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Versions: (draft-narayan-isms-sshsm-radius) 00 01 02 03 04 05 06 07 RFC 5608

Network Working Group                                         K. Narayan
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                               D. Nelson
Expires: March 4, 2008                             Elbrys Networks, Inc.
                                                       September 1, 2007


  Remote Authentication Dial-In User Service (RADIUS) Usage for Simple
          Network Management Protocol (SNMP) Transport Models
                  draft-ietf-isms-radius-usage-00.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on March 4, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This memo describes the use of a Remote Authentication Dial-In User
   Service (RADIUS) authentication and authorization service by Simple
   Network Management Protocol (SNMP) secure Transport Models to
   authenticate users and authorize creation of secure transport
   sessions.  While the recommendations of this memo are generally
   applicable to a broad class of SNMP Transport Models, the examples



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   focus on the Secure Shell Transport Model.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  General  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  RADIUS Operational Model . . . . . . . . . . . . . . . . .  3
     1.3.  RADIUS Usage With Secure Transports  . . . . . . . . . . .  5
     1.4.  SNMP Transport Models  . . . . . . . . . . . . . . . . . .  5
   2.  RADIUS Usage for SNMP Transport Models . . . . . . . . . . . .  6
     2.1.  RADIUS Authentication for Transport Protocols  . . . . . .  7
     2.2.  RADIUS Authorization for Transport Protocols . . . . . . .  7
     2.3.  SNMP Service Authorization . . . . . . . . . . . . . . . .  8
     2.4.  SNMP Access Control Authorization  . . . . . . . . . . . .  9
   3.  Table of Attributes  . . . . . . . . . . . . . . . . . . . . .  9
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
   Intellectual Property and Copyright Statements . . . . . . . . . . 14





















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1.  Introduction

1.1.  General

   This memo describes the use of a Remote Authentication Dial-In User
   Service (RADIUS) authentication and authorization service by Simple
   Network Management Protocol (SNMP) secure Transport Models to
   authenticate users and authorize creation of secure transport
   sessions.  While the recommendations of this memo are generally
   applicable to a broad class of SNMP Transport Models, the examples
   focus on the Secure Shell Transport Model.

   The RADIUS protocol is a widely deployed means of authentication and
   authorization for network access and administrative access to network
   devices.  The RADIUS protocol enables centralized administration of
   user accounts and credentials thereby significantly improving
   manageability and scalability and reducing administrative overhead.
   The RADIUS protocol also provides the advantage of allowing a common
   identity to be used with or shared across disparate management
   protocols, since the other network management interfaces such as
   NETCONF are capable of authentication with the same RADIUS server.

   In the context of this document, a Network Access Server (NAS) is a
   network device or host that contains an SNMP engine implementation,
   utilizing SNMP Transport Models.  While it is customary in SNMP
   documents to indicate which subsystem performs specific processing
   tasks, in this document we leave such decisions to the implementer,
   as is customary for RADIUS documents, and simply specify NAS
   behavior.  Such processing might be implemented in the secure
   transport module or in one or more modules of the SNMP engine.

1.2.  RADIUS Operational Model

   The RADIUS protocol [RFC2865] provides authentication and
   authorization services for network access devices, usually refered to
   as a Network Access Server (NAS).  The RADIUS protocol operates, at
   the most simple level, as a request-response mechanism.  RADIUS
   Clients, within the NAS, initiate a transaction by sending a RADIUS
   Access-Request message to a RADIUS Server, with which the client
   shares credentials.  The RADIUS Server will respond with either an
   Access-Accept message or an Access-Reject message.

   RADIUS supports authentication methods compatible with plaintext
   username and password mechanisms, MD5 Challenge/Response mechanisms,
   Extensible Authentication Protocol (EAP) mechanisms, and HTTP Digest
   mechanisms.  Upon presentation of identity and credentials the user
   is either accepted or rejected.  RADIUS servers indicate a successful
   authentication by returning an Access-Accept message.  An Access-



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   Reject message indicates unsuccessful authentication.

