function externalLinks() {

    if (!document.getElementsByTagName) {
        return;
    }

    // set vars
    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;