Huawei Technologies, Co., Ltd

Yuhua District 101 Software Avenue Nanjing Jiangsu 210012 China lana.wubo@huawei.com

Huawei Technologies, Co., Ltd

Yuhua District 101 Software Avenue Nanjing Jiangsu 210012 China zhengguangying@huawei.com

Huawei Technologies, Co., Ltd

Yuhua District 101 Software Avenue Nanjing 210012 China wangzitao@huawei.com

Operations and Management OPSAWG Authentication Authorization Accounting Authentication, Authorization, and Accounting AAA (AAA) This document defines a Terminal Access Controller Access-Control System Plus (TACACS+) client YANG module that augments the System Management data model, defined in RFC 7317, to allow devices to make use of TACACS+ servers for centralized Authentication, Authorization, and Accounting (AAA). Though being a standard module, this module does not endorse the security mechanisms of the TACACS+ protocol (RFC 8907), and TACACS+ MUST be used within a secure deployment. The YANG module in this document conforms to the Network Management Datastore Architecture (NMDA) defined in RFC 8342.

Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at .

Copyright Notice Copyright (c) 2021 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 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

• .  Introduction
• .  Conventions Used in This Document
  • .  Tree Diagrams
• .  Design of the TACACS+ Data Model
• .  TACACS+ Client Module
• .  Security Considerations
• .  IANA Considerations
• .  References
  • .  Normative References
  • .  Informative References
• .  Example TACACS+ Authentication Configuration
• Acknowledgments
• Authors' Addresses

Introduction This document defines a YANG module that augments the System Management data model defined in to support the configuration and management of TACACS+ clients. TACACS+ provides device administration for routers, network access servers, and other networked devices via one or more centralized servers. The System Management data model defines separate functionality to support local and RADIUS authentication:

User Authentication Model:

Defines a list of usernames with associated passwords and a configuration leaf to decide the order in which local or RADIUS authentication is used.

RADIUS Client Model:

Defines a list of RADIUS servers used by a device for centralized user authentication.

The System Management data model is augmented with the TACACS+ YANG module defined in this document to allow the use of TACACS+ servers as an alternative to RADIUS servers. The YANG module can be used with network management protocols such as the Network Configuration Protocol (NETCONF) . The YANG module in this document conforms to the Network Management Datastore Architecture (NMDA) defined in .

Conventions Used in This Document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. The following terms are defined in and are used in this specification:

• configuration data
• state data

The following terms are defined in and are used in this specification:

• augment
• data model
• data node

The terminology for describing YANG data models is found in .

Tree Diagrams The tree diagram used in this document follows the notation defined in .

Design of the TACACS+ Data Model This module is used to configure a TACACS+ client on a device to support deployment scenarios with centralized authentication, authorization, and accounting servers. Authentication is used to validate a user's username and password, authorization allows the user to access and execute commands at various privilege levels assigned to the user, and accounting keeps track of the activity of a user who has accessed the device. The ietf-system-tacacs-plus module augments the "/sys:system" path defined in the ietf-system module with the contents of the "tacacs-plus" grouping. Therefore, a device can use local, RADIUS, or TACACS+ authentication 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, which is directly under the "tacacs-plus" container, holds a list of TACACS+ servers and uses server-type to distinguish between Authentication, Authorization, and Accounting (AAA) services. 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 IP address setting or derived from the outbound interface from the local Forwarding Information Base (FIB). For the TACACS+ server located in a Virtual Private Network (VPN), a VPN Routing and Forwarding (VRF) instance needs to be specified. The "statistics" container under the "server list" is a collection of read-only counters for sent and received messages from a configured server. The YANG module for TACACS+ client has the following structure: module: ietf-system-tacacs-plus augment /sys:system: +--rw tacacs-plus +--rw server* [name] +--rw name string +--rw server-type tacacs-plus-server-type +--rw address inet:host +--rw port? inet:port-number +--rw (security) | +--:(obfuscation) | +--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

