Network Working Group J. Davin Request for Comments: 1351 MIT Laboratory for Computer Science J. Galvin Trusted Information Systems, Inc. K. McCloghrie Hughes LAN Systems, Inc. July 1992
SNMP Administrative Model
Status of this Memo
This document specifies an IAB standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "IAB Official Protocol Standards" for the standardization state and status of this protocol. Distribution of this memo is unlimited.
This memo presents an elaboration of the SNMP administrative model set forth in [1]. This model provides a unified conceptual basis for administering SNMP protocol entities to support
o authentication and integrity,
o privacy,
o access control, and
o the cooperation of multiple protocol entities.
Please send comments to the SNMP Security Developers mailing list (snmp-sec-dev@tis.com).
2. Introduction
This memo presents an elaboration of the SNMP administrative model set forth in [1]. It describes how the elaborated administrative model is applied to realize effective network management in a variety of configurations and environments.
The model described here entails the use of distinct identities for peers that exchange SNMP messages. Thus, it represents a departure from the community-based administrative model set forth in [1]. By unambiguously identifying the source and intended recipient of each SNMP message, this new strategy improves upon the historical community scheme both by supporting a more convenient access control model and allowing for effective use of asymmetric (public key) security protocols in the future.
3. Elements of the Model
3.1 SNMP Party
A SNMP party is a conceptual, virtual execution context whose operation is restricted (for security or other purposes) to an administratively defined subset of all possible operations of a particular SNMP protocol entity (see Section 3.2). Whenever a SNMP protocol entity processes a SNMP message, it does so by acting as a SNMP party and is thereby restricted to the set of operations defined
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for that party. The set of possible operations specified for a SNMP party may be overlapping or disjoint with respect to the sets of other SNMP parties; it may also be a proper or improper subset of all possible operations of the SNMP protocol entity.
Architecturally, each SNMP party comprises
o a single, unique party identity,
o a single authentication protocol and associated parameters by which all protocol messages originated by the party are authenticated as to origin and integrity,
o a single privacy protocol and associated parameters by which all protocol messages received by the party are protected from disclosure,
o a single MIB view (see Section 3.6) to which all management operations performed by the party are applied, and
o a logical network location at which the party executes, characterized by a transport protocol domain and transport addressing information.
Conceptually, each SNMP party may be represented by an ASN.1 value with the following syntax:
For each SnmpParty value that represents a SNMP party, the following statements are true:
o Its partyIdentity component is the party identity.
o Its partyTDomain component is called the transport domain and indicates the kind of transport service by which the party receives network management traffic. An example of a transport domain is rfc1351Domain (SNMP over UDP, using SNMP parties).
o Its partyTAddr component is called the transport addressing information and represents a transport service address by which the party receives network management traffic.
o Its partyProxyFor component is called the proxied party and represents the identity of a second SNMP party or other management entity with which interaction may be necessary to satisfy received management requests. In this context, the value noProxy signifies that the party responds to received management requests by entirely local mechanisms.
o Its partyMaxMessageSize component is called the maximum message size and represents the length in octets of the largest SNMP message this party is prepared to accept.
o Its partyAuthProtocol component is called the authentication protocol and identifies a protocol and a mechanism by which all messages generated by the party
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are authenticated as to integrity and origin. In this context, the value noAuth signifies that messages generated by the party are not authenticated as to integrity and origin.
o Its partyAuthClock component is called the authentication clock and represents a notion of the current time that is specific to the party. The significance of this component is specific to the authentication protocol.
o Its partyAuthLastMsg component is called the last-timestamp and represents a notion of time associated with the most recent, authentic protocol message generated by the party. The significance of this component is specific to the authentication protocol.
o Its partyAuthNonce component is called the nonce and represents a monotonically increasing integer associated with the most recent, authentic protocol message generated by the party. The significance of this component is specific to the authentication protocol.
o Its partyAuthPrivate component is called the private authentication key and represents any secret value needed to support the authentication protocol. The significance of this component is specific to the authentication protocol.
o Its partyAuthPublic component is called the public authentication key and represents any public value that may be needed to support the authentication protocol. The significance of this component is specific to the authentication protocol.
o Its partyAuthLifetime component is called the lifetime and represents an administrative upper bound on acceptable delivery delay for protocol messages generated by the party. The significance of this component is specific to the authentication protocol.
o Its partyPrivProtocol component is called the privacy protocol and identifies a protocol and a mechanism by which all protocol messages received by the party are protected from disclosure. In this context, the value noPriv signifies that messages received by the party are not protected from disclosure.
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o Its partyPrivPrivate component is called the private privacy key and represents any secret value needed to support the privacy protocol. The significance of this component is specific to the privacy protocol.
o Its partyPrivPublic component is called the public privacy key and represents any public value that may be needed to support the privacy protocol. The significance of this component is specific to the privacy protocol.
If, for all SNMP parties realized by a SNMP protocol entity, the authentication protocol is noAuth and the privacy protocol is noPriv, then that protocol entity is called non-secure.
3.2 SNMP Protocol Entity
A SNMP protocol entity is an actual process which performs network management operations by generating and/or responding to SNMP protocol messages in the manner specified in [1]. When a protocol entity is acting as a particular SNMP party (see Section 3.1), the operation of that entity must be restricted to the subset of all possible operations that is administratively defined for that party.
By definition, the operation of a SNMP protocol entity requires no concurrency between processing of any single protocol message (by a particular SNMP party) and processing of any other protocol message (by a potentially different SNMP party). Accordingly, implementation of a SNMP protocol entity to support more than one party need not be multi-threaded. However, there may be situations where implementors may choose to use multi-threading.
