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DHCP Options

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Jul 19th, 2018
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  1. 1.1 Requirements
  2.  
  3. Throughout this document, the words that are used to define the significance of particular requirements are capitalized. These words are:
  4.  
  5. o "MUST"
  6. This word or the adjective "REQUIRED" means that the item is an absolute requirement of this specification.
  7.  
  8. o "MUST NOT"
  9. This phrase means that the item is an absolute prohibition of this specification.
  10.  
  11. o "SHOULD"
  12. This word or the adjective "RECOMMENDED" means that there may exist valid reasons in particular circumstances to ignore this item, but the full implications should be understood and the case carefully weighed before choosing a different course.
  13.  
  14. o "SHOULD NOT"
  15. This phrase means that there may exist valid reasons in particular circumstances when the listed behavior is acceptable or even useful, but the full implications should be understood and the case carefully weighed before implementing any behavior described with this label.
  16.  
  17. o "MAY"
  18. This word or the adjective "OPTIONAL" means that this item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because it enhances the product, for example; another vendor may omit the same item.
  19.  
  20. 1.2 Terminology
  21.  
  22. This document uses the following terms:
  23.  
  24. o "DHCP client"
  25. A DHCP client or "client" is an Internet host using DHCP to obtain configuration parameters such as a network address.
  26.  
  27. o "DHCP server"
  28. A DHCP server of "server"is an Internet host that returns configuration parameters to DHCP clients.
  29.  
  30. o "binding"
  31. A binding is a collection of configuration parameters, including at least an IP address, associated with or "bound to" a DHCP client. Bindings are managed by DHCP servers.
  32.  
  33. 2. BOOTP Extension/DHCP Option Field Format
  34.  
  35. DHCP options have the same format as the BOOTP 'vendor extensions' defined in RFC 1497 [2]. Options may be fixed length or variable length. All options begin with a tag octet, which uniquely identifies the option. Fixed-length options without data consist of only a tag octet. Only options 0 and 255 are fixed length. All other options are variable-length with a length octet following the tag octet. The value of the length octet does not include the two octets specifying the tag and length. The length octet is followed by "length" octets of data. Options containing NVT ASCII data SHOULD NOT include a trailing NULL; however, the receiver of such options MUST be prepared to delete trailing nulls if they exist. The receiver MUST NOT require that a trailing null be included in the data. In the case of some variable-length options the length field is a constant but must still be specified.
  36.  
  37. Any options defined subsequent to this document MUST contain a length octet even if the length is fixed or zero.
  38.  
  39. All multi-octet quantities are in network byte-order.
  40.  
  41. When used with BOOTP, the first four octets of the vendor information field have been assigned to the "magic cookie" (as suggested in RFC 951). This field identifies the mode in which the succeeding data is to be interpreted. The value of the magic cookie is the 4 octet dotted decimal 99.130.83.99 (or hexadecimal number 63.82.53.63) in network byte order.
  42.  
  43. All of the "vendor extensions" defined in RFC 1497 are also DHCP options.
  44.  
  45. Option codes 128 to 254 (decimal) are reserved for site-specific options.
  46.  
  47. Except for the options in section 9, all options may be used with either DHCP or BOOTP.
  48.  
  49. Many of these options have their default values specified in other documents. In particular, RFC 1122 [4] specifies default values for most IP and TCP configuration parameters.
  50.  
  51. Many options supply one or more 32-bit IP address. Use of IP addresses rather than fully-qualified Domain Names (FQDNs) may make future renumbering of IP hosts more difficult. Use of these addresses is discouraged at sites that may require renumbering.
  52.  
  53. 3. RFC 1497 Vendor Extensions
  54.  
  55. This section lists the vendor extensions as defined in RFC 1497. They are defined here for completeness.
  56.  
  57. 3.1. Pad Option
  58.  
  59. The pad option can be used to cause subsequent fields to align on word boundaries.
  60.  
  61. The code for the pad option is 0, and its length is 1 octet.
  62.  
  63. Code
  64. +-----+
  65. | 0 |
  66. +-----+
  67.  
  68. 3.2. End Option
  69.  
  70. The end option marks the end of valid information in the vendor field. Subsequent octets should be filled with pad options.
  71.  
  72. The code for the end option is 255, and its length is 1 octet.
  73.  
  74. Code
  75. +-----+
  76. | 255 |
  77. +-----+
  78.  
  79. 3.3 Subnet Mask
  80.  
  81. The subnet mask option specifies the client's subnet mask as per RFC 950 [5].
  82.  
  83. If both the subnet mask and the router option are specified in a DHCP reply, the subnet mask option MUST be first.
  84.  
  85. The code for the subnet mask option is 1, and its length is 4 octets.
  86.  
  87. Code Len Subnet Mask
  88. +-----+-----+-----+-----+-----+-----+
  89. | 1 | 4 | m1 | m2 | m3 | m4 |
  90. +-----+-----+-----+-----+-----+-----+
  91.  
  92. 3.4. Time Offset
  93.  
  94. The time offset field specifies the offset of the client's subnet in seconds from Coordinated Universal Time (UTC). The offset is expressed as a two's complement 32-bit integer. A positive offset indicates a location east of the zero meridian and a negative offset indicates a location west of the zero meridian.
  95.  
  96. The code for the time offset option is 2, and its length is 4 octets.
  97.  
  98. Code Len Time Offset
  99. +-----+-----+-----+-----+-----+-----+
  100. | 2 | 4 | n1 | n2 | n3 | n4 |
  101. +-----+-----+-----+-----+-----+-----+
  102.  
  103. 3.5. Router Option
  104.  
  105. The router option specifies a list of IP addresses for routers on the client's subnet. Routers SHOULD be listed in order of preference.
  106.  
  107. The code for the router option is @h@3. The minimum length for the router option is 4 octets, and the length MUST always be a multiple of 4.
  108.  
  109. Code Len Address 1 Address 2
  110. +-----+-----+-----+-----+-----+-----+-----+-----+--
  111. | 3 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  112. +-----+-----+-----+-----+-----+-----+-----+-----+--
  113.  
  114. 3.6. Time Server Option
  115.  
  116. The time server option specifies a list of RFC 868 [6] time servers available to the client. Servers SHOULD be listed in order of preference.
