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IETF RFC 5343

Simple Network Management Protocol (SNMP) Context EngineID Discovery

Last modified on Monday, September 15th, 2008

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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.

 Abstract

   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 top


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) [RFC 3410], it is necessary to know the identifier of the remote SNMP protocol engine, the so-called snmpEngineID [RFC 3411]. 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 [RFC 3414]. This document updates RFC 3411 [RFC 3411] by clarifying the IANA rules for the maintenance of the SnmpEngineID format registry. 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 [RFC 2119]. 2. Background Within an administrative domain, an SNMP engine is uniquely identified by an snmpEngineID value [RFC 3411]. 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 [RFC 3411]. 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 top

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 [RFC 3411]) 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 [RFC 3411]. The SNMP protocol operations and the protocol data units (PDUs) operate on OIDs and thus deal with object types and instances [RFC 3416]. The SNMP architecture [RFC 3411] 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 [RFC 3412]. 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 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) [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 [RFC 3411] using the special well-known local localEngineID value. Schoenwaelder Standards Track PAGE 3 top

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 [RFC 3412]. 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 [RFC 3412]. The SnmpEngineID textual convention [RFC 3411] 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 [RFC 3418] or in the msgAuthoritativeEngineID field in the securityParameters of the User-based Security Model (USM) [RFC 3414]. 3.2. EngineID Discovery Discovery of the snmpEngineID is done by sending a Read Class protocol operation (see Section 2.8 of [RFC 3411]) 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 top

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 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 [RFC 3411]. 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 [RFC 3418]. 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 [RFC 3411]: Format Description References ------- ----------- ---------- 0 reserved, unused [RFC 3411] 1 IPv4 address [RFC 3411] 2 IPv6 address [RFC 3411] 3 MAC address [RFC 3411] 4 administratively assigned text [RFC 3411] 5 administratively assigned octets [RFC 3411] 6-127 reserved, unused [RFC 3411] 128-255 enterprise specific [RFC 3411] 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 top

RFC 5343 SNMP Context EngineID Discovery September 2008 space, a specification is required as per [RFC 5226]. 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 [RFC 5343] 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 [RFC 3415] gives noAuthNoPriv read access to the snmpEngineID. Furthermore, the USM discovery mechanism defined in RFC 3414 [RFC 3414] 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 [RFC 3411]. 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 top

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 [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC 3411] Harrington, D., Presuhn, R., and B. Wijnen, "An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks", STD 62, RFC 3411, December 2002. [RFC 3412] 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. [RFC 3414] 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. [RFC 3416] Presuhn, R., "Version 2 of the Protocol Operations for the Simple Network Management Protocol (SNMP)", STD 62, RFC 3416, December 2002. [RFC 3418] Presuhn, R., "Management Information Base (MIB) for the Simple Network Management Protocol (SNMP)", STD 62, RFC 3418, December 2002. [RFC 5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. 7.2. Informative References [RFC 3410] 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 top

RFC 5343 SNMP Context EngineID Discovery September 2008 [RFC 3415] 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 Germany Phone: +49 421 200-3587 EMail: j.schoenwaelder@jacobs-university.de Schoenwaelder Standards Track PAGE 8 top

RFC 5343 SNMP Context EngineID Discovery September 2008 Full Copyright Statement Copyright © The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Schoenwaelder Standards Track PAGE 9 top

Simple Network Management Protocol (SNMP) Context EngineID Discovery RFC TOTAL SIZE: 19938 bytes PUBLICATION DATE: Monday, September 15th, 2008 LEGAL RIGHTS: The IETF Trust (see BCP 78)


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