I recently had a project at work where the project's requirements forced me to think outside the box. They were:
The built-in VirtualBox networking options really wouldn't fit the bill. Here's why:
So what's the solution? Routed networking of course! What's that? There's no routed network option? Sure there is. It's called host-only networking with a local DHCP server. Next, you enable routing at the OS level. In Linux, set net.ipv4.ip_forward to 1 (for v4).
That solves the problem with VM's getting outside the hypervisor. The next problem is the return traffic knowing how to get back to the VM. Which hypervisor sent it? It's not bridged so it can't ARP. It's not NAT'd so it's not the hypervisor MAC. How do you point return traffic to the right server?
Your answer here is a dynamic routing protocol. We use OSPF, and bird is the word. A simple bird instance can advertise the IP network from vboxnet0 to the rest of your network over OSPF. Simple enough way to bring up and tear down a lot of networks with little effort. You don't have to use OSPF; you could use something else like IS-IS (which we also use but not here).
In summary, host-only network + DHCP server on hypervisor + routing + OSPF = routed VM networking in VirtualBox. It works really well and is being used for our in-house Selenium testing (taste.weebly.com).
A NOLA native just trying to get by. I live in San Francisco and work as a digital plumber for the joint that runs this thing. (Square/Weebly) Thoughts are mine, not my company's.