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Versions: (draft-dahm-opsawg-tacacs) 00 01 02 03 04 05 06 07

Operations                                                       T. Dahm
Internet-Draft                                                    A. Ota
Intended status: Informational                                Google Inc
Expires: February 22, 2018                                D. Medway Gash
                                                     Cisco Systems, Inc.
                                                               D. Carrel
                                                           vIPtela, Inc.
                                                                L. Grant
                                                         August 21, 2017


                          The TACACS+ Protocol
                      draft-ietf-opsawg-tacacs-07

Abstract

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

Status of This Memo

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

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

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

   This Internet-Draft will expire on February 22, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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



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

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Technical Definitions . . . . . . . . . . . . . . . . . . . .   4
   3.  TACACS+ Connections and Sessions  . . . . . . . . . . . . . .   4
     3.1.  Connection  . . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Session . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Single Connect Mode . . . . . . . . . . . . . . . . . . .   5
     3.4.  Session Completion  . . . . . . . . . . . . . . . . . . .   5
     3.5.  Treatment of Enumerated Protocol Values . . . . . . . . .   6
     3.6.  Text Encoding . . . . . . . . . . . . . . . . . . . . . .   7
     3.7.  Data Obfuscation  . . . . . . . . . . . . . . . . . . . .   7
     3.8.  The TACACS+ Packet Header . . . . . . . . . . . . . . . .   9
     3.9.  The TACACS+ Packet Body . . . . . . . . . . . . . . . . .  11
   4.  Authentication  . . . . . . . . . . . . . . . . . . . . . . .  11
     4.1.  The Authentication START Packet Body  . . . . . . . . . .  11
     4.2.  The Authentication REPLY Packet Body  . . . . . . . . . .  14
     4.3.  The Authentication CONTINUE Packet Body . . . . . . . . .  15
     4.4.  Description of Authentication Process . . . . . . . . . .  16
       4.4.1.  Version Behaviour . . . . . . . . . . . . . . . . . .  17
       4.4.2.  Common Authentication Flows . . . . . . . . . . . . .  17
       4.4.3.  Aborting an Authentication Session  . . . . . . . . .  21
   5.  Authorization . . . . . . . . . . . . . . . . . . . . . . . .  22
     5.1.  The Authorization REQUEST Packet Body . . . . . . . . . .  23
     5.2.  The Authorization REPLY Packet Body . . . . . . . . . . .  26
   6.  Accounting  . . . . . . . . . . . . . . . . . . . . . . . . .  27
     6.1.  The Account REQUEST Packet Body . . . . . . . . . . . . .  28
     6.2.  The Accounting REPLY Packet Body  . . . . . . . . . . . .  29
   7.  Attribute-Value Pairs . . . . . . . . . . . . . . . . . . . .  30
     7.1.  Authorization Attributes  . . . . . . . . . . . . . . . .  31
     7.2.  Accounting Attributes . . . . . . . . . . . . . . . . . .  34



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   8.  Privilege Levels  . . . . . . . . . . . . . . . . . . . . . .  35
   9.  TACACS+ Security Considerations . . . . . . . . . . . . . . .  36
     9.1.  Overall Security of The Protocol  . . . . . . . . . . . .  36
     9.2.  Security of Authentication Sessions . . . . . . . . . . .  38
     9.3.  Security of Authorization Sessions  . . . . . . . . . . .  38
     9.4.  Security of Accounting Sessions . . . . . . . . . . . . .  38
     9.5.  TACACS+ Deployment Recommendations  . . . . . . . . . . .  39
     9.6.  TACACS+ Client Implementation Recommendations . . . . . .  39
     9.7.  TACACS+ Server Implementation Recommendations . . . . . .  40
     9.8.  TACACS+ Security and Operational Concerns . . . . . . . .  40
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  41

1.  Introduction

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

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

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

      This document officially removes SENDPASS for security reasons.

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

   The TACACS+ protocol separates the functions of Authentication,
   Authorization and Accounting.  It allows for arbitrary length and
   content authentication exchanges, which will support any
   authentication mechanism to be utilized with TACACS+ clients.  It is
   extensible to provide for site customization and future development
   features, and it uses TCP to ensure reliable delivery.  The protocol
   allows the TACACS+ client to request very fine-grained access control
   and allows the server to respond to each component of that request.

   The separation of authentication, authorization and accounting is a
   fundamental component of the design of TACACS+. The distinction



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   between them is very important so this document will address each one
   separately.  It is important to note that TACACS+ provides for all
   three, but an implementation or configuration is not required to
   employ all three.  Each one serves a unique purpose that alone is
   useful, and together can be quite powerful.

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

2.  Technical Definitions

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

   Client

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

   Server

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

   Packet

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

3.  TACACS+ Connections and Sessions

3.1.  Connection

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

3.2.  Session

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

   An accounting and authorization session will consist of a single pair
   of packets (the request and its reply).  An authentication session
   may involve an arbitrary number of packets being exchanged.  The



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

3.3.  Single Connect Mode

   Single Connection Mode is intended to improve performance by allowing
   a client to multiplex multiple session on a single TCP connection.

   The packet header contains the TAC_PLUS_SINGLE_CONNECT_FLAG used by
   the client and server to negotiate the use of Single Connect Mode.

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

   To indicate it will support Single Connect 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 Connect Mode.
   This protocol does not define a procedure for changing Single Connect
   Mode after the first two packets.

