So this is the diagram of my home network. I have main internet gateway router from ISP. Then there are bunch of IoT stuff, laptop, mobiles, etc… hidden behind my home network (B) via my 2nd router. Trivial stuff or guest can connect directly to (public) network A.

Now I run my smart home service on a Pi server, which obviously should not be exposed to the internet. I added this App server on which there’s VPN endpoint, so that I can connect to my home network (B) from my phone via this VPN overlay. That allows me to safely control the Pi server when I’m not home. This App server can also act as media player, web server, etc… and does not guaranteed running 24/7.

This App server has 2 ethernets (+ wifi, which is usually switched off), connected to both A and B.

I’d want to Internet traffic to go in/out App server via network A only, is it possible? And network B is only for intranet use.

I doubt this is the case, because App Server is connected to internet via both A and B, so I don’t know which one is used.

  1. If I remove network A from the private router, then the App server would become the sole internet gateway for the private network, which would increase the configuration complexity. Nah, I don’t want to turn the App server into a router. Perhaps, that’d probably be where 2 ethernets are useful?
  2. If I keep both A and B, then A is most likely useless perhaps I can just remove A and let the App Server to use a single ethernet.

https://preview.redd.it/6xcrewhscezb1.png?width=1080&format=png&auto=webp&s=891e7454828869c9ace6981229cd5113a8656470