1dhcpd-options(5) File Formats Manual dhcpd-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
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 int32 data type specifies a signed 32-bit integer. The uint32
31 data type specifies an unsigned 32-bit integer. The int16 and uint16
32 data types specify signed and unsigned 16-bit integers. The int8 and
33 uint8 data types specify signed and unsigned 8-bit integers. Unsigned
34 8-bit integers are also sometimes referred to as octets.
35 The text data type specifies an NVT ASCII string, which must be
37 enclosed in double quotes - for example, to specify a root-path option,
38 the syntax would be
39 option root-path "10.0.1.4:/var/tmp/rootfs";
41 The domain-name data type specifies a domain name, which must not
43 enclosed in double quotes. This data type is not used for any exist‐
44 ing DHCP options. The domain name is stored just as if it were a text
45 option.
46 The flag data type specifies a boolean value. Booleans can be either
48 true or false (or on or off, if that makes more sense to you).
49 The string data type specifies either an NVT ASCII string enclosed in
51 double quotes, or a series of octets specified in hexadecimal, sepa‐
52 rated by colons. For example:
53 option dhcp-client-identifier "CLIENT-FOO";
55 or
56 option dhcp-client-identifier 43:4c:49:45:54:2d:46:4f:4f;
57
59 Sometimes it's helpful to be able to set the value of a DHCP option
60 based on some value that the client has sent. To do this, you can use
61 expression evaluation. The dhcp-eval(5) manual page describes how to
62 write expressions. To assign the result of an evaluation to an
63 option, define the option as follows:
64 option my-option = expression ;
66 For example:
68 option hostname = binary-to-ascii (16, 8, "-",
70 substring (hardware, 1, 6));
71
73 The documentation for the various options mentioned below is taken from
74 the latest IETF draft document on DHCP options. Options not listed
75 below may not yet be implemented, but it is possible to use such
76 options by defining them in the configuration file. Please see the
77 DEFINING NEW OPTIONS heading later in this document for more informa‐
78 tion.
79 Some of the options documented here are automatically generated by the
81 DHCP server or by clients, and cannot be configured by the user. The
82 value of such an option can be used in the configuration file of the
83 receiving DHCP protocol agent (server or client), for example in condi‐
84 tional expressions. However, the value of the option cannot be used in
85 the configuration file of the sending agent, because the value is
86 determined only after the configuration file has been processed. In the
87 following documentation, such options will be shown as "not user con‐
88 figurable"
89 The standard options are:
91 option all-subnets-local flag;
93 This option specifies whether or not the client may assume that all
95 subnets of the IP network to which the client is connected use the
96 same MTU as the subnet of that network to which the client is
97 directly connected. A value of true indicates that all subnets share
98 the same MTU. A value of false means that the client should assume
99 that some subnets of the directly connected network may have smaller
100 MTUs.
101 option arp-cache-timeout uint32;
103 This option specifies the timeout in seconds for ARP cache entries.
105 option bootfile-name text;
107 This option is used to identify a bootstrap file. If supported by
109 the client, it should have the same effect as the filename declara‐
110 tion. BOOTP clients are unlikely to support this option. Some DHCP
111 clients will support it, and others actually require it.
112 option boot-size uint16;
114 This option specifies the length in 512-octet blocks of the default
116 boot image for the client.
117 option broadcast-address ip-address;
119 This option specifies the broadcast address in use on the client's
121 subnet. Legal values for broadcast addresses are specified in sec‐
122 tion 3.2.1.3 of STD 3 (RFC1122).
123 option cookie-servers ip-address [, ip-address... ];
125 The cookie server option specifies a list of RFC 865 cookie servers
127 available to the client. Servers should be listed in order of pref‐
128 erence.
129 option default-ip-ttl uint8;
131 This option specifies the default time-to-live that the client should
133 use on outgoing datagrams.
134 option default-tcp-ttl uint8;
136 This option specifies the default TTL that the client should use when
138 sending TCP segments. The minimum value is 1.
139 option dhcp-client-identifier string;
141 This option can be used to specify a DHCP client identifier in a host
143 declaration, so that dhcpd can find the host record by matching
144 against the client identifier.
145 Please be aware that some DHCP clients, when configured with client
147 identifiers that are ASCII text, will prepend a zero to the ASCII
148 text. So you may need to write:
149 option dhcp-client-identifier "\0foo";
151 rather than:
153 option dhcp-client-identifier "foo";
155 option dhcp-lease-time uint32;
157 This option is used in a client request (DHCPDISCOVER or DHCPREQUEST)
159 to allow the client to request a lease time for the IP address. In a
160 server reply (DHCPOFFER), a DHCP server uses this option to specify
161 the lease time it is willing to offer.
162 This option is not directly user configurable in the server; refer to
164 the max-lease-time and default-lease-time server options in
165 dhcpd.conf(5).
