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Negative Caching of DNS Resolution Failures
RFC 9520

Document Type RFC - Proposed Standard (December 2023)
Authors Duane Wessels , William Carroll , Matthew Thomas
Last updated 2023-12-22
RFC stream Internet Engineering Task Force (IETF)
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IESG Responsible AD Warren "Ace" Kumari
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RFC 9520


Internet Engineering Task Force (IETF)                        D. Wessels
Request for Comments: 9520                                    W. Carroll
Updates: 2308, 4035, 4697                                      M. Thomas
Category: Standards Track                                       Verisign
ISSN: 2070-1721                                            December 2023

              Negative Caching of DNS Resolution Failures

Abstract

   In the DNS, resolvers employ caching to reduce both latency for end
   users and load on authoritative name servers.  The process of
   resolution may result in one of three types of responses: (1) a
   response containing the requested data, (2) a response indicating the
   requested data does not exist, or (3) a non-response due to a
   resolution failure in which the resolver does not receive any useful
   information regarding the data's existence.  This document concerns
   itself only with the third type.

   RFC 2308 specifies requirements for DNS negative caching.  There,
   caching of TYPE 2 responses is mandatory and caching of TYPE 3
   responses is optional.  This document updates RFC 2308 to require
   negative caching for DNS resolution failures.

   RFC 4035 allows DNSSEC validation failure caching.  This document
   updates RFC 4035 to require caching for DNSSEC validation failures.

   RFC 4697 prohibits aggressive requerying for NS records at a failed
   zone's parent zone.  This document updates RFC 4697 to expand this
   requirement to all query types and to all ancestor zones.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9520.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Motivation
     1.2.  Related Work
     1.3.  Terminology
   2.  Conditions That Lead to DNS Resolution Failures
     2.1.  SERVFAIL Responses
     2.2.  REFUSED Responses
     2.3.  Timeouts and Unreachable Servers
     2.4.  Delegation Loops
     2.5.  Alias Loops
     2.6.  DNSSEC Validation Failures
     2.7.  FORMERR Responses
   3.  Requirements for Caching DNS Resolution Failures
     3.1.  Retries and Timeouts
     3.2.  Caching
     3.3.  Requerying Delegation Information
     3.4.  DNSSEC Validation Failures
   4.  IANA Considerations
   5.  Security Considerations
   6.  Privacy Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   Caching has always been a fundamental component of DNS resolution on
   the Internet.  For example, [RFC0882] states:

   |  The sheer size of the database and frequency of updates suggest
   |  that it must be maintained in a distributed manner, with local
   |  caching to improve performance.

   The early DNS RFCs ([RFC0882], [RFC0883], [RFC1034], and [RFC1035])
   primarily discuss caching in the context of what [RFC2308] calls
   "positive responses", that is, when the response includes the
   requested data.  In this case, a TTL is associated with each Resource
   Record (RR) in the response.  Resolvers can cache and reuse the data
   until the TTL expires.

   Section 4.3.4 of [RFC1034] describes negative response caching, but
   notes it is optional and only talks about name errors (NXDOMAIN).
   This is the origin of using the SOA MINIMUM field as a negative
   caching TTL.

   [RFC2308] updated [RFC1034] to specify new requirements for DNS
   negative caching, including making it mandatory for caching resolvers
   to cache name error (NXDOMAIN) and no data (NODATA) responses when an
   SOA record is available to provide a TTL.  [RFC2308] further
   specified optional negative caching for two DNS resolution failure
   cases: server failure and dead/unreachable servers.

   This document updates [RFC2308] to require negative caching of all
   DNS resolution failures and provides additional examples of
   resolution failures, [RFC4035] to require caching for DNSSEC
   validation failures, as well as [RFC4697] to expand the scope of
   prohibiting aggressive requerying for NS records at a failed zone's
   parent zone to all query types and to all ancestor zones.

1.1.  Motivation

   Operators of DNS services have known for some time that recursive
   resolvers become more aggressive when they experience resolution
   failures.  A number of different anecdotes, experiments, and
   incidents support this claim.

