APIs are a way that software can talk with other software and integrate and automate routines. The REpresentational State Transfer (REST), also known as the RESTful API, is a web API that uses HTTP requests. For example, it allows you to make calls to a web application to display information or execute commands on the web server's backend. The OPNsense framework supports APIs for most webGUI features to interact with the backend controller as a modern web application.

The following diagram shows OPNsense's framework architecture, which has been extracted from the official documentation:

Figure 20.1 – OPNsense architecture extracted from https://docs.opnsense.org/development/architecture.html

As we can see, OPNsense's architecture is composed of a frontend, where the webGUI resides, and a backend, where the configd service resides. Every time we use OPNsense's API, we are utilizing this architecture. One of the advantages...