166 option dhcp-max-message-size uint16;
168 This option, when sent by the client, specifies the maximum size of
170 any response that the server sends to the client. When specified on
171 the server, if the client did not send a dhcp-max-message-size
172 option, the size specified on the server is used. This works for
173 BOOTP as well as DHCP responses.
174 option dhcp-message text;
176 This option is used by a DHCP server to provide an error message to a
178 DHCP client in a DHCPNAK message in the event of a failure. A client
179 may use this option in a DHCPDECLINE message to indicate why the
180 client declined the offered parameters.
181 This option is not user configurable.
183 option dhcp-message-type uint8;
185 This option, sent by both client and server, specifies the type of
187 DHCP message contained in the DHCP packet. Possible values (taken
188 directly from RFC2132) are:
189 1 DHCPDISCOVER
191 2 DHCPOFFER
192 3 DHCPREQUEST
193 4 DHCPDECLINE
194 5 DHCPACK
195 6 DHCPNAK
196 7 DHCPRELEASE
197 8 DHCPINFORM
198 This option is not user configurable.
200 option dhcp-option-overload uint8;
202 This option is used to indicate that the DHCP 'sname' or 'file'
204 fields are being overloaded by using them to carry DHCP options. A
205 DHCP server inserts this option if the returned parameters will
206 exceed the usual space allotted for options.
207 If this option is present, the client interprets the specified addi‐
209 tional fields after it concludes interpretation of the standard
210 option fields.
211 Legal values for this option are:
213 1 the 'file' field is used to hold options
215 2 the 'sname' field is used to hold options
216 3 both fields are used to hold options
217 This option is not user configurable.
219 option dhcp-parameter-request-list uint16;
221 This option, when sent by the client, specifies which options the
223 client wishes the server to return. Normally, in the ISC DHCP
224 client, this is done using the request statement. If this option is
225 not specified by the client, the DHCP server will normally return
226 every option that is valid in scope and that fits into the reply.
227 When this option is specified on the server, the server returns the
228 specified options. This can be used to force a client to take
229 options that it hasn't requested, and it can also be used to tailor
230 the response of the DHCP server for clients that may need a more lim‐
231 ited set of options than those the server would normally return.
232 option dhcp-rebinding-time uint32;
234 This option specifies the number of seconds from the time a client
236 gets an address until the client transitions to the REBINDING state.
237 This option is not user configurable.
239 option dhcp-renewal-time uint32;
241 This option specifies the number of seconds from the time a client
243 gets an address until the client transitions to the RENEWING state.
244 This option is not user configurable.
246 option dhcp-requested-address ip-address;
248 This option is used by the client in a DHCPDISCOVER to request that a
250 particular IP address be assigned.
251 This option is not user configurable.
253 option dhcp-server-identifier ip-address;
255 This option is used in DHCPOFFER and DHCPREQUEST messages, and may
257 optionally be included in the DHCPACK and DHCPNAK messages. DHCP
258 servers include this option in the DHCPOFFER in order to allow the
259 client to distinguish between lease offers. DHCP clients use the
260 contents of the 'server identifier' field as the destination address
261 for any DHCP messages unicast to the DHCP server. DHCP clients also
262 indicate which of several lease offers is being accepted by including
263 this option in a DHCPREQUEST message.
264 The value of this option is the IP address of the server.
266 This option is not directly user configurable. See the server-identi‐
268 fier server option in dhcpd.conf(5).
269 option domain-name text;
271 This option specifies the domain name that client should use when
273 resolving hostnames via the Domain Name System.
274 option domain-name-servers ip-address [, ip-address... ];
276 The domain-name-servers option specifies a list of Domain Name System
278 (STD 13, RFC 1035) name servers available to the client. Servers
279 should be listed in order of preference.
280 option extensions-path text;
282 This option specifies the name of a file containing additional
284 options to be interpreted according to the DHCP option format as
285 specified in RFC2132.
286 option finger-server ip-address [, ip-address... ];
288 The Finger server option specifies a list of Finger servers available
290 to the client. Servers should be listed in order of preference.
291 option font-servers ip-address [, ip-address... ];
293 This option specifies a list of X Window System Font servers avail‐
295 able to the client. Servers should be listed in order of preference.
296 option host-name string;
298 This option specifies the name of the client. The name may or may
300 not be qualified with the local domain name (it is preferable to use
301 the domain-name option to specify the domain name). See RFC 1035 for
302 character set restrictions. This option is only honored by dhclient-
303 script(8) if the hostname for the client machine is not set.
304 option ieee802-3-encapsulation flag;
306 This option specifies whether or not the client should use Ethernet
308 Version 2 (RFC 894) or IEEE 802.3 (RFC 1042) encapsulation if the
309 interface is an Ethernet. A value of false indicates that the client
310 should use RFC 894 encapsulation. A value of true means that the
311 client should use RFC 1042 encapsulation.
312 option ien116-name-servers ip-address [, ip-address... ];
314 The ien116-name-servers option specifies a list of IEN 116 name
316 servers available to the client. Servers should be listed in order
317 of preference.
318 option impress-servers ip-address [, ip-address... ];
320 The impress-server option specifies a list of Imagen Impress servers
322 available to the client. Servers should be listed in order of pref‐
323 erence.
