net-snmp

Network Working Group U. Blumenthal Request for Comments: 3414 B. Wijnen STD: 62 Lucent Technologies Obsoletes: 2574 December 2002 Category: Standards Track User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3) 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. Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract This document describes the User-based Security Model (USM) for Simple Network Management Protocol (SNMP) version 3 for use in the SNMP architecture. It defines the Elements of Procedure for providing SNMP message level security. This document also includes a Management Information Base (MIB) for remotely monitoring/managing the configuration parameters for this Security Model. This document obsoletes RFC 2574. Table of Contents 1. Introduction.......................................... 4 1.1. Threats............................................... 4 1.2. Goals and Constraints................................. 6 1.3. Security Services..................................... 6 1.4. Module Organization................................... 7 1.4.1. Timeliness Module..................................... 8 1.4.2. Authentication Protocol............................... 8 1.4.3. Privacy Protocol...................................... 8 1.5. Protection against Message Replay, Delay and Redirection....................................... 9 1.5.1. Authoritative SNMP engine............................. 9 1.5.2. Mechanisms............................................ 9 1.6. Abstract Service Interfaces........................... 11 Blumenthal & Wijnen Standards Track [Page 1] RFC 3414 USM for SNMPv3 December 2002 1.6.1. User-based Security Model Primitives for Authentication.................................... 11 1.6.2. User-based Security Model Primitives for Privacy........................................... 12 2. Elements of the Model................................. 12 2.1. User-based Security Model Users....................... 12 2.2. Replay Protection..................................... 13 2.2.1. msgAuthoritativeEngineID.............................. 14 2.2.2. msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime............................ 14 2.2.3. Time Window........................................... 15 2.3. Time Synchronization.................................. 15 2.4. SNMP Messages Using this Security Model............... 16 2.5. Services provided by the User-based Security Model.... 17 2.5.1. Services for Generating an Outgoing SNMP Message...... 17 2.5.2. Services for Processing an Incoming SNMP Message...... 20 2.6. Key Localization Algorithm............................ 22 3. Elements of Procedure................................. 22 3.1. Generating an Outgoing SNMP Message................... 22 3.2. Processing an Incoming SNMP Message................... 26 4. Discovery............................................. 31 5. Definitions........................................... 32 6. HMAC-MD5-96 Authentication Protocol................... 51 6.1. Mechanisms............................................ 51 6.1.1. Digest Authentication Mechanism....................... 51 6.2. Elements of the Digest Authentication Protocol........ 52 6.2.1. Users................................................. 52 6.2.2. msgAuthoritativeEngineID.............................. 53 6.2.3. SNMP Messages Using this Authentication Protocol...... 53 6.2.4. Services provided by the HMAC-MD5-96 Authentication Module................................. 53 6.2.4.1. Services for Generating an Outgoing SNMP Message...... 53 6.2.4.2. Services for Processing an Incoming SNMP Message...... 54 6.3. Elements of Procedure................................. 55 6.3.1. Processing an Outgoing Message........................ 55 6.3.2. Processing an Incoming Message........................ 56 7. HMAC-SHA-96 Authentication Protocol................... 57 7.1. Mechanisms............................................ 57 7.1.1. Digest Authentication Mechanism....................... 57 7.2. Elements of the HMAC-SHA-96 Authentication Protocol... 58 7.2.1. Users................................................. 58 7.2.2. msgAuthoritativeEngineID.............................. 58 7.2.3. SNMP Messages Using this Authentication Protocol...... 59 7.2.4. Services provided by the HMAC-SHA-96 Authentication Module................................. 59 7.2.4.1. Services for Generating an Outgoing SNMP Message...... 59 7.2.4.2. Services for Processing an Incoming SNMP Message...... 60 7.3. Elements of Procedure................................. 61 Blumenthal & Wijnen Standards Track [Page 2] RFC 3414 USM for SNMPv3 December 2002 7.3.1. Processing an Outgoing Message........................ 61 7.3.2. Processing an Incoming Message........................ 61 8. CBC-DES Symmetric Encryption Protocol................. 63 8.1. Mechanisms............................................ 63 8.1.1. Symmetric Encryption Protocol......................... 63 8.1.1.1. DES key and Initialization Vector..................... 64 8.1.1.2. Data Encryption....................................... 65 8.1.1.3. Data Decryption....................................... 65 8.2. Elements of the DES Privacy Protocol.................. 65 8.2.1. Users................................................. 