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INTERNET STANDARD
Errata ExistInternet Engineering Task Force (IETF) A. BiermanRequest for Comments: 8341 YumaWorksSTD: 91 M. BjorklundObsoletes:6536 Tail-f SystemsCategory: Standards Track March 2018ISSN: 2070-1721Network Configuration Access Control ModelAbstract The standardization of network configuration interfaces for use with the Network Configuration Protocol (NETCONF) or the RESTCONF protocol requires a structured and secure operating environment that promotes human usability and multi-vendor interoperability. There is a need for standard mechanisms to restrict NETCONF or RESTCONF protocol access for particular users to a preconfigured subset of all available NETCONF or RESTCONF protocol operations and content. This document defines such an access control model. This document obsoletesRFC 6536.Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available inSection 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc8341.Bierman & Bjorklund Standards Track [Page 1]
RFC 8341 NACM March 2018Copyright Notice Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject toBCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.Table of Contents1. Introduction ....................................................41.1. Terminology ................................................41.2. Changes sinceRFC 6536 .....................................62. Access Control Design Objectives ................................72.1. Access Control Points ......................................72.2. Simplicity .................................................82.3. Procedural Interface .......................................82.4. Datastore Access ...........................................82.5. Users and Groups ...........................................82.6. Maintenance ................................................92.7. Configuration Capabilities .................................92.8. Identifying Security-Sensitive Content .....................93. NETCONF Access Control Model (NACM) ............................103.1. Overview ..................................................103.1.1. Features ...........................................103.1.2. External Dependencies ..............................113.1.3. Message Processing Model ...........................113.2. Datastore Access ..........................................143.2.1. Mapping New Datastores to NACM .....................143.2.2. Access Rights ......................................143.2.3. RESTCONF Methods ...................................153.2.4. <get> and <get-config> Operations ..................163.2.5. <edit-config> Operation ............................163.2.6. <copy-config> Operation ............................183.2.7. <delete-config> Operation ..........................183.2.8. <commit> Operation .................................193.2.9. <discard-changes> Operation ........................193.2.10. <kill-session> Operation ..........................19Bierman & Bjorklund Standards Track [Page 2]
RFC 8341 NACM March 20183.3. Model Components ..........................................193.3.1. Users ..............................................193.3.2. Groups .............................................203.3.3. Emergency Recovery Session .........................203.3.4. Global Enforcement Controls ........................203.3.4.1. enable-nacm Switch ........................203.3.4.2. read-default Switch .......................203.3.4.3. write-default Switch ......................213.3.4.4. exec-default Switch .......................213.3.4.5. enable-external-groups Switch .............223.3.5. Access Control Rules ...............................223.4. Access Control Enforcement Procedures .....................223.4.1. Initial Operation ..................................233.4.2. Session Establishment ..............................233.4.3. "access-denied" Error Handling .....................233.4.4. Incoming RPC Message Validation ....................243.4.5. Data Node Access Validation ........................263.4.6. Outgoing <notification> Authorization ..............293.5. Data Model Definitions ....................................313.5.1. Data Organization ..................................313.5.2. YANG Module ........................................324. IANA Considerations ............................................425. Security Considerations ........................................425.1. NACM Configuration and Monitoring Considerations ..........435.2. General Configuration Issues ..............................455.3. Data Model Design Considerations ..........................476. References .....................................................476.1. Normative References ......................................476.2. Informative References ....................................49Appendix A. Usage Examples ........................................50A.1. <groups> Example ...........................................50A.2. Module Rule Example ........................................51A.3. Protocol Operation Rule Example ............................53A.4. Data Node Rule Example .....................................55A.5. Notification Rule Example ..................................57 Authors' Addresses ................................................58Bierman & Bjorklund Standards Track [Page 3]
RFC 8341 NACM March 20181. Introduction The Network Configuration Protocol (NETCONF) and the RESTCONF protocol do not provide any standard mechanisms to restrict the protocol operations and content that each user is authorized to access. There is a need for interoperable management of the controlled access to administrator-selected portions of the available NETCONF or RESTCONF content within a particular server. This document addresses access control mechanisms for the Operations and Content layers of NETCONF, as defined in [RFC6241]; and RESTCONF, as defined in [RFC8040]. It contains three main sections: 1. Access Control Design Objectives 2. NETCONF Access Control Model (NACM) 3. YANG Data Model (ietf-netconf-acm.yang) YANG version 1.1 [RFC7950] adds two new constructs that need special access control handling. The "action" statement is similar to the "rpc" statement, except that it is located within a data node. The "notification" statement can also be located within a data node.1.1. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described inBCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. The following terms are defined in [RFC8342] and are not redefined here: o datastore o configuration datastore o conventional configuration datastore o candidate configuration datastore o running configuration datastore o startup configuration datastoreBierman & Bjorklund Standards Track [Page 4]
RFC 8341 NACM March 2018 o operational state datastore o client o server The following terms are defined in [RFC6241] and are not redefined here: o protocol operation o session o user The following terms are defined in [RFC7950] and are not redefined here: o action o data node o data definition statement The following terms are defined in [RFC8040] and are not redefined here: o data resource o datastore resource o operation resource o target resource The following term is defined in [RFC7230] and is not redefined here: o request URI The following terms are used throughout this document: access control: A security feature provided by the server that allows an administrator to restrict access to a subset of all protocol operations and data, based on various criteria. access control model (ACM): A conceptual model used to configure and monitor the access control procedures desired by the administrator to enforce a particular access control policy.Bierman & Bjorklund Standards Track [Page 5]
RFC 8341 NACM March 2018 access control rule: The criterion used to determine if a particular access operation will be permitted or denied. access operation: How a request attempts to access a conceptual object. One of "none", "read", "create", "delete", "update", or "execute". data node hierarchy: The hierarchy of data nodes that identifies the specific "action" or "notification" node in the datastore. recovery session: A special administrative session that is given unlimited NETCONF access and is exempt from all access control enforcement. The mechanism or mechanisms used by a server to control and identify whether or not a session is a recovery session are implementation specific and are outside the scope of this document. write access: A shorthand for the "create", "delete", and "update" access operations.1.2. Changes sinceRFC 6536 The NACM procedures and data model have been updated to support new data modeling capabilities in version 1.1 of the YANG data modeling language. The "action" and "notification" statements can be used within data nodes to define data-model-specific operations and notifications. An important use case for these new YANG statements is the increased access control granularity that can be achieved over top-level "rpc" and "notification" statements. The new "action" and "notification" statements are used within data nodes, and access to the action or notification can be restricted to specific instances of these data nodes. Support for the RESTCONF protocol has been added. The RESTCONF operations are similar to the NETCONF operations, so a simple mapping to the existing NACM procedures and data model is possible. The data node access behavior for path matches has been clarified to also include matching descendant nodes of the specified path. The <edit-config> operation access rights behavior has been clarified to indicate that write access is not required for data nodes that are implicitly modified through side effects (such as the evaluation of YANG when-stmts, or data nodes implicitly deleted when creating a data node under a different branch under a YANG choice-stmt).Bierman & Bjorklund Standards Track [Page 6]
RFC 8341 NACM March 2018 The Security Considerations section has been updated to comply with the "YANG module security guidelines" [YANG-SEC]. Note that the YANG module in this document does not define any RPC operations.2. Access Control Design Objectives This section documents the design objectives for the NETCONF access control model presented inSection 3.2.1. Access Control Points NETCONF allows server implementers to add new custom protocol operations, and the YANG data modeling language supports this feature. These operations can be defined in standard or proprietary YANG modules. It is not possible to design an ACM for NETCONF that only focuses on a static set of standard protocol operations defined by NETCONF itself, like some other protocols. Since few assumptions can be made about an arbitrary protocol operation, the NETCONF architectural server components need to be protected at three conceptual control points. These access control points, described in Figure 1, are as follows: protocol operation: Permission to invoke specific protocol operations. datastore: Permission to read and/or alter specific data nodes within any datastore. notification: Permission to receive specific notification event types. +-------------+ +-------------+ client | protocol | | data node | request --> | operation | -------------> | access | | allowed? | datastore | allowed? | +-------------+ or state +-------------+ data access +----------------+ | notification | event --> | allowed? | +----------------+ Figure 1Bierman & Bjorklund Standards Track [Page 7]
