if (-f /etc/nginx/html/server-error-pages/_site/$server_name-maintenance-page_on.html)
{
  return 503;
}
error_page 503 @maintenance;
location @maintenance
{
  rewrite ^(.*)$ /$server_name-maintenance-page_on.html break;
  root /etc/nginx/html/server-error-pages/_site;
}