In my previous post about Xen, I talked about how easy Xen is to configure and set up, particularly on Ubuntu and Debian. I'm still grateful that Xen remains easy; however, I've lately had a few Xen-related challenges that needed attention. In particular, I've needed to create some surprisingly messy solutions when using vif-route to route multiple IP numbers on the same network through the dom0 to a domU.
I tend to use vif-route rather than vif-bridge, as I like the control it gives me in the dom0. The dom0 becomes a very traditional packet-forwarding firewall that can decide whether or not to forward packets to each domU host. However, I recently found some deep weirdness in IP routing when I use this approach while needing multiple Ethernet interfaces on the domU. Here's an example: