Network Working Group T. Lemon
Request for Comments: 3396 Nominum, Inc.
Updates: 2131 S. Cheshire
Category: Standards Track Apple Computer, Inc.
November 2002
Encoding Long Options
in the Dynamic Host Configuration Protocol (DHCPv4)
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.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document specifies the processing rules for Dynamic Host
Configuration Protocol (DHCPv4) options that appear multiple times in
the same message. Multiple instances of the same option are
generated when an option exceeds 255 octets in size (the maximum size
of a single option) or when an option needs to be split apart in
order to take advantage of DHCP option overloading. When multiple
instances of the same option appear in the options, file and/or sname
fields in a DHCP packet, the contents of these options are
concatenated together to form a single option prior to processing.
1. Introduction
This document updates RFC 2131 [3] by clarifying the rules for option
concatenation specified in section 4.1. It is expected that the
reader will be familiar with this portion of RFC 2131. The text in
section 4.1 that reads "Options may appear only once, unless
otherwise specified in the options document." should be considered
as deleted.
The DHCP protocol [3] specifies objects called "options" that are
encoded in the DHCPv4 packet to pass information between DHCP
protocol agents. These options are encoded as a one-byte type code,
a one-byte length, and a buffer consisting of the number of bytes
specified in the length, from zero to 255.
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However, in some cases it may be useful to send options that are
longer than 255 bytes. RFC 2131 [3] specifies that when more than
one option with a given type code appears in the DHCP packet, all
such options should be concatenated together. It does not, however,
specify the order in which this concatenation should occur.
We specify here the ordering that MUST be used by DHCP protocol
agents when sending options with more than 255 bytes. This method
also MUST be used for splitting options that are shorter than 255
bytes, if for some reason the encoding agent needs to do so. DHCP
protocol agents MUST use this method whenever they receive a DHCP
packet containing more than one occurrence of a certain type of
option.
2. Terminology
DHCP
Throughout this document, the acronym "DHCP" is used to refer to
the Dynamic Host Configuration Protocol as specified in RFC 2131
[3] and RFC 2132 [4].
DHCPv4
We have used the term "DHCPv4" in the abstract for this document
to distinguish between the DHCP protocol for IPv4 as defined in
RFC 2131 and RFC 2132 and the DHCP protocol for IPv6, which, at
the time that this document was written, was still under
development.
DHCP protocol agents
This refers to any device on the network that sends or receives
DHCP packets - any DHCP client, server or relay agent. The nature
of these devices is not important to this specification.
Encoding agent
The DHCP protocol agent that is composing a DHCP packet to send.
Decoding agent
The DHCP protocol agent that is processing a DHCP packet it has
received.
Options
DHCP options are collections of data with type codes that indicate
how the options should be used. Options can specify information
that is required for the DHCP protocol, IP stack configuration
parameters for the client, information allowing the client to
rendezvous with DHCP servers, and so on.
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Option overload
The DHCP packet format is based on the BOOTP packet format defined
in RFC 951 [1]. When used by DHCP protocol agents, BOOTP packets
have three fields that can contain options. These are the
optional parameters field, the sname field, and the filename
field. The DHCP options specification [4] defines the DHCP
Overload option, which specifies which of these three fields is
actually being used in any given DHCP message to store DHCP
options.
3. Requirements Language
In this document, the key words "MAY", "MUST, "MUST NOT", "OPTIONAL",
"RECOMMENDED", "SHOULD", and "SHOULD NOT", are to be interpreted as
described in BCP 14, RFC 2119 [2].
4. Applicability
This specification applies when a DHCP agent is encoding a packet
containing options, where some of those options must be broken into
parts. This need can occur for two reasons. First, it can occur
because the value of an option that needs to be sent is longer than
255 bytes. In this case, the encoding agent MUST follow the
algorithm specified here. It can also occur because there is not
sufficient space in the current output buffer to store the option,
but there is space for part of the option, and there is space in
another output buffer for the rest. In this case, the encoding agent
MUST either use this algorithm or not send the option at all.
This specification also applies in any case where a DHCP protocol
agent has received a DHCP packet that contains more than one instance
of an option of a given type. In this case, the agent MUST
concatenate these separate instances of the same option in the way
that we specify here.
This option updates the Dynamic Host Configuration Protocol [3] and
DHCP Options and BOOTP vendor extensions [4] documents. However,
because many currently-deployed DHCP protocol agents do not implement
option concatenation, DHCP protocol agents should be careful not to
transmit split options unless either it will not matter if the
recipient cannot correctly reassemble the options, or it is certain
that the recipient implements concatenation.
Let us divide all DHCP options into two categories - those that, by
definition, require implementation of the mechanisms defined in this
document, and those that do not. We will refer to the former as
concatenation-requiring options, and the latter as non-
concatenation-requiring options. In order for an option to be a
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concatenation-requiring option, the protocol specification that
defines that option must require implementation of option splitting
and option concatenation as described in this document, by
specifically referencing this document.
A DHCP protocol agent SHOULD NOT split an option as described in this
document unless it has no choice, or it knows that its peer can
properly handle split options. A peer is assumed to properly handle
split options if it has provided or requested at least one
concatenation-requiring option. Alternatively, the administrator of
the agent generating the option can specifically configure the agent
to assume that the recipient can correctly concatenate options split
as described in this document.