324 option interface-mtu uint16;
326 This option specifies the MTU to use on this interface. The minimum
328 legal value for the MTU is 68.
329 option ip-forwarding flag;
331 This option specifies whether the client should configure its IP
333 layer for packet forwarding. A value of false means disable IP for‐
334 warding, and a value of true means enable IP forwarding.
335 option irc-server ip-address [, ip-address... ];
337 The IRC server option specifies a list of IRC servers available to
339 the client. Servers should be listed in order of preference.
340 option log-servers ip-address [, ip-address... ];
342 The log-server option specifies a list of MIT-LCS UDP log servers
344 available to the client. Servers should be listed in order of pref‐
345 erence.
346 option lpr-servers ip-address [, ip-address... ];
348 The LPR server option specifies a list of RFC 1179 line printer
350 servers available to the client. Servers should be listed in order
351 of preference.
352 option mask-supplier flag;
354 This option specifies whether or not the client should respond to
356 subnet mask requests using ICMP. A value of false indicates that the
357 client should not respond. A value of true means that the client
358 should respond.
359 option max-dgram-reassembly uint16;
361 This option specifies the maximum size datagram that the client
363 should be prepared to reassemble. The minimum legal value is 576.
364 option merit-dump text;
366 This option specifies the path-name of a file to which the client's
368 core image should be dumped in the event the client crashes. The
369 path is formatted as a character string consisting of characters from
370 the NVT ASCII character set.
371 option mobile-ip-home-agent ip-address [, ip-address... ];
373 This option specifies a list of IP addresses indicating mobile IP
375 home agents available to the client. Agents should be listed in
376 order of preference, although normally there will be only one such
377 agent.
378 option nds-context string;
380 The nds-context option specifies the name of the initial Netware
382 Directory Service for an NDS client.
383 option nds-servers ip-address [, ip-address... ];
385 The nds-servers option specifies a list of IP addresses of NDS
387 servers.
388 option nds-tree-name string;
390 The nds-tree-name option specifies NDS tree name that the NDS client
392 should use.
393 option netbios-dd-server ip-address [, ip-address... ];
395 The NetBIOS datagram distribution server (NBDD) option specifies a
397 list of RFC 1001/1002 NBDD servers listed in order of preference.
398 option netbios-name-servers ip-address [, ip-address...];
400 The NetBIOS name server (NBNS) option specifies a list of RFC
402 1001/1002 NBNS name servers listed in order of preference. NetBIOS
403 Name Service is currently more commonly referred to as WINS. WINS
404 servers can be specified using the netbios-name-servers option.
405 option netbios-node-type uint8;
407 The NetBIOS node type option allows NetBIOS over TCP/IP clients which
409 are configurable to be configured as described in RFC 1001/1002. The
410 value is specified as a single octet which identifies the client
411 type.
412 Possible node types are:
414 1 B-node: Broadcast - no WINS
416 2 P-node: Peer - WINS only
418 4 M-node: Mixed - broadcast, then WINS
420 8 H-node: Hybrid - WINS, then broadcast
422 option netbios-scope string;
424 The NetBIOS scope option specifies the NetBIOS over TCP/IP scope
426 parameter for the client as specified in RFC 1001/1002. See RFC1001,
427 RFC1002, and RFC1035 for character-set restrictions.
428 option nis-domain text;
430 This option specifies the name of the client's NIS (Sun Network
432 Information Services) domain. The domain is formatted as a character
433 string consisting of characters from the NVT ASCII character set.
434 option nis-servers ip-address [, ip-address... ];
436 This option specifies a list of IP addresses indicating NIS servers
438 available to the client. Servers should be listed in order of pref‐
439 erence.
440 option nisplus-domain text;
442 This option specifies the name of the client's NIS+ domain. The
444 domain is formatted as a character string consisting of characters
445 from the NVT ASCII character set.
446 option nisplus-servers ip-address [, ip-address... ];
448 This option specifies a list of IP addresses indicating NIS+ servers
450 available to the client. Servers should be listed in order of pref‐
451 erence.
452 option nntp-server ip-address [, ip-address... ];
454 The NNTP server option specifies a list of NNTP servesr available to
456 the client. Servers should be listed in order of preference.
457 option non-local-source-routing flag;
459 This option specifies whether the client should configure its IP
461 layer to allow forwarding of datagrams with non-local source routes
462 (see Section 3.3.5 of [4] for a discussion of this topic). A value
463 of false means disallow forwarding of such datagrams, and a value of
464 true means allow forwarding.
465 option ntp-servers ip-address [, ip-address... ];
467 This option specifies a list of IP addresses indicating NTP (RFC
469 1035) servers available to the client. Servers should be listed in
470 order of preference.
471 option nwip-domain string;
473 The name of the NetWare/IP domain that a NetWare/IP client should
475 use.
476 option nwip-suboptions string;
478 A sequence of suboptions for NetWare/IP clients - see RFC2242 for
480 details. Normally this option is set by specifying specific Net‐
481 Ware/IP suboptions - see the NETWARE/IP SUBOPTIONS section for more
482 information.