Architecturally, every SNMP entity maintains a local database that represents all SNMP parties known to it -- those whose operation is realized locally, those whose operation is realized by proxy interactions with remote parties or devices, and those whose operation is realized by remote entities. In addition, every SNMP protocol entity maintains a local database that represents an access control policy (see Section 3.11) that defines the access privileges accorded to known SNMP parties.
3.3 SNMP Management Station
A SNMP management station is the operational role assumed by a SNMP party when it initiates SNMP management operations by the generation of appropriate SNMP protocol messages or when it receives and processes trap notifications.
Sometimes, the term SNMP management station is applied to partial
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implementations of the SNMP (in graphics workstations, for example) that focus upon this operational role. Such partial implementations may provide for convenient, local invocation of management services, but they may provide little or no support for performing SNMP management operations on behalf of remote protocol users.
3.4 SNMP Agent
A SNMP agent is the operational role assumed by a SNMP party when it performs SNMP management operations in response to received SNMP protocol messages such as those generated by a SNMP management station (see Section 3.3).
Sometimes, the term SNMP agent is applied to partial implementations of the SNMP (in embedded systems, for example) that focus upon this operational role. Such partial implementations provide for realization of SNMP management operations on behalf of remote users of management services, but they may provide little or no support for local invocation of such services.
3.5 View Subtree
A view subtree is the set of all MIB object instances which have a common ASN.1 OBJECT IDENTIFIER prefix to their names. A view subtree is identified by the OBJECT IDENTIFIER value which is the longest OBJECT IDENTIFIER prefix common to all (potential) MIB object instances in that subtree.
3.6 MIB View
A MIB view is a subset of the set of all instances of all object types defined according to the Internet-standard SMI [2] (i.e., of the universal set of all instances of all MIB objects), subject to the following constraints:
o Each element of a MIB view is uniquely named by an ASN.1 OBJECT IDENTIFIER value. As such, identically named instances of a particular object type (e.g., in different agents) must be contained within different MIB views. That is, a particular object instance name resolves within a particular MIB view to at most one object instance.
o Every MIB view is defined as a collection of view subtrees.
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3.7 SNMP Management Communication
A SNMP management communication is a communication from one specified SNMP party to a second specified SNMP party about management information that is represented in the MIB view of the appropriate party. In particular, a SNMP management communication may be
o a query by the originating party about information in the MIB view of the addressed party (e.g., getRequest and getNextRequest),
o an indicative assertion to the addressed party about information in the MIB view of the originating party (e.g., getResponse or trapNotification), or
o an imperative assertion by the originating party about information in the MIB view of the addressed party (e.g., setRequest).
A management communication is represented by an ASN.1 value with the syntax
For each SnmpMgmtCom value that represents a SNMP management communication, the following statements are true:
o Its dstParty component is called the destination and identifies the SNMP party to which the communication is directed.
o Its srcParty component is called the source and identifies the SNMP party from which the communication is originated.
o Its pdu component has the form and significance attributed to it in [1].
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3.8 SNMP Authenticated Management Communication
A SNMP authenticated management communication is a SNMP management communication (see Section 3.7) for which the originating SNMP party is (possibly) reliably identified and for which the integrity of the transmission of the communication is (possibly) protected. An authenticated management communication is represented by an ASN.1 value with the syntax
SnmpAuthMsg ::= [1] IMPLICIT SEQUENCE { authInfo ANY, - defined by authentication protocol authData SnmpMgmtCom }
For each SnmpAuthMsg value that represents a SNMP authenticated management communication, the following statements are true:
o Its authInfo component is called the authentication information and represents information required in support of the authentication protocol used by the SNMP party originating the message. The detailed significance of the authentication information is specific to the authentication protocol in use; it has no effect on the application semantics of the communication other than its use by the authentication protocol in determining whether the communication is authentic or not.
o Its authData component is called the authentication data and represents a SNMP management communication.
3.9 SNMP Private Management Communication
A SNMP private management communication is a SNMP authenticated management communication (see Section 3.8) that is (possibly) protected from disclosure. A private management communication is represented by an ASN.1 value with the syntax
For each SnmpPrivMsg value that represents a SNMP private management communication, the following statements are true:
o Its privDst component is called the privacy destination and identifies the SNMP party to which the communication is directed.
o Its privData component is called the privacy data and represents the (possibly encrypted) serialization (according to the conventions of [3] and [1]) of a SNMP authenticated management communication (see Section 3.8).
3.10 SNMP Management Communication Class
A SNMP management communication class corresponds to a specific SNMP PDU type defined in [1]. A management communication class is represented by an ASN.1 INTEGER value according to the type of the identifying PDU (see Table 1).
Get 1 GetNext 2 GetResponse 4 Set 8 Trap 16
Table 1: Management Communication Classes
The value by which a communication class is represented is computed as 2 raised to the value of the ASN.1 context-specific tag for the appropriate SNMP PDU.
A set of management communication classes is represented by the ASN.1 INTEGER value that is the sum of the representations of the communication classes in that set. The null set is represented by the value zero.
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3.11 SNMP Access Control Policy
A SNMP access control policy is a specification of a local access policy in terms of the network management communication classes which are authorized between pairs of SNMP parties. Architecturally, such a specification comprises three parts:
o the targets of SNMP access control - the SNMP parties that may perform management operations as requested by management communications received from other parties,
o the subjects of SNMP access control - the SNMP parties that may request, by sending management communications to other parties, that management operations be performed, and
o the policy that specifies the classes of SNMP management communications that a particular target is authorized to accept from a particular subject.
