Delegation Revalidation by DNS Resolvers
draft-ietf-dnsop-ns-revalidation-11

Domain Name System Operations                                   S. Huque
Internet-Draft                                                Salesforce
Intended status: Standards Track                                P. Vixie
Expires: 22 April 2026                                  SIE Europe, U.G.
                                                               W. Toorop
                                                              NLnet Labs
                                                         19 October 2025

                Delegation Revalidation by DNS Resolvers
                  draft-ietf-dnsop-ns-revalidation-11

Abstract

   This document describes an optional algorithm for the processing of
   Name Server (NS) resource record (RR) sets (RRsets) during iterative
   resolution, and describes the benefits and considerations of using
   this approach.  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 delegation by re-querying the
   parent zone at the expiration of the TTL of either the parent or
   child NS RRset, whichever comes first.

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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Upgrading NS RRset Credibility  . . . . . . . . . . . . . . .   7
   4.  Limiting upgrading NS Credibility . . . . . . . . . . . . . .   8
   5.  Upgrading A and AAAA RRset Credibility  . . . . . . . . . . .   8
     5.1.  Upgrading glue  . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Upgrading additional address from authoritative NS
           responses . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Strict and opportunistic revalidation . . . . . . . . . . . .   9
     6.1.  Strictly revalidating referrals and authoritative NS RRset
           responses . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.2.  Opportunistic revalidating referral and authoritative NS
           RRset responses . . . . . . . . . . . . . . . . . . . . .  10
   7.  Delegation Revalidation . . . . . . . . . . . . . . . . . . .  10
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   9.  Security and Privacy Considerations . . . . . . . . . . . . .  11
     9.1.  DNSSEC protection of infrastructure data  . . . . . . . .  12
     9.2.  Cache poisoning protection  . . . . . . . . . . . . . . .  12
     9.3.  Other considerations  . . . . . . . . . . . . . . . . . .  13

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   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     10.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  16
   Appendix B.  Implementation status  . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   This document recommends improved DNS [RFC1034] [RFC1035] resolver
   behavior with respect to the processing of NS record sets during
   iterative resolution.

   In Upgrading NS RRset Credibility (Section 3) we recommend 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.

   Upgrading NS RRset Credibility (Section 3) works most reliably with
   good quality child NS RRsets, where the name servers referenced in
   the RDATA of the NS RRset correspond to IP addresses of nameservers
   which correctly serve the child zone.  We consider both the root, as
   well as the zones delegated from the root, to have good quality child
   NS RRset.  We also note that there may be numerous zones further down
   the DNS delegation hierarchy that may not be competently
   administered, and may have incorrect or stale authoritative NS and
   associated address records.  As a result they may require a resolver
   to fallback to data from the delegating side of the zone cut for
   successful resolution.  Section 4 describes limiting the upgrading of
   NS RRset credibility to address this.

   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 re-queried if they are cached
   non-authoritatively.  The authoritative answers from those queries
   should replace the cached non-authoritative A and AAAA RRsets.  This
   is described in (see Upgrading A and AAAA RRset Credibility
   (Section 5)).

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   In Strict and opportunistic revalidation (Section 6), we make a
   distinction between strict and opportunistic revalidation.  Strict
   revalidation provides DNSSEC protection of infrastructure data
   (Section 9.1) with DNSSEC signed infrastructure data and validating
   resolvers, but for it to work correctly, good quality child NS RRsets
   are a pre-requisite.  Opportunistic revalidation allows for fallback
   to the non-authoritative data returned in the referral responses, but
   therefore does not provide the same degree of protection as strict
   revalidation does.

   Finally, in Delegation Revalidation (Section 7), we recommend that
   resolvers 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.

   Throughout this document we will also use terminology with the
   meaning as defined below:

   Triggering query:
      the DNS query that caused ("triggered") a referral response.

