I tried Wireshark. It shows the source and destination IP address, port number, and packet contents, but nothing about which application is using the packets.
Apparently tcpdump is similar; network data but no info on which application.
Wireshark was showing much more activity than I expected (including destination IPs outside my home network) when the browser, email and other known internet-related programs were closed and other computers are off. But without knowing which program is sending or receiving, it's hard to do anything about it.
I'm using Ubuntu 11.04.
I'm not looking for a general network sniffer; I only want to see what is going in and out of the same machine on which the sniffer is running.
Is there anything like that on Linux? Even if it costs money? Or is it that I don't know how or where to look for the application name in Wireshark?