RFC 5953
Transport Layer Security (TLS) Transport Model for the Simple Network Management Protocol (SNMP)
Internet Engineering Task Force (IETF) W. Hardaker Request for Comments: 5953 SPARTA, Inc. Category: Standards Track August 2010 ISSN: 2070-1721 Transport Layer Security (TLS) Transport Model for the Simple Network Management Protocol (SNMP)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 the 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 is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc5953.
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 ....................................................4 1.1. Conventions ................................................7 2. The Transport Layer Security Protocol ...........................8 3. How the TLSTM Fits into the Transport Subsystem .................8 3.1. Security Capabilities of this Model .......................10 3.1.1. Threats ............................................10 3.1.2. Message Protection .................................11 3.1.3. (D)TLS Connections .................................12 3.2. Security Parameter Passing ................................13 3.3. Notifications and Proxy ...................................13 4. Elements of the Model ..........................................14 4.1. X.509 Certificates ........................................14 4.1.1. Provisioning for the Certificate ...................14 4.2. (D)TLS Usage ..............................................16 4.3. SNMP Services .............................................17 4.3.1. SNMP Services for an Outgoing Message ..............17 4.3.2. SNMP Services for an Incoming Message ..............18 4.4. Cached Information and References .........................19 4.4.1. TLS Transport Model Cached Information .............19
4.4.1.1. tmSecurityName ............................19
4.4.1.2. tmSessionID ...............................20
4.4.1.3. Session State .............................20
5. Elements of Procedure ..........................................20
5.1. Procedures for an Incoming Message ........................20
5.1.1. DTLS over UDP Processing for Incoming Messages .....21
5.1.2. Transport Processing for Incoming SNMP Messages ....22
5.2. Procedures for an Outgoing SNMP Message ...................24
5.3. Establishing or Accepting a Session .......................25
5.3.1. Establishing a Session as a Client .................25
5.3.2. Accepting a Session as a Server ....................27
5.4. Closing a Session .........................................28
6. MIB Module Overview ............................................29
6.1. Structure of the MIB Module ...............................29
6.2. Textual Conventions .......................................29
6.3. Statistical Counters ......................................29
6.4. Configuration Tables ......................................29
6.4.1. Notifications ......................................30
6.5. Relationship to Other MIB Modules .........................30
6.5.1. MIB Modules Required for IMPORTS ...................30
7. MIB Module Definition ..........................................30
8. Operational Considerations .....................................53
8.1. Sessions ..................................................53
8.2. Notification Receiver Credential Selection ................54
8.3. contextEngineID Discovery .................................54
8.4. Transport Considerations ..................................55
9. Security Considerations ........................................55
9.1. Certificates, Authentication, and Authorization ...........55
9.2. (D)TLS Security Considerations ............................56
9.2.1. TLS Version Requirements ...........................56
9.2.2. Perfect Forward Secrecy ............................56
9.3. Use with SNMPv1/SNMPv2c Messages ..........................56
9.4. MIB Module Security .......................................57
10. IANA Considerations ...........................................58
11. Acknowledgements ..............................................59
12. References ....................................................60
12.1. Normative References .....................................60
12.2. Informative References ...................................61
Appendix A. Target and Notification Configuration Example ........63
A.1. Configuring a Notification Originator .....................63
A.2. Configuring TLSTM to Utilize a Simple Derivation of
tmSecurityName ............................................64
A.3. Configuring TLSTM to Utilize Table-Driven Certificate
Mapping ...................................................64
1. Introduction
It is important to understand the modular SNMPv3 architecture as defined by [RFC3411] and enhanced by the Transport Subsystem [RFC5590]. 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]. This document describes a Transport Model that makes use of the Transport Layer Security (TLS) [RFC5246] and the Datagram Transport Layer Security (DTLS) Protocol [RFC4347], within a Transport Subsystem [RFC5590]. DTLS is the datagram variant of the Transport Layer Security (TLS) protocol [RFC5246]. 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]. 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], [RFC2579], and [RFC2580].
The diagram shown below gives a conceptual overview of two SNMP entities communicating using the TLS Transport Model (shown as "TLSTM"). 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 that is defined in [RFC5591].
+---------------------------------------------------------------------+
| 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 |
+---------------------------------+ +---------------------------------+
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 a 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] 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] 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] is a mandatory-to- implement Security Model in STD 62. While (D)TLS and USM frequently refer to a user, the terminology preferred in RFC 3411 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].2. The Transport Layer Security Protocol
(D)TLS provides authentication, data message integrity, and privacy at the transport layer (see [RFC4347]). 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] and [RFC4347] for complete descriptions of the protocols.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] 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 RFC 3411 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 | +-------------------------------------------------------------------+3.1. Security Capabilities of this Model
3.1.1. Threats
The TLS Transport Model provides protection against the threats identified by the RFC 3411 architecture [RFC3411]: 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 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 the 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 that 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 Message Authentication
Code (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]).
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] 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 DoS attacks
because they are more vulnerable to spoofed IP addresses. See
Section 4.2 for details on how the cookie mechanism is used.
Note, however, that this mechanism does not provide any defense
against DoS attacks mounted from valid IP addresses.
See Section 9 for more detail on the security considerations
associated with the TLSTM and these security threats.
3.1.2. Message Protection
The RFC 3411 architecture recognizes three levels of security:
o without authentication and without privacy (noAuthNoPriv)
o with authentication but without privacy (authNoPriv)
o 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.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. 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 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).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 addresses, and ports) 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 discuss these concepts in much greater detail.3.3. Notifications and Proxy
(D)TLS connections may be initiated by (D)TLS clients on behalf of SNMP applications 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] 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.4. Elements of the Model
This section contains definitions required to realize the (D)TLS Transport Model defined by this document.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.4.1.1. Provisioning for the Certificate
Authentication using (D)TLS will require that SNMP entities have certificates, either signed by trusted Certification Authorities (CAs), or self signed. Furthermore, SNMP entities will most commonly need to be provisioned with root certificates that represent the list of trusted CAs 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:
o Mapping a certificate's subjectAltName or CommonName components to
a tmSecurityName, or
o 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.
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], 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 View-Based Access Control Model (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 |
+-------------+
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].
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 they 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] 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.
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.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.
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.
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 (SNMP)" [RFC5590] defines general requirements for caches and references.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 for detailed processing instructions on the use of the tmStateReference fields by the TLS Transport Model.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. 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 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 security model through the tmStateReference. This tmSecurityName is typically a copy of or is derived from the securityName that was passed by 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.
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).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.
(page 20 continued on part 2)