   If single Connect 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 fit the specific deployment to allocate a long
   lasting TCP connection to a specific client.  Even if the server is
   configured to permit single Connection Mode for a specific client,
   the server may close the connection.  For example: a server may be
   configured to time out a Single Connection Mode TCP Connection after
   a specific period of inactivity to preserve its resources.  The
   client MUST accommodate such closures on a TCP session even after
   Single Conenction Mode has been established.

3.4.  Session Completion

   The REPLY packets defined for the packets types in the sections below
   (Authentication, Authorization and Accounting) contain a status
   field.  The complete set of options for this field depend upon the



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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 can not apply the result
   and it MUST behave as if the server could not be connected to.  For
   example, the client try 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 the section (Section 4.4.3) on Aborting Authentication
   Sessions for details on handling additional status options .

   When the session is complete, then the TCP connection should be
   handled as follows, according to whether Single Connect 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 (Section 3.3) ), but may still be closed after
   a timeout period to preserve deployment resources

   If Single Connection Mode was enabled, but an ERROR occurred due to
   connection issues (such as an incorrect secret, see section
   (Section 3.7) ), then any further new sessions MUST NOT be accepted
   on the connection.  If there are any sessions that have already been
   established then they MAY be completed.  Once all active sessions are
   completed then the connection MUST be closed.

3.5.  Treatment of Enumerated Protocol Values

   This document describes various enumerated values in the packet
   header and the headers for specific packet types. for example in the
   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



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   listed in this document for a header field, then it should respond
   with a ERROR and terminate the session.  This will allow the client
   to try a different option.

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

3.6.  Text Encoding

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

   To ensure interoperability of current deployments, the TACACS+ client
   and server MUST handle user fields and those data fields used for
   passwords as 8 bit octet strings.  The deployment operator MUST
   ensure that consistent character encoding is applied.  The encoding
   SHOULD be UTF-8, and other encodings outside US-ASCII SHOULD be
   deprecated.

3.7.  Data Obfuscation

   The body of packets may be obfuscated.  The following sections
   describe the obfuscation mechanism that is supported in the protocol.
   In 'The Draft' this process was actually referred to as Encryption,
   but by modern day standards the mechanims would not meet the
   requirements of an encryption mechanism.

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

   The flag field may be set as follows:

   TAC_PLUS_UNENCRYPTED_FLAG == 0x0

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

   ENCRYPTED {data} == data ^ pseudo_pad




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

   Whenever used in this document, MD5 refers to the "RSA Data Security,
   Inc. MD5 Message-Digest Algorithm" as specified in RFC 1321 [RFC1321]
   .

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

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

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

   The session id is used in network byte order.

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

   MD5_1 = MD5{session_id, key, version, seq_no} MD5_2 = MD5{session_id,
   key, version, seq_no, MD5_1} ....  MD5_n = MD5{session_id, key,
   version, seq_no, MD5_n-1}

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

   TAC_PLUS_UNENCRYPTED_FLAG == 0x1

   In this case, the entire packet body is in cleartext.  Obfuscation
   and de-obfuscation are null operations.  This method should be
   avoided unless absolutely required for debug purposes, when tooling
   does not permit de-obfuscation.

   If deployment is configured for obfuscating a connection then do no
   skip de-obfuscation simply because an incoming packet indicates that
   it is not obfuscated.  If the flag is not set when expected, then it
   must be dropped.

   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 signalled.  The underlying TCP connection MAY




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   also be closed, if it is not being used for other sessions in single-
   connect mode.

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

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

3.8.  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                               |
   +----------------+----------------+----------------+----------------+

   major_version

   This is the major TACACS+ version number.

      TAC_PLUS_MAJOR_VER := 0xc

   minor_version

   The minor TACACS+ version number.

      TAC_PLUS_MINOR_VER_DEFAULT := 0x0

      TAC_PLUS_MINOR_VER_ONE := 0x1

   type

   This is the packet type.  Legal values are:

      TAC_PLUS_AUTHEN := 0x01 (Authentication)




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      TAC_PLUS_AUTHOR := 0x02 (Authorization)

      TAC_PLUS_ACCT := 0x03 (Accounting)

   seq_no

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

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

   flags

   This field contains various bitmapped flags.

   The flag bit:

   TAC_PLUS_UNENCRYPTED_FLAG := 0x01

   This flag indicates that the sender did not obfuscate the bode of the
   packet.  The application of this flag will be covered in the security
   section (Section 9) .  section.

   This flag SHOULD be clear in all deployments.  Modern network traffic
   tools easily support encryted traffic when configured with the shared
   secret (see section below), so even in test scenarios, the obfuscated
   mode SHOULD be used.

   The single-connection flag:

   TAC_PLUS_SINGLE_CONNECT_FLAG := 0x04

   This flag is used to allow a client and server to negotiate Single
   Connection Mode.

   session_id

   The Id for this TACACS+ session.  This field does not change for the
   duration of the TACACS+ session.  This number MUST be generated by a
   cryptographically strong random number generation method.  Failure to
   do so will compromise security of the session.  For more details
   refer to RFC 1750 [RFC1750]




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   length

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

3.9.  The TACACS+ Packet Body

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

      - To signal that any variable length data fields are unused, their
      length value is set to zero.

      - the lengths of data and message fields in a packet are specified
      by their corresponding length fields, (and are not null
      terminated.)