   In December 2009, a secondary server for a number of in-addr.arpa
   subdomains saw its traffic suddenly double, and queries of type
   DNSKEY in particular increase by approximately two orders of
   magnitude, coinciding with a DNSSEC key rollover by the zone operator
   [DNSSEC-ROLLOVER].  This predated a signed root zone, and an
   operating system vendor was providing non-root trust anchors to the
   recursive resolver, which became out of date following the rollover.
   Unable to validate responses for the affected in-addr.arpa zones,
   recursive resolvers aggressively retried their queries.

   In 2016, the Internet infrastructure company Dyn experienced a large
   attack that impacted many high-profile customers.  As documented in a
   technical presentation detailing the attack (see [RETRY-STORM]), Dyn
   staff wrote:

   |  At this point we are now experiencing botnet attack traffic and
   |  what is best classified as a "retry storm"
   |  
   |  Looking at certain large recursive platforms > 10x normal volume

   In 2018, the root zone Key Signing Key (KSK) was rolled over
   [KSK-ROLLOVER].  Throughout the rollover period, the root servers
   experienced a significant increase in DNSKEY queries.  Before the
   rollover, a.root-servers.net and j.root-servers.net together received
   about 15 million DNSKEY queries per day.  At the end of the
   revocation period, they received 1.2 billion per day: an 80x
   increase.  Removal of the revoked key from the zone caused DNSKEY
   queries to drop to post-rollover but pre-revoke levels, indicating
   there is still a population of recursive resolvers using the previous
   root trust anchor and aggressively retrying DNSKEY queries.

   In 2021, Verisign researchers used botnet query traffic to
   demonstrate that certain large public recursive DNS services exhibit
   very high query rates when all authoritative name servers for a zone
   return refused (REFUSED) or server failure (SERVFAIL) responses (see
   [BOTNET]).  When the authoritative servers were configured normally,
   query rates for a single botnet domain averaged approximately 50
   queries per second.  However, with the servers configured to return
   SERVFAIL, the query rate increased to 60,000 per second.
   Furthermore, increases were also observed at the root and Top-Level
   Domain (TLD) levels, even though delegations at those levels were
   unchanged and continued operating normally.

   Later that same year, on October 4, Facebook experienced a widespread
   and well-publicized outage [FB-OUTAGE].  During the 6-hour outage,
   none of Facebook's authoritative name servers were reachable and did
   not respond to queries.  Recursive name servers attempting to resolve
   Facebook domains experienced timeouts.  During this time, query
   traffic on the .COM/.NET infrastructure increased from 7,000 to
   900,000 queries per second [OUTAGE-RESOLVER].

1.2.  Related Work

   [RFC2308] describes negative caching for four types of DNS queries
   and responses: name errors, no data, server failures, and dead/
   unreachable servers.  It places the strongest requirements on
   negative caching for name errors and no data responses, while server
   failures and dead servers are left as optional.

   [RFC4697] is a Best Current Practice that documents observed
   resolution misbehaviors.  It describes a number of situations that
   can lead to excessive queries from recursive resolvers, including
   requerying for delegation data, lame servers, responses blocked by
   firewalls, and records with zero TTL.  [RFC4697] makes a number of
   recommendations, varying from "SHOULD" to "MUST".

   [THUNDERING-HERD] describes "The DNS thundering herd problem" as a
   situation arising when cached data expires at the same time for a
   large number of users.  Although that document is not focused on
   negative caching, it does describe the benefits of combining multiple
   identical queries to upstream name servers.  That is, when a
   recursive resolver receives multiple queries for the same name,
   class, and type that cannot be answered from cached data, it should
   combine or join them into a single upstream query rather than emit
   repeated identical upstream queries.

