Network Configuration Protocol Access Control Model

The standardization of network configuration interfaces for use with the NETCONF protocol requires a structured and secure operating environment, which promotes human usability and multi-vendor interoperability. There is a need for standard mechanisms to restrict NETCONF protocol access for particular users to a pre-configured subset of all available NETCONF operations and content. This document discusses requirements for a suitable access control model, and provides one solution which meets these requirements.

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1. Introduction
    1.1. Terminology
        1.1.1. Requirements Notation
        1.1.2. NETCONF Terms
        1.1.3. NACM Terms
2. Authentication Requirements
3. Access Control Requirements
    3.1. Protocol Control Points
    3.2. Simplicity
    3.3. Procedural Interface
    3.4. Database Access
        3.4.1. Access Rights
        3.4.2. <get> and <get-config> Operations
        3.4.3. <edit-config> Operation
        3.4.4. <copy-config> Operation
    3.5. Users and Groups
    3.6. Maintenance
    3.7. Configuration Capabilities
    3.8. Identifying Security Holes
    3.9. Data Shadowing
    3.10. NETCONF Specific Requirements
4. NETCONF Authentication and Authorization Model
    4.1. SSH Public Key Authentication
    4.2. Local User Password Authentication
    4.3. RADIUS Password Authentication and Service Authorization
        4.3.1. Operation
5. NETCONF Access Control Model (NACM)
    5.1. Introduction
        5.1.1. Features
        5.1.2. External Dependencies
        5.1.3. Message Processing Model
    5.2. Model Components
        5.2.1. Users
        5.2.2. Groups
        5.2.3. Sessions
        5.2.4. Access Permissions
        5.2.5. Global Enforcement Controls
        5.2.6. Access Control Rules
    5.3. Access Control Enforcement Procedures
        5.3.1. Initial Operation
        5.3.2. Session Establishment
        5.3.3. 'access-denied' Error Handling
        5.3.4. Incoming RPC Message Validation
        5.3.5. Data Node Access Validation
        5.3.6. Outgoing <rpc-reply> Authorization
        5.3.7. Outgoing <notification> Authorization
    5.4. Data Model Definitions
        5.4.1. High Level Procedures
        5.4.2. Data Organization
        5.4.3. YANG Module
    5.5. IANA Considerations
    5.6. Security Considerations
6. Normative References
Appendix A. Usage Examples
    A.1. <groups> Example
    A.2. <module-rule> Example
    A.3. <rpc-rule> Example
    A.4. <data-rule> Example
    A.5. <notification-rule> Example
Appendix B. Open Issues
Appendix C. Change Log
    C.1. 00-01
    C.2. 00
§ Authors' Addresses

1. Introduction

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

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

1.1. Terminology

1.1.1. Requirements Notation

1.1.2. NETCONF Terms

1.1.3. NACM Terms

2. Authentication Requirements

The authentication mechanism must support password authentication over RADIUS, to support deployment scenarios with centralized authentication servers. Additionally, local users must be supported, for scenarios when no centralized authentication server exists, or for situations where the centralized authentication server cannot be reached from the device.

The model for authentication configuration should be flexible enough to support authentication methods defined by other standard documents or by vendors.

3. Access Control Requirements

3.1. Protocol Control Points

The NETCONF protocol allows new operations to be added at any time, and the YANG data modeling language supports this feature. It is not possible to design an ACM for NETCONF which only focuses on a static set of operations, like some other protocols. Since few assumptions can be made about an arbitrary protocol operation, the NETCONF architectural server components must be protected at several conceptual control points.

3.2. Simplicity

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

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

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

Protocol access should be defined with a small and familiar set of permissions, while still allowing full control of NETCONF database access.

3.3. Procedural Interface

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

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

YANG modules should be designed so that different access levels for input parameters to RPC operations is not required.

3.4. Database Access

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

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

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

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

The non-volatile startup configuration needs to be loaded into the running configuration without applying any access control rules.

Only a privileged user should be able to alter the factory-default access control rules.

3.4.1. Access Rights

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

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

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

3.4.3. <edit-config> Operation

The NACM access rights are not directly coupled to the NETCONF operation attribute, although they are similar. Instead, a NACM access right applies to all operations which would result in a particular access operation to the target database. This section describes how these access rights apply to the specific database operations supported by the <edit-config> operation.

If the effective operation is 'none' (i.e., default-operation='none') for a particular data node, then no access control is applied to that data node.