65 8.2.2. msgAuthoritativeEngineID.............................. 66 8.2.3. SNMP Messages Using this Privacy Protocol............. 66 8.2.4. Services provided by the DES Privacy Module........... 66 8.2.4.1. Services for Encrypting Outgoing Data................. 66 8.2.4.2. Services for Decrypting Incoming Data................. 67 8.3. Elements of Procedure................................. 68 8.3.1. Processing an Outgoing Message........................ 68 8.3.2. Processing an Incoming Message........................ 69 9. Intellectual Property................................. 69 10. Acknowledgements...................................... 70 11. Security Considerations............................... 71 11.1. Recommended Practices................................. 71 11.2. Defining Users........................................ 73 11.3. Conformance........................................... 74 11.4. Use of Reports........................................ 75 11.5. Access to the SNMP-USER-BASED-SM-MIB.................. 75 12. References............................................ 75 A.1. SNMP engine Installation Parameters................... 78 A.2. Password to Key Algorithm............................. 80 A.2.1. Password to Key Sample Code for MD5................... 81 A.2.2. Password to Key Sample Code for SHA................... 82 A.3. Password to Key Sample Results........................ 83 A.3.1. Password to Key Sample Results using MD5.............. 83 A.3.2. Password to Key Sample Results using SHA.............. 83 A.4. Sample encoding of msgSecurityParameters.............. 83 A.5. Sample keyChange Results.............................. 84 A.5.1. Sample keyChange Results using MD5.................... 84 A.5.2. Sample keyChange Results using SHA.................... 85 B. Change Log............................................ 86 Editors' Addresses.................................... 87 Full Copyright Statement.............................. 88 Blumenthal & Wijnen Standards Track [Page 3] RFC 3414 USM for SNMPv3 December 2002 1. Introduction The Architecture for describing Internet Management Frameworks [RFC3411] describes that an SNMP engine is composed of: 1) a Dispatcher, 2) a Message Processing Subsystem, 3) a Security Subsystem, and 4) an Access Control Subsystem. Applications make use of the services of these subsystems. It is important to understand the SNMP architecture and the terminology of the architecture to understand where the Security Model described in this document fits into the architecture and interacts with other subsystems within the architecture. The reader is expected to have read and understood the description of the SNMP architecture, as defined in [RFC3411]. This memo describes the User-based Security Model as it is used within the SNMP Architecture. The main idea is that we use the traditional concept of a user (identified by a userName) with which to associate security information. This memo describes the use of HMAC-MD5-96 and HMAC-SHA-96 as the authentication protocols and the use of CBC-DES as the privacy protocol. The User-based Security Model however allows for other such protocols to be used instead of or concurrent with these protocols. Therefore, the description of HMAC-MD5-96, HMAC-SHA-96 and CBC-DES are in separate sections to reflect their self-contained nature and to indicate that they can be replaced or supplemented in the future. 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 [RFC2119]. 1.1. Threats Several of the classical threats to network protocols are applicable to the network management problem and therefore would be applicable to any SNMP Security Model. Other threats are not applicable to the network management problem. This section discusses principal threats, secondary threats, and threats which are of lesser importance. The principal threats against which this SNMP Security Model should provide protection are: Blumenthal & Wijnen Standards Track [Page 4] RFC 3414 USM for SNMPv3 December 2002 - 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. - Masquerade The masquerade threat is the danger that management operations not authorized for some user may be attempted by assuming the identity of another user that has the appropriate authorizations. Two secondary threats are also identified. The Security Model defined in this memo provides limited protection against: - Disclosure The disclosure threat is the danger of eavesdropping on the exchanges between managed agents and a management station. Protecting against this threat may be required as a matter of local policy. - Message Stream Modification The SNMP protocol is typically based upon a connection-less transport service which may operate over any sub-network service. The re-ordering, delay or replay of messages can and does occur through the natural operation of many such sub- network 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 sub-network service, in order to effect unauthorized management operations. There are at least two threats that an SNMP Security Model need not protect against. The security protocols defined in this memo do not provide protection against: - Denial of Service This SNMP Security Model does 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 network management protocol must cope as a matter of course. - Traffic Analysis This SNMP Security Model does not attempt to address traffic analysis attacks. Indeed, many traffic patterns are predictable - devices may be managed on a regular basis by a relatively small number of management applications - and therefore there is no significant advantage afforded by protecting against traffic analysis. Blumenthal & Wijnen Standards Track [Page 5] RFC 3414 USM for SNMPv3 December 2002 1.2. Goals and Constraints Based on the foregoing account of threats in the SNMP network management environment, the goals of this SNMP Security Model are as follows. 