draft-ietf-dnsext-dnssec-opt-in-07.txt

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   prove the non-existence of an applicable wildcard in non-existent
   name responses.  This NSEC record can be described as a "negative
   wildcard proof".  The use of Opt-In NSEC records changes the
   necessity for this practice.  For non-existent name responses when
   the query name (qname) is covered by an Opt-In tagged NSEC record,
   servers MAY choose to omit the wildcard proof record, and clients
   MUST NOT treat the absence of this NSEC record as a validation error.

   The intent of the standard DNSSEC 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"
       NSEC record.
   2.  No applicable wildcard exists.  This is done by returning a NSEC
       record proving that the wildcard does not exist (this is the
       negative wildcard proof).

   However, if the NSEC record covering the exact qname is an Opt-In
   NSEC 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 NSEC record is not useful.

   The negative wildcard proof is 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 NSEC record does not change the
   practice of returning a NSEC along with a wildcard expansion.  Even



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   though the Opt-In NSEC 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.

4.1.4  Dynamic Update

   Opt-In changes the semantics of Secure DNS Dynamic Update [9].  In
   particular, it introduces the need for rules that describe when to
   add or remove a delegation name from the NSEC chain.  This document
   does not attempt to define these rules.  Until these rules are
   defined, servers MUST NOT process DNS Dynamic Update requests against
   zones that use Opt-In NSEC records.  Servers SHOULD return responses
   to update requests with RCODE=REFUSED.

4.2  Client Considerations

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

4.2.1  Delegations Only

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

4.2.2  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 NSEC 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 standard DNSSEC, the security status is proven by the existence
      or absence of a DS RRset at the same name as the delegation.  The
      existence of the DS RRset indicates that the referred-to zone is
      signed.  The absence of the DS RRset is proven using a verified
      NSEC record of the same name that does not have the DS bit set in
      the type map.  This NSEC record MAY also be tagged as Opt-In.
   o  Using Opt-In, the security status is proven by the existence of a
      DS record (for signed) or the presence of a verified Opt-In tagged
      NSEC record that covers the delegation name.  That is, the NSEC
      record does not have the NSEC bit set in the type map, and the
      delegation name falls between the NSEC's owner and "next" name.




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   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 referred-to subzone is signed
   or unsigned.

   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 NSEC record, the resolver MUST NOT require proof (in the form
   of a NSEC record) that a wildcard did not exist.

4.2.3  NSEC Record Caching

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

4.2.4  Use of the AD bit

   The AD bit, as defined by [2] and [5], MUST NOT be set when:

   o  sending a Name Error (RCODE=3) response where the covering NSEC is
      tagged as Opt-In.
   o  sending an Opt-In insecure delegation response, unless the
      covering (Opt-In) NSEC record's owner name equals the delegation
      name.

   This rule is based on what the Opt-In NSEC record actually proves:
   for names that exist between the Opt-In NSEC record's owner and
   "next" names, the Opt-In NSEC 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.

5.  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 NSEC 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 NSEC record chain.  Zones that are frequently updating insecure
   delegations (e.g., TLDs) can avoid the substantial overhead of



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   modifying and resigning the affected NSEC records.

6.  Example

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

   Example A: Fully Opt-In Zone.

         EXAMPLE.               SOA    ...
         EXAMPLE.               RRSIG  SOA ...
         EXAMPLE.               NS     FIRST-SECURE.EXAMPLE.
         EXAMPLE.               RRSIG  NS ...
         EXAMPLE.               DNSKEY ...
         EXAMPLE.               RRSIG  DNSKEY ...
         EXAMPLE.               NSEC   FIRST-SECURE.EXAMPLE. (
                                       SOA NS RRSIG DNSKEY )
         EXAMPLE.               RRSIG  NSEC ...

