  DNS is one of the core building blocks of the Internet. In this paper, we
investigate DNS resolution in a strict IPv6-only scenario and find that a
substantial fraction of zones cannot be resolved. We point out, that the
presence of an AAAA resource record for a zone's nameserver does not
necessarily imply that it is resolvable in an IPv6-only environment since the
full DNS delegation chain must resolve via IPv6 as well. Hence, in an IPv6-only
setting zones may experience an effect similar to what is commonly referred to
as lame delegation. Our longitudinal study shows that the continuing
centralization of the Internet has a large impact on IPv6 readiness, i.e., a
small number of large DNS providers has, and still can, influence IPv6
readiness for a large number of zones. A single operator that enabled IPv6 DNS
resolution -- by adding IPv6 glue records -- was responsible for around 20.3%
of all zones in our dataset not resolving over IPv6 until January 2017. Even
today, 10% of DNS operators are responsible for more than 97.5% of all zones
that do not resolve using IPv6.
