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") == "external")
        {
            anchor.target = "_blank";
        }
    }
}

window.onload = externalLinks;