tcpdump

tcpdump is a tool to capture IP Packets at a command line level, its similar to the PCAP used by Wireshark (Wireshark is just a GUI to control PCAP). I say just a GUI, its rather good and saying it is 'just' a GUI is a little harsh.
On the MFEL, we can use tcpdump to capture either the IP Input or the IP Output. There are many commands available for tcpdump, and I will list some later on this page, but first lets just look at a working example, because this might be all you need for now.