Originally Posted By: wfaulk
Edit: I take it back. It has one really neat hardware feature. It functions as an ethernet-to-ethernet bridge. It doesn't route. If the system reboots or loses power or in any other way isn't active, it still passes traffic. I don't know how it does that, honestly. An 8PDT normally-closed relay comes to mind.

I've made a Ethernet tap before and it'll do exactly what you want. You need to dedicate two NICs to monitor the traffic however.