  117.  
  118. The code for the time server option is 4. The minimum length for this option is 4 octets, and the length MUST always be a multiple of 4.
  119.  
  120. Code Len Address 1 Address 2
  121. +-----+-----+-----+-----+-----+-----+-----+-----+--
  122. | 4 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  123. +-----+-----+-----+-----+-----+-----+-----+-----+--
  124.  
  125. 3.7. Name Server Option
  126.  
  127. The name server option specifies a list of IEN 116 [7] name servers available to the client. Servers SHOULD be listed in order of preference.
  128.  
  129. The code for the name server option is 5. The minimum length for this option is 4 octets, and the length MUST always be a multiple of 4.
  130.  
  131. Code Len Address 1 Address 2
  132. +-----+-----+-----+-----+-----+-----+-----+-----+--
  133. | 5 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  134. +-----+-----+-----+-----+-----+-----+-----+-----+--
  135.  
  136. 3.8. Domain Name Server Option
  137.  
  138. The domain name server option specifies a list of Domain Name System (STD 13, RFC 1035 [8]) name servers available to the client. Servers SHOULD be listed in order of preference.
  139.  
  140. The code for the domain name server option is 6. The minimum length for this option is 4 octets, and the length MUST always be a multiple of 4.
  141.  
  142. Code Len Address 1 Address 2
  143. +-----+-----+-----+-----+-----+-----+-----+-----+--
  144. | 6 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  145. +-----+-----+-----+-----+-----+-----+-----+-----+--
  146.  
  147. 3.9. Log Server Option
  148.  
  149. The log server option specifies a list of MIT-LCS UDP log servers available to the client. Servers SHOULD be listed in order of preference.
  150.  
  151. The code for the log server option is 7. The minimum length for this option is 4 octets, and the length MUST always be a multiple of 4.
  152.  
  153. Code Len Address 1 Address 2
  154. +-----+-----+-----+-----+-----+-----+-----+-----+--
  155. | 7 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  156. +-----+-----+-----+-----+-----+-----+-----+-----+--
  157.  
  158.  
  159. 3.10. Cookie Server Option
  160.  
  161. The cookie server option specifies a list of RFC 865 [9] cookie servers available to the client. Servers SHOULD be listed in order of preference.
  162.  
  163. The code for the log server option is 8. The minimum length for this option is 4 octets, and the length MUST always be a multiple of 4.
  164.  
  165. Code Len Address 1 Address 2
  166. +-----+-----+-----+-----+-----+-----+-----+-----+--
  167. | 8 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  168. +-----+-----+-----+-----+-----+-----+-----+-----+--
  169.  
  170. 3.11. LPR Server Option
  171.  
  172. The LPR server option specifies a list of RFC 1179 [10] line printer servers available to the client. Servers SHOULD be listed in order of preference.
  173.  
  174. The code for the LPR server option is 9. The minimum length for this option is 4 octets, and the length MUST always be a multiple of 4.
  175.  
  176. Code Len Address 1 Address 2
  177. +-----+-----+-----+-----+-----+-----+-----+-----+--
  178. | 9 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  179. +-----+-----+-----+-----+-----+-----+-----+-----+--
  180.  
  181. 3.12. Impress Server Option
  182.  
  183. The Impress server option specifies a list of Imagen Impress servers available to the client. Servers SHOULD be listed in order of preference.
  184.  
  185. The code for the Impress server option is 10. The minimum length for this option is 4 octets, and the length MUST always be a multiple of 4.
  186.  
  187. Code Len Address 1 Address 2
  188. +-----+-----+-----+-----+-----+-----+-----+-----+--
  189. | 10 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  190. +-----+-----+-----+-----+-----+-----+-----+-----+--
  191.  
  192. 3.13 Resource Location Server Option
  193.  
  194. This option specifies a list of RFC 887 [11] Resource Location servers available to the client. Servers SHOULD be listed in order of preference.
  195.  
  196. The code for this option is 11. The minimum length for this option
  197. is 4 octets, and the length MUST always be a multiple of 4.
  198.  
  199. Code Len Address 1 Address 2
  200. +-----+-----+-----+-----+-----+-----+-----+-----+--
  201. | 11 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  202. +-----+-----+-----+-----+-----+-----+-----+-----+--
  203.  
  204. 3.14. Host Name Option
  205.  
  206. This option specifies the name of the client. The name may or may not be qualified with the local domain name (see section @[email protected] for the preferred way to retrieve the domain name). See RFC 1035 for character set restrictions.
  207.  
  208. The code for this option is 12, and its minimum length is 1.
  209.  
  210. Code Len Host Name
  211. +-----+-----+-----+-----+-----+-----+-----+-----+--
  212. | 12 | n | h1 | h2 | h3 | h4 | h5 | h6 | ...
  213. +-----+-----+-----+-----+-----+-----+-----+-----+--
  214.  
  215. 3.15. Boot File Size Option
  216.  
  217. This option specifies the length in 512-octet blocks of the default boot image for the client. The file length is specified as an unsigned 16-bit integer.
  218.  
  219. The code for this option is 13, and its length is 2.
  220.  
  221. Code Len File Size
  222. +-----+-----+-----+-----+
  223. | 13 | 2 | l1 | l2 |
  224. +-----+-----+-----+-----+
  225.  
  226. 3.16. Merit Dump File
  227.  
  228. This option specifies the path-name of a file to which the client's core image should be dumped in the event the client crashes. The path is formatted as a character string consisting of characters from the NVT ASCII character set.
  229.  
  230. The code for this option is 14. Its minimum length is 1.
  231.  
  232. Code Len Dump File Pathname
  233. +-----+-----+-----+-----+-----+-----+---
  234. | 14 | n | n1 | n2 | n3 | n4 | ...
  235. +-----+-----+-----+-----+-----+-----+---
  236.  
  237. 3.17. Domain Name
  238.  
  239. This option specifies the domain name that client should use when resolving hostnames via the Domain Name System.
  240.  
  241. The code for this option is 15. Its minimum length is 1.
  242.  
  243. Code Len Domain Name
  244. +-----+-----+-----+-----+-----+-----+--
  245. | 15 | n | d1 | d2 | d3 | d4 | ...
