GetExternalIPAsync() does not return public IP sitting behind router about mono.nat

[alanmcgovern]

Unfortunately not within the NAT-PMP or uPnP spec. The API you're looking at returns whatever the device uses as it's external IP address.

In your case it sounds like you're double-NATd, which means the router you call "routeb" is getting assigned an "external" address which is as actually just one of RouteA's internal addresses.

While you can implement your own "GetExternalIPAddress" by parsing data from sites like whatismyipaddress.com, it won't be much use from a port forwarding perspective.

If you connect to RouteB then you'll only be able to forward ports between RouteB and RouteA. You won't be able to forward ports automatically between RouteA and the actual internet.

Typically the way to solve this is to put your ISP provided router into "Bridge" mode. This disables all advanced networking functionality and passes through all connections to the device connected to the primary Ethernet port - in your case it would be RouteB. That device is then responsible for DHCP, port forwarding, etc, and then you sidestep all the complications of a double-NAT.

LMK if you need anything else or I've misunderstood the issue.