Access to individual MIB object instances is determined implicitly since by definition each (target) SNMP party performs operations on exactly one MIB view. Thus, defining the permitted access of a (reliably) identified subject party to a particular target party effectively defines the access permitted by that subject to that target's MIB view and, accordingly, to particular MIB object instances.
Conceptually, a SNMP access policy is represented by a collection of ASN.1 values with the following syntax:
For each such value that represents one part of a SNMP access policy, the following statements are true:
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o Its aclTarget component is called the target and identifies the SNMP party to which the partial policy permits access.
o Its aclSubject component is called the subject and identifies the SNMP party to which the partial policy grants privileges.
o Its aclPrivileges component is called the privileges and represents a set of SNMP management communication classes that are authorized to be processed by the specified target party when received from the specified subject party.
3.12 SNMP Proxy Party
A SNMP proxy party is a SNMP party that performs management operations by communicating with another, logically remote party.
When communication between a logically remote party and a SNMP proxy party is via the SNMP (over any transport protocol), then the proxy party is called a SNMP native proxy party. Deployment of SNMP native proxy parties is a means whereby the processing or bandwidth costs of management may be amortized or shifted -- thereby facilitating the construction of large management systems.
When communication between a logically remote party and a SNMP proxy party is not via the SNMP, then the proxy party is called a SNMP foreign proxy party. Deployment of foreign proxy parties is a means whereby otherwise unmanageable devices or portions of an internet may be managed via the SNMP.
The transparency principle that defines the behavior of a SNMP party in general applies in particular to a SNMP proxy party:
The manner in which one SNMP party processes SNMP protocol messages received from another SNMP party is entirely transparent to the latter.
The transparency principle derives directly from the historical SNMP philosophy of divorcing architecture from implementation. To this dichotomy are attributable many of the most valuable benefits in both the information and distribution models of the management framework, and it is the architectural cornerstone upon which large management systems may be built. Consistent with this philosophy, although the implementation of SNMP proxy agents in certain environments may resemble that of a transport-layer bridge, this particular implementation strategy (or any other!) does not merit special
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recognition either in the SNMP management architecture or in standard mechanisms for proxy administration.
Implicit in the transparency principle is the requirement that the semantics of SNMP management operations are preserved between any two SNMP peers. In particular, the "as if simultaneous" semantics of a Set operation are extremely difficult to guarantee if its scope extends to management information resident at multiple network locations. For this reason, proxy configurations that admit Set operations that apply to information at multiple locations are discouraged, although such operations are not explicitly precluded by the architecture in those rare cases where they might be supported in a conformant way.
Also implicit in the transparency principle is the requirement that, throughout its interaction with a proxy agent, a management station is supplied with no information about the nature or progress of the proxy mechanisms by which its requests are realized. That is, it should seem to the management station -- except for any distinction in underlying transport address -- as if it were interacting via SNMP directly with the proxied device. Thus, a timeout in the communication between a proxy agent and its proxied device should be represented as a timeout in the communication between the management station and the proxy agent. Similarly, an error response from a proxied device should -- as much as possible -- be represented by the corresponding error response in the interaction between the proxy agent and management station.
3.13 Procedures
This section describes the procedures followed by a SNMP protocol entity in processing SNMP messages. These procedures are independent of the particular authentication and privacy protocols that may be in use.
3.13.1 Generating a Request
This section describes the procedure followed by a SNMP protocol entity whenever either a management request or a trap notification is to be transmitted by a SNMP party.
1. An ASN.1 SnmpMgmtCom value is constructed for which the srcParty component identifies the originating party, for which the dstParty component identifies the receiving party, and for which the other component represents the desired management operation.
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2. The local database is consulted to determine the authentication protocol and other relevant information for the originating SNMP party.
3. An ASN.1 SnmpAuthMsg value is constructed with the following properties:
o Its authInfo component is constructed according to the authentication protocol specified for the originating party.
In particular, if the authentication protocol for the originating SNMP party is identified as noAuth, then this component corresponds to the OCTET STRING value of zero length.
o Its authData component is the constructed SnmpMgmtCom value.
4. The local database is consulted to determine the privacy protocol and other relevant information for the receiving SNMP party.
5. An ASN.1 SnmpPrivMsg value is constructed with the following properties:
o Its privDst component identifies the receiving SNMP party.
o Its privData component is the (possibly encrypted) serialization of the SnmpAuthMsg value according to the conventions of [3] and [1].
In particular, if the privacy protocol for the receiving SNMP party is identified as noPriv, then the privData component is unencrypted. Otherwise, the privData component is processed according to the privacy protocol.
6. The constructed SnmpPrivMsg value is serialized according to the conventions of [3] and [1].
7. The serialized SnmpPrivMsg value is transmitted using the transport address and transport domain for the receiving SNMP party.
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3.13.2 Processing a Received Communication
This section describes the procedure followed by a SNMP protocol entity whenever a management communication is received.
1. If the received message is not the serialization (according to the conventions of [3] and [1]) of an ASN.1 SnmpPrivMsg value, then that message is discarded without further processing.
2. The local database is consulted for information about the receiving SNMP party identified by the privDst component of the SnmpPrivMsg value.
3. If information about the receiving SNMP party is absent from the local database, or specifies a transport domain and address which indicates that the receiving party's operation is not realized by the local SNMP protocol entity, then the received message is discarded without further processing.
4. An ASN.1 OCTET STRING value is constructed (possibly by decryption, according to the privacy protocol in use) from the privData component of said SnmpPrivMsg value.
In particular, if the privacy protocol recorded for the party is noPriv, then the OCTET STRING value corresponds exactly to the privData component of the SnmpPrivMsg value.
5. If the OCTET STRING value is not the serialization (according to the conventions of [3] and [1]) of an ASN.1 SnmpAuthMsg value, then the received message is discarded without further processing.
