the client. Servers should be listed in order of preference.
656 option static-routes ip-address ip-address
658 [, ip-address ip-address...];
659 This option specifies a list of static routes that the client should
661 install in its routing cache. If multiple routes to the same desti‐
662 nation are specified, they are listed in descending order of prior‐
663 ity.
664 The routes consist of a list of IP address pairs. The first address
666 is the destination address, and the second address is the router for
667 the destination.
668 The default route (0.0.0.0) is an illegal destination for a static
670 route. To specify the default route, use the routers option. Also,
671 please note that this option is not intended for classless IP routing
672 - it does not include a subnet mask. Since classless IP routing is
673 now the most widely deployed routing standard, this option is virtu‐
674 ally useless, and is not implemented by any of the popular DHCP
675 clients, for example the Microsoft DHCP client.
676 NOTE to Fedora dhclient users:
678 dhclient-script interprets trailing 0 octets of the target as indi‐
679 cating the subnet class of the route, so for the following static-
680 routes value:
681 option static-routes 172.0.0.0 172.16.2.254,
682 192.168.0.0 192.168.2.254;
683 dhclient-script will create routes:
684 172/8 via 172.16.2.254 dev $interface
685 192.168/16 via 192.168.2.254 dev $interface
686 option classless-static-routes destination-descriptor ip-address
688 [, destination-descriptor ip-address...];
689 This option (see RFC3442) specifies a list of classless static routes
691 that the client should install in its routing cache.
692 This option can contain one or more static routes, each of which con‐
694 sists of a destination descriptor and the IP address of the router
695 that should be used to reach that destination.
696 Many clients may not implement the Classless Static Routes option.
698 DHCP server administrators should therefore configure their DHCP
699 servers to send both a Router option and a Classless Static Routes
700 option, and should specify the default router(s) both in the Router
701 option and in the Classless Static Routes option.
702 If the DHCP server returns both a Classless Static Routes option and
704 a Router option, the DHCP client ignores the Router option.
705 option streettalk-directory-assistance-server ip-address
707 [, ip-address...];
708 The StreetTalk Directory Assistance (STDA) server option specifies a
710 list of STDA servers available to the client. Servers should be
711 listed in order of preference.
712 option streettalk-server ip-address [, ip-address... ];
714 The StreetTalk server option specifies a list of StreetTalk servers
716 available to the client. Servers should be listed in order of pref‐
717 erence.
718 option subnet-mask ip-address;
720 The subnet mask option specifies the client's subnet mask as per RFC
722 950. If no subnet mask option is provided anywhere in scope, as a
723 last resort dhcpd will use the subnet mask from the subnet declara‐
724 tion for the network on which an address is being assigned. However,
725 any subnet-mask option declaration that is in scope for the address
726 being assigned will override the subnet mask specified in the subnet
727 declaration.
728 option subnet-selection string;
730 Sent by the client if an address is required in a subnet other than
732 the one that would normally be selected (based on the relaying
733 address of the connected subnet the request is obtained from). See
734 RFC3011. Note that the option number used by this server is 118; this
735 has not always been the defined number, and some clients may use a
736 different value. Use of this option should be regarded as slightly
737 experimental!
738 This option is not user configurable in the server.
740 option swap-server ip-address;
742 This specifies the IP address of the client's swap server.
744 option tcp-keepalive-garbage flag;
746 This option specifies whether or not the client should send TCP
748 keepalive messages with an octet of garbage for compatibility with
749 older implementations. A value of false indicates that a garbage
750 octet should not be sent. A value of true indicates that a garbage
751 octet should be sent.
752 option tcp-keepalive-interval uint32;
754 This option specifies the interval (in seconds) that the client TCP
756 should wait before sending a keepalive message on a TCP connection.
757 The time is specified as a 32-bit unsigned integer. A value of zero
758 indicates that the client should not generate keepalive messages on
759 connections unless specifically requested by an application.
760 option tftp-server-name text;
762 This option is used to identify a TFTP server and, if supported by
764 the client, should have the same effect as the server-name declara‐
765 tion. BOOTP clients are unlikely to support this option. Some DHCP
766 clients will support it, and others actually require it.
767 option time-offset int32;
769 The time-offset option specifies the offset of the client's subnet in
771 seconds from Coordinated Universal Time (UTC).
772 option time-servers ip-address [, ip-address... ];
774 The time-server option specifies a list of RFC 868 time servers
776 available to the client. Servers should be listed in order of pref‐
777 erence.
778 option trailer-encapsulation flag;
780 This option specifies whether or not the client should negotiate the
782 use of trailers (RFC 893 [14]) when using the ARP protocol. A value
783 of false indicates that the client should not attempt to use trail‐
784 ers. A value of true means that the client should attempt to use
785 trailers.
