UDP flood attack can be initiated by sending a large number of UDP packets to random ports on a remote host. As a result, the distant host will:
- Check for the application listening at that port;
- See that no application listens at that port;
- Reply with an ICMP Destination Unreachable packet.
Thus, for a large number of UDP packets, the victimized system will be forced into sending many ICMP packets, eventually leading it to be unreachable by other clients.
To block it you can see the following link
http://kb.cyberoam.com/default.asp?id=1232