tcpdump fu

“Packet capture is one of the most fundamental and powerful ways
to do network analysis. You can learn virtually anything about what
is going on within a network by intercepting and examining the raw
data that crosses it. Modern network analysis tools are able to
capture, interpret and describe this network traffic in a
human-friendly manner.

tcpdump is one of the original packet capture (or “sniffing”)
tools that provide these analysis capabilities, and even though it
now shares the field with many other utilities, it remains one of
the most powerful and flexible.