Per-to-per chat problem

[Timil]

I have a C++ server that keep data on users using the chat applet (username + ip + room).
Two client can speak to each other just by sending message to the target IP (other connected, recipient of the message)

The problem is that ip resolution is just impossible throught Proxy and Firewall.

That's okay I know I can't beat them

But I need to know how can I solve IP translation problem?
People whith just a false IP adress send their own (internal) IP adress to the server, but when anybody want to send them a message, they are kicked by the rooter that can't understand this adress as it's a local adress only.

So how can I get this "translated" adress?

Thanks