1) Provide for verification that each received SNMP message has not been modified during its transmission through the network. 2) Provide for verification of the identity of the user on whose behalf a received SNMP message claims to have been generated. 3) Provide for detection of received SNMP messages, which request or contain management information, whose time of generation was not recent. 4) Provide, when necessary, that the contents of each received SNMP message are protected from disclosure. In addition to the principal goal of supporting secure network management, the design of this SNMP Security Model is also influenced by the following constraints: 1) When the requirements of effective management in times of network stress are inconsistent with those of security, the design of USM has given preference to the former. 2) Neither the security protocol nor its underlying security mechanisms should depend upon the ready availability of other network services (e.g., Network Time Protocol (NTP) or key management protocols). 3) A security mechanism should entail no changes to the basic SNMP network management philosophy. 1.3. Security Services The security services necessary to support the goals of this SNMP Security Model are as follows: - Data Integrity is the provision of the property that data has not been altered or destroyed in an unauthorized manner, nor have data sequences been altered to an extent greater than can occur non- maliciously. - Data Origin Authentication is the provision of the property that the claimed identity of the user on whose behalf received data was originated is corroborated. Blumenthal & Wijnen Standards Track [Page 6] RFC 3414 USM for SNMPv3 December 2002 - Data Confidentiality is the provision of the property that information is not made available or disclosed to unauthorized individuals, entities, or processes. - Message timeliness and limited replay protection is the provision of the property that a message whose generation time is outside of a specified time window is not accepted. Note that message reordering is not dealt with and can occur in normal conditions too. For the protocols specified in this memo, it is not possible to assure the specific originator of a received SNMP message; rather, it is the user on whose behalf the message was originated that is authenticated. For these protocols, it not possible to obtain data integrity without data origin authentication, nor is it possible to obtain data origin authentication without data integrity. Further, there is no provision for data confidentiality without both data integrity and data origin authentication. The security protocols used in this memo are considered acceptably secure at the time of writing. However, the procedures allow for new authentication and privacy methods to be specified at a future time if the need arises. 1.4. Module Organization The security protocols defined in this memo are split in three different modules and each has its specific responsibilities such that together they realize the goals and security services described above: - The authentication module MUST provide for: - Data Integrity, - Data Origin Authentication, - The timeliness module MUST provide for: - Protection against message delay or replay (to an extent greater than can occur through normal operation). - The privacy module MUST provide for - Protection against disclosure of the message payload. Blumenthal & Wijnen Standards Track [Page 7] RFC 3414 USM for SNMPv3 December 2002 The timeliness module is fixed for the User-based Security Model while there is provision for multiple authentication and/or privacy modules, each of which implements a specific authentication or privacy protocol respectively. 1.4.1. Timeliness Module Section 3 (Elements of Procedure) uses the timeliness values in an SNMP message to do timeliness checking. The timeliness check is only performed if authentication is applied to the message. Since the complete message is checked for integrity, we can assume that the timeliness values in a message that passes the authentication module are trustworthy. 1.4.2. Authentication Protocol Section 6 describes the HMAC-MD5-96 authentication protocol which is the first authentication protocol that MUST be supported with the User-based Security Model. Section 7 describes the HMAC-SHA-96 authentication protocol which is another authentication protocol that SHOULD be supported with the User-based Security Model. In the future additional or replacement authentication protocols may be defined as new needs arise. The User-based Security Model prescribes that, if authentication is used, then the complete message is checked for integrity in the authentication module. For a message to be authenticated, it needs to pass authentication check by the authentication module and the timeliness check which is a fixed part of this User-based Security model. 