A open dns resolver provides name resolution to any clients outside of its LAN / WAN or authoritative domains. Open DNS resolvers are a bad as they are usually involved in attacking DDoS attacks, spoofing, DNS cache poisoning, and much more. It allows everyone to use your network resources and bandwidth. Run the following command to test your BIND DNS (or any other DNS software) server for open recursion:

dig www.google.com @your.domain.com

or if you on a windows box, you can run “nslookup”
at the greater than prompt, type “your.domain.com”
at the next greater than prompt, type in www.google.com.

If it comes back with googles address, it means you have an open dns resolver or your server have recursion enabled.

If you are running bind, open named.conf and make sure the recursion setting is set to no.

recursion no;

If you running windows dns server.

Open DNS Manager.

In the console tree, right-click the applicable DNS server, then click Properties.

Click the Advanced tab.

In Server options, select the Disable recursion check box, and then click OK.