Network Working Group                                          S. Weiler
Internet-Draft                                              SPARTA, Inc.
Updates: 4033, 4034, 4035, 5155                                D. Blacka
(if approved)                                             VeriSign, Inc.
Intended status: Standards Track                           July 26, 2011
Expires:                        January 27, 13, 2012
Expires: July 16, 2012

         Clarifications and Implementation Notes for DNSSECbis


   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.

   This document updates the core DNSSECbis documents (RFC4033, RFC4034,
   and RFC4035) as well as the NSEC3 specification (RFC5155).  It also
   defines NSEC3 and SHA-2 as core parts of the DNSSECbis specification.

Status of this Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on January 27, July 16, 2012.

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Table of Contents

   1.  Introduction and Terminology . . . . . . . . . . . . . . . . .  3  4
     1.1.  Structure of this Document . . . . . . . . . . . . . . . .  3  4
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3  4
   2.  Important Additions to DNSSSECbis  . . . . . . . . . . . . . .  3  4
     2.1.  NSEC3 Support  . . . . . . . . . . . . . . . . . . . . . .  3  4
     2.2.  SHA-2 Support  . . . . . . . . . . . . . . . . . . . . . .  4  5
   3.  Scaling Concerns . . . . . . . . . . . . . . . . . . . . . . .  4  5
     3.1.  Implement a BAD cache  . . . . . . . . . . . . . . . . . .  4  5
   4.  Security Concerns  . . . . . . . . . . . . . . . . . . . . . .  4  5
     4.1.  Clarifications on Non-Existence Proofs . . . . . . . . . .  4  5
     4.2.  Validating Responses to an ANY Query . . . . . . . . . . .  5  6
     4.3.  Check for CNAME  . . . . . . . . . . . . . . . . . . . . .  5  6
     4.4.  Insecure Delegation Proofs . . . . . . . . . . . . . . . .  5  6
   5.  Interoperability Concerns  . . . . . . . . . . . . . . . . . .  5  6
     5.1.  Errors in Canonical Form Type Code List  . . . . . . . . .  6  7
     5.2.  Unknown DS Message Digest Algorithms . . . . . . . . . . .  6  7
     5.3.  Private Algorithms . . . . . . . . . . . . . . . . . . . .  6  7
     5.4.  Caution About Local Policy and Multiple RRSIGs . . . . . .  7  8
     5.5.  Key Tag Calculation  . . . . . . . . . . . . . . . . . . .  7  8
     5.6.  Setting the DO Bit on Replies  . . . . . . . . . . . . . .  8  9
     5.7.  Setting the AD Bit on Queries  . . . . . . . . . . . . . .  8  9
     5.8.  Setting the AD Bit on Replies  . . . . . . . . . . . . . .  8  9
     5.9.  Always set the CD bit on Queries . . . . . . . . . . . . .  8  9
     5.10. Nested Trust Anchors . . . . . . . . . . . . . . . . . . .  9 10
       5.10.1.  Closest Encloser  . . . . . . . . . . . . . . . . . .  9 10
       5.10.2.  Accept Any Success  . . . . . . . . . . . . . . . . .  9 11
       5.10.3.  Preference Based on Source  . . . . . . . . . . . . . 10 11
     5.11. Mandatory Algorithm Rules  . . . . . . . . . . . . . . . . 10 11
     5.12. Expect Extra Signatures From Strange Keys  . . . . . . . . 11 12
   6.  Minor Corrections and Clarifications . . . . . . . . . . . . . 11 12
     6.1.  Finding Zone Cuts  . . . . . . . . . . . . . . . . . . . . 11 12
     6.2.  Clarifications on DNSKEY Usage . . . . . . . . . . . . . . 12 13
     6.3.  Errors in Examples . . . . . . . . . . . . . . . . . . . . 12 13
     6.4.  Errors in RFC 5155 . . . . . . . . . . . . . . . . . . . . 12 14
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13 14
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13 14
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 15
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 13 15
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 14 15
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 14 16
   Appendix B.  Discussion of Setting the CD Bit  . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15 19

1.  Introduction and Terminology

   This document lists some additions, clarifications and corrections to
   the core DNSSECbis specification, as originally described in
   [RFC4033], [RFC4034], and [RFC4035], and later amended by [RFC5155].
   (See section Section 2 for more recent additions to that core
   document set.)