   Infrastructure RRsets (data):
      the NS and address (A and AAAA) RRsets used by resolvers to
      contact the authoritative name servers

   Revalidation:
      the process of obtaining the authoritative infrastructure data

   Validation (validating) query:
      the extra query that is performed to get the authoritative version
      of infrastructure RRsets

   Delegation revalidation:
      re-establishing the existence and validity of the parent-side NS
      RRset of a delegation

   Revalidation point:
      a delegation under revalidation

   Re-delegation:

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      the process of changing a delegation information to another set of
      authoritative name servers, potentially under different
      administrative control

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].

   While this variability is expected to continue in deployed
   implementations, this document specifies an algorithm that can be
   adopted by resolvers to achieve several benefits.

   It preferentially and predictably prefers the authoritative NS set at
   the apex of the child zone, which better comports with the the DNS
   protocol's data ranking rules.  (Note that the proposed redesign of
   the DNS delegation mechanism in [I-D.ietf-deleg] is expected to
   fundamentally alter where authoritative delegation information will
   reside.  This document deals with the DNS delegation mechanism as
   currently deployed.)

   It offers seeveral security advantages.  The mechanisms described in
   this document help against GHOST domain attacks and a variety of
   cache poisoning attacks with resolvers that adhere to Sections 5.4.1
   (Ranking data) and 6.1 (Zone authority) of [RFC2181].  Strictly
   revalidating referral and authoritative NS RRset responses, enables
   the resolver to defend itself against query redirection attacks, see
   Security and Privacy considerations (Section 9).

   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
   (Sections 5.4.1 (Ranking data) and 6.1 (Zone authority) of
   [RFC2181]).  Hence the NS RRset "below the zone cut" should
   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

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   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 end user
   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 name server 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 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 name servers, 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 name servers 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].

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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 one of the delegated name servers for the newly
   discovered zone cut.  Note that DNSSEC validating resolvers today,
   when following a secure referral, already need to dispatch a query to
   one of the delegated name servers 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 one of the newly discovered delegation's name servers.
   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 MAY be cached with an
   authoritative data ranking.  Successive queries directed to the same
   zone SHOULD 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 SHOULD remain the one with the highest
   ranking and SHOULD be used for successive queries.

   A response to 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 MAY be stored in the cache
   eventually.

   When a resolver detects that the child's apex NS RRset contains
   different name servers 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 8).

   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 SHOULD remain the one with the highest ranking
   and SHOULD be used for successive queries.  All resolution failures
   MUST be cached as directed in [RFC9520], to prevent aggressive
   requeries.

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4.  Limiting upgrading NS Credibility

   Upgrading NS RRset Credibility (Section 3) works most reliable with
   good quality child NS RRsets, where the name servers referenced in
   the RDATA of the NS RRset do result in IP addresses which serve the
   child zone.  We consider both the root, as well as the zones
   delegated from the root, to have good quality child NS RRset, but
   recognize that, because of the less transparently administered
   further delegations (among others by parties that are perhaps less
   devoted to DNS administration), some of those further delegations may
   be sub-optimal for upgrading NS RRset credibility.. An implementation
   MAY limit 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".

5.  Upgrading A and AAAA RRset Credibility

5.1.  Upgrading glue

   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 SHOULD be cached with authoritative data ranking.  The non-
   authoritative "glue" MAY be cached with non-authoritative data
   ranking for fallback purposes.  Successive queries directed to the
   same zone SHOULD be directed to the authoritative nameservers denoted
   in the referral response.

   The names from the NS RRset in a validating NS response may differ
   from the names from the NS RRset in the referral response.
   Outstanding validation queries for "glue" address RRs that do not
   match names in a newly discovered authoritative NS RRset may be
   discarded, or they may be left running to completion.  Their result
   MUST 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 5).

   Validated "glue" may result in unreachable destinations if they are
   obtained from poorly managed zones with incorrect address records.  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.

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5.2.  Upgrading additional address from authoritative NS responses

   Authoritative responses for a zone's NS RRset at the apex can contain
   nameserver addresses in the Additional section.  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].

