systemd doesn't predictably online network interfaces

[jimsalterjrs]

I don't have a link for this in the style of your other "why not systemd" bullet points, but this one has been griping me for YEARS since Ubuntu switched to systemd: router builds have become immensely more problematic because of systemd's tendency to not online network interfaces if it thinks something is or isn't plugged in in the wrong order.

With upstart, if you configure your interface, your interface will be online, every time. with systemd, I've ended up having to deal with weird hacks trying to force interfaces online in router builds; if you don't hold your mouth just right, systemd tends to not online either the LAN or the WAN interface immediately. If the LAN interface doesn't come online immediately as the system boots, isc-dhcp-server won't come online either, and your machines on the LAN don't get IP addresses.

I haven't been able to find a solution for this in the last several years, and it's not for lack of trying and asking!

[ghost]

I think it's related to the combination of NetworkManager and this new interface naming stuff (that I personally hate). If it's what I'm thinking, it can be "fixed" using net.ifnames=0 with your boot parameters.
I had an issue in the past that even the wifi refused to connect or not showing wifi APs at all and this boot parameter fixed.