786 option uap-servers text;
788 This option specifies a list of URLs, each pointing to a user authen‐
790 tication service that is capable of processing authentication
791 requests encapsulated in the User Authentication Protocol (UAP). UAP
792 servers can accept either HTTP 1.1 or SSLv3 connections. If the list
793 includes a URL that does not contain a port component, the normal
794 default port is assumed (i.e., port 80 for http and port 443 for
795 https). If the list includes a URL that does not contain a path com‐
796 ponent, the path /uap is assumed. If more than one URL is specified
797 in this list, the URLs are separated by spaces.
798 option user-class string;
800 This option is used by some DHCP clients as a way for users to spec‐
802 ify identifying information to the client. This can be used in a
803 similar way to the vendor-class-identifier option, but the value of
804 the option is specified by the user, not the vendor. Most recent
805 DHCP clients have a way in the user interface to specify the value
806 for this identifier, usually as a text string.
807 option vendor-class-identifier string;
809 This option is used by some DHCP clients to identify the vendor type
811 and possibly the configuration of a DHCP client. The information is
812 a string of bytes whose contents are specific to the vendor and are
813 not specified in a standard. To see what vendor class identifier
814 clients are sending, you can write the following in your DHCP server
815 configuration file:
816 set vendor-string = option vendor-class-identifier;
818 This will result in all entries in the DHCP server lease database
820 file for clients that sent vendor-class-identifier options having a
821 set statement that looks something like this:
822 set vendor-string = "SUNW.Ultra-5_10";
824 The vendor-class-identifier option is normally used by the DHCP
826 server to determine the options that are returned in the vendor-
827 encapsulated-options option. Please see the VENDOR ENCAPSULATED
828 OPTIONS section later in this manual page for further information.
829 option vendor-encapsulated-options string;
831 The vendor-encapsulated-options option can contain either a single
833 vendor-specific value or one or more vendor-specific suboptions.
834 This option is not normally specified in the DHCP server configura‐
835 tion file - instead, a vendor class is defined for each vendor, ven‐
836 dor class suboptions are defined, values for those suboptions are
837 defined, and the DHCP server makes up a response on that basis.
838 Some default behaviours for well-known DHCP client vendors (cur‐
840 rently, the Microsoft Windows 2000 DHCP client) are configured auto‐
841 matically, but otherwise this must be configured manually - see the
842 VENDOR ENCAPSULATED OPTIONS section later in this manual page for
843 details.
844 option vivso string;
846 The vivso option can contain multiple separate options, one for each
848 32-bit Enterprise ID. Each Enterprise-ID discriminated option then
849 contains additional options whose format is defined by the vendor who
850 holds that ID. This option is usually not configured manually, but
851 rather is configured via intervening option definitions. Please also
852 see the VENDOR ENCAPSULATED OPTIONS section later in this manual page
853 for details.
854 option www-server ip-address [, ip-address... ];
856 The WWW server option specifies a list of WWW servers available to
858 the client. Servers should be listed in order of preference.
859 option x-display-manager ip-address [, ip-address... ];
861 This option specifies a list of systems that are running the X Window
863 System Display Manager and are available to the client. Addresses
864 should be listed in order of preference.
865
867 An IETF draft, draft-ietf-dhc-agent-options-11.txt, defines a series of
868 encapsulated options that a relay agent can add to a DHCP packet when
869 relaying it to the DHCP server. The server can then make address
870 allocation decisions (or whatever other decisions it wants) based on
871 these options. The server also returns these options in any replies
872 it sends through the relay agent, so that the relay agent can use the
873 information in these options for delivery or accounting purposes.
874 The current draft defines two options. To reference these options in
876 the dhcp server, specify the option space name, "agent", followed by a
877 period, followed by the option name. It is not normally useful to
878 define values for these options in the server, although it is permissi‐
879 ble. These options are not supported in the client.
880 option agent.circuit-id string;
882 The circuit-id suboption encodes an agent-local identifier of the
884 circuit from which a DHCP client-to-server packet was received. It
885 is intended for use by agents in relaying DHCP responses back to the
886 proper circuit. The format of this option is currently defined to
887 be vendor-dependent, and will probably remain that way, although the
888 current draft allows for for the possibility of standardizing the
889 format in the future.
890 option agent.remote-id string;
892 The remote-id suboption encodes information about the remote host end
894 of a circuit. Examples of what it might contain include caller ID
895 information, username information, remote ATM address, cable modem
896 ID, and similar things. In principal, the meaning is not well-spec‐
897 ified, and it should generally be assumed to be an opaque object that
898 is administratively guaranteed to be unique to a particular remote
899 end of a circuit.
900 option agent.DOCSIS-device-class uint32;
902 The DOCSIS-device-class suboption is intended to convey information
904 about the host endpoint, hardware, and software, that either the host
905 operating system or the DHCP server may not otherwise be aware of
906 (but the relay is able to distinguish). This is implemented as a
907 32-bit field (4 octets), each bit representing a flag describing the
908 host in one of these ways. So far, only bit zero (being the least
909 significant bit) is defined in RFC3256. If this bit is set to one,
910 the host is considered a CPE Controlled Cable Modem (CCCM). All
911 other bits are reserved.
