Network Working Group                                       S. Alexander
Request for Comments: 1533                      Lachman Technology, Inc.
Obsoletes: 1497, 1395, 1084, 1048                               R. Droms
Category: Standards Track                            Bucknell University
                                                            October 1993

DHCP Options and BOOTP Vendor Extensions

Status of this Memo

   This RFC specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" for the standardization state and status
   of this protocol.  Distribution of this memo is unlimited.

Abstract

   The Dynamic Host Configuration Protocol (DHCP) [1] provides a
   framework for passing configuration information to hosts on a TCP/IP
   network.  Configuration parameters and other control information are
   carried in tagged data items that are stored in the "options" field
   of the DHCP message.  The data items themselves are also called
   "options."

   This document specifies the current set of DHCP options.  This
   document will be periodically updated as new options are defined.
    Each superseding document will include the entire current list of
   valid options.

   All of the vendor information extensions defined in RFC 1497 [2] may
   be used as DHCP options.  The definitions given in RFC 1497 are
   included in this document, which supersedes RFC 1497.  All of the
   DHCP options defined in this document, except for those specific to
   DHCP as defined in section 9, may be used as BOOTP vendor information
   extensions.

    1.  Introduction ..............................................  2
    2.  BOOTP Extension/DHCP Option Field Format ..................  2
    3.  RFC 1497 Vendor Extensions ................................  3
    4.  IP Layer Parameters per Host .............................. 10
    5.  IP Layer Parameters per Interface ........................  13
    6.  Link Layer Parameters per Interface ....................... 16
    7.  TCP Parameters ............................................ 17
    8.  Application and Service Parameters ........................ 18

    9.  DHCP Extensions ........................................... 23
   10.  Extensions ................................................ 27
   11.  Acknowledgements .......................................... 28
   12.  References ................................................ 28
   13.  Security Considerations ................................... 19
   14.  Authors' Addresses ........................................ 30

1. Introduction

   This document specifies options for use with both the Dynamic Host
   Configuration Protocol and the Bootstrap Protocol.

   The full description of DHCP packet formats may be found in the DHCP
   specification document [1], and the full description of BOOTP packet
   formats may be found in the BOOTP specification document [3].  This
   document defines the format of information in the last field of DHCP
   packets ('options') and of BOOTP packets ('vend').  The remainder of
   this section defines a generalized use of this area for giving
   information useful to a wide class of machines, operating systems and
   configurations. Sites with a single DHCP or BOOTP server that is
   shared among heterogeneous clients may choose to define other, site-
   specific formats for the use of the 'options' field.

   Section 2 of this memo describes the formats of DHCP options and
   BOOTP vendor extensions.  Section 3 describes options defined in
   previous documents for use with BOOTP (all may also be used with
   DHCP).  Sections 4-8 define new options intended for use with both
   DHCP and BOOTP. 9">Section 9 defines options used only in DHCP.

   References further describing most of the options defined in sections
   2-6 can be found in 12">section 12.  The use of the options defined in
   9">section 9 is described in the DHCP specification [1].

   Information on registering new options is contained in section 10.

2. BOOTP Extension/DHCP Option Field Format

   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.  In the case of some variable-length
   options the length field is a constant but must still be specified.

   Any options defined subsequent to this document should contain a
   length octet even if the length is fixed or zero.

   All multi-octet quantities are in network byte-order.

   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.

   All of the "vendor extensions" defined in RFC 1497 are also DHCP
   options.

   Option codes 128 to 254 (decimal) are reserved for site-specific
   options.

   Except for the options in section 9, all options may be used with
   either DHCP or BOOTP.

   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.

3. RFC 1497 Vendor Extensions

This section lists the vendor extensions as defined in RFC 1497.
They are defined here for completeness.

3.1. Pad Option

   The pad option can be used to cause subsequent fields to align on
   word boundaries.

   The code for the pad option is 0, and its length is 1 octet.

    Code
   +-----+
   |  0  |
   +-----+

3.2. End Option

   The end option marks the end of valid information in the vendor
   field.  Subsequent octets should be filled with pad options.

