



Internet Engineering Task Force (IETF)                       D. Lawrence
Request for Comments: 8767                                        Oracle
Updates: 1034, 1035, 2181                                      W. Kumari
Category: Standards Track                                        P. Sood
ISSN: 2070-1721                                                   Google
                                                              March 2020


              Serving Stale Data to Improve DNS Resiliency

Abstract

   This document defines a method (serve-stale) for recursive resolvers
   to use stale DNS data to avoid outages when authoritative nameservers
   cannot be reached to refresh expired data.  One of the motivations
   for serve-stale is to make the DNS more resilient to DoS attacks and
   thereby make them less attractive as an attack vector.  This document
   updates the definitions of TTL from RFCs 1034 and 1035 so that data
   can be kept in the cache beyond the TTL expiry; it also updates RFC
   2181 by interpreting values with the high-order bit set as being
   positive, rather than 0, and suggests a cap of 7 days.

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/rfc8767.

Copyright Notice

   Copyright (c) 2020 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
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   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.  Terminology
   3.  Background
   4.  Standards Action
   5.  Example Method
   6.  Implementation Considerations
   7.  Implementation Caveats
   8.  Implementation Status
   9.  EDNS Option
   10. Security Considerations
   11. Privacy Considerations
   12. NAT Considerations
   13. IANA Considerations
   14. References
     14.1.  Normative References
     14.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   Traditionally, the Time To Live (TTL) of a DNS Resource Record (RR)
   has been understood to represent the maximum number of seconds that a
   record can be used before it must be discarded, based on its
   description and usage in [RFC1035] and clarifications in [RFC2181].

   This document expands the definition of the TTL to explicitly allow
   for expired data to be used in the exceptional circumstance that a
   recursive resolver is unable to refresh the information.  It is
   predicated on the observation that authoritative answer
   unavailability can cause outages even when the underlying data those
   servers would return is typically unchanged.

   We describe a method below for this use of stale data, balancing the
   competing needs of resiliency and freshness.

   This document updates the definitions of TTL from [RFC1034] and
   [RFC1035] so that data can be kept in the cache beyond the TTL
   expiry; it also updates [RFC2181] by interpreting values with the
   high-order bit set as being positive, rather than 0, and also
   suggests a cap of 7 days.

2.  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.

   For a glossary of DNS terms, please see [RFC8499].

3.  Background

   There are a number of reasons why an authoritative server may become
   unreachable, including Denial-of-Service (DoS) attacks, network
   issues, and so on.  If a recursive server is unable to contact the
   authoritative servers for a query but still has relevant data that
   has aged past its TTL, that information can still be useful for
   generating an answer under the metaphorical assumption that "stale
   bread is better than no bread."

   [RFC1035], Section 3.2.1 says that the TTL "specifies the time
   interval that the resource record may be cached before the source of
   the information should again be consulted."  [RFC1035], Section 4.1.3
   further says that the TTL "specifies the time interval (in seconds)
   that the resource record may be cached before it should be
   discarded."

   A natural English interpretation of these remarks would seem to be
   clear enough that records past their TTL expiration must not be used.
   However, [RFC1035] predates the more rigorous terminology of
   [RFC2119], which softened the interpretation of "may" and "should".

   [RFC2181] aimed to provide "the precise definition of the Time to
   Live," but Section 8 of [RFC2181] was mostly concerned with the
   numeric range of values rather than data expiration behavior.  It
   does, however, close that section by noting, "The TTL specifies a
   maximum time to live, not a mandatory time to live."  This wording
   again does not contain BCP 14 key words [RFC2119], but it does convey
   the natural language connotation that data becomes unusable past TTL
   expiry.

   As of the time of this writing, several large-scale operators use
   stale data for answers in some way.  A number of recursive resolver
   packages, including BIND, Knot Resolver, OpenDNS, and Unbound,
   provide options to use stale data.  Apple macOS can also use stale
   data as part of the Happy Eyeballs algorithms in mDNSResponder.  The
   collective operational experience is that using stale data can
   provide significant benefit with minimal downside.

