1.\" $OpenBSD: dhcp-options.5,v 1.31 2019/05/08 22:00:55 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: May 8 2019 $ 40.Dt DHCP-OPTIONS 5 41.Os 42.Sh NAME 43.Nm dhcp-options 44.Nd DHCP 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.Pp 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 cidr 77data type specifies a network in CIDR notion. 78e.g. 1.2.3/24. 79.Pp 80The 81.Ar int32 82data type specifies a signed 32-bit integer. 83The 84.Ar uint32 85data type specifies an unsigned 32-bit integer. 86The 87.Ar uint16 88data type specifies unsigned 16-bit integers. 89The 90.Ar uint8 91data type specifies unsigned 8-bit integers, 92sometimes referred to as octets. 93.Pp 94The 95.Ar string 96data type specifies an 97.Tn NVT 98.Pq Network Virtual Terminal 99.Tn ASCII 100string, which must be enclosed in double quotes \- for example, 101to specify a domain-name option, the syntax would be 102.Pp 103.Dl option domain-name \&"isc.org\&"; 104.Pp 105The 106.Ar flag 107data type specifies a boolean value. 108Booleans can be either true or false 109(or on or off, if that makes more sense to you). 110.Pp 111The 112.Ar data-string 113data type specifies either an 114.Tn NVT ASCII 115string enclosed in double quotes, or a series of octets specified in 116hexadecimal, separated by colons. 117For example: 118.Pp 119.Dl option dhcp-client-identifier \&"CLIENT-FOO\&"; 120or 121.Dl option dhcp-client-identifier 43:4c:49:45:4e:54:2d:46:4f:4f; 122.Pp 123The documentation for the various options mentioned below is taken 124from the IETF draft document on DHCP options, RFC 2132. 125Options which are not listed by name may be defined by the name 126.Pf option- Ar nnn , 127where 128.Ar nnn 129is the decimal number of the option code. 130These options may be followed either by a string, enclosed in quotes, or by 131a series of octets, expressed as two-digit hexadecimal numbers separated 132by colons. 133For example: 134.Bd -literal -offset indent 135option option-133 "my-option-133-text"; 136option option-129 1:54:c9:2b:47; 137.Ed 138.Pp 139Because 140.Xr dhcpd 8 141does not know the format of these undefined option codes, 142no checking is done to ensure the correctness of the entered data. 143.Pp 144The defined options are: 145.Bl -tag -width Ds 146.It Ic option all-subnets-local Ar flag ; 147This option specifies whether or not the client may assume that all subnets 148of the IP network to which the client is connected use the same MTU as the 149subnet of that network to which the client is directly connected. 150A value of 1 indicates that all subnets share the same MTU. 151A value of 0 means that the client should assume that some subnets of the 152directly connected network may have smaller MTUs. 153.It Ic option arp-cache-timeout Ar uint32 ; 154This option specifies the timeout in seconds for ARP cache entries. 155.It Ic option autoproxy-script Ar string ; 156Site-specific as of RFC 3942. 157Widely used for Web Proxy Autodiscovery Protocol (WPAD). 158.It Ic option boot-size Ar uint16 ; 159This option specifies the length in 512-octet blocks of the default 160boot image for the client. 161.It Ic option bootfile-name Ar string ; 162This option is used to identify a bootstrap file. 163If supported by the client, it should have the same effect as the 164.Ic filename 165declaration. 166BOOTP clients are unlikely to support this option. 167Some DHCP clients will support it, and others actually require it. 168.It Ic option broadcast-address Ar ip-address ; 169This option specifies the broadcast address in use on the client's subnet. 170Legal values for broadcast addresses are specified in section 3.2.1.3 of 171RFC 1122. 172.It Ic option classless-static-routes Ar cidr ip-address Oo , Ar cidr ip-address ... Oc ; 173This option specifies a list of destination networks and the 174associated gateways. 175This option is defined in RFC 3442. 176The RFC says that clients supporting this option must ignore the 177.Ic Routers 178option when both are present. 179Thus default routes, if any, 180must be included in the list. 181The 182.Ar cidr 183of a default route is 0/0. 