| TOC |
|
Transport Layer Security (TLS) Transport Model for the Simple
Network Management Protocol (SNMP)
draft-ietf-isms-dtls-tm-14.txt
Abstract
This document describes a Transport Model for the Simple Network Management Protocol (SNMP), that uses either the Transport Layer Security protocol or the Datagram Transport Layer Security (DTLS) protocol. The TLS and DTLS protocols provide authentication and privacy services for SNMP applications. This document describes how the TLS Transport Model (TLSTM) implements the needed features of a SNMP Transport Subsystem to make this protection possible in an interoperable way.
This transport model is designed to meet the security and operational needs of network administrators. It supports sending of SNMP messages over TLS/TCP and DTLS/UDP. The TLS mode can make use of TCP's improved support for larger packet sizes and the DTLS mode provides potentially superior operation in environments where a connectionless (e.g. UDP) transport is preferred. Both TLS and DTLS integrate well into existing public keying infrastructures.
This document also defines a portion of the Management Information Base (MIB) for use with network management protocols. In particular it defines objects for managing the TLS Transport Model for SNMP.
Status of this Memo
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress.”
This Internet-Draft will expire on November 8, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.
Table of Contents
1.
Introduction
1.1.
Conventions
2.
The Transport Layer Security Protocol
3.
How the TLSTM fits into the Transport Subsystem
3.1.
Security Capabilities of this Model
3.1.1.
Threats
3.1.2.
Message Protection
3.1.3.
(D)TLS Connections
3.2.
Security Parameter Passing
3.3.
Notifications and Proxy
4.
Elements of the Model
4.1.
X.509 Certificates
4.1.1.
Provisioning for the Certificate
4.2.
(D)TLS Usage
4.3.
SNMP Services
4.3.1.
SNMP Services for an Outgoing Message
4.3.2.
SNMP Services for an Incoming Message
4.4.
Cached Information and References
4.4.1.
TLS Transport Model Cached Information
4.4.1.1.
tmSecurityName
4.4.1.2.
tmSessionID
4.4.1.3.
Session State
5.
Elements of Procedure
5.1.
Procedures for an Incoming Message
5.1.1.
DTLS over UDP Processing for Incoming Messages
5.1.2.
Transport Processing for Incoming SNMP Messages
5.2.
Procedures for an Outgoing SNMP Message
5.3.
Establishing or Accepting a Session
5.3.1.
Establishing a Session as a Client
5.3.2.
Accepting a Session as a Server
5.4.
Closing a Session
6.
MIB Module Overview
6.1.
Structure of the MIB Module
6.2.
Textual Conventions
6.3.
Statistical Counters
6.4.
Configuration Tables
6.4.1.
Notifications
6.5.
Relationship to Other MIB Modules
6.5.1.
MIB Modules Required for IMPORTS
7.
MIB Module Definition
8.
Operational Considerations
8.1.
Sessions
8.2.
Notification Receiver Credential Selection
8.3.
contextEngineID Discovery
8.4.
Transport Considerations
9.
Security Considerations
9.1.
Certificates, Authentication, and Authorization
9.2.
(D)TLS Security Considerations
9.2.1.
TLS Version Requirements
9.2.2.
Perfect Forward Secrecy
9.3.
Use with SNMPv1/SNMPv2c Messages
9.4.
MIB Module Security
10.
IANA Considerations
11.
Acknowledgements
12.
References
12.1.
Normative References
12.2.
Informative References
Appendix A.
Target and Notification Configuration Example
A.1.
Configuring a Notification Originator
A.2.
Configuring TLSTM to Utilize a Simple Derivation of tmSecurityName
A.3.
Configuring TLSTM to Utilize Table-Driven Certificate Mapping
§
Author's Address
| TOC |
1. Introduction
It is important to understand the modular SNMPv3 architecture as defined by [RFC3411] (Harrington, D., Presuhn, R., and B. Wijnen, “An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks,” December 2002.) and enhanced by the Transport Subsystem [RFC5590] (Harrington, D. and J. Schoenwaelder, “Transport Subsystem for the Simple Network Management Protocol (SNMP),” June 2009.). It is also important to understand the terminology of the SNMPv3 architecture in order to understand where the Transport Model described in this document fits into the architecture and how it interacts with the other architecture subsystems. For a detailed overview of the documents that describe the current Internet-Standard Management Framework, please refer to Section 7 of [RFC3410] (Case, J., Mundy, R., Partain, D., and B. Stewart, “Introduction and Applicability Statements for Internet-Standard Management Framework,” December 2002.).
This document describes a Transport Model that makes use of the Transport Layer Security (TLS) [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.) and the Datagram Transport Layer Security (DTLS) Protocol [RFC4347] (Rescorla, E. and N. Modadugu, “Datagram Transport Layer Security,” April 2006.), within a transport subsystem [RFC5590] (Harrington, D. and J. Schoenwaelder, “Transport Subsystem for the Simple Network Management Protocol (SNMP),” June 2009.). DTLS is the datagram variant of the Transport Layer Security (TLS) protocol [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.). The Transport Model in this document is referred to as the Transport Layer Security Transport Model (TLSTM). TLS and DTLS take advantage of the X.509 public keying infrastructure [RFC5280] (Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” May 2008.). While (D)TLS supports multiple authentication mechanisms, this document only discusses X.509 certificate based authentication. Although other forms of authentication are possible they are outside the scope of this specification. This transport model is designed to meet the security and operational needs of network administrators, operating in both environments where a connectionless (e.g. UDP) transport is preferred and in environments where large quantities of data need to be sent (e.g. over a TCP based stream). Both TLS and DTLS integrate well into existing public keying infrastructures. This document supports sending of SNMP messages over TLS/TCP and DTLS/UDP.
This document also defines a portion of the Management Information Base (MIB) for use with network management protocols. In particular it defines objects for managing the TLS Transport Model for SNMP.
Managed objects are accessed via a virtual information store, termed the Management Information Base or MIB. MIB objects are generally accessed through the Simple Network Management Protocol (SNMP). Objects in the MIB are defined using the mechanisms defined in the Structure of Management Information (SMI). This memo specifies a MIB module that is compliant to the SMIv2, which is described in STD 58: [RFC2578] (McCloghrie, K., Ed., Perkins, D., Ed., and J. Schoenwaelder, Ed., “Structure of Management Information Version 2 (SMIv2),” April 1999.), [RFC2579] (McCloghrie, K., Ed., Perkins, D., Ed., and J. Schoenwaelder, Ed., “Textual Conventions for SMIv2,” April 1999.) and [RFC2580] (McCloghrie, K., Perkins, D., and J. Schoenwaelder, “Conformance Statements for SMIv2,” April 1999.).
The diagram shown below gives a conceptual overview of two SNMP entities communicating using the TLS Transport Model (shown as "TLS TM"). One entity contains a command responder and notification originator application, and the other a command generator and notification receiver application. It should be understood that this particular mix of application types is an example only and other combinations are equally valid. Note: this diagram shows the Transport Security Model (TSM) being used as the security model which is defined in [RFC5591] (Harrington, D. and W. Hardaker, “Transport Security Model for the Simple Network Management Protocol (SNMP),” June 2009.).
+---------------------------------------------------------------------+
| Network |
+---------------------------------------------------------------------+
^ | ^ |
|Notifications |Commands |Commands |Notifications
+---|---------------------|-------+ +--|---------------|--------------+
| | V | | | V |
| +------------+ +------------+ | | +-----------+ +----------+ |
| | (D)TLS | | (D)TLS | | | | (D)TLS | | (D)TLS | |
| | (Client) | | (Server) | | | | (Client) | | (Server) | |
| +------------+ +------------+ | | +-----------+ +----------+ |
| ^ ^ | | ^ ^ |
| | | | | | | |
| +-------------+ | | +--------------+ |
| +-----|------------+ | | +-----|------------+ |
| | V | | | | V | |
| | +--------+ | +-----+ | | | +--------+ | +-----+ |
| | | TLS TM |<--------->|Cache| | | | | TLS TM |<--------->|Cache| |
| | +--------+ | +-----+ | | | +--------+ | +-----+ |
| |Transport Subsys. | ^ | | |Transport Subsys. | ^ |
| +------------------+ | | | +------------------+ | |
| ^ | | | ^ | |
| | +--+ | | | +--+ |
| v | | | V | |
| +-----+ +--------+ +-------+ | | | +-----+ +--------+ +-------+ | |
| | | |Message | |Securi.| | | | | | |Message | |Securi.| | |
| |Disp.| |Proc. | |Subsys.| | | | |Disp.| |Proc. | |Subsys.| | |
| | | |Subsys. | | | | | | | | |Subsys. | | | | |
| | | | | | | | | | | | | | | | | |
| | | | +----+ | | +---+ | | | | | | | +----+ | | +---+ | | |
| | <--->|v3MP|<--> |TSM|<--+ | | | <--->|v3MP|<--->|TSM|<--+ |
| | | | +----+ | | +---+ | | | | | | +----+ | | +---+ | |
| | | | | | | | | | | | | | | |
| +-----+ +--------+ +-------+ | | +-----+ +--------+ +-------+ |
| ^ | | ^ |
| | | | | |
| +-+------------+ | | +-+----------+ |
| | | | | | | |
| v v | | v V |
| +-------------+ +-------------+ | | +-------------+ +-------------+ |
| | COMMAND | | NOTIFICAT. | | | | COMMAND | | NOTIFICAT. | |
| | RESPONDER | | ORIGINATOR | | | | GENERATOR | | RECEIVER | |
| | application | | application | | | | application | | application | |
| +-------------+ +-------------+ | | +-------------+ +-------------+ |
| SNMP entity | | SNMP entity |
+---------------------------------+ +---------------------------------+
| TOC |
1.1. Conventions
For consistency with SNMP-related specifications, this document favors terminology as defined in STD 62, rather than favoring terminology that is consistent with non-SNMP specifications. This is consistent with the IESG decision to not require the SNMPv3 terminology be modified to match the usage of other non-SNMP specifications when SNMPv3 was advanced to Full Standard.
"Authentication" in this document typically refers to the English meaning of "serving to prove the authenticity of" the message, not data source authentication or peer identity authentication.
The terms "manager" and "agent" are not used in this document because, in the [RFC3411] (Harrington, D., Presuhn, R., and B. Wijnen, “An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks,” December 2002.) architecture, all SNMP entities have the capability of acting as manager, agent, or both depending on the SNMP application types supported in the implementation. Where distinction is required, the application names of command generator, command responder, notification originator, notification receiver, and proxy forwarder are used. See "SNMP Applications" [RFC3413] (Levi, D., Meyer, P., and B. Stewart, “Simple Network Management Protocol (SNMP) Applications,” December 2002.) for further information.
Large portions of this document simultaneously refer to both TLS and DTLS when discussing TLSTM components that function equally with either protocol. "(D)TLS" is used in these places to indicate that the statement applies to either or both protocols as appropriate. When a distinction between the protocols is needed they are referred to independently through the use of "TLS" or "DTLS". The Transport Model, however, is named "TLS Transport Model" and refers not to the TLS or DTLS protocol but to the specification in this document, which includes support for both TLS and DTLS.
Throughout this document, the terms "client" and "server" are used to refer to the two ends of the (D)TLS transport connection. The client actively opens the (D)TLS connection, and the server passively listens for the incoming (D)TLS connection. An SNMP entity may act as a (D)TLS client or server or both, depending on the SNMP applications supported.
The User-Based Security Model (USM) [RFC3414] (Blumenthal, U. and B. Wijnen, “User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3),” December 2002.) is a mandatory-to-implement Security Model in STD 62. While (D)TLS and USM frequently refer to a user, the terminology preferred in RFC3411 and in this memo is "principal". A principal is the "who" on whose behalf services are provided or processing takes place. A principal can be, among other things, an individual acting in a particular role; a set of individuals, with each acting in a particular role; an application or a set of applications, or a combination of these within an administrative domain.
Throughout this document, the term "session" is used to refer to a secure association between two TLS Transport Models that permits the transmission of one or more SNMP messages within the lifetime of the session. The (D)TLS protocols also have an internal notion of a session and although these two concepts of a session are related, when the term "session" is used this document is referring to the TLSTM's specific session and not directly to the (D)TLS protocol's session.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).
| TOC |
2. The Transport Layer Security Protocol
(D)TLS provides authentication, data message integrity, and privacy at the transport layer. (See [RFC4347] (Rescorla, E. and N. Modadugu, “Datagram Transport Layer Security,” April 2006.))
The primary goals of the TLS Transport Model are to provide privacy, peer identity authentication and data integrity between two communicating SNMP entities. The TLS and DTLS protocols provide a secure transport upon which the TLSTM is based. Please refer to [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.) and [RFC4347] (Rescorla, E. and N. Modadugu, “Datagram Transport Layer Security,” April 2006.) for complete descriptions of the protocols.
| TOC |
3. How the TLSTM fits into the Transport Subsystem
A transport model is a component of the Transport Subsystem. The TLS Transport Model thus fits between the underlying (D)TLS transport layer and the Message Dispatcher [RFC3411] (Harrington, D., Presuhn, R., and B. Wijnen, “An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks,” December 2002.) component of the SNMP engine.
The TLS Transport Model will establish a session between itself and the TLS Transport Model of another SNMP engine. The sending transport model passes unencrypted and unauthenticated messages from the Dispatcher to (D)TLS to be encrypted and authenticated, and the receiving transport model accepts decrypted and authenticated/integrity-checked incoming messages from (D)TLS and passes them to the Dispatcher.