6. If the dstParty component of the authData component of the obtained SnmpAuthMsg value is not the same as the privDst component of the SnmpPrivMsg value, then the received message is discarded without further processing.
7. The local database is consulted for information about the originating SNMP party identified by the srcParty component of the authData component of the SnmpAuthMsg value.
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8. If information about the originating SNMP party is absent from the local database, then the received message is discarded without further processing.
9. The obtained SnmpAuthMsg value is evaluated according to the authentication protocol and other relevant information associated with the originating SNMP party in the local database.
In particular, if the authentication protocol is identified as noAuth, then the SnmpAuthMsg value is always evaluated as authentic.
10. If the SnmpAuthMsg value is evaluated as unauthentic, then the received message is discarded without further processing, and an authentication failure is noted.
11. The ASN.1 SnmpMgmtCom value is extracted from the authData component of the SnmpAuthMsg value.
12. The local database is consulted for access privileges permitted by the local access policy to the originating SNMP party with respect to the receiving SNMP party.
13. The management communication class is determined from the ASN.1 tag value associated with the SnmpMgmtCom value.
14. If the management communication class of the received message is either 16 or 4 (i.e., Trap or GetResponse) and this class is not among the access privileges, then the received message is discarded without further processing.
15. If the management communication class of the received message is not among the access privileges, then the received message is discarded without further processing after generation and transmission of a response message. This response message is directed to the originating SNMP party on behalf of the receiving SNMP party. Its var-bind-list and request-id components are identical to those of the received request. Its error-index component is zero and its error-status component is readOnly.
16. If the proxied party associated with the receiving SNMP party in the local database is identified as noProxy,
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then the management operation represented by the SnmpMgmtCom value is performed by the receiving SNMP protocol entity with respect to the MIB view identified with the receiving SNMP party according to the procedures set forth in [1].
17. If the proxied party associated with the receiving SNMP party in the local database is not identified as noProxy, then the management operation represented by the SnmpMgmtCom value is performed through appropriate cooperation between the receiving SNMP party and the identified proxied party.
In particular, if the transport domain associated with the identified proxied party in the local database is rfc1351Domain, then the operation requested by the received message is performed by the generation of a corresponding request to the proxied party on behalf of the receiving party. If the received message requires a response from the local SNMP protocol entity, then that response is subsequently generated from the response (if any) received from the proxied party corresponding to the newly generated request.
3.13.3 Generating a Response
This section describes the procedure followed by a SNMP protocol entity whenever a response to a management request is generated.
The procedure for generating a response to a SNMP management request is identical to the procedure for transmitting a request (see Section 3.13.1), except for the derivation of the transport domain and address information. In this case, the response is transmitted using the transport domain and address from which the corresponding request originated -- even if that is different from the transport information recorded in the local database.
4. Application of the Model
This section describes how the administrative model set forth above is applied to realize effective network management in a variety of configurations and environments. Several types of administrative configurations are identified, and an example of each is presented.
4.1 Non-Secure Minimal Agent Configuration
This section presents an example configuration for a minimal, non- secure SNMP agent that interacts with one or more SNMP management
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stations. Table 2 presents information about SNMP parties that is known both to the minimal agent and to the manager, while Table 3 presents similarly common information about the local access policy.
As represented in Table 2, the example agent party operates at UDP port 161 at IP address 1.2.3.4 using the party identity gracie; the example manager operates at UDP port 2001 at IP address 1.2.3.5 using the identity george. At minimum, a non-secure SNMP agent implementation must provide for administrative configuration (and non-volatile storage) of the identities and transport addresses of two SNMP parties: itself and a remote peer. Strictly speaking, other information about these two parties (including access policy information) need not be configurable.
Suppose that the managing party george wishes to interrogate the agent named gracie by issuing a SNMP GetNext request message. The manager consults its local database of party information. Because the authentication protocol for the party george is recorded as noAuth, the GetNext request message generated by the manager is not
Target Subject Privileges gracie george 3 george gracie 20
Table 3: Access Information for Minimal Agent
authenticated as to origin and integrity. Because, according to the manager's database, the privacy protocol for the party gracie is noPriv, the GetNext request message is not protected from disclosure.
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Rather, it is simply assembled, serialized, and transmitted to the transport address (IP address 1.2.3.4, UDP port 161) associated in the manager's database with the party gracie.
When the GetNext request message is received at the agent, the identity of the party to which it is directed (gracie) is extracted from the message, and the receiving protocol entity consults its local database of party information. Because the privacy protocol for the party gracie is recorded as noPriv, the received message is assumed not to be protected from disclosure. Similarly, the identity of the originating party (george) is extracted, and the local party database is consulted. Because the authentication protocol for the party george is recorded as noAuth, the received message is immediately accepted as authentic.
The received message is fully processed only if the access policy database local to the agent authorizes GetNext request communications by the party george with respect to the agent party gracie. The access policy database presented as Table 3 authorizes such communications (as well as Get operations).
When the received request is processed, a GetResponse message is generated with gracie as the source party and george, the party from which the request originated, as the destination party. Because the authentication protocol for gracie is recorded in the local party database as noAuth, the generated GetResponse message is not authenticated as to origin or integrity. Because, according to the local database, the privacy protocol for the party george is noPriv, the response message is not protected from disclosure. The response message is transmitted to the transport address from which the corresponding request originated -- without regard for the transport address associated with george in the local database.