483 option path-mtu-aging-timeout uint32;
485 This option specifies the timeout (in seconds) to use when aging Path
487 MTU values discovered by the mechanism defined in RFC 1191.
488 option path-mtu-plateau-table uint16 [, uint16... ];
490 This option specifies a table of MTU sizes to use when performing
492 Path MTU Discovery as defined in RFC 1191. The table is formatted as
493 a list of 16-bit unsigned integers, ordered from smallest to largest.
494 The minimum MTU value cannot be smaller than 68.
495 option perform-mask-discovery flag;
497 This option specifies whether or not the client should perform subnet
499 mask discovery using ICMP. A value of false indicates that the
500 client should not perform mask discovery. A value of true means that
501 the client should perform mask discovery.
502 option policy-filter ip-address ip-address
504 [, ip-address ip-address...];
505 This option specifies policy filters for non-local source routing.
507 The filters consist of a list of IP addresses and masks which specify
508 destination/mask pairs with which to filter incoming source routes.
509 Any source routed datagram whose next-hop address does not match one
511 of the filters should be discarded by the client.
512 See STD 3 (RFC1122) for further information.
514 option pop-server ip-address [, ip-address... ];
516 The POP3 server option specifies a list of POP3 servers available to
518 the client. Servers should be listed in order of preference.
519 option resource-location-servers ip-address
521 [, ip-address...];
522 This option specifies a list of RFC 887 Resource Location servers
524 available to the client. Servers should be listed in order of pref‐
525 erence.
526 option root-path text;
528 This option specifies the path-name that contains the client's root
530 disk. The path is formatted as a character string consisting of
531 characters from the NVT ASCII character set.
532 option router-discovery flag;
534 This option specifies whether or not the client should solicit
536 routers using the Router Discovery mechanism defined in RFC 1256. A
537 value of false indicates that the client should not perform router
538 discovery. A value of true means that the client should perform
539 router discovery.
540 option router-solicitation-address ip-address;
542 This option specifies the address to which the client should transmit
544 router solicitation requests.
545 option routers ip-address [, ip-address... ];
547 The routers option specifies a list of IP addresses for routers on
549 the client's subnet. Routers should be listed in order of prefer‐
550 ence.
551 option slp-directory-agent boolean ip-address [, ip-address... ];
553 This option specifies two things: the IP addresses of one or more
555 Service Location Protocol Directory Agents, and whether the use of
556 these addresses is mandatory. If the initial boolean value is true,
557 the SLP agent should just use the IP addresses given. If the value
558 is false, the SLP agent may additionally do active or passive multi‐
559 cast discovery of SLP agents (see RFC2165 for details).
560 Please note that in this option and the slp-service-scope option, the
562 term "SLP Agent" is being used to refer to a Service Location Proto‐
563 col agent running on a machine that is being configured using the
564 DHCP protocol.
565 Also, please be aware that some companies may refer to SLP as NDS.
567 If you have an NDS directory agent whose address you need to config‐
568 ure, the slp-directory-agent option should work.
569 option slp-service-scope boolean text;
571 The Service Location Protocol Service Scope Option specifies two
573 things: a list of service scopes for SLP, and whether the use of this
574 list is mandatory. If the initial boolean value is true, the SLP
575 agent should only use the list of scopes provided in this option;
576 otherwise, it may use its own static configuration in preference to
577 the list provided in this option.
578 The text string should be a comma-separated list of scopes that the
580 SLP agent should use. It may be omitted, in which case the SLP
581 Agent will use the aggregated list of scopes of all directory agents
582 known to the SLP agent.
583 option smtp-server ip-address [, ip-address... ];
585 The SMTP server option specifies a list of SMTP servers available to
587 the client. Servers should be listed in order of preference.
588 option static-routes ip-address ip-address
590 [, ip-address ip-address...];
591 This option specifies a list of static routes that the client should
593 install in its routing cache. If multiple routes to the same desti‐
594 nation are specified, they are listed in descending order of prior‐
595 ity.
596 The routes consist of a list of IP address pairs. The first address
598 is the destination address, and the second address is the router for
599 the destination.
600 The default route (0.0.0.0) is an illegal destination for a static
602 route. To specify the default route, use the routers option. Also,
603 please note that this option is not intended for classless IP routing
604 - it does not include a subnet mask. Since classless IP routing is
605 now the most widely deployed routing standard, this option is virtu‐
606 ally useless, and is not implemented by any of the popular DHCP
607 clients, for example the Microsoft DHCP client.
608 NOTE to Red Hat dhclient users:
610 The RedHat dhclient-script interprets trailing 0 octets of the tar‐
611 get as indicating the subnet class of the route - so for this
612 static-routes value:
613 option static-routes 172.0.0.0 172.16.2.254,
614 192.168.0.0 192.168.2.254;
615 the Red Hat dhclient-script will create routes:
616 172/8 via 172.16.2.254 dev $interface
617 192.168/16 via 192.168.2.254 dev $interface
618 which slightly increases the usefulness of the static-routes option.
