Network Working Group K. Narayan
Request for Comments: 5608 Cisco Systems, Inc.
Category: Standards Track D. Nelson
Elbrys Networks, Inc.
August 2009
Remote Authentication Dial-In User Service (RADIUS) Usage for
Simple Network Management Protocol (SNMP) Transport Models
Abstract
This memo describes the use of a Remote Authentication Dial-In User
Service (RADIUS) authentication and authorization service with 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 (SSH) Transport Model.
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 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 in effect on the date of
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Please review these documents carefully, as they describe your rights
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Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
Narayan & Nelson Standards Track [Page 1]
RFC 5608 RADIUS Usage for SNMP Transport Models August 2009
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Table of Contents
1. Introduction ....................................................2
1.1. General ....................................................2
1.2. Requirements Language ......................................3
1.3. System Block Diagram .......................................3
1.4. RADIUS Operational Model ...................................3
1.5. RADIUS Usage with Secure Transports ........................5
1.6. Domain of Applicability ....................................5
1.7. SNMP Transport Models ......................................6
2. RADIUS Usage for SNMP Transport Models ..........................7
2.1. RADIUS Authentication for Transport Protocols ..............8
2.2. RADIUS Authorization for Transport Protocols ...............8
2.3. SNMP Service Authorization .................................9
3. Table of Attributes ............................................11
4. Security Considerations ........................................12
5. Acknowledgements ...............................................13
6. References .....................................................13
6.1. Normative References ......................................13
6.2. Informative References ....................................13
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.
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. 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 would quite likely be implemented in the secure
transport module.
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1.2. 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].
1.3. System Block Diagram
A block diagram of the major system components referenced in this
document may be useful to understanding the text that follows.
+--------+
+......................... |RADIUS |....+
. |Server | .
Shared +--------+ .
User | .
Credentials RADIUS | Shared
. | RADIUS
. | Secret
. | .
+-------------+ +-----------------+
| Network | | RADIUS Client / |
| Management | SNMP | SNMP Engine / |
| Application |------------------| Network Device |
+-------------+ SSH +-----------------+
Block Diagram
This diagram illustrates that a network management application
communicates with a network device, the managed entity, using SNMP
over SSH. The network devices uses RADIUS to communicate with a
RADIUS server to authenticate the network management application (or
the user whose credentials that application provides) and to obtain
authorization information related to access via SNMP for purpose of
device management. Other secure transport protocols might be used
instead of SSH.
1.4. RADIUS Operational Model
The RADIUS protocol [RFC2865] provides authentication and
authorization services for network access devices, usually referred
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.
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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-
Reject message indicates unsuccessful authentication.
Access-Accept messages are populated with one or more service
provisioning attributes, which 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. 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 with secure transports and SNMP Transport
Models.
RADIUS servers are often deployed on an enterprise-wide 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 a
small 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.
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This document recommends appropriate "hint" attributes for the SNMP
service type.
1.5. RADIUS Usage with Secure Transports
Some secure transport protocols that can be used with SNMP Transport
Models have defined authentication protocols supporting several
authentication methods. For example, the Secure Shell (SSH)
Authentication protocol [RFC4252] supports multiple methods
(including public key, password, and host-based) to authenticate SSH
clients.
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 example, 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+ (Terminal Access
Controller Access-Control System Plus), Kerberos, etc.
Secure transports do not typically specify how to utilize
authorization information obtained from a 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.
1.6. Domain of Applicability
Most of the RADIUS Attributes referenced in this document have broad
applicability for provisioning remote management access to NAS
devices using SNMP. However, the selection of secure transport
protocols has special considerations. This document does not specify
details of the integration of secure transport protocols with a
RADIUS client in the NAS implementation. However, there are
functional requirements for correct application of framed management
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protocols and secure transport protocols that will limit the
selection of such protocols that can be considered for use with
RADIUS. Since the RADIUS user credentials are obtained by the RADIUS
client from the secure transport protocol server, or in some cases
directly from the SNMP engine, the secure transport protocol, and its
implementation in the NAS, MUST support forms of credentials that are
compatible with the authentication methods supported by RADIUS.
RADIUS currently supports the following user authentication methods,
although others may be added in the future:
o Password - RFC 2865
o CHAP (Challenge Handshake Authentication Protocol) - RFC 2865
o ARAP (Apple Remote Access Protocol) - RFC 2869
o EAP (Extensible Authentication Protocol) - RFC 2869, RFC 3579
o HTTP Digest - RFC 5090
The secure transport protocols selected for use with RADIUS and SNMP
obviously need to support user authentication methods that are
compatible with those that exist in RADIUS. The RADIUS
authentication methods most likely usable with these protocols are
Password, CHAP, and possibly HTTP Digest, with Password being the
distinct common denominator. There are many secure transports that
support other, more robust, authentication mechanisms, such as public
key. RADIUS has no support for public key authentication, except
within the context of an EAP Method. The applicability statement for
EAP indicates that it is not intended for use as an application-layer
authentication mechanism, so its use with the mechanisms described in
this document is NOT RECOMMENDED. In some cases, Password may be the
only compatible RADIUS authentication method available.
