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;