Network Configuration Protocol Access Control Model
draft-bierman-netconf-access-control-01

Abstract

The standardization of network configuration interfaces for use with the NETCONF protocol requires a structured and secure operating environment, which 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 operations and content. This document discusses requirements for a suitable access control model, and provides one solution which meets these requirements.

Status of this Memo

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This Internet-Draft will expire on August 29, 2010.

Copyright Notice

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Table of Contents

1.  Introduction
    1.1.  Terminology
        1.1.1.  Requirements Notation
        1.1.2.  NETCONF Terms
        1.1.3.  NACM Terms
2.  Access Control Requirements
    2.1.  Protocol Control Points
    2.2.  Simplicity
    2.3.  Procedural Interface
    2.4.  Database Access
    2.5.  Users and Groups
    2.6.  Maintenance
    2.7.  Configuration Capabilities
    2.8.  Identifying Security Holes
    2.9.  Data Shadowing
    2.10.  NETCONF Specific Requirements
3.  NETCONF Access Control Model (NACM)
    3.1.  Introduction
        3.1.1.  Features
        3.1.2.  External Dependencies
        3.1.3.  Message Processing Model
    3.2.  Model Components
        3.2.1.  Users
        3.2.2.  Groups
        3.2.3.  Sessions
        3.2.4.  Access Permissions
        3.2.5.  Global Enforcement Controls
        3.2.6.  Access Control Rules
    3.3.  Access Control Enforcement Procedures
        3.3.1.  Initial Operation
        3.3.2.  Session Establishment
        3.3.3.  'access-denied' Error Handling
        3.3.4.  Incoming RPC Message Validation
        3.3.5.  Data Node Access Validation
        3.3.6.  Outgoing <rpc-reply> Authorization
        3.3.7.  Outgoing <notification> Authorization
    3.4.  Data Model Definitions
        3.4.1.  High Level Procedures
        3.4.2.  Data Organization
        3.4.3.  YANG Module
    3.5.  IANA Considerations
    3.6.  Security Considerations
4.  Normative References
Appendix A.  Usage Examples
    A.1.  <groups> Example
    A.2.  <module-rule> Example
    A.3.  <rpc-rule> Example
    A.4.  <data-rule> Example
    A.5.  <notification-rule> Example
Appendix B.  Open Issues
§  Author's Address

1.  Introduction

The NETCONF protocol does not provide any standard mechanisms to restrict the operations and content that each user is authorized to use. Any user that is able to start a NETCONF session is allowed to access every protocol operation, all database content, and all notification content.

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

This document addresses NETCONF protocol access control mechanisms for the RPC, Operations, and Content layers, as defined in [RFC4741] (Enns, R., “NETCONF Configuration Protocol,” December 2006.), and [RFC5277] (Chisholm, S. and H. Trevino, “NETCONF Event Notifications,” July 2008.). It contains three main sections:

1. Access Control Requirements
2. NETCONF Access Control Model (NACM)
3. YANG Data Model (nacm.yang)

1.1.  Terminology

1.1.1.  Requirements Notation

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

1.1.2.  NETCONF Terms

The following terms are defined in RFC 4741 and are not redefined here:

• client
• operation
• RPC operation
• server
• session
• user

1.1.3.  NACM Terms

The following terms are used throughout this documentation:

access control:

A security feature provided by the NETCONF server, which allows an operator 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 operator to enforce a particular access control policy.

access control rule:

The conceptual criteria used to determine if a particular NETCONF protocol operation should be permitted or denied.

superuser:

The special administrative user account which is given unlimited NETCONF access, and is exempt from all access control enforcement.

2.  Access Control Requirements

2.1.  Protocol Control Points

The NETCONF protocol allows new 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 which only focuses on a static set of operations, like some other protocols. Since few assumptions can be made about an arbitrary protocol operation, the NETCONF architectural server components must be protected at several conceptual control points.

              +-------------+             +-------------+
 client       |    RPC      |             |   prune     |      client
 request -->  |  operation  |             | restricted  | ---> reply
              |  allowed?   |             | <rpc-reply> |
              +-------------+             |   nodes?    |
                    |                     +-------------+
                    | if any database or
                    | state data is accessed
                    | by the operation
                    V
              +-------------+             +----------------+
              |  data node  |             |   prune        |
              |   access    |             | restricted     |
              |  allowed?   |             | <notification> | ---> client
              +-------------+             | event or data? |     session
                                          +----------------+

The following access control points are defined:

RPC operation:

Configurable permission to invoke specific RPC operations is required. Wildcard or multiple target mechanisms to reduce configuration and effort are also required.

NETCONF database:

Configurable permission to read and/or alter specific data nodes within any conceptual database is required. Wildcard or multiple target mechanisms to reduce configuration and effort are also required.

RPC Reply Content:

Configurable permission to read specific data nodes within any conceptual RPC output section is required. Unauthorized data is silently omitted from the reply, instead of dropping the reply or sending an 'access-denied' error.

Notification Content:

Configurable permission to receive specific notification event types is required.

2.2.  Simplicity

Experience has shown that a complicated ACM will not be widely deployed, because it is too hard to use. The key factor that is ignored in such solutions is the concept of 'localized cost'. It should be easy to do simple things, and hard to do complex things, instead of hard to do everything.

Configuration of the access control system must be simple to use. Simple and common tasks should be easy to configure, and require little expertise or domain-specific knowledge. Complex tasks should be possible using additional mechanisms which may require additional expertise.

A single set of access control rules should be able to control all types of NETCONF RPC operation invocation, all conceptual database access, and all NETCONF session output.