  246. +-----+-----+-----+-----+-----+-----+--
  247.  
  248. 3.18. Swap Server
  249.  
  250. This specifies the IP address of the client's swap server.
  251.  
  252. The code for this option is 16 and its length is 4.
  253.  
  254. Code Len Swap Server Address
  255. +-----+-----+-----+-----+-----+-----+
  256. | 16 | n | a1 | a2 | a3 | a4 |
  257. +-----+-----+-----+-----+-----+-----+
  258.  
  259. 3.19. Root Path
  260.  
  261. This option specifies the path-name that contains the client's root disk. The path is formatted as a character string consisting of characters from the NVT ASCII character set.
  262.  
  263. The code for this option is 17. Its minimum length is 1.
  264.  
  265. Code Len Root Disk Pathname
  266. +-----+-----+-----+-----+-----+-----+---
  267. | 17 | n | n1 | n2 | n3 | n4 | ...
  268. +-----+-----+-----+-----+-----+-----+---
  269.  
  270. 3.20. Extensions Path
  271.  
  272. A string to specify a file, retrievable via TFTP, which contains information which can be interpreted in the same way as the 64-octet vendor-extension field within the BOOTP response, with the following exceptions:
  273.  
  274. - the length of the file is unconstrained;
  275. - all references to Tag 18 (i.e., instances of the BOOTP Extensions Path field) within the file are ignored.
  276.  
  277. The code for this option is 18. Its minimum length is 1.
  278.  
  279. Code Len Extensions Pathname
  280. +-----+-----+-----+-----+-----+-----+---
  281. | 18 | n | n1 | n2 | n3 | n4 | ...
  282. +-----+-----+-----+-----+-----+-----+---
  283.  
  284. 4. IP Layer Parameters per Host
  285.  
  286. This section details the options that affect the operation of the IP layer on a per-host basis.
  287.  
  288. 4.1. IP Forwarding Enable/Disable Option
  289.  
  290. This option specifies whether the client should configure its IP layer for packet forwarding. A value of 0 means disable IP forwarding, and a value of 1 means enable IP forwarding.
  291.  
  292. The code for this option is 19, and its length is 1.
  293.  
  294. Code Len Value
  295. +-----+-----+-----+
  296. | 19 | 1 | 0/1 |
  297. +-----+-----+-----+
  298.  
  299. 4.2. Non-Local Source Routing Enable/Disable Option
  300.  
  301. This option specifies whether the client should configure its IP layer to allow forwarding of datagrams with non-local source routes (see Section 3.3.5 of [4] for a discussion of this topic). A value of 0 means disallow forwarding of such datagrams, and a value of 1 means allow forwarding.
  302.  
  303. The code for this option is 20, and its length is 1.
  304.  
  305. Code Len Value
  306. +-----+-----+-----+
  307. | 20 | 1 | 0/1 |
  308. +-----+-----+-----+
  309.  
  310. 4.3. Policy Filter Option
  311.  
  312. This option specifies policy filters for non-local source routing. The filters consist of a list of IP addresses and masks which specify destination/mask pairs with which to filter incoming source routes.
  313.  
  314. Any source routed datagram whose next-hop address does not match one of the filters should be discarded by the client. See [4] for further information.
  315.  
  316. The code for this option is 21. The minimum length of this option is 8, and the length MUST be a multiple of 8.
  317.  
  318. Code Len Address 1 Mask 1
  319. +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
  320. | 21 | n | a1 | a2 | a3 | a4 | m1 | m2 | m3 | m4 |
  321. +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
  322. Address 2 Mask 2
  323. +-----+-----+-----+-----+-----+-----+-----+-----+---
  324. | a1 | a2 | a3 | a4 | m1 | m2 | m3 | m4 | ...
  325. +-----+-----+-----+-----+-----+-----+-----+-----+---
  326.  
  327. 4.4. Maximum Datagram Reassembly Size
  328.  
  329. This option specifies the maximum size datagram that the client should be prepared to reassemble. The size is specified as a 16-bit unsigned integer. The minimum value legal value is 576.
  330.  
  331. The code for this option is 22, and its length is 2.
  332.  
  333. Code Len Size
  334. +-----+-----+-----+-----+
  335. | 22 | 2 | s1 | s2 |
  336. +-----+-----+-----+-----+
  337.  
  338. 4.5. Default IP Time-to-live
  339.  
  340. This option specifies the default time-to-live that the client should use on outgoing datagrams. The TTL is specified as an octet with a value between 1 and 255.
  341.  
  342. The code for this option is 23, and its length is 1.
  343.  
  344. Code Len TTL
  345. +-----+-----+-----+
  346. | 23 | 1 | ttl |
  347. +-----+-----+-----+
  348.  
  349. 4.6. Path MTU Aging Timeout Option
  350.  
  351. This option specifies the timeout (in seconds) to use when aging Path MTU values discovered by the mechanism defined in RFC 1191 [12]. The timeout is specified as a 32-bit unsigned integer.
  352.  
  353. The code for this option is 24, and its length is 4.
  354.  
  355. Code Len Timeout
  356. +-----+-----+-----+-----+-----+-----+
  357. | 24 | 4 | t1 | t2 | t3 | t4 |
  358. +-----+-----+-----+-----+-----+-----+
  359.  
  360. 4.7. Path MTU Plateau Table Option
  361.  
  362. This option specifies a table of MTU sizes to use when performing Path MTU Discovery as defined in RFC 1191. The table is formatted as a list of 16-bit unsigned integers, ordered from smallest to largest. The minimum MTU value cannot be smaller than 68.
  363.  
  364. The code for this option is 25. Its minimum length is 2, and the length MUST be a multiple of 2.
  365.  
  366. Code Len Size 1 Size 2
  367. +-----+-----+-----+-----+-----+-----+---
  368. | 25 | n | s1 | s2 | s1 | s2 | ...
  369. +-----+-----+-----+-----+-----+-----+---
  370.  
  371. 5. IP Layer Parameters per Interface
  372.  
  373. This section details the options that affect the operation of the IP layer on a per-interface basis. It is expected that a client can issue multiple requests, one per interface, in order to configure interfaces with their specific parameters.
  374.  
  375. 5.1. Interface MTU Option
  376.  
