code: public static SocketFactory getDefaultSocketFactory(Configuration conf) {
  
      String propValue = conf.get("hadoop.rpc.socket.factory.class.default");
      if ((propValue == null) || (propValue.length() == 0))
        return SocketFactory.getDefault();
  
      return getSocketFactoryFromProperty(conf, propValue);
    }
docs: Get the default socket factory as specified by the configuration
  parameter <tt>hadoop.rpc.socket.factory.default</tt>
  
  @param conf the configuration
  @return the default socket factory as specified in the configuration or
  the JVM default socket factory if the configuration does not
  contain a default socket factory property.