


Internet Engineering Task Force                               A. Bierman
Internet-Draft                                                   Brocade
Intended status: Standards Track                            M. Bjorklund
Expires: June 25, 2012                                    Tail-f Systems
                                                       December 23, 2011


     Network Configuration Protocol (NETCONF) Access Control Model
                  draft-ietf-netconf-access-control-07

Abstract

   The standardization of network configuration interfaces for use with
   the NETCONF 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 protocol access for particular users to a pre-
   configured subset of all available NETCONF protocol operations and
   content.  This document defines such an access control model.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 25, 2012.

Copyright Notice

   Copyright (c) 2011 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
   (http://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



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   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
   2.  Access Control Design Objectives . . . . . . . . . . . . . . .  6
     2.1.  Access Control Points  . . . . . . . . . . . . . . . . . .  6
     2.2.  Simplicity . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.3.  Procedural Interface . . . . . . . . . . . . . . . . . . .  7
     2.4.  Datastore Access . . . . . . . . . . . . . . . . . . . . .  7
     2.5.  Users and Groups . . . . . . . . . . . . . . . . . . . . .  7
     2.6.  Maintenance  . . . . . . . . . . . . . . . . . . . . . . .  8
     2.7.  Configuration Capabilities . . . . . . . . . . . . . . . .  8
     2.8.  Identifying Security-Sensitive Content . . . . . . . . . .  8
   3.  NETCONF Access Control Model (NACM)  . . . . . . . . . . . . . 10
     3.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . 10
       3.1.1.  Features . . . . . . . . . . . . . . . . . . . . . . . 10
       3.1.2.  External Dependencies  . . . . . . . . . . . . . . . . 11
       3.1.3.  Message Processing Model . . . . . . . . . . . . . . . 11
     3.2.  Datastore Access . . . . . . . . . . . . . . . . . . . . . 13
       3.2.1.  Access Rights  . . . . . . . . . . . . . . . . . . . . 13
       3.2.2.  <get> and <get-config> Operations  . . . . . . . . . . 14
       3.2.3.  <edit-config> Operation  . . . . . . . . . . . . . . . 14
       3.2.4.  <copy-config> Operation  . . . . . . . . . . . . . . . 15
       3.2.5.  <delete-config> Operation  . . . . . . . . . . . . . . 16
       3.2.6.  <commit> Operation . . . . . . . . . . . . . . . . . . 16
       3.2.7.  <discard-changes> Operation  . . . . . . . . . . . . . 16
       3.2.8.  <kill-session> Operation . . . . . . . . . . . . . . . 16
     3.3.  Model Components . . . . . . . . . . . . . . . . . . . . . 16
       3.3.1.  Users  . . . . . . . . . . . . . . . . . . . . . . . . 17
       3.3.2.  Groups . . . . . . . . . . . . . . . . . . . . . . . . 17
       3.3.3.  Emergency Recovery Session . . . . . . . . . . . . . . 17
       3.3.4.  Global Enforcement Controls  . . . . . . . . . . . . . 17
         3.3.4.1.  enable-nacm Switch . . . . . . . . . . . . . . . . 17
         3.3.4.2.  read-default Switch  . . . . . . . . . . . . . . . 17
         3.3.4.3.  write-default Switch . . . . . . . . . . . . . . . 18
         3.3.4.4.  exec-default Switch  . . . . . . . . . . . . . . . 18
         3.3.4.5.  enable-external-groups Switch  . . . . . . . . . . 18
       3.3.5.  Access Control Rules . . . . . . . . . . . . . . . . . 19
     3.4.  Access Control Enforcement Procedures  . . . . . . . . . . 19
       3.4.1.  Initial Operation  . . . . . . . . . . . . . . . . . . 19
       3.4.2.  Session Establishment  . . . . . . . . . . . . . . . . 20
       3.4.3.  "access-denied" Error Handling . . . . . . . . . . . . 20
       3.4.4.  Incoming RPC Message Validation  . . . . . . . . . . . 20



