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Internet Engineering Task Force (IETF)                            Y. CuiRequest for Comments: 7283                                        Q. SunUpdates:3315                                        Tsinghua UniversityCategory: Standards Track                                       T. LemonISSN: 2070-1721                                            Nominum, Inc.                                                               July 2014Handling Unknown DHCPv6 MessagesAbstract   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 updatesRFC 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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7283.Cui, et al.                  Standards Track                    [Page 1]

RFC 7283            Handling Unknown DHCPv6 Messages           July 2014Copyright Notice   Copyright (c) 2014 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .32.  Requirements Language . . . . . . . . . . . . . . . . . . . .33.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .34.  Relay Agent Behavior Update . . . . . . . . . . . . . . . . .3     4.1.  A Valid Message for Constructing a New Relay-forward           Message . . . . . . . . . . . . . . . . . . . . . . . . .44.2.  Relaying a Message toward the Server  . . . . . . . . . .54.3.  Relaying a Message toward the Client  . . . . . . . . . .55.  Client and Server Behavior Update . . . . . . . . . . . . . .56.  Security Considerations . . . . . . . . . . . . . . . . . . .57.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .68.  Normative References  . . . . . . . . . . . . . . . . . . . .6Cui, et al.                  Standards Track                    [Page 2]

RFC 7283            Handling Unknown DHCPv6 Messages           July 20141.  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.  SinceRFC 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 inRFC 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 notCui, et al.                  Standards Track                    [Page 3]

RFC 7283            Handling Unknown DHCPv6 Messages           July 2014   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 MessageSection 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.Cui, et al.                  Standards Track                    [Page 4]

RFC 7283            Handling Unknown DHCPv6 Messages           July 20144.2.  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 inSection 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 inSection 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 inRFC 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 withSection 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.  SinceRFC 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.Cui, et al.                  Standards Track                    [Page 5]

RFC 7283            Handling Unknown DHCPv6 Messages           July 2014   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.Cui, et al.                  Standards Track                    [Page 6]

RFC 7283            Handling Unknown DHCPv6 Messages           July 2014Authors' 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.comCui, et al.                  Standards Track                    [Page 7]