Its not a DDoS, its just a exploit in the fonline-server vulnerability, so a single malicious packet just crashes the server.
For the current time I've moved the game port to 4001, hopefully the assailant doesn't read our forums while I pinpoint the problem. Thanks for your patients and continued support!
Also if any IT / security experts want to have a crack at problem feel free to message me. I have collected the traffic of the attack and have started to filter through the packets to find the offending one, from 300k packets, i've narrowed it to around 3k :p.