[nsp] Filtering network content based on User Subscription

[Joe Shen]

Hi,
 
To guarantee their staff/chirldren are not affected by 'dirty' content, some
of our customers want to filter web content.  To avoid the cost of software
maintenance, they want us to provide content filtering service based on
subscription. Also, customers want to be abel to administrate filtering
rules by themselves.
 
At current stage, we planned to set up content filtering by following
criterion:
 
1. By white list,  web site not listed in a white list will be blocked
2. By black list, web site listed in a black list will be blocked
3. By time period, in a predefined time period ( as. 8:00 am -- 5:00 pm )
some of web sites are not accessible
 
 
I have no experience with service like this, and I don't know how to set up
such service while mainteining the scability & network performance. Is there
anybody would do me a favor to tell me which company provides such products?
and what's the working mechanism behind?
 
 
Each word will be high appreciated.
 
Regards
 
Joe Shen