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

4.  Authentication

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

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

4.1.  The Authentication START 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
   +----------------+----------------+----------------+----------------+
   |    action      |    priv_lvl    |  authen_type   | authen_service |
   +----------------+----------------+----------------+----------------+
   |    user_len    |    port_len    |  rem_addr_len  |    data_len    |
   +----------------+----------------+----------------+----------------+
   |    user ...
   +----------------+----------------+----------------+----------------+
   |    port ...
   +----------------+----------------+----------------+----------------+
   |    rem_addr ...
   +----------------+----------------+----------------+----------------+
   |    data...
   +----------------+----------------+----------------+----------------+

   Packet fields are as follows:

   action

   This indicates the authentication action.  Legal values are listed
   below.

      TAC_PLUS_AUTHEN_LOGIN := 0x01

      TAC_PLUS_AUTHEN_CHPASS := 0x02

      TAC_PLUS_AUTHEN_SENDAUTH := 0x04

   priv_lvl

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

   authen_type

   The type of authentication.  Legal values are:

      TAC_PLUS_AUTHEN_TYPE_ASCII := 0x01

      TAC_PLUS_AUTHEN_TYPE_PAP := 0x02

      TAC_PLUS_AUTHEN_TYPE_CHAP := 0x03

      TAC_PLUS_AUTHEN_TYPE_ARAP := 0x04 (deprecated)

      TAC_PLUS_AUTHEN_TYPE_MSCHAP := 0x05

      TAC_PLUS_AUTHEN_TYPE_MSCHAPV2 := 0x06



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   authen_service

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

      TAC_PLUS_AUTHEN_SVC_NONE := 0x00

      TAC_PLUS_AUTHEN_SVC_LOGIN := 0x01

      TAC_PLUS_AUTHEN_SVC_ENABLE := 0x02

      TAC_PLUS_AUTHEN_SVC_PPP := 0x03

      TAC_PLUS_AUTHEN_SVC_ARAP := 0x04

      TAC_PLUS_AUTHEN_SVC_PT := 0x05

      TAC_PLUS_AUTHEN_SVC_RCMD := 0x06

      TAC_PLUS_AUTHEN_SVC_X25 := 0x07

      TAC_PLUS_AUTHEN_SVC_NASI := 0x08

      TAC_PLUS_AUTHEN_SVC_FWPROXY := 0x09

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

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

   The TAC_PLUS_AUTHEN_SVC_ENABLE option identifies the ENABLE
   authen_service, which refers to a service requesting authentication
   in order to grant the user different privileges.  This is comparable
   to the Unix "su(1)" command.  An authen_service value of NONE is only
   to be used when none of the other authen_service values are
   appropriate.  ENABLE may be requested independently, no requirements
   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 US-ASCII name of the client port on which the authentication is
   taking place, and its length in bytes.  The value of this field is
   client specific.  (For example, Cisco uses "tty10" to denote the
   tenth tty line and "Async10" to denote the tenth async interface).
   The port_len indicates the length of the port field, in bytes.

   rem_addr, rem_addr_len

   An US-ASCII string indicating the remote location from which the user
   has connected to the client.  It is intended to hold a network
   address if the user is connected via a network, a caller ID is the
   user is connected via ISDN or a POTS, or any other remote location
   information that is available.  This field is optional (since the
   information may not be available).  The rem_addr_len indicates the
   length of the user field, in bytes.

   data, data_len

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

4.2.  The Authentication REPLY Packet Body

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

    1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8
   +----------------+----------------+----------------+----------------+
   |     status     |      flags     |        server_msg_len           |
   +----------------+----------------+----------------+----------------+
   |           data_len              |        server_msg ...
   +----------------+----------------+----------------+----------------+
   |           data ...
   +----------------+----------------+

   status

   The current status of the authentication.  Legal values are:

      TAC_PLUS_AUTHEN_STATUS_PASS := 0x01




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      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.  If
   it exists, it is intended to be presented to the user.  US-ASCII
   charset MUST be used.  The server_msg_len indicates the length of the
   server_msg field, in bytes.

   data, data_len

   This field holds data that is a part of the authentication exchange
   and is intended for the client, not the user.  Examples of its use
   are shown in the section Common Authentication flows (Section 4.4.2)
   . The data_len indicates the length of the data field, in bytes.

4.3.  The Authentication CONTINUE Packet Body

   This packet is sent from the client to the server following the
   receipt of a REPLY packet.

    1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8
   +----------------+----------------+----------------+----------------+
   |          user_msg len           |            data_len             |
   +----------------+----------------+----------------+----------------+
   |     flags      |  user_msg ...
   +----------------+----------------+----------------+----------------+
   |    data ...
   +----------------+



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   user_msg, user_msg_len

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

   data, data_len

   This field carries information that is specific to the action and the
   authen_type for this session.  Valid uses of this field are described
   below.  The data_len indicates the length of the data field, in
   bytes.

   flags

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

      TAC_PLUS_CONTINUE_FLAG_ABORT := 0x01

4.4.  Description of Authentication Process

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

   This document defines a standard set of the kinds of authentication
   supported by TACACS+.  Each authentication flow consists of a START
   packet.  The server responds either with a request for more
   information (GETDATA, GETUSER or GETPASS) or a termination PASS,
   FAIL, ERROR, RESTART or FOLLOW.  The actions and meanings when the
   server sends a RESTART, ERROR or FOLLOW are common and are described
   further below.