After a TLS Transport Model session is established, SNMP messages can conceptually be sent through the session from one SNMP message Dispatcher to another SNMP Message Dispatcher. If multiple SNMP messages are needed to be passed between two SNMP applications they MAY be passed through the same session. A TLSTM implementation engine MAY choose to close the session to conserve resources.
The TLS Transport Model of an SNMP engine will perform the translation between (D)TLS-specific security parameters and SNMP- specific, model-independent parameters.
The diagram below depicts where the TLS Transport Model (shown as "(D)TLS TM") fits into the architecture described in RFC3411 and the Transport Subsystem:
+------------------------------+
| Network |
+------------------------------+
^ ^ ^
| | |
v v v
+-------------------------------------------------------------------+
| +--------------------------------------------------+ |
| | Transport Subsystem | +--------+ |
| | +-----+ +-----+ +-------+ +-------+ | | | |
| | | UDP | | SSH | |(D)TLS | . . . | other |<--->| Cache | |
| | | | | TM | | TM | | | | | | |
| | +-----+ +-----+ +-------+ +-------+ | +--------+ |
| +--------------------------------------------------+ ^ |
| ^ | |
| | | |
| Dispatcher v | |
| +--------------+ +---------------------+ +----------------+ | |
| | Transport | | Message Processing | | Security | | |
| | Dispatch | | Subsystem | | Subsystem | | |
| | | | +------------+ | | +------------+ | | |
| | | | +->| v1MP |<--->| | USM | | | |
| | | | | +------------+ | | +------------+ | | |
| | | | | +------------+ | | +------------+ | | |
| | | | +->| v2cMP |<--->| | Transport | | | |
| | Message | | | +------------+ | | | Security |<--+ |
| | Dispatch <---->| +------------+ | | | Model | | |
| | | | +->| v3MP |<--->| +------------+ | |
| | | | | +------------+ | | +------------+ | |
| | PDU Dispatch | | | +------------+ | | | Other | | |
| +--------------+ | +->| otherMP |<--->| | Model(s) | | |
| ^ | +------------+ | | +------------+ | |
| | +---------------------+ +----------------+ |
| v |
| +-------+-------------------------+---------------+ |
| ^ ^ ^ |
| | | | |
| v v v |
| +-------------+ +---------+ +--------------+ +-------------+ |
| | COMMAND | | ACCESS | | NOTIFICATION | | PROXY | |
| | RESPONDER |<->| CONTROL |<->| ORIGINATOR | | FORWARDER | |
| | application | | | | applications | | application | |
| +-------------+ +---------+ +--------------+ +-------------+ |
| ^ ^ |
| | | |
| v v |
| +----------------------------------------------+ |
| | MIB instrumentation | SNMP entity |
+-------------------------------------------------------------------+
| TOC |
3.1. Security Capabilities of this Model
| TOC |
3.1.1. Threats
The TLS Transport Model provides protection against the threats identified by the RFC 3411 architecture [RFC3411] (Harrington, D., Presuhn, R., and B. Wijnen, “An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks,” December 2002.):
- 1.
- Modification of Information - The modification threat is the danger that an unauthorized entity may alter in-transit SNMP messages generated on behalf of an authorized principal in such a way as to effect unauthorized management operations, including falsifying the value of an object.
(D)TLS provides verification that the content of each received message has not been modified during its transmission through the network, data has not been altered or destroyed in an unauthorized manner, and data sequences have not been altered to an extent greater than can occur non-maliciously.- 2.
- Masquerade - The masquerade threat is the danger that management operations unauthorized for a given principal may be attempted by assuming the identity of another principal that has the appropriate authorizations.
The TLSTM verifies of the identity of the (D)TLS server through the use of the (D)TLS protocol and X.509 certificates. A TLS Transport Model implementation MUST support authentication of both the server and the client.- 3.
- Message stream modification - The re-ordering, delay or replay of messages can and does occur through the natural operation of many connectionless transport 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 connectionless transport services, in order to effect unauthorized management operations.
(D)TLS provides replay protection with a MAC that includes a sequence number. Since UDP provides no sequencing ability, DTLS uses a sliding window protocol with the sequence number used for replay protection (see [RFC4347] (Rescorla, E. and N. Modadugu, “Datagram Transport Layer Security,” April 2006.)).- 4.
- Disclosure - The disclosure threat is the danger of eavesdropping on the exchanges between SNMP engines.
(D)TLS provides protection against the disclosure of information to unauthorized recipients or eavesdroppers by allowing for encryption of all traffic between SNMP engines. A TLS Transport Model implementation MUST support message encryption to protect sensitive data from eavesdropping attacks.- 5.
- Denial of Service - the RFC 3411 architecture [RFC3411] (Harrington, D., Presuhn, R., and B. Wijnen, “An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks,” December 2002.) states that denial of service (DoS) attacks need not be addressed by an SNMP security protocol. However, connectionless transports (like DTLS over UDP) are susceptible to a variety of denial of service attacks because they are more vulnerable to spoofed IP addresses. See Section 4.2 ((D)TLS Usage) for details how the cookie mechanism is used. Note, however, that this mechanism does not provide any defense against denial of service attacks mounted from valid IP addresses.
See Section 9 (Security Considerations) for more detail on the security considerations associated with the TLSTM and these security threats.
| TOC |
3.1.2. Message Protection
The RFC 3411 architecture recognizes three levels of security:
- without authentication and without privacy (noAuthNoPriv)
- with authentication but without privacy (authNoPriv)
- with authentication and with privacy (authPriv)
The TLS Transport Model determines from (D)TLS the identity of the authenticated principal, the transport type and the transport address associated with an incoming message. The TLS Transport Model provides the identity and destination type and address to (D)TLS for outgoing messages.
When an application requests a session for a message it also requests a security level for that session. The TLS Transport Model MUST ensure that the (D)TLS connection provides security at least as high as the requested level of security. How the security level is translated into the algorithms used to provide data integrity and privacy is implementation-dependent. However, the NULL integrity and encryption algorithms MUST NOT be used to fulfill security level requests for authentication or privacy. Implementations MAY choose to force (D)TLS to only allow cipher_suites that provide both authentication and privacy to guarantee this assertion.
If a suitable interface between the TLS Transport Model and the (D)TLS Handshake Protocol is implemented to allow the selection of security level dependent algorithms (for example a security level to cipher_suites mapping table) then different security levels may be utilized by the application.
The authentication, integrity and privacy algorithms used by the (D)TLS Protocols may vary over time as the science of cryptography continues to evolve and the development of (D)TLS continues over time. Implementers are encouraged to plan for changes in operator trust of particular algorithms. Implementations SHOULD offer configuration settings for mapping algorithms to SNMPv3 security levels.
| TOC |
3.1.3. (D)TLS Connections
(D)TLS connections are opened by the TLS Transport Model during the elements of procedure for an outgoing SNMP message. Since the sender of a message initiates the creation of a (D)TLS connection if needed, the (D)TLS connection will already exist for an incoming message.
Implementations MAY choose to instantiate (D)TLS connections in anticipation of outgoing messages. This approach might be useful to ensure that a (D)TLS connection to a given target can be established before it becomes important to send a message over the (D)TLS connection. Of course, there is no guarantee that a pre-established session will still be valid when needed.
DTLS connections, when used over UDP, are uniquely identified within the TLS Transport Model by the combination of transportDomain, transportAddress, tmSecurityName, and requestedSecurityLevel associated with each session. Each unique combination of these parameters MUST have a locally-chosen unique tlstmSessionID for each active session. For further information see Section 5 (Elements of Procedure). TLS over TCP sessions, on the other hand, do not require a unique pairing of address and port attributes since their lower layer protocols (TCP) already provide adequate session framing. But they must still provide a unique tlstmSessionID for referencing the session.
The tlstmSessionID identifier MUST NOT change during the entire duration of the session from the TLSTM's perspective, and MUST uniquely identify a single session. As an implementation hint: note that the (D)TLS internal SessionID does not meet these requirements, since it can change over the life of the connection as seen by the TLSTM (for example, during renegotiation), and does not necessarily uniquely identify a TLSTM session (there can be multiple TLSTM sessions sharing the same D(TLS) internal SessionID).
| TOC |
3.2. Security Parameter Passing
For the (D)TLS server-side, (D)TLS-specific security parameters (i.e., cipher_suites, X.509 certificate fields, IP address and port) are translated by the TLS Transport Model into security parameters for the TLS Transport Model and security model (e.g., tmSecurityLevel, tmSecurityName, transportDomain, transportAddress). The transport-related and (D)TLS-security-related information, including the authenticated identity, are stored in a cache referenced by tmStateReference.
For the (D)TLS client-side, the TLS Transport Model takes input provided by the Dispatcher in the sendMessage() Abstract Service Interface (ASI) and input from the tmStateReference cache. The (D)TLS Transport Model converts that information into suitable security parameters for (D)TLS and establishes sessions as needed.
The elements of procedure in Section 5 (Elements of Procedure) discuss these concepts in much greater detail.
| TOC |
3.3. Notifications and Proxy
(D)TLS connections may be initiated by (D)TLS clients on behalf of SNMP appplications that initiate communications, such as command generators, notification originators, proxy forwarders. Command generators are frequently operated by a human, but notification originators and proxy forwarders are usually unmanned automated processes. The targets to whom notifications and proxied requests should be sent is typically determined and configured by a network administrator.
The SNMP-TARGET-MIB module [RFC3413] (Levi, D., Meyer, P., and B. Stewart, “Simple Network Management Protocol (SNMP) Applications,” December 2002.) contains objects for defining management targets, including transportDomain, transportAddress, securityName, securityModel, and securityLevel parameters, for notification originator, proxy forwarder, and SNMP-controllable command generator applications. Transport domains and transport addresses are configured in the snmpTargetAddrTable, and the securityModel, securityName, and securityLevel parameters are configured in the snmpTargetParamsTable. This document defines a MIB module that extends the SNMP-TARGET-MIB's snmpTargetParamsTable to specify a (D)TLS client-side certificate to use for the connection.
When configuring a (D)TLS target, the snmpTargetAddrTDomain and snmpTargetAddrTAddress parameters in snmpTargetAddrTable SHOULD be set to the snmpTLSTCPDomain or snmpDTLSUDPDomain object and an appropriate snmpTLSAddress value. When used with the SNMPv3 message processing model, the snmpTargetParamsMPModel column of the snmpTargetParamsTable SHOULD be set to a value of 3. The snmpTargetParamsSecurityName SHOULD be set to an appropriate securityName value and the snmpTlstmParamsClientFingerprint parameter of the snmpTlstmParamsTable SHOULD be set a value that refers to a locally held certificate (and the corresponding private key) to be used. Other parameters, for example cryptographic configuration such as which cipher suites to use, must come from configuration mechanisms not defined in this document.
The securityName defined in the snmpTargetParamsSecurityName column will be used by the access control model to authorize any notifications that need to be sent.
| TOC |
4. Elements of the Model
This section contains definitions required to realize the (D)TLS Transport Model defined by this document.
| TOC |
4.1. X.509 Certificates
(D)TLS can make use of X.509 certificates for authentication of both sides of the transport. This section discusses the use of X.509 certificates in the TLSTM.
While (D)TLS supports multiple authentication mechanisms, this document only discusses X.509 certificate based authentication; other forms of authentication are outside the scope of this specification. TLSTM implementations are REQUIRED to support X.509 certificates.
| TOC |
4.1.1. Provisioning for the Certificate
Authentication using (D)TLS will require that SNMP entities have certificates, either signed by trusted certification authorities, or self-signed. Furthermore, SNMP entities will most commonly need to be provisioned with root certificates which represent the list of trusted certificate authorities that an SNMP entity can use for certificate verification. SNMP entities SHOULD also be provisioned with a X.509 certificate revocation mechanism which can be used to verify that a certificate has not been revoked. Trusted public keys from either CA certificates and/or self-signed certificates MUST be installed into the server through a trusted out of band mechanism and their authenticity MUST be verified before access is granted.
Having received a certificate from a connecting TLSTM client, the authenticated tmSecurityName of the principal is derived using the snmpTlstmCertToTSNTable. This table allows mapping of incoming connections to tmSecurityNames through defined transformations. The transformations defined in the SNMP-TLS-TM-MIB include:
- Mapping a certificate's subjectAltName or CommonName components to a tmSecurityName, or
- Mapping a certificate's fingerprint value to a directly specified tmSecurityName
As an implementation hint: implementations may choose to discard any connections for which no potential snmpTlstmCertToTSNTable mapping exists before performing certificate verification to avoid expending computational resources associated with certificate verification.
Deployments SHOULD map the "subjectAltName" component of X.509 certificates to the TLSTM specific tmSecurityNames. The authenticated identity can be obtained by the TLS Transport Model by extracting the subjectAltName(s) from the peer's certificate. The receiving application will then have an appropriate tmSecurityName for use by other SNMPv3 components like an access control model.
An example of this type of mapping setup can be found in Appendix A (Target and Notification Configuration Example).
This tmSecurityName may be later translated from a TLSTM specific tmSecurityName to a SNMP engine securityName by the security model. A security model, like the TSM security model [RFC5591] (Harrington, D. and W. Hardaker, “Transport Security Model for the Simple Network Management Protocol (SNMP),” June 2009.), may perform an identity mapping or a more complex mapping to derive the securityName from the tmSecurityName offered by the TLS Transport Model.
The standard VACM access control model constrains securityNames to be 32 octets or less in length. A TLSTM generated tmSecurityName, possibly in combination with a messaging or security model that increases the length of the securityName, might cause the securityName length to exceed 32 octets. For example, a 32 octet tmSecurityName derived from an IPv6 address, paired with a TSM prefix, will generate a 36 octet securityName. Such a securityName will not be able to be used with standard VACM or TARGET MIB modules. Operators should be careful to select algorithms and subjectAltNames to avoid this situation.