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       3.4.5.  Data Node Access Validation  . . . . . . . . . . . . . 23
       3.4.6.  Outgoing <notification> Authorization  . . . . . . . . 25
     3.5.  Data Model Definitions . . . . . . . . . . . . . . . . . . 27
       3.5.1.  Data Organization  . . . . . . . . . . . . . . . . . . 27
       3.5.2.  YANG Module  . . . . . . . . . . . . . . . . . . . . . 28
     3.6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . 38
     3.7.  Security Considerations  . . . . . . . . . . . . . . . . . 39
       3.7.1.  NACM Configuration and Monitoring Considerations . . . 39
       3.7.2.  General Configuration Issues . . . . . . . . . . . . . 40
       3.7.3.  Data Model Design Considerations . . . . . . . . . . . 42
   4.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 43
     4.1.  Normative References . . . . . . . . . . . . . . . . . . . 43
     4.2.  Informative References . . . . . . . . . . . . . . . . . . 43
   Appendix A.  Usage Examples  . . . . . . . . . . . . . . . . . . . 44
     A.1.  <groups> Example . . . . . . . . . . . . . . . . . . . . . 44
     A.2.  Module Rule Example  . . . . . . . . . . . . . . . . . . . 45
     A.3.  RPC Rule Example . . . . . . . . . . . . . . . . . . . . . 46
     A.4.  Data Rule Example  . . . . . . . . . . . . . . . . . . . . 48
     A.5.  Notification Rule Example  . . . . . . . . . . . . . . . . 50
   Appendix B.  Change Log  . . . . . . . . . . . . . . . . . . . . . 52
     B.1.  06-07  . . . . . . . . . . . . . . . . . . . . . . . . . . 52
     B.2.  05-06  . . . . . . . . . . . . . . . . . . . . . . . . . . 52
     B.3.  04-05  . . . . . . . . . . . . . . . . . . . . . . . . . . 52
     B.4.  03-04  . . . . . . . . . . . . . . . . . . . . . . . . . . 52
     B.5.  02-03  . . . . . . . . . . . . . . . . . . . . . . . . . . 53
     B.6.  01-02  . . . . . . . . . . . . . . . . . . . . . . . . . . 53
     B.7.  00-01  . . . . . . . . . . . . . . . . . . . . . . . . . . 53
     B.8.  00 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 54






















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1.  Introduction

   The NETCONF protocol does not provide any standard mechanisms to
   restrict the protocol operations and content that each user is
   authorized to access.

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

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

   1.  Access Control Design Objectives

   2.  NETCONF Access Control Model (NACM)

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

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

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

   o  client

   o  datastore

   o  protocol operation

   o  server

   o  session

   o  user

   The following terms are defined in [RFC6020] and are not redefined
   here:

   o  data node

   o  data definition statement




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   The following terms are used throughout this documentation:

   access control:  A security feature provided by the NETCONF server,
      that allows an administrator to restrict access to a subset of all
      NETCONF 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.

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

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

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

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


























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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 new protocol operations to be added at any time, and
   the YANG data modeling language supports this feature.  It is not
   possible to design an ACM for NETCONF that only focuses on a static
   set of protocol operations, 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.


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


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



                                 Figure 1

   The following access control points, described in Figure 1, are
   identified:

   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.







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2.2.  Simplicity

   There is concern that a complicated ACM will not be widely deployed,
   because it is too hard to use.  It needs to be easy to do simple
   things, and possible to do complex things, instead of hard to do
   everything.

   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, which 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 NETCONF datastore
   access.

2.3.  Procedural Interface

   The NETCONF protocol uses a remote procedure call 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 NETCONF 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 NETCONF transport layer, and
   RADIUS performs authentication and service authorization at the same
   time, the underlying NETCONF transport needs to be able to report a



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   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.

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 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 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 can
      study some more RFC text, and determine how to mitigate the
      security risk(s).

   o  The ACM on each server can 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 pre-configured, 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.




