TACACS+ is a security protocol designed by Cisco Systems*, Inc. that is used to control dial-up access into networks. An unsupported but freely available implementation of TACACS+ is available in ftpeng.cisco.com in pub/tacacs.
You can install Platform Services on your server and modify TACACS+ to use the AS Client API to authenticate users.
Directions for modifying TACACS+ to use the AS Client API for authentication can be found in the ASAM/bin/PlatformServices/PlatformClient/TACACS directory created by the Platform Services installation process.