A pictorial view of the complete transformation process (using the TSM security model for the example) is shown below:
+-------------+ +-------+ +-----+
| Certificate | | | | |
| Path | | TLSTM | tmSecurityName | TSM |
| Validation | --> | | ----------------->| |
+-------------+ +-------+ +-----+
|
| securityName
V
+-------------+
| application |
+-------------+
| TOC |
4.2. (D)TLS Usage
(D)TLS MUST negotiate a cipher suite that uses X.509 certificates for authentication, and MUST authenticate both the client and the server. The mandatory-to-implement cipher suite is specified in the TLS specification [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.).
TLSTM verifies the certificates when the connection is opened (see Section 5.3). For this reason, TLS renegotiation with different certificates MUST NOT be done. That is, implementations MUST either disable renegotiation completely (RECOMMENDED), or MUST present the same certificate during renegotiation (and MUST verify that the other end presented the same certificate).
For DTLS over UDP, each SNMP message MUST be placed in a single UDP datagram; it MAY be split to multiple DTLS records. In other words, if a single datagram contains multiple DTLS application_data records, they are concatenated when received. The TLSTM implementation SHOULD return an error if the SNMP message does not fit in the UDP datagram, and thus cannot be sent.
For DTLS over UDP, the DTLS server implementation MUST support DTLS cookies ([RFC4347] (Rescorla, E. and N. Modadugu, “Datagram Transport Layer Security,” April 2006.) already requires that clients support DTLS cookies). Implementations are not required to perform the cookie exchange for every DTLS handshake; however, enabling it by default is RECOMMENDED.
For DTLS, replay protection MUST be used.
| TOC |
4.3. SNMP Services
This section describes the services provided by the TLS Transport Model with their inputs and outputs. The services are between the Transport Model and the Dispatcher.
The services are described as primitives of an abstract service interface (ASI) and the inputs and outputs are described as abstract data elements as they are passed in these abstract service primitives.
| TOC |
4.3.1. SNMP Services for an Outgoing Message
The Dispatcher passes the information to the TLS Transport Model using the ASI defined in the transport subsystem:
statusInformation =
sendMessage(
IN destTransportDomain -- transport domain to be used
IN destTransportAddress -- transport address to be used
IN outgoingMessage -- the message to send
IN outgoingMessageLength -- its length
IN tmStateReference -- reference to transport state
)
The abstract data elements returned from or passed as parameters into the abstract service primitives are as follows:
- statusInformation:
- An indication of whether the sending of the message was successful. If not, it is an indication of the problem.
- destTransportDomain:
- The transport domain for the associated destTransportAddress. The Transport Model uses this parameter to determine the transport type of the associated destTransportAddress. This document specifies the snmpTLSTCPDomain and the snmpDTLSUDPDomain transport domains.
- destTransportAddress:
- The transport address of the destination TLS Transport Model in a format specified by the SnmpTLSAddress TEXTUAL-CONVENTION.
- outgoingMessage:
- The outgoing message to send to (D)TLS for encapsulation and transmission.
- outgoingMessageLength:
- The length of the outgoingMessage.
- tmStateReference:
- A reference used to pass model-specific and mechanism-specific parameters between the Transport Subsystem and transport-aware Security Models.
| TOC |
4.3.2. SNMP Services for an Incoming Message
The TLS Transport Model processes the received message from the network using the (D)TLS service and then passes it to the Dispatcher using the following ASI:
statusInformation =
receiveMessage(
IN transportDomain -- origin transport domain
IN transportAddress -- origin transport address
IN incomingMessage -- the message received
IN incomingMessageLength -- its length
IN tmStateReference -- reference to transport state
)
The abstract data elements returned from or passed as parameters into the abstract service primitives are as follows:
- statusInformation:
- An indication of whether the passing of the message was successful. If not, it is an indication of the problem.
- transportDomain:
- The transport domain for the associated transportAddress. This document specifies the snmpTLSTCPDomain and the snmpDTLSUDPDomain transport domains.
- transportAddress:
- The transport address of the source of the received message in a format specified by the SnmpTLSAddress TEXTUAL-CONVENTION.
- incomingMessage:
- The whole SNMP message after being processed by (D)TLS.
- incomingMessageLength:
- The length of the incomingMessage.
- tmStateReference:
- A reference used to pass model-specific and mechanism-specific parameters between the Transport Subsystem and transport-aware Security Models.
| TOC |
4.4. Cached Information and References
When performing SNMP processing, there are two levels of state information that may need to be retained: the immediate state linking a request-response pair, and potentially longer-term state relating to transport and security. "Transport Subsystem for the Simple Network Management Protocol" (Harrington, D. and J. Schoenwaelder, “Transport Subsystem for the Simple Network Management Protocol (SNMP),” June 2009.) [RFC5590] defines general requirements for caches and references.
| TOC |
4.4.1. TLS Transport Model Cached Information
The TLS Transport Model has specific responsibilities regarding the cached information. See the Elements of Procedure in Section 5 (Elements of Procedure) for detailed processing instructions on the use of the tmStateReference fields by the TLS Transport Model.
| TOC |
4.4.1.1. tmSecurityName
The tmSecurityName MUST be a human-readable name (in snmpAdminString format) representing the identity that has been set according to the procedures in Section 5 (Elements of Procedure). The tmSecurityName MUST be constant for all traffic passing through a single TLSTM session. Messages MUST NOT be sent through an existing (D)TLS connection that was established using a different tmSecurityName.
On the (D)TLS server side of a connection the tmSecurityName is derived using the procedures described in Section 5.3.2 (Accepting a Session as a Server) and the SNMP-TLS-TM-MIB's snmpTlstmCertToTSNTable DESCRIPTION clause.
On the (D)TLS client side of a connection the tmSecurityName is presented to the TLS Transport Model by the application (possibly because of configuration specified in the SNMP-TARGET-MIB). The Security Model likely derived the tmSecurityName from the securityName presented to the Security Model by the application (possibly because of configuration specified in the SNMP-TARGET-MIB).
Transport-model-aware security models derive tmSecurityName from a securityName, possibly configured in MIB modules for notifications and access controls. Transport Models SHOULD use predictable tmSecurityNames so operators will know what to use when configuring MIB modules that use securityNames derived from tmSecurityNames. The TLSTM generates predictable tmSecurityNames based on the configuration found in the SNMP-TLS-TM-MIB's snmpTlstmCertToTSNTable and relies on the network operators to have configured this table appropriately.
| TOC |
4.4.1.2. tmSessionID
The tmSessionID MUST be recorded per message at the time of receipt. When tmSameSecurity is set, the recorded tmSessionID can be used to determine whether the (D)TLS connection available for sending a corresponding outgoing message is the same (D)TLS connection as was used when receiving the incoming message (e.g., a response to a request).
| TOC |
4.4.1.3. Session State
The per-session state that is referenced by tmStateReference may be saved across multiple messages in a Local Configuration Datastore. Additional session/connection state information might also be stored in a Local Configuration Datastore.
| TOC |
5. Elements of Procedure
Abstract service interfaces have been defined by [RFC3411] (Harrington, D., Presuhn, R., and B. Wijnen, “An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks,” December 2002.) and further augmented by [RFC5590] (Harrington, D. and J. Schoenwaelder, “Transport Subsystem for the Simple Network Management Protocol (SNMP),” June 2009.) to describe the conceptual data flows between the various subsystems within an SNMP entity. The TLSTM uses some of these conceptual data flows when communicating between subsystems.
To simplify the elements of procedure, the release of state information is not always explicitly specified. As a general rule, if state information is available when a message gets discarded, the message-state information should also be released. If state information is available when a session is closed, the session state information should also be released. Sensitive information, like cryptographic keys, should be overwritten appropriately prior to being released.
An error indication in statusInformation will typically include the Object Identifier (OID) and value for an incremented error counter. This may be accompanied by the requested securityLevel and the tmStateReference. Per-message context information is not accessible to Transport Models, so for the returned counter OID and value, contextEngine would be set to the local value of snmpEngineID and contextName to the default context for error counters.
| TOC |
5.1. Procedures for an Incoming Message
This section describes the procedures followed by the (D)TLS Transport Model when it receives a (D)TLS protected packet. The required functionality is broken into two different sections.
Section 5.1.1 (DTLS over UDP Processing for Incoming Messages) describes the processing required for de-multiplexing multiple DTLS connections, which is specifically needed for DTLS over UDP sessions. It is assumed that TLS protocol implementations already provide appropriate message demultiplexing.
Section 5.1.2 (Transport Processing for Incoming SNMP Messages) describes the transport processing required once the (D)TLS processing has been completed. This will be needed for all (D)TLS-based connections.
| TOC |
5.1.1. DTLS over UDP Processing for Incoming Messages
Demultiplexing of incoming packets into separate DTLS sessions MUST be implemented. For connection-oriented transport protocols, such as TCP, the transport protocol takes care of demultiplexing incoming packets to the right connection. Depending on the DTLS implementation, for DTLS over UDP, this demultiplexing will either need to be done within the TLS implementation or by the TLSTM implementation.
Like TCP, DTLS over UDP uses the four-tuple <source IP, destination IP, source port, destination port> for identifying the connection (and relevant DTLS connection state). This means that when establishing a new session, implementations MUST use a different UDP source port number for each active connection to a remote destination IP-address/port-number combination to ensure the remote entity can disambiguate between multiple connections.
If demultiplexing received UDP datagrams to DTLS connection state is done by the TLSTM implementation (instead of the DTLS implementation), the steps below describe one possible method to accomplish this.
The important output results from the steps in this process are the remote transport address, incomingMessage, incomingMessageLength, and the tlstmSessionID.
- 1)
- The TLS Transport Model examines the raw UDP message, in an implementation-dependent manner.
- 2)
- The TLS Transport Model queries the Local Configuration Datastore (LCD) (see [RFC3411] (Harrington, D., Presuhn, R., and B. Wijnen, “An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks,” December 2002.) Section 3.4.2) using the transport parameters (source and destination IP addresses and ports) to determine if a session already exists.
- 2a)
- If a matching entry in the LCD does not exist, then the UDP packet is passed to the DTLS implementation for processing. If the DTLS implementation decides to continue with the connection and allocate state for it, it returns a new DTLS connection handle (an implementation dependent detail). In this case, TLSTM selects a new tlstmSessionId, and caches this and the DTLS connection handle as a new entry in the LCD (indexed by the transport parameters). If the DTLS implementation returns an error or does not allocate connection state (which can happen with the stateless cookie exchange), processing stops.
- 2b)
- If a session does exist in the LCD then its DTLS connection handle (an implementation dependent detail) and its tlstmSessionId is extracted from the LCD. The UDP packet and the connection handle is passed to the DTLS implementation. If the DTLS implementation returns success but does not return an incomingMessage and an incomingMessageLength then processing stops (this is the case when the UDP datagram contained DTLS handshake messages, for example). If the DTLS implementation returns an error then processing stops.
- 3)
- Retrieve the incomingMessage and an incomingMessageLength from DTLS. These results and the tlstmSessionID are used below in Section 5.1.2 (Transport Processing for Incoming SNMP Messages) to complete the processing of the incoming message.
| TOC |
5.1.2. Transport Processing for Incoming SNMP Messages
The procedures in this section describe how the TLS Transport Model should process messages that have already been properly extracted from the (D)TLS stream. Note that care must be taken when processing messages originating from either TLS or DTLS to ensure they're complete and single. For example, multiple SNMP messages can be passed through a single DTLS message and partial SNMP messages may be received from a TLS stream. These steps describe the processing of a singular SNMP message after it has been delivered from the (D)TLS stream.
- 1)
- Determine the tlstmSessionID for the incoming message. The tlstmSessionID MUST be a unique session identifier for this (D)TLS connection. The contents and format of this identifier are implementation-dependent as long as it is unique to the session. A session identifier MUST NOT be reused until all references to it are no longer in use. The tmSessionID is equal to the tlstmSessionID discussed in Section 5.1.1 (DTLS over UDP Processing for Incoming Messages). tmSessionID refers to the session identifier when stored in the tmStateReference and tlstmSessionID refers to the session identifier when stored in the LCD. They MUST always be equal when processing a given session's traffic.
If this is the first message received through this session and the session does not have an assigned tlstmSessionID yet then the snmpTlstmSessionAccepts counter is incremented and a tlstmSessionID for the session is created. This will only happen on the server side of a connection because a client would have already assigned a tlstmSessionID during the openSession() invocation. Implementations may have performed the procedures described in Section 5.3.2 (Accepting a Session as a Server) prior to this point or they may perform them now, but the procedures described in Section 5.3.2 (Accepting a Session as a Server) MUST be performed before continuing beyond this point.- 2)
- Create a tmStateReference cache for the subsequent reference and assign the following values within it:
- tmTransportDomain
- = snmpTLSTCPDomain or snmpDTLSUDPDomain as appropriate.
- tmTransportAddress
- = The address the message originated from.
- tmSecurityLevel
- = The derived tmSecurityLevel for the session, as discussed in Section 3.1.2 (Message Protection) and Section 5.3 (Establishing or Accepting a Session).
- tmSecurityName
- = The derived tmSecurityName for the session as discussed in Section 5.3 (Establishing or Accepting a Session). This value MUST be constant during the lifetime of the session.
- tmSessionID
- = The tlstmSessionID described in step 1 above.
- 3)
- The incomingMessage and incomingMessageLength are assigned values from the (D)TLS processing.
