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;