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I'd like to set up something that I can plug into my network that will track network sources, destinations, ports, packet types, etc. and allow me to graph the results. Presumably the network card and port will be in promiscuous mode to capture everything. 

What software should I be looking at? Free/libre software only, please. Also, what kind of hardware would be needed to support this (CPU, RAM, etc.)? I don't want to capture entire packets, just the key header info.