RFC 8341 NACM March 20182.2. Simplicity There is concern that a complicated ACM will not be widely deployed because it is too hard to use. Configuration of the access control system needs to be as simple as possible. Simple and common tasks need to be easy to configure and require little expertise or domain-specific knowledge. Complex tasks are possible using additional mechanisms that may require additional expertise. A single set of access control rules ought to be able to control all types of NETCONF protocol operation invocation, all datastore access, and all notification events. Access control ought to be defined with a small and familiar set of permissions, while still allowing full control of datastore access.2.3. Procedural Interface NETCONF uses a Remote Procedure Call (RPC) model and an extensible set of protocol operations. Access control for any possible protocol operation is necessary.2.4. Datastore Access It is necessary to control access to specific nodes and subtrees within the datastore, regardless of which protocol operation -- standard or proprietary -- was used to access the datastore.2.5. Users and Groups It is necessary that access control rules for a single user or a configurable group of users can be configured. The ACM needs to support the concept of administrative groups, to support the well-established distinction between a root account and other types of less-privileged conceptual user accounts. These groups need to be configurable by the administrator. It is necessary that the user-to-group mapping can be delegated to a central server, such as a RADIUS server [RFC2865] [RFC5607]. Since authentication is performed by the transport layer and RADIUS performs authentication and service authorization at the same time, the underlying transport protocol needs to be able to report a set of group names associated with the user to the server. It is necessary that the administrator can disable the usage of these group names within the ACM.Bierman & Bjorklund Standards Track [Page 8]
RFC 8341 NACM March 20182.6. Maintenance It ought to be possible to disable part or all of the access control model enforcement procedures without deleting any access control rules.2.7. Configuration Capabilities Suitable configuration and monitoring mechanisms are needed to allow an administrator to easily manage all aspects of the ACM's behavior. A standard data model, suitable for use with the <edit-config> protocol operation, needs to be available for this purpose. Access control rules to restrict access operations on specific subtrees within the configuration datastore need to be supported.2.8. Identifying Security-Sensitive Content One of the most important aspects of the data model documentation, and one of the biggest concerns during deployment, is the identification of security-sensitive content. This applies to protocol operations in NETCONF, not just data and notifications. It is mandatory for security-sensitive objects to be documented in the Security Considerations section of an RFC. This is nice, but it is not good enough, for the following reasons: o This documentation-only approach forces administrators to study the RFC and determine if there are any potential security risks introduced by a new data model. o If any security risks are identified, then the administrator must study some more RFC text and determine how to mitigate the security risk(s). o The ACM on each server must be configured to mitigate the security risks, e.g., require privileged access to read or write the specific data identified in the Security Considerations section. o If the ACM is not preconfigured, then there will be a time window of vulnerability after the new data model is loaded and before the new access control rules for that data model are configured, enabled, and debugged. Often, the administrator just wants to disable default access to the secure content so that no inadvertent or malicious changes can be made to the server. This allows the default rules to be more lenient, without significantly increasing the security risk.Bierman & Bjorklund Standards Track [Page 9]
RFC 8341 NACM March 2018 A data model designer needs to be able to use machine-readable statements to identify content that needs to be protected by default. This will allow client and server tools to automatically identify data-model-specific security risks, by denying access to sensitive data unless the user is explicitly authorized to perform the requested access operation.3. NETCONF Access Control Model (NACM)3.1. Overview This section provides a high-level overview of the access control model structure. It describes the NETCONF protocol message processing model and the conceptual access control requirements within that model.3.1.1. Features The NACM data model provides the following features: o Independent control of RPC, action, data, and notification access is provided. o The concept of an emergency recovery session is supported, but configuration of the server for this purpose is beyond the scope of this document. An emergency recovery session will bypass all access control enforcement, in order to allow it to initialize or repair the NACM configuration. o A simple and familiar set of datastore permissions is used. o Support for YANG security tagging (e.g., a "nacm:default-deny-write" statement) allows default security modes to automatically exclude sensitive data. o Separate default access modes for read, write, and execute permissions are provided. o Access control rules are applied to configurable groups of users. o The access control enforcement procedures can be disabled during operation, without deleting any access control rules, in order to debug operational problems.Bierman & Bjorklund Standards Track [Page 10]
RFC 8341 NACM March 2018 o The number of denied protocol operation requests and denied datastore write requests can be monitored by the client. o Simple unconstrained YANG instance-identifiers are used to configure access control rules for specific data nodes.3.1.2. External Dependencies NETCONF [RFC6241] and RESTCONF [RFC8040] are used for network management purposes within this document. The YANG data modeling language [RFC7950] is used to define the data models for use with NETCONF or RESTCONF. YANG is also used to define the data model in this document.3.1.3. Message Processing Model The following diagram shows the conceptual message flow model, including the points at which access control is applied during NETCONF message processing. RESTCONF operations are mapped to the access control model based on the HTTP method and resource class used in the operation. For example, a POST method on a data resource is considered "write data node" access, but a POST method on an operation resource is considered "operation" access. The new "pre-read data node acc. ctl" boxes in the diagram below refer to group read access as it relates to data node ancestors of an action or notification. As an example, if an action is defined as /interfaces/interface/reset-interface, the group must be authorized to (1) read /interfaces and /interfaces/interface and (2) execute on /interfaces/interface/reset-interface.Bierman & Bjorklund Standards Track [Page 11]
RFC 8341 NACM March 2018 +-------------------------+ | session | | (username) | +-------------------------+ | ^ V | +--------------+ +---------------+ | message | | message | | dispatcher | | generator | +--------------+ +---------------+ | | ^ ^ | V | | | +=============+ | | | | pre-read | | | | | data node | | | | | acc. ctl | | | | +=============+ | | | | | | V V | | +===========+ +-------------+ +----------------+ | operation |---> | reply | | <notification> | | acc. ctl | | generator | | generator | +===========+ +-------------+ +----------------+ | ^ ^ ^ V +------+ | | +-----------+ | +=============+ +================+ | operation | | | read | | <notification> | | processor |-+ | data node | | access ctl | | | | acc. ctl | | | +-----------+ +=============+ +================+ | | ^ ^ ^ V +----------------+ | | | +===========+ | | | +============+ | write | | | | | pre-read | | data node | | | | | data node | | acc. ctl | -----------+ | | | | acc. ctl | +===========+ | | | | +============+ | | | | | ^ V V V | | | +---------------+ +-------------------+ | configuration | ---> | server | | datastore | | instrumentation | | | <--- | | +---------------+ +-------------------+ Figure 2Bierman & Bjorklund Standards Track [Page 12]
RFC 8341 NACM March 2018 The following high-level sequence of conceptual processing steps is executed for each received <rpc> message, if access control enforcement is enabled: o For each active session, access control is applied individually to all <rpc> messages (except <close-session>) received by the server, unless the session is identified as a recovery session. o If the <action> operation defined in [RFC7950] is invoked, then read access is required for all instances in the hierarchy of data nodes that identifies the specific action in the datastore, and execute access is required for the action node. If the user is not authorized to read all the specified data nodes and execute the action, then the request is rejected with an "access-denied" error. o Otherwise, if the user is not authorized to execute the specified protocol operation, then the request is rejected with an "access-denied" error. o If a datastore is accessed by the protocol operation, then the server checks to see if the client is authorized to access the nodes in the datastore. If the user is not authorized to perform the requested access operation on the requested data, then the request is rejected with an "access-denied" error. The following sequence of conceptual processing steps is executed for each generated notification event, if access control enforcement is enabled: o Server instrumentation generates a notification for a particular subscription. o If the "notification" statement is specified within a data subtree, as specified in [RFC7950], then read access is required for all instances in the hierarchy of data nodes that identifies the specific notification in the datastore, and read access is required for the notification node. If the user is not authorized to read all the specified data nodes and the notification node, then the notification is dropped for that subscription. o If the "notification" statement is a top-level statement, the notification access control enforcer checks the notification event type, and if it is one that the user is not authorized to read, then the notification is dropped for that subscription.Bierman & Bjorklund Standards Track [Page 13]
