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;