Network Working Group                                          S. Weiler
Internet-Draft                                              SPARTA, Inc.
Updates: 4033, 4034, 4035, 5155                                D. Blacka
(if approved)                                             VeriSign, Inc.
Intended status: Standards Track                        January 14,                       September 5, 2009
Expires: July 18, 2009 March 9, 2010

         Clarifications and Implementation Notes for DNSSECbis

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

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Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document. document (
   Please review these documents carefully, as they describe your rights
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   This document is a collection of technical clarifications to the
   DNSSECbis document set.  It is meant to serve as a resource to
   implementors as well as a repository of DNSSECbis errata.

Table of Contents

   1.  Introduction and Terminology . . . . . . . . . . . . . . . . .  3
     1.1.  Structure of this Document . . . . . . . . . . . . . . . .  3
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Important Additions to DNSSSECbis  . . . . . . . . . . . . . .  3
     2.1.  NSEC3 Support  . . . . . . . . . . . . . . . . . . . . . .  3
     2.2.  SHA-256 Support  . . . . . . . . . . . . . . . . . . . . .  3
   3.  Significant  Security Concerns  . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Clarifications on Non-Existence Proofs . . . . . . . . . .  4
     3.2.  Validating Responses to an ANY Query . . . . . . . . . . .  4
     3.3.  Check for CNAME  . . . . . . . . . . . . . . . . . . . . .  5
     3.4.  Insecure Delegation Proofs . . . . . . . . . . . . . . . .  5
     3.5.  Errors in Canonical Form Type Code List
   4.  Interoperability Concerns  . . . . . . . . .  5
   4.  Interoperability Concerns . . . . . . . . .  5
     4.1.  Errors in Canonical Form Type Code List  . . . . . . . . .  5
     4.2.  Unknown DS Message Digest Algorithms . . . . . . . . . . .  5
     4.3.  Private Algorithms . . . . . . . . . . . . . . . . . . . .  6
     4.4.  Caution About Local Policy and Multiple RRSIGs . . . . . .  6
     4.4.  7
     4.5.  Key Tag Calculation  . . . . . . . . . . . . . . . . . . .  7
     4.6.  Setting the DO Bit on Replies  . . . . . . . . . . . . . .  7
     4.7.  Setting the AD bit on Replies  . . . . . . . . . . . . . .  7
     4.8.  Setting the CD bit on Requests . . . . . . . . . . . . . .  8
     4.9.  Nested Trust Anchors . . . . . . . . . . . . . . . . . . .  8
   5.  Minor Corrections and Clarifications . . . . . . . . . . . . .  8
     5.1.  Finding Zone Cuts  . . . . . . . . . . . . . . . . . . . .  8
     5.2.  Clarifications on DNSKEY Usage . . . . . . . . . . . . . .  8  9
     5.3.  Errors in Examples . . . . . . . . . . . . . . . . . . . .  9
     5.4.  Errors in RFC 5155 . . . . . . . . . . . . . . . . . . . .  9
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

1.  Introduction and Terminology

   This document lists some additions, clarifications and corrections to DNSSECbis,
   the core DNSSECbis specification, as originally 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.

1.1.  Structure of this Document

   The clarifications to DNSSECbis are sorted according to their
   importance, starting with ones which could, if ignored, lead to
   security and stability problems and progressing down to clarifications that are
   expected 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.  Important Additions to DNSSSECbis

   This section provides updates the set of core DNSSEC protocol documents
   originally specified in Section 10 of [RFC4033].

2.1.  NSEC3 Support

   [RFC5155] describes the use and behavior of the NSEC3 and NSEC3PARAM
   records for hashed denial of existence.  Validator implementations
   are strongly encouraged to include support for NSEC3 as because a number
   of highly visible zones are expected to use it.  Validators that do
   not support validation of responses using NSEC3 will likely be
   hampered in validating large portions of the DNS space.

   [RFC5155] should be considered part of the DNS Security Document
   Family as described by [RFC4033], Section 10.

