Re: UDP checksums broken in Dom0 -> DomU vif transfer

[Konrad Rzeszutek Wilk <konrad@darnok.org>]

On Mon, Dec 19, 2011 at 10:27:36AM +0100, Michal Suchanek wrote:
> Hello,
> 
> when I boot DomU which uses DHCP to configure IPv4 address it does

You didn't say what version of DomU you are running? Is it 3.1?
> never get a lease.
> 
> The packets travel to Dom0 where the dhcp server receives them, sends
> a reply, that travels to DomU where dhclient receives it, says the
> checksum is invalid, and discards it.
> 
> The problem is documented here:
> 
> http://old-list-archives.xen.org/archives/html/xen-users/2006-02/msg00152.html
> http://old-list-archives.xen.org/archives/html/xen-devel/2011-04/msg01235.html
> http://bugzilla.xensource.com/bugzilla/show_bug.cgi?id=1655
> 
> The fix is to turn off UDP checksum offloading on the vif interface in
> Dom0 as documented in the above mail:
> 
> I edited /etc/xen/scripts/network-bridge,
> adding this command to the end of the op_start() function:
> 
>         add_to_bridge2 ${bridge} ${pdev}
>         do_ifup ${netdev}
> +       # disable ip checksum offloading for veth device
> +       ethtool -K ${netdev} tx off
>     else
>         # old style without ${vdev}
> 
> Note: I am not sure which path is taken through the script, I set the
> parameter manually with ethtool before I found this patch.
> 
> It some solutions suggest to turn off UDP checksum offloading in the
> DomU as well but it does not seem to be necessary since the packets
> would travel to the dhcp server and it would reply to them.
> 
> Some people say this is working for them.
> 
> I suspect this is because some Linux distributions already carry this patch.
> 
> Any reason why this can't be fixed in Xen upstream?

It should be fixed in the kernel and I think it is fixed. Did you have
this problem with a 3.1 or 3.0 kernel?
> 
> This issue is years old and has been discovered, solved, re-discovered
> and re-solved numerous times already.
> 
> Thanks
> 
> Michal
> 
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