kea-dhcp4-server orders after network-online.target, which under networkd
only waits for the WAN carrier: the vlan-* links are RequiredForOnline=no.
On stellio Kea started before networkd had addressed vlan-lan/vlan-mgmt,
failed both binds and, per its default, kept running with no socket at
all (ss -ulnp showed nothing on :67). Wireless and wired clients'
DISCOVERs reached vlan-lan and were never answered.
Set service-sockets-require-all with a bounded retry so Kea keeps trying
while networkd catches up, and Restart=on-failure on the unit for the
case it still gives up.
Verified with checks.x86_64-linux.router.
kurogeek