Yang data model for TACACS+
draft-ietf-opsawg-tacacs-yang-07

Abstract

This document defines a YANG module 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 module 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 working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

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

This Internet-Draft will expire on December 22, 2020.

Copyright Notice

Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

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

1. Introduction

This document defines a YANG module 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 defines two YANG features to support local or RADIUS authentication:

• User Authentication Model: Defines a list of usernames and passwords and control the order in which local or RADIUS authentication is used.
• 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 model 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "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 specification:

• client
• configuration data
• server
• state data

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

• augment
• data model
• 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 [RFC8340].

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 the contents of the"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, 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 session.

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?              tcsplus-server-type
          +--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

This YANG module imports typedefs from [RFC6991].

<CODE BEGINS> file "ietf-system-tacacsplus@2020-05-22.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;
    reference
      "RFC 6991: Common YANG Data Types";
  }
  import ietf-yang-types {
    prefix yang;
    reference
      "RFC 6991: Common YANG Data Types";
  }
  import ietf-network-instance {
    prefix ni;
    reference
      "RFC 8529: YANG Data Model for Network Instances";
  }
  import ietf-interfaces {
    prefix if;
    reference
      "RFC 8343: A YANG Data Model for Interface Management";
  }
  import ietf-system {
    prefix sys;
    reference
      "RFC 7317: A YANG Data Model for System Management";
  }
  import ietf-netconf-acm {
    prefix nacm;
    reference
      "RFC 8341: Network Configuration Access Control Model";
  }

  organization
    "IETF Opsawg (Operations and Management Area Working Group)";
  contact
    "WG Web:   <http://tools.ietf.org/wg/opsawg/>
     WG List:  <mailto:opsawg@ietf.org>

     Editor: Bo Wu <lana.wubo@huawei.com>
     Editor: Guangying Zheng <zhengguangying@huawei.com>";
  description
    "This module provides configuration of TACACS+ client.

     Copyright (c) 2020 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
     (http://trustee.ietf.org/license-info).

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

  // RFC Ed.: update the date below with the date of RFC 
  // publication and remove this note.
  // RFC Ed.: replace XXXX with actual RFC number and remove 
  // this note, and the TACACS+ Protocol refers to
  // draft-ietf-opsawg-tacacs.

  revision 2020-05-22 {
    description
      "Initial revision.";
    reference
      "RFC XXXX: A Yang Data Model for TACACS+";
  }

  typedef tcsplus-server-type {
    type bits {
      bit authentication {
        description
          "When set, the server is an authentication server.";
      }
      bit authorization {
        description
          "When set, the server is an authorization server.";
      }
      bit accounting {
        description
          "When set, the server is an accounting server.";
      }
    }
    description
      "tcsplus-server-type can be set to
       authentication/authorization/accounting
       or any combination of the three types. When all three types are
       supported, all the three bits are set.";
  }

  feature tacacsplus {
    description
      "Indicates that the device can be configured as a TACACS+
       client.";
    reference
      "RFC XXXX : The TACACS+ Protocol ";
  }

  identity tacacsplus {
    base sys:authentication-method;
    description
      "Indicates AAA operation using TACACS+.";
    reference
      "RFC XXXX: The TACACS+ Protocol";
  }

  grouping statistics {
    description
      "Grouping for TACACS+ statistics attributes";
    container statistics {
      config false;
      description
        "A collection of server-related statistics objects";
      leaf connection-opens {
        type yang:counter64;
        description
          "Number of new connection requests sent to the server, e.g.
           socket open";
      }
      leaf connection-closes {
        type yang:counter64;
        description
          "Number of connection close requests sent to the server, e.g.
           socket close";
      }
      leaf connection-aborts {
        type yang:counter64;
        description
          "Number of aborted connections to the server.  These do
           not include connections that are close gracefully.";
      }
      leaf connection-failures {
        type yang:counter64;
        description
          "Number of connection failures to the server";
      }
      leaf connection-timeouts {
        type yang:counter64;
        description
          "Number of connection timeouts to the server";
      }
      leaf messages-sent {
        type yang:counter64;
        description
          "Number of messages sent to the server";
      }
      leaf messages-received {
        type yang:counter64;
        description
          "Number of messages received from the server";
      }
      leaf errors-received {
        type yang:counter64;
        description
          "Number of error messages received from the server";
      }
      leaf sessions {
        type yang:counter64;
        description
          "Number of TACACS+ sessions completed with the server.
           If the Single Connection Mode was NOT enabled, the number of
           sessions is the same as the number of 'connection-closes'.
           If the Mode was enabled, a single TCP connection may contain
           multiple TACACS+ sessions.";
      }
    }
  }