   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 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",
   "OPTIONAL" in this document are to be interpreted as described in

2.  Important Additions to DNSSSECbis

   This section lists some documents that should be considered core
   DNSSEC protocol documents in addition to those 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 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.

   Note that the algorithm identifiers defined in RFC5155 (DSA-NSEC3-
   SHA1 and RSASHA1-NSEC3-SHA1) and RFC5702 (RSASHA256 and RSASHA512)
   signal that a zone MAY be using NSEC3, rather than NSEC.  The zone
   MAY indeed be using either and validators supporting these algorithms
   MUST support both NSEC3 and NSEC responses.

2.2.  SHA-2 Support

   [RFC4509] describes the use of SHA-256 as a digest algorithm in
   Delegation Signer (DS) RRs.  [RFC5702] describes the use of the
   RSASHA256 and RSASHA512 algorithms in DNSKEY and RRSIG RRs.
   Validator implementations are strongly encouraged to include support
   for these algorithms for DS, DNSKEY, and RRSIG records.

   Both [RFC4509] and [RFC5702] should also be considered part of the
   DNS Security Document Family as described by [RFC4033], Section 10.

3.  Scaling Concerns

3.1.  Implement a BAD cache

   Section 4.7 of RFC4035 permits security-aware resolvers to implement
   a BAD cache.  Because of scaling concerns not discussed in this
   document, that guidance has changed: security-aware resolvers SHOULD
   implement a BAD cache, as described in RFC4035.

4.  Security Concerns

   This section provides clarifications that, if overlooked, could lead
   to security issues.

4.1.  Clarifications on Non-Existence Proofs

   [RFC4035] Section 5.4 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 RRs 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.

4.2.  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.  That is,
   it is not necessary to include all RRsets at the QNAME in the

   When validating a response to QTYPE=*, all received RRsets that match
   QNAME and QCLASS MUST be validated.  If any of those RRsets fail
   validation, the answer is considered Bogus.  If there are no RRsets
   matching QNAME and QCLASS, that fact 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=*.

4.3.  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 in addition to the bit for the query
   type.  Without this check, an attacker could successfully transform a
   positive CNAME response into a NOERROR/NODATA response.

4.4.  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), 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.

5.  Interoperability Concerns
5.1.  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

5.2.  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 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

5.3.  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 5.2).  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 Bogus data, as
   discussed in [RFC4035].

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

5.4.  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

5.5.  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.

5.6.  Setting the DO Bit on Replies

   As stated in [RFC3225], the DO bit of the query MUST be copied in the
   response.  At least one implementation has done something different,
   so it may be wise for resolvers to be liberal in what they accept.

5.7.  Setting the AD Bit on Queries

   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 bit.

5.8.  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.

5.9.  Always set the CD bit on Queries

   When processing a request with the CD bit set, a resolver SHOULD
   attempt to return all responsive data, even data that has failed
   DNSSEC validation.  RFC4035 section 3.2.2 requires a resolver
   processing a request with the CD bit set to set the CD bit on its
   upstream queries.

   Prevailing wisdom suggests that a validating resolver SHOULD set the
   CD bit on every upstream query regardless of whether the CD bit was
   set on the incoming query or whether it has a trust anchor at or
   above the QNAME.  In other words, a validating resolver should
   attempt to retrieve all possible data -- even that which it can not
   validate itself -- on the grounds that a later query might come with
   the CD bit set.

   RFC4035 is ambiguous about what to do when a cached response was
   obtained with the CD bit not set, a case that only arises when the
   resolver chooses not to set the CD bit on all upstream queries, as
   suggested above.  In the typical case, no new query is required, nor
   does the cache need to track the state of the CD bit used to make a
   given query.  The problem arises when the cached response is a server
   failure (RCODE 2), which may indicate that the requested data failed
   DNSSEC validation at an upstream validating resolver.  (RFC2308
   permits caching of server failures for up to five minutes.)  In these
   cases, a new query with the CD bit set is required.

   Appendix B discusses more of the logic behind the recommendation
   presented in this section.

