Comment 4 for bug 1664818

I'm kind of concerned about any non-declarative functionality here. For example, DHCP client and receiving/updating routes are all handled by netwokrd already.

You can bind a systemd unit to e.g. sys-subsystem-net-devices-eth0.device and then any number of systemd units should be started and stopped in concert.

One can also block and wait until the relevant interface is up with /lib/systemd/systemd-networkd-wait-online --help

However, I'd rather avoid running arbitrary code as all the examples provided at the moment can be achieve as specifications to the networkd units. (either as generated by netplan, or as drop-in snippets in /etc or /run).