When i tried out Wake on Lan (WoL) i wanted to know if the machine i wanted to start actually receives the WoL Packet. When you ssh into a machine and then try see tcp traffic with tcpdump you have a nice infinitive loop at hand. Your ssh connection generates traffic which will be seen in tcpdump which outputs this which generates ssh traffic which you see in tcpdump ... run tcpdump with a simple filter a la: tcpdump 'port ! 22' and you're happy.