TACACS+ Client Module This YANG module imports typedefs from . This module also uses the interface typedef from , the leafref to VRF instance from , and the "default-deny-all" extension statement from . module ietf-system-tacacs-plus { yang-version 1.1; namespace "urn:ietf:params:xml:ns:yang:ietf-system-tacacs-plus"; prefix sys-tcs-plus; 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://datatracker.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. The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document are to be interpreted as described in BCP 14 (RFC 2119) (RFC 8174) when, and only when, they appear in all capitals, as shown here. Copyright (c) 2021 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 (https://trustee.ietf.org/license-info). This version of this YANG module is part of RFC 9105; see the RFC itself for full legal notices."; revision 2021-08-05 { description "Initial revision."; reference "RFC 9105: A YANG Data Model for Terminal Access Controller Access-Control System Plus (TACACS+)"; } typedef tacacs-plus-server-type { type bits { bit authentication { description "Indicates that the TACACS+ server is providing authentication services."; } bit authorization { description "Indicates that the TACACS+ server is providing authorization services."; } bit accounting { description "Indicates that the TACACS+ server is providing accounting services."; } } description "tacacs-plus-server-type can be set to authentication/authorization/accounting or any combination of the three types."; } identity tacacs-plus { base sys:authentication-method; description "Indicates AAA operation using TACACS+."; reference "RFC 8907: 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 closed 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 tacacs-plus { description "Grouping for TACACS+ attributes."; container tacacs-plus { must "not(derived-from-or-self(../sys:authentication" + "/sys:user-authentication-order, 'tacacs-plus'))" + " or bit-is-set(server/server-type,'authentication')" { error-message "When 'tacacs-plus' is used as a system" + " authentication method, a TACACS+" + " authentication server must be configured."; description "When 'tacacs-plus' 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 tacacs-plus-server-type; mandatory true; description "Server type: authentication/authorization/accounting and various combinations."; } 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."; } choice security { mandatory true; description "Security mechanism between TACACS+ client and server. This is modeled as a YANG 'choice' so that it can be augmented by a YANG module in a backwards-compatible manner."; case obfuscation { leaf shared-secret { type string { length "1..max"; } nacm:default-deny-all; description "The shared secret, which is known to both the TACACS+ client and server. TACACS+ server administrators SHOULD configure a shared secret with a minimum length of 16 characters. It is highly recommended that this shared secret is at least 32 characters long and sufficiently complex with a mix of different character types, i.e., upper case, lower case, numeric, and punctuation. Note that this security mechanism is best described as 'obfuscation' and not 'encryption' as it does not provide any meaningful integrity, privacy, or replay protection."; reference "RFC 8907: 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."; reference "RFC 8529: YANG Data Model for Network Instances"; } leaf single-connection { type boolean; default "false"; description "Indicates whether the Single Connection Mode is enabled for the server. By default, the Single Connection Mode is disabled."; } leaf timeout { type uint16 { range "1..max"; } 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 "Augments the system model with the tacacs-plus model."; uses tacacs-plus; } }

Security Considerations The YANG module specified in this document defines a schema for data that is designed to be accessed via network management protocols such as NETCONF or RESTCONF . The lowest NETCONF layer is the secure transport layer, and the mandatory-to-implement secure transport is Secure Shell (SSH) . The lowest RESTCONF layer is HTTPS, and the mandatory-to-implement secure transport is TLS . The Network Configuration Access Control Model (NACM) 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/tacacs-plus/server:

This list contains the data nodes used to control the TACACS+ servers used by the device. Unauthorized access to this list could enable an attacker to assume complete control over the device by pointing to a compromised TACACS+ server, or to modify the counters to hide attacks against the device.

/system/tacacs-plus/server/shared-secret:

This leaf controls the key known to both the TACACS+ client and server. Unauthorized access to this leaf could make the device vulnerable to attacks; therefore, it has been restricted using the "default-deny-all" access control defined in . When setting, it is highly recommended that the leaf is at least 32 characters long and sufficiently complex with a mix of different character types, i.e., upper case, lower case, numeric, and punctuation.

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 the TACACS+ protocol.

IANA Considerations IANA has registered the following URI in the "ns" subregistry within the "IETF XML Registry" :

URI:

urn:ietf:params:xml:ns:yang:ietf-system-tacacs-plus

Registrant Contact:

The IESG.

XML:

N/A, the requested URI is an XML namespace.