1.4.3. Privacy Protocol Section 8 describes the CBC-DES Symmetric Encryption Protocol which is the first privacy protocol to be used with the User-based Security Model. In the future additional or replacement privacy protocols may be defined as new needs arise. The User-based Security Model prescribes that the scopedPDU is protected from disclosure when a message is sent with privacy. The User-based Security Model also prescribes that a message needs to be authenticated if privacy is in use. Blumenthal & Wijnen Standards Track [Page 8] RFC 3414 USM for SNMPv3 December 2002 1.5. Protection against Message Replay, Delay and Redirection 1.5.1. Authoritative SNMP Engine In order to protect against message replay, delay and redirection, one of the SNMP engines involved in each communication is designated to be the authoritative SNMP engine. When an SNMP message contains a payload which expects a response (those messages that contain a Confirmed Class PDU [RFC3411]), then the receiver of such messages is authoritative. When an SNMP message contains a payload which does not expect a response (those messages that contain an Unconfirmed Class PDU [RFC3411]), then the sender of such a message is authoritative. 1.5.2. Mechanisms The following mechanisms are used: 1) To protect against the threat of message delay or replay (to an extent greater than can occur through normal operation), a set of timeliness indicators (for the authoritative SNMP engine) are included in each message generated. An SNMP engine evaluates the timeliness indicators to determine if a received message is recent. An SNMP engine may evaluate the timeliness indicators to ensure that a received message is at least as recent as the last message it received from the same source. A non-authoritative SNMP engine uses received authentic messages to advance its notion of the timeliness indicators at the remote authoritative source. An SNMP engine MUST also use a mechanism to match incoming Responses to outstanding Requests and it MUST drop any Responses that do not match an outstanding request. For example, a msgID can be inserted in every message to cater for this functionality. These mechanisms provide for the detection of authenticated messages whose time of generation was not recent. This protection against the threat of message delay or replay does not imply nor provide any protection against unauthorized deletion or suppression of messages. Also, an SNMP engine may not be able to detect message reordering if all the messages involved are sent within the Time Window interval. Other mechanisms defined independently of the security protocol can also be used to detect the re-ordering replay, deletion, or suppression of messages containing Set operations (e.g., the MIB variable snmpSetSerialNo [RFC3418]). Blumenthal & Wijnen Standards Track [Page 9] RFC 3414 USM for SNMPv3 December 2002 2) Verification that a message sent to/from one authoritative SNMP engine cannot be replayed to/as-if-from another authoritative SNMP engine. Included in each message is an identifier unique to the authoritative SNMP engine associated with the sender or intended recipient of the message. A message containing an Unconfirmed Class PDU sent by an authoritative SNMP engine to one non-authoritative SNMP engine can potentially be replayed to another non-authoritative SNMP engine. The latter non-authoritative SNMP engine might (if it knows about the same userName with the same secrets at the authoritative SNMP engine) as a result update its notion of timeliness indicators of the authoritative SNMP engine, but that is not considered a threat. In this case, A Report or Response message will be discarded by the Message Processing Model, because there should not be an outstanding Request message. A Trap will possibly be accepted. Again, that is not considered a threat, because the communication was authenticated and timely. It is as if the authoritative SNMP engine was configured to start sending Traps to the second SNMP engine, which theoretically can happen without the knowledge of the second SNMP engine anyway. Anyway, the second SNMP engine may not expect to receive this Trap, but is allowed to see the management information contained in it. 3) Detection of messages which were not recently generated. A set of time indicators are included in the message, indicating the time of generation. Messages without recent time indicators are not considered authentic. In addition, an SNMP engine MUST drop any Responses that do not match an outstanding request. This however is the responsibility of the Message Processing Model. This memo allows the same user to be defined on multiple SNMP engines. Each SNMP engine maintains a value, snmpEngineID, which uniquely identifies the SNMP engine. This value is included in each message sent to/from the SNMP engine that is authoritative (see section 1.5.1). On receipt of a message, an authoritative SNMP engine checks the value to ensure that it is the intended recipient, and a non-authoritative SNMP engine uses the value to ensure that the message is processed using the correct state information. Each