A 'create', 'merge', or 'replace' operation on a database node which would result in the creation of a new data node instance, for which the user does not have 'create' access permission, is rejected with an 'access-denied' error.

A 'merge' or 'replace' operation on a database node which would result in the modification of an existing data node instance, for which the user does not have 'update' access permission, is rejected with an 'access-denied' error.

A 'replace' or 'delete' operation on a database node which would result in the deletion of an existing data node instance, for which the user does not have 'delete' access permission, is rejected with an 'access-denied' error.

A 'merge' operation may include data nodes which do not alter portions of the existing database. For example, a container or list nodes may be present for naming purposes, which do not actually alter the corresponding database node. These unaltered data nodes within the scope of a 'merge' operation are ignored by the server, and do not require any access rights by the client.

A 'merge' operation may include data nodes, but not include particular child data nodes that are present in the database. These missing data nodes within the scope of a 'merge' operation are ignored by the server, and do not require any access rights by the client.

The contents of specific restricted database nodes must not be exposed in any <rpc-error> elements within the reply.

3.4.4. <copy-config> Operation

Access control for the <copy-config> operation requires special consideration because the operator is replacing the entire target database. Write access to the entire database is needed for this operation to succeed.

A client must have access to every database node, even ones that are not present in the source configuration data.

For example, consider a common use-case such as a simple backup and restore procedure. The operator must have full read access to the database in order to receive a complete copy of its contents. If not, the server will simply omit these sub-trees from the reply. If that copy is later used to restore the server database, the server will interpret the missing nodes as a request to delete those nodes, and return an error.

3.5. Users and Groups

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

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

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

The ACM must support the concept of administrative groups, to support the well-established distinction between a root account and other types of less-privileged conceptual user accounts. These groups must be configurable by the operator.

3.6. Maintenance

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

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

3.7. Configuration Capabilities

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

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

3.8. Identifying Security Holes

One of the most important aspects of the data model documentation, and biggest concerns during deployment, is the identification of security-sensitive content. This applies to operations in NETCONF, not just data and notifications.

It is customary for security-sensitive objects to be documented in the Security Considerations section of an RFC. This is nice, but it is not good enough, for the following reasons:

Often, the operator just wants to disable default access to the secure content, so no inadvertent or malicious changes can be made to the server. This allows the default rules to be more lenient, without significantly increasing the security risk.

A data model designer should be able to use machine-readable statements to identity NETCONF content which should be protected by default. This will allow client and server tools to automatically close data-model specific security holes, by denying access to sensitive data unless the user is explicitly authorized to perform the requested operation.

3.9. Data Shadowing

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

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

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

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

A NETCONF server must insure than unauthorized access to its conceptual databases and non-configuration data nodes is prevented.

It is beyond the scope of this document to define access control enforcement procedures for underlying device instrumentation that may exist to support the NETCONF server operation. An operator must identify each operation that the server provides, and decide if it needs any access control applied to it.

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

3.10. NETCONF Specific Requirements

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

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

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

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

4. NETCONF Authentication and Authorization Model

Conceptually, the NETCONF transport subsystem authenticates the user, and passes the name of the authenticated user to the NETCONF server. The NETCONF server authorizes the user by mapping it to one or more groups. Access to specific operations and content is then controlled by access control rules as described in Section 5 (NETCONF Access Control Model (NACM)).

Some protocols, such as RADIUS, performs both authentication and authorization, and has a mechanism to report authorization attributes to the client. These attributes are made available to the NETCONF server in an implementation specific manner.

This document defines two optional YANG features, 'local-users' and 'radius', which the server advertises to indicate support for configuring local users on the device, and for configuring RADIUS access, respectively.

4.1. SSH Public Key Authentication

If the NETCONF server advertises the 'local-users' feature, configuration of local users and their SSH public keys is supported in the /nacm/authentication/user list.

4.2. Local User Password Authentication

If the NETCONF server advertises the 'local-users' feature, configuration of local users and their passwords is supported in the /nacm/authentication/user list.

For NETCONF transport protocols that support password authentication, the leaf-list 'user-authentication-order' is used to control if local user password authentication should be used.

In SSH, password authentication is requested by the client. Other NETCONF transport protocols may also support password authentication.

When local user password authentication is requested, the NETCONF transport looks up the user name provided by the client in the /nacm/authentication/user list, and verifies the password.

