Understanding HttpDNS: Principles, Workflow, and Implementation in Xueersi Online School

The article begins by outlining the problems of traditional DNS, such as cache poisoning and multiple network hops, which can lead to DNS hijacking and service disruption.

It then presents HttpDNS as a solution, describing its basic principle: clients request domain‑to‑IP mappings via HTTP/HTTPS, receiving the optimal IP based on their location.

The advantages of HttpDNS are highlighted, including elimination of Local DNS risks, more accurate IP selection using client IP, and rapid propagation of DNS changes.

A detailed workflow diagram shows how HttpDNS interacts with authoritative DNS, utilizes the edns-client-subnet extension for precise routing, and caches results while falling back to traditional DNS when necessary.

The implementation section focuses on Alibaba Cloud HttpDNS, explaining the two‑layer IP structure (startup IP list and service IP list), the role of the scheduling service, and best‑practice recommendations such as rotating IPs and periodic refreshes.

Client integration is examined for three platforms: the PC client uses a libcurl‑based HttpDNS library that caches results for 1800 seconds and refreshes service IPs every four hours; the Android client adopts the official SDK with OkHttp, requiring only initialization and pre‑resolution configuration; the iOS client currently lacks HttpDNS support due to SNI and certificate‑validation constraints, requiring low‑level socket work to overcome.

The article concludes by emphasizing the need for a fallback to traditional DNS when HttpDNS is unavailable and reiterates the security‑first principle in deployment.