Delegation Revalidation by DNS Resolvers
draft-ietf-dnsop-ns-revalidation-08

Domain Name System Operations                                   S. Huque
Internet-Draft                                                Salesforce
Intended status: Standards Track                                P. Vixie
Expires: 12 July 2025                                   SIE Europe, U.G.
                                                               W. Toorop
                                                              NLnet Labs
                                                          8 January 2025

                Delegation Revalidation by DNS Resolvers
                  draft-ietf-dnsop-ns-revalidation-08

Abstract

   This document recommends improved DNS resolver behavior with respect
   to the processing of Name Server (NS) resource record (RR) sets
   (RRsets) during iterative resolution.  When following a referral
   response from an authoritative server to a child zone, DNS resolvers
   should explicitly query the authoritative NS RRset at the apex of the
   child zone and cache this in preference to the NS RRset on the parent
   side of the zone cut.  The (A and AAAA) address RRsets in the
   additional section from referral responses and authoritative NS
   answers for the names of the NS RRset, should similarly be re-queried
   and used to replace the entries with the lower trustworthiness
   ranking in cache.  Resolvers should also periodically revalidate the
   child delegation by re-querying the parent zone at the expiration of
   the TTL of the parent side NS RRset.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-dnsop-ns-revalidation/.

   Discussion of this document takes place on the DNSOP Working Group
   mailing list (mailto:dnsop@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/dnsop/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/dnsop/.

   Source for this draft and an issue tracker can be found at
   https://github.com/shuque/ns-revalidation.

Status of This Memo

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

Huque, et al.             Expires 12 July 2025                  [Page 1]
Internet-Draft         DNS Delegation Revalidation          January 2025

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 12 July 2025.

Copyright Notice

   Copyright (c) 2025 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 (https://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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Upgrading NS RRset Credibility  . . . . . . . . . . . . . . .   5
   4.  Upgrading A and AAAA RRset Credibility  . . . . . . . . . . .   6
     4.1.  Strictly revalidating referral and authoritative NS RRset
           responses . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Delegation Revalidation . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Security and Privacy Considerations . . . . . . . . . . . . .   8
     7.1.  DNSSEC protection of infrastructure data  . . . . . . . .   9
     7.2.  Cache poisoning protection  . . . . . . . . . . . . . . .   9
     7.3.  Other considerations  . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  12
   Appendix B.  Implementation status  . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

Huque, et al.             Expires 12 July 2025                  [Page 2]
Internet-Draft         DNS Delegation Revalidation          January 2025

1.  Introduction

   This document recommends improved DNS [RFC1034] [RFC1035] resolver
   behavior with respect to the processing of NS record sets during
   iterative resolution.  The first recommendation is that resolvers,
   when following a referral response from an authoritative server to a
   child zone, should explicitly query the authoritative NS RRset at the
   apex of the child zone and cache this in preference to the NS RRset
   on the parent side of the zone cut.  The address records in the
   additional section from the referral response (as glue) or
   authoritative NS response that match the names of the NS RRset should
   similarly be required if they are cached non-authoritatively.  The
   authoritative answers from those queries should replace the cached
   non-authoritative A and AAAA RRsets.

   The second recommendation is to revalidate the delegation by re-
   querying the parent zone at the expiration of the TTL of the parent
   side NS RRset.

1.1.  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 BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Motivation

   There is wide variability in the behavior of deployed DNS resolvers
   today with respect to how they process delegation records.  Some of
   them prefer the parent NS set, some prefer the child, and for others,
   what they preferentially cache depends on the dynamic state of
   queries and responses they have processed [SOMMESE].  This document
   aims to bring more commonality and predictability by standardizing
   the behavior in a way that comports with the DNS protocol.  Another
   goal is to improve DNS security.