RFC 8341 NACM March 20183.2. Datastore Access The same access control rules apply to all datastores that support the NACM -- for example, the candidate configuration datastore or the running configuration datastore. All conventional configuration datastores and the operational state datastore are controlled by the NACM. Local files, remote files, or datastores accessed via the <url> parameter are not controlled by the NACM.3.2.1. Mapping New Datastores to NACM It is possible that new datastores will be defined over time for use with NETCONF. The NACM MAY be applied to other datastores that have similar access rights as defined in the NACM. To apply the NACM to a new datastore, the new datastore specification needs to define how it maps to the NACM CRUDX (Create, Read, Update, Delete, eXec) access rights. It is possible that only a subset of the NACM access rights would be applicable. For example, only retrieval access control would be needed for a read-only datastore. Operations and access rights not supported by the NACM CRUDX model are outside the scope of this document. A datastore does not need to use the NACM, e.g., the datastore specification defines something else or does not use access control.3.2.2. Access Rights A small set of hard-wired datastore access rights is needed to control access to all possible protocol operations, including vendor extensions to the standard protocol operation set. The CRUDX model can support all protocol operations: o Create: allows the client to add a new data node instance to a datastore. o Read: allows the client to read a data node instance from a datastore or receive the notification event type. o Update: allows the client to update an existing data node instance in a datastore. o Delete: allows the client to delete a data node instance from a datastore. o eXec: allows the client to execute the operation.Bierman & Bjorklund Standards Track [Page 14]
RFC 8341 NACM March 20183.2.3. RESTCONF Methods The RESTCONF protocol utilizes HTTP methods to perform datastore operations, similar to NETCONF. The NACM procedures were originally written for NETCONF protocol operations, so the RESTCONF methods are mapped to NETCONF operations for the purpose of access control processing. The enforcement procedures described within this document apply to both protocols unless explicitly stated otherwise. The request URI needs to be considered when processing RESTCONF requests on data resources: o For HEAD and GET requests, any data nodes that are ancestor nodes of the target resource are considered to be part of the retrieval request for access control purposes. o For PUT, PATCH, and DELETE requests, any data nodes that are ancestor nodes of the target resource are not considered to be part of the edit request for access control purposes. The access operation for these nodes is considered to be "none". The edit begins at the target resource. o For POST requests on data resources, any data nodes that are specified in the request URI, including the target resource, are not considered to be part of the edit request for access control purposes. The access operation for these nodes is considered to be "none". The edit begins at a child node of the target resource, specified in the message body.Bierman & Bjorklund Standards Track [Page 15]
RFC 8341 NACM March 2018 Not all RESTCONF methods are subject to access control. The following table specifies how each method is mapped to NETCONF protocol operations. The value "none" indicates that the NACM is not applied at all to the specific RESTCONF method. +---------+-----------------+---------------------+-----------------+ | Method | Resource class | NETCONF operation | Access | | | | | operation | +---------+-----------------+---------------------+-----------------+ | OPTIONS | all | none | none | | HEAD | all | <get>, <get-config> | read | | GET | all | <get>, <get-config> | read | | POST | datastore, data | <edit-config> | create | | POST | operation | specified operation | execute | | PUT | data | <edit-config> | create, update | | PUT | datastore | <copy-config> | update | | PATCH | data, datastore | <edit-config> | update | | DELETE | data | <edit-config> | delete | +---------+-----------------+---------------------+-----------------+ Table 1: Mapping RESTCONF Methods to NETCONF3.2.4. <get> and <get-config> Operations The NACM access rights are not directly coupled to the <get> and <get-config> protocol operations but apply to all <rpc> operations that would result in a "read" access operation to the target datastore. This section describes how these access rights apply to the specific access operations supported by the <get> and <get-config> protocol operations. Data nodes to which the client does not have read access are silently omitted, along with any descendants, from the <rpc-reply> message. This is done to allow NETCONF filters for <get> and <get-config> to function properly, instead of causing an "access-denied" error because the filter criteria would otherwise include unauthorized read access to some data nodes. For NETCONF filtering purposes, the selection criteria are applied to the subset of nodes that the user is authorized to read, not the entire datastore.3.2.5. <edit-config> Operation The NACM access rights are not directly coupled to the <edit-config> "operation" attribute, although they are similar. Instead, a NACM access right applies to all protocol operations that would result in a particular access operation to the target datastore. This section describes how these access rights apply to the specific access operations supported by the <edit-config> protocol operation.Bierman & Bjorklund Standards Track [Page 16]
RFC 8341 NACM March 2018 If the effective access operation is "none" (i.e., default-operation="none") for a particular data node, then no access control is applied to that data node. This is required to allow access to a subtree within a larger data structure. For example, a user may be authorized to create a new "/interfaces/interface" list entry but not be authorized to create or delete its parent container ("/interfaces"). If the "/interfaces" container already exists in the target datastore, then the effective operation will be "none" for the "/interfaces" node if an "/interfaces/interface" list entry is edited. If the protocol operation would result in the creation of a datastore node and the user does not have "create" access permission for that node, the protocol operation is rejected with an "access-denied" error. If the protocol operation would result in the deletion of a datastore node and the user does not have "delete" access permission for that node, the protocol operation is rejected with an "access-denied" error. If the protocol operation would result in the modification of a datastore node and the user does not have "update" access permission for that node, the protocol operation is rejected with an "access-denied" error. A "merge" or "replace" <edit-config> operation may include data nodes that do not alter portions of the existing datastore. For example, a container or list node may be present for naming purposes but does not actually alter the corresponding datastore node. These unaltered data nodes are ignored by the server and do not require any access rights by the client. A "merge" <edit-config> operation may include data nodes but not include particular child data nodes that are present in the datastore. These missing data nodes within the scope of a "merge" <edit-config> operation are ignored by the server and do not require any access rights by the client. The contents of specific restricted datastore nodes MUST NOT be exposed in any <rpc-error> elements within the reply. An <edit-config> operation may cause data nodes to be implicitly created or deleted as an implicit side effect of a requested operation. For example, a YANG when-stmt expression may evaluate to a different result, causing data nodes to be deleted, or created with default values; or if a data node is created under one branch of a YANG choice-stmt, then all data nodes under the other branches areBierman & Bjorklund Standards Track [Page 17]
RFC 8341 NACM March 2018 implicitly removed. No NACM access rights are required on any data nodes that are implicitly changed as a side effect of another allowed operation.3.2.6. <copy-config> Operation Access control for the <copy-config> protocol operation requires special consideration because the administrator may be replacing the entire target datastore. If the source of the <copy-config> protocol operation is the running configuration datastore and the target is the startup configuration datastore, the client is only required to have permission to execute the <copy-config> protocol operation. Otherwise: o If the source of the <copy-config> operation is a datastore, then data nodes to which the client does not have read access are silently omitted. o If the target of the <copy-config> operation is a datastore, the client needs access to the modified nodes. Specifically: * If the protocol operation would result in the creation of a datastore node and the user does not have "create" access permission for that node, the protocol operation is rejected with an "access-denied" error. * If the protocol operation would result in the deletion of a datastore node and the user does not have "delete" access permission for that node, the protocol operation is rejected with an "access-denied" error. * If the protocol operation would result in the modification of a datastore node and the user does not have "update" access permission for that node, the protocol operation is rejected with an "access-denied" error.3.2.7. <delete-config> Operation Access to the <delete-config> protocol operation is denied by default. The "exec-default" leaf does not apply to this protocol operation. Access control rules must be explicitly configured to allow invocation by a non-recovery session.Bierman & Bjorklund Standards Track [Page 18]
