function externalLinks() {
 if(!document.getElementsByTagName)
   return;
   
 var anchors = document.getElementsByTagName("a");

 for(var i = 0; i < anchors.length; i++) {
  if(anchors[i].getAttribute("href") &&
     anchors[i].getAttribute("rel") == "external")  {
   anchors[i].target = "_blank";
  }
 }
}

window.onload = externalLinks;