   The delegation NS RRset at the bottom of the parent zone and the apex
   NS RRset in the child zone are unsynchronized in the DNS protocol.
   Section 4.2.2 of [RFC1034] says "The administrators of both zones
   should insure that the NS and glue RRs which mark both sides of the
   cut are consistent and remain so.".  But for a variety of reasons
   they could not be [SOMMESE].  Officially, a child zone's apex NS
   RRset is authoritative and thus has a higher cache credibility than
   the parent's delegation NS RRset, which is non-authoritative glue
   (Sections 5.4.1 (Ranking data) and 6.1 (Zone authority) of
   [RFC2181]).  Hence the NS RRset "below the zone cut" should

Huque, et al.             Expires 12 July 2025                  [Page 3]
Internet-Draft         DNS Delegation Revalidation          January 2025

   immediately replace the parent's delegating NS RRset in cache when an
   iterative caching DNS resolver crosses a zone boundary.  However,
   this can only happen if (1) the resolver receives the authoritative
   NS RRset in the Authority section of a response from the child zone,
   which is not mandatory, or (2) if the resolver explicitly issues an
   NS RRset query to the child zone as part of its iterative resolution
   algorithm.  In the absence of this, it is possible for an iterative
   caching resolver to never learn the authoritative NS RRset for a
   zone, unless a downstream client of the resolver explicitly issues
   such an NS query, which is not something that normal enduser
   applications do, and thus cannot be relied upon to occur with any
   regularity.

   Increasingly, there is a trend towards minimizing unnecessary data in
   DNS responses.  Several popular DNS implementations default to such a
   configuration (see "minimal-responses" in BIND and NSD).  So, they
   may never include the authoritative NS RRset in the Authority section
   of their responses.

   A common reason that zone owners want to ensure that resolvers place
   the authoritative NS RRset preferentially in their cache is that the
   TTLs may differ between the parent and child side of the zone cut.
   Some DNS Top Level Domains (TLDs) only support long fixed TTLs in
   their delegation NS sets.  This inhibits a child zone owner's ability
   to make more rapid changes to their nameserver configuration using a
   shorter TTL, if resolvers have no systematic mechanism to observe and
   cache the child NS RRset.

   Similarly, a child zone owner may also choose to have longer TTLs in
   their delegation NS sets and address records to decrease the attack
   window for cache poisoning attacks.  For example, at the time of
   writing, root-servers.net has a TTL of 6 weeks for the root server
   identifier address records, where the TTL in the priming response is
   6 days.

   A child zone's delegation still needs to be periodically revalidated
   at the parent to make sure that the parent zone has not legitimately
   re-delegated the zone to a different set of nameservers, or even
   removed the delegation.  Otherwise, resolvers that refresh the TTL of
   a child NS RRset on subsequent queries or due to pre-fetching, may
   cling to those nameservers long after they have been re-delegated
   elsewhere.  This leads to the second recommendation in this document,
   "Delegation Revalidation" - Resolvers should record the TTL of the
   parent's delegating NS RRset, and use it to trigger a revalidation
   action.  Attacks exploiting lack of this revalidation have been
   described in [GHOST1], [GHOST2].

Huque, et al.             Expires 12 July 2025                  [Page 4]
Internet-Draft         DNS Delegation Revalidation          January 2025

3.  Upgrading NS RRset Credibility

   When a referral response is received during iteration, a validation
   query SHOULD be sent in parallel with the resolution of the
   triggering query, to the delegated nameservers for the newly
   discovered zone cut.  Note that validating resolvers today, when
   following a secure referral, already need to dispatch a query to the
   delegated nameservers for the DNSKEY RRset, so this validation query
   could be sent in parallel with that DNSKEY query.

   A validation query consists of a query for the child's apex NS RRset,
   sent to the newly discovered delegation's nameservers.  Normal
   iterative logic applies to the processing of responses to validation
   queries, including storing the results in cache, trying the next
   server on SERVFAIL or timeout, and so on.  Positive responses to this
   validation query will be cached with an authoritative data ranking.
   Successive queries directed to the same zone will be directed to the
   nameservers listed in the child's apex, due to the ranking of this
   answer.  If the validation query fails, the parent NS RRset will
   remain the one with the highest ranking and will be used for
   successive queries.

   The triggering query to the child may contain the NS RRset in the
   authority section as well.  This NS RRset however has a lower
   trustworthiness than the set from the direct query (Section 5.4.1 of
   [RFC2181]), so regardless of the order in which the responses are
   received, the NS RRset from the answer section from the direct
   child's apex NS RRset query will be stored in the cache eventually.

   When a resolver detects that the child's apex NS RRset contains
   different nameservers than the non-authoritative version at the
   parent side of the zone cut, it MAY report the mismatch using DNS
   Error Reporting [RFC9567] on the Report-Channel for the child zone,
   as well as on the Report-Channel for the parent zone, with an
   extended DNS error code of TBD (See Section 6).

