Network Working Group D. Perkins Internet-Draft SNMPInfo Expires: August 15, 2004 February 15, 2004 A Consolidated Overview of Version 3 of the Simple Network Management Protocol (SNMPv3) draft-perkins-snmpv3-overview-00.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 August 15, 2004. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This document describes a Session Based Security Model (SBSM) for use within version 3 of the Simple Network Management Protocol (SNMPv3). The security model is designed to establish a "session" between two interacting SNMPv3 entities, over which SNMP operations can be sent securely. It provides a number of security properties not previously available in defined SNMPv3 security models, such as public key based identity authentication, limited life-time keying, and the ability to make use of previously implemented and deployed security infrastructures for purposes of identification and authentication. Perkins Expires August 15, 2004 [Page 1] Internet-Draft An Consolidated Overview of the SNMPv3 Protocol February 2004 Table of Contents 1. SNMPv3 Overview . . . . . . . . . . . . . . . . . . . . . . 3 1.1 SNMPv3 Message Format . . . . . . . . . . . . . . . . . . . 4 1.2 SNMPv3 Security . . . . . . . . . . . . . . . . . . . . . . 7 1.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 7 1.2.2 Security Model Threats . . . . . . . . . . . . . . . . . . . 7 1.2.3 View-based Access Control Model (VACM) . . . . . . . . . . . 8 1.3 SNMPv3 Engines and Applications . . . . . . . . . . . . . . 9 Normative References . . . . . . . . . . . . . . . . . . . . 9 Author's Address . . . . . . . . . . . . . . . . . . . . . . 10 Intellectual Property and Copyright Statements . . . . . . . 11 Perkins Expires August 15, 2004 [Page 2] Internet-Draft An Consolidated Overview of the SNMPv3 Protocol February 2004 1. SNMPv3 Overview The SNMP model of a managed network contains four basic components: o several (typically many) managed nodes, each with an SNMP entity which provides remote access to management instrumentation (traditionally called an agent); o at least one SNMP entity with management applications (typically called a manager); o a management protocol used to convey management information between the SNMP entities (such as managers and agents); and o management information. The management messages are either a request-response pair between a manager and agent, or a notification with potentially a confirmation from an agent to a manager. There are three additional types of SNMP entities: proxy: forwards SNMP messages from one entity to another. It may be used for translation from one version of SNMP protocol to another, or as a transport layer or security gateway; mid-level-manager: used in a multi-layer management system. Interacts with a higher level manager (which is also called a north-bound manager) using more abstract management information and communicates with many managed systems (which are also called network elements (NEs)) for specific management information. In basic SNMP entity-speak, a mid-level-manager contains elements of both an SNMP agent and manager; observer: used to watch one or more managed systems and when an event of importance occurs, then it will tell a manager using an SNMP notification operation Each instance of management information is uniquely identified by the triple: SNMP entity engineID: uniquely identifies, within an administrative domain, an SNMP entity which has direct access to management information (typically the management information is contained in the system containing the SNMP entity); context Name: uniquely identifies for an SNMP entity a collection of management information from the one more collections supported by the SNMP entity; Perkins Expires August 15, 2004 [Page 3] Internet-Draft An Consolidated Overview of the SNMPv3 Protocol February 2004 instanceID: uniquely identifies an instance of management information in a collection. The instanceID is specified as an ASN.1 object identifier value. The prefix is an ASN.1 object identifier value that identifies a class (called an SNMP object type), which is defined with the OBJECT-TYPE construct in SNMP MIB modules. The suffix is an ASN.1 object identifier value fragment, which is the encoded index values for columnar object types, or a single sub-identifier with value zero for scalar object types. 1.1 SNMPv3 Message Format SNMP messages are defined in ASN.1-1988, and serialized to a sequence of octets using the BER-1988. A message is sent over a connection oriented transport (such as TCP), or a connectionless transport (such as UDP). The connectionless transport is preferred due to its low overhead and robustness in a network with packet loss, high variation of packet delay, or packet duplication. An SNMPv3 message is the ASN.1 sequence SNMPv3Message [RFC3412:section 6] with the format: ---------------------------------------------------------------- | msgVersion | msgGlobalData | msgSecurityParameters | msgData | ---------------------------------------------------------------- where: msgVersion - an integer that indicates the format of the message,which can be: - snmpv1(0) [RFC1157] - snmpv2c(1) [RFC1901] - snmpv2u(2) [RFC1910] - snmpv3(3) [RFC3412] msgGlobalData - a sequence of type HeaderData (see below) msgSecurityParameters - an octet string, which is the serialization using the BER of a security model dependent sequence msgData - a choice of type ScopedPduData (see below) The field "msgGlobalData" is the ASN.1 sequence HeaderData [RFC3412:section 6] with the format: ---------------------------------------------------- | msgID | msgMaxSize | msgFlags | msgSecurityModel | ---------------------------------------------------- where: msgID - an integer, which is used to pair request and response messages msgMaxSize - an integer, which specifies the maximum Perkins Expires August 15, 2004 [Page 4] Internet-Draft An Consolidated Overview of the SNMPv3 Protocol February 2004 size of a message that can be received by the sender msgFlags - a bit string with the following legal values: '100'b - a noAuthNoPriv request (GET, GETNEXT, SET, GETBULK) '000'b - a noAuthNoPriv response (RESPONSE or REPORT) or unacknowledged notification (TRAP) '101'b - an authNoPriv request (GET, GETNEXT, SET, GETBULK) '001'b - an authNoPriv response (RESPONSE or REPORT) or unacknowledged notification (TRAP) '111'b - an authPriv request (GET, GETNEXT, SET, GETBULK) '011'b - an authPriv response (RESPONSE or REPORT) or unacknowledged notification (TRAP) where: noAuthNoPriv - means that the entire message is not authenticated, nor is the contents of field msgData encrypted authNoPriv - means that the entire message is authenticated using the mechanism specified by the value of field msgSecurityModel using, if needed, values from the sequence encoded in the msgSecurityParmeters field, and the contents of field msgData is not encrypted authPriv - means that the entire message is authenticated using the mechanism specified by the value of field msgSecurityModel using, if needed, values from the sequence encoded in the msgSecurityParmeters field, and the contents of field msgData is encrypted using, if needed, values from the sequence encoded in the msgSecurityParmeters field msgSecurityModel - a nonnegative integer, which identifies a security model, and implicitly the format of the content of field msgSecurityParameters. The rules for values is specified by textual convention SnmpSecurityModel in RFC 3411. The field "scopedPDUdata" is the ASN.1 choice ScopedPDUdata [RFC3412:section 6] and can be either the field called "encryptedPDU" (with ASN.1 data type of OCTET STRING) or the field called "plaintext" (which is sequence ScopedPDU). The format of the field "plaintext" is: --------------------------------------------------------- | an OCTET STRING, which is the encrypted serialization | | using BER of a scopedPDU sequence | --------------------------------------------------------- And the format of the sequence ScopedPDU is: Perkins Expires August 15, 2004 [Page 5] Internet-Draft An Consolidated Overview of the SNMPv3 Protocol February 2004 ------------------------------ | engineID | contextID | pdu | ------------------------------ where: engineID - an octet string, which identifies the system containing the management information specified in the PDU sequence contextID - an octet string, which further qualifies which collection of management information is specified in the PDU sequence. Note that the value that is a zero length string is a "required" contextID and contains instances of objects that are used for managing the SNMPv3 protocol pdu - a tagged sequence of pairs. The tag identifies the PDU type and corresponds to the operation. The most recent definitions are specified in section 3 of RFC 3416. And where each pair contains the identification of management information (either exact or an approximation depending on the PDU type), and value (or placeholder for the value) of the identified management information. For USM, the value of field msgSecurityParmeters is the BER serialization of sequence UsmSecurityParameters [RFC3414: section 2.4], which has format: ----------------------------------------------------------------- | engID | engBoots | engTime | userName | authParms | encrParms | ----------------------------------------------------------------- where: engID - identifies SNMP entity (either the message sender or receiver) that contains the "authoritative copy" of the security information shared by the sender and receiver, and is called the "authoritative engine". Note that USM specifies for each PDU type whether the sender or the receiver is authoritative. engBoots - specifies the sender's notion of the value of the single instance of object snmpEngineBoots of the authoritative engine engTime - specifies the sender's notion of the value of the single instance of object snmpEngineTime of the authoritative engine userName - identifies a USM user whose security information will be used, if needed, to authenticate and encrypt the message authParms - values, if needed, by the USM authentication protocol used to authenticate the message encrParms - values, if needed, by the USM encryption protocol to Perkins Expires August 15, 2004 [Page 6] Internet-Draft An Consolidated Overview of the SNMPv3 Protocol February 2004 encrypt the serialization using BER of the scopedPDU value of the message 1.2 SNMPv3 Security 1.2.1 Introduction SNMPv3 views security as being applied at two different stages, either in the tranmission/receipt of messages, and in the processing of the contents of messages. SNMPv3 uses the term "security model" to refer to message-level security and the term "access control model" to the security applied to message content processing. In message level processing, the identity of the sender must be determined and authenticated, and each message must be authenticated. The message level processing may also provide encryption and decryption of message content. SNMPv3 assigns a label to the message level security, which is one of "noAuthNoPriv", "authNoPriv", and "authPriv" (see message description for definitions.) Message content processing is concerned with access to instances of management information. Authorization rules determine for each identity the allowed access operations for each instance of management information. Authorization rules may also require that access is only allowed via SNMP messages at a specified or higher level of security. In summary, message level security requires authentication of the sender's identity, and athentication of messages, and potentially message content encryption. Authorization for message processing to access management information requires the authenticated identity from a message, and the message security level (that was or will be applied to the message) as input to a rules database. 