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 RFC1497 [2] may
be used as DHCP options. The definitions given in RFC1497 are
included in this document, which supersedes RFC1497. 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. RFC1497 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
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.
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.
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 RFC1497 [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 RFC1497 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, RFC1122 [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. RFC1497 Vendor Extensions
This section lists the vendor extensions as defined in RFC1497.
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 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 RFC868 [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, RFC1035 [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 RFC865 [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 RFC1179 [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 RFC887 [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 RFC1035 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 RFC1191 [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 RFC1191. 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 RFC1256
[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 (RFC893 [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 (RFC894 [15]) or IEEE 802.3 (RFC1042 [16]) encapsulation
if the interface is an Ethernet. A value of 0 indicates that the
client should use RFC894 encapsulation. A value of 1 means that the
client should use RFC1042 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 vendor class identifier option.
Servers not equipped to interpret the vendor-specific information
sent by a client MUST ignore it (although it may be reported).
Clients which do not receive desired vendor-specific information
SHOULD make an attempt to operate without it, although they may do so
(and announce they are doing so) in a degraded mode.
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 RFC1001/1002 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 RFC1001/1002. 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 RFC1001/1002. 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 ...
+-----+-----+-----+-----+-----+-----+-----+-----+---
8.11. 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 64. Its minimum length is 1.
Code Len NIS Client Domain Name
+-----+-----+-----+-----+-----+-----+---
64 n n1 n2 n3 n4 ...
+-----+-----+-----+-----+-----+-----+---
8.12. Network Information Service+ 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 65. Its minimum length is 4, and the
length MUST be a multiple of 4.
Code Len Address 1 Address 2
+-----+-----+-----+-----+-----+-----+-----+-----+--
65 n a1 a2 a3 a4 a1 a2 ...
+-----+-----+-----+-----+-----+-----+-----+-----+--
8.13. Mobile IP Home Agent option
This option specifies a list of IP addresses indicating mobile IP
home agents available to the client. Agents SHOULD be listed in
order of preference.
The code for this option is 68. Its minimum length is 0 (indicating
no home agents are available) and the length MUST be a multiple of 4.
It is expected that the usual length will be four octets, containing
a single home agent's address.
Code Len Home Agent Addresses (zero or more)
+-----+-----+-----+-----+-----+-----+--
68 n a1 a2 a3 a4 ...
+-----+-----+-----+-----+-----+-----+--
8.14. Simple Mail Transport Protocol (SMTP) Server Option
The SMTP server option specifies a list of SMTP servers available to
the client. Servers SHOULD be listed in order of preference.
The code for the SMTP server option is 69. The minimum length for
this option is 4 octets, and the length MUST always be a multiple of
4.
Code Len Address 1 Address 2
+-----+-----+-----+-----+-----+-----+-----+-----+--
69 n a1 a2 a3 a4 a1 a2 ...
+-----+-----+-----+-----+-----+-----+-----+-----+--
8.15. Post Office Protocol (POP3) Server Option
The POP3 server option specifies a list of POP3 available to the
client. Servers SHOULD be listed in order of preference.
The code for the POP3 server option is 70. The minimum length for
this option is 4 octets, and the length MUST always be a multiple of
4.
Code Len Address 1 Address 2
+-----+-----+-----+-----+-----+-----+-----+-----+--
70 n a1 a2 a3 a4 a1 a2 ...
+-----+-----+-----+-----+-----+-----+-----+-----+--
8.16. Network News Transport Protocol (NNTP) Server Option
The NNTP server option specifies a list of NNTP available to the
client. Servers SHOULD be listed in order of preference.
The code for the NNTP server option is 71. The minimum length for
this option is 4 octets, and the length MUST always be a multiple of
4.
Code Len Address 1 Address 2
+-----+-----+-----+-----+-----+-----+-----+-----+--
71 n a1 a2 a3 a4 a1 a2 ...
+-----+-----+-----+-----+-----+-----+-----+-----+--
8.17. Default World Wide Web (WWW) Server Option
The WWW server option specifies a list of WWW available to the
client. Servers SHOULD be listed in order of preference.
The code for the WWW server option is 72. The minimum length for
this option is 4 octets, and the length MUST always be a multiple of
4.
Code Len Address 1 Address 2
+-----+-----+-----+-----+-----+-----+-----+-----+--
72 n a1 a2 a3 a4 a1 a2 ...
