A sinkhole is a technique that redirects malicious network traffic to a controlled destination, such as a fake server or a black hole. A sinkhole can be used to stop malicious communications with a command-and-control server by preventing the malware from reaching its intended destination. A high entropy level means that the sinkhole can generate random domain names that match the changing domain name used by the malware for callback. Blocking TCP/443 at the edge router, disabling TCP/53 at the perimeter firewall, or configuring the DNS forwarders to use recursion are other possible actions that could stop malicious communications, but they could also disrupt legitimate services that use those protocols or settings. Reference: https://www.cisco.com/c/en/us/about/security-center/dns-sinkholing.html