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Versions: (draft-dahm-opsawg-tacacs) 00 01 02 03 04 05 06 07 08 09 10 11 12 13

Operations                                                       T. Dahm
Internet-Draft                                                    A. Ota
Intended status: Informational                                Google Inc
Expires: February 21, 2017                                D. Medway Gash
                                                     Cisco Systems, Inc.
                                                               D. Carrel
                                                           vIPtela, Inc.
                                                                L. Grant
                                                         August 20, 2016


                          The TACACS+ Protocol
                      draft-ietf-opsawg-tacacs-05

Abstract

   TACACS+ provides Device Administration for routers, network access
   servers and other networked computing devices via one or more
   centralized servers.  This document describes the protocol that is
   used by TACACS+.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on February 21, 2017.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Technical Definitions . . . . . . . . . . . . . . . . . . . .   4
   3.  TACACS+ Connections and Sessions  . . . . . . . . . . . . . .   5
     3.1.  Connection  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Session . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Single Connect Mode . . . . . . . . . . . . . . . . . . .   6
     3.4.  The TACACS+ Packet Header . . . . . . . . . . . . . . . .   6
     3.5.  The TACACS+ Packet Body . . . . . . . . . . . . . . . . .   8
     3.6.  Encryption  . . . . . . . . . . . . . . . . . . . . . . .   9
     3.7.  Text Encoding . . . . . . . . . . . . . . . . . . . . . .  10
   4.  Authentication  . . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  The Authentication START Packet Body  . . . . . . . . . .  11
     4.2.  The Authentication REPLY Packet Body  . . . . . . . . . .  13
     4.3.  The Authentication CONTINUE Packet Body . . . . . . . . .  15
     4.4.  Description of Authentication Process . . . . . . . . . .  15
       4.4.1.  Version Behaviour . . . . . . . . . . . . . . . . . .  16
       4.4.2.  Common Authentication Flows . . . . . . . . . . . . .  17
       4.4.3.  Aborting an Authentication Session  . . . . . . . . .  20
   5.  Authorization . . . . . . . . . . . . . . . . . . . . . . . .  21
     5.1.  The Authorization REQUEST Packet Body . . . . . . . . . .  22
     5.2.  The Authorization RESPONSE Packet Body  . . . . . . . . .  24
   6.  Accounting  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     6.1.  The Account REQUEST Packet Body . . . . . . . . . . . . .  26
     6.2.  The Accounting REPLY Packet Body  . . . . . . . . . . . .  27
   7.  Attribute-Value Pairs . . . . . . . . . . . . . . . . . . . .  29
     7.1.  Authorization Attributes  . . . . . . . . . . . . . . . .  30
     7.2.  Accounting Attributes . . . . . . . . . . . . . . . . . .  32
   8.  Privilege Levels  . . . . . . . . . . . . . . . . . . . . . .  34
   9.  TACACS+ Security Considerations . . . . . . . . . . . . . . .  35



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   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36

1.  Introduction

   Terminal Access Controller Access-Control System Plus (TACACS+) was
   originally conceived as a general Authentication, Authorization and
   Accounting protocol.  It is primarily used today for Device
   Administration: authenticating access to network devices, providing
   central authorization of operations, and audit of those operations.

   A wide range of TACACS+ clients and servers are already deployed in
   the field.  The TACACS+ protocol they are based on is defined in a
   draft document that was originally intended for IETF publication.
   This document is known as `The Draft' [TheDraft] .

   It is intended that all implementations which conform to this
   document will conform to `The Draft'.  However, attention is drawn to
   the following specific adjustments of the protocol specification from
   'The Draft':

      This document officially removes SENDPASS for security reasons.

      The normative description of Legacy features such as ARAP and
      outbound authentication have been removed, however the required
      enumerations are kept.

   The TACACS+ protocol separates the functions of Authentication,
   Authorization and Accounting.  It allows for arbitrary length and
   content authentication exchanges, which will support 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 server 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
   useful, and together can be quite powerful.

   This document restricts itself to a description of the protocol that
   is used by TACACS+. It does not cover deployment or best practices.





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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.  However, fixed passwords have
   limitations, mainly in the area of security.  Many modern
   authentication mechanisms utilize "one-time" passwords or a
   challenge-response query.  TACACS+ is designed to support all of
   these, and be powerful enough to handle any future mechanisms.
   Authentication generally takes place when the user first logs in to a
   machine or requests a service of it.

