systemd networking basics: cannot switch from DHCP to static IP address

I'm just starting with systemd networking, and I can't set up a new system with a static IP address. This is a default Ubuntu 20.04 server, and network-manager and ifupdown are not installed (so no /etc/network/interfaces, etc).

By default, the server boots up with DHCP enabled, and gets an IP address, and everything works. I've now added an /etc/systemd/network/static.network file which contains a basic setup:

[Match]
Name=e*

[Network]
Address=192.168.1.20/24
Gateway=192.168.1.40

If I do a systemctl restart systemd-networkd this is ignored, and systemctl status shows that an address of 192.168.1.10 was assigned from DHCP (enp1s0: DHCPv4 address 192.168.1.10/24 via 192.168.1.40). I get the same result if I reboot; the address is still the DHCP one. It seems as if the static.network file is being completely ignored, but it is being processed - if I add an obvious error to the file it is reported by the status command. Any ideas what I'm doing wrong?

The server is actually a VM running under RHEL 8, which I'm hoping is not relevant.

Well, this looks like a bug on 20.04. .network files are meant to be processed in lexical order, according to this extract from the systemd-networkd docs:

The .network files are read from the files located in the system network directories /usr/lib/systemd/network and /usr/local/lib/systemd/network, the volatile runtime network directory /run/systemd/network and the local administration network directory /etc/systemd/network. All configuration files are collectively sorted and processed in lexical order, regardless of the directories in which they live.

However, if I turn on systemd-networkd debugging (see here if you don't know how), it turns out that the lowest-lexically-sorted file has precedence. I can't be entirely sure, because the logs don't show the processing order, and only stop at the first hit for the relevant network interface.

This is what I know, for the plain Ubuntu 20.04 server install:

1. There are 5 .network files across the system
2. One of these is created by netplan at runtime, and is /run/systemd/network/10-netplan-enp1s0.network
3. My new file is /etc/systemd/network/static.network
4. On systemctl restart systemd-networkd the netplan file takes precedence over my own
5. If I manually remove the netplan file, and restart networking, my file is acted on. However, this is only temporary till the next reboot
6. If I rename my file to 05-static.network then it takes precedence over the netplan file

Ergo, processing order broken, unless I'm doing something stupid.

As a temporary fix you need to give your file a low lexical order by prefixing with something like 05-. However, this might break on a future update if the processing order is reversed. A better fix would be to disable netplan (not sure how to do this), but this might also break if something else then decides it has to create a .network file to make up for the missing netplan one.

Edit - removing netplan

You can remove netplan entirely with apt purge netplan.io. However, this may not be a good idea - it also removes cloud-init, for example, which is required by DigitalOcean. So:

# cd /etc/netplan
# rm 00-installer-config.yaml
# netplan apply

After a reboot, you no longer get the auto-generated .network file, and can use normal file naming in your .network files without a conflict.