DNSOP Working Group                                          D. Lawrence
Internet-Draft                                                    Oracle
Updates: 1034, 1035 (if approved)                              W. Kumari
Intended status: Standards Track                                 P. Sood
Expires: September 10, 2019                                       Google
                                                          March 09, 2019

              Serving Stale Data to Improve DNS Resiliency


   This draft 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.  It updates the definition
   of TTL from [RFC1034], [RFC1035], and [RFC2181] to make it clear that
   data can be kept in the cache beyond the TTL expiry and used for
   responses when a refreshed answer is not readily available.  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

Ed note

   Text inside square brackets ([]) is additional background
   information, answers to frequently asked questions, general musings,
   etc.  They will be removed before publication.  This document is
   being collaborated on in GitHub at <https://github.com/vttale/serve-
   stale>.  The most recent version of the document, open issues, etc
   should all be available here.  The authors gratefully accept pull

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 https://datatracker.ietf.org/drafts/current/.

   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 September 10, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Background  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Standards Action  . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Example Method  . . . . . . . . . . . . . . . . . . . . . . .   4
   6.  Implementation Considerations . . . . . . . . . . . . . . . .   6
   7.  Implementation Caveats  . . . . . . . . . . . . . . . . . . .   8
   8.  Implementation Status . . . . . . . . . . . . . . . . . . . .   9
   9.  EDNS Option . . . . . . . . . . . . . . . . . . . . . . . . .   9
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  10
   11. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  10
   12. NAT Considerations  . . . . . . . . . . . . . . . . . . . . .  10
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  11
     15.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Traditionally the Time To Live (TTL) of a DNS resource record 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 proposes that the definition of the TTL be explicitly
   expanded to allow for expired data to be used in the exceptional
   circumstance that a recursive resolver is unable to refresh the

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   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.

2.  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.

   For a comprehensive treatment of DNS terms, please see [RFC7719].

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 the 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", and Section 4.1.3 further
   says 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 in Section 8 was mostly concerned with the numeric range
   of values and the possibility that very large values should be
   capped.  (It also has the curious suggestion that a value in the
   range 2147483648 to 4294967295 should be treated as zero.)  It closes
   that section by noting, "The TTL specifies a maximum time to live,
   not a mandatory time to live."  This is again not [RFC2119]-normative
   language, but does convey the natural language connotation that data
   becomes unusable past TTL expiry.

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   Several major recursive resolver operators currently use stale data
   for answers in some way, including Akamai (in three different
   resolver implementations), BIND, Knot, OpenDNS, and Unbound.  Apple
   MacOS can also use stale data as part of the Happy Eyeballs
   algorithms in mDNSResponder.  The collective operational experience
   is that it provides significant benefit with minimal downside.

4.  Standards Action

   The definition of TTL in [RFC1035] Sections 3.2.1 and 4.1.3 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 orders of days to weeks, with a recommended cap
      of 604,800 seconds.  If the data is unable to be authoritatively
      refreshed when the TTL expires, the record MAY be used as though
      it is unexpired.

   Interpreting values which have the high order bit set as being
   positive, rather than 0, is a change from [RFC2181].  Suggesting a
   cap of seven days, rather than the 68 years allowed by [RFC2181],
   reflects the current practice of major modern DNS resolvers.

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

   Answers from authoritative servers that have a DNS Response Code of
   either 0 (NoError) or 3 (NXDomain) and the Authoritative Answers (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
   therefor leave any previous state intact.

5.  Example Method

   There is conceivably 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, as follows:

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   o  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.

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

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

   o  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 request is received by the recursive resolver, 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 data that
   satisfies the request and of course 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

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   request.  If so, it adds that data to the response message with a TTL
   greater than 0 per 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
   during one 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 above.  The maximum stale timer SHOULD be
   configurable, and defines the length of time after a record expires
   that it SHOULD be retained in the cache.  The suggested value is 7
   days, which gives time for monitoring to notice the resolution
   problem and for human intervention to fix it.

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.  Increased memory consumption could be mitigated by
   prioritizing removal of stale records over non-expired records during
   cache exhaustion.  Implementations may also wish to consider whether
   to track the names in requests for their last time of use or their
   popularity, using that as an additional factor when considering cache
   eviction.  A feature to manually flush only stale records could also
   be useful.

   The client response timer is another variable which 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 sub-optimal answers

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   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 five minutes.  If this variable is too small,
   authoritative servers may be rapidly hit with a significant amount of
   traffic when they become reachable again.

   Regarding the TTL to set on stale records in the response,
   historically TTLs of zero 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.

   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 TLD 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 which a resolver normally encounters makes
   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 pre-fetching) doesn't impact the
   existing cache state.  Some authoritative servers 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.

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   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

   During discussion in dnsop it was suggested that Refused from all
   authorities should be treated, from a serve-stale perspective, as
   though it were equivalent to NXDomain because it represents 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 which 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 behaviour
   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.  As modern resolvers
   use techniques like pre-fetching 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) records 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

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   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

   [RFC Editor: per RFC 6982 this section should be removed prior to

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

   Unbound has a similar feature for serving stale answers, but will
   respond with stale data immediately if it has recently tried and
   failed to refresh the answer by pre-fetching.

   Knot Resolver has a demo module here: https://knot-

   Details of Apple's implementation are not currently known.

   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 dnsop working group,
   it was suggested that an EDNS option should be available to either
   explicitly opt-in to getting data that is possibly stale, or at least
   as a debugging tool to indicate when stale data has been used for a

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

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   The reporting case was ultimately also rejected as working group
   participants determined that 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.  This would only be an issue
   if the authoritative servers are unreachable, the only time the
   techniques in this document are used, and thus 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.  This
   potentially makes that easier, although without introducing a new

   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 such as to get
   domain-validated certificates fraudulently issued to an attacker.
   While this RFC does not create a new vulnerability in this area, it
   does potentially enlarge the window in which such an attack could be
   made.  An obvious mitigation is that not only should a certificate
   authority not use a resolver that has this feature enabled, it should
   probably not use a caching resolver at all and instead fully look up
   each name freshly from the root.

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

   There are no IANA considerations.

14.  Acknowledgements

   The authors wish to thank 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.

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15.  References

15.1.  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, <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,

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,

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

   [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>.

15.2.  Informative References

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

              Moura, G., Heidemann, J., Mueller, M., Schmidt, R., and M.
              Davids, "When the Dike Breaks: Dissecting DNS Defenses
              During DDos", ACM 2018 Internet Measurement Conference,
              DOI 10.1145/3278532.3278534, October 2018,

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   [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", RFC 7719, DOI 10.17487/RFC7719, December
              2015, <https://www.rfc-editor.org/info/rfc7719>.

Authors' Addresses

   David C Lawrence

   Email: tale@dd.org

   Warren "Ace" Kumari
   1600 Amphitheatre Parkway
   Mountain View  CA 94043

   Email: warren@kumari.net

   Puneet Sood

   Email: puneets@google.com

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