   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 himself or herself 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.
   Examples 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+ server 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 (Section 5) .

   Accounting

   Accounting is typically the third action after authentication and
   authorization.  But again, neither authentication nor authorization



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   is 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 as an auditing tool for security services.
   It may also be used to account for services used, such as in a
   billing environment.  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
   (Section 6) .

   Client

   The client is any device, (often a Network Access Server) that
   provides access services.  The clients usually provide a character
   mode front end and then allow the user to telnet or rlogin to another
   host.  A client may also support protocol based access services.

   Server

   The server receives TACACS+ protocol requests, and replies according
   to its business model, in accordance with the flows defined in this
   document.

   Packet

   All uses of the word packet in this document refer to TACACS+
   protocol packets unless explicitly noted otherwise.

3.  TACACS+ Connections and Sessions

3.1.  Connection

   TACACS+ uses TCP for its transport.  Server port 49 is allocated for
   TACACS+ traffic.

3.2.  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.

   An accounting and authorization session will consist of a single pair
   of packets (the request and its reply).  An authentication session



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   may involve an arbitrary number of packets being exchanged.  The
   session is an operational concept that is maintained between the
   TACACS+ client and server.  It does not necessarily correspond to a
   given user or user action.

3.3.  Single Connect Mode

   The packet header (see below) contains a flag to allow sessions to be
   multiplexed on a connection.

   If a client sets this flag, this indicates that it supports
   multiplexing TACACS+ sessions over a single TCP connection.  The
   client MUST NOT send a second packet on a connection until single-
   connect status has been established.

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

   The flag is only relevant for the first two packets on a connection,
   to allow the client and server to establish single connection mode.
   The flag MUST be ignored after these two packets since the single-
   connect status of a connection, once established, must not be
   changed.  The connection must instead be closed and a new connection
   opened, if required.

   When single-connect status is established, multiple sessions MUST be
   allowed simultaneously and/or consecutively on a single TCP
   connection.  If single-connect status has not been established in the
   first two packets of a TCP connection, then the connection must be
   closed at the end of the first session.

3.4.  The TACACS+ Packet Header

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













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    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.

      TAC_PLUS_MINOR_VER_DEFAULT := 0x0

      TAC_PLUS_MINOR_VER_ONE := 0x1

   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 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+ servers 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.



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   flags

   This field contains various bitmapped flags.

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

   TAC_PLUS_UNENCRYPTED_FLAG := 0x01

   If this flag is set, then body encryption is not used.  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

   This flag is used to allow a client and server to agree whether
   multiple sessions may be multiplexed onto a single connection.

   session_id

   The Id for this TACACS+ session.  The session id is to be selected
   randomly.  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, see RFC 1750
   [RFC1750] )

   length

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

3.5.  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:

      - 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 packet MUST NOT be null
      terminated.



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

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

3.6.  Encryption

   The body of packets may be encrypted.  The following sections
   describe the encryption mechanism that is supported to enable
   backwards compatibility with 'The Draft'.

   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
   server.  This document does not discuss the management and storage of
   those keys.  It is an implementation detail of the server and client,
   as to whether they will maintain only one key, or a different key for
   each client or server with which they communicate.  For security
   reasons, the latter options MUST be available, but it is a site
   dependent 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 RFC 1321 [RFC1321]
   .

   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
   running 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 server.

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




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   The session id is used in network byte order.

   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}

   When a server detects that the secret(s) it has configured for the
   device mismatch, it MUST return ERROR.  The handling of the TCP
   connection by the server is implementation independent.

   TAC_PLUS_UNENCRYPTED_FLAG == 0x1

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

   If deployment is configured for encrypting a connection then do no
   skip decryption simply because an incoming packet indicates that it
   is not encrypted.  If the unencrypted flag is not set when expected,
   then it must be dropped.

   After a packet body is decrypted, the lengths of the component values
   in the packet are summed.  If the sum is not identical to the
   cleartext datalength value from the header, the packet MUST be
   discarded, and an error signalled.  The underlying TCP connection MAY
   also be closed, if it is not being used for other sessions in single-
   connect mode.

   Commonly such failures are seen when the keys are mismatched between
   the client and the TACACS+ server.

