    Internet Engineering Task Force (IETF)                        A. Bierman
    Request for Comments: 8341                                     YumaWorks
    STD: 91                                                     M. Bjorklund
    Obsoletes: 6536                                           Tail-f Systems
    Category: Standards Track                                     March 2018
    ISSN: 2070-1721

                   Network Configuration Access Control Model

    Abstract

       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.

    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/rfc8341.

    Bierman & Bjorklund          Standards Track                    [Page 1]

    RFC 8341                          NACM                        March 2018

    Copyright Notice

       Copyright (c) 2018 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 ....................................................4
          1.1. Terminology ................................................4
          1.2. Changes since RFC 6536 .....................................6
       2. Access Control Design Objectives ................................7
          2.1. Access Control Points ......................................7
          2.2. Simplicity .................................................8
          2.3. Procedural Interface .......................................8
          2.4. Datastore Access ...........................................8
          2.5. Users and Groups ...........................................8
          2.6. Maintenance ................................................9
          2.7. Configuration Capabilities .................................9
          2.8. Identifying Security-Sensitive Content .....................9
       3. NETCONF Access Control Model (NACM) ............................10
          3.1. Overview ..................................................10
               3.1.1. Features ...........................................10
               3.1.2. External Dependencies ..............................11
               3.1.3. Message Processing Model ...........................11
          3.2. Datastore Access ..........................................14
               3.2.1. Mapping New Datastores to NACM .....................14
               3.2.2. Access Rights ......................................14
               3.2.3. RESTCONF Methods ...................................15
               3.2.4. <get> and <get-config> Operations ..................16
               3.2.5. <edit-config> Operation ............................16
               3.2.6. <copy-config> Operation ............................18
               3.2.7. <delete-config> Operation ..........................18
               3.2.8. <commit> Operation .................................19
               3.2.9. <discard-changes> Operation ........................19
               3.2.10. <kill-session> Operation ..........................19

    Bierman & Bjorklund          Standards Track                    [Page 2]

    RFC 8341                          NACM                        March 2018

          3.3. Model Components ..........................................19
               3.3.1. Users ..............................................19
               3.3.2. Groups .............................................20
               3.3.3. Emergency Recovery Session .........................20
               3.3.4. Global Enforcement Controls ........................20
                      3.3.4.1. enable-nacm Switch ........................20
                      3.3.4.2. read-default Switch .......................20
                      3.3.4.3. write-default Switch ......................21
                      3.3.4.4. exec-default Switch .......................21
                      3.3.4.5. enable-external-groups Switch .............22
               3.3.5. Access Control Rules ...............................22
          3.4. Access Control Enforcement Procedures .....................22
               3.4.1. Initial Operation ..................................23
               3.4.2. Session Establishment ..............................23
               3.4.3. "access-denied" Error Handling .....................23
               3.4.4. Incoming RPC Message Validation ....................24
               3.4.5. Data Node Access Validation ........................26
               3.4.6. Outgoing <notification> Authorization ..............29
          3.5. Data Model Definitions ....................................31
               3.5.1. Data Organization ..................................31
               3.5.2. YANG Module ........................................32
       4. IANA Considerations ............................................42
       5. Security Considerations ........................................42
          5.1. NACM Configuration and Monitoring Considerations ..........43
          5.2. General Configuration Issues ..............................45
          5.3. Data Model Design Considerations ..........................47
       6. References .....................................................47
          6.1. Normative References ......................................47
          6.2. Informative References ....................................49
       Appendix A. Usage Examples ........................................50
         A.1. <groups> Example ...........................................50
         A.2. Module Rule Example ........................................51
         A.3. Protocol Operation Rule Example ............................53
         A.4. Data Node Rule Example .....................................55
         A.5. Notification Rule Example ..................................57
       Authors' Addresses ................................................58

    Bierman & Bjorklund          Standards Track                    [Page 3]

    RFC 8341                          NACM                        March 2018

    1.  Introduction

       The Network Configuration Protocol (NETCONF) and the RESTCONF
       protocol do not provide any standard mechanisms to restrict the
       protocol operations and content that each user is authorized to
       access.

       There is a need for interoperable management of the controlled access
       to administrator-selected portions of the available NETCONF or
       RESTCONF content within a particular server.

       This document addresses access control mechanisms for the Operations
       and Content layers of NETCONF, as defined in [RFC6241]; and RESTCONF,
       as defined in [RFC8040].  It contains three main sections:

       1.  Access Control Design Objectives

       2.  NETCONF Access Control Model (NACM)

       3.  YANG Data Model (ietf-netconf-acm.yang)

       YANG version 1.1 [RFC7950] adds two new constructs that need special
       access control handling.  The "action" statement is similar to the
       "rpc" statement, except that it is located within a data node.  The
       "notification" statement can also be located within a data node.

    1.1.  Terminology

       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 [RFC8342] and are not redefined
       here:

       o  datastore

       o  configuration datastore

       o  conventional configuration datastore

       o  candidate configuration datastore

       o  running configuration datastore

       o  startup configuration datastore

    Bierman & Bjorklund          Standards Track                    [Page 4]

    RFC 8341                          NACM                        March 2018

       o  operational state datastore

       o  client

       o  server

       The following terms are defined in [RFC6241] and are not redefined
       here:

       o  protocol operation

       o  session

       o  user

       The following terms are defined in [RFC7950] and are not redefined
       here:

       o  action

       o  data node

       o  data definition statement

       The following terms are defined in [RFC8040] and are not redefined
       here:

       o  data resource

       o  datastore resource

       o  operation resource

       o  target resource

       The following term is defined in [RFC7230] and is not redefined here:

       o  request URI

       The following terms are used throughout this document:

       access control:  A security feature provided by the server that
          allows an administrator to restrict access to a subset of all
          protocol operations and data, based on various criteria.

       access control model (ACM):  A conceptual model used to configure and
          monitor the access control procedures desired by the administrator
          to enforce a particular access control policy.

    Bierman & Bjorklund          Standards Track                    [Page 5]

    RFC 8341                          NACM                        March 2018

       access control rule:  The criterion used to determine if a particular
          access operation will be permitted or denied.

       access operation:  How a request attempts to access a conceptual
          object.  One of "none", "read", "create", "delete", "update", or
          "execute".

       data node hierarchy:  The hierarchy of data nodes that identifies the
          specific "action" or "notification" node in the datastore.

       recovery session:  A special administrative session that is given
          unlimited NETCONF access and is exempt from all access control
          enforcement.  The mechanism or mechanisms used by a server to
          control and identify whether or not a session is a recovery
          session are implementation specific and are outside the scope of
          this document.

       write access:  A shorthand for the "create", "delete", and "update"
          access operations.

    1.2.  Changes since RFC 6536

       The NACM procedures and data model have been updated to support new
       data modeling capabilities in version 1.1 of the YANG data modeling
       language.  The "action" and "notification" statements can be used
       within data nodes to define data-model-specific operations and
       notifications.

       An important use case for these new YANG statements is the increased
       access control granularity that can be achieved over top-level "rpc"
       and "notification" statements.  The new "action" and "notification"
       statements are used within data nodes, and access to the action or
       notification can be restricted to specific instances of these data
       nodes.

       Support for the RESTCONF protocol has been added.  The RESTCONF
       operations are similar to the NETCONF operations, so a simple mapping
       to the existing NACM procedures and data model is possible.

       The data node access behavior for path matches has been clarified to
       also include matching descendant nodes of the specified path.

       The <edit-config> operation access rights behavior has been clarified
       to indicate that write access is not required for data nodes that are
       implicitly modified through side effects (such as the evaluation of
       YANG when-stmts, or data nodes implicitly deleted when creating a
       data node under a different branch under a YANG choice-stmt).

    Bierman & Bjorklund          Standards Track                    [Page 6]

    RFC 8341                          NACM                        March 2018

       The Security Considerations section has been updated to comply with
       the "YANG module security guidelines" [YANG-SEC].  Note that the YANG
       module in this document does not define any RPC operations.

    2.  Access Control Design Objectives

       This section documents the design objectives for the NETCONF access
       control model presented in Section 3.

    2.1.  Access Control Points

       NETCONF allows server implementers to add new custom protocol
       operations, and the YANG data modeling language supports this
       feature.  These operations can be defined in standard or proprietary
       YANG modules.

       It is not possible to design an ACM for NETCONF that only focuses on
       a static set of standard protocol operations defined by NETCONF
       itself, like some other protocols.  Since few assumptions can be made
       about an arbitrary protocol operation, the NETCONF architectural
       server components need to be protected at three conceptual control
       points.

       These access control points, described in Figure 1, are as follows:

       protocol operation:  Permission to invoke specific protocol
          operations.

       datastore:  Permission to read and/or alter specific data nodes
          within any datastore.

       notification:  Permission to receive specific notification event
          types.

                     +-------------+                 +-------------+
        client       |  protocol   |                 |  data node  |
        request -->  |  operation  | ------------->  |   access    |
                     |  allowed?   |   datastore     |  allowed?   |
                     +-------------+   or state      +-------------+
                                       data access

                     +----------------+
                     |  notification  |
        event -->    |  allowed?      |
                     +----------------+

                                     Figure 1

    Bierman & Bjorklund          Standards Track                    [Page 7]

    RFC 8341                          NACM                        March 2018

    2.2.  Simplicity

       There is concern that a complicated ACM will not be widely deployed
       because it is too hard to use.  Configuration of the access control
       system needs to be as simple as possible.  Simple and common tasks
       need to be easy to configure and require little expertise or
       domain-specific knowledge.  Complex tasks are possible using
       additional mechanisms that may require additional expertise.

       A single set of access control rules ought to be able to control all
       types of NETCONF protocol operation invocation, all datastore access,
       and all notification events.

       Access control ought to be defined with a small and familiar set of
       permissions, while still allowing full control of datastore access.