Protocol access should be defined with a small and familiar set of permissions such as 'read', 'write', and 'execute'.

Default access control policy needs to be as secure as possible.

Access control does not need to be applied to NETCONF <hello> messages.

2.3.  Procedural Interface

The NETCONF protocol uses a procedural interface model, and an extensible set of protocol operations, including vendor-specific 'actions'. Access control for any possible protocol operation is required.

It must be possible to configure the ACM to permit or deny access to specific NETCONF operations.

YANG modules should be designed so that different access levels for input parameters to RPC operations is not required. However, since this cannot always be avoided, then it should be possible to control access to specific input parameters. If a restricted parameter is used, then the request is rejected with an 'access-denied' error.

2.4.  Database Access

It must be possible control access to specific nodes and sub-trees within the conceptual NETCONF database.

In order for a user to obtain access to a particular database node, the user must be authorized to have the same requested access to the specified node, and all of its ancestors.

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

Only the standard NETCONF databases (candidate, running, and startup) are controlled y the ACM. Local or remote files or databases accessed via the <url> parameter are optional to support.

The non-volatile startup configuration needs to be loaded into the running configuration without applying any access control rules. This operation is conceptually performed by the superuser account.

Only the superuser should be able to alter the factory-default access control rules.

Read operations for restricted configuration data, either directly or via wildcard access, are silently omitted from the <rpc-reply>.

Create, merge, replace, and delete operations on a database node for which the user does not have 'write' access permission is rejected with an 'access-denied' error. The specific restricted database nodes must not be exposed in any <rpc-error> elements within the reply.

2.5.  Users and Groups

The server must obtain a user name from the underlying NETCONF transport, such as an SSH user name.

It must be possible to specify access control rules for a single user or a configurable group of users.

A configurable superuser account is needed which bypasses all access control rules. This is needed in case the access control rules are mis-configured, and all access is denied.

The ACM must 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 must be configurable by the operator.

2.6.  Maintenance

It should be possible to disable part or all of the access control model without deleting any configuration. By default, only the 'superuser' should be able to perform this task.

It should be possible to configure a 'superuser' account so that all access control is disabled for just this user. This allows the access control rules to always be modified without completely disabling access control for all users.

2.7.  Configuration Capabilities

Suitable control and monitoring mechanisms are needed to allow an operator to easily manage all aspects of the ACM behavior. A standard XML data model, suitable for use with the <edit-config> operation must be available for this purpose.

All standard access control parameters must persist in non-volatile storage, and be used upon the next reboot.

Access control rules to restrict operations on specific sub-trees within the configuration database must be supported. Existing mechanisms should be used to identify the sub-tree(s) for this purpose.

2.8.  Identifying Security Holes

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 commands in NETCONF, not just data and notifications.

It is customary 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:

• This documentation-only forces operators to study the RFC and determine if there are any potential security holes introduced by a new YANG module.
• If any security holes are identified, then the operator must study some more RFC text, and determine out how to close the security hole(s).
• The ACM on each server must be configured to close the security holes, e.g., require privileged access to read or write the specific data identified in the Security Considerations section.
• If the ACM is not pre-configured, then there will be a time window of vulnerability, after the new module is loaded, and before the new access control rules for that module are configured, enabled, and debugged.

Often, the operator 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 should be able to use machine-readable statements to identity NETCONF content which should be protected by default. This will allow client and server tools to automatically close data-model specific security holes by requiring 'superuser' access unless an access control rule is explicitly configured to allow the requested operation.

2.9.  Data Shadowing

One of the more complicated security administration problems is identifying data nodes which shadow or mirror the content of another data node. An access control rule to prevent read operations for a particular node may be insufficient to prevent access to the data node with the copied value.

If the YANG leafref data type is used, then this data shadowing can be detected by applications (and the server stack), and prevented.

If the description statement, other documentation, or no documentation exists to identify a data shadow problem, then it may not be detected.

Since NETCONF allows any vendor operation to be added to the protocol, there is no way to reliably identify all of the operations that may expose copies of sensitive data nodes in <rpc-reply> messages.

A NETCONF server must insure than unauthorized access to its conceptual databases and non-configuration data nodes is prevented. However, if a NETCONF operation accesses device instrumentation instead of the protected data node, then this access will not be protected.

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 operator must identify each operation that the server provides, and decide if it needs any access control applied to it.

Proprietary protocol operations should be properly documented by the vendor, so it is clear to operators what data nodes (if any) are affected by the operation, and what information (if any) is returned in the <rpc-reply> message.

2.10.  NETCONF Specific Requirements

The server must be able to identify the specific protocol access request at the 4 access control points defined above.

The server must be able to identify any database access request, even for proprietary operations.

A session must always be authorized to invoke the <close-session> operation, defined in [RFC4741] (Enns, R., “NETCONF Configuration Protocol,” December 2006.).

A session must always be authorized to receive the <replayComplete> and <notificationComplete> notification events, defined in [RFC5277] (Chisholm, S. and H. Trevino, “NETCONF Event Notifications,” July 2008.)

The set of module name strings used within one particular server must be unique.

Within a single server, the module namespace URI associated with a specific module name string must persist across a reboot, and never change, once assigned.

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:

• Independent control of RPC, data, and notification access.
• Very simple access control rules configuration data model which is easy to use.
• The concept of a 'superuser' type of account is supported, but configuration of user accounts is beyond the scope of this document. The server must be able to determine if a superuser account is available, and if so, the actual user name for this account. A session associated with the superuser account will bypass all access control enforcement.
• A simple and familiar set of permissions is used:

  read:

  If granted, then the session is authorized to receive the associated data within an <rpc-reply> or <notification> message.

  write:

  If granted, then the session is authorized to alter the associated data node.

  exec:

  If granted, then the session is authorized to invoke the associated RPC operation.

