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Versions: (draft-kosters-dnsext-dnssec-opt-in) 00 01 02 03 04 05 06 07 08 09 RFC 4956

Network Working Group                                          R. Arends
Internet-Draft
Expires: May 2, 2003                                          M. Kosters
                                                               D. Blacka
                                                          Verisign, Inc.
                                                        November 1, 2002


                             DNSSEC Opt-In
                   draft-ietf-dnsext-dnssec-opt-in-04

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   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 May 2, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   In RFC 2535, delegations to unsigned subzones are cryptographically
   secured.  Maintaining this cryptography is not practical or
   necessary.  This document describes an "Opt-In" model that allows
   administrators to omit this cryptography and manage the cost of
   adopting DNSSEC with large zones.







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Table of Contents

   1.    Definitions and Terminology  . . . . . . . . . . . . . . . .  3
   2.    Overview . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.    Protocol Additions . . . . . . . . . . . . . . . . . . . . .  5
   3.1   Server Considerations  . . . . . . . . . . . . . . . . . . .  6
   3.1.1 Delegations Only . . . . . . . . . . . . . . . . . . . . . .  6
   3.1.2 Insecure Delegation Responses  . . . . . . . . . . . . . . .  6
   3.1.3 Wildcards and Opt-In . . . . . . . . . . . . . . . . . . . .  6
   3.2   Client Considerations  . . . . . . . . . . . . . . . . . . .  7
   3.2.1 Delegations Only . . . . . . . . . . . . . . . . . . . . . .  7
   3.2.2 Validation Process Changes . . . . . . . . . . . . . . . . .  7
   3.2.3 NXT Record Caching . . . . . . . . . . . . . . . . . . . . .  8
   3.2.4 Use of the AD bit  . . . . . . . . . . . . . . . . . . . . .  8
   4.    Benefits . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   5.    Example  . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   6.    Transition Issues  . . . . . . . . . . . . . . . . . . . . . 12
   7.    Security Considerations  . . . . . . . . . . . . . . . . . . 13
   8.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 14
   9.    Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 15
         References . . . . . . . . . . . . . . . . . . . . . . . . . 16
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 16
   A.    Implementing Opt-In using "Views"  . . . . . . . . . . . . . 18
   B.    Changes from Prior Versions  . . . . . . . . . . . . . . . . 20
         Full Copyright Statement . . . . . . . . . . . . . . . . . . 21


























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1. Definitions and Terminology

   Throughout this document, familiarity with the DNS system, RFC 1035
   [1], DNS security extensions, RFC 2535 [4], and DNSSEC terminology
   RFC 3090 [5] is assumed.

   The following abbreviations and terms are used in this document:

   RR: is used to refer to a DNS resource record.

   RRset: refers to a Resource Record Set, as defined by [3].  In this
      document, the RRset is also defined to include the covering SIG
      records, if any exist.

   covering NXT record/RRset: is the NXT record used to prove
      (non)existence of a particular name or RRset.  This means that for
      a RRset or name 'N', the covering NXT record has the name 'N', or
      has an owner name less than 'N' and "next" name greater than 'N'.

   delegation: refers to a NS RRset with a name different from the
      current zone apex (non-zone-apex), signifying a delegation to a
      subzone.

   secure delegation: refers to the NS, DS, NXT and SIG RRsets for a
      non-zone-apex owner name, signifying a delegation to a DNSSEC
      signed subzone.

   2535/DS insecure delegation: refers to the NS, NXT, and SIG RRsets
      for a non-zone-apex owner name, signifying a delegation to an
      unsigned subzone.  This differs from the secured delegation by the
      absence of a DS RRset, marked by the zero value for the DS type
      code in the NXT type map.

   Opt-In insecure delegation: refers to the NS RRset for a non-zone-
      apex owner name where the covering NXT record uses the Opt-In
      methodology described in this document.

   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 [2].











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2. Overview

   The cost to cryptographically secure delegations to unsigned zones is
   high for large delegation-centric zones and zones where insecure
   delegations will be updated rapidly.  For these zones, the costs of
   maintaining the NXT record chain may be extremely high relative to
   the gain of cryptographically authenticating existence of unsecured
   zones.

   This document describes a method of eliminating the superfluous
   cryptography present in secure delegations to insecure zones.  Using
   "Opt-In", a zone administrator can choose to remove insecure
   delegations from the NXT chain.  This is accomplished by extending
   the semantics of the NXT record by using a redundant bit in the type
   map.




