   When additional addresses in authoritative NS RRset responses are
   DNSSEC verifiable (because the complete RRset is included, including
   a verifiable signature for the RRset) and DNSSEC secure, they MAY 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.

6.  Strict and opportunistic revalidation

6.1.  Strictly revalidating referrals 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.

   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 any responses received from
   sending the triggering query to the parent's delegated nameservers
   SHOULD be discarded, and this query should be sent again to one of
   the child's apex nameservers.

   It is RECOMMENDED, to scope *strict* upgrading of NS, A and AAAA
   RRset credibility, to the root zone and zones delegated from the root
   zone only (see Section 4).

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6.2.  Opportunistic revalidating referral and authoritative NS RRset
      responses

   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.

   One may consider to only limit _strict_ upgrading of NS, A and AAAA
   RRset credibility to the root zone and zones delegated from the root
   zone, and perform _opportunistic_ revalidations for further
   delegations (see Section 4).

7.  Delegation Revalidation

   The essence of this mechanism is revalidation 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 revalidation is called a "revalidation point" and
   is "still valid" if its parent zone's servers still respond to an in-
   zone question with a referral to the revalidation 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 revalidation 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.

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

   Revalidation 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 revalidated with
   no further work.  However, when re-delegation or take-down occurs, a
   revalidating cache SHOULD discover this within one delegation TTL
   period, allowing the rapid expulsion of old data from the cache.

8.  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

9.  Security and Privacy Considerations

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9.1.  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 6), 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.

9.2.  Cache poisoning protection

   In [DNS-CACHE-INJECTIONS] an overview is given of 18 cache poisoning
   attacks of 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].

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   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 7)
   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 re-delegated zone [GHOST1], [GHOST2].

9.3.  Other considerations

   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.

   Revalidating referral and authoritative NS RRset responses will
   induce more traffic from the resolver to the authoritative name
   servers.  The traffic increase may be substantial if the address
   RRsets for the names in the NS RRset's RDATA were provided non-
   authoritatively (as glue or as additional addresses) and need
   revalidation too [REDIRECTED-QUERY-TRAFFIC].  Resolvers SHOULD take
   care to limit the amount of work they are willing to do to resolve a
   query to a sensible amount.

10.  References

10.1.  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>.

   [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>.

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   [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>.

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

   [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>.

   [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>.

   [RFC9520]  Wessels, D., Carroll, W., and M. Thomas, "Negative Caching
              of DNS Resolution Failures", RFC 9520,
              DOI 10.17487/RFC9520, December 2023,
              <https://www.rfc-editor.org/info/rfc9520>.

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

10.2.  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>.

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   [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>.

   [I-D.ietf-deleg]
              April, T., Špaček, P., Weber, R., and Lawrence,
              "Extensible Delegation for DNS", Work in Progress,
              Internet-Draft, draft-ietf-deleg-04, 16 October 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-deleg-
              04>.

   [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>.

   [REDIRECTED-QUERY-TRAFFIC]
              Toorop, W., Thessalonikefs, Y., Overeinder, B., Müller,
              M., and M. Davids, "The reduced risk of redirected query
              traffic with signed root name server data", n.d.,
              <https://www.icann.org/en/system/files/files/reduced-risk-
              redirected-query-traffic-signed-root-name-server-data-
              22may24-en.pdf#h.8mh7wvmas7vi>.

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   [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 delegation was originally proposed in
   [I-D.vixie-dnsext-resimprove], by Paul Vixie, Rodney Joffe, and
   Frederico Neves.

   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 opportunistic revalidating of
      referral and authoritative NS RRset responses, as described in
      Section 3, Section 5 and Section 6.2 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)

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   *  Section 7 has been implemented in the Unbound resolver since
      version 1.4.17 (released May 24, 2012).

Authors' Addresses

   Shumon Huque
   Salesforce
   415 Mission Street, 3rd Floor
   San Francisco, CA 94105
   United States of America
   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

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