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rfc:rfc6840

Internet Engineering Task Force (IETF) S. Weiler, Ed. Request for Comments: 6840 SPARTA, Inc. Updates: 4033, 4034, 4035, 5155 D. Blacka, Ed. Category: Standards Track Verisign, Inc. ISSN: 2070-1721 February 2013

 Clarifications and Implementation Notes for DNS Security (DNSSEC)

Abstract

 This document is a collection of technical clarifications to the DNS
 Security (DNSSEC) document set.  It is meant to serve as a resource
 to implementors as well as a collection of DNSSEC errata that existed
 at the time of writing.
 This document updates the core DNSSEC documents (RFC 4033, RFC 4034,
 and RFC 4035) as well as the NSEC3 specification (RFC 5155).  It also
 defines NSEC3 and SHA-2 (RFC 4509 and RFC 5702) as core parts of the
 DNSSEC specification.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6840.

Weiler & Blacka Standards Track [Page 1] RFC 6840 DNSSEC Implementation Notes February 2013

Copyright Notice

 Copyright (c) 2013 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
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Weiler & Blacka Standards Track [Page 2] RFC 6840 DNSSEC Implementation Notes February 2013

Table of Contents

 1. Introduction and Terminology ....................................4
    1.1. Structure of This Document .................................4
    1.2. Terminology ................................................4
 2. Important Additions to DNSSEC ...................................4
    2.1. NSEC3 Support ..............................................4
    2.2. SHA-2 Support ..............................................5
 3. Scaling Concerns ................................................5
    3.1. Implement a BAD Cache ......................................5
 4. Security Concerns ...............................................5
    4.1. Clarifications on Nonexistence Proofs ......................5
    4.2. Validating Responses to an ANY Query .......................6
    4.3. Check for CNAME ............................................6
    4.4. Insecure Delegation Proofs .................................7
 5. Interoperability Concerns .......................................7
    5.1. Errors in Canonical Form Type Code List ....................7
    5.2. Unknown DS Message Digest Algorithms .......................7
    5.3. Private Algorithms .........................................8
    5.4. Caution about Local Policy and Multiple RRSIGs .............9
    5.5. Key Tag Calculation ........................................9
    5.6. Setting the DO Bit on Replies ..............................9
    5.7. Setting the AD Bit on Queries .............................10
    5.8. Setting the AD Bit on Replies .............................10
    5.9. Always Set the CD Bit on Queries ..........................10
    5.10. Nested Trust Anchors .....................................11
    5.11. Mandatory Algorithm Rules ................................11
    5.12. Ignore Extra Signatures from Unknown Keys ................12
 6. Minor Corrections and Clarifications ...........................12
    6.1. Finding Zone Cuts .........................................12
    6.2. Clarifications on DNSKEY Usage ............................12
    6.3. Errors in Examples ........................................13
    6.4. Errors in RFC 5155 ........................................13
 7. Security Considerations ........................................13
 8. References .....................................................14
    8.1. Normative References ......................................14
    8.2. Informative References ....................................15
 Appendix A. Acknowledgments .......................................16
 Appendix B. Discussion of Setting the CD Bit ......................16
 Appendix C. Discussion of Trust Anchor Preference Options .........19
    C.1. Closest Encloser ..........................................19
    C.2. Accept Any Success ........................................20
    C.3. Preference Based on Source ................................20

Weiler & Blacka Standards Track [Page 3] RFC 6840 DNSSEC Implementation Notes February 2013

1. Introduction and Terminology

 This document lists some additions, clarifications, and corrections
 to the core DNSSEC specification, as originally described in
 [RFC4033], [RFC4034], and [RFC4035], and later amended by [RFC5155].
 (See 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 existing at the time of writing that need to be
 addressed when advancing the DNSSEC documents along the Standards
 Track.

1.1. Structure of This Document

 The clarifications and changes to DNSSEC 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",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 [RFC2119].