  377. This option specifies the MTU to use on this interface. The MTU is specified as a 16-bit unsigned integer. The minimum legal value for the MTU is 68.
  378.  
  379. The code for this option is 26, and its length is 2.
  380.  
  381. Code Len MTU
  382. +-----+-----+-----+-----+
  383. | 26 | 2 | m1 | m2 |
  384. +-----+-----+-----+-----+
  385.  
  386. 5.2. All Subnets are Local Option
  387.  
  388. This option specifies whether or not the client may assume that all subnets of the IP network to which the client is connected use the same MTU as the subnet of that network to which the client is directly connected. A value of 1 indicates that all subnets share the same MTU. A value of 0 means that the client should assume that some subnets of the directly connected network may have smaller MTUs.
  389.  
  390. The code for this option is 27, and its length is 1.
  391.  
  392. Code Len Value
  393. +-----+-----+-----+
  394. | 27 | 1 | 0/1 |
  395. +-----+-----+-----+
  396.  
  397. 5.3. Broadcast Address Option
  398.  
  399. This option specifies the broadcast address in use on the client's subnet. Legal values for broadcast addresses are specified in section 3.2.1.3 of [4].
  400.  
  401. The code for this option is 28, and its length is 4.
  402.  
  403. Code Len Broadcast Address
  404. +-----+-----+-----+-----+-----+-----+
  405. | 28 | 4 | b1 | b2 | b3 | b4 |
  406. +-----+-----+-----+-----+-----+-----+
  407.  
  408. 5.4. Perform Mask Discovery Option
  409.  
  410. This option specifies whether or not the client should perform subnet mask discovery using ICMP. A value of 0 indicates that the client should not perform mask discovery. A value of 1 means that the client should perform mask discovery.
  411.  
  412. The code for this option is 29, and its length is 1.
  413.  
  414. Code Len Value
  415. +-----+-----+-----+
  416. | 29 | 1 | 0/1 |
  417. +-----+-----+-----+
  418.  
  419. 5.5 Mask Supplier Option
  420.  
  421. This option specifies whether or not the client should respond to subnet mask requests using ICMP. A value of 0 indicates that the client should not respond. A value of 1 means that the client should respond.
  422.  
  423. The code for this option is 30, and its length is 1.
  424.  
  425. Code Len Value
  426. +-----+-----+-----+
  427. | 30 | 1 | 0/1 |
  428. +-----+-----+-----+
  429.  
  430. 5.6. Perform Router Discovery Option
  431.  
  432. This option specifies whether or not the client should solicit routers using the Router Discovery mechanism defined in RFC 1256 [13]. A value of 0 indicates that the client should not perform router discovery. A value of 1 means that the client should perform router discovery.
  433.  
  434. The code for this option is 31, and its length is 1.
  435.  
  436. Code Len Value
  437. +-----+-----+-----+
  438. | 31 | 1 | 0/1 |
  439. +-----+-----+-----+
  440.  
  441. 5.7. Router Solicitation Address Option
  442.  
  443. This option specifies the address to which the client should transmit router solicitation requests.
  444.  
  445. The code for this option is 32, and its length is 4.
  446.  
  447. Code Len Address
  448. +-----+-----+-----+-----+-----+-----+
  449. | 32 | 4 | a1 | a2 | a3 | a4 |
  450. +-----+-----+-----+-----+-----+-----+
  451.  
  452. 5.8. Static Route Option
  453.  
  454. This option specifies a list of static routes that the client should install in its routing cache. If multiple routes to the same destination are specified, they are listed in descending order of priority.
  455.  
  456. The routes consist of a list of IP address pairs. The first address is the destination address, and the second address is the router for the destination.
  457.  
  458. The default route (0.0.0.0) is an illegal destination for a static route. See section 3.5 for information about the router option.
  459.  
  460. The code for this option is 33. The minimum length of this option is 8, and the length MUST be a multiple of 8.
  461.  
  462. Code Len Destination 1 Router 1
  463. +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
  464. | 33 | n | d1 | d2 | d3 | d4 | r1 | r2 | r3 | r4 |
  465. +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
  466. Destination 2 Router 2
  467. +-----+-----+-----+-----+-----+-----+-----+-----+---
  468. | d1 | d2 | d3 | d4 | r1 | r2 | r3 | r4 | ...
  469. +-----+-----+-----+-----+-----+-----+-----+-----+---
  470.  
  471. 6. Link Layer Parameters per Interface
  472.  
  473. This section lists the options that affect the operation of the data link layer on a per-interface basis.
  474.  
  475. 6.1. Trailer Encapsulation Option
  476.  
  477. This option specifies whether or not the client should negotiate the use of trailers (RFC 893 [14]) when using the ARP protocol. A value of 0 indicates that the client should not attempt to use trailers. A value of 1 means that the client should attempt to use trailers.
  478.  
  479. The code for this option is 34, and its length is 1.
  480.  
  481. Code Len Value
  482. +-----+-----+-----+
  483. | 34 | 1 | 0/1 |
  484. +-----+-----+-----+
  485.  
  486. 6.2. ARP Cache Timeout Option
  487.  
  488. This option specifies the timeout in seconds for ARP cache entries. The time is specified as a 32-bit unsigned integer.
  489.  
  490. The code for this option is 35, and its length is 4.
  491.  
  492. Code Len Time
  493. +-----+-----+-----+-----+-----+-----+
  494. | 35 | 4 | t1 | t2 | t3 | t4 |
  495. +-----+-----+-----+-----+-----+-----+
  496.  
  497. 6.3. Ethernet Encapsulation Option
  498.  
  499. This option specifies whether or not the client should use Ethernet Version 2 (RFC 894 [15]) or IEEE 802.3 (RFC 1042 [16]) encapsulation if the interface is an Ethernet. A value of 0 indicates that the client should use RFC 894 encapsulation. A value of 1 means that the client should use RFC 1042 encapsulation.
  500.  
  501. The code for this option is 36, and its length is 1.
  502.  
  503. Code Len Value
  504. +-----+-----+-----+
  505. | 36 | 1 | 0/1 |
  506. +-----+-----+-----+
  507.  
  508. 7. TCP Parameters
  509.  
