1.\" $OpenBSD: dhcp-options.5,v 1.14 2013/01/03 16:35:22 krw Exp $ 2.\" 3.\" Copyright (c) 1995, 1996, 1997, 1998 The Internet Software Consortium. 4.\" All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. Neither the name of The Internet Software Consortium nor the names 16.\" of its contributors may be used to endorse or promote products derived 17.\" from this software without specific prior written permission. 18.\" 19.\" THIS SOFTWARE IS PROVIDED BY THE INTERNET SOFTWARE CONSORTIUM AND 20.\" CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, 21.\" INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF 22.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE 23.\" DISCLAIMED. IN NO EVENT SHALL THE INTERNET SOFTWARE CONSORTIUM OR 24.\" CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 25.\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 26.\" LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF 27.\" USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND 28.\" ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, 29.\" OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT 30.\" OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 31.\" SUCH DAMAGE. 32.\" 33.\" This software has been written for the Internet Software Consortium 34.\" by Ted Lemon <mellon@fugue.com> in cooperation with Vixie 35.\" Enterprises. To learn more about the Internet Software Consortium, 36.\" see ``http://www.isc.org/isc''. To learn more about Vixie 37.\" Enterprises, see ``http://www.vix.com''. 38.\" 39.Dd $Mdocdate: January 3 2013 $ 40.Dt DHCP-OPTIONS 5 41.Os 42.Sh NAME 43.Nm dhcp-options 44.Nd Dynamic Host Configuration Protocol options 45.Sh DESCRIPTION 46The Dynamic Host Configuration protocol allows the client to receive 47.Ic options 48from the DHCP server describing the network configuration and various 49services that are available on the network. 50When configuring 51.Xr dhcpd 8 52or 53.Xr dhclient 8 , 54options must often be declared. 55The syntax for declaring options, and the names and formats of the options 56that can be declared, are documented here. 57.Sh REFERENCE: OPTION STATEMENTS 58DHCP 59.Ic option 60statements always start with the 61.Ic option 62keyword, followed by an option name, followed by option data. 63The option names and data formats are described below. 64It is not necessary to exhaustively specify all DHCP options \- 65only those options which are needed by clients must be specified. 66.Pp 67Option data comes in a variety of formats, as defined below: 68.Pp 69The 70.Ar ip-address 71data type can be entered either as an explicit IP address 72(e.g., 239.254.197.10) or as a domain name (e.g., haagen.isc.org). 73A domain name must resolve to a single IP address. 74.Pp 75The 76.Ar int32 77data type specifies a signed 32-bit integer. 78The 79.Ar uint32 80data type specifies an unsigned 32-bit integer. 81The 82.Ar int16 83and 84.Ar uint16 85data types specify signed and unsigned 16-bit integers. 86The 87.Ar int8 88and 89.Ar uint8 90data types specify signed and unsigned 8-bit integers. 91Unsigned 8-bit integers are also sometimes referred to as octets. 92.Pp 93The 94.Ar string 95data type specifies an 96.Tn NVT 97.Pq Network Virtual Terminal 98.Tn ASCII 99string, which must be enclosed in double quotes \- for example, 100to specify a domain-name option, the syntax would be 101.Pp 102.Dl option domain-name \&"isc.org\&"; 103.Pp 104The 105.Ar flag 106data type specifies a boolean value. 107Booleans can be either true or false 108(or on or off, if that makes more sense to you). 109.Pp 110The 111.Ar data-string 112data type specifies either an 113.Tn NVT ASCII 114string enclosed in double quotes, or a series of octets specified in 115hexadecimal, separated by colons. 116For example: 117.Pp 118.Dl option dhcp-client-identifier \&"CLIENT-FOO\&"; 119or 120.Dl option dhcp-client-identifier 43:4c:49:45:4e:54:2d:46:4f:4f; 121.Pp 122The documentation for the various options mentioned below is taken 123from the IETF draft document on DHCP options, RFC 2132. 