UDP

This stands for User Datagram Protocol, a connectionless and usually unreliable (no packet sequencing, no resending of missing or bad packets) protocol that relies on IP in pretty much the way that its more famous sibling TCP does. It has a fairly simple algorithm - simply send a UDP packet and wait for a response. If the response is ICMP, the port is closed. If UDP, it is open.

Hacks using the UDP are almost always inextricably linked with the other two protocols discussed here. One of the most common UDP-based attacks is the UDP flood, which, like the Smurf Attack can cause Denial of Service to the victim. While not as straightforward as a TCP DoS attack, the UDP flood is fairly simple to execute. Simply scan the open networks using Nmap, and send a huge number of UDP packets towards the random open ports. This will cause the distant host to check for applications listening at the port, try to shut out those apps, and reply with an ICMP unreachable packet. This will ultimately cause the service to be unreachable by any other clients.