   The code for the end option is 255, and its length is 1 octet.

    Code
   +-----+
   | 255 |
   +-----+

3.3. Subnet Mask

   The subnet mask option specifies the client's subnet mask as per RFC
   950 [5].

   If both the subnet mask and the router option are specified in a DHCP
   reply, the subnet mask option MUST be first.

   The code for the subnet mask option is 1, and its length is 4 octets.

    Code   Len        Subnet Mask
   +-----+-----+-----+-----+-----+-----+
   |  1  |  4  |  m1 |  m2 |  m3 |  m4 |
   +-----+-----+-----+-----+-----+-----+

3.4. Time Offset

   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 signed 32-bit integer.

   The code for the time offset option is 2, and its length is 4 octets.

    Code   Len        Time Offset
   +-----+-----+-----+-----+-----+-----+
   |  2  |  4  |  n1 |  n2 |  n3 |  n4 |
   +-----+-----+-----+-----+-----+-----+

3.5. Router Option

   The router option specifies a list of IP addresses for routers on the
   client's subnet.  Routers SHOULD be listed in order of preference.

   The code for the router option is 3.  The minimum length for the
   router option is 4 octets, and the length MUST always be a multiple
   of 4.

    Code   Len         Address 1               Address 2
   +-----+-----+-----+-----+-----+-----+-----+-----+--
   |  3  |  n  |  a1 |  a2 |  a3 |  a4 |  a1 |  a2 |  ...
   +-----+-----+-----+-----+-----+-----+-----+-----+--

3.6. Time Server Option

   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.

   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.

    Code   Len         Address 1               Address 2
   +-----+-----+-----+-----+-----+-----+-----+-----+--
   |  4  |  n  |  a1 |  a2 |  a3 |  a4 |  a1 |  a2 |  ...
   +-----+-----+-----+-----+-----+-----+-----+-----+--

3.7. Name Server Option

   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.

   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.

    Code   Len         Address 1               Address 2
   +-----+-----+-----+-----+-----+-----+-----+-----+--
   |  5  |  n  |  a1 |  a2 |  a3 |  a4 |  a1 |  a2 |  ...
   +-----+-----+-----+-----+-----+-----+-----+-----+--

3.8. Domain Name Server Option

   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.

   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.

    Code   Len         Address 1               Address 2
   +-----+-----+-----+-----+-----+-----+-----+-----+--
   |  6  |  n  |  a1 |  a2 |  a3 |  a4 |  a1 |  a2 |  ...
   +-----+-----+-----+-----+-----+-----+-----+-----+--

3.9. Log Server Option

   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.

   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.

    Code   Len         Address 1               Address 2
   +-----+-----+-----+-----+-----+-----+-----+-----+--
   |  7  |  n  |  a1 |  a2 |  a3 |  a4 |  a1 |  a2 |  ...
   +-----+-----+-----+-----+-----+-----+-----+-----+--

3.10. Cookie Server Option

   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.

   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.

    Code   Len         Address 1               Address 2
   +-----+-----+-----+-----+-----+-----+-----+-----+--
   |  8  |  n  |  a1 |  a2 |  a3 |  a4 |  a1 |  a2 |  ...
   +-----+-----+-----+-----+-----+-----+-----+-----+--

3.11. LPR Server Option

   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.

   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.

    Code   Len         Address 1               Address 2
   +-----+-----+-----+-----+-----+-----+-----+-----+--
   |  9  |  n  |  a1 |  a2 |  a3 |  a4 |  a1 |  a2 |  ...
   +-----+-----+-----+-----+-----+-----+-----+-----+--

3.12. Impress Server Option

   The Impress server option specifies a list of Imagen Impress servers
   available to the client.  Servers SHOULD be listed in order of
   preference.

   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.

