Using Authentication, Authorization, and Accounting services to Dynamically Provision View-based Access Control Model User-to-Group Mappings
draft-ietf-isms-radius-vacm-11.txt

Abstract

This memo defines a portion of the Management Information Base (MIB) for use with network management protocols. It describes the use of information provided by Authentication, Authorization, and Accounting (AAA) services, such as the Remote Authentication Dial-In User Service (RADIUS), to dynamically update user-to-group mappings in the View-Based Access Control Model (VACM).

Status of this Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress.”

This Internet-Draft will expire on March 18, 2011.

Copyright Notice

Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

1.  Introduction
2.  The Internet-Standard Management Framework
3.  Conventions
4.  Overview
    4.1.  Using AAA services with SNMP
    4.2.  Applicability
5.  Structure of the MIB Module
    5.1.  Textual Conventions
    5.2.  The Table Structure
6.  Relationship to Other MIB Modules
    6.1.  Relationship to the VACM MIB
    6.2.  MIB modules required for IMPORTS
    6.3.  Documents required for REFERENCE clauses
7.  Elements of Procedure
    7.1.  Sequencing Requirements
    7.2.  Actions Upon Session Establishment Indication
        7.2.1.  Required Information
        7.2.2.  Creation of Entries in vacmAaaSecurityToGroupTable
        7.2.3.  Creation of Entries in vacmSecurityToGroupTable
        7.2.4.  Update of vacmGroupName
    7.3.  Actions Upon Session Termination Indication
        7.3.1.  Deletion of Entries from vacmAaaSecurityToGroupTable
        7.3.2.  Deletion of Entries from vacmSecurityToGroupTable
8.  Definitions
9.  Security Considerations
    9.1.  Principal Identity Naming
    9.2.  Management Information Considerations
10.  IANA Considerations
11.  Contributors
12.  References
    12.1.  Normative References
    12.2.  Informative References
§  Authors' Addresses

1.  Introduction

This memo specifies a way to dynamically provision selected View-Based Access Control Model (VACM) [RFC3415] (Wijnen, B., Presuhn, R., and K. McCloghrie, “View-based Access Control Model (VACM) for the Simple Network Management Protocol (SNMP),” December 2002.) Management Information Base (MIB) objects, based on information received from an Authentication, Authorization, and Accounting (AAA) service, such as RADIUS [RFC2865] (Rigney, C., Willens, S., Rubens, A., and W. Simpson, “Remote Authentication Dial In User Service (RADIUS),” June 2000.) and [RFC5607] (Nelson, D. and G. Weber, “Remote Authentication Dial-In User Service (RADIUS) Authorization for Network Access Server (NAS) Management,” July 2009.). It reduces the need for security administrators to manually update VACM configurations due to user churn, allowing a centralized AAA service to provide the information associating a given user with the access control policy (known as a "group" in VACM) governing that user's access to management information.

This memo requires no changes to the Abstract Service Interface for the Access Control Subsystem, and requires no changes to the Elements of Procedure for VACM. It provides a MIB module that reflects the information provided by the AAA service, along with elements of procedure for maintaining that information and performing corresponding updates to VACM MIB data.

The reader is expected to be familiar with [RFC3415] (Wijnen, B., Presuhn, R., and K. McCloghrie, “View-based Access Control Model (VACM) for the Simple Network Management Protocol (SNMP),” December 2002.), [RFC5607] (Nelson, D. and G. Weber, “Remote Authentication Dial-In User Service (RADIUS) Authorization for Network Access Server (NAS) Management,” July 2009.), [RFC5608] (Narayan, K. and D. Nelson, “Remote Authentication Dial-In User Service (RADIUS) Usage for Simple Network Management Protocol (SNMP) Transport Models,” August 2009.), and their supporting specifications.

2.  The Internet-Standard Management Framework

For a detailed overview of the documents that describe the current Internet-Standard Management Framework, please refer to section 7 of RFC 3410 [RFC3410] (Case, J., Mundy, R., Partain, D., and B. Stewart, “Introduction and Applicability Statements for Internet-Standard Management Framework,” December 2002.).