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

3.7.  Text Encoding

   All text fields in TACACS+ MUST be US-ASCII, excepting special
   consideration given to user field and data fields used for passwords.

   To ensure interoperability of current deployments, the TACACS+ client
   and server MUST handle user field and data fields used for passwords



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   as 8 bit octet strings.  The deployment operator MUST ensure that
   consistent character encoding is applied.  The encoding SHOULD be
   UTF-8, and other encodings outside US-ASCII SHOULD be deprecated.

4.  Authentication

4.1.  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   | authen_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_SENDAUTH := 0x04

   priv_lvl

   This indicates the privilege level that the user is authenticating
   as.  Please refer to the Privilege Level section (Section 8) below.

   authen_type

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

      TAC_PLUS_AUTHEN_TYPE_ASCII := 0x01




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      TAC_PLUS_AUTHEN_TYPE_PAP := 0x02

      TAC_PLUS_AUTHEN_TYPE_CHAP := 0x03

      TAC_PLUS_AUTHEN_TYPE_ARAP := 0x04 (deprecated)

      TAC_PLUS_AUTHEN_TYPE_MSCHAP := 0x05

      TAC_PLUS_AUTHEN_TYPE_MSCHAPV2 := 0x06

   authen_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 TAC_PLUS_AUTHEN_SVC_NONE option is intended for the authorization
   application of this field that indicates that no authentication was
   performed by the device.

   The TAC_PLUS_AUTHEN_SVC_LOGIN option is identifies regular login (as
   opposed to ENABLE) to a client device.

   The TAC_PLUS_AUTHEN_SVC_ENABLE option identifies the ENABLE
   authen_service, which refers to a service requesting authentication
   in order to grant the user different privileges.  This is comparable
   to the Unix "su(1)" command.  An authen_service value of NONE is only
   to be used when none of the other authen_service values are
   appropriate.  ENABLE may be requested independently, no requirements



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   for previous authentications or authorizations are imposed by the
   protocol.

   Other options are included for legacy/backwards compatibility.

   user, user_len

   The username is optional in this packet, depending upon the class of
   authentication.  If it is absent, user_len will be 0, if included,
   the user_len MUST indicate the length of the user field, in bytes.

   port, port_len

   The US-ASCII name of the client port on which the authentication is
   taking place, and its length in bytes.  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).
   The port_len MUST indicate the length of the port field, in bytes.

   rem_addr, rem_addr_len

   An US-ASCII string this is a "best effort" description of the remote
   location from which the user has connected to the client.  It is
   intended to hold a network address if the user is connected via a
   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
   is optional (since the information may not be available).  The
   rem_addr_len MUST indicate the length of the user field, in bytes.

   data, data_len

   This field is used to send data appropriate for the action and
   authen_type.  It is described in more detail in the section Common
   Authentication flows (Section 4.4.2) . The data_len MUST indicate the
   length of the data field, in bytes.

4.2.  The Authentication REPLY Packet Body

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











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    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

   flags

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

      TAC_PLUS_REPLY_FLAG_NOECHO := 0x01

   server_msg, server_msg_len

   c A message to be displayed to the user.  This field is optional.  If
   it exists, it is intended to be presented to the user.  US-ASCII
   charset MUST be used.  The server_msg_len MUST indicate the length of
   the server_msg field, in bytes.

   data, data_len

   This field holds data that is a part of the authentication exchange
   and is intended for the client, not the user.  It is described in
   more detail in the section Common Authentication flows



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   (Section 4.4.2) . The data_len MUST indicate the length of the data
   field, in bytes.

4.3.  The Authentication CONTINUE Packet Body

   This packet is sent from the client to the server 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, user_msg_len

   This field is the string that the user entered, or the client
   provided on behalf of the user, in response to the server_msg from a
   REPLY packet.  The user_len MUST indicate the length of the user
   field, in bytes.

   data, data_len

   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.  The data_len MUST indicate the length of the data field, in
   bytes.

   flags

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

      TAC_PLUS_CONTINUE_FLAG_ABORT := 0x01

4.4.  Description of Authentication Process

   The action, authen_type and authen_service fields (described above)
   combine to determine what kind of authentication is to be performed
   Every authentication START, REPLY and CONTINUE packet includes a data
   field.  The use of this field is dependent upon the kind of the
   Authentication.

