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;