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The Terminal Access Controller Access-Control System Plus (TACACS+) Protocol
draft-ietf-opsawg-tacacs-18

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8907.
Authors Thorsten Dahm , Andrej Ota , dcmgash@cisco.com , David Carrel , Lol Grant
Last updated 2020-09-30 (Latest revision 2020-03-20)
Replaces draft-dahm-opsawg-tacacs
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Informational
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Stream WG state Submitted to IESG for Publication
Document shepherd Joe Clarke
Shepherd write-up Show Last changed 2019-03-27
IESG IESG state Became RFC 8907 (Informational)
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Consensus boilerplate Yes
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Responsible AD Warren "Ace" Kumari
Send notices to opsawg-chairs@ietf.org, Joe Clarke <jclarke@cisco.com>
IANA IANA review state Version Changed - Review Needed
IANA action state No IANA Actions
draft-ietf-opsawg-tacacs-18
Operations                                                       T. Dahm
Internet-Draft                                                    A. Ota
Intended status: Informational                                Google Inc
Expires: September 21, 2020                               D. Medway Gash
                                                     Cisco Systems, Inc.
                                                               D. Carrel
                                                           vIPtela, Inc.
                                                                L. Grant
                                                          March 20, 2020

                          The TACACS+ Protocol
                      draft-ietf-opsawg-tacacs-18

Abstract

   This document describes the Terminal Access Controller Access-Control
   System Plus (TACACS+) protocol which is widely deployed today to
   provide Device Administration for routers, network access servers and
   other networked computing devices via one or more centralized
   servers.

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/.

   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 September 21, 2020.

Copyright Notice

   Copyright (c) 2020 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents

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   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.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Technical Definitions . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Client  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Server  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Packet  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.4.  Connection  . . . . . . . . . . . . . . . . . . . . . . .   5
     3.5.  Session . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.6.  Treatment of Enumerated Protocol Values . . . . . . . . .   5
     3.7.  Treatment of Text Strings . . . . . . . . . . . . . . . .   6
   4.  TACACS+ Packets and Sessions  . . . . . . . . . . . . . . . .   6
     4.1.  The TACACS+ Packet Header . . . . . . . . . . . . . . . .   6
     4.2.  The TACACS+ Packet Body . . . . . . . . . . . . . . . . .   8
     4.3.  Single Connection Mode  . . . . . . . . . . . . . . . . .   8
     4.4.  Session Completion  . . . . . . . . . . . . . . . . . . .   9
     4.5.  Data Obfuscation  . . . . . . . . . . . . . . . . . . . .  11
   5.  Authentication  . . . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  The Authentication START Packet Body  . . . . . . . . . .  13
     5.2.  The Authentication REPLY Packet Body  . . . . . . . . . .  15
     5.3.  The Authentication CONTINUE Packet Body . . . . . . . . .  17
     5.4.  Description of Authentication Process . . . . . . . . . .  17
       5.4.1.  Version Behavior  . . . . . . . . . . . . . . . . . .  18
       5.4.2.  Common Authentication Flows . . . . . . . . . . . . .  19
       5.4.3.  Aborting an Authentication Session  . . . . . . . . .  22
   6.  Authorization . . . . . . . . . . . . . . . . . . . . . . . .  23
     6.1.  The Authorization REQUEST Packet Body . . . . . . . . . .  23
     6.2.  The Authorization REPLY Packet Body . . . . . . . . . . .  27
   7.  Accounting  . . . . . . . . . . . . . . . . . . . . . . . . .  29

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     7.1.  The Account REQUEST Packet Body . . . . . . . . . . . . .  29
     7.2.  The Accounting REPLY Packet Body  . . . . . . . . . . . .  30
   8.  Argument-Value Pairs  . . . . . . . . . . . . . . . . . . . .  32
     8.1.  Value Encoding  . . . . . . . . . . . . . . . . . . . . .  32
     8.2.  Authorization Arguments . . . . . . . . . . . . . . . . .  33
     8.3.  Accounting Arguments  . . . . . . . . . . . . . . . . . .  35
   9.  Privilege Levels  . . . . . . . . . . . . . . . . . . . . . .  36
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  37
     10.1.  General Security of the Protocol . . . . . . . . . . . .  38
     10.2.  Security of Authentication Sessions  . . . . . . . . . .  39
     10.3.  Security of Authorization Sessions . . . . . . . . . . .  39
     10.4.  Security of Accounting Sessions  . . . . . . . . . . . .  40
     10.5.  TACACS+ Best Practices . . . . . . . . . . . . . . . . .  40
       10.5.1.  Shared Secrets . . . . . . . . . . . . . . . . . . .  40
       10.5.2.  Connections and Obfuscation  . . . . . . . . . . . .  41
       10.5.3.  Authentication . . . . . . . . . . . . . . . . . . .  42
       10.5.4.  Authorization  . . . . . . . . . . . . . . . . . . .  43
       10.5.5.  Redirection Mechanism  . . . . . . . . . . . . . . .  43
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  43
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  43
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  44
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  44
     13.2.  Informative References . . . . . . . . . . . . . . . . .  45
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  45

