Client Identifier Option in DHCP Server Replies
draft-ietf-dhc-client-id-03

Abstract

This document updates RFC2131 [RFC2131]. The changes to [RFC2131] defined in this draft clarifies the use of 'client identifier' option by the DHCP servers. The clarification addresses the issues arising out of the point specified by [RFC2131] that the server 'MUST NOT' return 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 January 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. Proposed Modification To [RFC2131]
• 4. IANA Considerations
• 5. Security Considerations
• 6. Acknowledgements
• 7. References
• Authors' Addresses

1. Introduction

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

The changes to [RFC2131] defined in this document clarifies the use of '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. DHCP relay agents and servers, following these recommendations MAY drop the DHCP packets in the absence of both 'client identifier' and 'chaddr'.

In some cases, client may not be having valid hardware address value to be filled in 'chaddr' field of the packet and hence may set this field as zero. 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 proposed solution is in line with DHCPv6 [RFC3315] where the server always includes the Client Identifier option in the Reply messages.

3. Proposed Modification To [RFC2131]

If the 'client identifier' option is set 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)
  

Client MAY use 'client identifier' or 'chaddr' received from server along with 'xid' to map the response to request. This will guarantee that a particular response from the server is meant for the particular client.

4. IANA Considerations

This memo asks the IANA for no new parameters.

5. Security Considerations

No known security considerations.

6. Acknowledgements

The authors would like to thank Bernie Volz, Ted Lemon, Barr Hibbs for their insightful discussions on the previous version of this document.

7. References

Authors' Addresses