Netplan renderer transition from NetworkManager to systemd-networkd

Answer to the Core Questions

1. Conflict resolution

• Netplan first reads all /etc/netplan/*.yaml files, merges them, and then applies the renderer key.
• The renderer specified at the top level of a file is considered the *global* renderer. Any interface that has its own renderer key overrides the global value.
• When two renderers are enabled simultaneously, Netplan will still apply the chosen renderer, but the other renderer may start configuring the same interface. This leads to race conditions, duplicated routes, and unpredictable interface states.
• Therefore, the recommended practice is to disable the non‑selected renderer entirely (e.g., stop and mask NetworkManager.service when using systemd-networkd, or vice‑versa).

2. Existing DHCP leases when switching

• When netplan apply is executed, the target renderer restarts its networking daemon. For systemd-networkd, this means the daemon reloads its configuration, releases any existing DHCP lease on the interface, and immediately requests a new lease.
• The transition is usually atomic: the interface goes down for a brief moment while the lease is released, then comes up again with the new lease. The old lease is not preserved.
• If the interface was already up and had a valid lease, you may see a short period of IP address change, which can affect running services that rely on static IPs.

Confirmed Steps for a Production Server

1. Backup the current configuration:

   sudo cp /etc/netplan/*.yaml /etc/netplan/*.yaml.bak
   

2. Open the YAML file(s) you use for the interface you want to switch. Add or change the global renderer:

   network:
     version: 2
     renderer: systemd-networkd
     ethernets:
       eth0:
         dhcp4: true
         renderer: systemd-networkd   # optional, overrides global
   

3. Validate the syntax:

   sudo netplan try
   

   This will apply the config for 120 s and revert if you abort. If the test succeeds, proceed.

4. Disable the other renderer:

   sudo systemctl stop NetworkManager
   sudo systemctl disable NetworkManager
   sudo systemctl mask NetworkManager
   

5. Ensure systemd-networkd is installed and enabled:

   sudo apt-get install systemd-networkd   # if missing
   sudo systemctl enable --now systemd-networkd
   

6. Apply the configuration permanently:

   sudo netplan apply
   

7. Verify the interface:

   ip addr show eth0
   systemctl status systemd-networkd
   

8. Reboot to confirm the state persists across reboots.

Things to Watch For

• Make sure no nmcli or NetworkManager‑specific settings (e.g., VPN, Wi‑Fi) remain in the YAML. They will be ignored by systemd-networkd but can cause confusion.
• If you use systemd-networkd-wait-online.service, confirm that it starts after the network is up; otherwise, services that depend on networking may start too early.
• Check /var/log/syslog for errors from systemd-networkd after the switch.

Diagnostic Prompt

To tailor the recommendation precisely, could you confirm whether systemd-networkd is already installed and enabled on your machine?

Assumptions and Uncertainty

• Assumes Ubuntu 18.04 LTS or newer, where Netplan is the default network configuration tool.
• Assumes the interface in question is Ethernet; Wi‑Fi or mobile broadband interfaces require additional configuration (e.g., systemd-networkd-wifi or systemd-networkd-wifi.service).
• Uncertainty remains around the exact timing of DHCP lease release for very large networks; the described behavior is typical for the current Ubuntu releases.