184.It Ic option classless-ms-static-routes Ar cidr ip-address Oo , Ar cidr ip-address ... Oc ; 185This option does the same as 186.Ic classless-static-routes , 187but uses option code 249 instead of 121, 188since Windows XP and Windows Server 2003 ignore option 121. 189.It Ic option cookie-servers Ar ip-address Oo , Ar ip-address ... Oc ; 190The 191.Ic cookie-servers 192option specifies a list of RFC 865 cookie servers available to the client. 193Servers should be listed in order of preference. 194.It Ic option default-ip-ttl Ar uint8 ; 195This option specifies the default time-to-live that the client should 196use on outgoing datagrams. 197.It Ic option default-tcp-ttl Ar uint8 ; 198This option specifies the default TTL that the client should use when 199sending TCP segments. 200The minimum value is 1. 201.It Ic option dhcp-class-identifier Ar string ; 202This option is used by DHCP clients to optionally identify the vendor type 203and configuration of a DHCP client. 204The information is a string of n octets, interpreted by servers. 205Vendors may choose to define specific vendor class identifiers to convey 206particular configuration or other identification information about a client. 207For example, the identifier may encode the client's hardware configuration. 208Servers not equipped to interpret the class-specific information sent by a 209client must ignore it (although it may be reported). 210Servers that respond should only use option 43 211.Pq Ic vendor-encapsulated-options 212to return the vendor-specific information to the client. 213.It Ic option dhcp-client-identifier Ar data-string ; 214This option can be used to specify a DHCP client identifier in a 215host declaration, so that 216.Xr dhcpd 8 217can find the host record by matching against the client identifier. 218.It Ic option dhcp-lease-time Ar uint32 ; 219This option is used in a client request (DHCPDISCOVER or DHCPREQUEST) to 220allow the client to request a lease time for the IP address. 221In a server reply (DHCPOFFER), a DHCP server uses this option to specify the 222lease time it is willing to offer. 223.It Ic option dhcp-max-message-size Ar uint16 ; 224This option specifies the maximum length 225.Ic option dhcp-message 226that the DHCP client is willing to accept. 227The length is specified as an unsigned 16-bit integer. 228A client may use the maximum DHCP message size option in DHCPDISCOVER or 229DHCPREQUEST messages, but should not use the option in DHCPDECLINE messages. 230.It Ic option dhcp-message Ar string ; 231This option is used to send an error message to a DHCP client in 232the event of a failure. 233The maximum acceptable length of the message can be set with 234.Ic option dhcp-max-message-size . 235.It Ic option dhcp-message-type Ar uint8 ; 236This option is used to convey the type of the DHCP message. 237Values: 1=DHCPDISCOVER, 2=DHCPOFFER, 3=DHCPREQUEST, 4=DHCPDECLINE, 5=DHCPACK, 2386=DHCPNAK, 7=DHCPRELEASE, 8=DHCPINFORM. 239.It Ic option dhcp-option-overload Ar uint8 ; 240This option is used to indicate that the DHCP 'sname' or 'file' fields are 241being overloaded by using them to carry DHCP options. 242A DHCP server inserts this option if the returned parameters will exceed 243the usual space allotted for options. 244If this option is present, the client interprets the specified additional 245fields after it concludes interpretation of the standard option fields. 246A value of 1 means the 'file' field is used to hold options. 247A value of 2 means the 'sname' field is used to hold options. 248A value of 3 means both fields are used to hold options. 249.It Ic option dhcp-parameter-request-list Ar uint8 Oo , Ar uint8 ... Oc ; 250This option is used by a DHCP client to request values for specified 251configuration parameters. 252The list of requested parameters is specified as n octets, where each 253octet is a valid DHCP option code as defined in this document. 254The client MAY list the options in order of preference. 255The DHCP server is not required to return the options in the requested 256order, but MUST try to insert the requested options in the order requested 257by the client. 