Network Working Group                                          S. Weiler
Internet-Draft                                              SPARTA, Inc Inc.
Updates: 4034, 4035                                           R. Austein
(if approved)                                                        ISC
Expires: September 5, 2007                                 March 4, May 22, 2008                                  November 19, 2007

         Clarifications and Implementation Notes for DNSSECbis

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document is a collection of minor technical clarifications to
   the DNSSECbis document set.  It is meant to serve as a resource to
   implementors as well as an interim repository of DNSSECbis errata.

Table of Contents

   1.  Introduction and Terminology . . . . . . . . . . . . . . . . .  3
     1.1.  Structure of this Document . . . . . . . . . . . . . . . .  3
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Significant Concerns . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Clarifications on Non-Existence Proofs . . . . . . . . . .  3
     2.2.  Validating Responses to an ANY Query . . . . . . . . . . .  4
     2.3.  Check for CNAMEa CNAME  . . . . . . . . . . . . . . . . . . . . .  4
     2.4.  Unsecure Delegation Proofs . . . . . . . . . . . . . . . .  4
     2.5.  Errors in Canonical Form Type Code List  . . . . . . . . .  4
   3.  Interoperability Concerns  . . . . . . . . . . . . . . . . . .  4  5
     3.1.  Unknown DS Message Digest Algorithms . . . . . . . . . . .  4  5
     3.2.  Private Algorithms . . . . . . . . . . . . . . . . . . . .  5
     3.3.  Caution About Local Policy and Multiple RRSIGs . . . . . .  5  6
     3.4.  Key Tag Calculation  . . . . . . . . . . . . . . . . . . .  6
     3.5.  Setting the DO Bit on Replies  . . . . . . . . . . . . . .  6
   4.  Minor Corrections and Clarifications . . . . . . . . . . . . .  6  7
     4.1.  Finding Zone Cuts  . . . . . . . . . . . . . . . . . . . .  6  7
     4.2.  Clarifications on DNSKEY Usage . . . . . . . . . . . . . .  6  7
     4.3.  Errors in Examples . . . . . . . . . . . . . . . . . . . .  7
     4.4.  Errors in Canonical Form Type Code List  . . . . . . . . .  7
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     7.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
     7.2.  Informative References . . . . . . . . . . . . . . . . . .  8  9
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  9
   Intellectual Property and Copyright Statements . . . . . . . . . . 11

1.  Introduction and Terminology

   This document lists some minor clarifications and corrections to
   DNSSECbis, as described in [RFC4033], [RFC4034], and [RFC4035].

   It is intended to serve as a resource for implementors and as a
   repository of items that need to be addressed when advancing the
   DNSSECbis documents from Proposed Standard to Draft Standard.

   Proposed substantive additions to this document should be sent to the
   namedroppers mailing list as well as to the editors of this document.
   The editors would greatly prefer contributions of text suitable for
   direct inclusion in this document.

1.1.  Structure of this Document

   The clarifications to DNSSECbis are sorted according to the editors'
   impression of their importance, starting with ones which could, if
   ignored, lead to security and stability problems and progressing down
   to clarifications that are likely to have little operational impact.

1.2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Significant Concerns

   This section provides clarifications that, if overlooked, could lead
   to security issues or major interoperability problems.

2.1.  Clarifications on Non-Existence Proofs

   [RFC4035] Section 5.4 slightly underspecifies the algorithm for
   checking non-existence proofs.  In particular, the algorithm there
   might incorrectly allow the NSEC from an ancestor zone to prove the
   non-existence of either other RRs at that name in the child zone or other
   names in the child zone.  It might also allow a NSEC at the same name
   as a DNAME to prove the non-existence of names beneath that DNAME.

   An ancestor delegation NSEC (one with the NS bit set, but no SOA bit
   set, and with a singer signer field that's shorter than the owner name) must
   not MUST
   NOT be used to assume non-existence of any RRs below that zone cut
   (both RRs at that ownername and at ownernames with more leading
   labels, no matter their content).  Similarly, an NSEC with the DNAME
   bit set must not be used to assume the non-existence of any
   descendant subdomain
   of that NSEC's owner name.

