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")
         && anchor.getAttribute("rel").indexOf("external") != -1 )
      anchor.target = "_blank";
  }
}