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3. Protocol Additions

   In RFC 2535, delegation NS RRsets are not signed, but instead are
   accompanied by a NXT RRset of the same name, and possibly a ("no-
   key") KEY RR [4] or DS record [7].  The security status of the
   subzone is determined by the presence of the KEY or DS records,
   cryptographically proven by the NXT record.  Opt-In expands this
   definition by allowing insecure delegations to exist within an
   otherwise signed zone without the corresponding NXT record at the
   delegation's owner name.  These insecure delegations are proven
   insecure by using a covering NXT record.

   Since this represents a change of the interpretation of NXT records,
   resolvers must be able to distinguish between RFC 2535 NXT records
   and Opt-In NXT records.  This is accomplished by "tagging" the NXT
   records that cover (or potentially cover) insecure delegation nodes.
   This tag is indicated by the absence of the NXT bit in the type map.
   Since the NXT bit in the type map merely indicates the existence of
   the record itself, this bit is redundant and safe for use as a tag.

   An Opt-In tagged NXT record does not assert the (non)existence of the
   delegations that it covers.  This allows for the addition or removal
   of these delegations without recalculating the resigning the NXT
   chain.  However, Opt-In tagged NXT records do assert the
   (non)existence of other signed RRsets.

   Zones using Opt-In MAY contain a mixture of Opt-In tagged NXT records
   and RFC 2535 NXT records.  If a NXT record is not Opt-In, there MUST
   NOT be any insecure delegations between it and the RRsets indicated
   by the 'next domain name' in the NXT RDATA.  If it is Opt-In, there
   MUST only be insecure delegations between it and the next node
   indicated by the 'next domain name' in the NXT RDATA.

   In summary,

   o  An Opt-In NXT type is identified by a zero-valued (or not-
      specified) NXT bit in the type bit map of the NXT record.

   o  A RFC2535 NXT type is identified by a one-valued NXT bit in the
      type bit map of the NXT record.

   and,

   o  An Opt-In NXT record does not assert the non-existence of a name
      between its owner name and "next" name, although it does assert
      that any name in this span MUST be an insecure delegation.

   o  An Opt-In NXT record does assert the (non)existence of RRsets with



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      the same owner name.


3.1 Server Considerations

   Opt-In imposes some new requirements on authoritative DNS servers.

3.1.1 Delegations Only

   This specification dictates that only insecure delegations may exist
   between the owner and "next" names of an Opt-In tagged NXT record.
   Servers and signing tools MUST enforce this restriction.

3.1.2 Insecure Delegation Responses

   When returning an Opt-In insecure delegation, the server MUST return
   the covering NXT RRset in the Authority section.

   This presents a change from RFC 2535, where the "no-key" KEY RRset
   would be returned instead.  However, in the delegation signer
   proposal, NXT records already must be returned along with the
   insecure delegation.  The primary difference that this proposal
   introduces is that the Opt-In tagged NXT record will have a different
   owner name from the delegation RRset.  This may require
   implementations to do a NXT search on cached responses.

3.1.3 Wildcards and Opt-In

   RFC 2535, in section 5.3, describes the practice of returning NXT
   records to prove the non-existence of an applicable wildcard in non-
   existent name responses.  This NXT record can be described as a
   "negative wildcard proof".  The use of Opt-In NXT records changes the
   necessity for this practice.  For non-existent name (NXDOMAIN)
   responses when the query name (qname) is covered by an Opt-In tagged
   NXT record, servers MUST NOT return negative wildcard proof records.

   The intent of the RFC 2535 negative wildcard proof requirement is to
   prevent malicious users from undetectably removing valid wildcard
   responses.  In order for this cryptographic proof to work, the
   resolver must be able to prove:

   1.  The exact qname does not exist.  This is done by the "normal" NXT
       record.

   2.  No applicable wildcard exists.  This is done by returning one or
       more NXT records proving that the wildcards do not exist
       (negative wildcard proofs).




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   However, if the NXT record covering the exact qname is an Opt-In NXT
   record, the resolver will not be able to prove the first part of this
   equation, as the qname might exist as an insecure delegation.  Thus,
   since the total proof cannot be completed, the negative wildcard
   proof records are not useful.

   The negative wildcard proofs are also not useful when returned as
   part of an Opt-In insecure delegation response for a similar reason:
   the resolver cannot prove that the qname does or does not exist, and
   therefore cannot prove that a wildcard expansion is valid.

   The presence of an Opt-In tagged NXT record does not change the
   practice of returning a NXT along with a wildcard expansion.  Even
   though the Opt-In NXT will not be able to prove that the wildcard
   expansion is valid, it will prove that the wildcard expansion is not
   masking any signed records.