   A set of standard kinds of authentication is defined in this
   document.  Each authentication flow consists of a START packet.  The



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   server responds either with a request for more information (GETDATA,
   GETUSER or GETPASS) or a termination (PASS or FAIL).  The actions and
   meanings when the server 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 SHOULD provide server_msg
   content for 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.

   All three cause the same action to be performed, but the use of
   TAC_PLUS_AUTHEN_STATUS_GETUSER, indicates to the client that the user
   response will be interpreted as a username, and for
   TAC_PLUS_AUTHEN_STATUS_GETPASS, that the user response represents
   will be interpreted as a password.  TAC_PLUS_AUTHEN_STATUS_GETDATA is
   the generic request for more information to flexibly support future
   requirements.  If the TAC_PLUS_REPLY_FLAG_NOECHO flag is set in the
   REPLY, then the user response must not be echoed as it is entered.
   The data field is only used in the REPLY where explicitly defined
   below.

4.4.1.  Version Behaviour

   The TACACS+ protocol is versioned to allow revisions while
   maintaining backwards compatibility.  The version number is in every
   packet header.  The changes between minor_version 0 and 1 apply only
   to the authentication process, and all deal with the way that CHAP
   and PAP authentications are handled. minor_version 1 may only be used
   for authentication kinds that explicitly call for it in the table
   below:

                LOGIN    CHPASS   SENDAUTH
   ASCII          v0         v0       -
   PAP            v1         -        v1
   CHAP           v1         -        v1
   MS-CHAPv1/2    v1         -        v1


   The '-' symbol represents that the option is not valid.

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






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   In minor_version 0, Inbound PAP performed a normal LOGIN, sending the
   username in the START packet and then waiting for a GETPASS and
   sending the password in a CONTINUE packet.

   In minor_version 1, CHAP and inbound PAP use LOGIN to perform inbound
   authentication and the exchanges use the data field so that the
   client only sends a single START packet and expects to receive a PASS
   or FAIL.  SENDAUTH is only used for PPP when performing outbound
   authentication.

   NOTE: Only those requests which have changed from their minor_version
   0 implementation (i.e.  CHAP, MS-CHAP and PAP authentications) will
   use the new minor_version number of 1.  All other requests (i.e.  all
   authorisation and accounting and ASCII authentication) MUST continue
   to use the same minor_version number of 0.  The removal of SENDPASS
   was prompted by security concerns, and is no longer considered part
   of the TACACS+ protocol.

4.4.2.  Common Authentication Flows

   This section describes common authentication flows.  If the options
   are implemented, they MUST follow the description.  If the server
   does not implement an option, it will respond with
   TAC_PLUS_AUTHEN_STATUS_ERROR.

   Inbound ASCII Login

       action = TAC_PLUS_AUTHEN_LOGIN
       authen_type = TAC_PLUS_AUTHEN_TYPE_ASCII
       minor_version = 0x0


   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, followed by a terminating REPLY (PASS or FAIL).

   Inbound PAP Login

       action = TAC_PLUS_AUTHEN_LOGIN
       authen_type = TAC_PLUS_AUTHEN_TYPE_PAP
       minor_version = 0x1


   The entire exchange MUST consist of a single START packet and a
   single REPLY.  The START packet MUST contain a username and the data
   field MUST contain the PAP ASCII password.  A PAP authentication only



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   consists of a username and password RFC 1334 [RFC1334] . The REPLY
   from the server MUST be either a PASS or FAIL.

   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
   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, 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 server will run MD5 over the id,
   the user's secret and the challenge, as defined in the PPP
   Authentication RFC RFC 1334 [RFC1334] and then compare that value
   with the response.  The REPLY from the server MUST be a PASS or FAIL.

   Inbound MS-CHAP v1 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
   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, the
   MS-CHAP challenge and the MS-CHAP 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 49 octets).

   To perform the authentication, the server will use a combination of
   MD4 and DES on the user's secret and the challenge, as defined in RFC
   2433 [RFC2433] and then compare the resulting value with the
   response.  The REPLY from the server MUST be a PASS or FAIL.

   For best practices, please refer to RFC 2433 [RFC2433]

   Inbound MS-CHAP v2 login



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       action = TAC_PLUS_AUTHEN_LOGIN
       authen_type = TAC_PLUS_AUTHEN_TYPE_MSCHAPV2
       minor_version = 0x1


   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, the
   MS-CHAP challenge and the MS-CHAP 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 49 octets).