    Code   Len         Address 1               Address 2
   +-----+-----+-----+-----+-----+-----+-----+-----+--
   |  10 |  n  |  a1 |  a2 |  a3 |  a4 |  a1 |  a2 |  ...
   +-----+-----+-----+-----+-----+-----+-----+-----+--

3.13. Resource Location Server Option

   This option specifies a list of RFC 887 [11] Resource Location
   servers available to the client.  Servers SHOULD be listed in order
   of preference.

   The code for this option is 11.  The minimum length for this option
   is 4 octets, and the length MUST always be a multiple of 4.

    Code   Len         Address 1               Address 2
   +-----+-----+-----+-----+-----+-----+-----+-----+--
   |  11 |  n  |  a1 |  a2 |  a3 |  a4 |  a1 |  a2 |  ...
   +-----+-----+-----+-----+-----+-----+-----+-----+--

3.14. Host Name Option

   This option specifies the name of the client.  The name may or may
   not be qualified with the local domain name (see section 3.17 for the
   preferred way to retrieve the domain name).  See RFC 1035 for
   character set restrictions.

   The code for this option is 12, and its minimum length is 1.

    Code   Len                 Host Name
   +-----+-----+-----+-----+-----+-----+-----+-----+--
   |  12 |  n  |  h1 |  h2 |  h3 |  h4 |  h5 |  h6 |  ...
   +-----+-----+-----+-----+-----+-----+-----+-----+--

3.15. Boot File Size Option

   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.

   The code for this option is 13, and its length is 2.

    Code   Len   File Size
   +-----+-----+-----+-----+
   |  13 |  2  |  l1 |  l2 |
   +-----+-----+-----+-----+

3.16. Merit Dump File

   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.

   The code for this option is 14.  Its minimum length is 1.

    Code   Len      Dump File Pathname
   +-----+-----+-----+-----+-----+-----+---
   |  14 |  n  |  n1 |  n2 |  n3 |  n4 | ...
   +-----+-----+-----+-----+-----+-----+---

3.17. Domain Name

   This option specifies the domain name that client should use when
   resolving hostnames via the Domain Name System.

   The code for this option is 15.  Its minimum length is 1.

    Code   Len        Domain Name
   +-----+-----+-----+-----+-----+-----+--
   |  15 |  n  |  d1 |  d2 |  d3 |  d4 |  ...
   +-----+-----+-----+-----+-----+-----+--

3.18. Swap Server

   This specifies the IP address of the client's swap server.

   The code for this option is 16 and its length is 4.

    Code   Len    Swap Server Address
   +-----+-----+-----+-----+-----+-----+
   |  16 |  n  |  a1 |  a2 |  a3 |  a4 |
   +-----+-----+-----+-----+-----+-----+

3.19. Root Path

   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.

   The code for this option is 17.  Its minimum length is 1.

    Code   Len      Root Disk Pathname
   +-----+-----+-----+-----+-----+-----+---
   |  17 |  n  |  n1 |  n2 |  n3 |  n4 | ...
   +-----+-----+-----+-----+-----+-----+---

3.20. Extensions Path

   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:

          - the length of the file is unconstrained;
          - all references to Tag 18 (i.e., instances of the
            BOOTP Extensions Path field) within the file are
            ignored.

   The code for this option is 18.  Its minimum length is 1.

    Code   Len      Extensions Pathname
   +-----+-----+-----+-----+-----+-----+---
   |  18 |  n  |  n1 |  n2 |  n3 |  n4 | ...
   +-----+-----+-----+-----+-----+-----+---

4. IP Layer Parameters per Host

This section details the options that affect the operation of the IP
layer on a per-host basis.

4.1. IP Forwarding Enable/Disable Option

   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.

   The code for this option is 19, and its length is 1.

    Code   Len  Value
   +-----+-----+-----+
   |  19 |  1  | 0/1 |
   +-----+-----+-----+

4.2. Non-Local Source Routing Enable/Disable Option

   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.

   The code for this option is 20, and its length is 1.

    Code   Len  Value
   +-----+-----+-----+
   |  20 |  1  | 0/1 |
   +-----+-----+-----+

4.3. Policy Filter Option

   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.

   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.

