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;