A Worldwide View on the Reachability of Encrypted DNS Services

To protect user DNS privacy, four DNS over Encryption (DoE) protocols have been proposed, including DNS over TLS (DoT), DNS over HTTPS (DoH), DNS over QUIC (DoQ), and DNS over HTTP/3 (DoH3). Ensuring reachability stands as a prominent prerequisite for the proper functionality of these DoE protocols, driving considerable efforts in this domain. However, existing studies predominantly concentrate on a limited number of DoT/DoH domains or employ a restricted subset of vantage points (VPs). In this paper, we present the first comprehensive worldwide view of DoE service reachability. By collecting data from our 15month-long scan, we elaborately built a list of 1302 operational DoE domains as measurement targets, 448 of which support IPv6. Then we performed 10M DoE over IPv4 (DoEv4) and 570K DoE over IPv6 (DoEv6) queries from 5K VPs over two months, encompassing 102 countries/regions. Our results reveal that the reachability of DoE services is poor in some countries/regions. Specifically, 592K (5.92%) DoEv4 queries and 28K (4.91%) DoEv6 queries are blocked. In countries/regions with strict Internet control, DoEv4 service blocking often occurs during TCP connection and QUIC version negotiation. Compared to DoEv4, the reachability of DoEv6 services is better. In particular, some DoE blocking policies target only specific IP addresses or DoE protocols, providing clients with the opportunity to access blocked DoE domains. Our study highlights the need for the DNS community to pay attention and improve the reachability of DoE services. CCS CONCEPTS • General and reference → Measurement; • Security and privacy → Security protocols.