Terminal Access Controller Access-Control System (TACACS) is a remote authentication

Authentication

Authentication is the act of confirming the truth of an attribute of a datum or entity...

 protocol that is used to communicate with an authentication server commonly used in UNIX

Unix

Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...

 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

Transmission Control Protocol

The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...

 or UDP

User Datagram Protocol

The User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...

) 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

Cisco Systems

Cisco Systems, Inc. is an American multinational corporation headquartered in San Jose, California, United States, that designs and sells consumer electronics, networking, voice, and communications technology and services. Cisco has more than 70,000 employees and annual revenue of US$...

.

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+

TACACS+

TACACS+ is a Cisco Systems proprietary protocol which provides access control for routers, network access servers and other networked computing devices via one or more centralized servers...

 and RADIUS

RADIUS

Remote Authentication Dial In User Service is a networking protocol that provides centralized Authentication, Authorization, and Accounting management for computers to connect and use a network service...

 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.
See also=

• List of authentication protocols
• Diameter
• Kerberos