server_name _; doesn't work

[kleinchris]

Hello, when I set a default vhosts with server_name _; it doesn't work. Here the example file:
server {
        listen 80;
        server_name _ ; # underdash means for all hosts

        location / {
                root /var/www/html;
                index index.html index.htm;
        }
        error_page   500 502 503 504  /50x.html;
        location = /50x.html {
                root   /var/www/html;
        }
}

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,61579,61579#msg-61579