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Bandwidth/Traffic Monitoring Options
(beehaw.org)
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
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Might be a little old? Not sure, you would have to research it.
I'm not well versed in what pfSense/opnSense needs, which is why I threw r630s at a project that mattered.
Some cheapo refubed i3 with an Intel NIC card would do. I just suggested the SFF refurb because a lot of people like low power (and SFFs are generally low power)