4.  Standards Action

   The definition of TTL in Sections 3.2.1 and 4.1.3 of [RFC1035] is
   amended to read:

   TTL  a 32-bit unsigned integer number of seconds that specifies the
        duration that the resource record MAY be cached before the
        source of the information MUST again be consulted.  Zero values
        are interpreted to mean that the RR can only be used for the
        transaction in progress, and should not be cached.  Values
        SHOULD be capped on the order of days to weeks, with a
        recommended cap of 604,800 seconds (7 days).  If the data is
        unable to be authoritatively refreshed when the TTL expires, the
        record MAY be used as though it is unexpired.  See Sections 5
        and 6 of [RFC8767] for details.

   Interpreting values that have the high-order bit set as being
   positive, rather than 0, is a change from [RFC2181], the rationale
   for which is explained in Section 6.  Suggesting a cap of 7 days,
   rather than the 68 years allowed by the full 31 bits of Section 8 of
   [RFC2181], reflects the current practice of major modern DNS
   resolvers.

   When returning a response containing stale records, a recursive
   resolver MUST set the TTL of each expired record in the message to a
   value greater than 0, with a RECOMMENDED value of 30 seconds.  See
   Section 6 for explanation.

   Answers from authoritative servers that have a DNS response code of
   either 0 (NoError) or 3 (NXDomain) and the Authoritative Answer (AA)
   bit set MUST be considered to have refreshed the data at the
   resolver.  Answers from authoritative servers that have any other
   response code SHOULD be considered a failure to refresh the data and
   therefore leave any previous state intact.  See Section 6 for a
   discussion.

5.  Example Method

   There is more than one way a recursive resolver could responsibly
   implement this resiliency feature while still respecting the intent
   of the TTL as a signal for when data is to be refreshed.

   In this example method, four notable timers drive considerations for
   the use of stale data:

   *  A client response timer, which is the maximum amount of time a
      recursive resolver should allow between the receipt of a
      resolution request and sending its response.

   *  A query resolution timer, which caps the total amount of time a
      recursive resolver spends processing the query.

   *  A failure recheck timer, which limits the frequency at which a
      failed lookup will be attempted again.

   *  A maximum stale timer, which caps the amount of time that records
      will be kept past their expiration.

   Most recursive resolvers already have the query resolution timer and,
   effectively, some kind of failure recheck timer.  The client response
   timer and maximum stale timer are new concepts for this mechanism.

   When a recursive resolver receives a request, it should start the
   client response timer.  This timer is used to avoid client timeouts.
   It should be configurable, with a recommended value of 1.8 seconds as
   being just under a common timeout value of 2 seconds while still
   giving the resolver a fair shot at resolving the name.

   The resolver then checks its cache for any unexpired records that
   satisfy the request and returns them if available.  If it finds no
   relevant unexpired data and the Recursion Desired flag is not set in
   the request, it should immediately return the response without
   consulting the cache for expired records.  Typically, this response
   would be a referral to authoritative nameservers covering the zone,
   but the specifics are implementation dependent.

   If iterative lookups will be done, then the failure recheck timer is
   consulted.  Attempts to refresh from non-responsive or otherwise
   failing authoritative nameservers are recommended to be done no more
   frequently than every 30 seconds.  If this request was received
   within this period, the cache may be immediately consulted for stale
   data to satisfy the request.

   Outside the period of the failure recheck timer, the resolver should
   start the query resolution timer and begin the iterative resolution
   process.  This timer bounds the work done by the resolver when
   contacting external authorities and is commonly around 10 to 30
   seconds.  If this timer expires on an attempted lookup that is still
   being processed, the resolution effort is abandoned.

   If the answer has not been completely determined by the time the
   client response timer has elapsed, the resolver should then check its
   cache to see whether there is expired data that would satisfy the
   request.  If so, it adds that data to the response message with a TTL
   greater than 0 (as specified in Section 4).  The response is then
   sent to the client while the resolver continues its attempt to
   refresh the data.

   When no authorities are able to be reached during a resolution
   attempt, the resolver should attempt to refresh the delegation and
   restart the iterative lookup process with the remaining time on the
   query resolution timer.  This resumption should be done only once per
   resolution effort.

   Outside the resolution process, the maximum stale timer is used for
   cache management and is independent of the query resolution process.
   This timer is conceptually different from the maximum cache TTL that
   exists in many resolvers, the latter being a clamp on the value of
   TTLs as received from authoritative servers and recommended to be
   7 days in the TTL definition in Section 4.  The maximum stale timer
   should be configurable.  It defines the length of time after a record
   expires that it should be retained in the cache.  The suggested value
   is between 1 and 3 days.

