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Negative Caching of DNS Resolution Failures

Document Type Active Internet-Draft (dnsop WG)
Authors Duane Wessels , William Carroll , Matthew Thomas
Last updated 2023-09-21
Replaces draft-dwmtwc-dnsop-caching-resolution-failures
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
Stream WG state Waiting for WG Chair Go-Ahead
Document shepherd Andrew McConachie
Shepherd write-up Show Last changed 2023-07-22
IESG IESG state RFC Ed Queue
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Warren "Ace" Kumari
Send notices to
IANA IANA review state IANA OK - No Actions Needed
IANA action state No IANA Actions
RFC Editor RFC Editor state EDIT
Internet Engineering Task Force                               D. Wessels
Internet-Draft                                                W. Carroll
Updates: 2308, 4035, 4697 (if approved)                        M. Thomas
Intended status: Standards Track                                Verisign
Expires: 24 March 2024                                 21 September 2023

              Negative Caching of DNS Resolution Failures


   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 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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on 24 March 2024.

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 (
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Related Work  . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
   2.  Conditions That Lead to DNS Resolution Failures . . . . . . .   6
     2.1.  SERVFAIL Responses  . . . . . . . . . . . . . . . . . . .   7
     2.2.  REFUSED Responses . . . . . . . . . . . . . . . . . . . .   7
     2.3.  Timeouts and Unreachable Servers  . . . . . . . . . . . .   8
     2.4.  Delegation Loops  . . . . . . . . . . . . . . . . . . . .   8
     2.5.  Alias Loops . . . . . . . . . . . . . . . . . . . . . . .   9
     2.6.  DNSSEC Validation Failures  . . . . . . . . . . . . . . .   9
     2.7.  FORMERR Responses . . . . . . . . . . . . . . . . . . . .   9
   3.  Requirements for Caching DNS Resolution Failures  . . . . . .  10
     3.1.  Retries and Timeouts  . . . . . . . . . . . . . . . . . .  10
     3.2.  Caching . . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.3.  Requerying Delegation Information . . . . . . . . . . . .  11
     3.4.  DNSSEC Validation Failures  . . . . . . . . . . . . . . .  11
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  12
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   8.  Change Log  . . . . . . . . . . . . . . . . . . . . . . . . .  12
   9.  Implementation Status . . . . . . . . . . . . . . . . . . . .  15
     9.1.  BIND  . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     10.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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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 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 a
   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.  This document also updates [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

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
   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
   [roll-over-and-die].  This predated a signed root zone and an

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   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 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 [dyn-attack], 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
   [root-ksk-roll].  Throughout the rollover period, the root servers
   experienced a significant increase in DNSKEY queries.  Before the
   rollover, and 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 or SERVFAIL [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 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 [fb-outage-verisign].

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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."

   An expired Internet-Draft describes "The DNS thundering herd problem"
   [thundering-herd] 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 makes 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.  DNSSEC, of course, 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."

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1.3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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 this
      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.

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   Note that NXDOMAIN and NOERROR/NODATA responses are not conditions
   for resolution failure.  In these cases, the server is providing a
   useful response, either indicating 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 zone,
   may return REFUSED in response to a query for

   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.

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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 which 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

   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 the popular 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.  If, however, 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:



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   In this example, no names under foo.example or 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:


   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 signaled 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

   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 EDNS(0)
   Extensions [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.

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   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 timeout
   value), which may be implementation- or configuration-dependent.  It
   is generally expected that typical timeout values range from 3 to 30

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.

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   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

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

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   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 2.6 of [RFC4732] for a similar "data
   corruption attack").  However, this would require continued spoofing
   throughout the backoff period and required 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.  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.

8.  Change Log

   RFC Editor: Please remove this section before publication.

   This section lists substantial changes to the document as it is being
   worked on.

   From -00 to -01:

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   *  use phrase "the initial TTL for negatively caching a resolution
      failure" instead of "negative cache TTL"

   *  typos, etc

   From dwmtwc-01 to ietf-00:

   *  Adopted by WG

   From -00 to -01:

   *  Clarify retries and timeouts to apply on a per-query basis.