When the generated response is received by the manager, the identity of the party to which it is directed (george) is extracted from the message, and the manager consults its local database of party information. Because the privacy protocol for the party george is recorded as noPriv, the received response is assumed not to be protected from disclosure. Similarly, the identity of the originating party (gracie) is extracted, and the local party database is consulted. Because the authentication protocol for the party gracie is recorded as noAuth, the received response is immediately accepted as authentic.
The received message is fully processed only if the access policy database local to the manager authorizes GetResponse communications by the party gracie with respect to the manager party george. The access policy database presented as Table 3 authorizes such response
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messages (as well as Trap messages).
4.2 Secure Minimal Agent Configuration
This section presents an example configuration for a secure, minimal SNMP agent that interacts with a single SNMP management station. Table 4 presents information about SNMP parties that is known both to the minimal agent and to the manager, while Table 5 presents similarly common information about the local access policy.
The interaction of manager and agent in this configuration is very similar to that sketched above for the non-secure minimal agent -- except that all protocol messages are authenticated as to origin and integrity and protected from disclosure. This example requires encryption in order to support distribution of secret keys via the SNMP itself. A more elaborate example comprising an additional pair of SNMP parties could support the exchange of non-secret information in authenticated messages without incurring the cost of encryption.
An actual secure agent configuration may require SNMP parties for which the authentication and privacy protocols are noAuth and noPriv, respectively, in order to support clock synchronization (see [4]). For clarity, these additional parties are not represented in this example.
Table 4: Party Information for Secure Minimal Agent
Target Subject Privileges ollie stan 3 stan ollie 20
Table 5: Access Information for Secure Minimal Agent
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As represented in Table 4, the example agent party operates at UDP port 161 at IP address 1.2.3.4 using the party identity ollie; the example manager operates at UDP port 2001 at IP address 1.2.3.5 using the identity stan. At minimum, a secure SNMP agent implementation must provide for administrative configuration (and non-volatile storage) of relevant information about two SNMP parties: itself and a remote peer. Both ollie and stan authenticate all messages that they generate by using the SNMP authentication protocol md5AuthProtocol and their distinct, private authentication keys. Although these private authentication key values ("0123456789ABCDEF" and "GHIJKL0123456789") are presented here for expository purposes, knowledge of private authentication keys is not normally afforded to human beings and is confined to those portions of the protocol implementation that require it.
When using the md5AuthProtocol, the public authentication key for each SNMP party is never used in authentication and verification of SNMP exchanges. Also, because the md5AuthProtocol is symmetric in character, the private authentication key for each party must be known to another SNMP party with which authenticated communication is desired. In contrast, asymmetric (public key) authentication protocols would not depend upon sharing of a private key for their operation.
All protocol messages originated by the party stan are encrypted on transmission using the desPrivProtocol privacy protocol and the private key "STUVWX0123456789"; they are decrypted upon reception according to the same protocol and key. Similarly, all messages originated by the party ollie are encrypted on transmission using the desPrivProtocol protocol and private privacy key "MNOPQR0123456789"; they are correspondingly decrypted on reception. As with authentication keys, knowledge of private privacy keys is not normally afforded to human beings and is confined to those portions of the protocol implementation that require it.
4.3 Proxy Configuration
This section presents examples of SNMP proxy configurations. On one hand, foreign proxy configurations provide the capability to manage non-SNMP devices. On the other hand, native proxy configurations allow an administrator to shift the computational burden of rich management functionality away from network devices whose primary task is not management. To the extent that SNMP proxy agents function as points of aggregation for management information, proxy configurations may also reduce the bandwidth requirements of large- scale management activities.
The example configurations in this section are simplified for
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clarity: actual configurations may require additional parties in order to support clock synchronization and distribution of secrets.
4.3.1 Foreign Proxy Configuration
This section presents an example configuration by which a SNMP management station may manage network elements that do not themselves support the SNMP. This configuration centers on a SNMP proxy agent that realizes SNMP management operations by interacting with a non- SNMP device using a proprietary protocol.
Table 6 presents information about SNMP parties that is recorded in the local database of the SNMP proxy agent. Table 7 presents information about SNMP parties that is recorded in the local database of the SNMP management station. Table 8 presents information about the access policy specified by the local administration.
As represented in Table 6, the proxy agent party operates at UDP port 161 at IP address 1.2.3.5 using the party identity moe; the example manager operates at UDP port 2002 at IP address 1.2.3.4 using the identity larry. Both larry and moe authenticate all messages that they generate by using the protocol md5AuthProtocol and their distinct, private authentication keys. Although these private authentication key values ("0123456789ABCDEF" and
Target Subject Privileges moe larry 3 larry moe 20
Table 8: Access Information for Foreign Proxy
"GHIJKL0123456789") are presented here for expository purposes, knowledge of private keys is not normally afforded to human beings and is confined to those portions of the protocol implementation that require it.
Although all SNMP agents that use cryptographic keys in their communication with other protocol entities will almost certainly engage in private SNMP exchanges to distribute those keys, in order to simplify this example, neither the management station nor the proxy agent sends or receives private SNMP communications. Thus, the privacy protocol for each of them is recorded as noPriv.
The party curly does not send or receive SNMP protocol messages; rather, all communication with that party proceeds via a hypothetical proprietary protocol identified by the value acmeMgmtPrtcl. Because the party curly does not participate in the SNMP, many of the attributes recorded for that party in a local database are ignored.
In order to interrogate the proprietary device associated with the party curly, the management station larry constructs a SNMP GetNext request and transmits it to the party moe operating (see Table 7) at UDP port 161, and IP address 1.2.3.5. This request is authenticated using the private authentication key "0123456789ABCDEF."