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   Often, the administrator just wants to disable default access to the
   secure content, so 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.

   A data model designer needs to be able to use machine-readable
   statements to identify NETCONF content which 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.








































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3.  NETCONF Access Control Model (NACM)

3.1.  Introduction

   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, data, and notification access.

   o  Simple access control rules configuration data model that is easy
      to use.

   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., "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.

   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.

   o  Access control rules are simple to configure.

   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.





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3.1.2.  External Dependencies

   The NETCONF [RFC6241] protocol is used for all management purposes
   within this document.

   The YANG Data Modeling Language [RFC6020] is used to define the
   NETCONF data models specified 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.






































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



                                 Figure 2

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

   o  Access control is applied to all <rpc> messages (except <close-
      session>) received by the server, individually, for each active



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      session, unless the session is identified as a "recovery session".

   o  If the user is authorized to execute the specified protocol
      operation, then processing continues, otherwise the request is
      rejected with an "access-denied" error.

   o  If the configuration datastore or conceptual state data is
      accessed by the protocol operation, then the server checks if the
      client is authorized to access the nodes in the data store.  If
      the user is authorized to perform the requested access operation
      on the requested data, then processing continues.

   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  The notification access control enforcer checks the notification
      event type, and if it is one which the user is not authorized to
      read, then the notification is dropped for that subscription.

3.2.  Datastore Access

   The same access control rules apply to all datastores.  For example,
   the candidate configuration datastore or the running configuration
   datastore.

   Only the standard NETCONF datastores (candidate, running, and
   startup) are controlled by NACM.  Local or remote files or datastores
   accessed via the <url> parameter are not controlled by NACM.

3.2.1.  Access Rights

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

   The "CRUDX" model can support all NETCONF 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.





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   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 protocol operation.

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

   Data nodes to which the client does not have read access are silently
   omitted 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 is applied to the
   subset of nodes that the user is authorized to read, not the entire
   datastore.

3.2.3.  <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 which 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.

   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
   sub-tree within 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 data
   store 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 data
   store node, and the user does not have "delete" access permission for
   that node, the protocol operation is rejected with an "access-denied"
   error.



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   If the protocol operation would result in the modification of a data
   store 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
   which 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.

3.2.4.  <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
         data store 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
         data store node, and the user does not have "delete" access



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         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
         data store node, and the user does not have "update" access
         permission for that node, the protocol operation is rejected
         with an "access-denied" error.

3.2.5.  <delete-config> Operation

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

3.2.6.  <commit> Operation

   The server MUST determine the exact nodes in the running
   configuration datastore which 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.

3.2.7.  <discard-changes> Operation

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

3.2.8.  <kill-session> Operation

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

   Access to the <kill-session> protocol operation is denied by default.
   The 'exec-default' parameter 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 access
   control model.




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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 which is unique within the server.

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

3.3.2.  Groups

   Access to a specific NETCONF protocol operation is granted to a
   session, associated with a group, not a user.

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

   A group member is identified by a user name 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", then 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", then all access
   requested 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", then this global



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   switch is relevant, if no matching access control rule is found to
   explicitly permit or deny read access to the requested NETCONF
   datastore data or notification event type.

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

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

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", then this global switch is relevant,
   if no matching access control rule is found to explicitly permit or
   deny write access to the requested NETCONF datastore data.

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

   When this global switch is set to "deny", and no matching access
   control rule is found for the NETCONF datastore write requested, then
   access is denied.

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", then 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,
   then 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,
   then access is denied.

3.3.4.5.  enable-external-groups Switch

   When this global switch is set to "true", the group names reported by
   the NETCONF transport layer for a session are used together with the



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   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
   NETCONF transport layer are ignored by NACM.

3.3.5.  Access Control Rules

   There are 4 types of rules available in 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, 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 seven separate phases that need to be addressed, four of
   which are related to the NETCONF message processing model.  In
   addition, the initial start-up mode for a NETCONF server, session
   establishment, and "access-denied" error handling procedures also
   need to be considered.

