DNS/キャッシュサーバ/PowerDNS/TTLの扱いについて、ここに記述してください。
max-cache-ttl Maximum number of seconds to cache an item in the DNS cache, no matter what the original TTL specified. Available since 3.2. max-negative-ttl A query for which there is authoritatively no answer is cached to quickly deny a record's existence later on, without putting a heavy load on the remote server. In practice, caches can become saturated with hundreds of thousands of hosts which are tried only once. This setting, which defaults to 3600 seconds, puts a maximum on the amount of time negative entries are cached.