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;