Originally designed by the Defense Data Network (DDN), TACACS is a centralized (Client-server) authentication system similar to Kerberos or RADIUS, but less substantial than both.

TACACS was used in early dialup servers for AAA, but due to its limited functionality compared to its peers, the original protocol saw little use. An enhanced version called "Extended TACACS" was developed (mostly by Cisco) and standardized in RFC 1492¹. Eventually, both standards were scrapped in favor of TACACS+.

¹ If you live in North or South America, you should recognize that number.