   The code for this option is 21.  The minimum length of this option is
   8, and the length MUST be a multiple of 8.

    Code   Len         Address 1                  Mask 1
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
   |  21 |  n  |  a1 |  a2 |  a3 |  a4 |  m1 |  m2 |  m3 |  m4 |
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
           Address 2                  Mask 2
   +-----+-----+-----+-----+-----+-----+-----+-----+---
   |  a1 |  a2 |  a3 |  a4 |  m1 |  m2 |  m3 |  m4 | ...
   +-----+-----+-----+-----+-----+-----+-----+-----+---

4.4. Maximum Datagram Reassembly Size

   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.

   The code for this option is 22, and its length is 2.

    Code   Len      Size
   +-----+-----+-----+-----+
   |  22 |  2  |  s1 |  s2 |
   +-----+-----+-----+-----+

4.5. Default IP Time-to-live

   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.

   The code for this option is 23, and its length is 1.

    Code   Len   TTL
   +-----+-----+-----+
   |  23 |  1  | ttl |
   +-----+-----+-----+

4.6. Path MTU Aging Timeout Option

   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.

   The code for this option is 24, and its length is 4.

    Code   Len           Timeout
   +-----+-----+-----+-----+-----+-----+
   |  24 |  4  |  t1 |  t2 |  t3 |  t4 |
   +-----+-----+-----+-----+-----+-----+

4.7. Path MTU Plateau Table Option

   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.

   The code for this option is 25.  Its minimum length is 2, and the
   length MUST be a multiple of 2.

    Code   Len     Size 1      Size 2
   +-----+-----+-----+-----+-----+-----+---
   |  25 |  n  |  s1 |  s2 |  s1 |  s2 | ...
   +-----+-----+-----+-----+-----+-----+---

5. IP Layer Parameters per Interface

   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.

5.1. Interface MTU Option

   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.

   The code for this option is 26, and its length is 2.

    Code   Len      MTU
   +-----+-----+-----+-----+
   |  26 |  2  |  m1 |  m2 |
   +-----+-----+-----+-----+

5.2. All Subnets are Local Option

   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.

   The code for this option is 27, and its length is 1.

    Code   Len  Value
   +-----+-----+-----+
   |  27 |  1  | 0/1 |
   +-----+-----+-----+

5.3. Broadcast Address Option

   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].

   The code for this option is 28, and its length is 4.

    Code   Len     Broadcast Address
   +-----+-----+-----+-----+-----+-----+
   |  28 |  4  |  b1 |  b2 |  b3 |  b4 |
   +-----+-----+-----+-----+-----+-----+

5.4. Perform Mask Discovery Option

   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.

   The code for this option is 29, and its length is 1.

    Code   Len  Value
   +-----+-----+-----+
   |  29 |  1  | 0/1 |
   +-----+-----+-----+

5.5. Mask Supplier Option

   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.

   The code for this option is 30, and its length is 1.

    Code   Len  Value
   +-----+-----+-----+
   |  30 |  1  | 0/1 |
   +-----+-----+-----+

5.6. Perform Router Discovery Option

   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.

   The code for this option is 31, and its length is 1.

    Code   Len  Value
   +-----+-----+-----+
   |  31 |  1  | 0/1 |
   +-----+-----+-----+

5.7. Router Solicitation Address Option

   This option specifies the address to which the client should transmit
   router solicitation requests.

   The code for this option is 32, and its length is 4.

    Code   Len            Address
   +-----+-----+-----+-----+-----+-----+
   |  32 |  4  |  a1 |  a2 |  a3 |  a4 |
   +-----+-----+-----+-----+-----+-----+

5.8. Static Route Option

   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.

   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.

   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.

   The code for this option is 33.  The minimum length of this option is
   8, and the length MUST be a multiple of 8.