• Support for YANG security tagging (e.g., nacm:secure extension) allows default security modes to automatically exclude sensitive data.
• Separate default access modes for read, write, and execute permissions.
• Access control rules are applied to configurable groups of users.
• The entire ACM can be disabled or downgraded during operation, in order to debug operational problems.
• Access control rule are simple to configure.
• The number of denied RPC operation requests and denied database write requests can be monitored by the client.
• Simple unconstrained YANG instance identifiers are used to configure access control rules for specific data nodes, or child nodes within specific RPC input, RPC output, and notification event type content.

3.1.2.  External Dependencies

The NETCONF (Enns, R., “NETCONF Configuration Protocol,” December 2006.) [RFC4741] protocol is used for all management purposes within this document. The server must support the features identified by the 'NETCONF-base' capability. It is expected that the mandatory transport mapping NETCONF Over SSH (Wasserman, M. and T. Goddard, “Using the NETCONF Configuration Protocol over Secure SHell (SSH),” December 2006.) [RFC4742] is also supported by the server, and that the server has access to the user name associated with each session.

The YANG Data Modeling Language (Bjorklund, M., “YANG - A data modeling language for NETCONF,” April 2010.) [I‑D.ietf‑netmod‑yang] is used to define the NETCONF data models specified in this document. The YANG instance-identifier data type can be used to configure data-node-specific access control rules.

3.1.3.  Message Processing Model

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

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

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

• Access control is applied to all <rpc> messages (except <close-session>) received by the server, individually, for each active session, unless the user identity for the session is the 'superuser'.
• If the session is authorized to execute the specified RPC operation, then processing continues, otherwise the request is rejected with an 'access-denied' error.
• If the configuration database or conceptual state data is accessed by the RPC operation, then the configuration access must be authorized first. If the session is authorized to perform the requested operation on the requested data, then processing continues.

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

• Server instrumentation generates a conceptual notification, for a particular subscription.
• The notification access control enforcer checks the notification event type, and if it is one which the session is not authorized to read, then the notification is dropped for that subscription.

3.2.  Model Components

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

3.2.1.  Users

A 'user' is the conceptual identity, which is associated with the access permissions granted to a particular session. A user is identified by a string which must be unique within the server. Configuration of users is beyond the scope of this document.

The user name string is usually derived from the transport layer during session establishment. A server is required to have an authenticated user name for a session before <rpc> requests will be accepted. Otherwise all requests must be rejected with an 'access-denied' error-tag value.

The server should support a 'superuser' administrative user account, which will bypass all access control enforcement. This is useful for restricting initial access and repairing a broken access control configuration. This account may be configurable to use a specific user, or disabled completely. Some systems have factory-selected superuser account names. There is no need to standardize the exact user name for the superuser account. If no such account, then all NETCONF access will be controlled.

3.2.2.  Groups

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

A group is identified by its YANG group identity, which must use the 'nacm-groups' identity as its base.

All group names must be unique within the server.

A group member is identified by a user name string.

The same user may be configured in multiple groups.

The server should support the 3 default group identities defined in this document (admin, monitor, guest), however these roles are just unique identities, provided for operator convenience. There is no standard behavior defined for each group identity. That is up to the operator who configures the groups.

3.2.3.  Sessions

A session is simply a NETCONF session, which is the entity which is granted access to specific NETCONF protocol mechanisms.

A session is associated with a single user name for the lifetime of the session.

3.2.4.  Access Permissions

The access permissions are the NETCONF protocol specific set of permissions that have been assigned to a particular session role or group. The same access permissions should stay in affect for the lifetime of a session.

The access control model treats RPC operation execution separately from configuration database access and outgoing messages:

read:

Read access to conceptual server data, <rpc-reply> and <notification> content.

write:

Write access to any configuration database.

exec:

Permission to invoke an RPC operation.

3.2.5.  Global Enforcement Controls

A global on/off switch is provided to enable or disable all access control enforcement.

An on/off switch is provided to enable or disable default access to invoke RPC operations.

An on/off switch is provided to enable or disable default permission to receive data in replies and notifications.

An on/off switch is provided to enable or disable default access to alter configuration data.

3.2.6.  Access Control Rules

There are 4 types of rules available in NACM:

module rule:

Controls access for definitions in a specific module, identified by its name.

RPC operation rule:

Controls access for a specific RPC operation, identified by its 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 module and name.

3.3.  Access Control Enforcement Procedures

There are seven separate phases that must 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 must also be considered.

3.3.1.  Initial Operation

Upon the very first start-up of the NETCONF server, the access control configuration will probably not be present. If not, a server should not allow any write access to any session role except 'superuser' type of account in this state.

There is no requirement to enforce access control rules before or while the non-volatile configuration data is processed and loaded into the running configuration.

3.3.2.  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.

A server should not include any sensitive information in any <capability> elements within the <hello> exchange.

Once session establishment is completed, and a user identity has been authenticated, a NETCONF server will enforce the access control rules, based on the supplied user identity and the configuration data stored on the server.

3.3.3.  'access-denied' Error Handling

The 'access-denied' error-tag is generated when the access control system denies access to either a request to invoke an RPC operation or a request to perform a particular operation on the configuration database.

A server must not include any sensitive information in any <error-info> elements within the <rpc-error> response.

