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I had a very weird experience recently that I'm trying to debug in my home network.

I was on my mac looking at the computers on my home network that were auto-discovered. I saw a computer I didn't recognize, which spooked me a bit. I was able to access that rogue computer's files. I was connected via password protected (WPA2) wifi at the time to my own network.

I then ran a traceroute to determine what was the first hop to any ip address. It turns out the first hop was not my own router. Instead it was a neighbor's wifi router. I was very surprised.

Here is my home setup:

Cable Modem for internet, plugged into WAN of Buffalo 300N router (with DD-WRT firmware). The 2 of the LAN ports on the router are in use: 1 for a desktop computer and 1 other for a Powerline device (an Actiontec MegaPlug A/V 200). Upstairs I have an output for the Powerline networking, which is a TrendNet device with 3 RJ-45 female connections, one of which is in use connected to my Panasonic TV for the purpose of streaming Netflix faster than Wifi affords.

While debugging why I was connected to someone else's router on my own network I noticed a few things:

  • If I unplugged the powerline device from my router's LAN, my internet would die.
  • If I unplugged the cable modem from my router's WAN, my internet connection would persist fine.
  • When visiting, I was presented a login screen for my neighbor's Linksys E1000 router, instead of my own admin screen.
  • When I power cycled everything and did an ipconfig /refresh on all my computers, I was connected back on my own network, but now my neighbor's computer was on my network (instead of my computers on his network).

My router was not configured to be in bridge mode. My only logical conclusion is that my router lost a DHCP war with my neighbor's router, and my neighbor must also be using a Powerline device in his home network. We live in an apartment building, so it's conceivable that the Powerline is jumping apartment units.

So my question: is my own conclusion correct given the symptons? If so, what can I do to prevent my neighbor's router from winning this DHCP battle, while still using Powerline networking in conjunction with my wifi router? Any advice on how to secure my network and not accidentally connect to my neighbor's network again?

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migrated from serverfault.com Feb 12 '13 at 16:22


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3 Answers

up vote 2 down vote accepted

Sounds like both you and your neighbor are using powerline networking and neither of you has encryption enabled on the powerline devices. There is nothing to stop the powerline data signals from leaving your house and visiting your neighbor. (there would be if your houses were fed from different transformers, or were far enough apart.)

If you and your neighbor are using different subnets for your LANs and you get a DHCP address from your neighbor’s router, you end up using your neighbor’s router as your gateway to the Internet instead of your own. That is why you could unplug your router and still have Internet, but unplugging the powerline network adapter terminated your Internet access.

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thanks a bunch. –  AndrewParker Feb 12 '13 at 21:51
Perhaps you should install some kind of filter at your fuse box? –  vonbrand Feb 13 '13 at 3:26
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Power-line devices normally come with software for setting up encryption. By doing this (you do it for each adapter) you will "lock out" your neighbor and stop the cross channel interference your seeing (if that IS whats happening).

There's also the possibility that your neighbor was up to something naughty and trying to get you connected to their wifi without you knowing, however I think that is doubtful.

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So do you think my hypothesis is correct? Is it possible to connect to a neighbor's router over the LAN ports via Powerline? –  AndrewParker Feb 12 '13 at 21:28
Sure, as far as I know the signal can travel relatively far most of the time being stopped by fuse boxes. Its quite possible for it to end up with in a neighbors apartment. My guess is your router stopped doing DHCP at some point (or became slow) and theirs replied faster (thats simply how dhcp works - first reply wins). When you rebooted everything yours became the faster system, and they ended up getting a IP from your dhcp server instead. –  djsmiley2k Feb 13 '13 at 8:35
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It is very clear that the Buffalo 300N is the problem there.

But why the Linksys E1000 is on your home network? Are you guys sharing the internet?


Never mind, I found the answer for my question from your post.

But back to your first point, you should find out why the internet would die if you unplug the powerline from the router lan?

Can you confirm that you are able to get internet connection from the cable modem --> Buffalo router first.

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