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                                                           
                                                                            
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    document authors.  All rights reserved.                                 
                                                                            
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 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                                                      
                                                                            
 .  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.                                               
                                                                            
 .  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].                      
                                                                            
 .  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.                      
                                                                            
 .  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.                                                             
                                                                            
 .  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.                                                
                                                                            
 .  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.      
                                                                            
 .  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.       
                                                                            
 .  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.                                                       
                                                                            
 .  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.                               
                                                                            
 .  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.                                
                                                                            
 .  Privacy Considerations                                                
                                                                            
    This document does not add any practical new privacy issues.            
                                                                            
 .  NAT Considerations                                                    
                                                                            
    The method described here is not affected by the use of NAT devices.    
                                                                            
 .  IANA Considerations                                                   
                                                                            
    This document has no IANA actions.                                      
                                                                            
 .  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