IANA has registered the following YANG module in the "YANG Module Names" registry :

Name:

ietf-system-tacacs-plus

Maintained by IANA:

N

Namespace:

urn:ietf:params:xml:ns:yang:ietf-system-tacacs-plus

Prefix:

sys-tcs-plus

Reference:

RFC 9105

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. The Network Configuration Protocol (NETCONF) defined in this document provides mechanisms to install, manipulate, and delete the configuration of network devices. It uses an Extensible Markup Language (XML)-based data encoding for the configuration data as well as the protocol messages. The NETCONF protocol operations are realized as remote procedure calls (RPCs). This document obsoletes RFC 4741. [STANDARDS-TRACK] This document describes a method for invoking and running the Network Configuration Protocol (NETCONF) within a Secure Shell (SSH) session as an SSH subsystem. This document obsoletes RFC 4742. [STANDARDS-TRACK] This document introduces a collection of common data types to be used with the YANG data modeling language. This document obsoletes RFC 6021. This document defines a YANG data model for the configuration and identification of some common system properties within a device containing a Network Configuration Protocol (NETCONF) server. This document also includes data node definitions for system identification, time-of-day management, user management, DNS resolver configuration, and some protocol operations for system management. YANG is a data modeling language used to model configuration data, state data, Remote Procedure Calls, and notifications for network management protocols. This document describes the syntax and semantics of version 1.1 of the YANG language. YANG version 1.1 is a maintenance release of the YANG language, addressing ambiguities and defects in the original specification. There are a small number of backward incompatibilities from YANG version 1. This document also specifies the YANG mappings to the Network Configuration Protocol (NETCONF). This document describes an HTTP-based protocol that provides a programmatic interface for accessing data defined in YANG, using the datastore concepts defined in the Network Configuration Protocol (NETCONF). RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This document captures the current syntax used in YANG module tree diagrams. The purpose of this document is to provide a single location for this definition. This syntax may be updated from time to time based on the evolution of the YANG language. The standardization of network configuration interfaces for use with the Network Configuration Protocol (NETCONF) or the RESTCONF protocol requires a structured and secure operating environment that promotes human usability and multi-vendor interoperability. There is a need for standard mechanisms to restrict NETCONF or RESTCONF protocol access for particular users to a preconfigured subset of all available NETCONF or RESTCONF protocol operations and content. This document defines such an access control model. This document obsoletes RFC 6536. Datastores are a fundamental concept binding the data models written in the YANG data modeling language to network management protocols such as the Network Configuration Protocol (NETCONF) and RESTCONF. This document defines an architectural framework for datastores based on the experience gained with the initial simpler model, addressing requirements that were not well supported in the initial model. This document updates RFC 7950. This document defines a YANG data model for the management of network interfaces. It is expected that interface-type-specific data models augment the generic interfaces data model defined in this document. The data model includes definitions for configuration and system state (status information and counters for the collection of statistics). The YANG data model in this document conforms to the Network Management Datastore Architecture (NMDA) defined in RFC 8342. This document obsoletes RFC 7223. This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. This document defines a network instance module. This module can be used to manage the virtual resource partitioning that may be present on a network device. Examples of common industry terms for virtual resource partitioning are VPN Routing and Forwarding (VRF) instances and Virtual Switch Instances (VSIs). The YANG data model in this document conforms to the Network Management Datastore Architecture (NMDA) defined in RFC 8342. This document describes the Terminal Access Controller Access-Control System Plus (TACACS+) protocol, which is widely deployed today to provide Device Administration for routers, network access servers, and other networked computing devices via one or more centralized servers. Informative References This document describes an IANA maintained registry for IETF standards which use Extensible Markup Language (XML) related items such as Namespaces, Document Type Declarations (DTDs), Schemas, and Resource Description Framework (RDF) Schemas.

Example TACACS+ Authentication Configuration The following shows an example where a TACACS+ authentication server instance is configured. { "ietf-system:system": { "authentication": { "user-authentication-order": [tacacs-plus, local-users] } "tacacs-plus": { "server": [ { "name": "tac_plus1", "server-type": "authentication", "address": "192.0.2.2", "shared-secret": "QaEfThUkO198010075460923+h3TbE8n", "source-ip": "192.0.2.12", "timeout": "10" } ] } } }

Acknowledgments The authors wish to thank , , , , , , , and many others for their helpful comments and suggestions.

Authors' Addresses Huawei Technologies, Co., Ltd

Yuhua District 101 Software Avenue Nanjing Jiangsu 210012 China lana.wubo@huawei.com

Huawei Technologies, Co., Ltd

Yuhua District 101 Software Avenue Nanjing Jiangsu 210012 China zhengguangying@huawei.com

Huawei Technologies, Co., Ltd

Yuhua District 101 Software Avenue Nanjing 210012 China wangzitao@huawei.com