  510. This section lists the options that affect the operation of the TCP layer on a per-interface basis.
  511.  
  512. 7.1. TCP Default TTL Option
  513.  
  514. This option specifies the default TTL that the client should use when sending TCP segments. The value is represented as an 8-bit unsigned integer. The minimum value is 1.
  515.  
  516. The code for this option is 37, and its length is 1.
  517.  
  518. Code Len TTL
  519. +-----+-----+-----+
  520. | 37 | 1 | n |
  521. +-----+-----+-----+
  522.  
  523. 7.2. TCP Keepalive Interval Option
  524.  
  525. This option specifies the interval (in seconds) that the client TCP should wait before sending a keepalive message on a TCP connection. The time is specified as a 32-bit unsigned integer. A value of zero indicates that the client should not generate keepalive messages on connections unless specifically requested by an application.
  526.  
  527. The code for this option is 38, and its length is 4.
  528.  
  529. Code Len Time
  530. +-----+-----+-----+-----+-----+-----+
  531. | 38 | 4 | t1 | t2 | t3 | t4 |
  532. +-----+-----+-----+-----+-----+-----+
  533.  
  534. 7.3. TCP Keepalive Garbage Option
  535.  
  536. This option specifies the whether or not the client should send TCP keepalive messages with a octet of garbage for compatibility with older implementations. A value of 0 indicates that a garbage octet should not be sent. A value of 1 indicates that a garbage octet should be sent.
  537.  
  538. The code for this option is 39, and its length is 1.
  539.  
  540. Code Len Value
  541. +-----+-----+-----+
  542. | 39 | 1 | 0/1 |
  543. +-----+-----+-----+
  544.  
  545. 8. Application and Service Parameters
  546.  
  547. This section details some miscellaneous options used to configure miscellaneous applications and services.
  548.  
  549. 8.1. Network Information Service Domain Option
  550.  
  551. This option specifies the name of the client's NIS [17] domain. The domain is formatted as a character string consisting of characters from the NVT ASCII character set.
  552.  
  553. The code for this option is 40. Its minimum length is 1.
  554.  
  555. Code Len NIS Domain Name
  556. +-----+-----+-----+-----+-----+-----+---
  557. | 40 | n | n1 | n2 | n3 | n4 | ...
  558. +-----+-----+-----+-----+-----+-----+---
  559.  
  560. 8.2. Network Information Servers Option
  561.  
  562. This option specifies a list of IP addresses indicating NIS servers available to the client. Servers SHOULD be listed in order of preference.
  563.  
  564. The code for this option is 41. Its minimum length is 4, and the length MUST be a multiple of 4.
  565.  
  566. Code Len Address 1 Address 2
  567. +-----+-----+-----+-----+-----+-----+-----+-----+--
  568. | 41 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  569. +-----+-----+-----+-----+-----+-----+-----+-----+--
  570.  
  571. 8.3. Network Time Protocol Servers Option
  572.  
  573. This option specifies a list of IP addresses indicating NTP [18] servers available to the client. Servers SHOULD be listed in order of preference.
  574.  
  575. The code for this option is 42. Its minimum length is 4, and the length MUST be a multiple of 4.
  576.  
  577. Code Len Address 1 Address 2
  578. +-----+-----+-----+-----+-----+-----+-----+-----+--
  579. | 42 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  580. +-----+-----+-----+-----+-----+-----+-----+-----+--
  581.  
  582. 8.4. Vendor Specific Information
  583.  
  584. This option is used by clients and servers to exchange vendor- specific information. The information is an opaque object of n octets, presumably interpreted by vendor-specific code on the clients and servers. The definition of this information is vendor specific. The vendor is indicated in the vendor class identifier option. Servers not equipped to interpret the vendor-specific information sent by a client MUST ignore it (although it may be reported). Clients which do not receive desired vendor-specific information SHOULD make an attempt to operate without it, although they may do so (and announce they are doing so) in a degraded mode.
  585.  
  586. If a vendor potentially encodes more than one item of information in this option, then the vendor SHOULD encode the option using "Encapsulated vendor-specific options" as described below:
  587.  
  588. The Encapsulated vendor-specific options field SHOULD be encoded as a sequence of code/length/value fields of identical syntax to the DHCP options field with the following exceptions:
  589.  
  590. 1) There SHOULD NOT be a "magic cookie" field in the encapsulated vendor-specific extensions field.
  591.  
  592. 2) Codes other than 0 or 255 MAY be redefined by the vendor within the encapsulated vendor-specific extensions field, but SHOULD conform to the tag-length-value syntax defined in section 2.
  593.  
  594. 3) Code 255 (END), if present, signifies the end of the encapsulated vendor extensions, not the end of the vendor extensions field. If no code 255 is present, then the end of the enclosing vendor-specific information field is taken as the end of the encapsulated vendor-specific extensions field.
  595.  
  596. The code for this option is 43 and its minimum length is 1.
  597.  
  598. Code Len Vendor-specific information
  599. +-----+-----+-----+-----+---
  600. | 43 | n | i1 | i2 | ...
  601. +-----+-----+-----+-----+---
  602.  
  603. When encapsulated vendor-specific extensions are used, the information bytes 1-n have the following format:
  604.  
  605. Code Len Data item Code Len Data item Code
  606. +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
  607. | T1 | n | d1 | d2 | ... | T2 | n | D1 | D2 | ... | ... |
  608. +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
  609.  
  610. 8.5. NetBIOS over TCP/IP Name Server Option
  611.  
  612. The NetBIOS name server (NBNS) option specifies a list of RFC 1001/1002 [19] [20] NBNS name servers listed in order of preference.
  613.  
  614. The code for this option is 44. The minimum length of the option is 4 octets, and the length must always be a multiple of 4.
  615.  
  616. Code Len Address 1 Address 2
  617. +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----
  618. | 44 | n | a1 | a2 | a3 | a4 | b1 | b2 | b3 | b4 | ...
  619. +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----
  620.  
  621. 8.6. NetBIOS over TCP/IP Datagram Distribution Server Option
  622.  
  623. The NetBIOS datagram distribution server (NBDD) option specifies a list of RFC 1001/1002 NBDD servers listed in order of preference. The code for this option is 45. The minimum length of the option is 4 octets, and the length must always be a multiple of 4.
