Network Working Group                                           G. Zheng
Internet-Draft                                                   M. Wang
Intended status: Standards Track                                   B. Wu
Expires: May 6, September 9, 2020                                        Huawei
                                                        November 3, 2019
                                                           March 8, 2020

                      Yang data model for TACACS+


   This document defines YANG modules that augment the System Management
   data model defined in the RFC 7317 with TACACS+ client model.  The
   data model of Terminal Access Controller Access Control System Plus
   (TACACS+) client allows the configuration of TACACS+ servers for
   centralized Authentication, Authorization and Accounting.

   The YANG modules in this document conforms to the Network Management
   Datastore Architecture (NMDA) defined in RFC 8342.

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
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   Drafts is at

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   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 May 6, September 9, 2020.

Copyright Notice

   Copyright (c) 2019 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
   ( 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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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions used in this document . . . . . . . . . . . . . .   3
     2.1.  Tree Diagrams . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Design of the Data Model  . . . . . . . . . . . . . . . . . .   3
   4.  TACACS+ Client Module . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11  12
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  12
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  TACACS+ Authentication Configuration . . . . . . . .  13  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   This document defines YANG modules that augment the System Management
   data model defined in the [RFC7317] with TACACS+ client model.

   TACACS+ provides Device Administration for routers, network access
   servers and other networked computing devices via one or more
   centralized servers which is defined in the TACACS+ Protocol.

   The System Management Model [RFC7317] defines two YANG features to
   support local or RADIUS authentication:

   o  User Authentication Model: Defines a list of usernames and
      passwords and control the order in which local or RADIUS
      authentication is used.

   o  RADIUS Client Model: Defines a list of RADIUS servers that a
      device uses.

   Since TACACS+ is also used for device management and the feature is
   not contained in the System Management model, this document defines a
   YANG data model that allows users to configure TACACS+ client
   functions on a device for centralized Authentication, Authorization
   and Accounting provided by TACACS+ servers.

   The YANG models can be used with network management protocols such as
   NETCONF[RFC6241] to install, manipulate, and delete the configuration
   of network devices.

   The YANG data model in this document conforms to the Network
   Management Datastore Architecture (NMDA) defined in [RFC8342].

2.  Conventions used in this document

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP14, [RFC2119], [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The following terms are defined in [RFC6241] and are used in this

   o  client

   o  configuration data

   o  server

   o  state data

   The following terms are defined in [RFC7950] and are used in this

   o  augment

   o  data model

   o  data node

   The terminology for describing YANG data models is found in

2.1.  Tree Diagrams

   Tree diagrams used in this document follow the notation defined in

3.  Design of the Data Model

   This model is used to configure TACACS+ client on the device to
   support deployment scenarios with centralized authentication,
   authorization, and accounting servers.  Authentication is used to
   validate a user's name and password, authorization allows the user to
   access and execute commands at various command levels assigned to the
   user and accounting keeps track of the activity of a user who has
   accessed the device.

   The ietf-system-tacacsplus module is intended to augment the
   "/sys:system" path defined in the ietf-system module with
   "tacacsplus" grouping.  Therefore, a device can use local, Remote
   Authentication Dial In User Service (RADIUS), or Terminal Access
   Controller Access Control System Plus (TACACS+) to validate users who
   attempt to access the router by several mechanisms, e.g. a command
   line interface or a web-based user interface.

   The "server" list is directly under the "tacacsplus" container, which
   holds a list of TACACS+ servers and uses server-type to distinguish
   between the three protocols.  The list of servers is for redundancy.

   Most of the parameters in the "server" list are taken directly from
   the TACACS+ protocol [I-D.ietf-opsawg-tacacs], and some are derived
   from the various implementations by network equipment manufacturers.
   For example, when there are multiple interfaces connected to the
   TACACS+ client or server, the source address of outgoing TACACS+
   packets could be specified, or the source address could be specified
   through the interface setting, or derived from the out-bound
   interface from the local FIB.  For the TACACS+ server located in a
   Virtual Private Network(VPN), a VRF instance needs to be specified.

   The "statistics" container under the "server list" is to record
   session statistics and usage information during user access which
   include the amount of data a user has sent and/or received during a

   The data model for TACACS+ client has the following structure:

   module: ietf-system-tacacsplus
     augment /sys:system:
       +--rw tacacsplus {tacacsplus}?
          +--rw server* [name]
             +--rw name                      string
             +--rw server-type?              enumeration
             +--rw address                   inet:host
             +--rw port?                     inet:port-number
             +--rw shared-secret             string
             +--rw (source-type)?
             |  +--:(source-ip)
             |  |  +--rw source-ip?          inet:ip-address
             |  +--:(source-interface)
             |     +--rw source-interface?   if:interface-ref
             +--rw vrf-instance?
             |       -> /ni:network-instances/network-instance/name
             +--rw single-connection?        boolean
             +--rw timeout?                  uint16
             +--ro statistics
                +--ro connection-opens?      yang:counter64
                +--ro connection-closes?     yang:counter64
                +--ro connection-aborts?     yang:counter64
                +--ro connection-failures?   yang:counter64
                +--ro connection-timeouts?   yang:counter64
                +--ro messages-sent?         yang:counter64
                +--ro messages-received?     yang:counter64
                +--ro errors-received?       yang:counter64
                +--ro sessions?              yang:counter64