SNMP engine maintains two values, snmpEngineBoots and snmpEngineTime, which taken together provide an indication of time at that SNMP engine. Both of these values are included in an authenticated message sent to/received from that SNMP engine. On receipt, the values are checked to ensure that the indicated Blumenthal & Wijnen Standards Track [Page 10] RFC 3414 USM for SNMPv3 December 2002 timeliness value is within a Time Window of the current time. The Time Window represents an administrative upper bound on acceptable delivery delay for protocol messages. For an SNMP engine to generate a message which an authoritative SNMP engine will accept as authentic, and to verify that a message received from that authoritative SNMP engine is authentic, such an SNMP engine must first achieve timeliness synchronization with the authoritative SNMP engine. See section 2.3. 1.6. Abstract Service Interfaces Abstract service interfaces have been defined to describe the conceptual interfaces between the various subsystems within an SNMP entity. Similarly a set of abstract service interfaces have been defined within the User-based Security Model (USM) to describe the conceptual interfaces between the generic USM services and the self-contained authentication and privacy services. These abstract service interfaces are defined by a set of primitives that define the services provided and the abstract data elements that must be passed when the services are invoked. This section lists the primitives that have been defined for the User-based Security Model. 1.6.1. User-based Security Model Primitives for Authentication The User-based Security Model provides the following internal primitives to pass data back and forth between the Security Model itself and the authentication service: statusInformation = authenticateOutgoingMsg( IN authKey -- secret key for authentication IN wholeMsg -- unauthenticated complete message OUT authenticatedWholeMsg -- complete authenticated message ) statusInformation = authenticateIncomingMsg( IN authKey -- secret key for authentication IN authParameters -- as received on the wire IN wholeMsg -- as received on the wire OUT authenticatedWholeMsg -- complete authenticated message ) Blumenthal & Wijnen Standards Track [Page 11] RFC 3414 USM for SNMPv3 December 2002 1.6.2. User-based Security Model Primitives for Privacy The User-based Security Model provides the following internal primitives to pass data back and forth between the Security Model itself and the privacy service: statusInformation = encryptData( IN encryptKey -- secret key for encryption IN dataToEncrypt -- data to encrypt (scopedPDU) OUT encryptedData -- encrypted data (encryptedPDU) OUT privParameters -- filled in by service provider ) statusInformation = decryptData( IN decryptKey -- secret key for decrypting IN privParameters -- as received on the wire IN encryptedData -- encrypted data (encryptedPDU) OUT decryptedData -- decrypted data (scopedPDU) ) 2. Elements of the Model This section contains definitions required to realize the security model defined by this memo. 2.1. User-based Security Model Users Management operations using this Security Model make use of a defined set of user identities. For any user on whose behalf management operations are authorized at a particular SNMP engine, that SNMP engine must have knowledge of that user. An SNMP engine that wishes to communicate with another SNMP engine must also have knowledge of a user known to that engine, including knowledge of the applicable attributes of that user. A user and its attributes are defined as follows: userName A string representing the name of the user. securityName A human-readable string representing the user in a format that is Security Model independent. There is a one-to-one relationship between userName and securityName. Blumenthal & Wijnen Standards Track [Page 12] RFC 3414 USM for SNMPv3 December 2002 authProtocol An indication of whether messages sent on behalf of this user can be authenticated, and if so, the type of authentication protocol which is used. Two such protocols are defined in this memo: - the HMAC-MD5-96 authentication protocol. - the HMAC-SHA-96 authentication protocol. authKey If messages sent on behalf of this user can be authenticated, the (private) authentication key for use with the authentication protocol. Note that a user's authentication key will normally be different at different authoritative SNMP engines. The authKey is not accessible via SNMP. The length requirements of the authKey are defined by the authProtocol in use. authKeyChange and authOwnKeyChange The only way to remotely update the authentication key. Does that in a secure manner, so that the update can be completed without the need to employ privacy protection. privProtocol An indication of whether messages sent on behalf of this user can be protected from disclosure, and if so, the type of privacy protocol which is used. One such protocol is defined in this memo: the CBC-DES Symmetric Encryption Protocol. privKey If messages sent on behalf of this user can be en/decrypted, the (private) privacy key for use with the privacy protocol. Note that a user's privacy key will normally be different at different authoritative SNMP engines. The privKey is not accessible via SNMP. The length requirements of the privKey are defined by the privProtocol in use. privKeyChange and privOwnKeyChange The only way to remotely update the encryption key. Does that in a secure manner, so that the update can be completed without the need to employ privacy protection. 