Internet Engineering Task Force                                 S. Huque
Internet-Draft                                                Salesforce
Intended status: Standards Track                                P. Vixie
Expires: March 11, 2021                                Farsight Security
                                                              R. Dolmans
                                                              NLnet Labs
                                                       September 7, 2020


                Delegation Revalidation by DNS Resolvers
                  draft-ietf-dnsop-ns-revalidation-00

Abstract

   This document recommends improved DNS [RFC1034] [RFC1035] resolver
   behavior with respect to the processing of Name Server (NS) resource
   record sets (RRset) 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.  Resolvers should also
   periodically revalidate the child delegation by re-quering the parent
   zone at the expiration of the TTL of the parent side NS RRset.

Status of This Memo

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

   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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 11, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.






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   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 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Upgrading NS RRset Credibility  . . . . . . . . . . . . . . .   4
   4.  Delegation Revalidation . . . . . . . . . . . . . . . . . . .   5
     4.1.  Using the DS Record TTL . . . . . . . . . . . . . . . . .   6
   5.  Optimizations . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Re-delegations and Delegation Removals  . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   RFC EDITOR: PLEASE REMOVE THIS PARAGRAPH BEFORE PUBLISHING: The
   source for this draft is maintained in GitHub at:
   https://github.com/shuque/ns-revalidation

   This document recommends improved DNS 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 second recommendation is to revalidate the
   delegation by re-quering the parent zone at the expiration of the TTL
   of the parent side NS RRset.










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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.  This document aims to
   bring more commonality and predictability by standardizing the
   behavior in a way that comports with the DNS protocol.

   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.
   [RFC1034] Section 4.2.2 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.  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 ([RFC2181], Section 5.4.1.
   "Ranking data", and Section 6.1.  "Zone authority").  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 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 Unbound).  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.  In fact, the Extensible Provisioning
   Protocol (EPP) [RFC5731], that is often used by TLDs to configure
   delegation parameters has no provision to set the TTL.  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.



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   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.

3.  Upgrading NS RRset Credibility

   *  When a delegation 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
      answers 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.

   *  Some resolvers may choose to delay the response to the triggering
      query until both the triggering query and the validation query
      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
      number of nameservers in the field that (incorrectly) fail to
      answer explicit queries for NS records, and thus the revalidation
      logic may need to be applied lazily and opportunistically to deal
      with them.









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   *  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.  Delegation Revalidation

   This documents proposes two mechanisms to perform delegation
   revalidation: an extensive and a simple mechanism.  [TODO: should we
   keep just the simple mechanism?]

   The extensive mechanism:

   *  The lowest TTL found in a parent zone's delegating NS RRset should
      be stored in the cache and used to trigger delegation revalidation
      as follows: Whenever a cached RRset is being considered for use in
      a response, the cache should be walked upward toward the root,
      looking for expired delegations.  At the first expired delegation
      encountered while walking upward toward the root, revalidation
      should be triggered, putting the processing of dependent queries
      on hold until validation is complete.

   *  To revalidate a delegation, the iterative caching DNS resolver
      will redo resolution of the triggering query at the closest
      enclosing zone cut above the revalidation point.  If Query-name
      Minimization [RFC7816] is being employed, this may not be the full
      name of the triggering query, but the query name with some number
      of left most labels excised as dictated by the qname minimization
      algorithm.  While searching for these nameservers, additional
      revalidations may occur, perhaps placing a chain of dependent
      queries on hold, unwinding in downward order as revalidations
      closer to the root must be complete before revalidations further
      from the root can begin.

   *  If a delegation can be revalidated at the same node, then the old
      apex NS RRset should be deleted from cache and then the new
      delegating NS RRset should be stored in cache.  The minimum TTL
      from the new delegating NS RRset should also be stored in cache to
      facilitate future revalidations.  This order of operations ensures
      that the RRset credibility rules do not prevent the new delegating
      NS RRset from entering the cache.  It is expected that the child's
      apex NS RRset will rapidly replace the parent's delegating NS
      RRset as soon as iteration restarts after the revalidation event.






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   *  If the new delegating NS RRset cannot be found (RCODE=NXDOMAIN) or
      if there is a new zone cut at some different level of the
      hierarchy (insertion or deletion of a delegation point above the
      revalidation point) or if the new RRset shares no nameserver names
      in common with the old one (indicating some kind of redelegation,
      which is rare) then the cache should be purged of all names and
      RRsets at or below the revalidation point.  This facilitates
      redelegation or revocation of a zone by a parent zone
      administrator, and also conserves cache storage by deleting
      unreachable data.

   The simple mechanism:

   *  Cap the time to cache the child NS RRset to the lower of child and
      parent NS RRset TTL.  The normal iterative resolution algorithm
      will then cause delegation revalidation to naturally occur at the
      expiration of the capped child NS TTL, along with dispatching of
      the validation query to upgrade NS RRset credibility.

