Network Working Group J. Schoenwaelder Internet-Draft Jacobs University Bremen Updates: 3411 (if approved) January 21, 2008 Intended status: Standards Track Expires: July 24, 2008 Simple Network Management Protocol (SNMP) Context EngineID Discovery draft-ietf-opsawg-snmp-engineid-discovery-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 July 24, 2008. Copyright Notice Copyright (C) The IETF Trust (2008). Abstract The Simple Network Management Protocol (SNMP) version three (SNMPv3) requires that an application knows 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 which can be used to learn the snmpEngineID of a remote Schoenwaelder Expires July 24, 2008 [Page 1]
Internet-Draft SNMP Context EngineID Discovery January 2008 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Local EngineID . . . . . . . . . . . . . . . . . . . . . . 4 3.2. EngineID Discovery . . . . . . . . . . . . . . . . . . . . 5 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 7 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 7.1. Normative References . . . . . . . . . . . . . . . . . . . 7 7.2. Informative References . . . . . . . . . . . . . . . . . . 8 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8 Intellectual Property and Copyright Statements . . . . . . . . . . 9 Schoenwaelder Expires July 24, 2008 [Page 2]
Internet-Draft SNMP Context EngineID Discovery January 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 which 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 co-exist 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]. 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 (also called a contextEngineID) and a context name which 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, 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 Schoenwaelder Expires July 24, 2008 [Page 3]
Internet-Draft SNMP Context EngineID Discovery January 2008 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 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 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. 3.1. Local EngineID An SNMP command responder implementing this specification MUST register their pduTypes using the localEngineID snmpEngineID value (defined below) using the registerContextEngineID() Abstract Service Schoenwaelder Expires July 24, 2008 [Page 4]
Internet-Draft SNMP Context EngineID Discovery January 2008 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 to register 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) 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. 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. 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. Schoenwaelder Expires July 24, 2008 [Page 5]
Internet-Draft SNMP Context EngineID Discovery January 2008 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 IANA has to create 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 defined 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 has to create a registry to assign new format values out of the originally assigned and reserved number space 1-127. For new assignments, a specification is required as per [RFC2434]. This document requested the following assignment: Format Description References ------- ----------- ---------- 6 local engine [RFCXXXX] [RFC Ed.: replace XXXX with RFC number assigned to the document] 5. 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 protect the discovery exchange. In situations where SNMPv3 is used without security (i.e., the Schoenwaelder Expires July 24, 2008 [Page 6]
Internet-Draft SNMP Context EngineID Discovery January 2008 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 snmpEngineID values by sniffing on the wire and any attempts to keep snmpEngineID values private will not lead to strong security. The usage of SNMPv3 without security is therefore generally NOT RECOMMENDED. If a device configuration permits non-secure SNMPv1/v2c access to a target system, then reading the snmpEngineID variable of the SNMP- FRAMEWORK-MIB will also reveal a suitable contextEngineID value for subsequent SNMPv3 usage. However, implementations should not rely on non-secure SNMPv1/v2c access and therefore MUST implement this specification to enable secure contextEngineID discovery. 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 to the access control decision. 6. Acknowledgments Dave Perkins suggested to introduce a "local" contextEngineID during the interim meeting of the ISMS working group in Boston, 2006. Joe Fernandez, David Harrington, and Dan Romascanu 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. Schoenwaelder Expires July 24, 2008 [Page 7]
Internet-Draft SNMP Context EngineID Discovery January 2008 [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. [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. 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. [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-07.txt (work in progress), November 2007. 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 Expires July 24, 2008 [Page 8]
Internet-Draft SNMP Context EngineID Discovery January 2008 Full Copyright Statement Copyright (C) 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. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Schoenwaelder Expires July 24, 2008 [Page 9]
