ECS

networkingprivacy

Stands for: EDNS Client Subnet

An EDNS option that forwards part of the client's address to the authoritative server.

EDNS Client Subnet lets a recursive resolver tell an authoritative server roughly where the client is, so a content network can answer with a nearby server rather than one near the resolver. It improves routing for users of large public resolvers and leaks the client's approximate location to every authoritative server queried.

ECS exists because anycast solved the wrong half of the problem: it puts the resolver near the user, but the authoritative server answers based on where the resolver is, not where the user is. Forwarding a truncated client subnet fixes the answer and creates a privacy question, which is why public resolvers differ on it - some send it for performance, some refuse on principle, and the choice is invisible to the person whose location is or is not being shared.

Also known as: ecs, edns client subnet

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