RFCs in HTML Format


RFC 1910

                  User-based Security Model for SNMPv2

Table of Contents

   1. Introduction ................................................    2
   1.1 Threats ....................................................    3
   1.2 Goals and Constraints ......................................    4
   1.3 Security Services ..........................................    5
   1.4 Mechanisms .................................................    5
   1.4.1 Digest Authentication Protocol ...........................    7
   1.4.2 Symmetric Encryption Protocol ............................    8
   2. Elements of the Model .......................................   10
   2.1 SNMPv2 Users ...............................................   10
   2.2 Contexts and Context Selectors .............................   11
   2.3 Quality of Service (qoS) ...................................   13
   2.4 Access Policy ..............................................   13
   2.5 Replay Protection ..........................................   13
   2.5.1 agentID ..................................................   14
   2.5.2 agentBoots and agentTime .................................   14
   2.5.3 Time Window ..............................................   15
   2.6 Error Reporting ............................................   15
   2.7 Time Synchronization .......................................   16
   2.8 Proxy Error Propagation ....................................   16
   2.9 SNMPv2 Messages Using this Model ...........................   16
   2.10 Local Configuration Datastore (LCD) .......................   18
   3. Elements of Procedure .......................................   19
   3.1 Generating a Request or Notification .......................   19
   3.2 Processing a Received Communication ........................   20
   3.2.1 Additional Details .......................................   28
   3.2.1.1 ASN.1 Parsing Errors ...................................   28
   3.2.1.2 Incorrectly Encoded Parameters .........................   29
   3.2.1.3 Generation of a Report PDU .............................   29
   3.2.1.4 Cache Timeout ..........................................   29
   3.3 Generating a Response ......................................   30
   4. Discovery ...................................................   30
   5. Definitions .................................................   31



Waters                        Experimental                      [Page 1]

RFC 1910 User-based Security Model for SNMPv2 February 1996 4.1 The USEC Basic Group ....................................... 32 4.2 Conformance Information .................................... 35 4.2.1 Compliance Statements .................................... 35 4.2.2 Units of Conformance ..................................... 35 6. Security Considerations ..................................... 36 6.1 Recommended Practices ...................................... 36 6.2 Defining Users ............................................. 37 6.3 Conformance ................................................ 38 7. Editor's Address ............................................ 38 8. Acknowledgements ............................................ 39 9. References .................................................. 39 Appendix A Installation ........................................ 41 Appendix A.1 Agent Installation Parameters ..................... 41 Appendix A.2 Password to Key Algorithm ......................... 43 Appendix A.3 Password to Key Sample ............................ 44 1. Introduction A management system contains: several (potentially many) nodes, each with a processing entity, termed an agent, which has access to management instrumentation; at least one management station; and, a management protocol, used to convey management information between the agents and management stations. Operations of the protocol are carried out under an administrative framework which defines authentication, authorization, access control, and privacy policies. Management stations execute management applications which monitor and control managed elements. Managed elements are devices such as hosts, routers, terminal servers, etc., which are monitored and controlled via access to their management information. The Administrative Infrastructure for SNMPv2 document [1] defines an administrative framework which realizes effective management in a variety of configurations and environments. In this administrative framework, a security model defines the mechanisms used to achieve an administratively-defined level of security for protocol interactions. Although many such security models might be defined, it is the purpose of this document, User- based Security Model for SNMPv2, to define the first, and, as of this writing, only, security model for this administrative framework. This administrative framework includes the provision of an access control model. The enforcement of access rights requires the means to identify the entity on whose behalf a request is generated. This SNMPv2 security model identifies an entity on whose behalf an SNMPv2 message is generated as a "user". Waters Experimental [Page 2]
