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.

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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.

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   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
   RFC 4034 [2] Section 6.1.  This relaxes the requirement in Section
   4.1.1 of RFC 4034 that the "next name" field contains the next owner
   name in the zone.  This change is expected to be fully compatible
   with all existing DNSSEC validators.  These NSEC records are returned
   whenever proving something specifically about the owner name (e.g.,
   that no resource records of a given type appear at that name).

   Whenever an NSEC record is needed to prove the non-existence of a
   name, a new NSEC record is dynamically produced and signed.  The new
   NSEC record has an owner name lexically before the QNAME but
   lexically following any existing name and a "next name" lexically
   following the QNAME but before any existing name.

   The generated NSEC record's type bitmap MUST have the RRSIG and NSEC
   bits set and SHOULD NOT have any other bits set.  This relaxes the
   requirement in Section 2.3 of RFC4035 that NSEC RRs not appear at
   names that did not exist before the zone was signed.

   The functions to generate the lexically following and proceeding
   names need not be perfect or consistent, but the generated NSEC
   records must not cover any existing names.  Furthermore, this
   technique works best when the generated NSEC records cover as few
   names as possible.  In this document, the functions that generate the
   nearby names are called "epsilon" functions, a reference to the
   mathematical convention of using the greek letter epsilon to
   represent small deviations.

   An NSEC record denying the existence of a wildcard may be generated
   in the same way.  Since the NSEC record covering a non-existent
   wildcard is likely to be used in response to many queries,
   authoritative name servers using the techniques described here may
   want to pregenerate or cache that record and its corresponding RRSIG.

   For example, a query for an A record at the non-instantiated name
   example.com might produce the following two NSEC records, the first
   denying the existence of the name example.com and the second denying
   the existence of a wildcard:

          exampld.com 3600 IN NSEC example-.com ( RRSIG NSEC )

          \).com 3600 IN NSEC +.com ( RRSIG NSEC )

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   Before answering a query with these records, an authoritative server
   must test for the existence of names between these endpoints.  If the
   generated NSEC would cover existing names (e.g., exampldd.com or
   *bizarre.example.com), a better epsilon function may be used or the
   covered name closest to the QNAME could be used as the NSEC owner
   name or next name, as appropriate.  If an existing name is used as
   the NSEC owner name, that name's real NSEC record MUST be returned.
   Using the same example, assuming an exampldd.com delegation exists,
   this record might be returned from the parent:

          exampldd.com 3600 IN NSEC example-.com ( NS DS RRSIG NSEC )

   Like every authoritative record in the zone, each generated NSEC
   record MUST have corresponding RRSIGs generated using each algorithm
   (but not necessarily each DNSKEY) in the zone's DNSKEY RRset, as
   described in RFC 4035 [3] Section 2.2.  To minimize the number of
   signatures that must be generated, a zone may wish to limit the
   number of algorithms in its DNSKEY RRset.

4.  Better Epsilon Functions

   Section 6.1 of RFC 4034 defines a strict ordering of DNS names.
   Working backward from that definition, it should be possible to
   define epsilon functions that generate the immediately following and
   preceding names, respectively.  This document does not define such
   functions.  Instead, this section presents functions that come
   reasonably close to the perfect ones.  As described above, an
   authoritative server should still ensure than no generated NSEC
   covers any existing name.

   To increment a name, add a leading label with a single null (zero-
   value) octet.

   To decrement a name, decrement the last character of the leftmost
   label, then fill that label to a length of 63 octets with octets of
   value 255.  To decrement a null (zero-value) octet, remove the octet
   -- if an empty label is left, remove the label.  Defining this
   function numerically: fill the leftmost label to its maximum length
   with zeros (numeric, not ASCII zeros) and subtract one.

