Client Identifier Option in DHCP Server Replies
DHCP servers must now echo the client identifier option back to clients that sent it.
Swamy, N., Halwasia, G., and P. Jhingran, "Client Identifier Option in DHCP Server Replies", RFC 6842, DOI 10.17487/RFC6842, January 2013, <https://www.rfc-editor.org/info/rfc6842>.
@misc{rfc6842,
series = {Request for Comments},
number = 6842,
howpublished = {RFC 6842},
publisher = {RFC Editor},
doi = {10.17487/RFC6842},
url = {https://www.rfc-editor.org/info/rfc6842},
author = {N. Swamy and G. Halwasia and P. Jhingran},
title = {{Client Identifier Option in DHCP Server Replies}},
pagetotal = 5,
year = 2013,
month = jan,
} RFC text
Contents
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.
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
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.
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.
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
AI ELI5
On this page
§2The Problem with Replies
In RFC 2131, a combination of the client identifier option or chaddr and the assigned network address identifies a client's lease. The same RFC says a server MUST NOT return the client identifier option in DHCPOFFER and DHCPACK messages, and that relay agents and servers MAY drop packets that have neither client identifier nor chaddr.
Some clients, such as a mobile phone or tablet, have no valid hardware address and set chaddr to all zeroes. They set the client identifier instead. Replies without it may be dropped at a relay agent, and the client cannot be sure a reply is meant for it, because xid values from different clients in a subnet need not be unique. Multiple DHCP clients sharing one host and one chaddr are affected too.
Some devices have no proper name tag, so they write a nickname instead. If replies leave the nickname off, nobody can tell who the reply is for.
§3The Modification to RFC 2131
If the client identifier option is present in a message from a client, the server MUST return it, unaltered, in its response. If the client did not send it, the server still MUST NOT include it. This applies to DHCPOFFER, DHCPACK and DHCPNAK, and the RFC revises the table from Section 4.3.1 of RFC 2131 accordingly.
On the client side, when a DHCP message contains a client identifier option, the client MUST compare it to the one it is configured to send. If they do not match, the client MUST silently discard the message.
If you gave a nickname, the clerk must say it back exactly. If you hear a different nickname, you ignore that answer.
| Option | DHCPOFFER | DHCPACK | DHCPNAK |
|---|---|---|---|
| Client identifier (if sent by client) THIS RFC | MUST | MUST | MUST |
| Client identifier (if not sent by client) | MUST NOT | MUST NOT | MUST NOT |
§2.1Why the Old Rule Existed
The RFC explains that the old requirement was made purely to conserve the limited space in the packet. The new behavior is in line with DHCPv6 (RFC 3315), where the server always includes the Client Identifier option in Reply messages.
It admits that clients might drop packets containing the formerly unexpected option, though this is unlikely. No known client implementations do so, and the authors judge that more harm comes from leaving the option out than from adding it.
The old rule was just to save a little room in the envelope. The authors decided the room is worth spending.
§4Security Considerations
The document adds no new security considerations beyond those in RFC 2131. It notes that clients connect to many networks without knowing which, so the client identifier is routinely shared with network operators and could be used in ways that violate the user's privacy. That problem existed in RFC 2131, and this document does nothing to address it.
Telling every network your nickname can be a privacy problem. This RFC does not fix that.