912 option agent.link-selection ip-address;
914 The link-selection suboption is provided by relay agents to inform
916 servers what subnet the client is actually attached to. This is use‐
917 ful in those cases where the giaddr (where responses must be sent to
918 the relay agent) is not on the same subnet as the client. When this
919 option is present in a packet from a relay agent, the DHCP server
920 will use its contents to find a subnet declared in configuration, and
921 from here take one step further backwards to any shared-network the
922 subnet may be defined within...the client may be given any address
923 within that shared network, as normally appropriate.
924
926 The Client FQDN option, currently defined in the Internet Draft draft-
927 ietf-dhc-fqdn-option-00.txt is not a standard yet, but is in suffi‐
928 ciently wide use already that we have implemented it. Due to the com‐
929 plexity of the option format, we have implemented it as a suboption
930 space rather than a single option. In general this option should not
931 be configured by the user - instead it should be used as part of an
932 automatic DNS update system.
933 option fqdn.no-client-update flag;
935 When the client sends this, if it is true, it means the client will
937 not attempt to update its A record. When sent by the server to the
938 client, it means that the client should not update its own A record.
939 option fqdn.server-update flag;
941 When the client sends this to the server, it is requesting that the
943 server update its A record. When sent by the server, it means that
944 the server has updated (or is about to update) the client's A record.
945 option fqdn.encoded flag;
947 If true, this indicates that the domain name included in the option
949 is encoded in DNS wire format, rather than as plain ASCII text. The
950 client normally sets this to false if it doesn't support DNS wire
951 format in the FQDN option. The server should always send back the
952 same value that the client sent. When this value is set on the con‐
953 figuration side, it controls the format in which the fqdn.fqdn subop‐
954 tion is encoded.
955 option fqdn.rcode1 flag;
957 option fqdn.rcode2 flag;
959 These options specify the result of the updates of the A and PTR
961 records, respectively, and are only sent by the DHCP server to the
962 DHCP client. The values of these fields are those defined in the DNS
963 protocol specification.
964 option fqdn.fqdn text;
966 Specifies the domain name that the client wishes to use. This can
968 be a fully-qualified domain name, or a single label. If there is no
969 trailing generally update that name in some locally-defined domain.
970 option fqdn.hostname --never set--;
972 This option should never be set, but it can be read back using the
974 option and config-option operators in an expression, in which case it
975 returns the first label in the fqdn.fqdn suboption - for example, if
976 the value of fqdn.fqdn is "foo.example.com.", then fqdn.hostname will
977 be "foo".
978 option fqdn.domainname --never set--;
980 This option should never be set, but it can be read back using the
982 option and config-option operators in an expression, in which case it
983 returns all labels after the first label in the fqdn.fqdn suboption -
984 for example, if the value of fqdn.fqdn is "foo.example.com.", then
985 fqdn.hostname will be "example.com.". If this suboption value is
986 not set, it means that an unqualified name was sent in the fqdn
987 option, or that no fqdn option was sent at all.
988 If you wish to use any of these suboptions, we strongly recommend that
990 you refer to the Client FQDN option draft (or standard, when it becomes
991 a standard) - the documentation here is sketchy and incomplete in com‐
992 parison, and is just intended for reference by people who already
993 understand the Client FQDN option specification.
994
996 RFC2242 defines a set of encapsulated options for Novell NetWare/IP
997 clients. To use these options in the dhcp server, specify the option
998 space name, "nwip", followed by a period, followed by the option name.
999 The following options can be specified:
1000 option nwip.nsq-broadcast flag;
1002 If true, the client should use the NetWare Nearest Server Query to
1004 locate a NetWare/IP server. The behaviour of the Novell client if
1005 this suboption is false, or is not present, is not specified.
1006 option nwip.preferred-dss ip-address [, ip-address... ];
1008 This suboption specifies a list of up to five IP addresses, each of
1010 which should be the IP address of a NetWare Domain SAP/RIP server
1011 (DSS).
1012 option nwip.nearest-nwip-server ip-address
1014 [, ip-address...];
1015 This suboption specifies a list of up to five IP addresses, each of
1017 which should be the IP address of a Nearest NetWare IP server.
1018 option nwip.autoretries uint8;
1020 Specifies the number of times that a NetWare/IP client should attempt
1022 to communicate with a given DSS server at startup.
1023 option nwip.autoretry-secs uint8;
1025 Specifies the number of seconds that a Netware/IP client should wait
1027 between retries when attempting to establish communications with a
1028 DSS server at startup.
1029 option nwip.nwip-1-1 uint8;
1031 If true, the NetWare/IP client should support NetWare/IP version 1.1
1033 compatibility. This is only needed if the client will be contacting
1034 Netware/IP version 1.1 servers.
1035 option nwip.primary-dss ip-address;
1037 Specifies the IP address of the Primary Domain SAP/RIP Service server
1039 (DSS) for this NetWare/IP domain. The NetWare/IP administration
1040 utility uses this value as Primary DSS server when configuring a sec‐
1041 ondary DSS server.
