When you use the Best Practice Analyzer for the Domain controller. it’s possible it says that the following root-server (l.root-server.net.) is not anymore available. You see the following error in the DNS
To solve this issue you need to lookup what’s the new l.root-servers.net. You can find this information on IANA:
http://www.iana.org/domains/root/servers
Change the address of the root server in the DNS this will be replicated to the other dns servers in the domain.