+-----+-----+-----+-----+-----+-----+-----+-----+--
8.18. Default Finger Server Option
The Finger server option specifies a list of Finger available to the
client. Servers SHOULD be listed in order of preference.
The code for the Finger server option is 73. The minimum length for
this option is 4 octets, and the length MUST always be a multiple of
4.
Code Len Address 1 Address 2
+-----+-----+-----+-----+-----+-----+-----+-----+--
73 n a1 a2 a3 a4 a1 a2 ...
+-----+-----+-----+-----+-----+-----+-----+-----+--
8.19. Default Internet Relay Chat (IRC) Server Option
The IRC server option specifies a list of IRC available to the
client. Servers SHOULD be listed in order of preference.
The code for the IRC server option is 74. The minimum length for
this option is 4 octets, and the length MUST always be a multiple of
4.
Code Len Address 1 Address 2
+-----+-----+-----+-----+-----+-----+-----+-----+--
74 n a1 a2 a3 a4 a1 a2 ...
+-----+-----+-----+-----+-----+-----+-----+-----+--
8.20. StreetTalk Server Option
The StreetTalk server option specifies a list of StreetTalk servers
available to the client. Servers SHOULD be listed in order of
preference.
The code for the StreetTalk server option is 75. The minimum length
for this option is 4 octets, and the length MUST always be a multiple
of 4.
Code Len Address 1 Address 2
+-----+-----+-----+-----+-----+-----+-----+-----+--
75 n a1 a2 a3 a4 a1 a2 ...
+-----+-----+-----+-----+-----+-----+-----+-----+--
8.21. StreetTalk Directory Assistance (STDA) Server Option
The StreetTalk Directory Assistance (STDA) server option specifies a
list of STDA servers available to the client. Servers SHOULD be
listed in order of preference.
The code for the StreetTalk Directory Assistance server option is 76.
The minimum length for this option is 4 octets, and the length MUST
always be a multiple of 4.
Code Len Address 1 Address 2
+-----+-----+-----+-----+-----+-----+-----+-----+--
76 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 TFTP server name
This option is used to identify a TFTP server when the 'sname' field
in the DHCP header has been used for DHCP options.
The code for this option is 66, and its minimum length is 1.
Code Len TFTP server
+-----+-----+-----+-----+-----+---
66 n c1 c2 c3 ...
+-----+-----+-----+-----+-----+---
9.5 Bootfile name
This option is used to identify a bootfile when the 'file' field in
the DHCP header has been used for DHCP options.
The code for this option is 67, and its minimum length is 1.
Code Len Bootfile name
+-----+-----+-----+-----+-----+---
67 n c1 c2 c3 ...
+-----+-----+-----+-----+-----+---
9.6. 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
8 DHCPINFORM
Code Len Type
+-----+-----+-----+
53 1 1-9
+-----+-----+-----+
9.7. 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 use the
contents of the 'server identifier' field as the destination address
for any DHCP messages unicast to the DHCP server. DHCP clients also
indicate which of several lease offers is being accepted by including
this option in a DHCPREQUEST message.
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
+-----+-----+-----+-----+-----+-----+
9.8. 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 ...
+-----+-----+-----+-----+---
9.9. 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 ...
+-----+-----+-----+-----+---
9.10. 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
+-----+-----+-----+-----+
9.11. 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
+-----+-----+-----+-----+-----+-----+
9.12. 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
+-----+-----+-----+-----+-----+-----+
9.13. Vendor class identifier
This option is used by DHCP clients to optionally identify the vendor
type and configuration of a DHCP client. The information is a string
of n octets, interpreted by servers. Vendors may choose to define
specific vendor class identifiers to convey particular configuration
or other identification information about a client. For example, the
identifier may encode the client's hardware configuration. Servers
not equipped to interpret the class-specific information sent by a
client MUST ignore it (although it may be reported). Servers that
respond SHOULD only use option 43 to return the vendor-specific
information to the client.
The code for this option is 60, and its minimum length is 1.
Code Len Vendor class Identifier
+-----+-----+-----+-----+---
60 n i1 i2 ...
+-----+-----+-----+-----+---
9.14. 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 SHOULD be treated as opaque objects by DHCP servers.
The client identifier MAY consist of type-value pairs similar to the
'htype'/'chaddr' fields defined in [3]. For instance, it MAY consist
of a hardware type and hardware address. In this case the type field
SHOULD be one of the ARP hardware types defined in STD2 [22]. A
hardware type of 0 (zero) should be used when the value field
contains an identifier other than a hardware address (e.g. a fully
qualified domain name).