   In response to a query for the non-existent name foo.example.com,
   these functions produce NSEC records of the following:

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     fon\255\255\255\255\255\255\255\255\255\255\255\255\255\255
     \255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
     \255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
     \255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
     \255.example.com 3600 IN NSEC \000.foo.example.com ( NSEC RRSIG )

     \)\255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
     \255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
     \255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
     \255\255\255\255\255\255\255\255\255\255\255\255\255\255\255
     \255\255.example.com 3600 IN NSEC \000.*.example.com ( NSEC RRSIG )

   The first of these NSEC RRs proves that no exact match for
   foo.example.com exists, and the second proves that there is no
   wildcard in example.com.

   Both of these functions are imperfect: they do not take into account
   constraints on number of labels in a name nor total length of a name.
   As noted in the previous section, though, this technique does not
   depend on the use of perfect epsilon functions: it is sufficient to
   test whether any instantiated names fall into the span covered by the
   generated NSEC and, if so, substitute those instantiated owner names
   for the NSEC owner name or next name, as appropriate.

5.  Security Considerations

   This approach requires on-demand generation of RRSIG records.  This
   creates several new vulnerabilities.

   First, on-demand signing requires that a zone's authoritative servers
   have access to its private keys.  Storing private keys on well-known
   Internet-accessible servers may make them more vulnerable to
   unintended disclosure.

   Second, since generation of digital signatures tends to be
   computationally demanding, the requirement for on-demand signing
   makes authoritative servers vulnerable to a denial of service attack.

   Last, if the epsilon functions are predictable, on-demand signing may
   enable a chosen-plaintext attack on a zone's private keys.  Zones
   using this approach should attempt to use cryptographic algorithms
   that are resistant to chosen-plaintext attacks.  It is worth noting
   that although DNSSEC has a "mandatory to implement" algorithm, that
   is a requirement on resolvers and validators -- there is no
   requirement that a zone be signed with any given algorithm.

   The success of using minimally covering NSEC records to prevent zone
   walking depends greatly on the quality of the epsilon functions

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RFC 4470                      NSEC Epsilon                    April 2006

   chosen.  An increment function that chooses a name obviously derived
   from the next instantiated name may be easily reverse engineered,
   destroying the value of this technique.  An increment function that
   always returns a name close to the next instantiated name is likewise
   a poor choice.  Good choices of epsilon functions are the ones that
   produce the immediately following and preceding names, respectively,
   though zone administrators may wish to use less perfect functions
   that return more human-friendly names than the functions described in
   Section 4 above.

   Another obvious but misguided concern is the danger from synthesized
   NSEC records being replayed.  It is possible for an attacker to
   replay an old but still validly signed NSEC record after a new name
   has been added in the span covered by that NSEC, incorrectly proving
   that there is no record at that name.  This danger exists with DNSSEC
   as defined in [3].  The techniques described here actually decrease
   the danger, since the span covered by any NSEC record is smaller than
   before.  Choosing better epsilon functions will further reduce this
   danger.

6.  Acknowledgements

   Many individuals contributed to this design.  They include, in
   addition to the authors of this document, Olaf Kolkman, Ed Lewis,
   Peter Koch, Matt Larson, David Blacka, Suzanne Woolf, Jaap Akkerhuis,
   Jakob Schlyter, Bill Manning, and Joao Damas.

   In addition, the editors would like to thank Ed Lewis, Scott Rose,
   and David Blacka for their careful review of the document.

7.  Normative References

   [1]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
        "DNS Security Introduction and Requirements", RFC 4033, March
        2005.

   [2]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
        "Resource Records for the DNS Security Extensions", RFC 4034,
        March 2005.

   [3]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
        "Protocol Modifications for the DNS Security Extensions", RFC
        4035, March 2005.

   [4]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

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Authors' Addresses

   Samuel Weiler
   SPARTA, Inc.
   7075 Samuel Morse Drive
   Columbia, Maryland  21046
   US

   EMail: weiler@tislabs.com

   Johan Ihren
   Autonomica AB
   Bellmansgatan 30
   Stockholm  SE-118 47
   Sweden

   EMail: johani@autonomica.se

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RFC 4470                      NSEC Epsilon                    April 2006

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