Re: [Xen-devel] ARP problems in -testing?

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

> I just tried this on a dedicated server located at thePlanet in Texas; I 
> think this is an issue that combines the following:
>
>   - No MAC address specified causes a random MAC address to be selected at 
> domain create time
>   - The upstream router has a local ARP cache
>   - The host system's ARP cache for addresses on the virtual NIC for the 
> instance is blown out when the virtual interface disappears when the 
> domain is destroyed
>
> This seems to explain why tcpdump shows packets going to the VM's IP, but 
> TCP sessions and ICMP packets don't get a response - the switch still 
> knows the "old" MAC address, but the first outbound packet to the outside 
> world implicitly eradicates the old ARP-cache entry. I don't think this is 
> Xen's fault at all, just a property of the implementation of ARP 
> resolution. I just happened to notice that the MAC shown in incoming 
> packets didn't match the current MAC address (based on ifconfig's output), 
> so this is my current theory on what's happening.

Thanks for the detailed answer -- I've added it to the Xen FAQ
(http://www.cl.cam.ac.uk/netos/xen/faq.html). I guessed it would
probably be something like this... 

 Cheers,
 Keir


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