Say I have a router with IP 42.98.1.70, with 2 NiCs connected to it with IPs 192.168.1.200 and 192.168.1.300. The router has port forwarding on port 10433 to redirect packets to 192.168.1.200. The routers internal network IP is 192.168.1.100.
When NIC 192.168.1.300 sends a packet to socket 42.98.1.70:10433. the host 192.168.1.200 gets a packet from socket 192.168.1.100:48900, which as far as I know, looks like a hole punched socket setup by the router.
So theoretically, if host 192.168.1.200 replies with a packet to socket 192.168.1.100:48900, that packet should eventually get back to host 192.168.1.300, since the router should bridge the two via its internal table mapping, aka 'UDP hole punching'.
However, the packet sent back to 192.168.1.100:48900 from 192.168.1.200 never reaches 192.168.1.300.
i'm suspecting that what could be going on is that UDP hole punching doesn't work in between NICs on the same network. It only works between a source that is external to the network and one that is within it. Is that the case?
After reading this RFC - https://www.rfc-editor.org/rfc/rfc5128, it looks like what i was trying to do is called "hair pinning". Although some routers support it, not all do. Apparently mine is one of those.