[AfrICANN-discuss] Ten Million DNS Resolvers on the Internet
annerachel at gmail.com
Mon Mar 26 13:24:50 SAST 2012
Ten Million DNS Resolvers on the Internet
by Joe Abley on March 22, 2012
Resolvers are servers on the Internet which use the Domain Name System
(DNS) protocol <http://www.ietf.org/rfc/rfc1035.txt> [TXT, 120 KB] to
retrieve information from authoritative servers and return answers to
end-user applications. They’re often found in enterprise and ISP networks,
and there are a number of public resolver services provided by people like
Google <http://code.google.com/speed/public-dns/> and
It’s also possible to configure your own computer to be a resolver, or to
deploy your own in your own network using free software like ISC
Labs’ unbound <http://www.nlnetlabs.nl/projects/unbound/>.
So, all in all, how many resolvers are there? Given that anybody can run
one, it seems like a difficult thing to measure. It turns out, however,
that all resolvers that talk directly to authoritative servers on the
Internet leave a trail, and with a little data crunching we can come up
with a number.
Back in 2010, ICANN, VeriSign and NTIA concluded a successful
DNSSEC <http://www.ietf.org/rfc/rfc4033.txt> [TXT, 52 KB] in the root zone
of the DNS. As part of that project, Root Server
Operators<http://www.root-servers.org/>collected DNS requests that
were delivered to their individual Root Server
infrastructure, and deposited the resulting data with
The goal of this data collection exercise was to try and identify any
potential problems for DNS clients due to DNSSEC deployment. The by-product
of this exercise, however, is a data set which provides insight into DNS
traffic between a highly-representative set of DNS resolvers and DNS
authority servers (almost all resolvers talk to a root server every once in
One of the data collection exercises carried out had a particularly long
time-base. The collection is referred to as "LTQC" (Long-Term Query
Collection) and it concerned itself just with priming queries, that is, the
initial query that every resolver sends to a root server when it starts up
in order to obtain an up-to-date set of DNS root server names.11 of the 13
root servers contributed data to this collection, including L-Root, the
root server operated by ICANN. Data was collected between November 2009 and
So, here’s our methodology: we look at every request contained in the LTQC
packet-capture, and count the number of unique IPv4 and IPv6 source
During the collection period, we saw 9,945,017 unique source addresses, of
which 59,489 (0.60%) were IPv6 and and 9,885,528 (99.40%) were IPv4.
So which resolvers won’t we see?
We won’t see internal resolvers that don’t send queries to authoritative
servers on the Internet directly, but instead send them via other
intermediate resolvers. Included in this class of resolver are any that are
hidden behind middleboxes <http://en.wikipedia.org/wiki/Middlebox> that
redirect DNS queries to a central cache, or otherwise change normal priming
We won’t necessarily see internal resolvers that are deployed behind a Network
least, in such a situation we might see only some of them.
We won’t see resolvers that started (and primed) before the data collection
period started, and then never primed again before the end of that period.
We obviously won’t see any resolvers that were brought live after the
collection period ended, and we assume that the number of resolvers is
probably increasing due to the general growth of the Internet.
Any resolver that was renumbered during the collection period (and primed
before and after the renumbering event) would be counted twice.
Intuitively, this seems like a minor effect; we think most resolvers are
renumbered fairly infrequently, since they are generally referred to by
address rather than name.
Given the expected errors in the number we measured due to the effects
described above, it seems appropriate to round the answer to a single
significant figure; this at least gives us an order of magnitude for a
What we are left with? That there are at least 10 million DNS resolvers on
the Internet today.
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