6.  Implementation Considerations

   This document mainly describes the issues behind serving stale data
   and intentionally does not provide a formal algorithm.  The concept
   is not overly complex, and the details are best left to resolver
   authors to implement in their codebases.  The processing of serve-
   stale is a local operation, and consistent variables between
   deployments are not needed for interoperability.  However, we would
   like to highlight the impact of various implementation choices,
   starting with the timers involved.

   The most obvious of these is the maximum stale timer.  If this
   variable is too large, it could cause excessive cache memory usage,
   but if it is too small, the serve-stale technique becomes less
   effective, as the record may not be in the cache to be used if
   needed.  Shorter values, even less than a day, can effectively handle
   the vast majority of outages.  Longer values, as much as a week, give
   time for monitoring systems to notice a resolution problem and for
   human intervention to fix it; operational experience has been that
   sometimes the right people can be hard to track down and
   unfortunately slow to remedy the situation.

   Increased memory consumption could be mitigated by prioritizing
   removal of stale records over non-expired records during cache
   exhaustion.  Eviction strategies could consider additional factors,
   including the last time of use or the popularity of a record, to
   retain active but stale records.  A feature to manually flush only
   stale records could also be useful.

   The client response timer is another variable that deserves
   consideration.  If this value is too short, there exists the risk
   that stale answers may be used even when the authoritative server is
   actually reachable but slow; this may result in undesirable answers
   being returned.  Conversely, waiting too long will negatively impact
   user experience.

   The balance for the failure recheck timer is responsiveness in
   detecting the renewed availability of authorities versus the extra
   resource use for resolution.  If this variable is set too large,
   stale answers may continue to be returned even after the
   authoritative server is reachable; per [RFC2308], Section 7, this
   should be no more than 5 minutes.  If this variable is too small,
   authoritative servers may be targeted with a significant amount of
   excess traffic.

   Regarding the TTL to set on stale records in the response,
   historically TTLs of 0 seconds have been problematic for some
   implementations, and negative values can't effectively be
   communicated to existing software.  Other very short TTLs could lead
   to congestive collapse as TTL-respecting clients rapidly try to
   refresh.  The recommended value of 30 seconds not only sidesteps
   those potential problems with no practical negative consequences, it
   also rate-limits further queries from any client that honors the TTL,
   such as a forwarding resolver.

   As for the change to treat a TTL with the high-order bit set as
   positive and then clamping it, as opposed to [RFC2181] treating it as
   zero, the rationale here is basically one of engineering simplicity
   versus an inconsequential operational history.  Negative TTLs had no
   rational intentional meaning that wouldn't have been satisfied by
   just sending 0 instead, and similarly there was realistically no
   practical purpose for sending TTLs of 2^25 seconds (1 year) or more.
   There's also no record of TTLs in the wild having the most
   significant bit set in the DNS Operations, Analysis, and Research
   Center's (DNS-OARC's) "Day in the Life" samples [DITL].  With no
   apparent reason for operators to use them intentionally, that leaves
   either errors or non-standard experiments as explanations as to why
   such TTLs might be encountered, with neither providing an obviously
   compelling reason as to why having the leading bit set should be
   treated differently from having any of the next eleven bits set and
   then capped per Section 4.

   Another implementation consideration is the use of stale nameserver
   addresses for lookups.  This is mentioned explicitly because, in some
   resolvers, getting the addresses for nameservers is a separate path
   from a normal cache lookup.  If authoritative server addresses are
   not able to be refreshed, resolution can possibly still be successful
   if the authoritative servers themselves are up.  For instance,
   consider an attack on a top-level domain that takes its nameservers
   offline; serve-stale resolvers that had expired glue addresses for
   subdomains within that top-level domain would still be able to
   resolve names within those subdomains, even those it had not
   previously looked up.

   The directive in Section 4 that only NoError and NXDomain responses
   should invalidate any previously associated answer stems from the
   fact that no other RCODEs that a resolver normally encounters make
   any assertions regarding the name in the question or any data
   associated with it.  This comports with existing resolver behavior
   where a failed lookup (say, during prefetching) doesn't impact the
   existing cache state.  Some authoritative server operators have said
   that they would prefer stale answers to be used in the event that
   their servers are responding with errors like ServFail instead of
   giving true authoritative answers.  Implementers MAY decide to return
   stale answers in this situation.