1.7. SNMP Transport Models
The Transport Subsystem for SNMP [RFC5590] defines a mechanism for
providing transport layer security (TLS) 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 [RFC5592]
defines the use of the Secure Shell protocol as the basis for a
Transport Model.
2. RADIUS Usage for SNMP Transport Models
There are two use cases for RADIUS support of management access via
SNMP. These are (a) service authorization and (b) access control
authorization. RADIUS almost always involves user authentication as
prerequisite to authorization, and there is a user authentication
phase for each of these two use cases. The first use case is
discussed in detail in this memo, while the second use case 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 and service authorization via
RADIUS are undertaken by the secure transport module, that underlies
the SNMP Transport Model.
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 an
authenticated principal to use SNMP, optionally over a secure
transport, typically using an SNMP Transport Model. This memo
describes mechanisms by which such information can be requested from
a RADIUS server and enforced within the NAS. An SNMP architecture,
[RFC3411], does not make a distinction between user authentication
and service authorization. In the case of existing, deployed
security models, such as the User-based Security Model (USM), this
distinction is not significant. For SNMP Transport Models, this
distinction is relevant and important.
It is relevant because of the way in which SSH implementations have
traditionally integrated with RADIUS clients. Those SSH
implementations traditionally seek to obtain user authentication
(e.g., validation of a username and password) from an outside
authentication service, often via a PAM-style interface. The service
authorization in traditional SSH server implementations comes via the
restrictions that the operating system (OS) shell (and file system,
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etc.) place on the user by means of access controls tied to the
username or the username's membership in various user groups. These
OS-style access controls are distinct from the service provisioning
features of RADIUS. If we wish to use existing SSH server
implementations, or slightly adapt them, for use with SNMP Transport
Models, and we wish to support RADIUS-provisioned service
authorization, we need to be aware that the RADIUS service
authorization information will need to be obtained by the relevant
SNMP models from the SSH module.
One reason that RADIUS-provisioned service authorization is important
is that in many deployments, the RADIUS server's back-end
authentication database contains credentials for many classes of
users, only a small portion of which may be authorized to access the
management interfaces of managed entities (NASes) via SNMP. This is
in contrast to the way USM for SNMP works, in which all principals
entered to the local configuration data-store are authorized for
access to the managed entity. In the absence of RADIUS-provisioned
service authorization, network management access may be granted to
unauthorized, but properly authenticated, users. With SNMPv3, an
appropriately configured Access Control Model would serve to
alleviate the risk of unauthorized access.
2.1. RADIUS Authentication for Transport Protocols
This document will rely on implementation specific integration of the
transport protocols with RADIUS clients for user authentication.
It is REQUIRED that the integration of RADIUS clients with transport
protocols utilize appropriate "hint" attributes in RADIUS Access-
Request messages, to signal to the RADIUS server the 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 [RFC5080] for additional details.
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A NAS that is compliant to this specification MUST treat any RADIUS
Access-Accept message that provisions a level of transport protection
(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.
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. [RFC5607] describes further RADIUS
service provisioning attributes for management access to the NAS,
including SNMP access.
2.3. SNMP Service Authorization
The Transport Subsystem for SNMP [RFC5590] 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 MUST use RADIUS
service provisioning attributes, as described herein, to specify SNMP
access over a secure transport. 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 they MUST enforce the service
provisioning decisions of the RADIUS server.
The following RADIUS attributes MUST be used, as "hint" attributes
included in the Access-Request message to signal use of SNMP over a
secure transport (i.e., authPriv) to the RADIUS server:
1. Service-Type with a value of Framed-Management.
2. Framed-Management-Protocol with a value of SNMP.
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3. Management-Transport-Protection with a value of Integrity-
Confidentiality-Protection.
The following RADIUS attributes MUST be used in an Access-Accept
message to provision SNMP over a secure transport that provides both
integrity and confidentiality (i.e., authPriv):
1. Service-Type with a value of Framed-Management.
2. Framed-Management-Protocol with a value of SNMP.
3. Management-Transport-Protection with a value of Integrity-
Confidentiality-Protection.
The following RADIUS attributes MUST be optionally used, to authorize
use of SNMP without protection (i.e., authNoPriv):
1. Service-Type with a value of Framed-Management.
2. Framed-Management-Protocol with a value of SNMP.
3. Management-Transport-Protection with a value of No-Protection.
There are no combinations of RADIUS attributes that denote the
equivalent of SNMP noAuthNoPriv access, as RADIUS always involves the
authentication of a user (i.e., a principal) as a prerequisite for
authorization. RADIUS can be used to provide an "Authorize-Only"
service, but only when the request contains a "cookie" from a
previous successful authentication with the same RADIUS server (i.e.,
the RADIUS State Attribute).
