Subject: [OPSAWG] TACACS+ Information Document Diffs Version 6-10

From: "Douglas Gash (dcmgash)" <[email-masked]>
Date: Sat, 12 May 2018 17:11:50 +0000

Dear OPSAWG,

Please find below a first attempt to run through the differences between the document version 6 (Feb 10 2017) and version 10 (April 15 2018).

The Diff was generated using the “Change Bar” option of the Document History page. (https://datatracker.ietf.org/doc/draft-ietf-opsawg-tacacs/history/)

Each highlighted difference has a short, capitalized description. The security section is removed as this is being handled in a different thread.

Please let us know:

-if clarifications are required for the changed. They are pretty terse.
-if the changes need adjustments.

There will be another upload shortly, at least for the security section. When we do the next upload we’ll provide a similar treatment.

If this treatment itself is insufficient, please let us know.

Many thanks.
 
 Operations                                                       T. Dahm
 Internet-Draft                                                    A. Ota
 Intended status: Informational                                Google Inc
|Expires: October 17, 2018                                 D. Medway Gash
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                                                      Cisco Systems, Inc.
                                                                D. Carrel
                                                            vIPtela, Inc.
                                                                 L. Grant
|                                                          April 15, 2018
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                           The TACACS+ Protocol
|                      draft-ietf-opsawg-tacacs-10
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 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 https://datatracker.ietf.org/drafts/current/.
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    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 October 17, 2018.
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 Copyright Notice
 
|   Copyright (c) 2018 IETF Trust and the persons identified as the
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    document authors.  All rights reserved.
 
    This document is subject to BCP 78 and the IETF Trust's Legal
    Provisions Relating to IETF Documents
|   (https://trustee.ietf.org/license-info) in effect on the date of
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    publication of this document.  Please review these documents
    carefully, as they describe your rights and restrictions with respect
    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  . . . . . . . . . . . . . .   4
      3.1.  Connection  . . . . . . . . . . . . . . . . . . . . . . .   4
|     3.2.  Session . . . . . . . . . . . . . . . . . . . . . . . . .   5
|     3.3.  Single Connection Mode  . . . . . . . . . . . . . . . . .   5
|     3.4.  Session Completion  . . . . . . . . . . . . . . . . . . .   6
|     3.5.  Treatment of Enumerated Protocol Values . . . . . . . . .   7
      3.6.  Text Encoding . . . . . . . . . . . . . . . . . . . . . .   7
      3.7.  Data Obfuscation  . . . . . . . . . . . . . . . . . . . .   7
      3.8.  The TACACS+ Packet Header . . . . . . . . . . . . . . . .   9
      3.9.  The TACACS+ Packet Body . . . . . . . . . . . . . . . . .  11
    4.  Authentication  . . . . . . . . . . . . . . . . . . . . . . .  11
|     4.1.  The Authentication START Packet Body  . . . . . . . . . .  12
      4.2.  The Authentication REPLY Packet Body  . . . . . . . . . .  14
|     4.3.  The Authentication CONTINUE Packet Body . . . . . . . . .  16
      4.4.  Description of Authentication Process . . . . . . . . . .  16
        4.4.1.  Version Behaviour . . . . . . . . . . . . . . . . . .  17
|       4.4.2.  Common Authentication Flows . . . . . . . . . . . . .  18
        4.4.3.  Aborting an Authentication Session  . . . . . . . . .  21
    5.  Authorization . . . . . . . . . . . . . . . . . . . . . . . .  22
|     5.1.  The Authorization REQUEST Packet Body . . . . . . . . . .  22
      5.2.  The Authorization REPLY Packet Body . . . . . . . . . . .  26
    6.  Accounting  . . . . . . . . . . . . . . . . . . . . . . . . .  27
      6.1.  The Account REQUEST Packet Body . . . . . . . . . . . . .  28
      6.2.  The Accounting REPLY Packet Body  . . . . . . . . . . . .  29
    7.  Attribute-Value Pairs . . . . . . . . . . . . . . . . . . . .  30
|     7.1.  Value Encoding  . . . . . . . . . . . . . . . . . . . . .  31
|     7.2.  Authorization Attributes  . . . . . . . . . . . . . . . .  31
|     7.3.  Accounting Attributes . . . . . . . . . . . . . . . . . .  34
    8.  Privilege Levels  . . . . . . . . . . . . . . . . . . . . . .  35
    9.  TACACS+ Security Considerations . . . . . . . . . . . . . . .  36
|     9.1.  General Security of The Protocol  . . . . . . . . . . . .  36
|     9.2.  Security of Authentication Sessions . . . . . . . . . . .  38
|     9.3.  Security of Authorization Sessions  . . . . . . . . . . .  38
|     9.4.  Security of Accounting Sessions . . . . . . . . . . . . .  39
|     9.5.  TACACS+ Client Implementation Recommendations . . . . . .  39
|     9.6.  TACACS+ Server Implementation Recommendations . . . . . .  39
|     9.7.  TACACS+ Deployment Best Practices . . . . . . . . . . . .  40
|   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  41
|   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  41
|   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  42
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 1.  Introduction
 
