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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Copyright Notice

  Copyright (c) 2014 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.

  This document may contain material from IETF Documents or IETF
  Contributions published or made publicly available before November
  10, 2008.  The person(s) controlling the copyright in some of this
  material may not have granted the IETF Trust the right to allow
  modifications of such material outside the IETF Standards Process.
  Without obtaining an adequate license from the person(s) controlling
  the copyright in such materials, this document may not be modified
  outside the IETF Standards Process, and derivative works of it may
  not be created outside the IETF Standards Process, except to format
  it for publication as an RFC or to translate it into languages other
  than English.

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










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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



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  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.




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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.




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  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.





























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Authors' Addresses

  Yong Cui
  Tsinghua University
  Beijing  100084
  P.R. China

  Phone: +86-10-6260-3059
  EMail: [email protected]


  Qi Sun
  Tsinghua University
  Beijing  100084
  P.R. China

  Phone: +86-10-6278-5822
  EMail: [email protected]


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

  Phone: +1-650-381-6000
  EMail: [email protected]























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