1.  Introduction

   This document describes the Terminal Access Controller Access-Control
   System Plus (TACACS+) protocol.  It was conceived initially as a
   general Authentication, Authorization and Accounting (AAA) protocol.
   It is widely deployed today but is mainly confined for a specific
   subset of AAA: Device Administration, that is: 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, but
   was never standardized.  The draft document is known as `The Draft'
   [TheDraft].

   This Draft was a product of its time, and did not address all of the
   key security concerns which are considered when designing modern
   standards.  Deployment must therefore be executed with care.  These
   concerns are addressed in the security section (Section 10).

   The primary intent of this informational document is to clarify the
   subset of `The Draft' which is common to implementations supporting

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   Device Administration.  It is intended that all implementations which
   conform to this document will conform to `The Draft'.  However, it is
   not intended that all implementations which conform to 'The Draft'
   will conform to this document.  The following features from `The
   Draft' have been removed:

      This document officially removes SENDPASS for security reasons.

      The normative description of Legacy features such as ARAP and
      outbound authentication has been removed.

      The Support for forwarding to an alternative daemon
      (TAC_PLUS_AUTHEN_STATUS_FOLLOW) has been deprecated.

   The TACACS+ protocol allows for arbitrary length and content
   authentication exchanges, to support alternative 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 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
   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, an
   implementation or deployment 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 authorization requests.

2.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Technical Definitions

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

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3.1.  Client

   The client is any device which initiates TACACS+ protocol requests to
   mediate access, mainly for the Device Administration use case.

3.2.  Server

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

3.3.  Packet

   All uses of the word packet in this document refer to TACACS+
   protocol data units unless explicitly noted otherwise.  The informal
   term "Packet" has become an established part of the definition.

3.4.  Connection

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

3.5.  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.

3.6.  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 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 an ERROR and terminate the session.  This will allow the client
   to try a different option.

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   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.

3.7.  Treatment of Text Strings

   The TACACS+ protocol makes extensive use of text strings.  The
   original draft intended that these strings would be treated as byte
   arrays where each byte would represent a US-ASCII character.

   More recently, server implementations have been extended to interwork
   with external identity services, and so a more nuanced approach is
   needed.  Usernames MUST be encoded and handled using the
   UsernameCasePreserved Profile specified in RFC 8265 [RFC8265].  The
   security considerations in Section 8 of that RFC apply.

   Where specifically mentioned, data fields contain arrays of arbitrary
   bytes as required for protocol processing.  These are not intended to
   be made visible through user interface to users.

   All other text fields in TACACS+ MUST be treated as printable byte
   arrays of US-ASCII as defined by RFC 20 [RFC0020].  The term
   "printable" used here means the fields MUST exclude the "Control
   Characters" defined in section 5.2 of RFC 20 [RFC0020].

4.  TACACS+ Packets and Sessions

4.1.  The TACACS+ Packet Header

   All TACACS+ packets begin with the following 12-byte header.  The
   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                               |
   +----------------+----------------+----------------+----------------+

   The following general rules apply to all TACACS+ packet types:

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      - To signal that any variable length data fields are unused, the
      corresponding length values are set to zero.  Such fields MUST be
      ignored, and treated as if not present.

      - 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.

   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.  Options 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.  TACACS+ 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

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   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
   packet.  This option MUST NOT be used in production.  The application
   of this flag will be covered in the security section (Section 10).

   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.

   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 (Section 4.3).

   All other bits MUST be ignored when reading, and SHOULD be set to
   zero when writing.

   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 4086 [RFC4086].

   length

   The total length of the packet body (not including the header).

4.2.  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.

4.3.  Single Connection Mode

   Single Connection Mode is intended to improve performance where there
   is a lot of traffic between a client and a server by allowing the
   client to multiplex multiple session on a single TCP connection.

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   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.

   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.

   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.

   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
   MUST 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.