    Terminal Access Controller Access-Control System Plus (TACACS+) was
|   conceived initially as a general Authentication, Authorization and
LANGUAGE CHANGED

    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 has been removed, however, the required
LANGUAGE CHANGED
       enumerations are kept.
 
|      The Support for forwarding to an alternative daemon
|      (TAC_PLUS_AUTHEN_STATUS_FOLLOW) has been deprecated.
ADDED
|
    The TACACS+ protocol separates the functions of Authentication,
    Authorization and Accounting.  It allows for arbitrary length and
|   content authentication exchanges, to support future authentication
|   mechanisms.  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.
LANGUAGE CHANGED, CLARIFICATION
 
|   The separation of authentication, authorization and accounting was a
|   key element of the design of TACACS+ protocol.  Essentially it makes
|   TACACS+ a suite of three protocols.  This document will address each
|   one in separate sections.  Although TACACS+ defines all three, but an
|   implementation or configuration is not required to employ all three.
|   Separating the elements is useful for Device Administration use case,
|   specifically, for authorization of individual commands in a session.
|   Note that there is no provision made at the protocol level for
|   association of an authentication to each authorization request.
LANGUAGE CHANGED, CLARIFICATIONS, TYPOS CORRECTED

    This document restricts itself to a description of the protocol that
    is used by TACACS+. It does not cover deployment or best practices.
 
 2.  Technical Definitions
 
    This section provides a few basic definitions that are applicable to
    this document
 
    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.
DE-EMPHASISED NETWORK ACCESS USE CASE.
 
    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
    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 Connection Mode
 
    Single Connection Mode is intended to improve performance by allowing
    a client to multiplex multiple session on a single TCP connection.
 
    The packet header contains the TAC_PLUS_SINGLE_CONNECT_FLAG used by
    the client and server to negotiate the use of Single Connect Mode.
 
    The client sets this flag, to indicate 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.
 
|   To indicate it will support Single Connection Mode, the server sets
|   this flag in the first reply packet in response to the first request
|   from a client.  The server may set this flag even if the client does
|   not set it, but the client may ignore the flag and close the
|   connection after the session completes.
LANGUAGE CHANGE FOR CONSISTENCY
 
    The flag is only relevant for the first two packets on a connection,
|   to allow the client and server to establish Single Connection Mode.
|   No provision is made for changing Single Connection Mode after the
|   first two packets: the client and server MUST ignore the flag after
|   the second packet on a connection.
TYPOS, ADDED DETEIL ON FLAGS IN SUBSEQUENT PACKETS 
 
|   If single Connection Mode has not been established in the first two
    packets of a TCP connection, then both the client and the server
    close the connection at the end of the first session.
LANGUAGE CHANGE FOR CONSISTENCY
 
|   The client negotiates Single Connection Mode to improve efficiency.
|   The server may refuse to allow Single Connection Mode for the client.
|   For example, it may not be appropriate to allocate a long-lasting TCP
|   connection to a specific client in some deployments.  Even if the
|   server is configured to permit single Connection Mode for a specific
|   client, the server may close the connection.  For example: a server
|   may be configured to time out a Single Connection Mode TCP Connection
|   after a specific period of inactivity to preserve its resources.  The
    client MUST accommodate such closures on a TCP session even after
|   Single Connection Mode has been established.
LANGUAGE CHANGE FOR CONSISTENCY, TYPOS
 
 3.4.  Session Completion
 
    The REPLY packets defined for the packets types in the sections below
    (Authentication, Authorization and Accounting) contain a status
    field.  The complete set of options for this field depend upon the
    packet type, but all three REPLY packet types define values
    representing PASS, ERROR and FAIL, which indicate the last packet of
    a regular session (one which is not aborted).
 
    The server responds with a PASS or a FAIL to indicate that the
    processing of the request completed and the client can apply the
    result (PASS or FAIL) to control the execution of the action which
    prompted the request to be sent to the server.
 
    The server responds with an ERROR to indicate that the processing of
    the request did not complete.  The client can not apply the result
    and it MUST behave as if the server could not be connected to.  For
|   example, the client tries alternative methods, if they are available,
GRAMMAR TYPO
    such as sending the request to a backup server, or using local
    configuration to determine whether the action which prompted the
    request should be executed.
 
    Refer to the section (Section 4.4.3) on Aborting Authentication
    Sessions for details on handling additional status options .
 
    When the session is complete, then the TCP connection should be
|   handled as follows, according to whether Single Connection Mode was
LANGUAGE CHANGE FOR CONSISTENCY
    negotiated:
 
    If Single Connection Mode was not negotiated, then the connection
    should be closed
 
    If Single Connection Mode was enabled, then the connection SHOULD be
    left open (see section (Section 3.3) ), but may still be closed after
    a timeout period to preserve deployment resources
 
    If Single Connection Mode was enabled, but an ERROR occurred due to
    connection issues (such as an incorrect secret, see section
    (Section 3.7) ), then any further new sessions MUST NOT be accepted
    on the connection.  If there are any sessions that have already been
    established then they MAY be completed.  Once all active sessions are
    completed then the connection MUST be closed.
 
|   It is recommended that client implementations provide robust schemes
|   for dealing with servers which cannot be connected to.  Options
|   include providing a list of servers for redundancy, and an option for
|   a local fallback configuration if no servers can be reached.  Details
|   will be implementation specific.
|
|   The client should manage connections and handle the case of a server
|   which establishes a connection, but does not respond.  The exact
|   behavior is implementation specific.  It is recommended that the
|   client should close the connection after a configurable timeout.

