[Xen-devel] UDP checksums broken in Dom0 -> DomU vif transfer

[Michal Suchanek <hramrach@xxxxxxxxxx>]

Hello,

when I boot DomU which uses DHCP to configure IPv4 address it does
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?

This issue is years old and has been discovered, solved, re-discovered
and re-solved numerous times already.

Thanks

Michal

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