In an earlier blog post I claimed that I was seeing problems with VMs using the virtio driver for networking on an Ubuntu 12.04 LTS KVM host using DHCP.
However, as far as I am concerned, this claim was wrong. I now figured out, that the messages about bad UDP checksums had nothing to do with my problem. I was rather experiencing the problems caused by a configuration that did not list the VLAN network interface (eth0.X) on which the DHCP relay agent received the answers from the DHCP server.
The mean thing is, that switch away from virtio fixed this problem. However, this was not because of the UDP checksums now being right (this was merely a side effect). It fixed the problem, because when not using the virtio driver, the DHCP relay agent would receive the answer packets, even if they were received on a VLAN interface it was not listening to. I can only guess that the implementation for VLAN-tagged interfaces is slightly different when using the virtio driver.
After adding the interface to the list of interfaces used by the DHCP relay agent, the DHCP packets are relayed correctly, even if using the virtio driver. The messages about bad UDP checksums now reappeared in the log file, but obviously this is not causing any trouble.
On the other hand, according to a bug report some users really seem to have problems with DHCP when using the virtio driver. However, this might only affect Ubuntu 12.04 LTS guests but not VMs on a Ubuntu 12.04 LTS host.