ADDED: CLARIFIED DETAILS FOR BEHAVIOUR

|
 3.5.  Treatment of Enumerated Protocol Values
 
    This document describes various enumerated values in the packet
|   header and the headers for specific packet types.  For example in the
TYPO
    Authentication start packet type, this document defines the action
    field with three values TAC_PLUS_AUTHEN_LOGIN, TAC_PLUS_AUTHEN_CHPASS
    and TAC_PLUS_AUTHEN_SENDAUTH.
 
    If the server does not implement one of the defined options in a
    packet that it receives, or it encounters an option that is not
    listed in this document for a header field, then it should respond
    with a ERROR and terminate the session.  This will allow the client
    to try a different option.
 
    If an error occurs 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.
 
 3.6.  Text Encoding
 
|   All text fields in TACACS+ MUST be printable US-ASCII, excepting
|   special consideration given to user field and data fields used for
|   passwords.
CLARIFIED PRINTABLE

    To ensure interoperability of current deployments, the TACACS+ client
    and server MUST handle user fields and those data fields used for
|   passwords as 8-bit octet strings.  The deployment operator MUST
|   ensure that consistent character encoding is applied from the end
|   client to the server.  The encoding SHOULD be UTF-8, and other
|   encodings outside printable US-ASCII SHOULD be deprecated.
CHANGED LANGUAGE, CLARIFICATION ON PRINTABLE
 
 3.7.  Data Obfuscation
 
    The body of packets may be obfuscated.  The following sections
|   describe the obfuscation method that is supported in the protocol.
    In 'The Draft' this process was actually referred to as Encryption,
|   but the algorithm would not meet modern standards, and so will not be
|   termed as encryption in this document.
TYPOS, LANGUAGE CHANGE
 
|   The obfuscation mechanism relies on a secret key, 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, other than
|   to require that the secret keys MUST remain secret.
REMOVED AMBIGUITY ON UNIQUE SECRETS, MOVED TO NEXT PARA

|
|   Server implementations MUST allow a unique secret key to be
|   associated with every client.  It is a site-dependent decision as to
|   whether the use of separate keys is appropriate.
ADDED CLARIFICATION
 
    The flag field may be set as follows:
 
|   TAC_PLUS_UNENCRYPTED_FLAG = 0x0
TYPO
 
    In this case, the packet body is obfuscated by XOR-ing it byte-wise
|   with a pseudo-random pad.
CLARIFICATION
|
|   ENCRYPTED {data} = data ^ pseudo_pad
CLARIFICATION
|
|   The packet body can then be de-obfuscated by XOR-ing it byte-wise
    with a pseudo random pad.
 
|   data = ENCRYPTED {data} ^ pseudo_pad
|
TYPO (== -> =) CLARIFICATION

    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 and session_id are used as extracted from the
|   header
CLARIFICATION
 
    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.  For details of TCP connection
|   handling on ERROR, refer to section (Section 3.4)
REFER TO NEW SECTION ON SESSION TERMINATION
 
    TAC_PLUS_UNENCRYPTED_FLAG == 0x1
 
    In this case, the entire packet body is in cleartext.  Obfuscation
    and de-obfuscation are null operations.  This method should be
    avoided unless absolutely required for debug purposes, when tooling
    does not permit de-obfuscation.
 
|   If deployment is configured for obfuscating a connection then the
|   request MUST be dropped if TAC_PLUS_UNENCRYPTED_FLAG is set to true.
REMOVED AMBIGUOUS BEHAVIOR
 
    After a packet body is de-obfuscated, 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 signaled.  For details of TCP connection
|   handling on ERROR, refer to section (Section 3.4)
REFER TO NEW SECTION ON SESSION TERMINATION
 
    Commonly such failures are seen when the keys are mismatched between
    the client and the TACACS+ server.
 
 3.8.  The TACACS+ Packet Header
 
|   All TACACS+ packets begin with the following 12-byte header.  The
TYPO
    header 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.
 
       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.  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.
 
    flags
 
    This field contains various bitmapped flags.
 
    The flag bit:
 
    TAC_PLUS_UNENCRYPTED_FLAG := 0x01
 
|   This flag indicates that the sender did not obfuscate the body of the
TYPO
    packet.  The application of this flag will be covered in the security
|   section (Section 9) .
TYPO
 
    This flag SHOULD be clear in all deployments.  Modern network traffic
|   tools support encrypted traffic when configured with the shared
|   secret (see section below), so obfuscated mode can and SHOULD be used
|   even during test.
LANGUAGE CHANGE FOR CLARIFICATION
 
    The single-connection flag:
 
    TAC_PLUS_SINGLE_CONNECT_FLAG := 0x04
 
    This flag is used to allow a client and server to negotiate Single
    Connection Mode.
 
    session_id
 
    The Id for this TACACS+ session.  This field does not change for the
    duration of the TACACS+ session.  This number MUST be generated by a
    cryptographically strong random number generation method.  Failure to
    do so will compromise security of the session.  For more details
    refer to RFC 1750 [RFC1750]

    length
 
    The total length of the packet body (not including the header).
 
