Is it possible to use a protocol such as bittorrent to distribute the list of blacklisted hosts? For example, on the sync server compile and share the full list of hosts and their metadata, once a day. Compile and share deltas from the daily file every five minutes. The client automatically downloads from the bittorrent seed and optionally shares what it has downloaded. Use checksums to ensure malicious and/or corrupted data is not inserted into the daily or delta lists.
This should reduce processing and bandwidth requirements on the sync server, as well as supporting more-frequent queries of the blacklist data by clients. This idea is an alternate solution to the dilemma posed in ticket 3286240.