1042
1044 DHCPv6 options differ from DHCPv4 options partially due to using 16-bit
1045 code and length tags, but semantically zero-length options are legal in
1046 DHCPv6, and multiple options are treated differently. Whereas in
1047 DHCPv4 multiple options would be concatenated to form one option, in
1048 DHCPv6 they are expected to be individual instantiations. Understand‐
1049 ably, many options are not "allowed" to have multiple instances in a
1050 packet - normally these are options which are digested by the DHCP pro‐
1051 tocol software, and not by users or applications.
1052 option dhcp6.client-id string;
1054 This option specifies the client's DUID identifier. DUIDs are simi‐
1056 lar but different from DHCPv4 client identifiers - there are docu‐
1057 mented duid types:
1058 duid-llt
1060 duid-en
1062 duid-ll
1064 This value should not be configured, but rather is provided by
1066 clients and treated as an opaque identifier key blob by servers.
1067 option dhcp6.server-id string;
1069 This option specifies the server's DUID identifier. One may use this
1071 option to configure an opaque binary blob for your server's identi‐
1072 fier.
1073 option dhcp6.ia-na string;
1075 The Identity Association for Non-temporary Addresses (ia-na) carries
1077 assigned addresses that are not temporary addresses for use by the
1078 DHCPv6 client. This option is produced by the DHCPv6 server soft‐
1079 ware, and should not be configured.
1080 option dhcp6.ia-ta string;
1082 The Identity Association for Temporary Addresses (ia-ta) carries tem‐
1084 porary addresses, which may change upon every renewal. There is no
1085 support for this in the current DHCPv6 software.
1086 option dhcp6.ia-addr string;
1088 The Identity Association Address option is encapsulated inside ia-na
1090 or ia-ta options in order to represent addresses associated with
1091 those IA's. These options are manufactured by the software, so
1092 should not be configured.
1093 option dhcp6.oro uint16 [ , uint16, ... ];
1095 The Option Request Option ("ORO") is the DHCPv6 equivalent of the
1097 parameter-request-list. Clients supply this option to ask servers to
1098 reply with options relevant to their needs and use. This option must
1099 not be directly configured, the request syntax in dhclient.conf (5)
1100 should be used instead.
1101 option dhcp6.preference uint8;
1103 The preference option informs a DHCPv6 client which server is applied
1105 during the initial stages of configuration - once a client is bound
1106 to an IA, it will remain bound to that IA until it is no longer valid
1107 or has expired. This value may be configured on the server, and is
1108 digested by the client software.
1109 option dhcp6.elapsed-time uint16;
1111 The elapsed-time option is constructed by the DHCPv6 client software,
1113 and is potentially consumed by intermediaries. This option should
1114 not be configured.
1115 option dhcp6.relay-msg string;
1117 The relay-msg option is constructed by intervening DHCPv6 relay agent
1119 software. This option is entirely used by protocol software, and is
1120 not meant for user configuration.
1121 option dhcp6.unicast ip6-address;
1123 The unicast option is provided by DHCPv6 servers which are willing
1125 (or prefer) to receive Renew packets from their clients by exchanging
1126 UDP unicasts with them. Normally, DHCPv6 clients will multicast
1127 their Renew messages. This may be configured on the server, and
1128 should be configured as an address the server is ready to reply to.
1129 option dhcp6.status-code status-code [ string ] ;
1131 The status-code option is provided by DHCPv6 servers to inform
1133 clients of error conditions during protocol communication. This
1134 option is manufactured and digested by protocol software, and should
1135 not be configured.
1136 option dhcp6.rapid-commit ;
1138 The rapid-commit option is a zero-length option that clients use to
1140 indicate their desire to enter into rapid-commit with the server.
1141 This option is not supported by the client at this time, and is
1142 digested by the server when present, so should not be configured.
1143 option dhcp6.vendor-opts string;
1145 The vendor-opts option is actually an encapsulated sub-option space,
1147 in which each Vendor-specific Information Option (VSIO) is identified
1148 by a 32-bit Enterprise-ID number. The encapsulated option spaces
1149 within these options are defined by the vendors.
1150 To make use of this option, the best way is to examine the section
1152 titled VENDOR ENCAPSULATED OPTIONS below, in particular the bits
1153 about the "vsio" option space.
1154 option dhcp6.interface-id string;
1156 The interface-id option is manufactured by relay agents, and may be
1158 used to guide configuration differentiating clients by the interface
1159 they are remotely attached to. It does not make sense to configure a
1160 value for this option, but it may make sense to inspect its contents.
1161 option dhcp6.reconf-msg dhcpv6-message;
1163 The reconf-msg option is manufactured by servers, and sent to clients
1165 in Reconfigure messages to inform them of what message the client
1166 should Reconfigure using. There is no support for DHCPv6 Reconfigure
1167 extensions, and this option is documented informationally only.
