RFC 6842

Client Identifier Option in DHCP Server Replies

📅 published January 2013, from ietf

RFC 6842: Client Identifier Option in DHCP Server Replies 🎀

Stream: Internet Engineering Task Force (IETF) 🌸
Request for Comments: 6842
Authors: N. Swamy (Samsung India), G. Halwasia (Cisco Systems), P. Jhingran (Cisco Systems) 👑
Category: Standards Track ✨
ISSN: 2070-1721
Date: January 2013 🦋
Updates: 2131 (RFC 2131, "Dynamic Host Configuration Protocol") 💜

Abstract ✨

okay so this doc is literally fixing [RFC2131] (the Dynamic Host Configuration Protocol) 💅 bc that spec says the server MUST NOT return the 'client identifier' option to the client. this document is all about cleaning up the issues that rule causes 🧹

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 ©️

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
   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] hands out configuration parameters to hosts on an IP-based network. 🍓 DHCP runs on a client-server model: designated DHCP servers allocate network addresses and deliver configuration parameters to dynamically configured hosts. ✨

The changes to [RFC2131] in this document clarify how DHCP servers use the 'client identifier' option. This clarification handles the issues (as covered in the Problem Statement) that come from the point in [RFC2131] saying the server MUST NOT return the 'client identifier' option to the client. 💅

1.1. Requirements Language 🦋

okay so the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are read the way [RFC2119] describes them 🫶

2. Problem Statement 😭

[RFC2131] says that a combination of 'client identifier' or 'chaddr' and the assigned network address make up a unique identifier for the client's lease. Both the client and server use that combo to identify a lease in any DHCP messages. 🔑 [RFC2131] also says the server MUST NOT return the 'client identifier' option in DHCPOFFER and DHCPACK messages. And on top of that, DHCP relay agents and servers implementing [RFC2131] MAY drop the DHCP packets if both the 'client identifier' and 'chaddr' options are missing. 💀

sometimes a client doesn't have a valid hardware address to fill in the 'chaddr' field, so it sets the field to all zeroes 🙈 this happens when DHCP is used to give an IP address to a mobile phone or a tablet, for example, where 'chaddr' is zero in DHCP request packets. in those cases the client usually sets the 'client identifier' option (to a value [RFC2131] allows), and both the client and server use that field to uniquely identify the client within a subnet ✨

here's the tricky part: because of the aforementioned recommendations in [RFC2131], valid downstream DHCP packets (DHCPOFFER, DHCPACK, and DHCPNAK) from the server MAY get dropped at the DHCP relay agent when the 'client identifier' option is missing and 'chaddr' is set to zero. 😱

it gets worse 🥲 when a client gets a response from the server with no 'client identifier' and 'chaddr' set to zero, the client can't be sure the response is meant for it. the 'xid' field is there to match responses with requests, but it can't prove a response belongs to a particular client on its own, since 'xid' values generated by multiple clients within a subnet don't have to be unique. 🫠

missing 'client identifier' in DHCP reply messages also causes trouble when several DHCP clients run on the same host and share the same 'chaddr' 👯

this document tackles these problems for the DHCP relay agent and client by changing how DHCP servers behave 💅 the solution follows DHCPv6 [RFC3315], where the server always includes the Client Identifier option in Reply messages. 🧚

the rule that servers not return the 'client identifier' option was put in purely to save the limited space in the packet. it's possible (though unlikely) that clients will drop packets containing this formerly unexpected option. no known client implementations drop packets for this, but the benefit of this change outweighs any small risk of that behavior. more harm comes from not having the 'client identifier' option present than from adding it now. 🔥

3. Modification to RFC 2131 👑

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

the table below is pulled from Section 4.3.1 of [RFC2131], with the relevant fields modified to fix the problems covered 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 gets 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 don't match, the client MUST silently discard the message. 🙅‍♀️

4. Security Considerations 🔐

⚠️ this one is serious bestie, read it fully 😭

This specification doesn't add any new security considerations beyond the ones already mentioned in [RFC2131]. but it's worth noting that DHCP clients routinely connect to different IP networks run by different network providers. DHCP clients have no a priori knowledge of which network they're connecting to. so the client identifier will, by definition, be routinely shared with network operators, and it could be used in ways that violate the user's privacy. this is a problem that already existed in [RFC2131], and this document does nothing to address it. 🕵️‍♀️💀

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.

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

📦 where this page comes from

ietf: rfc6842 2013-01
💖 girlified by claude-haiku-5-5 on 2026-10-17: a language model's rewrite of the original, under its terms