   A No Data response (see Section 2.2 of [RFC2308]) for the validating
   NS query should be treated the same as a failed validating NS query.
   The parent NS RRset will remain the one with the highest ranking and
   will be used for successive queries.

   Additional validation queries for the "glue" address RRs of referral
   responses (if not already authoritatively present in cache) SHOULD be
   sent with the validation query for the NS RRset as well.  Positive
   responses will be cached authoritatively and replace the non
   authoritative "glue" entries.  Successive queries directed to the
   same zone will be directed to the authoritative nameservers denoted
   in the referral response.

Huque, et al.             Expires 12 July 2025                  [Page 5]
Internet-Draft         DNS Delegation Revalidation          January 2025

   The names from the NS RRset from a validating NS query may differ
   from the names in NS RRset in the referral response.  Outstanding
   validation queries for "glue" address RRs that do not match names in
   the authoritative NS RRset may be discarded, or they may be left
   running to completion.  Their result will no longer be used in
   queries for the zone.  Outstanding validation queries for "glue"
   address RRs that do match names in the authoritative NS RRset MUST be
   left running to completion.  They do not need to be re-queried after
   reception of the authoritative NS RRset (see Section 4).

   Validated "glue" may result in unreachable destinations.  A resolver
   MAY choose to keep the non-authoritative value for the "glue" next to
   the preferred authoritative value for fallback purposes.  Such a
   resolver MAY choose to fallback to use the non-authoritative value as
   a last resort, but SHOULD do so only if all other authoritative
   "glue" led to unreachable destinations as well.

   Resolvers may choose to delay the response to the triggering query
   until both the triggering query and the validation queries have been
   answered.  In practice, we expect many implementations may answer the
   triggering query in advance of the validation query for performance
   reasons.  An additional reason is that there are unfortunately a
   number of nameservers in the field that (incorrectly) fail to
   properly answer explicit queries for zone apex NS records, and thus
   the revalidation logic may need to be applied lazily and
   opportunistically to deal with them.  In cases where the delegated
   nameservers respond incorrectly to an NS query, the resolver SHOULD
   abandon this algorithm for the zone in question and fall back to
   using only the information from the parent's referral response.

   If the resolver chooses to delay the response, and there are no
   nameserver names in common between the child's apex NS RRset and the
   parent's delegation NS RRset, then the responses received from
   forwarding the triggering query to the parent's delegated nameservers
   SHOULD be discarded after validation, and this query should be
   forwarded again to the child's apex nameservers.

4.  Upgrading A and AAAA RRset Credibility

   Authoritative responses for a zone's NS RRset at the apex can contain
   additional addresses.  An NS RRset validation response is an example
   of such a response.  A priming response is another example of an
   authoritative zone's NS RRset response [RFC8109].

   Additional addresses in authoritative NS RRset responses SHOULD be
   validated if they are not already authoritatively in cache.  Only
   when such additional addresses are DNSSEC verifiable, (because the
   complete RRset is included, including a verifiable signature for the

Huque, et al.             Expires 12 July 2025                  [Page 6]
Internet-Draft         DNS Delegation Revalidation          January 2025

   RRset), such additional addresses can be cached authoritatively
   immediately without additional validation queries.  DNSSEC validation
   is enough validation in those cases.  Otherwise, the addresses cannot
   be assumed to be complete or even authoritatively present in the same
   zone, and additional validation queries SHOULD be made for these
   addresses.

   Note that there may be outstanding address validation queries for the
   names of the authoritative NS RRset (from referral address validation
   queries).  In those cases no new validation queries need to be made.

4.1.  Strictly revalidating referral and authoritative NS RRset
      responses

   Resolvers may choose to delay the response to a triggering query
   until it can be verified that the answer came from a name server
   listening on an authoritatively acquired address for an
   authoritatively acquired name.  This would offer the most trustworthy
   responses with the least risk for forgery or eavesdropping, however
   without fallback to lower ranked NS RRsets and addresses, there is no
   failure mitigation and a failed NS RRset validation query, due to a
   broken child NS RRset or to malfunctioning child zone's authoritative
   servers, will then lead to a hard failure to query the referred to
   child zone.

5.  Delegation Revalidation

   The essence of this mechanism is re-validation of all delegation
   metadata that directly or indirectly supports an owner name in cache.
   This requires a cache to remember the delegated name server names for
   each zone cut as received from the parent (delegating) zone's name
   servers, and also the TTL of that NS RRset, and the TTL of the
   associated DS RRset (if seen).

