function externalLinks() {
  if (!document.getElementsByTagName)
    return;
  var arrAnchors = document.getElementsByTagName("a");
  for (var x=0;x<arrAnchors.length;x++) {
    var anchor = arrAnchors[x];
    if (anchor.getAttribute("href") && anchor.getAttribute("rel") == "external")
      anchor.target = "_blank";
  }
}

window.onload = externalLinks;