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;