    2.3.  Procedural Interface

       NETCONF uses a Remote Procedure Call (RPC) model and an extensible
       set of protocol operations.  Access control for any possible protocol
       operation is necessary.

    2.4.  Datastore Access

       It is necessary to control access to specific nodes and subtrees
       within the datastore, regardless of which protocol operation --
       standard or proprietary -- was used to access the datastore.

    2.5.  Users and Groups

       It is necessary that access control rules for a single user or a
       configurable group of users can be configured.

       The ACM needs to support the concept of administrative groups, to
       support the well-established distinction between a root account and
       other types of less-privileged conceptual user accounts.  These
       groups need to be configurable by the administrator.

       It is necessary that the user-to-group mapping can be delegated to a
       central server, such as a RADIUS server [RFC2865] [RFC5607].  Since
       authentication is performed by the transport layer and RADIUS
       performs authentication and service authorization at the same time,
       the underlying transport protocol needs to be able to report a set of
       group names associated with the user to the server.  It is necessary
       that the administrator can disable the usage of these group names
       within the ACM.

    Bierman & Bjorklund          Standards Track                    [Page 8]

    RFC 8341                          NACM                        March 2018

    2.6.  Maintenance

       It ought to be possible to disable part or all of the access control
       model enforcement procedures without deleting any access control
       rules.

    2.7.  Configuration Capabilities

       Suitable configuration and monitoring mechanisms are needed to allow
       an administrator to easily manage all aspects of the ACM's behavior.
       A standard data model, suitable for use with the <edit-config>
       protocol operation, needs to be available for this purpose.

       Access control rules to restrict access operations on specific
       subtrees within the configuration datastore need to be supported.

    2.8.  Identifying Security-Sensitive Content

       One of the most important aspects of the data model documentation,
       and one of the biggest concerns during deployment, is the
       identification of security-sensitive content.  This applies to
       protocol operations in NETCONF, not just data and notifications.

       It is mandatory for security-sensitive objects to be documented in
       the Security Considerations section of an RFC.  This is nice, but it
       is not good enough, for the following reasons:

       o  This documentation-only approach forces administrators to study
          the RFC and determine if there are any potential security risks
          introduced by a new data model.

       o  If any security risks are identified, then the administrator must
          study some more RFC text and determine how to mitigate the
          security risk(s).

       o  The ACM on each server must be configured to mitigate the security
          risks, e.g., require privileged access to read or write the
          specific data identified in the Security Considerations section.

       o  If the ACM is not preconfigured, then there will be a time window
          of vulnerability after the new data model is loaded and before the
          new access control rules for that data model are configured,
          enabled, and debugged.

       Often, the administrator just wants to disable default access to the
       secure content so that no inadvertent or malicious changes can be
       made to the server.  This allows the default rules to be more
       lenient, without significantly increasing the security risk.

    Bierman & Bjorklund          Standards Track                    [Page 9]

    RFC 8341                          NACM                        March 2018

       A data model designer needs to be able to use machine-readable
       statements to identify content that needs to be protected by default.
       This will allow client and server tools to automatically identify
       data-model-specific security risks, by denying access to sensitive
       data unless the user is explicitly authorized to perform the
       requested access operation.

    3.  NETCONF Access Control Model (NACM)

    3.1.  Overview

       This section provides a high-level overview of the access control
       model structure.  It describes the NETCONF protocol message
       processing model and the conceptual access control requirements
       within that model.

    3.1.1.  Features

       The NACM data model provides the following features:

       o  Independent control of RPC, action, data, and notification access
          is provided.

       o  The concept of an emergency recovery session is supported, but
          configuration of the server for this purpose is beyond the scope
          of this document.  An emergency recovery session will bypass all
          access control enforcement, in order to allow it to initialize or
          repair the NACM configuration.

       o  A simple and familiar set of datastore permissions is used.

       o  Support for YANG security tagging (e.g., a
          "nacm:default-deny-write" statement) allows default security modes
          to automatically exclude sensitive data.

       o  Separate default access modes for read, write, and execute
          permissions are provided.

       o  Access control rules are applied to configurable groups of users.

       o  The access control enforcement procedures can be disabled during
          operation, without deleting any access control rules, in order to
          debug operational problems.

    Bierman & Bjorklund          Standards Track                   [Page 10]

    RFC 8341                          NACM                        March 2018

       o  The number of denied protocol operation requests and denied
          datastore write requests can be monitored by the client.

       o  Simple unconstrained YANG instance-identifiers are used to
          configure access control rules for specific data nodes.

    3.1.2.  External Dependencies

       NETCONF [RFC6241] and RESTCONF [RFC8040] are used for network
       management purposes within this document.

       The YANG data modeling language [RFC7950] is used to define the data
       models for use with NETCONF or RESTCONF.  YANG is also used to define
       the data model in this document.

    3.1.3.  Message Processing Model

       The following diagram shows the conceptual message flow model,
       including the points at which access control is applied during
       NETCONF message processing.

       RESTCONF operations are mapped to the access control model based on
       the HTTP method and resource class used in the operation.  For
       example, a POST method on a data resource is considered "write data
       node" access, but a POST method on an operation resource is
       considered "operation" access.

       The new "pre-read data node acc. ctl" boxes in the diagram below
       refer to group read access as it relates to data node ancestors of an
       action or notification.  As an example, if an action is defined as
       /interfaces/interface/reset-interface, the group must be authorized
       to (1) read /interfaces and /interfaces/interface and (2) execute on
       /interfaces/interface/reset-interface.

    Bierman & Bjorklund          Standards Track                   [Page 11]

    RFC 8341                          NACM                        March 2018

                        +-------------------------+
                        |       session           |
                        |      (username)         |
                        +-------------------------+
                           |                 ^
                           V                 |
                 +--------------+     +---------------+
                 |   message    |     |   message     |
                 | dispatcher   |     |   generator   |
                 +--------------+     +---------------+
                   |      |               ^         ^
                   |      V               |         |
                   |  +=============+     |         |
                   |  | pre-read    |     |         |
                   |  | data node   |     |         |
                   |  | acc. ctl    |     |         |
                   |  +=============+     |         |
                   |    |                 |         |
                   V    V                 |         |
             +===========+     +-------------+   +----------------+
             | operation |---> |    reply    |   | <notification> |
             | acc. ctl  |     |  generator  |   |  generator     |
             +===========+     +-------------+   +----------------+
                   |              ^    ^                ^
                   V       +------+    |                |
             +-----------+ |   +=============+  +================+
             | operation | |   |    read     |  | <notification> |
             | processor |-+   | data node   |  |  access ctl    |
             |           |     | acc. ctl    |  |                |
             +-----------+     +=============+  +================+
                   |   |                  ^       ^     ^
                   V   +----------------+ |       |     |
             +===========+              | |       | +============+
             |  write    |              | |       | | pre-read   |
             | data node |              | |       | | data node  |
             | acc. ctl  | -----------+ | |       | | acc. ctl   |
             +===========+            | | |       | +============+
                   |                  | | |       |   ^
                   V                  V V |       |   |
             +---------------+      +-------------------+
             | configuration | ---> |      server       |
             |   datastore   |      |  instrumentation  |
             |               | <--- |                   |
             +---------------+      +-------------------+

                                     Figure 2

    Bierman & Bjorklund          Standards Track                   [Page 12]

    RFC 8341                          NACM                        March 2018

       The following high-level sequence of conceptual processing steps is
       executed for each received <rpc> message, if access control
       enforcement is enabled:

       o  For each active session, access control is applied individually to
          all <rpc> messages (except <close-session>) received by the
          server, unless the session is identified as a recovery session.

       o  If the <action> operation defined in [RFC7950] is invoked, then
          read access is required for all instances in the hierarchy of data
          nodes that identifies the specific action in the datastore, and
          execute access is required for the action node.  If the user is
          not authorized to read all the specified data nodes and execute
          the action, then the request is rejected with an "access-denied"
          error.

       o  Otherwise, if the user is not authorized to execute the specified
          protocol operation, then the request is rejected with an
          "access-denied" error.

       o  If a datastore is accessed by the protocol operation, then the
          server checks to see if the client is authorized to access the
          nodes in the datastore.  If the user is not authorized to perform
          the requested access operation on the requested data, then the
          request is rejected with an "access-denied" error.

       The following sequence of conceptual processing steps is executed for
       each generated notification event, if access control enforcement is
       enabled:

       o  Server instrumentation generates a notification for a particular
          subscription.

       o  If the "notification" statement is specified within a data
          subtree, as specified in [RFC7950], then read access is required
          for all instances in the hierarchy of data nodes that identifies
          the specific notification in the datastore, and read access is
          required for the notification node.  If the user is not authorized
          to read all the specified data nodes and the notification node,
          then the notification is dropped for that subscription.

       o  If the "notification" statement is a top-level statement, the
          notification access control enforcer checks the notification event
          type, and if it is one that the user is not authorized to read,
          then the notification is dropped for that subscription.

    Bierman & Bjorklund          Standards Track                   [Page 13]

    RFC 8341                          NACM                        March 2018

    3.2.  Datastore Access

       The same access control rules apply to all datastores that support
       the NACM -- for example, the candidate configuration datastore or the
       running configuration datastore.

       All conventional configuration datastores and the operational state
       datastore are controlled by the NACM.  Local files, remote files, or
       datastores accessed via the <url> parameter are not controlled by
       the NACM.

    3.2.1.  Mapping New Datastores to NACM

       It is possible that new datastores will be defined over time for use
       with NETCONF.  The NACM MAY be applied to other datastores that have
       similar access rights as defined in the NACM.  To apply the NACM to a
       new datastore, the new datastore specification needs to define how it
       maps to the NACM CRUDX (Create, Read, Update, Delete, eXec) access
       rights.  It is possible that only a subset of the NACM access rights
       would be applicable.  For example, only retrieval access control
       would be needed for a read-only datastore.  Operations and access
       rights not supported by the NACM CRUDX model are outside the scope of
       this document.  A datastore does not need to use the NACM, e.g., the
       datastore specification defines something else or does not use access
       control.

