tcpdump

Also written: tcpdump -w, tcpdump -r, filter

A command-line tool that captures and shows the packets crossing a network interface. Filters such as `host`, `port`, `tcp` and `udp`, combined with `and`, `or` and `not`, choose which packets to show; `-w` saves a capture to a file and `-r` reads one back. It is often the final judge when other tools disagree, because it shows what really crossed the wire.