   Since the goal of serve-stale is to provide resiliency for all
   obvious errors to refresh data, these other RCODEs are treated as
   though they are equivalent to not getting an authoritative response.
   Although NXDomain for a previously existing name might well be an
   error, it is not handled that way because there is no effective way
   to distinguish operator intent for legitimate cases versus error
   cases.

   During discussion in the IETF, it was suggested that, if all
   authorities return responses with an RCODE of Refused, it may be an
   explicit signal to take down the zone from servers that still have
   the zone's delegation pointed to them.  Refused, however, is also
   overloaded to mean multiple possible failures that could represent
   transient configuration failures.  Operational experience has shown
   that purposely returning Refused is a poor way to achieve an explicit
   takedown of a zone compared to either updating the delegation or
   returning NXDomain with a suitable SOA for extended negative caching.
   Implementers MAY nonetheless consider whether to treat all
   authorities returning Refused as preempting the use of stale data.

7.  Implementation Caveats

   Stale data is used only when refreshing has failed in order to adhere
   to the original intent of the design of the DNS and the behavior
   expected by operators.  If stale data were to always be used
   immediately and then a cache refresh attempted after the client
   response has been sent, the resolver would frequently be sending data
   that it would have had no trouble refreshing.  Because modern
   resolvers use techniques like prefetching and request coalescing for
   efficiency, it is not necessary that every client request needs to
   trigger a new lookup flow in the presence of stale data, but rather
   that a good-faith effort has been recently made to refresh the stale
   data before it is delivered to any client.

   It is important to continue the resolution attempt after the stale
   response has been sent, until the query resolution timeout, because
   some pathological resolutions can take many seconds to succeed as
   they cope with unavailable servers, bad networks, and other problems.
   Stopping the resolution attempt when the response with expired data
   has been sent would mean that answers in these pathological cases
   would never be refreshed.

   The continuing prohibition against using data with a 0-second TTL
   beyond the current transaction explicitly extends to it being
   unusable even for stale fallback, as it is not to be cached at all.

   Be aware that Canonical Name (CNAME) and DNAME records [RFC6672]
   mingled in the expired cache with other records at the same owner
   name can cause surprising results.  This was observed with an initial
   implementation in BIND when a hostname changed from having an IPv4
   Address (A) record to a CNAME.  The version of BIND being used did
   not evict other types in the cache when a CNAME was received, which
   in normal operations is not a significant issue.  However, after both
   records expired and the authorities became unavailable, the fallback
   to stale answers returned the older A instead of the newer CNAME.

8.  Implementation Status

   The algorithm described in Section 5 was originally implemented as a
   patch to BIND 9.7.0.  It has been in use on Akamai's production
   network since 2011; it effectively smoothed over transient failures
   and longer outages that would have resulted in major incidents.  The
   patch was contributed to the Internet Systems Consortium, and the
   functionality is now available in BIND 9.12 and later via the options
   stale-answer-enable, stale-answer-ttl, and max-stale-ttl.

   Unbound has a similar feature for serving stale answers and will
   respond with stale data immediately if it has recently tried and
   failed to refresh the answer by prefetching.  Starting from version
   1.10.0, Unbound can also be configured to follow the algorithm
   described in Section 5.  Both behaviors can be configured and fine-
   tuned with the available serve-expired-* options.

   Knot Resolver has a demo module here: <https://knot-
   resolver.readthedocs.io/en/stable/modules-serve_stale.html>.

   Apple's system resolvers are also known to use stale answers, but the
   details are not readily available.

   In the research paper "When the Dike Breaks: Dissecting DNS Defenses
   During DDoS" [DikeBreaks], the authors detected some use of stale
   answers by resolvers when authorities came under attack.  Their
   research results suggest that more widespread adoption of the
   technique would significantly improve resiliency for the large number
   of requests that fail or experience abnormally long resolution times
   during an attack.

9.  EDNS Option

   During the discussion of serve-stale in the IETF, it was suggested
   that an EDNS option [RFC6891] should be available.  One proposal was
   to use it to opt in to getting data that is possibly stale, and
   another was to signal when stale data has been used for a response.