         FIRST-SECURE.EXAMPLE.  A      ...
         FIRST-SECURE.EXAMPLE.  RRSIG  A ...
         FIRST-SECURE.EXAMPLE.  NSEC   NOT-SECURE-2.EXAMPLE. A RRSIG
         FIRST-SECURE.EXAMPLE.  RRSIG  NSEC ...

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

         NOT-SECURE-2.EXAMPLE.  NS     NS.NOT-SECURE.EXAMPLE.
         NOT-SECURE-2.EXAMPLE   NSEC   SECOND-SECURE.EXAMPLE NS RRSIG
         NOT-SECURE-2.EXAMPLE   RRSIG  NSEC ...

         SECOND-SECURE.EXAMPLE. NS     NS.ELSEWHERE.
         SECOND-SECURE.EXAMPLE. DS     ...
         SECOND-SECURE.EXAMPLE. RRSIG  DS ...
         SECOND-SECURE.EXAMPLE. NSEC   EXAMPLE. NS RRSIG DNSKEY
         SECOND-SECURE.EXAMPLE. RRSIG  NSEC ...

         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 DNSSEC response.

   A query for a nonexistent RRset will result in a response that
   differs from standard DNSSEC by: the NSEC record will be tagged as
   Opt-In, there may be no NSEC record proving the non-existence of a



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   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 NSEC record to prove that it is not secure.

   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. NSEC   EXAMPLE. NS RRSIG DS
         SECOND-SECURE.EXAMPLE. RRSIG  NSEC ...

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

   In the Example A.1 zone, the EXAMPLE. node MAY use either style of
   NSEC 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 NSEC record for EXAMPLE.
   was changed to the following RR:

         EXAMPLE.               NSEC   FIRST-SECURE.EXAMPLE. (SOA NS
                                       RRSIG DNSKEY NSEC )

   However, the other NSEC 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).

   NOT-SECURE-2.EXAMPLE. is an example of an insecure delegation that is
   part of the NSEC chain and also covered by an Opt-In tagged NSEC
   record.  Because NOT-SECURE-2.EXAMPLE. is a signed name, it cannot be
   removed from the zone without modifying and resigning the prior NSEC
   record.  Delegations with names that fall between NOT-SECURE-
   2.EXAMPLE. and SECOND-SECURE.EXAMPLE. may be added or removed without
   resigning any NSEC records.

7.  Transition Issues

   Opt-In is not backwards compatible with standard DNSSEC and is
   considered experimental.  Standard DNSSEC compliant implementations
   would not recognize Opt-In tagged NSEC records as different from



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   standard NSEC records.  Because of this, standard DNSSEC
   implementations, if they were to validate Opt-In style responses,
   would reject all Opt-In insecure delegations within a zone as
   invalid.  However, by only signing with private algorithms, standard
   DNSSEC implementations will treat Opt-In responses as unsigned.

   It should be noted that all elements in the resolution path between
   (and including) the validator and the authoritative name server must
   be aware of the Opt-In experiment and implement the Opt-In semantics
   for successful validation to be possible.  In particular, this
   includes any caching middleboxes between the validator and
   authoritative name server.

8.  Security Considerations

   Opt-In allows for unsigned names, in the form of delegations to
   unsigned subzones, to exist within an otherwise signed zone.  All
   unsigned names are, by definition, insecure, and their validity or
   existence cannot by cryptographically proven.

   In general:

   o  Records with unsigned names (whether existing or not) suffer from
      the same vulnerabilities as records in an unsigned zone.  These
      vulnerabilities are described in more detail in [12] (note in
      particular sections 2.3, "Name Games" and 2.6, "Authenticated
      Denial").
   o  Records with signed names have the same security whether or not
      Opt-In is used.

   Note that with or without Opt-In, an insecure delegation may have its
   contents undetectably altered by an attacker.  Because of this, the
   primary difference in security that Opt-In introduces is the loss of
   the ability to prove the existence or nonexistence of an insecure
   delegation within the span of an Opt-In NSEC record.