   A delegation under re-validation is called a "re-validation point"
   and is "still valid" if its parent zone's servers still respond to an
   in-zone question with a referral to the re-validation point, and if
   that referral overlaps with the previously cached referral by at
   least one name server name, and the DS RRset (if seen) overlaps the
   previously cached DS RRset (if also seen) by at least one delegated
   signer.

   If the response is not a referral or refers to a different zone than
   before, then the shape of the delegation hierarchy has changed.  If
   the response is a referral to the re-validation point but to a wholly
   novel NS RRset or a wholly novel DS RRset, then the authority for
   that zone has changed.  For clarity, this includes transitions
   between empty and non-empty DS RRsets.

Huque, et al.             Expires 12 July 2025                  [Page 7]
Internet-Draft         DNS Delegation Revalidation          January 2025

   If the shape of the delegation hierarchy or the authority for a zone
   has been found to change, then currently cached data whose owner
   names are at or below that re-validation point MUST NOT be used.
   Such non-use can be by directed garbage collection or lazy
   generational garbage collection or some other method befitting the
   architecture of the cache.  What matters is that the cache behave as
   though this data was removed.

   Since re-validation can discover changes in the shape of the
   delegation hierarchy it is more efficient to re-validate from the top
   (root) downward (to the owner name) since an upper level re-
   validation may obviate lower level re-validations.  What matters is
   that the supporting chain of delegations from the root to the owner
   name be demonstrably valid; further specifics are implementation
   details.

   Re-validation MUST be triggered when delegation meta-data has been
   cached for a period at most exceeding the delegating NS or DS (if
   seen) RRset TTL.  If the corresponding child zone's apex NS RRset TTL
   is smaller than the delegating NS RRset TTL, revalidation MUST happen
   at that interval instead.  However, resolvers SHOULD impose a
   sensible minimum TTL floor they are willing to endure to avoid
   potential computational DoS attacks inflicted by zones with very
   short TTLs.

   In normal operations this meta-data can be quickly re-validated with
   no further work.  However, when re-delegation or take-down occurs, a
   re-validating cache will discover this within one delegation TTL
   period, allowing the rapid expulsion of old data from the cache.

6.  IANA Considerations

   IANA is requested to assign a value to the "Extended DNS Error Codes"
   registry [RFC8914].

       +===========+============================+=================+
       | INFO-CODE | Purpose                    | Reference       |
       +===========+============================+=================+
       | TBD       | referral NS RRset mismatch | [this document] |
       +-----------+----------------------------+-----------------+

                                 Table 1

7.  Security and Privacy Considerations

Huque, et al.             Expires 12 July 2025                  [Page 8]
Internet-Draft         DNS Delegation Revalidation          January 2025

7.1.  DNSSEC protection of infrastructure data

   Referral response NS RRsets and glue, and the additional addresses
   from authoritative NS RRset responses (such as the root priming
   response), are not protected with DNSSEC signatures.  An attacker
   that is able to alter the unsigned A and AAAA RRsets in the
   additional section of referral and authoritative NS RRset responses,
   can fool a resolver into taking addresses under the control of the
   attacker to be authoritative for the zone.  Such an attacker can
   redirect all traffic to the zone (of the referral or authoritative NS
   RRset response) to a rogue name server.

   A rogue name server can view all queries from the resolver to that
   zone and alter all unsigned parts of responses, such as the parent
   side NS RRsets and glue of further referral responses.  Resolvers
   following referrals from a rogue name server, that do not revalidate
   those referral responses, can subsequently be fooled into taking
   addresses under the control of the attacker to be authoritative for
   those delegations as well.  The higher up the DNS tree, the more
   impact such an attack has.  An attacker controlling a rogue name
   server for the root has potentially complete control over the entire
   domain name space and can alter all unsigned parts undetected.

   Strictly revalidating referral and authoritative NS RRset responses
   (see Section 4.1), enables the resolver to defend itself against the
   above described attack with DNSSEC signed infrastructure RRsets.
   Unlike cache poisoning defences that leverage increase entropy to
   protect the transaction, revalidation of NS RRsets and addresses also
   provides protection against on-path attacks.

