[dns-operations] Testing edns client subnet

Albert Braden abraden at about.com
Fri Dec 9 23:24:07 UTC 2016

I am curious about how edns client subnet works, and I wanted to see it in action, so I setup a geo-routed A record test.abote.com, and two BIND 9.11.0-P1 recursive resolvers in different geographic regions. If I pull test.abote.com from different places using as a resolver, I get the IP appropriate to the client's location. If I explicitly send a subnet to my recursive resolver using +subnet= then I get a reply appropriate to the subnet provided. If I query my recursive servers without explicitly sending client subnet information, I get the IP appropriate to the resolver's location. If I run tcpdump on my recursive resolver and query it without explicitly sending subnet information, I do not see it sending any client subnet information.

What is required for a recursive resolver to send client subnet information to the auth. server when it is not explicitly provided by the client? Do I need to setup anything in the config? Are any compile options required?
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