INTERNET-DRAFT                                             Samuel Weiler
Expires: November 2003                                      May 12, 2003


         Legacy Resolver Compatibility for Delegation Signer
              draft-weiler-dnsext-dnssec-2535-compat-01

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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Abstract

   As the DNS Security (DNSSEC) specifications have evolved, the
   syntax and semantics of the DNSSEC resource records (RRs) have
   changed.  Many deployed nameservers understand variants of these
   semantics.  Dangerous interactions can occur when a resolver that
   understands an earlier version of these semantics queries an
   authoritative server that understands the new delegation signer
   semantics, including at least one failure scenario that will cause
   an unsecured zone to be unresolvable.  This document proposes that
   these interactions be avoided by changing the type codes and
   mnemonics of the DNSSEC RRs (SIG, KEY, and NXT).

. Introduction

   The DNSSEC protocol has been through many iterations whose syntax
   and semantics are not completely compatible.  This has occurred as
   part of the ordinary process of proposing a protocol, implementing
   it, testing it in the increasingly complex and diverse environment
   of the Internet, and refining the definitions of the initial
   Proposed Standard.  In the case of DNSSEC, the process has been
   complicated by DNS's criticality and wide deployment and the need
   to add security while minimizing daily operational complexity.

   A weak area for previous DNS specifications has been lack of detail
   in specifying resolver behavior, leaving implementors largely on
   their own to determine many details of resolver function.  This,
   combined with the number of iterations the DNSSEC spec has been
   through, has resulted in fielded code with a wide variety of
   behaviors.  This variety makes it difficult to predict how a
   protocol change will be handled by all deployed resolvers.  The
   risk that a change will cause unacceptable or even catastrophic
   failures makes it difficult to design and deploy a protocol change.
   One strategy for managing that risk is to structure protocol
   changes so that existing resolvers can completely ignore input that
   might confuse them or trigger undesirable failure modes.

   This document addresses a specific problem caused by Delegation
   Signer's [DS] introduction of new semantics for the NXT RR that are
   incompatible with the semantics in [RFC2535].  Answers provided by
   DS-aware servers can trigger an unacceptable failure mode in some
   resolvers that implement RFC 2535, which provides a great
   disincentive to sign zones with DS.  The proposed solution allows
   for the incremental deployment of DS.

1.1 The Problem

   Delegation signer [DS] introduces new semantics for the NXT RR that
   are incompatible with the semantics in [RFC2535].  In [RFC2535],
   NXT records were only required to be returned as part of a
   non-existence proof.  In [DS], an unsecure referral returns, in
   addition to the NS, a proof of non-existence of a DS RR in the form
   of an NXT and SIG(NXT).  RFC 2535 didn't specify how a resolver was
   to interpret a response with both an NS and an NXT in the authority
   section and with NOERROR or NODATA set.  Some widely deployed
   2535-aware resolvers interpret any answer with an NXT as a proof of
   non-existence of the requested record.  This results in unsecure
   delegations being invisible to 2535-aware resolvers and violates
   the basic architectural principle that DNSSEC must do no harm --
   the signing of zones must not prevent the resolution of unsecured
   names.

. Possible Solutions

   This section presents several possible solutions.  Section 3
   recommends one and describes it in more detail.

2.1. Change SIG, KEY, and NXT

   To avoid the problem described above, legacy (RFC2535-aware)
   resolvers need to be kept from seeing unsecure referrals that
   include NXT records in the authority section.  The simplest way to
   do that is to change the type codes for SIG, KEY, and NXT.

   The obvious drawback to this is that new resolvers will not be able
   to validate zones signed with the old RRs.  This problem already
   exists, however, because of the changes made by DS, and resolvers
   that understand the old RRs (and have compatibility issues with DS)
   are far more prevalent than 2535-signed zones.

2.2. Change a subset of type codes

   The observed problem with unsecure referrals could be addressed by
   changing only the NXT type code or another subset of the type codes
   that includes NXT.  This has the virtue of apparent simplicity, but
   it risks introducing new problems or not going far enough.  It's
   quite possible that more incompatibilities exist between DS and
   earlier semantics.  Legacy resolvers may also be confused by seeing
   records they recognize (SIG and KEY) while being unable to find
   NXTs.  Although it may seem unnecessary to fix that which is not
   obviously broken, it's far cleaner to change all of the type codes
   at once.  This will leave legacy resolvers and tools completely
   blinded to DNSSEC -- they will see only unknown RRs.

2.3. Replace the DO bit

   Another way to keep legacy resolvers from ever seeing DNSSEC
   records with DS semantics is to have authoritative servers only
   send that data to DS-aware resolvers.  It's been proposed that
   assigning a new EDNS0 flag bit to signal DS-awareness (tentatively
   called "DA"), and having authoritative servers send DNSSEC data
   only in response to queries with the DA bit set, would accomplish
   this.  This bit would presumably supplant the DO bit described in
   [RFC3225].

   This solution is sufficient only if all 2535-aware resolvers zero
   out EDNS0 flags that they don't understand.  If one passed through
   the DA bit unchanged, it would still see the new semantics, and it
   would probably fail to see unsecure delegations.  Since it's
   impractical to know how every DNS implementation handles unknown
   EDNS0 flags, this is not a universal solution.  It could, though,
   be considered in addition to changing the RR type codes.