1.2.2 Security Model Threats The SNMPv3 architecture specifies requirements for a security model based on type of threat, which are: The principal threats against which any SNMPv3 security model SHOULD provide protection are: Modification of Information: The modification threat is the danger that some unauthorized entity may alter in-transit SNMP messages generated on behalf of an authorized principal in such a way as to effect unauthorized management operations, including falsifying the value of an object. Perkins Expires August 15, 2004 [Page 7] Internet-Draft An Consolidated Overview of the SNMPv3 Protocol February 2004 Masquerade: The masquerade threat is the danger that management operations not authorized for some principal may be attempted by assuming the identity of another principal that has the appropriate authorizations. Secondary threats against which any SNMPv3 security model SHOULD provide protection are: Message Stream Modification: The SNMP protocol is typically based upon a connectionless transport service which may operate over any subnetwork service. The re-ordering, delay or replay of messages can and does occur through the natural operation of many such subnetwork services. The message stream modification threat is the danger that messages may be maliciously re-ordered, delayed or replayed to an extent which is greater than can occur through the natural operation of a subnetwork service, in order to effect unauthorized management operations. Disclosure: The disclosure threat is the danger of eavesdropping on the contents of SNMP messages. There are at least two threats against which any SNMPv3 security model need not protect, since they are deemed to be of lesser importance in this context: Denial of Service: A security model need not attempt to address the broad range of attacks by which service on behalf of authorized users is denied. Indeed, such denial-of-service attacks are in many cases indistinguishable from the type of network failures with which any viable management protocol must cope as a matter of course. Traffic Analysis: A security model need not attempt to address traffic analysis attacks. Many traffic patterns are predictable - entities may be managed on a regular basis by a relatively small number of management stations - and therefore there is no significant advantage afforded by protecting against traffic analysis. 1.2.3 View-based Access Control Model (VACM) Presently, a single access control model has been defined for SNMPv3[RFC 3415] called VACM. Conceptually, it is quite simple and powerful. It has an abstact function called isAccessAllowed to determine if access is allowed using the inputs: Perkins Expires August 15, 2004 [Page 8] Internet-Draft An Consolidated Overview of the SNMPv3 Protocol February 2004 security model ID: the message level security model security name: the identity for the operation security level: one of noAuthNoPriv, authNoPriv, or authPriv operation type: one of read, write, or notify context ID: the context which contains the instance of management information instance ID: the ID of the instance of management information for the operation The result of the function is an indication that the access is allowed or one of several error codes. Internally, the abstract function isAccessAllowed maps the pairing of security models and security names to a security group name. The access rules are based on group names and not directly with security names. A security name can be contained in at most one security group. 1.3 SNMPv3 Engines and Applications The SNMPv3 architecture models SNMPv3 entities of consisting of a messaging level processing component called the SNMP engine, and one or more higher level users of the engine called SNMPv3 applications. The applications are: command generators: send requests to get or set the values of management information command responders: process get and set requests and send back a response notification originators: send notifications (traps and/or informs) notification receivers: receive notifications proxy forwarders: forward SNMP messages Normative References [refs.RFC3412] Case, J., "Message Processing and Dispatching for the Simple Network Management Protocol (SNMP)", RFC 3412, STD 62, December 2002. Perkins Expires August 15, 2004 [Page 9] Internet-Draft An Consolidated Overview of the SNMPv3 Protocol February 2004 [refs.RFC3415] Wijen, B., Presuhn, R. and K. McCloghrie, "View-based Access Control Model (VACM) for the Simple Network Management Protocol (SNMP)", RFC 3415, STD 62, December 2002. [refs.RFC3414] Wijen, B. and U. Blumenthal, "User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3)", RFC 3414, STD 62, December 2002. Author's Address David T. Perkins SNMPInfo 548 Quailbrook Ct San Jose 95110 US EMail: dperkins@snmpinfo.com Perkins Expires August 15, 2004 [Page 10] Internet-Draft An Consolidated Overview of the SNMPv3 Protocol February 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property 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; neither does it represent that it has made any effort to identify any such rights. 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