prefetch

    http.init(0);
 
    http.req_copy_headers(0)
    http.req_set_method(0, "HEAD");

    #    // Generate the next URL by incrementing the first
    #    // number sequence found and send it back to Varnish.
    #    // If no number sequence is found, the request is skipped.
    http.req_set_url(0, http.prefetch_next_url());
    http.req_send_and_finish(0);