The TACACS+ Protocol

IETF protocol: http://tools.ietf.org/html/draft-grant-tacacs-02
A free tacacs+ server: http://tacacs.net/

The TACACS+ Protocol
                              Version 1.78


Status of This Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as ``work in progress.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet- Drafts
   Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).

Abstract

   TACACS+ provides access control for routers, network access servers
   and other networked computing devices via one or more centralized
   servers.  TACACS+ provides separate authentication, authorization and
   accounting services.  This document describes the protocol that is
   used by TACACS+.


   1.  Introduction

   The TACACS+ protocol is the latest generation of TACACS. TACACS is a
   simple UDP based access control protocol originally developed by BBN
   for the MILNET. Cisco has enhanced (extended) TACACS several times,
   and Cisco's implementation, based on the original TACACS, is referred



Carrel/Grant                                                    [Page 1]

DRAFT                      expires June 1998                January 1998


   to as XTACACS. The TACACS protocol is described in [2].

   TACACS+ improves on TACACS and XTACACS by separating the functions of
   Authentication, Authorization and Accounting and by encrypting all
   traffic between the NAS and the daemon.  It allows for arbitrary
   length and content authentication exchanges which will allow any
   authentication mechanism to be utilized with TACACS+ clients.  It is
   extensible to provide for site customization and future development
   features, and it uses TCP to ensure reliable delivery. The protocol
   allows the TACACS+ client to request very fine grained access control
   and allows the daemon to respond to each component of that request.

   The separation of authentication, authorization and accounting is a
   fundamental component of the design of TACACS+. The distinction
   between them is very important so this document will address each one
   separately. It is important to note that TACACS+ provides for all
   three, but an implementation or configuration is not required to
   employ all three. Each one serves a unique purpose that alone is use-
   ful, and together can be quite powerful. A very important benefit to
   separating authentication from authorization is that authorization
   (and per-user profiles) can be a dynamic process. Instead of a one-
   shot user profile, TACACS+ can be integrated with other negotiations,
   such as a PPP negotiation, for far greater flexibility. The account-
   ing portion can serve to provide security auditing or accounting/
   billing services.

   TACACS+ uses TCP for its transport. The daemon should listen at port
   49 which is the "LOGIN" port assigned for the TACACS protocol. This
   port is reserved in the assigned numbers RFC for both UDP and TCP.
   Current TACACS and extended TACACS implementations use port 49.

   2.  Technical Definitions

   This section provides a few basic definitions that are applicable to
   this document.

Authentication

   Authentication is the action of determining who a user (or entity)
   is. Authentication can take many forms. Traditional authentication
   utilizes a name and a fixed password. Most computers work this way,
   and TACACS+ can also work this way. However, fixed passwords have
   limitations, mainly in the area of security. Many modern authentica-
   tion mechanisms utilize "one-time" passwords or a challenge-response
   query. TACACS+ is designed to support all of these, and should be
   powerful enough to handle any future mechanisms. Authentication gen-
   erally takes place when the user first logs in to a machine or
   requests a service of it.



Carrel/Grant                                                    [Page 2]

DRAFT                      expires June 1998                January 1998


   Authentication is not mandatory, it is a site configured option. Some
   sites do not require it. Others require it only for certain services
   (see authorization below). Authentication may also take place when a
   user attempts to gain extra privileges, and must identify themselves
   as someone who possesses the required information (passwords, etc.)
   for those privileges.

Authorization

   It is important to distinguish Authorization from Authentication.
   Authorization is the action of determining what a user is allowed to
   do. Generally authentication precedes authorization, but again, this
   is not required. An authorization request may indicate that the user
   is not authenticated (we don't know who they are). In this case it is
   up to the authorization agent to determine if an unauthenticated user
   is allowed the services in question.

   In TACACS+, authorization does not merely provide yes or no answers,
   but it may also customize the service for the particular user. Exam-
   ples of when authorization would be performed are: When a user first
   logs in and wants to start a shell, or when a user starts PPP and
   wants to use IP over PPP with a particular IP address. The TACACS+
   daemon might respond to these requests by allowing the service, but
   placing a time restriction on the login shell, or by requiring IP
   access lists on the PPP connection. For a list of authorization
   attributes, see the authorization section below.

Accounting

   Accounting is typically the third action after authentication and
   authorization. But again, neither authentication nor authorization
   are required. Accounting is the action of recording what a user is
   doing, and/or has done. Accounting in TACACS+ can serve two purposes:
   It may be used to account for services used, such as in a billing
   environment. It may also be used as an auditing tool for security
   services. To this end, TACACS+ supports three types of accounting
   records. Start records indicate that a service is about to begin.
   Stop records indicate that a service has just terminated, and Update
   records are intermediate notices that indicate that a service is
   still being performed. TACACS+ accounting records contain all the
   information used in the authorization records, and also contain
   accounting specific information such as start and stop times (when
   appropriate) and resource usage information. A list of accounting
   attributes is defined in the accounting section.







Carrel/Grant                                                    [Page 3]

DRAFT                      expires June 1998                January 1998


Session

   The concept of a session is used throughout this document. A TACACS+
   session is a single authentication sequence, a single authorization
   exchange, or a single accounting exchange.

   The session concept is important because a session identifier is used
   as a part of the encryption, and it is used by both ends to distin-
   guish between packets belonging to multiple sessions.

   Multiple sessions may be supported simultaneously and/or consecu-
   tively on a single TCP connection if both the daemon and client sup-
   port this.  If multiple sessions are not being multiplexed over a
   single tcp connection, a new connection should be opened for each
   TACACS+ session and closed at the end of that session. For accounting
   and authorization, this implies just a single pair of packets
   exchanged over the connection (the request and its reply). For
   authentication, a single session may involve an arbitrary number of
   packets being exchanged.

   The session is an operational concept that is maintained between the
   TACACS+ client and daemon. It does not necessarily correspond to a
   given user or user action.

NAS

   A NAS is a Network Access Server. This is any device that provides
   access services. Nowadays, a NAS is typically more than just a termi-
   nal server. Terminal servers usually provide a character mode front
   end and then allow the user to telnet or rlogin to another host. A
   NAS may also support protocol based access services and may support
   PPP, ARAP, LAT, XREMOTE, and others.

MUST

   This word means that the definition is an absolute requirement of the
   specification.

MUST NOT

   This phrase means that the definition is an absolute prohibition of
   the specification.

SHOULD

   This word, or the adjective "recommended", means that there may exist
   valid reasons in particular circumstances to ignore this item, but
   the full implications should be understood and carefully weighed



Carrel/Grant                                                    [Page 4]

DRAFT                      expires June 1998                January 1998


   before choosing a different course.

MAY

   This word, or the adjective "optional", means that this item is one
   of an allowed set of alternatives. An implementation which does not
   include this option MUST be prepared to interoperate with another
   implementation which does include the option.

   3.  The TACACS+ packet header

   All TACACS+ packets always begin with the following 12  byte  header.
   The  header  is  always  cleartext and describes the remainder of the
   packet:


         1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8

        +----------------+----------------+----------------+----------------+
        |major  | minor  |                |                |                |
        |version| version|      type      |     seq_no     |   flags        |
        +----------------+----------------+----------------+----------------+
        |                                                                   |
        |                            session_id                             |
        +----------------+----------------+----------------+----------------+
        |                                                                   |
        |                              length                               |
        +----------------+----------------+----------------+----------------+

major_version

   This is the major TACACS+ version number.

   TAC_PLUS_MAJOR_VER      := 0xc


minor_version

   The minor TACACS+ version number. This is intended to allow revisions
   to the TACACS+ protocol while maintaining backwards compatibility.