619 option streettalk-directory-assistance-server ip-address
621 [, ip-address...];
622 The StreetTalk Directory Assistance (STDA) server option specifies a
624 list of STDA servers available to the client. Servers should be
625 listed in order of preference.
626 option streettalk-server ip-address [, ip-address... ];
628 The StreetTalk server option specifies a list of StreetTalk servers
630 available to the client. Servers should be listed in order of pref‐
631 erence.
632 option subnet-mask ip-address;
634 The subnet mask option specifies the client's subnet mask as per RFC
636 950. If no subnet mask option is provided anywhere in scope, as a
637 last resort dhcpd will use the subnet mask from the subnet declara‐
638 tion for the network on which an address is being assigned. However,
639 any subnet-mask option declaration that is in scope for the address
640 being assigned will override the subnet mask specified in the subnet
641 declaration.
642 option subnet-selection string;
644 Sent by the client if an address is required in a subnet other than
646 the one that would normally be selected (based on the relaying
647 address of the connected subnet the request is obtained from). See
648 RFC3011. Note that the option number used by this server is 118; this
649 has not always been the defined number, and some clients may use a
650 different value. Use of this option should be regarded as slightly
651 experimental!
652 This option is not user configurable in the server.
654 option swap-server ip-address;
656 This specifies the IP address of the client's swap server.
658 option tcp-keepalive-garbage flag;
660 This option specifies whether or not the client should send TCP
662 keepalive messages with an octet of garbage for compatibility with
663 older implementations. A value of false indicates that a garbage
664 octet should not be sent. A value of true indicates that a garbage
665 octet should be sent.
666 option tcp-keepalive-interval uint32;
668 This option specifies the interval (in seconds) that the client TCP
670 should wait before sending a keepalive message on a TCP connection.
671 The time is specified as a 32-bit unsigned integer. A value of zero
672 indicates that the client should not generate keepalive messages on
673 connections unless specifically requested by an application.
674 option tftp-server-name text;
676 This option is used to identify a TFTP server and, if supported by
678 the client, should have the same effect as the server-name declara‐
679 tion. BOOTP clients are unlikely to support this option. Some DHCP
680 clients will support it, and others actually require it.
681 option time-offset int32;
683 The time-offset option specifies the offset of the client's subnet in
685 seconds from Coordinated Universal Time (UTC).
686 option time-servers ip-address [, ip-address... ];
688 The time-server option specifies a list of RFC 868 time servers
690 available to the client. Servers should be listed in order of pref‐
691 erence.
692 option trailer-encapsulation flag;
694 This option specifies whether or not the client should negotiate the
696 use of trailers (RFC 893 [14]) when using the ARP protocol. A value
697 of false indicates that the client should not attempt to use trail‐
698 ers. A value of true means that the client should attempt to use
699 trailers.
700 option uap-servers text;
702 This option specifies a list of URLs, each pointing to a user authen‐
704 tication service that is capable of processing authentication
705 requests encapsulated in the User Authentication Protocol (UAP). UAP
706 servers can accept either HTTP 1.1 or SSLv3 connections. If the list
707 includes a URL that does not contain a port component, the normal
708 default port is assumed (i.e., port 80 for http and port 443 for
709 https). If the list includes a URL that does not contain a path com‐
710 ponent, the path /uap is assumed. If more than one URL is specified
711 in this list, the URLs are separated by spaces.
712 option user-class string;
714 This option is used by some DHCP clients as a way for users to spec‐
716 ify identifying information to the client. This can be used in a
717 similar way to the vendor-class-identifier option, but the value of
718 the option is specified by the user, not the vendor. Most recent
719 DHCP clients have a way in the user interface to specify the value
720 for this identifier, usually as a text string.
721 option vendor-class-identifier string;
723 This option is used by some DHCP clients to identify the vendor type
725 and possibly the configuration of a DHCP client. The information is
726 a string of bytes whose contents are specific to the vendor and are
727 not specified in a standard. To see what vendor class identifier
728 clients are sending, you can write the following in your DHCP server
729 configuration file:
730 set vendor-string = option vendor-class-identifier;
732 This will result in all entries in the DHCP server lease database
734 file for clients that sent vendor-class-identifier options having a
735 set statement that looks something like this:
736 set vendor-string = "SUNW.Ultra-5_10";
738 The vendor-class-identifier option is normally used by the DHCP
740 server to determine the options that are returned in the vendor-
741 encapsulated-options option. Please see the VENDOR ENCAPSULATED
742 OPTIONS section later in this manual page for further information.
743 option vendor-encapsulated-options string;
745 The vendor-encapsulated-options option can contain either a single
747 vendor-specific value or one or more vendor-specific suboptions.
748 This option is not normally specified in the DHCP server configura‐
749 tion file - instead, a vendor class is defined for each vendor, ven‐
750 dor class suboptions are defined, values for those suboptions are
751 defined, and the DHCP server makes up a response on that basis.
752 Some default behaviours for well-known DHCP client vendors (cur‐
754 rently, the Microsoft Windows 2000 DHCP client) are configured auto‐
755 matically, but otherwise this must be configured manually - see the
756 VENDOR ENCAPSULATED OPTIONS section later in this manual page for
757 details.