3.3.4.  Incoming RPC Message Validation

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

                NETCONF server
               +------------+
               |    XML     |
               |   message  |
               | dispatcher |
               +------------+
                      |
                      |
                      V
               +------------+
               | NC-base NS |
               |   <rpc>    |
               +------------+
                 |   |  |
                 |   |  +-------------------------+
                 |   +------------+               |
                 V                V               V
            +-----------+ +---------------+ +------------+
            | acme NS   | | NC-base NS    | | NC-base NS |
            | <my-edit> | | <edit-config> | | <unlock>   |
            +-----------+ +---------------+ +------------+
                   |               |
                   |               |
                   V               V
                 +----------------------+
                 |                      |
                 |    configuration     |
                 |      database        |
                 +----------------------+

Access control begins with the message dispatcher. Only well-formed XML messages should be processed by the server.

A server should not allow access to configuration databases through any top level element except the <rpc> element in the NETCONF-base namespace. If it does, the server should still enforce access control to configuration data, even if the access is from some mechanism outside the standard protocol operations.

After the server validates the <rpc> element, and determines the namespace URI and the element name of the RPC operation being requested, the RPC access control enforcer verifies that the session is authorized to invoke the RPC operation.

The RPC operation is authorized by following these steps:

1. If the <enable-nacm> parameter is set to 'false', then the RPC operation is permitted.
2. If the session is associated with the 'superuser' account, then the RPC operation is permitted.
3. If the requested operation is the NETCONF <close-session> operation, then the RPC operation is permitted.
4. Check all the <group> entries for ones that contain a <user-name> entry that matches the user name for the session making the request.
5. If no groups are found:
   • If the requested RPC operation is associated with a YANG module advertised in the server capabilities, and the rpc statement contains a nacm:secure or nacm:very-secure extension, then the RPC operation is denied.
   • If the <exec-default> parameter is set to 'permit', then permit the RPC operation, otherwise deny the request.
6. Check if there are any matching <rpc-rule> entries for the requested RPC operation. Any matching rules are processing in user-defined order, in case there are multiple <rpc-rule> entries for the requested RPC operation.
7. If an <rpc-rule> entry is found, then check the <allowed-rights> bits field for the entry, otherwise continue. If the 'exec' bit is present in the <allowed-rights> bits field then the RPC operation is permitted, otherwise it is denied.
8. Check if there are any matching <module-rule> entries for the same module as the requested RPC operation. Any matching rules are processing in user-defined order, in case there are multiple <module-rule> entries for the module containing the requested RPC operation.
9. If a <module-rule> entry is found, then check the <allowed-rights> bits field for the entry, otherwise continue. If the 'exec' bit is present in the <allowed-rights> bits field then the RPC operation is permitted, otherwise it is denied.
10. If the requested operation is identified an a nacm:secure or nacm:very-secure RPC operation, then the RPC operation is denied.
11. If the <exec-default> parameter is set to 'permit', then permit the RPC operation, otherwise deny the request.

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

error-tag:

access-denied

error-path:

/rpc/method-QName, where 'method-QName' is a qualified name identifying the actual RPC operation name. For example, '/rpc/edit-config' represents the <edit-config> operation in the NETCONF base namespace..

If the configuration database is accessed, either directly or as a side effect of the RPC operation, then the server must intercept the operation and make sure the session is authorized to perform the requested operation on the specified data.

3.3.5.  Data Node Access Validation

If a data node within a configuration database is accessed, or a conceptual non-configuration node is accessed, then the server must ensure that the client session is authorized to perform the requested operation (read or write) on the specified data node.

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

1. If the <enable-nacm> parameter is set to 'false', then the data node access request is permitted.
2. If the session is associated with the 'superuser' account, then the data node access request is permitted.
3. Check all the <group> entries for ones that contain a <user-name> entry that matches the user name for the session making the request.
4. If no groups are found:
   • If the requested data node is associated with a YANG module advertised in the server capabilities, and the data definition statements (or any of its ancestors) contains a nacm:secure or nacm:very-secure extension, then the data node access request is denied.
   • For a read request, if the <read-default> parameter is set to 'permit', then permit the data node access request, otherwise deny the request.
   • For a write request, if the <write-default> parameter is set to 'permit', then permit the data node access request, otherwise deny the request.
5. Check if there are any matching <data-rule> entries for the requested data node access request. Any matching rules are processing in user-defined order, in case there are multiple <data-rule> entries for the requested data node.
6. If an <data-rule> entry is found, then check the <allowed-rights> bits field for the entry, otherwise continue.
   1. For a read operation, if the 'read' bit is present in the <allowed-rights> bits field then the request is permitted, otherwise it is denied.
   2. For a write operation, if the 'write' bit is present in the <allowed-rights> bits field then the request is permitted, otherwise it is denied.
7. Check if there are any matching <module-rule> entries for the same module as the requested data node. Any matching rules are processing in user-defined order, in case there are multiple <module-rule> entries for the module containing the requested data node.
8. If a <module-rule> entry is found, then check the <allowed-rights> bits field for the entry, otherwise continue.
   1. For a read operation, if the 'read' bit is present in the <allowed-rights> bits field then the request is permitted, otherwise it is denied.
   2. For a write operation, if the 'write' bit is present in the <allowed-rights> bits field then the request is permitted, otherwise it is denied.
9. For a read request, if the requested data node is identified an a nacm:very-secure definition, then the data node access request is denied.
10. For a write request, if the requested data node is identified an a nacm:secure or nacm:very-secure definition, then the data node access request is denied.
11. For a read request, if the <read-default> parameter is set to 'permit', then permit the data node access request, otherwise deny the request.
12. For a write request, if the <write-default> parameter is set to 'permit', then permit the data node access request, otherwise deny the request.