    Code   Len         Destination 1           Router 1
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
   |  33 |  n  |  d1 |  d2 |  d3 |  d4 |  r1 |  r2 |  r3 |  r4 |
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
           Destination 2           Router 2
   +-----+-----+-----+-----+-----+-----+-----+-----+---
   |  d1 |  d2 |  d3 |  d4 |  r1 |  r2 |  r3 |  r4 | ...
   +-----+-----+-----+-----+-----+-----+-----+-----+---

6. Link Layer Parameters per Interface

This section lists the options that affect the operation of the data
link layer on a per-interface basis.

6.1. Trailer Encapsulation Option

   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.

   The code for this option is 34, and its length is 1.

    Code   Len  Value
   +-----+-----+-----+
   |  34 |  1  | 0/1 |
   +-----+-----+-----+

6.2. ARP Cache Timeout Option

   This option specifies the timeout in seconds for ARP cache entries.
   The time is specified as a 32-bit unsigned integer.

   The code for this option is 35, and its length is 4.

    Code   Len           Time
   +-----+-----+-----+-----+-----+-----+
   |  35 |  4  |  t1 |  t2 |  t3 |  t4 |
   +-----+-----+-----+-----+-----+-----+

6.3. Ethernet Encapsulation Option

   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.

   The code for this option is 36, and its length is 1.

    Code   Len  Value
   +-----+-----+-----+
   |  36 |  1  | 0/1 |
   +-----+-----+-----+

7. TCP Parameters

This section lists the options that affect the operation of the TCP
layer on a per-interface basis.

7.1. TCP Default TTL Option

   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.

   The code for this option is 37, and its length is 1.

    Code   Len   TTL
   +-----+-----+-----+
   |  37 |  1  |  n  |
   +-----+-----+-----+

7.2. TCP Keepalive Interval Option

   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.

   The code for this option is 38, and its length is 4.

    Code   Len           Time
   +-----+-----+-----+-----+-----+-----+
   |  38 |  4  |  t1 |  t2 |  t3 |  t4 |
   +-----+-----+-----+-----+-----+-----+

7.3. TCP Keepalive Garbage Option

   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.

   The code for this option is 39, and its length is 1.

    Code   Len  Value
   +-----+-----+-----+
   |  39 |  1  | 0/1 |
   +-----+-----+-----+

8. Application and Service Parameters

This section details some miscellaneous options used to configure
miscellaneous applications and services.

8.1. Network Information Service Domain Option

   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.

   The code for this option is 40.  Its minimum length is 1.

    Code   Len      NIS Domain Name
   +-----+-----+-----+-----+-----+-----+---
   |  40 |  n  |  n1 |  n2 |  n3 |  n4 | ...
   +-----+-----+-----+-----+-----+-----+---

8.2. Network Information Servers Option

   This option specifies a list of IP addresses indicating NIS servers
   available to the client.  Servers SHOULD be listed in order of
   preference.

   The code for this option is 41.  Its minimum length is 4, and the
   length MUST be a multiple of 4.

    Code   Len         Address 1               Address 2
   +-----+-----+-----+-----+-----+-----+-----+-----+--
   |  41 |  n  |  a1 |  a2 |  a3 |  a4 |  a1 |  a2 |  ...
   +-----+-----+-----+-----+-----+-----+-----+-----+--

8.3. Network Time Protocol Servers Option

   This option specifies a list of IP addresses indicating NTP [18]
   servers available to the client.  Servers SHOULD be listed in order
   of preference.

   The code for this option is 42.  Its minimum length is 4, and the
   length MUST be a multiple of 4.

    Code   Len         Address 1               Address 2
   +-----+-----+-----+-----+-----+-----+-----+-----+--
   |  42 |  n  |  a1 |  a2 |  a3 |  a4 |  a1 |  a2 |  ...
   +-----+-----+-----+-----+-----+-----+-----+-----+--

8.4. Vendor Specific Information

   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 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.

   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:

   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:

      1) There SHOULD NOT be a "magic cookie" field in the encapsulated
         vendor-specific extensions field.

      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.

      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.

   The code for this option is 43 and its minimum length is 1.