- 4)
- The TLS Transport Model passes the transportDomain, transportAddress, incomingMessage, and incomingMessageLength to the Dispatcher using the receiveMessage ASI:
statusInformation = receiveMessage( IN transportDomain -- snmpTLSTCPDomain or snmpDTLSUDPDomain, IN transportAddress -- address for the received message IN incomingMessage -- the whole SNMP message from (D)TLS IN incomingMessageLength -- the length of the SNMP message IN tmStateReference -- transport info )
| TOC |
5.2. Procedures for an Outgoing SNMP Message
The Dispatcher sends a message to the TLS Transport Model using the following ASI:
statusInformation =
sendMessage(
IN destTransportDomain -- transport domain to be used
IN destTransportAddress -- transport address to be used
IN outgoingMessage -- the message to send
IN outgoingMessageLength -- its length
IN tmStateReference -- transport info
)
This section describes the procedure followed by the TLS Transport Model whenever it is requested through this ASI to send a message.
- 1)
- If tmStateReference does not refer to a cache containing values for tmTransportDomain, tmTransportAddress, tmSecurityName, tmRequestedSecurityLevel, and tmSameSecurity, then increment the snmpTlstmSessionInvalidCaches counter, discard the message, and return the error indication in the statusInformation. Processing of this message stops.
- 2)
- Extract the tmSessionID, tmTransportDomain, tmTransportAddress, tmSecurityName, tmRequestedSecurityLevel, and tmSameSecurity values from the tmStateReference. Note: The tmSessionID value may be undefined if no session exists yet over which the message can be sent.
- 3)
- If tmSameSecurity is true and either tmSessionID is undefined or refers to a session that is no longer open then increment the snmpTlstmSessionNoSessions counter, discard the message and return the error indication in the statusInformation. Processing of this message stops.
- 4)
- If tmSameSecurity is false and tmSessionID refers to a session that is no longer available then an implementation SHOULD open a new session using the openSession() ASI (described in greater detail in step 5b). Instead of opening a new session an implementation MAY return a snmpTlstmSessionNoSessions error to the calling module and stop processing of the message.
- 5)
- If tmSessionID is undefined, then use tmTransportDomain, tmTransportAddress, tmSecurityName and tmRequestedSecurityLevel to see if there is a corresponding entry in the LCD suitable to send the message over.
- 5a)
- If there is a corresponding LCD entry, then this session will be used to send the message.
- 5b)
- If there is no corresponding LCD entry, then open a session using the openSession() ASI (discussed further in Section 5.3.1 (Establishing a Session as a Client)). Implementations MAY wish to offer message buffering to prevent redundant openSession() calls for the same cache entry. If an error is returned from openSession(), then discard the message, discard the tmStateReference, increment the snmpTlstmSessionOpenErrors, return an error indication to the calling module and stop processing of the message.
- 6)
- Using either the session indicated by the tmSessionID if there was one or the session resulting from a previous step (4 or 5), pass the outgoingMessage to (D)TLS for encapsulation and transmission.
| TOC |
5.3. Establishing or Accepting a Session
Establishing a (D)TLS connection as either a client or a server requires slightly different processing. The following two sections describe the necessary processing steps.
| TOC |
5.3.1. Establishing a Session as a Client
The TLS Transport Model provides the following primitive for use by a client to establish a new (D)TLS connection:
statusInformation = -- errorIndication or success openSession( IN tmStateReference -- transport information to be used OUT tmStateReference -- transport information to be used IN maxMessageSize -- of the sending SNMP entity )
The following describes the procedure to follow when establishing a SNMP over (D)TLS connection between SNMP engines for exchanging SNMP messages. This process is followed by any SNMP client's engine when establishing a session for subsequent use.
This procedure MAY be done automatically for an SNMP application that initiates a transaction, such as a command generator, a notification originator, or a proxy forwarder.
- 1)
- The snmpTlstmSessionOpens counter is incremented.
- 2)
- The client selects the appropriate certificate and cipher_suites for the key agreement based on the tmSecurityName and the tmRequestedSecurityLevel for the session. For sessions being established as a result of a SNMP-TARGET-MIB based operation, the certificate will potentially have been identified via the snmpTlstmParamsTable mapping and the cipher_suites will have to be taken from system-wide or implementation-specific configuration. If no row in the snmpTlstmParamsTable exists then implementations MAY choose to establish the connection using a default client certificate available to the application. Otherwise, the certificate and appropriate cipher_suites will need to be passed to the openSession() ASI as supplemental information or configured through an implementation-dependent mechanism. It is also implementation-dependent and possibly policy-dependent how tmRequestedSecurityLevel will be used to influence the security capabilities provided by the (D)TLS connection. However this is done, the security capabilities provided by (D)TLS MUST be at least as high as the level of security indicated by the tmRequestedSecurityLevel parameter. The actual security level of the session is reported in the tmStateReference cache as tmSecurityLevel. For (D)TLS to provide strong authentication, each principal acting as a command generator SHOULD have its own certificate.
- 3)
- Using the destTransportDomain and destTransportAddress values, the client will initiate the (D)TLS handshake protocol to establish session keys for message integrity and encryption.
If the attempt to establish a session is unsuccessful, then snmpTlstmSessionOpenErrors is incremented, an error indication is returned, and processing stops. If the session failed to open because the presented server certificate was unknown or invalid then the snmpTlstmSessionUnknownServerCertificate or snmpTlstmSessionInvalidServerCertificates MUST be incremented and a snmpTlstmServerCertificateUnknown or snmpTlstmServerInvalidCertificate notification SHOULD be sent as appropriate. Reasons for server certificate invalidation includes, but is not limited to, cryptographic validation failures and an unexpected presented certificate identity.- 4)
- The (D)TLS client MUST then verify that the (D)TLS server's presented certificate is the expected certificate. The (D)TLS client MUST NOT transmit SNMP messages until the server certificate has been authenticated, the client certificate has been transmitted and the TLS connection has been fully established.
If the connection is being established from configuration based on SNMP-TARGET-MIB configuration, then the snmpTlstmAddrTable DESCRIPTION clause describes how the verification is done (using either a certificate fingerprint, or an identity authenticated via certification path validation).
If the connection is being established for reasons other than configuration found in the SNMP-TARGET-MIB then configuration and procedures outside the scope of this document should be followed. Configuration mechanisms SHOULD be similar in nature to those defined in the snmpTlstmAddrTable to ensure consistency across management configuration systems. For example, a command-line tool for generating SNMP GETs might support specifying either the server's certificate fingerprint or the expected host name as a command line argument.- 5)
- (D)TLS provides assurance that the authenticated identity has been signed by a trusted configured certification authority. If verification of the server's certificate fails in any way (for example because of failures in cryptographic verification or the presented identity did not match the expected named entity) then the session establishment MUST fail, the snmpTlstmSessionInvalidServerCertificates object is incremented. If the session can not be opened for any reason at all, including cryptographic verification failures and snmpTlstmCertToTSNTable lookup failures, then the snmpTlstmSessionOpenErrors counter is incremented and processing stops.
- 6)
- The TLSTM-specific session identifier (tlstmSessionID) is set in the tmSessionID of the tmStateReference passed to the TLS Transport Model to indicate that the session has been established successfully and to point to a specific (D)TLS connection for future use. The tlstmSessionID is also stored in the LCD for later lookup during processing of incoming messages (Section 5.1.2 (Transport Processing for Incoming SNMP Messages)).
| TOC |
5.3.2. Accepting a Session as a Server
A (D)TLS server should accept new session connections from any client that it is able to verify the client's credentials for. This is done by authenticating the client's presented certificate through a certificate path validation process (e.g. [RFC5280] (Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, “Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile,” May 2008.)) or through certificate fingerprint verification using fingerprints configured in the snmpTlstmCertToTSNTable. Afterward the server will determine the identity of the remote entity using the following procedures.
The (D)TLS server identifies the authenticated identity from the (D)TLS client's principal certificate using configuration information from the snmpTlstmCertToTSNTable mapping table. The (D)TLS server MUST request and expect a certificate from the client and MUST NOT accept SNMP messages over the (D)TLS connection until the client has sent a certificate and it has been authenticated. The resulting derived tmSecurityName is recorded in the tmStateReference cache as tmSecurityName. The details of the lookup process are fully described in the DESCRIPTION clause of the snmpTlstmCertToTSNTable MIB object. If any verification fails in any way (for example because of failures in cryptographic verification or because of the lack of an appropriate row in the snmpTlstmCertToTSNTable) then the session establishment MUST fail, and the snmpTlstmSessionInvalidClientCertificates object is incremented. If the session can not be opened for any reason at all, including cryptographic verification failures, then the snmpTlstmSessionOpenErrors counter is incremented and processing stops.
Servers that wish to support multiple principals at a particular port SHOULD make use of a (D)TLS extension that allows server-side principal selection like the Server Name Indication extension defined in Section 3.1 of [RFC4366] (Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and T. Wright, “Transport Layer Security (TLS) Extensions,” April 2006.). Supporting this will allow, for example, sending notifications to a specific principal at a given TCP or UDP port.
| TOC |
5.4. Closing a Session
The TLS Transport Model provides the following primitive to close a session:
statusInformation = closeSession( IN tmSessionID -- session ID of the session to be closed )
The following describes the procedure to follow to close a session between a client and server. This process is followed by any SNMP engine closing the corresponding SNMP session.
- 1)
- Increment either the snmpTlstmSessionClientCloses or the snmpTlstmSessionServerCloses counter as appropriate.
- 2)
- Look up the session using the tmSessionID.
- 3)
- If there is no open session associated with the tmSessionID, then closeSession processing is completed.
- 4)
- Have (D)TLS close the specified connection. This MUST include sending a close_notify TLS Alert to inform the other side that session cleanup may be performed.
| TOC |
6. MIB Module Overview
This MIB module provides management of the TLS Transport Model. It defines needed textual conventions, statistical counters, notifications and configuration infrastructure necessary for session establishment. Example usage of the configuration tables can be found in Appendix A (Target and Notification Configuration Example).
| TOC |
6.1. Structure of the MIB Module
Objects in this MIB module are arranged into subtrees. Each subtree is organized as a set of related objects. The overall structure and assignment of objects to their subtrees, and the intended purpose of each subtree, is shown below.
| TOC |
6.2. Textual Conventions
Generic and Common Textual Conventions used in this module can be found summarized at http://www.ops.ietf.org/mib-common-tcs.html
This module defines the following new Textual Conventions:
- A new TransportAddress format for describing (D)TLS connection addressing requirements.
- A certificate fingerprint allowing MIB module objects to generically refer to a stored X.509 certificate using a cryptographic hash as a reference pointer.
| TOC |
6.3. Statistical Counters
The SNMP-TLS-TM-MIB defines counters that provide network management stations with information about session usage and potential errors that a device may be experiencing.
| TOC |
6.4. Configuration Tables
The SNMP-TLS-TM-MIB defines configuration tables that an administrator can use for configuring a device for sending and receiving SNMP messages over (D)TLS. In particular, there are MIB tables that extend the SNMP-TARGET-MIB for configuring (D)TLS certificate usage and a MIB table for mapping incoming (D)TLS client certificates to SNMPv3 securityNames.
| TOC |
6.4.1. Notifications
The SNMP-TLS-TM-MIB defines notifications to alert management stations when a (D)TLS connection fails because a server's presented certificate did not meet an expected value (snmpTlstmServerCertificateUnknown) or because cryptographic validation failed (snmpTlstmServerInvalidCertificate).
| TOC |
6.5. Relationship to Other MIB Modules
Some management objects defined in other MIB modules are applicable to an entity implementing the TLS Transport Model. In particular, it is assumed that an entity implementing the SNMP-TLS-TM-MIB will implement the SNMPv2-MIB [RFC3418] (Presuhn, R., “Management Information Base (MIB) for the Simple Network Management Protocol (SNMP),” December 2002.), the SNMP-FRAMEWORK-MIB [RFC3411] (Harrington, D., Presuhn, R., and B. Wijnen, “An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks,” December 2002.), the SNMP-TARGET-MIB [RFC3413] (Levi, D., Meyer, P., and B. Stewart, “Simple Network Management Protocol (SNMP) Applications,” December 2002.), the SNMP-NOTIFICATION-MIB [RFC3413] (Levi, D., Meyer, P., and B. Stewart, “Simple Network Management Protocol (SNMP) Applications,” December 2002.) and the SNMP-VIEW-BASED-ACM-MIB [RFC3415] (Wijnen, B., Presuhn, R., and K. McCloghrie, “View-based Access Control Model (VACM) for the Simple Network Management Protocol (SNMP),” December 2002.).
