On a machine that isn't behind a firewall already you will want to set up strict firewall rules, including blocking inbound DNS from anywhere except trusted add

On a machine that isn't behind a firewall already you will want to set up strict firewall rules, including blocking inbound DNS from anywhere except trusted addresses. Other hardening steps should be taken as well but they're not specific to DNS resolvers. Recursive DNS resolvers on the open Internet start getting abused really fast for DNS amplification DDoS attacks (for anyone unaware reading this).