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;