Managed objects are accessed via a virtual information store, termed the Management Information Base or MIB. MIB objects are generally accessed through the Simple Network Management Protocol (SNMP). Objects in the MIB are defined using the mechanisms defined in the Structure of Management Information (SMI). This memo specifies a MIB module that is compliant to the SMIv2, which is described in STD 58, RFC 2578 [RFC2578] (McCloghrie, K., Ed., Perkins, D., Ed., and J. Schoenwaelder, Ed., “Structure of Management Information Version 2 (SMIv2),” April 1999.), STD 58, RFC 2579 [RFC2579] (McCloghrie, K., Ed., Perkins, D., Ed., and J. Schoenwaelder, Ed., “Textual Conventions for SMIv2,” April 1999.) and STD 58, RFC 2580 [RFC2580] (McCloghrie, K., Perkins, D., and J. Schoenwaelder, “Conformance Statements for SMIv2,” April 1999.).

3.  Conventions

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 in RFC 2119 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

4.  Overview

4.1.  Using AAA services with SNMP

There are two use cases for AAA support of management access via SNMP. These are (a) service authorization and (b) access control authorization. The former is discussed in detail in [RFC5608] (Narayan, K. and D. Nelson, “Remote Authentication Dial-In User Service (RADIUS) Usage for Simple Network Management Protocol (SNMP) Transport Models,” August 2009.). The latter is the subject of this memo.

The use case assumption here is that roles within an organization, which are reflected in VACM as groups, naming access control policies, change infrequently, while the users assigned to those roles change much more frequently. This memo describes how the user-to-role (group) mapping can be delegated to the RADIUS server, avoiding the need to re-provision managed devices as users are added, deleted, or assigned new roles in an organization.

This memo assumes that the detailed access control policies are pre-configured in VACM, and does not attempt to address the question of how the policy associated with a given role is put in place.

The only additional information obtained from the AAA service is the mapping of the authenticated user's identifier to a specific role (or "group" in VACM terminology) in the access control policy. Dynamic user authorization for MIB database access control, as defined herein, is limited to mapping the authenticated user to a group, which in turn is mapped to whatever access control policies are already in place in VACM.

The SNMP architecture [RFC3411] (Harrington, D., Presuhn, R., and B. Wijnen, “An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks,” December 2002.) maintains strong modularity and separation of concerns, separating user identity (authentication) from user database access rights (authorization). RADIUS, on the other hand, allows for no such separation of authorization from authentication. Consequently, the approach here is to leverage existing RADIUS usage for identifying a principal, documented in [RFC5608] (Narayan, K. and D. Nelson, “Remote Authentication Dial-In User Service (RADIUS) Usage for Simple Network Management Protocol (SNMP) Transport Models,” August 2009.), along with the RADIUS Management-Policy-Id Attribute [RFC5607] (Nelson, D. and G. Weber, “Remote Authentication Dial-In User Service (RADIUS) Authorization for Network Access Server (NAS) Management,” July 2009.).

A unique identifier is needed for each AAA-authorized "session", corresponding to a communication channel, such as a transport session, for which a principal has been AAA-authenticated and which is authorized to offer SNMP service. How these identifiers are assigned is implementation-dependent. When a RADIUS Management-Policy-Id Attribute (or equivalent) is bound to such a session and principal authentication, this binding provides sufficient information to compute dynamic updates to VACM. How this information is communicated within an implementation is implementation dependent; this memo is only concerned with externally observable behavior.

The key concept here is that what we will informally call a "AAA binding" binds:

1. a communications channel
2. an authenticated principal
3. service authorization
4. an access control policy name

Some of the binding is done via other specifications. A transport model, such as the Secure Shell Transport Model [RFC5592] (Harrington, D., Salowey, J., and W. Hardaker, “Secure Shell Transport Model for the Simple Network Management Protocol (SNMP),” June 2009.), provides a binding between 1) and 2) and 3), providing a securityName. In turn, [RFC5607] (Nelson, D. and G. Weber, “Remote Authentication Dial-In User Service (RADIUS) Authorization for Network Access Server (NAS) Management,” July 2009.) provides a binding between (1+2+3) and 4). This document extends that further, to create a binding between (1+2+3+4) and the local (VACM MIB) definition of the named policy, called a group in VACM.