258.It Ic option dhcp-rebinding-time Ar uint32 ; 259This option specifies the time interval from address assignment until the 260client transitions to the REBINDING state. 261The value is in seconds. 262.It Ic option dhcp-renewal-time Ar uint32 ; 263This option specifies the time interval from address assignment until the 264client transitions to the RENEWING state. 265The value is in seconds. 266.It Ic option dhcp-requested-address Ar ip-address ; 267This option is used in a client request (DHCPDISCOVER) to allow the client 268to request that a particular IP address be assigned. 269.It Ic option dhcp-server-identifier Ar ip-address ; 270This option is used in DHCPOFFER and DHCPREQUEST messages, and may 271optionally be included in the DHCPACK and DHCPNAK messages. 272DHCP servers include this option in the DHCPOFFER in order to allow the 273client to distinguish between lease offers. 274DHCP clients use the contents of the 'server identifier' field as the 275destination address for any DHCP messages unicast to the DHCP server. 276DHCP clients also indicate which of several lease offers is being accepted 277by including this option in a DHCPREQUEST message. 278The identifier is the IP address of the selected server. 279.It Ic option domain-name Ar string ; 280This option specifies the domain name that the client should use when 281resolving hostnames via the Domain Name System. 282.It Ic option domain-name-servers Ar ip-address Oo , Ar ip-address ... Oc ; 283The 284.Ic domain-name-servers 285option specifies a list of Domain Name System name servers 286available to the client. 287Servers should be listed in order of preference. 288.It Ic option domain-search Ar string Oo , Ar string ... Oc ; 289The 290.Ic domain-search 291option specifies a list of the domain names that should be 292used during DNS name resolution. 293.Pp 294When 295.Xr dhclient 8 296constructs 297.Xr resolv.conf 5 298it will use this list of domains in preference to any information 299provided by the 300.Ic domain-name 301option. 302.It Ic option extensions-path Ar string ; 303A string to specify a file, retrievable via TFTP, which contains information 304which can be interpreted in the same way as the 64-octet vendor-extension 305field within the BOOTP response, with exceptions; see RFC 2132, Section 3.20 306for details. 307.It Ic option finger-server Ar ip-address Oo , Ar ip-address ... Oc ; 308The 309.Ic finger-server 310option specifies a list of 311.Xr finger 1 312servers available to the client. 313Servers should be listed in order of preference. 314.It Ic option font-servers Ar ip-address Oo , Ar ip-address ... Oc ; 315This option specifies a list of X Window System Font servers available 316to the client. 317Servers should be listed in order of preference. 318.It Ic option host-name Ar string ; 319This option specifies the name of the client. 320The name may or may not be qualified with the local domain name 321(it is preferable to use the 322.Ic domain-name 323option to specify the domain name). 324See RFC 1035 for character set restrictions. 325.It Ic option ieee802-3-encapsulation Ar flag ; 326This option specifies whether or not the client should use Ethernet 327Version 2 (RFC 894) or IEEE 802.3 (RFC 1042) encapsulation if the 328interface is an Ethernet. 329A value of 0 indicates that the client should use RFC 894 encapsulation. 330A value of 1 means that the client should use RFC 1042 encapsulation. 331.It Ic option ien116-name-servers Ar ip-address Oo , Ar ip-address ... Oc ; 332The 333.Ic ien116-name-servers 334option specifies a list of IEN 116 name servers available to the client. 335Servers should be listed in order of preference. 336.It Ic option impress-servers Ar ip-address Oo , Ar ip-address ... Oc ; 337The 338.Ic impress-servers 339option specifies a list of Imagen Impress servers available to the client. 340Servers should be listed in order of preference. 341.It Ic option interface-mtu Ar uint16 ; 342This option specifies the MTU to use on this interface. 343The minimum legal value for the MTU is 68. 