    3.2.2.  Access Rights

       A small set of hard-wired datastore access rights is needed to
       control access to all possible protocol operations, including vendor
       extensions to the standard protocol operation set.

       The CRUDX model can support all protocol operations:

       o  Create: allows the client to add a new data node instance to a
          datastore.

       o  Read: allows the client to read a data node instance from a
          datastore or receive the notification event type.

       o  Update: allows the client to update an existing data node instance
          in a datastore.

       o  Delete: allows the client to delete a data node instance from a
          datastore.

       o  eXec: allows the client to execute the operation.

    Bierman & Bjorklund          Standards Track                   [Page 14]

    RFC 8341                          NACM                        March 2018

    3.2.3.  RESTCONF Methods

       The RESTCONF protocol utilizes HTTP methods to perform datastore
       operations, similar to NETCONF.  The NACM procedures were originally
       written for NETCONF protocol operations, so the RESTCONF methods are
       mapped to NETCONF operations for the purpose of access control
       processing.  The enforcement procedures described within this
       document apply to both protocols unless explicitly stated otherwise.

       The request URI needs to be considered when processing RESTCONF
       requests on data resources:

       o  For HEAD and GET requests, any data nodes that are ancestor nodes
          of the target resource are considered to be part of the retrieval
          request for access control purposes.

       o  For PUT, PATCH, and DELETE requests, any data nodes that are
          ancestor nodes of the target resource are not considered to be
          part of the edit request for access control purposes.  The access
          operation for these nodes is considered to be "none".  The edit
          begins at the target resource.

       o  For POST requests on data resources, any data nodes that are
          specified in the request URI, including the target resource, are
          not considered to be part of the edit request for access control
          purposes.  The access operation for these nodes is considered to
          be "none".  The edit begins at a child node of the target
          resource, specified in the message body.

    Bierman & Bjorklund          Standards Track                   [Page 15]

    RFC 8341                          NACM                        March 2018

       Not all RESTCONF methods are subject to access control.  The
       following table specifies how each method is mapped to NETCONF
       protocol operations.  The value "none" indicates that the NACM is not
       applied at all to the specific RESTCONF method.

       +---------+-----------------+---------------------+-----------------+
       | Method  | Resource class  | NETCONF operation   | Access          |
       |         |                 |                     | operation       |
       +---------+-----------------+---------------------+-----------------+
       | OPTIONS | all             | none                | none            |
       | HEAD    | all             | <get>, <get-config> | read            |
       | GET     | all             | <get>, <get-config> | read            |
       | POST    | datastore, data | <edit-config>       | create          |
       | POST    | operation       | specified operation | execute         |
       | PUT     | data            | <edit-config>       | create, update  |
       | PUT     | datastore       | <copy-config>       | update          |
       | PATCH   | data, datastore | <edit-config>       | update          |
       | DELETE  | data            | <edit-config>       | delete          |
       +---------+-----------------+---------------------+-----------------+

                   Table 1: Mapping RESTCONF Methods to NETCONF

    3.2.4.  <get> and <get-config> Operations

       The NACM access rights are not directly coupled to the <get> and
       <get-config> protocol operations but apply to all <rpc> operations
       that would result in a "read" access operation to the target
       datastore.  This section describes how these access rights apply to
       the specific access operations supported by the <get> and
       <get-config> protocol operations.

       Data nodes to which the client does not have read access are silently
       omitted, along with any descendants, from the <rpc-reply> message.
       This is done to allow NETCONF filters for <get> and <get-config> to
       function properly, instead of causing an "access-denied" error
       because the filter criteria would otherwise include unauthorized read
       access to some data nodes.  For NETCONF filtering purposes, the
       selection criteria are applied to the subset of nodes that the user
       is authorized to read, not the entire datastore.

    3.2.5.  <edit-config> Operation

       The NACM access rights are not directly coupled to the <edit-config>
       "operation" attribute, although they are similar.  Instead, a NACM
       access right applies to all protocol operations that would result in
       a particular access operation to the target datastore.  This section
       describes how these access rights apply to the specific access
       operations supported by the <edit-config> protocol operation.

    Bierman & Bjorklund          Standards Track                   [Page 16]

    RFC 8341                          NACM                        March 2018

       If the effective access operation is "none" (i.e.,
       default-operation="none") for a particular data node, then no access
       control is applied to that data node.  This is required to allow
       access to a subtree within a larger data structure.  For example, a
       user may be authorized to create a new "/interfaces/interface" list
       entry but not be authorized to create or delete its parent container
       ("/interfaces").  If the "/interfaces" container already exists in
       the target datastore, then the effective operation will be "none" for
       the "/interfaces" node if an "/interfaces/interface" list entry is
       edited.

       If the protocol operation would result in the creation of a datastore
       node and the user does not have "create" access permission for that
       node, the protocol operation is rejected with an "access-denied"
       error.

       If the protocol operation would result in the deletion of a datastore
       node and the user does not have "delete" access permission for that
       node, the protocol operation is rejected with an "access-denied"
       error.

       If the protocol operation would result in the modification of a
       datastore node and the user does not have "update" access permission
       for that node, the protocol operation is rejected with an
       "access-denied" error.

       A "merge" or "replace" <edit-config> operation may include data nodes
       that do not alter portions of the existing datastore.  For example, a
       container or list node may be present for naming purposes but does
       not actually alter the corresponding datastore node.  These unaltered
       data nodes are ignored by the server and do not require any access
       rights by the client.

       A "merge" <edit-config> operation may include data nodes but not
       include particular child data nodes that are present in the
       datastore.  These missing data nodes within the scope of a "merge"
       <edit-config> operation are ignored by the server and do not require
       any access rights by the client.

       The contents of specific restricted datastore nodes MUST NOT be
       exposed in any <rpc-error> elements within the reply.

       An <edit-config> operation may cause data nodes to be implicitly
       created or deleted as an implicit side effect of a requested
       operation.  For example, a YANG when-stmt expression may evaluate to
       a different result, causing data nodes to be deleted, or created with
       default values; or if a data node is created under one branch of a
       YANG choice-stmt, then all data nodes under the other branches are

    Bierman & Bjorklund          Standards Track                   [Page 17]

    RFC 8341                          NACM                        March 2018

       implicitly removed.  No NACM access rights are required on any data
       nodes that are implicitly changed as a side effect of another allowed
       operation.

    3.2.6.  <copy-config> Operation

       Access control for the <copy-config> protocol operation requires
       special consideration because the administrator may be replacing the
       entire target datastore.

       If the source of the <copy-config> protocol operation is the running
       configuration datastore and the target is the startup configuration
       datastore, the client is only required to have permission to execute
       the <copy-config> protocol operation.

       Otherwise:

       o  If the source of the <copy-config> operation is a datastore, then
          data nodes to which the client does not have read access are
          silently omitted.

       o  If the target of the <copy-config> operation is a datastore, the
          client needs access to the modified nodes.  Specifically:

          *  If the protocol operation would result in the creation of a
             datastore node and the user does not have "create" access
             permission for that node, the protocol operation is rejected
             with an "access-denied" error.

          *  If the protocol operation would result in the deletion of a
             datastore node and the user does not have "delete" access
             permission for that node, the protocol operation is rejected
             with an "access-denied" error.

          *  If the protocol operation would result in the modification of a
             datastore node and the user does not have "update" access
             permission for that node, the protocol operation is rejected
             with an "access-denied" error.

    3.2.7.  <delete-config> Operation

       Access to the <delete-config> protocol operation is denied by
       default.  The "exec-default" leaf does not apply to this protocol
       operation.  Access control rules must be explicitly configured to
       allow invocation by a non-recovery session.

    Bierman & Bjorklund          Standards Track                   [Page 18]

    RFC 8341                          NACM                        March 2018

    3.2.8.  <commit> Operation

       The server MUST determine the exact nodes in the running
       configuration datastore that are actually different and only check
       "create", "update", and "delete" access permissions for this set of
       nodes, which could be empty.

       For example, if a session can read the entire datastore but only
       change one leaf, that session needs to be able to edit and commit
       that one leaf.

    3.2.9.  <discard-changes> Operation

       The client is only required to have permission to execute the
       <discard-changes> protocol operation.  No datastore permissions are
       needed.

    3.2.10.  <kill-session> Operation

       The <kill-session> operation does not directly alter a datastore.
       However, it allows one session to disrupt another session that is
       editing a datastore.

       Access to the <kill-session> protocol operation is denied by default.
       The "exec-default" leaf does not apply to this protocol operation.
       Access control rules must be explicitly configured to allow
       invocation by a non-recovery session.

    3.3.  Model Components

       This section defines the conceptual components related to the access
       control model.

    3.3.1.  Users

       A "user" is the conceptual entity that is associated with the access
       permissions granted to a particular session.  A user is identified by
       a string that is unique within the server.

       As described in [RFC6241], the username string is derived from the
       transport layer during session establishment.  If the transport layer
       cannot authenticate the user, the session is terminated.

    Bierman & Bjorklund          Standards Track                   [Page 19]

    RFC 8341                          NACM                        March 2018

    3.3.2.  Groups

       Access to a specific NETCONF protocol operation is granted to a
       session.  The session is associated with a group (i.e., not with
       a user).

       A group is identified by its name.  All group names are unique within
       the server.

       Access control is applied at the level of groups.  A group contains
       zero or more group members.

       A group member is identified by a username string.

       The same user can be a member of multiple groups.

    3.3.3.  Emergency Recovery Session

       The server MAY support a recovery session mechanism, which will
       bypass all access control enforcement.  This is useful for
       restricting initial access and repairing a broken access control
       configuration.

