ASUS RT-N66U – WAN Connection: Ethernet link down.

A few weeks ago I began to have a recurring and incredibly frustrating problem with my Internet connection. Multiple times a day my Internet connection would drop for a couple of minutes at a time. My cable modem would show I had a good link. Upon checking my ASUS RT-N66U router I would see this in the system log:

WAN Connection: Ethernet link down.

Then a few minutes later the connection would come back on its own. I would see this in the log:

WAN Connection: Ethernet link up.

I work from home, mostly using SSH to log into remote servers. My connection would flake out just long enough for my SSH connection to drop. It drove me up the wall.

I did a lot of Google searching and couldn’t find much of anything about the problem. What I did find wasn’t productive.

Going strictly off of the error message I figured I had one of three problems, bad network cable between the router and modem, bad WAN port on the router, or bad WAN port on the modem. I changed network cables with no luck. I replaced my ASUS router with an old Netgear router I had. The problem stopped. I went back to the old cable, still no problem. I put the ASUS router back into play, the problem started again. I knew it had to be something with the ASUS router. I had owned it for too long to send it back for warranty coverage. I also thought it odd that the hardware in the WAN port could be flakey enough to cause an intermittent problem like this while otherwise working fine. I figured it was software related.

In order to simplify things I turned off every feature of the router I didn’t %100 need. Guest wifi network, QoS,  USB support, DDNS, cloud support, and so on and so forth were turned off. I even shut down the second access point I have in my house. After simplifying everything something wonderful happened. The problem stopped. I gave it a few days to be certain and sure enough, no more drops.

I then began to turn on each service again, one per day. When I turned on DDNS via the drops started again within a few hours. I turned off DDNS and the drops stopped. The culprit had been found.

I don’t have a good idea why the DDNS client causes the WAN Ethernet port to drop out. Its some sort of bug in the firmware I’m running.

In the end my solution was to leave DDNS turned off in the router and then to install a update client on my computer.


My ASUS RT-N66U router started dropping my Internet connection. It had something to do with updating Turning off the DDNS client solved the problem.

Update 9/10/14: I switch to the Merlin Firmware and even with DDNS turned on the router has been fine with no drops. That aside the new firmware is working great, all of the features I would expect and I have not noticed any change in my speed.

Update 5/3/16: I’m still running the Merlin Firmware, updating it as they become available. No problems at all.

16 thoughts on “ASUS RT-N66U – WAN Connection: Ethernet link down.

  1. Hi,

    Any luck on finding what is the real problem behind this? Or a possible fix?

    I had the same problem which was fixed by turning off the DDNS service inside the router.

  2. Thanks for the post! This problem was driving me nuts and I hope that this has solved it. (Too soon to tell.)

  3. I’m having the same issue and just got off the phone with ASUS tech support. They don’t seem to know anything about it. I’m turning the DDNS off as well and hopefully this will solve my problem as well. Thank you so much for the post. Fingers crossed.

    1. I have the same issue, but turning off DDNS seems not working. Actually, I turned it on before, but I turned off short after. The funny thing is that even if I turned off the DDNS, I am still able to use the address to log onto Aicloud. I am using the latest Merlin Firmware.
      Any suggestions?
      Thanks a lot!

  4. It turns out that the issue is related to my modem, i.e. the modem firmware issue. It’s been 42 hours since I replaced the modem, there is no internet drop any more. Hopefully this helps people with the same issue.

    1. Heya Paul I’m having the same problem (and DDNS is off as well). Did you have a Motorola SB6121? Did you literally have to replace the modem to fix the issue?

  5. 1 day and counting… your fix appears to be working so far!! ASSus should fire their entire customer support division and hire your blog… What a $200 hassle.

    1. I’m glad it’s working for you. When I first had the problem I didn’t even bother calling, I figured it would be a fight to prove it wasn’t my modem or ISP.

  6. 2 days ago during the night there was a power outage. After that my Asus RT-AC66U lost connection with cable modem. Resetting router and/or modem didn’t help. Actually, after resetting modem router got WAN IP just for a several seconds and during that time it was possible to access Internet and modem admin page on
    But after several seconds everything stopoed to work. I couldn’t access Internet nor modem admin page. LAN access and router admin page and file transfers between devices connected on router (both cable and wireless) worked fine so I assumed my router is fine. I called my cable provider to check modem and technitian came and connected it to his laptop and concluded that modem is fine but now router and UTP cable were remaining suspects. I’ve changed cable without any improvement. Meanwhile I’ve googled for similar router problem and found your post.
    Good news … turning DDNS also solved my problem. It’s also visible in router logs that after router gets WAN IP it starts DDNS and after that it loses WAN connection.
    But the question now is why that started to happen out of nowhere after using actively DDNS for more than a year. And I’m on Merlin fiware for which you wrote it helped in your case.

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