  DNS has always been criticized for its inherent design flaws, making the
system vulnerable to kinds of attacks. Besides, DNS domain names are not fully
controlled by the users, which can be easily taken down by the authorities and
registrars. Since blockchain has its unique properties like immutability and
decentralization, it seems to be promising to build a decentralized name
service on blockchain. Ethereum Name Service (ENS), as a novel name service
built atop Etheruem, has received great attention from the community. Yet, no
existing work has systematically studied this emerging system, especially the
security issues and misbehaviors in ENS. To fill the void, we present the first
large-scale study of ENS by collecting and analyzing millions of event logs
related to ENS. We characterize the ENS system from a number of perspectives.
Our findings suggest that ENS is showing gradually popularity during its four
years' evolution, mainly due to its distributed and open nature that ENS domain
names can be set to any kinds of records, even censored and malicious contents.
We have identified several security issues and misbehaviors including
traditional DNS security issues and new issues introduced by ENS smart
contracts. Attackers are abusing the system with thousands of squatting ENS
names, a number of scam blockchain addresses and malicious websites, etc. Our
exploration suggests that our community should invest more effort into the
detection and mitigation of issues in Blockchain-based Name Services towards
building an open and trustworthy name service.
