code: public static void doTrackback( Object it, StaplerRequest req, StaplerResponse rsp ) throws IOException, ServletException {
  
          String url = req.getParameter("url");
  
          rsp.setStatus(HttpServletResponse.SC_OK);
          rsp.setContentType("application/xml; charset=UTF-8");
          try (PrintWriter pw = rsp.getWriter()) {
              pw.println("<response>");
              pw.println("<error>" + (url != null ? 0 : 1) + "</error>");
              if (url == null) {
                  pw.println("<message>url must be specified</message>");
              }
              pw.println("</response>");
          }
      }
docs: Parses trackback ping.