[CentOS] what options do I have?

Dear All,

If I want make a lan users (with private IP) can access the internet
just after passing the verification, what options do I have?

What I want is:
* If I user want to access the internet
* He (must) run the browser
* whatever the address he typed on the address bar, he will be
   brought to the verification location, which will be 'force' him
   to enter his username and password
* if he passed the verification the internet access is available for
   him, but if not the internet keep unavailable for him.

The method will be the for all users, either he use the wire or wireless 
connection.

Will the iptables help me to solve this? How?

Thanks in advance for the help


- Tanu -