function external()
{
    if(document.getElementsByTagName)
    {
        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;