I have customer who controls access to the internet via TACACS server, basically a PIX firewall uses authentication from the TACACS to say if traffic is allowed to pass out of the gateway. I can't find anything on how to configure a linux client of TACACS authentication only how to set up a linux TACACS server.
TACAS appears to be a Cisco thing these days, but there's software implementations too, some commercial, some not, see TACAS+ on Wikipedia