   Minor version 1 is currently defined for some commands.  It may only
   be used for commands that explicitly call for it in this document.
   All other requests must use the default value.

   TAC_PLUS_MINOR_VER_DEFAULT      := 0x0

   TAC_PLUS_MINOR_VER_ONE          := 0x1



Carrel/Grant                                                    [Page 5]

DRAFT                      expires June 1998                January 1998


   See the compatibility section at the end of the document.

   When a daemon receives a packet with a minor_version that it does not
   support, it should return an ERROR status with the minor_version set
   to the closest supported value.

type
   This is the packet type. Legal values are:

   TAC_PLUS_AUTHEN := 0x01 (Authentication)

   TAC_PLUS_AUTHOR := 0x02 (Authorization)

   TAC_PLUS_ACCT   := 0x03 (Accounting)

seq_no

   This is the sequence number of the current packet for the current
   session. The first TACACS+ packet in a session MUST have the sequence
   number 1 and each subsequent packet will increment the sequence
   number by one. Thus clients only send packets containing odd sequence
   numbers, and TACACS+ daemons only send packets containing even
   sequence numbers.

   The sequence number must never wrap i.e. if the sequence number 2^8-1
   is ever reached, that session must terminate and be restarted with a
   sequence number of 1.

flags

   This field contains various bitmapped flags.

   The unencrypted flag bit says whether encryption is being used on the
   body of the TACACS+ packet (the entire portion after the header).

   TAC_PLUS_UNENCRYPTED_FLAG := 0x01


   If this flag is set, the packet is not encrypted. If this flag is
   cleared, the packet is encrypted.

   Unencrypted packets are intended for testing, and are not recommended
   for normal use.

   The single-connection flag:

   TAC_PLUS_SINGLE_CONNECT_FLAG := 0x04




Carrel/Grant                                                    [Page 6]

DRAFT                      expires June 1998                January 1998


   If a NAS sets this flag, this indicates that it supports multiplexing
   TACACS+ sessions over a single tcp connection.  The flag need only be
   examined on the first two packets for any given connection since the
   single-connect status of a connection, once established, should not
   be changed. The connection must instead be closed and a new connec-
   tion opened, if required.

   If the daemon sets this flag in the first reply packet in response to
   the first packet from a NAS, this indicates its willingness to sup-
   port single-connection over the current connection. The daemon may
   set this flag even if the NAS does not set it, but the NAS is under
   no obligation to honor it.

session_id

   The Id for this TACACS+ session. The session id should be randomly
   chosen. This field does not change for the duration of the TACACS+
   session. (If this value is not a cryptographically strong random
   number, it will compromise the protocol's security. [6])

length

   The total length of the TACACS+ packet body (not including the
   header).  This value is in network byte order. Packets are never pad-
   ded beyond this length.

   4.  The TACACS+ packet body

   The TACACS+ body types are defined in the packet header. The
   remainder of this document will address the contents of the different
   TACACS+ bodies. The following general rules apply to all TACACS+ body
   types:

   - The entire body is protected by the encryption mechanism indicated
   in the header.

   - Any variable length data fields which are unused MUST have a length
   value equal to zero.

   - Unused fixed length fields SHOULD have values of zero.

   - All data and message fields in a TACACS+ packet MUST NOT be null
   terminated.

   - All length values are unsigned and in network byte order.

   - There should be no padding in any of the fields or at the end of a
   packet.



Carrel/Grant                                                    [Page 7]

DRAFT                      expires June 1998                January 1998


   5.  Body Encryption

   The body of TACACS+ packets may be encrypted. The following sections
   describe the encryption mechanisms that are supported. Only one
   encryption mechanism SHOULD be used within a single session.

   When the encryption mechanism relies on a secret key, it is referring
   to a shared secret value that is known to both the client and the
   daemon. This document does not discuss the management and storage of
   those keys. It is an implementation detail of the daemon and client,
   as to whether they will maintain only one key, or a different key for
   each client or daemon with which they communicate. For security rea-
   sons, the latter options should be available, but it is a site depen-
   dent decision as to whether the use of separate keys is appropriate.

   The encrypted flag field may be set as follows:

TAC_PLUS_UNENCRYPTED_FLAG == 0x0

   In this case, the packet body is encrypted by XOR-ing it byte-wise
   with a pseudo random pad.


   ENCRYPTED {data} == data ^ pseudo_pad


   The pad is generated by concatenating a series of MD5 hashes (each 16
   bytes long) and truncating it to the length of the input data.

   Whenever used in this document, MD5 refers to the "RSA Data Security,
   Inc. MD5 Message-Digest Algorithm" as specified in [3].


   pseudo_pad = {MD5_1 [,MD5_2 [ ... ,MD5_n]]} truncated to len(data)



   The first MD5 hash is generated by concatenating the session_id, the
   secret key, the version number and the sequence number and then run-
   ning MD5 over that stream. All of those input values are available in
   the packet header, except for the secret key which is a shared secret
   between the TACACS+ client and daemon.

   The version number is the one byte combination of the major and minor
   version numbers.

   The session id is used in the byte order in which it appears in the
   TACACS+ header. (i.e. in network byte order, not host byte order).



Carrel/Grant                                                    [Page 8]

DRAFT                      expires June 1998                January 1998


   Subsequent hashes are generated by using the same input stream, but
   concatenating the previous hash value at the end of the input stream.


   MD5_1 = MD5{session_id, key, version, seq_no}

   MD5_2 = MD5{session_id, key, version, seq_no, MD5_1}

    ....

   MD5_n = MD5{session_id, key, version, seq_no, MD5_n-1}


TAC_PLUS_UNENCRYPTED_FLAG == 0x1

   In this case, the entire packet body is in cleartext. Encryption and
   decryption are null operations. This method should only be used for
   debugging. It does not provide data protection or authentication and
   is highly susceptible to packet spoofing. Implementing this encryp-
   tion method is optional.

   NOTE: implementations should take care not to skip decryption simply
   because an incoming packet indicates that it is not encrypted.

   After a packet body is decrypted, the lengths of the component values
   in the packet should be summed and checked against the cleartext
   datalength value from the header. Any packets which fail this check
   should be discarded and an error signalled. Commonly such failures
   may be expected to be seen when there are mismatched keys between the
   NAS and the TACACS+ server.

   If an error must be declared but the type of the incoming packet can-
   not be determined, a packet with the identical cleartext header but
   with a sequence number incremented by one and the length set to zero
   may be returned to indicate an error.

   6.  Body types

   All further discussions of TACACS+ packet bodies assumes that any
   encryption/decryption has already been performed. From here on, we
   are only concerned with the cleartext data.

   There are several constant fields in many of the following bodies. A
   few merit mention here as they apply to most packet bodies.

   The user is the username or user id that is authenticated or being
   authenticated.




Carrel/Grant                                                    [Page 9]

DRAFT                      expires June 1998                January 1998


   The port is an ascii description of the port on which the user is
   connected.

   The rem_addr is a "best effort" description of the remote location
   from which the user has connected to the client. In many cases, the
   remote address will not be available or will be unreliable at best,
   but it may be useful when included.

   The user_msg is always the ASCII input from the user.

   The server_msg is always used to hold a message that is intended to
   be presented to the user. In some contexts it may be optional as to
   whether to actually present it.

   The data field has several uses but is not used in all packets.

   6.1.  Authentication

   TACACS+ authentication has three packet types: START, CONTINUE and
   REPLY.  START and CONTINUE are always sent by the client and REPLY is
   always sent by the daemon.