   To perform the authentication, the server will use a the algorithm
   specified RFC2759 [RFC2759] on the user's secret and challenge and
   then compare the resulting value with the response.  The REPLY from
   the server MUST be a PASS or FAIL.

   For best practices for MS-CHAP v2, please refer to RFC2759 [RFC2759]

   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 server 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.

   In order to readily distinguish enable requests from other types of
   request, the value of the authen_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.

   ASCII change password request

   action = TAC_PLUS_AUTHEN_CHPASS
   authen_type = TAC_PLUS_AUTHEN_TYPE_ASCII


   This exchange consists of multiple messages while the server collects
   the information it requires in order to change the user's password.



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   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.

4.4.3.  Aborting an Authentication 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
   terminated 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 is terminated.  In each case, the server_msg may contain a
   message to be displayed to the user.

   When the status equals TAC_PLUS_AUTHEN_STATUS_FOLLOW the packet
   indicates that the TACACS+ server requests that authentication is
   performed with an alternate server.  The data field MUST contain
   ASCII text describing one or more servers.  A server description
   appears like this:

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

   If more than one host is specified, they MUST be separated into rows
   by an ASCII Carriage Return (0x0D).

   The protocol and key are optional, and apply only to host in the same
   row.  The protocol can describe an 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 IPV4 address specified as octets separated by dots
   ('.'), or IPV6 address test representation defined in RFC 4291.

   If a key is supplied, the client MAY use the key in order to
   authenticate to that host.  The client may use a preconfigured key
   for the host if it has one.  If not then the client may communicate
   with the host using unencrypted option.





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   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 will proceed as if that host could not be contacted.
   The data field may contain a message to be printed on an
   administrative console or log.

   If the status equals TAC_PLUS_AUTHEN_STATUS_RESTART, then the
   authentication sequence is 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.

   If a client chooses not to accept the TAC_PLUS_AUTHEN_STATUS_RESTART
   packet, then it is TREATED as if the status was
   TAC_PLUS_AUTHEN_STATUS_FAIL.

5.  Authorization

   This part of the TACACS+ protocol provides an extensible way of
   providing remote 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
   indicate how the user was authenticated or processed and a variable
   set of arguments that describe the services and options for which
   authorization is requested.

   The RESPONSE contains a variable set of response arguments
   (attribute-value pairs) that 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.



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5.1.  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

      TAC_PLUS_AUTHEN_METH_GUEST := 0x08

      TAC_PLUS_AUTHEN_METH_RADIUS := 0x10




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      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 terminal line used to gain access.
   LOCAL is a client 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 authentication provided via the R-command
   protocols from Berkeley Unix.

   priv_lvl

   This field is used in the same way as the priv_lvl field in
   authentication request and is described in the Privilege Level
   section (Section 8) below.  It indicates the users current privilege
   level.

   authen_type

   This field matches the authen_type field in the authentication
   section (Section 4) above.  It indicates the type of authentication
   that was performed.  If this information is not available, then the
   client will set authen_type to: TAC_PLUS_AUTHEN_TYPE_NOT_SET := 0x00.
   This value is valid only in authorization and accounting requests.

   authen_service

   This field matches the authen_service field in the authentication
   section (Section 4) above.  It indicates the service through which
   the user authenticated.

   user, user_len

   This field contains the user's account name.  The user_len MUST
   indicate the length of the user field, in bytes.

   port, port_len

   This field matches the port field in the authentication section
   (Section 4) above.  The port_len MUST indicate the length of the port
   field, in bytes.

   rem_addr, rem_addr_len





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   This field matches the rem_addr field in the authentication section
   (Section 4) above.  The rem_addr_len MUST indicate the length of the
   port field, in bytes.

   arg_cnt

   The number of authorization arguments to follow

   arg_1 ... arg_N, arg_1_len .... arg_N_len

   The attribute-value pair that describes the authorization to be
   performed.  (see below), and their corresponding length fields (which
   MUST indicate the length of each argument in bytes).

   The authorization arguments in both the REQUEST and the RESPONSE are
   attribute-value pairs.  The attribute and the value are in a single
   US-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.

