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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 2013Clarifications 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,   andRFC 4035) as well as the NSEC3 specification (RFC 5155).  It also   defines NSEC3 and SHA-2 (RFC 4509 andRFC 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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6840.Weiler & Blacka              Standards Track                    [Page 1]

RFC 6840               DNSSEC Implementation Notes         February 2013Copyright Notice   Copyright (c) 2013 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 2013Table of Contents1. Introduction and Terminology ....................................41.1. Structure of This Document .................................41.2. Terminology ................................................42. Important Additions to DNSSEC ...................................42.1. NSEC3 Support ..............................................42.2. SHA-2 Support ..............................................53. Scaling Concerns ................................................53.1. Implement a BAD Cache ......................................54. Security Concerns ...............................................54.1. Clarifications on Nonexistence Proofs ......................54.2. Validating Responses to an ANY Query .......................64.3. Check for CNAME ............................................64.4. Insecure Delegation Proofs .................................75. Interoperability Concerns .......................................75.1. Errors in Canonical Form Type Code List ....................75.2. Unknown DS Message Digest Algorithms .......................75.3. Private Algorithms .........................................85.4. Caution about Local Policy and Multiple RRSIGs .............95.5. Key Tag Calculation ........................................95.6. Setting the DO Bit on Replies ..............................95.7. Setting the AD Bit on Queries .............................105.8. Setting the AD Bit on Replies .............................105.9. Always Set the CD Bit on Queries ..........................105.10. Nested Trust Anchors .....................................115.11. Mandatory Algorithm Rules ................................115.12. Ignore Extra Signatures from Unknown Keys ................126. Minor Corrections and Clarifications ...........................126.1. Finding Zone Cuts .........................................126.2. Clarifications on DNSKEY Usage ............................126.3. Errors in Examples ........................................136.4. Errors inRFC 5155 ........................................137. Security Considerations ........................................138. References .....................................................148.1. Normative References ......................................148.2. Informative References ....................................15Appendix A. Acknowledgments .......................................16Appendix B. Discussion of Setting the CD Bit ......................16Appendix C. Discussion of Trust Anchor Preference Options .........19C.1. Closest Encloser ..........................................19C.2. Accept Any Success ........................................20C.3. Preference Based on Source ................................20Weiler & Blacka              Standards Track                    [Page 3]

RFC 6840               DNSSEC Implementation Notes         February 20131.  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].   (SeeSection 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 inSection 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 bySection 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 bySection 10 of [RFC4033].3.  Scaling Concerns3.1.  Implement a BAD CacheSection 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 ProofsSection 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, andWeiler & 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 inSection 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 inSection 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 CNAMESection 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 20134.4.  Insecure Delegation ProofsSection 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 Concerns5.1.  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 ofSection 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 AlgorithmsSection 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 inAppendix 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 inSection 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 inSection 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 inSection 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 CalculationAppendix 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 inSection 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 20135.7.  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 RepliesSection 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   inSection 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 20135.10.  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 inAppendix 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 ofSection 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 leastWeiler & 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 inRFC 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 Clarifications6.1.  Finding Zone CutsAppendix 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 inSection 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 inSection 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 bySection 2.1.2 of [RFC4034].  It isWeiler & 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 inAppendix C.1 of [RFC4035] refers to the examples inAppendix 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 ofAppendix 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 inRFC 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 inSection 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 leadWeiler & Blacka              Standards Track                   [Page 13]

RFC 6840               DNSSEC Implementation Notes         February 2013   to security failures.  In particular, the validation algorithm   clarifications inSection 4 are critical for preserving the security   properties DNSSEC offers.  Furthermore, failure to address some of   the interoperability concerns inSection 5 could limit the ability to   later change or expand DNSSEC, including adding new algorithms.   The recommendation inSection 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.  References8.1.  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 20138.2.  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 2013Appendix 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 inSection 5.3, and the lack of specificity in handling ANY   queries, as described inSection 4.2, were discovered by David   Blacka.   The error in algorithm 1 key tag calculation, as described inSection 5.5, was found by Abhijit Hayatnagarkar.  Donald Eastlake   contributed text forSection 5.5.   The bug relating to delegation NSEC RR's inSection 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 forSection 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 neitherWeiler & 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.   InSection 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 inSection 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 thisWeiler & 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 inSection 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 upstreamWeiler & 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                          TAAppendix 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 isWeiler & 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.comWeiler & Blacka              Standards Track                   [Page 21]