   [RFC5452], "Measures for Making DNS More Resilient against Forged
   Answers", includes a section that describes the phenomenon known as
   "Birthday Attacks".  Here, again, the problem arises when a recursive
   resolver emits multiple identical upstream queries.  Multiple
   outstanding queries make it easier for an attacker to guess and
   correctly match some of the DNS message parameters, such as the port
   number and ID field.  This situation is further exacerbated in the
   case of timeout-based resolution failures.  Of course, DNSSEC is a
   suitable defense to spoofing attacks.

   [RFC8767] describes "Serving Stale Data to Improve DNS Resiliency".
   This permits a recursive resolver to return possibly stale data when
   it is unable to refresh cached, expired data.  It introduces the idea
   of a failure recheck timer and says:

   |  Attempts to refresh from non-responsive or otherwise failing
   |  authoritative nameservers are recommended to be done no more
   |  frequently than every 30 seconds.

1.3.  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.

   DNS transport:  In this document, "DNS transport" means a protocol
      used to transport DNS messages between a client and a server.
      This includes "classic DNS" transports, i.e., DNS-over-UDP and
      DNS-over-TCP [RFC1034] [RFC7766], as well as newer encrypted DNS
      transports, such as DNS-over-TLS [RFC7858], DNS-over-HTTPS
      [RFC8484], DNS-over-QUIC [RFC9250], and similar communication of
      DNS messages using other protocols.  Note: at the time of writing,
      not all DNS transports are standardized for all types of servers
      but may become standardized in the future.

2.  Conditions That Lead to DNS Resolution Failures

   A DNS resolution failure occurs when none of the servers available to
   a resolver client provide any useful response data for a particular
   query name, type, and class.  A response is considered useful when it
   provides either the requested data, a referral to a descendant zone,
   or an indication that no data exists at the given name.

   It is common for resolvers to have multiple servers from which to
   choose for a particular query.  For example, in the case of stub-to-
   recursive, the stub resolver may be configured with multiple
   recursive resolver addresses.  In the case of recursive-to-
   authoritative, a given zone usually has more than one name server (NS
   record), each of which can have multiple IP addresses and multiple
   DNS transports.

   Nothing in this document prevents a resolver from retrying a query at
   a different server or the same server over a different DNS transport.
   In the case of timeouts, a resolver can retry the same server and DNS
   transport a limited number of times.

   If any one of the available servers provides a useful response, then
   it is not considered a resolution failure.  However, if none of the
   servers for a given query tuple <name, type, class> provide a useful
   response, the result is a resolution failure.

   Note that NXDOMAIN and NOERROR/NODATA responses are not conditions
   for resolution failure.  In these cases, the server is providing a
   useful response, indicating either that a name does not exist or that
   no data of the requested type exists at the name.  These negative
   responses can be cached as described in [RFC2308].

   The remainder of this section describes a number of different
   conditions that can lead to resolution failure.  This section is not
   exhaustive.  Additional conditions may be expected to cause similar
   resolution failures.

2.1.  SERVFAIL Responses

   Server failure is defined in [RFC1035] as: "The name server was
   unable to process this query due to a problem with the name server."
   A server failure is signaled by setting the RCODE field to SERVFAIL.

   Authoritative servers return SERVFAIL when they don't have any valid
   data for a zone.  For example, a secondary server has been configured
   to serve a particular zone but is unable to retrieve or refresh the
   zone data from the primary server.

   Recursive servers return SERVFAIL in response to a number of
   different conditions, including many described below.

   Although the extended DNS errors method exists "primarily to extend
   SERVFAIL to provide additional information," it "does not change the
   processing of RCODEs" [RFC8914].  This document operates at the level
   of resolution failure and does not concern particular causes.

2.2.  REFUSED Responses

   A name server returns a message with the RCODE field set to REFUSED
   when it refuses to process the query, e.g., for policy or other
   reasons [RFC1035].

   Authoritative servers generally return REFUSED when processing a
   query for which they are not authoritative.  For example, a server
   that is configured to be authoritative for only the example.net zone
   may return REFUSED in response to a query for example.com.

