Many thanks to @PowerDNS for this blog post on how to properly serve #datagrams while listening on a wildcard address. Particularly useful as "Unix Network Programming" from #Stevens and al. is unhelpful in that regard (their approach can be summarized as "Who cares if the reply does not come out of the same address as that of the destination of the request, as long as the destination of the reply is correct?" I care, that's who; because of stateful firewalls for a start)
https://blog.powerdns.com/2012/10/08/on-binding-datagram-udp-sockets-to-the-any-addresses