tempmj built a passive ethernet tap for monitoring and debugging network connections:
I basically took 4 Panduit brand modular ethernet jacks and wired between two of them normally, other than the fact that the TX pairs from “Side A” looped through an extra jack and the TX pairs from “Side B” looped through the remaining jack. On the two tap jacks (the extra jacks) that created, it was the TX pair that i hooked it to (so the RX on my laptop would see the traffic). To protect it i encased it in a LOT of hot glue :P