   Recursive servers generally return REFUSED for query sources that do
   not match configured access control lists.  For example, a server
   that is configured to allow queries from only 2001:db8:1::/48 may
   return REFUSED in response to a query from 2001:db8:5::1.

2.3.  Timeouts and Unreachable Servers

   A timeout occurs when a resolver fails to receive any response from a
   server within a reasonable amount of time.  Additionally, a DNS
   transport may more quickly indicate lack of reachability in a way
   that wouldn't be considered a timeout: for example, an ICMP port
   unreachable message, a TCP "connection refused" error, or a TLS
   handshake failure.  [RFC2308] refers to these conditions collectively
   as "dead / unreachable servers".

   Note that resolver implementations may have two types of timeouts: a
   smaller timeout that might trigger a query retry and a larger timeout
   after which the server is considered unresponsive.  Section 3.1
   discusses the requirements for resolvers when retrying queries.

   Timeouts can present a particular problem for negative caching,
   depending on how the resolver handles multiple outstanding queries
   for the same <query name, type, class> tuple.  For example, consider
   a very popular website in a zone whose name servers are all
   unresponsive.  A recursive resolver might receive tens or hundreds of
   queries per second for that website.  If the recursive server
   implementation joins these outstanding queries together, then it only
   sends one recursive-to-authoritative query for the numerous pending
   stub-to-recursive queries.  However, if the implementation does not
   join outstanding queries together, then it sends one recursive-to-
   authoritative query for each stub-to-recursive query.  If the
   incoming query rate is high and the timeout is large, this might
   result in hundreds or thousands of recursive-to-authoritative queries
   while waiting for an authoritative server to time out.

   A recursive resolver that does not join outstanding queries together
   is more susceptible to Birthday Attacks ([RFC5452], Section 5),
   especially when those queries result in timeouts.

2.4.  Delegation Loops

   A delegation loop, or cycle, can occur when one domain utilizes name
   servers in a second domain, and the second domain uses name servers
   in the first.  For example:

   FOO.EXAMPLE.    NS      NS1.EXAMPLE.COM.
   FOO.EXAMPLE.    NS      NS2.EXAMPLE.COM.

   EXAMPLE.COM.    NS      NS1.FOO.EXAMPLE.
   EXAMPLE.COM.    NS      NS2.FOO.EXAMPLE.

   In this example, no names under foo.example or example.com can be
   resolved because of the delegation loop.  Note that a delegation loop
   may involve more than two domains.  A resolver that does not detect
   delegation loops may generate DDoS-levels of attack traffic to
   authoritative name servers, as documented in the TsuNAME
   vulnerability [TsuNAME].

2.5.  Alias Loops

   An alias loop, or cycle, can occur when one CNAME or DNAME RR refers
   to a second name, which, in turn, is specified as an alias for the
   first.  For example:

   APP.FOO.EXAMPLE.        CNAME   APP.EXAMPLE.NET.
   APP.EXAMPLE.NET.        CNAME   APP.FOO.EXAMPLE.

   The need to detect CNAME loops has been known since at least
   [RFC1034], which states in Section 3.6.2:

   |  Of course, by the robustness principle, domain software should not
   |  fail when presented with CNAME chains or loops; CNAME chains
   |  should be followed and CNAME loops signalled as an error.

2.6.  DNSSEC Validation Failures

   For zones that are signed with DNSSEC, a resolution failure can occur
   when a security-aware resolver believes it should be able to
   establish a chain of trust for an RRset but is unable to do so,
   possibly after trying multiple authoritative name servers.  DNSSEC
   validation failures may be due to signature mismatch, missing DNSKEY
   RRs, problems with denial-of-existence records, clock skew, or other
   reasons.

   Section 4.7 of [RFC4035] already discusses the requirements and
   reasons for caching validation failures.  Section 3.4 of this
   document strengthens those requirements.

