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Versions: (draft-weiler-dnsext-dnssec-bis-updates) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 RFC 6840

Network Working Group                                          S. Weiler
Internet-Draft                                               SPARTA, Inc
Updates: 4034, 4035 (if approved)                           May 23, 2005
Expires: November 24, 2005

         Clarifications and Implementation Notes for DNSSECbis

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   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 possible DNSSECbis

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Proposed additions in future versions

   An index sorted by the section of DNSSECbis being clarified.

   A list of proposed protocol changes being made in other documents,
   such as NSEC3 and Epsilon.  This document would not make those
   changes, merely provide an index into the documents that are making

Changes between -00 and -01

   Document significantly restructured.

   Added section on QTYPE=ANY.

Changes between personal submission and first WG draft

   Added Section 2.1 based on namedroppers discussions from March 9-10,

   Added Section 3.4, Section 3.3, Section 4.3, and Section 2.2.

   Added the DNSSECbis RFC numbers.

   Figured out the confusion in Section 4.1.

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Table of Contents

   1.  Introduction and Terminology . . . . . . . . . . . . . . . . .  4
     1.1   Structure of this Document . . . . . . . . . . . . . . . .  4
     1.2   Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Significant Concerns . . . . . . . . . . . . . . . . . . . . .  4
     2.1   Clarifications on Non-Existence Proofs . . . . . . . . . .  4
     2.2   Empty Non-Terminal Proofs  . . . . . . . . . . . . . . . .  5
     2.3   Validating Responses to an ANY Query . . . . . . . . . . .  5
   3.  Interoperability Concerns  . . . . . . . . . . . . . . . . . .  5
     3.1   Unknown DS Message Digest Algorithms . . . . . . . . . . .  5
     3.2   Private Algorithms . . . . . . . . . . . . . . . . . . . .  6
     3.3   Caution About Local Policy and Multiple RRSIGs . . . . . .  6
     3.4   Key Tag Calculation  . . . . . . . . . . . . . . . . . . .  7
   4.  Minor Corrections and Clarifications . . . . . . . . . . . . .  7
     4.1   Finding Zone Cuts  . . . . . . . . . . . . . . . . . . . .  7
     4.2   Clarifications on DNSKEY Usage . . . . . . . . . . . . . .  7
     4.3   Errors in Examples . . . . . . . . . . . . . . . . . . . .  8
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     7.1   Normative References . . . . . . . . . . . . . . . . . . .  8
     7.2   Informative References . . . . . . . . . . . . . . . . . .  9
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  9
   A.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
       Intellectual Property and Copyright Statements . . . . . . . . 11

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1.  Introduction and Terminology

   This document lists some minor clarifications and corrections to
   DNSSECbis, as described in [1], [2], and [3].

   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.

   In this version (-01 of the WG document), feedback is particularly
   solicited on the structure of the document and whether the text in
   the recently added sections is correct and sufficient.

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

1.1  Structure of this Document

   The clarifications to DNSSECbis are sorted according to the editor's
   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.
   Mere typos and awkward phrasings are not addressed unless they could
   lead to misinterpretation of the DNSSECbis documents.

1.2  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [4].

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 the parent side of a zone cut
   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.

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   A parent-side delegation NSEC (one with the NS bit set, but no SOA
   bit set, and with a singer field that's shorter than the owner name)
   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 of that NSEC's owner name.

2.2  Empty Non-Terminal Proofs

   To be written, based on Roy Arends' May 11th message to namedroppers.

2.3  Validating Responses to an ANY Query

   RFC4035 does not address now 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 all records at the QNAME in response to QTYPE=*.

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.

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   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 [5].

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

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   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 [6] might not work reliably.

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.

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 the
   X" 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

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   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
   "x.w.example.com" 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
   "x.w.example.com", 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.

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

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

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

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   [3]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
        "Protocol Modifications for the DNS Security Extensions",
        RFC 4035, March 2005.

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

7.2  Informative References

   [5]  Blacka, D., "DNSSEC Experiments",
        draft-blacka-dnssec-experiments-00 (work in progress),
        December 2004.

   [6]  Gieben, R. and O. Kolkman, "DNSSEC Operational Practices",
        draft-ietf-dnsop-dnssec-operational-practices-04 (work in
        progress), May 2005.

Author's Address

   Samuel Weiler
   SPARTA, Inc
   7075 Samuel Morse Drive
   Columbia, Maryland  21046

   Email: weiler@tislabs.com

Appendix A.  Acknowledgments

   The editor is 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 2.3, 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.

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   The editor would like to thank Olafur Gudmundsson and Scott Rose for
   their substantive comments on the text of this document.

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