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When that request is received by the party moe, the originator of the message is verified as being the party larry by using local knowledge (see Table 6) of the private authentication key "0123456789ABCDEF." Because party larry is authorized to issue GetNext requests with respect to party moe by the relevant access control policy (Table 8), the request is accepted. Because the local database records the proxied party for party moe as curly, the request is satisfied by its translation into appropriate operations of the acmeMgmtPrtcl directed at party curly. These new operations are transmitted to the party curly at the address 0x98765432 in the acmeMgmtPrtcl domain.
When and if the proprietary protocol exchange between the proxy agent and the proprietary device concludes, a SNMP GetResponse management operation is constructed by the SNMP party moe to relay the results to party larry. This response communication is authenticated as to origin and integrity using the authentication protocol md5AuthProtocol and private authentication key "GHIJKL0123456789" specified for transmissions from party moe. It is then transmitted to the SNMP party larry operating at the management station at IP address 1.2.3.4 and UDP port 2002 (the source address for the corresponding request).
When this response is received by the party larry, the originator of the message is verified as being the party moe by using local knowledge (see Table 7) of the private authentication key "GHIJKL0123456789." Because party moe is authorized to issue GetResponse communications with respect to party larry by the relevant access control policy (Table 8), the response is accepted, and the interrogation of the proprietary device is complete.
It is especially useful to observe that the database of SNMP parties recorded at the proxy agent (Table 6) need be neither static nor configured exclusively by the management station. For instance, suppose that, in this example, the acmeMgmtPrtcl was a proprietary, MAC-layer mechanism for managing stations attached to a local area network. In such an environment, the SNMP party moe would reside at a SNMP proxy agent attached to such a LAN and could, by participating in the LAN protocols, detect the attachment and disconnection of various stations on the LAN. In this scenario, the SNMP proxy agent could easily adjust its local database of SNMP parties to support indirect management of the LAN stations by the SNMP management station. For each new LAN station detected, the SNMP proxy agent would add to its database both an entry analogous to that for party curly (representing the new LAN station itself) and an entry analogous to that for party moe (representing a proxy for that new station in the SNMP domain).
By using the SNMP to interrogate the database of parties held locally
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by the SNMP proxy agent, a SNMP management station can discover and interact with new stations as they are attached to the LAN.
4.3.2 Native Proxy Configuration
This section presents an example configuration that supports SNMP native proxy operations -- indirect interaction between a SNMP agent and a management station that is mediated by a second SNMP (proxy) agent.
This example configuration is similar to that presented in the discussion of SNMP foreign proxy above. In this example, however, the party associated with the identity curly receives messages via the SNMP, and, accordingly interacts with the SNMP proxy agent moe using authenticated SNMP communications.
Table 9 presents information about SNMP parties that is recorded in the local database of the SNMP proxy agent. Table 7 presents information about SNMP parties that is recorded in the local database of the SNMP management station. Table 10 presents information about the access policy specified by the local administration.
As represented in Table 9, the proxy party operates at UDP port 161 at IP address 1.2.3.5 using the party identity moe;
Target Subject Privileges moe larry 3 larry moe 20 curly moe 3 moe curly 20
Table 10: Access Information for Native Proxy
the example manager operates at UDP port 2002 at IP address 1.2.3.4 using the identity larry; the proxied party operates at UDP port 161 at IP address 1.2.3.6 using the party identity curly. Messages generated by all three SNMP parties are authenticated as to origin and integrity by using the authentication protocol md5AuthProtocol and distinct, private authentication keys. Although these private key values ("0123456789ABCDEF," "GHIJKL0123456789," and "MNOPQR0123456789") are presented here for expository purposes, knowledge of private keys is not normally afforded to human beings and is confined to those portions of the protocol implementation that require it.
In order to interrogate the proxied device associated with the party curly, the management station larry constructs a SNMP GetNext request and transmits it to the party moe operating (see Table 7) at UDP port 161 and IP address 1.2.3.5. This request is authenticated using the private authentication key "0123456789ABCDEF."
When that request is received by the party moe, the originator of the message is verified as being the party larry by using local knowledge (see Table 9) of the private authentication key "0123456789ABCDEF." Because party larry is authorized to issue GetNext (and Get) requests with respect to party moe by the relevant access control policy (Table 10), the request is accepted. Because the local database records the proxied party for party moe as curly, the request is satisfied by its translation into a corresponding SNMP GetNext request directed from party moe to party curly. This new communication is authenticated using the private authentication key "GHIJKL0123456789" and transmitted to party curly at the IP address 1.2.3.6.
When this new request is received by the party curly, the originator of the message is verified as being the party moe by using local knowledge (see Table 9) of the private authentication key "GHIJKL0123456789." Because party moe is authorized to issue GetNext (and Get) requests with respect to party curly by the relevant access control policy (Table 10), the request is accepted. Because the local database records the proxied party for party curly as noProxy, the GetNext request is satisfied by local mechanisms. A SNMP GetResponse message representing the results of the query is then generated by
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party curly. This response communication is authenticated as to origin and integrity using the private authentication key "MNOPQR0123456789" and transmitted to party moe at IP address 1.2.3.5 (the source address for the corresponding request).
When this response is received by party moe, the originator of the message is verified as being the party curly by using local knowledge (see Table 9) of the private authentication key "MNOPQR0123456789." Because party curly is authorized to issue GetResponse communications with respect to party moe by the relevant access control policy (Table 10), the response is not rejected. Instead, it is translated into a response to the original GetNext request from party larry. This response is authenticated as to origin and integrity using the private authentication key "GHIJKL0123456789" and is transmitted to the party larry at IP address 1.2.3.4 (the source address for the original request).
When this response is received by the party larry, the originator of the message is verified as being the party moe by using local knowledge (see Table 7) of the private authentication key "GHIJKL0123456789." Because party moe is authorized to issue GetResponse communications with respect to party larry by the relevant access control policy (Table 10), the response is accepted, and the interrogation is complete.