4.2.  Applicability

Though this memo was motivated to support the use of specific Transport Models, such as the Secure Shell Transport Model [RFC5592] (Harrington, D., Salowey, J., and W. Hardaker, “Secure Shell Transport Model for the Simple Network Management Protocol (SNMP),” June 2009.), it MAY be used with other implementation environments satisfying these requirements:

• use an AAA service for sign-on service and data access authorization;
• provide an indication of the start of a session for a particular authenticated principal, identified using an SNMP securityName [RFC3411] (Harrington, D., Presuhn, R., and B. Wijnen, “An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks,” December 2002.), and provide the corresponding value to be used to identify a VACM group to be used, based on information provided by the AAA service in use;
• provide an indication of the end of the need for being able to make access decisions for a particular authenticated principal, as at the end of a session, whether due to disconnection, termination due to timeout, or any other reason.

Likewise, although this memo specifically refers to RADIUS, it MAY be used with other AAA services satisfying these requirements:

• the service provides information semantically equivalent to the RADIUS Management-Policy-Id Attribute [RFC5607] (Nelson, D. and G. Weber, “Remote Authentication Dial-In User Service (RADIUS) Authorization for Network Access Server (NAS) Management,” July 2009.), which corresponds to the name of a VACM group;
• the service provides an authenticated principal identifier (e.g., the RADIUS User-Name Attribute [RFC2865] (Rigney, C., Willens, S., Rubens, A., and W. Simpson, “Remote Authentication Dial In User Service (RADIUS),” June 2000.)) which can be transformed to an equivalent principal identifier in the form of a securityName [RFC3411] (Harrington, D., Presuhn, R., and B. Wijnen, “An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks,” December 2002.).

5.  Structure of the MIB Module

5.1.  Textual Conventions

This MIB module makes use of the SnmpAdminString [RFC3411] (Harrington, D., Presuhn, R., and B. Wijnen, “An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks,” December 2002.) and SnmpSecurityModel [RFC3411] (Harrington, D., Presuhn, R., and B. Wijnen, “An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks,” December 2002.) textual conventions.

5.2.  The Table Structure

This MIB module defines a single table, the vacmAaaSecurityToGroupTable. This table is indexed by the integer assigned to each security model, the protocol-independent securityName corresponding to a principal, and the unique identifier of a session.

6.  Relationship to Other MIB Modules

This MIB module has a close operational relationship with the SNMP-VIEW-BASED-ACM-MIB (more commonly known as the "VACM MIB") from [RFC3415] (Wijnen, B., Presuhn, R., and K. McCloghrie, “View-based Access Control Model (VACM) for the Simple Network Management Protocol (SNMP),” December 2002.). It also relies on IMPORTS from several other modules.

6.1.  Relationship to the VACM MIB

Although the MIB module defined here has a close relationship with the VACM MIB's vacmSecurityToGroupTable, it in no way changes the elements of procedure for VACM, nor does it affect any other tables defined in VACM. See the elements of procedure (below) for details of how the contents of the vacmSecurityToGroupTable are affected by this MIB module.

6.2.  MIB modules required for IMPORTS

This MIB module employs definitions from [RFC2578] (McCloghrie, K., Ed., Perkins, D., Ed., and J. Schoenwaelder, Ed., “Structure of Management Information Version 2 (SMIv2),” April 1999.), [RFC2579] (McCloghrie, K., Ed., Perkins, D., Ed., and J. Schoenwaelder, Ed., “Textual Conventions for SMIv2,” April 1999.) and [RFC3411] (Harrington, D., Presuhn, R., and B. Wijnen, “An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks,” December 2002.).