The SNMP-TLS-TM-MIB module contained in this document is for managing TLS Transport Model information.
| TOC |
6.5.1. MIB Modules Required for IMPORTS
The SNMP-TLS-TM-MIB module imports items from SNMPv2-SMI [RFC2578] (McCloghrie, K., Ed., Perkins, D., Ed., and J. Schoenwaelder, Ed., “Structure of Management Information Version 2 (SMIv2),” April 1999.), SNMPv2-TC [RFC2579] (McCloghrie, K., Ed., Perkins, D., Ed., and J. Schoenwaelder, Ed., “Textual Conventions for SMIv2,” April 1999.), SNMP-FRAMEWORK-MIB [RFC3411] (Harrington, D., Presuhn, R., and B. Wijnen, “An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks,” December 2002.), SNMP-TARGET-MIB [RFC3413] (Levi, D., Meyer, P., and B. Stewart, “Simple Network Management Protocol (SNMP) Applications,” December 2002.) and SNMPv2-CONF [RFC2580] (McCloghrie, K., Perkins, D., and J. Schoenwaelder, “Conformance Statements for SMIv2,” April 1999.).
| TOC |
7. MIB Module Definition
SNMP-TLS-TM-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-TYPE,
OBJECT-IDENTITY, mib-2, snmpDomains,
Counter32, Unsigned32, Gauge32, NOTIFICATION-TYPE
FROM SNMPv2-SMI -- RFC2578 or any update thereof
TEXTUAL-CONVENTION, TimeStamp, RowStatus, StorageType,
AutonomousType
FROM SNMPv2-TC -- RFC2579 or any update thereof
MODULE-COMPLIANCE, OBJECT-GROUP, NOTIFICATION-GROUP
FROM SNMPv2-CONF -- RFC2580 or any update thereof
SnmpAdminString
FROM SNMP-FRAMEWORK-MIB -- RFC3411 or any update thereof
snmpTargetParamsName, snmpTargetAddrName
FROM SNMP-TARGET-MIB -- RFC3413 or any update thereof
;
snmpTlstmMIB MODULE-IDENTITY
LAST-UPDATED "201005070000Z"
ORGANIZATION "ISMS Working Group"
CONTACT-INFO "WG-EMail: isms@lists.ietf.org
Subscribe: isms-request@lists.ietf.org
Chairs:
Juergen Schoenwaelder
Jacobs University Bremen
Campus Ring 1
28725 Bremen
Germany
+49 421 200-3587
j.schoenwaelder@jacobs-university.de
Russ Mundy
SPARTA, Inc.
7110 Samuel Morse Drive
Columbia, MD 21046
USA
Editor:
Wes Hardaker
Sparta, Inc.
P.O. Box 382
Davis, CA 95617
USA
ietf@hardakers.net
"
DESCRIPTION "
The TLS Transport Model MIB
Copyright (c) 2010 IETF Trust and the persons identified as
the document authors. All rights reserved.
Redistribution and use in source and binary forms, with or
without modification, is permitted pursuant to, and subject
to the license terms contained in, the Simplified BSD License
set forth in Section 4.c of the IETF Trust's Legal Provisions
Relating to IETF Documents
(http://trustee.ietf.org/license-info)."
REVISION "201005070000Z"
DESCRIPTION "This version of this MIB module is part of
RFC XXXX; see the RFC itself for full legal
notices."
-- NOTE to RFC editor: replace XXXX with actual RFC number
-- for this document and change the date to the
-- current date and remove this note
::= { mib-2 www }
-- RFC Ed.: replace www with IANA-assigned number under the mib-2
-- SNMP OID tree and remove this note
-- ************************************************
-- subtrees of the SNMP-TLS-TM-MIB
-- ************************************************
snmpTlstmNotifications OBJECT IDENTIFIER ::= { snmpTlstmMIB 0 }
snmpTlstmIdentities OBJECT IDENTIFIER ::= { snmpTlstmMIB 1 }
snmpTlstmObjects OBJECT IDENTIFIER ::= { snmpTlstmMIB 2 }
snmpTlstmConformance OBJECT IDENTIFIER ::= { snmpTlstmMIB 3 }
-- ************************************************
-- snmpTlstmObjects - Objects
-- ************************************************
snmpTLSTCPDomain OBJECT-IDENTITY
STATUS current
DESCRIPTION
"The SNMP over TLS transport domain. The corresponding
transport address is of type SnmpTLSAddress.
The securityName prefix to be associated with the
snmpTLSTCPDomain is 'tls'. This prefix may be used by
security models or other components to identify which secure
transport infrastructure authenticated a securityName."
REFERENCE
"RFC 2579: Textual Conventions for SMIv2"
::= { snmpDomains xx }
-- RFC Ed.: replace xx with IANA-assigned number and
-- remove this note
-- RFC Ed.: replace 'tls' with the actual IANA assigned prefix string
-- if 'tls' is not assigned to this document.
snmpDTLSUDPDomain OBJECT-IDENTITY
STATUS current
DESCRIPTION
"The SNMP over DTLS/UDP transport domain. The corresponding
transport address is of type SnmpTLSAddress.
The securityName prefix to be associated with the
snmpDTLSUDPDomain is 'dtls'. This prefix may be used by
security models or other components to identify which secure
transport infrastructure authenticated a securityName."
REFERENCE
"RFC 2579: Textual Conventions for SMIv2"
::= { snmpDomains yy }
-- RFC Ed.: replace yy with IANA-assigned number and
-- remove this note
-- RFC Ed.: replace 'dtls' with the actual IANA assigned prefix string
-- if 'dtls' is not assigned to this document.
SnmpTLSAddress ::= TEXTUAL-CONVENTION
DISPLAY-HINT "1a"
STATUS current
DESCRIPTION
"Represents a IPv4 address, an IPv6 address or an US-ASCII
encoded hostname and port number.
An IPv4 address must be in dotted decimal format followed by a
colon ':' (US-ASCII character 0x3A) and a decimal port number
in US-ASCII.
An IPv6 address must be a colon separated format (as described
in I-D.ietf-6man-text-addr-representation), surrounded by
square brackets ('[', US-ASCII character 0x5B, and ']',
US-ASCII character 0x5D), followed by a colon ':' (US-ASCII
character 0x3A) and a decimal port number in US-ASCII.
A hostname is always in US-ASCII (as per RFC1033);
internationalized hostnames are encoded in US-ASCII as domain
names after transformation via the ToASCII operation specified
in RFC 3490. The ToASCII operation MUST be performed with the
UseSTD3ASCIIRules flag set. The hostname is followed by a
colon ':' (US-ASCII character 0x3A) and a decimal port number
in US-ASCII. The name SHOULD be fully qualified whenever
possible.
Values of this textual convention may not be directly usable
as transport-layer addressing information, and may require
run-time resolution. As such, applications that write them
must be prepared for handling errors if such values are not
supported, or cannot be resolved (if resolution occurs at the
time of the management operation).
The DESCRIPTION clause of TransportAddress objects that may
have SnmpTLSAddress values must fully describe how (and
when) such names are to be resolved to IP addresses and vice
versa.
This textual convention SHOULD NOT be used directly in object
definitions since it restricts addresses to a specific
format. However, if it is used, it MAY be used either on its
own or in conjunction with TransportAddressType or
TransportDomain as a pair.
When this textual convention is used as a syntax of an index
object, there may be issues with the limit of 128
sub-identifiers specified in SMIv2 (STD 58). It is RECOMMENDED
that all MIB documents using this textual convention make
explicit any limitations on index component lengths that
management software must observe. This may be done either by
including SIZE constraints on the index components or by
specifying applicable constraints in the conceptual row
DESCRIPTION clause or in the surrounding documentation."
REFERENCE
"RFC 1033: DOMAIN ADMINISTRATORS OPERATIONS GUIDE
RFC 3490: Internationalizing Domain Names in Applications
I-D.ietf-6man-text-addr-representation:
A Recommendation for IPv6 Address Text Representation
"
SYNTAX OCTET STRING (SIZE (1..255))
-- RFC Editor: if I-D.ietf-6man-text-addr-representation fails to get
-- published then replace the reference to
-- I-D.ietf-6man-text-addr-representation with a reference to
-- "RFC3513: Internet Protocol Version 6 (IPv6) Addressing Architecture"
-- instead.
SnmpTLSFingerprint ::= TEXTUAL-CONVENTION
DISPLAY-HINT "1x:254x"
STATUS current
DESCRIPTION
"A fingerprint value that can be used to uniquely reference
other data of potentially arbitrary length.
A SnmpTLSFingerprint value is composed of a 1-octet hashing
algorithm identifier followed by the fingerprint value. The
octet value encoded is taken from the IANA TLS HashAlgorithm
Registry (RFC5246). The remaining octets are filled using the
results of the hashing algorithm.
This TEXTUAL-CONVENTION allows for a zero-length (blank)
SnmpTLSFingerprint value for use in tables where the
fingerprint value may be optional. MIB definitions or
implementations may refuse to accept a zero-length value as
appropriate."
REFERENCE "RFC 5246: The Transport Layer
Security (TLS) Protocol Version 1.2
http://www.iana.org/assignments/tls-parameters/
"
SYNTAX OCTET STRING (SIZE (0..255))
-- Identities for use in the snmpTlstmCertToTSNTable
snmpTlstmCertToTSNMIdentities OBJECT IDENTIFIER
::= { snmpTlstmIdentities 1 }
snmpTlstmCertSpecified OBJECT-IDENTITY
STATUS current
DESCRIPTION "Directly specifies the tmSecurityName to be used for
this certificate. The value of the tmSecurityName
to use is specified in the snmpTlstmCertToTSNData
column. The snmpTlstmCertToTSNData column must
contain a non-zero length SnmpAdminString compliant
value or the mapping described in this row must be
considered a failure."
::= { snmpTlstmCertToTSNMIdentities 1 }
snmpTlstmCertSANRFC822Name OBJECT-IDENTITY
STATUS current
DESCRIPTION "Maps a subjectAltName's rfc822Name to a
tmSecurityName. The local part of the rfc822Name is
passed unaltered but the host-part of the name must
be passed in lower case. This mapping results in a
1:1 correspondence between equivalent subjectAltName
rfc822Name values and tmSecurityName values except
that the host-part of the name MUST be passed in
lower case.
Example rfc822Name Field: FooBar@Example.COM
is mapped to tmSecurityName: FooBar@example.com"
::= { snmpTlstmCertToTSNMIdentities 2 }
snmpTlstmCertSANDNSName OBJECT-IDENTITY
STATUS current
DESCRIPTION "Maps a subjectAltName's dNSName to a
tmSecurityName after first converting it to all
lower case (RFC5280 does not specify converting to
lower case so this involves an extra step). This
mapping results in a 1:1 correspondence between
subjectAltName dNSName values and the tmSecurityName
values."
REFERENCE "RFC5280 - Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation
List (CRL) Profile"
::= { snmpTlstmCertToTSNMIdentities 3 }
snmpTlstmCertSANIpAddress OBJECT-IDENTITY
STATUS current
DESCRIPTION "Maps a subjectAltName's iPAddress to a
tmSecurityName by transforming the binary encoded
address as follows:
1) for IPv4 the value is converted into a decimal
dotted quad address (e.g. '192.0.2.1')
2) for IPv6 addresses the value is converted into a
32-character all lowercase hexadecimal string
without any colon separators.
This mapping results in a 1:1 correspondence between
subjectAltName iPAddress values and the
tmSecurityName values.
The resulting length is the maximum length supported
by the View-Based Access Control Model (VACM).
Using both the Transport Security Model's support
for transport prefixes (see the SNMP-TSM-MIB's
snmpTsmConfigurationUsePrefix object for details)
will result in securityName lengths that exceed what
VACM can handle."
::= { snmpTlstmCertToTSNMIdentities 4 }
snmpTlstmCertSANAny OBJECT-IDENTITY
STATUS current
DESCRIPTION "Maps any of the following fields using the
corresponding mapping algorithms:
|------------+----------------------------|
| Type | Algorithm |
|------------+----------------------------|
| rfc822Name | snmpTlstmCertSANRFC822Name |
| dNSName | snmpTlstmCertSANDNSName |
| iPAddress | snmpTlstmCertSANIpAddress |
|------------+----------------------------|
The first matching subjectAltName value found in the
certificate of the above types MUST be used when
deriving the tmSecurityName. The mapping algorithm
specified in the 'Algorithm' column MUST be used to
derive the tmSecurityName.
This mapping results in a 1:1 correspondence between
subjectAltName values and tmSecurityName values. The
three sub-mapping algorithms produced by this
combined algorithm cannot produce conflicting
results between themselves."
::= { snmpTlstmCertToTSNMIdentities 5 }
snmpTlstmCertCommonName OBJECT-IDENTITY
STATUS current
DESCRIPTION "Maps a certificate's CommonName to a tmSecurityName
after converting it to a UTF-8 encoding. The usage
of CommonNames is deprecated and users are
encouraged to use subjectAltName mapping methods
instead. This mapping results in a 1:1
correspondence between certificate CommonName values
and tmSecurityName values."
::= { snmpTlstmCertToTSNMIdentities 6 }
-- The snmpTlstmSession Group
snmpTlstmSession OBJECT IDENTIFIER ::= { snmpTlstmObjects 1 }
snmpTlstmSessionOpens OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times an openSession() request has been executed
as an (D)TLS client, regardless of whether it succeeded or
failed."
::= { snmpTlstmSession 1 }
snmpTlstmSessionClientCloses OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times a closeSession() request has been
executed as an (D)TLS client, regardless of whether it
succeeded or failed."
::= { snmpTlstmSession 2 }
snmpTlstmSessionOpenErrors OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times an openSession() request failed to open a
session as a (D)TLS client, for any reason."
::= { snmpTlstmSession 3 }
snmpTlstmSessionAccepts OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times a (D)TLS server has accepted a new
connection from a client and has received at least one SNMP
message through it."
::= { snmpTlstmSession 4 }
snmpTlstmSessionServerCloses OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times a closeSession() request has been
executed as an (D)TLS server, regardless of whether it
succeeded or failed."
::= { snmpTlstmSession 5 }
snmpTlstmSessionNoSessions OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times an outgoing message was dropped because
the session associated with the passed tmStateReference was no
longer (or was never) available."
::= { snmpTlstmSession 6 }
snmpTlstmSessionInvalidClientCertificates OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times an incoming session was not established
on an (D)TLS server because the presented client certificate
was invalid. Reasons for invalidation include, but are not
limited to, cryptographic validation failures or lack of a
suitable mapping row in the snmpTlstmCertToTSNTable."
