These best practices cover two categories of public resolvers:
Closed and Public Resolvers – Access to this type of resolver services is determined either by the source IP address or by some other mechanism (TSIG key, TLS certificate, etc.). These service providers are typically NOT Internet Service Providers, and the clients sending the queries are located on remote networks. Note that some operators of closed and public resolvers may also offer a free tier service, which also makes them open and public resolvers (for example, commercial DNS filtering/scrubbing services).
Open and Public Resolvers – “Fully open” public DNS resolvers are available to any users on the Internet freely to use, whether they are stub resolvers (clients) or recursive servers using the open resolver as a forwarding service.
There are two types of best practices for public resolver operators: DNS security, and DNS availability and resilience. In addition to these two categories specific to the core DNS, all operators must pay careful attention to practices related to hardening their core system security.
DNS Security: These best practices are aimed at improving the security of your DNS service itself, helping prevent users from being served malicious data, and lowering the chances of data corruption going unnoticed.