   Authentication begins with the client sending a START message to the
   daemon. The START message describes the type of authentication to be
   performed, and may contain the username and some authentication data.
   The START packet is only ever sent as the first message in a TACACS+
   authentication session, or as the packet immediately following a res-
   tart. (A restart may be requested by the daemon in a REPLY packet). A
   START packet always has seq_no equal to 1.

   In response to a START packet, the daemon sends a REPLY. The REPLY
   message indicates whether the authentication is finished, or whether
   it should continue. If the REPLY indicates that authentication should
   continue, then it will also indicate what new information is
   requested. The client will get that information and return it in a
   CONTINUE message.

   The daemon MUST always send a REPLY to both the START and the CON-
   TINUE messages, the only exception being if the client indicates an
   abort in the CONTINUE, in which case the session is immediately
   aborted.

   Thus, there are zero or more pairs of packets where the client sends
   a CONTINUE and the daemon sends a REPLY.







Carrel/Grant                                                   [Page 10]

DRAFT                      expires June 1998                January 1998



The authentication START packet body


      1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8

     +----------------+----------------+----------------+----------------+
     |    action      |    priv_lvl    |  authen_type   |     service    |
     +----------------+----------------+----------------+----------------+
     |    user len    |    port len    |  rem_addr len  |    data len    |
     +----------------+----------------+----------------+----------------+
     |    user ...
     +----------------+----------------+----------------+----------------+
     |    port ...
     +----------------+----------------+----------------+----------------+
     |    rem_addr ...
     +----------------+----------------+----------------+----------------+
     |    data...
     +----------------+----------------+----------------+----------------+



   Packet fields are as follows:

action

   This describes the authentication action to be performed. Legal
   values are:

   TAC_PLUS_AUTHEN_LOGIN    := 0x01

   TAC_PLUS_AUTHEN_CHPASS   := 0x02

   TAC_PLUS_AUTHEN_SENDPASS := 0x03 (deprecated)

   TAC_PLUS_AUTHEN_SENDAUTH := 0x04















Carrel/Grant                                                   [Page 11]

DRAFT                      expires June 1998                January 1998



priv_lvl

   This indicates the privilege level that the  user  is  authenticating
   as.  Privilege levels are ordered values from 0 to 15 with each level
   representing a privilege level that is a superset of the  next  lower
   value.  If a NAS client uses a different privilege level scheme, then
   mapping must be provided. Pre-defined values are:

   TAC_PLUS_PRIV_LVL_MAX   := 0x0f

   TAC_PLUS_PRIV_LVL_ROOT  := 0x0f

   TAC_PLUS_PRIV_LVL_USER  := 0x01

   TAC_PLUS_PRIV_LVL_MIN   := 0x00


authen_type

   The type of authentication that is being performed. Legal values are:

   TAC_PLUS_AUTHEN_TYPE_ASCII      := 0x01

   TAC_PLUS_AUTHEN_TYPE_PAP        := 0x02

   TAC_PLUS_AUTHEN_TYPE_CHAP       := 0x03

   TAC_PLUS_AUTHEN_TYPE_ARAP       := 0x04

   TAC_PLUS_AUTHEN_TYPE_MSCHAP     := 0x05




















Carrel/Grant                                                   [Page 12]

DRAFT                      expires June 1998                January 1998



service

   This is the service that  is  requesting  the  authentication.  Legal
   values are:

   TAC_PLUS_AUTHEN_SVC_NONE        := 0x00

   TAC_PLUS_AUTHEN_SVC_LOGIN       := 0x01

   TAC_PLUS_AUTHEN_SVC_ENABLE      := 0x02

   TAC_PLUS_AUTHEN_SVC_PPP         := 0x03

   TAC_PLUS_AUTHEN_SVC_ARAP        := 0x04

   TAC_PLUS_AUTHEN_SVC_PT          := 0x05

   TAC_PLUS_AUTHEN_SVC_RCMD        := 0x06

   TAC_PLUS_AUTHEN_SVC_X25         := 0x07

   TAC_PLUS_AUTHEN_SVC_NASI        := 0x08

   TAC_PLUS_AUTHEN_SVC_FWPROXY     := 0x09


   The ENABLE service refers to a service requesting authentication in order
   to grant the user different privileges. This is comparable to the Unix
   "su(1)" command. A service value of NONE should only be used when none of
   the other service values are appropriate.

user

   The username. It is optional in this packet.

port

   The ASCII name of the client port on which the authentication is tak-
   ing place. The value of this field is client specific. (For example,
   Cisco uses "tty10" to denote the tenth tty line and "Async10" to
   denote the tenth async interface).

rem_addr

   An ASCII string that describes the user's remote location. This field
   is optional (since the information may not be available). It is
   intended to hold a network address if the user is connected via a



Carrel/Grant                                                   [Page 13]

DRAFT                      expires June 1998                January 1998


   network, a caller ID is the user is connected via ISDN or a POTS, or
   any other remote location information that is available. This field
   value is client specified.

data

   This field is used to send data appropriate for the action and
   authen_type. It is described in more detail below.

   7.  The authentication REPLY packet body

   The TACACS+ daemon sends only one type of  authentication  packet  (a
   REPLY packet) to the client. The REPLY packet body looks as follows:


         1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8

        +----------------+----------------+----------------+----------------+
        |     status     |      flags     |        server_msg len           |
        +----------------+----------------+----------------+----------------+
        |           data len              |        server_msg ...
        +----------------+----------------+----------------+----------------+
        |           data ...
        +----------------+----------------+


status

   The current status of the authentication. Legal values are:

   TAC_PLUS_AUTHEN_STATUS_PASS     := 0x01

   TAC_PLUS_AUTHEN_STATUS_FAIL     := 0x02

   TAC_PLUS_AUTHEN_STATUS_GETDATA  := 0x03

   TAC_PLUS_AUTHEN_STATUS_GETUSER  := 0x04

   TAC_PLUS_AUTHEN_STATUS_GETPASS  := 0x05

   TAC_PLUS_AUTHEN_STATUS_RESTART  := 0x06

   TAC_PLUS_AUTHEN_STATUS_ERROR    := 0x07

   TAC_PLUS_AUTHEN_STATUS_FOLLOW   := 0x21






Carrel/Grant                                                   [Page 14]

DRAFT                      expires June 1998                January 1998


flags

   Bitmapped flags that modify the action to be taken. The following
   values are defined:

   TAC_PLUS_REPLY_FLAG_NOECHO      := 0x01


server_msg

   A message to be displayed to the user. This field is optional.  If it
   exists, it is intended to be presented to the user.

data

   This field holds data that is a part of the authentication exchange
   and is intended for the NAS, not the user. Valid uses of this field
   are described below.




   8.  The authentication CONTINUE packet body

   This packet is sent from the NAS to the daemon following the  receipt
   of a REPLY packet.


         1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8

        +----------------+----------------+----------------+----------------+
        |          user_msg len           |            data len             |
        +----------------+----------------+----------------+----------------+
        |     flags      |  user_msg ...
        +----------------+----------------+----------------+----------------+
        |    data ...
        +----------------+



user_msg

   This field is the string that the user entered, or the NAS provided
   on behalf of the user, in response to the server_msg from a REPLY
   packet.






Carrel/Grant                                                   [Page 15]

DRAFT                      expires June 1998                January 1998


data

   This field carries information that is specific to the action and the
   authen_type for this session. Valid uses of this field are described
   below.

flags

   This holds the bitmapped flags that modify the action to be taken.
   The following values are defined:

   TAC_PLUS_CONTINUE_FLAG_ABORT    := 0x01


   9.  The authentication process

   The flavor of the authentication is determined by the action and the
   authen_type fields in the START packet. First we should discuss some
   general fields that apply to all flavors of authentication exchanges.
   The user and data fields in the START packet are defined below for
   each flavor.