  grouping tacacsplus {
    description
      "Grouping for TACACS+ attributes";
    container tacacsplus {
      if-feature "tacacsplus";
      must "not(derived-from-or-self(../sys:authentication"
         + "/sys:user-authentication-order, 'tacacsplus')) or server" {
        error-message "When 'tacacsplus' is used as a system"
                    + " authentication method, a TACACS+ server"
                    + " must be configured.";
        description
          "When 'tacacsplus' is used as an authentication method,
           a TACACS+ server must be configured.";
      }
      description
        "Container for TACACS+ configurations and operations.";
      list server {
        key "name";
        ordered-by user;
        description
          "List of TACACS+ servers used by the device.";
        leaf name {
          type string;
          description
            "An arbitrary name for the TACACS+ server.";
        }
        leaf server-type {
          type tcsplus-server-type;
          description
            "Server type: authentication/authorization/accounting and
             various combinations.
             When all three types are supported, all the three bits
             are set.";
        }
        leaf address {
          type inet:host;
          mandatory true;
          description
            "The address of the TACACS+ server.";
        }
        leaf port {
          type inet:port-number;
          default "49";
          description
            "The port number of TACACS+ Server port.";
        }
        leaf shared-secret {
          type string {
            length "16..max";
          }
          mandatory true;
          nacm:default-deny-all;
          description
            "The shared secret, which is known to both the
             TACACS+ client and server. TACACS+ server administrators
             should configure shared secret of minimum 16 characters
             length.
             It is highly recommended that shared keys are at least 32
             characters long.";
          reference
            "RFC XXXX: The TACACS+ Protocol";
        }
        choice source-type {
          description
            "The source address type for outbound TACACS+ packets.";
          case source-ip {
            leaf source-ip {
              type inet:ip-address;
              description
                "Specifies source IP address for TACACS+ outbound
                 packets.";
            }
          }
          case source-interface {
            leaf source-interface {
              type if:interface-ref;
              description
                "Specifies the interface from which the IP address is
                 derived for use as the source for the outbound TACACS+
                 packet";
            }
          }
        }
        leaf vrf-instance {
          type leafref {
            path "/ni:network-instances/ni:network-instance/ni:name";
          }
          description
            "Specifies the VPN Routing and Forwarding (VRF) instance to
             use to communicate with the TACACS+ server.";
        }
        leaf single-connection {
          type boolean;
          default "false";
          description
            "Whether the single connection mode is enabled for the
             server. By default, the single connection mode is
             disabled.";
        }
        leaf timeout {
          type uint16 {
            range "1..300";
          }
          units "seconds";
          default "5";
          description
            "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" {
    description
      "Augment the system model with the tacacsplus model";
    uses tacacsplus;
  }
}

<CODE ENDS>

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. These are the subtrees and data nodes and their sensitivity/vulnerability:

/system/tacacsplus/server:

This list contains the objects used to control the TACACS+ servers used by the device. Unauthorized access to this list could cause a user management failure on the device.

/system/tacacsplus/server/shared-secret:

This leaf controls the key known to both the TACACS+ client and server. Unauthorized access to this leaf could cause the device vulnerable to attacks, therefore has been restricted using the "default-deny-all" access control defined in [RFC8341].

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 (RFC Ed.: replace XXXX with actual 
     RFC number and remove this note.)

7. Acknowledgments

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

8. References

8.1. Normative References

8.2. Informative References

Appendix A. Example Tacacs+ Authentication Configuration

The following shows an example where a tacacs+ authentication server instance is configured.

   {
     "ietf-system:system": {
       "authentication": {
         "user-authentication-order": [tacacsplus, local-users]  
       }
       "tacacsplus": {
         "server": [
           {
             "name": "tac_plus1",
             "server-type": "authentication",
             "address": "192.0.2.2",
             "shared-secret": "QaEfThUkO1980100754609236h3TbE8n",
             "source-ip": "192.0.2.12",
             "single-connection": "false",
             "timeout": "10"
           }
         ]
       }   
     }
   }

Authors' Addresses