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Thread: Ubuntu not taking ARP request/answers into account

  1. #1
    Join Date
    Apr 2012

    Ubuntu not taking ARP request/answers into account

    Hello all,

    I have a strange problem on a small network of mine.

    It's composed of 2 machines and a switch.

    PC1 is Ubuntu 9.04 (old I know) with IP It is connected to PC2 through a switch (both machines are belonging to the same VLAN).

    PC2 is Windows Seven with IP

    Both machines cannot ping each other. A bit of wireshark showed me a strange behaviour in the ARP protocol.

    Ping from PC 1 (ubuntu) creates an ARP request that is received and answered by the PC2. The answered however is not taken into account by PC1 : i'm still still unable to ping

    From icmp_seq=58764 Destination Host Unreachable
    From icmp_seq=58765 Destination Host Unreachable
    From icmp_seq=58766 Destination Host Unreachable
    And the ARP cache is not "complete":

    satem@satem-desktop:~$ arp -a
    ? ( à <incomplet> sur eth1
    On the other hand, pings from the PC2 generate ARP requests that reach PC1 but are not answered nor taken into account into the ARP cache of PC1.

    Removing the VLAN tagging seems to solve the problem but I can't see why it would cause a problem (and I can't afford to remove it permanently). Also sysctl parameters arp_filter and rp_filter have been set to 0 with no apparent effects.

    Thank you for your help.

  2. #2
    Join Date
    Apr 2012

    Re: Ubuntu not taking ARP request/answers into account

    Update: after additional tests, the result is still the same when 9.04 is replaced by 11.10.

    Also, I noticed that the ARP request/answers received by the PC1 (Ubuntu machine) are still VLAN tagged. On the other hand, the Windows machine gets untagged packets.

    Also, same configuration with Windows Only machines works perfectly fine.

  3. #3
    Join Date
    May 2012

    Re: Ubuntu not taking ARP request/answers into account

    Have the same problem when trying to create a hotspot with hostap and isc-dhcp-server.My Symbian phone gets IP but thats all.
    I'm desperate can't find solution nowhere.

    Sorry to bother

  4. #4
    Join Date
    Dec 2011
    The Netherlands
    Ubuntu 11.10 Oneiric Ocelot

    Re: Ubuntu not taking ARP request/answers into account

    What is it you're trying to do?
    You could try using a proper router. (if the switch is a managed one, i think it is since you're talking about vlans, it usually has one build in, that mode usually enables when it boots w/o anything connected to it)
    I think the windows<->windows combi enables some custom/propriatary protocol and
    windows<->ubuntu misses that, thus disabling the network.

    Enable ipforwarding.
    sudo sh -c "echo 1 > /proc/sys/net/ipv4/ip_forward"
    "Everything that has a beginning, has an end." --an unexpectedly wise wallpaper


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