Little Snitch

Little Snitch is a host-based application firewall for macOS. It can be used to monitor applications, preventing or permitting them to connect to attached networks through advanced rules. It is produced and maintained by the Austrian firm Objective Development Software GmbH.
Unlike a stateful firewall, which is designed primarily to protect a system from external attacks by restricting inbound traffic, Little Snitch is designed to protect privacy by limiting outbound traffic. Little Snitch controls network traffic by registering kernel extensions through the standard application programming interface (API) provided by Apple.
If an application or process attempts to establish a network connection, Little Snitch prevents the connection. A dialog is presented to the user which allows one to deny or permit the connection on a one-time or permanent basis. The dialog allows one to restrict the parameters of the connection, restricting it to a specific port, protocol or domain. Little Snitch's integral network monitor allows one to see ongoing traffic in real time with domain names and traffic direction displayed.
The application (version 4) received a positive 4.5/5 review from "Macworld".