   Access-Accept messages are typically populated with one or more
   service provisioning attributes, that control the type and extent of
   service provided to the user at the NAS.  The authorization portion
   may be thought of as service provisioning.  Based on the
   configuration of the user's account on the RADIUS Server, upon
   authentication the NAS is provided with instructions as to what type
   of service to provide to the user.  When that service provisioning
   does not match the capabilities of the NAS, or of the particular
   interface to the NAS over which the user is requesting access, RFC
   2865 [RFC2865] requires that the NAS MUST reject the access request.
   For a description of the basic set of attributes, refer to [RFC2865].
   RFC 2865 describes service provisioning attributes for management
   access to a NAS, as well as various terminal emulation and packet
   forwarding services on the NAS.  This memo describes specific RADIUS
   service provisioning attributes that are useful for use with secure
   transports and SNMP Transport Models.

   RADIUS servers are often deployed on an enterprise- or organization-
   wide basis, covering a variety of disparate use cases.  In such
   deployments, all NASes and all users are serviced by a common pool of
   RADIUS servers.  In many deployments, the RADIUS Server will handle
   requests from many different types of NASes with different
   capabilities, and different types of interfaces, services and
   protocol support.

   In order for a RADIUS server to make the correct authorization
   decision in all cases, the server will often need to know something
   about the type of NAS at which the user is requesting access, the
   type of service that the user is requesting, and the role of the user
   in the organization.  For example, many users may be authorized to
   receive network access via a Remote Access Server (RAS), Virtual
   Private Network (VPN) server, or LAN access switch.  Typically only
   small a sub-set of all users are authorized to access the
   administrative interfaces of network infrastructure devices, e.g. the
   Command Line Interface (CLI) or SNMP engine of switches and routers.

   In order for the RADIUS server to have information regarding the type
   of access being requested, it is common for the NAS (i.e. the RADIUS
   client) to include "hint" attributes in the RADIUS Access-Request
   message, describing the NAS and the type of service being requested.
   This document recommends appropriate "hint" attributes for the SNMP
   Transport Model service type.







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1.3.  RADIUS Usage With Secure Transports

   Secure transport protocols used with SNMP Transport Models have
   defined authentication protocols that allows for authentication by
   various methods.  For example, the Secure Shell (SSH) Authentication
   protocol [RFC4252] describes an authentication protocol and support
   multiple methods that can be used SSH servers to authenticate the SSH
   client, these methods include Public Key, Password and Host
   (e.g.hosts.equiv).

   SSH Server integration with RADIUS traditionally uses the username
   and password mechanism.

   Secure transport protocols do not, however, specify how the transport
   interfaces to authentication clients, leaving such as implementation
   specific.  For e.g., the "password" method of SSH authentication
   primarily describes how passwords are acquired from the SSH client
   and transported to the SSH server, the interpretation of the password
   and validation against password databases is left to SSH server
   implementations.  SSH server implementations often use the Pluggable
   Authentication Modules (PAM) interface provided by operating systems
   such as Linux and Solaris to integrate with password based network
   authentication mechanisms such as RADIUS, TACACS+, Kerberos, etc.

   Secure transports do not typically specify how to utilize
   authorization information obtained from an AAA service, such as
   RADIUS.  More often, user authentication is sufficient to cause the
   secure transport server to begin delivering service to the user.
   Access control in these situations is supplied by the application to
   which the secure transport server session is attached.  For example,
   if the application is a Linux shell, the user's access rights are
   controlled by that user account's group membership and the file
   system access protections.  This behavior does not closely follow the
   traditional service provisioning model of AAA systems, such as
   RADIUS.

1.4.  SNMP Transport Models

   The Transport Subsystem for SNMP [tmsm] defines a mechanism for
   providing transport layer security for SNMP, allowing protocols such
   as SSH and TLS to be used to secure SNMP communication.  The
   Transport Subsystem allows the modular definition of Transport Models
   for multiple secure transport protocols.  Transport Models rely upon
   the underlying secure transport for user authentication services.
   The Transport Model (TM) then maps the authenticated identity to a
   model-independent principal, which it stores in the tmStateReference.
   When the selected security model is the Transport Security Model
   (TSM), the expected behavior is for the securityName to be set by the



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   TSM from the authenticated principal information stored in the
   tmStateReference by the TM.

   The Secure Shell protocol provides a secure transport channel with
   support for channel authentication via local accounts and integration
   with various external authentication and authorization services such
   as RADIUS, Kerberos, etc.  The Secure Shell Transport Model [sshtm]
   defines the use of the Secure Shell protocol as the basis for a
   Transport Model.