    3.3.4.  Global Enforcement Controls

       There are five global controls that are used to help control how
       access control is enforced.

    3.3.4.1.  enable-nacm Switch

       A global "enable-nacm" on/off switch is provided to enable or disable
       all access control enforcement.  When this global switch is set to
       "true", all requests are checked against the access control rules and
       only permitted if configured to allow the specific access request.
       When this global switch is set to "false", all access requests are
       permitted.

    3.3.4.2.  read-default Switch

       An on/off "read-default" switch is provided to enable or disable
       default access to receive data in replies and notifications.  When
       the "enable-nacm" global switch is set to "true", this global switch
       is relevant if no matching access control rule is found to explicitly
       permit or deny read access to the requested datastore data or
       notification event type.

    Bierman & Bjorklund          Standards Track                   [Page 20]

    RFC 8341                          NACM                        March 2018

       When this global switch is set to "permit" and no matching access
       control rule is found for the datastore read or notification event
       requested, access is permitted.

       When this global switch is set to "deny" and no matching access
       control rule is found for the datastore read or notification event
       requested, access is denied.  This means that the requested data is
       not sent to the client.  See step 11 in Section 3.4.5 for details.

    3.3.4.3.  write-default Switch

       An on/off "write-default" switch is provided to enable or disable
       default access to alter configuration data.  When the "enable-nacm"
       global switch is set to "true", this global switch is relevant if no
       matching access control rule is found to explicitly permit or deny
       write access to the requested datastore data.

       When this global switch is set to "permit" and no matching access
       control rule is found for the datastore write requested, access is
       permitted.

       When this global switch is set to "deny" and no matching access
       control rule is found for the datastore write requested, access is
       denied.  See step 12 in Section 3.4.5 for details.

    3.3.4.4.  exec-default Switch

       An on/off "exec-default" switch is provided to enable or disable
       default access to execute protocol operations.  When the
       "enable-nacm" global switch is set to "true", this global switch is
       relevant if no matching access control rule is found to explicitly
       permit or deny access to the requested NETCONF protocol operation.

       When this global switch is set to "permit" and no matching access
       control rule is found for the NETCONF protocol operation requested,
       access is permitted.

       When this global switch is set to "deny" and no matching access
       control rule is found for the NETCONF protocol operation requested,
       access is denied.  See step 12 in Section 3.4.4 and step 13 in
       Section 3.4.5 for details.

    Bierman & Bjorklund          Standards Track                   [Page 21]

    RFC 8341                          NACM                        March 2018

    3.3.4.5.  enable-external-groups Switch

       When this global switch is set to "true", the group names reported by
       the transport layer for a session are used together with the locally
       configured group names to determine the access control rules for the
       session.

       When this switch is set to "false", the group names reported by the
       transport layer are ignored by the NACM.

    3.3.5.  Access Control Rules

       There are four types of rules available in the NACM:

       module rule:  controls access for definitions in a specific YANG
          module, identified by its name.

       protocol operation rule:  controls access for a specific protocol
          operation, identified by its YANG module and name.

       data node rule:  controls access for a specific data node and its
          descendants, identified by its path location within the conceptual
          XML document for the data node.

       notification rule:  controls access for a specific notification event
          type, identified by its YANG module and name.

    3.4.  Access Control Enforcement Procedures

       There are six separate phases that need to be addressed, four of
       which are related to the NETCONF message processing model
       (Section 3.1.3):

       1.  Initial operation

       2.  Session establishment

       3.  "access-denied" error handling

       4.  Incoming RPC message validation

       5.  Data node access validation

       6.  Outgoing <notification> authorization

       In addition, the initial startup mode for a NETCONF server, session
       establishment, and "access-denied" error-handling procedures also
       need to be considered.

    Bierman & Bjorklund          Standards Track                   [Page 22]

    RFC 8341                          NACM                        March 2018

       The server MUST use the access control rules in effect at the time it
       starts processing the message.  The same access control rules MUST
       stay in effect for the processing of the entire message.

    3.4.1.  Initial Operation

       Upon the very first startup of the NETCONF server, the access control
       configuration will probably not be present.  If it isn't, a server
       MUST NOT allow any write access to any session role except a recovery
       session.

       Access rules are enforced any time a request is initiated from a user
       session.  Access control is not enforced for server-initiated access
       requests, such as the initial load of the running configuration
       datastore, during bootup.

    3.4.2.  Session Establishment

       The access control model applies specifically to the well-formed XML
       content transferred between a client and a server after session
       establishment has been completed and after the <hello> exchange has
       been successfully completed.

       Once session establishment is completed and a user has been
       authenticated, the transport layer reports the username and a
       possibly empty set of group names associated with the user to the
       NETCONF server.  The NETCONF server will enforce the access control
       rules, based on the supplied username, group names, and the
       configuration data stored on the server.

    3.4.3.  "access-denied" Error Handling

       The "access-denied" error-tag is generated when the access control
       system denies access to either a request to invoke a protocol
       operation or a request to perform a particular access operation on
       the configuration datastore.

       A server MUST NOT include any information the client is not allowed
       to read in any <error-info> elements within the <rpc-error> response.

    Bierman & Bjorklund          Standards Track                   [Page 23]

    RFC 8341                          NACM                        March 2018

    3.4.4.  Incoming RPC Message Validation

       The diagram below shows the basic conceptual structure of the access
       control processing model for incoming NETCONF <rpc> messages within a
       server.

                  NETCONF server
                  +------------+
                  |    XML     |
                  |   message  |
                  | dispatcher |
                  +------------+
                         |
                         |
                         V
                 +---------------+
                 | <rpc> message |
                 +---------------+
                   |    |     |
                   |    |     +--------------------------------+
                   |    +---------------+                      |
                   V                    V                      V
         +------------------+ +--------------------+ +--------------------+
         | vendor operation | | standard operation | | standard operation |
         |    <my-edit>     | |   <edit-config>    | |      <unlock>      |
         +------------------+ +--------------------+ +--------------------+
                     |                 |
                     |                 |
                     V                 V
                    +----------------------+
                    |    configuration     |
                    |      datastore       |
                    +----------------------+

                                     Figure 3

       Access control begins with the message dispatcher.

       After the server validates the <rpc> element and determines the
       namespace URI and the element name of the protocol operation being
       requested, the server verifies that the user is authorized to invoke
       the protocol operation.

    Bierman & Bjorklund          Standards Track                   [Page 24]

    RFC 8341                          NACM                        March 2018

       The server MUST separately authorize every protocol operation by
       following these steps:

       1.   If the "enable-nacm" leaf is set to "false", then the protocol
            operation is permitted.

       2.   If the requesting session is identified as a recovery session,
            then the protocol operation is permitted.

       3.   If the requested operation is the NETCONF <close-session>
            protocol operation, then the protocol operation is permitted.

       4.   Check all the "group" entries to see if any of them contain a
            "user-name" entry that equals the username for the session
            making the request.  If the "enable-external-groups" leaf is
            "true", add to these groups the set of groups provided by the
            transport layer.

       5.   If no groups are found, continue with step 10.

       6.   Process all rule-list entries, in the order they appear in the
            configuration.  If a rule-list's "group" leaf-list does not
            match any of the user's groups, proceed to the next rule-list
            entry.

       7.   For each rule-list entry found, process all rules, in order,
            until a rule that matches the requested access operation is
            found.  A rule matches if all of the following criteria are met:

            *  The rule's "module-name" leaf is "*" or equals the name of
               the YANG module where the protocol operation is defined.

            *  Either (1) the rule does not have a "rule-type" defined or
               (2) the "rule-type" is "protocol-operation" and the
               "rpc-name" is "*" or equals the name of the requested
               protocol operation.

            *  The rule's "access-operations" leaf has the "exec" bit set or
               has the special value "*".

       8.   If a matching rule is found, then the "action" leaf is checked.
            If it is equal to "permit", then the protocol operation is
            permitted; otherwise, it is denied.

       9.   At this point, no matching rule was found in any rule-list
            entry.

    Bierman & Bjorklund          Standards Track                   [Page 25]

    RFC 8341                          NACM                        March 2018

       10.  If the requested protocol operation is defined in a YANG module
            advertised in the server capabilities and the "rpc" statement
            contains a "nacm:default-deny-all" statement, then the protocol
            operation is denied.

       11.  If the requested protocol operation is the NETCONF
            <kill-session> or <delete-config>, then the protocol operation
            is denied.

       12.  If the "exec-default" leaf is set to "permit", then permit the
            protocol operation; otherwise, deny the request.

       If the user is not authorized to invoke the protocol operation, then
       an <rpc-error> is generated with the following information:

       error-tag:  access-denied

       error-path:  Identifies the requested protocol operation.  The
          following example represents the <edit-config> protocol operation
          in the NETCONF base namespace:

             <error-path
               xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
                 /nc:rpc/nc:edit-config
             </error-path>

       If a datastore is accessed, either directly or as a side effect of
       the protocol operation, then the server MUST intercept the access
       operation and make sure that the user is authorized to perform the
       requested access operation on the specified data, as defined in
       Section 3.4.5.

    3.4.5.  Data Node Access Validation

       If (1) a data node within a datastore is accessed or (2) an action or
       notification is tied to a data node, then the server MUST ensure that
       the user is authorized to perform the requested "read", "create",
       "update", "delete", or "execute" access operation on the specified
       data node.

       If an action is requested to be executed, the server MUST ensure that
       the user is authorized to perform the "execute" access operation on
       the requested action.

       If a notification tied to a data node is generated, the server MUST
       ensure that the user is authorized to perform the "read" access
       operation on the requested notification.