RFC 8341 NACM March 20183.2.8. <commit> Operation The server MUST determine the exact nodes in the running configuration datastore that are actually different and only check "create", "update", and "delete" access permissions for this set of nodes, which could be empty. For example, if a session can read the entire datastore but only change one leaf, that session needs to be able to edit and commit that one leaf.3.2.9. <discard-changes> Operation The client is only required to have permission to execute the <discard-changes> protocol operation. No datastore permissions are needed.3.2.10. <kill-session> Operation The <kill-session> operation does not directly alter a datastore. However, it allows one session to disrupt another session that is editing a datastore. Access to the <kill-session> protocol operation is denied by default. The "exec-default" leaf does not apply to this protocol operation. Access control rules must be explicitly configured to allow invocation by a non-recovery session.3.3. Model Components This section defines the conceptual components related to the access control model.3.3.1. Users A "user" is the conceptual entity that is associated with the access permissions granted to a particular session. A user is identified by a string that is unique within the server. As described in [RFC6241], the username string is derived from the transport layer during session establishment. If the transport layer cannot authenticate the user, the session is terminated.Bierman & Bjorklund Standards Track [Page 19]
RFC 8341 NACM March 20183.3.2. Groups Access to a specific NETCONF protocol operation is granted to a session. The session is associated with a group (i.e., not with a user). A group is identified by its name. All group names are unique within the server. Access control is applied at the level of groups. A group contains zero or more group members. A group member is identified by a username string. The same user can be a member of multiple groups.3.3.3. Emergency Recovery Session The server MAY support a recovery session mechanism, which will bypass all access control enforcement. This is useful for restricting initial access and repairing a broken access control configuration.3.3.4. Global Enforcement Controls There are five global controls that are used to help control how access control is enforced.3.3.4.1. enable-nacm Switch A global "enable-nacm" on/off switch is provided to enable or disable all access control enforcement. When this global switch is set to "true", all requests are checked against the access control rules and only permitted if configured to allow the specific access request. When this global switch is set to "false", all access requests are permitted.3.3.4.2. read-default Switch An on/off "read-default" switch is provided to enable or disable default access to receive data in replies and notifications. When the "enable-nacm" global switch is set to "true", this global switch is relevant if no matching access control rule is found to explicitly permit or deny read access to the requested datastore data or notification event type.Bierman & Bjorklund Standards Track [Page 20]
RFC 8341 NACM March 2018 When this global switch is set to "permit" and no matching access control rule is found for the datastore read or notification event requested, access is permitted. When this global switch is set to "deny" and no matching access control rule is found for the datastore read or notification event requested, access is denied. This means that the requested data is not sent to the client. See step 11 inSection 3.4.5 for details.3.3.4.3. write-default Switch An on/off "write-default" switch is provided to enable or disable default access to alter configuration data. When the "enable-nacm" global switch is set to "true", this global switch is relevant if no matching access control rule is found to explicitly permit or deny write access to the requested datastore data. When this global switch is set to "permit" and no matching access control rule is found for the datastore write requested, access is permitted. When this global switch is set to "deny" and no matching access control rule is found for the datastore write requested, access is denied. See step 12 inSection 3.4.5 for details.3.3.4.4. exec-default Switch An on/off "exec-default" switch is provided to enable or disable default access to execute protocol operations. When the "enable-nacm" global switch is set to "true", this global switch is relevant if no matching access control rule is found to explicitly permit or deny access to the requested NETCONF protocol operation. When this global switch is set to "permit" and no matching access control rule is found for the NETCONF protocol operation requested, access is permitted. When this global switch is set to "deny" and no matching access control rule is found for the NETCONF protocol operation requested, access is denied. See step 12 inSection 3.4.4 and step 13 inSection 3.4.5 for details.Bierman & Bjorklund Standards Track [Page 21]
RFC 8341 NACM March 20183.3.4.5. enable-external-groups Switch When this global switch is set to "true", the group names reported by the transport layer for a session are used together with the locally configured group names to determine the access control rules for the session. When this switch is set to "false", the group names reported by the transport layer are ignored by the NACM.3.3.5. Access Control Rules There are four types of rules available in the NACM: module rule: controls access for definitions in a specific YANG module, identified by its name. protocol operation rule: controls access for a specific protocol operation, identified by its YANG module and name. data node rule: controls access for a specific data node and its descendants, identified by its path location within the conceptual XML document for the data node. notification rule: controls access for a specific notification event type, identified by its YANG module and name.3.4. Access Control Enforcement Procedures There are six separate phases that need to be addressed, four of which are related to the NETCONF message processing model (Section 3.1.3): 1. Initial operation 2. Session establishment 3. "access-denied" error handling 4. Incoming RPC message validation 5. Data node access validation 6. Outgoing <notification> authorization In addition, the initial startup mode for a NETCONF server, session establishment, and "access-denied" error-handling procedures also need to be considered.Bierman & Bjorklund Standards Track [Page 22]
RFC 8341 NACM March 2018 The server MUST use the access control rules in effect at the time it starts processing the message. The same access control rules MUST stay in effect for the processing of the entire message.3.4.1. Initial Operation Upon the very first startup of the NETCONF server, the access control configuration will probably not be present. If it isn't, a server MUST NOT allow any write access to any session role except a recovery session. Access rules are enforced any time a request is initiated from a user session. Access control is not enforced for server-initiated access requests, such as the initial load of the running configuration datastore, during bootup.3.4.2. Session Establishment The access control model applies specifically to the well-formed XML content transferred between a client and a server after session establishment has been completed and after the <hello> exchange has been successfully completed. Once session establishment is completed and a user has been authenticated, the transport layer reports the username and a possibly empty set of group names associated with the user to the NETCONF server. The NETCONF server will enforce the access control rules, based on the supplied username, group names, and the configuration data stored on the server.3.4.3. "access-denied" Error Handling The "access-denied" error-tag is generated when the access control system denies access to either a request to invoke a protocol operation or a request to perform a particular access operation on the configuration datastore. A server MUST NOT include any information the client is not allowed to read in any <error-info> elements within the <rpc-error> response.Bierman & Bjorklund Standards Track [Page 23]
RFC 8341 NACM March 20183.4.4. Incoming RPC Message Validation The diagram below shows the basic conceptual structure of the access control processing model for incoming NETCONF <rpc> messages within a server. NETCONF server +------------+ | XML | | message | | dispatcher | +------------+ | | V +---------------+ | <rpc> message | +---------------+ | | | | | +--------------------------------+ | +---------------+ | V V V +------------------+ +--------------------+ +--------------------+ | vendor operation | | standard operation | | standard operation | | <my-edit> | | <edit-config> | | <unlock> | +------------------+ +--------------------+ +--------------------+ | | | | V V +----------------------+ | configuration | | datastore | +----------------------+ Figure 3 Access control begins with the message dispatcher. After the server validates the <rpc> element and determines the namespace URI and the element name of the protocol operation being requested, the server verifies that the user is authorized to invoke the protocol operation.Bierman & Bjorklund Standards Track [Page 24]
RFC 8341 NACM March 2018 The server MUST separately authorize every protocol operation by following these steps: 1. If the "enable-nacm" leaf is set to "false", then the protocol operation is permitted. 2. If the requesting session is identified as a recovery session, then the protocol operation is permitted. 3. If the requested operation is the NETCONF <close-session> protocol operation, then the protocol operation is permitted. 4. Check all the "group" entries to see if any of them contain a "user-name" entry that equals the username for the session making the request. If the "enable-external-groups" leaf is "true", add to these groups the set of groups provided by the transport layer. 5. If no groups are found, continue with step 10. 6. Process all rule-list entries, in the order they appear in the configuration. If a rule-list's "group" leaf-list does not match any of the user's groups, proceed to the next rule-list entry. 7. For each rule-list entry found, process all rules, in order, until a rule that matches the requested access operation is found. A rule matches if all of the following criteria are met: * The rule's "module-name" leaf is "*" or equals the name of the YANG module where the protocol operation is defined. * Either (1) the rule does not have a "rule-type" defined or (2) the "rule-type" is "protocol-operation" and the "rpc-name" is "*" or equals the name of the requested protocol operation. * The rule's "access-operations" leaf has the "exec" bit set or has the special value "*". 8. If a matching rule is found, then the "action" leaf is checked. If it is equal to "permit", then the protocol operation is permitted; otherwise, it is denied. 9. At this point, no matching rule was found in any rule-list entry.Bierman & Bjorklund Standards Track [Page 25]