   When the REPLY status equals TAC_PLUS_AUTHEN_STATUS_GETDATA,
   TAC_PLUS_AUTHEN_STATUS_GETUSER or TAC_PLUS_AUTHEN_STATUS_GETPASS,
   then authentication continues and the 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.



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   If the information being requested by the server form the client is
   sensitive, then the server should set the TAC_PLUS_REPLY_FLAG_NOECHO
   flag.  When the client queries the user for the information, the
   response MUST NOT be echoed as it is entered.

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

4.4.1.  Version Behaviour

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

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


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

   All authorisation and accounting and ASCII authentication use
   minor_version number of 0.

   PAP, CHAP and MS-CHAP login use minor_version 1.  The normal exchange
   is a single START packet from the client and a single REPLY from the
   server.

   SENDAUTH is only used for PPP when performing outbound
   authentication.

   The removal of SENDPASS was prompted by security concerns, and is no
   longer considered part of the TACACS+ protocol.

4.4.2.  Common Authentication Flows

   This section describes common authentication flows.  If the server
   does not implement an option, it MUST respond with
   TAC_PLUS_AUTHEN_STATUS_FAIL.

   Inbound ASCII Login




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       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.  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.

   Inbound PAP Login

       action = TAC_PLUS_AUTHEN_LOGIN
       authen_type = TAC_PLUS_AUTHEN_TYPE_PAP
       minor_version = 0x1


   The entire exchange MUST consist of a single START packet and a
   single REPLY.  The START packet MUST contain a username and the data
   field MUST contain the PAP ASCII password.  A PAP authentication only
   consists of a username and password RFC 1334 [RFC1334] . The REPLY
   from the server MUST be either a PASS, FAIL or ERROR.

   Inbound CHAP login

       action = TAC_PLUS_AUTHEN_LOGIN
       authen_type = TAC_PLUS_AUTHEN_TYPE_CHAP
       minor_version = 0x1


   The entire exchange MUST consist of a single START packet and a
   single REPLY.  The START packet MUST contain the username in the user
   field and the data field 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 PAP hash as
   defined in the PPP Authentication RFC RFC 1334 [RFC1334] and then



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   compare that value with the response.  The REPLY from the server MUST
   be a PASS, FAIL or ERROR.

   The client condcuts the exchange with the endstation and then sends
   the resulting materials (challenge and responsee) to the server.  So
   although the selection of the challenge and its length are not an
   aspect of the TACACS+ protocol, it is strongly recommended that the
   client/endstation interaction is configured with a secure challenge
   in mind, and the TACACS+ server can help by rejecting authentications
   where the challenge is below a minimum length (for example, 8 bytes).

   Inbound MS-CHAP v1 login

       action = TAC_PLUS_AUTHEN_LOGIN
       authen_type = TAC_PLUS_AUTHEN_TYPE_MSCHAP
       minor_version = 0x1


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

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

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

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

   Inbound MS-CHAP v2 login

       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.






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   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]

   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 Inbound ASCII login.

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

   ASCII change password request

   action = TAC_PLUS_AUTHEN_CHPASS
   authen_type = TAC_PLUS_AUTHEN_TYPE_ASCII


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








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4.4.3.  Aborting an Authentication Session

   The client may prematurely terminate a session by setting the
   TAC_PLUS_CONTINUE_FLAG_ABORT flag in the CONTINUE message.  If this
   flag is set, the data portion of the message may contain an ASCII
   message explaining the reason for the abort.  This information will
   be handled by the server according to the requirements of the
   deployment.  The session is terminated, for more details about
   session temrination, oplease refer to section (Section 3.4)

   In the case of PALL, FAIL or ERROR, the server can insert a message
   into server_msg to be displayed to the user.

   The Draft `The Draft' [TheDraft] defined a mechanism to direct
   authentication requests to an alternative server.  This mechanism is
   regarded as legacy and its implementation is optional.

   If this feature is not implemented, then the client should treat
   TAC_PLUS_AUTHEN_STATUS_FOLLOW as TAC_PLUS_AUTHEN_STATUS_FAIL

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

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

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

   The protocol and key are optional, and apply only to host in the same
   row.  The protocol can describe an alternate way of performing the
   authentication, other than TACACS+.  If the protocol is not present,
   then TACACS+ is assumed.

   Protocols are ASCII numbers corresponding to the methods listed in
   the authen_method field of authorization packets (defined below).
   The host is specified as either a fully qualified domain name, or an
   ASCII numeric IPV4 address specified as octets separated by dots
   ('.'), or IPV6 address text representation defined in RFC 4291.

   If a key is supplied, the client MAY use the key in order to
   authenticate to that host.  The client may use a preconfigured key
   for the host if it has one.

   Use of the hosts in a TAC_PLUS_AUTHEN_STATUS_FOLLOW packet is at the
   discretion of the TACACS+ client.  It may choose to use any one, all



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   or none of these hosts.  If it chooses to use none, then it MUST
   treat the authentication as if the return status was
   TAC_PLUS_AUTHEN_STATUS_FAIL.

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

   If the status equals TAC_PLUS_AUTHEN_STATUS_RESTART, then the
   authentication sequence is restarted with a new START packet from the
   client, with new session Id, and seq_no set to 1.  This REPLY packet
   indicates that the current authen_type value (as specified in the
   START packet) is not acceptable for this session.  The client may try
   an alternative authen_type.

