PCs unable to resolve MBP's MAC address

Discussion in 'Mac OS X Server, Xserve, and Networking' started by bjdraw, Dec 30, 2009.

  1. bjdraw macrumors 6502a

    Joined:
    Jan 24, 2008
    Location:
    Tampa FL
    #1
    I'm not sure when this started, maybe with Snow leopard, but every once in a while my MBP is unable to communicate with either of my Windows PCs. If I run 'arp -an' on the MBP, I can see the MAC address of my PCs, but if I run arp -a on the PCs I don't see the MAC of the MBP.

    So I ran wireshark on the PC and 'tcpdump -i en1 arp' on the MBP (en1 is Airport) and while I see the PC broadcasting a who-has, I don't see them on the MBP side. I do see who-has requests on the MBP coming from my router, to which my MBP responds, just not from either of my PCs.

    If I disable and re-enable the Airport on the MBP communication starts working again, or I can enter in a static arp entry for the MBP on the PC.

    MacBook Pro5,5 <-> 802.11G <-> ActionTec MI424-WR C <-> CAT5 <-> Netgear FS116 <-> CAT5 <-> PC

    Any suggestions would be greatly appreciated.
     
  2. hmmfe macrumors regular

    Joined:
    Feb 28, 2003
    #2
    That is indeed strange. If I can summarize, the MBP occasionally does not see an ARP request via wireless from PCs. However, you do see the PCs sending the ARP requests. I'd try two things to start...

    1) use a separate device to sniff the wireless network. This will eliminate the MBPs airport card as a possible point of failure. If the second device can see the ARP requests but the MBP cannot then I'd suspect the airport card as the culprit. On the other hand, if the "sniffer" cannot either, then I'd suggest the Actiontec is to blame.

    2) Put the MBP on the wired network (i.e. on the switch) with the PCs to determine if the same behavior exists. This would give us information on whether it is the TCP/IP stack on the MBP to blame.
     
  3. bjdraw thread starter macrumors 6502a

    Joined:
    Jan 24, 2008
    Location:
    Tampa FL
    #3
    Fantastic suggestions, I'll give it a try. I don't use a wired connection with my MBP much, but when I do I've never experienced the problem.

    Thanks again.
     
  4. bjdraw thread starter macrumors 6502a

    Joined:
    Jan 24, 2008
    Location:
    Tampa FL
    #4
    So I've been putting up with this for months and just disabling and re-enabling the Airport whenever it arrises. But it also affects my iPhone when connecting to my Windows PCs.

    So tonight I did some more investigating and ran a capture on my Windows Home Server and my Macbook Pro while trying to ping my Windows 7 PC from my MBP.

    I can see the ARP broadcast requests for my MBP's IP on the WHS, but not on the capture running on my MBP. I do however see ARP broadcasts from my wireless router on the capture on my MBP, to which my Mac responds immediately.

    Disabling and re-enabling the Airport fixes it because the Mac broadcasts a gratuitous arp when the NIC is enabled.

    So I still have no idea why I don't see the arp requests in the capture on my MBP, but I did disable the OSX firewall for the test. I've also never been able to reproduce this on my wired network.
     
  5. bjdraw thread starter macrumors 6502a

    Joined:
    Jan 24, 2008
    Location:
    Tampa FL
    #5
    Finally broke down and called FiOS about this today and they were able to fix it so I figured I'd post it.

    Basically I had my DHCP scope configured to 192.168.1.150-200 and the tech told me that out of the box only STBs are supposed to be that high. So I switched it to 70-99 and the problem went away.

    Evidently there is some sort of broadcast isolation in the router that prevents broadcast arp packets from making it from the lan to the wan if the IP is over 100. Very odd, but I'm glad FiOS has great support and was able to help me.

    Thanks for the suggestions, hopefully google helps others find this.
     
  6. dampfdruck macrumors member

    Joined:
    Oct 20, 2008
    #6
    Hello,

    It could be that your netmasks dont' match. APR is only used when the destination IP is within the same subnet as the source IP. If not, the packets are sent to the default gateway instead.
     

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