4.3. RADIUS Password Authentication and Service Authorization

If the NETCONF server advertises the 'radius' feature, it supports user authentication and service authorization with RADIUS, as described in this section.

For NETCONF transport protocols that support password authentication, the leaf-list 'user-authentication-order' is used to control if RADIUS password authentication should be used.

In SSH, password authentication is requested by the client. Other NETCONF transport protocols may also support password authentication.

4.3.1. Operation

[Editor's Note: I prefer to keep this section short, and just refer to the relevant rfcs which have detailed information on radius usage, instead of duplicating this info here...]

As described in RFC 5607, if an Access-Accept message is received which does not authorize the requested service, access MUST be denied.

If any Management-Policy-Id attributes are present in the Access-Accept message, they are treated as group names in the access control procedure, as described in Section 5 (NETCONF Access Control Model (NACM)).

5. NETCONF Access Control Model (NACM)

5.1. 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.

5.1.1. Features

5.1.2. External Dependencies

5.1.3. Message Processing Model

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

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

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

5.2. Model Components

5.2.1. Users

A 'user' is the conceptual identity, which is associated with the access permissions granted to a particular session. A user is identified by a string which must be unique within the server.

The user name string is usually derived from the transport layer during session establishment. A server is required to have an authenticated user name for a session before <rpc> requests will be accepted. Otherwise all write requests must be rejected with an 'access-denied' error-tag value. If a read operation is not authorized, then the requested data is silently dropped from the reply.

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

5.2.2. Groups

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

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

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

5.2.3. Sessions

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

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

5.2.4. Access Permissions

The access permissions are the NETCONF protocol specific set of permissions that have been assigned to a particular session role or group.

The same access permissions MUST stay in effect for the processing of a particular message.

The server MUST use the access control rules in effect at the time the message is processed.

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

5.2.5. Global Enforcement Controls

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

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

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

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

5.2.6. Access Control Rules

5.3. Access Control Enforcement Procedures

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

5.3.1. Initial Operation

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

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

5.3.2. Session Establishment

The access control model applies specifically to the well-formed XML content transferred between a client and a server, after session establishment has been completed, and after the <hello> exchange has been successfully completed.

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

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

5.3.3. 'access-denied' Error Handling

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

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

5.3.4. Incoming RPC Message Validation

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

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

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

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

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

5.3.5. Data Node Access Validation

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

5.3.6. Outgoing <rpc-reply> Authorization

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

For RPC operations which do not access any data nodes, then any client authorized to invoke the RPC operation is also authorized to receive the <rpc-reply> for that RPC operation.

5.3.7. Outgoing <notification> Authorization

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

Configuration of access control rules specifically for descendent nodes of the notification event type element are outside the scope of this document. If the session is authorized to receive the notification event type, then it is also authorized to receive any data it contains.

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

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

5.4. Data Model Definitions

5.4.1. High Level Procedures

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

5.4.2. Data Organization

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

5.4.3. YANG Module

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

// RFC Ed.: please update the date to the date of publication
<CODE BEGINS> file="ietf-nacm@2010-10-25.yang"

module ietf-nacm {

  namespace "urn:ietf:params:xml:ns:yang:ietf-nacm";

  prefix "nacm";

  import ietf-yang-types {
    prefix yang;
  }

  import ietf-inet-types {
    prefix inet;
  }

  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.bierman@brocade.com>

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

  description
    "NETCONF Server Access Control Model.

     Copyright (c) 2010 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 XXX; see
     the RFC itself for full legal notices.";
  // RFC Ed.: replace XXXX with actual RFC number and remove this note

  //reference "RFC XXXX";

  // RFC Ed.: remove this note
  // Note: extracted from draft-ietf-netconf-access-control-01.txt

  // RFC Ed.: please update the date to the date of publication
  revision "2010-10-25" {
    description
      "Initial version";
    reference
      "RFC XXXX: Network Configuration Protocol Access Control Model";
  }

  /*
   * Extension statements
   */

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

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

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

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

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

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

  /*
   * Features
   */

  feature authentication {
    description
      "Indicates that the NETCONF server can be configured
       to do authentication of users.";
  }

  feature radius {
    if-feature authentication;
    description
      "Indicates that the NETCONF server can be
       configured to act as a NAS and authenticate users
       with RADIUS.";
    reference
      "RFC 2865: Remote Authentication Dial In User Service (RADIUS)
       RFC 5607: Remote Authentication Dial-In User Service (RADIUS)
                 Authorization for Network Access Server (NAS)
                 Management";
  }

  feature local-users {
    if-feature authentication;
    description
      "Indicates that the NETCONF server supports
       local user authentication.";
  }