5.10.  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 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 have a choice
   of which trust anchor(s) to use.  Which to use is a matter of
   implementation choice.  It is possible and perhaps advisable to
   expose the choice of policy as a configuration option.  The rest of
   this section discusses some possible policies.  As a default, we
   suggest that validators implement the "Accept Any Success" policy
   described below in Section 5.10.2 while exposing other policies as
   configuration options.

5.10.1.  Closest Encloser

   One policy is to choose the trust anchor closest to the QNAME of the
   response.  In our example, that would be the "zone.example." trust

   This policy has the advantage of allowing the operator to trivially
   override a parent zone's trust anchor with one that the operator can
   validate in a stronger way, perhaps because the resolver operator is
   affiliated with the zone in question.  This policy also minimizes the
   number of public key operations needed, which may be of benefit in
   resource-constrained environments.

   This policy has the disadvantage of possibly giving the user some
   unexpected and unnecessary validation failures when sub-zone trust
   anchors are neglected.  As a concrete example, consider a validator
   that configured a trust anchor for "zone.example." in 2009 and one
   for "example." in 2011.  In 2012, "zone.example." rolls its KSK and
   updates its DS records, but the validator operator doesn't update its
   trust anchor.  With the "closest encloser" policy, the validator gets
   validation failures.

5.10.2.  Accept Any Success

   Another policy is to try all applicable trust anchors until one gives
   a validation result of Secure, in which case the final validation
   result is Secure.  If and only if all applicable trust anchors give a
   result of Insecure, the final validation result is Insecure.  If one
   or more trust anchors lead to a Bogus result and there is no Secure
   result, then the final validation result is Bogus.

   This has the advantage of causing the fewer validation failures,
   which may deliver a better user experience.  If one trust anchor is
   out of date (as in our above example), the user may still be able to
   get a Secure validation result (and see DNS responses).

   This policy has the disadvantage of making the validator subject to
   compromise of the weakest of these trust anchors while making its
   relatively painless to keep old trust anchors configured in

5.10.3.  Preference Based on Source

   When the trust anchors have come from different sources (e.g.
   automated updates ([RFC5011]), one or more DLV registries
   ([RFC5074]), and manually configured), a validator may wish to choose
   between them based on the perceived reliability of those sources.
   The order of precedence might be exposed as a configuration option.

   For example, a validator might choose to prefer trust anchors found
   in a DLV registry over those manually configured on the theory that
   the manually configured ones will not be as aggressively maintained.

   Conversely, a validator might choose to prefer manually configured
   trust anchors over those obtained from a DLV registry on the theory
   that the manually configured ones have been more carefully

   Or the validator might do something more complicated: prefer a sub-
   set of manually configured trust anchors (based on a configuration
   option), then trust anchors that have been updated using the RFC5011
   mechanism, then trust anchors from one DLV registry, then trust
   anchors from a different DLV registry, then the rest of the manually
   configured trust anchors.

5.11.  Mandatory Algorithm Rules

   The last paragraph of RFC4035 Section 2.2 includes rules for which
   algorithms must be used to sign a zone.  Since these rules have been
   confusing, we restate them in different language here:

      The DS RRset and DNSKEY RRset are used to signal which algorithms
      are used to sign a zone.  The pressence of an algorithm in a
      zone's DS or DNSKEY RRset set signals that that algorithm is used
      to sign the entire zone.

      A signed zone MUST include a DNSKEY for each algorithm present in
      the zone's DS RRset and expected trust anchors for the zone.  The
      zone MUST also be signed with each algorithm (though not each key)
      present in the DNSKEY RRset.  It is possible to add algorithms at
      the DNSKEY that aren't in the DS record, but not vice-versa.  If
      more than one key of the same algorithm is in the DNSKEY RRset, it
      is sufficient to sign each RRset with any subset of these DNSKEYs.
      It is acceptable to sign some RRsets with one subset of keys (or
      key) and other RRsets with a different subset, so long as at least
      one DNSKEY of each algorithm is used to sign each RRset.
      Likewise, if there are DS records for multiple keys of the same
      algorithm, any subset of those may appear in the DNSKEY RRset.

   Lastly, note that this a requirement at the server side, not the
   client side.  Validators SHOULD accept any single valid path.  They
   SHOULD NOT insist that all algorithms signalled in the DS RRset work,
   and they MUST NOT insist that all algorithms signalled in the DNSKEY
   RRset work.  A validator MAY have a configuration option to perform a
   signature completeness test to support troubleshooting.