   The priv_lvl, service, port and rem_addr in the START packet are all
   provided to help identify the conditions on the NAS. In the CONTINUE
   packet, the user_msg and data fields are defined below for each fla-
   vor.  For all REPLY packets, the server_msg may contain a message to
   be displayed to the user, but the data field usage varies and is
   described below.

   The descriptions below first describe "normal" authentication where,
   in response to a START packet, the daemon either sends a request for
   more information (GETDATA, GETUSER or GETPASS) or a termination (PASS
   or FAIL). The actions and meanings when the daemon sends a RESTART,
   ERROR or FOLLOW are common and are described further below.

   When the REPLY status equals TAC_PLUS_AUTHEN_STATUS_GETDATA,
   TAC_PLUS_AUTHEN_STATUS_GETUSER or TAC_PLUS_AUTHEN_STATUS_GETPASS,
   then authentication continues and the server_msg may be used by the
   client to prompt the user for more information. The client MUST then
   return a CONTINUE packet containing the requested information in the
   user_msg field.

   TAC_PLUS_AUTHEN_STATUS_GETDATA is the generic request for more infor-
   mation. All three cause the same action to be performed, but in the
   case of TAC_PLUS_AUTHEN_STATUS_GETUSER, the client can know that the
   information that the user responds with is a username, and for
   TAC_PLUS_AUTHEN_STATUS_GETPASS, that the user response represents a
   password.



Carrel/Grant                                                   [Page 16]

DRAFT                      expires June 1998                January 1998


   If the TAC_PLUS_REPLY_FLAG_NOECHO flag is set in the REPLY, then the
   user response should not be echoed as it is entered. The data field
   is only used in the REPLY where explicitly defined below.

   9.0.1.  Enable Requests


      action      = TAC_PLUS_AUTHEN_LOGIN

      priv_lvl    = implementation dependent

      authen_type = not used

      service     = TAC_PLUS_AUTHEN_SVC_ENABLE


   This is an ENABLE request, used to change the current running
   privilege level of a principal. The exchange MAY consist of multiple
   messages while the daemon collects the information it requires in
   order to allow changing the principal's privilege level. This
   exchange is very similar to an Inbound ASCII login (which see).

   In order to readily distinguish enable requests from other types of
   request, the value of the service field MUST be set to
   TAC_PLUS_AUTHEN_SVC_ENABLE when requesting an ENABLE. It MUST NOT be
   set to this value when requesting any other operation.

   9.0.2.  Inbound ASCII Login


           action      =   TAC_PLUS_AUTHEN_LOGIN

           authen_type =   TAC_PLUS_AUTHEN_TYPE_ASCII


   This is a standard ASCII authentication. The START packet may contain
   the username, but need not do so. The data fields in both the START
   and CONTINUE packets are not used for ASCII logins. There is a single
   START followed by zero or more pairs of REPLYs and CONTINUEs, fol-
   lowed by a terminating REPLY (PASS or FAIL).

   9.0.3.  Inbound PAP Login


           action        =   TAC_PLUS_AUTHEN_LOGIN

           authen_type   =   TAC_PLUS_AUTHEN_TYPE_PAP




Carrel/Grant                                                   [Page 17]

DRAFT                      expires June 1998                January 1998


           minor_version =   0x1


   The entire exchange MUST consist of a single START packet and a sin-
   gle REPLY. The START packet MUST contain a username and the data
   field MUST contain the PAP ASCII password. A PAP authentication only
   consists of a username and password [4]. The REPLY from the daemon
   MUST be either a PASS or FAIL.

   9.0.4.  Inbound CHAP login


           action        =   TAC_PLUS_AUTHEN_LOGIN

           authen_type   =   TAC_PLUS_AUTHEN_TYPE_CHAP

           minor_version =   0x1


   The entire exchange MUST consist of a single START packet and a sin-
   gle REPLY. The START packet MUST contain the username in the user
   field and the data field will be a concatenation of the PPP id, the
   challenge and the response.

   The length of the challenge value can be determined from the length
   of the data field minus the length of the id (always 1 octet) and the
   length of the response field (always 16 octets).

   To perform the authentication, the daemon will run MD5 over the id,
   the user's secret and the challenge, as defined in the PPP Authenti-
   cation RFC [4] and then compare that value with the response. The
   REPLY from the daemon MUST be a PASS or FAIL.


   9.0.5.  Inbound MS-CHAP login


           action        =   TAC_PLUS_AUTHEN_LOGIN

           authen_type   =   TAC_PLUS_AUTHEN_TYPE_MSCHAP

           minor_version =   0x1


   The entire exchange MUST consist of a single START packet and a sin-
   gle REPLY. The START packet MUST contain the username in the user
   field and the data field will be a concatenation of the PPP id, the
   MS-CHAP challenge and the MS-CHAP response.



Carrel/Grant                                                   [Page 18]

DRAFT                      expires June 1998                January 1998


   The length of the challenge value can be determined from the length
   of the data field minus the length of the id (always 1 octet) and the
   length of the response field (always 49 octets).

   To perform the authentication, the daemon will use a combination of
   MD4 and DES on the user's secret and the challenge, as defined in [7]
   and then compare the resulting value with the response. The REPLY
   from the daemon MUST be a PASS or FAIL.


   9.0.6.  Outbound MS-CHAP request


           action        = TAC_PLUS_AUTHEN_SENDAUTH

           authen_type   = TAC_PLUS_AUTHEN_TYPE_MSCHAP

           minor_version = 0x1


   This is used when the NAS needs to provide MS-CHAP authentication
   credentials to the remote PPP peer. The entire exchange MUST consist
   of a single START packet and a single REPLY. The START packet MUST
   contain the username in the user field and the data field will be a
   concatenation of the PPP id and the challenge.

   The length of the challenge value can be determined from the length
   of the data field minus the length of the id (always 1 octet). The
   daemon will use MD4 and DES to process the user's secret and the
   challenge, as defined in [7].

   The REPLY from the daemon MUST be a PASS or FAIL. If the status is
   PASS, then the data field MUST contain the 49-octet output, in which
   24 octets are MD4 output for the Microsoft LAN Manager compatible
   challenge response, 24 octets are MD4 output for the Microsoft Win-
   dows NT compatible challenge response and 1 octet is the flag to
   determine which part of the response packet should be utilized.



   9.0.7.  Inbound ARAP login


           action        =   TAC_PLUS_AUTHEN_LOGIN

           authen_type   =   TAC_PLUS_AUTHEN_TYPE_ARAP

           minor_version =   0x1



Carrel/Grant                                                   [Page 19]

DRAFT                      expires June 1998                January 1998


   The entire exchange MUST consist of a single START packet and a sin-
   gle REPLY. The START packet MUST contain the username in the user
   field and the data field will be a concatenation of the NAS's chal-
   lenge to the remote peer (8 octets) the remote peer's challenge to
   the NAS (8 octets) and the remote peer's response to the NAS's chal-
   lenge (8 octets).

   The daemon must run DES encryption over both the challenges using the
   user's secret as the DES key, as described in the ARAP specification
   [5]. For a successful authentication, the encrypted NAS challenge
   MUST be identical to the peer's response. The REPLY from the daemon
   MUST be a PASS or FAIL. The encrypted peer challenge (8 octets) is
   returned in the data field of a REPLY packet if the status is set to
   PASS.