RFC 1910 User-based Security Model for SNMPv2 February 1996 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 SNMPv2 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 SNMPv2 security model should provide protection are: Modification of Information The modification threat is the danger that some unauthorized entity may alter in-transit SNMPv2 messages generated on behalf of an authorized user 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 protocols defined in this memo do provide protection against: Message Stream Modification The SNMPv2 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 exchanges between managed agents and a management station. Protecting against this threat may be required as a matter of local policy. There are at least two threats that an SNMPv2 security protocol need not protect against. The security protocols defined in this memo do not provide protection against: Waters Experimental [Page 3]
RFC 1910 User-based Security Model for SNMPv2 February 1996 Denial of Service An SNMPv2 security protocol 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 network management protocol must cope as a matter of course. Traffic Analysis In addition, an SNMPv2 security protocol need not attempt to address traffic analysis attacks. Indeed, many traffic patterns are predictable - agents 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. Goals and Constraints Based on the foregoing account of threats in the SNMP network management environment, the goals of this SNMPv2 security model are as follows. (1) The protocol should provide for verification that each received SNMPv2 message has not been modified during its transmission through the network in such a way that an unauthorized management operation might result. (2) The protocol should provide for verification of the identity of the user on whose behalf a received SNMPv2 message claims to have been generated. (3) The protocol should provide for detection of received SNMPv2 messages, which request or contain management information, whose time of generation was not recent. (4) The protocol should provide, when necessary, that the contents of each received SNMPv2 message are protected from disclosure. In addition to the principal goal of supporting secure network management, the design of this SNMPv2 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 should prefer 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 Waters Experimental [Page 4]
RFC 1910 User-based Security Model for SNMPv2 February 1996 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 an SNMPv2 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. Data Confidentiality is the provision of the property that information is not made available or disclosed to unauthorized individuals, entities, or processes. For the protocols specified in this memo, it is not possible to assure the specific originator of a received SNMPv2 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. Mechanisms The security protocols defined in this memo employ several types of mechanisms in order to realize the goals and security services described above: Waters Experimental [Page 5]
RFC 1910 User-based Security Model for SNMPv2 February 1996 - In support of data integrity, a message digest algorithm is required. A digest is calculated over an appropriate portion of an SNMPv2 message and included as part of the message sent to the recipient. - In support of data origin authentication and data integrity, a secret value is both inserted into, and appended to, the SNMPv2 message prior to computing the digest; the inserted value overwritten prior to transmission, and the appended value is not transmitted. The secret value is shared by all SNMPv2 entities authorized to originate messages on behalf of the appropriate user. - To protect against the threat of message delay or replay (to an extent greater than can occur through normal operation), a set of time (at the agent) indicators and a request-id are included in each message generated. An SNMPv2 agent evaluates the time indicators to determine if a received message is recent. An SNMPv2 manager evaluates the time indicators to ensure that a received message is at least as recent as the last message it received from the same source. An SNMPv2 manager uses received authentic messages to advance its notion of time (at the agent). An SNMPv2 manager also evaluates the request-id in received Response messages and discards messages which do not correspond to outstanding requests. These mechanisms provide for the detection of messages whose time of generation was not recent in all but one circumstance; this circumstance is the delay or replay of a Report message (sent to a manager) when the manager has has not recently communicated with the source of the Report message. In this circumstance, the detection guarantees only that the Report message is more recent than the last communication between source and destination of the Report message. However, Report messages do not request or contain management information, and thus, goal #3 in Section 1.2 above is met; further, Report messages can at most cause the manager to advance its notion of time (at the agent) by less than the proper amount. This protection against the threat of message delay or replay does not imply nor provide any protection against unauthorized deletion or suppression of messages. 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 [15]). - In support of data confidentiality, an encryption algorithm is required. An appropriate portion of the message is encrypted prior to being transmitted. Waters Experimental [Page 6]
