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Network Working Group J. Schoenwaelder Request for Comments: 5343 Jacobs University Bremen Updates: 3411 September 2008 Category: Standards Track

Simple Network Management Protocol (SNMP) Context EngineID Discovery

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.


 The Simple Network Management Protocol (SNMP) version three (SNMPv3)
 requires that an application know the identifier (snmpEngineID) of
 the remote SNMP protocol engine in order to retrieve or manipulate
 objects maintained on the remote SNMP entity.
 This document introduces a well-known localEngineID and a discovery
 mechanism that can be used to learn the snmpEngineID of a remote SNMP
 protocol engine.  The proposed mechanism is independent of the
 features provided by SNMP security models and may also be used by
 other protocol interfaces providing access to managed objects.
 This document updates RFC 3411.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
 2.  Background  . . . . . . . . . . . . . . . . . . . . . . . . . . 2
 3.  Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.1.  Local EngineID  . . . . . . . . . . . . . . . . . . . . . . 4
   3.2.  EngineID Discovery  . . . . . . . . . . . . . . . . . . . . 4
 4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
 5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
 6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7
 7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
   7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
   7.2.  Informative References  . . . . . . . . . . . . . . . . . . 7

Schoenwaelder Standards Track [Page 1] RFC 5343 SNMP Context EngineID Discovery September 2008

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
 snmpEngineID can in principle be configured on each management
 application for each SNMP agent, it is often desirable to discover
 the snmpEngineID automatically.
 This document introduces a discovery mechanism that can be used to
 learn the snmpEngineID of a remote SNMP protocol engine.  The
 proposed mechanism is independent of the features provided by SNMP
 security models.  The mechanism has been designed to coexist with
 discovery mechanisms that may exist in SNMP security models, such as
 the authoritative engine identifier discovery of the User-based
 Security Model (USM) of SNMP [RFC3414].
 This document updates RFC 3411 [RFC3411] by clarifying the IANA rules
 for the maintenance of the SnmpEngineID format registry.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 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 (also called a
 contextEngineID) and a context name that identifies the specific
 context (also 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,

Schoenwaelder Standards Track [Page 2] RFC 5343 SNMP Context EngineID Discovery September 2008

 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 character string (following the
 SnmpAdminString textual convention of the SNMP-FRAMEWORK-MIB
 [RFC3411]) while the contextEngineID is an octet string 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 addresses used to communicate with an SNMP engine.  This
 property can be used to correlate management information easily, even
 in situations where multiple different transports were used to
 retrieve the information or where transport addresses can change
 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) [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 default 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.

Schoenwaelder Standards Track [Page 3] RFC 5343 SNMP Context EngineID Discovery September 2008

3.1. Local EngineID

 An SNMP command responder implementing this specification MUST
 register their pduTypes using the localEngineID snmpEngineID value
 (defined below) by invoking 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 snmpEngineID.  This is consistent with the SNMPv3
 specifications since they explicitly allow registration of multiple
 engineIDs and multiple pduTypes [RFC3412].
 The SnmpEngineID textual convention [RFC3411] defines that an
 snmpEngineID value MUST be between 5 and 32 octets long.  This
 specification proposes to use the variable length format 3) of the
 SnmpEngineID textual convention 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 default context
 of an SNMP engine.  Note that the localEngineID value is intended to
 be used as a special value for the contextEngineID field in the
 ScopedPDU.  It MUST NOT be used as a value to identify an SNMP
 engine; that is, this value MUST NOT be used in the snmpEngineID.0
 scalar [RFC3418] or in the msgAuthoritativeEngineID field in the
 securityParameters of the User-based Security Model (USM) [RFC3414].

3.2. EngineID Discovery

 Discovery of the snmpEngineID is done by sending a 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 knows a suitable
 snmpEngineID value.
 The procedure to discover the snmpEngineID of a remote SNMP engine
 can be described as follows:
 1.  Check whether a suitable contextEngineID value is already known.
     If yes, use the provided contextEngineID value and stop the
     discovery procedure.

