DNS Extensions Working Group                                   R. Arends
Internet-Draft                                      Telematica Instituut
Expires: December 7, 2004 April 25, 2005                                          P. Koch
                                                  Universitaet Bielefeld
                                                             J. Schlyter
                                                             June 8,
                                                        October 25, 2004

                Evaluating DNSSEC Transition Mechanisms

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   Copyright (C) The Internet Society (2004). All Rights Reserved.


   This document collects and summarizes different proposals for
   alternative and additional strategies for authenticated denial in DNS
   responses, evaluates these proposals and gives a recommendation for a
   way forward.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Transition Mechanisms  . . . . . . . . . . . . . . . . . . . .  3
     2.1   Mechanisms Updating DNSSEC-bis . . . . . . . . . . . . . . .  4
       2.1.1   Dynamic NSEC Synthesis . . . . . . . . . . . . . . . . . . .  4
       2.1.2   Add Versioning/Subtyping to Current NSEC . . . . . . . . . .  4
       2.1.3   Type Bit Map NSEC Indicator  . . . . . . . . . . . . . . . .  5
       2.1.4   New Apex Type  . . . . . . . . . . . . . . . . . . . . . . .  6
       2.1.5   NSEC White Lies  . . . . . . . . . . . . . . . . . . . . . .  7
       2.1.6   NSEC Optional via DNSSKEY Flag . . . . . . . . . . . . . . .  8
     2.2   Mechanisms not Updating DNSSEC-bis . . . . . . . . . . . . .  9
       2.2.1   Partial Type-code and Signal Rollover  . . . . . . . . . . .  9
       2.2.2   A Complete Type-code and Signal Rollover . . . . . . . . . .  9
       2.2.3   Unknown Algorithm in RRSIG . . . . . . . . . . . . . . . . . 10
   3.  Recommendation . . . . . . . . . . . . . . . . . . . . . . . . 11
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
       Intellectual Property and Copyright Statements . . . . . . . . 13

1.  Introduction

   The working group consents on not including NSEC-alt in the
   DNSSEC-bis documents.  The working group considers to take up
   "prevention of zone enumeration" as a work item.

   There may be multiple mechanisms to allow for co-existence with
   DNSSEC-bis.  The chairs allow the working group a little over a week
   (up to June 12) to come to consensus on a possible modification to
   the document to enable gentle rollover.  If that consensus cannot be
   reached the DNSSEC-bis documents will go out as-is.

   To ease the process of getting consensus, a summary of the proposed
   solutions and analysis of the pros and cons were written during the

   This summary includes:

      An inventory of the proposed mechanisms to make a transition to
      future work on authenticated denial of existence.
      List the known Pros and Cons, possibly provide new arguments, and
      possible security considerations of these mechanisms.
      Provide a recommendation on a way forward that is least disruptive
      to the DNSSEC-bis specifications as they stand and keep an open
      path to other methods for authenticated denial existence.

   The descriptions of the proposals in this document are coarse and do
   not cover every detail necessary for implementation.  In any case,
   documentation and further study is needed before implementaion and/or
   deployment, including those which seem to be solely operational in

2.  Transition Mechanisms

   In the light of recent discussions and past proposals, we have found
   several ways to allow for transition to future expansion of
   authenticated denial.  We tried to illuminate the paths and pitfalls
   in these ways forward.  Some proposals lead to a versioning of
   DNSSEC, where DNSSEC-bis may co-exist with DNSSEC-ter, other
   proposals are 'clean' but may cause delay, while again others may be
   plain hacks.

   Some paths do not introduce versioning, and might require the current
   DNSSEC-bis documents to be fully updated to allow for extensions to
   authenticated denial mechanisms.  Other paths introduce versioning
   and do not (or minimally) require DNSSEC-bis documents to be updated,
   allowing DNSSEC-bis to be deployed, while future versions can be
   drafted independent from or partially depending on DNSSEC-bis.

