Virtual Bridge Setup

Bridge interfaces are helpful in virtual and container environments since they can be easily put in promiscuous mode thus allowing multiple IP and MAC addresses to send traffic over a single physical or virtual interface.

In our lab, we set up virtual bridge interface br0 on every VM and on the phyiscal KVM host.

Identify netplan config file

Ubuntu uses netplan to configure and manage network interfaces. Depending on how your KVM host or VM were set up, you might end up with interface config files that are named differently. For instance, in this lab, the KVM host has it's configuration in /etc/netplan/00-installer-config.yaml and VMs have their netplan config stored in /etc/netplan/01-netcfg.yaml.

New nodes will typically have their interfaces configured with a dynamic IP address assigned by DHCP so your netplan config file will look similar to this:

Typical netplan yaml file configured for DHCP

# This file describes the network interfaces available on your system
# For more information, see netplan(5).
network:
  version: 2
  renderer: networkd
  ethernets:
    enp1s0:
      dhcp4: yes

Update netplan config

To set up the br0 virtual bridge with a static IP, modify the contents of netplan config file identified in the previous step, to bridge your physical or network adapter to a virtual bridge interface. Replace the variables shown in <...> with correct values.

Netplan yaml config file (template)

network:
  ethernets:
    <PHYSICAL_NIC>:
      dhcp4: no
      dhcp6: no
  bridges:
    br0:
      interfaces: [<PHYSICAL_NIC>]
      addresses: [<HOST_IP>/<HOST_NETMASK>]
      gateway4: <GATEWAY_IP>
      nameservers:
        addresses: [<DNS_SERVER_IP>]
      parameters:
        stp: true
        forward-delay: 4
      dhcp4: no
      dhcp6: no
  version: 2

For instance:

Network

Make sure to replace the name of the physical interface with the one that is correct for your system. If necessary, update IP addresses to the ones specific to your environment.

Netplan yaml config file (sample)

network:
  ethernets:
    enp37s0: 
      dhcp4: no
      dhcp6: no
  bridges:
    br0:
      interfaces: [enp37s0]
      addresses: [10.0.0.235/24]
      gateway4: 10.0.0.1
      nameservers:
        addresses: [10.0.0.1]
      parameters:
        stp: true
        forward-delay: 4
      dhcp4: no
      dhcp6: no
  version: 2

Apply new config

Important

Make sure you have set up your virtual bridge correctly in the previous step. At the moment, you are connected to the host via SSH on a dynamic IP address. As soon as you issue the netplan apply command, your old IP and network interface will become inaccessible and your SSH session will hang or terminate. You will need to reconnect on the static IP address that is now attached to the new br0 interface. If there is a misconfiguration, you might not be able to reconnect to your VM.

sudo netplan apply

Validate setup

SSH back into the system on the newly configured static IP and validate that the bridge interface is correctly set up:

ip link show

You should see your virtual interface br0 as the master of the phyiscal interface, for instance:

enp37s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel master br0 state UP mode DEFAULT group default qlen 1000
br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default qlen 1000

Alternatively, you can check the br0 interface status with the brctl show command

sudo apt install bridge-utils
brctl show

This should show br0 mapped to the physical interface, such as enp1s0:

bridge name    bridge id            STP enabled    interfaces
br0            8000.5254006e4b2d    yes            enp1s0