   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 start-up 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 datastore, during
   bootup.





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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 NETCONF transport layer reports the user name 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 user name, 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.

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.






















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                   NETCONF server
                  +------------+
                  |    XML     |
                  |   message  |
                  | dispatcher |
                  +------------+
                         |
                         |
                         V
                  +------------+
                  | NC-base NS |
                  |   <rpc>    |
                  +------------+
                    |   |  |
                    |   |  +-------------------------+
                    |   +------------+               |
                    V                V               V
               +-----------+ +---------------+ +------------+
               | Vendor NS | | NC-base NS    | | NC-base NS |
               | <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.

   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.





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   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 for ones that contain a "user-
        name" entry that equals the user name 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.

        *  The rule does not have a "rule-type" defined, or 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.   Otherwise, no matching rule was found in any rule-list entry.

   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.





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   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.  For
      example:


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

      represents the <edit-config> protocol operation in the NETCONF
      base namespace.

   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 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 a data node within a datastore is accessed, then the server MUST
   ensure that the user is authorized to perform the requested read,
   create, update, or delete access operation on the specified data
   node.

   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 for ones that contain a "user-
        name" entry that equals the user name for the session making the
        request.  If the the "enable-external-groups" leaf is "true",
        add to these groups the set of groups provided by the transport
        layer.





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   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.

        *  The rule does not have a "rule-type" defined, or the "rule-
           type" is "data-node" and the "path" matches the requested
           data node.

        *  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 "*".

        *  For an update access operation, the rule's "access-
           operations" leaf has the "update" 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.   Otherwise, 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 is 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-



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        write" or a "nacm:default-deny-all" statement, then the data
        node access request is denied.

   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 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.

3.4.6.  Outgoing <notification> Authorization

   Configuration of access control rules specifically for descendant
   nodes of the notification event type element 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.

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






























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



                                 Figure 4

   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.




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   4.   Check all the "group" entries for ones that contain a "user-
        name" entry that equals the user name 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.

        *  The rule does not have a "rule-type" defined, or the "rule-
           type" is "notification" and the "notification-name" is "*",
           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.

   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.




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      +--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

3.5.2.  YANG Module

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

   The "ietf-netconf-acm" YANG module imports typedefs from [RFC6021].


   // RFC Ed.: please update the date to the date of publication
   <CODE BEGINS> file="ietf-netconf-acm@2011-12-23.yang"

   module ietf-netconf-acm {

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

     prefix "nacm";

     import ietf-yang-types {



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       prefix yang;
     }

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


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

        WG Chair: Mehmet Ersue
                  <mailto:mehmet.ersue@nsn.com>

        WG Chair: Bert Wijnen
                  <mailto:bertietf@bwijnen.net>

        Editor:   Andy Bierman
                  <mailto:andy@netconfcentral.org>

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

     description
       "NETCONF Access Control Model.

        Copyright (c) 2011 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
        (http://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC XXXX; see
        the RFC itself for full legal notices.";
     // RFC Ed.: replace XXXX with actual RFC number and
     // remove this note

     // RFC Ed.: remove this note
     // Note: extracted from draft-ietf-netconf-access-control-07.txt

     // RFC Ed.: please update the date to the date of publication
     revision "2011-12-23" {
       description
         "Initial version";



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       reference
         "RFC XXXX: Network Configuration Protocol
                    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, and the NACM module is enabled (i.e.,
          /nacm/enable-nacm object equals 'true'), 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.

          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, and the NACM module is enabled (i.e.,
          /nacm/enable-nacm object equals 'true'), 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.

          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";
       }



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       description
         "General Purpose User Name 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.";
         }
         bit delete {
           description
             "Any protocol operation that removes a data node.";
         }
         bit exec {
           description
             "Execution access to the specified protocol operation.";
         }
       }
       description
         "NETCONF Access Operation.";
     }

     typedef group-name-type {
       type string {
         length "1..max";
         pattern "[^\*].*";



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       }
       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 instance identifier string.