1168 option dhcp6.reconf-accept ;
1170 The reconf-accept option is included by DHCPv6 clients that support
1172 the Reconfigure extentions, advertising that they will respond if the
1173 server were to ask them to Reconfigure. There is no support for
1174 DHCPv6 Reconfigure extensions, and this option is documented informa‐
1175 tionally only.
1176 option dhcp6.sip-servers-names domain-list;
1178 The sip-servers-names option allows SIP clients to locate a local SIP
1180 server that is to be used for all outbound SIP requests, a so-
1181 called"outbound proxy server." If you wish to use manually entered
1182 IPv6 addresses instead, please see the sip-servers-addresses option
1183 below.
1184 option dhcp6.sip-servers-addresses ip6-address [, ip6-address ... ] ;
1186 The sip-servers-addresses option allows SIP clients to locate a local
1188 SIP server that is to be used for all outbound SIP requests, a so-
1189 called "outbound proxy servers." If you wish to use domain names
1190 rather than IPv6 addresses, please see the sip-servers-names option
1191 above.
1192 option dhcp6.name-servers ip6-address [, ip6-address ... ] ;
1194 The name-servers option instructs clients about locally available
1196 recursive DNS servers. It is easiest to describe this as the "name‐
1197 server" line in /etc/resolv.conf.
1198 option dhcp6.domain-search domain-list;
1200 The domain-search option specifies the client's domain search path to
1202 be applied to recursive DNS queries. It is easiest to describe this
1203 as the "search" line in /etc/resolv.conf.
1204 option dhcp6.ia-pd string;
1206 The ia-pd option is manufactured by clients and servers to create a
1208 Prefix Delegation binding - to delegate an IPv6 prefix to the client.
1209 There is not yet any support for prefix delegation in this software,
1210 and this option is provided informationally only.
1211 option dhcp6.ia-prefix string;
1213 The ia-prefix option is placed inside ia-pd options in order to iden‐
1215 tify the prefix(es) allocated to the client. There is not yet any
1216 suport for prefix delegation in this software, and this option is
1217 provided informationally only.
1218 option dhcp6.nis-servers ip6-address [, ip6-address ... ] ;
1220 The nis-servers option identifies, in order, NIS servers available to
1222 the client.
1223 option dhcp6.nisp-servers ip6-address [, ip6-address ... ] ;
1225 The nisp-servers option identifies, in order, NIS+ servers available
1227 to the client.
1228 option nis-domain-name domain-list;
1230 The nis-domain-name option specifies the NIS domain name the client
1232 is expected to use, and is related to the nis-servers option.
1233 option nisp-domain-name domain-list;
1235 The nisp-domain-name option specifies the NIS+ domain name the client
1237 is expected to use, and is related to the nisp-servers option.
1238 option dhcp6.sntp-servers ip6-address [, ip6-address ... ] ;
1240 The sntp-servers option specifies a list of local SNTP servers avail‐
1242 able for the client to synchronize their clocks.
1243 option dhcp6.info-refresh-time uint32;
1245 The info-refresh-time option gives DHCPv6 clients using Information-
1247 request messages a hint as to how long they should between refreshing
1248 the information they were given. Note that this option will only be
1249 delivered to the client, and be likely to affect the client's behav‐
1250 iour, if the client requested the option.
1251 option dhcp6.bcms-server-d domain-list;
1253 The bcms-server-d option contains the domain names of local BCMS
1255 (Broadcast and Multicast Control Services) controllers which the
1256 client may use.
1257 option dhcp6.bcms-server-a ip6-address [, ip6-address ... ] ;
1259 The bcms-server-a option contains the IPv6 addresses of local BCMS
1261 (Broadcast and Multicast Control Services) controllers which the
1262 client may use.
1263 option dhcp6.remote-id string;
1265 The remote-id option is constructed by relay agents, to inform the
1267 server of details pertaining to what the relay knows about the client
1268 (such as what port it is attached to, and so forth). The contents of
1269 this option have some vendor-specific structure (similar to VSIO),
1270 but we have chosen to treat this option as an opaque field.
1271 option dhcp6.subscriber-id;
1273 The subscriber-id option is an opaque field provided by the relay
1275 agent, which provides additional information about the subscriber in
1276 question. The exact contents of this option depend upon the vendor
1277 and/or the operator's configuration of the remote device, and as such
1278 is an opaque field.
1279 option dhcp6.fqdn string;
1281 The fqdn option is normally constructed by the client or server, and
1283 negotiates the client's Fully Qualified Domain Name, as well as which
1284 party is responsible for Dynamic DNS Updates. See the section on the
1285 Client FQDN SubOptions for full details (the DHCPv4 and DHCPv6 FQDN
1286 options use the same "fqdn." encapsulated space, so are in all ways
1287 identical).
