By default, KVM provides a NAT network for VMs. This documents the setup of a bridge interface, to allow VMs on the host to join the network of the host, instead of being behind a NAT (which is default).

In 18.04 this is accomplished by using the netplan network configuration tool, which was not used in the previous LTS.

Most installations will have a 50-cloud-init.yaml by default, but you may have to adjust if not.

The bridge interface will replace the main interface, by default.

First, install bridge-utils:

sudo apt update && sudo apt install -y bridge-utils

Open the file:

sudo nano /etc/netplan/50-cloud-init.yaml

By default, it will look something like:

network:
    ethernets:
        enp1s0:
            addresses: []
            dhcp4: true
        enp2s0:
            addresses: []
            dhcp4: true
            optional: true
    version: 2

First, disable DHCP on the interface you wish to bridge. Set dhcp4: true to dhcp4: no.

Add this to the bottom, noting the interface which it should be bridged to. To bridge with a static IP: (NOTE: UNTESTED)

    bridges:
        br0:
            interfaces: [enp0s3]
            dhcp4: no
            optional: true
            addresses: [172.168.0.10/24]
            gateway4: 172.168.0.1
            nameservers:
              addresses: [172.168.0.1]

Or, to bridge with DHCP:

    bridges:
        br0:
            interfaces: [enp1s0]
            dhcp4: true
            optional: true

The final file will look like:

network:
    ethernets:
        enp1s0:
            addresses: []
            dhcp4: no   
        enp2s0:
            addresses: []
            dhcp4: true
            optional: true
    version: 2

    bridges:
        br0:
            interfaces: [enp1s0]
            dhcp4: true
            optional: true

To apply the configuration:

sudo netplan apply

Or to apply with debugging:

sudo netplan --debug  apply

This will disable the nic you mention, and enable the new bridge nic. This will change the new mac address, and the IP address, so you will have to update the DHCP reservation if this is being used.

Next, confirm the bridge is set up properly:

sudo networkctl status -a

br0 should be in state: routable (configured).

Similarly, check the ip addresses:

ip addr

br0 should have an ip address associated.

Finally, to connect a VM to the bridge network, when creating the VM in virt-manager, select “Specify shared device name”, and enter br0 in the text field.