The local machine would need to be the gateway router in order to be authoritative on traffic source/destination. This is a corner case that probably should not be handled by TDE, as it is a rather stupid idea to run a GUI on your main gateway/firewall machines for security reasons.
I remember years ago when I added my second computer. Good noob years as I started learning more about so many things computer related.
My choice then was to configure my primary machine as my gateway. Simple ip forwarding. I ran iptables on both machines. No set top router entered this picture for a long while thereafter. In that picture, the primary system was a desktop too.
In the past many years a common question I see is how to configure two computers as I described. Lots of people using old hardware and hand-me-downs and can't afford a set top router. Many lack experience and skills to configure a set top router or install improved firmware.
Many ISPs today impose bandwidth caps. Knemo on a gateway desktop can't provide the details users need to monitor those limits.
KNemo could fill those needs.
Corner case? Possibly.
Not demanding. Just talking out loud. :)
Darrell