RFC 1910 User-based Security Model for SNMPv2 February 1996 1.4.1. Digest Authentication Protocol The Digest Authentication Protocol defined in this memo provides for: - verifying the integrity of a received message (i.e., the message received is the message sent). The integrity of the message is protected by computing a digest over an appropriate portion of a message. The digest is computed by the originator of the message, transmitted with the message, and verified by the recipient of the message. - verifying the user on whose behalf the message was generated. A secret value known only to SNMPv2 entities authorized to generate messages on behalf of this user is both inserted into, and appended to, the message prior to the digest computation. Thus, the verification of the user is implicit with the verification of the digest. (Note that the use of two copies of the secret, one near the start and one at the end, is recommended by [14].) - verifying that a message sent to/from one SNMPv2 entity cannot be replayed to/as-if-from another SNMPv2 entity. Included in each message is an identifier unique to the SNMPv2 agent associated with the sender or intended recipient of the message. Also, each message containing a Response PDU contains a request-id which associates the message to a recently generated request. A Report message sent by one SNMPv2 agent to one SNMPv2 manager can potentially be replayed to another SNMPv2 manager. However, Report messages do not request or contain management information, and thus, goal #3 in Section 1.2 above is met; further, Report messages can at most cause the manager to advance its notion of time (at the agent) by less than the correct amount. - detecting messages which were not recently generated. A set of time indicators are included in the message, indicating the time of generation. Messages (other than those containing Report PDUs) without recent time indicators are not considered authentic. In addition, messages containing Response PDUs have a request-id; if the request-id does not match that of a recently generated request, then the message is not considered to be authentic. Waters Experimental [Page 7]
RFC 1910 User-based Security Model for SNMPv2 February 1996 A Report message sent by an SNMPv2 agent can potentially be replayed at a later time to an SNMPv2 manager which has not recently communicated with that agent. However, Report messages do not request or contain management information, and thus, goal #3 in Section 1.2 above is met; further, Report messages can at most cause the manager to advance its notion of time (at the agent) by less than the correct amount. This protocol uses the MD5 [3] message digest algorithm. A 128-bit digest is calculated over the designated portion of an SNMPv2 message and included as part of the message sent to the recipient. The size of both the digest carried in a message and the private authentication key is 16 octets. This memo allows the same user to be defined on multiple SNMPv2 agents and managers. Each SNMPv2 agent maintains a value, agentID, which uniquely identifies the agent. This value is included in each message sent to/from that agent. Messages sent from a SNMPv2 dual- role entity [1] to a SNMPv2 manager include the agentID value maintained by the dual-role entity's agent. On receipt of a message, an agent checks the value to ensure it is the intended recipient, and a manager uses the value to ensure that the message is processed using the correct state information. Each SNMPv2 agent maintains two values, agentBoots and agentTime, which taken together provide an indication of time at that agent. Both of these values are included in an authenticated message sent to/received from that agent. Authenticated messages sent from a SNMPv2 dual-role entity to a SNMPv2 manager include the agentBoots and agentTime values maintained by the dual-role entity's agent. On receipt, the values are checked to ensure that the indicated time 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 SNMPv2 manager to generate a message which an agent will accept as authentic, and to verify that a message received from that agent is authentic, that manager must first achieve time synchronization with that agent. Similarly, for a manger to verify that a message received from an SNMPv2 dual-role entity is authentic, that manager must first achieve time synchronization with the dual- role entity's agent. 1.4.2. Symmetric Encryption Protocol The Symmetric Encryption Protocol defined in this memo provides support for data confidentiality through the use of the Data Encryption Standard (DES) in the Cipher Block Chaining mode of Waters Experimental [Page 8]
RFC 1910 User-based Security Model for SNMPv2 February 1996 operation. The designated portion of an SNMPv2 message is encrypted and included as part of the message sent to the recipient. Two organizations have published specifications defining the DES: the National Institute of Standards and Technology (NIST) [5] and the American National Standards Institute [6]. There is a companion Modes of Operation specification for each definition (see [7] and [8], respectively). The NIST has published three additional documents that implementors may find useful. - There is a document with guidelines for implementing and using the DES, including functional specifications for the DES and its modes of operation [9]. - There is a specification of a validation test suite for the DES [10]. The suite is designed to test all aspects of the DES and is useful for pinpointing specific problems. - There is a specification of a maintenance test for the DES [11]. The test utilizes a minimal amount of data and processing to test all components of the DES. It provides a simple yes-or-no indication of correct operation and is useful to run as part of an initialization step, e.g., when a computer reboots. This Symmetric Encryption Protocol specifies that the size of the privacy key is 16 octets, of which the first 8 octets are a DES key and the second 8 octets are a DES Initialization Vector. The 64-bit DES key in the first 8 octets of the private key is a 56 bit quantity used directly by the algorithm plus 8 parity bits - arranged so that one parity bit is the least significant bit of each octet. The setting of the parity bits is ignored by this protocol. The length of an octet sequence to be encrypted by the DES must be an integral multiple of 8. When encrypting, the data is padded at the end as necessary; the actual pad value is irrelevant. If the length of the octet sequence to be decrypted is not an integral multiple of 8 octets, the processing of the octet sequence is halted and an appropriate exception noted. When decrypting, the padding is ignored. Waters Experimental [Page 9]