Schoenwaelder Standards Track [Page 4] RFC 5343 SNMP Context EngineID Discovery September 2008

 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
 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
     not be discovered.
 The procedure outlined above is an example 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. IANA Considerations

 RFC 3411 requested that IANA create a registry for SnmpEngineID
 formats.  However, RFC 3411 did not ask IANA to record the initial
 assignments made by RFC 3411 nor did RFC 3411 spell out the precise
 allocation rules.  To address this issue, the following rules are
 hereby established.
 IANA maintains a registry for SnmpEngineID formats.  The first four
 octets of an SnmpEngineID carry an enterprise number, while the fifth
 octet in a variable length SnmpEngineID value, called the format
 octet, indicates how the following octets are formed.  The following
 format values were allocated in [RFC3411]:
   Format    Description                     References
   -------   -----------                     ----------
        0    reserved, unused                 [RFC3411]
        1    IPv4 address                     [RFC3411]
        2    IPv6 address                     [RFC3411]
        3    MAC address                      [RFC3411]
        4    administratively assigned text   [RFC3411]
        5    administratively assigned octets [RFC3411]
     6-127   reserved, unused                 [RFC3411]
   128-255   enterprise specific              [RFC3411]
 IANA can assign new format values out of the originally assigned and
 reserved number space 1-127.  For new assignments in this number

Schoenwaelder Standards Track [Page 5] RFC 5343 SNMP Context EngineID Discovery September 2008

 space, a specification is required as per [RFC5226].  The number
 space 128-255 is enterprise specific and is not controlled by IANA.
 Per this document, IANA has made the following assignment:
   Format    Description                     References
   -------   -----------                     ----------
        6    local engine                     [RFC5343]

5. Security Considerations

 SNMP version 3 (SNMPv3) provides cryptographic security to protect
 devices from unauthorized access.  This specification recommends use
 of the security services provided by SNMPv3.  In particular, it is
 RECOMMENDED to protect the discovery exchange.
 An snmpEngineID can contain information such as a device's MAC
 address, IPv4 address, IPv6 address, or administratively assigned
 text.  An attacker located behind a router / firewall / network
 address translator may not be able to obtain this information
 directly, and he therefore might discover snmpEngineID values in
 order to obtain this kind of device information.
 In many environments, making snmpEngineID values accessible via a
 security level of noAuthNoPriv will benefit legitimate tools that try
 to algorithmically determine some basic information about a device.
 For this reason, the default View-based Access Control Model (VACM)
 configuration in Appendix A of RFC 3415 [RFC3415] gives noAuthNoPriv
 read access to the snmpEngineID.  Furthermore, the USM discovery
 mechanism defined in RFC 3414 [RFC3414] uses unprotected messages and
 reveals snmpEngineID values.
 In highly secure environments, snmpEngineID values can be protected
 by using the discovery mechanism described in this document together
 with a security model that does not exchange cleartext SNMP messages,
 such as the Transport Security Model (TSM) [TSM].
 The isAccessAllowed() abstract service primitive of the SNMP access
 control subsystem does not take the contextEngineID into account when
 checking access rights [RFC3411].  As a consequence, it is not
 possible to define a special view for context engineID discovery.  A
 request with a localEngineID is thus treated like a request with the
 correct snmpEngineID by the access control subsystem.  This is inline
 with the SNMPv3 design where the authenticated identity is the
 securityName (together with the securityModel and securityLevel
 information), and transport addresses or knowledge of contextEngineID
 values do not impact the access-control decision.

Schoenwaelder Standards Track [Page 6] RFC 5343 SNMP Context EngineID Discovery September 2008

6. Acknowledgments

 Dave Perkins suggested the introduction of a "local" contextEngineID
 during the interim meeting of the ISMS (Integrated Security Model for
 SNMP) working group in Boston, 2006.  Joe Fernandez, David
 Harrington, Dan Romascanu, and Bert Wijnen provided helpful review
 and feedback, which helped to improve this document.

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.
 [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
            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.
 [RFC3418]  Presuhn, R., "Management Information Base (MIB) for the
            Simple Network Management Protocol (SNMP)", STD 62,
            RFC 3418, December 2002.
 [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
            IANA Considerations Section in RFCs", BCP 26, RFC 5226,
            May 2008.

7.2. 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.

Schoenwaelder Standards Track [Page 7] RFC 5343 SNMP Context EngineID Discovery September 2008

 [RFC3415]  Wijnen, B., Presuhn, R., and K. McCloghrie, "View-based
            Access Control Model (VACM) for the Simple Network
            Management Protocol (SNMP)", STD 62, RFC 3415,
            December 2002.
 [TSM]      Harrington, D., "Transport Security Model for SNMP", Work
            in Progress, July 2008.

Author's Address

 Juergen Schoenwaelder
 Jacobs University Bremen
 Campus Ring 1
 28725 Bremen
 Phone: +49 421 200-3587

Schoenwaelder Standards Track [Page 8] RFC 5343 SNMP Context EngineID Discovery September 2008

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Schoenwaelder Standards Track [Page 9]

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