Creating an RJ45 Ethernet loopback cable

Sometimes it’s handy to make a PC think that it is connected to a network, even if there isn’t one physically present (e.g. in a test environment where not all services are replicated). This is quite easy to achieve, with an RJ45 Ethernet loopback cable. By using 6″ lengths of the core from a CAT5 Ethernet cable to connecting pin 1 to pin 3 and pin 2 to pin 6, a simple device is created which will fool a network interface card into thinking it is connected to a network.

RJ45 Ethernet loopback cable

27 thoughts on “Creating an RJ45 Ethernet loopback cable


  1. I found one of these lying around in a test lab at work earlier this week – it turns out that HP manufacture them as part number 317463-001 (spare part number 314765-001).


  2. In theory, it should work regardless of the operating system because this is a physical layer device; however I guess the higher levels in the network stack could interpret a loopback differently (and in the case of Linux, each distribution is likely to differ slightly).


  3. Anonymous said…
    Will it work in Linux? I tried using it windows and it works, but not in linux.

    Are you using a gigabit port? It should work with 10/100 Ethernet but higher speed ports require additional connections. For Gigabit ports, in addition to shorting pins 1 & 3 and pins 2 & 6, short pin 4 to pin 7 and pin 5 to pin 8.


  4. Anonymous said…
    For Gigabit ports, in addition to shorting pins 1 & 3 and pins 2 & 6, short pin 4 to pin 7 and pin 5 to pin 8.

    I’ve tried this in several gigabit ports … on PC’s (running Linux none using windows) and on Cisco Swithced and it doesn’t appear to work.


  5. I’m curious… would this work sufficiently to make a VM think it’s seeing a real network – and allow it to access the file system of the host?
    I’ve been having fun making a virtualbox linux VM access a Vista host.


  6. I create 1GB loopback and plug in linux system and it does not work. The link led is not on. But the amber led is on that tell me 1GB port. You think the motherboard does not support loopback?


  7. I would like to use something like this to test data cabling continuity in walls.
    Does anyone know how to do this ? I only need it to run at 10 /100.
    So, instead of shorting the two pairs, can I just wire in two led’s to the plug,patch my laptop into the datapoint run a ping -t and stick the loopback plug into the patch panel to see it light up. without burning my nic.
    many thanks in advance.


  8. Hi every body,

    i was make a loopback ping 1-3, 2-6, but it doesnt works, in my laptop lenovo t400 i test it in my brother laptop and work, somebody know with it doesnt work for me, i use OS win xp sp3. regards.

    juan


  9. I have tried this on LINUX +WINDOWS………both…..

    What I found is that…..The system will show u connected if NIC (Ethernet card) is/or set to 100Mbps (Fast Ethernet)……………on gig ethernets port will not be up…..either ur cable is cat6 or cat 5e………………….

    Good LUCK.


  10. Hi,
    I found that Loopback cable doesn’t work when the network adapter is set to 1000 Mbps, to make it working i set it to 100 Mbps

    Thanks!


  11. Loopback connectors won’t work with Gigabit ethernet, because there is an echo cancellor built into chip on nic that senses 100% crosstalk on pin 4,7, 5,8 if you can disable this function then it will work. I’ve been experimenting but can’t find setting’s to disable “NEXT”
    regards-


  12. Thanks for all the comments on this (old) post.

    Judging by the number of people saying that it is/isn’t possible to do this on gigabit Ethernet it does sound as though it’s vendor specific. That wold also fit with Evan’s comment about echo cancellation on the NIC – depending on the exact hardware used on the adapter your mileage may vary…

    I was certainly only using 100Mbps when I wrote this back in 2006…


  13. The do-it-yourself nature of this is perfect for those of us who like to improve ideas for our benefit.

    For example, if you made 2 or 3 of these cause you worry about losing them, make one into a keychain (hopefully you don’t lose your keys often) or make the cable long enough to clip onto a carbinar. I have a carbinar on my toolbag and keep a usb key and other tools clipped into it to prevent me from losing them.

    Thanks for a great post!


  14. Thanks for the excellent info, you saved me over 10,000 euro on a specific contract that wasn’t going well! I have had to make hundreds of the little buggers, but it sure did save my skin!


  15. I’ve try this both on 100M nic and 1G nic ,both RJ45 led lights up. but both there are just sending packets , not a single receiving packet!
    Can you help me.
    thank you

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