   If a client does not implement TAC_PLUS_AUTHEN_STATUS_RESTART option,
   then it MUST process the response as if the status was
   TAC_PLUS_AUTHEN_STATUS_FAIL.

5.  Authorization

   In the TACACS+ Protocol, authorization is the action of determining
   what a user is allowed to do.  Generally authentication precedes
   authorization, though it is not mandatory that a client use the same
   service for authentication that it will use for authorization.  An
   authorization request may indicate that the user is not authenticated
   (we don't know who they are).  In this case it is up to the server to
   determine, according to its configuration, if an unauthenticated user
   is allowed the services in question.

   Authorization does not merely provide yes or no answers, but it may
   also customize the service for the particular user.  A common use of
   authorization is to provision a shell session when a user first logs
   in to a device to administer it.  The TACACS+ server might respond to
   the request by allowing the service, but placing a time restriction
   on the login shell.  For a list of common attributes used in
   authorization, see the Authorization Attributes section (Section 7.1)
   .

   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




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   arguments that describe the services and options for which
   authorization is requested.

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

5.1.  The Authorization REQUEST Packet Body

     1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8
   +----------------+----------------+----------------+----------------+
   |  authen_method |    priv_lvl    |  authen_type   | authen_service |
   +----------------+----------------+----------------+----------------+
   |    user_len    |    port_len    |  rem_addr_len  |    arg_cnt     |
   +----------------+----------------+----------------+----------------+
   |   arg_1_len    |   arg_2_len    |      ...       |   arg_N_len    |
   +----------------+----------------+----------------+----------------+
   |   user ...
   +----------------+----------------+----------------+----------------+
   |   port ...
   +----------------+----------------+----------------+----------------+
   |   rem_addr ...
   +----------------+----------------+----------------+----------------+
   |   arg_1 ...
   +----------------+----------------+----------------+----------------+
   |   arg_2 ...
   +----------------+----------------+----------------+----------------+
   |   ...
   +----------------+----------------+----------------+----------------+
   |   arg_N ...
   +----------------+----------------+----------------+----------------+

   authen_method

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

      TAC_PLUS_AUTHEN_METH_NOT_SET := 0x00

      TAC_PLUS_AUTHEN_METH_NONE := 0x01

      TAC_PLUS_AUTHEN_METH_KRB5 := 0x02

      TAC_PLUS_AUTHEN_METH_LINE := 0x03

      TAC_PLUS_AUTHEN_METH_ENABLE := 0x04

      TAC_PLUS_AUTHEN_METH_LOCAL := 0x05




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      TAC_PLUS_AUTHEN_METH_TACACSPLUS := 0x06

      TAC_PLUS_AUTHEN_METH_GUEST := 0x08

      TAC_PLUS_AUTHEN_METH_RADIUS := 0x10

      TAC_PLUS_AUTHEN_METH_KRB4 := 0x11

      TAC_PLUS_AUTHEN_METH_RCMD := 0x20

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

   priv_lvl

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

   authen_type

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

   authen_service

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

   user, user_len

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

   port, port_len





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

   rem_addr, rem_addr_len

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

   arg_cnt

   The number of authorization arguments to follow

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

   The arguments are the primary elements of the authorization
   interaction.  In the request packet they describe the specifics of
   the authorization that is being requested.  Each argument is encoded
   in the packet as a single arg filed (arg_1...  arg_N) with a
   corresponding length fields (which indicates the length of each
   argument in bytes).

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

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

   Optional arguments are ones that may be disregarded by either client
   or server.  Mandatory arguments require that the receiving side can
   handle the attribute, that is: its implementation and configuration
   includes the details of how to act on it.  If the client receives a
   mandatory argument that it cannot handle, it MUST consider the
   authorization to have failed.  It is legal to send an attribute-value
   pair with a zero length value.

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

   Though the attributes allow extensibility, a common core set of
   authorization attributes SHOULD be supported by clients and servers,



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   these are listed in the Authorization Attributes (Section 7.1)
   section below.

5.2.  The Authorization REPLY Packet Body

    1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8
   +----------------+----------------+----------------+----------------+
   |    status      |     arg_cnt    |         server_msg len          |
   +----------------+----------------+----------------+----------------+
   +            data_len             |    arg_1_len   |    arg_2_len   |
   +----------------+----------------+----------------+----------------+
   |      ...       |   arg_N_len    |         server_msg ...
   +----------------+----------------+----------------+----------------+
   |   data ...
   +----------------+----------------+----------------+----------------+
   |   arg_1 ...
   +----------------+----------------+----------------+----------------+
   |   arg_2 ...
   +----------------+----------------+----------------+----------------+
   |   ...
   +----------------+----------------+----------------+----------------+
   |   arg_N ...
   +----------------+----------------+----------------+----------------+

   status This field indicates the authorization status

      TAC_PLUS_AUTHOR_STATUS_PASS_ADD := 0x01

      TAC_PLUS_AUTHOR_STATUS_PASS_REPL := 0x02

      TAC_PLUS_AUTHOR_STATUS_FAIL := 0x10

      TAC_PLUS_AUTHOR_STATUS_ERROR := 0x11

      TAC_PLUS_AUTHOR_STATUS_FOLLOW := 0x21

   server_msg, server_msg_len

   This is an US-ASCII string that may be presented to the user.  The
   server_msg_len indicates the length of the server_msg field, in
   bytes.

   data, data_len

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



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   arg_cnt

   The number of authorization arguments to follow.

