DHC Working Group                                                 Y. Cui
Internet-Draft                                                    Q. Sun
Updates: 3315 (if approved)                          Tsinghua University
Intended status: Standards Track                                T. Lemon
Expires: August 8, 2014                                    Nominum, Inc.
                                                        February 4, 2014

                    Handling Unknown DHCPv6 Messages


   DHCPv6 is not specific about handling messages with unknown types.
   This memo describes the problems 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 defining new messages sent from DHCP servers to DHCP relay
   agents, and should be read by potential authors of such documents.
   This document updates RFC3315.

Status of This Memo

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   This Internet-Draft will expire on August 8, 2014.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
   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 Server  . . . . . . . . . . . .   5
     4.3.  Relaying a Message toward Client  . . . . . . . . . . . .   5
   5.  Client and Server Behavior Update . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Contributors List . . . . . . . . . . . . . . . . . . . . . .   6
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   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 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",
   document are to be interpreted as described in [RFC2119].

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3.  Problem Statement

   When a relay agent receives a message, it decides to send the message
   either toward the server or toward the client.  However, RFC 3315
   does not explicitly describe how the relay agent can determine
   whether it should send a message toward the server or the client,
   although this is implied by the message definitions in RFC3315.

   Another issue is that RFC3315 does not specify what a relay agent
   should do if it does not recognize a received message; the relay
   agent is not required to relay the message, nor advised to drop the
   message.  If relaying an unknown message, the relay agent is given no
   guidance about whether to send it toward the server or the client.

   In addition, there is no specific requirement for dealing with
   unknown messages by the client or server in RFC3315.

   Note 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.

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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 receives the message for which it is not the
     target according to the message type.

   New DHCP message types may be defined in future that are intended to
   convey information from DHCP servers to relay agents.  Relay agents
   that do not implement these messages will not recognize that such
   messages are intended for them.  A relay agent that implements this
   specification will necessarily forward such messages to the DHCP
   servers to which it is configured to relay client messages.

   At this time, no messages of this variety have been specified.  If
   such a message is specified in the future, the specification could
   include text something like the following:

     DHCP servers that send this new message type MAY, when they receive
     a relayed message of this type, mark the relay agent to which the
     message was sent as not implementing messages of this type.  In
     this case, the DHCP server MAY implement a strategy of probing the
     relay agent occasionally to see if it has been updated.

   However, this is not strictly necessary, since DHCP does not provide
   a signaling message for rejecting unexpected message types, and
   therefore DHCP servers are not expected to respond to such messages.

   Documents specifying new message types should use different types for
   communicating *to* relay agents than are used for communicating
   *from* relay agents, so that no confusion can occur where a message
   sent to a relay agent is sent back to the DHCP server, which then
   tries to process it as if it came from a relay agent.

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4.2.  Relaying a Message toward 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 is described in Section 20.1.1 of [RFC3315].

4.3.  Relaying a Message toward Client

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

   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 arbitrary type code.  The same
   problem may happen 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 RFC3315 did not specify either 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 might be made vulnerable to
   client attacks through the relay agent.

   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 due to vulnerable clients.

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

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7.  IANA Considerations

   This document does not include an IANA request.

8.  Contributors List

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

9.  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

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

   Qi Sun
   Tsinghua University
   Beijing  100084

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

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

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

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