2.1  Mechanisms Updating DNSSEC-bis

2.1.1  Dynamic NSEC Synthesis

   This proposal assumes that NSEC RRs and the authenticating RRSIG will
   be generated dynamically to just cover the (non existent) query name.
   The owner name is (the) one preceding the name queried for, the Next
   Owner Name Field has the value of the Query Name Field + 1 (first
   successor in canonical ordering).  A separate key (the normal ZSK or
   a separate ZSK per authoritative server) would be used for RRSIGs on
   NSEC RRs.  This is a defense against enumeration, though it has the
   presumption of online signing.  Coexistence and Migration

   There is no change in interpretation other then that the next owner
   name might or might not exist.  Limitations

   This introduces an unbalanced cost between query and response
   generation due to dynamic generation of signatures.  Amendments to DNSSEC-bis

   The current DNSSEC-bis documents might need to be updated to indicate
   that the next owner name might not be an existing name in the zone.
   This is not a real change to the spec since implementers have been
   warned not to synthesize with previously cached NSEC records.  A
   specific bit to identify the dynamic signature generating Key might
   be useful as well, to prevent it from being used to fake positive
   data.  Cons

   Unbalanced cost is a ground for DDoS.  Though this protects against
   enumeration, it is not really a path for versioning.  Pros

   Hardly any amendments to DNSSEC-bis.

2.1.2  Add Versioning/Subtyping to Current NSEC

   This proposal introduces versioning for the NSEC RR type (a.k.a.
   subtyping) by adding a (one octet) version field to the NSEC RDATA.
   Version number 0 is assigned to the current (DNSSEC-bis) meaning,
   making this an 'Must Be Zero' (MBZ) for the to be published docset.  Coexistence and Migration

   Since the versioning is done inside the NSEC RR, different versions
   may coexist.  However, depending on future methods, that may or may
   not be useful inside a single zone.  Resolvers cannot ask for
   specific NSEC versions but may be able to indicate version support by
   means of a to be defined EDNS option bit.  Limitations

   There are no technical limitations, though it will cause delay to
   allow testing of the (currently unknown) new NSEC interpretation.

   Since the versioning and signaling is done inside the NSEC RR, future
   methods will likely be restricted to a single RR type authenticated
   denial (as opposed to e.g.  NSEC-alt, which currently proposes three
   RR types).  Amendments to DNSSEC-bis

   Full Update of the current DNSSEC-bis documents to provide for new
   fields in NSEC, while specifying behavior in case of unknown field
   values.  Cons

   Though this is a clean and clear path without versioning DNSSEC, it
   takes some time to design, gain consensus, update the current
   dnssec-bis document, test and implement a new authenticated denial
   record.  Pros

   Does not introduce an iteration to DNSSEC while providing a clear and
   clean migration strategy.

2.1.3  Type Bit Map NSEC Indicator

   Bits in the type-bit-map are reused or allocated to signify the
   interpretation of NSEC.

   This proposal assumes that future extensions make use of the existing
   NSEC RDATA syntax, while it may need to change the interpretation of
   the RDATA or introduce an alternative denial mechanism, invoked by
   the specific type-bit-map-bits.  Coexistence and migration

   Old and new NSEC meaning could coexist, depending how the signaling
   would be defined.  The bits for NXT, NSEC, RRSIG or other outdated RR
   types are available  as well as those covering meta/query types or
   types to be specifically allocated.  Limitations

   This mechanism uses an NSEC field that was not designed for that
   purpose.  Similar methods were discussed during the Opt-In discussion
   and the Silly-State discussion.  Amendments to DNSSEC-bis

   The specific type-bit-map-bits must be allocated and they need to be
   specified as 'Must Be Zero' (MBZ) when used for standard (dnssec-bis)
   interpretation.  Also, behaviour of the resolver and validator must
   be documented in case unknown values are encountered for the MBZ
   field.  Currently the protocol document specifies that the validator
   MUST ignore the setting of the NSEC and the RRSIG bits, while other
   bits are only used for the specific purpose of the type-bit-map field  Cons

   The type-bit-map was not designed for this purpose.  It is a
   straightforward hack.  Text in protocol section 5.4 was put in
   specially to defend against this usage.  Pros

   No change needed to the on-the-wire protocol as specified in the
   current docset.

