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';
 	}
}

//is called at body onload
//window.onload = externalLinks;