RFC 8341 NACM March 2018 10. If the requested protocol operation is defined in a YANG module advertised in the server capabilities and the "rpc" statement contains a "nacm:default-deny-all" statement, then the protocol operation is denied. 11. If the requested protocol operation is the NETCONF <kill-session> or <delete-config>, then the protocol operation is denied. 12. If the "exec-default" leaf is set to "permit", then permit the protocol operation; otherwise, deny the request. If the user is not authorized to invoke the protocol operation, then an <rpc-error> is generated with the following information: error-tag: access-denied error-path: Identifies the requested protocol operation. The following example represents the <edit-config> protocol operation in the NETCONF base namespace: <error-path xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0"> /nc:rpc/nc:edit-config </error-path> If a datastore is accessed, either directly or as a side effect of the protocol operation, then the server MUST intercept the access operation and make sure that the user is authorized to perform the requested access operation on the specified data, as defined inSection 3.4.5.3.4.5. Data Node Access Validation If (1) a data node within a datastore is accessed or (2) an action or notification is tied to a data node, then the server MUST ensure that the user is authorized to perform the requested "read", "create", "update", "delete", or "execute" access operation on the specified data node. If an action is requested to be executed, the server MUST ensure that the user is authorized to perform the "execute" access operation on the requested action. If a notification tied to a data node is generated, the server MUST ensure that the user is authorized to perform the "read" access operation on the requested notification.Bierman & Bjorklund Standards Track [Page 26]
RFC 8341 NACM March 2018 The data node access request is authorized by following these steps: 1. If the "enable-nacm" leaf is set to "false", then the access operation is permitted. 2. If the requesting session is identified as a recovery session, then the access operation is permitted. 3. Check all the "group" entries to see if any of them contain a "user-name" entry that equals the username for the session making the request. If the "enable-external-groups" leaf is "true", add to these groups the set of groups provided by the transport layer. 4. If no groups are found, continue with step 9. 5. Process all rule-list entries, in the order they appear in the configuration. If a rule-list's "group" leaf-list does not match any of the user's groups, proceed to the next rule-list entry. 6. For each rule-list entry found, process all rules, in order, until a rule that matches the requested access operation is found. A rule matches if all of the following criteria are met: * The rule's "module-name" leaf is "*" or equals the name of the YANG module where the requested data node is defined. * Either (1) the rule does not have a "rule-type" defined or (2) the "rule-type" is "data-node" and the "path" matches the requested data node, action node, or notification node. A path is considered to match if the requested node is the node specified by the path or is a descendant node of the path. * For a "read" access operation, the rule's "access-operations" leaf has the "read" bit set or has the special value "*". * For a "create" access operation, the rule's "access-operations" leaf has the "create" bit set or has the special value "*". * For a "delete" access operation, the rule's "access-operations" leaf has the "delete" bit set or has the special value "*".Bierman & Bjorklund Standards Track [Page 27]
RFC 8341 NACM March 2018 * For an "update" access operation, the rule's "access-operations" leaf has the "update" bit set or has the special value "*". * For an "execute" access operation, the rule's "access-operations" leaf has the "exec" bit set or has the special value "*". 7. If a matching rule is found, then the "action" leaf is checked. If it is equal to "permit", then the data node access is permitted; otherwise, it is denied. For a "read" access operation, "denied" means that the requested data is not returned in the reply. 8. At this point, no matching rule was found in any rule-list entry. 9. For a "read" access operation, if the requested data node is defined in a YANG module advertised in the server capabilities and the data definition statement contains a "nacm:default-deny-all" statement, then the requested data node and all its descendants are not included in the reply. 10. For a "write" access operation, if the requested data node is defined in a YANG module advertised in the server capabilities and the data definition statement contains a "nacm:default-deny-write" or a "nacm:default-deny-all" statement, then the access request is denied for the data node and all its descendants. 11. For a "read" access operation, if the "read-default" leaf is set to "permit", then include the requested data node in the reply; otherwise, do not include the requested data node or any of its descendants in the reply. 12. For a "write" access operation, if the "write-default" leaf is set to "permit", then permit the data node access request; otherwise, deny the request. 13. For an "execute" access operation, if the "exec-default" leaf is set to "permit", then permit the request; otherwise, deny the request.Bierman & Bjorklund Standards Track [Page 28]
RFC 8341 NACM March 20183.4.6. Outgoing <notification> Authorization Configuration of access control rules specifically for descendant nodes of the notification event type are outside the scope of this document. If the user is authorized to receive the notification event type, then it is also authorized to receive any data it contains. If the notification is specified within a data subtree, as specified in [RFC7950], then read access to the notification is required. Processing continues as described inSection 3.4.5. The following figure shows the conceptual message processing model for outgoing <notification> messages. NETCONF server +------------+ | XML | | message | | generator | +------------+ ^ | +----------------+ | <notification> | | generator | +----------------+ ^ | +=================+ | <notification> | | access control | | <eventType> | +=================+ ^ | +------------------------+ | server instrumentation | +------------------------+ | ^ V | +----------------------+ | configuration | | datastore | +----------------------+ Figure 4Bierman & Bjorklund Standards Track [Page 29]
RFC 8341 NACM March 2018 The generation of a notification for a specific subscription [RFC5277] is authorized by following these steps: 1. If the "enable-nacm" leaf is set to "false", then the notification is permitted. 2. If the session is identified as a recovery session, then the notification is permitted. 3. If the notification is the NETCONF <replayComplete> or <notificationComplete> event type [RFC5277], then the notification is permitted. 4. Check all the "group" entries to see if any of them contain a "user-name" entry that equals the username for the session making the request. If the "enable-external-groups" leaf is "true", add to these groups the set of groups provided by the transport layer. 5. If no groups are found, continue with step 10. 6. Process all rule-list entries, in the order they appear in the configuration. If a rule-list's "group" leaf-list does not match any of the user's groups, proceed to the next rule-list entry. 7. For each rule-list entry found, process all rules, in order, until a rule that matches the requested access operation is found. A rule matches if all of the following criteria are met: * The rule's "module-name" leaf is "*" or equals the name of the YANG module where the notification is defined. * Either (1) the rule does not have a "rule-type" defined or (2) the "rule-type" is "notification" and the "notification-name" is "*" or equals the name of the notification. * The rule's "access-operations" leaf has the "read" bit set or has the special value "*". 8. If a matching rule is found, then the "action" leaf is checked. If it is equal to "permit", then permit the notification; otherwise, drop the notification for the associated subscription. 9. Otherwise, no matching rule was found in any rule-list entry.Bierman & Bjorklund Standards Track [Page 30]
RFC 8341 NACM March 2018 10. If the requested notification is defined in a YANG module advertised in the server capabilities and the "notification" statement contains a "nacm:default-deny-all" statement, then the notification is dropped for the associated subscription. 11. If the "read-default" leaf is set to "permit", then permit the notification; otherwise, drop the notification for the associated subscription.3.5. Data Model Definitions3.5.1. Data Organization The following diagram highlights the contents and structure of the NACM YANG module. module: ietf-netconf-acm +--rw nacm +--rw enable-nacm? boolean +--rw read-default? action-type +--rw write-default? action-type +--rw exec-default? action-type +--rw enable-external-groups? boolean +--ro denied-operations yang:zero-based-counter32 +--ro denied-data-writes yang:zero-based-counter32 +--ro denied-notifications yang:zero-based-counter32 +--rw groups | +--rw group* [name] | +--rw name group-name-type | +--rw user-name* user-name-type +--rw rule-list* [name] +--rw name string +--rw group* union +--rw rule* [name] +--rw name string +--rw module-name? union +--rw (rule-type)? | +--:(protocol-operation) | | +--rw rpc-name? union | +--:(notification) | | +--rw notification-name? union | +--:(data-node) | +--rw path node-instance-identifier +--rw access-operations? union +--rw action action-type +--rw comment? stringBierman & Bjorklund Standards Track [Page 31]
RFC 8341 NACM March 20183.5.2. YANG Module The following YANG module specifies the normative NETCONF content that MUST be supported by the server. The "ietf-netconf-acm" YANG module imports typedefs from [RFC6991]. <CODE BEGINS> file "ietf-netconf-acm@2018-02-14.yang" module ietf-netconf-acm { namespace "urn:ietf:params:xml:ns:yang:ietf-netconf-acm"; prefix nacm; import ietf-yang-types { prefix yang; } organization "IETF NETCONF (Network Configuration) Working Group"; contact "WG Web: <https://datatracker.ietf.org/wg/netconf/> WG List: <mailto:netconf@ietf.org> Author: Andy Bierman <mailto:andy@yumaworks.com> Author: Martin Bjorklund <mailto:mbj@tail-f.com>"; description "Network Configuration Access Control Model. Copyright (c) 2012 - 2018 IETF Trust and the persons identified as authors of the code. All rights reserved. Redistribution and use in source and binary forms, with or without modification, is permitted pursuant to, and subject to the license terms contained in, the Simplified BSD License set forth inSection 4.c of the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info). This version of this YANG module is part ofRFC 8341; see the RFC itself for full legal notices.";Bierman & Bjorklund Standards Track [Page 32]
