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18 Nov 09

Poptop Questions & Answers

  • Cannot determine ethernet address for proxy ARP



    This is due to an issue with the pppd program, which attempts to find a
    hardware interface on the subnet to which the pppd client has been
    assigned. In this case its looking for a hardware interface on the
    192.168.5.0 subnet. It will fail to find one, and will drop the
    proxyarp request.



    The simplest way around this problem, and the one that is suggested in
    the pppd documentation, is to set the pppd client IP assignment to be on
    the local subnet. An example in this case might be 192.168.56.129.
    However, it may not be possible to do that. In the case of a fully
    loaded subnet, there may not be any addresses to spare. Or there may be
    some security issues with giving out local subnet addresses. What to
    do?



    The place to look is in the arp table. If you run tcpdump on host
    (192.168.56.12) during the time when client is pinging, you will see
    unanswered arp requests from host attempting to find the hardware
    address for 192.168.5.12. You need to proxy the hardware address of the
    pptp_srvr for client in order for this request to be fulfilled. This is
    the job of proxyarp. However, proxyarp has let us down in this
    instance, and we need to find a workaround.



    This can be done manually using the arp command on pptp_srvr. For
    example, if the ethernet card on pptp_srvr is
    eth0, you could force the arp to proxy the client pptp
    address by saying

    	arp --use-device --set 192.168.5.12 eth0 pub


    You should now be able to ping from client to host through the pptp
    connection.

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