::= { snmpTlstmSession 7 }
snmpTlstmSessionUnknownServerCertificate OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times an outgoing session was not established
on an (D)TLS client because the server certificate presented
by a SNMP over (D)TLS server was invalid because no
configured fingerprint or CA was acceptable to validate it.
This may result because there was no entry in the
snmpTlstmAddrTable or because no path could be found to a
known certification authority."
::= { snmpTlstmSession 8 }
snmpTlstmSessionInvalidServerCertificates OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of times an outgoing session was not established
on an (D)TLS client because the server certificate presented
by an SNMP over (D)TLS server could not be validated even if
the fingerprint or expected validation path was known. I.E.,
a cryptographic validation error occurred during certificate
validation processing.
Reasons for invalidation include, but are not
limited to, cryptographic validation failures."
::= { snmpTlstmSession 9 }
snmpTlstmSessionInvalidCaches OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The number of outgoing messages dropped because the
tmStateReference referred to an invalid cache."
::= { snmpTlstmSession 10 }
-- Configuration Objects
snmpTlstmConfig OBJECT IDENTIFIER ::= { snmpTlstmObjects 2 }
-- Certificate mapping
snmpTlstmCertificateMapping OBJECT IDENTIFIER ::= { snmpTlstmConfig 1 }
snmpTlstmCertToTSNCount OBJECT-TYPE
SYNTAX Gauge32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"A count of the number of entries in the
snmpTlstmCertToTSNTable"
::= { snmpTlstmCertificateMapping 1 }
snmpTlstmCertToTSNTableLastChanged OBJECT-TYPE
SYNTAX TimeStamp
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The value of sysUpTime.0 when the snmpTlstmCertToTSNTable was
last modified through any means, or 0 if it has not been
modified since the command responder was started."
::= { snmpTlstmCertificateMapping 2 }
snmpTlstmCertToTSNTable OBJECT-TYPE
SYNTAX SEQUENCE OF SnmpTlstmCertToTSNEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"This table is used by a (D)TLS server to map the (D)TLS
client's presented X.509 certificate to a tmSecurityName.
On an incoming (D)TLS/SNMP connection the client's presented
certificate must either be validated based on an established
trust anchor, or it must directly match a fingerprint in this
table. This table does not provide any mechanisms for
configuring the trust anchors; the transfer of any needed
trusted certificates for path validation is expected to occur
through an out-of-band transfer.
Once the certificate has been found acceptable (either by path
validation or directly matching a fingerprint in this table),
this table is consulted to determine the appropriate
tmSecurityName to identify with the remote connection. This
is done by considering each active row from this table in
prioritized order according to its snmpTlstmCertToTSNID value.
Each row's snmpTlstmCertToTSNFingerprint value determines
whether the row is a match for the incoming connection:
1) If the row's snmpTlstmCertToTSNFingerprint value
identifies the presented certificate then consider the
row as a successful match.
2) If the row's snmpTlstmCertToTSNFingerprint value
identifies a locally held copy of a trusted CA
certificate and that CA certificate was used to
validate the path to the presented certificate then
consider the row as a successful match.
Once a matching row has been found, the
snmpTlstmCertToTSNMapType value can be used to determine how
the tmSecurityName to associate with the session should be
determined. See the snmpTlstmCertToTSNMapType column's
DESCRIPTION for details on determining the tmSecurityName
value. If it is impossible to determine a tmSecurityName from
the row's data combined with the data presented in the
certificate then additional rows MUST be searched looking for
another potential match. If a resulting tmSecurityName mapped
from a given row is not compatible with the needed
requirements of a tmSecurityName (e.g., VACM imposes a
32-octet-maximum length and the certificate derived
securityName could be longer) then it must be considered an
invalid match and additional rows MUST be searched looking for
another potential match.
If no matching and valid row can be found, the connection MUST
be closed and SNMP messages MUST NOT be accepted over it.
Missing values of snmpTlstmCertToTSNID are acceptable and
implementations should continue to the next highest numbered
row. It is recommended that administrators skip index values
to leave room for the insertion of future rows (E.G., use values
of 10 and 20 when creating initial rows).
Users are encouraged to make use of certificates with
subjectAltName fields that can be used as tmSecurityNames so
that a single root CA certificate can allow all child
certificate's subjectAltName to map directly to a
tmSecurityName via a 1:1 transformation. However, this table
is flexible to allow for situations where existing deployed
certificate infrastructures do not provide adequate
subjectAltName values for use as tmSecurityNames.
Certificates may also be mapped to tmSecurityNames using the
CommonName portion of the Subject field. However, the usage
of the CommonName field is deprecated and thus this usage is
NOT RECOMMENDED. Direct mapping from each individual
certificate fingerprint to a tmSecurityName is also possible
but requires one entry in the table per tmSecurityName and
requires more management operations to completely configure a
device."
::= { snmpTlstmCertificateMapping 3 }
snmpTlstmCertToTSNEntry OBJECT-TYPE
SYNTAX SnmpTlstmCertToTSNEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A row in the snmpTlstmCertToTSNTable that specifies a mapping
for an incoming (D)TLS certificate to a tmSecurityName to use
for a connection."
INDEX { snmpTlstmCertToTSNID }
::= { snmpTlstmCertToTSNTable 1 }
SnmpTlstmCertToTSNEntry ::= SEQUENCE {
snmpTlstmCertToTSNID Unsigned32,
snmpTlstmCertToTSNFingerprint SnmpTLSFingerprint,
snmpTlstmCertToTSNMapType AutonomousType,
snmpTlstmCertToTSNData OCTET STRING,
snmpTlstmCertToTSNStorageType StorageType,
snmpTlstmCertToTSNRowStatus RowStatus
}
snmpTlstmCertToTSNID OBJECT-TYPE
SYNTAX Unsigned32 (1..4294967295)
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A unique, prioritized index for the given entry. Lower
numbers indicate a higher priority."
::= { snmpTlstmCertToTSNEntry 1 }
snmpTlstmCertToTSNFingerprint OBJECT-TYPE
SYNTAX SnmpTLSFingerprint (SIZE(1..255))
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"A cryptographic hash of a X.509 certificate. The results of
a successful matching fingerprint to either the trusted CA in
the certificate validation path or to the certificate itself
is dictated by the snmpTlstmCertToTSNMapType column."
::= { snmpTlstmCertToTSNEntry 2 }
snmpTlstmCertToTSNMapType OBJECT-TYPE
SYNTAX AutonomousType
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"Specifies the mapping type for deriving a tmSecurityName from
a certificate. Details for mapping of a particular type SHALL
be specified in the DESCRIPTION clause of the OBJECT-IDENTITY
that describes the mapping. If a mapping succeeds it will
return a tmSecurityName for use by the TLSTM model and
processing stops.
If the resulting mapped value is not compatible with the
needed requirements of a tmSecurityName (e.g., VACM imposes a
32-octet-maximum length and the certificate derived
securityName could be longer) then future rows MUST be
searched for additional snmpTlstmCertToTSNFingerprint matches
to look for a mapping that succeeds.
Suitable values for assigning to this object that are defined
within the SNMP-TLS-TM-MIB can be found in the
snmpTlstmCertToTSNMIdentities portion of the MIB tree."
DEFVAL { snmpTlstmCertSpecified }
::= { snmpTlstmCertToTSNEntry 3 }
snmpTlstmCertToTSNData OBJECT-TYPE
SYNTAX OCTET STRING (SIZE(0..1024))
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"Auxiliary data used as optional configuration information for
a given mapping specified by the snmpTlstmCertToTSNMapType
column. Only some mapping systems will make use of this
column. The value in this column MUST be ignored for any
mapping type that does not require data present in this
column."
DEFVAL { "" }
::= { snmpTlstmCertToTSNEntry 4 }
snmpTlstmCertToTSNStorageType OBJECT-TYPE
SYNTAX StorageType
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The storage type for this conceptual row. Conceptual rows
having the value 'permanent' need not allow write-access to
any columnar objects in the row."
DEFVAL { nonVolatile }
::= { snmpTlstmCertToTSNEntry 5 }
snmpTlstmCertToTSNRowStatus OBJECT-TYPE
SYNTAX RowStatus
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The status of this conceptual row. This object may be used
to create or remove rows from this table.
To create a row in this table, an administrator must set this
object to either createAndGo(4) or createAndWait(5).
Until instances of all corresponding columns are appropriately
configured, the value of the corresponding instance of the
snmpTlstmParamsRowStatus column is notReady(3).
In particular, a newly created row cannot be made active until
the corresponding snmpTlstmCertToTSNFingerprint,
snmpTlstmCertToTSNMapType, and snmpTlstmCertToTSNData columns
have been set.
The following objects may not be modified while the
value of this object is active(1):
- snmpTlstmCertToTSNFingerprint
- snmpTlstmCertToTSNMapType
- snmpTlstmCertToTSNData
An attempt to set these objects while the value of
snmpTlstmParamsRowStatus is active(1) will result in
an inconsistentValue error."
::= { snmpTlstmCertToTSNEntry 6 }
-- Maps tmSecurityNames to certificates for use by the SNMP-TARGET-MIB
snmpTlstmParamsCount OBJECT-TYPE
SYNTAX Gauge32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"A count of the number of entries in the snmpTlstmParamsTable"
::= { snmpTlstmCertificateMapping 4 }
snmpTlstmParamsTableLastChanged OBJECT-TYPE
SYNTAX TimeStamp
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The value of sysUpTime.0 when the snmpTlstmParamsTable
was last modified through any means, or 0 if it has not been
modified since the command responder was started."
::= { snmpTlstmCertificateMapping 5 }
snmpTlstmParamsTable OBJECT-TYPE
SYNTAX SEQUENCE OF SnmpTlstmParamsEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"This table is used by a (D)TLS client when a (D)TLS
connection is being set up using an entry in the
SNMP-TARGET-MIB. It extends the SNMP-TARGET-MIB's
snmpTargetParamsTable with a fingerprint of a certificate to
use when establishing such a (D)TLS connection."
::= { snmpTlstmCertificateMapping 6 }
snmpTlstmParamsEntry OBJECT-TYPE
SYNTAX SnmpTlstmParamsEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A conceptual row containing a fingerprint hash of a locally
held certificate for a given snmpTargetParamsEntry. The
values in this row should be ignored if the connection that
needs to be established, as indicated by the SNMP-TARGET-MIB
infrastructure, is not a certificate and (D)TLS based
connection. The connection SHOULD NOT be established if the
certificate fingerprint stored in this entry does not point to
a valid locally held certificate or if it points to an
unusable certificate (such as might happen when the
certificate's expiration date has been reached)."
INDEX { IMPLIED snmpTargetParamsName }
::= { snmpTlstmParamsTable 1 }
SnmpTlstmParamsEntry ::= SEQUENCE {
snmpTlstmParamsClientFingerprint SnmpTLSFingerprint,
snmpTlstmParamsStorageType StorageType,
snmpTlstmParamsRowStatus RowStatus
}
snmpTlstmParamsClientFingerprint OBJECT-TYPE
SYNTAX SnmpTLSFingerprint
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"This object stores the hash of the public portion of a
locally held X.509 certificate. The X.509 certificate, its
public key, and the corresponding private key will be used
when initiating a (D)TLS connection as a (D)TLS client."
::= { snmpTlstmParamsEntry 1 }
snmpTlstmParamsStorageType OBJECT-TYPE
SYNTAX StorageType
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The storage type for this conceptual row. Conceptual rows
having the value 'permanent' need not allow write-access to
any columnar objects in the row."
DEFVAL { nonVolatile }
::= { snmpTlstmParamsEntry 2 }
snmpTlstmParamsRowStatus OBJECT-TYPE
SYNTAX RowStatus
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The status of this conceptual row. This object may be used
to create or remove rows from this table.
To create a row in this table, an administrator must set this
object to either createAndGo(4) or createAndWait(5).
Until instances of all corresponding columns are appropriately
configured, the value of the corresponding instance of the
snmpTlstmParamsRowStatus column is notReady(3).
In particular, a newly created row cannot be made active until
the corresponding snmpTlstmParamsClientFingerprint column has
been set.
The snmpTlstmParamsClientFingerprint object may not be modified
while the value of this object is active(1).
An attempt to set these objects while the value of
snmpTlstmParamsRowStatus is active(1) will result in
an inconsistentValue error."
::= { snmpTlstmParamsEntry 3 }
snmpTlstmAddrCount OBJECT-TYPE
SYNTAX Gauge32
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"A count of the number of entries in the snmpTlstmAddrTable"
::= { snmpTlstmCertificateMapping 7 }
snmpTlstmAddrTableLastChanged OBJECT-TYPE
SYNTAX TimeStamp
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The value of sysUpTime.0 when the snmpTlstmAddrTable
was last modified through any means, or 0 if it has not been
modified since the command responder was started."
::= { snmpTlstmCertificateMapping 8 }
snmpTlstmAddrTable OBJECT-TYPE
SYNTAX SEQUENCE OF SnmpTlstmAddrEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"This table is used by a (D)TLS client when a (D)TLS
connection is being set up using an entry in the
SNMP-TARGET-MIB. It extends the SNMP-TARGET-MIB's
snmpTargetAddrTable so that the client can verify that the
correct server has been reached. This verification can use
either a certificate fingerprint, or an identity
authenticated via certification path validation.
If there is an active row in this table corresponding to the
entry in the SNMP-TARGET-MIB that was used to establish the
connection, and the row's snmpTlstmAddrServerFingerprint
column has non-empty value, then the server's presented
certificate is compared with the
snmpTlstmAddrServerFingerprint value (and the
snmpTlstmAddrServerIdentity column is ignored). If the
fingerprint matches, the verification has succeeded. If the
fingerprint does not match then the connection MUST be
closed.