RFC 1910 User-based Security Model for SNMPv2 February 1996 2. Elements of the Model This section contains definitions required to realize the security model defined by this memo. 2.1. SNMPv2 Users Management operations using this security model make use of a defined set of user identities. For any SNMPv2 user on whose behalf management operations are authorized at a particular SNMPv2 agent, that agent must have knowledge of that user. A SNMPv2 manager that wishes to communicate with a particular agent must also have knowledge of a user known to that agent, including knowledge of the applicable attributes of that user. Similarly, a SNMPv2 manager that wishes to receive messages from a SNMPv2 dual-role entity must have knowledge of the user on whose behalf the dual-role entity sends the message. A user and its attributes are defined as follows: <userName> An octet string representing the name of the user. <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. One such protocol is defined in this memo: the Digest Authentication Protocol. <authPrivateKey> 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 agents. <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 Symmetric Encryption Protocol. <privPrivateKey> If messages sent on behalf of this user can be protected from disclosure, the (private) privacy key for use with the privacy protocol. Note that a user's privacy key will normally be different at different agents. Waters Experimental [Page 10]
RFC 1910 User-based Security Model for SNMPv2 February 1996 2.2. Contexts and Context Selectors An SNMPv2 context is a collection of management information accessible (locally or via proxy) by an SNMPv2 agent. An item of management information may exist in more than one context. An SNMPv2 agent potentially has access to many contexts. Each SNMPv2 message contains a context selector which unambiguously identifies an SNMPv2 context accessible by the SNMPv2 agent to which the message is directed or by the SNMPv2 agent associated with the sender of the message. For a local SNMPv2 context which is realized by an SNMPv2 entity, that SNMPv2 entity uses locally-defined mechanisms to access the management information identified by the SNMPv2 context. For a proxy SNMPv2 context, the SNMPv2 entity acts as a proxy SNMPv2 agent to access the management information identified by the SNMPv2 context. The term remote SNMPv2 context is used at an SNMPv2 manager to indicate a SNMPv2 context (either local or proxy) which is not realized by the local SNMPv2 entity (i.e., the local SNMPv2 entity uses neither locally-defined mechanisms, nor acts as a proxy SNMPv2 agent to access the management information identified by the SNMPv2 context). Proxy SNMPv2 contexts are further categorized as either local-proxy contexts or remote-proxy contexts. A proxy SNMPv2 agent receives Get/GetNext/GetBulk/Set operations for a local-proxy context, and forwards them with a remote-proxy context; it receives SNMPv2-Trap and Inform operations for a remote-proxy context, and forwards them with a local-proxy context; for Response operations, a proxy SNMPv2 agent receives them with either a local-proxy or remote-proxy context, and forwards them with a remote-proxy or local-proxy context, respectively. Waters Experimental [Page 11]
RFC 1910 User-based Security Model for SNMPv2 February 1996 For the non-proxy situation: context-A Manager <----------------> Agent the type of context is: +-----------------+ | context-A | +-----------------+-----------------+ | Manager | remote | +-----------------+-----------------+ | Agent | local | +-----------------+-----------------+ | agentID | of Agent | +-----------------+-----------------+ | contextSelector | locally unique | +-----------------+-----------------+ For proxy: context-B context-C Manager <----------------> Proxy <----------------> Agent Agent the type and identity of the contexts are: +-----------------+-----------------+ | context-B | context-C | +-----------------+-----------------+-----------------+ | Manager | remote | -- | +-----------------+-----------------+-----------------+ | Proxy-Agent | local-proxy | remote-proxy | +-----------------+-----------------+-----------------+ | Agent | -- | local | +-----------------+-----------------+-----------------+ | agentID | of Proxy agent | of Agent | +-----------------+-----------------+-----------------+ | contextSelector | locally unique | locally unique | +-----------------+-----------------+-----------------+ The combination of an agentID value and a context selector provides a globally-unique identification of a context. When a context is accessible by multiple agents (e.g., including by proxy SNMPv2 agents), it has multiple such globally-unique identifications, one associated with each agent which can access it. In the example above, "context-B" and "context-C" are different names for the same context. Waters Experimental [Page 12]
