1. Install reverse_proxy module & libxml

shell> sudo apt-get install sudo apt-get install libapache2-mod-proxy-html libxml2-dev

2. Config proxy.conf

cd /etc/apache2/mods-enabled
sudo vi proxy.conf

change from
Deny from all
to
Allow from all
3. run the proxy modue
shell > a2enmod proxy
4. restart apache
sudo /etc/init.d/apache2 restart

แถม install mod_rewrite
sudo a2enmod rewrite

REF:http://www.livingubuntu.com/77
and http://blog.plathong.net/2009/05/mod_proxy-in-ubuntu/