/etc/network/interfaces, systemd-networkd and NetworkManager: how do they coexist together?

Question

Is it sufficient to have one of these three in Ubuntu 19.04? Is it possible to have all three and how to configure them so they were not conflicting?

Does systemd-networkd started by networking.service?

Answer 1

As you might know /etc/network/interfaces has been replaced with netplan. And netplan works fine with both NetworkManager and systemd-networkd.

So you can have them all work together!

If you're running Ubuntu as your desktop, then you should use NetworkManager or config netplan so it uses NetworkManager as its default renderer:

network:
  version: 2
  renderer: NetworkManager

Otherwise you can easily setup your interfaces in netplan itself, for example to have interfaces like configuration and network-manager work together:

network:
  version: 2
  renderer: NetworkManager
  wifis:
    wlp4s0:
      dhcp4: yes
      access-points:     
        my-access-point:
          password: passw00rd
network:
  version: 2
  renderer: networkd
  ethernets:
    enp3s0:
      addresses:
        - 192.168.1.10/24
      dhcp4: false
      dhcp6: false
      gateway4: "192.168.1.1"
      nameservers:
        addresses:
          - "8.8.4.4"
          - "8.8.8.8"

Configuration

To configure netplan, save configuration files under /etc/netplan/ with a .yaml extension (e.g. /etc/netplan/config.yaml), then run sudo netplan apply. This command parses and applies the configuration to the system. Configuration written to disk under /etc/netplan/ will persist between reboots.

_{Netplan configuration examples}

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Replacing netplan with ifupdown is still possible but something that I wouldn't suggests.

1. Install ifupdown
2. Remove netplan
3. Configure interfaces
4. Configure /etc/NetworkManager/NetworkManager.conf so it would be able to manage your interfaces.
5. Restart networking services

You can even run dhclient directly to bring an interface up.