RFC 1910 User-based Security Model for SNMPv2 February 1996 2.3. Quality of Service (qoS) Messages are generated with a particular Quality of Service (qoS), either: - without authentication and privacy, - with authentication but not privacy, - with authentication and privacy. All users are capable of having messages without authentication and privacy generated on their behalf. Users having an authentication protocol and an authentication key can have messages with authentication but not privacy generated on their behalf. Users having an authentication protocol, an authentication key, a privacy protocol and a privacy key can have messages with authentication and privacy generated on their behalf. In addition to its indications of authentication and privacy, the qoS may also indicate that the message contains an operation that may result in a report PDU being generated (see Section 2.6 below). 2.4. Access Policy An administration's access policy determines the access rights of users. For a particular SNMPv2 context to which a user has access using a particular qoS, that user's access rights are given by a list of authorized operations, and for a local context, a read-view and a write-view. The read-view is the set of object instances authorized for the user when reading objects. Reading objects occurs when processing a retrieval (get, get-next, get-bulk) operation and when sending a notification. The write-view is the set of object instances authorized for the user when writing objects. Writing objects occurs when processing a set operation. A user's access rights may be different at different agents. 2.5. Replay Protection Each SNMPv2 agent (or dual-role entity) maintains three objects: - agentID, which is an identifier unique among all agents in (at least) an administrative domain; - agentBoots, which is a count of the number of times the agent has rebooted/re-initialized since agentID was last configured; and, Waters Experimental [Page 13]
RFC 1910 User-based Security Model for SNMPv2 February 1996 - agentTime, which is the number of seconds since agentBoots was last incremented. An SNMPv2 agent is always authoritative with respect to these variables. It is the responsibility of an SNMPv2 manager to synchronize with the agent, as appropriate. In the case of an SNMPv2 dual-role entity sending an Inform-Request, it is that entity acting in an agent role which is authoritative with respect to these variables for the Inform-Request. An agent is required to maintain the values of agentID and agentBoots in non-volatile storage. 2.5.1. agentID The agentID value contained in an authenticated message is used to defeat attacks in which messages from a manager are replayed to a different agent and/or messages from one agent (or dual-role entity) are replayed as if from a different agent (or dual-role entity). When an agent (or dual-role entity) is first installed, it sets its local value of agentID according to a enterprise-specific algorithm (see the definition of agentID in Section 4.1). 2.5.2. agentBoots and agentTime The agentBoots and agentTime values contained in an authenticated message are used to defeat attacks in which messages are replayed when they are no longer valid. Through use of agentBoots and agentTime, there is no requirement for an SNMPv2 agent to have a non-volatile clock which ticks (i.e., increases with the passage of time) even when the agent is powered off. Rather, each time an SNMPv2 agent reboots, it retrieves, increments, and then stores agentBoots in non-volatile storage, and resets agentTime to zero. When an agent (or dual-role entity) is first installed, it sets its local values of agentBoots and agentTime to zero. If agentTime ever reaches its maximum value (2147483647), then agentBoots is incremented as if the agent has rebooted and agentTime is reset to zero and starts incrementing again. Each time an agent (or dual-role entity) reboots, any SNMPv2 managers holding that agent's values of agentBoots and agentTime need to re- synchronize prior to sending correctly authenticated messages to that agent (see Section 2.7 for re-synchronization procedures). Note, however, that the procedures do provide for a notification to be accepted as authentic by a manager, when sent by an agent which has rebooted since the manager last re-synchronized. Waters Experimental [Page 14]
RFC 1910 User-based Security Model for SNMPv2 February 1996 If an agent (or dual-role entity) is ever unable to determine its latest agentBoots value, then it must set its agentBoots value to 0xffffffff. Whenever the local value of agentBoots has the value 0xffffffff, it latches at that value and an authenticated message always causes an usecStatsNotInWindows authentication failure. In order to reset an agent whose agentBoots value has reached the value 0xffffffff, manual intervention is required. The agent must be physically visited and re-configured, either with a new agentID value, or with new secret values for the authentication and privacy keys of all users known to that agent. 2.5.3. 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. 