These may help:
Barbarians at the Gate: An Introduction to Distributed Denial of Service Attacks
Closing the Floodgates: DDoS Mitigation Techniques
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You didnt post the attack' ip profile. If attack is coming from a korea host, it is not a DDOS attack. it is a regular DOS attack.
How many unique ips are attacking? ip are changing as the time pass or not?
You must identify:
how many host are attacking
if IP from those hosts are valid (yeap, on a DDOS packet may be crafted with random ip address)
if you can identify true hosts, just ask YOUR isp to block it at ISP side of your connection.
But if you cant identify hosts (fake ips), your are dammed.
Another alternative might be, taking the server offline completly till the skiddies realize they aren't attacking anything and hopefully stop attacking? or, you can change servers (for example if it was irc.blah.com change it to something else like irc.blahblah.com) and then change the IP.