function externalLinks() {
  if (!document.getElementsByTagName && document.getElementById) return;

  var anchors = document.getElementsByTagName("a");

  for (var i=0; i<anchors.length; i++) {
   var anchor = anchors[i];
   if (anchor.getAttribute("href") &&
       anchor.getAttribute("rel") == "external")
       anchor.target = "_blank";
 }
}
window.onload = externalLinks;