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;