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+Network Working Group                                  Brian A. Anderson
+Request for Comments: 927                                            BBN
+                                                           December 1984
+
+                TACACS User Identification Telnet Option
+
+
+Status of this Memo
+
+   This RFC suggests a proposed protocol for the ARPA-Internet
+   community, and requests discussion and suggestions for improvements.
+   Distribution of this memo is unlimited.
+
+Introduction
+
+   The following is the description of a TELNET option designed to
+   facilitate double login avoidance.  It is intended primarily for TAC
+   connections to target hosts on behalf of TAC users, but it can be
+   used between any two consenting hosts.  For example, all hosts at one
+   site (e.g., BBN) can use this option to avoid double login when
+   TELNETing to one another.
+
+1. Command name and code
+
+   TUID     26
+
+2. Command Meanings
+
+   IAC WILL TUID
+
+      The sender (the TELNET user) proposes to authenticate the user and
+      send the identifing UUID; or, the sender (the TELNET user) agrees
+      to authenticate the user on whose behalf the connection is
+      initiated.
+
+   IAC WON'T TUID
+
+      The sender (the TELNET user) refuses to authenticate the user on
+      whose behalf the connection is initiated.
+
+   IAC DO TUID
+
+      The sender (the TELNET server) proposes that the recipient (the
+      TELNET user) authenticate the user and send the identifing UUID;
+      or, the sender (the TELNET server) agrees to accept the
+      recipient's (the TELNET user's) authentication of the user
+      identified by his UUID.
+
+
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+
+
+Anderson                                                        [Page 1]
+
+
+
+RFC 927                                                    December 1984
+TUID Telnet Option
+
+
+   IAC DON'T TUID
+
+      The sender (the TELNET server) refuses to accept the recipient's
+      (the TELNET user) authentication of the user.
+
+   IAC SB TUID <uuid> IAC SE
+
+      The sender (the TELNET user) sends the UUID <uuid> of the user on
+      whose behalf the connection is established to the host to which he
+      is connected.  The <uuid> is a 32 bit binary number.
+
+3. Default
+
+   WON'T TUID
+
+      A TELNET user host (the initiator of a TELNET connection) not
+      implementing or using the TUID option will reply WON'T TUID to a
+      DO TUID.
+
+   DON'T TUID
+
+      A TELNET server host (the recipient of a TELNET connection) not
+      implementing or using the TUID option reply DON'T TUID to a WILL
+      TUID.
+
+4. Motivation for the Option
+
+   Under TACACS (the TAC Access Control System) a user must be
+   authenticated (give a correct name/password pair) to a TAC before he
+   can connect to a host via the TAC.  To avoid a second authentication
+   by the target host, the TAC can pass along the user's proven identity
+   (his UUID) to the that host.  Hosts may accept the TAC's
+   authentication of the user or not, at their option.
+
+   The same option can be used between any pair of cooperating hosts for
+   the purpose of double login avoidance.
+
+5. Description for the Option
+
+   At the time that a host establishes a TELNET connection for a user to
+   another host, if the latter supports the TUID option and wants to
+   receive the user's UUID, it sends an IAC DO TUID to the the user's
+   host.  If the user's host supports the TUID option and wants to
+   authenticate the user by sending the user's UUID, it responds IAC
+   WILL TUID; otherwise it responds with IAC WON'T TUID.  If both the
+   user and server TELNETs agree, the user TELNET will then send the
+   UUID to the server TELNET by sub-negotiation.
+
+
+Anderson                                                        [Page 2]
+
+
+
+RFC 927                                                    December 1984
+TUID Telnet Option
+
+
+6. Examples
+
+   There are two possible negotiations that result in the double login
+   avoidance authentication of a user.  Both the server and the user
+   TELNET support the TUID option.
+
+      S = Server, U = User
+
+      Case 1:
+
+         S-> IAC DO TUID
+         U-> IAC WILL TUID
+         U-> IAC SB TUID <32-bit UUID> IAC SE
+
+      Case 2:
+
+         U-> IAC WILL TUID
+         S-> IAC DO TUID
+         U-> IAC SB TUID <32-bit UUID> IAC SE
+
+   There are also two possible negoitiations that do not result in the
+   authentication of a user.  In the first example the server supports
+   TUID and the user TELNET doesn't.  In the second example the user
+   TELNET supports TUID but the server TELNET doesn't.
+
+      S = Server, U = User
+
+      Case 3:
+
+         S-> IAC DO TUID
+         U-> IAC WONT TUID
+
+      Case 4:
+
+         U-> IAC WILL TUID
+         S-> IAC DONT TUID
+
+   The TUID is transmitted with the subnegotiation command.  For
+   example, if the UUID had the value 1 the following string of octets
+   would be transmitted:
+
+      IAC SB TUID 0 0 0 1 IAC SE
+
+   If the UUID had the value 255 the following string of octets would be
+   transmitted:
+
+      IAC SB TUID 0 0 0 IAC IAC IAC SE
+
+
+Anderson                                                        [Page 3]
+
+
+
+RFC 927                                                    December 1984
+TUID Telnet Option
+
+
+   If the UUID had the value of all ones the following string of octets
+   would be transmitted:
+
+      IAC SB TUID IAC IAC IAC IAC IAC IAC IAC IAC IAC SE
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+Anderson                                                        [Page 4]
+