344.It Ic option ip-forwarding Ar flag ; 345This option specifies whether the client should configure its IP layer 346for packet forwarding. 347A value of 0 means disable IP forwarding, and a value of 1 means enable 348IP forwarding. 349.It Ic option irc-server Ar ip-address Oo , Ar ip-address ... Oc ; 350The 351.Ic irc-server 352option specifies a list of IRC servers available to the client. 353Servers should be listed in order of preference. 354.It Ic option log-servers Ar ip-address Oo , Ar ip-address ... Oc ; 355The 356.Ic log-servers 357option specifies a list of MIT-LCS UDP log servers available to the client. 358Servers should be listed in order of preference. 359.It Ic option lpr-servers Ar ip-address Oo , Ar ip-address ... Oc ; 360The 361.Ic lpr-servers 362option specifies a list of RFC 1179 line printer servers available to the 363client. 364Servers should be listed in order of preference. 365.It Ic option mask-supplier Ar flag ; 366This option specifies whether or not the client should respond to subnet mask 367requests using ICMP. 368A value of 0 indicates that the client should not respond. 369A value of 1 means that the client should respond. 370.It Ic option max-dgram-reassembly Ar uint16 ; 371This option specifies the maximum size datagram that the client should be 372prepared to reassemble. 373The minimum legal value is 576. 374.It Ic option merit-dump Ar string ; 375This option specifies the pathname of a file to which the client's 376core image should be dumped in the event the client crashes. 377The path is formatted as a character string consisting of characters from 378the 379.Tn NVT ASCII 380character set. 381.It Ic option mobile-ip-home-agent Ar ip-address Oo , Ar ip-address ... Oc ; 382This option specifies a list of IP addresses indicating Mobile IP 383home agents available to the client. 384Agents should be listed in order of preference, although normally there 385will be only one such agent. 386.It Ic option nds-context Ar data-string ; 387This option specifies the initial NDS context the client should use. 388NDS contexts are 16-bit Unicode strings. 389For transmission in the NDS Context Option, an NDS context is 390transformed into octets using UTF-8. 391The string should NOT be zero terminated. 392A single DHCP option can only contain 255 octets. 393Since an NDS context name can be longer than that, this option can 394appear more than once in the DHCP packet. 395The contents of all NDS Context options in the packet should be 396concatenated as suggested in the DHCP specification to get the 397complete NDS context. 398A single encoded character could be split between two NDS Context Options. 399See RFC 2241 for details. 400.It Ic option nds-servers Ar ip-address Oo , Ar ip-address ... Oc ; 401This option specifies one or more NDS servers for the client to contact 402for access to the NDS database. 403Servers should be listed in order of preference. 404See RFC 2241 for details. 405.It Ic option nds-tree-name Ar data-string ; 406This option specifies the name of the NDS tree the client will be contacting. 407NDS tree names are 16-bit Unicode strings. 408For transmission in the NDS Tree Name Option, an NDS tree name is 409transformed into octets using UTF-8. 410The string should not be zero terminated. 411See RFC 2241 for details. 412.It Ic option netbios-dd-server Ar ip-address Oo , Ar ip-address ... Oc ; 413The NetBIOS datagram distribution server (NBDD) option specifies a 414list of RFC 1001/1002 NBDD servers listed in order of preference. 415.It Ic option netbios-name-servers Ar ip-address Oo , Ar ip-address ... Oc ; 416The NetBIOS name server (NBNS) option specifies a list of RFC 1001/1002 417NBNS name servers listed in order of preference. 418NetBIOS Name Service is currently more commonly referred to as WINS. 419WINS servers can be specified using the 420.Ic netbios-name-servers 421option. 422.It Ic option netbios-node-type Ar uint8 ; 423The NetBIOS node type option allows NetBIOS over TCP/IP clients which 424are configurable to be configured as described in RFC 1001/1002. 