  /*
   * Identities
   */

  identity authentication-method {
    description
      "Base identity for NETCONF authentication methods.";
  }

  identity radius {
    base authentication-method;
    description
      "Indicates NETCONF authentication using RADIUS.";
    reference
      "RFC 2865: Remote Authentication Dial In User Service (RADIUS)
       RFC 5607: Remote Authentication Dial-In User Service (RADIUS)
                 Authorization for Network Access Server (NAS)
                 Management";
  }

  identity local-users {
    base authentication-method;
    description
      "Indicates password-based NETCONF authentication using locally
       configured users.";
  }

  /*
   * Derived types
   */

  typedef nacm-user-name-type {
    type string {
      length "1..max";
    }
    description
      "General Purpose User Name string.";
  }

  typedef nacm-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 nacm-rights-type {
    type union {
      type nacm-matchall-string-type;

      type bits {
        bit create {
          description
            "Create access allowed to all specified data.
             Any protocol operation that creates a
             new instance of the specified data is a create
             operation.";
        }
        bit read {
          description
            "Read access allowed to all specified data.
             Any protocol operation or notification that
             returns data to an application is a read
             operation.";
        }
        bit update {
          description
            "Update access allowed to all specified data.
             Any protocol operation that alters an existing
             data node is an update operation.";
        }
        bit delete {
          description
            "Delete access allowed to all specified data.
             Any protocol operation that removes a database
             node instance is a delete operation.";
        }
        bit exec {
          description
            "Execution access to the specified RPC operation.
             Any RPC operation invocation is an exec operation.";
        }
      }
    }
    description
      "NETCONF Access Rights.
       The string '*' indicates that all possible access
       rights apply to the access rule.  Otherwise, only
       the specific access rights represented by the bit names
       that are present apply to the access rule.";
  }

  typedef nacm-group-name-type {
    type string {
      length "1..max";
      pattern "[^\*].*";
    }
    description
      "Name of administrative group that can be
       assigned to the user, and specified in
       an access control rule.";
  }

  typedef nacm-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 schema-instance-identifier {
    type yang:xpath1.0;
    description
      "Path expression used to represent a special
       schema-instance identifier string.

       A schema-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 schema-instance-identifier
       represents all possible server instances for that key.

       This XPath expression is evaluated in the following context:

         o  The set of namespace declarations are those in scope on
            the leaf element where this type is used.

         o  The set of variable bindings contains one variable,
            'USER', which contains the name of user of the current
             session.

         o  The function library is the core function library, but
            note that due to the syntax restrictions of an
            instance-identifier, no functions are allowed.

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

  typedef md5-crypt {
    type string {
      pattern "$0$.* | $1$[a-zA-Z0-9./]{2,8}$.*";
    }
    description
      "The md5-crypt type is used to store a password hash based on the
       MD5 message digest algorithm.  When a clear text value is set to
       a leaf of this type, the server calculates a MD5 password hash,
       and stores the result in the datastore.  Thus, the password is
       never stored in clear text.

       When a leaf of this type is read, the stored password hash is
       returned.

       A value of this type matches one of the forms:

         $0$<clear text password>
         $1$<salt>$<password hash>

      The '$0$' prefix signals that the value is clear text.  When
      such a value is received by the server, an MD5 digest is
      calculated, and the string '$1$<salt>$' is prepended to the
      result, where <salt> is a random 2-8 characters long salt used
      to generate the digest.  This value is stored in the
      configuration data store.

      If a value starting with '$1$<salt>$' is received, the server
      knows that the value already represents an MD5 digest, and
      stores it as is in the data store.

      When a server needs to verify a password given by a user, it
      finds the stored password hash string for that user, extracts
      the salt, and calculates the hash with the salt and given
      password as input.  If the calculated hash value is the same as
      the stored value, the password given by the client is correct.