          A node-instance-identifier value is an
          unrestricted YANG instance-identifier expression.
          All the same rules as an instance-identifier apply
          except 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 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 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.




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            o  The context node is the root node in the data tree.";
     }

     /*
      * Data definition statements
      */

     container nacm {
       nacm:default-deny-all;

       description
         "Parameters for NETCONF Access Control Model.";

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

       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.";
       }



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       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 a protocol operation request was denied
            since the server last restarted.";
       }

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

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

       container groups {
         description
           "NETCONF Access Control Groups.";

         list group {
           key name;




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           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 user name of
                a member of the group associated with
                this entry.";
           }
         }
       }

       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.";
         }




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         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' matches the request.  If a rule
              matches, the 'action' leaf determines if access is granted
              or not.";

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

           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
                matches 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.";



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               }
             }
             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.";
               }
             }
             case data-node {
               leaf path {
                 type node-instance-identifier;
                 mandatory true;
                 description
                   "Data Node Instance Identifier associated with the
                    data node 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 data
                    store 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;



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             description
               "The access control action associated with the
                rule.  If a rule is determined to match a
                particular request, then this object is used
                to determine whether to permit or deny the
                request.";
           }

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


   <CODE ENDS>

                                 Figure 5

3.6.  IANA Considerations

   There are two actions that are requested of IANA: This document
   registers one URI in "The IETF XML Registry".  Following the format
   in [RFC3688], the following has been registered.


        URI: urn:ietf:params:xml:ns:yang:ietf-netconf-acm
        Registrant Contact: The IESG.
        XML: N/A, the requested URI is an XML namespace.

   This document registers one module in the "YANG Module Names"
   registry.  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 XXXX
           // RFC Ed.: Replace XXX with actual RFC number
           // and remove this note







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3.7.  Security Considerations

   This entire document discusses access control requirements and
   mechanisms for restricting NETCONF protocol behavior within a given
   session.

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

3.7.1.  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 which 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
   NACM is enabled, and also decide if the default access parameters are
   set appropriately.  Make sure 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.

   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 dependant user identity to a NACM user
   name is unique and correct.  This requirement is specified in detail
   in section 2.2 of [RFC6241].




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   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 datastore configuration.

   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 NETCONF 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 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".

   An administrator needs to restrict read access to the following
   objects within this data model, which reveal access control
   configuration which could be considered sensitive.

   o  /nacm/enable-nacm

   o  /nacm/read-default

   o  /nacm/write-default

   o  /nacm/exec-default

   o  /nacm/enable-external-groups

   o  /nacm/groups

   o  /nacm/rule-list

3.7.2.  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



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   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 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., 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.

   There is a risk that non-standard protocol operations, or even the
   standard <get> protocol operation, may return data which "aliases" or
   "copies" sensitive data from a different data object.  There may
   simply be multiple data model definitions which 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 which 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 include 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 which disable all
   access to the server.  The configuration and identification of such a
   recovery session mechanism are implementation-specific and outside
   the scope of this document.  An administrator needs to be aware of
   any "recovery session" mechanisms available on the device, and make
   sure they are used appropriately.




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   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 insure 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>

3.7.3.  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 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 is not required.  Use of
   generic protocol operations should be avoided, and separate protocol
   operations defined instead, if different access levels are needed.


















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4.  References

4.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004.

   [RFC5277]  Chisholm, S. and H. Trevino, "NETCONF Event
              Notifications", RFC 5277, July 2008.

   [RFC6020]  Bjorklund, M., "YANG - A Data Modeling Language for the
              Network Configuration Protocol (NETCONF)", RFC 6020,
              October 2010.

   [RFC6021]  Schoenwaelder, J., "Common YANG Data Types", RFC 6021,
              October 2010.

   [RFC6241]  Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
              Bierman, "Network Configuration Protocol (NETCONF)",
              RFC 6241, June 2011.

4.2.  Informative References

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, June 2000.