124Options which are not listed by name may be defined by the name 125.Pf option\- Ns Ar nnn , 126where 127.Ar nnn 128is the decimal number of the option code. 129These options may be followed either by a string, enclosed in quotes, or by 130a series of octets, expressed as two-digit hexadecimal numbers separated 131by colons. 132For example: 133.Bd -literal -offset indent 134option option-133 "my-option-133-text"; 135option option-129 1:54:c9:2b:47; 136.Ed 137.Pp 138Because 139.Xr dhcpd 8 140does not know the format of these undefined option codes, 141no checking is done to ensure the correctness of the entered data. 142.Pp 143The defined options are: 144.Bl -tag -width Ds 145.It Ic option all-subnets-local Ar flag ; 146This option specifies whether or not the client may assume that all subnets 147of the IP network to which the client is connected use the same MTU as the 148subnet of that network to which the client is directly connected. 149A value of 1 indicates that all subnets share the same MTU. 150A value of 0 means that the client should assume that some subnets of the 151directly connected network may have smaller MTUs. 152.It Ic option arp-cache-timeout Ar uint32 ; 153This option specifies the timeout in seconds for ARP cache entries. 154.It Ic option autoproxy-script Ar string ; 155Site-specific as of RFC 3942. 156Widely used for Web Proxy Autodiscovery Protocol (WPAD). 157.It Ic option boot-size Ar uint16 ; 158This option specifies the length in 512-octet blocks of the default 159boot image for the client. 160.It Ic option bootfile-name Ar string ; 161This option is used to identify a bootstrap file. 162If supported by the client, it should have the same effect as the 163.Ic filename 164declaration. 165BOOTP clients are unlikely to support this option. 166Some DHCP clients will support it, and others actually require it. 167.It Ic option broadcast-address Ar ip-address ; 168This option specifies the broadcast address in use on the client's subnet. 169Legal values for broadcast addresses are specified in section 3.2.1.3 of 170RFC 1122. 171.It Ic option cookie-servers Ar ip-address Oo , Ar ip-address ... Oc ; 172The 173.Ic cookie-servers 174option specifies a list of RFC 865 cookie servers available to the client. 175Servers should be listed in order of preference. 176.It Ic option default-ip-ttl Ar uint8 ; 177This option specifies the default time-to-live that the client should 178use on outgoing datagrams. 179.It Ic option default-tcp-ttl Ar uint8 ; 180This option specifies the default TTL that the client should use when 181sending TCP segments. 182The minimum value is 1. 183.It Ic option dhcp-class-identifier Ar string ; 184This option is used by DHCP clients to optionally identify the vendor type 185and configuration of a DHCP client. 186The information is a string of n octets, interpreted by servers. 187Vendors may choose to define specific vendor class identifiers to convey 188particular configuration or other identification information about a client. 189For example, the identifier may encode the client's hardware configuration. 190Servers not equipped to interpret the class-specific information sent by a 191client must ignore it (although it may be reported). 192Servers that respond should only use option 43 193.Pq Ic vendor-encapsulated-options 194to return the vendor-specific information to the client. 195.It Ic option dhcp-client-identifier Ar data-string ; 196This option can be used to specify a DHCP client identifier in a 197host declaration, so that 198.Xr dhcpd 8 199can find the host record by matching against the client identifier. 200.It Ic option dhcp-lease-time Ar uint32 ; 201This option is used in a client request (DHCPDISCOVER or DHCPREQUEST) to 202allow the client to request a lease time for the IP address. 203In a server reply (DHCPOFFER), a DHCP server uses this option to specify the 204lease time it is willing to offer. 205.It Ic option dhcp-max-message-size Ar uint16 ; 206This option specifies the maximum length DHCP message that it is willing 207to accept. 208The length is specified as an unsigned 16-bit integer. 209A client may use the maximum DHCP message size option in DHCPDISCOVER or 210DHCPREQUEST messages, but should not use the option in DHCPDECLINE messages. 