   It is not legal for an attribute name to contain either of the
   separators.  It is legal for attribute values to contain the
   separators.

   Optional arguments are ones that may be disregarded by either client
   or server.  Mandatory arguments require that the receiving side
   understands 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 minimum is two characters (one name
   value and the separator)

5.2.  The Authorization RESPONSE Packet Body














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    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, server_msg_len

   This is an US-ASCII string that may be presented to the user.  The
   decision to present this message is client specific.  The
   server_msg_len MUST indicate the length of the server_msg field, in
   bytes.

   data, data_len

   This is an US-ASCII string that may be presented on an administrative
   display, console or log.  The decision to present this message is
   client specific.  The data_len MUST indicate the length of the data
   field, in bytes.

   arg_cnt

   The number of authorization arguments to follow.



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   arg_1 ... arg_N, arg_1_len .... arg_N_len

   The attribute-value pair that describes the authorization to be
   performed.  (see below), and their corresponding length fields (which
   MUST indicate the length of each argument in bytes).

   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 is set to
   TAC_PLUS_AUTHOR_STATUS_PASS_ADD and the arg_cnt is set to 0.

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

   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, and must be
   ignored.

6.  Accounting

6.1.  The Account REQUEST Packet Body

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












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    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_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.

   See section 12 Accounting Attribute-value Pairs for the dictionary of
   attributes relevant to accounting.

6.2.  The Accounting REPLY Packet Body

   The response to an accounting message is used to indicate that the
   accounting function on the server has completed.  The server will
   reply with success only when the record has been committed to the
   required level of security, relieving the burden on the client from
   ensuring any better form of accounting is required.




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    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, server_msg_len

   This is a US-ASCII string that may be presented to the user.  The
   decision to present this message is client specific.  The
   server_msg_len MUST indicate the length of the server_msg field, in
   bytes.

   data, data_len

   This is a US-ASCII string that may be presented on an administrative
   display, console or log.  The decision to present this message is
   client specific.  The data_len MUST indicate the length of the data
   field, in bytes.

   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
   TAC_PLUS_AUTHEN_STATUS_FOLLOW status for Authentication.

   The server 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 will 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.

   Summary of Accounting Packets



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   +----------+-------+-------+-------------+-------------------------+
   | Watchdog | Stop  | Start | Flags & 0xE | Meaning                 |
   +----------+-------+-------+-------------+-------------------------+
   |    0     |   0   |   0   |      0      | INVALID                 |
   |    0     |   0   |   1   |      2      | Start Accounting Record |
   |    0     |   1   |   0   |      4      | Stop Accounting Record  |
   |    0     |   1   |   1   |      6      | INVALID                 |
   |    1     |   0   |   0   |      8      | Watchdog, no update     |
   |    1     |   0   |   1   |      A      | Watchdog, with update   |
   |    1     |   1   |   0   |      C      | INVALID                 |
   |    1     |   1   |   1   |      E      | INVALID                 |
   +----------+-------+-------+-------------+-------------------------+

   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.

   The Server MUST respond with TAC_PLUS_ACCT_STATUS_ERROR if the client
   requests an INVALID option.

7.  Attribute-Value Pairs

   TACACS+ is intended to be an extensible protocol.  The attributes
   used in Authorization and Accounting are not fixed.  Some attributes
   are defined below for common use cases, clients MUST use these
   attributes when supporting the corresponding use cases.

   All numeric values in an attribute-value string are provided as
   decimal US-ASCII numbers, unless otherwise stated.

   All boolean attributes are encoded with values "true" or "false".

   It is recommended that hosts be specified as a numeric address so as
   to avoid any ambiguities.

   Absolute times are specified in seconds since the epoch, 12:00am Jan
   1 1970.  The timezone MUST be UTC unless a timezone attribute is
   specified.

   Attributes may be submitted with no value, in which case they consist
   of the name and the mandatory or optional separator.  For example,
   the attribute "cmd" which has no value is transmitted as a string of
   4 characters "cmd="





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7.1.  Authorization Attributes

   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", "shell", "tty-server",
   "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".