   [RFC5607]  Nelson, D. and G. Weber, "Remote Authentication Dial-In
              User Service (RADIUS) Authorization for Network Access
              Server (NAS) Management", RFC 5607, July 2009.


















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Appendix A.  Usage Examples

   The following XML snippets are provided as examples only, to
   demonstrate how 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 3 groups:

   1.  The "admin" group contains 2 users named "admin" and "andy".

   2.  The "limited" group contains 2 users named "wilma" and "bam-bam".

   3.  The "guest" group contains 2 users named "guest" and
       "guest@example.com".



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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 case in the "rule-type" choice.


   <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>



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     </rule-list>

     <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 4 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.

A.3.  RPC Rule Example

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









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   <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 or 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 limited or guest group
           to delete any configurations.
         </comment>
       </rule>
     </rule-list>

     <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>




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   This example shows 3 protocol operation rules:

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

   deny-delete-config:  This rule prevents the "limited" or "guest"
      groups 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".

A.4.  Data Rule Example

   Data 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>



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           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>

     <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 admin full access to all acme interfaces.
         </comment>
       </rule>
     </rule-list>
   </nacm>





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   This example shows 4 data rules:

   deny-nacm:  This rule denies the "guest" group any access to the
      <nacm> subtree.  Note that the default namespace is only
      applicable because this subtree is defined in the same namespace
      as the <data-rule> element.

   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,
      just 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 or limited groups
           to receive config change events.
         </comment>
       </rule>
     </rule-list>
   </nacm>



   This example shows 1 notification rule:




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

















































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Appendix B.  Change Log

   -- RFC Ed.: remove this section before publication.

B.1.  06-07

   Added the leaf "enable-external-groups".

   Removed dependency to RFC 6242.

   Some editorial changes after IESG review.

B.2.  05-06

   Added clarification to Security Considerations section about
   ordered-by user lists (/nacm/rule-list and /nacm/rule-list/rule).

   Added clarifications to security considerations wrt/ user names and
   NETCONF capability changes.

   Fixed typos found in review.

B.3.  04-05

   Updated Security Considerations section.

   Changed term 'operator' to 'administrator'.

   Used the terms "access operation" and "protocol operation"
   consistently.

   Moved some normative text from section 2 to section 3.  Also made it
   more clear that section 2 is not a requirements section, but
   documentation of the objectives for NACM.

   Renamed "nacm:secure" to "nacm:default-deny-write", and "nacm:very-
   secure" to "nacm:default-deny-all".  Explained that "nacm:default-
   deny-write" is ignored on rpc statements.

   Described that <kill-session> and <delete-config> behave as if
   specified with "nacm:default-deny-all".

B.4.  03-04

   Introduced rule-lists to group related rules together.

   Moved "module-rule", "rpc-rule", "notification-rule", and "data-rule"
   into one common "rule", with a choice to select between the four



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   variants.

   Changed "superuser" to "recovery session", and adjusted text
   throughout document for this change.

   Clarified behavior of global default NACM parameters, enable-nacm,
   read-default, write-default, exec-default.

   Clarified when access control is applied during system
   initialization.

B.5.  02-03

   Fixed improper usage of RFC 2119 keywords.

   Changed term usage of "database" to "datastore".

   Clarified that "secure" and "very-secure" extensions only apply if
   the /nacm/enable-nacm object is "true".

B.6.  01-02

   Removed authentication text and objects.

   Changed module name from ietf-nacm to ietf-netconf-acm.

   Updated NETCONF and YANG terminology.

   Removed open issues section.

   Changed some must to MUST in requirements section.

B.7.  00-01

   Updated YANG anf YANG Types references.

   Updated module namespace URI to standard format.

   Updated module header meta-data to standard format.

   Filled in IANA section.

B.8.  00

   Initial version cloned from
   draft-bierman-netconf-access-control-02.txt.





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Authors' Addresses

   Andy Bierman
   Brocade

   Email: andy@netconfcentral.org


   Martin Bjorklund
   Tail-f Systems

   Email: mbj@tail-f.com







































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