code: private void validateRequest( DownloadRequest dreq )
              throws ErrorResponseException
      {
          String path = dreq.getPath();
          if ( path.endsWith( ResourceCatalog.VERSION_XML_FILENAME ) || path.indexOf( "__" ) != -1 )
          {
              throw new ErrorResponseException( DownloadResponse.getNoContentResponse() );
          }
      }
docs: Make sure that it is a valid request. This is also the place to implement the
  reverse IP lookup