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;