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

   The arguments describe the specifics of the authorization that is
   being requested.  For details of the content of the args, refer to:
   Authorization Attributes (Section 7.1) 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 attribute-value pairs (if any) in the
   response, instead of the authorization attribute-value pairs from the
   request.

   To approve the authorization with no modifications, the server sets
   the status to TAC_PLUS_AUTHOR_STATUS_PASS_ADD and the arg_cnt to 0.

   A status of TAC_PLUS_AUTHOR_STATUS_ERROR indicates an error occurred
   on the server.  For the differences between ERROR and FAIL, refer to
   section Session Completion (Section 3.4) . None of the arg values
   have any relevance if an ERROR is set, and must be ignored.

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

6.  Accounting

   Accounting is typically the third action after authentication and
   authorization.  But again, neither authentication nor authorization
   is required.  Accounting is the action of recording what a user is
   doing, and/or has done.  Accounting in TACACS+ can serve two
   purposes: It may be used as an auditing tool for security services.
   It may also be used to account for services used, such as in a
   billing environment.  To this end, TACACS+ supports three types of



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   accounting records.  Start records indicate that a service is about
   to begin.  Stop records indicate that a service has just terminated,
   and Update records are intermediate notices that indicate that a
   service is still being performed.  TACACS+ accounting records contain
   all the information used in the authorization records, and also
   contain accounting specific information such as start and stop times
   (when appropriate) and resource usage information.  A list of
   accounting attributes is defined in the accounting section
   (Section 6) .

6.1.  The Account REQUEST Packet Body

    1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8
   +----------------+----------------+----------------+----------------+
   |      flags     |  authen_method |    priv_lvl    |  authen_type   |
   +----------------+----------------+----------------+----------------+
   | authen_service |    user_len    |    port_len    |  rem_addr_len  |
   +----------------+----------------+----------------+----------------+
   |    arg_cnt     |   arg_1_len    |   arg_2_len    |      ...       |
   +----------------+----------------+----------------+----------------+
   |   arg_N_len    |    user ...
   +----------------+----------------+----------------+----------------+
   |   port ...
   +----------------+----------------+----------------+----------------+
   |   rem_addr ...
   +----------------+----------------+----------------+----------------+
   |   arg_1 ...
   +----------------+----------------+----------------+----------------+
   |   arg_2 ...
   +----------------+----------------+----------------+----------------+
   |   ...
   +----------------+----------------+----------------+----------------+
   |   arg_N ...
   +----------------+----------------+----------------+----------------+


   flags

   This holds bitmapped flags.

      TAC_PLUS_ACCT_FLAG_START := 0x02

      TAC_PLUS_ACCT_FLAG_STOP := 0x04

      TAC_PLUS_ACCT_FLAG_WATCHDOG := 0x08

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



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   the conditions on the client, and authentication context, so that
   these details may be logged for accounting purposes.

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

6.2.  The Accounting REPLY Packet Body

   The purpose of accounting is to record the action that has occurred
   on the client.  The server MUST reply with success only when the
   accounting request has been recorded.  If the server did not record
   the accounting request then it MUST reply with ERROR.

    1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8  1 2 3 4 5 6 7 8
   +----------------+----------------+----------------+----------------+
   |         server_msg len          |            data_len             |
   +----------------+----------------+----------------+----------------+
   |     status     |         server_msg ...
   +----------------+----------------+----------------+----------------+
   |     data ...
   +----------------+

   status

   This is the return status.  Values are:

      TAC_PLUS_ACCT_STATUS_SUCCESS := 0x01

      TAC_PLUS_ACCT_STATUS_ERROR := 0x02

      TAC_PLUS_ACCT_STATUS_FOLLOW := 0x21

   server_msg, server_msg_len

   This is a US-ASCII string that may be presented to the user.  The
   server_msg_len indicates the length of the server_msg field, in
   bytes.

   data, data_len

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

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



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   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.
   Update records are sent at the client's discretion if the task has
   not finished.

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

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

7.  Attribute-Value Pairs

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

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




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

   It is recommended that hosts be specified as a IP address so as to
   avoid any ambiguities.  ASCII numeric IPV4 address are specified as
   octets separated by dots ('.'), IPV6 address text representation
   defined in RFC 4291.

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

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

7.1.  Authorization Attributes

   service

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

   protocol

   the ptotocol field may be used to indicate a subset of a setvice.

   cmd

   a shell (exec) command.  This indicates the command name of the
   command that is to be run.  The "cmd" attribute MUST be specified if
   service equals "shell".

   Authorization of shell commands is a common use-case for the TACACS+
   protocol.  Command Authorization generally takes one of two forms:
   session-based and command-based.

   For session-based shell authorization, the "cmd" argument will have
   an empty value.  The client determines which commands are allowed in
   a session according to the arguments present in the authorization.

   In command-based authorization, the client requests that the server
   determine whether a command is allowed by making an authorization
   request for each command.  The "cmd" argument will have the command
   name as its value.




