DHC Working Group Y. Cui Internet-Draft Q. Sun Updates: 3315 (if approved) Tsinghua University Intended status: Standards Track T. Lemon Expires: September 28, 2014 Nominum, Inc. March 27, 2014 Handling Unknown DHCPv6 Messages draft-ietf-dhc-dhcpv6-unknown-msg-08 Abstract 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 This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on September 28, 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 Cui, et al. Expires September 28, 2014 [Page 1] Internet-Draft Handling Unknown DHCPv6 Messages March 2014 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 . . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3 4.2. Relaying a Message toward Server . . . . . . . . . . . . 4 4.3. Relaying a Message toward Client . . . . . . . . . . . . 4 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Cui, et al. Expires September 28, 2014 [Page 2] Internet-Draft Handling Unknown DHCPv6 Messages March 2014 3. Problem Statement When a relay agent receives a message, it decides to send the message either toward the server or toward the client. It decides on the direction to forward based on the message type. Since RFC3315 was published new message types have been defined. More such messages may be defined in the future. RFC3315 does not specify the 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 such messages, and to other DHCP agents inappropriately processing such messages. 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. 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." Cui, et al. Expires September 28, 2014 [Page 3] Internet-Draft Handling Unknown DHCPv6 Messages March 2014 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 future that are sent, unsolicited, to relay agents. Relay agents that do not implement these messages will not recognize such messages as being intended for them. A relay agent that implements this specification will therefore 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 some strategy for identifying non-conforming relays and not sending such messages to them. However, since DHCP servers do not respond to unknown messages, this is unlikely to create significant load, and therefore is likely to be unnecessary. 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. Cui, et al. Expires September 28, 2014 [Page 4] Internet-Draft Handling Unknown DHCPv6 Messages March 2014 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 RFC3315. 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 issues that could already be 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 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 firewall). Cui, et al. Expires September 28, 2014 [Page 5] Internet-Draft Handling Unknown DHCPv6 Messages March 2014 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 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 Cui, et al. Expires September 28, 2014 [Page 6]