3.3.6.  Outgoing <rpc-reply> Authorization

The <rpc-reply> message should be checked by the server to make sure no unauthorized data is contained within it. If so, the restricted data must be removed from the message before it is sent to the client.

Configuration of access control rules exclusively for the RPC output section, for an <rpc-reply> message that does not return data from a conceptual database or non-configuration data suitable for retrieval with the <get> operation is outside the scope of this document. For RPC operations which do not access any data nodes, then any client authorized to invoke the RPC operation is also authorized to receive the <rpc-reply> for that RPC operation.

3.3.7.  Outgoing <notification> Authorization

The <notification> message should be checked by the server to make sure no unauthorized data is contained within it. If so, the restricted data must be removed from the message before it is sent to the client.

Configuration of access control rules specifically for descendent nodes of the notification event type element are outside the scope of this document. If the session 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.

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

The generation of a notification event for a specific subscription is authorized by following these steps:

1. If the <enable-nacm> parameter is set to 'false', then the notification event is permitted.
2. If the session is associated with the 'superuser' account, then the notification event is permitted.
3. If the requested operation is the NETCONF <replayComplete> or <notificationComplete> event type, then the notification event is permitted.
4. Check all the <group> entries for ones that contain a <user-name> entry that matches the user name for the session that started the notification subscription.
5. If no groups are found:
   • If the requested notification is associated with a YANG module advertised in the server capabilities, and the notification statement contains a nacm:secure or nacm:very-secure extension, then the notification event is dropped for the associated subscription.
   • If the <read-default> parameter is set to 'permit', then permit the notification event, otherwise drop this event type for the associated subscription.
6. Check if there are any matching <notification-rule> entries for the specific notification event type being delivered to the subscription. Any matching rules are processing in user-defined order, in case there are multiple <notification-rule> entries for the requested notification event type.
7. If a <notification-rule> entry is found, then check the <allowed-rights> bits field for the entry, otherwise continue. If the 'read' bit is present in the <allowed-rights> bits field then the notification event type is permitted, otherwise it is dropped for the associated subscription.
8. Check if there are any matching <module-rule> entries for the same module as the notification event type. Any matching rules are processing in user-defined order, in case there are multiple <module-rule> entries for the module containing the notification event type.
9. If a <module-rule> entry is found, then check the <allowed-rights> bits field for the entry, otherwise continue. If the 'read' bit is present in the <allowed-rights> bits field then the notification event type is permitted, otherwise it is dropped for the associated subscription.
10. If the requested event type is identified an a nacm:very-secure notification definition, then the notification event type is denied.
11. If the <read-default> parameter is set to 'permit', then permit the notification event type, otherwise it is dropped for the associated subscription.`<

3.4.  Data Model Definitions

This section defines the semantics of the conceptual data structures found in the data model in Section 3.4 (Data Model Definitions).

3.4.1.  High Level Procedures

There are some high level management procedures that an administrator needs to consider before using this access control model:

1. Configure the global settings.
2. Configure one or more user groups.
3. Configure zero or more access control rules for specific modules.
4. Configure zero or more access control rules for specific RPC operations.
5. Configure zero or more access control rules for data node access.
6. Configure zero or more access control rules for notification event type access.

3.4.2.  Data Organization

The top-level element is called <nacm>, and it is defined the 'nacm' module namespace.

There are several data structures defined as child nodes of the <nacm> element:

leaf <enable-nacm>:

On/off boolean switch to enable or disable access control enforcement.

leaf <read-default>:

Enumeration to permit or deny default read access requests.

leaf <write-default>:

Enumeration to permit or deny default write access requests.

leaf <exec-default>:

Enumeration to permit or deny default RPC operation execution requests.

leaf <denied-rpcs>:

Read-only counter of the number of times the server has denied an RPC operation request, since the last reboot of the server.

leaf <denied-data-writes>:

Read-only counter of the number of times the server has denied a data node write request, since the last reboot of the server.

container <groups>:

Configures the groups used within the access control system.

list <group>:

A list of user names belonging to the same administrative group.

container <rules>:

Configures the access control rules used within the server.

list <module-rule>:

Configures the access control rules for a specific module.

list <rpc-rule>:

Configures the access control rules for RPC operation invocation.

list <data-rule>:

Configures the access control rules for configuration database access.

list <notification-rule>:

Configures the access control rules for controlling delivery of <notification> events.

3.4.3.  YANG Module

The following YANG module is provided to specify the normative NETCONF content that must by supported by the server.

<CODE BEGINS> file="nacm.yang"

module nacm {

    namespace "file://draft-bierman-netconf-access-control-01.txt";

    prefix "nacm";

    import ietf-yang-types { prefix yang; }

    organization  "Netconf Central, Inc.";

    contact "Andy Bierman <andy@netconfcentral.org>.";

    description
      "NETCONF Server Access Control Model";

    revision 2010-02-21 {
        description
          "Initial version (work-in-progress).";
    }

    typedef nacm-user-name {
        description "General Purpose User Name string.";
        type string {
            length "1..64";
            pattern '[a-z,A-Z,_][a-z,A-Z,0-9,\-,_,@,.]{0,63}';
        }
    }

    typedef nacm-rights {
      description
        "NETCONF Access Rights";

      type bits {
	bit read {
           description
              "Read access allowed to all specified data.
               Any protocol operation or notification that
               returns data to an application is a read
               operation.";
        }
	bit write {
           description
              "Write access allowed to all specified data.
               Any protocol operation that alters a database
               is a write operation.";

        }
	bit exec {
           description
              "Execution access to the specified RPC operation.
               Any RPC operation invocation is an exec operation.";
        }
      }
    }

    typedef nacm-group {
      description
        "Type of administrative group that can be
         assigned to the user, and specified in
         an access control rule.