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   cmd-arg

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

   acl

   US-ASCII number representing a connection access list.  Applicable
   only to session-based shell authorization.

   inacl

   US-ASCII identifier for an interface input access list.

   outacl

   US-ASCII identifier for an interface output access list.

   addr

   a network address

   addr-pool

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

   routing

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

   route

   Indicates a route that is to be applied to this interface.  Values
   MUST be of the form "<dst_address> <mask> [<routing_addr>]".  If a
   <routing_addr> is not specified, the resulting route is via the
   requesting peer.

   timeout

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

   idletime





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

   autocmd

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

   priv-lvl

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

   remote_user

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

   remote_host

   remote host (authen_method must have the value
   TAC_PLUS_AUTHEN_METH_RCMD)

   callback-dialstring

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

   callback-line

   The line number to use for a callback.

   callback-rotary



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   The rotary number to use for a callback.

   nocallback-verify

   Do not require authentication after callback.

7.2.  Accounting Attributes

   The following attributes are defined for TACACS+ accounting only.
   They MUST precede any attribute-value pairs that are defined in the
   authorization section (Section 5) above.

   task_id

   Start and stop records for the same event MUST have matching task_id
   attribute values.  The client MUST ensure that active task_ids are
   not duplicated: a client MUST NOT reuse a task_id a start record
   until it has sent a stop record for that task_id.  Servers MUST not
   make assumptions about the format of a task_id.

   start_time

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

   stop_time

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

   elapsed_time

   The elapsed time in seconds for the action.

   timezone

   The timezone abbreviation for all timestamps included in this packet.

   event

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

   reason

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

   bytes



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   The number of bytes transferred by this action

   bytes_in

   The number of input bytes transferred by this action to the port

   bytes_out

   The number of output bytes transferred by this action from the port

   paks

   The number of packets transferred by this action.

   paks_in

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

   paks_out

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

   status

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

   err_msg

   An US-ASCII string describing the status of the action.

8.  Privilege Levels

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

   One scheme is built in to the protocol 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.  Pre-defined values
   are:




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      TAC_PLUS_PRIV_LVL_MAX := 0x0f

      TAC_PLUS_PRIV_LVL_ROOT := 0x0f

      TAC_PLUS_PRIV_LVL_USER := 0x01

      TAC_PLUS_PRIV_LVL_MIN := 0x00

   A Privilege level can be assigned to a shell (EXEC) session when it
   starts starts (for example, TAC_PLUS_PRIV_LVL_USER).  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 perfrom actions that are mapped to a higher
   privilege level, then an ENABLE type reuthentication can be initiated
   by the client, in a way similar to su in unix.  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, and
   so it is no longer mandatory for clients to use it.  However, it is
   still common enough that it SHOULD be supported by servers.

9.  TACACS+ Security Considerations

   Although in widespread use, the TACACS+ protocol (as defined in "the
   Draft") does not meet modern security standards on its own.  For this
   reason, the authors intend to follow up this document with a more
   secure version of the protocol.

   TACACS+ was originally specified in "The Draft" (1998) is incomplete,
   and leaves key points unspecified.  As a result, software authors
   have had to make implementation choices about what should, or should
   not, be done in certain situations.  These implementation choices are
   somewhat constrained by ad hoc interoperability tests.  That is, all
   TACACS+ clients and servers interoperate, so there is a rough
   consensus on how the protocol works.

9.1.  Overall Security of The Protocol

   TACACS+ protocol does not include a security mechanism that would
   meet modern day requirements.  Support for MD5-based crypto pad
   encryption fails to provide any kind of transport integrity, which
   presents at least the following risks:



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      Accounting information may be modified by the man-in-the-middle
      attacker, making such logs unsuitable and untrustable for auditing
      purposes.

      Only the body of the request is encrypted which leaves all header
      fields open to trivial modification by the man-in-the-middle
      attacker.

      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 encrypted 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 means that original data may be revealed at the
      later time and still provide valuable information to the attacker.

   Even though, to the best knowledge of authors, this method of
   encryption wasn't rigorously tested, authors feel that enough
   information is available that it is best referred to as "obfuscation"
   and not "encryption" and as such it MUST NOT BE relied upon to
   provide privacy.

   For example, a "session_id" can be replaced by an alternate one,
   which could allow an unprivileged administrator to "steal" the
   authorization from a session for a privileged administrator.  An
   attacker could also update the "flags" field to indicate that one or
   the other end of a connection requires TAC_PLUS_UNENCRYPTED_FLAG,
   which would subvert the obfuscation mechanism.

   For this reasons, users deploying TACACS+ protocol in their
   environments MUST limit access to known clients and MUST control the
   security of the entire transmission path.  Attacks who can guess the
   key or otherwise break the obfuscation WILL gain unrestricted and
   undetected access to all TACACS+ traffic.  The security risk of such
   attack succeeding against a centralised AAA system like TACACS+
   cannot be overstated.



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9.2.  Security of Authentication Sessions

   The authentication options include options which MUST NOT be used
   outside a secured deployment.  Specifically, options which permit the
   exchange of clear-text passwords or MSCHAPv1 and MS-CHAPv2.  As of
   the publication of this document, there has been no similar attacks
   on the CHAP protocol.

   Section 4.4.3 permits the redirection of a session to another server
   via the TAC_PLUS_AUTHEN_STATUS_FOLLOW mechanism.  As part of this
   process, the secret key for a new server can be sent to the client.
   This public exchange of secret keys means that once one session is
   broken, it may be possible to leverage that attack to attacking
   connections to other servers.  This option MUST NOT be used outside a
   secured deployment.