5.12.  Expect Extra Signatures From Strange Keys

   Validating resolvers should not be surprised to find RRSIGs in a zone
   that do not (currently) have a corresponding DNSKEY in the zone.
   Likewise, a validating resolver should not be surprised to find
   RRSIGs with algorithm types that don't exist in the DNSKEY RRset or
   DNSKEYs with algorithm types that don't appear in the zone's DS

   Good key rollover and algorithm rollover practices, as discussed in
   RFC4641 and its successor documents and as suggested by the rules in
   the previous section, may require that such RRSIGs be present in a

6.  Minor Corrections and Clarifications

6.1.  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.

6.2.  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 is
   also possible to use a single DNSKEY, with or without the SEP bit
   set, to sign the entire zone, including the DNSKEY RRset itself.

6.3.  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.

6.4.  Errors in RFC 5155

   A NSEC3 record that matches an Empty 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 )*

7.  IANA Considerations

   This document specifies no IANA Actions.

8.  Security Considerations

   This document adds two cryptographic features to the core DNSSEC
   protocol.  Security considerations for those features are discussed
   in the documents defining them.  Additionally, this document
   addresses some ambiguities and omissions in 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 4 are critical for preserving the security
   properties DNSSEC offers.  Furthermore, failure to address some of
   the interoperability concerns in Section 5 could limit the ability to
   later change or expand DNSSEC, including adding new algorithms.

   The recommendation in Section 5.9 to always set the CD bit has
   security implications.  By setting the CD bit, a resolver will not
   benefit from more stringent validation rules or a more complete set
   of trust anchors at an upstream validator.

9.  References
9.1.  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.

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

   [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.

   [RFC5702]  Jansen, J., "Use of SHA-2 Algorithms with RSA in DNSKEY
              and RRSIG Resource Records for DNSSEC", RFC 5702,
              October 2009.

9.2.  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.

   [RFC5011]  StJohns, M., "Automated Updates of DNS Security (DNSSEC)
              Trust Anchors", RFC 5011, September 2007.

   [RFC5074]  Weiler, S., "DNSSEC Lookaside Validation (DLV)", RFC 5074,
              November 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 5.3, and the lack of specificity in handling ANY
   queries, as described in Section 4.2, were discovered by David

   The error in algorithm 1 key tag calculation, as described in
   Section 5.5, was found by Abhijit Hayatnagarkar.  Donald Eastlake
   contributed text for Section 5.5.

   The bug relating to delegation NSEC RR's in Section 4.1 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 6.3 of this document.

   Text on the mandatory algorithm rules was derived from suggestions by
   Matthijs Mekking and Ed Lewis.

   The CD bit logic was analyzed in depth by David Blacka, Olafur
   Gudmundsson, Mike St. Johns, and Andrew Sullivan.

   The editors would like to thank Alfred Hoenes, Ed Lewis, Danny Mayer,
   Olafur Gudmundsson, Suzanne Woolf, Rickard Bellgrim, Mike St. Johns,
   Mark Andrews, Wouter Wijngaards, Matthijs Mekking, Andrew Sullivan,
   and Scott Rose for their substantive comments on the text of this

Appendix B.  Discussion of Setting the CD Bit

   RFC 4035 may be read as relying on the implicit assumption that there
   is (at least usually) at most one validating system between the stub
   resolver and the authoritative server for a given zone.  It is
   entirely possible, however, for more than one validator to stand
   between a stub resolver and an authoritative server.  If these
   different validators have disjoint trust anchors configured, then it
   will be possible that each would be able to validate some portion of
   the DNS tree but neither will be able to validate all of it.
   Accordingly, it might be argued that it is desirable not to set the
   CD bit on upstream queries, because that will allow for maximal

   In Section 5.9 of the present memo, it is recommended to set the CD
   bit on an upstream query even when the incoming query arrives with
   CD=0.  This is for two reasons: it encourages a more predictable
   validation experience (because it means that one validator is always
   doing the validation), and it ensures that all DNSSEC data that
   exists may be available from the local cache should a query with CD=1

   As a matter of policy, it is possible to set the CD bit differently
   than suggested in Section 5.9.  A different choice will, of course,
   not always yield the benefits listed above.  It is beyond the scope
   of this memo to outline all of the considerations and counter
   considerations for all possible policies.  Nevertheless, it is
   possible to describe three approaches and their underlying philosophy
   of operation.  These are laid out in the tables below.