   *  Say more about the 5 second caching requirement in TTLs section.

   *  Expanded opening paragraphs of section 2, now titled "Conditions
      That Lead To DNS Resolution Failures".

   *  Text from the former section 3.3 ("Scope") moved to top of section

   *  Section 3.2 was formerly "TTLs" and is now "Caching".  The draft
      no longer requires e.g. caching by tuples, but now just requires
      caching failures so that repeated queries are not sent out.

   *  State that resolvers should protect themselves from cache resource
      exhaustion attacks.

   From -01 to -02:

   *  Added cache poisoning attack to Security Considerations.

   From -02 to -03:

   *  Added missing reference to Verisign blog post.

   From -03 to -04:

   *  Address most of Peter van Dijk's DNS Directorate review comments.

   *  Removed "For Discussion" section from introduction referencing
      apparent inconsistent RFC2119 keyword use in RFC2308.

   *  Replaced "For Discussion" section from "Requerying Delegation
      Information" to generalize RFC 4697 requirements not to requery
      parent zones to cover all query types.

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   *  Replaced "For Discussion" section from "DNSSEC Validation
      Failures" to strengthen RFC 4035 to require caching of DNSSEC
      validation failures.

   *  Added RFC 4035 and RFC 4697 to updated RFCs list.

   *  Added (empty) Implementation Status section.

   From -04 to -05:

   *  Expanded abstract to include updates to RFCs 4035 and 4697.

   *  Removed reference to unused terms from RFC 8126.

   *  Reworded "server transport" to "a server address over a given

   *  Added explanatory text in "Server Failure" section for exclusion
      of extended DNS errors

   *  Changed "Timeouts" section to "Timeouts and Unreachable Servers"
      and added reference to transport layer indicators from RFC 2308.

   *  Clarified meaning of "timeout value".

   From -05 to -06:

   *  Changed minimum 5 second caching to 1 second, with other changes
      to give implementors and operators more leeway.

   *  Changed "exponential backoff" to more general concept of
      increasing backoff.

   *  Added some implementation status notes for BIND, from dnsop list

   From -06 to -07:

   *  Artart review: minor editorial clarifications

   *  Genart review: remove confusing and superfluous section

   *  Genart review: clarify resolution failure caching time range.

   *  Genart review: better define DNS transports

   *  Dnsdir review: clarify FORMERR response retries.

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   From -07 to -08:

   *  "only exacerbated" -> "further exacerbated"

   *  lowercase IPv6 addresses

   *  lowercase example domain in text

   *  updated introduction to include all updated RFCs

   *  change 3.2 SHOULD to should

   *  section 3.4: say a little about "some restrictions" from RFC 4035

   *  Intdir telechat review: a few grammatical nits

   *  Various IESG reviewer suggestions

9.  Implementation Status

   RFC Editor: Please remove this section before publication.

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in RFC 7942.
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may

9.1.  BIND

   The following is excerpted from a message to the dnsop mailing list
   regarding how BIND caches resolution failures:

   BIND implemented a SERVFAIL cache in 2014 with a default cache
   duration of 10 seconds; after a slew of complaints, in 2015 we
   lowered it to 1 second, and also reduced the configurable maximum
   from 5 minutes to 30 seconds.  The reason was that certain common
   failure conditions are transitory, and it's not unreasonable to
   prioritize rapid recovery.

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   Now, to be clear, the comparison isn't exactly apples to apples: the
   BIND SERVFAIL cache is a somewhat stupider mechanism than the one
   outlined in the draft.  It caches *all* SERVFAIL responses,
   regardless of the reason they were generated.  For example: when the
   cache is cold, a query may time out or hit DDoS mitigation limits
   before it's finished getting through the whole iteration process; an
   immediate retry would start further along the delegation chain and
   would succeed.  Such problems weren't noticeable until we implemented
   the 10-second cache, but became very noticeable afterward.

