[libvirt-users] network checksum offloading broken using virtio

Hi, with recent guest installs (both centos 5 and 7) on centos 7 hosts I seem to have to disable checksum offloading using "ethtool -K eth0 tx off" in order to allow traffic to flow a specific route. Basically the guest is installed with IP 192.168.21.10 and a default gateway of 192.168.21.254. Up until that point I can ssh into the system normally. There exists an OpenVPN System with the IP 192.168.21.1 that choses client IP's for the vpn connections from the pool 192.168.20.0/24. In order to pass the reponses back to the OpenVPN system I installed the route "192.168.20.0/24 via 192.168.21.1 dev eth0". When I now ping the IP 192.168.21.10 through the VPN connection this works fine but when I try to ssh into that system the connection just hangs. Looking at a tcpdump I noticed that the checksum for the packets weren't quite right so I issued a "ethtool -K eth0 tx off" and suddenly everything worked as expected. What is strange here is that I'm seeing this with both CentOS 5 and CentOS 7 guests and only when dealing with th routed traffic and not the regular one. Does anyone have an idea what is going on here? Could this be an issue with the virtio driver? Regards, Dennis