425The value is specified as a single octet which identifies the client type. 426.Pp 427Possible node types are: 428.Bl -tag -width Ds 429.It 1 430B-node: Broadcast - no WINS 431.It 2 432P-node: Peer - WINS only 433.It 4 434M-node: Mixed - broadcast, then WINS 435.It 8 436H-node: Hybrid - WINS, then broadcast 437.El 438.It Ic option netbios-scope Ar string ; 439The NetBIOS scope option specifies the NetBIOS over TCP/IP scope 440parameter for the client as specified in RFC 1001/1002. 441See RFC 1001, RFC 1002, and RFC 1035 for character-set restrictions. 442.It Ic option nis-domain Ar string ; 443This option specifies the name of the client's NIS (Sun Network Information 444Services) domain. 445The domain is formatted as a character string consisting of characters 446from the 447.Tn NVT ASCII 448character set. 449.It Ic option nis-servers Ar ip-address Oo , Ar ip-address ... Oc ; 450This option specifies a list of IP addresses indicating NIS servers 451available to the client. 452Servers should be listed in order of preference. 453.It Ic option nisplus-domain Ar string ; 454This option specifies the name of the client's NIS+ domain. 455The domain is formatted as a character string consisting of characters 456from the 457.Tn NVT ASCII 458character set. 459.It Ic option nisplus-servers Ar ip-address Oo , Ar ip-address ... Oc ; 460This option specifies a list of IP addresses indicating NIS+ servers 461available to the client. 462Servers should be listed in order of preference. 463.It Ic option nntp-server Ar ip-address Oo , Ar ip-address ... Oc ; 464The 465.Ic nntp-server 466option specifies a list of NNTP servers available to the client. 467Servers should be listed in order of preference. 468.It Ic option non-local-source-routing Ar flag ; 469This option specifies whether the client should configure its IP 470layer to allow forwarding of datagrams with non-local source routes. 471A value of 0 means disallow forwarding of such datagrams, and a value of 1 472means allow forwarding. 473.It Ic option ntp-servers Ar ip-address Oo , Ar ip-address ... Oc ; 474This option specifies a list of IP addresses indicating NTP (RFC 5905) 475servers available to the client. 476Servers should be listed in order of preference. 477.It Ic option path-mtu-aging-timeout Ar uint32 ; 478This option specifies the timeout (in seconds) to use when aging Path 479MTU values discovered by the mechanism defined in RFC 1191. 480.It Ic option path-mtu-plateau-table Ar uint16 Oo , Ar uint16 ... Oc ; 481This option specifies a table of MTU sizes to use when performing 482Path MTU Discovery as defined in RFC 1191. 483The table is formatted as a list of 16-bit unsigned integers, 484ordered from smallest to largest. 485The minimum MTU value cannot be smaller than 68. 486.It Ic option perform-mask-discovery Ar flag ; 487This option specifies whether or not the client should perform subnet mask 488discovery using ICMP. 489A value of 0 indicates that the client should not perform mask discovery. 490A value of 1 means that the client should perform mask discovery. 491.It Ic option policy-filter Ar ip-address ip-address Oo , Ar ip-address ip-address ... Oc ; 492This option specifies policy filters for non-local source routing. 493The filters consist of a list of IP addresses and masks which specify 494destination/mask pairs with which to filter incoming source routes. 495.Pp 496Any source-routed datagram whose next-hop address does not match one 497of the filters should be discarded by the client. 498.Pp 499See RFC 1122 for further information. 500.It Ic option pop-server Ar ip-address Oo , Ar ip-address ... Oc ; 501The 502.Ic pop-server 503option specifies a list of POP3 servers available to the client. 504Servers should be listed in order of preference. 505.It Ic option relay-agent-information Ar data-string ; 506This is a "container" option for specific agent-supplied sub-options. 507See RFC 3046 for details. 508.It Ic option resource-location-servers Ar ip-address Oo , Ar ip-address ... Oc ; 509This option specifies a list of RFC 887 Resource Location servers available 510to the client. 