NXDOMAIN: There Really Is Nothing Underneath
RFC 8020

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Internet Engineering Task Force (IETF)                     S. Bortzmeyer
Request for Comments: 8020                                         AFNIC
Updates: 1034, 2308                                             S. Huque
Category: Standards Track                                  Verisign Labs
ISSN: 2070-1721                                            November 2016

              NXDOMAIN: There Really Is Nothing Underneath

Abstract

   This document states clearly that when a DNS resolver receives a
   response with a response code of NXDOMAIN, it means that the domain
   name which is thus denied AND ALL THE NAMES UNDER IT do not exist.

   This document clarifies RFC 1034 and modifies a portion of RFC 2308:
   it updates both of them.

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 7841.

   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/rfc8020.

Copyright Notice

   Copyright (c) 2016 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.

Bortzmeyer & Huque           Standards Track                    [Page 1]
RFC 8020                      NXDOMAIN Cut                 November 2016

Table of Contents

   1. Introduction and Background .....................................2
      1.1. Terminology ................................................3
   2. Rules ...........................................................3
   3. Updates to RFCs .................................................5
      3.1. Updates to RFC 1034 ........................................5
      3.2. Updates to RFC 2308 ........................................5
   4. Benefits ........................................................5
   5. Possible Issues .................................................6
   6. Implementation Considerations ...................................6
   7. Security Considerations .........................................7
   8. References ......................................................7
      8.1. Normative References .......................................7
      8.2. Informative References .....................................8
   Appendix A. Why can't we just use the owner name of the returned
               SOA? ...................................................9
   Appendix B. Related Approaches .....................................9
   Acknowledgments ....................................................9
   Authors' Addresses ................................................10

1.  Introduction and Background

   The DNS protocol [RFC1035] defines response code 3 as "Name Error",
   or "NXDOMAIN" [RFC2308], which means that the queried domain name
   does not exist in the DNS.  Since domain names are represented as a
   tree of labels ([RFC1034], Section 3.1), nonexistence of a node
   implies nonexistence of the entire subtree rooted at this node.

   The DNS iterative resolution algorithm precisely interprets the
   NXDOMAIN signal in this manner.  If it encounters an NXDOMAIN
   response code from an authoritative server, it immediately stops
   iteration and returns the NXDOMAIN response to the querier.

   However, in most known existing resolvers today, a cached
   nonexistence for a domain is not considered "proof" that there can be
   no child domains underneath.  This is due to an ambiguity in
   [RFC1034] that failed to distinguish Empty Non-Terminal (ENT) names
   ([RFC7719]) from nonexistent names (Section 3.1).  The distinction
   became especially important for the development of DNSSEC, which
   provides proof of nonexistence.  [RFC4035], Section 3.1.3.2,
   describes how security-aware authoritative name servers make the
   distinction, but no existing RFCs describe the behavior for recursive
   name servers.

Bortzmeyer & Huque           Standards Track                    [Page 2]
RFC 8020                      NXDOMAIN Cut                 November 2016

   This document specifies that an NXDOMAIN response for a domain name