From: David T. Perkins [dperkins@dsperkins.com]
Sent: Monday, February 23, 2004 1:58 AM
To: Wes Hardaker; sbsm@machshav.com
Subject: Re: [Sbsm] new draft
At 09:18 PM 2/21/2004 -0800, Wes Hardaker wrote:
>Dave? Can you post the complimentary draft here as well?
>
>--
Here it is....
Network Working Group D. Perkins
Internet-Draft SNMPInfo
Expires: August 16, 2004 February 16, 2004
A Consolidated Overview of Version 3 of the Simple Network Management
Protocol (SNMPv3)
draft-perkins-snmpv-overview-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on August 16, 2004.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document provides a concise overview of the SNMPv3 protocol. It
is designed to quickly bring a new reader up to speed with the
internal workings of the SNMPv3 protocol and its security features.
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Table of Contents
1. SNMPv3 Overview . . . . . . . . . . . . . . . . . . . . . . 3
1.1 SNMPv3 Message Format . . . . . . . . . . . . . . . . . . . 4
1.2 SNMPv3 Security . . . . . . . . . . . . . . . . . . . . . . 7
1.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2.2 Security Model Threats . . . . . . . . . . . . . . . . . . . 7
1.2.3 View-based Access Control Model (VACM) . . . . . . . . . . . 8
1.3 SNMPv3 Engines and Applications . . . . . . . . . . . . . . 9
Normative References . . . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . 11
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1. SNMPv3 Overview
The SNMP model of a managed network contains four basic components:
o several (typically many) managed nodes, each with an SNMP entity
which provides remote access to management instrumentation
(traditionally called an agent);
o at least one SNMP entity with management applications (typically
called a manager);
o a management protocol used to convey management information
between the SNMP entities (such as managers and agents); and
o management information.
The management messages are either a request-response pair between a
manager and agent, or a notification with potentially a confirmation
from an agent to a manager. There are three additional types of SNMP
entities:
proxy: forwards SNMP messages from one entity to another. It may be
used for translation from one version of SNMP protocol to another,
or as a transport layer or security gateway;
mid-level-manager: used in a multi-layer management system. Interacts
with a higher level manager (which is also called a north-bound
manager) using more abstract management information and
communicates with many managed systems (which are also called
network elements (NEs)) for specific management information. In
basic SNMP entity-speak, a mid-level-manager contains elements of
both an SNMP agent and manager;
observer: used to watch one or more managed systems and when an event
of importance occurs, then it will tell a manager using an SNMP
notification operation
Each instance of management information is uniquely identified by the
triple:
SNMP entity engineID: uniquely identifies, within an administrative
domain, an SNMP entity which has direct access to management
information (typically the management information is contained in
the system containing the SNMP entity);
context Name: uniquely identifies for an SNMP entity a collection of
management information from the one more collections supported by
the SNMP entity;
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instanceID: uniquely identifies an instance of management information
in a collection. The instanceID is specified as an ASN.1 object
identifier value. The prefix is an ASN.1 object identifier value
that identifies a class (called an SNMP object type), which is
defined with the OBJECT-TYPE construct in SNMP MIB modules. The
suffix is an ASN.1 object identifier value fragment, which is the
encoded index values for columnar object types, or a single
sub-identifier with value zero for scalar object types.
1.1 SNMPv3 Message Format
SNMP messages are defined in ASN.1-1988, and serialized to a sequence
of octets using the BER-1988. A message is sent over a connection
oriented transport (such as TCP), or a connectionless transport (such
as UDP). The connectionless transport is preferred due to its low
overhead and robustness in a network with packet loss, high variation
of packet delay, or packet duplication.
