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Internet Engineering Task Force                                 S. Huque
Internet-Draft                                                Salesforce
Intended status: Standards Track                                P. Vixie
Expires: May 7, 2020                                   Farsight Security
                                                        November 4, 2019


                Delegation Revalidation by DNS Resolvers
                  draft-huque-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
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   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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   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 May 7, 2020.

Copyright Notice

   Copyright (c) 2019 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



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   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  . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Upgrading NS RRset Credibility  . . . . . . . . . . . . . . .   4
   4.  Delegation Revalidation . . . . . . . . . . . . . . . . . . .   4
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   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.

2.  Motivation

   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).  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



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   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.

   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).

   Qname Minimisation [RFC7816], a more privacy preserving mode of
   iterative resolution, specifies the use of the NS query type at every
   step of the resolution process until the full query name has been
   reconstructed at the leaf zone.  This would provide a way to
   definitively learn the child zone's authoritative NS RRset.  In
   practice however, many (most?) implementations of Qname Minimisation
   currently employ the original query type or the A query type.  Thus,
   regardless of whether Qname Minimisation is in use or not, this
   document recommends explicitly fetching the authoritative NS RRset at
   the child zone when following a referral.

   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, and this inhibits the zone owner's ability
   to make more rapid changes to their nameserver configuration, if
   resolvers have no systematic mechanism to observe the child NS RRset.

   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.  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".
   Essentially, the resolver should record the TTL of the parent's
   delegating NS RRset, and use it to trigger a revalidation action.
   Technically, if both parent and child zone are DNSSEC [RFC4033]
   [RFC4034] [RFC4035] signed with a corresponding secure delegation
   between them, then expiration of the DS record will cause
   revalidation of the current child zone's DNSKEY set, so responses
   from the orphaned child nameservers would no longer be trusted.
   However, delegation revalidation is still necessary to locate the
   current nameserver addresses.





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3.  Upgrading NS RRset Credibility

   o  When a delegation response is received during iteration, a
      validation query should be sent in parallel with the forwarding of
      the triggering query to the delegated nameservers for the newly
      discovered zone cut.  The response to the triggering query should
      be delayed until both the forwarded query and the validation query
      have been answered.

   o  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,
      propagating NXDOMAIN back to the triggering query, trying the next
      server on SERVFAIL or timeout, and so on.

   o  If 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.

   o  [TBD: There are a small but non trivial number of authoritative
      DNS services that timeout on explicit NS queries.  Should we
      accommodate them by treating this practice lazily and
      opportunistically?  Or do we expect the DNS Flag Day efforts will
      effectively banish them in the near future?]

4.  Delegation Revalidation

   o  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.

   o  To revalidate a delegation, the iterative caching DNS resolver
      will forward the query that triggered revalidation to the
      nameservers at the closest enclosing zone cut above the
      revalidation point.  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.



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   o  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.

   o  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.

   o  To make the timing of a revalidation event unpredictable from the
      point of view of a potential cache-spoof attacker, the parent's
      delegating NS RRset TTL should be reduced by a random fraction of
      its value before being stored for use in revalidation activities.

   o  This section describes a precise algorithm for delegation
      revalidation.  A simpler resolver implementation may choose
      alternative methods, e.g. it may choose to perform the upward
      search for expired delegations at scheduled intervals rather than
      for every response decision.  Or it may have a fixed maximum TTL
      for child zones before they are expired from the cache and
      requeried at the parent.

5.  Acknowledgements

   The practices described in this document were originally proposed in
   [I-D.vixie-dnsext-resimprove], by Vixie, Joffe, and Neves.

6.  IANA Considerations

   This document includes no request to IANA.

7.  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 very important



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   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].

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>.

   [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>.

8.2.  Informative References

   [I-D.vixie-dnsext-resimprove]
              Vixie, P., Joffe, R., and F. Neves, "Improvements to DNS
              Resolvers for Resiliency, Robustness, and Responsiveness",
              draft-vixie-dnsext-resimprove-00 (work in progress), June
              2010.

   [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>.

Authors' Addresses

   Shumon Huque
   Salesforce

   Email: shuque@gmail.com


   Paul Vixie
   Farsight Security

   Email: paul@redbarn.org


































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