758 option www-server ip-address [, ip-address... ];
760 The WWW server option specifies a list of WWW servers available to
762 the client. Servers should be listed in order of preference.
763 option x-display-manager ip-address [, ip-address... ];
765 This option specifies a list of systems that are running the X Window
767 System Display Manager and are available to the client. Addresses
768 should be listed in order of preference.
769
771 An IETF draft, draft-ietf-dhc-agent-options-11.txt, defines a series of
772 encapsulated options that a relay agent can add to a DHCP packet when
773 relaying it to the DHCP server. The server can then make address
774 allocation decisions (or whatever other decisions it wants) based on
775 these options. The server also returns these options in any replies
776 it sends through the relay agent, so that the relay agent can use the
777 information in these options for delivery or accounting purposes.
778 The current draft defines two options. To reference these options in
780 the dhcp server, specify the option space name, "agent", followed by a
781 period, followed by the option name. It is not normally useful to
782 define values for these options in the server, although it is permissi‐
783 ble. These options are not supported in the client.
784 option agent.circuit-id string;
786 The circuit-id suboption encodes an agent-local identifier of the
788 circuit from which a DHCP client-to-server packet was received. It
789 is intended for use by agents in relaying DHCP responses back to the
790 proper circuit. The format of this option is currently defined to
791 be vendor-dependent, and will probably remain that way, although the
792 current draft allows for for the possibility of standardizing the
793 format in the future.
794 option agent.remote-id string;
796 The remote-id suboption encodes information about the remote host end
798 of a circuit. Examples of what it might contain include caller ID
799 information, username information, remote ATM address, cable modem
800 ID, and similar things. In principal, the meaning is not well-spec‐
801 ified, and it should generally be assumed to be an opaque object that
802 is administratively guaranteed to be unique to a particular remote
803 end of a circuit.
804 option agent.DOCSIS-device-class uint32;
806 The DOCSIS-device-class suboption is intended to convey information
808 about the host endpoint, hardware, and software, that either the host
809 operating system or the DHCP server may not otherwise be aware of
810 (but the relay is able to distinguish). This is implemented as a
811 32-bit field (4 octets), each bit representing a flag describing the
812 host in one of these ways. So far, only bit zero (being the least
813 significant bit) is defined in RFC3256. If this bit is set to one,
814 the host is considered a CPE Controlled Cable Modem (CCCM). All
815 other bits are reserved.
816
818 The Client FQDN option, currently defined in the Internet Draft draft-
819 ietf-dhc-fqdn-option-00.txt is not a standard yet, but is in suffi‐
820 ciently wide use already that we have implemented it. Due to the com‐
821 plexity of the option format, we have implemented it as a suboption
822 space rather than a single option. In general this option should not
823 be configured by the user - instead it should be used as part of an
824 automatic DNS update system.
825 option fqdn.no-client-update flag;
827 When the client sends this, if it is true, it means the client will
829 not attempt to update its A record. When sent by the server to the
830 client, it means that the client should not update its own A record.
831 option fqdn.server-update flag;
833 When the client sends this to the server, it is requesting that the
835 server update its A record. When sent by the server, it means that
836 the server has updated (or is about to update) the client's A record.
837 option fqdn.encoded flag;
839 If true, this indicates that the domain name included in the option
841 is encoded in DNS wire format, rather than as plain ASCII text. The
842 client normally sets this to false if it doesn't support DNS wire
843 format in the FQDN option. The server should always send back the
844 same value that the client sent. When this value is set on the con‐
845 figuration side, it controls the format in which the fqdn.fqdn subop‐
846 tion is encoded.
847 option fqdn.rcode1 flag;
849 option fqdn.rcode2 flag;
851 These options specify the result of the updates of the A and PTR
853 records, respectively, and are only sent by the DHCP server to the
854 DHCP client. The values of these fields are those defined in the DNS
855 protocol specification.
856 option fqdn.fqdn text;
858 Specifies the domain name that the client wishes to use. This can
860 be a fully-qualified domain name, or a single label. If there is no
861 trailing generally update that name in some locally-defined domain.
862 option fqdn.hostname --never set--;
864 This option should never be set, but it can be read back using the
866 option and config-option operators in an expression, in which case it
867 returns the first label in the fqdn.fqdn suboption - for example, if
868 the value of fqdn.fqdn is "foo.example.com.", then fqdn.hostname will
869 be "foo".
870 option fqdn.domainname --never set--;
872 This option should never be set, but it can be read back using the
874 option and config-option operators in an expression, in which case it
875 returns all labels after the first label in the fqdn.fqdn suboption -
876 for example, if the value of fqdn.fqdn is "foo.example.com.", then
877 fqdn.hostname will be "example.com.". If this suboption value is
878 not set, it means that an unqualified name was sent in the fqdn
879 option, or that no fqdn option was sent at all.
880 If you wish to use any of these suboptions, we strongly recommend that
882 you refer to the Client FQDN option draft (or standard, when it becomes
883 a standard) - the documentation here is sketchy and incomplete in com‐
884 parison, and is just intended for reference by people who already
885 understand the Client FQDN option specification.
