function setExternalLinks() {
    if (!document.getElementsByTagName) {
        return null;
    }
    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.setAttribute("target", "blank");
        }
    }
}
window.onload = setExternalLinks;