211.It Ic option dhcp-message-type Ar uint8 ; 212This option is used to convey the type of the DHCP message. 213Values: 1=DHCPDISCOVER, 2=DHCPOFFER, 3=DHCPREQUEST, 4=DHCPDECLINE, 5=DHCPACK, 2146=DHCPNAK, 7=DHCPRELEASE, 8=DHCPINFORM. 215.It Ic option dhcp-option-overload Ar uint8 Oo , Ar uint8 ... Oc ; 216This option is used to indicate that the DHCP 'sname' or 'file' fields are 217being overloaded by using them to carry DHCP options. 218A DHCP server inserts this option if the returned parameters will exceed 219the usual space allotted for options. 220If this option is present, the client interprets the specified additional 221fields after it concludes interpretation of the standard option fields. 222A value of 1 means the 'file' field is used to hold options. 223A value of 2 means the 'sname' field is used to hold options. 224A value of 3 means both fields are used to hold options. 225.It Ic option dhcp-parameter-request-list Ar uint8 Oo , Ar uint8 ... Oc ; 226This option is used by a DHCP client to request values for specified 227configuration parameters. 228The list of requested parameters is specified as n octets, where each 229octet is a valid DHCP option code as defined in this document. 230The client MAY list the options in order of preference. 231The DHCP server is not required to return the options in the requested 232order, but MUST try to insert the requested options in the order requested 233by the client. 234.It Ic option dhcp-rebinding-time Ar uint32 ; 235This option specifies the time interval from address assignment until the 236client transitions to the REBINDING state. 237The value is in seconds. 238.It Ic option dhcp-renewal-time Ar uint32 ; 239This option specifies the time interval from address assignment until the 240client transitions to the RENEWING state. 241The value is in seconds. 242.It Ic option dhcp-requested-address Ar ip-address ; 243This option is used in a client request (DHCPDISCOVER) to allow the client 244to request that a particular IP address be assigned. 245.It Ic option dhcp-server-identifier Ar ip-address ; 246This option is used in DHCPOFFER and DHCPREQUEST messages, and may 247optionally be included in the DHCPACK and DHCPNAK messages. 248DHCP servers include this option in the DHCPOFFER in order to allow the 249client to distinguish between lease offers. 250DHCP clients use the contents of the 'server identifier' field as the 251destination address for any DHCP messages unicast to the DHCP server. 252DHCP clients also indicate which of several lease offers is being accepted 253by including this option in a DHCPREQUEST message. 254The identifier is the IP address of the selected server. 255.It Ic option domain-name Ar string ; 256This option specifies the domain name that the client should use when 257resolving hostnames via the Domain Name System. 258.It Ic option domain-name-servers Ar ip-address Oo , Ar ip-address ... Oc ; 259The 260.Ic domain-name-servers 261option specifies a list of Domain Name System name servers 262available to the client. 263Servers should be listed in order of preference. 264.It Ic option extensions-path Ar string ; 265A string to specify a file, retrievable via TFTP, which contains information 266which can be interpreted in the same way as the 64-octet vendor-extension 267field within the BOOTP response, with exceptions; see RFC 2132, Section 3.20 268for details. 269.It Ic option finger-server Ar ip-address Oo , Ar ip-address ... Oc ; 270The 271.Ic finger-server 272option specifies a list of 273.Xr finger 1 274servers available to the client. 275Servers should be listed in order of preference. 276.It Ic option font-servers Ar ip-address Oo , Ar ip-address ... Oc ; 277This option specifies a list of X Window System Font servers available 278to the client. 279Servers should be listed in order of preference. 280.It Ic option host-name Ar string ; 281This option specifies the name of the client. 282The name may or may not be qualified with the local domain name 283(it is preferable to use the 284.Ic domain-name 285option to specify the domain name). 286See RFC 1035 for character set restrictions. 