   Code   Len   Vendor-specific information
   +-----+-----+-----+-----+---
   |  43 |  n  |  i1 |  i2 | ...
   +-----+-----+-----+-----+---

   When encapsulated vendor-specific extensions are used, the
   information bytes 1-n have the following format:

    Code   Len   Data item        Code   Len   Data item       Code
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
   |  T1 |  n  |  d1 |  d2 | ... |  T2 |  n  |  D1 |  D2 | ... | ... |
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+

8.5. NetBIOS over TCP/IP Name Server Option

   The NetBIOS name server (NBNS) option specifies a list of RFC
   1001/1002 [19] [20] NBNS name servers listed in order of preference.

   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.

    Code   Len           Address 1              Address 2
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----
   |  44 |  n  |  a1 |  a2 |  a3 |  a4 |  b1 |  b2 |  b3 |  b4 | ...
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----

8.6. NetBIOS over TCP/IP Datagram Distribution Server Option

   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.

    Code   Len           Address 1              Address 2
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----
   |  45 |  n  |  a1 |  a2 |  a3 |  a4 |  b1 |  b2 |  b3 |  b4 | ...
   +-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+----

8.7. NetBIOS over TCP/IP Node Type Option

   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:

      Value         Node Type
      -----         ---------
      0x1           B-node
      0x2           P-node
      0x4           M-node
      0x8           H-node

   In the above chart, the notation '0x' indicates a number in base-16
   (hexadecimal).

   The code for this option is 46.  The length of this option is always
   1.

    Code   Len  Node Type
   +-----+-----+-----------+
   |  46 |  1  | see above |
   +-----+-----+-----------+

8.8. NetBIOS over TCP/IP Scope Option

   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.

   The code for this option is 47.  The minimum length of this option is
   1.

    Code   Len       NetBIOS Scope
   +-----+-----+-----+-----+-----+-----+----
   |  47 |  n  |  s1 |  s2 |  s3 |  s4 | ...
   +-----+-----+-----+-----+-----+-----+----

8.9. X Window System Font Server Option

   This option specifies a list of X Window System [21] Font servers
   available to the client. Servers SHOULD be listed in order of
   preference.

   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.

    Code   Len         Address 1               Address 2
   +-----+-----+-----+-----+-----+-----+-----+-----+---
   |  48 |  n  |  a1 |  a2 |  a3 |  a4 |  a1 |  a2 |   ...
   +-----+-----+-----+-----+-----+-----+-----+-----+---

8.10. X Window System Display Manager Option

   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.

   Addresses SHOULD be listed in order of preference.

   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.

    Code   Len         Address 1               Address 2

   +-----+-----+-----+-----+-----+-----+-----+-----+---
   |  49 |  n  |  a1 |  a2 |  a3 |  a4 |  a1 |  a2 |   ...
   +-----+-----+-----+-----+-----+-----+-----+-----+---

9. DHCP Extensions

This section details the options that are specific to DHCP.

9.1. Requested IP Address

   This option is used in a client request (DHCPDISCOVER) to allow the
   client to request that a particular IP address be assigned.

   The code for this option is 50, and its length is 4.

    Code   Len          Address
   +-----+-----+-----+-----+-----+-----+
   |  50 |  4  |  a1 |  a2 |  a3 |  a4 |
   +-----+-----+-----+-----+-----+-----+

9.2. IP Address Lease Time

   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.

   The time is in units of seconds, and is specified as a 32-bit
   unsigned integer.

   The code for this option is 51, and its length is 4.

    Code   Len         Lease Time
   +-----+-----+-----+-----+-----+-----+
   |  51 |  4  |  t1 |  t2 |  t3 |  t4 |
   +-----+-----+-----+-----+-----+-----+

9.3. Option Overload

   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.

   If this option is present, the client interprets the specified
   additional fields after it concludes interpretation of the standard
   option fields.

