tcpdump

very convenient packet inspecting software.

References

Manual

http://www.tcpdump.org/tcpdump_man.html

Filter expression

http://www.manpagez.com/man/7/pcap-filter/

Basics

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