For correct identification of clients, each client's client-
identifier MUST be unique among the client-identifiers used on the
subnet to which the client is attached. Vendors and system
administrators are responsible for choosing client-identifiers that
meet this requirement for uniqueness.
The code for this option is 61, and its minimum length is 2.
Code Len Type Client-Identifier
+-----+-----+-----+-----+-----+---
61 n t1 i1 i2 ...
+-----+-----+-----+-----+-----+---
10. Defining new extensions
The author of a new DHCP option will follow these steps to obtain
acceptance of the option as a part of the DHCP Internet Standard:
1. The author devises the new option.
2. The author requests a number for the new option from IANA 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@iana.org
3. The author documents the new option, using the newly obtained
option number, as an Internet Draft.
4. The author submits the Internet Draft for review through the IETF
standards process as defined in "Internet Official Protocol
Standards" (STD 1). The new option will be submitted for eventual
acceptance as an Internet Standard.
5. The new option progresses through the IETF standards process; the
new option will be reviewed by the Dynamic Host Configuration
Working Group (if that group still exists), or as an Internet
Draft not submitted by an IETF working group.
6. If the new option fails to gain acceptance as an Internet
Standard, the assigned option number will be returned to IANA for
reassignment.
This procedure for defining new extensions will ensure that:
* allocation of new option numbers is coordinated from a single
authority,
* new options are reviewed for technical correctness and
appropriateness, and
* documentation for new options is complete and published.
11. Acknowledgements
The author thanks the many (and too numerous to mention!) members of
the DHC WG for their tireless and ongoing efforts in the development
of DHCP and this document.
The efforts of J Allard, Mike Carney, Dave Lapp, Fred Lien and John
Mendonca in organizing DHCP interoperability testing sessions are
gratefully acknowledged.
The development of this document was supported in part by grants from
the Corporation for National Research Initiatives (CNRI), Bucknell
University and Sun Microsystems.
12. References
[1] Droms, R., "Dynamic Host Configuration Protocol", RFC2131,
Bucknell University, March 1997.
[2] Reynolds, J., "BOOTP Vendor Information Extensions", RFC1497,
USC/Information Sciences Institute, August 1993.
[3] Croft, W., and J. Gilmore, "Bootstrap Protocol", RFC951,
Stanford University and Sun Microsystems, September 1985.
[4] Braden, R., Editor, "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC1122, USC/Information Sciences
Institute, October 1989.
[5] Mogul, J., and J. Postel, "Internet Standard Subnetting
Procedure", STD 5, RFC950, 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, RFC1035, USC/Information Sciences
Institute, November 1987.
[9] Postel, J., "Quote of the Day Protocol", STD 23, RFC865,
USC/Information Sciences Institute, May 1983.
[10] McLaughlin, L., "Line Printer Daemon Protocol", RFC1179, The
Wollongong Group, August 1990.
[11] Accetta, M., "Resource Location Protocol", RFC887, CMU,
December 1983.
[12] Mogul, J. and S. Deering, "Path MTU Discovery", RFC1191,
DECWRL, Stanford University, November 1990.
[13] Deering, S., "ICMP Router Discovery Messages", RFC1256,
Xerox PARC, September 1991.
[14] Leffler, S. and M. Karels, "Trailer Encapsulations", RFC893,
U. C. Berkeley, April 1984.
[15] Hornig, C., "Standard for the Transmission of IP Datagrams over
Ethernet Networks", RFC894, Symbolics, April 1984.
[16] Postel, J. and J. Reynolds, "Standard for the Transmission of
IP Datagrams Over IEEE 802 Networks", RFC1042, 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", RFC1305, UDEL, March 1992.
[19] NetBIOS Working Group, "Protocol Standard for a NetBIOS Service
on a TCP/UDP transport: Concepts and Methods", STD 19, RFC1001,
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, RFC1198,
MIT Laboratory for Computer Science, January 1991.
[22] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC1700,
USC/Information Sciences Institute, July 1992.
13. Security Considerations
Security issues are not discussed in this memo.
14. Authors' Addresses
Steve Alexander
Silicon Graphics, Inc.
2011 N. Shoreline Boulevard
Mailstop 510
Mountain View, CA 94043-1389
Phone: (415) 933-6172
EMail: sca@engr.sgi.com
Ralph Droms
Bucknell University
Lewisburg, PA 17837
Phone: (717) 524-1145
EMail: droms@bucknell.edu
