Stop. Take 5 minutes to save the internet

Hey fellow sysadmins! Please take 5 minutes to make sure your DNS servers aren't open to the world for recursive queries. They can be used as amplifiers in DDOS attacks.

https://www.isc.org/wordpress/is-your-open-dns-resolver-part-of-a-criminal-conspiracy/

The short version of what you need to do is here.