   Since December 6, 2023, the root zone contains a DNSSEC signed
   cryptographic message digest [RFC8976][ROOT-ZONEMD], covering all
   root zone data.  This includes all non-authoritative data such as the
   A and AAAA RRsets for the IP addresses of the root server
   identifiers, as well as the NS RRsets and glue that make up the
   delegations.  A root zone local to the resolver [RFC8806] with a
   verified and validated ZONEMD RRset, would provide protection
   similarly strong to strictly revalidating the root and the top level
   domains.

7.2.  Cache poisoning protection

   In [DNS-CACHE-INJECTIONS] an overview is given of 18 cache poisoning
   attacks from which 13 can be remedied with delegation revalidation.
   The paper provides recommendations for handling records in DNS
   responses with respect to an earlier version of the idea presented in
   this document [I-D.wijngaards-dnsext-resolver-side-mitigation].

Huque, et al.             Expires 12 July 2025                  [Page 9]
Internet-Draft         DNS Delegation Revalidation          January 2025

   Upgrading NS RRset Credibility (Section 3) allows resolvers to cache
   and utilize the authoritative child apex NS RRset in preference to
   the non-authoritative parent NS RRset.  However, it is important to
   implement the steps described in Delegation Revalidation (Section 5)
   at the expiration of the parent's delegating TTL.  Otherwise, the
   operator of a malicious child zone, originally delegated to, but
   subsequently delegated away from, can cause resolvers that refresh
   TTLs on subsequent NS set queries, or that pre-fetch NS queries, to
   never learn of the redelegated zone [GHOST1], [GHOST2].

7.3.  Other considerations

   An implementation may wish to consider limiting revalidation to
   delegations that cross administrative boundaries such as anywhere in
   ".ip6.arpa" and ".in-addr.arpa" as well as any so-called "public
   suffix" such as the root zone, top level zones such as ".com" or
   ".net", and effective top level zones such as ".ad.jp" or ".co.uk".

   Some resolvers do not adhere to Sections 5.4.1 and 6.1 of [RFC2181],
   and only use the non-authoritative parent side NS RRsets and glue
   returned in referral responses for contacting authoritative name
   servers [I-D.fujiwara-dnsop-resolver-update].  As a consequence, they
   are not susceptible to many of the cache poisoning attacks enumerated
   in [DNS-CACHE-INJECTIONS] that are based upon the relative
   trustworthiness of DNS data.  Such resolvers are also not susceptible
   to the GHOST domain attacks [GHOST1], [GHOST2].  Such resolvers will
   however never benefit from DNSSEC protection of infrastructure RRsets
   and are susceptible to query redirection attacks.

8.  References

8.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
              <https://www.rfc-editor.org/info/rfc2181>.

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
              <https://www.rfc-editor.org/info/rfc2308>.

Huque, et al.             Expires 12 July 2025                 [Page 10]
Internet-Draft         DNS Delegation Revalidation          January 2025

   [RFC8109]  Koch, P., Larson, M., and P. Hoffman, "Initializing a DNS
              Resolver with Priming Queries", BCP 209, RFC 8109,
              DOI 10.17487/RFC8109, March 2017,
              <https://www.rfc-editor.org/info/rfc8109>.

   [RFC8806]  Kumari, W. and P. Hoffman, "Running a Root Server Local to
              a Resolver", RFC 8806, DOI 10.17487/RFC8806, June 2020,
              <https://www.rfc-editor.org/info/rfc8806>.

   [RFC8914]  Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
              Lawrence, "Extended DNS Errors", RFC 8914,
              DOI 10.17487/RFC8914, October 2020,
              <https://www.rfc-editor.org/info/rfc8914>.

   [RFC8976]  Wessels, D., Barber, P., Weinberg, M., Kumari, W., and W.
              Hardaker, "Message Digest for DNS Zones", RFC 8976,
              DOI 10.17487/RFC8976, February 2021,
              <https://www.rfc-editor.org/info/rfc8976>.

   [RFC9567]  Arends, R. and M. Larson, "DNS Error Reporting", RFC 9567,
              DOI 10.17487/RFC9567, April 2024,
              <https://www.rfc-editor.org/info/rfc9567>.

8.2.  Informative References

   [DNS-CACHE-INJECTIONS]
              Klein, A., Shulman, H., and M. Waidner, "Internet-Wide
              Study of DNS Cache Injections", n.d.,
              <https://ieeexplore.ieee.org/abstract/document/8057202>.

