function externalLinks() {
 if (!document.getElementsByTagName) 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;