2.6. Error Reporting While processing a received communication, an SNMPv2 entity may determine that the message is unacceptable (see Section 3.2). In this case, the appropriate counter from the snmpGroup [15] or usecStatsGroup object groups is incremented and the received message is discarded without further processing. If an SNMPv2 entity acting in the agent role makes such a determination and the qoS indicates that a report may be generated, then after incrementing the appropriate counter, it is required to generate a message containing a report PDU, with the same user and context as the received message, and to send it to the transport address which originated the received message. For all report PDUs, except those generated due to incrementing the usecStatsNotInWindows counter, the report PDU is unauthenticated. For those generated due to incrementing usecStatsNotInWindows, the report PDU is authenticated only if the received message was authenticated. The report flag in the qoS may only be set if the message contains a Get, GetNext, GetBulk, Set operation. The report flag should never be set for a message that contains a Response, Inform, SNMPv2-Trap or Report operation. Furthermore, a report PDU is never sent by an SNMPv2 entity acting in a manager role. Waters Experimental [Page 15]
RFC 1910 User-based Security Model for SNMPv2 February 1996
RFC 1910 User-based Security Model for SNMPv2 February 1996 ::= { usecStats 7 } -- conformance information usecMIBConformance OBJECT IDENTIFIER ::= { usecMIB 2 } usecMIBCompliances OBJECT IDENTIFIER ::= { usecMIBConformance 1 } usecMIBGroups OBJECT IDENTIFIER ::= { usecMIBConformance 2 } -- compliance statements usecMIBCompliance MODULE-COMPLIANCE STATUS current DESCRIPTION "The compliance statement for SNMPv2 entities which implement the SNMPv2 USEC model." MODULE -- this module MANDATORY-GROUPS { usecBasicGroup, usecStatsGroup } ::= { usecMIBCompliances 1 } -- units of conformance usecBasicGroup OBJECT-GROUP OBJECTS { agentID, agentBoots, agentTime, agentSize } STATUS current DESCRIPTION "A collection of objects providing identification, clocks, and capabilities of an SNMPv2 entity which implements the SNMPv2 USEC model." ::= { usecMIBGroups 1 } usecStatsGroup OBJECT-GROUP OBJECTS { usecStatsUnsupportedQoS, usecStatsNotInWindows, usecStatsUnknownUserNames, usecStatsWrongDigestValues, usecStatsUnknownContexts, usecStatsBadParameters, usecStatsUnauthorizedOperations } Waters Experimental [Page 35]
RFC 1910 User-based Security Model for SNMPv2 February 1996 STATUS current DESCRIPTION "A collection of objects providing basic error statistics of an SNMPv2 entity which implements the SNMPv2 USEC model." ::= { usecMIBGroups 2 } END 6. Security Considerations 6.1. Recommended Practices This section describes practices that contribute to the secure, effective operation of the mechanisms defined in this memo. - A management station must discard SNMPv2 responses for which neither the request-id component nor the represented management information corresponds to any currently outstanding request. Although it would be typical for a management station to do this as a matter of course, when using these security protocols it is significant due to the possibility of message duplication (malicious or otherwise). - A management station must generate unpredictable request-ids in authenticated messages in order to protect against the possibility of message duplication (malicious or otherwise). - A management station should perform time synchronization using authenticated messages in order to protect against the possibility of message duplication (malicious or otherwise). - When sending state altering messages to a managed agent, a management station should delay sending successive messages to the managed agent until a positive acknowledgement is received for the previous message or until the previous message expires. No message ordering is imposed by the SNMPv2. Messages may be received in any order relative to their time of generation and each will be processed in the ordered received. Note that when an authenticated message is sent to a managed agent, it will be valid for a period of time of approximately 150 seconds under normal circumstances, and is subject to replay during this period. Indeed, a management station must cope with the loss and re- ordering of messages resulting from anomalies in the network as a matter of course. Waters Experimental [Page 36]
RFC 1910 User-based Security Model for SNMPv2 February 1996 However, a managed object, snmpSetSerialNo [15], is specifically defined for use with SNMPv2 set operations in order to provide a mechanism to ensure the processing of SNMPv2 messages occurs in a specific order. - The frequency with which the secrets of an SNMPv2 user should be changed is indirectly related to the frequency of their use. Protecting the secrets from disclosure is critical to the overall security of the protocols. Frequent use of a secret provides a continued source of data that may be useful to a cryptanalyst in exploiting known or perceived weaknesses in an algorithm. Frequent changes to the secret avoid this vulnerability. Changing a secret after each use is generally regarded as the most secure practice, but a significant amount of overhead may be associated with that approach. Note, too, in a local environment the threat of disclosure may be less significant, and as such the changing of secrets may be less frequent. However, when public data networks are the communication paths, more caution is prudent. 