Physical loopback plug (RJ-45)

For whatever reason one might to want a physical RJ-45 loopback plug instead of using a virtual loopback. Be it hunting errors or popping in two of these onto a network to generate a large amount of traffic. The plug will pretty much receive traffic and send all traffic straight back onto the network. This will work for 100 Mbps but I’m not sure with higher speeds. I’ll make sure to update if I find/figure that out.

You’ll need the following:


* RJ-45 plug
* Some cable. Either cut an unused CAT5 cable or use any similar cable.
* RJ-45 Ethernet crimping tool.

While you might not own a crimping tool I know there are guides on how to do it without one. However, I’ll leave that to you to find one if you’re interested.



In the supersized image above we can see the pins on the plug. We’re going to connect send & receive on two places in the plug with the two cables. Namely pin 1 to 3 and 2 to 6. For more information about the pins there are some good articles on the net, Wikipedia for example.

Let’s connect pin 1 and 3 with the first cable. Make sure to press them all the way down into the hole.


Let’s connect pin 2 and 6 with the second cable. Make sure to press them all the way down into the hole.


Great, now it is almost done. Last we need to use the crimping tool to finish our plug. Just put it in the socket on the tool and give it a squeeze.


Now put it into a network card and make sure it is registered. You can also run Wireshark to verify that it works as intended.


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