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
AbstractDHCPv6 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.
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. 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
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. RFC2119]. 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).
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. 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.
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]. Section 20.2 of [RFC3315], regardless of the type of message encapsulated in the Relay Message option. 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.
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). [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.