根
顶级域
递归
权威
RR
security
Extension
DANE
privacy
qos
Local
DDoS
Service
software
app
Attack
STD
other
draft-ietf-dnsop-no-response-issue
无应答的区分
NOTIMP, FORMERR, NXDOMAIN, NOERROR(no data), REFUSED, BADVERS