  624.  
  625. Code Len Address 1 Address 2
  626. +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----
  627. | 45 | n | a1 | a2 | a3 | a4 | b1 | b2 | b3 | b4 | ...
  628. +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----
  629.  
  630. 8.7. NetBIOS over TCP/IP Node Type Option
  631.  
  632. The NetBIOS node type option allows NetBIOS over TCP/IP clients which are configurable to be configured as described in RFC 1001/1002. The value is specified as a single octet which identifies the client type as follows:
  633.  
  634. Value Node Type
  635. ----- ---------
  636. 0x1 B-node
  637. 0x2 P-node
  638. 0x4 M-node
  639. 0x8 H-node
  640.  
  641.  
  642. In the above chart, the notation '0x' indicates a number in base-16 (hexadecimal).
  643.  
  644. The code for this option is 46. The length of this option is always 1.
  645.  
  646. Code Len Node Type
  647. +-----+-----+-----------+
  648. | 46 | 1 | see above |
  649. +-----+-----+-----------+
  650.  
  651. 8.8. NetBIOS over TCP/IP Scope Option
  652.  
  653. The NetBIOS scope option specifies the NetBIOS over TCP/IP scope parameter for the client as specified in RFC 1001/1002. See [19], [20], and [8] for character-set restrictions.
  654.  
  655. The code for this option is 47. The minimum length of this option is 1.
  656.  
  657. Code Len NetBIOS Scope
  658. +-----+-----+-----+-----+-----+-----+----
  659. | 47 | n | s1 | s2 | s3 | s4 | ...
  660. +-----+-----+-----+-----+-----+-----+----
  661.  
  662. 8.9. X Window System Font Server Option
  663.  
  664. This option specifies a list of X Window System [21] Font servers available to the client. Servers SHOULD be listed in order of preference.
  665.  
  666. The code for this option is 48. The minimum length of this option is 4 octets, and the length MUST be a multiple of 4.
  667.  
  668. Code Len Address 1 Address 2
  669. +-----+-----+-----+-----+-----+-----+-----+-----+---
  670. | 48 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  671. +-----+-----+-----+-----+-----+-----+-----+-----+---
  672.  
  673. 8.10. X Window System Display Manager Option
  674.  
  675. This option specifies a list of IP addresses of systems that are running the X Window System Display Manager and are available to the client.
  676.  
  677. Addresses SHOULD be listed in order of preference.
  678.  
  679. The code for the this option is 49. The minimum length of this option is 4, and the length MUST be a multiple of 4.
  680.  
  681. Code Len Address 1 Address 2
  682.  
  683. +-----+-----+-----+-----+-----+-----+-----+-----+---
  684. | 49 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  685. +-----+-----+-----+-----+-----+-----+-----+-----+---
  686.  
  687. 8.11. Network Information Service+ Domain Option
  688.  
  689. This option specifies the name of the client's NIS+ [17] domain. The domain is formatted as a character string consisting of characters from the NVT ASCII character set.
  690.  
  691. The code for this option is 64. Its minimum length is 1.
  692.  
  693. Code Len NIS Client Domain Name
  694. +-----+-----+-----+-----+-----+-----+---
  695. | 64 | n | n1 | n2 | n3 | n4 | ...
  696. +-----+-----+-----+-----+-----+-----+---
  697.  
  698. 8.12. Network Information Service+ Servers Option
  699.  
  700. This option specifies a list of IP addresses indicating NIS+ servers available to the client. Servers SHOULD be listed in order of preference.
  701.  
  702. The code for this option is 65. Its minimum length is 4, and the length MUST be a multiple of 4.
  703.  
  704. Code Len Address 1 Address 2
  705. +-----+-----+-----+-----+-----+-----+-----+-----+--
  706. | 65 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  707. +-----+-----+-----+-----+-----+-----+-----+-----+--
  708.  
  709. 8.13. Mobile IP Home Agent option
  710.  
  711. This option specifies a list of IP addresses indicating mobile IP home agents available to the client. Agents SHOULD be listed in order of preference.
  712.  
  713. The code for this option is 68. Its minimum length is 0 (indicating no home agents are available) and the length MUST be a multiple of 4. It is expected that the usual length will be four octets, containing a single home agent's address.
  714.  
  715. Code Len Home Agent Addresses (zero or more)
  716. +-----+-----+-----+-----+-----+-----+--
  717. | 68 | n | a1 | a2 | a3 | a4 | ...
  718. +-----+-----+-----+-----+-----+-----+--
  719.  
  720. 8.14. Simple Mail Transport Protocol (SMTP) Server Option
  721.  
  722. The SMTP server option specifies a list of SMTP servers available to the client. Servers SHOULD be listed in order of preference.
  723.  
  724. The code for the SMTP server option is 69. The minimum length for this option is 4 octets, and the length MUST always be a multiple of 4.
  725.  
  726. Code Len Address 1 Address 2
  727. +-----+-----+-----+-----+-----+-----+-----+-----+--
  728. | 69 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  729. +-----+-----+-----+-----+-----+-----+-----+-----+--
  730.  
  731. 8.15. Post Office Protocol (POP3) Server Option
  732.  
  733. The POP3 server option specifies a list of POP3 available to the client. Servers SHOULD be listed in order of preference.
  734.  
  735. The code for the POP3 server option is 70. The minimum length for this option is 4 octets, and the length MUST always be a multiple of 4.
  736.  
  737. Code Len Address 1 Address 2
  738. +-----+-----+-----+-----+-----+-----+-----+-----+--
  739. | 70 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  740. +-----+-----+-----+-----+-----+-----+-----+-----+--
  741.  
  742. 8.16. Network News Transport Protocol (NNTP) Server Option
  743.  
  744. The NNTP server option specifies a list of NNTP available to the client. Servers SHOULD be listed in order of preference.
  745.  
  746. The code for the NNTP server option is 71. The minimum length for this option is 4 octets, and the length MUST always be a multiple of 4.
  747.  
  748. Code Len Address 1 Address 2
  749. +-----+-----+-----+-----+-----+-----+-----+-----+--
  750. | 71 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  751. +-----+-----+-----+-----+-----+-----+-----+-----+--
  752.  
