





Internet Engineering Task Force (IETF)                        A. Bierman
Request for Comments: 6536                                     YumaWorks
Category: Standards Track                                   M. Bjorklund
ISSN: 2070-1721                                           Tail-f Systems
                                                              March 2012


     Network Configuration Protocol (NETCONF) Access Control Model

Abstract

   The standardization of network configuration interfaces for use with
   the Network Configuration Protocol (NETCONF) 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 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 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6536.

Copyright Notice

   Copyright (c) 2012 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
   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.



Bierman & Bjorklund          Standards Track                    [Page 1]

RFC 6536                          NACM                        March 2012


Table of Contents

   1. Introduction ....................................................3
      1.1. Terminology ................................................3
   2. Access Control Design Objectives ................................4
      2.1. Access Control Points ......................................5
      2.2. Simplicity .................................................5
      2.3. Procedural Interface .......................................6
      2.4. Datastore Access ...........................................6
      2.5. Users and Groups ...........................................6
      2.6. Maintenance ................................................6
      2.7. Configuration Capabilities .................................7
      2.8. Identifying Security-Sensitive Content .....................7
   3. NETCONF Access Control Model (NACM) .............................8
      3.1. Introduction ...............................................8
           3.1.1. Features ............................................8
           3.1.2. External Dependencies ...............................9
           3.1.3. Message Processing Model ............................9
      3.2. Datastore Access ..........................................11
           3.2.1. Access Rights ......................................11
           3.2.2. <get> and <get-config> Operations ..................12
           3.2.3. <edit-config> Operation ............................12
           3.2.4. <copy-config> Operation ............................13
           3.2.5. <delete-config> Operation ..........................14
           3.2.6. <commit> Operation .................................14
           3.2.7. <discard-changes> Operation ........................14
           3.2.8. <kill-session> Operation ...........................14
      3.3. Model Components ..........................................15
           3.3.1. Users ..............................................15
           3.3.2. Groups .............................................15
           3.3.3. Emergency Recovery Session .........................15
           3.3.4. Global Enforcement Controls ........................15
                  3.3.4.1. enable-nacm Switch ........................15
                  3.3.4.2. read-default Switch .......................16
                  3.3.4.3. write-default Switch ......................16
                  3.3.4.4. exec-default Switch .......................16
                  3.3.4.5. enable-external-groups Switch .............17
           3.3.5. Access Control Rules ...............................17
      3.4. Access Control Enforcement Procedures .....................17
           3.4.1. Initial Operation ..................................17
           3.4.2. Session Establishment ..............................18
           3.4.3. "access-denied" Error Handling .....................18
           3.4.4. Incoming RPC Message Validation ....................18
           3.4.5. Data Node Access Validation ........................21
           3.4.6. Outgoing <notification> Authorization ..............23
      3.5. Data Model Definitions ....................................26
           3.5.1. Data Organization ..................................26
           3.5.2. YANG Module ........................................26



Bierman & Bjorklund          Standards Track                    [Page 2]

RFC 6536                          NACM                        March 2012


      3.6. IANA Considerations .......................................36
      3.7. Security Considerations ...................................36
           3.7.1. NACM Configuration and Monitoring Considerations ...37
           3.7.2. General Configuration Issues .......................38
           3.7.3. Data Model Design Considerations ...................40
   4. References .....................................................40
      4.1. Normative References ......................................40
      4.2. Informative References ....................................41
   Appendix A.  Usage Examples .......................................42
     A.1.  <groups> Example ..........................................42
     A.2.  Module Rule Example .......................................43
     A.3.  Protocol Operation Rule Example ...........................44
     A.4.  Data Node Rule Example ....................................46
     A.5.  Notification Rule Example .................................48

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 interoperable 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 Operations
   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




Bierman & Bjorklund          Standards Track                    [Page 3]

RFC 6536                          NACM                        March 2012


   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

   The following terms are used throughout this document:

   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 criterion 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", or
      "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 or not a session is a recovery session are
      implementation specific and outside the scope of this document.

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

2.  Access Control Design Objectives

   This section documents the design objectives for the NETCONF Access
   Control Model presented in Section 3.






Bierman & Bjorklund          Standards Track                    [Page 4]

RFC 6536                          NACM                        March 2012


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.

   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

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.



Bierman & Bjorklund          Standards Track                    [Page 5]

RFC 6536                          NACM                        March 2012


   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 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.







Bierman & Bjorklund          Standards Track                    [Page 6]

RFC 6536                          NACM                        March 2012


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 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 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.

   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





Bierman & Bjorklund          Standards Track                    [Page 7]

RFC 6536                          NACM                        March 2012


   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.  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 remote procedure call (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.






Bierman & Bjorklund          Standards Track                    [Page 8]

RFC 6536                          NACM                        March 2012


   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

   The NETCONF protocol [RFC6241] 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.
































Bierman & Bjorklund          Standards Track                    [Page 9]

RFC 6536                          NACM                        March 2012


                    +-------------------------+
                    |       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  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.




Bierman & Bjorklund          Standards Track                   [Page 10]

RFC 6536                          NACM                        March 2012


   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 datastore.  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 that 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.

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




Bierman & Bjorklund          Standards Track                   [Page 11]

RFC 6536                          NACM                        March 2012


   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 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.

   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.






Bierman & Bjorklund          Standards Track                   [Page 12]

RFC 6536                          NACM                        March 2012


   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.

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






Bierman & Bjorklund          Standards Track                   [Page 13]

RFC 6536                          NACM                        March 2012


      *  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.

3.2.5.  <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.