   The TCP connection underlying the Single Connection Mode will close
   eventually, either because of the timeout from the server or from an
   intermediate link.  If a session is in progress when the client
   detects disconnect then the client should handle it as described in
   Section 4.4.  If a session is not in progress, then the client will
   need to detect this, and restart the single connection mode when the
   it initiates the next session.

4.4.  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

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   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 cannot 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,
   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 Section 5.4.3 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
   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 4.3), 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 4.5),
   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.

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4.5.  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.

   The obfuscation mechanism relies on a secret key, a shared secret
   value that is known to both the client and the server.  The secret
   keys MUST remain secret.

   Server implementations MUST allow a unique secret key to be
   associated with each client.  It is a site-dependent decision as to
   whether the use of separate keys is appropriate.

   The flag field MUST be configured with the following bit as follows:

   TAC_PLUS_UNENCRYPTED_FLAG = 0x0

   So that the packet body is obfuscated by XOR-ing it byte-wise with a
   pseudo-random pad.

   ENCRYPTED {data} = data ^ pseudo_pad

   The packet body can then be de-obfuscated by XOR-ing it byte-wise
   with a pseudo random pad.

   data = ENCRYPTED {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 and session_id are extracted from the header

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   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 4.4.

   TAC_PLUS_UNENCRYPTED_FLAG == 0x1

   This option is deprecated and MUST NOT be used in production.  In
   this case, the entire packet body is in cleartext.  A request MUST be
   dropped if TAC_PLUS_UNENCRYPTED_FLAG is set to true.

   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 4.4.

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

5.  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.

   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.

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5.1.  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.  Valid 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 9) below.

   authen_type

   The type of authentication.  Please see section Common Authentication
   Flows (Section 5.4.2).  Valid values are:

      TAC_PLUS_AUTHEN_TYPE_ASCII := 0x01

      TAC_PLUS_AUTHEN_TYPE_PAP := 0x02

      TAC_PLUS_AUTHEN_TYPE_CHAP := 0x03

      TAC_PLUS_AUTHEN_TYPE_MSCHAP := 0x05

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      TAC_PLUS_AUTHEN_TYPE_MSCHAPV2 := 0x06

   authen_service

   This is the service that is requesting the authentication.  Valid
   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_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
   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.
   ENABLE may be requested independently, no requirements 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, the client MUST set user_len to 0.

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   If included, the user_len indicates the length of the user field, in
   bytes.

   port, port_len

   The name of the client port on which the authentication is taking
   place.  The value of this field is free format text and is client
   specific.  Examples of this this argument include "tty10" to denote
   the tenth tty line and "async10" to denote the tenth async interface.
   The client documentation SHOULD define the values and their meanings
   for this field.  For details of text encoding, see (Section 3.7).
   port_len indicates the length of the port field, in bytes.

   rem_addr, rem_addr_len

   A string indicating the remote location from which the user has
   connected to the client.  For details of text encoding, see
   (Section 3.7).

   When TACACS+ was used for dial-up services, this value contained the
   caller ID

   When TACACS+ is used for Device Administration, the user is normally
   connected via a network, and in this case the value is intended to
   hold a network address, IPv4 or IPv6.  For IPv6 address text
   representation defined please see RFC 5952 [RFC5952].

   This field is optional (since the information may not be available).
   The rem_addr_len indicates 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 5.4.2).  The data_len indicates the
   length of the data field, in bytes.

5.2.  The Authentication REPLY Packet Body

   The TACACS+ server sends only one type of authentication packet (a
   REPLY packet) to the client.

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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.  Valid 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
   server_msg_len indicates the length of the server_msg field, in
   bytes.  For details of text encoding, see (Section 3.7).

   data, data_len

   This field holds data that is a part of the authentication exchange
   and is intended for client processing, not the user.  It is not a
   printable text encoding.  Examples of its use are shown in the

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   section Common Authentication flows (Section 5.4.2).  The data_len
   indicates the length of the data field, in bytes.

5.3.  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.  It is not a printable text encoding.  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

5.4.  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
   supported by TACACS+.  Each authentication flow consists of a START

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   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.

   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 reflected in the user interface as it is
   entered.

   The data field is only used in the REPLY where explicitly defined
   below.

5.4.1.  Version Behavior

   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 authorization and accounting and ASCII authentication use
   minor_version number of 0.

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   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.

5.4.2.  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.

5.4.2.1.  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.

5.4.2.2.  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] (Obsolete).
   The REPLY from the server MUST be either a PASS, FAIL or ERROR.