    Bierman & Bjorklund          Standards Track                   [Page 26]

    RFC 8341                          NACM                        March 2018

       The data node access request is authorized by following these steps:

       1.   If the "enable-nacm" leaf is set to "false", then the access
            operation is permitted.

       2.   If the requesting session is identified as a recovery session,
            then the access operation is permitted.

       3.   Check all the "group" entries to see if any of them contain a
            "user-name" entry that equals the username for the session
            making the request.  If the "enable-external-groups" leaf is
            "true", add to these groups the set of groups provided by the
            transport layer.

       4.   If no groups are found, continue with step 9.

       5.   Process all rule-list entries, in the order they appear in the
            configuration.  If a rule-list's "group" leaf-list does not
            match any of the user's groups, proceed to the next rule-list
            entry.

       6.   For each rule-list entry found, process all rules, in order,
            until a rule that matches the requested access operation is
            found.  A rule matches if all of the following criteria are met:

            *  The rule's "module-name" leaf is "*" or equals the name of
               the YANG module where the requested data node is defined.

            *  Either (1) the rule does not have a "rule-type" defined or
               (2) the "rule-type" is "data-node" and the "path" matches the
               requested data node, action node, or notification node.  A
               path is considered to match if the requested node is the node
               specified by the path or is a descendant node of the path.

            *  For a "read" access operation, the rule's "access-operations"
               leaf has the "read" bit set or has the special value "*".

            *  For a "create" access operation, the rule's
               "access-operations" leaf has the "create" bit set or has the
               special value "*".

            *  For a "delete" access operation, the rule's
               "access-operations" leaf has the "delete" bit set or has the
               special value "*".

    Bierman & Bjorklund          Standards Track                   [Page 27]

    RFC 8341                          NACM                        March 2018

            *  For an "update" access operation, the rule's
               "access-operations" leaf has the "update" bit set or has the
               special value "*".

            *  For an "execute" access operation, the rule's
               "access-operations" leaf has the "exec" bit set or has the
               special value "*".

       7.   If a matching rule is found, then the "action" leaf is checked.
            If it is equal to "permit", then the data node access is
            permitted; otherwise, it is denied.  For a "read" access
            operation, "denied" means that the requested data is not
            returned in the reply.

       8.   At this point, no matching rule was found in any rule-list
            entry.

       9.   For a "read" access operation, if the requested data node is
            defined in a YANG module advertised in the server capabilities
            and the data definition statement contains a
            "nacm:default-deny-all" statement, then the requested data node
            and all its descendants are not included in the reply.

       10.  For a "write" access operation, if the requested data node is
            defined in a YANG module advertised in the server capabilities
            and the data definition statement contains a
            "nacm:default-deny-write" or a "nacm:default-deny-all"
            statement, then the access request is denied for the data node
            and all its descendants.

       11.  For a "read" access operation, if the "read-default" leaf is set
            to "permit", then include the requested data node in the reply;
            otherwise, do not include the requested data node or any of its
            descendants in the reply.

       12.  For a "write" access operation, if the "write-default" leaf is
            set to "permit", then permit the data node access request;
            otherwise, deny the request.

       13.  For an "execute" access operation, if the "exec-default" leaf is
            set to "permit", then permit the request; otherwise, deny the
            request.

    Bierman & Bjorklund          Standards Track                   [Page 28]

    RFC 8341                          NACM                        March 2018

    3.4.6.  Outgoing <notification> Authorization

       Configuration of access control rules specifically for descendant
       nodes of the notification event type are outside the scope of this
       document.  If the user is authorized to receive the notification
       event type, then it is also authorized to receive any data it
       contains.

       If the notification is specified within a data subtree, as specified
       in [RFC7950], then read access to the notification is required.
       Processing continues as described in Section 3.4.5.

       The following figure shows the conceptual message processing model
       for outgoing <notification> messages.

                                   NETCONF server
                                  +------------+
                                  |    XML     |
                                  |   message  |
                                  | generator  |
                                  +------------+
                                        ^
                                        |
                                +----------------+
                                | <notification> |
                                |  generator     |
                                +----------------+
                                        ^
                                        |
                               +=================+
                               | <notification>  |
                               |  access control |
                               |  <eventType>    |
                               +=================+
                                        ^
                                        |
                            +------------------------+
                            | server instrumentation |
                            +------------------------+
                                      |     ^
                                      V     |
                             +----------------------+
                             |    configuration     |
                             |      datastore       |
                             +----------------------+

                                     Figure 4

    Bierman & Bjorklund          Standards Track                   [Page 29]

    RFC 8341                          NACM                        March 2018

       The generation of a notification for a specific subscription
       [RFC5277] is authorized by following these steps:

       1.   If the "enable-nacm" leaf is set to "false", then the
            notification is permitted.

       2.   If the session is identified as a recovery session, then the
            notification is permitted.

       3.   If the notification is the NETCONF <replayComplete> or
            <notificationComplete> event type [RFC5277], then the
            notification is permitted.

       4.   Check all the "group" entries to see if any of them contain a
            "user-name" entry that equals the username for the session
            making the request.  If the "enable-external-groups" leaf is
            "true", add to these groups the set of groups provided by the
            transport layer.

       5.   If no groups are found, continue with step 10.

       6.   Process all rule-list entries, in the order they appear in the
            configuration.  If a rule-list's "group" leaf-list does not
            match any of the user's groups, proceed to the next rule-list
            entry.

       7.   For each rule-list entry found, process all rules, in order,
            until a rule that matches the requested access operation is
            found.  A rule matches if all of the following criteria are met:

            *  The rule's "module-name" leaf is "*" or equals the name of
               the YANG module where the notification is defined.

            *  Either (1) the rule does not have a "rule-type" defined or
               (2) the "rule-type" is "notification" and the
               "notification-name" is "*" or equals the name of the
               notification.

            *  The rule's "access-operations" leaf has the "read" bit set or
               has the special value "*".

       8.   If a matching rule is found, then the "action" leaf is checked.
            If it is equal to "permit", then permit the notification;
            otherwise, drop the notification for the associated
            subscription.

       9.   Otherwise, no matching rule was found in any rule-list entry.

    Bierman & Bjorklund          Standards Track                   [Page 30]

    RFC 8341                          NACM                        March 2018

       10.  If the requested notification is defined in a YANG module
            advertised in the server capabilities and the "notification"
            statement contains a "nacm:default-deny-all" statement, then the
            notification is dropped for the associated subscription.

       11.  If the "read-default" leaf is set to "permit", then permit the
            notification; otherwise, drop the notification for the
            associated subscription.

    3.5.  Data Model Definitions

    3.5.1.  Data Organization

       The following diagram highlights the contents and structure of the
       NACM YANG module.

       module: ietf-netconf-acm
         +--rw nacm
            +--rw enable-nacm?              boolean
            +--rw read-default?             action-type
            +--rw write-default?            action-type
            +--rw exec-default?             action-type
            +--rw enable-external-groups?   boolean
            +--ro denied-operations         yang:zero-based-counter32
            +--ro denied-data-writes        yang:zero-based-counter32
            +--ro denied-notifications      yang:zero-based-counter32
            +--rw groups
            |  +--rw group* [name]
            |     +--rw name         group-name-type
            |     +--rw user-name*   user-name-type
            +--rw rule-list* [name]
               +--rw name     string
               +--rw group*   union
               +--rw rule* [name]
                  +--rw name                 string
                  +--rw module-name?         union
                  +--rw (rule-type)?
                  |  +--:(protocol-operation)
                  |  |  +--rw rpc-name?            union
                  |  +--:(notification)
                  |  |  +--rw notification-name?   union
                  |  +--:(data-node)
                  |     +--rw path                 node-instance-identifier
                  +--rw access-operations?   union
                  +--rw action               action-type
                  +--rw comment?             string

    Bierman & Bjorklund          Standards Track                   [Page 31]

    RFC 8341                          NACM                        March 2018

    3.5.2.  YANG Module

       The following YANG module specifies the normative NETCONF content
       that MUST be supported by the server.

       The "ietf-netconf-acm" YANG module imports typedefs from [RFC6991].

       <CODE BEGINS> file "ietf-netconf-acm@2018-02-14.yang"
       module ietf-netconf-acm {

         namespace "urn:ietf:params:xml:ns:yang:ietf-netconf-acm";

         prefix nacm;

         import ietf-yang-types {
           prefix yang;
         }

         organization
           "IETF NETCONF (Network Configuration) Working Group";

         contact
           "WG Web:   <https://datatracker.ietf.org/wg/netconf/>
            WG List:  <mailto:netconf@ietf.org>

            Author:   Andy Bierman
                      <mailto:andy@yumaworks.com>

            Author:   Martin Bjorklund
                      <mailto:mbj@tail-f.com>";

         description
           "Network Configuration Access Control Model.

            Copyright (c) 2012 - 2018 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 8341; see
            the RFC itself for full legal notices.";

    Bierman & Bjorklund          Standards Track                   [Page 32]

    RFC 8341                          NACM                        March 2018

         revision "2018-02-14" {
           description
             "Added support for YANG 1.1 actions and notifications tied to
              data nodes.  Clarified how NACM extensions can be used by
              other data models.";
           reference
             "RFC 8341: Network Configuration Access Control Model";
         }

         revision "2012-02-22" {
           description
             "Initial version.";
           reference
             "RFC 6536: Network Configuration Protocol (NETCONF)
                        Access Control Model";
         }

         /*
          * Extension statements
          */

         extension default-deny-write {
           description
             "Used to indicate that the data model node
              represents a sensitive security system parameter.

              If present, the NETCONF server will only allow the designated
              'recovery session' to have write access to the node.  An
              explicit access control rule is required for all other users.

              If the NACM module is used, then it must be enabled (i.e.,
              /nacm/enable-nacm object equals 'true'), or this extension
              is ignored.