   If we were able to selectively cache *only* those SERVFAILs that are
   unlikely to recover soon, then five seconds might indeed be a good
   starting point.  But, with our relatively dumb cache, we found that
   one second did a fairly good job reducing the processing burden from
   repeated queries, and eliminated the user complaints about the
   resolver taking forever to recover from short-lived problems.  It's
   been working well enough that it hasn't been a priority to develop a
   more complex failure cache.

10.  References

10.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
              NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,

   [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,

   [RFC4697]  Larson, M. and P. Barber, "Observed DNS Resolution
              Misbehavior", BCP 123, RFC 4697, DOI 10.17487/RFC4697,
              October 2006, <>.

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

10.2.  Informative References

   [botnet]   Wessels, D. and M. Thomas, "Botnet Traffic Observed at
              Various Levels of the DNS Hierarchy", May 2021,

              Sullivan, A., "Dyn, DDoS, and DNS", March 2017,

              Janardhan, S., "More details about the October 4 outage",
              October 2021, <

              Verisign, "Observations on Resolver Behavior During DNS
              Outages", 20 January 2022,

   [RFC0882]  Mockapetris, P., "Domain names: Concepts and facilities",
              RFC 882, DOI 10.17487/RFC0882, November 1983,

   [RFC0883]  Mockapetris, P., "Domain names: Implementation
              specification", RFC 883, DOI 10.17487/RFC0883, November
              1983, <>.

   [RFC4686]  Fenton, J., "Analysis of Threats Motivating DomainKeys
              Identified Mail (DKIM)", RFC 4686, DOI 10.17487/RFC4686,
              September 2006, <>.

   [RFC4732]  Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet
              Denial-of-Service Considerations", RFC 4732,
              DOI 10.17487/RFC4732, December 2006,

   [RFC5452]  Hubert, A. and R. van Mook, "Measures for Making DNS More
              Resilient against Forged Answers", RFC 5452,
              DOI 10.17487/RFC5452, January 2009,

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   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,

   [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,

   [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, <>.

   [RFC7873]  Eastlake 3rd, D. and M. Andrews, "Domain Name System (DNS)
              Cookies", RFC 7873, DOI 10.17487/RFC7873, May 2016,

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,

   [RFC8767]  Lawrence, D., Kumari, W., and P. Sood, "Serving Stale Data
              to Improve DNS Resiliency", RFC 8767,
              DOI 10.17487/RFC8767, March 2020,

   [RFC8914]  Kumari, W., Hunt, E., Arends, R., Hardaker, W., and D.
              Lawrence, "Extended DNS Errors", RFC 8914,
              DOI 10.17487/RFC8914, October 2020,

   [RFC9250]  Huitema, C., Dickinson, S., and A. Mankin, "DNS over
              Dedicated QUIC Connections", RFC 9250,
              DOI 10.17487/RFC9250, May 2022,

              Michaleson, G., Wallström, P., Arends, R., and G. Huston,
              "Roll Over and Die?", February 2010,

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              Müller, M., Thomas, M., Wessels, D., Hardaker, W., Chung,
              T., Toorop, W., and R.v. Rijswijk-Deij, "Roll, Roll, Roll
              Your Root: A Comprehensive Analysis of the First Ever
              DNSSEC Root KSK Rollover", October 2019,

              Sivaraman, M. and C. Liu, "The DNS thundering herd problem
              (expired Internet-Draft)", June 2020,

   [TsuNAME]  Moura, G. C. M., Castro, S., Heidemann, J., and W.
              Hardaker, "TsuNAME: exploiting misconfiguration and
              vulnerability to DDoS DNS", November 2021,

Authors' Addresses

   Duane Wessels
   12061 Bluemont Way
   Reston, VA 20190
   United States of America
   Phone: +1 703 948-3200

   William Carroll
   12061 Bluemont Way
   Reston, VA 20190
   United States of America
   Phone: +1 703 948-3200

   Matthew Thomas
   12061 Bluemont Way
   Reston, VA 20190
   United States of America
   Phone: +1 703 948-3200

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