const server = dgram.createSocket('udp4');
      
      server.on('error', (err) => {
          console.log(`server error:\n${err.stack}`);
          server.close();
      });
      
      server.on('message', (msg, rinfo) => {
          console.log(msg);
          //把收到的送回去
          server.send(msg, 0, msg.length, rinfo.port, rinfo.address, function(err, bytes) {
              if(err) { console.log(err); return; }
          });
      });
      
      server.on('listening', () => {
          var address = server.address();
          console.log(`server listening ${address.address}:${address.port}`);
      });
      server.bind(2009);