6.3.  Documents required for REFERENCE clauses

This MIB module contains REFERENCE clauses making reference to [RFC2865] (Rigney, C., Willens, S., Rubens, A., and W. Simpson, “Remote Authentication Dial In User Service (RADIUS),” June 2000.), [RFC3411] (Harrington, D., Presuhn, R., and B. Wijnen, “An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks,” December 2002.), and [RFC5590] (Harrington, D. and J. Schoenwaelder, “Transport Subsystem for the Simple Network Management Protocol (SNMP),” June 2009.).

7.  Elements of Procedure

The following elements of procedure are formulated in terms of two types of events: an indication of the establishment of a session, and an indication that one has ended. These can result in the creation of entries in the vacmAaaSecurityToGroupTable, which can in turn trigger creation, update, or deletion of entries in the vacmSecurityToGroupTable.

There are various possible implementation-dependent error cases not spelled out here, such as running out of memory. By their nature, recovery in such cases will be implementation-dependent. Implementors are advised to consider fail-safe strategies, e.g., prematurely terminating access in preference to erroneously perpetuating access.

7.1.  Sequencing Requirements

These procedures assume that a transport model, such as [RFC5592] (Harrington, D., Salowey, J., and W. Hardaker, “Secure Shell Transport Model for the Simple Network Management Protocol (SNMP),” June 2009.), coordinates session establishment with AAA authentication and authorization. They rely on the receipt by the AAA client of the RADIUS Management-Policy-Id [RFC5607] (Nelson, D. and G. Weber, “Remote Authentication Dial-In User Service (RADIUS) Authorization for Network Access Server (NAS) Management,” July 2009.) Attribute (or its equivalent) from the RADIUS Access-Accept message (or equivalent). They also assume that the User-Name [RFC2865] (Rigney, C., Willens, S., Rubens, A., and W. Simpson, “Remote Authentication Dial In User Service (RADIUS),” June 2000.) from the RADIUS Access-Request message (or equivalent) corresponds to a securityName [RFC3411] (Harrington, D., Presuhn, R., and B. Wijnen, “An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks,” December 2002.).

To ensure correct processing of SNMP PDUs, the handling of the indication of the establishment of a session in accordance with the elements of procedure below MUST be completed before the isAccessAllowed() abstract service interface [RFC3415] (Wijnen, B., Presuhn, R., and K. McCloghrie, “View-based Access Control Model (VACM) for the Simple Network Management Protocol (SNMP),” December 2002.) is invoked for any SNMP PDUs from that session.

If a session termination indication occurs before all invocations of the isAccessAllowed() abstract service interface [RFC3415] (Wijnen, B., Presuhn, R., and K. McCloghrie, “View-based Access Control Model (VACM) for the Simple Network Management Protocol (SNMP),” December 2002.) have completed for all SNMP PDUs from that session, those remaining invocations MAY result in denial of access.

7.2.  Actions Upon Session Establishment Indication

7.2.1.  Required Information

Four pieces of information are needed to process the session establishment indication:

• the SnmpSecurityModel [RFC3411] (Harrington, D., Presuhn, R., and B. Wijnen, “An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks,” December 2002.) needed as an index into the vacmSecurityToGroupTable;
• the RADIUS User-Name Attribute;
• a session identifier, as a unique, definitive identifier of the session that the AAA authorization is tied to;
• the RADIUS Management-Policy-Id Attribute.

All four of these pieces of information are REQUIRED. In particular, if either the User-Name or Management-Policy-Id is absent, invalid, or a zero-length string, no further processing of the session establishment indication is undertaken.

As noted in Section 4.2 (Applicability), the above text refers specifically to RADIUS attributes. Other AAA services can be substituted, but the requirements imposed on the User-Name and the Management-Policy-Id-Attribute MUST be satisfied using the equivalent fields for those services.

