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 Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 14, 2005. 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. Arends Expires April 14, 2005 [Page 1] Internet-Draft QR Clarify October 2004 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 Arends Expires April 14, 2005 [Page 2] Internet-Draft QR Clarify October 2004 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]. Arends Expires April 14, 2005 [Page 3] Internet-Draft QR Clarify October 2004 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. Arends Expires April 14, 2005 [Page 4] Internet-Draft QR Clarify October 2004 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. Arends Expires April 14, 2005 [Page 5] Internet-Draft QR Clarify October 2004 4. IANA Considerations None. Arends Expires April 14, 2005 [Page 6] Internet-Draft QR Clarify October 2004 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 Arends Expires April 14, 2005 [Page 7] Internet-Draft QR Clarify October 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. 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Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Arends Expires April 14, 2005 [Page 8]