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 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on September 2, 2007. Copyright Notice 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 Schoenwaelder Expires September 2, 2007 [Page 1] Internet-Draft SNMP EngineID Discovery March 2007 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 Schoenwaelder Expires September 2, 2007 [Page 2] Internet-Draft SNMP EngineID Discovery March 2007 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 Schoenwaelder Expires September 2, 2007 [Page 3] Internet-Draft SNMP EngineID Discovery March 2007 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 Schoenwaelder Expires September 2, 2007 [Page 4] Internet-Draft SNMP EngineID Discovery March 2007 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 Schoenwaelder Expires September 2, 2007 [Page 5] Internet-Draft SNMP EngineID Discovery March 2007 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 Schoenwaelder Expires September 2, 2007 [Page 6] Internet-Draft SNMP EngineID Discovery March 2007 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 Schoenwaelder Expires September 2, 2007 [Page 7] Internet-Draft SNMP EngineID Discovery March 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). 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. 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