4.4 Public Key Configuration
This section presents an example configuration predicated upon a hypothetical security protocol. This hypothetical protocol would be based on asymmetric (public key) cryptography as a means for providing data origin authentication (but not protection against disclosure). This example illustrates the consistency of the administrative model with public key technology, and the extension of the example to support protection against disclosure should be apparent.
The example configuration comprises a single SNMP agent that interacts with a single SNMP management station. Tables 11 and 12 present information about SNMP parties that is by the agent and manager, respectively, while Table 5 presents information about the local access policy that is known to both manager and agent.
As represented in Table 11, the example agent party operates at UDP port 161 at IP address 1.2.3.4 using the party identity ollie; the example manager operates at UDP port 2004 at IP address 1.2.3.5 using the identity stan. Both ollie and stan authenticate all messages that they generate as to origin and integrity by using the hypothetical SNMP authentication protocol pkAuthProtocol and their distinct, private
Table 12: Party Information for Public Key Management Station
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authentication keys. Although these private authentication key values ("0123456789ABCDEF" and "GHIJKL0123456789") are presented here for expository purposes, knowledge of private keys is not normally afforded to human beings and is confined to those portions of the protocol implementation that require it.
In most respects, the interaction between manager and agent in this configuration is almost identical to that in the example of the minimal, secure SNMP agent described above. The most significant difference is that neither SNMP party in the public key configuration has knowledge of the private key by which the other party authenticates its transmissions. Instead, for each received authenticated SNMP communication, the identity of the originator is verified by applying an asymmetric cryptographic algorithm to the received message together with the public authentication key for the originating party. Thus, in this configuration, the agent knows the manager's public key ("ghijkl0123456789") but not its private key ("GHIJKL0123456789"); similarly, the manager knows the agent's public key ("0123456789abcdef") but not its private key ("0123456789ABCDEF").
For simplicity, privacy protocols are not addressed in this example configuration, although their use would be necessary to the secure, automated distribution of secret keys.
4.5 MIB View Configurations
This section describes a convention for the definition of MIB views and, using that convention, presents example configurations of MIB views for SNMP parties.
A MIB view is defined by a collection of view subtrees (see Section 3.6), and any MIB view may be represented in this way. Because MIB view definitions may, in certain cases, comprise a very large number of view subtrees, a convention for abbreviating MIB view definitions is desirable.
The convention adopted in [5] supports abbreviation of MIB view definitions in terms of families of view subtrees that are either included in or excluded from the definition of the relevant MIB view. By this convention, a table locally maintained by each SNMP entity defines the MIB view associated with each SNMP party realized by that entity. Each entry in the table represents a family of view subtrees that (according to the status of that entry) is either included in or excluded from the MIB view of some SNMP party. Each table entry represents a subtree family as a pairing of an OBJECT IDENTIFIER value (called the family name) together with a bitstring value (called the family mask). The family mask indicates which
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subidentifiers of the associated family name are significant to the definition of the represented subtree family. For each possible MIB object instance, that instance belongs to the view subtree family represented by a particular table entry if
o the OBJECT IDENTIFIER name of that MIB object instance comprises at least as many subidentifiers as does the family name for said table entry, and
o each subidentifier in the name of said MIB object instance matches the corresponding subidentifier of the relevant family name whenever the corresponding bit of the associated family mask is non-zero.
The appearance of a MIB object instance in the MIB view for a particular SNMP party is related to the membership of that instance in the subtree families associated with that party in local table entries:
o If a MIB object instance belongs to none of the relevant subtree families, then that instance is not in the MIB view for the relevant SNMP party.
o If a MIB object instance belongs to the subtree family represented by exactly one of the relevant table entries, then that instance is included in, or excluded from, the relevant MIB view according to the status of that entry.
o If a MIB object instance belongs to the subtree families represented by more than one of the relevant table entries, then that instance is included in, or excluded from, the relevant MIB view according to the status of the single such table entry for which, first, the associated family name comprises the greatest number of subidentifiers, and, second, the associated family name is lexicographically greatest.
The subtree family represented by a table entry for which the associated family mask is all ones corresponds to the single view subtree identified by the family name for that entry. Because the convention of [5] provides for implicit extension of family mask values with ones, the subtree family represented by a table entry with a family mask of zero length always corresponds to a single view subtree.
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Party Identity Status Family Name Family Mask lucy include internet ""h
Table 13: View Definition for Minimal Agent
Using this convention for abbreviating MIB view definitions, some of the most common definitions of MIB views may be conveniently expressed. For example, Table 13 illustrates the MIB view definitions required for a minimal SNMP entity that locally realizes a single SNMP party for which the associated MIB view embraces all instances of all MIB objects defined within the internet network management framework. The represented table has a single entry. The SNMP party (lucy) for which that entry defines the MIB view is identified in the first column. The status of that entry (include) signifies that any MIB object instance belonging to the subtree family represented by that entry may appear in the MIB view for party lucy. The family name for that entry is internet, and the zero-length family mask value signifies that the relevant subtree family corresponds to the single view subtree rooted at that node.
Another example of MIB view definition (see Table 14) is that of a SNMP protocol entity that locally realizes multiple SNMP parties with distinct MIB views. The MIB view associated with the party lucy comprises all instances of all MIB objects defined within the internet network management framework, except those pertaining to the administration of SNMP parties. In contrast, the MIB view attributed to the party ricky contains only MIB object instances defined in the system group of the internet-standard MIB together with those object instances by which SNMP parties are administered.
