How to check and close your open DNS servers ?
Open DNS servers
1) Go to http://www.dnsreport.com/tools/dnsreport.ch?domain=mydomain.com
2) Check for BIG RED “FAIL” (7th box, going down from up) saying “Open DNS servers”.
If you see the message, you can correct it the following way:
1) login to your server as root
2) nano -w /etc/named.conf (if you are running Bind)
3) Look for // query-source address * port 53; and right below write the following line:
recursion no;
4) Save and exit, restart Bind by typing service named restart
5) Check DNSReport.com to make sure it went right. You should not see the error message anymore.
If you run something else than Bind then please check http://www.dnsreport.com/info/opendns.htm for instructions.