Internet DRAFT - draft-arends-dnsext-qr-clarification
draft-arends-dnsext-qr-clarification
Network Working Group R. Arends
Internet-Draft Telematica Instituut
Expires: April 14, 2005 October 14, 2004
DNS Response clarification.
draft-arends-dnsext-qr-clarification-00
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document clarifies DNS response message interpretation to avoid
denial of service attacks using DNS responses.
In a recent DNS software assessment it has come to light that some
implementations respond to DNS response messages. A loop occurs if
the receiver of this response responds with a response.
It was never explicitly stated that response messages must not be
answered. This draft makes the statement explicit.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Reserved Words . . . . . . . . . . . . . . . . . . . . . . 3
2. DNS Message processing . . . . . . . . . . . . . . . . . . . . 4
3. Security Considerations . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
6. Normative References . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . 8
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1. Introduction
In a recent DNS software assessment it has come to light that some
implementations respond to DNS response messages. A loop occurs if
the receiver responds as well with a DNS response message.
A DNS message has a header which includes a flag (QR) to denote the
message as a request message or a response message. If the QR flag
is clear, the message is a request message. If the QR flag is set,
the message is a response message.
A typical DNS message exchange exists of a request message from a
resolver to a server, followed by a response message from a server to
a resolver. A response message is never sent unsolicited. A
response message is never answered with a response message.
It is not explicitly stated in [RFC1035] that response messages must
not be answered. This draft makes the statement explicit.
The author does not believe this to be a change to the DNS protocol
set, but since some implementers believed this wasn't a requirement,
it is hereby documented explicitly as a requirement.
1.1 Reserved Words
The key words "MUST" and "MUST NOT" in this document are to be
interpreted as described in [RFC2119].
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2. DNS Message processing
A DNS response message is a DNS message with the QR flag set in the
header, independent of other content of the DNS message.
An unsolicited response message is a DNS response message which is
not invoked by a request message.
A DNS implementation MUST ignore unsolicited DNS response messages.
A DNS implementation MUST NOT send a DNS response message in response
to a DNS response message.
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3. Security Considerations
This draft acknowledges that a Denial of Service scenario can occur
when a DNS implementation responds to DNS response messages. It
explicitly forbids this behavior.
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4. IANA Considerations
None.
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5. Acknowledgments
Thanks to Steven M. Bellovin and Harald Tveit Alvestrand for their
help with this shortest draft ever. We've set a record!
6 Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Author's Address
Roy Arends
Telematica Instituut
Drienerlolaan 5
7522 NB Enschede
NL
EMail: roy.arends@telin.nl
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