Client Subnet in DNS Queries
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C. Contavalli, W. van der Gaast, D. Lawrence, W. Kumari · 2016-05
Abstract
RFC 7871 specifies the EDNS(0) Client Subnet (ECS) option, which allows a recursive resolver to include a truncated source IP prefix of the original client in DNS queries sent to authoritative nameservers. Authoritative servers can use this information to return geographically or topologically optimal responses (e.g., CDN edge node selection) tailored to the client's network location.
Why This RFC Matters
Before ECS, CDN and anycast DNS providers could only use the recursive resolver's IP for geo-based DNS steering, which was inaccurate when large public resolvers (Google 8.8.8.8, Cloudflare 1.1.1.1) served clients from distant locations. ECS allows authoritative servers to see a /24 prefix of the actual client, enabling accurate geo-routing and dramatically improving CDN performance for users of third-party resolvers. It is widely deployed by major CDNs and public resolvers, though it raises privacy considerations addressed in RFC 9156.