A YANG Data Model for Terminal Access
Controller Access-Control System Plus (TACACS+)
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.
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:
/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 .
WG List:
Editor: Bo Wu
Editor: Guangying Zheng ";
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
Informative References
Example TACACS+ Authentication Configuration
The following shows an example where a TACACS+ authentication server
instance is configured.
Acknowledgments
The authors wish to thank ,
, ,
, ,
, ,
and many others for their helpful comments and suggestions.