qemu Guest-Only Networking

Background

qemu offers a basic NAT functionality to get guests on the internet through the host. It’s very nice but setting up networking between guests is not very intuitive.

I know, just setup VDE and be done with it. I’m looking for something more portable that doesn’t have any extra software requirements.

My first decent in this rabbit hole started with hubport. It seems like the obvious but hubport can only network within a single guest. I guess it’s for some strange use case.

Thanks to a wonderful post from lacsaP Patatetom, I believe I’ve found the answer using sockets and a shared multicast address:

qemu -drive file=hd1.img,media=disk -nic socket,mac=52:54:00:11:11:11,mcast=239.192.168.1:1101
qemu -drive file=hd2.img,media=disk -nic socket,mac=52:54:00:22:22:22,mcast=239.192.168.1:1101
qemu -drive file=hd3.img,media=disk -nic socket,mac=52:54:00:33:33:33,mcast=239.192.168.1:1101

This is not as straight forward to setup as the default NAT.

You must specify a unique mac address per guest.
No DHCP is available, each guest must have it’s own static IP addresses assigned.
The multicast 239.0.0.0/8 range is assigned for private use within an organization.
The port number can be anything you wish so long as it’s the same across guests.

Created: 2025-03-30

Modified: 2025-03-30