Read-Only network cable
From RADION OpenLab
No IP address usually is adequate stealthiness, but the second NIC still can respond to low-level, layer 2 requests, such as ARP requests. To make it totally invisible, use a read-only cable. The wiring diagram is in Figure 3. These cables can be made easily with a length of Cat5 cable, two RJ45 connectors and a crimping tool. First, strip off a half inch of the outer insulation, and insert each wire into a slot on the RJ45 connector and crimp. There does not seem to be a consistent standard for which pin gets which color wire, so eyeball a commercial cable and follow its example. Label this the hub end.
Next, cut two inches off the other end of the cable to get a bit of wire to make the jumper between pins 1 and 2 of the Snort end. You can do this by bending it in half and shoving it in the RJ45 connector. Again, strip a half inch off the outer insulation. Strip a quarter inch of the inner insulation off the wires from pins 1, 2, 3 and 6. With care and solid conductor Cat5 cable, you can push the wires from pins 1 and 3 into the slot for pin 3 and the wires for pins 2 and 6 into slot 6. Be sure all wires are inserted fully into the slots, then crimp the end. See Resources for links to pictures of the result. (A word of warning: trying this with stranded wire is a task for masochists.)
If your cable is stranded or if you can't find a crimper and are handy with a soldering iron, simply cut the patch cord in the middle and connect the appropriate wires together. Use heat shrink tubing to insulate and protect your handiwork.
