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;