2.2. Replay Protection Each SNMP engine maintains three objects: - snmpEngineID, which (at least within an administrative domain) uniquely and unambiguously identifies an SNMP engine. Blumenthal & Wijnen Standards Track [Page 13] RFC 3414 USM for SNMPv3 December 2002 - snmpEngineBoots, which is a count of the number of times the SNMP engine has re-booted/re-initialized since snmpEngineID was last configured; and, - snmpEngineTime, which is the number of seconds since the snmpEngineBoots counter was last incremented. Each SNMP engine is always authoritative with respect to these objects in its own SNMP entity. It is the responsibility of a non- authoritative SNMP engine to synchronize with the authoritative SNMP engine, as appropriate. An authoritative SNMP engine is required to maintain the values of its snmpEngineID and snmpEngineBoots in non-volatile storage. 2.2.1. msgAuthoritativeEngineID The msgAuthoritativeEngineID value contained in an authenticated message is used to defeat attacks in which messages from one SNMP engine to another SNMP engine are replayed to a different SNMP engine. It represents the snmpEngineID at the authoritative SNMP engine involved in the exchange of the message. When an authoritative SNMP engine is first installed, it sets its local value of snmpEngineID according to a enterprise-specific algorithm (see the definition of the Textual Convention for SnmpEngineID in the SNMP Architecture document [RFC3411]). 2.2.2. msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime The msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime values contained in an authenticated message are used to defeat attacks in which messages are replayed when they are no longer valid. They represent the snmpEngineBoots and snmpEngineTime values at the authoritative SNMP engine involved in the exchange of the message. Through use of snmpEngineBoots and snmpEngineTime, there is no requirement for an SNMP engine to have a non-volatile clock which ticks (i.e., increases with the passage of time) even when the SNMP engine is powered off. Rather, each time an SNMP engine re-boots, it retrieves, increments, and then stores snmpEngineBoots in non-volatile storage, and resets snmpEngineTime to zero. When an SNMP engine is first installed, it sets its local values of snmpEngineBoots and snmpEngineTime to zero. If snmpEngineTime ever reaches its maximum value (2147483647), then snmpEngineBoots is incremented as if the SNMP engine has re-booted and snmpEngineTime is reset to zero and starts incrementing again. Blumenthal & Wijnen Standards Track [Page 14] RFC 3414 USM for SNMPv3 December 2002 Each time an authoritative SNMP engine re-boots, any SNMP engines holding that authoritative SNMP engine's values of snmpEngineBoots and snmpEngineTime need to re-synchronize prior to sending correctly authenticated messages to that authoritative SNMP engine (see Section 2.3 for (re-)synchronization procedures). Note, however, that the procedures do provide for a notification to be accepted as authentic by a receiving SNMP engine, when sent by an authoritative SNMP engine which has re-booted since the receiving SNMP engine last (re- )synchronized. If an authoritative SNMP engine is ever unable to determine its latest snmpEngineBoots value, then it must set its snmpEngineBoots value to 2147483647. Whenever the local value of snmpEngineBoots has the value 2147483647 it latches at that value and an authenticated message always causes an notInTimeWindow authentication failure. In order to reset an SNMP engine whose snmpEngineBoots value has reached the value 2147483647, manual intervention is required. The engine must be physically visited and re-configured, either with a new snmpEngineID value, or with new secret values for the authentication and privacy protocols of all users known to that SNMP engine. Note that even if an SNMP engine re-boots once a second that it would still take approximately 68 years before the max value of 2147483647 would be reached. 2.2.3. Time Window The Time Window is a value that specifies the window of time in which a message generated on behalf of any user is valid. This memo specifies that the same value of the Time Window, 150 seconds, is used for all users. 2.3. Time Synchronization Time synchronization, required by a non-authoritative SNMP engine in order to proceed with authentic communications, has occurred when the non-authoritative SNMP engine has obtained a local notion of the authoritative SNMP engine's values of snmpEngineBoots and snmpEngineTime from the authoritative SNMP engine. These values must be (and remain) within the authoritative SNMP engine's Time Window. So the local notion of the authoritative SNMP engine's values must be kept loosely synchronized with the values stored at the authoritative SNMP engine. In addition to keeping a local copy of snmpEngineBoots and snmpEngineTime from the authoritative SNMP engine, a non-authoritative SNMP engine must also keep one Blumenthal & Wijnen Standards Track [Page 15] RFC 3414 USM for SNMPv3 December 