7.2.2.  Creation of Entries in vacmAaaSecurityToGroupTable

Whenever an indication arrives that a new session has been established, determine whether a corresponding entry exists in the vacmAaaSecurityToGroupTable. If one does not, create a new row with the columns populated as follows:

• vacmAaaSecurityModel = value of SnmpSecurityModel corresponding to the security model in use;
• vacmAaaSecurityName = RADIUS User-Name Attribute or equivalent, the securityName that will be used in invocations of the isAccessAllowed() abstract service interface [RFC3415] (Wijnen, B., Presuhn, R., and K. McCloghrie, “View-based Access Control Model (VACM) for the Simple Network Management Protocol (SNMP),” December 2002.);
• vacmAaaSessionID = session identifier, unique across all open sessions of all of this SNMP engine's transport models;
• vacmAaaGroupName = RADIUS Management-Policy-Id Attribute or equivalent.

Otherwise, if the row already exists, update the vacmAaaGroupName with the the RADIUS Management-Policy-Id Attribute or equivalent supplied.

7.2.3.  Creation of Entries in vacmSecurityToGroupTable

Whenever an entry is created in the vacmAaaSecurityToGroupTable, the vacmSecurityToGroupTable is examined to determine whether a corresponding entry exists there, using the value of vacmAaaSecurityModel for vacmSecurityModel, and the value of vacmAaaSecurityName for vacmSecurityName. If no corresponding entry exists, create one, using the vacmAaaGroupName of the newly created entry to fill in vacmGroupName, using a value of "volatile" for the row's StorageType, and a value of "active" for its RowStatus.

7.2.4.  Update of vacmGroupName

Whenever the value of an instance of vacmAaaGroupName is updated, if a corresponding entry exists in the vacmSecurityToGroupTable, and that entry's StorageType is "volatile" and its RowStatus is "active", update the value of vacmGroupName with the value from vacmAaaGroupName.

If a corresponding entry already exists in the vacmSecurityToGroupTable, and that row's StorageType is anything other than "volatile", or its RowStatus is anything other than "active", then that instance of vacmGroupName MUST NOT be modified.

The operational assumption here is that if the row's StorageType is "volatile", then this entry was probably dynamically created; an entry created by a security administrator would not normally be given a StorageType of "volatile". If value being provided by RADIUS (or other AAA service) is the same as what is already there, this is a no-op. If the value is different, the new information is understood as a more recent role (group) assignment for the user, which should supersede the one currently held there. The structure of the vacmSecurityToGroupTable makes it impossible for a (vacmSecurityModel, vacmSecurityName) tuple to map to more than one group.

7.3.  Actions Upon Session Termination Indication

Whenever a RADIUS (or other AAA) authenticated session ends for any reason, an indication is provided. This indication MUST provide means of determining the SnmpSecurityModel, and an identifier for the transport session tied to the AAA authorization. The manner in which this occurs is implementation dependent.

7.3.1.  Deletion of Entries from vacmAaaSecurityToGroupTable

Entries in the vacmAaaSecurityToGroupTable MUST NOT persist across system reboots.

When a session has been terminated, the vacmAaaSecurityToGroupTable is searched for a corresponding entry. A "matching" entry is any entry for which the SnmpSecurityModel and session ID match the information associated with the session termination indication. Any matching entries are deleted. It is possible that no entries will match; this is not an error, and no special processing is required in this case.

7.3.2.  Deletion of Entries from vacmSecurityToGroupTable

Whenever the last remaining row bearing a particular (vacmAaaSecurityModel, vacmAaaSecurityName) pair is deleted from the vacmAaaSecurityToGroupTable, the vacmSecurityToGroupTable is examined for a corresponding row. If one exists, and if its StorageType is "volatile" and its RowStatus is "active", that row MUST be deleted as well. The mechanism to accomplish this task is implementation-dependent.

8.  Definitions

SNMP-VACM-AAA-MIB DEFINITIONS ::= BEGIN

IMPORTS
    MODULE-COMPLIANCE, OBJECT-GROUP       FROM SNMPv2-CONF
    MODULE-IDENTITY, OBJECT-TYPE,
    mib-2,
    Unsigned32                            FROM SNMPv2-SMI
    SnmpAdminString,
    SnmpSecurityModel                     FROM SNMP-FRAMEWORK-MIB;

vacmAaaMIB    MODULE-IDENTITY
    LAST-UPDATED "201009010000Z"          -- 1 September, 2010
    ORGANIZATION "ISMS Working Group"
    CONTACT-INFO "WG-email:   isms@ietf.org"

    DESCRIPTION  "The management and local datastore information
                  definitions for the AAA-Enabled View-based Access
                  Control Model for SNMP.

