Wireless hotspot servlet - Linux

I am looking for a servlet that redirects users to an authentication
page before allowing them to connect to the network/internet. I
believe they call this a Application Proxy Server. What I would favor
is that it would authenticate to a radius server which will be on the
local box which in turn will look up the user data on a sql table.

I am knowledgable of the mikrotik OS and how it Redirects to the
router and then authenticates to a radius server. What I want is that
servlet they use so that I can make it all run on the same box.
Pretty much trying to intigrate the NAS with the radius server. Kinda
looking into emulating the router OS within the linux OS but I'm
thinking that might be a bad idea. Thanks!