              The 'default-deny-write' extension MAY appear within a data
              definition statement.  It is ignored otherwise.";
         }

         extension default-deny-all {
           description
             "Used to indicate that the data model node
              controls a very sensitive security system parameter.

              If present, the NETCONF server will only allow the designated
              'recovery session' to have read, write, or execute access to
              the node.  An explicit access control rule is required for all
              other users.

    Bierman & Bjorklund          Standards Track                   [Page 33]

    RFC 8341                          NACM                        March 2018

              If the NACM module is used, then it must be enabled (i.e.,
              /nacm/enable-nacm object equals 'true'), or this extension
              is ignored.

              The 'default-deny-all' extension MAY appear within a data
              definition statement, 'rpc' statement, or 'notification'
              statement.  It is ignored otherwise.";
         }

         /*
          * Derived types
          */

         typedef user-name-type {
           type string {
             length "1..max";
           }
           description
             "General-purpose username string.";
         }

         typedef matchall-string-type {
           type string {
             pattern '\*';
           }
           description
             "The string containing a single asterisk '*' is used
              to conceptually represent all possible values
              for the particular leaf using this data type.";
         }

         typedef access-operations-type {
           type bits {
             bit create {
               description
                 "Any protocol operation that creates a
                  new data node.";
             }
             bit read {
               description
                 "Any protocol operation or notification that
                  returns the value of a data node.";
             }
             bit update {
               description
                 "Any protocol operation that alters an existing
                  data node.";
             }

    Bierman & Bjorklund          Standards Track                   [Page 34]

    RFC 8341                          NACM                        March 2018

             bit delete {
               description
                 "Any protocol operation that removes a data node.";
             }
             bit exec {
               description
                 "Execution access to the specified protocol operation.";
             }
           }
           description
             "Access operation.";
         }

         typedef group-name-type {
           type string {
             length "1..max";
             pattern '[^\*].*';
           }
           description
             "Name of administrative group to which
              users can be assigned.";
         }

         typedef action-type {
           type enumeration {
             enum permit {
               description
                 "Requested action is permitted.";
             }
             enum deny {
               description
                 "Requested action is denied.";
             }
           }
           description
             "Action taken by the server when a particular
              rule matches.";
         }

         typedef node-instance-identifier {
           type yang:xpath1.0;
           description
             "Path expression used to represent a special
              data node, action, or notification instance-identifier
              string.

              A node-instance-identifier value is an
              unrestricted YANG instance-identifier expression.

    Bierman & Bjorklund          Standards Track                   [Page 35]

    RFC 8341                          NACM                        March 2018

              All the same rules as an instance-identifier apply,
              except that predicates for keys are optional.  If a key
              predicate is missing, then the node-instance-identifier
              represents all possible server instances for that key.

              This XML Path Language (XPath) expression is evaluated in the
              following context:

                 o  The set of namespace declarations are those in scope on
                    the leaf element where this type is used.

                 o  The set of variable bindings contains one variable,
                    'USER', which contains the name of the user of the
                    current session.

                 o  The function library is the core function library, but
                    note that due to the syntax restrictions of an
                    instance-identifier, no functions are allowed.

                 o  The context node is the root node in the data tree.

              The accessible tree includes actions and notifications tied
              to data nodes.";
         }

         /*
          * Data definition statements
          */

         container nacm {
           nacm:default-deny-all;

           description
             "Parameters for NETCONF access control model.";

           leaf enable-nacm {
             type boolean;
             default "true";
             description
               "Enables or disables all NETCONF access control
                enforcement.  If 'true', then enforcement
                is enabled.  If 'false', then enforcement
                is disabled.";
           }

    Bierman & Bjorklund          Standards Track                   [Page 36]

    RFC 8341                          NACM                        March 2018

           leaf read-default {
             type action-type;
             default "permit";
             description
               "Controls whether read access is granted if
                no appropriate rule is found for a
                particular read request.";
           }

           leaf write-default {
             type action-type;
             default "deny";
             description
               "Controls whether create, update, or delete access
                is granted if no appropriate rule is found for a
                particular write request.";
           }

           leaf exec-default {
             type action-type;
             default "permit";
             description
               "Controls whether exec access is granted if no appropriate
                rule is found for a particular protocol operation request.";
           }

           leaf enable-external-groups {
             type boolean;
             default "true";
             description
               "Controls whether the server uses the groups reported by the
                NETCONF transport layer when it assigns the user to a set of
                NACM groups.  If this leaf has the value 'false', any group
                names reported by the transport layer are ignored by the
                server.";
           }

           leaf denied-operations {
             type yang:zero-based-counter32;
             config false;
             mandatory true;
             description
               "Number of times since the server last restarted that a
                protocol operation request was denied.";
           }

    Bierman & Bjorklund          Standards Track                   [Page 37]

    RFC 8341                          NACM                        March 2018

           leaf denied-data-writes {
             type yang:zero-based-counter32;
             config false;
             mandatory true;
             description
               "Number of times since the server last restarted that a
                protocol operation request to alter
                a configuration datastore was denied.";
           }

           leaf denied-notifications {
             type yang:zero-based-counter32;
             config false;
             mandatory true;
             description
               "Number of times since the server last restarted that
                a notification was dropped for a subscription because
                access to the event type was denied.";
           }

           container groups {
             description
               "NETCONF access control groups.";

             list group {
               key name;

               description
                 "One NACM group entry.  This list will only contain
                  configured entries, not any entries learned from
                  any transport protocols.";

               leaf name {
                 type group-name-type;
                 description
                   "Group name associated with this entry.";
               }

               leaf-list user-name {
                 type user-name-type;
                 description
                   "Each entry identifies the username of
                    a member of the group associated with
                    this entry.";
               }
             }
           }

    Bierman & Bjorklund          Standards Track                   [Page 38]

    RFC 8341                          NACM                        March 2018

           list rule-list {
             key name;
             ordered-by user;
             description
               "An ordered collection of access control rules.";

             leaf name {
               type string {
                 length "1..max";
               }
               description
                 "Arbitrary name assigned to the rule-list.";
             }
             leaf-list group {
               type union {
                 type matchall-string-type;
                 type group-name-type;
               }
               description
                 "List of administrative groups that will be
                  assigned the associated access rights
                  defined by the 'rule' list.

                  The string '*' indicates that all groups apply to the
                  entry.";
             }

             list rule {
               key name;
               ordered-by user;
               description
                 "One access control rule.

                  Rules are processed in user-defined order until a match is
                  found.  A rule matches if 'module-name', 'rule-type', and
                  'access-operations' match the request.  If a rule
                  matches, the 'action' leaf determines whether or not
                  access is granted.";

               leaf name {
                 type string {
                   length "1..max";
                 }
                 description
                   "Arbitrary name assigned to the rule.";
               }

    Bierman & Bjorklund          Standards Track                   [Page 39]

    RFC 8341                          NACM                        March 2018

               leaf module-name {
                 type union {
                   type matchall-string-type;
                   type string;
                 }
                 default "*";
                 description
                   "Name of the module associated with this rule.

                    This leaf matches if it has the value '*' or if the
                    object being accessed is defined in the module with the
                    specified module name.";
               }
               choice rule-type {
                 description
                   "This choice matches if all leafs present in the rule
                    match the request.  If no leafs are present, the
                    choice matches all requests.";
                 case protocol-operation {
                   leaf rpc-name {
                     type union {
                       type matchall-string-type;
                       type string;
                     }
                     description
                       "This leaf matches if it has the value '*' or if
                        its value equals the requested protocol operation
                        name.";
                   }
                 }
                 case notification {
                   leaf notification-name {
                     type union {
                       type matchall-string-type;
                       type string;
                     }
                     description
                       "This leaf matches if it has the value '*' or if its
                        value equals the requested notification name.";
                   }
                 }

    Bierman & Bjorklund          Standards Track                   [Page 40]

    RFC 8341                          NACM                        March 2018

                 case data-node {
                   leaf path {
                     type node-instance-identifier;
                     mandatory true;
                     description
                       "Data node instance-identifier associated with the
                        data node, action, or notification controlled by
                        this rule.

                        Configuration data or state data
                        instance-identifiers start with a top-level
                        data node.  A complete instance-identifier is
                        required for this type of path value.

                        The special value '/' refers to all possible
                        datastore contents.";
                   }
                 }
               }

               leaf access-operations {
                 type union {
                   type matchall-string-type;
                   type access-operations-type;
                 }
                 default "*";
                 description
                   "Access operations associated with this rule.

                    This leaf matches if it has the value '*' or if the
                    bit corresponding to the requested operation is set.";
               }

               leaf action {
                 type action-type;
                 mandatory true;
                 description
                   "The access control action associated with the
                    rule.  If a rule has been determined to match a
                    particular request, then this object is used
                    to determine whether to permit or deny the
                    request.";
               }

    Bierman & Bjorklund          Standards Track                   [Page 41]

    RFC 8341                          NACM                        March 2018

               leaf comment {
                 type string;
                 description
                   "A textual description of the access rule.";
               }
             }
           }
         }
       }

       <CODE ENDS>

    4.  IANA Considerations

       This document reuses the URI for "ietf-netconf-acm" in the "IETF XML
       Registry".

       This document updates the module registration in the "YANG Module
       Names" registry to reference this RFC instead of RFC 6536 for
       "ietf-netconf-acm".  Following the format in [RFC6020], the following
       has been registered.

            Name: ietf-netconf-acm
            Namespace: urn:ietf:params:xml:ns:yang:ietf-netconf-acm
            Prefix: nacm
            Reference: RFC 8341

    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
       [RFC5246].

