I'm trying to set up a VPN on Windows 7 and I started by forwarding port 1723. However, the exact moment that my router saves the port forwarding, I lose internet access on the PC being forwarded to. That's the PC where I plan to run the VPN server. As soon as I undo the port forwarding setup, internet is restored.
In that time Windows 7 can find no problems and it doesn't display the yellow exclamation mark. I can still connect to PCs on the local network and the router itself.
I've tried forwarding to another device instead and it doesn't break the connection, so it isn't a router problem.
Note that there is no active VPN server at this time. That is, there's no receiving end for this port forward of death.
Edit: Looks like I can still connect to IP addresses, so it's the DNS that fails.
Edit 2: It only fails if the DNS is set to Google's. If I set it to automatic, it works fine.