         The identityref data type is used to allow as
         many groups to be added as needed.  There are
         no standard semantics for each identity.
         It simply represents a unique group name.";

      type identityref {
         base nacm-groups;
      }
    }

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

    typedef schema-instance-identifier {
      description
        "Path expression used to represent a special
         schema-instance identifier string.

         A schema-instance-identifier value string 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 schema-instance-identifier
         represents all possible server instances for that key.";
      type string;
    }

    extension secure {
      description
        "Used to indicate that the data model node
         represents a sensitive security system parameter.

         If present, the NETCONF server will only allow
         the designated 'superuser' to have write or execute
         default nacm-rights for the node.  An explicit access
         control rule is required for all other users.

         The 'secure' extension may appear within a data, rpc,
         or notification node definition.  It is ignored
         otherwise.";
    }

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

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

         The 'very-secure' extension may appear within a data, rpc,
         or notification node definition.  It is ignored
         otherwise.";
    }

    identity nacm-groups {
       description
          "Root of all NETCONF Administrative Groups";
    }

    identity admin {
       description
          "Example Administrator group.";
       base nacm-groups;
    }

    identity monitor {
       description
          "Example Monitoring group.";
       base nacm-groups;
    }

    identity guest {
       description
          "Example Guest group.";
       base nacm-groups;
    }

    container nacm {
        nacm:very-secure;

        description
          "Parameters for NETCONF Access Control Model.";

        presence
         "An empty nacm container indicates that the
          NACM service is running, and possibly using
          all default parameters.";

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

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

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

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

        leaf denied-rpcs {
            description
              "Number of times an RPC operation request was denied";
            type yang:zero-based-counter32;
            config false;
        }

        leaf denied-data-writes {
            description
              "Number of times a request to alter a data node
               was denied.";
            type yang:zero-based-counter32;
            config false;
        }

        container groups {
            description
              "NACM Group Table";

            list group {
                description "One NACM Group Entry";

                key group-identity;

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

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

        container rules {
            description
              "NETCONF Access Control Rules.";

            grouping common-rule-parms {
                leaf rule-name {
                    description
                      "Arbitrary name assigned to the
                       access control rule.";
                    type string {
                       length "1..1023";
                    }
                }

                leaf allowed-rights {
                    description
                      "List of access rights granted to
                       specified administrative groups for the
                       content specified by the associated path.";
                    type nacm-rights;
                    mandatory true;
                }

                leaf-list allowed-group {
                    description
                      "List of administrative groups which will be
                       assigned the associated access rights
                       for the content specified by the associated
                       path.";
                    type nacm-group;
                    min-elements 1;
                }

                leaf comment {
                    description
                      "A textual description of the access rule.";
                    type string {
                      length "0..4095";
                    }
                }
            }

            list module-rule {
                description
                   "One Module Access Rule.
                    Rules are processed in user-defined order.
                    A module rule is considered a match if
                    the XML namespace for the specified module
                    name matches the XML namespace used within
                    a NETCONF PDU, and the administrative group
                    associated with the requesting session is
                    specified in the 'allowed-group' leaf-list.";

                ordered-by user;

                key "module-name rule-name";

                leaf module-name {
                    description
                      "Name of the module associated with
                       this rule.";
                    type string {
			length "1..max";
                    }
                }

                uses common-rule-parms;
            }

            list rpc-rule {
                description
                   "One RPC Operation Access Rule.
                    Rules are processed in user-defined order.
                    An RPC rule is considered a match if
                    the module name of the requested RPC
                    operation matches 'rpc-module-name',
                    the requested RPC operation matches
                    'rpc-name', and an administrative group
                    associated with the session user
                    is listed in the 'allowed-group'
                    leaf-list";

                ordered-by user;

                key "rpc-module-name
                     rpc-name
                     rule-name";

                leaf rpc-module-name {
                    description
                      "Name of the module defining this
                       RPC operation.";
                    type string {
			length "1..max";
                    }
                }

                leaf rpc-name {
                    description
                      "Name of the RPC operation.";
                    type string {
			length "1..max";
                    }
                }

                uses common-rule-parms;
            }

            list data-rule {
                description
                  "One Data Access Control Rule.
                   Rules are processed in user-defined order.
                   A data rule is considered to match when
                   the path expression identifies
                   the same node that is being accessed
                   in the NETCONF database, and the administrative
                   group associated with the session is identified
                   in the 'allowed-group' leaf-list.";

                key "rule-name";

                ordered-by user;

		leaf path {
                  description
                    "Schema 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 database contents.";

		  type schema-instance-identifier;
                  mandatory true;
                }

                uses common-rule-parms;
            }

            list notification-rule {
                description
                   "One Notification Access Rule.
                    A notification is considered a match if
                    the module name of the requested
                    event type matches 'notification-module-name',
                    the requested event type matches the
                    'notification-name', and the administrative group
                    associated with the requesting session
                    is listed in the 'allowed-group' leaf-list.";

                ordered-by user;

                key "notification-module-name
                     notification-name
                     rule-name";

                leaf notification-module-name {
                    description
                      "Name of the module defining this
                       notification event type.";
                    type string {
			length "1..max";
                    }
                }

                leaf notification-name {
                    description
                      "Name of the notification event.";
                    type string {
			length "1..max";
                    }
                }

                uses common-rule-parms;
            }
        }
    }

}





<CODE ENDS>

3.5.  IANA Considerations

There are two actions that are requested of IANA:

1. register data model schema namespace URI (TBD)
2. register data model name ('nacm')

3.6.  Security Considerations

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

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.

