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.setAttribute('target','_blank') ;
   }
 }
}
window.onload = externalLinks;