   9.0.8.  Outbound PAP request


           action        =   TAC_PLUS_AUTHEN_SENDAUTH

           authen_type   =   TAC_PLUS_AUTHEN_TYPE_PAP

           minor_version =   0x1


   This is used when the NAS needs to provide PAP authentication creden-
   tials to the remote PPP peer. The entire exchange MUST consist of a
   single START packet and a single REPLY. The START packet contains a
   username in the user field. A REPLY with status set to PASS MUST con-
   tain a cleartext password in the data field. Caution is urged when
   using this. By sending a cleartext password to the NAS, that password
   will then be passed to the remote PPP peer. It should be ensured that
   the provided password can never be used to authenticate back to the
   NAS. Use of this is discouraged, but supported for complete intero-
   perability with the PPP protocol.


   9.0.9.  Outbound CHAP request


           action        = TAC_PLUS_AUTHEN_SENDAUTH

           authen_type   = TAC_PLUS_AUTHEN_TYPE_CHAP

           minor_version = 0x1


   This is used when the NAS needs to provide CHAP authentication



Carrel/Grant                                                   [Page 20]

DRAFT                      expires June 1998                January 1998


   credentials to the remote PPP peer. The entire exchange MUST consist
   of a single START packet and a single REPLY. The START packet MUST
   contain the username in the user field and the data field will be a
   concatenation of the PPP id and the challenge.

   The length of the challenge value can be determined from the length
   of the data field minus the length of the id (always 1 octet). The
   daemon will run MD5 over the id, the user's secret and the challenge,
   as defined in the PPP Authentication RFC [4].

   The REPLY from the daemon MUST be a PASS or FAIL. If the status is
   PASS, then the data field MUST contain the 16 octet MD5 output


   9.0.10.  Outbound ASCII and ARAP request


           action      =   TAC_PLUS_AUTHEN_SENDAUTH

           authen_type =   TAC_PLUS_AUTHEN_TYPE_ASCII

           authen_type =   TAC_PLUS_AUTHEN_TYPE_ARAP

   This is an error. This action is not supported for ASCII logins and
   in not needed for ARAP since ARAP authentication is already a two way
   protocol.


   9.0.11.  ASCII change password request


           action      =   TAC_PLUS_AUTHEN_CHPASS

           authen_type =   TAC_PLUS_AUTHEN_TYPE_ASCII


   This exchange consists of multiple messages while the daemon collects
   the information it requires in order to change the user's password.
   It is very similar to an ASCII login. The status value
   TAC_PLUS_AUTHEN_STATUS_GETPASS MUST only be used when requesting the
   "new" password. It MAY be sent multiple times. When requesting the
   "old" password, the status value MUST be set to
   TAC_PLUS_AUTHEN_STATUS_GETDATA.


   9.0.12.  PPP change password request





Carrel/Grant                                                   [Page 21]

DRAFT                      expires June 1998                January 1998


           action      =   TAC_PLUS_AUTHEN_CHPASS

           authen_type =   TAC_PLUS_AUTHEN_TYPE_PAP

           authen_type =   TAC_PLUS_AUTHEN_TYPE_CHAP


   This is never valid. The PPP protocol does not support password
   changing.


   9.0.13.  ARAP change password request


           action      =   TAC_PLUS_AUTHEN_CHPASS

           authen_type =   TAC_PLUS_AUTHEN_TYPE_ARAP


   This exchange consists of a single START packet and a single REPLY.
   The START packet MUST contain the username and the data field con-
   tains both the old and the new passwords encrypted (**FORMAT NOT
   KNOWN AT THIS TIME **). The reply is a PASS or FAIL and the data
   field is unused.

   10.  Aborting a session

   The client may prematurely terminate a session by setting the
   TAC_PLUS_CONTINUE_FLAG_ABORT flag in the CONTINUE message. If this
   flag is set, the data portion of the message may contain an ASCII
   message explaining the reason for the abort. The session is ter-
   minated and no REPLY message is sent.

   There are three other possible return status values that can be used
   in a REPLY packet. These can be sent regardless of the action or
   authen_type. Each of these indicates that the TACACS+ authentication
   session should be terminated. In each case, the server_msg may con-
   tain a message to be displayed to the user.

   When the status equals TAC_PLUS_AUTHEN_STATUS_FOLLOW the packet indi-
   cates that the TACACS+ daemon requests that authentication should be
   performed with an alternate daemon. The data field MUST contain ASCII
   text describing one or more daemons. A daemon description appears
   like this:

           [@<protocol>@]<host>[@<key>]

   The protocol and key are optional. The protocol can describe an



Carrel/Grant                                                   [Page 22]

DRAFT                      expires June 1998                January 1998


   alternate way of performing the authentication, other than TACACS+.
   If the protocol is not present, then TACACS+ is assumed.

   Protocols are ASCII numbers corresponding to the methods listed in
   the authen_method field of authorization packets (defined below). The
   host is specified as either a fully qualified domain name, or an
   ASCII numeric IP address specified as octets separated by dots (`.').

   If a key is supplied, the client MAY use the key in order to authen-
   ticate to that host. If more than one host is specified, they MUST be
   separated by an ASCII <CR> (0x0D).

   Use of the hosts in a TAC_PLUS_AUTHEN_STATUS_FOLLOW packet is at the
   discretion of the TACACS+ client. It may choose to use any one, all
   or none of these hosts. If it chooses to use none, then it MUST treat
   the authentication as if the return status was
   TAC_PLUS_AUTHEN_STATUS_FAIL.

   While the order of hosts in this packet indicates a preference, but
   the client is not obliged to use that ordering.

   If the status equals TAC_PLUS_AUTHEN_STATUS_ERROR, then the host is
   indicating that it is experiencing an unrecoverable error and the
   authentication should proceed as if that host could not be contacted.
   The data field may contain a message to be printed on an administra-
   tive console or log.

   If the status equals TAC_PLUS_AUTHEN_STATUS_RESTART, then the authen-
   tication sequence should be restarted with a new START packet from
   the client. This REPLY packet indicates that the current authen_type
   value (as specified in the START packet) is not acceptable for this
   session, but that others may be.

   The TAC_PLUS_AUTHEN_STATUS_RESTART REPLY packet may contain a list of
   valid authen_type values in the data portion of the packet. The
   authen_type values are a single byte in length so the data_len value
   indicates the number of authen_type values included. This packet is
   only currently intended for PPP authentication when multiple authen-
   tication mechanisms are available and can be negotiated between the
   client and the remote peer. This also requires future PPP authentica-
   tion extensions which have not yet been passed through the IETF. If a
   client chooses not to accept the TAC_PLUS_AUTHEN_STATUS_RESTART
   packet, then it should be TREATED as if the status was
   TAC_PLUS_AUTHEN_STATUS_FAIL.

   11.  Authorization

   TACACS+ authorization is an extensible way of providing remote



Carrel/Grant                                                   [Page 23]

DRAFT                      expires June 1998                January 1998


   authorization services.  An authorization session is defined as a
   single pair of messages, a REQUEST followed by a RESPONSE.

   The authorization REQUEST message contains a fixed set of fields that
   describe the authenticity of the user or process, and a variable set
   of arguments that describes the services and options for which
   authorization is requested.

   The RESPONSE contains a variable set of response arguments
   (attribute-value pairs) which can restrict or modify the clients
   actions.

   The arguments in both a REQUEST and a RESPONSE can be specified as
   either mandatory or optional. An optional argument is one that may or
   may not be used, modified or even understood by the recipient.

   A mandatory argument MUST be both understood and used. This allows
   for extending the attribute list while providing secure backwards
   compatibility.
































