Internet DRAFT - draft-ietf-dhc-dhcpv6-unknown-msg
draft-ietf-dhc-dhcpv6-unknown-msg
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
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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].
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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."
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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.
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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).
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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
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