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5.4.2.3.  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 1334 [RFC1334] and then
   compares that value with the response.  The MD5 algorithm option is
   always used.  The REPLY from the server MUST be a PASS, FAIL or
   ERROR.

   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).

5.4.2.4.  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.

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   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.

5.4.2.5.  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 reject authentications where the challenge
   deviates from 16 bytes as defined in the RFC.

5.4.2.6.  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 5.4.2.1).

   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.

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5.4.2.7.  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.

5.4.3.  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 a message
   explaining the reason for the abort.  For details of text encoding,
   see (Section 3.7).  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 4.4.

   In cases of PASS, 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

   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.

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   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.

6.  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
   into 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 arguments used in
   authorization, see the Authorization Arguments section (Section 8.2).

   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 (argument-
   value pairs) that can restrict or modify the client's actions.

6.1.  The Authorization REQUEST 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
   +----------------+----------------+----------------+----------------+
   |  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 filed allows the client to indicate the authentication method
   used by the 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

      TAC_PLUS_AUTHEN_METH_KRB4 := 0x11

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      TAC_PLUS_AUTHEN_METH_RCMD := 0x20

   As this information is not always subject to verification, it is
   recommended that this field is in policy evaluastion.  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.
   RADIUS is the Radius authentication protocol.  RCMD refers to
   authentication provided via the R-command protocols from Berkeley
   Unix.  KRB5 and KRB4 are Kerberos version 5 and 4.

   As mentioned above, this field is used by the client to indicate how
   it performed the authentication.  One of the options
   (TAC_PLUS_AUTHEN_METH_TACACSPLUS := 0x06) is TACACS+ itself, and so
   the detail of how the client performed this option is given in
   Authentication Section (Section 5).  For all other options, such as
   KRB and RADIUS, then TACACS+ protocol did not play any part in the
   authentication phase; as those interactions were not conducted using
   the TACACS+ protocol they will not be documented here.  For
   implementers of clients who need details of the other protocols,
   please refer to the respective Kerberos [RFC4120] and RADIUS
   [RFC3579] RFCs.

   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 9) below.  It indicates the users current privilege
   level.

   authen_type

   This field corresponds to the authen_type field in the authentication
   section (Section 5) 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 5) above.  It indicates the service
   through which the user authenticated.

   user, user_len

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   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 5) 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 5) 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 field (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
   argument-value pairs.  The argument and the value are in a single
   string and are separated by either a "=" (0X3D) or a "*" (0X2A).  The
   equals sign indicates a mandatory argument.  The asterisk indicates
   an optional one.  For details of text encoding, see (Section 3.7).

   An argument name MUST NOT contain either of the separators.  An
   argument value MAY 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.

   Optional arguments are ones that may be disregarded by either client
   or server.  Mandatory arguments require that the receiving side can
   handle the argument, 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.  The value part of an argument-value
   pair may be empty, that is: the length of the value may be zero.

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   Argument-value strings are not NULL terminated, rather their length
   value indicates their end.  The maximum length of an argument-value
   string is 255 characters.  The minimum is two characters (one name-
   value character and the separator)

   Though the arguments allow extensibility, a common core set of
   authorization arguments SHOULD be supported by clients and servers,
   these are listed in the Authorization Arguments (Section 8.2) section
   below.

6.2.  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 string that may be presented to the user.  The
   server_msg_len indicates the length of the server_msg field, in
   bytes.  For details of text encoding, see (Section 3.7).

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   data, data_len

   This is a 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.  For details of text encoding, see (Section 3.7).

   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 Arguments (Section 8.2) section below.  Each argument
   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 argument-value pairs (if any) in the
   response, instead of the authorization argument-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
   Session Completion (Section 4.4).  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.

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7.  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 arguments is defined in the accounting section
   (Section 7).

7.1.  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.

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      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 the Accounting Arguments section (Section 8.3) for the dictionary
   of arguments relevant to accounting.

7.2.  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 string that may be presented to the user.  The
   server_msg_len indicates the length of the server_msg field, in
   bytes.  For details of text encoding, see (Section 3.7).

   data, data_len

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   This is a 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.  For details of text encoding, see (Section 3.7).

   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.

