function externalLinks()
{
  if (!document.getElementsByTagName)
  {
   return;
  }
  
  var anchors = document.getElementsByTagName("a");
  
  for (var idx=0; idx<anchors.length; idx++)
  {
    var anchor = anchors[idx];
   
    if (anchor.getAttribute("href") && anchor.getAttribute("rel") == "external")
    {
      anchor.target = "_blank";
    }
  }
}

window.onload = externalLinks;