       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 is a risk related to the lack of access control enforcement for
       the RESTCONF OPTIONS and PATCH methods.  The risk here is that the
       response to OPTIONS and PATCH may vary based on the presence or
       absence of a resource corresponding to the URL's path.  If this is
       the case, then it can be used to trivially probe for the presence or
       absence of values within a tree.  Therefore, a server MUST NOT vary

    Bierman & Bjorklund          Standards Track                   [Page 42]

    RFC 8341                          NACM                        March 2018

       its responses based on the existence of the underlying resource,
       which would indicate the presence or absence of resource instances.
       In particular, servers should not expose any instance information
       before ensuring that the client has the necessary access permissions
       to obtain that information.  In such cases, servers are expected to
       always return the "access-denied" error response.

       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:

       o  /nacm: The entire /nacm subtree is related to security.  Refer to
          the following sections for more details.

       This section highlights the issues for an administrator to consider
       when configuring a NETCONF server with the NACM.

    5.1.  NACM Configuration and Monitoring Considerations

       Configuration of the access control system is highly sensitive to
       system security.  A server may choose not to allow any user
       configuration to some portions of it, such as the global security
       level or the groups that allowed access to system resources.

       By default, NACM enforcement is enabled.  By default, "read" access
       to all datastore contents is enabled (unless "nacm:default-deny-all"
       is specified for the data definition), and "exec" access is enabled
       for safe protocol operations.  An administrator needs to ensure that
       the NACM is enabled and also decide if the default access parameters
       are set appropriately.  Make sure that the following data nodes are
       properly configured:

       o  /nacm/enable-nacm (default "true")

       o  /nacm/read-default (default "permit")

       o  /nacm/write-default (default "deny")

       o  /nacm/exec-default (default "permit")

       An administrator needs to restrict write access to all configurable
       objects within this data model.

    Bierman & Bjorklund          Standards Track                   [Page 43]

    RFC 8341                          NACM                        March 2018

       If write access is allowed for configuration of access control rules,
       then care needs to be taken not to disrupt the access control
       enforcement.  For example, if the NACM access control rules are
       edited directly within the running configuration datastore (i.e.,
       :writable-running capability is supported and used), then care needs
       to be taken not to allow unintended access while the edits are being
       done.

       An administrator needs to make sure that the translation from a
       transport- or implementation-dependent user identity to a NACM
       username is unique and correct.  This requirement is specified in
       detail in Section 2.2 of [RFC6241].

       An administrator needs to be aware that the YANG data structures
       representing access control rules (/nacm/rule-list and
       /nacm/rule-list/rule) are ordered by the client.  The server will
       evaluate the access control rules according to their relative
       conceptual order within the running configuration datastore.

       Note that the /nacm/groups data structure contains the administrative
       group names used by the server.  These group names may be configured
       locally and/or provided through an external protocol, such as RADIUS
       [RFC2865] [RFC5607].

       An administrator needs to be aware of the security properties of any
       external protocol used by the transport layer to determine group
       names.  For example, if this protocol does not protect against
       man-in-the-middle attacks, an attacker might be able to inject group
       names that are configured in the NACM so that a user gets more
       permissions than it should.  In such cases, the administrator may
       wish to disable the usage of such group names by setting
       /nacm/enable-external-groups to "false".

       Some of the readable data nodes in this YANG module may be considered
       sensitive or vulnerable in some network environments.  It is thus
       important to control read access (e.g., via get, get-config, or
       notification) to these data nodes.  These are the subtrees and data
       nodes and their sensitivity/vulnerability:

       o  /nacm/enable-nacm

       o  /nacm/read-default

       o  /nacm/write-default

       o  /nacm/exec-default

       o  /nacm/enable-external-groups

    Bierman & Bjorklund          Standards Track                   [Page 44]

    RFC 8341                          NACM                        March 2018

       o  /nacm/groups

       o  /nacm/rule-list

       An administrator needs to restrict read access to the above-listed
       objects within this data model, as they reveal access control
       configuration that could be considered sensitive.

    5.2.  General Configuration Issues

       There is a risk that invocation of non-standard protocol operations
       will have undocumented side effects.  An administrator needs to
       construct access control rules such that the configuration datastore
       is protected from such side effects.

       It is possible for a session with some write access (e.g., allowed to
       invoke <edit-config>), but without any access to a particular
       datastore subtree containing sensitive data, to determine the
       presence or non-presence of that data.  This can be done by
       repeatedly issuing some sort of edit request (create, update, or
       delete) and possibly receiving "access-denied" errors in response.
       These "fishing" attacks can identify the presence or non-presence of
       specific sensitive data even without the "error-path" field being
       present within the <rpc-error> response.

       It may be possible for the set of NETCONF capabilities on the server
       to change over time.  If so, then there is a risk that new protocol
       operations, notifications, and/or datastore content have been added
       to the device.  An administrator needs to be sure that the access
       control rules are correct for the new content in this case.
       Mechanisms to detect NETCONF capability changes on a specific device
       are outside the scope of this document.

       It is possible that the data model definition itself (e.g., a YANG
       when-stmt) will help an unauthorized session determine the presence
       or even value of sensitive data nodes by examining the presence and
       values of different data nodes.

       It is possible that the data model definition itself (e.g., a YANG
       when-stmt or choice-stmt) will allow a session to implicitly create
       or delete nodes that the session does not have write access to as an
       implicit side effect from the processing of an allowed <edit-config>
       operation.

    Bierman & Bjorklund          Standards Track                   [Page 45]

    RFC 8341                          NACM                        March 2018

       There is a risk that non-standard protocol operations, or even the
       standard <get> protocol operation, may return data that "aliases" or
       "copies" sensitive data from a different data object.  There may
       simply be multiple data model definitions that expose or even
       configure the same underlying system instrumentation.

       A data model may contain external keys (e.g., YANG leafref), which
       expose values from a different data structure.  An administrator
       needs to be aware of sensitive data models that contain leafref
       nodes.  This entails finding all the leafref objects that "point" at
       the sensitive data (i.e., "path-stmt" values) that implicitly or
       explicitly includes the sensitive data node.

       It is beyond the scope of this document to define access control
       enforcement procedures for underlying device instrumentation that may
       exist to support the NETCONF server operation.  An administrator can
       identify each protocol operation that the server provides and decide
       if it needs any access control applied to it.

       This document incorporates the optional use of a recovery session
       mechanism, which can be used to bypass access control enforcement in
       emergencies such as NACM configuration errors that disable all access
       to the server.  The configuration and identification of such a
       recovery session mechanism are implementation specific and are
       outside the scope of this document.  An administrator needs to be
       aware of any recovery session mechanisms available on the device and
       make sure that they are used appropriately.

       It is possible for a session to disrupt configuration management,
       even without any write access to the configuration, by locking the
       datastore.  This may be done to ensure that all or part of the
       configuration remains stable while it is being retrieved, or it may
       be done as a "denial-of-service" attack.  There is no way for the
       server to know the difference.  An administrator may wish to restrict
       "exec" access to the following protocol operations:

       o  <lock>

       o  <unlock>

       o  <partial-lock>

       o  <partial-unlock>

    Bierman & Bjorklund          Standards Track                   [Page 46]

    RFC 8341                          NACM                        March 2018

    5.3.  Data Model Design Considerations

       Designers need to clearly identify any sensitive data, notifications,
       or protocol operations defined within a YANG module.  For such
       definitions, a "nacm:default-deny-write" or "nacm:default-deny-all"
       statement ought to be present, in addition to a clear description of
       the security risks.

       Protocol operations need to be properly documented by the data model
       designer so that it is clear to administrators what data nodes (if
       any) are affected by the protocol operation and what information (if
       any) is returned in the <rpc-reply> message.

       Data models ought to be designed so that different access levels for
       input parameters to protocol operations are not required.  The use of
       generic protocol operations should be avoided, and if different
       access levels are needed, separate protocol operations should be
       defined instead.

    6.  References

    6.1.  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>.

       [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
                  (TLS) Protocol Version 1.2", RFC 5246,
                  DOI 10.17487/RFC5246, August 2008,
                  <https://www.rfc-editor.org/info/rfc5246>.

       [RFC5277]  Chisholm, S. and H. Trevino, "NETCONF Event
                  Notifications", RFC 5277, DOI 10.17487/RFC5277, July 2008,
                  <https://www.rfc-editor.org/info/rfc5277>.

       [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
                  the Network Configuration Protocol (NETCONF)", RFC 6020,
                  DOI 10.17487/RFC6020, October 2010,
                  <https://www.rfc-editor.org/info/rfc6020>.

       [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>.

    Bierman & Bjorklund          Standards Track                   [Page 47]

    RFC 8341                          NACM                        March 2018

       [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>.

       [RFC7230]  Fielding, R., Ed., and J. Reschke, Ed., "Hypertext
                  Transfer Protocol (HTTP/1.1): Message Syntax and Routing",
                  RFC 7230, DOI 10.17487/RFC7230, June 2014,
                  <https://www.rfc-editor.org/info/rfc7230>.

       [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>.

       [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>.

       [W3C.REC-xml-20081126]
                  Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
                  F. Yergeau, "Extensible Markup Language (XML) 1.0
                  (Fifth Edition)", World Wide Web Consortium Recommendation
                  REC-xml-20081126, November 2008,
                  <https://www.w3.org/TR/2008/REC-xml-20081126>.

    Bierman & Bjorklund          Standards Track                   [Page 48]

    RFC 8341                          NACM                        March 2018

    6.2.  Informative References

       [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
                  "Remote Authentication Dial In User Service (RADIUS)",
                  RFC 2865, DOI 10.17487/RFC2865, June 2000,
                  <https://www.rfc-editor.org/info/rfc2865>.

       [RFC5607]  Nelson, D. and G. Weber, "Remote Authentication Dial-In
                  User Service (RADIUS) Authorization for Network Access
                  Server (NAS) Management", RFC 5607, DOI 10.17487/RFC5607,
                  July 2009, <https://www.rfc-editor.org/info/rfc5607>.

       [YANG-SEC] IETF, "YANG Security Guidelines", <https://trac.ietf.org/
                  trac/ops/wiki/yang-security-guidelines>.

