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;