function external() {

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

window.onload = external;