An SNMPv3 message is the ASN.1 sequence SNMPv3Message
[RFC3412:section 6] with the format:
----------------------------------------------------------------
| msgVersion | msgGlobalData | msgSecurityParameters | msgData |
----------------------------------------------------------------
where:
msgVersion - an integer that indicates the format of the
message,which can be:
- snmpv1(0) [RFC1157]
- snmpv2c(1) [RFC1901]
- snmpv2u(2) [RFC1910]
- snmpv3(3) [RFC3412]
msgGlobalData - a sequence of type HeaderData (see below)
msgSecurityParameters - an octet string, which is the
serialization using the BER of a security model
dependent sequence
msgData - a choice of type ScopedPduData (see below)
The field "msgGlobalData" is the ASN.1 sequence HeaderData
[RFC3412:section 6] with the format:
----------------------------------------------------
| msgID | msgMaxSize | msgFlags | msgSecurityModel |
----------------------------------------------------
where:
msgID - an integer, which is used to pair request and
response messages
msgMaxSize - an integer, which specifies the maximum
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size of a message that can be received by the sender
msgFlags - a bit string with the following legal values:
'100'b - a noAuthNoPriv request (GET, GETNEXT, SET, GETBULK)
'000'b - a noAuthNoPriv response (RESPONSE or REPORT) or
unacknowledged notification (TRAP)
'101'b - an authNoPriv request (GET, GETNEXT, SET, GETBULK)
'001'b - an authNoPriv response (RESPONSE or REPORT) or
unacknowledged notification (TRAP)
'111'b - an authPriv request (GET, GETNEXT, SET, GETBULK)
'011'b - an authPriv response (RESPONSE or REPORT) or
unacknowledged notification (TRAP)
where:
noAuthNoPriv - means that the entire message is not
authenticated, nor is the contents of field
msgData encrypted
authNoPriv - means that the entire message is
authenticated using the mechanism specified by the
value of field msgSecurityModel using, if needed,
values from the sequence encoded in the
msgSecurityParmeters field, and the contents
of field msgData is not encrypted
authPriv - means that the entire message is
authenticated using the mechanism specified by the
value of field msgSecurityModel using, if needed,
values from the sequence encoded in the
msgSecurityParmeters field, and the contents
of field msgData is encrypted using, if needed,
values from the sequence encoded in the
msgSecurityParmeters field
msgSecurityModel - a nonnegative integer, which identifies
a security model, and implicitly the format of the
content of field msgSecurityParameters. The rules
for values is specified by textual convention
SnmpSecurityModel in RFC 3411.
The field "scopedPDUdata" is the ASN.1 choice ScopedPDUdata
[RFC3412:section 6] and can be either the field called "encryptedPDU"
(with ASN.1 data type of OCTET STRING) or the field called
"plaintext" (which is sequence ScopedPDU). The format of the field
"plaintext" is:
---------------------------------------------------------
| an OCTET STRING, which is the encrypted serialization |
| using BER of a scopedPDU sequence |
---------------------------------------------------------
And the format of the sequence ScopedPDU is:
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------------------------------
| engineID | contextID | pdu |
------------------------------
where:
engineID - an octet string, which identifies the system
containing the management information specified in
the PDU sequence
contextID - an octet string, which further qualifies
which collection of management information is specified
in the PDU sequence. Note that the value that is
a zero length string is a "required" contextID and
contains instances of objects that are used for
managing the SNMPv3 protocol
pdu - a tagged sequence of pairs. The tag identifies the
PDU type and corresponds to the operation. The most
recent definitions are specified in section 3 of
RFC 3416. And where each pair contains the
identification of management information (either
exact or an approximation depending on the
PDU type), and value (or placeholder for the value)
of the identified management information.
For USM, the value of field msgSecurityParmeters is the BER
serialization of sequence UsmSecurityParameters [RFC3414: section
2.4], which has format:
-----------------------------------------------------------------
| engID | engBoots | engTime | userName | authParms | encrParms |
-----------------------------------------------------------------
where:
engID - identifies SNMP entity (either the message sender
or receiver) that contains the "authoritative copy" of
the security information shared by the sender and receiver,
and is called the "authoritative engine". Note that
USM specifies for each PDU type whether the sender
or the receiver is authoritative.
engBoots - specifies the sender's notion of the value of the
single instance of object snmpEngineBoots of the
authoritative engine
engTime - specifies the sender's notion of the value of the
single instance of object snmpEngineTime of the
authoritative engine
userName - identifies a USM user whose security information
will be used, if needed, to authenticate and encrypt the
message
authParms - values, if needed, by the USM authentication protocol
used to authenticate the message
encrParms - values, if needed, by the USM encryption protocol to
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encrypt the serialization using BER of the scopedPDU value
of the message
1.2 SNMPv3 Security
1.2.1 Introduction
SNMPv3 views security as being applied at two different stages,
either in the tranmission/receipt of messages, and in the processing
of the contents of messages. SNMPv3 uses the term "security model"
to refer to message-level security and the term "access control
model" to the security applied to message content processing.
In message level processing, the identity of the sender must be
determined and authenticated, and each message must be authenticated.