511Servers should be listed in order of preference. 512.It Ic option root-path Ar string ; 513This option specifies the pathname that contains the client's root disk. 514The path is formatted as a character string consisting of characters from 515the 516.Tn NVT ASCII 517character set. 518.It Ic option router-discovery Ar flag ; 519This option specifies whether or not the client should solicit routers using 520the Router Discovery mechanism defined in RFC 1256. 521A value of 0 indicates that the client should not perform router discovery. 522A value of 1 means that the client should perform router discovery. 523.It Ic option router-solicitation-address Ar ip-address ; 524This option specifies the address to which the client should transmit 525router solicitation requests. 526.It Ic option routers Ar ip-address Oo , Ar ip-address ... Oc ; 527The 528.Ic routers 529option specifies a list of IP addresses for routers on the client's subnet. 530Routers should be listed in order of preference. 531.It Ic option smtp-server Ar ip-address Oo , Ar ip-address ... Oc ; 532The 533.Ic smtp-server 534option specifies a list of SMTP servers available to the client. 535Servers should be listed in order of preference. 536.It Ic option static-routes Ar ip-address ip-address Oo , Ar ip-address ip-address ... Oc ; 537This option specifies a list of static routes that the client should 538install in its routing cache. 539If multiple routes to the same destination are specified, they are listed 540in descending order of priority. 541.Pp 542The routes consist of a list of IP address pairs. 543The first address is the destination address, 544and the second address is the router for the destination. 545.Pp 546The default route (0.0.0.0) is an illegal destination for a static route. 547To specify the default route, use the 548.Ic routers 549option. 550Note that this option is obsolete and should be replaced by the 551.Ic classless-static-routes 552option. 553.Xr dhclient 8 554ignores this option. 555.It Ic option streettalk-directory-assistance-server Ar ip-address Oo , Ar ip-address ... Oc ; 556The StreetTalk Directory Assistance (STDA) server option specifies a 557list of STDA servers available to the client. 558Servers should be listed in order of preference. 559.It Ic option streettalk-server Ar ip-address Oo , Ar ip-address ... Oc ; 560The 561.Ic streettalk-server 562option specifies a list of StreetTalk servers available to the client. 563Servers should be listed in order of preference. 564.It Ic option subnet-mask Ar ip-address ; 565The 566.Ic subnet-mask 567option specifies the client's subnet mask as per RFC 950. 568If no subnet-mask option is provided anywhere in scope, as a last resort 569.Xr dhcpd 8 570will use the subnet mask from the subnet declaration for the network on 571which an address is being assigned. 572However, 573.Em any 574subnet-mask option declaration that is in scope for the address being 575assigned will override the subnet mask specified in the subnet declaration. 576.It Ic option swap-server Ar ip-address ; 577This specifies the IP address of the client's swap server. 578.It Ic option tcp-keepalive-garbage Ar flag ; 579This option specifies whether or not the client should send TCP keepalive 580messages with an octet of garbage for compatibility with older implementations. 581A value of 0 indicates that a garbage octet should not be sent. 582A value of 1 indicates that a garbage octet should be sent. 583.It Ic option tcp-keepalive-interval Ar uint32 ; 584This option specifies the interval (in seconds) that the client TCP 585should wait before sending a keepalive message on a TCP connection. 586The time is specified as a 32-bit unsigned integer. 587A value of zero indicates that the client should not generate keepalive 588messages on connections unless specifically requested by an application. 589.It Ic option tftp-config-file Ar string ; 590Option 144. 591Per RFC 2132 options 128 \- 254 are site-specific. 592RFC 3942 reclassifies options 128 to 223 as publicly defined options and 593puts them in "Unavailable" state by IANA. 594See RFC 3679 for "Unused DHCP Option Codes to be Reassigned to Future 595DHCP Options". 