287.It Ic option ieee802-3-encapsulation Ar flag ; 288This option specifies whether or not the client should use Ethernet 289Version 2 (RFC 894) or IEEE 802.3 (RFC 1042) encapsulation if the 290interface is an Ethernet. 291A value of 0 indicates that the client should use RFC 894 encapsulation. 292A value of 1 means that the client should use RFC 1042 encapsulation. 293.It Ic option ien116-name-servers Ar ip-address Oo , Ar ip-address ... Oc ; 294The 295.Ic ien116-name-servers 296option specifies a list of IEN 116 name servers available to the client. 297Servers should be listed in order of preference. 298.It Ic option impress-servers Ar ip-address Oo , Ar ip-address ... Oc ; 299The 300.Ic impress-servers 301option specifies a list of Imagen Impress servers available to the client. 302Servers should be listed in order of preference. 303.It Ic option interface-mtu Ar uint16 ; 304This option specifies the MTU to use on this interface. 305The minimum legal value for the MTU is 68. 306.It Ic option ip-forwarding Ar flag ; 307This option specifies whether the client should configure its IP layer 308for packet forwarding. 309A value of 0 means disable IP forwarding, and a value of 1 means enable 310IP forwarding. 311.It Ic option irc-server Ar ip-address Oo , Ar ip-address ... Oc ; 312The 313.Ic irc-server 314option specifies a list of IRC servers available to the client. 315Servers should be listed in order of preference. 316.It Ic option log-servers Ar ip-address Oo , Ar ip-address ... Oc ; 317The 318.Ic log-servers 319option specifies a list of MIT-LCS UDP log servers available to the client. 320Servers should be listed in order of preference. 321.It Ic option lpr-servers Ar ip-address Oo , Ar ip-address ... Oc ; 322The 323.Ic lpr-servers 324option specifies a list of RFC 1179 line printer servers available to the 325client. 326Servers should be listed in order of preference. 327.It Ic option mask-supplier Ar flag ; 328This option specifies whether or not the client should respond to subnet mask 329requests using ICMP. 330A value of 0 indicates that the client should not respond. 331A value of 1 means that the client should respond. 332.It Ic option max-dgram-reassembly Ar uint16 ; 333This option specifies the maximum size datagram that the client should be 334prepared to reassemble. 335The minimum legal value is 576. 336.It Ic option merit-dump Ar string ; 337This option specifies the pathname of a file to which the client's 338core image should be dumped in the event the client crashes. 339The path is formatted as a character string consisting of characters from 340the 341.Tn NVT ASCII 342character set. 343.It Ic option mobile-ip-home-agent Ar ip-address Oo , Ar ip-address ... Oc ; 344This option specifies a list of IP addresses indicating mobile IP 345home agents available to the client. 346Agents should be listed in order of preference, although normally there 347will be only one such agent. 348.It Ic option nds-context Ar string ; 349This option specifies the initial NDS context the client should use. 350NDS contexts are 16-bit Unicode strings. 351For transmission in the NDS Context Option, an NDS context is 352transformed into octets using UTF-8. 353The string should NOT be zero terminated. 354A single DHCP option can only contain 255 octets. 355Since an NDS context name can be longer than that, this option can 356appear more than once in the DHCP packet. 357The contents of all NDS Context options in the packet should be 358concatenated as suggested in the DHCP specification to get the 359complete NDS context. 360A single encoded character could be split between two NDS Context Options. 361See RFC 2241 for details. 362.It Ic option nds-servers Ar ip-address Oo , Ar ip-address ... Oc ; 363This option specifies one or more NDS servers for the client to contact 364for access to the NDS database. 365Servers should be listed in order of preference. 366See RFC 2241 for details. 367.It Ic option nds-tree-name Ar string ; 368This option specifies the name of the NDS tree the client will be contacting. 369NDS tree names are 16-bit Unicode strings. 370For transmission in the NDS Tree Name Option, an NDS tree name is 371transformed into octets using UTF-8. 