listen system only when bind addr not specified

transport/internet/system_dialer.go

@@ -39,8 +39,12 @@ func resolveSrcAddr(network net.Network, src net.Address) net.Addr {
 	}
 }
 
+func hasBindAddr(sockopt *SocketConfig) bool {
+	return len(sockopt.BindAddress) > 0 && sockopt.BindPort > 0
+}
+
 func (d *DefaultSystemDialer) Dial(ctx context.Context, src net.Address, dest net.Destination, sockopt *SocketConfig) (net.Conn, error) {
-	if dest.Network == net.Network_UDP {
+	if dest.Network == net.Network_UDP && !hasBindAddr(sockopt) {
 		srcAddr := resolveSrcAddr(net.Network_UDP, src)
 		if srcAddr == nil {
 			srcAddr = &net.UDPAddr{
@@ -75,7 +79,7 @@ func (d *DefaultSystemDialer) Dial(ctx context.Context, src net.Address, dest ne
 					if err := applyOutboundSocketOptions(network, address, fd, sockopt); err != nil {
 						newError("failed to apply socket options").Base(err).WriteToLog(session.ExportIDToError(ctx))
 					}
-					if dest.Network == net.Network_UDP && len(sockopt.BindAddress) > 0 && sockopt.BindPort > 0 {
+					if dest.Network == net.Network_UDP && hasBindAddr(sockopt) {
 						if err := bindAddr(fd, sockopt.BindAddress, sockopt.BindPort); err != nil {
 							newError("failed to bind source address to ", sockopt.BindAddress).Base(err).WriteToLog(session.ExportIDToError(ctx))
 						}