function ExternalLinks()
{
    if (!document.getElementsByTagName) return;
    var links = document.getElementsByTagName("a");
    var linkCount = links.length;

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

window.onload = ExternalLinks;