function externalLinks() {
  if( !document.getElementsByTagName ) return;
  var anchors = document.getElementsByTagName("a");
  for( var i=0 ; i<anchors.length ; i++ ) {
    var anchor = anchors[i];
    try{
      if( anchor.getAttribute("href") && anchor.getAttribute("rel") == "external" )
        anchor.target = "_blank";
    }
    catch(e) { alert("This link has an invalid href: "+anchor.outerHTML); }
  }
}
window.onload = externalLinks;