2.7.  FORMERR Responses

   A name server returns a message with the RCODE field set to FORMERR
   when it is unable to interpret the query [RFC1035].  FORMERR
   responses are often associated with problems processing Extension
   Mechanisms for DNS (EDNS(0)) [RFC6891].  Authoritative servers may
   return FORMERR when they do not implement EDNS(0), or when EDNS(0)
   option fields are malformed, but not for unknown EDNS(0) options.

   Upon receipt of a FORMERR response, some recursive clients will retry
   their queries without EDNS(0), while others will not.  Nonetheless,
   resolution failures from FORMERR responses are rare.

3.  Requirements for Caching DNS Resolution Failures

3.1.  Retries and Timeouts

   A resolver MUST NOT retry a given query to a server address over a
   given DNS transport more than twice (i.e., three queries in total)
   before considering the server address unresponsive over that DNS
   transport for that query.

   A resolver MAY retry a given query over a different DNS transport to
   the same server if it has reason to believe the DNS transport is
   available for that server and is compatible with the resolver's
   security policies.

   This document does not place any requirements on how long an
   implementation should wait before retrying a query (aka a timeout
   value), which may be implementation or configuration dependent.  It
   is generally expected that typical timeout values range from 3 to 30
   seconds.

3.2.  Caching

   Resolvers MUST implement a cache for resolution failures.  The
   purpose of this cache is to eliminate repeated upstream queries that
   cannot be resolved.  When an incoming query matches a cached
   resolution failure, the resolver MUST NOT send any corresponding
   outgoing queries until after the cache entries expire.

   Implementation details for such a cache are not specified in this
   document.  The implementation might cache different resolution
   failure conditions differently.  For example, DNSSEC validation
   failures might be cached according to the queried name, class, and
   type, whereas unresponsive servers might be cached only according to
   the server's IP address.  Developers should document their
   implementation choices so that operators know what behaviors to
   expect when resolution failures are cached.

   Resolvers MUST cache resolution failures for at least 1 second.
   Resolvers MAY cache different types of resolution failures for
   different (i.e., longer) amounts of time.  Consistent with [RFC2308],
   resolution failures MUST NOT be cached for longer than 5 minutes.

   The minimum cache duration SHOULD be configurable by the operator.  A
   longer cache duration for resolution failures will reduce the
   processing burden from repeated queries but may also increase the
   time to recover from transitory issues.

   Resolvers SHOULD employ an exponential or linear backoff algorithm to
   increase the cache duration for persistent resolution failures.  For
   example, the initial time for negatively caching a resolution failure
   might be set to 5 seconds and increased after each retry that results
   in another resolution failure, up to a configurable maximum, not to
   exceed the 5-minute upper limit.

   Notwithstanding the above, resolvers SHOULD implement measures to
   mitigate resource exhaustion attacks on the failed resolution cache.
   That is, the resolver should limit the amount of memory and/or
   processing time devoted to this cache.

3.3.  Requerying Delegation Information

   Section 2.1 of [RFC4697] identifies circumstances in which:

   |  ...every name server in a zone's NS RRSet is unreachable (e.g.,
   |  during a network outage), unavailable (e.g., the name server
   |  process is not running on the server host), or misconfigured
   |  (e.g., the name server is not authoritative for the given zone,
   |  also known as "lame").

   It prohibits unnecessary "aggressive requerying" to the parent of a
   non-responsive zone by sending NS queries.

   The problem of aggressive requerying to parent zones is not limited
   to queries of type NS.  This document updates the requirement from
   Section 2.1.1 of [RFC4697] to apply more generally:

   |  Upon encountering a zone whose name servers are all non-
   |  responsive, a resolver MUST cache the resolution failure.
   |  Furthermore, the resolver MUST limit queries to the non-responsive
   |  zone's parent zone (and to other ancestor zones) just as it would
   |  limit subsequent queries to the non-responsive zone.

3.4.  DNSSEC Validation Failures

   Section 4.7 of [RFC4035] states:

   |  To prevent such unnecessary DNS traffic, security-aware resolvers
   |  MAY cache data with invalid signatures, with some restrictions.

