_service._proto.name TTL class SRV priority weight port target
如:_sip._tcp.example.com. 86400 IN SRV 0 5 5060 sipserver.example.com.
service: _sip
protocol: _tcp
name: example.com.
priority: 0
weight: 5
port: 5060
target: sipserver.example.com.
_sip._tcp.example.com.
_sip._tcp