Network Working Group J. Schoenwaelder
Internet-Draft Jacobs University Bremen
Intended status: Informational March 1, 2007
Expires: September 2, 2007
Simple Network Management Protocol (SNMP) EngineID Discovery
draft-schoenw-snmp-discover-01.txt
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Copyright (C) The IETF Trust (2007).
Abstract
The third version of the Simple Network Management Protocol (SNMP)
assumes that a manager knows the identifier of a remote SNMP protocol
engine (the so called snmpEngineID) in order to retrieve or
manipulate objects maintained locally on the remote engine. This
document introduces a well-known localEngineID and a discovery
mechanism which can be used to learn the engine identifier of a
remote SNMP protocol engine. The proposed mechanism is independent
of the features provided by SNMP security models and may also be used
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by other protocol interfaces providing access to managed objects.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Local EngineID . . . . . . . . . . . . . . . . . . . . . . 4
3.2. EngineID Discovery . . . . . . . . . . . . . . . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Normative References . . . . . . . . . . . . . . . . . . . 6
6.2. Informative References . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
Intellectual Property and Copyright Statements . . . . . . . . . . 8
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1. Introduction
To retrieve or manipulate management information using the third
version of the Simple Network Management Protocol (SNMPv3) [RFC3410],
it is necessary to know the identifier of the remote SNMP protocol
engine, the so called snmpEngineID [RFC3411]. While an appropriate
engine identifier can in principle be provided by an operator, it is
often desirable to discover the engine identifier automatically.
This document introduces a discovery mechanism which can be used to
learn the engine identifier of a remote SNMP protocol engine. The
proposed mechanism is independent of the features provided by SNMP
security models and may be used also by other protocol interfaces to
discover the engine identifier. The mechanism has been designed to
co-exists with discovery mechanisms, which may exist in security
models, such as the authoritative engine identifier discovery of the
User-based Security Model (USM) of SNMP [RFC3414].
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 RFC 2119 [RFC2119].
2. Background
Within an administrative domain, an SNMP engine is uniquely
identified by an snmpEngineID value [RFC3411]. An SNMP entity, which
consists of an SNMP engine and several SNMP applications, may provide
access to multiple contexts.
An SNMP context is a collection of management information accessible
by an SNMP entity. An item of management information may exist in
more than one context and an SNMP entity potentially has access to
many contexts [RFC3411]. A context is identified by the snmpEngineID
value of the entity hosting the management information (called a
contextEngineID) and a context name which identifies the specific
context (called a contextName).
To identify an individual item of management information within an
administrative domain, a four tuple is used consisting of
1. a contextEngineID,
2. a contextName,
3. an object type, and
4. its instance identification.
The last two elements are encoded in an object identifier (OID)
value. The contextName is a string while the contextEngineID is a
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binary value constructed according to the rules defined as part of
the SnmpEngineID textual convention of the SNMP-FRAMEWORK-MIB
[RFC3411].
The SNMP protocol operations and the protocol data units (PDUs)
operate on OIDs and thus deal with object types and instances
[RFC3416]. The SNMP architecture [RFC3411] introduces the concept of
a scopedPDU as a data structure containing a contextEngineID, a
contextName, and a PDU. The SNMP version 3 (SNMPv3) message format
uses ScopedPDUs to exchange management information [RFC3412].
Within the SNMP framework, contextEngineIDs serve as end-to-end
identifiers. This becomes important in situations where SNMP proxies
are deployed to translate between protocol versions or to cross
middleboxes such as network address translators. In addition,
snmpEngineIDs separate the identification of an SNMP engine from the
transport endpoints used to communicate with an SNMP engine. This
property allows to correlate management information easily even in
situations where multiple different transports were used to retrieve
the information or where transport endpoints can change dynamically.
To retrieve data from an SNMPv3 agent, it is necessary to know the
appropriate contextEngineID. The User-based Security Model (USM) of
SNMPv3 provides a mechanism to discover the snmpEngineID of the
remote SNMP engine since this is needed for security processing
reasons. The discovered snmpEngineID can subsequently be used as a
contextEngineID in a ScopedPDU to access management information local
to the remote SNMP engine. Other security models, such as the
Transport Security Model (TSM) [I-D.TSM], lack such a procedure and
may use the discovery mechanism defined in this memo.
3. Procedure
The proposed discovery mechanism consists of two parts, namely (i)
the definition of a special well-known snmpEngineID value, called the
localEngineID, which always refers to a local context, and (ii) the
definition of a procedure to acquire the snmpEngineID scalar of the
SNMP-FRAMEWORK-MIB [RFC3411] using the special well-known local
localEngineID value.