Some implementors may find it easiest to only split concatenation-
requiring options, and never split non-concatenation-requiring
options. This is permissible. However, an implementation which
supports any concatenation-requiring option MUST be capable of
concatenating received options for both concatenation-requiring and
non-concatenation-requiring options.
No restrictions apply to option concatenation when a DHCP agent
receives a DHCP message. Any DHCP protocol agent that implements the
mechanisms described in this document can assume that when it
receives two options of the same type, it should concatenate them.
5. The Aggregate Option Buffer
DHCP options can be stored in the DHCP packet in three separate
portions of the packet. These are the optional parameters field, the
sname field, and the file field, as described in RFC 2131 [3]. This
complicates the description of the option splitting mechanism because
there are three separate fields into which split options may be
placed.
To further complicate matters, an option that doesn't fit into one
field can't overlap the boundary into another field - the encoding
agent must instead break the option into two parts and store one part
in each buffer.
To simplify this discussion, we will talk about an aggregate option
buffer, which will be the aggregate of the three buffers. This is a
logical aggregation - the buffers MUST appear in the locations in the
DHCP packet described in RFC 2131 [3].
The aggregate option buffer is made up of the optional parameters
field, the file field, and the sname field, in that order.
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WARNING: This is not the physical ordering of these fields in the
DHCP packet.
Options MUST NOT be stored in the aggregate option buffer in such a
way that they cross either boundary between the three fields in the
aggregate buffer.
The encoding agent is free to choose to use either or both the sname
field and file field. If the encoding agent does not choose to use
either or both of these two fields, then they MUST NOT be considered
part of the aggregate option buffer in that case.
6. Encoding Agent Behavior
Encoding agents decide to split options based on the reasons we have
described in the preceding section entitled "applicability".
Options can be split on any octet boundary. No split portion of an
option that has been split can contain more than 255 octets. The
split portions of the option MUST be stored in the aggregate option
buffer in sequential order - the first split portion MUST be stored
first in the aggregate option buffer, then the second portion, and so
on. The encoding agent MUST NOT attempt to specify any semantic
information based on how the option is split.
Note that because the aggregate option buffer does not represent the
physical ordering of the DHCP packet, if an option were split into
three parts and each part went into one of the possible option
fields, the first part would go into the optional parameters field,
the second part would go into the file field, and the third part
would go into the sname field. This maintains consistency with
section 4.1 of RFC 2131 [3].
Each split portion of an option MUST be stored in the aggregate
option buffer as if it were a normal variable-length option as
described in RFC 2132 [4]. The length fields of each split portion
of the option MUST add up to the total length of the option data.
For any given option being split, the option code field in each split
portion MUST be the same.
7. Decoding Agent Behavior
When a decoding agent is scanning an incoming DHCP packet's option
buffer and finds two or more options with the same option code, it
MUST consider them to be split portions of an option as described in
the preceding section.
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In the case that a decoding agent finds a split option, it MUST treat
the contents of that option as a single option, and the contents MUST
be reassembled in the order that was described above under encoding
agent behavior.
The decoding agent should ensure that when the option's value is
used, any alignment issues that are particular to the machine
architecture on which the decoding agent is running are accounted for
- there is no requirement that the encoding agent align the options
in any particular way.
There is no semantic meaning to where an option is split - the
encoding agent is free to split the option at any point, and the
decoding agent MUST reassemble the split option parts into a single
object, and MUST NOT treat each split portion of the option as a
separate object.
8. Example
Consider an option, Bootfile name (option code 67), with a value of
"/diskless/foo". Normally, this would be encoded as a single option,
as follows:
+----+----+---+---+---+---+---+---+---+---+---+---+---+---+---+
| 67 | 13 | / | d | i | s | k | l | e | s | s | / | f | o | o |
+----+----+---+---+---+---+---+---+---+---+---+---+---+---+---+
If an encoding agent needed to split the option in order to fit it
into the option buffer, it could encode it as two separate options,
as follows, and store it in the aggregate option buffer in the
following sequence:
+----+---+---+---+---+---+---+---+---+
| 67 | 7 | / | d | i | s | k | l | e |
+----+---+---+---+---+---+---+---+---+
+----+---+---+---+---+---+---+---+
| 67 | 6 | s | s | / | f | o | o |
+----+---+---+---+---+---+---+---+
9. Security Considerations
This document raises no new security issues. Potential exposures to
attack in the DHCP protocol are discussed in section 7 of the DHCP
protocol specification [3] and in Authentication for DHCP Messages
[5].
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Note that the authentication option itself can be split; in such
cases implementations must be careful when setting the authentication
field to zero (prior to generation or verification of the MAC) as it
may be split across multiple options.
10. References
10.1. Normative References
[1] Croft, W. and J. Gilmore, "BOOTSTRAP PROTOCOL (BOOTP)", RFC 951,
September 1985.
[2] Bradner, S., "Key words for use in RFCs to indicate requirement
levels", BCP 14, RFC 2119, March 1997.
[3] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March
1997.
[4] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
10.2. Informative References
[5] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages", RFC
3118, June 2001.
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RFC 3396 Encoding Long Options in DHCPv4 November 2002
12. Authors' Addresses
Ted Lemon
Nominum, Inc.
2385 Bay Road
Redwood City, CA 94043
USA
EMail: mellon@nominum.com
Stuart Cheshire
Apple Computer, Inc.
1 Infinite Loop
Cupertino
California 95014
USA
Phone: +1 408 974 3207
EMail: rfc@stuartcheshire.org
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Acknowledgement
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