2.2.  Validating Responses to an ANY Query

   [RFC4035] does not address how to validate responses when QTYPE=*.
   As described in Section 6.2.2 of [RFC1034], a proper response to
   QTYPE=* may include a subset of the RRsets at a given name -- it is
   not necessary to include all RRsets at the QNAME in the response.

   When validating a response to QTYPE=*, validate all received RRsets
   that match QNAME and QCLASS.  If any of those RRsets fail validation,
   treat the answer as Bogus.  If there are no RRsets matching QNAME and
   QCLASS, validate that fact using the rules in [RFC4035] Section 5.4
   (as clarified in this document).  To be clear, a validator must not
   insist on receiving
   expect to receive all records at the QNAME in response to QTYPE=*.

2.3.  Check for CNAMEa CNAME

   Section 5 of [RFC4035] says little about validating responses based
   on (or that should be based on) CNAMEs.  When validating a NODATA
   response, it's important to check the CNAME bit in the NSEC bitmap.
   If the CNAME bit is set, the validator MUST validate the CNAME RR and
   follow it, as appropriate.

2.4.  Unsecure Delegation Proofs

   [RFC4035] Section 5.2 specifies that a validator, when proving a
   delegation is unsecure, needs to check for the absence of the DS and
   SOA bits in the NSEC type bitmap.  The validator also needs to check
   for the presence of the NS bit in the NSEC RR (proving that there is,
   indeed, a delegation).  If this is not checked, spoofed unsigned
   delegations might be used to claim that an existing signed record is
   not signed.

2.5.  Errors in Canonical Form Type Code List

   When canonicalizing DNS names, DNS names in the RDATA section of NSEC
   and RRSIG resource records are not downcased.

   [RFC4034] Section 6.2 item 3 has a list of resource record types for
   which DNS names in the RDATA are downcased for purposes of DNSSEC
   canonical form (for both ordering and signing).  That list
   erroneously contains NSEC and RRSIG.  According to [RFC3755], DNS
   names in the RDATA of NSEC and RRSIG should not be downcased.

   The same section also lists HINFO twice.  The implementor is
   encouraged to exercise good discretion and professional judgment when
   deciding whether to downcase such DNS names once or twice.  [RFC3597]
   contained the same error and, since it predated RFC3755, it doesn't
   mention RRSIG or NSEC.

3.  Interoperability Concerns

3.1.  Unknown DS Message Digest Algorithms

   Section 5.2 of [RFC4035] includes rules for how to handle delegations
   to zones that are signed with entirely unsupported algorithms, as
   indicated by the algorithms shown in those zone's DS RRsets.  It does
   not explicitly address how to handle DS records that use unsupported
   message digest algorithms.  In brief, DS records using unknown or
   unsupported message digest algorithms MUST be treated the same way as
   DS records referring to DNSKEY RRs of unknown or unsupported

   The existing text says:

      If the validator does not support any of the algorithms listed in
      an authenticated DS RRset, then the resolver has no supported
      authentication path leading from the parent to the child.  The
      resolver should treat this case as it would the case of an
      authenticated NSEC RRset proving that no DS RRset exists, as
      described above.

   To paraphrase the above, when determining the security status of a
   zone, a validator discards (for this purpose only) any DS records
   listing unknown or unsupported algorithms.  If none are left, the
   zone is treated as if it were unsigned.

   Modified to consider DS message digest algorithms, a validator also
   discards any DS records using unknown or unsupported message digest

3.2.  Private Algorithms

   As discussed above, section 5.2 of [RFC4035] requires that validators
   make decisions about the security status of zones based on the public
   key algorithms shown in the DS records for those zones.  In the case
   of private algorithms, as described in [RFC4034] Appendix A.1.1, the
   eight-bit algorithm field in the DS RR is not conclusive about what
   algorithm(s) is actually in use.