   The opt-in use case was rejected, as the technique was meant to be
   immediately useful in improving DNS resiliency for all clients.

   The reporting case was ultimately also rejected because even the
   simpler version of a proposed option was still too much bother to
   implement for too little perceived value.

10.  Security Considerations

   The most obvious security issue is the increased likelihood of DNSSEC
   validation failures when using stale data because signatures could be
   returned outside their validity period.  Stale negative records can
   increase the time window where newly published TLSA or DS RRs may not
   be used due to cached NSEC or NSEC3 records.  These scenarios would
   only be an issue if the authoritative servers are unreachable (the
   only time the techniques in this document are used), and thus serve-
   stale does not introduce a new failure in place of what would have
   otherwise been success.

   Additionally, bad actors have been known to use DNS caches to keep
   records alive even after their authorities have gone away.  The
   serve-stale feature potentially makes the attack easier, although
   without introducing a new risk.  In addition, attackers could combine
   this with a DDoS attack on authoritative servers with the explicit
   intent of having stale information cached for a longer period of
   time.  But if attackers have this capacity, they probably could do
   much worse than prolonging the life of old data.

   In [CloudStrife], it was demonstrated how stale DNS data, namely
   hostnames pointing to addresses that are no longer in use by the
   owner of the name, can be used to co-opt security -- for example, to
   get domain-validated certificates fraudulently issued to an attacker.
   While this document does not create a new vulnerability in this area,
   it does potentially enlarge the window in which such an attack could
   be made.  A proposed mitigation is that certificate authorities
   should fully look up each name starting at the DNS root for every
   name lookup.  Alternatively, certificate authorities should use a
   resolver that is not serving stale data.

11.  Privacy Considerations

   This document does not add any practical new privacy issues.

12.  NAT Considerations

   The method described here is not affected by the use of NAT devices.

13.  IANA Considerations

   This document has no IANA actions.

14.  References

14.1.  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>.

   [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>.

   [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>.

   [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>.

14.2.  Informative References

   [CloudStrife]
              Borgolte, K., Fiebig, T., Hao, S., Kruegel, C., and G.
              Vigna, "Cloud Strife: Mitigating the Security Risks of
              Domain-Validated Certificates",
              DOI 10.1145/3232755.3232859, ACM 2018 Applied Networking
              Research Workshop, July 2018, <https://www.ndss-
              symposium.org/wp-content/uploads/2018/02/ndss2018_06A-
              4_Borgolte_paper.pdf>.

   [DikeBreaks]
              Moura, G.C.M., Heidemann, J., Müller, M., Schmidt, R. de
              O., and M. Davids, "When the Dike Breaks: Dissecting DNS
              Defenses During DDoS", DOI 10.1145/3278532.3278534,
              ACM 2018 Internet Measurement Conference, October 2018,
              <https://www.isi.edu/~johnh/PAPERS/Moura18b.pdf>.

   [DITL]     DNS-OARC, "DITL Traces and Analysis", January 2018,
              <https://www.dns-oarc.net/oarc/data/ditl>.

   [RFC6672]  Rose, S. and W. Wijngaards, "DNAME Redirection in the
              DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,
              <https://www.rfc-editor.org/info/rfc6672>.

   [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>.

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

Acknowledgements

   The authors wish to thank Brian Carpenter, Vladimir Cunat, Robert
   Edmonds, Tony Finch, Bob Harold, Tatuya Jinmei, Matti Klock, Jason
   Moreau, Giovane Moura, Jean Roy, Mukund Sivaraman, Davey Song, Paul
   Vixie, Ralf Weber, and Paul Wouters for their review and feedback.
   Paul Hoffman deserves special thanks for submitting a number of Pull
   Requests.

   Thank you also to the following members of the IESG for their final
   review: Roman Danyliw, Benjamin Kaduk, Suresh Krishnan, Mirja
   Kühlewind, and Adam Roach.

Authors' Addresses

   David C Lawrence
   Oracle

   Email: tale@dd.org


   Warren "Ace" Kumari
   Google
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   United States of America

   Email: warren@kumari.net


   Puneet Sood
   Google

   Email: puneets@google.com
