WTH!?! Bizarre Ethernet Problem.

Discussion in 'Tech Talk' started by Bushflyr, Sep 28, 2012.

  1. Bushflyr

    Bushflyr ʇno uıƃuɐɥ ʇsnɾ
    Millennium Member

    OK, this is just F'ing weird. I have a server set up running Ubuntu 12.04. It's hooked to an Asus RT-n66u via ethernet cable. Also wired to the router is my 27" iMac.

    Twice in the last week (and once many moons ago) I've come in to find no internet connection on my Mac. The first time I went through every networking diagnostic thread I could find with zero success. No ethernet connection, but internet works fine when I fire up the wifi, and every other device that connects via wifi is fine. Finally, for some reason, I plugged a spare keyboard and monitor into the server only to find it non responsive. :dunno: OK. Hard shut down and ethernet is instantly working again on my Mac. :faint:

    It just happened again and I killed the server. Voila, ethernet fixed.

    So. WTH is going on and how is one device breaking the ethernet connection to another device but leaves wifi AOK?

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  3. There have been ongoing Ethernet problems with older Macs.
    Google your Mac and Ethernet problems and you will have lots to read.

  4. gemeinschaft

    gemeinschaft AKA Fluffy316

    Just curious, but is your Ubuntu server handing out DHCP or your router?

    I have seen where someone setup a server and did not realize that it was broadcasting and responding to DHCP requests. May not apply here, but thought I would mention since this is something I have run across before.
  5. Do a trace route and look at arp cache. Also turn off rip on the Linux box
  6. Bushflyr

    Bushflyr ʇno uıƃuɐɥ ʇsnɾ
    Millennium Member

    Yep. I read, literally, every one of those threads and did everything recommended. But the problem isn't the Mac. The problem is the server. As soon as i reboot the server everything works perfectly. I don't even have to log out on my Mac, it's instantly ok.

    I'm pretty sure it would be the router. I don't recall setting up DHCP on the server at all. Server, Mac, and router all use static ip's. And the router handles dhcp for wifi.

    Will do that when I get home. What's rip?
  7. Interesting.
    I have a client that has installed a bunch of headless servers running Ubuntu 12.04 and 10.04 and the problem he is having is that every so often they lose Ethernet connection and don't re up as they should. Headless so reboot required.

    Frankly I am not yet sure why the problem exists. The short term fix was a cron job that checks for internet connection every 5 minutes and does an ifup if it has failed.

    You have fixed IP addresses, I don't, so it would be easy to just write a little script to ping the Mac and if it fails to get a return then ifup.
    #6 Linux3, Sep 29, 2012
    Last edited: Sep 29, 2012
  8. Probably something you have already checked, but are you certain your statically assigned IP's are outside the range of your DHCP scope?
  9. I'd love to see a packet cap from this issue. Judicious use of Wireshark is your best bet to see exactly what's going on.
  10. Bushflyr

    Bushflyr ʇno uıƃuɐɥ ʇsnɾ
    Millennium Member

    I'll check out wireshark. I'm not sure how I'd go about getting a cap from a dead wire, self assigned ip, but I'll google it.
  11. Bushflyr

    Bushflyr ʇno uıƃuɐɥ ʇsnɾ
    Millennium Member

    OK, got back home and rebooted.

    arp cache returns "no entry"

    rip is not installed.

    traceroute returns:

    [/var/log]: traceroute
    traceroute to (, 30 hops max, 60 byte packets
     1  iMac-0023____04EA.local (  0.281 ms  0.271 ms  0.264 ms
    Does that help at all?

    A FoaF network guru said he suspected an ARP flood or Broadcast storm caused by a faulty NIC on the server, but I'm still researching what that means. :dunno:
    #10 Bushflyr, Oct 3, 2012
    Last edited: Oct 3, 2012
  12. Bushflyr

    Bushflyr ʇno uıƃuɐɥ ʇsnɾ
    Millennium Member

    Just checked. My DHCP range is from xxx.2 - xxx.99. Static IP's start at xxx.100.

    Mac networking is set to DHCP and the server returns:

    [~]: cat /etc/network/interfaces
    # This file describes the network interfaces available on your system
    # and how to activate them. For more information, see interfaces(5).
    # The loopback network interface
    auto lo
    iface lo inet loopback
    # The primary network interface
    auto eth0
    iface eth0 inet dhcp
    So I assume it's also set to DHCP with the router handing out all IP requests.
  13. Have you ensured latest firmware on your Asus RT-n66u?

    Try switching out the network card on your Ubuntu server ... or if it is built-in to the motherboard, just add a new card. Preferably something that has a very long history of reliably working with Linux (like an Intel "server" class NIC). I bought this one a couple years ago http://www.newegg.com/Product/Product.aspx?Item=N82E16833106015 and it has worked wonderfully for me under CentOS and Ubuntu. It is a dual port NIC ... but this single port one has a ton of good reviews that reference Linux: http://www.newegg.com/Product/Product.aspx?Item=N82E16833106121
    #12 douggmc, Oct 3, 2012
    Last edited: Oct 3, 2012
  14. Bushflyr

    Bushflyr ʇno uıƃuɐɥ ʇsnɾ
    Millennium Member

    Thanks, but no. Throwing parts at something without knowing the exact problem is the absolute last resort. Even if it does fix the problem I'll never know what it really was and won't learn anything.

    Half the reason i built this from the ground up was that nothing commercial comes close for the price. The other half was to learn Linux better. Nothing like starting from scratch to force an education. :supergrin:
  15. :thumbsup: Cool .. I hear ya. I'm the same way and have done same thing as you.

    What about the firmware on the router? Current? Are you on a beta and can you step it back to older version?

    Do you have any other protocols or services running on router like DLNA or Bittorrent server (not familiar with the model you have ... but I know some of the newer one's have a lot of cool features)? Perhaps turn everything off besides basic DHCP and routing services temporarily if so.

    Do you have an alternate router you can swap out temporarily so as to rule the Asus one out as the culprit (i.e., if same thing happens with a different router you can probably assume it is not the router)?

    Lastly ... just one more stab at the NIC. I've burned a lot of time with semi-supported NICs on Linux servers. A 30 buck NIC can work wonders in preventing hair loss if it is the culprit. And ... if you want to "learn linux", unless you mean being a kernel hacker and contributing source code to "fix" support for your NIC ... then you might be better served, again, to throw another NIC in there and move on.

    Anyhooo .... post back if you find the culprit/solution.

    Oh ... can't hurt to swap out CAT cables incase something is flaky there.
  16. I've seen bad NIC's cause broadcast storms. A dead giveaway of that is if the lights on the card or the switch port it's plugged into are constantly blinking.

    Usually after a few minutes of that the nic will shut down and a power cycle clears it's cache and the whole mess starts over again.

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