tcpdump

listen for icmp traffic

tcpdump -i eth0 icmp

check a port

tcpdump -i any port 25
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9