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IETF RFC 4243
Vendor-Specific Information Suboption for the Dynamic Host Configuration Protocol (DHCP) Relay Agent Option
Last modified on Wednesday, December 7th, 2005
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Network Working Group M. Stapp
Request for Comments: 4243 R. Johnson
Category: Standards Track T. Palaniappan
Cisco Systems, Inc.
December 2005
Vendor-Specific Information Suboption for the
Dynamic Host Configuration Protocol (DHCP) Relay Agent Option
 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 © The Internet Society (2005).
Abstract
This memo defines a new Vendor-Specific Information suboption for the
Dynamic Host Configuration Protocol's (DHCP) relay agent information
option. The suboption allows a DHCP relay agent to include vendor-
specific information in the DHCP messages it forwards, as configured
by its administrator.
Table of Contents
1. Introduction ....................................................2
2. Requirements Terminology ........................................2
3. The Vendor-Specific Suboption ...................................2
4. Relay Agent Behavior ............................................4
5. DHCP Server Behavior ............................................4
6. Security Considerations .........................................4
7. IANA Considerations .............................................5
8. Acknowledgements ................................................5
Normative References ...............................................5
Informative References .............................................5
Stapp, et al. Standards Track PAGE 1 
RFC 4243 Vendor-Specific Relay Suboption December 2005
1. Introduction
DHCP (RFC 2131 [2]) provides IP addresses and configuration
information for IPv4 clients. It includes a relay agent capability,
in which processes within the network infrastructure receive
broadcast messages from clients and forward them to DHCP servers as
unicast messages. In network environments like DOCSIS data-over-
cable and xDSL, for example, it has proven useful for the relay agent
to add information to the DHCP message before forwarding it, using
the relay agent information option (RFC 3046 [3]).
Servers that recognize the relay agent option echo it back in their
replies, and some of the information that relays add may be used to
help an edge device efficiently return replies to clients. The
information that relays supply can also be used in the server's
decision making about the addresses and configuration parameters that
the client should receive.
In many environments, it's desirable to associate some vendor- or
provider-specific information with the clients' DHCP messages. This
is often done using the relay agent information option. RFC 3046
defines Remote-ID and Circuit-ID sub-options that are used to carry
such information. The values of those suboptions, however, are
usually based on some network resource, such as an IP address of a
network access device, an ATM Virtual Circuit identifier, or a DOCSIS
cable-modem identifier. As a result, the values carried in these
suboptions are dependent on the physical network configuration. The
Vendor-Specific suboption allows administrators to associate other
useful data with relayed DHCP messages.
2. Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
3. The Vendor-Specific Suboption
This memo defines a new DHCP relay agent option suboption that
carries vendor-defined data. The suboption takes a form similar to
the Vendor-Identifying, Vendor-Specific Option [7].
Stapp, et al. Standards Track PAGE 2
RFC 4243 Vendor-Specific Relay Suboption December 2005
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Length | Enterprise Number1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | DataLen1 | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
\ Suboption Data1 \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Enterprise Number2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DataLen2 | Suboption Data2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Code for the suboption is 9.
The one-byte Length field is the length of the data carried in the
suboption, in bytes. The length includes the length of the first
Enterprise Number; the minimum length is 4 bytes.
"Enterprise NumberN" is a vendor's Enterprise Number as registered
with IANA [4]. It is a four-byte integer value in network byte-
order.
DataLenN is the length of the data associated with the Enterprise
Number.
The Suboption Data is an opaque sequence of bytes.
The Vendor-Specific suboption includes at least one Enterprise Number
and carries opaque data defined by the organization identified by the
Enterprise Number. A relay may include data associated with more
than one vendor's Enterprise Number within a single instance of the
Suboption.
Of course, the Vendor-Specific data are vendor-specific. This
specification does not establish any requirements on the data in the
suboption. Vendors who make use of this suboption are encouraged to
document their usage in order to make interoperability possible.