3.1. Local EngineID
SNMP command responder implementing this specification MUST register
their main local context using the localEngineID snmpEngineID value
(defined below) using the registerContextEngineID() Abstract Service
Interface (ASI) defined in RFC 3412 [RFC3412]. This registration is
done in addition to the normal registration under the SNMP engine's
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snmpEngineID. This is consistent with the SNMPv3 specifications
since they explicitly allow to register management information in
multiple contexts [RFC3412].
The SnmpEngineID textual convention defines that an snmpEngineID
value MUST be between 5 and 12 octets long. This specification
proposes to use the variable length format 3) and to allocate the
reserved, unused format value 6 using the enterprise ID 0 for the
localEngineID. An ASN.1 definition for localEngineID would look like
this:
localEngineID OCTET STRING ::= '8000000006'H
The localEngineID value always provides access to the main local
context of an SNMP engine.
3.2. EngineID Discovery
Discovery of the snmpEngineID is simply done by sending an Read Class
protocol operation (see section 2.8 of [RFC3411] to retrieve the
snmpEngineID scalar using the localEngineID defined above as a
contextEngineID value. Implementations SHOULD only perform this
discovery step when it is needed. In particular, if security models
are used that already discover the remote snmpEngineID (such as USM),
then no further discovery is necessary. The same is true in
situations where the application already supplies a suitable
snmpEngineID value (e.g., in proxy situations).
The procedure to discover the snmpEngineID of a remote SNMP engine
can be described as follows:
1) Check whether the application provided a contextEngineID value to
be used. If yes, use the provided contextEngineID value and stop the
discovery procedure.
2) Check whether the selected security model supports discovery of
the remote snmpEngineID (e.g., USM with its discovery mechanism). If
yes, let the security model perform the discovery. If the remote
snmpEngineID value has been successfully determined, assign it to the
contextEngineID and stop the discovery procedure.
3) Send a Read Class operation to the remote SNMP engine using the
localEngineID value as the contextEngineID in order to retrieve the
scalar snmpEngineID.0 of the SNMP-FRAMEWORK-MIB [RFC3411]. If
successful, set the contextEngineID to the retrieved value and stop
the discovery procedure.
4) Return an error indication that a suitable contextEngineID could
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not be discovered.
The procedure outlined above is exemplary and can be modified to
retrieve more variables in step 3), such as the sysObjectID.0 scalar
or the snmpSetSerialNo.0 scalar of the SNMPv2-MIB [RFC3418].
4. Security Considerations
SNMP version 3 (SNMPv3) provides cryptographic security to protect
devices from unauthorized access. This specification recommends to
use the security services provided by SNMPv3. In particular, it is
recommended to use the security services provided by an SNMP security
model to protect the discovery exchange.
In situations where SNMPv3 is used without security (i.e., the
security level of noAuthNoPriv is used), the introduction of a
localEngineID may make it slightly easier for an attacker to discover
suitable snmpEngineID values. However, since SNMP messages with a
security level of noAuthNoPriv are normally carried in clear-text
over the wire, it is usually easy for an attacker to discover a
contextEngineID by sniffing on the wire and any attempts to keep the
snmpEngineIDs private won't lead to strong security. The usage of
SNMPv3 without security is therefore generally not recommended.
5. Acknowledgments
Dave Perkins suggested to introduce a "local" contextEngineID during
the interim meeting of the ISMS working group in Boston, 2006. Joe
Fernandez and David Harrington provided helpful review and feedback,
which helped to improve this document.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, March 1997.
[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.
[RFC3412] Case, J., Harrington, D., Presuhn, R., and B. Wijnen,
"Message Processing and Dispatching for the Simple Network
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Management Protocol (SNMP)", STD 62, RFC 3412,
December 2002.
[RFC3414] Blumenthal, U. and B. Wijnen, "User-based Security Model
(USM) for version 3 of the Simple Network Management
Protocol (SNMPv3)", STD 62, RFC 3414, December 2002.
[RFC3416] Presuhn, R., "Version 2 of the Protocol Operations for the
Simple Network Management Protocol (SNMP)", STD 62,
RFC 3416, December 2002.
6.2. Informative References
[RFC3410] Case, J., Mundy, R., Partain, D., and B. Stewart,
"Introduction and Applicability Statements for Internet-
Standard Management Framework", RFC 3410, December 2002.
[RFC3418] Presuhn, R., "Management Information Base (MIB) for the
Simple Network Management Protocol (SNMP)", STD 62,
RFC 3418, December 2002.
[I-D.TSM] Harrington, D., "Transport Security Model for SNMP",
draft-ietf-isms-transport-security-model-03.txt (work in
progress), February 2007.
Author's Address
Juergen Schoenwaelder
Jacobs University Bremen
Campus Ring 1
28725 Bremen
Germany
Phone: +49 421 200-3587
Email: j.schoenwaelder@iu-bremen.de
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