Shutdown mDNS Connection once connected

Close the connection when we have connected. There is no
benefit to leaving it open, but some risk. This allows
us to avoid fingerprinting and port exhaustion that might
come up when running lots of peers

Relates to pion/webrtc#699

agent.go

@@ -459,6 +459,10 @@ func (a *Agent) setSelectedPair(p *candidatePair) {
 	a.selectedPair = p
 	a.updateConnectionState(ConnectionStateConnected)
 
+	// Close mDNS Conn. We don't need to do anymore querying
+	// and no reason to respond to others traffic
+	a.closeMulticastConn()
+
 	// Signal connected
 	a.onConnectedOnce.Do(func() { close(a.onConnected) })
 }
@@ -757,12 +761,7 @@ func (a *Agent) Close() error {
 			a.log.Warnf("failed to close buffer: %v", err)
 		}
 
-		if a.mDNSConn != nil {
-			if err := a.mDNSConn.Close(); err != nil {
-				a.log.Warnf("failed to close mDNS Conn: %v", err)
-			}
-		}
-
+		a.closeMulticastConn()
 	})
 	if err != nil {
 		return err
@@ -963,6 +962,14 @@ func (a *Agent) getSelectedPair() (*candidatePair, error) {
 	return out, nil
 }
 
+func (a *Agent) closeMulticastConn() {
+	if a.mDNSConn != nil {
+		if err := a.mDNSConn.Close(); err != nil {
+			a.log.Warnf("failed to close mDNS Conn: %v", err)
+		}
+	}
+}
+
 // Role represents ICE agent role, which can be controlling or controlled.
 type Role byte