   The code for this option is 52, and its length is 1.  Legal values
   for this option are:

           Value   Meaning
           -----   --------
             1     the "file" field is used to hold options
             2     the "sname" field is used to hold options
             3     both fields are used to hold options

    Code   Len  Value
   +-----+-----+-----+
   |  52 |  1  |1/2/3|
   +-----+-----+-----+

9.4. DHCP Message Type

   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:

           Value   Message Type
           -----   ------------
             1     DHCPDISCOVER
             2     DHCPOFFER
             3     DHCPREQUEST
             4     DHCPDECLINE
             5     DHCPACK
             6     DHCPNAK
             7     DHCPRELEASE

    Code   Len  Type
   +-----+-----+-----+
   |  53 |  1  | 1-7 |
   +-----+-----+-----+

9.5. Server Identifier

   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 indicate
   which of several lease offers is being accepted by including this
   option in a DHCPREQUEST message.

   The identifier is the IP address of the selected server.

   The code for this option is 54, and its length is 4.

    Code   Len            Address
   +-----+-----+-----+-----+-----+-----+
   |  54 |  4  |  a1 |  a2 |  a3 |  a4 |
   +-----+-----+-----+-----+-----+-----+

9.6. Parameter Request List

   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.

   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.

   The code for this option is 55.  Its minimum length is 1.

    Code   Len   Option Codes
   +-----+-----+-----+-----+---
   |  55 |  n  |  c1 |  c2 | ...
   +-----+-----+-----+-----+---

9.7. Message

   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.

   The code for this option is 56 and its minimum length is 1.

    Code   Len     Text
   +-----+-----+-----+-----+---
   |  56 |  n  |  c1 |  c2 | ...
   +-----+-----+-----+-----+---

9.8. Maximum DHCP Message Size

   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.

   The code for this option is 57, and its length is 2.  The minimum
   legal value is 576 octets.

    Code   Len     Length
   +-----+-----+-----+-----+
   |  57 |  2  |  l1 |  l2 |
   +-----+-----+-----+-----+

9.9. Renewal (T1) Time Value

   This option specifies the time interval from address assignment until
   the client transitions to the RENEWING state.

   The value is in units of seconds, and is specified as a 32-bit
   unsigned integer.

   The code for this option is 58, and its length is 4.

    Code   Len         T1 Interval
   +-----+-----+-----+-----+-----+-----+
   |  58 |  4  |  t1 |  t2 |  t3 |  t4 |
   +-----+-----+-----+-----+-----+-----+

9.10. Rebinding (T2) Time Value

   This option specifies the time interval from address assignment until
   the client transitions to the REBINDING state.

   The value is in units of seconds, and is specified as a 32-bit
   unsigned integer.

   The code for this option is 59, and its length is 4.

    Code   Len         T2 Interval
   +-----+-----+-----+-----+-----+-----+
   |  59 |  4  |  t1 |  t2 |  t3 |  t4 |
   +-----+-----+-----+-----+-----+-----+

9.11. Class-identifier

   This option is used by DHCP clients to optionally identify the type
   and configuration of a DHCP client.  The information is a string of n
   octets, interpreted by servers.  Vendors and sites may choose to
   define specific 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).

   The code for this option is 60, and its minimum length is 1.

   Code   Len   Class-Identifier
   +-----+-----+-----+-----+---
   |  60 |  n  |  i1 |  i2 | ...
   +-----+-----+-----+-----+---

9.12. Client-identifier

   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.

   Identifiers consist of a type-value pair, similar to the

   It is expected that this field will typically contain a hardware type
   and hardware address, but this is not required.  Current legal values
   for hardware types are defined in [22].

   The code for this option is 61, and its minimum length is 2.

   Code   Len   Type  Client-Identifier
   +-----+-----+-----+-----+-----+---
   |  61 |  n  |  t1 |  i1 |  i2 | ...
   +-----+-----+-----+-----+-----+---

10. Extensions

   Additional generic data fields may be registered by contacting:

      Internet Assigned Numbers Authority (IANA)
      USC/Information Sciences Institute
      4676 Admiralty Way
      Marina del Rey, California  90292-6695

      or by email as: iana@isi.edu

   Implementation specific use of undefined generic types (those in the
   range 61-127) may conflict with other implementations, and
   registration is required.