9.3.  Security of Authorization Sessions

   TACACS+ authorization is specifically separate from authentication.
   Careful consideration must be given to whether this mode is
   appropriate for the target deployment.  Authorization sessions are
   not cryptographically linked to any authentication sessions.
   Instead, sessions are tied together implicitly by the contents of the
   other fields, such as "use", "port", "rem_addr", etc.

   The specification allows for the exchange of attribute-value pairs.
   While a few such attributes are defined here, the protocol is
   extensible, and vendors can define their own attributes.  There is no
   registry for such attributes, and in the absence of a published
   specification, no way for a client or server to know the meaning of a
   new attribute.

   As a result, implemetors MUST ensure that new attribute-value pairs
   are used consistently to communicate between client and server
   implementations.

9.4.  Security of Accounting Sessions

   The security considerations for accounting sessions are largely the
   same as for authorization sessions.  This section describes
   additional issues specific to accounting sessions.

   There is no way in TACACS+ to signal that accounting is required.
   There is no way for a server to signal a client how often accounting
   is required.  The accounting packets are received solely at the
   clients discretion.  Adding such functionality would assist with
   auditing of user actions.




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   The "task_id" field is defined only for accounting packets, and not
   for authentication or authorization packets.  As such, it is
   difficult to correlate accounting data with a previous authentication
   or authorization request.

9.5.  TACACS+ Deployment Recommendations

   Due to the above concerns with the protocol, it is critical that it
   be deployed in a secure manner.  The following recommendations are
   made for those deploying and configuring TACACS+ as a solution for
   Device Administration:

      Secure the Deployment: TACACS+ does not provide modern security so
      TACACS+ MUST BE employed over networks which ensure privacy and
      integrity of the communication.  The way this is ensured will
      depend upon the organisational means: a dedicated and secure
      management network where available in enterprise deployments, or
      IPsec where dedicated networks are not available.

      Always set a secret key (recommended minimum 14 characters) on the
      client and server when configuring the connection between them.

      Servers MUST be configured with a list of known clients.  Servers
      MUST be configured to reject requests from clients not on the
      list.  A unique secret key SHOULD be configured for every
      individual client.

      Restrict to TAC_PLUS_AUTHEN_TYPE_CHAP for authen_type where
      possible.  Use other options only when unavoidable due to
      requirements of identity/password systems.

      Servers SHOULD be restricted to requiring TACACS+ authentication
      for authorization requests (i.e.  TAC_PLUS_AUTHEN_METH_TACACSPLUS
      is used).

      Avoid the use of the redirection mechanism.
      TAC_PLUS_AUTHEN_STATUS_FOLLOW, specifically avoid the option to
      send send secret keys in the server list.

      Take case when applying extensions to the dictionary of
      authorization/accounting arguments.  Ensure that the client and
      server use of new argument names are consistent.

9.6.  TACACS+ Client Implementation Recommendations

   When implementing TACACS+ Clients it is recommended:





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      Clients SHOULD not use TAC_PLUS_UNENCRYPTED_FLAG, even on networks
      that are considered secure.

      Ignore redirects to hosts which are outside of the pre-configured
      range or list.  A client SHOULD ignore any key provided via
      TAC_PLUS_AUTHEN_STATUS_FOLLOW, and SHOULD instead use a
      preconfigured key for that host.

      If receiving an unknown mandatory authorization attribute, behave
      as if it had received TAC_PLUS_AUTHOR_STATUS_FAIL.  For full
      details, refer to (Authorization attributes section).

9.7.  TACACS+ Server Implementation Recommendations

   When implementing TACACS+ Servers, it is recommended:

      Server SHOULD reject all connections which have the
      TAC_PLUS_UNENCRYPTED_FLAG with applicable ERROR response for type
      of packet.

      Servers MUST permit configuration of secret keys per individual
      client.  Servers SHOULD warn administrators if secret keys are not
      unique per client.

      On detection of an invalid shared secret: Servers SHOULD NOT
      accept any new sessions on a connection, and terminate the
      connection on completion of any sessions previously established
      with a valid shared secret.

      Allow the administrator to mandate : - TAC_PLUS_AUTHEN_TYPE_CHAP
      for authen_type - TAC_PLUS_AUTHEN_METH_TACACSPLUS for
      authen_method in authorization - Minimum length for shared secrets

9.8.  TACACS+ Security and Operational Concerns

   This section identifies some of the known security and operational
   concerns.  It is important to acknowledge that TACACS+ on its own
   does not provide modern levels of security, and that it MUST be used
   within a secure deployment.

      The "encryption" is based upon MD5.  In modern terms this can be
      regarded merely as "obfuscation".

      Only the packet body (not header) is obfuscated.  For example,
      session_id, flags containing TAC_PLUS_UNENCRYPTED_FLAG is exposed
      in cleartext.





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      Support of insecure authentication protocols such as plaintext,
      MS-CHAP.

      Difficulty to correlate authentication, authorization, and
      accounting requests for a single unit of end client activity.

      Potential confusion between clients and servers from different
      vendors of the meaning of specific argument attributes.

      Potential confusion between clients and servers from different
      vendors of the meaning of specific commands.

   In summary: It is strongly advised that TACACS+ MUST be used within a
   secure deployment.  Failure to do so may impact overall network
   security.

10.  References

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

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

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

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

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

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

Authors' Addresses







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

   EMail: thorstendlux@google.com


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

   EMail: aota@google.com


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

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


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

   EMail: dcarrel@viptela.com


   Lol Grant













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