draft-ietf-dnsop-ns-revalidation.xml

<?xml version='1.0' encoding='utf-8'?>

<!DOCTYPE rfc SYSTEM "rfc2629-xhtml.ent">

<?xml-stylesheet type='text/xsl' href='rfc2629.xslt' ?>

<!-- used by XSLT processors -->
<!-- For a complete list and description of processing instructions (PIs),
     please see http:https://xml.resource.org/authoring/README.html. -->
<!-- Below are generally applicable Processing Instructions (PIs) that most I-Ds might want to use.
     (Here they are set differently than their defaults in xml2rfc v1.32) -->
<?rfc strict="yes" ?>
<!-- give errors regarding ID-nits and DTD validation -->
<!-- control the table of contents (ToC) -->
<?rfc toc="yes"?>
<!-- generate a ToC -->
<?rfc tocdepth="4"?>
<!-- the number of levels of subsections in ToC. default: 3 -->
<!-- control references -->
<?rfc symrefs="yes"?>
<!-- use symbolic references tags, i.e, [RFC2119] instead of [1] -->
<?rfc sortrefs="yes" ?>
<!-- sort the reference entries alphabetically -->
<!-- control vertical white space
     (using these PIs as follows is recommended by the RFC Editor) -->
<?rfc compact="yes" ?>
<!-- do not start each main section on a new page -->
<?rfc subcompact="no" ?>
<!-- keep one blank line between list items -->
<!-- end of list of popular I-D processing instructions -->

<rfc xmlns:xi="http:https://www.w3.org/2001/XInclude"
        category="std" consensus="true"
        docName="draft-ietf-dnsop-ns-revalidation-02"
        ipr="trust200902" obsoletes="" updates=""
        submissionType="IETF" xml:lang="en"
        tocInclude="true" tocDepth="4"
        symRefs="true" sortRefs="true" version="3">

  <!-- ***** FRONT MATTER ***** -->

  <front>

    <title abbrev="DNS Delegation Revalidation">
	Delegation Revalidation by DNS Resolvers
    </title>
    <seriesInfo name="Internet-Draft"
        value="draft-ietf-dnsop-ns-revalidation-02"/>
    <author fullname="Shumon Huque" initials="S." surname="Huque">
      <organization>Salesforce</organization>
      <address>
        <email>shuque@gmail.com</email>
        <!-- uri and facsimile elements may also be added -->
      </address>
    </author>
    <author fullname="Paul Vixie" initials="P." surname="Vixie">
      <organization>Farsight Security</organization>
      <address>
        <email>paul@redbarn.org</email>
        <!-- uri and facsimile elements may also be added -->
      </address>
    </author>
    <author fullname="Ralph Dolmans" initials="R." surname="Dolmans">
      <organization>NLnet Labs</organization>
      <address>
        <email>ralph@nlnetlabs.nl</email>
        <!-- uri and facsimile elements may also be added -->
      </address>
    </author>
    <date day="07" month="3" year="2022"/>
    <!-- Meta-data Declarations -->

    <area>General</area>
    <workgroup>Internet Engineering Task Force</workgroup>
    <keyword>Internet-Draft</keyword>
    <keyword>DNS</keyword>
    <keyword>Resolver</keyword>
    <keyword>Delegation</keyword>
    <keyword>Revalidation</keyword>
    <keyword>Authoritative</keyword>
    <keyword>Name Server Record</keyword>
    <keyword>NS</keyword>
    <keyword>Parent</keyword>
    <keyword>Child</keyword>
    <keyword>Resource Record Set</keyword>

    <abstract>
      <t>
	This document recommends improved DNS <xref target="RFC1034" format="default"/>
        <xref target="RFC1035" format="default"/> 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.
      </t>
    </abstract>

  </front>

  <middle>

    <section anchor="intro" numbered="true" toc="default">
      <name>Introduction</name>
      <t>
        RFC EDITOR: PLEASE REMOVE THIS PARAGRAPH BEFORE PUBLISHING: The
        source for this draft is maintained in GitHub at:
        https://github.com/shuque/ns-revalidation
      </t>
      <t>
	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.
      </t>
    </section>

    <section anchor="motivation" numbered="true" toc="default">
      <name>Motivation</name>
      <t>
        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.
      </t>
      <t>
	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. <xref target="RFC1034" format="default"/> 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
	(<xref target="RFC2181" format="default"/>, 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.
      </t>
      <t>
	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.
      </t>
      <t>
	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
        <xref target="RFC5731" format="default">Extensible Provisioning
        Protocol (EPP)</xref>, 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.
      </t>
      <t>
	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.
      </t>
    </section>

    <section anchor="upgrade-ns" numbered="true" toc="default">
      <name>Upgrading NS RRset Credibility</name>
      <ul spacing="normal">
	<li>
	   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.
	</li>
        <li>
	   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.
	</li>
        <!-- Ralph: fwiw, I'm ok with completely dropping the next items. Let's
	     keep this simple. -->

	<li>
	   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.
	</li>
        <li>
	   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.
	</li>
      </ul>
    </section>

    <section anchor="revalidation" numbered="true" toc="default">
      <name>Delegation Revalidation</name>

      <t>
      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).
      </t>
      <t>
      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.
      </t>
      <t>
      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.
      </t>
      <t>
      If the shape of the delegation hierarchy or the authority for a zone
      has been found to change, then no currently cached data whose owner
      names are at or below that re-validation point can 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.
      </t>
      <t>
      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.
      </t>
      <t>
      Re-validation is 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 should happen at that
      interval instead. However, resolvers should impose a sensitive minimum
      TTL floor they are willing to endure to avoid potential computational
      DoS attacks inflicted by zones with very short TTLs.
      </t>
      <t>
      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.
      </t>

    </section>

    <section anchor="IANA" numbered="true" toc="default">
      <name>IANA Considerations</name>
      <t>This document includes no request to IANA.</t>
    </section>

    <section anchor="Security" numbered="true" toc="default">
      <name>Security Considerations</name>
      <t>
	<xref target="upgrade-ns" format="default">Upgrading NS RRset Credibility</xref>
	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
        <xref target="revalidation" format="default">Delegation Revalidation</xref> 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].
      </t>
    </section>

  </middle>

  <!--	*****BACK MATTER ***** -->

  <back>

    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.1034.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.1035.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2181.xml"/>
      </references>
      <references>
        <name>Informative References</name>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5731.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.draft-vixie-dnsext-resimprove-00.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.draft-wijngaards-dnsext-resolver-side-mitigation-01.xml"/>
      </references>
    </references>

    <section anchor="Acknowledgements" numbered="false" toc="default">
      <name>Acknowledgements</name>
      <t>
        Wouter Wijngaards proposed explicitly obtaining authoritative child
        NS data in <xref target="I-D.wijngaards-dnsext-resolver-side-mitigation" format="default"/>. 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 <xref target="I-D.vixie-dnsext-resimprove" format="default"/>, by Vixie, Joffe,
	and Neves.
      </t>
    </section>

  </back>
</rfc>