Q&A: What is EDNS0 Client Subnet and does Silverline support it when resolving FQDNs for backend servers?


What is EDNS0 Client Subnet (ECS)?

Does Silverline support ECS when resolving FQDNs when determining backend servers?



  • Proxy/Proxies
  • Proxy FQDN Backend



What is EDNS0 Client Subnet (ECS)?

EDNS0 Client Subnet is an extension to DNS allowing recursive nameservers to decorate queries with a summary of the client’s IP address. ECS allows GSLB nameservers to respond with answers that are optimized for the location of client systems when they are not physically close to recursive nameservers.


For example, a DNS client in San Jose, CA may query the recursive nameservers provided by its ISP which are located in New York, NY. Without ECS, a GSLB nameserver would answer using only the IP address of the recursive nameserver which could result in an answer far from the client. With ECS, the recursive nameserver can supply a summary of the client’s IP address to the authoritative nameserver which allows it to return an answer that is more local to the client.


Does Silverline Support ECS when resolving FQDNs for backend servers?

Silverline does not supply ECS options when resolving FQDNs for managed proxy backends.


When Silverline resolves FQDNs to determine proxy backend server addresses, the clients do so using private (RFC1918) IP addresses and using recursive nameservers that are geographically close to the relevant points of presence. Supplying private IP addresses in ECS options is inappropriate, and since the DNS client and recursive resolver are close to each other, the intent of ECS is already satisfied.


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