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Network Working Group S. Alexander
Request for Comments: 2132 Silicon Graphics, Inc.
Obsoletes: 1533 R. Droms
Category: Standards Track Bucknell University
March 1997
DHCP Options and BOOTP Vendor Extensions
Status of this memo
This document 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" (STD 1) 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. Future
options will be specified in separate RFCs. The current list of
valid options is also available in ftp://ftp.isi.edu/in-
notes/iana/assignments [22].
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.
Table of Contents
1. Introduction .............................................. 2
2. BOOTP Extension/DHCP Option Field Format .................. 4
3. RFC 1497 Vendor Extensions ................................ 5
4. IP Layer Parameters per Host .............................. 11
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 ........................................... 25
Alexander & Droms Standards Track [Page 1]
RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997
10. Defining new extensions ................................... 31
11. Acknowledgements .......................................... 31
12. References ................................................ 32
13. Security Considerations ................................... 33
14. Authors' Addresses ........................................ 34
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. Section 9 defines options used only in DHCP.
References further describing most of the options defined in sections
2-6 can be found in section 12. The use of the options defined in
section 9 is described in the DHCP specification [1].
Information on registering new options is contained in section 10.
This document updates the definition of DHCP/BOOTP options that
appears in RFC1533. The classing mechanism has been extended to
include vendor classes as described in section 8.4 and 9.13. The new
procedure for defining new DHCP/BOOTP options in described in section
10. Several new options, including NIS+ domain and servers, Mobile
IP home agent, SMTP server, TFTP server and Bootfile server, have
been added. Text giving definitions used throughout the document has
been added in section 1.1. Text emphasizing the need for uniqueness
of client-identifiers has been added to section 9.14.
Alexander & Droms Standards Track [Page 2]
RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997
1.1 Requirements
Throughout this document, the words that are used to define the
significance of particular requirements are capitalized. These words
are:
o "MUST"
This word or the adjective "REQUIRED" means that the item is an
absolute requirement of this specification.
o "MUST NOT"
This phrase means that the item is an absolute prohibition of
this specification.
o "SHOULD"
This word or the adjective "RECOMMENDED" means that there may
exist valid reasons in particular circumstances to ignore this
item, but the full implications should be understood and the case
carefully weighed before choosing a different course.
o "SHOULD NOT"
This phrase means that there may exist valid reasons in
particular circumstances when the listed behavior is acceptable
or even useful, but the full implications should be understood
and the case carefully weighed before implementing any behavior
described with this label.
o "MAY"
This word or the adjective "OPTIONAL" means that this item is
truly optional. One vendor may choose to include the item
because a particular marketplace requires it or because it
enhances the product, for example; another vendor may omit the
same item.
1.2 Terminology
This document uses the following terms:
o "DHCP client"
A DHCP client or "client" is an Internet host using DHCP to
obtain configuration parameters such as a network address.
Alexander & Droms Standards Track [Page 3]
RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997
o "DHCP server"
A DHCP server of "server"is an Internet host that returns
configuration parameters to DHCP clients.
o "binding"
A binding is a collection of configuration parameters, including
at least an IP address, associated with or "bound to" a DHCP
client. Bindings are managed by DHCP servers.
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. Options containing NVT ASCII data SHOULD
NOT include a trailing NULL; however, the receiver of such options
MUST be prepared to delete trailing nulls if they exist. The
receiver MUST NOT require that a trailing null be included in the
data. In the case of some variable-length options the length field
is a constant but must still be specified.
Any options defined subsequent to this document MUST 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.
Alexander & Droms Standards Track [Page 4]
RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997
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.
Many options supply one or more 32-bit IP address. Use of IP
addresses rather than fully-qualified Domain Names (FQDNs) may make
future renumbering of IP hosts more difficult. Use of these
addresses is discouraged at sites that may require renumbering.
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.
Alexander & Droms Standards Track [Page 5]
RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997
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 two's complement 32-bit integer. A positive offset
indicates a location east of the zero meridian and a negative offset
indicates a location west of the zero meridian.
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.
Alexander & Droms Standards Track [Page 6]
RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997
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 | ...
+-----+-----+-----+-----+-----+-----+-----+-----+--
Alexander & Droms Standards Track [Page 7]
RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997
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.
Alexander & Droms Standards Track [Page 8]
RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997
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 | ...
+-----+-----+-----+-----+-----+-----+---
Alexander & Droms Standards Track [Page 9]
RFC 2132 DHCP Options and BOOTP Vendor Extensions March 1997
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.
Alexander & Droms Standards Track [Page 10]
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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.
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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.
Alexander & Droms Standards Track [Page 12]
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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.
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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 |
+-----+-----+-----+
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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.
Alexander & Droms Standards Track [Page 15]
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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.
Alexander & Droms Standards Track [Page 16]
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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.
Alexander & Droms Standards Track [Page 17]
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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]