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;