1288 option dhcp6.lq-query string;
1290 The lq-query option is used internally by for lease query.
1292 option dhcp6.client-data string;
1294 The client-data option is used internally by for lease query.
1296 option dhcp6.clt-time uint32;
1298 The clt-time option is used internally by for lease query.
1300 option dhcp6.lq-relay-data ip6-address string;
1302 The lq-relay-data option is used internally by for lease query.
1304 option dhcp6.lq-client-link ip6-address [, ip6-address ... ] ;
1306 The lq-client-link option is used internally by for lease query.
1308 option dhcp6.bootfile-url string ;
1310 The server sends this option to inform the client about a URL to a
1312 boot file. Used primarily for UEFI network booting, it contains an
1313 RFC3986 compliant URI which the client may use to boot an operating
1314 system. This option is defined in RFC5970
1315 option dhcp6.arch-type arch-id [, arch-id...] ;
1317 A client will send this option to a server so that the server may
1319 make decisions on what options and addresses to offer the requesting
1320 client. The option consists of a list of 16 bit unsigned values that
1321 represent the architecture of the requesting client. These values
1322 corespond to the values available to the dhcpv4 option architecture-
1323 type, as defined in RFC4578, section 2.1. This option is defined in
1324 RFC5970
1325 option dhcp6.net-id uint8 uint8 uint8 ;
1327 A client will send this option to a server to inform it about the
1329 clients level of UNDI support. The option consists of 3 octets (a
1330 type, major and minor value). Specific meanings of these values are
1331 doumented in section 2.2 of RFC4578. This option is defined in
1332 RFC5970
1333
1335 The Internet Systems Consortium DHCP client and server provide the
1336 capability to define new options. Each DHCP option has a name, a
1337 code, and a structure. The name is used by you to refer to the
1338 option. The code is a number, used by the DHCP server and client to
1339 refer to an option. The structure describes what the contents of an
1340 option looks like.
1341 To define a new option, you need to choose a name for it that is not in
1343 use for some other option - for example, you can't use "host-name"
1344 because the DHCP protocol already defines a host-name option, which is
1345 documented earlier in this manual page. If an option name doesn't
1346 appear in this manual page, you can use it, but it's probably a good
1347 idea to put some kind of unique string at the beginning so you can be
1348 sure that future options don't take your name. For example, you might
1349 define an option, "local-host-name", feeling some confidence that no
1350 official DHCP option name will ever start with "local".
1351 Once you have chosen a name, you must choose a code. All codes between
1353 224 and 254 are reserved as 'site-local' DHCP options, so you can pick
1354 any one of these for your site (not for your product/application). In
1355 RFC3942, site-local space was moved from starting at 128 to starting at
1356 224. In practice, some vendors have interpreted the protocol rather
1357 loosely and have used option code values greater than 128 themselves.
1358 There's no real way to avoid this problem, and it was thought to be
1359 unlikely to cause too much trouble in practice. If you come across a
1360 vendor-documented option code in either the new or old site-local spa‐
1361 ces, please contact your vendor and inform them about rfc3942.
1362 The structure of an option is simply the format in which the option
1364 data appears. The ISC DHCP server currently supports a few simple
1365 types, like integers, booleans, strings and IP addresses, and it also
1366 supports the ability to define arrays of single types or arrays of
1367 fixed sequences of types.
1368 New options are declared as follows:
1370 option new-name code new-code = definition ;
1372 The values of new-name and new-code should be the name you have chosen
1374 for the new option and the code you have chosen. The definition
1375 should be the definition of the structure of the option.
1376 The following simple option type definitions are supported:
1378 BOOLEAN
1380 option new-name code new-code = boolean ;
1382 An option of type boolean is a flag with a value of either on or off
1384 (or true or false). So an example use of the boolean type would be:
1385 option use-zephyr code 180 = boolean;
1387 option use-zephyr on;
1388 INTEGER
1390 option new-name code new-code = sign integer width ;
1392 The sign token should either be blank, unsigned or signed. The width
1394 can be either 8, 16 or 32, and refers to the number of bits in the
1395 integer. So for example, the following two lines show a definition of
1396 the sql-connection-max option and its use:
1397 option sql-connection-max code 192 = unsigned integer 16;
1399 option sql-connection-max 1536;
1400 IP-ADDRESS
1402 option new-name code new-code = ip-address ;
1404 An option whose structure is an IP address can be expressed either as a
1406 domain name or as a dotted quad. So the following is an example use of
1407 the ip-address type:
1408 option sql-server-address code 193 = ip-address;
1410 option sql-server-address sql.example.com;
1411 IP6-ADDRESS
1413 option new-name code new-code = ip6-address ;
1415 An option whose structure is an IPv6 address must be expressed as a
1417 valid IPv6 address. The following is an example use of the ip6-address
1418 type:
1419 option dhcp6.some-server code 1234 = array of ip6-address;
1421 option dhcp6.some-server 3ffe:bbbb:aaaa:aaaa::1, 3ffe:bbbb:aaaa:aaaa::2;
1422 TEXT
1425 option new-name code new-code = text ;
1427 An option whose type is text will encode an ASCII text string. For
1429 example:
1430 option sql-default-connection-name code 194 = text;
1432 option sql-default-connection-name "PRODZA";
1433 DATA STRING
1436 option new-name code new-code = string ;
1438 An option whose type is a data string is essentially just a collection
1440 of bytes, and can be specified either as quoted text, like the text
1441 type, or as a list of hexadecimal contents separated by colons whose
1442 values must be between 0 and FF. For example:
1443 option sql-identification-token code 195 = string;
1445 option sql-identification-token 17:23:19:a6:42:ea:99:7c:22;
1446 DOMAIN-LIST
1449 option new-name code new-code = domain-list [compressed] ;
1451 An option whose type is domain-list is an RFC1035 formatted (on the
1453 wire, "DNS Format") list of domain names, separated by root labels.
