function externalLinks() {
  var d = document;
  if (!d.getElementsByTagName) return;
  var links = d.getElementsByTagName("a");
  for (var i=0; i<links.length; i++) {
    var link = links[i];
    if (link.getAttribute("href") && (link.getAttribute("rel") != null) && link.getAttribute("rel").indexOf("external") >= 0 ){
      link.target = "_blank";
    }
  }
}
window.onload = externalLinks;