Client Identifier Option in DHCP Server Replies
draft-ietf-dhc-client-id-07

Abstract

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.

Requirements

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].

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 09, 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 (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
• 2. Problem Statement
• 3. Modification To [RFC2131]
• 4. IANA Considerations
• 5. Security Considerations
• 6. Acknowledgments
• 7. Normative References
• Authors' Addresses

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.

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.

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

Authors' Addresses