ICMP

This stands for Internet Control Message Protocol, the most used in networking. This is a connectionless protocol, meaning it will not use any port number. It is usually meant for diagnostic purposes, server querying, or error reporting. As a hacker, knowledge about this is important because you will be using ICMP a lot to end payloads. Pinging, for example, uses ICMP - and this, just like any ICMP message, has inherent security holes.

In ICMP, for example, an error message is not sent in response to an error. When the error is sent, it will send the IP header and datagram, which details the error’s cause. This will cause the receiver to associate the error with the specific process. This means that when the Type 0 (echo reply) has been sent, the reply will not be a Type 8 (echo request), though a Type 8 will always elicit a Type 0.
This is taken advantage of by the “Smurf Attack”, which is nowhere near as cute as it sounds. In this technique, the attacker will spoof the ICMP packet’s source address, sending a broadcast to all computers in the network. If this traffic is not filtered, then the victim’s network will be congested, dropping its productivity. Aside from using this to Smurf the network, it can also be used to gather information by discovering all hosts on the network.