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+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
+
+Abstract
+
+   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
+   client.
+
+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
+   http://www.rfc-editor.org/info/rfc6842.
+
+Copyright Notice
+
+   Copyright (c) 2013 IETF Trust and the persons identified as the
+   document authors.  All rights reserved.
+
+   This document is subject to BCP 78 and the IETF Trust's Legal
+   Provisions Relating to IETF Documents
+   (http://trustee.ietf.org/license-info) in effect on the date of
+   publication of this document.  Please review these documents
+   carefully, as they describe your rights and restrictions with respect
+   to this document.  Code Components extracted from this document must
+   include Simplified BSD License text as described in Section 4.e of
+   the Trust Legal Provisions and are provided without warranty as
+   described in the Simplified BSD License.
+
+
+
+
+
+
+Swamy, et al.                Standards Track                    [Page 1]
+
+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",
+   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
+   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
+
+
+
+
+
+Swamy, et al.                Standards Track                    [Page 2]
+
+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.
+
+
+
+
+
+Swamy, et al.                Standards Track                    [Page 3]
+
+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.
+
+
+
+
+
+
+
+
+
+Swamy, et al.                Standards Track                    [Page 4]
+
+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
+   India
+
+   Phone: +91 80 4181 9999
+   EMail: nn.swamy@samsung.com
+
+
+   Gaurav Halwasia
+   Cisco Systems
+   SEZ Unit, Cessna Business Park
+   Sarjapur Marathalli Outer Ring Road
+   Bangalore, 560103
+   India
+
+   Phone: +91 80 4426 1321
+   EMail: ghalwasi@cisco.com
+
+
+   Prashant Jhingran
+   Cisco Systems
+   SEZ Unit, Cessna Business Park
+   Sarjapur Marathalli Outer Ring Road
+   Bangalore, 560103
+   India
+
+   Phone: +91 80 4426 1800
+   EMail: pjhingra@cisco.com
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+Swamy, et al.                Standards Track                    [Page 5]
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