Carrel/Grant                                                   [Page 24]

DRAFT                      expires June 1998                January 1998



   11.1.  The authorization REQUEST packet body



         1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8

        +----------------+----------------+----------------+----------------+
        |  authen_method |    priv_lvl    |  authen_type   | authen_service |
        +----------------+----------------+----------------+----------------+
        |    user len    |    port len    |  rem_addr len  |    arg_cnt     |
        +----------------+----------------+----------------+----------------+
        |   arg 1 len    |   arg 2 len    |      ...       |   arg N len    |
        +----------------+----------------+----------------+----------------+
        |   user ...
        +----------------+----------------+----------------+----------------+
        |   port ...
        +----------------+----------------+----------------+----------------+
        |   rem_addr ...
        +----------------+----------------+----------------+----------------+
        |   arg 1 ...
        +----------------+----------------+----------------+----------------+
        |   arg 2 ...
        +----------------+----------------+----------------+----------------+
        |   ...
        +----------------+----------------+----------------+----------------+
        |   arg N ...
        +----------------+----------------+----------------+----------------+


authen_method

   This indicates the authentication method used by the client to
   acquire the user information.

   TAC_PLUS_AUTHEN_METH_NOT_SET    := 0x00

   TAC_PLUS_AUTHEN_METH_NONE       := 0x01

   TAC_PLUS_AUTHEN_METH_KRB5       := 0x02

   TAC_PLUS_AUTHEN_METH_LINE       := 0x03

   TAC_PLUS_AUTHEN_METH_ENABLE     := 0x04

   TAC_PLUS_AUTHEN_METH_LOCAL      := 0x05

   TAC_PLUS_AUTHEN_METH_TACACSPLUS := 0x06



Carrel/Grant                                                   [Page 25]

DRAFT                      expires June 1998                January 1998


   TAC_PLUS_AUTHEN_METH_GUEST      := 0x08

   TAC_PLUS_AUTHEN_METH_RADIUS     := 0x10

   TAC_PLUS_AUTHEN_METH_KRB4       := 0x11

   TAC_PLUS_AUTHEN_METH_RCMD       := 0x20


   KRB5 and KRB4 are kerberos version 5 and 4. LINE refers to a fixed
   password associated with the line used to gain access. LOCAL is a NAS
   local user database. ENABLE is a command that authenticates in order
   to grant new privileges. TACACSPLUS is, of course, TACACS+. GUEST is
   an unqualified guest authentication, such as an ARAP guest login.
   RADIUS is the Radius authentication protocol. RCMD refers to authen-
   tication provided via the R-command protocols from Berkeley Unix.
   (One should be aware of the security limitations to R-command authen-
   tication.)

priv_lvl

   This field matches the priv_lvl field in the authentication section
   above. It indicates the users current privilege level.

authen_type

   This field matches the authen_type field in the authentication sec-
   tion above. It indicates the type of authentication that was per-
   formed.

authen_service

   This field matches the service field in the authentication section
   above. It indicates the service through which the user authenticated.

user

   This field contains the user's account name.

port

   This field matches the port field in the authentication section
   above.

rem_addr

   This field matches the rem_addr field in the authentication section
   above.



Carrel/Grant                                                   [Page 26]

DRAFT                      expires June 1998                January 1998


arg_cnt

   The number of authorization arguments to follow

arg

   An attribute-value pair that describes the command to be performed.
   (see below)

   The authorization arguments in both the REQUEST and the RESPONSE are
   attribute-value pairs. The attribute and the value are in a single
   ascii string and are separated by either a "=" (0X3D) or a "*"
   (0X2A). The equals sign indicates a mandatory argument. The asterisk
   indicates an optional one.

   Optional arguments are ones that may be disregarded by either client
   or daemon. Mandatory arguments require that the receiving side under-
   stands the attribute and will act on it. If the client receives a
   mandatory argument that it cannot oblige or does not understand, it
   MUST consider the authorization to have failed. It is legal to send
   an attribute-value pair with a NULL (zero length) value.

   Attribute-value strings are not NULL terminated, rather their length
   value indicates their end. The maximum length of an attribute-value
   string is 255 characters. The following attributes are defined:



   12.  Table 1: Attribute-value Pairs


service

   The primary service. Specifying a service attribute indicates that
   this is a request for authorization or accounting of that service.
   Current values are "slip", "ppp", "arap", "shell", "tty-daemon",
   "connection", "system" and "firewall". This attribute MUST always be
   included.

protocol

   a protocol that is a subset of a service. An example would be any PPP
   NCP. Currently known values are "lcp", "ip", "ipx", "atalk", "vines",
   "lat", "xremote", "tn3270", "telnet", "rlogin", "pad", "vpdn", "ftp",
   "http", "deccp", "osicp" and "unknown".






Carrel/Grant                                                   [Page 27]

DRAFT                      expires June 1998                January 1998


cmd

   a shell (exec) command. This indicates the command name for a shell
   command that is to be run. This attribute MUST be specified if ser-
   vice equals "shell". A NULL value indicates that the shell itself is
   being referred to.

cmd-arg

   an argument to a shell (exec) command. This indicates an argument for
   the shell command that is to be run. Multiple cmd-arg attributes may
   be specified, and they are order dependent.

acl

   ASCII number representing a connection access list. Used only when
   service=shell and cmd=NULL

inacl

   ASCII identifier for an interface input access list.

outacl

   ASCII identifier for an interface output access list.

zonelist

   A numeric zonelist value. (Applicable to AppleTalk only).

addr

   a network address

addr-pool

   The identifier of an address pool from which the NAS should assign an
   address.

routing

   A boolean. Specifies whether routing information is to be propagated
   to, and accepted from this interface.

route

   Indicates a route that is to be applied to this interface. Values
   MUST be of the form "<dst_address> <mask> [<routing_addr>]". If a



Carrel/Grant                                                   [Page 28]

DRAFT                      expires June 1998                January 1998


   <routing_addr> is not specified, the resulting route should be via
   the requesting peer.

timeout

   an absolute timer for the connection (in minutes). A value of zero
   indicates no timeout.

idletime

   an idle-timeout for the connection (in minutes). A value of zero
   indicates no timeout.

autocmd



   an auto-command to run. Used only when service=shell and cmd=NULL

noescape

   Boolean. Prevents user from using an escape character. Used only when
   service=shell and cmd=NULL

nohangup

   Boolean. Do no disconnect after an automatic command. Used only when
   service=shell and cmd=NULL

priv_lvl

   privilege level to be assigned.

remote_user

   remote userid (authen_method must have the value
   TAC_PLUS_AUTHEN_METH_RCMD)

remote_host

   remote host (authen_method must have the value
   TAC_PLUS_AUTHEN_METH_RCMD)

callback-dialstring

   Indicates that callback should be done. Value is NULL, or a dial-
   string. A NULL value indicates that the service MAY choose to get the
   dialstring through other means.



Carrel/Grant                                                   [Page 29]

DRAFT                      expires June 1998                January 1998


callback-line

   The line number to use for a callback.

callback-rotary

   The rotary number to use for a callback.

nocallback-verify

   Do not require authentication after callback.



For all boolean attributes, valid values are "true" or "false". A

value of NULL means an attribute with a zero length string for its value
i.e. cmd=NULL is actually transmitted as the string of 4 characters
"cmd=".

If a host is specified in a cmd-arg or addr, it is recommended that it
be specified as a numeric address so as to avoid any ambiguities.

In the case of rcmd authorizations, the authen_method will be set to
TAC_PLUS_AUTHEN_METH_RCMD and the remote_user and remote_host attributes
will provide the remote user and host information to enable rhost style
authorization. The response may request that a privilege level be set
for the user.

