Comment 4 for bug 1244589

Please note that this was fixed in nova-network last year by https://review.openstack.org/#/c/18336/4. See https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1029430 for an explanation of why this iptables mangling is necessary. The TL;DR is that some older, albeit widely used, DHCP clients are fickle about packet checksums. For example, this affects the DHCP clients in Ubuntu 12.04 and CirrOS.

I suggest making this a bug of high importance and further suggest that the fix is backported. Without the checksums filled in, VMs with affected DHCP clients are basically stillborn.