Is it possible to have multiple virtual machines with the same (guest) ip on the same host?

Question

What I try to achieve:

Multiple virtual machines (virtualbox) on a Debian host which communicate with another server (not with each other or the host). All VMs need to have the same static network settings (so that i can clone and use them without changing the settings in the guest). Every networking attempt of the VMs has to be redirected to the aforementioned server.

My current setup:

1. I create a tap on the host for each VM and use bridged networking to connect the VMs with the corresponding tap
2. All taps are connected with a bridge. The bridge has an ip address which is the gateway for the virtual machines.
3. The host as a dedicated interface which connects it to the server. This interface is also connected with the bridge
4. An iptables SNAT rule on this interface to connect the VMs with the server
5. An iptables DNAT rule redirects all traffic from the bridge to the server

This works so far. But with this setup the VMs cannot have the same ip address because they are connected via the bridge. I tried to SNAT the ip addresses on the taps but that didn't work.

Answer 1

You could set up NAT with each guest on its on private network. First, you'd need to stop bridging all of their network interfaces together (after all, if they're all on the same switch, they're hardly on their own private networks).

Then you'd set up NAT rules, and maybe it'd work. But probably not—the outgoing path should work, but the return path is a problem. The NAT machinery gets a reply packet, NATs it back to the source 192.168.0.2 (or whatever). Then it passes it to routing... which sends it where? All the guests have that same IP address, all their interfaces have the same address/subnet mask.

So we need to fix routing. Thankfully, that's doable, with policy routing.

If you add in -t mangle rules for each guest (based on the source port, e.g., the vnet0 or whatever) you could mark the connections. Then you can route the response back (using ip rule) based on that mark.

That's a lot of config, but once you get it working, it's scriptable. It'll probably work. I haven't tested it. Whether its sane or not, well… I'd try to get DHCP working right personally. Consider what the next person to have to administer that will think (which may well be you in a few months once you've forgotten exactly how it works.)

Answer 2

Simple answer: no. What you're trying to do is, from a networking perspective, identical to trying to have multiple physical hosts with the same IP. It won't work.