very convenient packet inspecting software.
Manual
http://www.tcpdump.org/tcpdump_man.html
Filter expression
http://www.manpagez.com/man/7/pcap-filter/
Listening: tcpdump -i eth0
More information to output: tcpdump -i eth0 -vv (or -vvv)
Filter by host / src / dest: tcpdump -i eth0 host 8.8.8.8 (or "dst" "src")
Ascii or Hex contents: -A or -x