6.2. Defining Users The mechanisms defined in this document employ the notion of "users" having access rights. How "users" are defined is subject to the security policy of the network administration. For example, users could be individuals (e.g., "joe" or "jane"), or a particular role (e.g., "operator" or "administrator"), or a combination (e.g., "joe- operator", "jane-operator" or "joe-admin"). Furthermore, a "user" may be a logical entity, such as a manager station application or set of manager station applications, acting on behalf of a individual or role, or set of individuals, or set of roles, including combinations. Appendix A describes an algorithm for mapping a user "password" to a 16 octet value for use as either a user's authentication key or privacy key (or both). Passwords are often generated, remembered, and input by a human. Human-generated passwords may be less than the 16 octets required by the authentication and privacy protocols, and brute force attacks can be quite easy on a relatively short ASCII character set. Therefore, the algorithm is Appendix A performs a transformation on the password. If the Appendix A algorithm is used, agent implementations (and agent configuration applications) must ensure that passwords are at least 8 characters in length. Because the Appendix A algorithm uses such passwords (nearly) directly, it is very important that they not be easily guessed. It Waters Experimental [Page 37]
RFC 1910 User-based Security Model for SNMPv2 February 1996 is suggested that they be composed of mixed-case alphanumeric and punctuation characters that don't form words or phrases that might be found in a dictionary. Longer passwords improve the security of the system. Users may wish to input multiword phrases to make their password string longer while ensuring that it is memorable. Note that there is security risk in configuring the same "user" on multiple systems where the same password is used on each system, since the compromise of that user's secrets on one system results in the compromise of that user on all other systems having the same password. The algorithm in Appendix A avoids this problem by including the agent's agentID value as well as the user's password in the calculation of a user's secrets; this results in the user's secrets being different at different agents; however, if the password is compromised the algorithm in Appendix A is not effective. 6.3. Conformance To be termed a "Secure SNMPv2 implementation", an SNMPv2 implementation: - must implement the Digest Authentication Protocol. - must, to the maximal extent possible, prohibit access to the secret(s) of each user about which it maintains information in a LCD, under all circumstances except as required to generate and/or validate SNMPv2 messages with respect to that user. - must implement the SNMPv2 USEC MIB. In addition, an SNMPv2 agent must provide initial configuration in accordance with Appendix A.1. Implementation of the Symmetric Encryption Protocol is optional. 7. Editor's Address Glenn W. Waters Bell-Northern Research Ltd. P.O. Box 3511, Station C Ottawa, Ontario K1Y 4H7 CA Phone: +1 613 763 3933 EMail: gwaters@bnr.ca Waters Experimental [Page 38]
RFC 1910 User-based Security Model for SNMPv2 February 1996 8. Acknowledgements This document is the result of significant work by three major contributors: Keith McCloghrie (Cisco Systems, kzm@cisco.com) Marshall T. Rose (Dover Beach Consulting, mrose@dbc.mtview.ca.us) Glenn W. Waters (Bell-Northern Research Ltd., gwaters@bnr.ca) The authors wish to acknowledge James M. Galvin of Trusted Information Systems who contributed significantly to earlier work on which this memo is based, and the general contributions of members of the SNMPv2 Working Group, and, in particular, Aleksey Y. Romanov and Steven L. Waldbusser. A special thanks is extended for the contributions of: Uri Blumenthal (IBM) Shawn Routhier (Epilogue) Barry Sheehan (IBM) Bert Wijnen (IBM) 9. References [1] McCloghrie, K., Editor, "An Administrative Infrastructure for SNMPv2", RFC 1909, Cisco Systems, January 1996. [2] Case, J., Fedor, M., Schoffstall, M., and J. Davin, "Simple Network Management Protocol", STD 15, RFC 1157, SNMP Research, Performance Systems International, MIT Laboratory for Computer Science, May 1990. [3] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, MIT Laboratory for Computer Science, April 1992. [4] The SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Coexistence between Version 1 and Version 2 of the Internet-standard Network Management Framework", RFC 1908, January 1996. [5] Data Encryption Standard, National Institute of Standards and Technology. Federal Information Processing Standard (FIPS) Publication 46-1. Supersedes FIPS Publication 46, (January, 1977; reaffirmed January, 1988). [6] Data Encryption Algorithm, American National Standards Institute. ANSI X3.92-1981, (December, 1980). Waters Experimental [Page 39]
RFC 1910 User-based Security Model for SNMPv2 February 1996 [7] DES Modes of Operation, National Institute of Standards and Technology. Federal Information Processing Standard (FIPS) Publication 81, (December, 1980). [8] Data Encryption Algorithm - Modes of Operation, American National Standards Institute. ANSI X3.106-1983, (May 1983). [9] Guidelines for Implementing and Using the NBS Data Encryption Standard, National Institute of Standards and Technology. Federal Information Processing Standard (FIPS) Publication 74, (April, 1981). [10] Validating the Correctness of Hardware Implementations of the NBS Data Encryption Standard, National Institute of Standards and Technology. Special Publication 500-20. [11] Maintenance Testing for the Data Encryption Standard, National Institute of Standards and Technology. Special Publication 500-61, (August, 1980). [12] The SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and S., Waldbusser, "Protocol Operations for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1905, January 1996. [13] The SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Transport Mappings for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1906, January 1996. [14] Krawczyk, H., "Keyed-MD5 for Message Authentication", Work in Progress, IBM, June 1995. [15] The SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Management Information Base for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1907 January 1996. Waters Experimental [Page 40]