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.
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.
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3. Table of Attributes
Table 1 provides a guide to which attributes may be found in which
kinds of packets, and in what quantity.
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 0 0 95 NAS-IPv6-Address [RFC3162]
0-1 * 0 0 0 32 NAS-Identifier [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 0 133 Framed-Management-Protocol
[RFC5607]
0-1 0-1 0 0 134 Management-Transport-Protection
[RFC5607]
Table 1
Table 2 defines the meaning of the entries in Table 1.
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.
* Only one of these attribute options SHOULD be included.
Table 2
SSH integration with RADIUS traditionally uses usernames and
passwords (with the User-Password Attribute), but other secure
transports could use other authentication mechanisms, and would
include RADIUS authentication attributes appropriate for that
mechanism instead of User-Password.
This document does not describe the usage of RADIUS Accounting or
Dynamic RADIUS Re-Authorization. Such RADIUS usages are not
currently envisioned for SNMP, and are beyond the scope of this
document.
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4. 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
Transport Models [RFC5590] and the Transport Security Model [RFC5591]
are found in the "Security Considerations" sections of the respective
documents.
If the SNMPv1 or SNMPv2c Security Model is used, then securityName
comes from the community name, as per RFC 3584. If the User-based
Security Model is selected, then securityName is determined using
USM. This may not be what is expected when using an SNMP secure
Transport Model with an external authentication service, such as
RADIUS.
Simultaneously using a secure transport with RADIUS authentication
and authorization, and the SNMPv1 or SNMPv2c or USM security models
is NOT RECOMMENDED. See the "Coexistence, Security Parameters, and
Access Control" section of [RFC5590].
There are good reasons to provision USM access to supplement AAA-
based access, however. When the network is under duress, or the AAA-
service is unreachable, for any reason, it is important to have
access credentials stored in the local configuration data-store of
the managed entity. USM credentials are a likely way to fulfill this
requirement. This is analogous to configuring a local "root"
password in the "/etc/passwd" file of a UNIX workstation, to be used
as a backup means of login, for times when the Network Information
Service (NIS) authentication service is unreachable.
The Message-Authenticator (80) Attribute [RFC3579] SHOULD be used
with RADIUS messages that are described in this memo. This is useful
because the Message-Authenticator Attribute is the best available
mechanism in RADIUS as it stands today to provide tamper-evident
integrity protection of the service provisioning attributes in an
Access-Accept packet. It is slightly less important for Access-
Request packets, although it may be desirable to protect any "hint"
attributes contained in those messages. This protection mitigates
the fact that RADIUS messages are not encrypted and that attributes
could be added, deleted or modified by an adversary in a position to
intercept the packet.
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5. Acknowledgements
The authors would like to acknowledge the contributions of David
Harrington and Juergen Schoenwaelder for numerous helpful discussions
in this space, and Wes Hardaker for his thoughtful review comments.
6. References
6.1. 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.
[RFC5080] Nelson, D. and A. DeKok, "Common Remote Authentication
Dial In User Service (RADIUS) Implementation Issues and
Suggested Fixes", RFC 5080, December 2007.
[RFC5590] Harrington, D. and J. Schoenwaelder, "Transport Subsystem
for the Simple Network Management Protocol (SNMP)",
RFC 5590, June 2009.
[RFC5591] Harrington, D. and W. Hardaker, "Transport Security Model
for Simple Network Management Protocol (SNMP)", RFC 5591,
June 2009.
[RFC5607] Nelson, D. and G. Weber, "Remote Authentication Dial-In
User Service (RADIUS) Authorization for Network Access
Server (NAS) Management", RFC 5607, July 2009.
6.2. Informative References
[PAM] Samar, V. and R. Schemers, "UNIFIED LOGIN WITH PLUGGABLE
AUTHENTICATION MODULES (PAM)", Open Group RFC 86.0,
October 1995,
<http://www.opengroup.org/rfc/mirror-rfc/rfc86.0.txt>.
[RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
Implementation in Roaming", RFC 2607, June 1999.
[RFC3162] Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6",
RFC 3162, August 2001.
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[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
December 2002.
[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.
[RFC4252] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Authentication Protocol", RFC 4252, January 2006.
[RFC5592] Harrington, D., Salowey, J., and W. Hardaker, "Secure
Shell Transport Model for Simple Network Management
Protocol (SNMP)", RFC 5592, June 2009.
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
David Nelson
Elbrys Networks, Inc.
282 Corporate Drive
Portsmouth, NH 03801
USA
Phone: +1.603.570.2636
EMail: dnelson@elbrysnetworks.com
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