   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
   service equals "shell".  If no value is present then 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

   US-ASCII number representing a connection access list.  Used only
   when value of service is "shell"" and cmd has no value.

   inacl

   US-ASCII identifier for an interface input access list.

   outacl

   US-ASCII identifier for an interface output access list.

   zonelist

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

   addr



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   a network address

   addr-pool

   The identifier of an address pool from which the client can 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
   <routing_addr> is not specified, the resulting route is 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 not disconnect after an automatic command.  Used only
   when service=shell and cmd=NULL

   priv-lvl

   privilege level to be assigned.  Please refer to the Privilege Level
   section (Section 8) below.




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   remote_user

   remote userid (authen_method must have the value
   TAC_PLUS_AUTHEN_METH_RCMD).  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.

   remote_host

   remote host (authen_method must have the value
   TAC_PLUS_AUTHEN_METH_RCMD)

   callback-dialstring

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

   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.

7.2.  Accounting Attributes

   The following new attributes are defined for TACACS+ accounting only.
   When these attribute-value pairs are included in the argument list,
   they will precede any attribute-value pairs that are defined in the
   authorization section (Section 5) above.

   task_id

   Start and stop records for the same event MUST have matching task_id
   attribute values.  The client must not reuse a specific task_id in a
   start record until it has sent a stop record for that task_id.

   start_time

   The time the action started ().



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   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

   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.




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   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 US-ASCII string describing the status of the action.

8.  Privilege Levels

   The TACACS+ Protocol supports flexible authorization schemes through
   the extensible attributes.  One scheme is built in to the protocol:
   Privilege Levels.  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.  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

   If a client uses a different privilege level scheme, then it must map
   the privilege level to scheme above.

   Privilege Levels are applied in two ways in the TACACS+ protocol:

      - As an argument in authorization EXEC phase (when service=shell
      and cmd=NULL), where it is primarily used to set the initial
      privilege level for the EXEC session.

      - In the packet headers for Authentication, Authorization and
      Accounting.  The privilege level in the header is primarily
      significant in the Authentication phase for enable authentication
      where a different privilege level is required.

   The use of Privilege levels to determine session-based access to
   commands and resources is not mandatory for clients, but it is in
   common use so SHOULD be supported by servers.






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9.  TACACS+ Security Considerations

   The protocol described in this document has been in widespread use
   since specified in "The Draft" (1998).  However it does not meet
   modern security standards, and faces vulnerabilities with privacy and
   authenticity.

   For current deployments, it is recommended:

      To separate the TACACS+ management traffic from the regular
      network access traffic.

      To use IPsec where available.

   Because of the security issues with TACACS+, the authors intend to
   follow up this document with an enhanced specification of the
   protocol employing modern security mechanisms.

10.  References

   [TheDraft]
              Carrel, D. and L. Grant, "The TACACS+ Protocol Version
              1.78", June 1997, <https://tools.ietf.org/html/draft-
              grant-tacacs-02>.

   [RFC1321]  Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
              April 1992.

   [RFC1334]  Lloyd, B. and W. Simpson, "PPP Authentication Protocols",
              RFC 1334, DOI 10.17487/RFC1334, October 1992,
              <http://www.rfc-editor.org/info/rfc1334>.

   [RFC1750]  Eastlake 3rd, D., Crocker, S., and J. Schiller,
              "Randomness Recommendations for Security", RFC 1750,
              DOI 10.17487/RFC1750, December 1994,
              <http://www.rfc-editor.org/info/rfc1750>.

   [RFC2433]  Zorn, G. and S. Cobb, "Microsoft PPP CHAP Extensions",
              RFC 2433, DOI 10.17487/RFC2433, October 1998,
              <http://www.rfc-editor.org/info/rfc2433>.

   [RFC2759]  Zorn, G., "Microsoft PPP CHAP Extensions, Version 2",
              RFC 2759, DOI 10.17487/RFC2759, January 2000,
              <http://www.rfc-editor.org/info/rfc2759>.







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Authors' Addresses

   Thorsten Dahm
   Google Inc
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   US

   EMail: thorstendlux@google.com


   Andrej Ota
   Google Inc
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   US

   EMail: aota@google.com


   Douglas C. Medway Gash
   Cisco Systems, Inc.
   170 West Tasman Dr.
   San Jose, CA  95134
   US

   Phone: +44 0208 8244508
   EMail: dcmgash@cisco.com


   David Carrel
   vIPtela, Inc.
   1732 North First St.
   San Jose, CA  95112
   US

   EMail: dcarrel@viptela.com


   Lol Grant











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