    Bierman & Bjorklund          Standards Track                   [Page 49]

    RFC 8341                          NACM                        March 2018

    Appendix A.  Usage Examples

       The following XML [W3C.REC-xml-20081126] snippets are provided as
       examples only, to demonstrate how the NACM can be configured to
       perform some access control tasks.

    A.1.  <groups> Example

       There needs to be at least one <group> entry in order for any of the
       access control rules to be useful.

       The following XML shows arbitrary groups and is not intended to
       represent any particular use case.

       <nacm xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-acm">
         <groups>
           <group>
             <name>admin</name>
             <user-name>admin</user-name>
             <user-name>andy</user-name>
           </group>

           <group>
             <name>limited</name>
             <user-name>wilma</user-name>
             <user-name>bam-bam</user-name>
           </group>

           <group>
             <name>guest</name>
             <user-name>guest</user-name>
             <user-name>guest@example.com</user-name>
           </group>
         </groups>
       </nacm>

       This example shows three groups:

       admin:  The "admin" group contains two users named "admin" and
          "andy".

       limited:  The "limited" group contains two users named "wilma" and
          "bam-bam".

       guest:  The "guest" group contains two users named "guest" and
          "guest@example.com".

    Bierman & Bjorklund          Standards Track                   [Page 50]

    RFC 8341                          NACM                        March 2018

    A.2.  Module Rule Example

       Module rules are used to control access to all the content defined in
       a specific module.  A module rule has the "module-name" leaf set but
       no nodes from the "rule-type" choice set.

       <nacm xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-acm">
         <rule-list>
           <name>guest-acl</name>
           <group>guest</group>

           <rule>
             <name>deny-ncm</name>
             <module-name>ietf-netconf-monitoring</module-name>
             <access-operations>*</access-operations>
             <action>deny</action>
             <comment>
                 Do not allow guests any access to the NETCONF
                 monitoring information.
             </comment>
           </rule>
         </rule-list>

         <rule-list>
           <name>limited-acl</name>
           <group>limited</group>

           <rule>
             <name>permit-ncm</name>
             <module-name>ietf-netconf-monitoring</module-name>
             <access-operations>read</access-operations>
             <action>permit</action>
             <comment>
                 Allow read access to the NETCONF
                 monitoring information.
             </comment>
           </rule>
           <rule>
             <name>permit-exec</name>
             <module-name>*</module-name>
             <access-operations>exec</access-operations>
             <action>permit</action>
             <comment>
                 Allow invocation of the
                 supported server operations.
             </comment>
           </rule>
         </rule-list>

    Bierman & Bjorklund          Standards Track                   [Page 51]

    RFC 8341                          NACM                        March 2018

         <rule-list>
           <name>admin-acl</name>
           <group>admin</group>

           <rule>
             <name>permit-all</name>
             <module-name>*</module-name>
             <access-operations>*</access-operations>
             <action>permit</action>
             <comment>
                 Allow the 'admin' group complete access to all
                 operations and data.
             </comment>
           </rule>
         </rule-list>
       </nacm>

       This example shows four module rules:

       deny-ncm:  This rule prevents the "guest" group from reading any
          monitoring information in the "ietf-netconf-monitoring" YANG
          module.

       permit-ncm:  This rule allows the "limited" group to read the
          "ietf-netconf-monitoring" YANG module.

       permit-exec:  This rule allows the "limited" group to invoke any
          protocol operation supported by the server.

       permit-all:  This rule allows the "admin" group complete access to
          all content in the server.  No subsequent rule will match for the
          "admin" group because of this module rule.

    Bierman & Bjorklund          Standards Track                   [Page 52]

    RFC 8341                          NACM                        March 2018

    A.3.  Protocol Operation Rule Example

       Protocol operation rules are used to control access to a specific
       protocol operation.

       <nacm xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-acm">
         <rule-list>
           <name>guest-limited-acl</name>
           <group>limited</group>
           <group>guest</group>

           <rule>
             <name>deny-kill-session</name>
             <module-name>ietf-netconf</module-name>
             <rpc-name>kill-session</rpc-name>
             <access-operations>exec</access-operations>
             <action>deny</action>
             <comment>
               Do not allow the 'limited' group or the 'guest' group
               to kill another session.
             </comment>
           </rule>
           <rule>
             <name>deny-delete-config</name>
             <module-name>ietf-netconf</module-name>
             <rpc-name>delete-config</rpc-name>
             <access-operations>exec</access-operations>
             <action>deny</action>
             <comment>
               Do not allow the 'limited' group or the 'guest' group
               to delete any configurations.
             </comment>
           </rule>
         </rule-list>

    Bierman & Bjorklund          Standards Track                   [Page 53]

    RFC 8341                          NACM                        March 2018

         <rule-list>
           <name>limited-acl</name>
           <group>limited</group>

           <rule>
             <name>permit-edit-config</name>
             <module-name>ietf-netconf</module-name>
             <rpc-name>edit-config</rpc-name>
             <access-operations>exec</access-operations>
             <action>permit</action>
             <comment>
               Allow the 'limited' group to edit the configuration.
             </comment>
           </rule>
         </rule-list>
       </nacm>

       This example shows three protocol operation rules:

       deny-kill-session:  This rule prevents the "limited" group or the
          "guest" group from invoking the NETCONF <kill-session> protocol
          operation.

       deny-delete-config:  This rule prevents the "limited" group or the
          "guest" group from invoking the NETCONF <delete-config> protocol
          operation.

       permit-edit-config:  This rule allows the "limited" group to invoke
          the NETCONF <edit-config> protocol operation.  This rule will have
          no real effect unless the "exec-default" leaf is set to "deny".

    Bierman & Bjorklund          Standards Track                   [Page 54]

    RFC 8341                          NACM                        March 2018

    A.4.  Data Node Rule Example

       Data node rules are used to control access to specific (config and
       non-config) data nodes within the NETCONF content provided by the
       server.

       <nacm xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-acm">
         <rule-list>
           <name>guest-acl</name>
           <group>guest</group>

           <rule>
             <name>deny-nacm</name>
             <path xmlns:n="urn:ietf:params:xml:ns:yang:ietf-netconf-acm">
               /n:nacm
             </path>
             <access-operations>*</access-operations>
             <action>deny</action>
             <comment>
               Deny the 'guest' group any access to the /nacm data.
             </comment>
           </rule>
         </rule-list>

         <rule-list>
           <name>limited-acl</name>
           <group>limited</group>

           <rule>
             <name>permit-acme-config</name>
             <path xmlns:acme="http://example.com/ns/netconf">
               /acme:acme-netconf/acme:config-parameters
             </path>
             <access-operations>
               read create update delete
             </access-operations>
             <action>permit</action>
             <comment>
               Allow the 'limited' group complete access to the acme
               NETCONF configuration parameters.  Showing long form
               of 'access-operations' instead of shorthand.
             </comment>
           </rule>
         </rule-list>

    Bierman & Bjorklund          Standards Track                   [Page 55]

    RFC 8341                          NACM                        March 2018

         <rule-list>
           <name>guest-limited-acl</name>
           <group>guest</group>
           <group>limited</group>

           <rule>
             <name>permit-dummy-interface</name>
             <path xmlns:acme="http://example.com/ns/itf">
               /acme:interfaces/acme:interface[acme:name='dummy']
             </path>
             <access-operations>read update</access-operations>
             <action>permit</action>
             <comment>
               Allow the 'limited' and 'guest' groups read
               and update access to the dummy interface.
             </comment>
           </rule>
         </rule-list>

         <rule-list>
           <name>admin-acl</name>
           <group>admin</group>
           <rule>
             <name>permit-interface</name>
             <path xmlns:acme="http://example.com/ns/itf">
               /acme:interfaces/acme:interface
             </path>
             <access-operations>*</access-operations>
             <action>permit</action>
             <comment>
               Allow the 'admin' group full access to all acme interfaces.
             </comment>
           </rule>
         </rule-list>
       </nacm>

    Bierman & Bjorklund          Standards Track                   [Page 56]

    RFC 8341                          NACM                        March 2018

       This example shows four data node rules:

       deny-nacm:  This rule denies the "guest" group any access to the
          /nacm subtree.

       permit-acme-config:  This rule gives the "limited" group read-write
          access to the acme <config-parameters>.

       permit-dummy-interface:  This rule gives the "limited" and "guest"
          groups read-update access to the acme <interface> entry named
          "dummy".  This entry cannot be created or deleted by these groups;
          it can only be altered.

       permit-interface:  This rule gives the "admin" group read-write
          access to all acme <interface> entries.

    A.5.  Notification Rule Example

       Notification rules are used to control access to a specific
       notification event type.

       <nacm xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-acm">
         <rule-list>
           <name>sys-acl</name>
           <group>limited</group>
           <group>guest</group>

           <rule>
             <name>deny-config-change</name>
             <module-name>acme-system</module-name>
             <notification-name>sys-config-change</notification-name>
             <access-operations>read</access-operations>
             <action>deny</action>
             <comment>
               Do not allow the 'guest' group or the 'limited' group
               to receive config change events.
             </comment>
           </rule>
         </rule-list>
       </nacm>

       This example shows one notification rule:

       deny-config-change:  This rule prevents the "limited" group or the
          "guest" group from receiving the acme <sys-config-change>
          event type.

    Bierman & Bjorklund          Standards Track                   [Page 57]

    RFC 8341                          NACM                        March 2018

    Authors' Addresses

       Andy Bierman
       YumaWorks
       685 Cochran St.
       Suite #160
       Simi Valley, CA  93065
       United States of America

       Email: andy@yumaworks.com

       Martin Bjorklund
       Tail-f Systems

       Email: mbj@tail-f.com

    Bierman & Bjorklund          Standards Track                   [Page 58]