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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 12, 2005
Expires: November 13, 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

Proposed additions in future versions

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   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 personal submission and first WG draft

   Added Section 6 based on namedroppers discussions from March 9-10,

   Added Section 7 through Section 10.

   Added the DNSSECbis RFC numbers.

   Figured out the confusion in Section 4.

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Table of Contents

   1.   Introduction and Terminology . . . . . . . . . . . . . . . .   4
   2.   Unknown DS Message Digest Algorithms . . . . . . . . . . . .   4
   3.   Private Algorithms . . . . . . . . . . . . . . . . . . . . .   5
   4.   Finding Zone Cuts  . . . . . . . . . . . . . . . . . . . . .   5
   5.   Clarifications on DNSKEY Usage . . . . . . . . . . . . . . .   5
   6.   Clarifications on Non-Existence Proofs . . . . . . . . . . .   6
   7.   Key Tag Calculation  . . . . . . . . . . . . . . . . . . . .   6
   8.   Caution About Local Policy and Multiple RRSIGs . . . . . . .   6
   9.   Minor Errors in Examples . . . . . . . . . . . . . . . . . .   7
   10.  Empty Non-Terminal Proofs  . . . . . . . . . . . . . . . . .   7
   11.  IANA Considerations  . . . . . . . . . . . . . . . . . . . .   7
   12.  Security Considerations  . . . . . . . . . . . . . . . . . .   7
   13.  References . . . . . . . . . . . . . . . . . . . . . . . . .   7
     13.1   Normative References . . . . . . . . . . . . . . . . . .   7
     13.2   Informative References . . . . . . . . . . . . . . . . .   8
        Author's Address . . . . . . . . . . . . . . . . . . . . . .   8
   A.   Acknowledgments  . . . . . . . . . . . . . . . . . . . . . .   8
        Intellectual Property and Copyright Statements . . . . . . .  10

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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 (-00 of the WG document), feedback is particularly
   solicited on the structure of the document and whether the text in
   the newly added sections (Section 6 through Section 10) is correct
   and sufficient.

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

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

2.  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 resolver 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.

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

3.  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 2).  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

   This clarification facilitates the broader use of private algorithms,
   as suggested by [5] .

4.  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.

5.  Clarifications on DNSKEY Usage

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

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   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.

6.  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.

   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).

7.  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.

8.  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

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   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 [6] might not work reliably.

9.  Minor 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.

10.  Empty Non-Terminal Proofs

   To be written.

11.  IANA Considerations

   This document specifies no IANA Actions.

12.  Security Considerations

13.  References

13.1  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,

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        March 2005.

   [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.

13.2  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, was discovered by David Blacka.

   The error in algorithm 1 key tag calculation, as described in
   Section 7, was found by Abhijit Hayatnagarkar.  Donald Eastlake
   contributed text for Section 7.

   The bug in the non-existence proof logic in RFC4035 Section 5.4 was
   found by Roy Badami.

   The errors in the RFC4035 examples were found by Roy Arends, who also
   contributed text for Section 9 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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