2. Important Additions to DNSSEC

 This section lists some documents that are now 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 use it.  Validators that do not support
 validation of responses using NSEC3 will be hampered in validating
 large portions of the DNS space.
 [RFC5155] is now considered part of the DNS Security Document Family
 as described by Section 10 of [RFC4033].

Weiler & Blacka Standards Track [Page 4] RFC 6840 DNSSEC Implementation Notes February 2013

 Note that the algorithm identifiers defined in [RFC5155] (DSA-NSEC3-
 SHA1 and RSASHA1-NSEC3-SHA1) and [RFC5702] (RSASHA256 and RSASHA512)
 signal that a zone might be using NSEC3, rather than NSEC.  The zone
 may 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] are now considered part of the DNS
 Security Document Family as described by Section 10 of [RFC4033].

3. Scaling Concerns

3.1. Implement a BAD Cache

 Section 4.7 of [RFC4035] permits security-aware resolvers to
 implement a BAD cache.  That guidance has changed: security-aware
 resolvers SHOULD implement a BAD cache as described in [RFC4035].
 This change in guidance is based on operational experience with
 DNSSEC administrative errors leading to significant increases in DNS
 traffic, with an accompanying realization that such events are more
 likely and more damaging than originally supposed.  An example of one
 such event is documented in "Rolling Over DNSSEC Keys" [Huston].

4. Security Concerns

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

4.1. Clarifications on Nonexistence Proofs

 Section 5.4 of [RFC4035] under-specifies the algorithm for checking
 nonexistence proofs.  In particular, the algorithm as presented would
 allow a validator to interpret an NSEC or NSEC3 RR from an ancestor
 zone as proving the nonexistence of an RR in a child zone.
 An "ancestor delegation" NSEC RR (or NSEC3 RR) is one with:
 o  the NS bit set,
 o  the Start of Authority (SOA) bit clear, and

Weiler & Blacka Standards Track [Page 5] RFC 6840 DNSSEC Implementation Notes February 2013

 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
 nonexistence 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 nonexistence of names beneath that
 DNAME.  An NSEC or NSEC3 RR with the DNAME bit set MUST NOT be used
 to assume the nonexistence 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
 response.
 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 Section 5.4 of [RFC4035] (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 nothing explicit 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 by (for
 example) simply stripping the CNAME RRset from the response.  A naive
 validator would then note that the QTYPE was not present in the
 matching NSEC/NSEC3 RR, but fail to notice that the CNAME bit was
 set; thus, the response should have been a positive CNAME response.

Weiler & Blacka Standards Track [Page 6] RFC 6840 DNSSEC Implementation Notes February 2013

4.4. Insecure Delegation Proofs

 Section 5.2 of [RFC4035] 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
 MUST 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.
 Without this check, an attacker could reuse an NSEC or NSEC3 RR
 matching a non-delegation name to spoof an unsigned delegation at
 that name.  This would claim that an existing signed RRset (or set of
 signed RRsets) is below an unsigned delegation, thus not signed and
 vulnerable to further attack.

5. Interoperability Concerns

5.1. Errors in Canonical Form Type Code List

 When canonicalizing DNS names (for both ordering and signing), DNS
 names in the RDATA section of NSEC resource records are not converted
 to lowercase.  DNS names in the RDATA section of RRSIG resource
 records are converted to lowercase.
 The guidance in the above paragraph differs from what has been
 published before but is consistent with current common practice.
 Item 3 of Section 6.2 of [RFC4034] says that names in both of these
 RR types should be converted to lowercase.  The earlier [RFC3755]
 says that they should not.  Current practice follows neither document
 fully.
 Section 6.2 of [RFC4034] also erroneously lists HINFO as a record
 that needs conversion to lowercase, and twice at that.  Since HINFO
 records contain no domain names, they are not subject to case
 conversion.

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 zones'
 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.