1454 The optional compressed keyword indicates if the option should be com‐
1455 pressed relative to the start of the option contents (not the packet
1456 contents).
1457 When in doubt, omit the compressed keyword. When the software recieves
1459 an option that is compressed and the compressed keyword is omitted, it
1460 will still decompress the option (relative to the option contents
1461 field). The keyword only controls whether or not transmitted packets
1462 are compressed.
1463 Note that when domain-list formatted options are output as environment
1465 variables to dhclient-script(8), the standard DNS -escape mechanism is
1466 used: they are decimal. This is appropriate for direct use in eg
1467 /etc/resolv.conf.
1468 ENCAPSULATION
1471 option new-name code new-code = encapsulate identifier ;
1473 An option whose type is encapsulate will encapsulate the contents of
1475 the option space specified in identifier. Examples of encapsulated
1476 options in the DHCP protocol as it currently exists include the vendor-
1477 encapsulated-options option, the netware-suboptions option and the
1478 relay-agent-information option.
1479 option space local;
1481 option local.demo code 1 = text;
1482 option local-encapsulation code 197 = encapsulate local;
1483 option local.demo "demo";
1484 ARRAYS
1487 Options can contain arrays of any of the above types except for the
1489 text and data string types, which aren't currently supported in arrays.
1490 An example of an array definition is as follows:
1491 option kerberos-servers code 200 = array of ip-address;
1493 option kerberos-servers 10.20.10.1, 10.20.11.1;
1494 RECORDS
1496 Options can also contain data structures consisting of a sequence of
1498 data types, which is sometimes called a record type. For example:
1499 option contrived-001 code 201 = { boolean, integer 32, text };
1501 option contrived-001 on 1772 "contrivance";
1502 It's also possible to have options that are arrays of records, for
1504 example:
1505 option new-static-routes code 201 = array of {
1507 ip-address, ip-address, ip-address, integer 8 };
1508 option static-routes
1509 10.0.0.0 255.255.255.0 net-0-rtr.example.com 1,
1510 10.0.1.0 255.255.255.0 net-1-rtr.example.com 1,
1511 10.2.0.0 255.255.224.0 net-2-0-rtr.example.com 3;
1512
1515 The DHCP protocol defines the vendor-encapsulated-options option, which
1516 allows vendors to define their own options that will be sent encapsu‐
1517 lated in a standard DHCP option. It also defines the Vendor Identified
1518 Vendor Sub Options option ("VIVSO"), and the DHCPv6 protocol defines
1519 the Vendor-specific Information Option ("VSIO"). The format of all of
1520 these options is usually internally a string of options, similarly to
1521 other normal DHCP options. The VIVSO and VSIO options differ in that
1522 that they contain options that correspond to vendor Enterprise-ID num‐
1523 bers (assigned by IANA), which then contain options according to each
1524 Vendor's specifications. You will need to refer to your vendor's docu‐
1525 mentation in order to form options to their specification.
1526 The value of these options can be set in one of two ways. The first
1528 way is to simply specify the data directly, using a text string or a
1529 colon-separated list of hexadecimal values. For help in forming these
1530 strings, please refer to RFC2132 for the DHCPv4 Vendor Specific Infor‐
1531 mation Option, RFC3925 for the DHCPv4 Vendor Identified Vendor Sub
1532 Options, or RFC3315 for the DHCPv6 Vendor-specific Information Option.
1533 For example:
1534 option vendor-encapsulated-options
1536 2:4:
1537 AC:11:41:1:
1538 3:12:
1539 73:75:6e:64:68:63:70:2d:73:65:72:76:65:72:31:37:2d:31:
1540 4:12:
1541 2f:65:78:70:6f:72:74:2f:72:6f:6f:74:2f:69:38:36:70:63;
1542 option vivso
1543 00:00:09:bf:0E:
1544 01:0c:
1545 48:65:6c:6c:6f:20:77:6f:72:6c:64:21;
1546 option dhcp6.vendor-opts
1547 00:00:09:bf:
1548 00:01:00:0c:
1549 48:65:6c:6c:6f:20:77:6f:72:6c:64:21;
1550 The second way of setting the value of these options is to have the
1552 DHCP server generate a vendor-specific option buffer. To do this, you
1553 must do four things: define an option space, define some options in
1554 that option space, provide values for them, and specify that that
1555 option space should be used to generate the relevant option.