372The string should not be zero terminated. 373See RFC 2241 for details. 374.It Ic option netbios-dd-server Ar ip-address Oo , Ar ip-address ... Oc ; 375The NetBIOS datagram distribution server (NBDD) option specifies a 376list of RFC 1001/1002 NBDD servers listed in order of preference. 377.It Ic option netbios-name-servers Ar ip-address Oo , Ar ip-address ... Oc ; 378The NetBIOS name server (NBNS) option specifies a list of RFC 1001/1002 379NBNS name servers listed in order of preference. 380NetBIOS Name Service is currently more commonly referred to as WINS. 381WINS servers can be specified using the 382.Ic netbios-name-servers 383option. 384.It Ic option netbios-node-type Ar uint8 ; 385The NetBIOS node type option allows NetBIOS over TCP/IP clients which 386are configurable to be configured as described in RFC 1001/1002. 387The value is specified as a single octet which identifies the client type. 388.Pp 389Possible node types are: 390.Bl -tag -width Ds 391.It 1 392B-node: Broadcast - no WINS 393.It 2 394P-node: Peer - WINS only 395.It 4 396M-node: Mixed - broadcast, then WINS 397.It 8 398H-node: Hybrid - WINS, then broadcast 399.El 400.It Ic option netbios-scope Ar string ; 401The NetBIOS scope option specifies the NetBIOS over TCP/IP scope 402parameter for the client as specified in RFC 1001/1002. 403See RFC 1001, RFC 1002, and RFC 1035 for character-set restrictions. 404.It Ic option nis-domain Ar string ; 405This option specifies the name of the client's NIS (Sun Network Information 406Services) domain. 407The domain is formatted as a character string consisting of characters 408from the 409.Tn NVT ASCII 410character set. 411.It Ic option nis-servers Ar ip-address Oo , Ar ip-address ... Oc ; 412This option specifies a list of IP addresses indicating NIS servers 413available to the client. 414Servers should be listed in order of preference. 415.It Ic option nisplus-domain Ar string ; 416This option specifies the name of the client's NIS+ domain. 417The domain is formatted as a character string consisting of characters 418from the 419.Tn NVT ASCII 420character set. 421.It Ic option nisplus-servers Ar ip-address Oo , Ar ip-address ... Oc ; 422This option specifies a list of IP addresses indicating NIS+ servers 423available to the client. 424Servers should be listed in order of preference. 425.It Ic option nntp-server Ar ip-address Oo , Ar ip-address ... Oc ; 426The 427.Ic nntp-server 428option specifies a list of NNTP servers available to the client. 429Servers should be listed in order of preference. 430.It Ic option non-local-source-routing Ar flag ; 431This option specifies whether the client should configure its IP 432layer to allow forwarding of datagrams with non-local source routes. 433A value of 0 means disallow forwarding of such datagrams, and a value of 1 434means allow forwarding. 435.It Ic option ntp-servers Ar ip-address Oo , Ar ip-address ... Oc ; 436This option specifies a list of IP addresses indicating NTP (RFC 5905) 437servers available to the client. 438Servers should be listed in order of preference. 439.It Ic option path-mtu-aging-timeout Ar uint32 ; 440This option specifies the timeout (in seconds) to use when aging Path 441MTU values discovered by the mechanism defined in RFC 1191. 442.It Ic option path-mtu-plateau-table Ar uint16 Oo , Ar uint16 ... Oc ; 443This option specifies a table of MTU sizes to use when performing 444Path MTU Discovery as defined in RFC 1191. 445The table is formatted as a list of 16-bit unsigned integers, 446ordered from smallest to largest. 447The minimum MTU value cannot be smaller than 68. 448.It Ic option perform-mask-discovery Ar flag ; 449This option specifies whether or not the client should perform subnet mask 450discovery using ICMP. 451A value of 0 indicates that the client should not perform mask discovery. 452A value of 1 means that the client should perform mask discovery. 453.It Ic option policy-filter Ar ip-address ip-address Oo , Ar ip-address ip-address ... Oc ; 454This option specifies policy filters for non-local source routing. 455The filters consist of a list of IP addresses and masks which specify 456destination/mask pairs with which to filter incoming source routes. 