How to add a routed NIC device to a virtual machine

When adding a routed NIC device to an instance, you must configure the instance to use the link-local gateway IPs as default routes. For containers, this is configured for you automatically. For virtual machines, the gateways must be configured manually or via a mechanism like cloud-init.

To configure the gateways with cloud-init, firstly initialize an instance:

lxc init ubuntu:22.04 jammy --vm

Then add the routed NIC device:

lxc config device add jammy eth0 nic nictype=routed parent=my-parent-network ipv4.address=192.0.2.2 ipv6.address=2001:db8::2

In this command, my-parent-network is your parent network, and the IPv4 and IPv6 addresses are within the subnet of the parent.

Next we will add some netplan configuration to the instance using the cloud-init.network-config configuration key:

cat <<EOF | lxc config set jammy cloud-init.network-config -
network:
  version: 2
  ethernets:
    enp5s0:
      routes:
      - to: default
        via: 169.254.0.1
        on-link: true
      - to: default
        via: fe80::1
        on-link: true
      addresses:
      - 192.0.2.2/32
      - 2001:db8::2/128
EOF

This netplan configuration adds the static link-local next-hop addresses (169.254.0.1 and fe80::1) that are required. For each of these routes we set on-link to true, which specifies that the route is directly connected to the interface. We also add the addresses that we configured in our routed NIC device. For more information on netplan, see their documentation.

Note

This netplan configuration does not include a name server. To enable DNS within the instance, you must set a valid DNS IP address. If there is a lxdbr0 network on the host, the name server can be set to that IP instead.

You can then start your instance with:

lxc start jammy

Note

Before you start your instance, make sure that you have configured the parent network to enable proxy ARP/NDP.