   This document updates [RFC4035] with the following, stronger,
   requirement:

   |  To prevent such unnecessary DNS traffic, security-aware resolvers
   |  MUST cache DNSSEC validation failures, with some restrictions.

   One of the restrictions mentioned in [RFC4035] is to use a small TTL
   when caching data that fails DNSSEC validation.  This is, in part,
   because the provided TTL cannot be trusted.  The advice from
   Section 3.2 herein can be used as guidance on TTLs for caching DNSSEC
   validation failures.

4.  IANA Considerations

   This document has no IANA actions.

5.  Security Considerations

   As noted in Section 3.2, an attacker might attempt a resource
   exhaustion attack by sending queries for a large number of names and/
   or types that result in resolution failure.  Resolvers SHOULD
   implement measures to protect themselves and bound the amount of
   memory devoted to caching resolution failures.

   A cache poisoning attack (see Section 2.2 of [RFC7873]) resulting in
   denial of service may be possible because failure messages cannot be
   signed.  An attacker might generate queries and send forged failure
   messages, causing the resolver to cease sending queries to the
   authoritative name server (see Section 2.6 of [RFC4732] for a similar
   "data corruption attack" and Section 5.2 of [TuDoor] for a "DNSDoS
   attack").  However, this would require continued spoofing throughout
   the backoff period and repeated attacks due to the 5-minute cache
   limit.  As in Section 4.1.12 of [RFC4686], this attack's effects
   would be "localized and of limited duration".

6.  Privacy Considerations

   This specification has no impact on user privacy.

7.  References

7.1.  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>.

   [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>.

   [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>.

   [RFC4697]  Larson, M. and P. Barber, "Observed DNS Resolution
              Misbehavior", BCP 123, RFC 4697, DOI 10.17487/RFC4697,
              October 2006, <https://www.rfc-editor.org/info/rfc4697>.

   [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>.

7.2.  Informative References

   [BOTNET]   Wessels, D. and M. Thomas, "Botnet Traffic Observed at
              Various Levels of the DNS Hierarchy", May 2021,
              <https://indico.dns-oarc.net/event/38/contributions/841/>.

   [DNSSEC-ROLLOVER]
              Michaleson, G., Wallström, P., Arends, R., and G. Huston,
              "Roll Over and Die?", February 2010,
              <https://www.potaroo.net/ispcol/2010-02/rollover.html>.

   [FB-OUTAGE]
              Janardhan, S., "More details about the October 4 outage",
              October 2021, <https://engineering.fb.com/2021/10/05/
              networking-traffic/outage-details/>.

   [KSK-ROLLOVER]
              Müller, M., Thomas, M., Wessels, D., Hardaker, W., Chung,
              T., Toorop, W., and R. van Rijswijk-Deij, "Roll, Roll,
              Roll Your Root: A Comprehensive Analysis of the First Ever
              DNSSEC Root KSK Rollover", IMC '19: Proceedings of the
              Internet Measurement Conference, Pages 1-14,
              DOI 10.1145/3355369.3355570, October 2019,
              <https://doi.org/10.1145/3355369.3355570>.

   [OUTAGE-RESOLVER]
              Verisign, "Observations on Resolver Behavior During DNS
              Outages", January 2022,
              <https://blog.verisign.com/security/facebook-dns-outage/>.

   [RETRY-STORM]
              Sullivan, A., "Dyn, DDoS, and DNS", March 2017,
              <https://ccnso.icann.org/sites/default/files/file/field-
              file-attach/2017-04/presentation-oracle-dyn-ddos-dns-
              13mar17-en.pdf>.

   [RFC0882]  Mockapetris, P., "Domain names: Concepts and facilities",
              RFC 882, DOI 10.17487/RFC0882, November 1983,
              <https://www.rfc-editor.org/info/rfc882>.

   [RFC0883]  Mockapetris, P., "Domain names: Implementation
              specification", RFC 883, DOI 10.17487/RFC0883, November
              1983, <https://www.rfc-editor.org/info/rfc883>.

