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+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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+Cui, et al.                  Standards Track                    [Page 1]
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+RFC 7283            Handling Unknown DHCPv6 Messages           July 2014
+
+
+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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+Cui, et al.                  Standards Track                    [Page 2]
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+RFC 7283            Handling Unknown DHCPv6 Messages           July 2014
+
+
+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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+Cui, et al.                  Standards Track                    [Page 3]
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+RFC 7283            Handling Unknown DHCPv6 Messages           July 2014
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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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+Cui, et al.                  Standards Track                    [Page 4]
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+RFC 7283            Handling Unknown DHCPv6 Messages           July 2014
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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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+Cui, et al.                  Standards Track                    [Page 5]
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+RFC 7283            Handling Unknown DHCPv6 Messages           July 2014
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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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+RFC 7283            Handling Unknown DHCPv6 Messages           July 2014
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+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
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+Cui, et al.                  Standards Track                    [Page 7]
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