2.2.  SHA-256 Support

   [RFC4509] describes the use of SHA-256 as a digest algorithm for use
   with Delegation Signer (DS) RRs.  [I-D.ietf-dnsext-dnssec-rsasha256]
   describes the use of the RSASHA256 algorthim algorithm for use in DNSKEY and
   RRSIG RRs.  Validator implementations are strongly encouraged to
   include support for this algorithm for DS, DNSKEY, and RRSIG records.

   Both [RFC4509] and [I-D.ietf-dnsext-dnssec-rsasha256] should also be
   considered part of the DNS Security Document Family as described by
   [RFC4033], Section 10.

3.  Significant  Security Concerns

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

3.1.  Clarifications on Non-Existence Proofs

   [RFC4035] Section 5.4 underspecifies under-specifies the algorithm for checking non-
   existence proofs.  In particular, the algorithm as presented would
   incorrectly allow an NSEC or NSEC3 RR from an ancestor zone to prove
   the non-existence of other RRs at that name in the child zone or
   other names in the child zone.

   An "ancestor delegation" NSEC RR (or NSEC3 RR) is one with:

   o  the NS bit set,
   o  the SOA bit clear, and
   o  a signer field that is shorter than the owner name of the NSEC RR,
      or the original owner name for the NSEC3 RR.

   Ancestor delegation NSEC or NSEC3 RRs MUST NOT be used to assume non-
   existence of any RRs below that zone cut, which include all RRs at
   that (original) owner name other than DS RRs, and all RRs below that
   owner name regardless of type.

   Similarly, the algorithm would also allow an NSEC RR at the same
   owner name as a DNAME RR, or an NSEC3 RR at the same original owner
   name as a DNAME, to prove the non-existence of names beneath that
   DNAME.  An NSEC or NSEC3 RR with the DNAME bit set MUST NOT be used
   to assume the non-existence of any subdomain of that NSEC/NSEC3 RR's
   (original) owner name.

3.2.  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 -- name.  That is,
   it is not necessary to include all RRsets at the QNAME in the

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

3.3.  Check for CNAME

   Section 5 of [RFC4035] says little about validating responses based
   on (or that should be based on) CNAMEs.  When validating a NOERROR/
   NODATA response, validators MUST check the CNAME bit in the matching
   NSEC or NSEC3 RR's type bitmap.  If bitmap in addition to the CNAME bit is set, the
   validator MUST validate for the query
   type.  Without this check, an attacker could successfully transform a
   positive CNAME RR and follow it, as appropriate. response into a NOERROR/NODATA response.

3.4.  Insecure Delegation Proofs

   [RFC4035] Section 5.2 specifies that a validator, when proving a
   delegation is not secure, needs to check for the absence of the DS
   and SOA bits in the NSEC (or NSEC3) type bitmap.  The validator also
   needs to check for the presence of the NS bit in the matching NSEC
   (or NSEC3) RR (proving that there is, indeed, a delegation). delegation), or
   alternately make sure that the delegation is covered by an NSEC3 RR
   with the Opt-Out flag set.  If this is not checked, spoofed unsigned
   delegations might be used to claim that an existing signed record is
   not signed.


4.  Interoperability Concerns

4.1.  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 erroneously lists HINFO, and twice at that.
   Since HINFO records contain no domain names, they are not subject to

4.  Interoperability Concerns


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 public key
   algorithms, as indicated by the key 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 public key algorithms.

   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) disregards 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
   disregards any DS records using unknown or unsupported message digest


4.3.  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 4.1). 4.2).  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 Bogus data, as
   discussed in [RFC4035].

   This clarification facilitates the broader use of private algorithms,
   as suggested by [RFC4955].


4.4.  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


4.5.  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.


4.6.  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 permitted.  To be clear, in replies to
   queries with the DO-bit set servers may or may not set the DO bit.


4.7.  Setting the AD bit on Replies

   Section 3.2.3 of [RFC4035] describes under which conditions a
   validating resolver should set or clear the AD bit in a response.  In
   order to protect legacy stub resolvers and middleboxes, validating
   resolvers SHOULD only set the AD bit when a response both meets the
   conditions listed in RFC 4035, section 3.2.3, and the request
   contained either a set DO bit or a set AD bit.

