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"; 
   }
 } 
} 
if(typeof window.addOnloadEvent == 'function') {
  // function exists, so we can now call it
  addOnloadEvent(externalLinks);
}