window.onload = externalLinks;

function externalLinks() {
    if(!document.getElementsByTagName) {
        return;
    }
    var links,a,i;
    links = document.getElementsByTagName("a");
    for(i = 0; i < links.length; i++) {
        a = links[i];
        if(a.getAttribute("rel") && a.getAttribute("rel").indexOf("external") > -1) {
            a.onclick = function() {
                window.open(this.href);
                return false;
            }
        }
    }
}