newwindow = false;

function fenster(url,breite,hoehe) {
  if(newwindow&&newwindow.closed==false)
    newwindow.close()
    var l = (screen.availWidth - breite) / 2;
    var t = (screen.availHeight - hoehe) / 2;
    newwindow = window.open(url,"newwindow", "menubar=no,status=no,resizable=yes,scrollbars=yes,width=" + breite +
                                            ",height=" + hoehe +
                                            ",left=" + l +
                                            ",top=" + t + ";");
  newwindow.focus();
}

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;