  753. 8.17. Default World Wide Web (WWW) Server Option
  754.  
  755. The WWW server option specifies a list of WWW available to the client. Servers SHOULD be listed in order of preference.
  756.  
  757. The code for the WWW server option is 72. The minimum length for this option is 4 octets, and the length MUST always be a multiple of 4.
  758.  
  759. Code Len Address 1 Address 2
  760. +-----+-----+-----+-----+-----+-----+-----+-----+--
  761. | 72 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  762. +-----+-----+-----+-----+-----+-----+-----+-----+--
  763.  
  764. 8.1@h@8. Default Finger Server Option
  765.  
  766. The Finger server option specifies a list of Finger available to the client. Servers SHOULD be listed in order of preference.
  767.  
  768. The code for the Finger server option is 73. The minimum length for this option is 4 octets, and the length MUST always be a multiple of 4.
  769.  
  770. Code Len Address 1 Address 2
  771. +-----+-----+-----+-----+-----+-----+-----+-----+--
  772. | 73 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  773. +-----+-----+-----+-----+-----+-----+-----+-----+--
  774.  
  775. 8.19. Default Internet Relay Chat (IRC) Server Option
  776.  
  777. The IRC server option specifies a list of IRC available to the client. Servers SHOULD be listed in order of preference.
  778.  
  779. The code for the IRC server option is 74. The minimum length for this option is 4 octets, and the length MUST always be a multiple of 4.
  780.  
  781. Code Len Address 1 Address 2
  782. +-----+-----+-----+-----+-----+-----+-----+-----+--
  783. | 74 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  784. +-----+-----+-----+-----+-----+-----+-----+-----+--
  785.  
  786. 8.20. StreetTalk Server Option
  787.  
  788. The StreetTalk server option specifies a list of StreetTalk servers available to the client. Servers SHOULD be listed in order of preference.
  789.  
  790. The code for the StreetTalk server option is 75. The minimum length for this option is 4 octets, and the length MUST always be a multiple of 4.
  791.  
  792. Code Len Address 1 Address 2
  793. +-----+-----+-----+-----+-----+-----+-----+-----+--
  794. | 75 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  795. +-----+-----+-----+-----+-----+-----+-----+-----+--
  796.  
  797. 8.21. StreetTalk Directory Assistance (STDA) Server Option
  798.  
  799. The StreetTalk Directory Assistance (STDA) server option specifies a list of STDA servers available to the client. Servers SHOULD be listed in order of preference.
  800.  
  801. The code for the StreetTalk Directory Assistance server option is 76. The minimum length for this option is 4 octets, and the length MUST always be a multiple of 4.
  802.  
  803. Code Len Address 1 Address 2
  804. +-----+-----+-----+-----+-----+-----+-----+-----+--
  805. | 76 | n | a1 | a2 | a3 | a4 | a1 | a2 | ...
  806. +-----+-----+-----+-----+-----+-----+-----+-----+--
  807.  
  808. 9. DHCP Extensions
  809.  
  810. This section details the options that are specific to DHCP.
  811.  
  812. 9.1. Requested IP Address
  813.  
  814. This option is used in a client request (DHCPDISCOVER) to allow the client to request that a particular IP address be assigned.
  815.  
  816. The code for this option is 50, and its length is 4.
  817.  
  818. Code Len Address
  819. +-----+-----+-----+-----+-----+-----+
  820. | 50 | 4 | a1 | a2 | a3 | a4 |
  821. +-----+-----+-----+-----+-----+-----+
  822.  
  823. 9.2. IP Address Lease Time
  824.  
  825. This option is used in a client request (DHCPDISCOVER or DHCPREQUEST) to allow the client to request a lease time for the IP address. In a server reply (DHCPOFFER), a DHCP server uses this option to specify the lease time it is willing to offer.
  826.  
  827. The time is in units of seconds, and is specified as a 32-bit unsigned integer.
  828.  
  829. The code for this option is 51, and its length is 4.
  830.  
  831. Code Len Lease Time
  832. +-----+-----+-----+-----+-----+-----+
  833. | 51 | 4 | t1 | t2 | t3 | t4 |
  834. +-----+-----+-----+-----+-----+-----+
  835.  
  836. 9.3. Option Overload
  837.  
  838. This option is used to indicate that the DHCP 'sname' or 'file' fields are being overloaded by using them to carry DHCP options. A DHCP server inserts this option if the returned parameters will exceed the usual space allotted for options.
  839.  
  840. If this option is present, the client interprets the specified additional fields after it concludes interpretation of the standard option fields.
  841.  
  842. The code for this option is 52, and its length is 1. Legal values for this option are:
  843.  
  844. Value Meaning
  845. ----- --------
  846. 1 the 'file' field is used to hold options
  847. 2 the 'sname' field is used to hold options
  848. 3 both fields are used to hold options
  849.  
  850. Code Len Value
  851. +-----+-----+-----+
  852. | 52 | 1 |1/2/3|
  853. +-----+-----+-----+
  854.  
  855. 9.4 TFTP server name
  856.  
  857. This option is used to identify a TFTP server when the 'sname' field in the DHCP header has been used for DHCP options.
  858.  
  859. The code for this option is 66, and its minimum length is 1.
  860.  
  861. Code Len TFTP server
  862. +-----+-----+-----+-----+-----+---
  863. | 66 | n | c1 | c2 | c3 | ...
  864. +-----+-----+-----+-----+-----+---
  865.  
  866. 9.5 Bootfile name
  867.  
  868. This option is used to identify a bootfile when the 'file' field in the DHCP header has been used for DHCP options.
  869.  
  870. The code for this option is 67, and its minimum length is 1.
  871.  
  872. Code Len Bootfile name
  873. +-----+-----+-----+-----+-----+---
  874. | 67 | n | c1 | c2 | c3 | ...
  875. +-----+-----+-----+-----+-----+---
  876.  
  877. 9.6. DHCP Message Type
  878.  
  879. This option is used to convey the type of the DHCP message. The code for this option is 53, and its length is 1. Legal values for this option are:
  880.  
