ISP Column - June 2026

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DNS Query Duplication

June 2026

Geoff Huston

The tendency for free resources to be exploited to the point of over-consumption and even exhaustion is a well-known economic and social concept, frequently described with the term the tragedy of the commons. Because use of the resource costs each individual user nothing, there is no financial incentive for any user to moderate their consumption of the common resource. Over-consumption inevitably ensues!

In the DNS name resolution space queries are free. To what extent do we see over-querying on the part of recursive resolvers in the DNS? There is certainly some motivation for a recursive resolver to query enthusiastically, given that a strategy of what one could term over-querying can work around slow or unresponsive nameservers, or repair UDP datagram loss. Over-querying describes a behaviour where a DNS resolution system does not wait for a local timer to expire to conclude that an original UDP-based query has been lost but repeats the query well before a reasonable timeout interval has elapsed. The apparent thinking behind this rapid-fire over-querying behaviour is that in the event of query loss, the subsequent query may be successful, and the response to this subsequent query will arrive back to the resolver soon after the response to the original query would've arrived, saving the resolver (and the end user) waiting for a query timeout interval. If both queries are successful, then the resolver will simply discard the responses to the later query. As queries are free, the cost to the authoritative server in generating the answer twice is borne by the authoritative server and not the recursive resolver. Such a behaviour can improve the responsiveness and resilience of a recursive resolver, and the cost of generating these additional responses is met by others. In other words, if there is no cost to the resolver in this behaviour then there is no motivation to adopt more conservative query stance!

The question here is: To what extent are DNS resolvers in today's Internet overly profligate in generating duplicate queries?

We'll use the APNIC Labs measurement data generated on a single day, the 1st of June 2026, for this study. This system uses an online ad presentation system to seed measurement tasks on a collection of users' browsers. The system generates some 35 million ad presentations for each 24-hour period, spread over users drawn from all parts of the Internet. Each individual measurement ad presentation contains up to 15 different component URL fetches (for different measurement tasks), where each URL contains a unique DNS name to resolve. The use of a unique label ensures that no DNS caching is used to resolve these names, so all end user DNS resolution requests are seen as queries at the measurements DNS authoritative servers and not answered by intermediate DNS caches held by resolvers.

There are various forms of DNS behaviours that are configured into the APNIC measurement set, some of which are deliberately intended to generate additional queries (such as, for example, a DNS name where the authoritative nameserver always returns a SERVFAIL response code). Here, we are looking atjust those DNS query names which are served by a dual stack nameserver that always promptly responds to A and AAAA query types. UDP is not a reliable transport protocol, and some datagram loss is expected, but it would be reasonable to anticipate an overall query loss rate of some 2% or lower in most circumstsances, so that we expect to see a comparable query duplication rate when looking at a random sample set of DNS data from this experiment.

The measurement experiment directs users located in each of these regions to a DNS server that is physically located in the same region as the user. We use a division of the Internet into six major areas, as shown Table 1.

CodeRegionServer Location<br>apEast Asia/OceaniaSingapore<br>amNorth AmericaDallas<br>laLatin AmericaSao Paulo<br>euEurope and Western AsiaFrankfurt<br>inIndian SubcontinentMumbai<br>hkChina, Hong Kong and MacaoHong Kong SAR

Table 1 – APNIC Lab's Measurement Regions

The data set that was gathered during this 24-hour period is shown in Table 2. Here, we are distinguishing between an original query, and successive duplicate queries, where a duplicate is defined as a query that has the same query name and query type.

RegionExpsUnique qnamesNon-Dup qnamesDup qnamesRateDup QueriesTotal QueriesDup Query Rate<br>ap10,615,19548,109,64430,106,04018,003,60437%37,843,95585,953,59944%<br>eu8,189,62436,891,29726,747,91610,145,38127%17,979,70854,871,00532%<br>in5,774,30226,242,26310,348,34015,893,92361%36,112,19662,354,45957%<br>am4,016,88020,707,63814,965,5715,742,06728%9,416,28030,123,91831%<br>la2,305,65910,212,1376,732,5923,479,54534%7,002,04917,214,18640%<br>hk1,198,3294,330,9362,280,0322,050,90447%4,763,7219,094,65752%<br>Total...