   If no private algorithms appear in the DS set or if any supported
   algorithm appears in the DS set, no special processing will be
   needed.  In the remaining cases, the security status of the zone
   depends on whether or not the resolver supports any of the private
   algorithms in use (provided that these DS records use supported hash
   functions, as discussed in Section 3.1).  In these cases, the
   resolver MUST retrieve the corresponding DNSKEY for each private
   algorithm DS record and examine the public key field to determine the
   algorithm in use.  The security-aware resolver MUST ensure that the
   hash of the DNSKEY RR's owner name and RDATA matches the digest in
   the DS RR.  If they do not match, and no other DS establishes that
   the zone is secure, the referral should be considered BAD data, as
   discussed in [RFC4035].

   This clarification facilitates the broader use of private algorithms,
   as suggested by [I-D.ietf-dnsext-dnssec-experiments]. [RFC4955].

3.3.  Caution About Local Policy and Multiple RRSIGs

   When multiple RRSIGs cover a given RRset, [RFC4035] Section 5.3.3
   suggests that "the local resolver security policy determines whether
   the resolver also has to test these RRSIG RRs and how to resolve
   conflicts if these RRSIG RRs lead to differing results."  In most
   cases, a resolver would be well advised to accept any valid RRSIG as
   sufficient.  If the first RRSIG tested fails validation, a resolver
   would be well advised to try others, giving a successful validation
   result if any can be validated and giving a failure only if all
   RRSIGs fail validation.

   If a resolver adopts a more restrictive policy, there's a danger that
   properly-signed data might unnecessarily fail validation, perhaps
   because of cache timing issues.  Furthermore, certain zone management
   techniques, like the Double Signature Zone-signing Key Rollover
   method described in section of [RFC4641] might not work

3.4.  Key Tag Calculation

   [RFC4034] Appendix B.1 incorrectly defines the Key Tag field
   calculation for algorithm 1.  It correctly says that the Key Tag is
   the most significant 16 of the least significant 24 bits of the
   public key modulus.  However, [RFC4034] then goes on to incorrectly
   say that this is 4th to last and 3rd to last octets of the public key
   modulus.  It is, in fact, the 3rd to last and 2nd to last octets.

3.5.  Setting the DO Bit on Replies

   [RFC4035] does not provide any instructions to servers as to how to
   set the DO bit.  Some authoritative server implementations have
   chosen to copy the DO bit settings from the incoming query to the
   outgoing response.  Others have chosen to never set the DO bit in
   responses.  Either behavior is permited.  To be clear, in replies to
   queries with the DO-bit set servers may or may not set the DO bit.

4.  Minor Corrections and Clarifications

4.1.  Finding Zone Cuts

   Appendix C.8 of [RFC4035] discusses sending DS queries to the servers
   for a parent zone.  To do that, a resolver may first need to apply
   special rules to discover what those servers are.

   As explained in Section of [RFC4035], security-aware name
   servers need to apply special processing rules to handle the DS RR,
   and in some situations the resolver may also need to apply special
   rules to locate the name servers for the parent zone if the resolver
   does not already have the parent's NS RRset.  Section 4.2 of
   [RFC4035] specifies a mechanism for doing that.

4.2.  Clarifications on DNSKEY Usage

   Questions of the form "can I use a different DNSKEY for signing this
   RRset" have occasionally arisen.

   The short answer is "yes, absolutely".  You can even use a different
   DNSKEY for each RRset in a zone, subject only to practical limits on
   the size of the DNSKEY RRset.  However, be aware that there is no way
   to tell resolvers what a particularly DNSKEY is supposed to be used
   for -- any DNSKEY in the zone's signed DNSKEY RRset may be used to
   authenticate any RRset in the zone.  For example, if a weaker or less
   trusted DNSKEY is being used to authenticate NSEC RRsets or all
   dynamically updated records, that same DNSKEY can also be used to
   sign any other RRsets from the zone.

   Furthermore, note that the SEP bit setting has no effect on how a
   DNSKEY may be used -- the validation process is specifically
   prohibited from using that bit by [RFC4034] section 2.1.2.  It
   possible to use a DNSKEY without the SEP bit set as the sole secure
   entry point to the zone, yet use a DNSKEY with the SEP bit set to
   sign all RRsets in the zone (other than the DNSKEY RRset).  It's also
   possible to use a single DNSKEY, with or without the SEP bit set, to
   sign the entire zone, including the DNSKEY RRset itself.