3.2 Client Considerations

   Opt-In imposes some new requirements on DNS resolvers (caching or
   otherwise).

3.2.1 Delegations Only

   As stated in the "Server Considerations" section above, this
   specification restricts the namespace covered by Opt-In tagged NXT
   records to insecure delegations only.  Thus, resolvers MUST reject as
   invalid any records that fall within an Opt-In NXT record's span that
   are not NS records or corresponding glue records.

3.2.2 Validation Process Changes

   This specification does not change the resolver's resolution
   algorithm.  However, it does change the DNSSEC validation process.
   Resolvers MUST be able to use Opt-In tagged NXT records to
   cryptographically prove the validity and security status (as
   insecure) of a referral.  Resolvers determine the security status of
   the referred-to zone as follows:

   o  In RFC 2535, the security status is proven by existence of a
      verified "no-key" KEY RRset.  The absence of the "no-key" KEY
      RRset indicates that the referred-to zone is secure.

   o  Using Delegation Signer, the security status is proven by the
      existence or absence of a DS record at the same name as the
      delegation.  The absence is proven using a verified NXT record of
      the same name that does not have the DS bit set in the type map.
      This NXT record MAY also be tagged as Opt-In.



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   o  Using Opt-In, the security status is proven by the existence of a
      DS record (for secure) or the presence of a verified Opt-In tagged
      NXT record that covers the delegation name.  That is, the NXT
      record does not have the NXT bit set in the type map, and the
      delegation name falls between the NXT's owner and "next" name.

   Using Opt-In does not substantially change the nature of following
   referrals within DNSSEC.  At every delegation point, the resolver
   will have cryptographic proof that the subzone is secure or insecure.

   When receiving either an Opt-In insecure delegation response or a
   non-existent name response where that name is covered by an Opt-In
   tagged NXT record, the resolver MUST NOT require proof (in the form
   of a NXT record) that a wildcard did not exist.

3.2.3 NXT Record Caching

   Caching resolvers MUST be able to retrieve the appropriate covering
   Opt-In NXT record when returning referrals that need them.  This
   requirement differs from Delegation Signer in that the covering NXT
   will not have the same owner name as the delegation.  Some
   implementations may have to use new methods for finding these NXT
   records.

3.2.4 Use of the AD bit

   The AD bit, as defined by [8], MUST NOT be set when:

   o  sending a non-existent name (NXDOMAIN) response where the covering
      NXT is tagged as Opt-In.

   o  sending an Opt-In insecure delegation response, unless the
      covering (Opt-In) NXT record's owner name equals the delegation
      name.

   This rule is based on what the Opt-In NXT record actually proves.
   For names that exist between the Opt-In NXT record's owner and "next"
   names, the Opt-In NXT record cannot prove the non-existence or
   existence of the name.  As such, not all data in the response has
   been cryptographically verified, so the AD bit cannot be set.











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4. Benefits

   Using Opt-In allows administrators of large and/or changing
   delegation-centric zones to minimize the overhead involved in
   maintaining the security of the zone.

   Opt-In accomplishes this by eliminating the need for both "no-key"
   KEY (in [4]) and NXT records for insecure delegations.  This, in a
   zone with a large number of delegations to unsigned subzones, can
   lead to substantial space savings (both in memory and on disk).
   Additionally, Opt-In allows for the addition or removal of insecure
   delegations without modifying the NXT record chain.  Zones that are
   frequently updating insecure delegations (e.g., TLDs) can avoid the
   substantial overhead of modifying and resigning the affected NXT
   records.




































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5. Example

   Consider the zone EXAMPLE, shown below.  This is a zone where all of
   the NXT records are tagged as Opt-In.

   Example A: Fully Opt-In Zone.

         EXAMPLE.               SOA   ...
         EXAMPLE.               SIG   SOA ...
         EXAMPLE.               NS    FIRST-SECURE.EXAMPLE.
         EXAMPLE.               SIG   NS ...
         EXAMPLE.               KEY   ...
         EXAMPLE.               SIG   KEY ...
         EXAMPLE.               NXT   FIRST-SECURE.EXAMPLE. SOA NS SIG KEY
         EXAMPLE.               SIG   NXT ...

         FIRST-SECURE.EXAMPLE.  A     ...
         FIRST-SECURE.EXAMPLE.  SIG   A ...
         FIRST-SECURE.EXAMPLE.  NXT   SECOND-SECURE.EXAMPLE. A SIG
         FIRST-SECURE.EXAMPLE.  SIG   NXT ...

