Large Scale Analysis of DoH Deployment on the Internet

Abstract

DNS over HTTPS (DoH) is one of the standards to protect the security and privacy of users. The choice of DoH provider has controversial consequences, from monopolisation of surveillance to lost visibility by network administrators and security providers. More importantly, it is a novel security business. Software products and organisations depend on users choosing well-known and trusted DoH resolvers. However, there is no comprehensive study on the number of DoH resolvers on the Internet, its growth, and the trustworthiness of the organisations behind them. This paper studies the deployment of DoH resolvers by (i) scanning the whole Internet for DoH resolvers in 2021 and 2022; (ii) creating lists of well-known DoH resolvers by the community; (iii) characterising what those resolvers are, (iv) comparing the growth and differences. Results show that (i) the number of DoH resolvers increased 4.8 times in the period 2021–2022, (ii) the number of organisations providing DoH services has doubled, and (iii) the number of DoH resolvers in 2022 is 28 times larger than the number of well-known DoH resolvers by the community. Moreover, 94% of the public DoH resolvers on the Internet are unknown to the community, 77% use certificates from free services, and 57% belong to unknown organisations or personal servers. We conclude that the number of DoH resolvers is growing at a fast rate; also that at least 30% of them are not completely trustworthy and users should be very careful when choosing a DoH resolver.