Quoting Chris Roubekas <croubekas@panafonet.gr>:

Dear all,

I am planning on implementing a SuSE machine as an internet gateway server for a small net cafe.

However, I would like to know if there exist utilities such as to control when a computer can enter the internet and when not , time spent on a machine etc...

See http://lessnetworks.com/. Upper left corner, "Download a Hotspot". HTH, Jeffrey P.S. No connection with the company. I am a user of their HotSpot network and heard their presentation at a LUG meeting.