                  Copyright (c) 2010 IETF Trust and the persons
                  identified as the document authors.  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 MIB module is part of RFC XXXX;
                  see the RFC itself for full legal notices."

    REVISION "201009010000Z"
    DESCRIPTION "Initial version, published as RFC XXXX."
     ::= { mib-2 XXX }
-- RFC Ed.: replace XXX with IANA-assigned number & remove this note
-- RFC Ed.: replace XXXX with the RFC number & remove this note

vacmAaaMIBObjects   OBJECT IDENTIFIER ::= { vacmAaaMIB 1 }

vacmAaaMIBConformance OBJECT IDENTIFIER ::= { vacmAaaMIB 2 }

vacmAaaSecurityToGroupTable OBJECT-TYPE
    SYNTAX       SEQUENCE OF VacmAaaSecurityToGroupEntry
    MAX-ACCESS   not-accessible
    STATUS       current
    DESCRIPTION "This table provides a listing of all currently active
                 sessions for which a mapping of the combination of
                 SnmpSecurityModel and securityName into the name of
                 a VACM group which has been provided by an AAA service.
                 The group name (in VACM) in turn identifies an access
                 control policy to be used for the corresponding
                 principals."
    REFERENCE   "RFC 3411 section 3.2.2 defines securityName"
    ::= { vacmAaaMIBObjects 1 }

vacmAaaSecurityToGroupEntry OBJECT-TYPE
    SYNTAX       VacmAaaSecurityToGroupEntry
    MAX-ACCESS   not-accessible
    STATUS       current
    DESCRIPTION "An entry in this table maps the combination of a
                 SnmpSecurityModel and securityName into the name
                 of a VACM group defining the access control policy
                 which is to govern a particular session.

                 Each entry corresponds to a session.

                 Entries do not persist across reboots.

                 An entry is created whenever an indication occurs
                 that a new session has been established that would
                 not have the same index values as an existing entry.

                 When a session is torn down, disconnected, timed out
                 (e.g., following the RADIUS Session-Timeout Attribute),
                 or otherwise terminated for any reason, the
                 corresponding vacmAaaSecurityToGroupEntry is deleted."
    REFERENCE   "RFC 3411 section 3.2.2 defines securityName"
    INDEX       {
                  vacmAaaSecurityModel,
                  vacmAaaSecurityName,
                  vacmAaaSessionID
                }
    ::= { vacmAaaSecurityToGroupTable 1 }

VacmAaaSecurityToGroupEntry ::= SEQUENCE
    {
        vacmAaaSecurityModel            SnmpSecurityModel,
        vacmAaaSecurityName             SnmpAdminString,
        vacmAaaSessionID                Unsigned32,
        vacmAaaGroupName                SnmpAdminString
    }

vacmAaaSecurityModel OBJECT-TYPE
    SYNTAX       SnmpSecurityModel(1..2147483647)
    MAX-ACCESS   not-accessible
    STATUS       current
    DESCRIPTION "The security model associated with the AAA binding
                 represented by this entry.

                 This object cannot take the 'any' (0) value."
    ::= { vacmAaaSecurityToGroupEntry 1 }

vacmAaaSecurityName OBJECT-TYPE
    SYNTAX       SnmpAdminString (SIZE(1..32))
    MAX-ACCESS   not-accessible
    STATUS       current
    DESCRIPTION "The securityName of the principal associated with the
                 AAA binding represented by this entry.  In RADIUS
                 environments, this corresponds to the User-Name
                 Attribute."
    REFERENCE   "RFC 3411 section 3.2.2 defines securityName, and
                 RFC 2865 section 5.1 defines User-Name."
    ::= { vacmAaaSecurityToGroupEntry 2 }

vacmAaaSessionID OBJECT-TYPE
    SYNTAX       Unsigned32
    MAX-ACCESS   not-accessible
    STATUS       current
    DESCRIPTION "An implementation-dependent identifier of the session.

