This passive Ethernet tap featured at greatscottgadgets is great.
Get a couple of USB Ethernet adapters for your laptop, set up a tun interface and you can see everything on both sides without any arp cache poisoning.
It is possible to make the device using some Krone punch down blocks and some cat5 cable.
Here is some pictures, I have not added the 220pf caps yet, they are to derate 1 Gig networks to 100 Meg.
The schematic below, shows what is going on.