Weiler & Blacka Standards Track [Page 7] RFC 6840 DNSSEC Implementation Notes February 2013

 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.
 In other words, when determining the security status of a zone, a
 validator disregards any authenticated DS records that specify
 unknown or unsupported DNSKEY algorithms.  If none are left, the zone
 is treated as if it were unsigned.
 This document modifies the above text to additionally disregard
 authenticated DS records using unknown or unsupported message digest
 algorithms.

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 Appendix A.1.1 of [RFC4034],
 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 RRset, or if any supported
 algorithm appears in the DS RRset, no special processing is needed.
 Furthermore, if the validator implementation does not support any
 private algorithms, or only supports private algorithms using an
 algorithm number not present in the DS RRset, no special processing
 is 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 message digest
 algorithms, as discussed in Section 5.2 of this document).  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 as described in Section 5.2 of [RFC4035],
 authenticating the DNSKEY.  If all of the retrieved and authenticated
 DNSKEY RRs use unknown or unsupported private algorithms, then the
 zone is treated as if it were unsigned.

Weiler & Blacka Standards Track [Page 8] RFC 6840 DNSSEC Implementation Notes February 2013

 Note that if none of the private algorithm DS RRs can be securely
 matched to DNSKEY RRs 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, Section 5.3.3 of [RFC4035]
 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".
 This document specifies that a resolver SHOULD accept any valid RRSIG
 as sufficient, and only determine that an RRset is Bogus 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 due to cache
 timing issues.  Furthermore, certain zone management techniques, like
 the Double Signature Zone Signing Key Rollover method described in
 Section 4.2.1.2 of [RFC6781], will not work reliably.  Such a
 resolver is also vulnerable to malicious insertion of gibberish
 signatures.

5.5. Key Tag Calculation

 Appendix B.1 of [RFC4034] 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 fourth-to-last and third-to-last octets of the
 public key modulus.  It is, in fact, the third-to-last and second-to-
 last octets.

5.6. Setting the DO Bit on Replies

 As stated in Section 3 of [RFC3225], the DNSSEC OK (DO) bit of the
 query MUST be copied in the response.  However, in order to
 interoperate with implementations that ignore this rule on sending,
 resolvers MUST ignore the DO bit in responses.

Weiler & Blacka Standards Track [Page 9] RFC 6840 DNSSEC Implementation Notes February 2013

5.7. Setting the AD Bit on Queries

 The semantics of the Authentic Data (AD) bit in the query were
 previously undefined.  Section 4.6 of [RFC4035] instructed resolvers
 to always clear the AD bit when composing queries.
 This document defines setting the AD bit in a query as a signal
 indicating that the requester understands and is interested in the
 value of the AD bit in the response.  This allows a requester 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 interoperate with legacy stub resolvers and middleboxes that
 neither understand nor ignore the AD bit, validating resolvers SHOULD
 only set the AD bit when a response both meets the conditions listed
 in Section 3.2.3 of [RFC4035], 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 Checking Disabled (CD) bit set, a
 resolver SHOULD attempt to return all response data, even data that
 has failed DNSSEC validation.  Section 3.2.2 of [RFC4035] requires a
 resolver processing a request with the CD bit set to set the CD bit
 on its upstream queries.
 This document further specifies that validating resolvers SHOULD set
 the CD bit on every upstream query.  This is regardless of whether
 the CD bit was set on the incoming query or whether it has a trust
 anchor at or above the QNAME.
 [RFC4035] is ambiguous about what to do when a cached response was
 obtained with the CD bit unset, a case that only arises when the
 resolver chooses not to set the CD bit on all upstream queries, as
 specified 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.

Weiler & Blacka Standards Track [Page 10] RFC 6840 DNSSEC Implementation Notes February 2013

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
 "www.sub.zone.example.", 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.  Appendix C discusses several possible
 algorithms.
 It is possible and advisable to expose the choice of policy as a
 configuration option.  As a default, it is suggested that validators
 implement the "Accept Any Success" policy described in Appendix C.2
 while exposing other policies as configuration options.
 The "Accept Any Success" 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.

