Make a Passive Network Tap





Introduction: Make a Passive Network Tap

This instructable will show you how to make an inexpensive network tap to monitor your network.

Companies like Network Optics make incredible taps, for all sorts of media, but if you have 10/100 home network then for $18 in parts from home depot you can make a tap and send the output to YAF/snort/tcpdump/wireshark and see if any data is leaking that should not be.

I have been doing Flow Analysis lately instead of using other tools. I like YAF . Then again I work on it...

If you want to see step by step instructions on setting up a flow collection infrastructure look at this wiki page.

Step 1: Parts

You will need:
3x Leviton Multi Use Cat 5e Jacks (5G108-W)
- I used 2 white and 1 blue, to let me know which one is the tap.
Leviton 3 port wall plate (#41080-3W)
Handy Box
5 inches of cat 5 cable

Step 2: Tools

You will need a wire stripper and a screw driver.

Step 3: Strip Wire

Cut 5 inches of cat 5 cable, and pull out the 8 strands of wire.

Step 4: Wire the First Jack

Separate the strands of wire and wire up the leviton jack. It comes with a little punchdown tool to make this job easy. I followed the color code on the side of the jack, it does not really matter though, as long as you are consistent the whole way through.

Step 5: Wire the Second Jack

To wire the second jack, you should put both the jacks in the wall plate.
Use the punchdown tool to put the wires in the jack using the color codes or same pattern as you did on the first jack. Make sure to leave enough wire left over to reach the third jack.

Step 6: Third Jack

To wire the third jack, drop the third jack in the panel then wire it up just like the 1st and 2nd.

Trim any excess wire.

Step 7: Close It Up

At this point you can close up the box and you are done.

Test it by hooking up the input in the top jack, snooping interface in the middle, and the destination on the bottom.

You can start up your snooping program and watch the traffic spin by. Make sure to have the snooping interface set to promiscuous mode and not assigned an ip.

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Sorry... stupid question. What exactly can you use this for? I can monitor my network using the network monitor application...

Even this article is too old to consider as 'Active' it seems Google still send some traffic for people looking for passive network monitoring solution.

To answer @bryanbrews question, you can monitor part of network data with network monitor application but when it comes to capture EVERY Bit of traffic and most impotently Stealthily device like this tap instrument is must. Even it act like some other 'bridge' equipment, it NEVER sent any data to trace back this device. It is something like wireless / radio scanner, it 'catch' all traffic without 'transmit' single bits if data ....


Hey rancidbry- This would used for looking at all the traffic on your network not just what is being sent to your nic. A way to use it would be say if you had a cheap firewall that did not have logging. Clearly you are dropping packets at the firewall for incoming smb requests, but you do not have a way to see where they are coming from. You could place this between the firewall and cable modem, fire up wireshark and see what was coming in. -Joe

Dude, that thing's going to be an EMI magnet . . . not to mention all the NEXT potential. Wouldn't it be better to try to maintain the twists to within 1/2" or better of the IDC blades? If it were me, I would put a hairpin bend in each wire at the tap jack and push the entire bend (both sides of the wire) into the IDC. That would allow both conductors of each set (solid and stripe) to be near one another to allow for twisting. Otherwise a great instructable.

With all the computing equipment it will be connecting with it , I t may not be notices, especially if it is used in a wall socket with a power socket nearby.

cheaper still use 2 cat5 RJ 45 Cables..
cable 1 is host.
cable 2 is tap.
Cut cable2 in half . use a lighter to burn off some of the plastic insulation on the ends
green wire and the green&white wires on both halves.
Remove some of the grey sleeve on cable 1. burn off the insulation on the green&white, green, orange&white and orange wires.
now connect the green&white wire from cable 2 to the green&white wire on cable 1. insulate with electrical tape.
connect the green wire from cable 2 to the green wire on cable 1
insulate with electrical tape.
connect the green wire from other half of cable 2 to the orange wire on cable 1
insulate with electrical tape
connect the green&white wire from other half of cable 2 to the Orange&white wire on cable 1.
insulate with electrical tape.
use marker pen to identify host , tap A and B
cheap and simple.
remove the cable from your PC and Router and replace with cable 1
plug end of cable 2 into another computer with wireshark etc. running . you now have a passive tap.
image grey cable host.
yellow cable tap.

Picture 004.jpgPicture 001.jpgPicture 002.jpgPicture 003.jpg

I don't get it: Make sure to have the snooping interface set to promiscuous mode and not assigned an ip. How can I do this on a windows machine?

When I plug the cable from a NB with wireshark, the connections were cut off on two machines. Could any explain? Thanks.

Ok try a normal run average of about 100' untwisted.

There are tolerances but not big ones.