   [GHOST1]   Jiang, J., Liang, J., Li, K., Li, J., Duan, H., and J. Wu,
              "Ghost Domain Names: Revoked Yet Still Resolvable", n.d.,
              <https://www.ndss-symposium.org/ndss2012/>.

   [GHOST2]   Li, X., Liu, B., Bai, X., Zhang, M., Zhang, Q., Li, Z.,
              Duan, H., and Q. Li, "Ghost Domain Reloaded: Vulnerable
              Links in Domain Name Delegation and Revocation", n.d.,
              <https://www.ndss-symposium.org/ndss-paper/ghost-domain-
              reloaded-vulnerable-links-in-domain-name-delegation-and-
              revocation/>.

   [I-D.fujiwara-dnsop-resolver-update]
              Fujiwara, K., "Updating Resolver Algorithm", Work in
              Progress, Internet-Draft, draft-fujiwara-dnsop-resolver-
              update-00, 31 October 2016,
              <https://datatracker.ietf.org/doc/html/draft-fujiwara-
              dnsop-resolver-update-00>.

Huque, et al.             Expires 12 July 2025                 [Page 11]
Internet-Draft         DNS Delegation Revalidation          January 2025

   [I-D.vixie-dnsext-resimprove]
              Vixie, P. A., Joffe, R., and F. Neves, "Improvements to
              DNS Resolvers for Resiliency, Robustness, and
              Responsiveness", Work in Progress, Internet-Draft, draft-
              vixie-dnsext-resimprove-00, 23 June 2010,
              <https://datatracker.ietf.org/doc/html/draft-vixie-dnsext-
              resimprove-00>.

   [I-D.wijngaards-dnsext-resolver-side-mitigation]
              Wijngaards, W., "Resolver side mitigations", Work in
              Progress, Internet-Draft, draft-wijngaards-dnsext-
              resolver-side-mitigation-01, 24 February 2009,
              <https://datatracker.ietf.org/doc/html/draft-wijngaards-
              dnsext-resolver-side-mitigation-01>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [ROOT-ZONEMD]
              Wessels, D., "Root zone operational announcement:
              introducing ZONEMD for the root zone", n.d.,
              <https://lists.dns-oarc.net/pipermail/dns-
              operations/2023-December/022388.html>.

   [SOMMESE]  Sommese, R., Moura, G. C. M., Jonker, M., van Rijswijk-
              Deij, R., Dainotti, A., Claffy, K. C., and A. Sperotto,
              "When parents and children disagree: Diving into DNS
              delegation inconsistency", n.d.,
              <https://par.nsf.gov/servlets/purl/10186683>.

Appendix A.  Acknowledgements

   Wouter Wijngaards proposed explicitly obtaining authoritative child
   NS data in [I-D.wijngaards-dnsext-resolver-side-mitigation].  This
   behavior has been implemented in the Unbound DNS resolver via the
   "harden-referral-path" option.  The combination of child NS fetch and
   revalidating the child delegation was originally proposed in
   [I-D.vixie-dnsext-resimprove], by Paul Vixie, Rodney Joffe, and
   Frederico Neves.

Huque, et al.             Expires 12 July 2025                 [Page 12]
Internet-Draft         DNS Delegation Revalidation          January 2025

   The authors would like to thank Ralph Dolmans who was an early
   collaborator on this work, as well as the many members of the IETF
   DNS Operations Working Group for helpful comments and discussion.

Appendix B.  Implementation status

   *Note to the RFC Editor*: please remove this entire appendix before
   publication.

   *  The Unbound resolver software has delegation revalidation as
      described in this document implemented since version 1.1 (released
      August 29, 2008).  It is enabled with a configuration option
      harden-referral-path: yes which is disabled by default.

      "Redhat Enterprise Linux has been running Unbound with the harden-
      referral-path: option set to yes for years without problems", as
      mentioned by Paul Wouters during dnsop workgroup session at the
      IETF 119.

   *  The Knot Resolver software revalidates the priming response as
      part of priming the root zone since version 1.5.1 (released
      December 12, 2017)

Authors' Addresses

   Shumon Huque
   Salesforce
   Email: shuque@gmail.com

   Paul Vixie
   SIE Europe, U.G.
   Email: paul@redbarn.org

   Willem Toorop
   NLnet Labs
   Science Park 400
   1098 XH Amsterdam
   Netherlands
   Email: willem@nlnetlabs.nl

Huque, et al.             Expires 12 July 2025                 [Page 13]