5.11. Mandatory Algorithm Rules

 The last paragraph of Section 2.2 of [RFC4035] includes rules
 describing which algorithms must be used to sign a zone.  Since these
 rules have been confusing, they are restated using different language
 here:
    The DS RRset and DNSKEY RRset are used to signal which algorithms
    are used to sign a zone.  The presence of an algorithm in either a
    zone's DS or DNSKEY RRset 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

Weiler & Blacka Standards Track [Page 11] RFC 6840 DNSSEC Implementation Notes February 2013

    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.
 This requirement applies to servers, not validators.  Validators
 SHOULD accept any single valid path.  They SHOULD NOT insist that all
 algorithms signaled in the DS RRset work, and they MUST NOT insist
 that all algorithms signaled in the DNSKEY RRset work.  A validator
 MAY have a configuration option to perform a signature completeness
 test to support troubleshooting.

5.12. Ignore Extra Signatures from Unknown Keys

 Validating resolvers MUST disregard RRSIGs in a zone that do not
 (currently) have a corresponding DNSKEY in the zone.  Similarly, a
 validating resolver MUST disregard RRSIGs with algorithm types that
 don't exist in the DNSKEY RRset.
 Good key rollover and algorithm rollover practices, as discussed in
 RFC 6781 and its successor documents and as suggested by the rules in
 the previous section, may require that such RRSIGs be present in a
 zone.

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 but does not state how to find those servers.
 Specific instructions can be found in Section 4.2 of [RFC4035].

6.2. Clarifications on DNSKEY Usage

 It is possible to use different DNSKEYs to sign different subsets of
 a zone, constrained only by the rules in Section 5.11.  It is even
 possible to use a different DNSKEY for each RRset in a zone, subject
 only to practical limits on the size of the DNSKEY RRset and the
 above rules.  However, be aware that there is no way to tell
 resolvers what a particular 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 Section 2.1.2 of [RFC4034].  It is

Weiler & Blacka Standards Track [Page 12] RFC 6840 DNSSEC Implementation Notes February 2013

 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 Appendix C.1 of [RFC4035] refers to the examples in
 Appendix 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 Appendix C.6 of [RFC4035] 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

 An 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
 Section 3.2.1 of [RFC5155] should be:
    Type Bit Maps Field = ( Window Block # | Bitmap Length | Bitmap )*

7. Security Considerations

 This document adds SHA-2 and NSEC3 support 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

Weiler & Blacka Standards Track [Page 13] RFC 6840 DNSSEC Implementation Notes February 2013

 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.

8. References

8.1. 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.

Weiler & Blacka Standards Track [Page 14] RFC 6840 DNSSEC Implementation Notes February 2013

8.2. Informative References

 [Huston]   Michaelson, G., Wallstrom, P., Arends, R., and G. Huston,
            "Rolling Over DNSSEC Keys", Internet Protocol
            Journal, Vol. 13, No.1, pp. 2-16, March 2010.
 [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
            NCACHE)", RFC 2308, March 1998.
 [RFC3755]  Weiler, S., "Legacy Resolver Compatibility for Delegation
            Signer (DS)", RFC 3755, May 2004.
 [RFC4955]  Blacka, D., "DNS Security (DNSSEC) Experiments", RFC 4955,
            July 2007.
 [RFC5011]  StJohns, M., "Automated Updates of DNS Security (DNSSEC)
            Trust Anchors", STD 74, RFC 5011, September 2007.
 [RFC5074]  Weiler, S., "DNSSEC Lookaside Validation (DLV)", RFC 5074,
            November 2007.
 [RFC6781]  Kolkman, O., Mekking, W., and R. Gieben, "DNSSEC
            Operational Practices, Version 2", RFC 6781,
            December 2012.

