Internet Engineering Task Force (IETF)                            Y. Cui
Request for Comments: 7283                                        Q. Sun
Updates: 3315                                        Tsinghua University
Category: Standards Track                                       T. Lemon
ISSN: 2070-1721                                            Nominum, Inc.
                                                               July 2014


                    Handling Unknown DHCPv6 Messages

Abstract

   DHCPv6 is not specific about handling messages with unknown types.
   This memo describes the problems associated with receiving DHCPv6
   messages with unknown types, and defines how a DHCPv6 server, client,
   or relay agent should behave when receiving unknown DHCPv6 messages.
   This document also provides advice for authors of future documents
   that define new messages to be sent from DHCP servers to DHCP relay
   agents.  This document updates RFC 3315.

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/rfc7283.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   3
   3.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Relay Agent Behavior Update . . . . . . . . . . . . . . . . .   3
     4.1.  A Valid Message for Constructing a New Relay-forward
           Message . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Relaying a Message toward the Server  . . . . . . . . . .   5
     4.3.  Relaying a Message toward the Client  . . . . . . . . . .   5
   5.  Client and Server Behavior Update . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   6
   8.  Normative References  . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   DHCPv6 [RFC3315] provides a framework for conveying IPv6
   configuration information to hosts on a TCP/IP network.  But
   [RFC3315] is not specific about how to deal with messages with
   unrecognized types.  This document describes the problems associated
   with receiving DHCPv6 messages with unknown types, and defines the
   behavior of a DHCPv6 server, client, or relay agent when handling
   unknown DHCPv6 messages.

2.  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].

3.  Problem Statement

   When a relay agent receives a message, it sends the message toward
   either the server or the client.  The relay agent decides on the
   direction to forward based on the message type.  Since RFC 3315 was
   published, new message types have been defined.  Additional message
   types may be defined in the future.  RFC 3315 does not specify what
   to do when a DHCP agent does not recognize the type of message it has
   received.  This may lead to relay agents inappropriately dropping
   these messages and to other DHCP agents inappropriately processing
   these messages.

   In addition, there is no specific requirement for dealing with
   unknown messages by the client or server in RFC 3315.

   Note that it is expected that most future DHCPv6 messages will not be
   used to communicate directly with relay agents (though they may need
   to be relayed by relay agents).

4.  Relay Agent Behavior Update

   Relay agents relay messages toward servers and clients according to
   the message type.  The Relay-reply message is sent toward the client.
   The Relay-forward message and other types of messages are sent toward
   the server.

   We say "toward the client" and "toward the server" because relay
   agents may be chained together, so a relay message may be sent
   through multiple relay agents along the path to its destination.
   Relay-reply messages specify a destination address; the relay agent
   extracts the encapsulated message and sends it to the specified
   destination address.  Any message other than a Relay-reply does not

   have such a specified destination, so it follows the default
   forwarding path configured on the relay agent, which is always toward
   the server.

   The sole purpose of requiring relay agents to relay unknown messages
   is to ensure that when legitimate new messages are defined in the
   protocol, relay agents (even if they were manufactured prior to the
   definition of these new messages) will, by default, succeed in
   relaying such messages.

4.1.  A Valid Message for Constructing a New Relay-forward Message

   Section 20.1 of [RFC3315] states that:

      When a relay agent receives a valid message to be relayed, it
      constructs a new Relay-forward message.

   It does not define which types of messages are valid for constructing
   Relay-forward messages.  In this document, we specify the definition
   as follows.

      The message is valid for constructing a new Relay-forward message:

      (a)  if the message is a Relay-forward message, or

      (b)  if the relay agent recognizes the message type and is not the
           intended target, or

      (c)  if the relay agent does not recognize the message type.

   New DHCP message types may be defined in the future that are sent,
   unsolicited, to relay agents.  Relay agents that do not implement
   these messages will not recognize the messages as being intended for
   them.  Therefore, a relay agent that implements this specification
   will forward such messages to the DHCP servers to which it is
   configured to relay client messages.

   At this time, no such message types have been specified.  If such a
   message is specified in the future, it is possible that this would
   result in needless load on DHCP servers.  If such a message type is
   defined in a future specification, authors may need to consider a
   strategy for identifying non-conforming relays and not sending such
   messages to those relay agents.

   However, since DHCP servers do not respond to unknown messages, this
   is unlikely to create significant load and is therefore likely to be
   unnecessary.

4.2.  Relaying a Message toward the Server

   If the relay agent receives a Relay-forward message, Section 20.1.2
   of [RFC3315] defines the required behavior.  If the relay agent
   receives messages other than Relay-forward and Relay-reply and the
   relay agent does not recognize its message type, it MUST forward them
   as described in Section 20.1.1 of [RFC3315].

4.3.  Relaying a Message toward the Client

   If the relay agent receives a Relay-reply message, it MUST process
   the message as defined in Section 20.2 of [RFC3315], regardless of
   the type of message encapsulated in the Relay Message option.

5.  Client and Server Behavior Update

   A client or server MUST silently discard any received DHCPv6 message
   with an unknown message type.

6.  Security Considerations

   This document creates no new security issues that are not already
   present in RFC 3315.  By explicitly documenting the correct handling
   of unknown messages, this document, if implemented, reduces any
   security exposure that might result from incorrect handling of
   unknown messages.  The following issues are already present with
   Section 23 of [RFC3315], but we discuss them in detail here as
   guidance for implementors.

   As the relay agent will forward all unknown types of DHCPv6 messages,
   a malicious attacker can interfere with the relaying function by
   constructing fake DHCPv6 messages with an arbitrary type code.  The
   same problem may occur in current DHCPv4 and DHCPv6 practice, where
   the attacker constructs the fake DHCP message with a known type code.

   Clients and servers that implement this specification will discard
   unknown DHCPv6 messages.  Since RFC 3315 did not specify relay agent,
   client, or server behavior in the presence of unknown messages, it is
   possible that some servers or clients that have not been updated to
   conform to this specification will become vulnerable to attacks
   through the relay agent as a result of this change.

   For this reason, we recommend that relay agents, clients, and servers
   be updated to follow this new specification.  However, in most
   deployment scenarios, it will be much easier to attack clients
   directly than through a relay agent.  Furthermore, attacks using
   unknown message types are already possible on the local wire.

   So, in most cases, if clients are not upgraded, there should be
   minimal additional risk.  At sites where only servers and relay
   agents can be upgraded, the incremental benefit of doing so most
   likely exceeds any risk of vulnerable clients.

   Nothing in this update should be construed to mean that relay agents
   may not be administratively configurable to drop messages based on
   the message type, for security reasons (e.g., in a firewall).

7.  Contributors

   Many thanks to Bernie Volz, Tomek Mrugalski, Sheng Jiang, Cong Liu,
   and Yuchi Chen for their contributions to the document.

8.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, 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

   Yong Cui
   Tsinghua University
   Beijing  100084
   P.R. China

   Phone: +86-10-6260-3059
   EMail: yong@csnet1.cs.tsinghua.edu.cn


   Qi Sun
   Tsinghua University
   Beijing  100084
   P.R. China

   Phone: +86-10-6278-5822
   EMail: sunqi@csnet1.cs.tsinghua.edu.cn


   Ted Lemon
   Nominum, Inc.
   2000 Seaport Blvd
   Redwood City, CA  94063
   USA

   Phone: +1-650-381-6000
   EMail: Ted.Lemon@nominum.com