If the server's presented certificate has passed
certification path validation [RFC5280] to a configured
trust anchor, and an active row exists with a zero-length
snmpTlstmAddrServerFingerprint value, then the
snmpTlstmAddrServerIdentity column contains the expected
host name. This expected host name is then compared against
the server's certificate as follows:
- Implementations MUST support matching the expected host
name against a dNSName in the subjectAltName extension
field and MAY support checking the name against the
CommonName portion of the subject distinguished name.
- The '*' (ASCII 0x2a) wildcard character is allowed in the
dNSName of the subjectAltName extension (and in common
name, if used to store the host name), but only as the
left-most (least significant) DNS label in that value.
This wildcard matches any left-most DNS label in the
server name. That is, the subject *.example.com matches
the server names a.example.com and b.example.com, but does
not match example.com or a.b.example.com. Implementations
MUST support wildcards in certificates as specified above,
but MAY provide a configuration option to disable them.
- If the locally configured name is an internationalized
domain name, conforming implementations MUST convert it to
the ASCII Compatible Encoding (ACE) format for performing
comparisons, as specified in Section 7 of [RFC5280].
If the expected host name fails these conditions then the
connection MUST be closed.
If there is no row in this table corresponding to the entry
in the SNMP-TARGET-MIB and the server can be authorized by
another, implementation dependent means, then the connection
MAY still proceed."
::= { snmpTlstmCertificateMapping 9 }
snmpTlstmAddrEntry OBJECT-TYPE
SYNTAX SnmpTlstmAddrEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION
"A conceptual row containing a copy of a certificate's
fingerprint for a given snmpTargetAddrEntry. The values in
this row should be ignored if the connection that needs to be
established, as indicated by the SNMP-TARGET-MIB
infrastructure, is not a (D)TLS based connection. If an
snmpTlstmAddrEntry exists for a given snmpTargetAddrEntry then
the presented server certificate MUST match or the connection
MUST NOT be established. If a row in this table does not
exist to match a snmpTargetAddrEntry row then the connection
SHOULD still proceed if some other certificate validation path
algorithm (e.g. RFC5280) can be used."
INDEX { IMPLIED snmpTargetAddrName }
::= { snmpTlstmAddrTable 1 }
SnmpTlstmAddrEntry ::= SEQUENCE {
snmpTlstmAddrServerFingerprint SnmpTLSFingerprint,
snmpTlstmAddrServerIdentity SnmpAdminString,
snmpTlstmAddrStorageType StorageType,
snmpTlstmAddrRowStatus RowStatus
}
snmpTlstmAddrServerFingerprint OBJECT-TYPE
SYNTAX SnmpTLSFingerprint
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"A cryptographic hash of a public X.509 certificate. This
object should store the hash of the public X.509 certificate
that the remote server should present during the (D)TLS
connection setup. The fingerprint of the presented
certificate and this hash value MUST match exactly or the
connection MUST NOT be established."
DEFVAL { "" }
::= { snmpTlstmAddrEntry 1 }
snmpTlstmAddrServerIdentity OBJECT-TYPE
SYNTAX SnmpAdminString
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The reference identity to check against the identity
presented by the remote system."
DEFVAL { "" }
::= { snmpTlstmAddrEntry 2 }
snmpTlstmAddrStorageType OBJECT-TYPE
SYNTAX StorageType
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The storage type for this conceptual row. Conceptual rows
having the value 'permanent' need not allow write-access to
any columnar objects in the row."
DEFVAL { nonVolatile }
::= { snmpTlstmAddrEntry 3 }
snmpTlstmAddrRowStatus OBJECT-TYPE
SYNTAX RowStatus
MAX-ACCESS read-create
STATUS current
DESCRIPTION
"The status of this conceptual row. This object may be used
to create or remove rows from this table.
To create a row in this table, an administrator must set this
object to either createAndGo(4) or createAndWait(5).
Until instances of all corresponding columns are
appropriately configured, the value of the
corresponding instance of the snmpTlstmAddrRowStatus
column is notReady(3).
In particular, a newly created row cannot be made active until
the corresponding snmpTlstmAddrServerFingerprint column has been
set.
Rows MUST NOT be active if the snmpTlstmAddrServerFingerprint
column is blank and the snmpTlstmAddrServerIdentity is set to
'*' since this would insecurely accept any presented
certificate.
The snmpTlstmAddrServerFingerprint object may not be modified
while the value of this object is active(1).
An attempt to set these objects while the value of
snmpTlstmAddrRowStatus is active(1) will result in
an inconsistentValue error."
::= { snmpTlstmAddrEntry 4 }
-- ************************************************
-- snmpTlstmNotifications - Notifications Information
-- ************************************************
snmpTlstmServerCertificateUnknown NOTIFICATION-TYPE
OBJECTS { snmpTlstmSessionUnknownServerCertificate }
STATUS current
DESCRIPTION
"Notification that the server certificate presented by a SNMP
over (D)TLS server was invalid because no configured
fingerprint or CA was acceptable to validate it. This may be
because there was no entry in the snmpTlstmAddrTable or
because no path could be found to known certificate
authority.
To avoid notification loops, this notification MUST NOT be
sent to servers that themselves have triggered the
notification."
::= { snmpTlstmNotifications 1 }
snmpTlstmServerInvalidCertificate NOTIFICATION-TYPE
OBJECTS { snmpTlstmAddrServerFingerprint,
snmpTlstmSessionInvalidServerCertificates}
STATUS current
DESCRIPTION
"Notification that the server certificate presented by an SNMP
over (D)TLS server could not be validated even if the
fingerprint or expected validation path was known. I.E., a
cryptographic validation error occurred during certificate
validation processing.
To avoid notification loops, this notification MUST NOT be
sent to servers that themselves have triggered the
notification."
::= { snmpTlstmNotifications 2 }
-- ************************************************
-- snmpTlstmCompliances - Conformance Information
-- ************************************************
snmpTlstmCompliances OBJECT IDENTIFIER ::= { snmpTlstmConformance 1 }
snmpTlstmGroups OBJECT IDENTIFIER ::= { snmpTlstmConformance 2 }
-- ************************************************
-- Compliance statements
-- ************************************************
snmpTlstmCompliance MODULE-COMPLIANCE
STATUS current
DESCRIPTION
"The compliance statement for SNMP engines that support the
SNMP-TLS-TM-MIB"
MODULE
MANDATORY-GROUPS { snmpTlstmStatsGroup,
snmpTlstmIncomingGroup,
snmpTlstmOutgoingGroup,
snmpTlstmNotificationGroup }
::= { snmpTlstmCompliances 1 }
-- ************************************************
-- Units of conformance
-- ************************************************
snmpTlstmStatsGroup OBJECT-GROUP
OBJECTS {
snmpTlstmSessionOpens,
snmpTlstmSessionClientCloses,
snmpTlstmSessionOpenErrors,
snmpTlstmSessionAccepts,
snmpTlstmSessionServerCloses,
snmpTlstmSessionNoSessions,
snmpTlstmSessionInvalidClientCertificates,
snmpTlstmSessionUnknownServerCertificate,
snmpTlstmSessionInvalidServerCertificates,
snmpTlstmSessionInvalidCaches
}
STATUS current
DESCRIPTION
"A collection of objects for maintaining
statistical information of an SNMP engine which
implements the SNMP TLS Transport Model."
::= { snmpTlstmGroups 1 }
snmpTlstmIncomingGroup OBJECT-GROUP
OBJECTS {
snmpTlstmCertToTSNCount,
snmpTlstmCertToTSNTableLastChanged,
snmpTlstmCertToTSNFingerprint,
snmpTlstmCertToTSNMapType,
snmpTlstmCertToTSNData,
snmpTlstmCertToTSNStorageType,
snmpTlstmCertToTSNRowStatus
}
STATUS current
DESCRIPTION
"A collection of objects for maintaining
incoming connection certificate mappings to
tmSecurityNames of an SNMP engine which implements the
SNMP TLS Transport Model."
::= { snmpTlstmGroups 2 }
snmpTlstmOutgoingGroup OBJECT-GROUP
OBJECTS {
snmpTlstmParamsCount,
snmpTlstmParamsTableLastChanged,
snmpTlstmParamsClientFingerprint,
snmpTlstmParamsStorageType,
snmpTlstmParamsRowStatus,
snmpTlstmAddrCount,
snmpTlstmAddrTableLastChanged,
snmpTlstmAddrServerFingerprint,
snmpTlstmAddrServerIdentity,
snmpTlstmAddrStorageType,
snmpTlstmAddrRowStatus
}
STATUS current
DESCRIPTION
"A collection of objects for maintaining
outgoing connection certificates to use when opening
connections as a result of SNMP-TARGET-MIB settings."
::= { snmpTlstmGroups 3 }
snmpTlstmNotificationGroup NOTIFICATION-GROUP
NOTIFICATIONS {
snmpTlstmServerCertificateUnknown,
snmpTlstmServerInvalidCertificate
}
STATUS current
DESCRIPTION
"Notifications"
::= { snmpTlstmGroups 4 }
END
| TOC |
8. Operational Considerations
This section discusses various operational aspects of deploying TLSTM.
| TOC |
8.1. Sessions
A session is discussed throughout this document as meaning a security association between two TLSTM instances. State information for the sessions are maintained in each TLSTM implementation and this information is created and destroyed as sessions are opened and closed. A "broken" session (one side up and one side down) can result if one side of a session is brought down abruptly (i.e., reboot, power outage, etc.). Whenever possible, implementations SHOULD provide graceful session termination through the use of TLS disconnect messages. Implementations SHOULD also have a system in place for detecting "broken" sessions through the use of heartbeats [I‑D.seggelmann‑tls‑dtls‑heartbeat] (Seggelmann, R., Tuexen, M., and M. Williams, “Transport Layer Security and Datagram Transport Layer Security Heartbeat Extension,” .) or other detection mechanisms.
Implementations SHOULD limit the lifetime of established sessions depending on the algorithms used for generation of the master session secret, the privacy and integrity algorithms used to protect messages, the environment of the session, the amount of data transferred, and the sensitivity of the data.
| TOC |
8.2. Notification Receiver Credential Selection
When an SNMP engine needs to establish an outgoing session for notifications, the snmpTargetParamsTable includes an entry for the snmpTargetParamsSecurityName of the target. Servers that wish to support multiple principals at a particular port SHOULD make use of the Server Name Indication extension defined in Section 3.1 of [RFC4366] (Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and T. Wright, “Transport Layer Security (TLS) Extensions,” April 2006.). Without the Server Name Indication the receiving SNMP engine (Server) will not know which (D)TLS certificate to offer to the Client so that the tmSecurityName identity-authentication will be successful.
Another solution is to maintain a one-to-one mapping between certificates and incoming ports for notification receivers. This can be handled at the notification originator by configuring the snmpTargetAddrTable (snmpTargetAddrTDomain and snmpTargetAddrTAddress) and requiring the receiving SNMP engine to monitor multiple incoming static ports based on which principals are capable of receiving notifications.
Implementations MAY also choose to designate a single Notification Receiver Principal to receive all incoming notifications or select an implementation specific method of selecting a server certificate to present to clients.
| TOC |
8.3. contextEngineID Discovery
SNMPv3 requires that an application know the identifier (snmpEngineID) of the remote SNMP protocol engine in order to retrieve or manipulate objects maintained on the remote SNMP entity.
[RFC5343] (Schoenwaelder, J., “Simple Network Management Protocol (SNMP) Context EngineID Discovery,” September 2008.) introduces a well-known localEngineID and a discovery mechanism that can be used to learn the snmpEngineID of a remote SNMP protocol engine. Implementations are RECOMMENDED to support and use the contextEngineID discovery mechanism defined in [RFC5343] (Schoenwaelder, J., “Simple Network Management Protocol (SNMP) Context EngineID Discovery,” September 2008.).
| TOC |
8.4. Transport Considerations
This document defines how SNMP messages can be transmitted over the TLS and DTLS based protocols. Each of these protocols are additionally based on other transports (TCP and UDP). These two base protocols also have operational considerations that must be taken into consideration when selecting a (D)TLS based protocol to use such as its performance in degraded or limited networks. It is beyond the scope of this document to summarize the characteristics of these transport mechanisms. Please refer to the base protocol documents for details on messaging considerations with respect to MTU size, fragmentation, performance in lossy-networks, etc.
| TOC |
9. Security Considerations
This document describes a transport model that permits SNMP to utilize (D)TLS security services. The security threats and how the (D)TLS transport model mitigates these threats are covered in detail throughout this document. Security considerations for DTLS are covered in [RFC4347] (Rescorla, E. and N. Modadugu, “Datagram Transport Layer Security,” April 2006.) and security considerations for TLS are described in Section 11 and Appendices D, E, and F of TLS 1.2 [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.). When run over a connectionless transport such as UDP, DTLS is more vulnerable to denial of service attacks from spoofed IP addresses; see Section 4.2 for details how the cookie exchange is used to address this issue.
| TOC |
9.1. Certificates, Authentication, and Authorization
Implementations are responsible for providing a security certificate installation and configuration mechanism. Implementations SHOULD support certificate revocation lists.
(D)TLS provides for authentication of the identity of both the (D)TLS server and the (D)TLS client. Access to MIB objects for the authenticated principal MUST be enforced by an access control subsystem (e.g. the VACM).
Authentication of the command generator principal's identity is important for use with the SNMP access control subsystem to ensure that only authorized principals have access to potentially sensitive data. The authenticated identity of the command generator principal's certificate is mapped to an SNMP model-independent securityName for use with SNMP access control.