4.  TACACS+ Client Module

   <CODE BEGINS> file "ietf-system-tacacsplus@2019-11-01.yang" "ietf-system-tacacsplus@2020-03-05.yang"

module ietf-system-tacacsplus {
  yang-version 1.1;
  namespace "urn:ietf:params:xml:ns:yang:ietf-system-tacacsplus";
  prefix sys-tcsplus;

  import ietf-inet-types {
    prefix inet;
      "RFC 6991: Common YANG Data Types";
  import ietf-yang-types {
    prefix yang;
      "RFC 6991: Common YANG Data Types";
  import ietf-network-instance {
    prefix ni;
      "RFC 8529: YANG Data Model for Network Instances";
  import ietf-interfaces {
    prefix if;
      "RFC 8343: A YANG Data Model for Interface Management";
  import ietf-system {
    prefix sys;
      "RFC 7317: A YANG Data Model for System Management";
  import ietf-netconf-acm {
    prefix nacm;
      "RFC 8341: Network Configuration Access Control Model";

    "IETF Opsawg (Operations and Management Area Working Group)";
    "WG Web:   <>
     WG List:  <>

     Editor:   Guangying Zheng
    "This module provides configuration of TACACS+ client.

     Copyright (c) 2019 IETF Trust and the persons identified as
     authors of the code.  All rights reserved.

     Redistribution and use in source and binary forms, with or
     without modification, is permitted pursuant to, and subject
     to the license terms contained in, the Simplified BSD License
     set forth in Section 4.c of the IETF Trust's Legal Provisions
     Relating to IETF Documents

     This version of this YANG module is part of RFC XXXX; see the
      RFC itself for full legal notices.";

  revision 2019-11-01 2020-03-05 {
      "Initial revision.";


  feature tacacsplus {
      "Indicates that the device can be configured as a TACACS+
      "draft-ietf-opsawg-tacacs-11: The TACACS+ Protocol";

  identity tacacsplus {
    base sys:authentication-method;
      "Indicates AAA operation using TACACS+.";
      "draft-ietf-opsawg-tacacs-11: The TACACS+ Protocol";

  grouping statistics {
      "Grouping for TACACS+ statistics attributes";
    container statistics {
      config false;
        "A collection of server-related statistics objects";
      leaf connection-opens {
        type yang:counter64;
          "Number of new connection requests sent to the server, e.g.
           socket open";
      leaf connection-closes {
        type yang:counter64;
          "Number of connection close requests sent to the server, e.g.
           socket close";
      leaf connection-aborts {
        type yang:counter64;
          "Number of aborted connections to the server.  These do
           not include connections that are close gracefully.";
      leaf connection-failures {
        type yang:counter64;
          "Number of connection failures to the server";
      leaf connection-timeouts {
        type yang:counter64;
          "Number of connection timeouts to the server";
      leaf messages-sent {
        type yang:counter64;
          "Number of messages sent to the server";
      leaf messages-received {
        type yang:counter64;
          "Number of messages received by the server";
      leaf errors-received {
        type yang:counter64;
          "Number of error messages received from the server";
      leaf sessions {
        type yang:counter64;
          "Total Number of sessions. A single-connection tacacs+
           connection may be >1 sessions.";

  grouping tacacsplus {
      "Grouping for TACACS+ attributes";
    container tacacsplus {
      must "not(derived-from-or-self(../sys:authentication"
         + "/sys:user-authentication-order, 'tacacsplus')) or server" {
        error-message "When 'tacacsplus' is used as a sysytem"
                    + " authentication method, a TACACS+ server"
                    + " must be configured.";
          "When 'tacacsplus' is used as an authentication method,
           a TACACS+ server must be configured.";
      if-feature "tacacsplus";
        "Container for TACACS+ configurations and operations.";
      list server {
        key "name";
        ordered-by user;
          "List of TACACS+ servers used by the device.";
        leaf name {
          type string;
            "An arbitrary name for the TACACS+ server.";
        leaf server-type {
          type enumeration {
            enum authentication {
                "The server is an authentication server.";
            enum authorization {
                "The server is an authorization server.";
            enum accounting {
                "The server is an accounting server.";
            enum all {
                "The group of all types of TACACS+ servers.";
            "Server type: authentication/authorization/accounting/all.";
        leaf address {
          type inet:host;
          mandatory true;
            "The address of the TACACS+ server.";
        leaf port {
          type inet:port-number;
          default "49";
            "The port number of TACACS+ Server port.";
        leaf shared-secret {
          type string;
          mandatory true;
            "The shared secret, which is known to both the
             TACACS+ client and server. TACACS+ server administrators
             should configure secret keys of minimum
             16 characters length.";
            "TACACS+ protocol:";
        choice source-type {
            "The source address type for outbound TACACS+ packets.";
          case source-ip {
            leaf source-ip {
              type inet:ip-address;
                "Specifies source IP address for TACACS+ outbound
          case source-interface {
            leaf source-interface {
              type if:interface-ref;
                "Specifies the interface from which the IP address is
                 derived for use as the source for the outbound TACACS+
        leaf vrf-instance {
          type leafref {
            path "/ni:network-instances/ni:network-instance/ni:name";
            "Specifies the VPN Routing and Forwarding (VRF) instance to
             use to communicate with the TACACS+ server.";
        leaf single-connection {
          type boolean;
          default "false";
            "Whether the single connection mode is enabled for the
             server. By default, the single connection mode is
        leaf timeout {
          type uint16 {
            range "1..300";
          units "seconds";
          default "5";
            "The number of seconds the device will wait for a
             response from each TACACS+ server before trying with a
             different server.";
        uses statistics;

