Server :: Iptables Configuration For UDP Flood?

Banning the IP is the best way to protect your server but of course, attacker can use another IP and use a lot of your bandwidth until you find and ban the IP. So the only thing we can do to prevent this is, block the packets my iptables length module.

I check the bandwidth usage through "iftop". Incoming traffic is always like 120kb/second and that has to be that way because the traffic enters my server no doubt that it gets dropped by iptables later.

The actual thing what the Ddos ( UDP Flood ) does it that it causes an outbound traffic that eats up like 5mb/second easily and my servers lag. Only if the IP is banned, the outbound traffic comes to an end.

Now I want to use the length module to block it but it just won't work. I've tried the following and shuffled them too but no help.

Code:
iptables -I INPUT -p udp -m length --length 15 -j DROP
iptables -A INPUT -p udp -m length --length 15 -j DROP
Packet length is 15 according to tcpdump:

Code:
19:49:34.504864 IP fms-02.colt.net.belgamanagement.be.56413 > nyc.v1servers.com.20100: UDP, length 15