packet cache:数据包缓存，优先级高
query cache：后台数据库查询缓存，packet cache无时查询
negquery-cache：无域名或域名无记录缓存
缓存操作：
PDNS Server:
pdns_control ccounts
pdns_control purge example.net
pdns_control purge
PDNS Recursor
rec_control get cache-entries packetcache-entries negcache-entries nsspeeds-entries
rec_control dump-cache /tmp/dns-cache
rec_control wipe-cache example.net标签：none