         NOT-SECURE.EXAMPLE.    NS    NS.NOT-SECURE.EXAMPLE.
         NS.NOT-SECURE.EXAMPLE. A     ...

         SECOND-SECURE.EXAMPLE. NS    NS.ELSEWHERE.
         SECOND-SECURE.EXAMPLE. KEY   ...
         SECOND-SECURE.EXAMPLE. SIG   KEY ...
         SECOND-SECURE.EXAMPLE. NXT   EXAMPLE. NS SIG KEY
         SECOND-SECURE.EXAMPLE. SIG   NXT ...

         UNSIGNED.EXAMPLE.      NS    NS.UNSIGNED.EXAMPLE.
         NS.UNSIGNED.EXAMPLE.   A     ...


   In this example, a query for a signed RRset (e.g., "FIRST-
   SECURE.EXAMPLE A"), or a secure delegation ("WWW.SECOND-
   SECURE.EXAMPLE A") will result in a standard RFC 2535 response.

   A query for a nonexistent RRset will result in a response that
   differs from RFC 2535 by: the NXT record will be tagged as Opt-In,
   there will be no NXT record proving the non-existence of a matching
   wildcard record, and the AD bit will not be set.

   A query for an insecure delegation RRset (or a referral) will return
   both the answer (in the Authority section) and the corresponding Opt-
   In NXT record to prove that it is not secure.





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   Example A.1: Response to query for WWW.UNSIGNED.EXAMPLE.  A


         RCODE=NOERROR, AD=0

         Answer Section:

         Authority Section:
         UNSIGNED.EXAMPLE.      NS    NS.UNSIGNED.EXAMPLE
         SECOND-SECURE.EXAMPLE. NXT   EXAMPLE. NS SIG KEY
         SECOND-SECURE.EXAMPLE. SIG   NXT ...

         Additional Section:
         NS.UNSIGNED.EXAMPLE    A     ...
         EXAMPLE.               KEY   ...
         EXAMPLE.               SIG   KEY ...

   In the Example A zone, the EXAMPLE.  node MAY use either style of NXT
   record, because there are no insecure delegations that occur between
   it and the next node, FIRST-SECURE.EXAMPLE.  In other words, Example
   A would still be a valid zone if the NXT record for EXAMPLE.  was
   changed to the following RR:

         EXAMPLE.               NXT   FIRST-SECURE.EXAMPLE. SOA NS SIG KEY NXT

   However, the other NXT records (FIRST-SECURE.EXAMPLE.  and SECOND-
   SECURE.EXAMPLE.) MUST be tagged as Opt-In because there are insecure
   delegations in the range they define.  (NOT-SECURE.EXAMPLE.  and
   UNSIGNED.EXAMPLE., respectively).






















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6. Transition Issues

   Opt-In is not backwards compatible with RFC 2535.  RFC 2535 compliant
   DNSSEC implementations will not recognize Opt-In tagged NXT records
   as different from RFC 2535 NXT records.  Because of this, RFC 2535
   implementations will reject all Opt-In insecure delegations within a
   zone as invalid.












































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7. Security Considerations

   Opt-In allows for unsigned names.  All unsigned names are, by
   definition, insecure, and their validity (or existence) can not be
   cryptographically proven.  With Opt-In, a malicious entity is able
   to: insert, modify, or delete insecure delegation RRsets within the
   Opt-In spans of a otherwise secured zone.  In addition, a malicious
   entity is able to replay or delete wildcard expansions (if there is
   an existing applicable wildcard) in the Opt-In spans of the zone.

   For example, if a resolver received the following response from the
   example zone above:

   Example S.1: Response to query for WWW.DOES-NOT-EXIST.EXAMPLE.  A

         RCODE=NOERROR

         Authority Section:
         DOES-NOT-EXIST.EXAMPLE. NS    NS.FORGED.
         EXAMPLE.                NXT   FIRST-SECURE.EXAMPLE. SOA NS SIG KEY
         EXAMPLE.                SIG   NXT ...

         Additional Section:
         EXAMPLE.                KEY   ...
         EXAMPLE.                SIG   KEY ...

   The resolver would have no choice but to believe that the referral to
   NS.FORGED.  is valid.

   While in particular cases, this issue may not present a significant
   security problem, in general it should not be lightly dismissed.  It
   is strongly RECOMMENDED that Opt-In be used sparingly.  In
   particular, zone signing tools SHOULD NOT default to Opt-In, and MAY
   choose to not support Opt-In at all.

















