I thought LAN cables were shielded
Okay, I agree that nothing is perfect, but it seems a bit of a stretch to say that you could detect LAN traffic from tens of meters away by "listening" to a shielded cable.
You might be able to do it from the other side of a wall, but if you want to target a specific cable, good luck.
I accept this could work if you can set it up in the server room, but then we're back to the Primary Rule : if you have access, security is gone.
Biting the hand that feeds IT