   [RFC4686]  Fenton, J., "Analysis of Threats Motivating DomainKeys
              Identified Mail (DKIM)", RFC 4686, DOI 10.17487/RFC4686,
              September 2006, <https://www.rfc-editor.org/info/rfc4686>.

   [RFC4732]  Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet
              Denial-of-Service Considerations", RFC 4732,
              DOI 10.17487/RFC4732, December 2006,
              <https://www.rfc-editor.org/info/rfc4732>.

   [RFC5452]  Hubert, A. and R. van Mook, "Measures for Making DNS More
              Resilient against Forged Answers", RFC 5452,
              DOI 10.17487/RFC5452, January 2009,
              <https://www.rfc-editor.org/info/rfc5452>.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,
              <https://www.rfc-editor.org/info/rfc6891>.

   [RFC7766]  Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
              D. Wessels, "DNS Transport over TCP - Implementation
              Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
              <https://www.rfc-editor.org/info/rfc7766>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC7873]  Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
              Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,
              <https://www.rfc-editor.org/info/rfc7873>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

   [RFC8767]  Lawrence, D., Kumari, W., and P. Sood, "Serving Stale Data
              to Improve DNS Resiliency", RFC 8767,
              DOI 10.17487/RFC8767, March 2020,
              <https://www.rfc-editor.org/info/rfc8767>.

   [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>.

   [RFC9250]  Huitema, C., Dickinson, S., and A. Mankin, "DNS over
              Dedicated QUIC Connections", RFC 9250,
              DOI 10.17487/RFC9250, May 2022,
              <https://www.rfc-editor.org/info/rfc9250>.

   [THUNDERING-HERD]
              Sivaraman, M. and C. Liu, "The DNS thundering herd
              problem", Work in Progress, Internet-Draft, draft-muks-
              dnsop-dns-thundering-herd-00, 25 June 2020,
              <https://datatracker.ietf.org/doc/html/draft-muks-dnsop-
              dns-thundering-herd-00>.

   [TsuNAME]  Moura, G. C. M., Castro, S., Heidemann, J., and W.
              Hardaker, "TsuNAME: exploiting misconfiguration and
              vulnerability to DDoS DNS", IMC '21: Proceedings of the
              21st ACM Internet Measurement Conference, Pages 398-418,
              DOI 10.1145/3487552.3487824, November 2021,
              <https://doi.org/10.1145/3487552.3487824>.

   [TuDoor]   Li, X., Xu, W., Liu, B., Zhang, M., Li, Z., Zhang, J.,
              Chang, D., Zheng, X., Wang, C., Chen, J., Duan, H., and Q.
              Li, "TuDoor Attack: Systematically Exploring and
              Exploiting Logic Vulnerabilities in DNS Response Pre-
              processing with Malformed Packets", IEEE Symposium on
              Security and Privacy (SP), DOI 10.1109/SP54263.2024.00046,
              2024, <https://doi.ieeecomputersociety.org/10.1109/
              SP54263.2024.00046>.

Acknowledgments

   The authors wish to thank Mukund Sivaraman, Petr Spacek, Peter van
   Dijk, Tim Wicinksi, Joe Abley, Evan Hunt, Barry Leiba, Lucas Pardue,
   Paul Wouters, and other members of the DNSOP Working Group for their
   feedback and contributions.

Authors' Addresses

   Duane Wessels
   Verisign
   12061 Bluemont Way
   Reston, VA 20190
   United States of America
   Phone: +1 703 948-3200
   Email: dwessels@verisign.com
   URI:   https://verisign.com

   William Carroll
   Verisign
   12061 Bluemont Way
   Reston, VA 20190
   United States of America
   Phone: +1 703 948-3200
   Email: wicarroll@verisign.com
   URI:   https://verisign.com

   Matthew Thomas
   Verisign
   12061 Bluemont Way
   Reston, VA 20190
   United States of America
   Phone: +1 703 948-3200
   Email: mthomas@verisign.com
   URI:   https://verisign.com