[CentOS-virt] Xen domU default gateway missing/ARP table full

Pasi Kärkkäinen pasik at iki.fi
Thu Nov 19 12:23:20 UTC 2009

On Wed, Nov 18, 2009 at 11:39:24AM -0500, Ken Bass wrote:
> I have been trying to figure out why my domU NIC becomes unreachable 
> (could not even ping) at various times. (Normally when the server was 
> trying to update clamav from the various busy mirrors at 4am). There 
> also seemed to be some latency when connecting which I chalked up to it 
> being a virtual machine.
> When I checked my logs, I found thousands of :
> Nov 17 04:07:52 nomad kernel: Neighbour table overflow.
> and applications reporting errors such as:
> Nov 17 04:08:05 nomad freshclam[4085]: nonblock_connect: connect(): fd=5 
> errno=105: No buffer space available
> I am running a routed (not bridged) configuration.
> What I figured out is that each Centos 5.4 domU is maintaining an ARP 
> table. That table is filling up which causes the network to be 
> unreachable until entries are purged from the cache. Since this is a 
> routed configuration, the ARP table should really only consist of two or 
> three entries, my domU, my dom0, and the gateway.
> It appears the networking-scripts until Centos are ignoring the GATEWAY 
> entry. I end up with route of:
>     *          U     0      0        0 eth0
> default         *              U     0      0        0 eth0
> The default route should be the specific IP address in my 
> /etc/sysconfig/network file. When I manually add the route, the arp 
> table issue
> is fixed. The network stack no longer trys to query an arp entry for 
> every IP address.
> I found this bug at Xen which was closed as INVALID saying 'Centos is 
> broken'. That was from 2006.
> http://bugzilla.xensource.com/bugzilla/show_bug.cgi?id=596
> Any ideas on what is broken and what the correct fix is? Right now, I 
> just added
> /sbin/route add -net netmask gw x.x.x.x
> to my /etc/rc.local which seems like a hack solution.

I usually specify the default gateway in
/etc/sysconfig/network-scripts/ifcfg-eth0 and it works just fine.

-- Pasi

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