function externalLinks() {
if (!document.getElementsByTagName && document.getElementById) 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;