draft-ietf-dnsext-dnssec-online-signing-02.txt

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Network Working Group                                          S. Weiler
Internet-Draft                                               SPARTA, Inc
Updates: 4034, 4035 (if approved)                               J. Ihren
Expires: July 24, 2006                                     Autonomica AB
                                                        January 20, 2006


       Minimally Covering NSEC Records and DNSSEC On-line Signing
               draft-ietf-dnsext-dnssec-online-signing-02

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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 RFC4034.  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.




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Changes from ietf-01 to ietf-02

   Clarified that a generated NSEC RR's type bitmap MUST have the RRSIG
   and NSEC bits set, to be consistent with DNSSECbis -- previous text
   said SHOULD.

   Made the applicability statement a little less oppressive.

Changes from ietf-00 to ietf-01

   Added an applicability statement, making reference to ongoing work on
   NSEC3.

   Added the phrase "epsilon functions", which has been commonly used to
   describe the technique and already appeared in the header of each
   page, in place of "increment and decrement functions".  Also added an
   explanatory sentence.

   Corrected references from 4034 section 6.2 to section 6.1.

   Fixed an out-of-date reference to [-bis] and other typos.

   Replaced IANA Considerations text.

   Escaped close parentheses in examples.

   Added some more acknowledgements.

Changes from weiler-01 to ietf-00

   Inserted RFC numbers for 4033, 4034, and 4035.

   Specified contents of bitmap field in synthesized NSEC RR's, pointing
   out that this relaxes a constraint in 4035.  Added 4035 to the
   Updates header.

Changes from weiler-00 to weiler-01

   Clarified that this updates RFC4034 by relaxing requirements on the
   next name field.

   Added examples covering wildcard names.

   In the 'better functions' section, reiterated that perfect functions
   aren't needed.

   Added a reference to RFC 2119.




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Table of Contents

   1.  Introduction and Terminology . . . . . . . . . . . . . . . . .  4
   2.  Applicability of This Technique  . . . . . . . . . . . . . . .  4
   3.  Minimally Covering NSEC Records  . . . . . . . . . . . . . . .  5
   4.  Better Epsilon Functions . . . . . . . . . . . . . . . . . . .  6
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   7.  Normative References . . . . . . . . . . . . . . . . . . . . .  8
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . .  8
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11







































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1.  Introduction and Terminology

   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.

   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 6.

   The key words "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 6, 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 6.

   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.





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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.

   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
   RFC4034 [2] section 6.1.  This relaxes the requirement in section
   4.1.1 of RFC4034 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 nor 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:



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

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

   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 RFC4035 [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