11. Acknowledgements

   The authors would like to thank Philip Almquist for his feedback on
   this document.  The comments of the DHCP Working Group are also
   gratefully acknowledged.  In particular, Mike Carney and Jon Dreyer
   from SunSelect suggested the current format of the Vendor-specific
   Information option.

   RFC 1497 is based on earlier work by Philip Prindeville, with help
   from Drew Perkins, Bill Croft, and Steve Deering.

12. References

   [1] Droms, R., "Dynamic Host Configuration Protocol", RFC 1531,
       Bucknell University, October 1993.

   [2] Reynolds, J., "BOOTP Vendor Information Extensions", RFC 1497,
       USC/Information Sciences Institute, August 1993.

   [3] Croft, W., and J. Gilmore, "Bootstrap Protocol", RFC 951,
       Stanford University and Sun Microsystems, September 1985.

   [4] Braden, R., Editor, "Requirements for Internet Hosts -
       Communication Layers", STD 3, RFC 1122, USC/Information Sciences
       Institute, October 1989.

   [5] Mogul, J., and J. Postel, "Internet Standard Subnetting
       Procedure", STD 5, RFC 950, USC/Information Sciences Institute,
       August 1985.

   [6] Postel, J., and K. Harrenstien, "Time Protocol", STD 26, RFC
       868, USC/Information Sciences Institute, SRI, May 1983.

   [7] Postel, J., "Name Server", IEN 116, USC/Information Sciences
       Institute, August 1979.

   [8] Mockapetris, P., "Domain Names - Implementation and
       Specification", STD 13, RFC 1035, USC/Information Sciences
       Institute, November 1987.

   [9] Postel, J., "Quote of the Day Protocol", STD 23, RFC 865,
       USC/Information Sciences Institute, May 1983.

   [10] McLaughlin, L., "Line Printer Daemon Protocol", RFC 1179, The
        Wollongong Group, August 1990.

   [11] Accetta, M., "Resource Location Protocol", RFC 887, CMU,
        December 1983.

   [12] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191,
        DECWRL,  Stanford University, November 1990.

   [13] Deering, S., "ICMP Router Discovery Messages", RFC 1256,
        Xerox PARC, September 1991.

   [14] Leffler, S. and M. Karels, "Trailer Encapsulations", RFC 893,
        U. C. Berkeley, April 1984.

   [15] Hornig, C., "Standard for the Transmission of IP Datagrams over
        Ethernet Networks", RFC 894, Symbolics, April 1984.

   [16] Postel, J. and J. Reynolds, "Standard for the Transmission of
        IP Datagrams Over IEEE 802 Networks", RFC 1042,  USC/Information
        Sciences Institute, February 1988.

   [17] Sun Microsystems, "System and Network Administration", March
        1990.

   [18] Mills, D., "Internet Time Synchronization: The Network Time
        Protocol", RFC 1305, UDEL, March 1992.

   [19] NetBIOS Working Group, "Protocol Standard for a NetBIOS Service
        on a TCP/UDP transport: Concepts and Methods", STD 19, RFC 1001,
        March 1987.

   [20] NetBIOS Working Group, "Protocol Standard for a NetBIOS Service
        on a TCP/UDP transport: Detailed Specifications", STD 19, RFC
        1002, March 1987.

   [21] Scheifler, R., "FYI On the X Window System", FYI 6, RFC 1198,
        MIT Laboratory for Computer Science, January 1991.

   [22] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1340,
        USC/Information Sciences Institute, July 1992.

13. Security Considerations

Security issues are not discussed in this memo.

14. Authors' Addresses

   Steve Alexander
   Lachman Technology, Inc.
   1901 North Naper Boulevard
   Naperville, IL 60563-8895

   Phone: (708) 505-9555 x256
   EMail: stevea@lachman.com

   Ralph Droms
   Computer Science Department
   323 Dana Engineering
   Bucknell University
   Lewisburg, PA 17837

   Phone: (717) 524-1145
   EMail: droms@bucknell.edu