2.1.4  New Apex Type

   This introduces a new Apex type (parallel to the zone's SOA)
   indicating the DNSSEC version (or authenticated denial) used in or
   for this zone.  Coexistence and Migration

   Depending on the design of this new RR type multiple denial
   mechanisms may coexist in a zone.  Old validators will not understand
   and thus ignore the new type, so interpretation of the new NSEC
   scheme may fail, negative responses may appear 'bogus'.  Limitations

   A record of this kind is likely to carry additional feature/
   feature/versioning indications unrelated to the current question of
   authenticated denial.  Amendments to DNSSEC-bis

   The current DNSSEC-bis documents need to be updated to indicate that
   the absence of this type indicates dnssec-bis, and that the (mere)
   presence of this type indicated unknown versions.  Cons

   The only other 'zone' or 'apex' record is the SOA record.  Though
   this proposal is not new, it is yet unknown how it might fulfill
   authenticated denial extensions.  This new RR type would only provide
   for a generalized signaling mechanism, not the new authenticated
   denial scheme.  Since it is likely to be general in nature, due to
   this generality consensus is not to be reached soon.  Pros

   This approach would allow for a lot of other per zone information to
   be transported or signaled to both (slave) servers and resolvers.

2.1.5  NSEC White Lies

   This proposal disables one part of NSEC (the pointer part) by means
   of a special target (root, apex, owner, ...), leaving intact only the
   ability to authenticate denial of existence of RR sets, not denial of
   existence of domain names (NXDOMAIN).  It may be necessary to have
   one working NSEC to prove the absence of a wildcard.  Coexistence and Migration

   The NSEC target can be specified per RR, so standard NSEC and 'white
   lie' NSEC can coexist in a zone.  There is no need for migration
   because no versioning is introduced or intended.  Limitations

   This proposal breaks the protocol and is applicable to certain types
   of zones only (no wildcard, no deep names, delegation only).  Most of
   the burden is put on the resolver side and operational consequences
   are yet to be studied.  Amendments to DNSSEC-bis

   The current DNSSEC-bis documents need to be updated to indicate that
   the NXDOMAIN responses may be insecure.  Cons

   Strictly speaking this breaks the protocol and doesn't fully fulfill
   the requirements for authenticated denial of existence.  Security
   implications need to be carefully documented: search path problems
   (forged denial of existence may lead to wrong expansion of non-FQDNs,
   cf.  RFC 1535); replay attacks to deny existence of records  Pros

   Hardly any amendments to DNSSEC-bis.  Operational "trick" that is
   available anyway.

2.1.6  NSEC Optional via DNSSKEY Flag

   A new DNSKEY may be defined to declare NSEC optional per zone.  Coexistence and Migration

   Current resolvers/validators will not understand the Flag bit and
   will have to treat negative responses as bogus.  Otherwise, no
   migration path is needed since NSEC is simply turned off.  Limitations

   NSEC can only be made completely optional at the cost of being unable
   to prove unsecure delegations (absence of DS RR).  A next to this
   approach would just disable authenticated denial for non-existence of
   nodes.  Amendments to DNSSEC-bis

   New DNSKEY Flag to be defined.  Resolver/Validator behaviour needs to
   be specified in the light of absence of authenticated denial.  Cons

   Doesn't fully meet requirements.  Operational consequences to be
   studied.  Pros

   Official version of the "trick" presented in (8).  Operational
   problems can be addressed during future work on validators.

2.2  Mechanisms not Updating DNSSEC-bis

2.2.1  Partial Type-code and Signal Rollover

   Carefully crafted type code/signal rollover to define a new
   authenticated denial space that extends/replaces DNSSEC-bis
   authenticated denial space.  This particular path is illuminated by
   Paul Vixie in a Message-Id <20040602070859.0F50913951@sa.vix.com>
   posted to <namedroppers@ops.ietf.org> 2004-06-02.  Coexistence and Migration

   To protect the current resolver for future versions, a new DNSSEC-OK
   bit must be allocated to make clear it does or does not understand
   the future version.  Also, a new DS type needs to be allocated to
   allow differentiation between a current signed delegation and a
   'future' signed delegation.  Also, current NSEC needs to be rolled
   into a new authenticated denial type.  Limitations

   None.  Amendments to DNSSEC-bis

   None.  Cons

   It is cumbersome to carefully craft an TCR that 'just fits'.  The
   DNSSEC-bis protocol has many 'borderline' cases that needs special
   consideration.  It might be easier to do a full TCR, since a few of
   the types and signals need upgrading anyway.  Pros

   Graceful adoption of future versions of NSEC, while there are no
   amendments to DNSSEC-bis.