The (D)TLS handshake only provides assurance that the certificate of the authenticated identity has been signed by an configured accepted certification authority. (D)TLS has no way to further authorize or reject access based on the authenticated identity. An Access Control Model (such as the VACM) provides access control and authorization of a command generator's requests to a command responder and a notification receiver's authorization to receive Notifications from a notification originator. However to avoid man-in-the-middle attacks both ends of the (D)TLS based connection MUST check the certificate presented by the other side against what was expected. For example, command generators must check that the command responder presented and authenticated itself with a X.509 certificate that was expected. Not doing so would allow an impostor, at a minimum, to present false data, receive sensitive information and/or provide a false belief that configuration was actually received and acted upon. Authenticating and verifying the identity of the (D)TLS server and the (D)TLS client for all operations ensures the authenticity of the SNMP engine that provides MIB data.
The instructions found in the DESCRIPTION clause of the snmpTlstmCertToTSNTable object must be followed exactly. It is also important that the rows of the table be searched in prioritized order starting with the row containing the lowest numbered snmpTlstmCertToTSNID value.
| TOC |
9.2. (D)TLS Security Considerations
This section discusses security considerations specific to the usage of (D)TLS.
| TOC |
9.2.1. TLS Version Requirements
Implementations of TLS typically support multiple versions of the Transport Layer Security protocol as well as the older Secure Sockets Layer (SSL) protocol. Because of known security vulnerabilities, TLSTM clients and servers MUST NOT request, offer, or use SSL 2.0. See Appendix E.2 of [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.) for further details.
| TOC |
9.2.2. Perfect Forward Secrecy
The use of Perfect Forward Secrecy is RECOMMENDED and can be provided by (D)TLS with appropriately selected cipher suites, as discussed in Appendix F of [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.).
| TOC |
9.3. Use with SNMPv1/SNMPv2c Messages
The SNMPv1 and SNMPv2c message processing described in [RFC3584] (Frye, R., Levi, D., Routhier, S., and B. Wijnen, “Coexistence between Version 1, Version 2, and Version 3 of the Internet-standard Network Management Framework,” August 2003.) (BCP 74) always selects the SNMPv1 or SNMPv2c Security Models, respectively. Both of these and the User-based Security Model typically used with SNMPv3 derive the securityName and securityLevel from the SNMP message received, even when the message was received over a secure transport. Access control decisions are therefore made based on the contents of the SNMP message, rather than using the authenticated identity and securityLevel provided by the TLS Transport Model. It is RECOMMENDED that only SNMPv3 messages using the Transport Security Model (TSM) or another secure-transport aware security model be sent over the TLSTM transport.
Using a non-transport-aware Security Model with a secure Transport Model is NOT RECOMMENDED. See [RFC5590] (Harrington, D. and J. Schoenwaelder, “Transport Subsystem for the Simple Network Management Protocol (SNMP),” June 2009.) Section 7.1 for additional details on the coexistence of security-aware transports and non-transport-aware security models.
| TOC |
9.4. MIB Module Security
There are a number of management objects defined in this MIB module with a MAX-ACCESS clause of read-write and/or read-create. Such objects may be considered sensitive or vulnerable in some network environments. The support for SET operations in a non-secure environment without proper protection can have a negative effect on network operations. These are the tables and objects and their sensitivity/vulnerability:
- The snmpTlstmParamsTable can be used to change the outgoing X.509 certificate used to establish a (D)TLS connection. Modification to objects in this table need to be adequately authenticated since modification to values in this table will have profound impacts to the security of outbound connections from the device. Since knowledge of authorization rules and certificate usage mechanisms may be considered sensitive, protection from disclosure of the SNMP traffic via encryption is also highly recommended.
- The snmpTlstmAddrTable can be used to change the expectations of the certificates presented by a remote (D)TLS server. Modification to objects in this table need to be adequately authenticated since modification to values in this table will have profound impacts to the security of outbound connections from the device. Since knowledge of authorization rules and certificate usage mechanisms may be considered sensitive, protection from disclosure of the SNMP traffic via encryption is also highly recommended.
- The snmpTlstmCertToTSNTable is used to specify the mapping of incoming X.509 certificates to tmSecurityNames which eventually get mapped to a SNMPv3 securityName. Modification to objects in this table need to be adequately authenticated since modification to values in this table will have profound impacts to the security of incoming connections to the device. Since knowledge of authorization rules and certificate usage mechanisms may be considered sensitive, protection from disclosure of the SNMP traffic via encryption is also highly recommended. When this table contains a significant number of rows it may affect the system performance when accepting new (D)TLS connections.
Some of the readable objects in this MIB module (i.e., objects with a MAX-ACCESS other than not-accessible) may be considered sensitive or vulnerable in some network environments. It is thus important to control even GET and/or NOTIFY access to these objects and possibly to even encrypt the values of these objects when sending them over the network via SNMP. These are the tables and objects and their sensitivity/vulnerability:
- This MIB contains a collection of counters that monitor the (D)TLS connections being established with a device. Since knowledge of connection and certificate usage mechanisms may be considered sensitive, protection from disclosure of the SNMP traffic via encryption is also highly recommended.
SNMP versions prior to SNMPv3 did not include adequate security. Even if the network itself is secure (for example by using IPsec), even then, there is no control as to who on the secure network is allowed to access and GET/SET (read/change/create/delete) the objects in this MIB module.
It is RECOMMENDED that implementers consider the security features as provided by the SNMPv3 framework (see [RFC3410] (Case, J., Mundy, R., Partain, D., and B. Stewart, “Introduction and Applicability Statements for Internet-Standard Management Framework,” December 2002.), section 8), including full support for the SNMPv3 cryptographic mechanisms (for authentication and privacy).
Further, deployment of SNMP versions prior to SNMPv3 is NOT RECOMMENDED. Instead, it is RECOMMENDED to deploy SNMPv3 and to enable cryptographic security. It is then a customer/operator responsibility to ensure that the SNMP entity giving access to an instance of this MIB module is properly configured to give access to the objects only to those principals (users) that have legitimate rights to indeed GET or SET (change/create/delete) them.
| TOC |
10. IANA Considerations
IANA is requested to assign:
-
Two TCP/UDP port numbers from the "Registered Ports" range
of the Port Numbers registry, with the following keywords
(where TBD1 and TBD2 correspond to the assigned port numbers):
These are the default ports for receipt of SNMP command messages (snmptls and snmpdtls) and SNMP notification messages (snmptls-trap and snmpdtls-trap) over a TLS Transport Model as defined in this document.
Keyword Decimal Description References ------- ------- ----------- ---------- snmptls TBD1/tcp SNMP-TLS [RFC-isms-dtls-tm] snmpdtls TBD1/udp SNMP-DTLS [RFC-isms-dtls-tm] snmptls-trap TBD2/tcp SNMP-Trap-TLS [RFC-isms-dtls-tm] snmpdtls-trap TBD2/udp SNMP-Trap-DTLS [RFC-isms-dtls-tm]
- An SMI number under snmpDomains for the snmpTLSTCPDomain object identifier,
- An SMI number under snmpDomains for the snmpDTLSUDPDomain object identifier,
- A SMI number under mib-2, for the MIB module in this document,
- "tls" as the corresponding prefix for the snmpTLSTCPDomain in the SNMP Transport Model registry,
- "dtls" as the corresponding prefix for the snmpDTLSUDPDomain in the SNMP Transport Model registry,
RFC Editor's note: this section should be replaced with appropriate descriptive assignment text after IANA assignments are made and prior to publication.
| TOC |
11. Acknowledgements
This document closely follows and copies the Secure Shell Transport Model for SNMP documented by David Harrington and Joseph Salowey in [RFC5592] (Harrington, D., Salowey, J., and W. Hardaker, “Secure Shell Transport Model for the Simple Network Management Protocol (SNMP),” June 2009.).
This document was reviewed by the following people who helped provide useful comments (in alphabetical order): Andy Donati, Pasi Eronen, David Harrington, Jeffrey Hutzelman, Alan Luchuk, Michael Peck, Tom Petch, Randy Presuhn, Ray Purvis, Peter Saint-Andre, Joseph Salowey, Jurgen Schonwalder, Dave Shield, Robert Story.
This work was supported in part by the United States Department of Defense. Large portions of this document are based on work by General Dynamics C4 Systems and the following individuals: Brian Baril, Kim Bryant, Dana Deluca, Dan Hanson, Tim Huemiller, John Holzhauer, Colin Hoogeboom, Dave Kornbau, Chris Knaian, Dan Knaul, Charles Limoges, Steve Moccaldi, Gerardo Orlando, and Brandon Yip.
| TOC |
12. References
| TOC |
12.1. Normative References
| TOC |
12.2. Informative References
| [RFC3410] | Case, J., Mundy, R., Partain, D., and B. Stewart, “Introduction and Applicability Statements for Internet-Standard Management Framework,” RFC 3410, December 2002 (TXT). |
| [RFC5343] | Schoenwaelder, J., “Simple Network Management Protocol (SNMP) Context EngineID Discovery,” RFC 5343, September 2008 (TXT). |
| [RFC5592] | Harrington, D., Salowey, J., and W. Hardaker, “Secure Shell Transport Model for the Simple Network Management Protocol (SNMP),” RFC 5592, June 2009 (TXT). |
| [I-D.seggelmann-tls-dtls-heartbeat] | Seggelmann, R., Tuexen, M., and M. Williams, “Transport Layer Security and Datagram Transport Layer Security Heartbeat Extension.” |
| TOC |
Appendix A. Target and Notification Configuration Example
The following sections describe example configuration for the SNMP-TLS-TM-MIB, the SNMP-TARGET-MIB, the NOTIFICATION-MIB and the SNMP-VIEW-BASED-ACM-MIB.
| TOC |
A.1. Configuring a Notification Originator
The following row adds the "Joe Cool" user to the "administrators" group:
vacmSecurityModel = 4 (TSM)
vacmSecurityName = "Joe Cool"
vacmGroupName = "administrators"
vacmSecurityToGroupStorageType = 3 (nonVolatile)
vacmSecurityToGroupStatus = 4 (createAndGo)
The following row configures the snmpTlstmAddrTable to use certificate path validation and to require the remote notification receiver to present a certificate for the "server.example.org" identity.
snmpTargetAddrName = "toNRAddr"
snmpTlstmAddrServerFingerprint = ""
snmpTlstmAddrServerIdentity = "server.example.org"
snmpTlstmAddrStorageType = 3 (nonVolatile)
snmpTlstmAddrRowStatus = 4 (createAndGo)
The following row configures the snmpTargetAddrTable to send notifications using TLS/TCP to the snmptls-trap port at 192.0.2.1:
snmpTargetAddrName = "toNRAddr"
snmpTargetAddrTDomain = snmpTLSTCPDomain
snmpTargetAddrTAddress = "192.0.2.1:XXXsnmptls-trap"
snmpTargetAddrTimeout = 1500
snmpTargetAddrRetryCount = 3
snmpTargetAddrTagList = "toNRTag"
snmpTargetAddrParams = "toNR" (MUST match above)
snmpTargetAddrStorageType = 3 (nonVolatile)
snmpTargetAddrColumnStatus = 4 (createAndGo)
RFC Editor's note: replace the string "XXXsnmptls-trap" above with the appropriately assigned "snmptls-trap" port.
The following row configures the snmpTargetParamsTable to send the notifications to "Joe Cool", using authPriv SNMPv3 notifications through the TransportSecurityModel [RFC5591]:
snmpTargetParamsName = toNR
snmpTargetParamsMPModel = SNMPv3
snmpTargetParamsSecurityModel = 4 (TransportSecurityModel)
snmpTargetParamsSecurityName = "Joe Cool"
snmpTargetParamsSecurityLevel = 3 (authPriv)
snmpTargetParamsStorageType = 3 (nonVolatile)
snmpTargetParamsRowStatus = 4 (createAndGo0
| TOC |
A.2. Configuring TLSTM to Utilize a Simple Derivation of tmSecurityName
The following row configures the snmpTlstmCertToTSNTable to map a validated client certificate, referenced by the client's public X.509 hash fingerprint, to a tmSecurityName using the subjectAltName component of the certificate.
snmpTlstmCertToTSNID = 1
(chosen by ordering preference)
snmpTlstmCertToTSNFingerprint = HASH (appropriate fingerprint)
snmpTlstmCertToTSNMapType = snmpTlstmCertSANAny
snmpTlstmCertToTSNData = "" (not used)
snmpTlstmCertToTSNStorageType = 3 (nonVolatile)
snmpTlstmCertToTSNRowStatus = 4 (createAndGo)
This type of configuration should only be used when the naming conventions of the (possibly multiple) certificate authorities are well understood, so two different principals cannot inadvertently be identified by the same derived tmSecurityName.
| TOC |
A.3. Configuring TLSTM to Utilize Table-Driven Certificate Mapping
The following row configures the snmpTlstmCertToTSNTable to map a validated client certificate, referenced by the client's public X.509 hash fingerprint, to the directly specified tmSecurityName of "Joe Cool".
snmpTlstmCertToTSNID = 1
(chosen by ordering preference)
snmpTlstmCertToTSNFingerprint = HASH (appropriate fingerprint)
snmpTlstmCertToTSNMapType = snmpTlstmCertSpecified
snmpTlstmCertToTSNSecurityName = "Joe Cool"
snmpTlstmCertToTSNStorageType = 3 (nonVolatile)
snmpTlstmCertToTSNRowStatus = 4 (createAndGo)
| TOC |
Author's Address
| Wes Hardaker | |
| Sparta, Inc. | |
| P.O. Box 382 | |
| Davis, CA 95617 | |
| USA | |
| Phone: | +1 530 792 1913 |
| Email: | ietf@hardakers.net |
