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
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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 32. Requirements Language . . . . . . . . . . . . . . . . . . . . 33. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 34. Relay Agent Behavior Update . . . . . . . . . . . . . . . . . 34.1. 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 . . . . . . . . . . . . . . . . . . . . 6
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
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
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 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
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
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.
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).
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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