2.  RADIUS Usage for SNMP Transport Models

   There are three ways in which RADIUS may be used to inform the use of
   SNMP Transport Models.  These include (a) user authentication, (b)
   service authorization and (c) access control authorization.  The
   first two items are discussed in detail in this memo, while the third
   item is a topic of current research, and beyond the scope of this
   document.  This document describes the way in which RADIUS attributes
   and messages are applied to the specific application area of SNMP
   Transport Models.

   User authentication for SNMP Transport Models has the same syntax and
   semantics as user authentication for any other network service.  In
   the context of SNMP the "user" is thought of as a "principal" and may
   represent a host, an application or a human.

   Service authorization allows a RADIUS server to authorize a
   authenticated principal to use SNMP over a specific secure Transport
   Model.  This memo describes mechanisms by which such information can
   be requested from a RADIUS server and enforced within the NAS.  The
   SNMP architecture, as described in RFC 3411, does not make a
   distinction between user authentication and service authorization.
   In the case of existing, deployed models, such as the User-based
   Security Model (USM), this distinction is not significant.  For the
   SNMP Transport Models and the SNMP Transport Security Model (TSM),
   this distinction is relevant, and perhaps important.

   Data object access control authorization in SNMP is handled by the
   Access Control Subsystem (ACS), instantiated as various Access
   Control Models.  The SNMP architecture, as described in RFC 3411,
   explicitly mandates the separation of authentication and
   authorization operations in order to retain modularity of the SNMP
   system.  The Abstract Service Interface (ASI) of the ACM uses method-
   independent parameters, including securityName, to determine access
   control rights.  A detailed description of how an Access Control
   Method (ACM) might utilize the services of a RADIUS client to obtain
   access control policy information is the topic of current research,



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   and beyond the scope of this document.

2.1.  RADIUS Authentication for Transport Protocols

   This document will rely of implementation specific integration of the
   transport protocols with RADIUS clients for user authentication.

   It is RECOMMENDED that the integration of RADIUS clients with
   transport protocols utilize appropriate "hint" attributes in RADIUS
   Access-Request messages, to signal to the RADIUS server they type of
   service being requested over the transport session.  Specific
   attributes for use with SNMP Transport Models are recommended in this
   document.

   RADIUS servers, compliant to this specification, MAY use RADIUS hint
   attributes, as described herein, to inform the decision whether to
   accept or reject the authentication request.

2.2.  RADIUS Authorization for Transport Protocols

   In compliance with RFC 2865, NASes MUST enforce implicitly mandatory
   attributes, such as Service-Type, within an Access-Accept message.
   NASes MUST treat Access-Accept Messages that attempt to provision
   unsupported services as if they were an Access-Reject.  NASes SHOULD
   treat unknown attributes as if they were provisioning unsupported
   services.  See [radius-fixes] for additional details.

   A NAS that is compliant to this specification, MUST treat any RADIUS
   Access-Accept message that provisions a transport protocol (e.g.
   SSH) that cannot be provided, and/or application service (e.g.  SNMP)
   that cannot be provided over that transport, as if an Access-Reject
   message had been received instead.  The RADIUS Service-Type attribute
   is the primary indicator of the service being provisioned, although
   other attributes may also convey service provisioning information.
   Specific attributes for use with SNMP Transport Models are
   recommended in this document.

   For traditional SSH usage, RADIUS servers typically provision
   management access service, as SSH is often used to access the command
   line shell of a host system, e.g. the NAS.  RFC 2865 defines two
   types of management access service attributes, one for privileged
   access to the Command Line Interface (CLI) of the NAS and one for
   non-privileged CLI access.  These traditional management access
   services are not used with SNMP. [radman] describes further RADIUS
   service provisioning attributes for management access to the NAS,
   including SNMP access.





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2.3.  SNMP Service Authorization

   The Transport Subsystem for SNMP [tmsm] defines the notion of a
   session, although the specifics of how sessions are managed is left
   to Transport Models.  The Transport Subsystem defines some basic
   requirements for transport protocols around creation and deletion of
   sessions.  This memo specifies additional requirements for transport
   protocols during session creation, and for session termination.