2.2.2  A Complete Type-code and Signal Rollover

   A new DNSSEC space is defined which can exist independent of current
   DNSSEC-bis space.

   This proposal assumes that all current DNSSEC type-codes (RRSIG/
   (RRSIG/DNSKEY/NSEC/DS) and signals (DNSSEC-OK) are not used in any
   future versions of DNSSEC.  Any future version of DNSSEC has its own
   types to allow for keys, signatures, authenticated denial, etcetera.  Coexistence and Migration

   Both spaces can co-exist.  They can be made completely orthogonal.  Limitations

   None.  Amendments to DNSSEC-bis

   None.  Cons

   With this path we abandon the current DNSSEC-bis.  Though it is easy
   to role specific well-known and well-tested parts into the re-write,
   once deployment has started this path is very expensive for
   implementers, registries, registrars and registrants as well as
   resolvers/users.  A TCR is not to be expected to occur frequently, so
   while a next generation authenticated denial may be enabled by a TCR,
   it is likely that that TCR will only be agreed upon if it serves a
   whole basket of changes or additions.  A quick introduction of
   NSEC-ng should not be expected from this path.  Pros

   No amendments/changes to current DNSSEC-bis docset needed.  It is
   always there as last resort.

2.2.3  Unknown Algorithm in RRSIG

   This proposal assumes that future extensions make use of the existing
   NSEC RDATA syntax, while it may need to change the interpretation of
   the RDATA or introduce an alternative denial mechanism, invoked by
   the specific unknown signing algorithm.  The different interpretation
   would be signaled by use of different signature algorithms in the
   RRSIG records covering the NSEC RRs.

   When an entire zone is signed with a single unknown algorithm, it
   will cause implementations that follow current dnssec-bis documents
   to treat individual RRsets as unsigned.  Coexistence and migration

   Old and new NSEC RDATA interpretation or known and unknown Signatures
   can NOT coexist in a zone since signatures cover complete (NSEC)
   RRSets.  Limitations

   Validating resolvers agnostic of new interpretation will treat the
   NSEC RRset as "not signed".  This affects wildcard and non-existence
   proof, as well as proof for (un)secured delegations.  Also, all
   positive signatures (RRSIGs on RRSets other than DS, NSEC) appear
   insecure/bogus to an old validator.

   The algorithm version space is split for each future version of
   DNSSEC.  Violation of the 'modular components' concept.  We use the
   'validator' to protect the 'resolver' from unknown interpretations.  Amendments to DNSSEC-bis

   None.  Cons

   The algorithm field was not designed for this purpose.  This is a
   straightforward hack.  Pros

   No amendments/changes to current DNSSEC-bis docset needed.

3.  Recommendation

   The authors recommend that the working group commits to and starts
   work on a partial TCR, allowing gracefull transition towards a future
   version of NSEC.  Meanwhile, to accomodate the need for an
   immediately, temporary, solution against zone-traversal, we recommend
   On-Demand NSEC synthesis.

   This approach does not require any mandatory changes to DNSSEC-bis,
   does not violate the protocol and fulfills the requirements.  As a
   side effect, it moves the cost of implementation and deployment to
   the users (zone owners) of this mechanism.

Authors' Addresses

   Roy Arends
   Telematica Instituut
   Drienerlolaan 5
   Enschede  7522 NB

   Phone: +31 534850485
   EMail: roy.arends@telin.nl

   Peter Koch
   Universitaet Bielefeld
   Bielefeld  33594

   Phone: +49 521 106 2902
   EMail: pk@TechFak.Uni-Bielefeld.DE

   Jakob Schlyter
   Box 5774
   Stockholm  SE-114 87

   EMail: jakob@nic.se
   URI:   http://www.nic.se/

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