 3.9.  The TACACS+ Packet Body
 
    The TACACS+ body types are defined in the packet header.  The next
    sections of this document will address the contents of the different
    TACACS+ bodies.  The following general rules apply to all TACACS+
    body types:
 
       - To signal that any variable length data fields are unused, their
|      length value is set to zero.  Such fields MUST be ignored, and
|      treated as if not present.
ADDED CLARIFICATION ON BEHAVIOUR
 
       - the lengths of data and message fields in a packet are specified
       by their corresponding length fields, (and are not null
       terminated.)
 
       - All length values are unsigned and in network byte order.
 
 4.  Authentication
 
    Authentication is the action of determining who a user (or entity)
    is.  Authentication can take many forms.  Traditional authentication
|   employs a name and a fixed password.  However, fixed passwords are
|   vulnerable security, so many modern authentication mechanisms utilize
|   "one-time" passwords or a challenge-response query.  TACACS+ is
|   designed to support all of these, and be flexible 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.
MINOR LANGUAGE CHANGES
 
    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.
 
 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 indicates the authentication action.  Legal values are listed
    below.
 
       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.  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 (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 indicates regular login (as
MINOR LANGUAGE CHANGE
    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, which substitutes the current user's
|   identity with another.  An authen_service value of NONE is only to be
|   used when none of the other authen_service values are appropriate.
CLARIFICATION ON SU

|   ENABLE may be requested independently, no requirements for previous
|   authentications or authorizations are imposed by the protocol.
FURTHER BEHAVIOR DETAIL
 
    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, the client MUST set user_len to 0.
    If included, the user_len indicates the length of the user field, in
    bytes.
 
    port, port_len
 
|   The printable 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 indicates the length of the port field, in
|   bytes.
CLARIFICATION ON PRINTABLE ASCII
 
    rem_addr, rem_addr_len
 
|   A printable US-ASCII string indicating 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
|   indicates the length of the user field, in bytes.
CLARIFICATION ON PRINTABLE ASCII
 
    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 indicates 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.
 
     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
 
|   A message to be displayed to the user.  This field is optional.  The
|   printable US-ASCII charset MUST be used.  The server_msg_len
|   indicates the length of the server_msg field, in bytes.
CLARIFICATION ON PRINTABLE
 
    data, data_len
 
    This field holds data that is a part of the authentication exchange
    and is intended for the client, not the user.  Examples of its use
    are shown in the section Common Authentication flows (Section 4.4.2)
    . The data_len indicates 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 indicates 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 indicates 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 indicate 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.
 
|   This document defines a core set of authentication flows to be
LANGUAGE CLARIFICATION
    supported by TACACS+.  Each authentication flow consists of a START
    packet.  The server responds either with a request for more
    information (GETDATA, GETUSER or GETPASS) or a termination PASS,
|   FAIL, ERROR or RESTART.  The actions and meanings when the server
|   sends a RESTART or ERROR are common and are described further below.
REMOVE FOLLOW
 
    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 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.
 
    The client should interpret TAC_PLUS_AUTHEN_STATUS_GETUSER as a
    request for username and TAC_PLUS_AUTHEN_STATUS_GETPASS as a request
    for password.  The TAC_PLUS_AUTHEN_STATUS_GETDATA is the generic
    request for more information to flexibly support future requirements.
 
    If the information being requested by the server form the client is
    sensitive, then the server should set the TAC_PLUS_REPLY_FLAG_NOECHO
    flag.  When the client queries the user for the information, the
    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.
 
    All authorisation and accounting and ASCII authentication use
    minor_version number of 0.
 
    PAP, CHAP and MS-CHAP login use minor_version 1.  The normal exchange
    is a single START packet from the client and a single REPLY from the
    server.
 
    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 server
    does not implement an option, it MUST respond with
    TAC_PLUS_AUTHEN_STATUS_FAIL.
 
|4.4.2.1.  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.  If the user does not include the username then
    the server MUST obtain it from the client with a CONTINUE
|   TAC_PLUS_AUTHEN_STATUS_GETUSER.  If the user does not provide a
|   username then the server can send another
|   TAC_PLUS_AUTHEN_STATUS_GETUSER request, but the server MUST limit the
|   number of retries that are permitted, recommended limit is three
|   attempts.  When the server has the username, it will obtain the
|   password using a continue with TAC_PLUS_AUTHEN_STATUS_GETPASS.  ASCII
|   login uses the user_msg field for both the username and password.
|   The data fields in both the START and CONTINUE packets are not used
|   for ASCII logins, any content MUST be ignored.  The session is
|   composed of a single START followed by zero or more pairs of REPLYs
|   and CONTINUEs, followed by a final REPLY indicating PASS, FAIL or
|   ERROR.
CLARIFICATION ON ABSENCE OF USERNAME
 
|4.4.2.2.  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
    consists of a username and password RFC 1334 [RFC1334] . The REPLY
    from the server MUST be either a PASS, FAIL or ERROR.
 
