Ok I managed to "start" a UDP "server" with java.NIO and it can read datagrams and see where they are coming from. Now I tried to send datagrams back to the origin, but my client couldnt see anything coming back. Is it me being to tired a monday night or am I doing it the wrong way? I know the client code works because Ive been querying game server with it for some time now.
Just to check the obvious - you need to ALSO run your server code on the client, and your client code on the server, if you want to send data from the server to the client.
As has been said before, UDP is connectionless. I.e. there is no connection. I.e. if someone sends you stuff, you cannot "reply" you can only send them stuff from scratch, as if you'd never heard from them.
But maybe that's not your problem?