Minimally Covering NSEC Records and DNSSEC On-line Signing
RFC 4470

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Network Working Group                                          S. Weiler
Request for Comments: 4470                                  SPARTA, Inc.
Updates: 4035, 4034                                             J. Ihren
Category: Standards Track                                  Autonomica AB
                                                              April 2006

       Minimally Covering NSEC Records and DNSSEC On-line Signing

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document describes how to construct DNSSEC NSEC resource records
   that cover a smaller range of names than called for by RFC 4034.  By
   generating and signing these records on demand, authoritative name
   servers can effectively stop the disclosure of zone contents
   otherwise made possible by walking the chain of NSEC records in a
   signed zone.

Table of Contents

   1. Introduction ....................................................1
   2. Applicability of This Technique .................................2
   3. Minimally Covering NSEC Records .................................2
   4. Better Epsilon Functions ........................................4
   5. Security Considerations .........................................5
   6. Acknowledgements ................................................6
   7. Normative References ............................................6

1.  Introduction

   With DNSSEC [1], an NSEC record lists the next instantiated name in
   its zone, proving that no names exist in the "span" between the
   NSEC's owner name and the name in the "next name" field.  In this
   document, an NSEC record is said to "cover" the names between its
   owner name and next name.

Weiler & Ihren              Standards Track                     [Page 1]
RFC 4470                      NSEC Epsilon                    April 2006

   Through repeated queries that return NSEC records, it is possible to
   retrieve all of the names in the zone, a process commonly called
   "walking" the zone.  Some zone owners have policies forbidding zone
   transfers by arbitrary clients; this side effect of the NSEC
   architecture subverts those policies.

   This document presents a way to prevent zone walking by constructing
   NSEC records that cover fewer names.  These records can make zone
   walking take approximately as many queries as simply asking for all
   possible names in a zone, making zone walking impractical.  Some of
   these records must be created and signed on demand, which requires
   on-line private keys.  Anyone contemplating use of this technique is
   strongly encouraged to review the discussion of the risks of on-line
   signing in Section 5.

1.2.  Keywords

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [4].

2.  Applicability of This Technique

   The technique presented here may be useful to a zone owner that wants
   to use DNSSEC, is concerned about exposure of its zone contents via
   zone walking, and is willing to bear the costs of on-line signing.

   As discussed in Section 5, on-line signing has several security
   risks, including an increased likelihood of private keys being
   disclosed and an increased risk of denial of service attack.  Anyone
   contemplating use of this technique is strongly encouraged to review
   the discussion of the risks of on-line signing in Section 5.

   Furthermore, at the time this document was published, the DNSEXT
   working group was actively working on a mechanism to prevent zone
   walking that does not require on-line signing (tentatively called
   NSEC3).  The new mechanism is likely to expose slightly more
   information about the zone than this technique (e.g., the number of
   instantiated names), but it may be preferable to this technique.

3.  Minimally Covering NSEC Records

   This mechanism involves changes to NSEC records for instantiated
   names, which can still be generated and signed in advance, as well as
   the on-demand generation and signing of new NSEC records whenever a
   name must be proven not to exist.

Weiler & Ihren              Standards Track                     [Page 2]
RFC 4470                      NSEC Epsilon                    April 2006

   In the "next name" field of instantiated names' NSEC records, rather
   than list the next instantiated name in the zone, list any name that
   falls lexically after the NSEC's owner name and before the next
   instantiated name in the zone, according to the ordering function in