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;