Duas NICs em uma máquina antiga executando o pfsense farão o truque.
Hardware considerations
When selecting hardware for a new build, carefully consider current and future hardware requirements. These include:
64-bit Intel or AMD CPU (x86-64, amd64) on pfSense 2.4 and later (Q2 2017) CPUs must have AES-NI support on pfSense 2.5 and later (TBD, 2018+) Must be able to boot from USB or optical drive and run the installer on pfSense 2.4 and later
Realmente dois NICs estariam bem. Ou você precisaria de um toque de rede ou de um comutador capaz de realizar o espelhamento de portas.