   RADIUS servers compliant to this specification SHOULD use RADIUS
   service provisioning attributes, as described herein, to specify SNMP
   access over a secure transport protocol.  Such RADIUS servers MAY use
   RADIUS hint attributes included in the Access-Request message, as
   described herein, in determining what, if any, service to provision.

   NASes compliant to this specification MUST use RADIUS service
   provisioning attributes, as described in this section, when they are
   present in a RADIUS Access-Accept message, to determine whether the
   session can be created and MUST enforce the service provisioning
   decisions of the RADIUS server.

   The following RADIUS attributes SHOULD be used, as hint attributes
   included in the Access-Request message to signal use of SNMP over a
   secure transport to the RADIUS server:

   1.  Service-Type with a value of Framed-Management.
   2.  Framed-Management-Protocol with a value of SNMP-Transport-Model.
   3.  Management-Transport with a value of SSH or TLS, as appropriate.

   Refer to [radman] for a detailed description of these attributes.
   From the perspective of the RADIUS Server, these attribute and value
   pairs indicate that the user is requesting to use SNMP over an SNMP
   Transport Model.

   The following RADIUS attributes are used in an Access-Accept message
   to provision SNMP over a secure transport:

   1.  Service-Type with a value of Framed-Management.
   2.  Framed-Management-Protocol with a value of SNMP-Transport-Model.

   Refer to [radman] for a detailed description of these attributes.
   From the perspective of the NAS, these two attribute and value pairs
   indicate that the user is authorized to use SNMP using an SNMP
   Transport Model.

   The following RADIUS attributes MAY be optionally be used, to
   authorize use of SNMP over a specific transport protocol:




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   1.  Management-Transport with a value of SSH or TLS.

   Refer to [radman] for a detailed description of this attribute.  From
   the perspective of the NAS, this attribute and value pair indicates
   that the user is authorized to use SNMP using the specific SNMP
   Transport Model.  In the case of a Management-Transport attribute
   with a value of SSH, together with a Framed-Management-Protocol
   attribute with a value of SNMP-Transport-Model, and a Service-Type
   attribute with a value of Framed-Management, use of the SSH Transport
   Model is indicated.

   The following RADIUS attributes are used to limit the extent of a
   secure transport session carrying SNMP traffic, in conjunction with
   an SNMP Transport Model:

   1.  Session-Timeout
   2.  Inactivity-Timeout.

   Refer to [RFC2865] for a detailed description of these attributes.
   From the perspective of the NAS, these attributes indicate session
   timeouts to be applied to the secure transport sessions.  The
   Session-Timeout attribute indicates the maximum number of seconds
   that a session may exist before it is unconditionally disconnected.
   The Inactivity-Timeout attribute indicates the maximum number of
   seconds that a transport session may exist without any protocol
   activity (messages sent or received) before the session is
   disconnected.  These timeouts are enforced by the NAS.

2.4.  SNMP Access Control Authorization

   [radman] describes a RADIUS attribute that can be used for SNMP
   access control authorization, however, the details of how an SNMP
   Access Control Model, such as the View-based Access Control Model
   (VACM), might utilize RADIUS authorization are the topic of current
   research, and beyond the scope of this document.


3.  Table of Attributes

   The following table provides a guide to which attributes may be found
   in which kinds of packets, and in what quantity.










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  Access-
  Request Accept Reject Challenge  #    Attribute
  ---------------------------------------------------------------------
  0-1     0        0        0       1   User-Name  [RFC2865]
  0-1     0        0        0       2   User-Password  [RFC2865]
  0-1     0        0        0       4   NAS-IP-Address  [RFC2865]
  0-1     0-1      0        0       6   Service-Type  [RFC2865]
  0-1     0-1      0        0-1    24   State  [RFC2865]
  0       0-1      0        0      27   Session-Timeout  [RFC2865]
  0       0-1      0        0      28   Idle-Timeout  [RFC2865]
  0-1     0-1      0-1      0-1    80   Message-Authenticator  [RFC3579]
  0-1     0-1      0        0     TBA   Framed-Management-Protocol
                                                                [radman]
  0-1     0-1      0        0     TBA   Transport-Protocol  [radman]
  0       0+       0        0     TBA   Management-Policy-Id  [radman]

  The following table defines the meaning of the above table entries.