2002 local variable, latestReceivedEngineTime. This value records the highest value of snmpEngineTime that was received by the non-authoritative SNMP engine from the authoritative SNMP engine and is used to eliminate the possibility of replaying messages that would prevent the non-authoritative SNMP engine's notion of the snmpEngineTime from advancing. A non-authoritative SNMP engine must keep local notions of these values (snmpEngineBoots, snmpEngineTime and latestReceivedEngineTime) for each authoritative SNMP engine with which it wishes to communicate. Since each authoritative SNMP engine is uniquely and unambiguously identified by its value of snmpEngineID, the non-authoritative SNMP engine may use this value as a key in order to cache its local notions of these values. Time synchronization occurs as part of the procedures of receiving an SNMP message (Section 3.2, step 7b). As such, no explicit time synchronization procedure is required by a non-authoritative SNMP engine. Note, that whenever the local value of snmpEngineID is changed (e.g., through discovery) or when secure communications are first established with an authoritative SNMP engine, the local values of snmpEngineBoots and latestReceivedEngineTime should be set to zero. This will cause the time synchronization to occur when the next authentic message is received. 2.4. SNMP Messages Using this Security Model The syntax of an SNMP message using this Security Model adheres to the message format defined in the version-specific Message Processing Model document (for example [RFC3412]). The field msgSecurityParameters in SNMPv3 messages has a data type of OCTET STRING. Its value is the BER serialization of the following ASN.1 sequence: USMSecurityParametersSyntax DEFINITIONS IMPLICIT TAGS ::= BEGIN UsmSecurityParameters ::= SEQUENCE { -- global User-based security parameters msgAuthoritativeEngineID OCTET STRING, msgAuthoritativeEngineBoots INTEGER (0..2147483647), msgAuthoritativeEngineTime INTEGER (0..2147483647), msgUserName OCTET STRING (SIZE(0..32)), -- authentication protocol specific parameters msgAuthenticationParameters OCTET STRING, -- privacy protocol specific parameters msgPrivacyParameters OCTET STRING Blumenthal & Wijnen Standards Track [Page 16] RFC 3414 USM for SNMPv3 December 2002 } END The fields of this sequence are: - The msgAuthoritativeEngineID specifies the snmpEngineID of the authoritative SNMP engine involved in the exchange of the message. - The msgAuthoritativeEngineBoots specifies the snmpEngineBoots value at the authoritative SNMP engine involved in the exchange of the message. - The msgAuthoritativeEngineTime specifies the snmpEngineTime value at the authoritative SNMP engine involved in the exchange of the message. - The msgUserName specifies the user (principal) on whose behalf the message is being exchanged. Note that a zero-length userName will not match any user, but it can be used for snmpEngineID discovery. - The msgAuthenticationParameters are defined by the authentication protocol in use for the message, as defined by the usmUserAuthProtocol column in the user's entry in the usmUserTable. - The msgPrivacyParameters are defined by the privacy protocol in use for the message, as defined by the usmUserPrivProtocol column in the user's entry in the usmUserTable). See appendix A.4 for an example of the BER encoding of field msgSecurityParameters. 2.5. Services provided by the User-based Security Model This section describes the services provided by the User-based Security Model with their inputs and outputs. The services are described as primitives of an abstract service interface and the inputs and outputs are described as abstract data elements as they are passed in these abstract service primitives. 2.5.1. Services for Generating an Outgoing SNMP Message When the Message Processing (MP) Subsystem invokes the User-based Security module to secure an outgoing SNMP message, it must use the appropriate service as provided by the Security module. These two services are provided: Blumenthal & Wijnen Standards Track [Page 17] RFC 3414 USM for SNMPv3 December 2002 1) A service to generate a Request message. The abstract service primitive is: statusInformation = -- success or errorIndication generateRequestMsg( IN messageProcessingModel -- typically, SNMP version IN globalData -- message header, admin data IN maxMessageSize -- of the sending SNMP entity IN securityModel -- for the outgoing message IN securityEngineID -- authoritative SNMP entity IN securityName -- on behalf of this principal IN securityLevel -- Level of Security requested IN scopedPDU -- message (plaintext) payload OUT securityParameters -- filled in by Security Module OUT wholeMsg -- complete generated message OUT wholeMsgLength -- length of generated message ) 2) A service to generate a Response message. The abstract service primitive is: statusInformation = -- success or errorIndication generateResponseMsg( IN messageProcessingModel -- typically, SNMP version IN globalData -- message header, admin data IN maxMessageSize -- of the sending SNMP entity IN securityModel -- for the outgoing message IN securityEngineID -- authoritative SNMP entity IN securityName -- on behalf of this principal IN securityLevel -- Level of Security requested IN scopedPDU -- message (plaintext) payload