|4.4.2.3.  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 is 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 calculates the PPP hash as
    defined in the PPP Authentication RFC RFC 1334 [RFC1334] and then
|   compare that value with the response.  The MD5 algorithm option is
|   always used.  The REPLY from the server MUST be a PASS, FAIL or
|   ERROR.
CLARIFICATION ON MD5
 
|   In cases where the client conducts the exchange with the endstation
|   and then sends the resulting materials (challenge and response) to
|   the server, the selection of the challenge and its length are not an
|   aspect of the TACACS+ protocol.  However, it is strongly recommended
|   that the client/endstation interaction is configured with a secure
|   challenge.  The TACACS+ server can help by rejecting authentications
|   where the challenge is below a minimum length (Minimum recommended is
|   8 bytes).
LANGUAGE CLARIFICATIONS
 
|   In cases where the TACACS+ Server generates the challenge then it
|   MUST change for every request and MUST be derived from a strong
|   cryptographic source.
ADDED PARA FOR CLARIFICATION

|4.4.2.4.  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] . The TACACS+
|   server MUST reject authentications where the challenge deviates from
|   8 bytes as defined in the RFC.
ADDED CLARIFICATION FOR REJECT
 
|4.4.2.5.  MS-CHAP v2 login
 
        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 the algorithm
    specified RFC 2759 [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]
|   . The TACACS+ server MUST rejects authentications where the challenge
|   deviates from 16 bytes as defined in the RFC.
ADDED CLARIFICATION FOR REJECT
 
|4.4.2.6.  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 user.  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 ASCII login (Section 4.4.2.1) .
ADDED REFERENCE
 
    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.
 
|4.4.2.7.  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.
    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.  This information will
    be handled by the server according to the requirements of the
    deployment.  The session is terminated, for more details about
|   session termination, refer to section (Section 3.4)
LANGUAGE CLEANUP
 
    In the case of PALL, FAIL or ERROR, the server can insert a message
    into server_msg to be displayed to the user.
 
    The Draft `The Draft' [TheDraft] defined a mechanism to direct
    authentication requests to an alternative server.  This mechanism is
|   regarded as insecure, is deprecated, and not covered here.  The
|   client should treat TAC_PLUS_AUTHEN_STATUS_FOLLOW as
|   TAC_PLUS_AUTHEN_STATUS_FAIL
REMOVED WHOLE SECTION ON FOLLOW.
 
    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, with new session Id, and seq_no set to 1.  This REPLY packet
    indicates that the current authen_type value (as specified in the
    START packet) is not acceptable for this session.  The client may try
    an alternative authen_type.
 
    If a client does not implement TAC_PLUS_AUTHEN_STATUS_RESTART option,
    then it MUST process the response as if the status was
    TAC_PLUS_AUTHEN_STATUS_FAIL.
 
 5.  Authorization
 
    In the TACACS+ Protocol, authorization is the action of determining
    what a user is allowed to do.  Generally authentication precedes
    authorization, though it is not mandatory that a client use the same
    service for authentication that it will use for authorization.  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 server to
    determine, according to its configuration, if an unauthenticated user
    is allowed the services in question.
 
    Authorization does not merely provide yes or no answers, but it may
    also customize the service for the particular user.  A common use of
    authorization is to provision a shell session when a user first logs
    in to a device to administer it.  The TACACS+ server might respond to
    the request by allowing the service, but placing a time restriction
    on the login shell.  For a list of common attributes used in
|   authorization, see the Authorization Attributes section (Section 7.2)
AUTO CHANGE
    .
 
    In the TACACS+ protocol an authorization is always a single pair of
    messages: a REQUEST from the client followed by a REPLY from the
    server.
 
    The authorization REQUEST message contains a fixed set of fields that
    indicate how the user was authenticated and a variable set of
    arguments that describe the services and options for which
    authorization is requested.
 
    The REPLY contains a variable set of response arguments (attribute-
|   value pairs) that can restrict or modify the client's actions.
LANGUAGE TYPO
 
 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.  As this information is not always
|   subject to verification, it is recommended that this field is
|   ignored.
ADDED CLARIFICTION
 
       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
       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 corresponds to 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 is the same as the authen_service field in the
|   authentication section (Section 4) above.  It indicates the service
|   through which the user authenticated.
DISAMBIGUATE
 
    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 indicates the length of the port
    field, in bytes.
 
    rem_addr, rem_addr_len
    This field matches the rem_addr field in the authentication section
    (Section 4) above.  The rem_addr_len indicates 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 arguments are the primary elements of the authorization
    interaction.  In the request packet they describe the specifics of
    the authorization that is being requested.  Each argument is encoded
    in the packet as a single arg filed (arg_1...  arg_N) with a
    corresponding length fields (which indicates the length of each
    argument in bytes).
 