  0    This attribute MUST NOT be present in a packet.
  0+   Zero or more instances of this attribute MAY be present in
       a packet.
  0-1  Zero or one instance of this attribute MAY be present in
       a packet.
  1    Exactly one instance of this attribute MUST be present in
       a packet.

   Note that this document does not describe the usage of RADIUS
   Accounting, nor Dynamic RADIUS Re-Authorization.  Such RADIUS usages
   are not currently envisioned for SNMP, and are beyond the scope of
   this document.


4.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an
   RFC.


5.  Security Considerations

   This specification describes the use of RADIUS for purposes of
   authentication and authorization.  Threats and security issues for
   this application are described in [RFC3579] and [RFC3580]; security
   issues encountered in roaming are described in [RFC2607].

   Additional security considerations for use of SNMP with secure



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   Transport Models [sshtm] and the Transport Security Model [sshtm] are
   found in the Security Considerations sections of the respective
   documents.

   Note that if the SNMP Message Processing Module selects the SNMPv1 or
   SNMPv2c Security Model as the security model to use (because the
   message is SNMPv1 or SNMPv2), then securityName comes from the
   community name, as per RFC3584.  This may not be what is expected
   when using an SNMP secure Transport Model.

   Note that if the SNMP User-based Security Model is selected (because
   the SNMPv3 message contains a msgSecurityModel=USM), then
   securityName is determined using USM (after performing USM
   authentication).  This may not ne what is expected when using an SNMP
   secure Transport Model with an external authentication service, such
   as RADIUS.

   The Message-Authenticator (80) attribute SHOULD be used with RADIUS
   messages that are described in this memo, as defined in [RFC3579].


6.  Acknowledgements

   The authors would like to acknowledge the contributions of Dave
   Harrington and Juergen Schoenwaelder for numerous helpful discussions
   in this space.


7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, June 2000.

   [RFC4252]  "The Secure Shell Authentication Protocol", 2005.

   [radman]   Nelson, D. and G. Weber, "Remote Authentication Dial-In
              User Service (RADIUS) Authorization for Network Access
              Server (NAS) Management",
              draft-ietf-radext-management-authorization-00.txt (work in
              progress), August 2007.

   [sshtm]    Harrington, D. and J. Salowey, "Secure Shell Transport



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              Model for SNMP", draft-ietf-isms-secshell-08.txt (work in
              progress), July 2007.

   [tmsm]     Harrington, D. and J. Schoenwaelder, "Transport Subsystem
              for the Simple Network Management Protocol (SNMP)",
              draft-ietf-isms-tmsm-09.txt (work in progress), July 2007.

   [tsm]      Harrington, D., "Transport Subsystem for the Simple
              Network Management Protocol (SNMP)",
              draft-ietf-isms-transport-security-model-05.txt (work in
              progress), July 2007.

7.2.  Informative References

   [RFC2607]  Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
              Implementation in Roaming", RFC 2607, June 1999.

   [RFC3579]  Aboba, B. and P. Calhoun, "RADIUS (Remote Authentication
              Dial In User Service) Support For Extensible
              Authentication Protocol (EAP)", RFC 3579, September 2003.

   [RFC3580]  Congdon, P., Aboba, B., Smith, A., Zorn, G., and J. Roese,
              "IEEE 802.1X Remote Authentication Dial In User Service
              (RADIUS) Usage Guidelines", RFC 3580, September 2003.

   [radius-fixes]
              Nelson, D. and A. DeKok, "Common RADIUS Implementation
              Issues and Suggested Fixes",
              draft-ietf-radext-fixes-06.txt (work in progress),
              August 2007.


Authors' Addresses

   Kaushik Narayan
   Cisco Systems, Inc.
   10 West Tasman Drive
   San Jose, CA  95134
   USA

   Phone: +1 408-526-8168
   Email: kaushik_narayan@yahoo.com









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   David Nelson
   Elbrys Networks, Inc.
   75 Rochester Ave, Unit #3,
   Portsmouth, NH  03801
   USA

   Phone: +1 (603) 570-2636
   Email: dnelson@comcast.net











































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Full Copyright Statement

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Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).





Narayan & Nelson          Expires March 4, 2008                [Page 14]


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