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

8.  Argument-Value Pairs

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

8.1.  Value Encoding

   All argument values are encoded as strings.  For details of text
   encoding, see (Section 3.7).  The following type representations
   SHOULD be followed

   Numeric

   All numeric values in an argument-value string are provided as
   decimal numbers, unless otherwise stated.  All arguments include a
   length field, and TACACS+ implementations MUST verify that they can
   accommodate the lengths of numeric arguments before attempting to
   process them.  If the length cannot be accommodated then the argument
   MUST be regarded as not handled and the logic in authorization
   section (Section 6.1) regarding the processing of arguments MUST be
   applied.

   Boolean

   All Boolean arguments are encoded with values "true" or "false".

   IP-Address

   It is recommended that hosts be specified as a IP address so as to
   avoid any ambiguities.  For details of text encoding, see
   (Section 3.7).  IPv4 address are specified as octet numerics
   separated by dots ('.'), IPv6 address text representation defined in
   RFC 5952 [RFC5952].

   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 argument is
   specified.

   String

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   Many values have no specific type representation and are interpreted
   as plain strings.

   Empty Values

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

8.2.  Authorization Arguments

   service (String)

   The primary service.  Specifying a service argument indicates that
   this is a request for authorization or accounting of that service.
   For example: "shell", "tty-server", "connection", "system" and
   "firewall", others may be chosen for the required application.  This
   argument MUST always be included.

   protocol (String)

   the protocol field may be used to indicate a subset of a service.

   cmd (String)

   a shell (exec) command.  This indicates the command name of the
   command that is to be run.  The "cmd" argument 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)

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

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   acl (Numeric)

   a number representing a connection access list.  Applicable only to
   session-based shell authorization.  For details of text encoding, see
   (Section 3.7).

   inacl (String)

   identifier (name) of an interface input access list.  For details of
   text encoding, see (Section 3.7).

   outacl (String)

   identifier (name) of an interface output access list.  For details of
   text encoding, see (Section 3.7).

   addr (IP-Address)

   a network address

   addr-pool (String)

   The identifier of an address pool from which the client can assign an
   address.

   timeout (Numeric)

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

   idletime (Numeric)

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

   autocmd (String)

   an auto-command to run.  Applicable only to session-based shell
   authorization.

   noescape (Boolean)

   Prevents user from using an escape character.  Applicable only to
   session-based shell authorization.

   nohangup (Boolean)

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   Boolean.  Do not disconnect after an automatic command.  Applicable
   only to session-based shell authorization.

   priv-lvl (Numeric)

   privilege level to be assigned.  Please refer to the Privilege Level
   section (Section 9) below.

8.3.  Accounting Arguments

   The following arguments are defined for TACACS+ accounting only.
   They MUST precede any argument-value pairs that are defined in the
   authorization section (Section 6) above.

   task_id (String)

   Start and stop records for the same event MUST have matching task_id
   argument 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)

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

   stop_time (Date Time)

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

   elapsed_time (Numeric)

   The elapsed time in seconds for the action.

   timezone (String)

   The timezone abbreviation for all timestamps included in this packet.
   A database of timezones is maintained here: TZDB [TZDB].

   event (String)

   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 (String)

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

   bytes (Numeric)

   The number of bytes transferred by this action

   bytes_in (Numeric)

   The number of bytes transferred by this action from the endstation to
   the client port

   bytes_out (Numeric)

   The number of bytes transferred by this action from the client to the
   endstation port

   paks (Numeric)

   The number of packets transferred by this action.

   paks_in (Numeric)

   The number of input packets transferred by this action from the
   endstation to the client port.

   paks_out (Numeric)

   The number of output packets transferred by this action from the
   client port to the endstation.

   err_msg (String)

   string describing the status of the action.  For details of text
   encoding, see (Section 3.7).

9.  Privilege Levels

   The TACACS+ Protocol supports flexible authorization schemes through
   the extensible arguments.

   One scheme is built into 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
   specific commands) to different privilege levels.  The allocation of
   commands to privilege levels is highly dependent upon the deployment.
   Common allocations are as follows:

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      TAC_PLUS_PRIV_LVL_MIN := 0x00.  The level normally allocated to an
      unauthenticated session.

      TAC_PLUS_PRIV_LVL_USER := 0x01.  The level normally allocated to a
      regular authenticated session

      TAC_PLUS_PRIV_LVL_ROOT := 0x0f.  The level normally allocated to a
      session authenticated by a highly privileged user to allow
      commands with significant system impact.

      TAC_PLUS_PRIV_LVL_MAX := 0x0f.  The highest privilege level.

   A Privilege level can be assigned to a shell (EXEC) session when it
   starts.  The client will permit the actions associated with this
   level to be executed.  This 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.