886
888 RFC2242 defines a set of encapsulated options for Novell NetWare/IP
889 clients. To use these options in the dhcp server, specify the option
890 space name, "nwip", followed by a period, followed by the option name.
891 The following options can be specified:
892 option nwip.nsq-broadcast flag;
894 If true, the client should use the NetWare Nearest Server Query to
896 locate a NetWare/IP server. The behaviour of the Novell client if
897 this suboption is false, or is not present, is not specified.
898 option nwip.preferred-dss ip-address [, ip-address... ];
900 This suboption specifies a list of up to five IP addresses, each of
902 which should be the IP address of a NetWare Domain SAP/RIP server
903 (DSS).
904 option nwip.nearest-nwip-server ip-address
906 [, ip-address...];
907 This suboption specifies a list of up to five IP addresses, each of
909 which should be the IP address of a Nearest NetWare IP server.
910 option nwip.autoretries uint8;
912 Specifies the number of times that a NetWare/IP client should attempt
914 to communicate with a given DSS server at startup.
915 option nwip.autoretry-secs uint8;
917 Specifies the number of seconds that a Netware/IP client should wait
919 between retries when attempting to establish communications with a
920 DSS server at startup.
921 option nwip.nwip-1-1 uint8;
923 If true, the NetWare/IP client should support NetWare/IP version 1.1
925 compatibility. This is only needed if the client will be contacting
926 Netware/IP version 1.1 servers.
927 option nwip.primary-dss ip-address;
929 Specifies the IP address of the Primary Domain SAP/RIP Service server
931 (DSS) for this NetWare/IP domain. The NetWare/IP administration
932 utility uses this value as Primary DSS server when configuring a sec‐
933 ondary DSS server.
934
936 The Internet Systems Consortium DHCP client and server provide the
937 capability to define new options. Each DHCP option has a name, a
938 code, and a structure. The name is used by you to refer to the
939 option. The code is a number, used by the DHCP server and client to
940 refer to an option. The structure describes what the contents of an
941 option looks like.
942 To define a new option, you need to choose a name for it that is not in
944 use for some other option - for example, you can't use "host-name"
945 because the DHCP protocol already defines a host-name option, which is
946 documented earlier in this manual page. If an option name doesn't
947 appear in this manual page, you can use it, but it's probably a good
948 idea to put some kind of unique string at the beginning so you can be
949 sure that future options don't take your name. For example, you might
950 define an option, "local-host-name", feeling some confidence that no
951 official DHCP option name will ever start with "local".
952 Once you have chosen a name, you must choose a code. For site-local
954 options, all codes between 128 and 254 are reserved for DHCP options,
955 so you can pick any one of these. In practice, some vendors have
956 interpreted the protocol rather loosely and have used option code val‐
957 ues greater than 128 themselves. There's no real way to avoid this
958 problem, but it's not likely to cause too much trouble in practice.
959 The structure of an option is simply the format in which the option
961 data appears. The ISC DHCP server currently supports a few simple
962 types, like integers, booleans, strings and IP addresses, and it also
963 supports the ability to define arrays of single types or arrays of
964 fixed sequences of types.
965 New options are declared as follows:
967 option new-name code new-code = definition ;
969 The values of new-name and new-code should be the name you have chosen
971 for the new option and the code you have chosen. The definition
972 should be the definition of the structure of the option.
973 The following simple option type definitions are supported:
975 BOOLEAN
977 option new-name code new-code = boolean ;
979 An option of type boolean is a flag with a value of either on or off
981 (or true or false). So an example use of the boolean type would be:
982 option use-zephyr code 180 = boolean;
984 option use-zephyr on;
985 INTEGER
987 option new-name code new-code = sign integer width ;
989 The sign token should either be blank, unsigned or signed. The width
991 can be either 8, 16 or 32, and refers to the number of bits in the
992 integer. So for example, the following two lines show a definition of
993 the sql-connection-max option and its use:
994 option sql-connection-max code 192 = unsigned integer 16;
996 option sql-connection-max 1536;
997 IP-ADDRESS
999 option new-name code new-code = ip-address ;
1001 An option whose structure is an IP address can be expressed either as a
1003 domain name or as a dotted quad. So the following is an example use of
1004 the ip-address type:
1005 option sql-server-address code 193 = ip-address;
1007 option sql-server-address sql.example.com;
1008 TEXT
1011 option new-name code new-code = text ;
1013 An option whose type is text will encode an ASCII text string. For
1015 example:
1016 option sql-default-connection-name code 194 = text;
1018 option sql-default-connection-name "PRODZA";
1019 DATA STRING
1022 option new-name code new-code = string ;
1024 An option whose type is a data string is essentially just a collection
1026 of bytes, and can be specified either as quoted text, like the text
1027 type, or as a list of hexadecimal contents separated by colons whose
1028 values must be between 0 and FF. For example:
1029 option sql-identification-token code 195 = string;
1031 option sql-identification-token 17:23:19:a6:42:ea:99:7c:22;
1032 ENCAPSULATION
1035 option new-name code new-code = encapsulate identifier ;
1037 An option whose type is encapsulate will encapsulate the contents of
1039 the option space specified in identifier. Examples of encapsulated
1040 options in the DHCP protocol as it currently exists include the vendor-
1041 encapsulated-options option, the netware-suboptions option and the
1042 relay-agent-information option.