A more complicated example of MIB view configuration illustrates the abbreviation of related collections of view subtrees by view subtree families (see Table 15). In this
Party Identity Status Family Name Family Mask lucy include internet ""h lucy exclude snmpParties ""h ricky include system ""h ricky include snmpParties ""h
Table 14: View Definition for Multiple Parties
example, the MIB view associated with party lucy includes all object instances in the system group of the internet-standard MIB together with some information related to the second network interface attached to the managed device. However, this interface-related information does not include the speed of the interface. The family
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mask value "FFA0"h in the second table entry signifies that a MIB object instance belongs to the relevant subtree family if the initial prefix of its name places it within the ifEntry portion of the registration hierarchy and if the eleventh subidentifier of its name is 2. The MIB object instance representing the speed of the second network interface belongs to the subtree families represented by both the second and third entries of the table, but that particular instance is excluded from the MIB view for party lucy because the lexicographically greater of the relevant family names appears in the table entry with status exclude.
The MIB view for party ricky is also defined in this example. The MIB view attributed to the party ricky includes all object instances in the icmp group of the internet-standard MIB, together with all information relevant to the fifth network interface attached to the managed device. In addition, the MIB view attributed to party ricky includes the number of octets received on the fourth attached network interface.
While, as suggested by the examples above, a wide range of MIB view configurations are efficiently supported by the abbreviated representation of [5], prudent MIB design can sometimes further reduce the size and complexity of the most
Party Identity Status Family Name Family Mask lucy include system ""h lucy include { ifEntry 0 2 } "FFA0"h lucy exclude { ifSpeed 2 } ""h ricky include icmp ""h ricky include { ifEntry 0 5 } "FFA0"h ricky include { ifInOctets 4 } ""h
Table 15: More Elaborate View Definitions
likely MIB view definitions. On one hand, it is critical that mechanisms for MIB view configuration impose no absolute constraints either upon the access policies of local administrations or upon the structure of MIB namespaces; on the other hand, where the most common access policies are known, the configuration costs of realizing those policies may be slightly reduced by assigning to distinct portions of the registration hierarchy those MIB objects for which local policies most frequently require distinct treatment. The relegation in [5] of certain objects to a distinct arc in the MIB namespace is an example of this kind of optimization.
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5. Compatibility
Ideally, all SNMP management stations and agents would communicate exclusively using the secure facilities described in this memo. In reality, many SNMP agents may implement only the insecure SNMP mechanisms described in [1] for some time to come.
New SNMP agent implementations should never implement both the insecure mechanisms of [1] and the facilities described here. Rather, consistent with the SNMP philosophy, the burden of supporting both sorts of communication should fall entirely upon managers. Perhaps the best way to realize both old and new modes of communication is by the use of a SNMP proxy agent deployed locally on the same system with a management station implementation. The management station implementation itself operates exclusively by using the newer, secure modes of communication, and the local proxy agent translates the requests of the manager into older, insecure modes as needed.
It should be noted that proxy agent implementations may require additional information beyond that described in this memo in order to accomplish the requisite translation tasks implicit in the definition of the proxy function. This information could easily be retrieved from a filestore.
6. Security Considerations
It is important to note that, in the example configuration for native proxy operations presented in this memo, the use of symmetric cryptography does not securely prevent direct communication between the SNMP management station and the proxied SNMP agent.
While secure isolation of the management station and the proxied agent can, according to the administrative model set forth in this memo, be realized using symmetric cryptography, the required configuration is more complex and is not described in this memo. Rather, it is recommended that native proxy configurations that require secure isolation of management station from proxied agent be implemented using security protocols based on asymmetric (or "public key") cryptography. However, no SNMP security protocols based on asymmetric cryptography are currently defined.
In order to participate in the administrative model set forth in this memo, SNMP implementations must support local, non-volatile storage of the local party database. Accordingly, every attempt has been made to minimize the amount of non-volatile storage required.
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7. References
[1] Case, J., M. Fedor, M. Schoffstall, and J. Davin, The Simple Network Management Protocol", RFC 1157, University of Tennessee at Knoxville, Performance Systems International, Performance Systems International, and the MIT Laboratory for Computer Science, May 1990. (Obsoletes RFC 1098.)
[2] Rose, M., and K. McCloghrie, "Structure and Identification of Management Information for TCP/IP based internets", RFC 1155, Performance Systems International, Hughes LAN Systems, May 1990. (Obsoletes RFC 1065.)
[3] Information Processing -- Open Systems Interconnection -- Specification of Basic Encoding Rules for Abstract Syntax Notation One (ASN.1), International Organization for Standardization/International Electrotechnical Institute, 1987, International Standard 8825.
[4] Galvin, J., McCloghrie, K., and J. Davin, "SNMP Security Protocols", RFC 1352, Trusted Information Systems, Inc., Hughes LAN Systems, Inc., MIT Laboratory for Computer Science, July 1992.
[5] McCloghrie, K., Davin, J., and J. Galvin, "Definitions of Managed Objects for Administration of SNMP Parties", RFC 1353, Hughes LAN Systems, Inc., MIT Laboratory for Computer Science, Trusted Information Systems, Inc., July 1992.
8. Authors' Addresses
James R. Davin MIT Laboratory for Computer Science 545 Technology Square Cambridge, MA 02139
Phone: (617) 253-6020 EMail: jrd@ptt.lcs.mit.edu
James M. Galvin Trusted Information Systems, Inc. 3060 Washington Road, Route 97 Glenwood, MD 21738
Phone: (301) 854-6889 EMail: galvin@tis.com
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Keith McCloghrie Hughes LAN Systems, Inc. 1225 Charleston Road Mountain View, CA 94043