                 This value MUST be unique among all currently open
                 sessions of all of this SNMP engine's transport models.
                 The value has no particular significance other than to
                 distinguish sessions.

                 Implementations in which tmSessionID has a compatible
                 syntax and is unique across all transport models MAY
                 use that value."
    REFERENCE   "The abstract service interface parameter tmSessionID
                 is defined in RFC 5590 section 5.2.4."
    ::= { vacmAaaSecurityToGroupEntry 3 }

vacmAaaGroupName    OBJECT-TYPE
    SYNTAX       SnmpAdminString (SIZE(1..32))
    MAX-ACCESS   read-only
    STATUS       current
    DESCRIPTION "The name of the group to which this entry is to belong.
                 In RADIUS environments this comes from the RADIUS
                 Management-Policy-Id Attribute.

                 When the appropriate conditions are met,
                 the value of this object is applied the vacmGroupName
                 in the corresponding vacmSecurityToGroupEntry."
    REFERENCE    "RFC 3415"
    ::= { vacmAaaSecurityToGroupEntry 4 }


-- Conformance information ******************************************

vacmAaaMIBCompliances
               OBJECT IDENTIFIER ::= {vacmAaaMIBConformance 1}
vacmAaaMIBGroups
               OBJECT IDENTIFIER ::= {vacmAaaMIBConformance 2}

-- compliance statements

vacmAaaMIBBasicCompliance MODULE-COMPLIANCE
    STATUS       current
    DESCRIPTION "The compliance statement for SNMP engines implementing
                 the AAA-Enabled View-based Access Control Model for
                 SNMP."
    MODULE    -- this module
        MANDATORY-GROUPS { vacmAaaGroup }

    ::= { vacmAaaMIBCompliances 1 }

-- units of conformance

vacmAaaGroup OBJECT-GROUP
    OBJECTS {
              vacmAaaGroupName
            }
    STATUS       current
    DESCRIPTION "A collection of objects for supporting the use of AAA
                 services to provide user / group mappings for VACM."
    ::= { vacmAaaMIBGroups 1 }


END

9.  Security Considerations

The algorithms in this memo make heuristic use of the StorageType of entries in the vacmSecurityToGroupTable to distinguish those provisioned by a security administrator (which would presumably not be configured as "volatile") from those dynamically generated. In making this distinction, it assumes that those entries explicitly provisioned by a security administrator and given a non-"volatile" status are not to be dynamically overridden. Furthermore, it assumes that any active entries with "volatile" status can be treated as dynamic, and deleted or updated as needed. Users of this memo need to be aware of this operational assumption, which, while reasonable, is not necessarily universally valid. For example, this situation could also occur if the SNMP security administrator had mistakenly created these non-volatile entries in error.

The design of VACM ensures that if an unknown policy (group name) is used in the vacmSecurityToGroupTable, no access is granted. A consequence of this is that no matter what information is provided by the AAA server, no user can gain SNMP access rights not already granted to some group through the VACM configuration.

9.1.  Principal Identity Naming

In order to ensure that the access control policy ultimately applied as a result of the mechanisms described here is indeed the intended policy for a given principal using a particular security model, care needs to be applied in the mapping of the authenticated user (principal) identity to the securityName used to make the access control decision. Broadly speaking, there are two approaches to ensure consistency of identity:

• Entries for the vacmSecurityToGroupTable corresponding to a given security model are created only through the operation of the procedures described in this memo. A consequence of this would be that all such entries would have been created using the RADIUS User-Name (or other AAA-authenticated identity) and RADIUS Management-Policy-Id Attribute (or equivalent).
• Administrative policy allows a matching pre-configured entry to exist in the vacmSecurityToGroupTable, i.e., an entry with the corresponding vacmSecurityModel and with a vacmSecurityName matching the authenticated principal's RADIUS User-Name. In this case, administrative policy also needs to ensure consistency of identity between each authenticated principal's RADIUS User-Name and the administratively configured vacmSecurityName in the vacmSecurityToGroupTable row entries for that particular security model.

