Comment 2 for bug 1773377

> instance -> dnsmasq -> bind9 (managed by designate-bind charm) -> upstream DNS server

This is actually:
instance -> dnsmasq -> Neutron router or provider router -> bind9 (managed by designate-bind) -> upstream DNS server

Because dnsmasq is in fixed IP ranges or VLAN provider ranges. In that case, I don't think there is a way to determine a proper IP address of designate-bind to be passed to dnsmasq if a designate-bind unit has multiple addresses. Because the network ranges of "Neutron router or provider router" are managed outside of Juju model so no relation or network bindings to detect that.