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8. IANA Considerations

   None.
















































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9. Acknowledgments

   The contributions, suggestions and remarks of the following persons
   (in alphabetic order) to this draft are acknowledged:

      Mats Dufberg, Miek Gieben, Olafur Gudmundsson, Bob Halley, Olaf
      Kolkman, Edward Lewis, Ted Lindgreen, Rip Loomis, Bill Manning,
      Dan Massey, Scott Rose, Mike Schiraldi, Jakob Schlyter, Brian
      Wellington.










































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References

   [1]  Mockapetris, P., "Domain names - implementation and
        specification", STD 13, RFC 1035, November 1987.

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

   [3]  Elz, R. and R. Bush, "Clarifications to the DNS Specification",
        RFC 2181, July 1997.

   [4]  Eastlake, D., "Domain Name System Security Extensions", RFC
        2535, March 1999.

   [5]  Lewis, E., "DNS Security Extension Clarification on Zone
        Status", RFC 3090, March 2001.

   [6]  Conrad, D., "Indicating Resolver Support of DNSSEC", RFC 3225,
        December 2001.

   [7]  Gudmundsson, O., "Delegation Signer Resource Record", draft-
        ietf-dnsext-delegation-signer-09 (work in progress), September
        2002.

   [8]  Gudmundsson, O. and B. Wellington, "Redefinition of DNS AD bit",
        draft-ietf-dnsext-ad-is-secure-06 (work in progress), June 2002.


Authors' Addresses

   Roy Arends
   Bankastraat 41-E
   1094 EB  Amsterdam
   NL

   Phone: +31206931681
   EMail: roy@logmess.com


   Mark Kosters
   Verisign, Inc.
   21355 Ridgetop Circle
   Dulles, VA  20166
   US

   Phone: +1 703 948 3200
   EMail: markk@verisign.com
   URI:   http://www.verisignlabs.com



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   David Blacka
   Verisign, Inc.
   21355 Ridgetop Circle
   Dulles, VA  20166
   US

   Phone: +1 703 948 3200
   EMail: davidb@verisign.com
   URI:   http://www.verisignlabs.com










































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Appendix A. Implementing Opt-In using "Views"

   In many cases, it may be convenient to implement an Opt-In zone by
   combining two separately maintained "views" of a zone at request
   time.  In this context, "view" refers to a particular version of a
   zone, not to any specific DNS implementation feature.

   In this scenario, one view is the secure view, the other is the
   insecure (or legacy) view.  The secure view consists of an entirely
   signed zone using Opt-In tagged NXT records.  The insecure view
   contains no DNSSEC information.  It is helpful, although not
   necessary, for the secure view to be a subset (minus DNSSEC records)
   of the insecure view.

   In addition, the only RRsets that may solely exist in the insecure
   view are non-zone-apex NS RRsets.  That is, all non-NS RRsets (and
   the zone apex NS RRset) MUST be signed and in the secure view.

   These two views may be combined at request time to provide a virtual,
   single opt-in zone.  The following algorithm is used when responding
   to each query:

      V_A is the secure view as described above.

      V_B is the insecure view as described above.

      R_A is a response generated from V_A, following RFC 2535 [4].

      R_B is a response generated from V_B, following DNS resolution as
      per RFC 1035 [1].

      R_C is the response generated by combining R_A with R_B, as
      described below.

      A query is DNSSEC-aware if it either has the DO bit [6] turned on,
      or is for a DNSSEC-specific record type.




   1.  If V_A is a subset of V_B and the query is not DNSSEC-aware,
       generate and return R_B, otherwise

   2.  Generate R_A.

   3.  If R_A's RCODE != NXDOMAIN, return R_A, otherwise

   4.  Generate R_B and combine it with R_A to form R_C:



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          For each section (ANSWER, AUTHORITY, ADDITIONAL), copy the
          records from R_A into R_B, EXCEPT the AUTHORITY section SOA
          record, if R_B's RCODE = NOERROR.

   5.  Return R_C.














































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Appendix B. Changes from Prior Versions

   Changes from version 03:

      Editorial changes for clarification only.

   Changes from version 02:

      Added text on changes to validation process, use of the AD bit,
      and interactions with wildcards.  Added wildcard caveats to the
      "Security Considerations" section.  Added "Transition Issues"
      section.

   Changes from version 01:

      Changed to "delegation only".  Strengthened "Security
      Considerations" section.  Added "Server Considerations" and
      "Client Considerations" sections.  Added AD bit requirement.

   Changes from version 00:

      Complete rewrite, altering approach from "views" to tagged NXT
      records




























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Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
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