A seed for the concept of a network router

[Marek Paśnikowski]

Dear All

I am struggling to identify the most basic information to take the first
step towards building a Guix-based network router.  I would like to ask
for a minimal service configuration to meet the following goal:

I have a fanless mini-computer with two ETH interfaces.  It currently
sits under a proper router device like all the other devices in my LAN.
I wish to be able to physically hook it up between the router and the
WAN modem in such a way, that all incoming traffic (except one SSH port)
gets immediately forwarded to the router, as if there was nothing else
between the modem and the router.  Bonus points if it is possible to
fake the MAC address.

The underlying idea is that I would then be able to experiment with all
the available options and concepts without fear of bricking my learning
process.  Thanks to the machine being an actual computer, I have the
ability to connect a display and a keyboard to really break the concept
of routing down to its atoms and come back safely to a previously
working configuration.

So, what is the first step in this journey?