Each updown location host it's own recursive DNS resolver (we use unbound for that) which caches responses accurately by honoring to the TTL configured in your DNS entries. This also means it can cache the absence of record, accoding to the negative TTL present in your SOA record.
By doing so we avoid any dependency on public DNS resolvers and the problems this could add: availability, poor cache retention, increased latency, cache poisoning, incorrect GeoDNS results, etc.