   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.
   However, it is still common enough that it SHOULD be supported by
   servers.

10.  Security Considerations

   The original TACACS+ Draft `The Draft' [TheDraft] from 1998 did not
   address all of the key security concerns which are considered when
   designing modern standards.  This section addresses known limitations
   and concerns which will impact overall security of the protocol and
   systems where this protocol is deployed to manage central
   authentication, authorization or accounting for network device
   administration.

   Multiple implementations of the protocol described in the original
   TACACS+ Draft `The Draft' [TheDraft] have been deployed.  As the
   protocol was never standardized, current implementations may be
   incompatible in non-obvious ways, giving rise to additional security
   risks.  This section does not claim to enumerate all possible
   security vulnerabilities.

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10.1.  General Security of the Protocol

   TACACS+ protocol does not include a security mechanism that would
   meet modern-day requirements.  These security mechanisms would be
   best referred to as "obfuscation" and not "encryption" since they
   provide no meaningful integrity, privacy or replay protection.  An
   attacker with access to the data stream should be assumed to be able
   to read and modify all TACACS+ packets.  Without mitigation, a range
   of risks such as the following are possible:

      Accounting information may be modified by the man-in-the-middle
      attacker, making such logs unsuitable and not trustable for
      auditing purposes.

      Invalid or misleading values may be inserted by the man-in-the-
      middle attacker in various fields at known offsets to try and
      circumvent the authentication or authorization checks even inside
      the obfuscated body.

   While the protocol provides some measure of transport privacy, it is
   vulnerable to at least the following attacks:

      Brute force attacks exploiting increased efficiency of MD5 digest
      computation.

      Known plaintext attacks which may decrease the cost of brute force
      attack.

      Chosen plaintext attacks which may decrease the cost of a brute
      force attack.

      No forward secrecy.

   Even though, to the best knowledge of authors, this method of
   encryption wasn't rigorously tested, enough information is available
   that it is best referred to as "obfuscation" and not "encryption".

   For these reasons, users deploying TACACS+ protocol in their
   environments MUST limit access to known clients and MUST control the
   security of the entire transmission path.  Attackers who can guess
   the key or otherwise break the obfuscation will gain unrestricted and
   undetected access to all TACACS+ traffic.  Ensuring that a
   centralized AAA system like TACACS+ is deployed on a secured
   transport is essential to managing the security risk of such an
   attack.

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   The following parts of this section enumerate only the session-
   specific risks which are in addition to general risk associated with
   bare obfuscation and lack of integrity checking.

10.2.  Security of Authentication Sessions

   Authentication sessions SHOULD be used via a secure transport (see
   Best Practices section (Section 10.5)) as the man-in-the-middle
   attack may completely subvert them.  Even CHAP, which may be
   considered resistant to password interception, is unsafe as it does
   not protect the username from a trivial man-in-the-middle attack.

   This document deprecates the redirection mechanism using the
   TAC_PLUS_AUTHEN_STATUS_FOLLOW option which was included in the
   original draft.  As part of this process, the secret key for a new
   server was sent to the client.  This public exchange of secret keys
   means that once one session is broken, it may be possible to leverage
   that key to attacking connections to other servers.  This mechanism
   MUST NOT be used in modern deployments.  It MUST NOT be used outside
   a secured deployment.

10.3.  Security of Authorization Sessions

   Authorization sessions SHOULD be used via a secure transport (see
   Best Practices section (Section 10.5)) as it's trivial to execute a
   successful man-in-the-middle attacks that changes well-known
   plaintext in either requests or responses.

   As an example, take the field "authen_method".  It's not unusual in
   actual deployments to authorize all commands received via the device
   local serial port (a console port) as that one is usually considered
   secure by virtue of the device located in a physically secure
   location.  If an administrator would configure the authorization
   system to allow all commands entered by the user on a local console
   to aid in troubleshooting, that would give all access to all commands
   to any attacker that would be able to change the "authen_method" from
   TAC_PLUS_AUTHEN_METH_TACACSPLUS to TAC_PLUS_AUTHEN_METH_LINE.  In
   this regard, the obfuscation provided by the protocol itself wouldn't
   help much, because:

      Lack of integrity means that any byte in the payload may be
      changed without either side detecting the change.

      Known plaintext means that an attacker would know with certainty
      which octet is the target of the attack (in this case, 1st octet
      after the header).

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      In combination with known plaintext, the attacker can determine
      with certainty the value of the crypto-pad octet used to obfuscate
      the original octet.