3.2.6.  <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.

3.2.7.  <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 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.







Bierman & Bjorklund          Standards Track                   [Page 14]

RFC 6536                          NACM                        March 2012


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.

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 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", 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.




Bierman & Bjorklund          Standards Track                   [Page 15]

RFC 6536                          NACM                        March 2012


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
   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.



Bierman & Bjorklund          Standards Track                   [Page 16]

RFC 6536                          NACM                        March 2012


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
   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 four 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
   (Section 3.1.3).  In addition, the initial startup 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 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.






Bierman & Bjorklund          Standards Track                   [Page 17]

RFC 6536                          NACM                        March 2012


   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.

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 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.

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.

















Bierman & Bjorklund          Standards Track                   [Page 18]

RFC 6536                          NACM                        March 2012


                   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.

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




Bierman & Bjorklund          Standards Track                   [Page 19]

RFC 6536                          NACM                        March 2012


   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 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.

        *  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.   At this point, 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.

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




Bierman & Bjorklund          Standards Track                   [Page 20]

RFC 6536                          NACM                        March 2012


   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 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 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.





Bierman & Bjorklund          Standards Track                   [Page 21]

RFC 6536                          NACM                        March 2012


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





Bierman & Bjorklund          Standards Track                   [Page 22]

RFC 6536                          NACM                        March 2012


   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.

































Bierman & Bjorklund          Standards Track                   [Page 23]

RFC 6536                          NACM                        March 2012


                   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.






Bierman & Bjorklund          Standards Track                   [Page 24]

RFC 6536                          NACM                        March 2012


   4.   Check all the "group" entries for ones that 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.

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










Bierman & Bjorklund          Standards Track                   [Page 25]

RFC 6536                          NACM                        March 2012


3.5.  Data Model Definitions

3.5.1.  Data Organization

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

      +--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].








Bierman & Bjorklund          Standards Track                   [Page 26]

RFC 6536                          NACM                        March 2012


   <CODE BEGINS> file "ietf-netconf-acm@2012-02-22.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:   <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@yumaworks.com>

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

     description
       "NETCONF Access Control Model.

        Copyright (c) 2012 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 6536; see
        the RFC itself for full legal notices.";

     revision "2012-02-22" {



Bierman & Bjorklund          Standards Track                   [Page 27]

RFC 6536                          NACM                        March 2012


       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, 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 {



Bierman & Bjorklund          Standards Track                   [Page 28]

RFC 6536                          NACM                        March 2012


         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.";
         }
         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 {



Bierman & Bjorklund          Standards Track                   [Page 29]

RFC 6536                          NACM                        March 2012


         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 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 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



Bierman & Bjorklund          Standards Track                   [Page 30]

RFC 6536                          NACM                        March 2012


              instance-identifier, no functions are allowed.

           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
           "Enables or disables 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



Bierman & Bjorklund          Standards Track                   [Page 31]

RFC 6536                          NACM                        March 2012


            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.";
       }

       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 {



Bierman & Bjorklund          Standards Track                   [Page 32]

RFC 6536                          NACM                        March 2012


           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.";
           }
         }
       }

       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.";



Bierman & Bjorklund          Standards Track                   [Page 33]

RFC 6536                          NACM                        March 2012


         }

         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 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
                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



Bierman & Bjorklund          Standards Track                   [Page 34]

RFC 6536                          NACM                        March 2012


                    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.";
               }
             }
             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
                    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 {



Bierman & Bjorklund          Standards Track                   [Page 35]

RFC 6536                          NACM                        March 2012


             type action-type;
             mandatory true;
             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>

3.6.  IANA Considerations

   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 6536

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.



Bierman & Bjorklund          Standards Track                   [Page 36]

RFC 6536                          NACM                        March 2012


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 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
   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-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 datastore configuration.







Bierman & Bjorklund          Standards Track                   [Page 37]

RFC 6536                          NACM                        March 2012


   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, as they reveal access control
   configuration that 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
   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.



Bierman & Bjorklund          Standards Track                   [Page 38]

RFC 6536                          NACM                        March 2012


   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 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 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 that 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.

   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 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:



Bierman & Bjorklund          Standards Track                   [Page 39]

RFC 6536                          NACM                        March 2012


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

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.





Bierman & Bjorklund          Standards Track                   [Page 40]

RFC 6536                          NACM                        March 2012


   [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.






































Bierman & Bjorklund          Standards Track                   [Page 41]

RFC 6536                          NACM                        March 2012


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 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 42]

RFC 6536                          NACM                        March 2012


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>
     </rule-list>



Bierman & Bjorklund          Standards Track                   [Page 43]

RFC 6536                          NACM                        March 2012


     <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.

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>



Bierman & Bjorklund          Standards Track                   [Page 44]

RFC 6536                          NACM                        March 2012


         <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>

   This example shows three 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.




Bierman & Bjorklund          Standards Track                   [Page 45]

RFC 6536                          NACM                        March 2012


   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 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 46]

RFC 6536                          NACM                        March 2012


     <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>

   This example shows four data node 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>.






Bierman & Bjorklund          Standards Track                   [Page 47]

RFC 6536                          NACM                        March 2012


   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 one notification rule:

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














Bierman & Bjorklund          Standards Track                   [Page 48]

RFC 6536                          NACM                        March 2012


Authors' Addresses

   Andy Bierman
   YumaWorks

   EMail: andy@yumaworks.com


   Martin Bjorklund
   Tail-f Systems

   EMail: mbj@tail-f.com







































Bierman & Bjorklund          Standards Track                   [Page 49]