RFC 8341 NACM March 2018 revision "2018-02-14" { description "Added support for YANG 1.1 actions and notifications tied to data nodes. Clarified how NACM extensions can be used by other data models."; reference "RFC 8341: Network Configuration Access Control Model"; } revision "2012-02-22" { description "Initial version."; reference "RFC 6536: Network Configuration Protocol (NETCONF) Access Control Model"; } /* * Extension statements */ extension default-deny-write { description "Used to indicate that the data model node represents a sensitive security system parameter. If present, the NETCONF server will only allow the designated 'recovery session' to have write access to the node. An explicit access control rule is required for all other users. If the NACM module is used, then it must be enabled (i.e., /nacm/enable-nacm object equals 'true'), or this extension is ignored. The 'default-deny-write' extension MAY appear within a data definition statement. It is ignored otherwise."; } extension default-deny-all { description "Used to indicate that the data model node controls a very sensitive security system parameter. If present, the NETCONF server will only allow the designated 'recovery session' to have read, write, or execute access to the node. An explicit access control rule is required for all other users.Bierman & Bjorklund Standards Track [Page 33]
RFC 8341 NACM March 2018 If the NACM module is used, then it must be enabled (i.e., /nacm/enable-nacm object equals 'true'), or this extension is ignored. The 'default-deny-all' extension MAY appear within a data definition statement, 'rpc' statement, or 'notification' statement. It is ignored otherwise."; } /* * Derived types */ typedef user-name-type { type string { length "1..max"; } description "General-purpose username string."; } typedef matchall-string-type { type string { pattern '\*'; } description "The string containing a single asterisk '*' is used to conceptually represent all possible values for the particular leaf using this data type."; } typedef access-operations-type { type bits { bit create { description "Any protocol operation that creates a new data node."; } bit read { description "Any protocol operation or notification that returns the value of a data node."; } bit update { description "Any protocol operation that alters an existing data node."; }Bierman & Bjorklund Standards Track [Page 34]
RFC 8341 NACM March 2018 bit delete { description "Any protocol operation that removes a data node."; } bit exec { description "Execution access to the specified protocol operation."; } } description "Access operation."; } typedef group-name-type { type string { length "1..max"; pattern '[^\*].*'; } description "Name of administrative group to which users can be assigned."; } typedef action-type { type enumeration { enum permit { description "Requested action is permitted."; } enum deny { description "Requested action is denied."; } } description "Action taken by the server when a particular rule matches."; } typedef node-instance-identifier { type yang:xpath1.0; description "Path expression used to represent a special data node, action, or notification instance-identifier string. A node-instance-identifier value is an unrestricted YANG instance-identifier expression.Bierman & Bjorklund Standards Track [Page 35]
RFC 8341 NACM March 2018 All the same rules as an instance-identifier apply, except that predicates for keys are optional. If a key predicate is missing, then the node-instance-identifier represents all possible server instances for that key. This XML Path Language (XPath) expression is evaluated in the following context: o The set of namespace declarations are those in scope on the leaf element where this type is used. o The set of variable bindings contains one variable, 'USER', which contains the name of the user of the current session. o The function library is the core function library, but note that due to the syntax restrictions of an instance-identifier, no functions are allowed. o The context node is the root node in the data tree. The accessible tree includes actions and notifications tied to data nodes."; } /* * Data definition statements */ container nacm { nacm:default-deny-all; description "Parameters for NETCONF access control model."; leaf enable-nacm { type boolean; default "true"; description "Enables or disables all NETCONF access control enforcement. If 'true', then enforcement is enabled. If 'false', then enforcement is disabled."; }Bierman & Bjorklund Standards Track [Page 36]
RFC 8341 NACM March 2018 leaf read-default { type action-type; default "permit"; description "Controls whether read access is granted if no appropriate rule is found for a particular read request."; } leaf write-default { type action-type; default "deny"; description "Controls whether create, update, or delete access is granted if no appropriate rule is found for a particular write request."; } leaf exec-default { type action-type; default "permit"; description "Controls whether exec access is granted if no appropriate rule is found for a particular protocol operation request."; } leaf enable-external-groups { type boolean; default "true"; description "Controls whether the server uses the groups reported by the NETCONF transport layer when it assigns the user to a set of NACM groups. If this leaf has the value 'false', any group names reported by the transport layer are ignored by the server."; } leaf denied-operations { type yang:zero-based-counter32; config false; mandatory true; description "Number of times since the server last restarted that a protocol operation request was denied."; }Bierman & Bjorklund Standards Track [Page 37]
RFC 8341 NACM March 2018 leaf denied-data-writes { type yang:zero-based-counter32; config false; mandatory true; description "Number of times since the server last restarted that a protocol operation request to alter a configuration datastore was denied."; } leaf denied-notifications { type yang:zero-based-counter32; config false; mandatory true; description "Number of times since the server last restarted that a notification was dropped for a subscription because access to the event type was denied."; } container groups { description "NETCONF access control groups."; list group { key name; description "One NACM group entry. This list will only contain configured entries, not any entries learned from any transport protocols."; leaf name { type group-name-type; description "Group name associated with this entry."; } leaf-list user-name { type user-name-type; description "Each entry identifies the username of a member of the group associated with this entry."; } } }Bierman & Bjorklund Standards Track [Page 38]
RFC 8341 NACM March 2018 list rule-list { key name; ordered-by user; description "An ordered collection of access control rules."; leaf name { type string { length "1..max"; } description "Arbitrary name assigned to the rule-list."; } leaf-list group { type union { type matchall-string-type; type group-name-type; } description "List of administrative groups that will be assigned the associated access rights defined by the 'rule' list. The string '*' indicates that all groups apply to the entry."; } list rule { key name; ordered-by user; description "One access control rule. Rules are processed in user-defined order until a match is found. A rule matches if 'module-name', 'rule-type', and 'access-operations' match the request. If a rule matches, the 'action' leaf determines whether or not access is granted."; leaf name { type string { length "1..max"; } description "Arbitrary name assigned to the rule."; }Bierman & Bjorklund Standards Track [Page 39]
RFC 8341 NACM March 2018 leaf module-name { type union { type matchall-string-type; type string; } default "*"; description "Name of the module associated with this rule. This leaf matches if it has the value '*' or if the object being accessed is defined in the module with the specified module name."; } choice rule-type { description "This choice matches if all leafs present in the rule match the request. If no leafs are present, the choice matches all requests."; case protocol-operation { leaf rpc-name { type union { type matchall-string-type; type string; } description "This leaf matches if it has the value '*' or if its value equals the requested protocol operation name."; } } case notification { leaf notification-name { type union { type matchall-string-type; type string; } description "This leaf matches if it has the value '*' or if its value equals the requested notification name."; } }Bierman & Bjorklund Standards Track [Page 40]
RFC 8341 NACM March 2018 case data-node { leaf path { type node-instance-identifier; mandatory true; description "Data node instance-identifier associated with the data node, action, or notification controlled by this rule. Configuration data or state data instance-identifiers start with a top-level data node. A complete instance-identifier is required for this type of path value. The special value '/' refers to all possible datastore contents."; } } } leaf access-operations { type union { type matchall-string-type; type access-operations-type; } default "*"; description "Access operations associated with this rule. This leaf matches if it has the value '*' or if the bit corresponding to the requested operation is set."; } leaf action { type action-type; mandatory true; description "The access control action associated with the rule. If a rule has been determined to match a particular request, then this object is used to determine whether to permit or deny the request."; }Bierman & Bjorklund Standards Track [Page 41]
RFC 8341 NACM March 2018 leaf comment { type string; description "A textual description of the access rule."; } } } } } <CODE ENDS>4. IANA Considerations This document reuses the URI for "ietf-netconf-acm" in the "IETF XML Registry". This document updates the module registration in the "YANG Module Names" registry to reference this RFC instead ofRFC 6536 for "ietf-netconf-acm". Following the format in [RFC6020], the following has been registered. Name: ietf-netconf-acm Namespace: urn:ietf:params:xml:ns:yang:ietf-netconf-acm Prefix: nacm Reference:RFC 83415. Security Considerations The YANG module specified in this document defines a schema for data that is designed to be accessed via network management protocols such as NETCONF [RFC6241] or RESTCONF [RFC8040]. The lowest NETCONF layer is the secure transport layer, and the mandatory-to-implement secure transport is Secure Shell (SSH) [RFC6242]. The lowest RESTCONF layer is HTTPS, and the mandatory-to-implement secure transport is TLS [RFC5246]. The NETCONF access control model [RFC8341] provides the means to restrict access for particular NETCONF or RESTCONF users to a preconfigured subset of all available NETCONF or RESTCONF protocol operations and content. There is a risk related to the lack of access control enforcement for the RESTCONF OPTIONS and PATCH methods. The risk here is that the response to OPTIONS and PATCH may vary based on the presence or absence of a resource corresponding to the URL's path. If this is the case, then it can be used to trivially probe for the presence or absence of values within a tree. Therefore, a server MUST NOT varyBierman & Bjorklund Standards Track [Page 42]
