function external_links()
{
    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") == "external")
        anchor.target = "_blank";
    }
}

window.onload = external_links;