4.3.  Errors in Examples

   The text in [RFC4035] Section C.1 refers to the examples in B.1 as
   "" while B.1 uses "x.w.example".  This is painfully
   obvious in the second paragraph where it states that the RRSIG labels
   field value of 3 indicates that the answer was not the result of
   wildcard expansion.  This is true for "x.w.example" but not for
   "", which of course has a label count of 4
   (antithetically, a label count of 3 would imply the answer was the
   result of a wildcard expansion).

   The first paragraph of [RFC4035] Section C.6 also has a minor error:
   the reference to "a.z.w.w.example" should instead be "a.z.w.example",
   as in the previous line.

4.4.  Errors in Canonical Form Type Code List

   [RFC4034] Section 6.2 item 3 has a list of resource record types for
   which DNS names in the RDATA are downcased for purposes of DNSSEC
   canonical form (for both ordering and signing).  That list contains
   HINFO twice.  The implementor is encouraged to exercise good
   discretion and professional judgement when deciding whether to
   downcase such DNS names once or twice.

   Further, it is worth noting that the list of RR types in [RFC3597]
   has the same bug and, as could be expected, doesn't include RRSIG or

5.  IANA Considerations

   This document specifies no IANA Actions.

6.  Security Considerations

   This document does not make fundamental changes to the DNSSEC
   protocol, as it was generally understood when DNSSECbis was
   published.  It does, however, address some ambiguities and omissions
   in those documents that, if not recognized and addressed in
   implementations, could lead to security failures.  In particular, the
   validation algorithm clarifications in Section 2 are critical for
   preserving the security properties DNSSEC offers.  Furthermore,
   failure to address some of the interoperability concerns in Section 3
   could limit the ability to later change or expand DNSSEC, including
   by adding new algorithms.

7.  References

7.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.

              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

7.2.  Informative References

              Blacka, D., "DNSSEC Experiments",
              draft-ietf-dnsext-dnssec-experiments-03 (work in
              progress), April 2006.

              Laurie, B., "DNSSEC Hashed Authenticated Denial of
              Existence", draft-ietf-dnsext-nsec3-09 (work in progress),
              January 2007.

   [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
              (RR) Types", RFC 3597, September 2003.


   [RFC3755]  Weiler, S. and J. Ihren, "Minimally Covering NSEC Records
              and DNSSEC On-line Signing", S., "Legacy Resolver Compatibility for Delegation
              Signer (DS)", RFC 4470, April 2006. 3755, May 2004.

   [RFC4641]  Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
              RFC 4641, September 2006.

   [RFC4955]  Blacka, D., "DNS Security (DNSSEC) Experiments", RFC 4955,
              July 2007.

Appendix A.  Acknowledgments

   The editors are extremely grateful to those who, in addition to
   finding errors and omissions in the DNSSECbis document set, have
   provided text suitable for inclusion in this document.

   The lack of specificity about handling private algorithms, as
   described in Section 3.2, and the lack of specificity in handling ANY
   queries, as described in Section 2.2, were discovered by David

   The error in algorithm 1 key tag calculation, as described in
   Section 3.4, was found by Abhijit Hayatnagarkar.  Donald Eastlake
   contributed text for Section 3.4.

   The bug relating to delegation NSEC RR's in Section 2.1 was found by
   Roy Badami.  Roy Arends found the related problem with DNAME.

   The errors in the [RFC4035] examples were found by Roy Arends, who
   also contributed text for Section 4.3 of this document.

   The editors would like to thank Ed Lewis, Danny Mayer, Olafur
   Gudmundsson, Suzanne Woolf, and Scott Rose for their substantive
   comments on the text of this document.

Authors' Addresses

   Samuel Weiler
   SPARTA, Inc Inc.
   7110 Samuel Morse Drive
   Columbia, Maryland  21046


   Rob Austein
   950 Charter Street
   Redwood City, CA  94063


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