Weiler & Blacka Standards Track [Page 15] RFC 6840 DNSSEC Implementation Notes February 2013

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 DNSSEC 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
 Blacka.
 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,
 Jeremy Reed, Paul Hoffman, Mohan Parthasarathy, Florian Weimer,
 Warren Kumari and Scott Rose for their contributions to this
 document.

Appendix B. Discussion of Setting the CD Bit

 [RFC4035] may be read as relying on the implicit assumption that
 there is 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 exist between a stub resolver
 and an authoritative server.  If these different validators have
 disjoint trust anchors configured, then it is possible that each
 would be able to validate some portion of the DNS tree, but neither

Weiler & Blacka Standards Track [Page 16] RFC 6840 DNSSEC Implementation Notes February 2013

 is 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 allows for maximal validation.
 In Section 5.9 of this document, 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 as only 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 arrive.
 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 document 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 second
 column indicates whether the query needs to be forwarded for
 resolution (F) or can be satisfied from a local cache (C).  The third
 column is a line number, so that it can be referred to later in the
 table.  The fourth column indicates any relevant conditions at the
 resolver, for example, whether the resolver has a covering trust
 anchor, and so on.  If there are no parameters here, the column is
 empty.  The fifth and final column indicates what action the resolver
 takes.
 The tables differentiate between "cached data" and "cached RCODE=2".
 This is a shorthand; the point is that one has to treat RCODE=2
 (server failure) 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 regardless of whether it has a covering trust
 anchor for the query.  The general philosophy represented by this

Weiler & Blacka Standards Track [Page 17] RFC 6840 DNSSEC Implementation Notes February 2013

 table is that only one resolver should be responsible for validation
 irrespective of the possibility that an upstream resolver may be
 present with trust anchors that cover different or additional QNAMEs.
 It is the model recommended in Section 5.9 of this document.
  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 is NOT
 RECOMMENDED.
  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 trust anchor configured that covers the query.  If there is no
 covering trust anchor, the resolver clears the CD bit in the upstream

Weiler & Blacka Standards Track [Page 18] RFC 6840 DNSSEC Implementation Notes February 2013

 query.  If there is a covering trust anchor, the resolver 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 it 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 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
                        CD=0
  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
                        TA

Appendix C. Discussion of Trust Anchor Preference Options

 This section presents several different policies for validating
 resolvers to use when they have a choice of trust anchors available
 for validating a given answer.

C.1. Closest Encloser

 One policy is to choose the trust anchor closest to the QNAME of the
 response.  For example, consider a validator configured with trust
 anchors for "example." and "zone.example."  When asked to validate a
 response for "www.sub.zone.example.", a validator using the "Closest
 Encloser" policy would choose the "zone.example." trust anchor.
 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

Weiler & Blacka Standards Track [Page 19] RFC 6840 DNSSEC Implementation Notes February 2013

 affiliated with the zone in question.  This policy also minimizes the
 number of public key operations needed, which is of benefit in
 resource-constrained environments.
 This policy has the disadvantage of 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 Key Signing
 Key (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.

C.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 fewest 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
 the compromise of the weakest of these trust anchors, while making it
 relatively painless to keep old trust anchors configured in
 perpetuity.

C.3. Preference Based on Source

 When the trust anchors have come from different sources (e.g.,
 automated updates ([RFC5011]), one or more DNSSEC Lookaside
 Validation (DLV) registries ([RFC5074]), and manual configuration), 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.

Weiler & Blacka Standards Track [Page 20] RFC 6840 DNSSEC Implementation Notes February 2013

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

Authors' Addresses

 Samuel Weiler (editor)
 SPARTA, Inc.
 7110 Samuel Morse Drive
 Columbia, MD  21046
 US
 EMail: weiler@tislabs.com
 David Blacka (editor)
 Verisign, Inc.
 12061 Bluemont Way
 Reston, VA  20190
 US
 EMail: davidb@verisign.com

Weiler & Blacka Standards Track [Page 21]

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