var d = document;

function externalAnchors()
{
  if (d.getElementsByTagName)
  {
    var anchors = d.getElementsByTagName("a");

    for (var i = 0; i < anchors.length; i++)
    {
      if (anchors[i].getAttribute("href") && anchors[i].getAttribute("rel") == "external")
      {
        anchors[i].target = "_blank";
      }
    }
  }
}

window.onload = function ()
{
  externalAnchors();
}