457.Pp 458Any source-routed datagram whose next-hop address does not match one 459of the filters should be discarded by the client. 460.Pp 461See RFC 1122 for further information. 462.It Ic option pop-server Ar ip-address Oo , Ar ip-address ... Oc ; 463The 464.Ic pop-server 465option specifies a list of POP3 servers available to the client. 466Servers should be listed in order of preference. 467.It Ic option relay-agent-information Ar string ; 468This is a "container" option for specific agent-supplied sub-options. 469See RFC 3046 for details. 470.It Ic option resource-location-servers Ar ip-address Oo , Ar ip-address ... Oc ; 471This option specifies a list of RFC 887 Resource Location servers available 472to the client. 473Servers should be listed in order of preference. 474.It Ic option root-path Ar string ; 475This option specifies the pathname that contains the client's root disk. 476The path is formatted as a character string consisting of characters from 477the 478.Tn NVT ASCII 479character set. 480.It Ic option router-discovery Ar flag ; 481This option specifies whether or not the client should solicit routers using 482the Router Discovery mechanism defined in RFC 1256. 483A value of 0 indicates that the client should not perform router discovery. 484A value of 1 means that the client should perform router discovery. 485.It Ic option router-solicitation-address Ar ip-address ; 486This option specifies the address to which the client should transmit 487router solicitation requests. 488.It Ic option routers Ar ip-address Oo , Ar ip-address ... Oc ; 489The 490.Ic routers 491option specifies a list of IP addresses for routers on the client's subnet. 492Routers should be listed in order of preference. 493.It Ic option smtp-server Ar ip-address Oo , Ar ip-address ... Oc ; 494The 495.Ic smtp-server 496option specifies a list of SMTP servers available to the client. 497Servers should be listed in order of preference. 498.It Ic option static-routes Ar ip-address ip-address Oo , Ar ip-address ip-address ... Oc ; 499This option specifies a list of static routes that the client should 500install in its routing cache. 501If multiple routes to the same destination are specified, they are listed 502in descending order of priority. 503.Pp 504The routes consist of a list of IP address pairs. 505The first address is the destination address, 506and the second address is the router for the destination. 507.Pp 508The default route (0.0.0.0) is an illegal destination for a static route. 509To specify the default route, use the 510.Ic routers 511option. 512.It Ic option streettalk-directory-assistance-server Ar ip-address Oo , Ar ip-address ... Oc ; 513The StreetTalk Directory Assistance (STDA) server option specifies a 514list of STDA servers available to the client. 515Servers should be listed in order of preference. 516.It Ic option streettalk-server Ar ip-address Oo , Ar ip-address ... Oc ; 517The 518.Ic streettalk-server 519option specifies a list of StreetTalk servers available to the client. 520Servers should be listed in order of preference. 521.It Ic option subnet-mask Ar ip-address ; 522The 523.Ic subnet-mask 524option specifies the client's subnet mask as per RFC 950. 525If no subnet-mask option is provided anywhere in scope, as a last resort 526.Xr dhcpd 8 527will use the subnet mask from the subnet declaration for the network on 528which an address is being assigned. 529However, 530.Em any 531subnet-mask option declaration that is in scope for the address being 532assigned will override the subnet mask specified in the subnet declaration. 533.It Ic option swap-server Ar ip-address ; 534This specifies the IP address of the client's swap server. 535.It Ic option tcp-keepalive-garbage Ar flag ; 536This option specifies whether or not the client should send TCP keepalive 537messages with an octet of garbage for compatibility with older implementations. 538A value of 0 indicates that a garbage octet should not be sent. 539A value of 1 indicates that a garbage octet should be sent. 540.It Ic option tcp-keepalive-interval Ar uint32 ; 541This option specifies the interval (in seconds) that the client TCP 542should wait before sending a keepalive message on a TCP connection. 