I have KVM set up to run a virtual machine (Windows Home Server 2011 acting as a build agent) on a dedicated server (CentOS 6.3). Recently, I ran updates on the host, and the virtual machine is now unable to connect to the internet.

The virtual network is running through NAT, the host has an interface (eth0:0) set up with a static IP (virt-manager shows the network and its IP correctly), and all connections to that IP should be sent to the guest.

The host and guest can ping one another, but the guest cannot ping anything above the host, nor can I ping the guest from anywhere else (I can ping the host). Results from the guest to another server under my control and from an external system to the guest both return "Destination port unreachable". Running tcpdump on the host and destination shows the host replying to the ping, but the destination never sees it (it doesn't even look like the host is bothering to send it on at all, which leads me to suspect iptables). The ping output matches that, listing replies from 192.168.100.1.

The guest can resolve DNS, however, which I find rather odd. The guest's network settings (connection TCP/IPv4 properties) are set up with a static local IP (192.168.100.128), mask of 255.255.255.0, and gateway and DNS at 192.168.100.1.

When originally setting up the vm/net, I had set up some iptables rules to enable bridging, but after my hosting company complained about the bridge, I set up a new virtual net using NAT and believe I removed all the rules.

The VM's network was working perfectly fine for the last few months, until yesterday. I haven't heard anything from the hosting company, didn't change anything on the guest, so as far as I know, nothing else has changed (unfortunately the list of packages updated has since fallen off scrollback and I didn't note it down).