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;