4.1.  Using the DS Record TTL

   If both parent and child zone are DNSSEC [RFC4033] [RFC4034]
   [RFC4035] signed with a corresponding secure delegation between them,
   then expiration of the Delegation Signer (DS) record set will cause
   revalidation of the current child zone's DNSKEY set.  According to
   RFC 4035, Section 2.4, "The TTL of a DS RRSet SHOULD match the TTL of
   the delegating NS RRset", so this revalidation should be triggered on
   the same time scale, and thus responses from the stale child
   nameservers would no longer be trusted.  However, delegation
   revalidation is still necessary to locate the current nameserver
   addresses to which subsequent DNS queries should be directed.

   In practice, the DS TTL often differs from the delegating NS TTL.
   For example, currently the root zone and the COM and NET TLDs all set
   an NS RRset TTL of 2 days, while the DS RRset TTL is just 1 day.
   RSSAC-003 [rssac-003] makes the following observation: "In the root
   zone, delegating NS records have a 2 day TTL.  However, the DS
   records have a 1 day TTL, against the advice of RFC 4035.  This is
   not particularly surprising since a mistake with a DS record can deny
   resolution for all names under a TLD.  Given the way that DS records
   are currently used in the root zone (e.g., usually matching just one
   TLD KSK) it is better for them to have a lower TTL in the event of an
   emergency change."

   If a secure delegation is present, resolvers may use the DS RRset's
   TTL as the revalidation interval in preference to to the delegating
   NS RRSet TTL.  (Question: should this be the recommendation instead?;
   after all the DS is signed so its TTL cannot be spoofed.?)



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5.  Optimizations

   TODO: mention possible optimizations: record whether certain
   nameservers return the child NS set in their authoritative section
   responses and subsequently forego the extra child NS query for a
   period of time.  Suggest that authoritative servers that do minimal
   responses return their NS sets in response to DNSKEY queries, and
   resolvers that see such behavior may also subsequently forego the
   extra child NS query (from Olafur G.).

6.  Re-delegations and Delegation Removals

   TODO: mention in more detail what to do when resolvers observe
   redelegations or removal of delegations at the parent.  A quick
   initial summary follows.

   If a delegation is removed (i.e. the parent returns NXDOMAIN), then
   cache contents should be treated as described in [RFC8020] -- ideally
   the resolver's cache should be entirely pruned at the delegation
   point, or the cached contents below the delegation may be allowed to
   be used until they expire.  Similar treatment of the cache should be
   followed if the child zone has been entirely re-delegated to a new
   set of nameservers.  If only a subset of nameservers have been re-
   delegated, then no new cache cleanup action is needed.

7.  IANA Considerations

   This document includes no request to IANA.

8.  Security Considerations

   Upgrading NS RRset Credibility (Section 3) allows resolvers to cache
   and utilize the authoritative child apex NS RRset in preference to
   the non-authoriative parent NS RRset.  However, it is important to
   implement the steps described in Delegation Revalidation (Section 4)
   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.  This problem has been seen in
   the wild [include reference to Ghost Domains paper here].

9.  References

9.1.  Normative References






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   [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>.

   [RFC7816]  Bortzmeyer, S., "DNS Query Name Minimisation to Improve
              Privacy", RFC 7816, DOI 10.17487/RFC7816, March 2016,
              <https://www.rfc-editor.org/info/rfc7816>.

   [RFC8020]  Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is
              Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020,
              November 2016, <https://www.rfc-editor.org/info/rfc8020>.

9.2.  Informative References

   [I-D.vixie-dnsext-resimprove]
              Vixie, P., Joffe, R., and F. Neves, "Improvements to DNS
              Resolvers for Resiliency, Robustness, and Responsiveness",
              Work in Progress, Internet-Draft, draft-vixie-dnsext-
              resimprove-00, June 23, 2010,
              <https://tools.ietf.org/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, February 24, 2009,
              <https://tools.ietf.org/html/draft-wijngaards-dnsext-
              resolver-side-mitigation-01>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <https://www.rfc-editor.org/info/rfc4033>.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,
              <https://www.rfc-editor.org/info/rfc4034>.





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   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
              <https://www.rfc-editor.org/info/rfc4035>.

   [RFC5731]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
              Domain Name Mapping", STD 69, RFC 5731,
              DOI 10.17487/RFC5731, August 2009,
              <https://www.rfc-editor.org/info/rfc5731>.

   [rssac-003]
              RSSAC_Caucus, "RSSAC003 Report on Root Zone TTLs", August
              2015, <https://www.icann.org/en/system/files/files/rssac-
              003-root-zone-ttls-21aug15-en.pdf>.

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 Vixie, Joffe, and Neves.

Authors' Addresses

   Shumon Huque
   Salesforce

   Email: shuque@gmail.com


   Paul Vixie
   Farsight Security

   Email: paul@redbarn.org


   Ralph Dolmans
   NLnet Labs

   Email: ralph@nlnetlabs.nl









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