ICMP
Internet Control Message Protocol
A network diagnostic protocol used by ping; abused for scanning and floods.
ICMP is a control and diagnostic protocol used for messages such as echo request and reply behind ping and for path discovery with traceroute. Attackers abuse it for host discovery and network mapping, for floods like ping floods and smurf attacks, and occasionally for covert tunneling of data. It matters for both troubleshooting and reconnaissance detection. A pitfall is blocking ICMP entirely, which can break path MTU discovery, so administrators usually rate limit and filter specific message types instead.
Looking ICMP up is step one. Getting tested on it is step two.
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