1556 To define a new option space in which vendor options can be stored, use
1558 the option space statement:
1559 option space name [ [ code width number ] [ length width number ] [
1561 hash size number ] ] ;
1562 Where the numbers following code width, length width, and hash size
1564 respectively identify the number of bytes used to describe option
1565 codes, option lengths, and the size in buckets of the hash tables to
1566 hold options in this space (most DHCPv4 option spaces use 1 byte codes
1567 and lengths, which is the default, whereas most DHCPv6 option spaces
1568 use 2 byte codes and lengths).
1569 The code and length widths are used in DHCP protocol - you must config‐
1571 ure these numbers to match the applicable option space you are config‐
1572 uring. They each default to 1. Valid values for code widths are 1, 2
1573 or 4. Valid values for length widths are 0, 1 or 2. Most DHCPv4
1574 option spaces use 1 byte codes and lengths, which is the default,
1575 whereas most DHCPv6 option spaces use 2 byte codes and lengths. A
1576 zero-byte length produces options similar to the DHCPv6 Vendor-specific
1577 Information Option - but not their contents!
1578 The hash size defaults depend upon the code width selected, and may be
1580 254 or 1009. Valid values range between 1 and 65535. Note that the
1581 higher you configure this value, the more memory will be used. It is
1582 considered good practice to configure a value that is slightly larger
1583 than the estimated number of options you plan to configure within the
1584 space. Previous versions of ISC DHCP (up to and including DHCP 3.0.*),
1585 this value was fixed at 9973.
1586 The name can then be used in option definitions, as described earlier
1588 in this document. For example:
1589 option space SUNW code width 1 length width 1 hash size 3;
1591 option SUNW.server-address code 2 = ip-address;
1592 option SUNW.server-name code 3 = text;
1593 option SUNW.root-path code 4 = text;
1594 option space ISC code width 1 length width 1 hash size 3;
1596 option ISC.sample code 1 = text;
1597 option vendor.ISC code 2495 = encapsulate vivso-sample;
1598 option vendor-class.ISC code 2495 = text;
1599 option ISC.sample "configuration text here";
1601 option vendor-class.ISC "vendor class here";
1602 option space docsis code width 2 length width 2 hash size 17;
1604 option docsis.tftp-servers code 32 = array of ip6-address;
1605 option docsis.cablelabs-configuration-file code 33 = text;
1606 option docsis.cablelabs-syslog-servers code 34 = array of ip6-address;
1607 option docsis.device-id code 36 = string;
1608 option docsis.time-servers code 37 = array of ip6-address;
1609 option docsis.time-offset code 38 = signed integer 32;
1610 option vsio.docsis code 4491 = encapsulate docsis;
1611 Once you have defined an option space and the format of some options,
1613 you can set up scopes that define values for those options, and you can
1614 say when to use them. For example, suppose you want to handle two
1615 different classes of clients. Using the option space definition shown
1616 in the previous example, you can send different option values to dif‐
1617 ferent clients based on the vendor-class-identifier option that the
1618 clients send, as follows:
1619 class "vendor-classes" {
1621 match option vendor-class-identifier;
1622 }
1623 subclass "vendor-classes" "SUNW.Ultra-5_10" {
1625 vendor-option-space SUNW;
1626 option SUNW.root-path "/export/root/sparc";
1627 }
1628 subclass "vendor-classes" "SUNW.i86pc" {
1630 vendor-option-space SUNW;
1631 option SUNW.root-path "/export/root/i86pc";
1632 }
1633 option SUNW.server-address 172.17.65.1;
1635 option SUNW.server-name "sundhcp-server17-1";
1636 option vivso-sample.sample "Hello world!";
1638 option docsis.tftp-servers ::1;
1640 As you can see in the preceding example, regular scoping rules apply,
1643 so you can define values that are global in the global scope, and only
1644 define values that are specific to a particular class in the local
1645 scope. The vendor-option-space declaration tells the DHCP server to
1646 use options in the SUNW option space to construct the DHCPv4 vendor-
1647 encapsulated-options option. This is a limitation of that option - the
1648 DHCPv4 VIVSO and the DHCPv6 VSIO options can have multiple vendor defi‐
1649 nitions all at once (even transmitted to the same client), so it is not
1650 necessary to configure this.
1651
1653 dhcpd.conf(5), dhcpd.leases(5), dhclient.conf(5), dhcp-eval(5),
1654 dhcpd(8), dhclient(8), RFC2132, RFC2131, RFC3046, RFC3315.
1655
1657 The Internet Systems Consortium DHCP Distribution was written by Ted
1658 Lemon under a contract with Vixie Labs. Funding for this project was
1659 provided through Internet Systems Consortium. Information about Inter‐
1660 net Systems Consortium can be found at https://www.isc.org.
1661 dhcpd-options(5)