unbelievable network problem

Discussion in 'Linux Networking' started by Amadeus W. M., Jul 23, 2005.

  1. I have a cable modem connection to the internet. I have a linksys router
    which goes into the modem, and my pcs connected to the router. The router
    gets the IP address through dhcp, always xx.yy.zz.226.

    Lately, due to bad weather, there were some power outages at the ISP, and
    the network went down. Since then, my connection drops every few hours or

    To rule out problems with the router, I plugged in my pc straight into the
    modem. I configure eth0 with dhcp. Now I get xx.yy.zz.8. If I still try
    the router I get the old xx.yy.zz.226. Don't know why I'm getting
    different addresses, when the ISP knows me by the mac address of the cable
    modem. Anyway...

    Still the connection drops every few hours, and I noticed it always
    happens when the ISP (or someone!) sends out arp packets (Who has ... tell ...).

    If I /etc/rc.d/init.d/network restart, the connection comes back up, with
    xx.yy.zz.8 (the same address that eth0 always gets through dhcp).

    Then I thought, maybe someone else on the network claims they have
    xx.yy.zz.8, so I said let's configure eth0 MANUALLY (not dhcp) with the
    STATIC IP that the router always gets: xx.yy.zz.226.

    And that works too!

    Don't know how, or for how long. The only explanation I can think of is
    that both xx.yy.zz.226 and xx.yy.zz.8 happened to be unassigned by the
    dhcp server. However, when the ISP updates its arp tables, it seems that
    someone else claims to have my ip, and I loose the connection.

    Does anybody have any insight about what might be going on? I'm even
    considering horror scenarios with hunt arp poisoning, but I suspect more
    misconfiguration on the ISP side.
    Amadeus W. M., Jul 23, 2005
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  2. Hello from the Eighth Doctor
    Look at it this way, depending on who your ISP is, ComCast, TWC, Cox, or really
    any Cable TV service provider who thinks they can deliver I'net access via their
    Cable TV lines, could be at fault.

    So, who's your provider? Without knowing that, we can suppose anything.
    The Eighth Doctor, Jul 23, 2005
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  3. Maybe your cable modem remember the mac id of the router.
    Then contact your ISP.

    Davide Bianchi, Jul 23, 2005
  4. Amadeus W. M.

    James Knott Guest

    The DHCP server is talking to your computer or router and not the cable
    modem. If it sees a different MAC address, it will assign a different IP
    address. To get the same address for both devices, clone the computer's
    MAC address to the router.

    When you say the connection drops, do you mean that eth0 no longer has an
    address? Has been turned down? A bit better description would come in
    James Knott, Jul 23, 2005
  5. So how come the connection works when I give my eth0 statically, the IP
    address that the router normally gets through dhcp? Could it be that
    since my router is out of the loop, its address remains unassigned, so I'm
    free to use it on eth0?
    eth0 still has an ip address, but I get no answer when I try to connect
    anywhere. As if the response packets to my IP are being routed to some
    other IP.

    Again, I assign eth0 a static ip (the one the router would get),
    and I put the appropriate default gateway in the routing table. Of course,
    things stay this way at my end, until I change them.
    Amadeus W. M., Jul 23, 2005
  6. Millenium.

    For the last few years I had ATT, SBC, Comcast, Qwest, Road Runner, and
    now Millenium. Without a doubt, Millenium is the worst nightmare. Aside
    from these problems that I'm having now, every time it rains, the modem
    goes offline, so I get cut off from the ISP.
    Amadeus W. M., Jul 23, 2005
  7. You think I didn't? They're useless, they don't know anything beyond
    unplug the modem, turn off the computer, and reboot.
    Amadeus W. M., Jul 23, 2005
  8. Look, as far as I can see from what you said, the problem is not on your
    side. You've paid for a service, you should get the service. If your ISP
    doesn't want to give the service terminate your contract and go to another
    ISP more willing to give you what you want. When enough peoples vote with
    their feet, the lusemonkey goes out of business and the world become a
    better place.

    Davide Bianchi, Jul 23, 2005
  9. Amadeus W. M.

    James Knott Guest

    Next time, reboot the modem out the window. ;-)
    James Knott, Jul 23, 2005
  10. Amadeus W. M.

    James Knott Guest

    You "own" the IP address for the duration of the DHCP lease. So, until the
    lease has expired, there should be no one else using the address. However,
    some systems might not like seeing a computer, with an unassigned address.
    What happens if you ping the ISPs router? Watch what happens, using
    something like Ethereal.
    A few years ago, I had a similar problem, but sending out an occasional ping
    cleared the problem, which I suspected was with their arp cache. If
    pinging the router clears the problem, you might configure your system to
    ping the ISPs router on a periodic basis.
    James Knott, Jul 23, 2005
  11. Amadeus W. M.

    Bill Marcum Guest

    Tell them you did, now it's their turn.
    Bill Marcum, Jul 23, 2005
  12. They don't allow ping. The senior technician told me a while back that
    ping is a major security risk, and that a person with my background should
    know better.
    Amadeus W. M., Jul 23, 2005
  13. Amadeus W. M.

    James Knott Guest

    Other than DOS attacks or revealing the existence of a system, did he say
    what other problems there might be? Anyway, you don't have to actually get
    a reply. You're only "reminding" the system you exist. You can even ping
    another system, such as www.yahoo.com.
    James Knott, Jul 23, 2005
  14. Amadeus W. M.

    gary Guest


    Just a thought! Call the cable company and mention the rain problem. This
    indicates you might have a bridge/amp creating a break. Also request to
    have the cable tested at the head to your home. Then test from the modem
    back. It could be something as simple as a crack/break with the cable

    As for the ISP reassignment. Most modems allow you to nail the communication
    from the modem to the ISP/router. This is usually just a checkbox setting
    in your modem setup screen(S).
    gary, Jul 24, 2005
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