Network Configuration Protocol (NETCONF) Access Control Model
RFC 6536
| Document | Type |
RFC - Proposed Standard
(March 2012)
Errata
Obsoleted by RFC 8341
|
|
|---|---|---|---|
| Authors | Andy Bierman , Martin Björklund | ||
| Last updated | 2020-01-21 | ||
| Replaces | draft-bierman-netconf-access-control | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Yang Validation | ☯ 2 errors, 2 warnings | ||
| Reviews | |||
| Additional resources |
Yang catalog entry for ietf-netconf-acm@2011-12-23.yang
Yang impact analysis for draft-ietf-netconf-access-control Mailing list discussion |
||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | RFC 6536 (Proposed Standard) | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | Dan Romascanu | ||
| IESG note | Bert Wijnen (bertietf@bwijnen.net) is the document shepherd. | ||
| Send notices to | (None) |
RFC 6536
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]