|Title||Client Identifier Option in DHCP Server Replies
|Author||N. Swamy, G.
Halwasia, P. Jhingran
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
Client Identifier Option in DHCP Server Replies
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
Status of This Memo
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(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.
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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
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
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.
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.
Narasimha Swamy Nelakuditi
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