This document incorporates the optional use of a superuser account, which can be used to bypass access control enforcement. Configuration user accounts is outside the scope of this document, however it is suggested that the 'root' account not be used for NETCONF over SSH servers, because 'root' SSH logins should be disabled in the SSH server.

If the server chooses to allow user configuration of the access control system, then only sessions using the 'superuser' administrative user should be allowed to have write access to the data model.

If the server chooses to allow user retrieval of the access control system configuration, then only sessions using the 'superuser' administrative user should be allowed to have read access to the data model.

There is a risk that invocation of non-standard RPC operations will have undocumented side effects. An administrator should construct access control rules such that the configuration database is protected from such side effects. Also, such RPC operations should never be invoked by a session using the 'superuser' administrative user.

There is a risk that non-standard RPC operations, or even the standard <get> operation, may return data which 'aliases' or 'copies' sensitive data from a different data object. In this case, the namespace and/or the element name will not match the values for the sensitive data, which is then fully or partially copied into a different namespace and/or element. An administrator should avoid using data models which use this practice.

An administrator should restrict write access to all configurable objects within this data model. It is suggested that only sessions using the 'superuser' administrative role be permitted to configure the data model defined in this document.

If write access is allowed for configuration of access control rules, then care must be taken not to disrupt the access control enforcement.

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

• enable-nacm
• read-default
• write-default
• exec-default
• groups
• rules

4. Normative References

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 must 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.

<nc:config xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
   <nacm:nacm
      xmlns:nacm="file://draft-bierman-netconf-access-control-01.txt">
      <nacm:groups>
         <nacm:group>
            <nacm:group-identity>nacm:admin</nacm:group-identity>
            <nacm:user-name>admin</nacm:user-name>
            <nacm:user-name>andy</nacm:user-name>
         </nacm:group>
         <nacm:group>
            <nacm:group-identity>nacm:monitor</nacm:group-identity>
            <nacm:user-name>wilma</nacm:user-name>
            <nacm:user-name>bam-bam</nacm:user-name>
         </nacm:group>
         <nacm:group>
            <nacm:group-identity>nacm:guest</nacm:group-identity>
            <nacm:user-name>guest</nacm:user-name>
            <nacm:user-name>guest@example.com</nacm:user-name>
         </nacm:group>
      </nacm:groups>
   </nc:config>
</nacm:nacm>

This example shows 3 groups:

1. The nacm:admin group contains 2 users named 'admin' and 'andy'.
2. The nacm:monitor group contains 2 users named 'wilma' and 'bam-bam'.
3. The nacm:guest group contains 2 users named 'guest' and 'guest@example.com'.

A.2.  <module-rule> Example

Module rules are used to control access to all the content defined in a specific module. These rules are checked after none of the specific rules (i.e., rpc-rule, data-rule, or notification-rule) matched the current access request.

<nc:config xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
   <nacm:nacm
      xmlns:nacm="file://draft-bierman-netconf-access-control-01.txt">
      <nacm:rules>
        <nacm:module-rule>
          <nacm:module-name>
            ietf-netconf-monitoring
          </nacm:module-name>
          <nacm:rule-name>1</nacm:rule-name>
          <nacm:allowed-rights>read</nacm:allowed-rights>
          <nacm:allowed-group>nacm:admin</nacm:allowed-group>
          <nacm:allowed-group>nacm:monitor</nacm:allowed-group>
          <nacm:comment>
            do not allow guests to read the netconf information
          </nacm:comment>
        </nacm:module-rule>
        <nacm:module-rule>
          <nacm:module-name>ietf-netconf</nacm:module-name>
          <nacm:rule-name>1</nacm:rule-name>
          <nacm:allowed-rights>read write exec</nacm:allowed-rights>
          <nacm:allowed-group>nacm:admin</nacm:allowed-group>
          <nacm:comment>
            allow admin complete access to the standard operations
          </nacm:comment>
        </nacm:module-rule>
      </nacm:rules>
   </nc:config>
</nacm:nacm>

This example shows 2 module rules:

1. This rule allows the admin and monitor groups (but not the guest group) to read the <netconf-state> sub-tree, which is defined in the ietf-netconf-monitoring YANG module.
2. This rule allows complete access to the ietf-netconf module for the admin group.

A.3.  <rpc-rule> Example

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

<nc:config xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
   <nacm:nacm
      xmlns:nacm="file://draft-bierman-netconf-access-control-01.txt">
      <nacm:rules>
        <nacm:rpc-rule>
          <nacm:rpc-module-name>
            ietf-netconf
          </nacm:rpc-module-name>
          <nacm:rpc-name>
            kill-session
          </nacm:rpc-name>
          <nacm:rule-name>1</nacm:rule-name>
          <nacm:allowed-rights/>
          <nacm:allowed-group>nacm:monitor</nacm:allowed-group>
          <nacm:allowed-group>nacm:guest</nacm:allowed-group>
          <nacm:comment>
            do not allow monitor or guest to kill another session
          </nacm:comment>
        </nacm:rpc-rule>
        <nacm:rpc-rule>
          <nacm:rpc-module-name>
            ietf-netconf
          </nacm:rpc-module-name>
          <nacm:rpc-name>
            delete-config
          </nacm:rpc-name>
          <nacm:rule-name>1</nacm:rule-name>
          <nacm:allowed-rights/>
          <nacm:allowed-group>nacm:monitor</nacm:allowed-group>
          <nacm:allowed-group>nacm:guest</nacm:allowed-group>
          <nacm:comment>
            do not allow monitor or guest to delete any configs
          </nacm:comment>
        </nacm:rpc-rule>
      </nacm:rules>
   </nc:config>
</nacm:nacm>

