Internet DRAFT - draft-ietf-opsawg-tacacs-yang
draft-ietf-opsawg-tacacs-yang
Opsawg B. Wu, Ed.
Internet-Draft G. Zheng
Intended status: Standards Track M. Wang, Ed.
Expires: November 14, 2021 Huawei
May 13, 2021
A YANG Module for TACACS+
draft-ietf-opsawg-tacacs-yang-12
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 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 November 14, 2021.
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
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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
2. Conventions used in this document . . . . . . . . . . . . . . 3
2.1. Tree Diagrams . . . . . . . . . . . . . . . . . . . . . . 3
3. Design of the TACACS+ Data Model . . . . . . . . . . . . . . 3
4. TACACS+ Client Module . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 12
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Normative References . . . . . . . . . . . . . . . . . . 13
8.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Example TACACS+ Authentication Configuration . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
This document defines a YANG module that augments the System
Management data model defined in the [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 Model [RFC7317] defines separate functionality
to support local and RADIUS authentication:
o 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.
o RADIUS Client Model: Defines a list of RADIUS servers used by a
device for centralized user authentication.
The System Management 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.
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The YANG module can be used with network management protocols such as
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 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:
o configuration data
o state data
The following terms are defined in [RFC7950] and are used in this
specification:
o augment
o data model
o 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.
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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+ 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 "tacacs-plus" container,
which holds a list of TACACS+ servers and uses server-type to
distinguish between Authentication, Authorization and Accounting
(AAA). 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:
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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 "ietf-system-tacacs-plus@2021-05-13.yang"
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;
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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) 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
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(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.
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.";
// 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.
revision 2021-05-13 {
description
"Initial revision.";
reference
"RFC XXXX: A YANG Module for 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 {
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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 {
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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 {
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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 modelled 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 of
minimum 16 characters length.
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, 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";
}
}
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}
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
"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
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response from each TACACS+ server before trying with a
different server.";
}
uses statistics;
}
}
}
augment "/sys:system" {
description
"Augment the system model with the tacacs-plus model";
uses tacacs-plus;
}
}
<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 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.
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/system/tacacsplus/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, 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
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-tacacs-plus
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-tacacs-plus
Namespace: urn:ietf:params:xml:ns:yang:ietf-system-tacacs-plus
Prefix: sys-tcs-plus
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, Robert Wilton, and
many others for their helpful comments and suggestions.
8. References
8.1. 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>.
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[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>.
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[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., 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>.
8.2. 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"
}
]
}
}
}
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Authors' Addresses
Bo Wu (editor)
Huawei Technologies, Co.,
Ltd
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China
Email: lana.wubo@huawei.com
Guangying Zheng
Huawei Technologies, Co.,
Ltd
101 Software Avenue, Yuhua District
Nanjing, Jiangsu 210012
China
Email: zhengguangying@huawei.com
Michael Wang (editor)
Huawei Technologies, Co.,
Ltd
101 Software Avenue, Yuhua District
Nanjing 210012
China
Email: wangzitao@huawei.com
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