      The digest algorithm is the md5 crypt function used for
      encrypting passwords for various UNIX systems.";
    reference
      "RFC 1321: The MD5 Message-Digest Algorithm
       http://en.wikipedia.org/wiki/Crypt_(Unix)";
         // FIXME: ref to wikipedia ok??
  }


  container nacm {
    nacm:very-secure;

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

    description
      "Parameters for NETCONF Access Control Model.";

    container authentication {
      nacm:very-secure;
      if-feature authentication;

      description
        "The authentication configuration for the
         NETCONF server.";

      leaf-list user-authentication-order {
        type identityref {
          base authentication-method;
        }
        must '(. = "nacm:radius" and ../radius/server) or'
           + '(. != "nacm:radius")' {
          error-message
            "When 'radius' is used, a radius server
             must be configured.";
        }
        ordered-by user;

        description
          "When the NETCONF server authenticates a user with
           a password, it tries the authentication methods in this
           leaf-list in order.  If authentication with one method
           fails, the next method is used.  If no method succeeds,
           the user is denied access.

           If the 'radius' feature is advertised by the NETCONF
           server, the 'radius' identity can be added to this
           list.

           If the 'local-users' feature is advertised by the
           NETCONF server, the 'local-users' identity can be
           added to this list.";
      }

      container radius {
        if-feature radius;

        description
          "The radius configuration for the NETCONF server.";

        list server {
          key address;

          description
            "The radius server configuration used by
             the NETCONF server.";

          leaf address {
            type inet:host;
            description
              "The address of the radius server.";
          }
          leaf port {
            type inet:port-number;
            default "1812";
            description
             "The port number of the radius server.";
          }
          leaf shared-secret {
            type string; // FIXME
/*
  We're using a special type aes-cfb-128-encrypted-string which works
  like the md5-crypt string, but encrypts the clear text value using a
  pre-provisioned password (not part of the config db!).

  We use $0$ for cleartext and $4$ for the encrypted value.
  (we also have a des-version which uses $3$).

  But I was thinking that maybe we could define a type for encrypted
  values without specifying the encryption algorithm, just specifying
  the format.  $0$<clear text> | $x$<encrypted value>, and how it is
  encrypted is implementation specific.

  One alternative is to store this shared secret in clear text.  It is
  transmitted over a secure transport, and marked as very-secure.  (The
  same argument could be made for user passwords, but these are
  personal and not even root should be able to read my passwd in clear
  text, so it makes more sense to keep them hidden.)
 */
            description
              "The shared secret which is known to both the RADIUS
               client and server.";
            reference
              "RFC 2865: Remote Authentication Dial In User Service";
          }
          /*
            How about configuration of number of retransmits
            and timeout?
           */
        }
      }

      list user {
        if-feature local-users;
        key name;

        description
          "The list of local users configured on this device.";

        leaf name {
          type nacm-user-name-type;
          description
           "The user name string identifying this entry.";
        }
        leaf password {
          type md5-crypt;
          description
            "The password for this entry.";
        }
        leaf ssh-dsa {
          type binary;
          description
            "The public DSA key for this entry.";
        }
        leaf ssh-rsa {
          type binary;
          description
            "The public RSA key for this entry.";
        }
      }
    }

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

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

    leaf denied-rpcs {
      type yang:zero-based-counter32;
      config false;
      mandatory true;
      description
        "Number of times an RPC operation request was denied
         since the server last restarted.";
    }

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

    container groups {
      list group {
        key name;

        description
          "One NACM Group Entry.";

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

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

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

      grouping common-rule-parms {
        leaf rule-name {
          type string {
            length "1..256";
          }
          description
            "Arbitrary name assigned to the
             access control rule.";
        }

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

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

             The string '*' indicates that all configured
             administrative groups apply to the entry.";
        }

        leaf nacm-action {
          type nacm-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 {
            length "1..4095";
          }
          description
            "A textual description of the access rule.";
        }
      }

      list module-rule {
        key "module-name rule-name";
        ordered-by user;

        description
          "One Module Access Rule.

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

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

        uses common-rule-parms {
          refine allowed-rights {
            mandatory true;
          }
        }
      }

      list rpc-rule {
        key "module-name rpc-name rule-name";
        ordered-by user;

        description
          "One RPC Operation Access Rule.

           Rules are processed in user-defined order.  An RPC rule is
           considered a match if the module name of the requested RPC
           operation matches 'module-name', the requested RPC
           operation matches 'rpc-name', and an administrative group
           associated with the session user is listed in the
           'allowed-group' leaf-list.  The 'allowed-rights' leaf
           is ignored by the server if it is present.
           Only the 'exec' bit can possibly cause
           a match for an RPC rule.";

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

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

        uses common-rule-parms;
      }

      list data-rule {
        key "rule-name";
        ordered-by user;

        description
          "One Data Access Control Rule.