The protocol attribute is intended for use with PPP. When service equals
"ppp" and protocol equals "lcp", the message describes the PPP link
layer service. For other values of protocol, this describes a PPP NCP
(network layer service). A single PPP session can support multiple NCPs.

The attributes addr, inacl, outacl, route and routing may be used for
all network protocol types that are supported. Their format and meaning
is determined by the values of the service or protocol attributes. Not
all are necessarily implemented for any given network protocol.













Carrel/Grant                                                   [Page 30]

DRAFT                      expires June 1998                January 1998



   12.1.  The authorization RESPONSE packet body



         1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8

        +----------------+----------------+----------------+----------------+
        |    status      |     arg_cnt    |         server_msg len          |
        +----------------+----------------+----------------+----------------+
        +            data len             |    arg 1 len   |    arg 2 len   |
        +----------------+----------------+----------------+----------------+
        |      ...       |   arg N len    |         server_msg ...
        +----------------+----------------+----------------+----------------+
        |   data ...
        +----------------+----------------+----------------+----------------+
        |   arg 1 ...
        +----------------+----------------+----------------+----------------+
        |   arg 2 ...
        +----------------+----------------+----------------+----------------+
        |   ...
        +----------------+----------------+----------------+----------------+
        |   arg N ...
        +----------------+----------------+----------------+----------------+



status
   This field indicates the authorization status

   TAC_PLUS_AUTHOR_STATUS_PASS_ADD  := 0x01

   TAC_PLUS_AUTHOR_STATUS_PASS_REPL := 0x02

   TAC_PLUS_AUTHOR_STATUS_FAIL      := 0x10

   TAC_PLUS_AUTHOR_STATUS_ERROR     := 0x11

   TAC_PLUS_AUTHOR_STATUS_FOLLOW    := 0x21


server_msg

   This is an ASCII string that may be presented to the user. The decision
   to present this message is client specific.






Carrel/Grant                                                   [Page 31]

DRAFT                      expires June 1998                January 1998


data

   This is an ASCII string that may be presented on an administrative
   display, console or log. The decision to present this message is client
   specific.

arg_cnt

   The number of authorization arguments to follow.

arg

   An attribute-value pair that describes the command to be performed. (see
   below)

   If the status equals TAC_PLUS_AUTHOR_STATUS_FAIL, then the appropriate
   action is to deny the user action.

   If the status equals TAC_PLUS_AUTHOR_STATUS_PASS_ADD, then the
   arguments specified in the request are authorized and the arguments in
   the response are to be used IN ADDITION to those arguments.

   If the status equals TAC_PLUS_AUTHOR_STATUS_PASS_REPL then the
   arguments in the request are to be completely replaced by the
   arguments in the response.

   If the intended action is to approve the authorization with no
   modifications, then the status should be set to
   TAC_PLUS_AUTHOR_STATUS_PASS_ADD and the arg_cnt should be set to
   0.

   A status of TAC_PLUS_AUTHOR_STATUS_ERROR indicates an error occurred
   on the daemon.

   When the status equals TAC_PLUS_AUTHOR_STATUS_FOLLOW, then the arg_cnt
   MUST be 0. In that case, the actions to be taken and the contents of
   the data field are identical to the TAC_PLUS_AUTHEN_STATUS_FOLLOW
   status for Authentication.

   None of the arg values have any relevance if an ERROR is set.

   13.  Accounting

   TACACS+ accounting is very similar to authorization. The packet for-
   mat is also similar. There is a fixed portion and an extensible por-
   tion. The extensible portion uses all the same attribute-value pairs
   that authorization uses, and adds several more.




Carrel/Grant                                                   [Page 32]

DRAFT                      expires June 1998                January 1998



   13.1.  The account REQUEST packet body


         1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8

        +----------------+----------------+----------------+----------------+
        |      flags     |  authen_method |    priv_lvl    |  authen_type   |
        +----------------+----------------+----------------+----------------+
        | authen_service |    user len    |    port len    |  rem_addr len  |
        +----------------+----------------+----------------+----------------+
        |    arg_cnt     |   arg 1 len    |   arg 2 len    |      ...       |
        +----------------+----------------+----------------+----------------+
        |   arg N len    |    user ...
        +----------------+----------------+----------------+----------------+
        |   port ...
        +----------------+----------------+----------------+----------------+
        |   rem_addr ...
        +----------------+----------------+----------------+----------------+
        |   arg 1 ...
        +----------------+----------------+----------------+----------------+
        |   arg 2 ...
        +----------------+----------------+----------------+----------------+
        |   ...
        +----------------+----------------+----------------+----------------+
        |   arg N ...
        +----------------+----------------+----------------+----------------+


flags

   This holds bitmapped flags.

   TAC_PLUS_ACCT_FLAG_MORE     := 0x01 (deprecated)

   TAC_PLUS_ACCT_FLAG_START    := 0x02

   TAC_PLUS_ACCT_FLAG_STOP     := 0x04

   TAC_PLUS_ACCT_FLAG_WATCHDOG := 0x08

   All other fields are defined in the authorization and authentication
   sections above and have the same semantics.

   The following new attributes are defined for TACACS+ accounting only.
   When these attribute-value pairs are included in the argument list,
   they should precede any attribute-value pairs that are defined in the
   authorization section above.



Carrel/Grant                                                   [Page 33]

DRAFT                      expires June 1998                January 1998


Table 2: Accounting Attribute-value Pairs


task_id

   Start and stop records for the same event MUST have matching (unique)
   task_id's

start_time

   The time the action started (in seconds since the epoch, 12:00am Jan
   1 1970).

stop_time

   The time the action stopped (in seconds since the epoch.)

elapsed_time

   The elapsed time in seconds for the action. Useful when the device
   does not keep real time.

timezone

   The timezone abbreviation for all timestamps included in this packet.

event

   Used only when "service=system". Current values are "net_acct",
   "cmd_acct", "conn_acct", "shell_acct" "sys_acct" and "clock_change".
   These indicate system level changes. The flags field SHOULD indicate
   whether the service started or stopped.

reason

   Accompanies an event attribute. It describes why the event occurred.

bytes

   The number of bytes transferred by this action

bytes_in

   The number of input bytes transferred by this action







Carrel/Grant                                                   [Page 34]

DRAFT                      expires June 1998                January 1998


bytes_out

   The number of output bytes transferred by this action

paks

   The number of packets transferred by this action.

paks_in

   The number of input packets transferred by this action.

paks_out

   The number of output packets transferred by this action.

status

   The numeric status value associated with the action. This is a signed
   four (4) byte word in network byte order. 0 is defined as success.
   Negative numbers indicate errors. Positive numbers indicate non-error
   failures. The exact status values may be defined by the client.



err_msg

   An ascii string describing the status of the action.

   NOTE: All numeric values in an attribute-value string are provided as
   decimal ASCII numbers.




















Carrel/Grant                                                   [Page 35]

DRAFT                      expires June 1998                January 1998



   13.2.  The accounting REPLY packet body

   The response to an accounting message is used to  indicate  that  the
   accounting   function  on  the  daemon  has  completed  and  securely
   committed the record. This provides  the  client  the  best  possible
   guarantee that the data is indeed logged.



         1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8

        +----------------+----------------+----------------+----------------+
        |         server_msg len          |            data len             |
        +----------------+----------------+----------------+----------------+
        |     status     |         server_msg ...
        +----------------+----------------+----------------+----------------+
        |     data ...
        +----------------+



status




   This is the return status. Values are:
   TAC_PLUS_ACCT_STATUS_SUCCESS    := 0x01

   TAC_PLUS_ACCT_STATUS_ERROR      := 0x02

   TAC_PLUS_ACCT_STATUS_FOLLOW     := 0x21


server_msg

   This is an ASCII string that may be presented to the user. The deci-
   sion to present this message is client specific.

data

   This is an ASCII string that may be presented on an administrative
   display, console or log. The decision to present this message is
   client specific.

