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DHC                                                             R. Droms
Internet-Draft                                       Cisco Systems, Inc.
Expires: February 27, 2004                               August 29, 2003

         Authentication of DHCP Relay Agent Options Using IPsec

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on February 27, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.


   The DHCP Relay Agent Information Option (RFC 3046) conveys
   information between a DHCP relay agent and a DHCP server. This
   specification defines a  mechanism for securing the messages
   exchanged between a relay agent and a server using IPsec (RFC 2401).

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Table of Contents

   1. Requirements Terminology . . . . . . . . . . . . . . . . . . . . 3
   2. DHCP Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
   3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   4. Use of IPsec to secure DHCP messages . . . . . . . . . . . . . . 3
   5. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . . 5
   6. Security Considerations  . . . . . . . . . . . . . . . . . . . . 5
   7. IPsec Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   8. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . . 5
      Normative references . . . . . . . . . . . . . . . . . . . . . . 5
      Informative References . . . . . . . . . . . . . . . . . . . . . 5
      Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
      Intellectual Property and Copyright Statements . . . . . . . . . 7

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1. Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [1].

2. DHCP Terminology

   This document uses the terms "DHCP server" (or "server") and "DHCP
   client" (or "client") as defined in RFC 2131.  The term "DHCP relay
   agent" refers to a "BOOTP relay agent" as defined in RFC 2131.

3. Introduction

   DHCP (RFC 2131 [5]) provides IP addresses and configuration
   information for DHCP clients. It includes a relay agent capability
   (RFC 951 [6], RFC 1542 [7]), in which processes within the network
   infrastructure receive broadcast messages from clients and forward
   them to 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
   [2]. The kind of information that a relay agent adds is often used in
   the server's decision making about the addresses and configuration
   parameters that the client should receive. The way that the relay
   agent data is used in server decision-making tends to make that data
   very important, and highlights the importance of the trust
   relationship between the relay agent and the server.

   The existing DHCP Authentication specification (RFC 3118) [8] only
   secures communication between the DHCP client and server. Because
   relay agent information is added after the client has signed its
   message, the DHCP Authentication specification explictly excludes
   relay agent data from that authentication.

   The goals of this specification is to define a method that a relay
   agent can use to:

      1.  protect the integrity of the data that the relay adds

      2.  provide replay protection for that data

      3.  leverage the existing IPsec mechanism

4. Use of IPsec to secure DHCP messages

   Relay agents and servers that exchange messages securely can use

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   IPsec mechanisms [3] as described in this section. If a client
   message is relayed through multiple relay agents, each of the relay
   agents must have established independent, pairwise trust
   relationships. That is, if messages from client C will be relayed by
   relay agent A to relay agent B and then to the server, relay agents A
   and B must be configured to use IPSec for the messages they exchange,
   and relay agent B and the server must be configured to use IPSec for
   the messages they exchange.

   Relay agents and servers that support secure relay agent to server or
   relay agent to relay agent communication use IPsec under the
   following conditions:

   Selectors:       Relay agents are manually configured with the
      addresses of the relay agent or server to which DHCP messages are
      to be forwarded.  Each relay agent and server that will be using
      IPsec for securing DHCP messages must also be configured with a
      list of the relay agents to which messages will be returned.  The
      selectors for the relay agents and servers will be the pairs of
      addresses defining relay agents and servers that exchange DHCP
      messages on the DHCP UDP ports 67 and 68.

   Mode:  Relay agents and servers use transport mode and ESP [4]. The
      information in DHCP messages is not generally considered
      confidential, so encryption need not be used (i.e., NULL
      encryption can be used).

   Key management:  Because the relay agents and servers are used within
      an organization, public key schemes are not necessary.  Because
      the relay agents and servers must be manually configured, manually
      configured key management may suffice, but does not provide
      defense against replayed messages.  Accordingly, IKE with
      preshared secrets SHOULD be supported. IKE with public keys MAY be

   Security policy: DHCP messages between relay agents and servers
      should only be accepted from DHCP peers as identified in the local

   Authentication:  Shared keys, indexed to the source IP address of the
      received DHCP message, are adequate in this application.

   Availability:    Appropriate IPsec implementations are likely to be
      available for servers and for relay agents in more featureful
      devices used in enterprise and core ISP networks.  IPsec is less
      likely to be available for relay agents in low end devices
      primarily used in the home or small office markets.

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5. IANA Considerations

   There are no IANA considerations for the authentication mechanisms
   described in this document.

6. Security Considerations

   This specification describes a mechanism that can be used to provide
   authentication and message integrity protection to the messages
   between DHCP relay agents and DHCP servers.

   The authentication sub-option protocol requires configuration of
   relay agents and servers with shared secret keys.

7. IPsec Considerations

   The use of IPsec for securing relay agent options in DHCP messages
   requires the existence of an IPsec implementation available to the
   relay agents and DHCP servers. It also requires manual configuration
   of the participants, including manual distribution of keys.

8. Acknowledgments

   The need for this specification was made clear by comments made by
   Thomas Narten and John Schnizlein at IETF 53.

Normative references

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", RFC 2119, March 1997.

   [2]  Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
        January 2001.

   [3]  Kent, S. and R. Atkinson, "Security Architecture for the
        Internet Protocol", RFC 2401, November 1998.

   [4]  Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
        (ESP)", RFC 2406, November 1998.

Informative References

   [5]  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
        March 1997.

   [6]  Croft, B. and J. Gilmore, "Bootstrap Protocol", RFC 951,
        September 1985.

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   [7]  Wimer, W., "Clarifications and Extensions for the Bootstrap
        Protocol", RFC 1542, October 1993.

   [8]  Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
        RFC 3118, June 2001.

Author's Address

   Ralph Droms
   Cisco Systems, Inc.
   1414 Massachusetts Ave.
   Boxborough, MA  01719

   Phone: +1 978.936.1674
   EMail: rdroms@cisco.com

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