           Rules are processed in user-defined order.  A data rule is
           considered to match when the path expression identifies
           the same node that is being accessed in the NETCONF
           database, and the administrative group associated with the
           session is identified in the 'allowed-group' leaf-list,
           and the requested operation is included in the
           'allowed-rights' leaf.";

        leaf path {
          type schema-instance-identifier;
          mandatory true;
          description
            "Schema Instance Identifier associated with the data node
             controlled by this rule.

             Configuration data or state data instance identifiers
             start with a top-level data node.  A complete instance
             identifier is required for this type of path value.

             The special value '/' refers to all possible database
             contents.";
        }

        uses common-rule-parms {
          refine allowed-rights {
            mandatory true;
          }
        }
      }

      list notification-rule {
        key "module-name
             notification-name
             rule-name";
        ordered-by user;

        description
          "One Notification Access Rule.

           A notification is considered a match if the module name of
           the requested event type matches
           'module-name', the requested event type
           matches the 'notification-name', and the administrative
           group associated with the requesting session is listed in
           the 'allowed-group' leaf-list.  If the 'allowed-rights'
           leaf is present, it is ignored by the server.
           Only the 'read' bit can possibly cause
           a match for a notification rule.";

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

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

        uses common-rule-parms;
      }
    }
  }
}


<CODE ENDS>

5.5. IANA Considerations

5.6. Security Considerations

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

Configuration of the access control system is highly sensitive to system security. A server may choose not to allow any user configuration to some portions of it, such as the global security level, or the groups which allowed access to system resources.

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

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

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

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

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

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

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

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

6. Normative References

Appendix A. Usage Examples

The following XML snippets are provided as examples only, to demonstrate how NACM can be configured to perform some access control tasks.

A.1. <groups> Example

There must be at least one <group> entry in order for any of the access control rules to be useful.

The following XML shows arbitrary groups, and is not intended to represent any particular use-case.

A.2. <module-rule> Example

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

A.3. <rpc-rule> Example

A.4. <data-rule> Example

Data rules are used to control access to specific (config and non-config) data nodes within the NETCONF content provided by the server.

A.5. <notification-rule> Example

Notification rules are used to control access to a specific notification event type.

[RFC2119]	Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).
[RFC2865]	Rigney, C., Willens, S., Rubens, A., and W. Simpson, “Remote Authentication Dial In User Service (RADIUS),” RFC 2865, June 2000 (TXT).
[RFC3688]	Mealling, M., “The IETF XML Registry,” BCP 81, RFC 3688, January 2004 (TXT).
[RFC4252]	Ylonen, T. and C. Lonvick, “The Secure Shell (SSH) Authentication Protocol,” RFC 4252, January 2006 (TXT).
[RFC4253]	Ylonen, T. and C. Lonvick, “The Secure Shell (SSH) Transport Layer Protocol,” RFC 4253, January 2006 (TXT).
[RFC4741]	Enns, R., “NETCONF Configuration Protocol,” RFC 4741, December 2006 (TXT).
[RFC4742]	Wasserman, M. and T. Goddard, “Using the NETCONF Configuration Protocol over Secure SHell (SSH),” RFC 4742, December 2006 (TXT).
[RFC5277]	Chisholm, S. and H. Trevino, “NETCONF Event Notifications,” RFC 5277, July 2008 (TXT).
[RFC5607]	Nelson, D. and G. Weber, “Remote Authentication Dial-In User Service (RADIUS) Authorization for Network Access Server (NAS) Management,” RFC 5607, July 2009 (TXT).
[RFC6020]	Bjorklund, M., “YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF),” RFC 6020, October 2010 (TXT).
[RFC6021]	Schoenwaelder, J., “Common YANG Data Types,” RFC 6021, October 2010 (TXT).
[W3C.REC-xml]	Bray, T., Paoli, J., Sperberg-McQueen, C., and E. Maler, “Extensible Markup Language (XML) 1.0 (2nd ed),” W3C REC-xml, October 2000.

	Andy Bierman
	Brocade
Email:	andy.bierman@brocade.com

	Martin Bjorklund
	Tail-f Systems
Email:	mbj@tail-f.com

Network Configuration Protocol Access Control Modeldraft-ietf-netconf-access-control-01

Abstract