The *vlan99* interface needs to be created and activated by
`systemd-networkd` before `dnsmasq` can start and bind to it. Ordering
the *dnsmasq.service* unit after *network.target* and
*network-online.target* should ensure that this is the case.
The production deployment of *dnsmasq* for Home Assistant has deviated
from how the *hass-dhcp* role configures it. Bringing the role back in
sync with how things really are.
The *hass-dhcp* role installs dnsmasq and configures it to serve DHCP
requests on the Home Assistant network. Since this network is not
routed, the regular DHCP relay/server setup will not work.
Dustin