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Networking · Protocols

ICMP

The control and diagnostic protocol of IP, carrying echo requests, unreachable messages and fragmentation needed signalling.

FoundationalUpdated 2026-09-01

Overview

ICMP is how the network reports problems: destination unreachable, TTL exceeded, and the fragmentation needed message that path MTU discovery depends on.

Blocking ICMP wholesale is a common but harmful hardening reflex, it breaks path MTU discovery and traceroute while doing little for security. Filter selectively by type instead.