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
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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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