1043 option space local;
1045 option local.demo code 1 = text;
1046 option local-encapsulation code 197 = encapsulate local;
1047 option local.demo "demo";
1048 ARRAYS
1051 Options can contain arrays of any of the above types except for the
1053 text and data string types, which aren't currently supported in arrays.
1054 An example of an array definition is as follows:
1055 option kerberos-servers code 200 = array of ip-address;
1057 option kerberos-servers 10.20.10.1, 10.20.11.1;
1058 RECORDS
1060 Options can also contain data structures consisting of a sequence of
1062 data types, which is sometimes called a record type. For example:
1063 option contrived-001 code 201 = { boolean, integer 32, text };
1065 option contrived-001 on 1772 "contrivance";
1066 It's also possible to have options that are arrays of records, for
1068 example:
1069 option new-static-routes code 201 = array of {
1071 ip-address, ip-address, ip-address, integer 8 };
1072 option static-routes
1073 10.0.0.0 255.255.255.0 net-0-rtr.example.com 1,
1074 10.0.1.0 255.255.255.0 net-1-rtr.example.com 1,
1075 10.2.0.0 255.255.224.0 net-2-0-rtr.example.com 3;
1076
1079 The DHCP protocol defines the vendor-encapsulated-options option,
1080 which allows vendors to define their own options that will be sent
1081 encapsulated in a standard DHCP option. The format of the vendor-
1082 encapsulated-options option is either a series of bytes whose format is
1083 not specified, or a sequence of options, each of which consists of a
1084 single-byte vendor-specific option code, followed by a single-byte
1085 length, followed by as many bytes of data as are specified in the
1086 length (the length does not include itself or the option code).
1087 The value of this option can be set in one of two ways. The first way
1089 is to simply specify the data directly, using a text string or a colon-
1090 separated list of hexadecimal values. For example:
1091 option vendor-encapsulated-options
1093 2:4:AC:11:41:1:
1094 3:12:73:75:6e:64:68:63:70:2d:73:65:72:76:65:72:31:37:2d:31:
1095 4:12:2f:65:78:70:6f:72:74:2f:72:6f:6f:74:2f:69:38:36:70:63;
1096 The second way of setting the value of this option is to have the DHCP
1098 server generate a vendor-specific option buffer. To do this, you must
1099 do four things: define an option space, define some options in that
1100 option space, provide values for them, and specify that that option
1101 space should be used to generate the vendor-encapsulated-options
1102 option.
1103 To define a new option space in which vendor options can be stored, use
1105 the option space statement:
1106 option space name ;
1108 The name can then be used in option definitions, as described earlier
1110 in this document. For example:
1111 option space SUNW;
1113 option SUNW.server-address code 2 = ip-address;
1114 option SUNW.server-name code 3 = text;
1115 option SUNW.root-path code 4 = text;
1116 Once you have defined an option space and the format of some options,
1118 you can set up scopes that define values for those options, and you can
1119 say when to use them. For example, suppose you want to handle two
1120 different classes of clients. Using the option space definition shown
1121 in the previous example, you can send different option values to dif‐
1122 ferent clients based on the vendor-class-identifier option that the
1123 clients send, as follows:
1124 class "vendor-classes" {
1126 match option vendor-class-identifier;
1127 }
1128 option SUNW.server-address 172.17.65.1;
1130 option SUNW.server-name "sundhcp-server17-1";
1131 subclass "vendor-classes" "SUNW.Ultra-5_10" {
1133 vendor-option-space SUNW;
1134 option SUNW.root-path "/export/root/sparc";
1135 }
1136 subclass "vendor-classes" "SUNW.i86pc" {
1138 vendor-option-space SUNW;
1139 option SUNW.root-path "/export/root/i86pc";
1140 }
1141 As you can see in the preceding example, regular scoping rules apply,
1143 so you can define values that are global in the global scope, and only
1144 define values that are specific to a particular class in the local
1145 scope. The vendor-option-space declaration tells the DHCP server to
1146 use options in the SUNW option space to construct the vendor-encapsu‐
1147 lated-options option.
1148
1150 dhcpd.conf(5), dhcpd.leases(5), dhclient.conf(5), dhcp-eval(5),
1151 dhcpd(8), dhclient(8), RFC2132, RFC2131, draft-ietf-dhc-agent-
1152 options-??.txt.
1153
1155 The Internet Systems Consortium DHCP Distribution was written by Ted
1156 Lemon under a contract with Vixie Labs. Funding for this project was
1157 provided through Internet Systems Consortium. Information about Inter‐
1158 net Systems Consortium can be found at http://www.isc.org.
1159 dhcpd-options(5)