10.4.  Security of Accounting Sessions

   Accounting sessions SHOULD be used via a secure transport (see Best
   Practices section (Section 10.5).  Although Accounting sessions are
   not directly involved in authentication or authorizing operations on
   the device, man-in-the-middle attacker may do any of the following:

      Replace accounting data with new valid or garbage which can
      confuse auditors or hide information related to their
      authentication and/or authorization attack attempts.

      Try and poison accounting log with entries designed to make
      systems behave in unintended ways (which includes TACACS+ server
      and any other systems that would manage accounting entries).

   In addition to these direct manipulations, different client
   implementations pass different fidelity of accounting data.  Some
   vendors have been observed in the wild that pass sensitive data like
   passwords, encryption keys and similar as part of the accounting log.
   Due to lack of strong encryption with perfect forward secrecy, this
   data may be revealed in future, leading to a security incident.

10.5.  TACACS+ Best Practices

   With respect to the observations about the security issues described
   above, a network administrator MUST NOT rely on the obfuscation of
   the TACACS+ protocol. TACACS+ MUST be used within a secure
   deployment: TACACS+ MUST be deployed over networks which ensure
   privacy and integrity of the communication, and MUST be deployed over
   a network which is separated from other traffic.  Failure to do so
   will impact overall network security.

   The following recommendations impose restrictions on how the protocol
   is applied.  These restrictions were not imposed in the original
   draft.  New implementations, and upgrades of current implementations,
   MUST implement these recommendations.  Vendors SHOULD provide
   mechanisms to assist the administrator to achieve these best
   practices.

10.5.1.  Shared Secrets

   TACACS+ servers and clients MUST treat shared secrets as sensitive
   data to be managed securely, as would be expected for other sensitive
   data such as identity credential information.  TACACS+ servers MUST

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   NOT leak sensitive data.  For example, TACACS+ servers MUST NOT
   expose shared secrets in logs.

   TACACS+ servers MUST allow a dedicated secret key to be defined for
   each client.

   TACACS+ server management systems MUST provide a mechanism to track
   secret key lifetimes and notify administrators to update them
   periodically.  TACACS+ server administrators SHOULD change secret
   keys at regular intervals.

   TACACS+ servers SHOULD warn administrators if secret keys are not
   unique per client.

   TACACS+ server administrators SHOULD always define a secret for each
   client.

   TACACS+ servers and clients MUST support shared keys that are at
   least 32 characters long.

   TACACS+ servers MUST support policy to define minimum complexity for
   shared keys.

   TACACS+ clients SHOULD NOT allow servers to be configured without
   shared secret key, or shared key that is less than 16 characters
   long.

   TACACS+ server administrators SHOULD configure secret keys of minimum
   16 characters length.

10.5.2.  Connections and Obfuscation

   TACACS+ servers MUST allow the definition of individual clients.  The
   servers MUST only accept network connection attempts from these
   defined, known clients.

   TACACS+ servers MUST reject connections with
   TAC_PLUS_UNENCRYPTED_FLAG set.  There MUST always be a shared secret
   set on the server for the client requesting the connection.

   If an invalid shared secret is detected when processing packets for a
   client, TACACS+ servers MUST NOT accept any new sessions on that
   connection.  TACACS+ servers MUST terminate the connection on
   completion of any sessions that were previously established with a
   valid shared secret on that connection.

   TACACS+ clients MUST NOT set TAC_PLUS_UNENCRYPTED_FLAG.  Clients MUST
   be implemented in a way that requires explicit configuration to

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   enable the use of TAC_PLUS_UNENCRYPTED_FLAG, this option MUST NOT be
   used when the client is in production

   When a TACACS+ client receives responses from servers where:

      the response packet was received from the server configured with
      shared key, but the packet has TAC_PLUS_UNENCRYPTED_FLAG set.

      the response packet was received from the server configured not to
      use obfuscation, but the packet has TAC_PLUS_UNENCRYPTED_FLAG not
      set.

   then the TACACS+ client MUST close TCP session, and process the
   response in the same way that a TAC_PLUS_AUTHEN_STATUS_FAIL
   (authentication sessions) or TAC_PLUS_AUTHOR_STATUS_FAIL
   (authorization sessions) was received.

10.5.3.  Authentication

   To help TACACS+ administrators select less weak authentication
   options, TACACS+ servers MUST allow the administrator to configure
   the server to only accept challenge/response options for
   authentication (TAC_PLUS_AUTHEN_TYPE_CHAP or
   TAC_PLUS_AUTHEN_TYPE_MSCHAP or TAC_PLUS_AUTHEN_TYPE_MSCHAPV2 for
   authen_type).

