OPNsense is a fork of popular pfSense network/router/SDN product/solution.

Interesting combo here, Phalcon and Python.

the frontend implemented with PHP/Phalcon and the backend using a custom service built in Python.

https://docs.opnsense.org/development/architecture.html

As for quality, all new features will be built using a solid framework with a Model View Controller. For this purpose we choose Phalcon as it is the fastest open source PHP framework available.

https://docs.opnsense.org/fork/thefork.html#quality

So both Volt and Jinja are used, which are identical to the FE dev :)

Full source code at: https://github.com/opnsense/core