EDNS Client Subnet

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EDNS Client Subnet (ECS) is an option in the Extension Mechanisms for DNS that allows a recursive DNS resolver to specify the subnetwork for the host or client on whose behalf it is making a DNS query. This is generally intended to help speed up the delivery of data from content delivery networks, by allowing better use of DNS-based load balancing to select a service address near the client when the client computer is not necessarily near the recursive resolver.[1][2]

When an authoritative name server receives a DNS query, it takes advantage of ECS DNS extension to resolve the hostname to a CDN which is geolocationally near to the client IP's subnet, hence the client makes further requests to a nearby CDN, thereby reducing latency. The EDNS client subnet mechanism is specified in RFC 7871.

As of 2019, DNS resolution service providers supporting EDNS Client Subnet include Google Public DNS, OpenDNS and Quad9.


  1. ^ "How it works". A Faster Internet. Archived from the original on 2018-03-28. Retrieved 2018-03-27.CS1 maint: unfit URL (link)
  2. ^ "EDNS Client Subnet (ECS) Guidelines | Public DNS | Google Developers". Google Developers. Retrieved 2018-04-02.