The message level processing may also provide encryption and
decryption of message content. SNMPv3 assigns a label to the message
level security, which is one of "noAuthNoPriv", "authNoPriv", and
"authPriv" (see message description for definitions.) Message
content processing is concerned with access to instances of
management information. Authorization rules determine for each
identity the allowed access operations for each instance of
management information. Authorization rules may also require that
access is only allowed via SNMP messages at a specified or higher
level of security.
In summary, message level security requires authentication of the
sender's identity, and athentication of messages, and potentially
message content encryption. Authorization for message processing to
access management information requires the authenticated identity
from a message, and the message security level (that was or will be
applied to the message) as input to a rules database.
1.2.2 Security Model Threats
The SNMPv3 architecture specifies requirements for a security model
based on type of threat, which are:
The principal threats against which any SNMPv3 security model
SHOULD provide protection are:
Modification of Information: The modification threat is the danger
that some unauthorized entity may alter in-transit SNMP
messages generated on behalf of an authorized principal in such
a way as to effect unauthorized management operations,
including falsifying the value of an object.
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Masquerade: The masquerade threat is the danger that management
operations not authorized for some principal may be attempted
by assuming the identity of another principal that has the
appropriate authorizations.
Secondary threats against which any SNMPv3 security model SHOULD
provide protection are:
Message Stream Modification: The SNMP protocol is typically based
upon a connectionless transport service which may operate over
any subnetwork service. The re-ordering, delay or replay of
messages can and does occur through the natural operation of
many such subnetwork services. The message stream modification
threat is the danger that messages may be maliciously
re-ordered, delayed or replayed to an extent which is greater
than can occur through the natural operation of a subnetwork
service, in order to effect unauthorized management operations.
Disclosure: The disclosure threat is the danger of eavesdropping
on the contents of SNMP messages.
There are at least two threats against which any SNMPv3 security
model need not protect, since they are deemed to be of lesser
importance in this context:
Denial of Service: A security model need not attempt to address
the broad range of attacks by which service on behalf of
authorized users is denied. Indeed, such denial-of-service
attacks are in many cases indistinguishable from the type of
network failures with which any viable management protocol must
cope as a matter of course.
Traffic Analysis: A security model need not attempt to address
traffic analysis attacks. Many traffic patterns are
predictable - entities may be managed on a regular basis by a
relatively small number of management stations - and therefore
there is no significant advantage afforded by protecting
against traffic analysis.
1.2.3 View-based Access Control Model (VACM)
Presently, a single access control model has been defined for
SNMPv3[RFC 3415] called VACM. Conceptually, it is quite simple and
powerful. It has an abstact function called isAccessAllowed to
determine if access is allowed using the inputs:
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security model ID: the message level security model
security name: the identity for the operation
security level: one of noAuthNoPriv, authNoPriv, or authPriv
operation type: one of read, write, or notify
context ID: the context which contains the instance of management
information
instance ID: the ID of the instance of management information for the
operation
The result of the function is an indication that the access is
allowed or one of several error codes.
Internally, the abstract function isAccessAllowed maps the pairing of
security models and security names to a security group name. The
access rules are based on group names and not directly with security
names. A security name can be contained in at most one security
group.
1.3 SNMPv3 Engines and Applications
The SNMPv3 architecture models SNMPv3 entities of consisting of a
messaging level processing component called the SNMP engine, and one
or more higher level users of the engine called SNMPv3 applications.
The applications are:
command generators: send requests to get or set the values of
management information
command responders: process get and set requests and send back a
response
notification originators: send notifications (traps and/or informs)
notification receivers: receive notifications
proxy forwarders: forward SNMP messages
Normative References
[refs.RFC3412]
Case, J., "Message Processing and Dispatching for the
Simple Network Management Protocol (SNMP)", RFC 3412, STD
62, December 2002.
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[refs.RFC3415]
Wijen, B., Presuhn, R. and K. McCloghrie, "View-based
Access Control Model (VACM) for the Simple Network
Management Protocol (SNMP)", RFC 3415, STD 62, December
2002.
[refs.RFC3414]
Wijen, B. and U. Blumenthal, "User-based Security Model
(USM) for version 3 of the Simple Network Management
Protocol (SNMPv3)", RFC 3414, STD 62, December 2002.
Author's Address
David T. Perkins
SNMPInfo
548 Quailbrook Ct
San Jose 95110
US
EMail: dperkins@snmpinfo.com
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