While switching NixOS configurations with both
networking.useNetworkd = true;
virtualisation.virtualbox.host.enable;
You often end up waiting for systemd-networkd-wait-online.service.
This happens because the vboxnet0 device doesn't have a carrier until
virtualbox machines are started, so networkd gets stuck in
"Configuring":
⇒ networkctl list
IDX LINK TYPE OPERATIONAL SETUP
1 lo loopback carrier unmanaged
2 wlp2s0 wlan routable unmanaged
3 vboxnet0 ether no-carrier configuring
This updates the NixOS virtualbox host module to include a
RequiredForOnline=no statement in the generated 40-vboxnet0.network
file, so networkd doesn't consider it necessary for
systemd-networkd-wait-online.service to finish.
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