Terminal Access Controller Access-Control System (TACACS) is a remote authentication protocol that is used to communicate with an authentication server commonly used in UNIX networks. TACACS allows a remote access server to communicate with an authentication server in order to determine if the user has access to the network. TACACS is defined in RFC 1492, and uses (either TCP or UDP) port 49 by default. A later version of TACACS introduced by Cisco in 1990 was called Extended TACACS (XTACACS). The XTACACS protocol was developed by and is proprietary to Cisco Systems.
TACACS allows a client to accept a username and password and send a query to a TACACS authentication server, sometimes called a TACACS daemon or simply TACACSD. This server was normally a program running on a host. The host would determine whether to accept or deny the request and send a response back. The TIP (routing node accepting dial-up line connections, which the user would normally want to log in into) would then allow access or not, based upon the response. In this way, the process of making the decision is "opened up" and the algorithms and data used to make the decision are under the complete control of whomever is running the TACACS daemon.
TACACS+ and RADIUS have generally replaced TACACS and XTACACS in more recently built or updated networks. TACACS+ is an entirely new protocol and not compatible with TACACS or XTACACS. TACACS+ uses the Transmission Control Protocol (TCP) and RADIUS uses the User Datagram Protocol (UDP). Some administrators recommend using TACACS+ because TCP is seen as a more reliable protocol. Whereas RADIUS combines authentication and authorization in a user profile, TACACS+ separates the two operations.