EDNS Client Subnet (ECS)
EDNS Client Subnet (ECS) is an EDNS0 option, defined in RFC 7871, that lets a recursive resolver forward a truncated portion of the original client's IP address -- not the resolver's own address -- to an authoritative nameserver along with a query. Without it, an authoritative server answering a query from a large public resolver like Google's or Cloudflare's only sees that resolver's own location, which can be far from the actual client, so a CDN or geo-load-balanced service has no reliable way to return the nearest server's address. ECS fixes that by exposing enough of the client's subnet for geo-steering to work, at the cost of forwarding a little of that client's location to every authoritative server in the resolution path. It's one of the ordinary, non-broken reasons the same domain can resolve to a different address depending on which resolver and which network a query comes from.
By AdminKit team, Netbase JSC · Updated