   The table that describes each model has five columns.  The first
   column indicates the value of the CD bit that the resolver receives
   (for instance, on the name server side in an iterative resolver, or
   as local policy or from the API in the case of a stub).  The next
   column indicates whether the query needs to be forwarded for
   resolution (F) or can be satisfied from a local cache (C).  The next
   column is a line number, so that it can be referred to later in the
   table.  The next column indicates any relevant conditions at the
   resolver: whether the resolver has a covering trust anchor and so on
   (if there are no parameters here, the column is empty).  The final
   column indicates what the resolver does.

   The tables differentiate between "cached data" and "cached RCODE=2".
   This is a shorthand; the point is that one has to treat RCODE=2 as
   special, because it might indicate a validation failure somewhere
   upstream.  The distinction is really between "cached RCODE=2" and
   "cached everything else".

   The tables are probably easiest to think of in terms of describing
   what happens when a stub resolver sends a query to an intermediate
   resolver, but they are perfectly general and can be applied to any
   validating resolver.

   Model 1: "always set"

   This model is so named because the validating resolver sets the CD
   bit on queries it makes reegardless of whether it has a covering
   trust anchor for the query.  It is the model recommended in
   Section 5.9 of this memo.  The general philosophy represented by this
   table is that only one resolver should be responsible for validation
   irrespective of the possibility that an upstream resolver may be
   present and with TAs that cover different or additional QNAMEs.

    CD F/C    line      conditions              action
    1   F      A1                             Set CD=1 on upstream query
    0   F      A2                             Set CD=1 on upstream query
    1   C      A3                             Return the cache contents
                                               (data or RCODE=2)
    0   C      A4       no covering TA        Return cache contents
                                               (data or RCODE=2)
    0   C      A5       covering TA           Validate cached result and
                                               return it.

   Model 2: "never set when receiving CD=0"

   This model is so named because it sets CD=0 on upstream queries for
   all received CD=0 queries even if it has a covering trust anchor.
   The general philosophy represented by this table is that more than
   one resolver may take responsibility for validating a QNAME and that
   a validation failure for a QNAME by any resolver in the chain is a
   validation failure for the query.  Using this model instead of model

    CD F/C    line       conditions              action
    1  F      N1                              Set CD=1 on upstream query
    0  F      N2                              Set CD=0 on upstream query
    1  C      N3         cached data          Return cached data
    1  C      N4         cached RCODE=2       Treat as line N1
    0  C      N5         no covering TA       Return cache contents
                                               (data or RCODE=2)
    0  C      N6         covering TA &        Treat as line N2
                          cached data was
                          generated with CD=1
    0  C      N7         covering TA &        Validate and return
                          cached data was
                          generated with CD=0

   Model 3: "sometimes set"

   This model is so named because it sets the CD bit on upstream queries
   triggered by received CD=0 queries based on whether the validator has
   a TA configured that covers the query.  If there is no covering TA,
   the resolver clears the CD bit in the upstream query.  If there is a
   covering TA, it sets CD=1 and performs validation itself.  The
   general philosophy represented by this table is that a resolver
   should try and validate QNAMEs for which is has trust anchors and
   should not preclude validation by other resolvers for QNAMEs for
   which it does not have covering trust anchors.  Using this model
   instead of model 1 is NOT RECOMMENDED.

    CD F/C    line       conditions              action
    1  F      S1                            Set CD=1 on upstream query
    0  F      S2         covering TA        Set CD=1 on upstream query
    0  F      S3         no covering TA     Set CD=0 on upstream query
    1  C      S4         cached data        Return cached data
    1  C      S5         cached RCODE=2     Treat as line S1
    0  C      S6         cached data was    Return cache contents
                          generated with
    0  C      S7         cached data was    Validate & return cache
                          generated with     contents
                          CD=1 &
                          covering TA
    0  C      S8         cached RCODE=2     Return cache contents
    0  C      S9         cached data        Treat as line S3
                          was generated
                          with CD=1 &
                          no covering

Authors' Addresses

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

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