Creating VMs quickly and painlessly in Ubuntu and using bridged networks

This guide is an update of an earler guide which was based on the virt-builder command. The virt-builder command is no longer the best way to quickly build an Ubuntu VM. The better approach with Ubuntu 22.04 and later is to use the virt-intall command.

Establish bridge networking host configuration (Ubuntu 22.04 host)

On the host machine, if the existing netplan looks like this:

network:
  ethernets:
    eno1:
      addresses:
      - 128.32.42.201/24
      gateway4: 128.32.42.1
      nameservers:
        addresses:
        - 8.8.8.8
  version: 2

Change it to be as follows:

network:
  version: 2
  ethernets:
    eno1:
      dhcp4: false
  bridges:
    br0:
      interfaces: [eno1]
      dhcp4: false
      addresses:
      - 128.32.42.201/24
      gateway4: 128.32.42.1
      nameservers:
        addresses:
        - 8.8.8.8

Bridge networking guest configuration

The guest will see its IP interface as normal:

network:
    ethernets:
        enp1s0:
            addresses:
            - 128.32.43.202/24
            gateway4: 128.32.43.1
            nameservers:
                addresses:
                - 8.8.8.8
    version: 2

Note that the interface name in the VM instance will depend on the particular backing image. enp1s0 is the correct interface in Ubuntu cloud images.

Cloud init

Ubuntu supports a method called cloud init to configure users, network and other features. One way to use cloud init is to create CDROM ISO files with the parameter. Fortunately the virt-install command handles this automatically so only a few options are needed.

disable=on disables cloud init after first boot.

clouduser-ssh-key=$SSH_KEY imports the given public key to the default user. The default user is named ubuntu.

network-config=cloud-init-network-config uses the cloud-init-network-config file. An example of this file is included.

Putting it together

Look at the example in vm-script.sh and cloud-init-network-config.