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;