TAC as in tacacs?
Yep. The original TACACS specification was in a BBN technical
memo, CC-0045; RFC 1492 contains an informal specification
of the extended version that Cisco implemented. The background
section of RFC 1492 gives a bit of the history:
There used to be a network called ARPANET. This network consisted of
end nodes (hosts), routing nodes (IMPs) and links. There were (at
least) two types of IMPs: those that connected dedicated lines only
and those that could accept dial up lines. The latter were called
People being what they were, there was a desire to control who could
use the dial up lines. Someone invented a protocol, called "TACACS"
(Terminal Access Controller Access Control System?), which allowed a
TIP 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 sent a
response back. The TIP would then allow access or not, based upon
While TIPs are -- shall we say? -- no longer a major presence on the
Internet, terminal servers are. Cisco Systems terminal servers
implement an extended version of this TACACS protocol. Thus, the
access control decision is delegated to a host. 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
whoever is running the TACACS daemon. For example, "anyone with a
first name of Joe can only login after 10:00 PM Mon-Fri, unless his
last name is Smith or there is a Susan already logged in."
The extensions to the protocol provide for more types of
authentication requests and more types of response codes than were in
the original specification.
The original TACACS protocol specification does exist. However, due
to copyright issues, I was not able to obtain a copy of this document
and this lack of access is the main reason for the writing of this
document. This version of the specification was developed with the
assistance of Cisco Systems, who has an implementation of the TACACS
protocol that is believed to be compatible with the original
specification. To be precise, the Cisco Systems implementation
supports both the simple (non-extended) and extended versions. It is
the simple version that would be compatible with the original.
Please keep in mind that this is an informational RFC and does not
specify a standard, and that more information may be uncovered in the
future (i.e., the original specification may become available) that
could cause parts of this document to be known to be incorrect.
This RFC documents the extended TACACS protocol use by the Cisco
Systems terminal servers. This same protocol is used by the
University of Minnesota's distributed authentication system.