How to setup netplan with additional loopback addresses but mainly use NetworkManager?

Question

I have recently installed Kubuntu 18.04 which uses netplan instead of the old networking subsystems. In the past I have set up a few extra loopback addresses for a variety of reasons using /etc/network/interfaces, however that doesn't work in 18.04 anymore. For my work development environment, I also need 172.16.123.1 added to a loopback interface.

What I noticed is that the current netplan configuration file just diverts all network management to NetworkManager, however from what I can tell NetworkManager has no ability to manage loopback devices or add loopback addresses.

So what is the correct way with netplan to configure some additional loopback addresses but still primarily use NetworkManager for interface configuration?

Update 2021-12-10 In newer versions of netplan, the format of the file in this answer has changed from the accepted answer, as has the default file location in later revisions of Ubuntu 20.04 as well as newer Ubuntu releases.

The file is now /etc/netplan/01-netcfg.yml and the format needs to look like

network:
version: 2
renderer: networkd
ethernets:
  lo:
    match:
      name: lo
    addresses: [ 172.16.123.1/32 ]

Netplan seems to be extremely fussy about the layout and format of this file, and I've taken down a few VMs trying to set them up the old way.

Answer 1

I'm not sure this is the "correct way" but it indeed works like you want it to.

You can set which renderer to be used for the loopback interface like below, edit the file /etc/netplan/01-network-manager-all.yaml (or your *.yaml file):

# Let NetworkManager manage all devices on this system
network:
  version: 2
  renderer: NetworkManager
  ethernets:
    lo:
      renderer: networkd
      match:
        name: lo
      addresses:
        - 172.16.123.1/32

Tested and works as expected on Ubuntu 18.04 Desktop.

Hope this helps.

Answer 2

You can either add an IP address with

lo:
   addresses: [192.168.1.1/24]

or

lo:
    addresses:
    - 192.168.1.1/24

To view all the IP's assigned to the loopback use the command ip address

Answer 3

I found after the latest update to ubuntu 18.4.1 i had to use the syntax:

lo:
  addresses: [IPADDRESS]

The YAML file is very fussy about spaces and indents. Note the square brackets are required.