Stapp, et al. Standards Track PAGE 3
RFC 4243 Vendor-Specific Relay Suboption December 2005
4. Relay Agent Behavior
DHCP relay agents MAY be configured to include Vendor-Specific
suboptions if they include a relay agent information option in
relayed DHCP messages. The suboptions' types and data are assigned
and configured through mechanisms that are outside the scope of this
memo.
Relay implementors are encouraged to offer their administrators a
means of configuring what data can be included in this suboption, and
to document what they are capable of.
5. DHCP Server Behavior
This suboption provides additional information to the DHCP server.
The DHCP server, if it is configured to support this suboption, may
use this information, in addition to other relay agent option data
and other options included in the DHCP client messages, in order to
assign an IP address and/or other configuration parameters to the
client. There is no special additional processing for this
suboption.
6. Security Considerations
Message authentication in DHCP for intradomain use, where the out-
of-band exchange of a shared secret is feasible, is defined in RFC
3118 [5]. Potential exposures to attack are discussed in section 7
of the DHCP protocol specification in RFC 2131 [2].
The DHCP relay agent option depends on a trusted relationship between
the DHCP relay agent and the server, as described in section 5 of RFC
3046. Fraudulent relay agent option data could potentially lead to
theft-of-service or exhaustion of limited resources (like IP
addresses) by unauthorized clients. A host that tampered with relay
agent data associated with another host's DHCP messages could deny
service to that host, or interfere with its operation by leading the
DHCP server to assign it inappropriate configuration parameters.
While the introduction of fraudulent relay agent options can be
prevented by a perimeter defense that blocks these options unless the
relay agent is trusted, a deeper defense using authentication for
relay agent options via the Authentication Suboption [6] SHOULD be
deployed as well.
There are several data in a DHCP message that convey information that
may identify an individual host on the network. These include the
chaddr, the client-id option, and the hostname and client-fqdn
options. Depending on the type of data included, the Vendor-Specific
suboption may also convey information that identifies a specific host
or a specific user on the network. In practice, this information
isn't exposed outside the internal service-provider network, where
DHCP messages are usually confined. Administrators who configure
data that will be used in DHCP Vendor-Specific suboptions should be
careful to use data that are appropriate for the types of networks
Stapp, et al. Standards Track PAGE 4
RFC 4243 Vendor-Specific Relay Suboption December 2005
they administer. If DHCP messages travel outside the service-
provider's own network, or if the suboption values may become visible
to other users, it may raise privacy concerns for the access provider
or service provider.
7. IANA Considerations
The IANA has assigned the suboption number 9 for the Vendor-Specific
Information Suboption from the DHCP Relay Agent Information Option
[3] suboption number space.
8. Acknowledgements
The authors are grateful to Andy Sudduth, Josh Littlefield, and Kim
Kinnear for their review and comments.
Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[3] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
January 2001.
[4] IANA, "Private Enterprise Numbers (http://www.iana.org/
assignments/enterprise-numbers.html)".
Informative References
[5] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[6] Stapp, M. and T. Lemon, "The Authentication Suboption for the
Dynamic Host Configuration Protocol (DHCP) Relay Agent Option",
RFC 4030, March 2005.
[7] Littlefield, J., "Vendor-Identifying Vendor Options for Dynamic
Host Configuration Protocol version 4 (DHCPv4)", RFC 3925,
October 2004.
Stapp, et al. Standards Track PAGE 5
RFC 4243 Vendor-Specific Relay Suboption December 2005
Authors' Addresses
Mark Stapp
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
USA
Phone: 978.936.0000
EMail: mjs@cisco.com
Richard Johnson
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
USA
Phone: 408.526.4000
EMail: raj@cisco.com
Theyn Palaniappan
Cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
USA
Phone: 408.526.4000
EMail: athenmoz@cisco.com
Stapp, et al. Standards Track PAGE 6
RFC 4243 Vendor-Specific Relay Suboption December 2005
Full Copyright Statement
Copyright © The Internet Society (2005).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Stapp, et al. Standards Track PAGE 7
Vendor-Specific Information Suboption for the Dynamic Host Configuration Protocol (DHCP) Relay Agent Option
RFC TOTAL SIZE: 14342 bytes
PUBLICATION DATE: Wednesday, December 7th, 2005
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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