   Note that the use of the AD bit in the query was previously
   undefined.  This document defines it as a signal indicating that the
   requester understands and is interested in the value of the AD bit in
   the response.  This allows a requestor to indicate that it
   understands the AD bit without also requesting DNSSEC data via the DO


4.8.  Setting the CD bit on Requests

   When processing a request with the CD bit set, the resolver MUST set
   the CD bit on its upstream queries.


4.9.  Nested Trust Anchors

   A DNSSEC validator may be configured such that, for a given response,
   more than one trust anchor could be used to validate the chain of
   trust to the response zone.  For example, imagine a validor validator
   configured with trust anchors for "example." and "zone.example."
   When the validator is asked to validate a response to
   "", either trust anchor could apply.

   When presented with this situation, DNSSEC validators SHOULD try all
   applicable trust anchors until one succeeds.

   There are some scenarios where different behaviors, such as choosing
   the trust anchor closest to the QNAME of the response, may be
   desired.  A DNSSEC validator MAY enable such behaviors as
   configurable overrides.

5.  Minor Corrections and Clarifications

5.1.  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.

5.2.  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 is
   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.

5.3.  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.

5.4.  Errors in RFC 5155

   A NSEC3 record, record that matches an Empty Non-Terminal, Non-Terminal effectively has no
   type associated with it.  This NSEC3 record has an empty type bit
   map.  Section 3.2.1 of [RFC5155] contains the statement:

      Blocks with no types present MUST NOT be included.

   However, the same section contains a regular expression:

      Type Bit Maps Field = ( Window Block # | Bitmap Length | Bitmap )+

   The plus sign in the regular expression indicates that there is one
   or more of the preceding element.  This means that there must be at
   least one window block.  If this window block has no types, it
   contradicts with the first statement.  Therefore, the correct text in
   RFC 5155 3.2.1 should be:

      Type Bit Maps Field = ( Window Block # | Bitmap Length | Bitmap )*

6.  IANA Considerations

   This document specifies no IANA Actions.

7.  Security Considerations

   This document does not make fundamental changes adds two cryptographic features to the core DNSSEC
   protocol, as
   protocol.  Additionally, it was generally understood when DNSSECbis was
   published.  It does, however, address addresses some ambiguities and omissions
   in those the core DNSSEC documents that, if not recognized and addressed in
   implementations, could lead to security failures.  In particular, the
   validation algorithm clarifications in Section 3 are critical for
   preserving the security properties DNSSEC offers.  Furthermore,
   failure to address some of the interoperability concerns in Section 4
   could limit the ability to later change or expand DNSSEC, including
   adding new algorithms.

8.  References

8.1.  Normative References

              Jansen, J., "Use of SHA-2 algorithms with RSA in DNSKEY
              and RRSIG Resource Records for DNSSEC",
              draft-ietf-dnsext-dnssec-rsasha256-14 (work in progress),
              June 2009.

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

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

   [RFC4509]  Hardaker, W., "Use of SHA-256 in DNSSEC Delegation Signer
              (DS) Resource Records (RRs)", RFC 4509, May 2006.

   [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
              Security (DNSSEC) Hashed Authenticated Denial of
              Existence", RFC 5155, March 2008.

8.2.  Informative References

   [RFC3755]  Weiler, S., "Legacy Resolver Compatibility for Delegation
              Signer (DS)", RFC 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 would like the thank Rob Austein for his previous work as
   an editor of this document.

   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 4.2, 4.3, and the lack of specificity in handling ANY
   queries, as described in Section 3.2, were discovered by David

   The error in algorithm 1 key tag calculation, as described in
   Section 4.4, 4.5, was found by Abhijit Hayatnagarkar.  Donald Eastlake
   contributed text for Section 4.4. 4.5.

   The bug relating to delegation NSEC RR's in Section 3.1 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 5.3 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.
   7110 Samuel Morse Drive
   Columbia, Maryland  21046


   David Blacka
   VeriSign, Inc.
   21345 Ridgetop Circle
   Dulles, VA  20166