Internet Engineering Task Force (IETF)                          N. Swamy
Request for Comments: 6842                                 Samsung India
Updates: 2131                                                G. Halwasia
Category: Standards Track                                    P. Jhingran
ISSN: 2070-1721                                            Cisco Systems
                                                            January 2013

            Client Identifier Option in DHCP Server Replies


   This document updates RFC 2131 "Dynamic Host Configuration Protocol"
   by addressing the issues arising from that document's specification
   that the server MUST NOT return the 'client identifier' option to the

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

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   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

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RFC 6842                Client Identifier Option            January 2013

Table of Contents

   1. Introduction ....................................................2
      1.1. Requirements Language ......................................2
   2. Problem Statement ...............................................2
   3. Modification to RFC 2131 ........................................3
   4. Security Considerations .........................................4
   5. Acknowledgments .................................................4
   6. Normative References ............................................4

1.  Introduction

   The Dynamic Host Configuration Protocol (DHCP) defined in [RFC2131]
   provides configuration parameters to hosts on an IP-based network.
   DHCP is built on a client-server model, where designated DHCP servers
   allocate network addresses and deliver configuration parameters to
   dynamically configured hosts.

   The changes to [RFC2131] defined in this document clarify the use of
   the 'client identifier' option by the DHCP servers.  The
   clarification addresses the issues (as mentioned in Problem
   Statement) arising out of the point specified by [RFC2131] that the
   server MUST NOT return the 'client identifier' option to the client.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Problem Statement

   [RFC2131] specifies that a combination of 'client identifier' or
   'chaddr' and assigned network address constitute a unique identifier
   for the client's lease and are used by both the client and server to
   identify a lease referred in any DHCP messages.  [RFC2131] also
   specifies that the server MUST NOT return the 'client identifier'
   option in DHCPOFFER and DHCPACK messages.  Furthermore, DHCP relay
   agents and servers implementing [RFC2131] MAY drop the DHCP packets
   in the absence of both the 'client identifier' and 'chaddr' option.

   In some cases, a client may not have a valid hardware address to
   populate the 'chaddr' field and may set the field to all zeroes.  One
   such example is when DHCP is used to assign an IP address to a mobile
   phone or a tablet and where the 'chaddr' field is set to zero in DHCP
   request packets.  In such cases, the client usually sets the 'client

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RFC 6842                Client Identifier Option            January 2013

   identifier' option field (to a value as permitted in [RFC2131]), and
   both the client and server use this field to uniquely identify the
   client with in a subnet.

   Note that due to aforementioned recommendations in [RFC2131], valid
   downstream DHCP packets (DHCPOFFER, DHCPACK, and DHCPNAK) from the
   server MAY get dropped at the DHCP relay agent in the absence of the
   'client identifier' option when the 'chaddr' field is set to zero.

   The problem may get aggravated when a client receives a response from
   the server without 'client identifier' and with the 'chaddr' value
   set to zero, as it cannot guarantee that the response is intended for
   it.  This is due to the fact that even though the 'xid' field is
   present to map responses with requests, this field alone cannot
   guarantee that a particular response is for a particular client, as
   'xid' values generated by multiple clients within a subnet need not
   be unique.

   Lack of the 'client identifier' option in DHCP reply messages also
   affects the scenario where multiple DHCP clients may be running on
   the same host sharing the same 'chaddr'.

   This document attempts to address these problems faced by the DHCP
   relay agent and client by proposing modification to DHCP server
   behavior.  The solution specified in this document is in line with
   DHCPv6 [RFC3315] where the server always includes the Client
   Identifier option in the Reply messages.

   The requirement for DHCP servers not to return the 'client
   identifier' option was made purely to conserve the limited space in
   the packet.  It is possible, though unlikely, that clients will drop
   packets that contain this formerly unexpected option.  There are no
   known client implementations that will drop packets, but the benefit
   provided by this change outweighs any small risk of such behavior.
   More harm is being done by not having the 'client identifier' option
   present than might be done by adding it now.

3.  Modification to RFC 2131

   If the 'client identifier' option is present in a message received
   from a client, the server MUST return the 'client identifier' option,
   unaltered, in its response message.

   The following table is extracted from Section 4.3.1 of [RFC2131] and
   relevant fields are modified accordingly to overcome the problems
   mentioned in this document.

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RFC 6842                Client Identifier Option            January 2013

   Option                    DHCPOFFER    DHCPACK            DHCPNAK
   ------                    ---------    -------            -------
   Client identifier (if     MUST         MUST               MUST
     sent by client)
   Client identifier (if     MUST NOT     MUST NOT           MUST NOT
     not sent by client)

   When a client receives a DHCP message containing a 'client
   identifier' option, the client MUST compare that client identifier to
   the one it is configured to send.  If the two client identifiers do
   not match, the client MUST silently discard the message.

4.  Security Considerations

   This specification does not add any new security considerations other
   than the ones already mentioned in [RFC2131].  It is worth noting
   that DHCP clients routinely connect to different IP networks managed
   by different network providers.  DHCP clients have no a priori
   knowledge of which network they are connecting to.  Consequently, the
   client identifier will, by definition, be routinely shared with
   network operators and could be used in ways that violate the user's
   privacy.  This is a problem that existed in [RFC2131].  This document
   does nothing to address this problem.

5.  Acknowledgments

   The authors would like to thank Bernie Volz, Ted Lemon, Barr Hibbs,
   Richard Johnson, Barry Leiba, Stephen Farrell, and Adrian Farrel for
   insightful discussions and review.

6.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, March 1997.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

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RFC 6842                Client Identifier Option            January 2013

Authors' Addresses

   Narasimha Swamy Nelakuditi
   Samsung India
   Block-B, Bagmane Lakeview,
   66/1, Bagmane Tech Park,
   Byrasandra, C.V. Raman Nagar, Bangalore, 560093

   Phone: +91 80 4181 9999

   Gaurav Halwasia
   Cisco Systems
   SEZ Unit, Cessna Business Park
   Sarjapur Marathalli Outer Ring Road
   Bangalore, 560103

   Phone: +91 80 4426 1321

   Prashant Jhingran
   Cisco Systems
   SEZ Unit, Cessna Business Park
   Sarjapur Marathalli Outer Ring Road
   Bangalore, 560103

   Phone: +91 80 4426 1800

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