RFC 1910 User-based Security Model for SNMPv2 February 1996 APPENDIX A - Installation A.1. Agent Installation Parameters During installation, an agent is configured with several parameters. These include: (1) a security posture The choice of security posture determines the extent of the view configured for unauthenticated access. One of three possible choices is selected: minimum-secure, semi-secure, or very-secure. (2) one or more transport service addresses These parameters may be specified explicitly, or they may be specified implicitly as the same set of network-layer addresses configured for other uses by the device together with the well- known transport-layer "port" information for the appropriate transport domain [13]. The agent listens on each of these transport service addresses for messages sent on behalf of any user it knows about. (3) one or more secrets These are the authentication/privacy secrets for the first user to be configured. One way to accomplish this is to have the installer enter a "password" for each required secret. The password is then algorithmically converted into the required secret by: - forming a string of length 1,048,576 octets by repeating the value of the password as often as necessary, truncating accordingly, and using the resulting string as the input to the MD5 algorithm. The resulting digest, termed "digest1", is used in the next step. - a second string of length 44 octets is formed by concatenating digest1, the agent's agentID value, and digest1. This string is used as input to the MD5 algorithm. The resulting digest is the required secret (see Appendix A.2). Waters Experimental [Page 41]
RFC 1910 User-based Security Model for SNMPv2 February 1996 With these configured parameters, the agent instantiates the following user, context, views and access rights. This configuration information should be readOnly (persistent). - One user: privacy not supported privacy supported --------------------- ----------------- <userName> "public" "public" <authProtocol> Digest Auth. Protocol Digest Auth. Protocol <authPrivateKey> authentication key authentication key <privProtocol> none Symmetric Privacy Protocol <privPrivateKey> -- privacy key - One local context with its <contextSelector> as the empty-string. - One view for authenticated access: - the <all> MIB view is the "internet" subtree. - A second view for unauthenticated access. This view is configured according to the selected security posture. For the "very-secure" posture: - the <restricted> MIB view is the union of the "snmp" [15], "usecAgent" and "usecStats" subtrees. For the "semi-secure" posture: - the <restricted> MIB view is the union of the "snmp" [15], "usecAgent", "usecStats" and "system" subtrees. For the "minimum-secure" posture: - the <restricted> MIB view is the "internet" subtree. - Access rights to allow: - read-only access for unauthenticated messages on behalf of the user "public" to the <restricted> MIB view of contextSelector "". - read-write access for authenticated but not private messages on behalf of the user "public" to the <all> MIB view of contextSelector "". - if privacy is supported, read-write access for authenticated and private messages on behalf of the user "public" to the Waters Experimental [Page 42]
RFC 1910 User-based Security Model for SNMPv2 February 1996 <all> MIB view of contextSelector "". A.2. Password to Key Algorithm The following code fragment demonstrates the password to key algorithm which can be used when mapping a password to an authentication or privacy key. (The calls to MD5 are as documented in RFC 1321.) void password_to_key(password, passwordlen, agentID, key) u_char *password; /* IN */ u_int passwordlen; /* IN */ u_char *agentID; /* IN - pointer to 12 octet long agentID */ u_char *key; /* OUT - caller supplies pointer to 16 octet buffer */ { MD5_CTX MD; u_char *cp, password_buf[64]; u_long password_index = 0; u_long count = 0, i; MD5Init (&MD); /* initialize MD5 */ /* loop until we've done 1 Megabyte */ while (count < 1048576) { cp = password_buf; for(i = 0; i < 64; i++) { *cp++ = password[ password_index++ % passwordlen ]; /* * Take the next byte of the password, wrapping to the * beginning of the password as necessary. */ } MDupdate (&MD, password_buf, 64); count += 64; } MD5Final (key, &MD); /* tell MD5 we're done */ /* localize the key with the agentID and pass through MD5 to produce final key */ memcpy (password_buf, key, 16); memcpy (password_buf+16, agentID, 12); memcpy (password_buf+28, key, 16); MD5Init (&MD); MDupdate (&MD, password_buf, 44); MD5Final (key, &MD); return; } Waters Experimental [Page 43]
RFC 1910 User-based Security Model for SNMPv2 February 1996 A.3. Password to Key Sample The following shows a sample output of the password to key algorithm. With a password of "maplesyrup" the output of the password to key algorithm before the key is localized with the agent's agentID is: '9f af 32 83 88 4e 92 83 4e bc 98 47 d8 ed d9 63'H After the intermediate key (shown above) is localized with the agentID value of: '00 00 00 00 00 00 00 00 00 00 00 02'H the final output of the password to key algorithm is: '52 6f 5e ed 9f cc e2 6f 89 64 c2 93 07 87 d8 2b'H



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