IN securityStateReference -- reference to security state -- information from original -- request OUT securityParameters -- filled in by Security Module OUT wholeMsg -- complete generated message OUT wholeMsgLength -- length of generated message ) The abstract data elements passed as parameters in the abstract service primitives are as follows: statusInformation An indication of whether the encoding and securing of the message was successful. If not it is an indication of the problem. Blumenthal & Wijnen Standards Track [Page 18] RFC 3414 USM for SNMPv3 December 2002 messageProcessingModel The SNMP version number for the message to be generated. This data is not used by the User-based Security module. globalData The message header (i.e., its administrative information). This data is not used by the User-based Security module. maxMessageSize The maximum message size as included in the message. This data is not used by the User-based Security module. securityParameters These are the security parameters. They will be filled in by the User-based Security module. securityModel The securityModel in use. Should be User-based Security Model. This data is not used by the User-based Security module. securityName Together with the snmpEngineID it identifies a row in the usmUserTablethat is to be used for securing the message. The securityName has a format that is independent of the Security Model. In case of a response this parameter is ignored and the value from the cache is used. securityLevel The Level of Security from which the User-based Security module determines if the message needs to be protected from disclosure and if the message needs to be authenticated. securityEngineID The snmpEngineID of the authoritative SNMP engine to which a dateRequest message is to be sent. In case of a response it is implied to be the processing SNMP engine's snmpEngineID and so if it is specified, then it is ignored. scopedPDU The message payload. The data is opaque as far as the User-based Security Model is concerned. securityStateReference A handle/reference to cachedSecurityData to be used when securing an outgoing Response message. This is the exact same handle/reference as it was generated by the User-based Security module when processing the incoming Request message to which this is the Response message. Blumenthal & Wijnen Standards Track [Page 19] RFC 3414 USM for SNMPv3 December 2002 wholeMsg The fully encoded and secured message ready for sending on the wire. wholeMsgLength The length of the encoded and secured message (wholeMsg). Upon completion of the process, the User-based Security module returns statusInformation. If the process was successful, the completed message with privacy and authentication applied if such was requested by the specified securityLevel is returned. If the process was not successful, then an errorIndication is returned. 2.5.2. Services for Processing an Incoming SNMP Message When the Message Processing (MP) Subsystem invokes the User-based Security module to verify proper security of an incoming message, it must use the service provided for an incoming message. The abstract service primitive is: statusInformation = -- errorIndication or success -- error counter OID/value if error processIncomingMsg( IN messageProcessingModel -- typically, SNMP version IN maxMessageSize -- of the sending SNMP entity IN securityParameters -- for the received message IN securityModel -- for the received message IN securityLevel -- Level of Security IN wholeMsg -- as received on the wire IN wholeMsgLength -- length as received on the wire OUT securityEngineID -- authoritative SNMP entity OUT securityName -- identification of the principal OUT scopedPDU, -- message (plaintext) payload OUT maxSizeResponseScopedPDU -- maximum size of the Response PDU OUT securityStateReference -- reference to security state ) -- information, needed for response The abstract data elements passed as parameters in the abstract service primitives are as follows: statusInformation An indication of whether the process was successful or not. If not, then the statusInformation includes the OID and the value of the error counter that was incremented. messageProcessingModel The SNMP version number as received in the message. This data is not used by the User-based Security module. Blumenthal & Wijnen Standards Track [Page 20] RFC 3414 USM for SNMPv3 December 2002 maxMessageSize The maximum message size as included in the message. The User-bas User-based Security module uses this value to calculate the maxSizeResponseScopedPDU. securityParameters These are the security parameters as received in the message. securityModel The securityModel in use. Should be the User-based Security Model. This data is not used by the User-based Security module. securityLevel The Level of Security from which the User-based Security module determines if the message needs to be protected from disclosure and if the message needs to be authenticated. wholeMsg The whole message as it was received. wholeMsgLength The length of the message as it was received (wholeMsg). securityEngineID The snmpEngineID that was extracted from the field msgAuthoritativeEngineID and that was used to lookup the secrets in the usmUserTable. securityName The security name representing the user on whose beha