543The time is specified as a 32-bit unsigned integer. 544A value of zero indicates that the client should not generate keepalive 545messages on connections unless specifically requested by an application. 546.It Ic option tftp-config-file Ar string ; 547Option 144. 548Per RFC 2132 options 128 \- 254 are site-specific. 549RFC 3942 reclassifies options 128 to 223 as publicly defined options and 550puts them in "Unavailable" state by IANA. 551See RFC 3679 for "Unused DHCP Option Codes to be Reassigned to Future 552DHCP Options". 553See RFC 2939 for procedures for definitions of new DHCP options. 554.It Ic option tftp-server-name Ar string ; 555This option is used to identify a TFTP server and, if supported by the 556client, should have the same effect as the 557.Ic server-name 558declaration. 559BOOTP clients are unlikely to support this option. 560Some DHCP clients will support it, and others actually require it. 561.It Ic option time-offset Ar int32 ; 562The 563.Ic time-offset 564option specifies the offset of the client's subnet in seconds from 565Coordinated Universal Time (UTC). 566.It Ic option time-servers Ar ip-address Oo , Ar ip-address ... Oc ; 567The 568.Ic time-server 569option specifies a list of RFC 868 time servers available to the client. 570Servers should be listed in order of preference. 571.It Ic option trailer-encapsulation Ar flag ; 572This option specifies whether or not the client should negotiate the 573use of trailers (RFC 893) when using the ARP protocol. 574A value of 0 indicates that the client should not attempt to use trailers. 575A value of 1 means that the client should attempt to use trailers. 576.It Ic option user-class Ar string ; 577This option is used by a DHCP client to optionally identify the type or 578category of user or applications it represents. 579A DHCP server uses the User Class option to choose the address pool it 580allocates an address from and/or to select any other configuration option. 581This option may carry multiple User Classes. 582Servers may interpret the meanings of multiple class specifications in an 583implementation dependent or configuration dependent manner, and so the use 584of multiple classes by a DHCP client should be based on the specific server 585implementation and configuration which will be used to process that User 586class option. 587See RFC 3004 for details. 588.It Ic option vendor-encapsulated-options Ar data-string ; 589This option is used by clients and servers to exchange vendor-specific 590information. 591The information is an opaque object of n octets, presumably interpreted by 592vendor-specific code on the clients and servers. 593See RFC 2132, Section 8.4 for details. 594.It Ic option voip-configuration-server Ar ip-address Oo , Ar ip-address ... Oc ; 595The 596.Ic voip-configuration-server 597option specifies a list of (normally TFTP) servers that VoIP clients 598may download their configuration information and software images 599from. 600Servers should be listed in order of preference. 601.It Ic option www-server Ar ip-address Oo , Ar ip-address ... Oc ; 602The 603.Ic www-server 604option specifies a list of WWW servers available to the client. 605.It Ic option x-display-manager Ar ip-address Oo , Ar ip-address ... Oc ; 606This option specifies a list of systems that are running the X Window 607System Display Manager and are available to the client. 608Addresses should be listed in order of preference. 609.El 610.Sh SEE ALSO 611.Xr dhclient.conf 5 , 612.Xr dhcpd.conf 5 , 613.Xr dhcpd.leases 5 , 614.Xr dhclient 8 , 615.Xr dhcpd 8 616.Sh STANDARDS 617.Rs 618.%A R. Droms 619.%D March 1997 620.%R RFC 2131 621.%T Dynamic Host Configuration Protocol 622.Re 623.Pp 624.Rs 625.%A S. Alexander 626.%A R. Droms 627.%D March 1997 628.%R RFC 2132 629.%T DHCP Options and BOOTP Vendor Extensions 630.Re 631.Sh AUTHORS 632.An -nosplit 633.Xr dhcpd 8 634was written by 635.An Ted Lemon Aq mellon@vix.com 636under a contract with Vixie Labs. 637.Pp 638The current implementation was reworked by 639.An Henning Brauer Aq henning@openbsd.org . 640