In the later case, inconsistent re-use of the same name for different entities or individuals (principals) can cause the incorrect access control policy to be applied for the authenticated principal, depending on whether the policy configured using SNMP, or the policy applied using the procedures of this memo, is the intended policy. This may result in greater or lesser access rights than the administrative policy intended. Inadvertent mis-identification in such cases may be undetectable by the SNMP engine or other software elements of the managed entity.

9.2.  Management Information Considerations

There are no management objects defined in this MIB module that have a MAX-ACCESS clause of read-write and/or read-create. So, if this MIB module is implemented correctly, then there is no risk that an intruder can alter or create any management objects of this MIB module via direct SNMP SET operations.

Some of the readable objects in this MIB module (including some objects with a MAX-ACCESS of not-accessible, whose values are exposed as a result access to indexed objects) may be considered sensitive or vulnerable in some network environments. It is thus important to control even GET and/or NOTIFY access to these objects and possibly to even encrypt the values of these objects when sending them over the network via SNMP. These are the tables and objects and their sensitivity/vulnerability:

• vacmAaaSecurityToGroupTable - the entire table is potentially sensitive, since walking the table will reveal user names, security models in use, session identifiers, and group names;
• vacmAaaSecurityModel - though not-accessible, this is exposed as an index of vacmAaaGroupName;
• vacmAaaSecurityName - though not-accessible, this is exposed as an index of vacmAaaGroupName;
• vacmAaaSessionID - though not-accessible, this is exposed as an index of vacmAaaGroupName;
• vacmAaaGroupName - since this identifies a security policy and associates it with a particular user, this is potentially sensitive.

SNMP versions prior to SNMPv3 did not include adequate security. Even if the network itself is secure (for example by using IPsec), even then, there is no control as to who on the secure network is allowed to access and GET/SET (read/change/create/delete) the objects in this MIB module.

It is RECOMMENDED that implementers consider the security features as provided by the SNMPv3 framework (see [RFC3410] (Case, J., Mundy, R., Partain, D., and B. Stewart, “Introduction and Applicability Statements for Internet-Standard Management Framework,” December 2002.), section 8), including full support for the SNMPv3 cryptographic mechanisms (for authentication and privacy).

Further, deployment of SNMP versions prior to SNMPv3 is NOT RECOMMENDED. Instead, it is RECOMMENDED to deploy SNMPv3 and to enable cryptographic security. It is then a customer/operator responsibility to ensure that the SNMP entity giving access to an instance of this MIB module is properly configured to give access to the objects only to those principals (users) that have legitimate rights to indeed GET or SET (change/create/delete) them.

10.  IANA Considerations

The MIB module in this document uses the following IANA-assigned OBJECT IDENTIFIER value recorded in the SMI Numbers registry:

   Descriptor        OBJECT IDENTIFIER value
   ----------        -----------------------
   vacmAaaMIB        { mib-2 XXX }

Editor's Note (to be removed prior to publication): the IANA is requested to assign a value for "XXX" under the 'mib-2' subtree and to record the assignment in the SMI Numbers registry. When the assignment has been made, the RFC Editor is asked to replace "XXX" (here and in the MIB module) with the assigned value and to remove this note.

11.  Contributors

The following participants from the isms working group contributed to the development of this document:

• Andrew Donati
• David Harrington
• Jeffrey Hutzelman
• Jürgen Schönwälder
• Tom Petch
• Wes Hardaker

During the IESG review additional comments were received from:

• Adrian Farrel
• Amanda Baber
• Dan Romescanu
• David Kessens
• Francis Dupont
• Glenn Keeni
• Jari Arkko
• Joel Jaeggli
• Magnus Nyström
• Mike Heard
• Robert Story
• Russ Housley
• Sean Turner
• Tim Polk

12.  References

12.1. Normative References

12.2. Informative References

Authors' Addresses