2.4. Increment the EDNS version

   Another proposed solution is to increment the EDNS version number
   as defined in [RFC2671], on the assumption that all existing
   implementations will reject higher versions than they support,
   and retain the DO bit as the signal for DNSSEC awareness.  This
   approach has not been tested.

2.5. Do nothing

   There is a large deployed base of DNS resolvers that understand
   DNSSEC as defined by the standards track [RFC2535] and [RFC2065]
   and, due to underspecification in those documents, interpret any
   answer with an NXT as a non-existence proof.  So long as that is
   the case, zone owners will have a strong incentive to not sign any
   zones that contain unsecure delegations, lest those delegations be
   invisible to such a large installed base.  This will dramatically
   slow DNSSEC adoption.

   Unfortunately, without signed zones there's no clear incentive for
   operators of resolvers to upgrade their software to support the new
   version of DNSSEC, as defined in [DS].  Historical data suggests
   that resolvers are rarely upgraded, and that old nameserver code
   never dies.

   Rather than wait years for resolvers to be upgraded through natural
   processes before signing zones with unsecure delegations,
   addressing this problem with a protocol change will immediately
   remove the disincentive for signing zones and allow widespread
   deployment of DNSSEC.

. Protocol changes

   This document proposes changing the type codes of SIG, KEY, and
   NXT.  This solution is the cleanest and safest, largely because the
   behavior of resolvers that receive unknown type codes is well
   understood.  This approach has also received the most testing.

   To avoid operational confusion, it's also necessary to change the
   mnemonics for these RRs.  DNSKEY will be the replacement for KEY,
   with the mnemonic indicating that these keys are not for
   application use, per [RFC3445].  RRSIG (Resource Record SIGnature)
   will replace SIG, and NSEC (Next SECure) will replace NXT.

   The new types will have exactly the same syntax and semantics as
   specified for SIG, KEY, and NXT in [RFC2535] and [DS], and they
   completely replace the old types.  A resolver, if it receives the
   old types, SHOULD treat them as unknown RRs, and SHOULD NOT assign
   any special semantic value to them.  It MUST NOT use them for
   DNSSEC validations or other DNS operational decision making.  For
   example, a resolver MUST NOT use DNSKEYs to validate SIGs or use
   KEYs to validate RRSIGs.  Authoritative servers SHOULD NOT serve
   SIG, KEY, or NXT records.  If those records are included, they MUST
   NOT receive special treatment.  As an example, if a SIG is included
   in a signed zone, there MUST be an RRSIG for it.

   As a clarification to previous documents, NSEC can appear in the
   authority section of an answer at any time, not just in negative
   answers.  The mere presence of an NSEC record in the answer or
   authority section of an answer with RCODE=NOERROR, MUST NOT be
   interpreted as a negative answer.  The NSEC must be validated.

. IANA Considerations

   This document updates the IANA registry for DNS Resource Record
   Types by assigning types 46, 47, and 48 to the DNSKEY, RRSIG, and
   NSEC RRs, respectively.

   Types 24, 25, and 30 (SIG, KEY, and NXT) should be marked as
   Obsolete.

. Security Considerations

   The change proposed here does not materially effect security.  The
   implications of trying to use both new and legacy types together
   are not well understood, and attempts to do so would probably lead
   to unintended and dangerous results.

   Changing type codes will leave code paths in legacy resolvers that
   are never exercised.  Unexercised code paths are a frequent source
   of security holes, largely because those code paths do not get
   frequent scrutiny.

   Doing nothing, as described in 3.1, will slow DNSSEC deployment.
   While this does not decrease security, it also fails to increase
   it.

. Normative references

   [RFC2065] Eastlake, D. and C. Kaufman, "Domain Name System Security
             Extensions", RFC 2065, January 1997.

   [RFC2535] Eastlake, D., "Domain Name System Security Extensions",
             RFC 2535, March 1999.

   [DS]      Gudmundsson, O., "Delegation Signer Resource Record",
             draft-ietf-dnsext-delegation-signer-14.txt, work in
             progress, May 2003.

. Informative References

   [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
             2671, August 1999.

   [RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC", RFC
             3225, December 2001.

   [RFC2929] Eastlake, D., E. Brunner-Williams, and B. Manning.
             Domain Name System (DNS) IANA Considerations. BCP 42,
             RFC 2929, September 2000.

   [RFC3445] Massey, D., and S. Rose.  Limiting the Scope of the KEY
             Resource Record (RR).  RFC 3445, December 2002.

. Acknowledgments

   The proposed solution and the analysis of alternatives had many
   contributors.  With apologies to anyone overlooked, those include:
   Micheal Graff, John Ihren, Olaf Kolkman, Mark Kosters, Ed Lewis,
   Bill Manning, and Suzanne Woolf.

   Thanks to Jakob Schlyter and Mark Andrews for identifying the
   incompatibility described in section 1.1.

   In addition to the above, the author would like to thank Scott
   Rose, Olafur Gudmundsson, and Sandra Murphy for their substantive
   comments.

. Author's Address

   Samuel Weiler
   Network Associates Laboratories
   15204 Omega Dr., Suite 300
   Rockville, MD  20850
   USA
   weiler@tislabs.com