RFC 8341 NACM March 2018 its responses based on the existence of the underlying resource, which would indicate the presence or absence of resource instances. In particular, servers should not expose any instance information before ensuring that the client has the necessary access permissions to obtain that information. In such cases, servers are expected to always return the "access-denied" error response. There are a number of data nodes defined in this YANG module that are writable/creatable/deletable (i.e., config true, which is the default). These data nodes may be considered sensitive or vulnerable in some network environments. Write operations (e.g., edit-config) to these data nodes without proper protection can have a negative effect on network operations. These are the subtrees and data nodes and their sensitivity/vulnerability: o /nacm: The entire /nacm subtree is related to security. Refer to the following sections for more details. This section highlights the issues for an administrator to consider when configuring a NETCONF server with the NACM.5.1. NACM Configuration and Monitoring Considerations Configuration of the access control system is highly sensitive to system security. A server may choose not to allow any user configuration to some portions of it, such as the global security level or the groups that allowed access to system resources. By default, NACM enforcement is enabled. By default, "read" access to all datastore contents is enabled (unless "nacm:default-deny-all" is specified for the data definition), and "exec" access is enabled for safe protocol operations. An administrator needs to ensure that the NACM is enabled and also decide if the default access parameters are set appropriately. Make sure that the following data nodes are properly configured: o /nacm/enable-nacm (default "true") o /nacm/read-default (default "permit") o /nacm/write-default (default "deny") o /nacm/exec-default (default "permit") An administrator needs to restrict write access to all configurable objects within this data model.Bierman & Bjorklund Standards Track [Page 43]
RFC 8341 NACM March 2018 If write access is allowed for configuration of access control rules, then care needs to be taken not to disrupt the access control enforcement. For example, if the NACM access control rules are edited directly within the running configuration datastore (i.e., :writable-running capability is supported and used), then care needs to be taken not to allow unintended access while the edits are being done. An administrator needs to make sure that the translation from a transport- or implementation-dependent user identity to a NACM username is unique and correct. This requirement is specified in detail inSection 2.2 of [RFC6241]. An administrator needs to be aware that the YANG data structures representing access control rules (/nacm/rule-list and /nacm/rule-list/rule) are ordered by the client. The server will evaluate the access control rules according to their relative conceptual order within the running configuration datastore. Note that the /nacm/groups data structure contains the administrative group names used by the server. These group names may be configured locally and/or provided through an external protocol, such as RADIUS [RFC2865] [RFC5607]. An administrator needs to be aware of the security properties of any external protocol used by the transport layer to determine group names. For example, if this protocol does not protect against man-in-the-middle attacks, an attacker might be able to inject group names that are configured in the NACM so that a user gets more permissions than it should. In such cases, the administrator may wish to disable the usage of such group names by setting /nacm/enable-external-groups to "false". Some of the readable data nodes in this YANG module may be considered sensitive or vulnerable in some network environments. It is thus important to control read access (e.g., via get, get-config, or notification) to these data nodes. These are the subtrees and data nodes and their sensitivity/vulnerability: o /nacm/enable-nacm o /nacm/read-default o /nacm/write-default o /nacm/exec-default o /nacm/enable-external-groupsBierman & Bjorklund Standards Track [Page 44]
RFC 8341 NACM March 2018 o /nacm/groups o /nacm/rule-list An administrator needs to restrict read access to the above-listed objects within this data model, as they reveal access control configuration that could be considered sensitive.5.2. General Configuration Issues There is a risk that invocation of non-standard protocol operations will have undocumented side effects. An administrator needs to construct access control rules such that the configuration datastore is protected from such side effects. It is possible for a session with some write access (e.g., allowed to invoke <edit-config>), but without any access to a particular datastore subtree containing sensitive data, to determine the presence or non-presence of that data. This can be done by repeatedly issuing some sort of edit request (create, update, or delete) and possibly receiving "access-denied" errors in response. These "fishing" attacks can identify the presence or non-presence of specific sensitive data even without the "error-path" field being present within the <rpc-error> response. It may be possible for the set of NETCONF capabilities on the server to change over time. If so, then there is a risk that new protocol operations, notifications, and/or datastore content have been added to the device. An administrator needs to be sure that the access control rules are correct for the new content in this case. Mechanisms to detect NETCONF capability changes on a specific device are outside the scope of this document. It is possible that the data model definition itself (e.g., a YANG when-stmt) will help an unauthorized session determine the presence or even value of sensitive data nodes by examining the presence and values of different data nodes. It is possible that the data model definition itself (e.g., a YANG when-stmt or choice-stmt) will allow a session to implicitly create or delete nodes that the session does not have write access to as an implicit side effect from the processing of an allowed <edit-config> operation.Bierman & Bjorklund Standards Track [Page 45]
RFC 8341 NACM March 2018 There is a risk that non-standard protocol operations, or even the standard <get> protocol operation, may return data that "aliases" or "copies" sensitive data from a different data object. There may simply be multiple data model definitions that expose or even configure the same underlying system instrumentation. A data model may contain external keys (e.g., YANG leafref), which expose values from a different data structure. An administrator needs to be aware of sensitive data models that contain leafref nodes. This entails finding all the leafref objects that "point" at the sensitive data (i.e., "path-stmt" values) that implicitly or explicitly includes the sensitive data node. It is beyond the scope of this document to define access control enforcement procedures for underlying device instrumentation that may exist to support the NETCONF server operation. An administrator can identify each protocol operation that the server provides and decide if it needs any access control applied to it. This document incorporates the optional use of a recovery session mechanism, which can be used to bypass access control enforcement in emergencies such as NACM configuration errors that disable all access to the server. The configuration and identification of such a recovery session mechanism are implementation specific and are outside the scope of this document. An administrator needs to be aware of any recovery session mechanisms available on the device and make sure that they are used appropriately. It is possible for a session to disrupt configuration management, even without any write access to the configuration, by locking the datastore. This may be done to ensure that all or part of the configuration remains stable while it is being retrieved, or it may be done as a "denial-of-service" attack. There is no way for the server to know the difference. An administrator may wish to restrict "exec" access to the following protocol operations: o <lock> o <unlock> o <partial-lock> o <partial-unlock>Bierman & Bjorklund Standards Track [Page 46]
RFC 8341 NACM March 20185.3. Data Model Design Considerations Designers need to clearly identify any sensitive data, notifications, or protocol operations defined within a YANG module. For such definitions, a "nacm:default-deny-write" or "nacm:default-deny-all" statement ought to be present, in addition to a clear description of the security risks. Protocol operations need to be properly documented by the data model designer so that it is clear to administrators what data nodes (if any) are affected by the protocol operation and what information (if any) is returned in the <rpc-reply> message. Data models ought to be designed so that different access levels for input parameters to protocol operations are not required. The use of generic protocol operations should be avoided, and if different access levels are needed, separate protocol operations should be defined instead.6. References6.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels",BCP 14,RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2",RFC 5246, DOI 10.17487/RFC5246, August 2008, <https://www.rfc-editor.org/info/rfc5246>. [RFC5277] Chisholm, S. and H. Trevino, "NETCONF Event Notifications",RFC 5277, DOI 10.17487/RFC5277, July 2008, <https://www.rfc-editor.org/info/rfc5277>. [RFC6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)",RFC 6020, DOI 10.17487/RFC6020, October 2010, <https://www.rfc-editor.org/info/rfc6020>. [RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed., and A. Bierman, Ed., "Network Configuration Protocol (NETCONF)",RFC 6241, DOI 10.17487/RFC6241, June 2011, <https://www.rfc-editor.org/info/rfc6241>.Bierman & Bjorklund Standards Track [Page 47]
