DHC Working Group                                               N. Swamy
Internet-Draft                                             Samsung India
Updates: 2131 (if approved)                                  G. Halwasia
Intended status: Standards Track                             P. Jhingran
Expires: May 9, 2013                                       Cisco Systems
                                                        November 5, 2012

            Client Identifier Option in DHCP Server Replies


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


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

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 9, 2013.

Copyright Notice

   Copyright (c) 2012 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

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   (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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Modification To [RFC2131] . . . . . . . . . . . . . . . . . . . 4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Normative References  . . . . . . . . . . . . . . . . . . . . . 5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . . 5

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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 'client identifier' option to the client.

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 'client identifier' in
   DHCPOFFER and DHCPACK messages.  Furthermore, DHCP relay agents and
   servers implementing [RFC2131] "MAY" drop the DHCP packets in the
   absence of both 'client identifier' and 'chaddr'.

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

   Note that due to above mentioned 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
   'client identifier' option when 'chaddr' field is set as zero.

   The problem may get aggravated when a client receives a response from
   the server without 'client identifier' and with 'chaddr' value set to
   zero, as it cannot guarantee that the response is intended for it.
   This is because 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.

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   Lack of '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 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 [RFC2131]

   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.

   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.

   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.  IANA Considerations

   This memo asks the IANA for no new parameters.

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5.  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.

6.  Acknowledgments

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

7.  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.

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
   Email: nn.swamy@samsung.com

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

   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

   Phone: +91 80 4426 1800
   Email: pjhingra@cisco.com

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