   TACACS+ server administrators SHOULD enable the option mentioned in
   the previous paragraph.  TACACS+ Server deployments SHOULD ONLY
   enable other options (such as TAC_PLUS_AUTHEN_TYPE_ASCII or
   TAC_PLUS_AUTHEN_TYPE_PAP) when unavoidable due to requirements of
   identity/password systems.

   TACACS+ server administrators SHOULD NOT allow the same credentials
   to be applied in challenge-based (TAC_PLUS_AUTHEN_TYPE_CHAP or
   TAC_PLUS_AUTHEN_TYPE_MSCHAP or TAC_PLUS_AUTHEN_TYPE_MSCHAPV2) and non
   challenge-based authen_type options as the insecurity of the latter
   will compromise the security of the former.

   TAC_PLUS_AUTHEN_SENDAUTH and TAC_PLUS_AUTHEN_SENDPASS options
   mentioned in the original draft SHOULD NOT be used, due to their
   security implications.  TACACS+ servers SHOULD NOT implement them.
   If they must be implemented, the servers MUST default to the options
   being disabled and MUST warn the administrator that these options are
   not secure.

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10.5.4.  Authorization

   The authorization and accounting features are intended to provide
   extensibility and flexibility.  There is a base dictionary defined in
   this document, but it may be extended in deployments by using new
   argument names.  The cost of the flexibility is that administrators
   and implementers MUST ensure that the argument and value pairs shared
   between the clients and servers have consistent interpretation.

   TACACS+ clients that receive an unrecognized mandatory argument MUST
   evaluate server response as if they received
   TAC_PLUS_AUTHOR_STATUS_FAIL.

10.5.5.  Redirection Mechanism

   The original draft described a redirection mechanism
   (TAC_PLUS_AUTHEN_STATUS_FOLLOW).  This feature is difficult to
   secure.  The option to send secret keys in the server list is
   particularly insecure, as it can reveal client shared secrets.

   TACACS+ servers MUST deprecate the redirection mechanism.

   If the redirection mechanism is implemented then TACACS+ servers MUST
   disable it by default, and MUST warn TACACS+ server administrators
   that it must only be enabled within a secure deployment due to the
   risks of revealing shared secrets.

   TACACS+ clients SHOULD deprecate this feature by treating
   TAC_PLUS_AUTHEN_STATUS_FOLLOW as TAC_PLUS_AUTHEN_STATUS_FAIL.

11.  IANA Considerations

   This informational document describes TACACS+ protocol and its common
   deployments.  There is no further consideration required from IANA.

12.  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, John Heasley, 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
   time and effort to help improve the document, identifying weaknesses
   and providing remediation.

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   The authors would also like to thank the support from the OPSAWG
   Chairs and advisors, especially Joe Clarke.

13.  References

13.1.  Normative References

   [RFC0020]  Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <https://www.rfc-editor.org/info/rfc20>.

   [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>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [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>.

   [RFC3579]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
              Dial In User Service) Support For Extensible
              Authentication Protocol (EAP)", RFC 3579,
              DOI 10.17487/RFC3579, September 2003,
              <https://www.rfc-editor.org/info/rfc3579>.

   [RFC4086]  Eastlake 3rd, D., Crocker, S., and J. Schiller,
              "Randomness Requirements for Security", RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <http://www.rfc-editor.org/info/rfc4086>.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              DOI 10.17487/RFC4120, July 2005,
              <https://www.rfc-editor.org/info/rfc4120>.

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   [RFC5952]  Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
              Address Text Representation", RFC 5952,
              DOI 10.17487/RFC5952, August 2010,
              <https://www.rfc-editor.org/info/rfc5952>.

   [RFC8265]  Saint-Andre, P. and A. Melnikov, "Preparation,
              Enforcement, and Comparison of Internationalized Strings
              Representing Usernames and Passwords", RFC 8265,
              DOI 10.17487/RFC8265, October 2017,
              <https://www.rfc-editor.org/info/rfc8265>.

13.2.  Informative References

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

   [TZDB]     Eggert, P. and A. Olson, "Sources for Time Zone and
              Daylight Saving Time Data", 1987,
              <https://www.iana.org/time-zones>.

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: andrej@ota.si

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

   EMail: dcmgash@cisco.com

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

   EMail: dcarrel@viptela.com

   Lol Grant

   EMail: lol.grant@gmail.com

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