    The authorization arguments in both the REQUEST and the REPLY are
    attribute-value pairs.  The attribute and the value are in a single
|   printable US-ASCII string and are separated by either a "=" (0X3D) or
|   a "*" (0X2A).  The equals sign indicates a mandatory argument.  The
CLARIFY PRINTABLE
    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.  This means that the arguments must be parsed until the
|   first separator is encountered, all characters in the argument, after
|   this separator, are interpreted as the argument value.
CLARIFIED BEHAVIOUR
 
    Optional arguments are ones that may be disregarded by either client
    or server.  Mandatory arguments require that the receiving side can
    handle the attribute, that is: its implementation and configuration
    includes the details of how to act on it.  If the client receives a
    mandatory argument that it cannot handle, it MUST consider the
    authorization to have failed.  It is legal to send an attribute-value
    pair with a 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-
AUTO FORMAT CHANGE
    value character and the separator)
 
    Though the attributes allow extensibility, a common core set of
    authorization attributes SHOULD be supported by clients and servers,
|   these are listed in the Authorization Attributes (Section 7.2)
AUTOGENERATED CHANGE
    section below.
 
 5.2.  The Authorization REPLY 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, server_msg_len
 
|   This is a printable US-ASCII string that may be presented to the
|   user.  The server_msg_len indicates the length of the server_msg
|   field, in bytes.
CLARIFIED PRINTABLE
 
    data, data_len
 
|   This is a printable 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 indicates the length of the
|   data field, in bytes.
CLARIFIED PRINTABLE
 
    arg_cnt
    The number of authorization arguments to follow.
 
    arg_1 ... arg_N, arg_1_len .... arg_N_len
 
    The arguments describe the specifics of the authorization that is
    being requested.  For details of the content of the args, refer to:
|   Authorization Attributes (Section 7.2) section below.  Each argument
AUTOGENERATED SECTION
    is encoded in the packet as a single arg field (arg_1... arg_N) with
    a corresponding length fields (which indicates the length of each
    argument in bytes).
 
    If the status equals TAC_PLUS_AUTHOR_STATUS_FAIL, then the requested
    authorization MUST be denied.
 
    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 MUST be applied according to the rules described
    above.
 
    If the status equals TAC_PLUS_AUTHOR_STATUS_PASS_REPL then the client
    MUST use the authorization attribute-value pairs (if any) in the
    response, instead of the authorization attribute-value pairs from the
    request.
 
    To approve the authorization with no modifications, the server sets
    the status to TAC_PLUS_AUTHOR_STATUS_PASS_ADD and the arg_cnt to 0.
 
    A status of TAC_PLUS_AUTHOR_STATUS_ERROR indicates an error occurred
    on the server.  For the differences between ERROR and FAIL, refer to
    section Session Completion (Section 3.4) . None of the arg values
    have any relevance if an ERROR is set, and must be ignored.
 
    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.
 
 6.  Accounting
 
    Accounting is typically the third action after authentication and
    authorization.  But again, neither authentication nor authorization
    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) .
 
 6.1.  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_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.  They provide details for
    the conditions on the client, and authentication context, so that
    these details may be logged for accounting purposes.
 
    See section 12 Accounting Attribute-value Pairs for the dictionary of
    attributes relevant to accounting.
 
 6.2.  The Accounting REPLY Packet Body
 
    The purpose of accounting is to record the action that has occurred
    on the client.  The server MUST reply with success only when the
    accounting request has been recorded.  If the server did not record
    the accounting request then it MUST reply with ERROR.
 
     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 printable US-ASCII string that may be presented to the
|   user.  The server_msg_len indicates the length of the server_msg
|   field, in bytes.
CLARIFIED PRINTABLE
 
    data, data_len
 
|   This is a printable 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 indicates the length of the
|   data field, in bytes.
CLARIFIED PRINTABLE
 
    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.
 
    TACACS+ accounting is intended to record various types of events on
    clients, for example: login sessions, command entry, and others as
    required by the client implementation.  These events are collectively
    referred to in `The Draft' [TheDraft] as "tasks".
 
    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.
 
    Summary of Accounting Packets
 
    +----------+-------+-------+-------------+-------------------------+
    | 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.
|
|   The WATCHDOG flag is used by the client to communicate ongoing status
|   of a long-running task.  Update records are sent at the client's
|   discretion.  The frequency of the update depends upon the intended
|   application: A watchdog to provide progress indication will require
|   higher frequency than a daily keep-alive.  When the WATCHDOG flag is
|   set along with the START flag, it indicates that the update record
|   provides additional or updated arguments from the original START
|   record.  If the START flag is not set, then this indicates only that
|   task is still running, and no new information is provided (servers
|   MUST ignore any arguments).  The STOP flag MUST NOT be set in
|   conjunction with the WATCHDOG flag.
SECTION REWRITE TO CLARIFY WATCHDOG BEHAVIOUR 

    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 limited by this
TYPO
    document.  Some attributes are defined below for common use cases,
    clients MUST use these attributes when supporting the corresponding
    use cases.
 
