Re: ARP problems in -testing?

[Derrik Pates <demon@devrandom.net>]

Keir Fraser wrote:
> The MAC addresses from the tcpdump log look sane. The problem must lie
> elsewhere. 

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.

-- 
Derrik Pates
demon@devrandom.net


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