Internet Engineering Task Force (IETF) B. Wu, Ed.

Internet Engineering Task Force (IETF) B. Wu, Ed.
Request for Comments: 9105 G. Zheng
Category: Standards Track M. Wang, Ed.
ISSN: 2070-1721 Huawei
August 2021

A YANG Data Model for Terminal Access Controller Access-Control System
Plus (TACACS+)

Abstract

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
https://www.rfc-editor.org/info/rfc9105.

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
(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 TACACS+ Data Model
4. TACACS+ Client Module
5. Security Considerations
6. IANA Considerations
7. References
7.1. Normative References
7.2. Informative References
Appendix A. Example TACACS+ Authentication Configuration
Acknowledgments
Authors' Addresses

1. Introduction

This document defines a YANG module that augments the System
Management data model defined in [RFC7317] to support the
configuration and management of TACACS+ clients.

TACACS+ [RFC8907] provides device administration for routers, network
access servers, and other networked devices via one or more
centralized servers.

The System Management data model [RFC7317] 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) [RFC6241].

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

2. 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 [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:

* configuration data

* state data

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

* augment

* data model

* data node

The terminology for describing YANG data models is found in
[RFC7950].

2.1. Tree Diagrams

The tree diagram used in this document follows the notation defined
in [RFC8340].

3. 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 [RFC8907], 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

4. TACACS+ Client Module

This YANG module imports typedefs from [RFC6991]. This module also
uses the interface typedef from [RFC8343], the leafref to VRF
instance from [RFC8529], and the "default-deny-all" extension
statement from [RFC8341].

<CODE BEGINS> file "[email protected]"
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:[email protected]>

Editor: Bo Wu <[email protected]>
Editor: Guangying Zheng <[email protected]>";
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;
}
}
<CODE ENDS>

5. 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 [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 Network Configuration Access Control Model (NACM) [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/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 [RFC8341]. 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 Section 10 of the TACACS+ protocol
[RFC8907].

6. IANA Considerations

IANA has registered the following URI in the "ns" subregistry within
the "IETF XML Registry" [RFC3688]:

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 [RFC7950]:

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

7. References

7.1. Normative References

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

[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,
<https://www.rfc-editor.org/info/rfc6241>.

[RFC6242] Wasserman, M., "Using the NETCONF Protocol over Secure
Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
<https://www.rfc-editor.org/info/rfc6242>.

[RFC6991] Schoenwaelder, J., Ed., "Common YANG Data Types",
RFC 6991, DOI 10.17487/RFC6991, July 2013,
<https://www.rfc-editor.org/info/rfc6991>.

[RFC7317] Bierman, A. and M. Bjorklund, "A YANG Data Model for
System Management", RFC 7317, DOI 10.17487/RFC7317, August
2014, <https://www.rfc-editor.org/info/rfc7317>.

[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<https://www.rfc-editor.org/info/rfc7950>.

[RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
<https://www.rfc-editor.org/info/rfc8040>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8340] Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
<https://www.rfc-editor.org/info/rfc8340>.

[RFC8341] Bierman, A. and M. Bjorklund, "Network Configuration
Access Control Model", STD 91, RFC 8341,
DOI 10.17487/RFC8341, March 2018,
<https://www.rfc-editor.org/info/rfc8341>.

[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,
<https://www.rfc-editor.org/info/rfc8342>.

[RFC8343] Bjorklund, M., "A YANG Data Model for Interface
Management", RFC 8343, DOI 10.17487/RFC8343, March 2018,
<https://www.rfc-editor.org/info/rfc8343>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

[RFC8529] Berger, L., Hopps, C., Lindem, A., Bogdanovic, D., and X.
Liu, "YANG Data Model for Network Instances", RFC 8529,
DOI 10.17487/RFC8529, March 2019,
<https://www.rfc-editor.org/info/rfc8529>.

[RFC8907] Dahm, T., Ota, A., Medway Gash, D.C., Carrel, D., and L.
Grant, "The Terminal Access Controller Access-Control
System Plus (TACACS+) Protocol", RFC 8907,
DOI 10.17487/RFC8907, September 2020,
<https://www.rfc-editor.org/info/rfc8907>.

7.2. Informative References

[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
DOI 10.17487/RFC3688, January 2004,
<https://www.rfc-editor.org/info/rfc3688>.

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": [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 Alex Campbell, John Heasley, Ebben Aries,
Alan DeKok, Joe Clarke, Tom Petch, Robert Wilton, and many others for
their helpful comments and suggestions.

Authors' Addresses

Bo Wu (editor)
Huawei Technologies, Co., Ltd
Yuhua District
101 Software Avenue
Nanjing
Jiangsu, 210012
China

Email: [email protected]

Guangying Zheng
Huawei Technologies, Co., Ltd
Yuhua District
101 Software Avenue
Nanjing
Jiangsu, 210012
China

Email: [email protected]

Michael Wang (editor)
Huawei Technologies, Co., Ltd
Yuhua District
101 Software Avenue
Nanjing
210012
China

Email: [email protected]