  881. Value Message Type
  882. ----- ------------
  883. 1 DHCPDISCOVER
  884. 2 DHCPOFFER
  885. 3 DHCPREQUEST
  886. 4 DHCPDECLINE
  887. 5 DHCPACK
  888. 6 DHCPNAK
  889. 7 DHCPRELEASE
  890. 8 DHCPINFORM
  891.  
  892. Code Len Type
  893. +-----+-----+-----+
  894. | 53 | 1 | 1-9 |
  895. +-----+-----+-----+
  896.  
  897. 9.7. Server Identifier
  898.  
  899. This option is used in DHCPOFFER and DHCPREQUEST messages, and may optionally be included in the DHCPACK and DHCPNAK messages. DHCP servers include this option in the DHCPOFFER in order to allow the client to distinguish between lease offers. DHCP clients use the contents of the 'server identifier' field as the destination address for any DHCP messages unicast to the DHCP server. DHCP clients also indicate which of several lease offers is being accepted by including this option in a DHCPREQUEST message.
  900.  
  901. The identifier is the IP address of the selected server.
  902.  
  903. The code for this option is 54, and its length is 4.
  904.  
  905. Code Len Address
  906. +-----+-----+-----+-----+-----+-----+
  907. | 54 | 4 | a1 | a2 | a3 | a4 |
  908. +-----+-----+-----+-----+-----+-----+
  909.  
  910. 9.8. Parameter Request List
  911.  
  912. This option is used by a DHCP client to request values for specified configuration parameters. The list of requested parameters is specified as n octets, where each octet is a valid DHCP option code as defined in this document.
  913.  
  914. The client MAY list the options in order of preference. The DHCP server is not required to return the options in the requested order, but MUST try to insert the requested options in the order requested by the client.
  915.  
  916. The code for this option is 55. Its minimum length is 1.
  917.  
  918. Code Len Option Codes
  919. +-----+-----+-----+-----+---
  920. | 55 | n | c1 | c2 | ...
  921. +-----+-----+-----+-----+---
  922.  
  923. 9.9 Message
  924.  
  925. This option is used by a DHCP server to provide an error message to a DHCP client in a DHCPNAK message in the event of a failure. A client may use this option in a DHCPDECLINE message to indicate the why the client declined the offered parameters. The message consists of n octets of NVT ASCII text, which the client may display on an available output device.
  926.  
  927. The code for this option is 56 and its minimum length is 1.
  928.  
  929. Code Len Text
  930. +-----+-----+-----+-----+---
  931. | 56 | n | c1 | c2 | ...
  932. +-----+-----+-----+-----+---
  933.  
  934. 9.10. Maximum DHCP Message Size
  935.  
  936. This option specifies the maximum length DHCP message that it is willing to accept. The length is specified as an unsigned 16-bit integer. A client may use the maximum DHCP message size option in DHCPDISCOVER or DHCPREQUEST messages, but should not use the option in DHCPDECLINE messages.
  937.  
  938. The code for this option is 57, and its length is 2. The minimum legal value is 576 octets.
  939.  
  940. Code Len Length
  941. +-----+-----+-----+-----+
  942. | 57 | 2 | l1 | l2 |
  943. +-----+-----+-----+-----+
  944.  
  945. 9.11. Renewal (T1) Time Value
  946.  
  947. This option specifies the time interval from address assignment until the client transitions to the RENEWING state.
  948.  
  949. The value is in units of seconds, and is specified as a 32-bit unsigned integer.
  950.  
  951. The code for this option is 58, and its length is 4.
  952.  
  953. Code Len T1 Interval
  954. +-----+-----+-----+-----+-----+-----+
  955. | 58 | 4 | t1 | t2 | t3 | t4 |
  956. +-----+-----+-----+-----+-----+-----+
  957.  
  958. 9.12. Rebinding (T2) Time Value
  959.  
  960. This option specifies the time interval from address assignment until the client transitions to the REBINDING state.
  961.  
  962. The value is in units of seconds, and is specified as a 32-bit unsigned integer.
  963.  
  964. The code for this option is 59, and its length is 4.
  965.  
  966. Code Len T2 Interval
  967. +-----+-----+-----+-----+-----+-----+
  968. | 59 | 4 | t1 | t2 | t3 | t4 |
  969. +-----+-----+-----+-----+-----+-----+
  970.  
  971. 9.13. Vendor class identifier
  972.  
  973. This option is used by DHCP clients to optionally identify the vendor type and configuration of a DHCP client. The information is a string of n octets, interpreted by servers. Vendors may choose to define specific vendor class identifiers to convey particular configuration or other identification information about a client. For example, the identifier may encode the client's hardware configuration. Servers not equipped to interpret the class-specific information sent by a client MUST ignore it (although it may be reported). Servers that respond SHOULD only use option 43 to return the vendor-specific information to the client.
  974.  
  975. The code for this option is 60, and its minimum length is 1.
  976.  
  977. Code Len Vendor class Identifier
  978. +-----+-----+-----+-----+---
  979. | 60 | n | i1 | i2 | ...
  980. +-----+-----+-----+-----+---
  981.  
  982. 9.14. Client-identifier
  983.  
  984. This option is used by DHCP clients to specify their unique identifier. DHCP servers use this value to index their database of address bindings. This value is expected to be unique for all clients in an administrative domain.
  985.  
  986. Identifiers SHOULD be treated as opaque objects by DHCP servers.
  987.  
  988. The client identifier MAY consist of type-value pairs similar to the 'htype'/'chaddr' fields defined in [3]. For instance, it MAY consist of a hardware type and hardware address. In this case the type field SHOULD be one of the ARP hardware types defined in STD2 [22]. A hardware type of 0 (zero) should be used when the value field contains an identifier other than a hardware address (e.g. a fully qualified domain name).
  989.  
  990. For correct identification of clients, each client's client- identifier MUST be unique among the client-identifiers used on the subnet to which the client is attached. Vendors and system administrators are responsible for choosing client-identifiers that meet this requirement for uniqueness.
  991.  
  992. The code for this option is 61, and its minimum length is 2.
  993.  
  994. Code Len Type Client-Identifier
  995. +-----+-----+-----+-----+-----+---
  996. | 61 | n | t1 | i1 | i2 | ...
  997. +-----+-----+-----+-----+-----+---
  998.  
  999. Source: https://tools.ietf.org/html/rfc2132
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