|7.1.  Value Encoding
NEW SECTION ADDED 
|
|   All attribute values are encoded as printable US-ASCII strings.  The
|   following type representations SHOULD be followed
|
|   Numeric
|
    All numeric values in an attribute-value string are provided as
|   decimal printable US-ASCII numbers, unless otherwise stated.
 
|   Boolean
|
|   All boolean attributes are encoded as printable US-ASCII with values
|   "true" or "false".
|
|   IP-Address
 
    It is recommended that hosts be specified as a IP address so as to
|   avoid any ambiguities.  IPV4 address are specified as US-ASCII octet
|   numerics separated by dots ('.'), IPV6 address text representation
    defined in RFC 4291.
 
|   Date Time
|
|   Absolute date/times are specified in seconds since the epoch, 12:00am
|   Jan 1 1970.  The timezone MUST be UTC unless a timezone attribute is
|   specified.  Stardate is canonically inconsistent and so SHOULD NOT be
|   used.
|
|   String
|
|   Many values have no specific type representation and so are
|   interpreted as plain strings.
|
|   Empty Values
 
    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
    four characters "cmd="
 
|7.2.  Authorization Attributes
 
|   service (String)
TYPE INFORMATION ADDED
    The primary service.  Specifying a service attribute indicates that
    this is a request for authorization or accounting of that service.
|
TYPE INFORMATION ADDED
    For example: "shell", "tty-server", "connection", "system" and
    "firewall".  This attribute MUST always be included.
 
|   protocol (String)
TYPE INFORMATION ADDED
|   the protocol field may be used to indicate a subset of a service.
TYPO
 
|   cmd (String)
TYPE INFORMATION ADDED
    a shell (exec) command.  This indicates the command name of the
    command that is to be run.  The "cmd" attribute MUST be specified if
    service equals "shell".
 
    Authorization of shell commands is a common use-case for the TACACS+
    protocol.  Command Authorization generally takes one of two forms:
    session-based and command-based.
 
    For session-based shell authorization, the "cmd" argument will have
    an empty value.  The client determines which commands are allowed in
    a session according to the arguments present in the authorization.
 
    In command-based authorization, the client requests that the server
    determine whether a command is allowed by making an authorization
    request for each command.  The "cmd" argument will have the command
    name as its value.
 
|   cmd-arg (String)
TYPE INFORMATION ADDED
    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 (Numeric)
TYPE INFORMATION ADDED
|   printable US-ASCII number representing a connection access list.
|   Applicable only to session-based shell authorization.
CLARIFIED ON PRINTABLE
 
|   inacl (String)
TYPE INFORMATION ADDED
|   printable US-ASCII identifier for an interface input access list.
 
|   outacl (String)
TYPE INFORMATION ADDED 
|   printable US-ASCII identifier for an interface output access list.
CLARIFIED ON PRINTABLE
|   addr (IP-Address)
TYPE INFORMATION ADDED 
    a network address
|   addr-pool (String)
TYPE INFORMATION ADDED 
    The identifier of an address pool from which the client can assign an
    address.
 
|   routing (Boolean)
TYPE INFORMATION ADDED 
|   Specifies whether routing information is to be propagated to, and
|   accepted from this interface.
 
|   route (String)
TYPE INFORMATION ADDED 
    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 (Numeric)
TYPE INFORMATION ADDED
    an absolute timer for the connection (in minutes).  A value of zero
    indicates no timeout.
 
|   idletime (Numeric)
TYPE INFORMATION ADDED
    an idle-timeout for the connection (in minutes).  A value of zero
    indicates no timeout.
 
|   autocmd (String)
TYPE INFORMATION ADDED
    an auto-command to run.  Applicable only to session-based shell
    authorization.
 
|   noescape (Boolean)
TYPE INFORMATION ADDED 
|   Prevents user from using an escape character.  Applicable only to
|   session-based shell authorization.
 
|   nohangup (Boolean)
TYPE INFORMATION ADDED
    Boolean.  Do not disconnect after an automatic command.  Applicable
|   only to session-based shell authorization.
 
|   priv-lvl (Numeric)
TYPE INFORMATION ADDED 
    privilege level to be assigned.  Please refer to the Privilege Level
    section (Section 8) below.
 
|   remote_user (String)
TYPE INFORMATION ADDED
    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 (String)
TYPE INFORMATION ADDED
    remote host (authen_method must have the value
    TAC_PLUS_AUTHEN_METH_RCMD)
 
|7.3.  Accounting Attributes
TYPE INFORMATION ADDED
    The following attributes are defined for TACACS+ accounting only.
    They MUST precede any attribute-value pairs that are defined in the
    authorization section (Section 5) above.
 
|   task_id (String)
TYPE INFORMATION ADDED
    Start and stop records for the same event MUST have matching task_id
    attribute values.  The client MUST ensure that active task_ids are
    not duplicated: a client MUST NOT reuse a task_id a start record
    until it has sent a stop record for that task_id.  Servers MUST not
    make assumptions about the format of a task_id.
 