This example shows 2 RPC rules:

1. This rule prevents the monitor or guest groups (but not the admin group) from invoking the NETCONF <kill-session> RPC operation.
2. This rule prevents the monitor or guest groups (but not the admin group) from invoking the NETCONF <delete-config> RPC operation.

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.

<nc:config xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
   <nacm:nacm
      xmlns:nacm="file://draft-bierman-netconf-access-control-01.txt">
      <nacm:rules>
        <nacm:data-rule>
          <nacm:rule-name>data-1</nacm:rule-name>
          <nacm:path>/nacm:nacm</nacm:path>
          <nacm:allowed-rights>read write</nacm:allowed-rights>
          <nacm:allowed-group>nacm:admin</nacm:allowed-group>
          <nacm:comment>
            allow admin access to /nacm
          </nacm:comment>
        </nacm:data-rule>
        <nacm:data-rule>
          <nacm:rule-name>data-acme-config</nacm:rule-name>
          <nacm:path
             xmlns:acme="http://example.com/ns/netconf">
            /acme:acme-netconf/acme:config-parameters
          </nacm:path>
          <nacm:allowed-rights>read write</nacm:allowed-rights>
          <nacm:allowed-group>nacm:admin</nacm:allowed-group>
          <nacm:allowed-group>nacm:monitor</nacm:allowed-group>
          <nacm:comment>
            allow admin and monitor to access acme
            netconf config parameters.
          </nacm:comment>
        </nacm:data-rule>
        <nacm:data-rule>
          <nacm:rule-name>dummy-itf</nacm:rule-name>
          <nacm:path
             xmlns:acme="http://example.com/ns/itf">
            /acme:interfaces/acme:interface[acme:name='dummy']
          </nacm:path>
          <nacm:allowed-rights>read write</nacm:allowed-rights>
          <nacm:allowed-group>nacm:monitor</nacm:allowed-group>
          <nacm:allowed-group>nacm:guest</nacm:allowed-group>
          <nacm:comment>
            allow monitor and guest full access to acme
            dummy interface
          </nacm:comment>
        </nacm:data-rule>
        <nacm:data-rule>
          <nacm:rule-name>admin-itf</nacm:rule-name>
          <nacm:path
             xmlns:acme="http://example.com/ns/itf">
            /acme:interfaces/acme:interface
          </nacm:path>
          <nacm:allowed-rights>read write</nacm:allowed-rights>
          <nacm:allowed-group>nacm:admin</nacm:allowed-group>
          <nacm:comment>
            allow admin full access to all acme interfaces
          </nacm:comment>
        </nacm:data-rule>
      </nacm:rules>
   </nc:config>
</nacm:nacm>

This example shows 4 data rules:

data-1:

This rule gives the admin group read-write access to the <nacm> sub-tree.

data-acme-config:

This rule gives the admin and monitor groups read-write access to the acme <config-parameters>.

dummy-itf:

This rule gives the monitor and guest groups read-write access to the acme <interface>. entry named 'dummy'.

admin-itf:

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.

<nc:config xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
   <nacm:nacm
      xmlns:nacm="file://draft-bierman-netconf-access-control-01.txt">
      <nacm:rules>
        <nacm:notification-rule>
          <nacm:notification-module-name>
            acme-system
          </nacm:notification-module-name>
          <nacm:notification-name>
            sys-config-change
          </nacm:notification-name>
          <nacm:rule-name>1</nacm:rule-name>
          <nacm:allowed-rights/>
          <nacm:allowed-group>nacm:monitor</nacm:allowed-group>
          <nacm:allowed-group>nacm:guest</nacm:allowed-group>
          <nacm:comment>
            only the admin group can receive config change events
          </nacm:comment>
        </nacm:notification-rule>
      </nacm:rules>
   </nc:config>
</nacm:nacm>

This example shows 1 notification rule:

1. This rule prevents the monitor or guest groups (but not the admin group) from receiving the acme <sys-config-change> event type.

Appendix B.  Open Issues

1. Do modules need to be identified by their XML namespace URI, or is the module name good enough?
2. Are any more wildcard mechanisms needed to specify the scope of an access control rule?
3. Should regular expressions (module='foo-*') be allowed in schema-instance-identifier strings?
4. Should XPath be allowed for specifying access control rules for data nodes?
5. Are any 'access-denied' notifications needed?
6. Should data rules support nodes that would not be eligible for retrieval with the <get> operation? If so, should schema nodes such as rpc 'input' or 'output' be in the path expression? How would notification content be identified?
7. Do any external access control models need to be supported somehow? For example, should the <groups> configuration be optionally read-only, so it can just mirror the internal (external or proprietary) group configuration?
8. Should the nacm:secure and nacm:very-secure extensions be optional to support, via a YANG feature?
9. Should the default access levels (e.g., read-default) be more restrictive by default? Shiuld these defaults be a vendor decision? An operator decision? It is important that the server be able to install a factory default <nacm> container if needed.

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