This Multicast User-Mode Linux network transport allows multiple
UMLs (even ones running on different host machines!) to talk to
each other over a virtual ethernet network. However, it requires
at least one UML with one of the other transports to act as a
bridge if any of them need to be able to talk to their hosts or any
other IP machines.

To use this, your host kernel(s) must support IP Multicasting.

For more information, see
<http://user-mode-linux.sourceforge.net/old/networking.html> That site
has examples of the UML command line to use to enable Multicast
networking, and notes about the security of this approach.

If you need UMLs on multiple physical hosts to communicate as if
they shared an Ethernet network, say Y. If you need to communicate
with other IP machines, make sure you select one of the other
transports (possibly in addition to Multicast; they're not
exclusive). If you don't need to network UMLs say N to each of
the transports.

source code:

/arch/um/drivers/