Diagnosing “+Limiting icmp unreach response from…” with tcpdump

Anyone who has run a FreeBSD server for any length of time has seen these messages in their daily security emails. (You do read those, right?)

+Limiting icmp unreach response from 296 to 200 packets/sec

+Limiting icmp unreach response from 337 to 200 packets/sec

+Limiting icmp unreach response from 318 to 200 packets/sec

+Limiting icmp unreach response from 535 to 200 packets/sec

+Limiting icmp unreach response from 332 to 200 packets/sec

+Limiting icmp unreach response from 328 to 200 packets/sec

W...