|   start_time (Date Time)
TYPE INFORMATION ADDED
    The time the action started (in seconds since the epoch.).
 
|   stop_time (Date Time)
TYPE INFORMATION ADDED
    The time the action stopped (in seconds since the epoch.)
 
|   elapsed_time (Numeric)
TYPE INFORMATION ADDED
    The elapsed time in seconds for the action.
 
|   timezone (String)
TYPE INFORMATION ADDED
    The timezone abbreviation for all timestamps included in this packet.
 
|   event (String)
TYPE INFORMATION ADDED
    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
TYPO
    whether the service started or stopped.
 
|   reason (String)
TYPE INFORMATION ADDED
    Accompanies an event attribute.  It describes why the event occurred.
 
|   bytes (Numeric)
TYPE INFORMATION ADDED
    The number of bytes transferred by this action
 
|   bytes_in (Numeric)
TYPE INFORMATION ADDED
|   The number of bytes transferred by this action from the endstation to
|   the client port
DIRECTION CLARIFICATION

|   bytes_out (Numeric)
TYPE INFORMATION ADDED
|   The number of bytes transferred by this action from the client to the
|   endstation port
DIRECTION CLARIFICATION

|   paks (Numeric)
TYPE INFORMATION ADDED
    The number of packets transferred by this action.
 
|   paks_in (Numeric)
TYPE INFORMATION ADDED 
|   The number of input packets transferred by this action from the
|   endstation to the client port.
DIRECTION CLARIFICATION
|   paks_out (Numeric)
TYPE INFORMATION ADDED
    The number of output packets transferred by this action from the
|   client port to the endstation.
DIRECTION CLARIFICATION
|   err_msg (String)
TYPE INFORMATION ADDED
|   A printable US-ASCII string describing the status of the action.
CLARIFICATION ON PRINTABLE
 
 8.  Privilege Levels
 
    The TACACS+ Protocol supports flexible authorization schemes through
    the extensible attributes.
 
    One scheme is built in to the protocol and has been extensively used
    for Session-based shell authorization: Privilege Levels.  Privilege
    Levels are ordered values from 0 to 15 with each level being a
    superset of the next lower value.  Configuration and implementation
|   of the client will map actions (such as the permission to execute of
TYPO
    specific commands) to different privilege levels.  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
 
    A Privilege level can be assigned to a shell (EXEC) session when it
|   starts (for example, TAC_PLUS_PRIV_LVL_USER).  The client will permit
|   the actions associated with this level to be executed.  This
TYPO (DUPLICATION)

    privilege level is returned by the Server in a session-based shell
    authorization (when "service" equals "shell" and "cmd" is empty).
|   When a user required to perform actions that are mapped to a higher
|   privilege level, then an ENABLE type reauthentication can be
|   initiated by the client.  The client will insert the required
|   privilege level into the authentication header for enable
|   authentication request.
TYPO, CLARIFACTION
 
    The use of Privilege levels to determine session-based access to
    commands and resources is not mandatory for clients.  Although the
    privilege level scheme is widely supported, its lack of flexibility
    in requiring a single monotonic hierarchy of permissions means that
    other session-based command authorization schemes have evolved, and
    so it is no longer mandatory for clients to use it.  However, it is
    still common enough that it SHOULD be supported by servers.
 
 9.  TACACS+ Security Considerations

<SNIP... SECURITY SECTION ON OTHERT THREAD>

|10.  Acknowledgements
 
|   The authors would like to thank the following reviewers whose
|   comments and contributions made considerable improvements to the
|   document: Alan DeKok, Alexander Clouter, Chris Janicki, Tom Petch,
|   Robert Drake, among many others.
|
|   The authors would particularly like to thank Alan DeKok, who provided
|   significant insights and recommendations on all aspects of the
|   document and the protocol.  Alan DeKok has dedicated considerable
|   effort to identify weaknesses and provide remedies to help improve
|   the document.
|
|   The authors would also like to thanks the support from the OPSAWG
|   Chairs and advisors.
|
CLARIFIED ACKNOWLEDGEMENTS

|11.  References
 
    [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>.
 
|   [TheDraft]
|              Carrel, D. and L. Grant, "The TACACS+ Protocol Version
|              1.78", June 1997,
|              <https://tools.ietf.org/html/draft-grant-tacacs-02>.
|
 Authors' Addresses
 
    Thorsten Dahm
    Google Inc
    1600 Amphitheatre Parkway
    Mountain View, CA  94043
    US
 
    EMail: [email-masked]
 
    Andrej Ota
    Google Inc
    1600 Amphitheatre Parkway
    Mountain View, CA  94043
    US
 
    EMail: [email-masked]
 
    Douglas C. Medway Gash
    Cisco Systems, Inc.
    170 West Tasman Dr.
    San Jose, CA  95134
    US
 
    Phone: +44 0208 8244508
    EMail: [email-masked]
    David Carrel
    vIPtela, Inc.
    1732 North First St.
    San Jose, CA  95112
    US
 
    EMail: [email-masked]
 
    Lol Grant
|
|   EMail: [email-masked]