  augment "/sys:system" {
      "Augment the system model with authorization and accounting
       Augment the system model with the tacacsplus model";
    uses tacacsplus;


5.  Security Considerations

   The YANG module defined in this document is designed to be accessed
   via network management protocols such as NETCONF [RFC6241] or
   RESTCONF [RFC8040].  The lowest NETCONF layer is the secure transport
   layer, and the mandatory-to-implement secure transport is Secure
   Shell (SSH) [RFC6242].  The lowest RESTCONF layer is HTTPS, and the
   mandatory-to-implement secure transport is TLS [RFC8446].

   The NETCONF access control model [RFC8341] provides the means to
   restrict access for particular NETCONF or RESTCONF users to a
   preconfigured subset of all available NETCONF or RESTCONF protocol
   operations and content.

   There are a number of data nodes defined in this YANG module that are
   writable/creatable/deletable (i.e., config true, which is the
   default).  These data nodes may be considered sensitive or vulnerable
   in some network environments.  Write operations (e.g., edit-config)
   to these data nodes without proper protection can have a negative
   effect on network operations.

   This document describes the use of TACACS+ for purposes of
   authentication, authorization and accounting, it is vulnerable to all
   of the threats that are present in TACACS+ applications.  For a
   discussion of such threats, see Section 9 of the TACACS+ Protocol

6.  IANA Considerations

   This document registers a URI in the IETF XML registry [RFC3688].
   Following the format in [RFC3688], the following registration is
   requested to be made:

      URI: urn:ietf:params:xml:ns:yang:ietf-system-tacacsplus
      Registrant Contact: The IESG.
      XML: N/A, the requested URI is an XML namespace.

   This document registers a YANG module in the YANG Module Names
   registry [RFC7950].

        Name: ietf-system-tacacsplus
               Namespace: urn:ietf:params:xml:ns:yang: ietf-tacacsplus
               Prefix: sys-tcsplus
               Reference: RFC XXXX

7.  Acknowledgments

   The authors wish to thank Alex Campbell, John Heasley, Ebben Aries,
   Alan DeKok, Joe Clarke, and many others for their helpful comments
   and suggestions.

8.  References

8.1.  Normative References

              Dahm, T., Ota, A.,, d., Carrel, D., and
              L. Grant, "The TACACS+ Protocol", draft-ietf-opsawg-
              tacacs-17 (work in progress), September November 2019.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,

   [RFC6242]  Wasserman, M., "Using the NETCONF Protocol over Secure
              Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,

   [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",
              RFC 6991, DOI 10.17487/RFC6991, July 2013,

   [RFC7317]  Bierman, A. and M. Bjorklund, "A YANG Data Model for
              System Management", RFC 7317, DOI 10.17487/RFC7317, August
              2014, <>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,

   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
              BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,

   [RFC8341]  Bierman, A. and M. Bjorklund, "Network Configuration
              Access Control Model", STD 91, RFC 8341,
              DOI 10.17487/RFC8341, March 2018,

   [RFC8342]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "Network Management Datastore Architecture
              (NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

8.2.  Informative References

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,

Appendix A.  TACACS+ Authentication Configuration

   The system management model defines two authentication configuration
   options and controls authentication methods by configuring "user-
   authentication-order" . One is "local-users", and the other is

   This draft defines the "tacacsplus" model extension and therefore
   needs to be configured in the same way.  The 'tacacsplus' identity is
   defined to control whether or not TACACS+ authentication should be
   used.  The current system authentication configuration model is as

   +--rw system
    +--rw authentication
       +--rw user-authentication-order*   identityref

Authors' Addresses

   Guangying Zheng
   101 Software Avenue, Yuhua District
   Nanjing, Jiangsu  210012


   Michael Wang
   Huawei Technologies, Co.,
   101 Software Avenue, Yuhua District
   Nanjing  210012


   Bo Wu
   101 Software Avenue, Yuhua District
   Nanjing, Jiangsu  210012