RFC 8341 NACM March 2018 [RFC6242] Wasserman, M., "Using the NETCONF Protocol over Secure Shell (SSH)",RFC 6242, DOI 10.17487/RFC6242, June 2011, <https://www.rfc-editor.org/info/rfc6242>. [RFC6991] Schoenwaelder, J., Ed., "Common YANG Data Types",RFC 6991, DOI 10.17487/RFC6991, July 2013, <https://www.rfc-editor.org/info/rfc6991>. [RFC7230] Fielding, R., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing",RFC 7230, DOI 10.17487/RFC7230, June 2014, <https://www.rfc-editor.org/info/rfc7230>. [RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",RFC 7950, DOI 10.17487/RFC7950, August 2016, <https://www.rfc-editor.org/info/rfc7950>. [RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF Protocol",RFC 8040, DOI 10.17487/RFC8040, January 2017, <https://www.rfc-editor.org/info/rfc8040>. [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase inRFC 2119 Key Words",BCP 14,RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>. [RFC8342] Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K., and R. Wilton, "Network Management Datastore Architecture (NMDA)",RFC 8342, DOI 10.17487/RFC8342, March 2018, <https://www.rfc-editor.org/info/rfc8342>. [W3C.REC-xml-20081126] Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth Edition)", World Wide Web Consortium Recommendation REC-xml-20081126, November 2008, <https://www.w3.org/TR/2008/REC-xml-20081126>.Bierman & Bjorklund Standards Track [Page 48]
RFC 8341 NACM March 20186.2. Informative References [RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson, "Remote Authentication Dial In User Service (RADIUS)",RFC 2865, DOI 10.17487/RFC2865, June 2000, <https://www.rfc-editor.org/info/rfc2865>. [RFC5607] Nelson, D. and G. Weber, "Remote Authentication Dial-In User Service (RADIUS) Authorization for Network Access Server (NAS) Management",RFC 5607, DOI 10.17487/RFC5607, July 2009, <https://www.rfc-editor.org/info/rfc5607>. [YANG-SEC] IETF, "YANG Security Guidelines", <https://trac.ietf.org/trac/ops/wiki/yang-security-guidelines>.Bierman & Bjorklund Standards Track [Page 49]
RFC 8341 NACM March 2018Appendix A. Usage Examples The following XML [W3C.REC-xml-20081126] snippets are provided as examples only, to demonstrate how the NACM can be configured to perform some access control tasks.A.1. <groups> Example There needs to be at least one <group> entry in order for any of the access control rules to be useful. The following XML shows arbitrary groups and is not intended to represent any particular use case. <nacm xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-acm"> <groups> <group> <name>admin</name> <user-name>admin</user-name> <user-name>andy</user-name> </group> <group> <name>limited</name> <user-name>wilma</user-name> <user-name>bam-bam</user-name> </group> <group> <name>guest</name> <user-name>guest</user-name> <user-name>guest@example.com</user-name> </group> </groups> </nacm> This example shows three groups: admin: The "admin" group contains two users named "admin" and "andy". limited: The "limited" group contains two users named "wilma" and "bam-bam". guest: The "guest" group contains two users named "guest" and "guest@example.com".Bierman & Bjorklund Standards Track [Page 50]
RFC 8341 NACM March 2018A.2. Module Rule Example Module rules are used to control access to all the content defined in a specific module. A module rule has the "module-name" leaf set but no nodes from the "rule-type" choice set. <nacm xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-acm"> <rule-list> <name>guest-acl</name> <group>guest</group> <rule> <name>deny-ncm</name> <module-name>ietf-netconf-monitoring</module-name> <access-operations>*</access-operations> <action>deny</action> <comment> Do not allow guests any access to the NETCONF monitoring information. </comment> </rule> </rule-list> <rule-list> <name>limited-acl</name> <group>limited</group> <rule> <name>permit-ncm</name> <module-name>ietf-netconf-monitoring</module-name> <access-operations>read</access-operations> <action>permit</action> <comment> Allow read access to the NETCONF monitoring information. </comment> </rule> <rule> <name>permit-exec</name> <module-name>*</module-name> <access-operations>exec</access-operations> <action>permit</action> <comment> Allow invocation of the supported server operations. </comment> </rule> </rule-list>Bierman & Bjorklund Standards Track [Page 51]
RFC 8341 NACM March 2018 <rule-list> <name>admin-acl</name> <group>admin</group> <rule> <name>permit-all</name> <module-name>*</module-name> <access-operations>*</access-operations> <action>permit</action> <comment> Allow the 'admin' group complete access to all operations and data. </comment> </rule> </rule-list> </nacm> This example shows four module rules: deny-ncm: This rule prevents the "guest" group from reading any monitoring information in the "ietf-netconf-monitoring" YANG module. permit-ncm: This rule allows the "limited" group to read the "ietf-netconf-monitoring" YANG module. permit-exec: This rule allows the "limited" group to invoke any protocol operation supported by the server. permit-all: This rule allows the "admin" group complete access to all content in the server. No subsequent rule will match for the "admin" group because of this module rule.Bierman & Bjorklund Standards Track [Page 52]
RFC 8341 NACM March 2018A.3. Protocol Operation Rule Example Protocol operation rules are used to control access to a specific protocol operation. <nacm xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-acm"> <rule-list> <name>guest-limited-acl</name> <group>limited</group> <group>guest</group> <rule> <name>deny-kill-session</name> <module-name>ietf-netconf</module-name> <rpc-name>kill-session</rpc-name> <access-operations>exec</access-operations> <action>deny</action> <comment> Do not allow the 'limited' group or the 'guest' group to kill another session. </comment> </rule> <rule> <name>deny-delete-config</name> <module-name>ietf-netconf</module-name> <rpc-name>delete-config</rpc-name> <access-operations>exec</access-operations> <action>deny</action> <comment> Do not allow the 'limited' group or the 'guest' group to delete any configurations. </comment> </rule> </rule-list>Bierman & Bjorklund Standards Track [Page 53]
RFC 8341 NACM March 2018 <rule-list> <name>limited-acl</name> <group>limited</group> <rule> <name>permit-edit-config</name> <module-name>ietf-netconf</module-name> <rpc-name>edit-config</rpc-name> <access-operations>exec</access-operations> <action>permit</action> <comment> Allow the 'limited' group to edit the configuration. </comment> </rule> </rule-list> </nacm> This example shows three protocol operation rules: deny-kill-session: This rule prevents the "limited" group or the "guest" group from invoking the NETCONF <kill-session> protocol operation. deny-delete-config: This rule prevents the "limited" group or the "guest" group from invoking the NETCONF <delete-config> protocol operation. permit-edit-config: This rule allows the "limited" group to invoke the NETCONF <edit-config> protocol operation. This rule will have no real effect unless the "exec-default" leaf is set to "deny".Bierman & Bjorklund Standards Track [Page 54]
RFC 8341 NACM March 2018A.4. Data Node Rule Example Data node rules are used to control access to specific (config and non-config) data nodes within the NETCONF content provided by the server. <nacm xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-acm"> <rule-list> <name>guest-acl</name> <group>guest</group> <rule> <name>deny-nacm</name> <path xmlns:n="urn:ietf:params:xml:ns:yang:ietf-netconf-acm"> /n:nacm </path> <access-operations>*</access-operations> <action>deny</action> <comment> Deny the 'guest' group any access to the /nacm data. </comment> </rule> </rule-list> <rule-list> <name>limited-acl</name> <group>limited</group> <rule> <name>permit-acme-config</name> <path xmlns:acme="http://example.com/ns/netconf"> /acme:acme-netconf/acme:config-parameters </path> <access-operations> read create update delete </access-operations> <action>permit</action> <comment> Allow the 'limited' group complete access to the acme NETCONF configuration parameters. Showing long form of 'access-operations' instead of shorthand. </comment> </rule> </rule-list>Bierman & Bjorklund Standards Track [Page 55]
RFC 8341 NACM March 2018 <rule-list> <name>guest-limited-acl</name> <group>guest</group> <group>limited</group> <rule> <name>permit-dummy-interface</name> <path xmlns:acme="http://example.com/ns/itf"> /acme:interfaces/acme:interface[acme:name='dummy'] </path> <access-operations>read update</access-operations> <action>permit</action> <comment> Allow the 'limited' and 'guest' groups read and update access to the dummy interface. </comment> </rule> </rule-list> <rule-list> <name>admin-acl</name> <group>admin</group> <rule> <name>permit-interface</name> <path xmlns:acme="http://example.com/ns/itf"> /acme:interfaces/acme:interface </path> <access-operations>*</access-operations> <action>permit</action> <comment> Allow the 'admin' group full access to all acme interfaces. </comment> </rule> </rule-list> </nacm>Bierman & Bjorklund Standards Track [Page 56]
RFC 8341 NACM March 2018 This example shows four data node rules: deny-nacm: This rule denies the "guest" group any access to the /nacm subtree. permit-acme-config: This rule gives the "limited" group read-write access to the acme <config-parameters>. permit-dummy-interface: This rule gives the "limited" and "guest" groups read-update access to the acme <interface> entry named "dummy". This entry cannot be created or deleted by these groups; it can only be altered. permit-interface: This rule gives the "admin" group read-write access to all acme <interface> entries.A.5. Notification Rule Example Notification rules are used to control access to a specific notification event type. <nacm xmlns="urn:ietf:params:xml:ns:yang:ietf-netconf-acm"> <rule-list> <name>sys-acl</name> <group>limited</group> <group>guest</group> <rule> <name>deny-config-change</name> <module-name>acme-system</module-name> <notification-name>sys-config-change</notification-name> <access-operations>read</access-operations> <action>deny</action> <comment> Do not allow the 'guest' group or the 'limited' group to receive config change events. </comment> </rule> </rule-list> </nacm> This example shows one notification rule: deny-config-change: This rule prevents the "limited" group or the "guest" group from receiving the acme <sys-config-change> event type.Bierman & Bjorklund Standards Track [Page 57]
RFC 8341 NACM March 2018Authors' Addresses Andy Bierman YumaWorks 685 Cochran St. Suite #160 Simi Valley, CA 93065 United States of America Email: andy@yumaworks.com Martin Bjorklund Tail-f Systems Email: mbj@tail-f.comBierman & Bjorklund Standards Track [Page 58]
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