geoip-enabled authoritative dns server is quite easy to find - bind9 with patch can do it, powerdns as well. but the problem starts when server receives DNS query from global public resolvers like google's 22.214.171.124. the request might come from ip in completely different geolocation location than the actual user. queries from google can carry that information.
do you have any experience with running servers that handle edns-client-subnet and provide answers depending on the geolocation of the client? what [reasonably stable] open source dns servers provide that functionality?
i've found so far one - gdnsd.org but i'm curious to hear about your experience and alternatives.