Domain Name System filtering (or DNS filtering) refers to a method of blocking access to certain IP addresses, websites or web pages that are potentially harmful, contain prohibited content or are being used for criminal activities. The DNS is used to translate commonly used website names into their equivalent numerical IP addresses that are used by web browsers to locate them and return their contents.
Filtering by the DNS subjects all requests for IP addresses to pass through certain controls such that any address that is known to be malicious is blocked. The DNS filter operates by comparing addresses against a blocklist that contains a list of all known malicious addresses. If the IP address associated with the website that the user is accessing matches any on the blocklist then the request will be redirected to a page that displays a block page which contains an explanation of why the site cannot be accessed.