TACACS (Terminal Access Controller Access Control System) is an older authentication protocol common to UNIX networks that allows a remote access server to forward a user's logon password to an authentication server to determine whether access can be allowed to a given system. TACACS is an encryption protocol and therefore less secure than the later TACACS+ and Remote Authentication Dial-In User Service protocols. A later version of TACACS is XTACACS (Extended TACACS). Both are described in Request for Comments 1492.
In spite of its name, TACACS+ is an entirely new protocol. TACACS+ and RADIUS have generally replaced the earlier protocols in more recently built or updated networks. 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.
TACACS and XTACACS are still running on many older systems.