function externalLinks() {
   if (!document.getElementsByTagName) return;

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

window.onload = externalLinks;