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;