I need to implement a gateway that will allow me to control the access to the internet of a network made up of a about half a dozen of workstations. My main obejectives are the following:

1- Monitoring of traffic 2- Logging of traffic 3- Access control - block websites (mainly adult) and certian traffic(example torrents) 4- Possibly cache content 5- Easy management interface 6- Preferable free and opensource

Serverfault users can you please suggest from your vast experience which software you think is the best to suit my needs?

Any suggestion is greatly appreciated. Thank you