596See RFC 2939 for procedures for definitions of new DHCP options. 597.It Ic option tftp-server-name Ar string ; 598This option is used to identify a TFTP server and, if supported by the 599client, should have the same effect as the 600.Ic server-name 601declaration. 602BOOTP clients are unlikely to support this option. 603Some DHCP clients will support it, and others actually require it. 604.It Ic option time-offset Ar int32 ; 605The 606.Ic time-offset 607option specifies the offset of the client's subnet in seconds from 608Coordinated Universal Time (UTC). 609.It Ic option time-servers Ar ip-address Oo , Ar ip-address ... Oc ; 610The 611.Ic time-server 612option specifies a list of RFC 868 time servers available to the client. 613Servers should be listed in order of preference. 614.It Ic option trailer-encapsulation Ar flag ; 615This option specifies whether or not the client should negotiate the 616use of trailers (RFC 893) when using the ARP protocol. 617A value of 0 indicates that the client should not attempt to use trailers. 618A value of 1 means that the client should attempt to use trailers. 619.It Ic option user-class Ar string ; 620This option is used by a DHCP client to optionally identify the type or 621category of user or applications it represents. 622A DHCP server uses the User Class option to choose the address pool it 623allocates an address from and/or to select any other configuration option. 624This option may carry multiple User Classes. 625Servers may interpret the meanings of multiple class specifications in an 626implementation dependent or configuration dependent manner, and so the use 627of multiple classes by a DHCP client should be based on the specific server 628implementation and configuration which will be used to process that User 629class option. 630See RFC 3004 for details. 631.It Ic option vendor-encapsulated-options Ar data-string ; 632This option is used by clients and servers to exchange vendor-specific 633information. 634The information is an opaque object of n octets, presumably interpreted by 635vendor-specific code on the clients and servers. 636See RFC 2132, Section 8.4 for details. 637.It Ic option voip-configuration-server Ar ip-address Oo , Ar ip-address ... Oc ; 638The 639.Ic voip-configuration-server 640option specifies a list of (normally TFTP) servers that VoIP clients 641may download their configuration information and software images 642from. 643Servers should be listed in order of preference. 644.It Ic option www-server Ar ip-address Oo , Ar ip-address ... Oc ; 645The 646.Ic www-server 647option specifies a list of WWW servers available to the client. 648.It Ic option x-display-manager Ar ip-address Oo , Ar ip-address ... Oc ; 649This option specifies a list of systems that are running the X Window 650System Display Manager and are available to the client. 651Addresses should be listed in order of preference. 652.El 653.Sh SEE ALSO 654.Xr dhclient.conf 5 , 655.Xr dhcpd.conf 5 , 656.Xr dhcpd.leases 5 , 657.Xr dhclient 8 , 658.Xr dhcpd 8 659.Sh STANDARDS 660.Rs 661.%A R. Droms 662.%D October 1993 663.%R RFC 1534 664.%T Interoperation Between DHCP and BOOTP 665.Re 666.Pp 667.Rs 668.%A R. Droms 669.%D March 1997 670.%R RFC 2131 671.%T Dynamic Host Configuration Protocol 672.Re 673.Pp 674.Rs 675.%A S. Alexander 676.%A R. Droms 677.%D March 1997 678.%R RFC 2132 679.%T DHCP Options and BOOTP Vendor Extensions 680.Re 681.Pp 682.Rs 683.%A T. Lemon 684.%A S. Cheshire 685.%D November 2002 686.%R RFC 3396 687.%T Encoding Long Options in the Dynamic Host Configuration Protocol (DHCPv4) 688.Re 689.Pp 690.Rs 691.%A T. Lemon 692.%A S. Cheshire 693.%A B. Volz 694.%D December 2002 695.%R RFC 3442 696.%T The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4 697.Re 698.Sh AUTHORS 699.An -nosplit 700.Xr dhcpd 8 701was written by 702.An Ted Lemon Aq Mt mellon@vix.com 703under a contract with Vixie Labs. 704.Pp 705The current implementation was reworked by 706.An Henning Brauer Aq Mt henning@openbsd.org . 707