   When the status equals TAC_PLUS_ACCT_STATUS_FOLLOW, then the actions
   to be taken and the contents of the data field are identical to the



Carrel/Grant                                                   [Page 36]

DRAFT                      expires June 1998                January 1998


   TAC_PLUS_AUTHEN_STATUS_FOLLOW status for Authentication.

   The daemon MUST terminate the session after sending a REPLY.

   The TAC_PLUS_ACCT_FLAG_START flag indicates that this is a start
   accounting message. Start messages should only be sent once when a
   task is started. The TAC_PLUS_ACCT_FLAG_STOP indicates that this is a
   stop record and that the task has terminated. The
   TAC_PLUS_ACCT_FLAG_WATCHDOG flag means that this is an update record.
   Update records are sent at the client's discretion when the task is
   still running.

   The START and STOP flags are mutually exclusive. When the WATCHDOG
   flag is set along with the START flag, it indicates that the update
   record is a duplicate of the original START record. If the START flag
   is not set, then this indicates a minimal record indicating only that
   task is still running. The STOP flag MUST NOT be set in conjunction
   with the WATCHDOG flag.


   14.  Compatibility between Minor Versions 0 and 1

   Whenever a TACACS+ daemon receives a packet with a minor_version that
   it does not support, it should return an ERROR status with the
   minor_version set to the supported value closest to the requested
   value.

   The changes between minor_version 0 and 1 all deal with the way that
   CHAP, ARAP and PAP authentications are handled.

   In minor_version 0, CHAP, ARAP and outbound PAP authentications were
   performed by the NAS sending a SENDPASS packet to the daemon. The
   SENDPASS requested a copy of the user's plaintext password so that
   the NAS could complete the authentication. The CHAP hashing and ARAP
   encryption were all performed on the NAS. Inbound PAP performed a
   normal LOGIN, sending the username in the START packet and then wait-
   ing for a GETPASS and sending the password in a CONTINUE packet.

   In minor_version 1, CHAP, ARAP and inbound PAP use LOGIN to perform
   inbound authentication and the exchanges use the data field so that
   the NAS only sends a single START packet and expects to receive a
   PASS or FAIL. SENDPASS has been deprecated and SENDAUTH introduced,
   so that the NAS can request authentication credentials for authenti-
   cating to a remote peer. SENDAUTH is only used for PPP when perform-
   ing outbound authentication.

   NOTE: Only those requests which have changed from their minor_version
   0 implementation (i.e. ARAP, CHAP and PAP) should use the new



Carrel/Grant                                                   [Page 37]

DRAFT                      expires June 1998                January 1998


   minor_version number of 1. All other requests (whose implementation
   has not changed) MUST continue to use the same minor_version number
   of 0 that they have always used.

   If a daemon or NAS implementation desires to provide support for
   minor_number 0 TACACS+ hosts, it MUST pay attention to the
   minor_version in the TACACS+ header (as it should anyway) and be
   prepared to support the SENDPASS operation.

   The removal of SENDPASS was prompted by security concerns, and imple-
   mentors should think very carefully about how they wish to provide
   this service. On a NAS, the minor_version 0 compatibility can be lay-
   ered such that higher layers only need to understand the
   minor_version 1 methodology, with the compatibility layer translating
   requests appropriately when contacting an older daemon.

   On a TACACS+ server, when detecting minor_number 0, the daemon should
   allow for PAP authentications that do not send the password in the
   data field, but instead expect to read the PAP password from a subse-
   quent CONTINUE packet.

   If the daemon supports SENDPASS, then it should be prepared to handle
   such requests for CHAP and ARAP and even PAP, when outbound authenti-
   cation takes place.


   15.  Notes to Implementors

   For those interested in integrating one-time password support into
   TACACS+ daemons, there are some subtleties to consider.  TACACS+ is
   designed to make this straightforward, but two cases require some
   extra care.

   One-time password support with ARAP and PPP's CHAP authentication
   protocols is NOT straightforward, but there are work arounds. The
   problem lies in the nature of ARAP and CHAP authentication. Both
   employ a challenge-response protocol that requires a copy of the
   cleartext password to be stored at both ends. Unfortunately, due to
   their cryptographic nature, one-time password systems can rarely pro-
   vide the cleartext version of the next password.

   A simple workaround is to have the user enter their username as a
   combination of the username and the one-time password, separated by a
   special character, and a fixed password can be used in the password
   field. The fixed password can be assigned on a per user basis or as a
   single site-wide password.

   For the separator character, Cisco Systems has been using the `*'



Carrel/Grant                                                   [Page 38]

DRAFT                      expires June 1998                January 1998


   (asterisk) character. After some deliberation, it was decided that it
   was the least likely character to be found in a username.

















































Carrel/Grant                                                   [Page 39]

DRAFT                      expires June 1998                January 1998


   16.  References


   [1] D. Carrel, L. Grant, "The TACACS+ API Definition"


   [2] C. Finseth, RFC 1492, "An Access Control Protocol, Sometimes
        Called TACACS", July 1993.


   [3] R. Rivest, RFC 1321, "The MD5 Message-Digest Algorithm", April
        1992.


   [4] B. Lloyd, W. Simpson, RFC 1334, "PPP Authentication Protocols",
        October 1992.


   [5] Apple Computer Corp.  AppleTalk Remote Access Protocol (ARAP)
        Version 2.0 External Reference Specification.  Preliminary docu-
        ment (no date available).


   [6] D. Eastlake, S. Crocker, J. Schiller, RFC 1750, "Randomness
        Recommendations for Security", December 1994.


   [7] S. Cobb, Microsoft PPP CHAP Extensions, Network working group
        Informational memo, Revision 1.2, March 1995.






















Carrel/Grant                                                   [Page 40]

DRAFT                      expires June 1998                January 1998


   17.  Revision History

   v1.75: first IETF submission

   v1.76: fix encrypted flag text

   v1.77: Add service 5 for Protocol translation

   v1.78: Add ms chap description










































Carrel/Grant                                                   [Page 41]

DRAFT                      expires June 1998                January 1998





                           Table of Contents



   Introduction ...................................................    1
   Technical Definitions ..........................................    2
   The TACACS+ packet header ......................................    5
   The TACACS+ packet body ........................................    7
   Body Encryption ................................................    8
   Body types .....................................................    9
   Authentication .................................................   10
   Enable Requests ................................................   17
   Inbound ASCII Login ............................................   17
   Inbound PAP Login ..............................................   17
   Inbound CHAP login .............................................   18
   Inbound MS-CHAP login ..........................................   18
   Outbound MS-CHAP request .......................................   19
   Inbound ARAP login .............................................   19
   Outbound PAP request ...........................................   20
   Outbound CHAP request ..........................................   20
   Outbound ASCII and ARAP request ................................   21
   ASCII change password request ..................................   21
   PPP change password request ....................................   21
   ARAP change password request ...................................   22
   Authorization ..................................................   23
   The authorization REQUEST packet body ..........................   24
   Table 1: Attribute-value Pairs .................................   27
   The authorization RESPONSE packet body .........................   30
   Accounting .....................................................   32
   The account REQUEST packet body ................................   32
   The accounting REPLY packet body ...............................   35
   Compatibility between Minor Versions 0 and 1 ...................   37
   Notes to Implementors ..........................................   38
   References .....................................................   40
   Revision History ...............................................   41